JET 11 ENERGO-ECONOMICS PAYBACK INVESTMENT CALCULATION MODELLING OF GAS-STEAM COMBINED CYCLE POWER PLANT ENERGETSKO-EKONOMSKO MODEL VRAČILNE DOBE PLINSKO-PARNE ELEKTRARNE S KOMBINIRANIM PROCESOM Dušan Strušnik 1ℜ , Jurij Avsec 2 Keywords: calculation, efficiency, investment, market, modelling, payback Abstract The paper deals with energo-economic payback calculation modelling of the combined gas- steam cycle operation, demonstrating the basic characteristic properties of cycle behaviour in different operating regimes and calculating the payback period of the investment. The calcula- tion of the payback period of the investment is based on the calculation of the net present value and with actual obtaining data sets from a recently built gas-steam combined cycle power plant. The results of the calculation modelling show that the gas-steam combined cycle power plant can achieve a useful efficiency of up to 88% in the back-pressure operation of the steam turbine. The useful efficiency of the gas turbine is up to 40%. The payback period of the investment de- pends on the investment costs, the quantity and market price of the consumed fuel, the quantity JET Volume 15 (2022) p.p. 11-28 Issue 4, 2022 Type of article: 1.01 www.fe.um.si/si/jet.html ℜ Corresponding author: Doc. Dr. Dušan Strušnik, Energetika Ljubljana d.o.o., TE-TOL Unit, Toplarniška 19, 1000 Ljubljana, E-mail: dusan.strusnik@gmail.com 1 Energetika Ljubljana d.o.o., TE-TOL Unit, Toplarniška 19, SI-1000 Ljubljana, Slovenija 2 University of Maribor, Faculty of Energy Technology, Hočevarjev trg 1, SI-8270 Krško, Slovenia Energo-economics payback investment calculation modelling of recently built gas-steam combined cycle power plant Energetsko-ekonomsko model vračilne dobe plinsko-parne elektrarne s kombiniranim procesom Dušan Strušnik, Jurij Avsec 12 JET 12 JET Dušan Strušnik, Jurij Avsec JET Vol. 15 (2022) Issue 4 and market price of the generated electricity and thermal energy. The results show that with a price ratio fuel/electricity of 0.36, the payback period of the investment is 4 years, with a price ratio fuel/electricity of 0.54, the payback period of the investment is as much as 17 years. Povzetek Prispevek obravnava energetsko-ekonomski model izračuna vračila delovanja kombiniranega plinsko-parnega cikla s prikazom osnovnih značilnih lastnosti obnašanja cikla v različnih režimih obratovanja in izračunom vračilne dobe investicije. Izračun vračilne dobe investicije temelji na izračunu neto sedanje vrednosti in ob dejanskem pridobivanju nizov podatkov iz nedavno zgra- jene plinsko-parne kombinirane elektrarne. Rezultati računskega modeliranja kažejo, da lahko plinsko-parna elektrarna pri protitlačnem obratovanju parne turbine doseže koristni izkoristek do 88 %. Koristni izkoristek plinske turbine je do 40 %. Vračilna doba investicije je odvisna od strošk- ov investicije, količine in tržne cene porabljenega goriva ter količine in tržne cene proizvedene električne in toplotne energije. Rezultati kažejo, da je pri cenovnem razmerju gorivo/elektrika 0,36 vračilna doba investicije 4 leta, pri cenovnem razmerju gorivo/elektrika 0,54 pa kar 17 let. 1 INTRODUCTION The construction of new thermal energy systems enables the conversion of internal fuel energy into electricity and thermal energy in a more environmentally friendly way. In energy conversion, ecological awareness in thermal power plants is mainly reflected in the appropriate choice of fuel. To this end, more environmentally friendly processes are increasingly being used in practi- cal applications, which enables the combined conversion of electricity and thermal energy using natural gas. [1] Such a cycle is called a gas-steam combined cycle power plant (GSCCP). GSCCP consists of a gas turbine (GT), a heat recovery steam generator (HRSG), a steam turbine (ST) and a thermal station for district heating (DH). [2] Some other authors also researched the environ- mental and ecological influences of using of different types of fuels for heat and power gener- ated by GSCCP. Luis et al. presented the energy-ecologic efficiency of waste-to-energy plants and carried out the influence of emission abatement and biogenic carbon offset due to biomass regrowth regarding waste-fired plants. [3] Skorek-Osikowska et al. analysed thermodynamic and ecological assessment of selected coal-fired power plants integrated with carbon dioxide capture where they discovered that the post-combustion system allowed for a reduction of the value of the average annual carbon dioxide (CO 2 ) emission rate aggravating the unit of net electricity produced for 735 kg CO 2 /MWh [4]. Silveira et al. studied the ecological efficiency and thermoeco- nomic analysis of a cogeneration system at a hospital. [5] In reviewing the scientific literature, we have not yet found a paper analysing the energo-economics payback investment model of GSCCP. The GT consists of a compressor part, combustion chambers, a turbine part and a generator of GT. The compressor part of the GT is used to compress the air, which then enters the combustion chambers. Combustion chambers are used for the combustion of natural gas or for the chemical process of converting the internal energy of natural gas into thermal energy. [6] Thermal energy is used to increase the enthalpy value of compressed gas. After the combustion process, com- pressed gases with increased enthalpy value or flue gases enter the turbine part of GT. In the turbine part of the GT, the thermal energy of the flue gases is converted into mechanical energy, which is converted into electrical energy by means of the GT generator. The flue gases are dis- charged from the turbine part of the GT to HRSG GT at a temperature of approx. 560 °C. [7] The JET 13 Energo-economics payback investment calculation modelling of recently built gas-steam combined cycle power plant main purpose of HRSG is to use the residual heat energy of flue gases for the production of high pressure (HP) steam, the production of low pressure (LP) steam, and for the production of heat for district heating (DH). The remaining unused flue gas heat is discharged from the HRSG to the surroundings via a chimney at a temperature of approx. 75 °C. [8] At a pressure of approx. 95 bar and a temperature of approx. 520 °C, HP steam is discharged from HRSG to a steam turbine (ST), where the thermal energy of HP steam is converted into mechan- ical energy, which is converted by means of generator ST into electricity. LP steam is discharged from HRSG to industrial consumers at a pressure of approx. 9 bar and a temperature of approx. 260 °C. The amount of DH thermal energy from the HRSG depends on the flue gas temperature in the chimney, as the HRSG DH system maintains the flue gas temperature above the condensing flue gas temperature which is approx. 75 °C. ST plant consists of an expansion cylinder, a generator part and a DH system. A special feature of the ST plant is the backpressure mode of operation, as the ST plant does not have a condenser. This means that all the outlet steam from the expansion cylinder is used to generate DH heat. A schematic representation of the operation of the GSCCP is shown in Fig. 1. Figure 1: A schematic representation of the operation of the GSCCP . Other authors have also researched the GSCCP thermodynamics operation concepts. Maheshwari et al. studied thermodynamic different configurations of gas-steam combined cycles employing 14 JET 14 JET Dušan Strušnik, Jurij Avsec JET Vol. 15 (2022) Issue 4 intercooling and different means of cooling in the topping cycle. [9] With vapour absorption inlet air-cooling, Shukla et al. researched thermodynamic investigation of parameters affecting the execution of steam injected cooled gas turbine-based combined cycle power plant. [10] Kafaei et al. researched the best angle of hot steam injection holes in the steam turbine blade cascade. [11] Srinivas et al. carried out sensitivity analysis of steam-injected gas turbine-based combined cycle with dual pressure HRSG [12]. When reviewing the literature, we found no paper describ- ing the payback investment models of GSCCP with actually obtaining data sets from the recently built plant. The data sets are obtained from the supervisory control and data acquisition (SCADA). [13] SCA- DA continuously, 24 hours a day and 365 days a year, records the most important data sets of the GSCCP operation. [14] The innovation, originality, and contribution to the new knowledge; however, are expressed in the validated energo-economics payback investment calculation modelling of GSCCP with actual- ly obtained data from SCADA by the recently-built plant. The recently-built plant is located in the middle of Slovenia, which lies in southern central Europe, Fig. 2. Figure 2: The recently-built GSCCP in the middle of Slovenia. This paper first presents the operation of the system, before turning its attention to the presenta- tion of the energo-economics payback investment calculation model, actual data sets filtration and model validation. Following this, the results are presented. Finally, the concluding part pre- sents the most important findings and discussion. 2 ENERGO-ECONOMICS PAYBACK INVESTMENT CALCULATION MODEL The energo-economics payback investment calculation model consists of auxiliary units and cal- culation units. The auxiliary unit of input data contains the database of electric power of the GT generator (P GTe ), which represents a set of input data to the energo-economic calculation model. JET 15 Energo-economics payback investment calculation modelling of recently built gas-steam combined cycle power plant Using a set of input data, the GT calculation unit calculates the characteristic properties of GT operation, such as consumption and power of natural gas GT useful efficiency, etc. The HRSG cal- culation unit calculates the amount of generated HP steam, the amount of generated LP steam and the amount of generated heat for DH using the data obtained from the input unit and the data obtained from the GT calculation unit. The results of the GT calculation unit and the results of the HRSG calculation unit enter the ST calculation unit and the calculation unit of the payback period of the investment. All the results of all calculation units are finally combined in a results report monitoring unit. A schematic representation of the operation of the energo-economics payback investment calculation model is shown in Fig. 3. Figure 3: Schematic representation of the energo-economics payback investment calculation model. The energo-economic calculation model is made using actual data sets obtained from SCADA. In addition to the consumption of natural gas for GSCCP operation, it also calculates the total amount of generated electricity and thermal energy of GSCCP, GT useful efficiency, ST useful efficiency, total GSCCP useful efficiency, etc. Beyond the stated values, the mathematical model also calculates the energy flows generated in 5400 hours of GSCCP operation. The auxiliary ST calculation unit contains an artificial neural network (ANN), that calculates the ST exhaust steam thermodynamic properties in dependence on the ST entering steam quantity and quality. The ANN, feed-forward type, is aimed at identifying and modelling the complex nonlinear relation- ships between the input and the output target of a system. [15] The ANN approach is an evolu- tionary and fast calculation methodology that does not require complex mathematical equations to explain a non-linear and multi-dimension system. [16] The ANN that was used in the auxiliary ST calculation unit was selected using a validation process. However, the ANN architecture that gave the best results in the validation procedure was used in the auxiliary ST calculation unit. The calculation of the payback period of the investment is based on the calculation of the net present value of cash flows and depends on the net cash flow, something which in turn varies according to investment costs, maintenance costs, tax rate, quantity and market price of fuel consumed, quantity and market price of generated electricity and thermal energy, etc. 16 JET 16 JET Dušan Strušnik, Jurij Avsec JET Vol. 15 (2022) Issue 4 The GT calculation model calculates natural gas consumption for GSCCP operation using an equa- tion generated based on GT manufacturer data GSCCP operation using an equation generated based on GT manufacturer data [17]: 6 Dušan Strušnik, Jurij Avsec JET Vol. 15 (2022) Issue 4 ---------- relationships between the input and the output target of a system. [15] The ANN approach is an evolutionary and fast calculation methodology that does not require complex mathematical equations to explain a non-linear and multi-dimension system. [16] The ANN that was used in the auxiliary ST calculation unit was selected using a validation process. However, the ANN architecture that gave the best results in the validation procedure was used in the auxiliary ST calculation unit. The calculation of the payback period of the investment is based on the calculation of the net present value of cash flows and depends on the net cash flow, something which in turn varies according to investment costs, maintenance costs, tax rate, quantity and market price of fuel consumed, quantity and market price of generated electricity and thermal energy, etc. The GT calculation model calculates natural gas consumption for GSCCP operation using an equation generated based on GT manufacturer data GSCCP operation using an equation generated based on GT manufacturer data [17]: 𝑚𝑚 ̇ �� = �� �,����� ∙� ��� � �� .����∙� ��� � �� .���� ∙� ��� �� .��� �� � ��� � � ∙� ��� �� .��� �/0.752� ∙ 3600 (1) where 𝑚𝑚 ̇ �� is natural gas flow and 𝑃𝑃 ��� is the power of GT generator. Now that the natural gas flow for GSCCP operation is known, the GT calculation unit calculates the power of the natural gas consumed in two different ways. The power of natural gas, taking into account higher calorific value (HHV), is calculated by the GT calculation unit using the equation: [18] 𝑃𝑃 ��� = 𝑚𝑚 ̇ �� ∙ 𝐻𝐻𝐻𝐻𝐻𝐻 = 𝑚𝑚 ̇ �� ∙ 0.011348 (2) where 𝑃𝑃 ��� is the power of the natural gas taking into account HHV. 𝑃𝑃 ��� is used in the economic calculation of natural gas consumption. The GT calculation unit calculates the power of the natural gas taking into account lower calorific value (LHV) using the equation: [18] 𝑃𝑃 ��� = 𝑚𝑚 ̇ �� ∙ 𝐿𝐿 𝐻𝐻𝐻𝐻 = 𝑚𝑚 ̇ �� ∙ 0.01028 (3) where 𝑃𝑃 ��� is the power of the natural gas taking into account LHV. 𝑃𝑃 ��� is used in all other process calculations, for example to calculate process useful efficiency, etc. The amount of generated HP steam, LP steam and generated heat for DH is calculated by the HRSG calculation unit using the generated equations below. calculations, for example to calculate process useful efficiency, etc. The amount of generated HP steam, LP steam and generated heat for DH is calculated by the HRSG calculation unit using the generated equations below. The equations are generated based on the data of the HRSG manufacturer. [19] 𝑚𝑚 ̇ �� = 0.00007994 ∙ 𝑃𝑃 ��� � − 0.01236 ∙ 𝑃𝑃 ��� � + 0.7599 ∙ 𝑃𝑃 ��� + 0.003202 (4) 𝑚𝑚 ̇ �� = 0.00002231 ∙ 𝑃𝑃 ��� � − 0.002278) ∙ 𝑃𝑃 ��� � + 0.1225 ∙ 𝑃𝑃 ��� + 0.001664 (5) 𝑃𝑃 ������� = 0.00006536 ∙ 𝑃𝑃 ��� � − 0.00707 ∙ 𝑃𝑃 ��� � + 0.3642 ∙ 𝑃𝑃 ��� + 0.003177 (6) where 𝑚𝑚 ̇ �� is HP steam mass flow from HRSG, 𝑚𝑚 ̇ �� is LP steam mass flow from HRSG and 𝑃𝑃 ������� is generated DH heat from HRSG. Now that the amount of generated HP steam and LP steam is known, the ST calculation unit can also calculate the power generated by ST generator: [20] 𝑃𝑃 ��� = 𝑚𝑚 ̇ �� ∙ (ℎ �� −ℎ ��� ) ∙ 0.9 (7) (1) where ṁ NG is natural gas flow and P GTe is the power of GT generator. Now that the natural gas flow for GSCCP operation is known, the GT calculation unit calculates the power of the natural gas consumed in two different ways. The power of natural gas, taking into account higher calorific value (HHV), is calculated by the GT calculation unit using the equation: [18] 6 Dušan Strušnik, Jurij Avsec JET Vol. 15 (2022) Issue 4 ---------- relationships between the input and the output target of a system. [15] The ANN approach is an evolutionary and fast calculation methodology that does not require complex mathematical equations to explain a non-linear and multi-dimension system. [16] The ANN that was used in the auxiliary ST calculation unit was selected using a validation process. However, the ANN architecture that gave the best results in the validation procedure was used in the auxiliary ST calculation unit. The calculation of the payback period of the investment is based on the calculation of the net present value of cash flows and depends on the net cash flow, something which in turn varies according to investment costs, maintenance costs, tax rate, quantity and market price of fuel consumed, quantity and market price of generated electricity and thermal energy, etc. The GT calculation model calculates natural gas consumption for GSCCP operation using an equation generated based on GT manufacturer data GSCCP operation using an equation generated based on GT manufacturer data [17]: 𝑚𝑚 ̇ �� = �� �,����� ∙� ��� � �� .����∙� ��� � �� .���� ∙� ��� �� .��� �� � ��� � � ∙� ��� �� .��� �/0.752� ∙ 3600 (1) where 𝑚𝑚 ̇ �� is natural gas flow and 𝑃𝑃 ��� is the power of GT generator. Now that the natural gas flow for GSCCP operation is known, the GT calculation unit calculates the power of the natural gas consumed in two different ways. The power of natural gas, taking into account higher calorific value (HHV), is calculated by the GT calculation unit using the equation: [18] 𝑃𝑃 ��� = 𝑚𝑚 ̇ �� ∙ 𝐻𝐻𝐻𝐻𝐻𝐻 = 𝑚𝑚 ̇ �� ∙ 0.011348 (2) where 𝑃𝑃 ��� is the power of the natural gas taking into account HHV. 𝑃𝑃 ��� is used in the economic calculation of natural gas consumption. The GT calculation unit calculates the power of the natural gas taking into account lower calorific value (LHV) using the equation: [18] 𝑃𝑃 ��� = 𝑚𝑚 ̇ �� ∙ 𝐿𝐿 𝐻𝐻𝐻𝐻 = 𝑚𝑚 ̇ �� ∙ 0.01028 (3) where 𝑃𝑃 ��� is the power of the natural gas taking into account LHV. 𝑃𝑃 ��� is used in all other process calculations, for example to calculate process useful efficiency, etc. The amount of generated HP steam, LP steam and generated heat for DH is calculated by the HRSG calculation unit using the generated equations below. calculations, for example to calculate process useful efficiency, etc. The amount of generated HP steam, LP steam and generated heat for DH is calculated by the HRSG calculation unit using the generated equations below. The equations are generated based on the data of the HRSG manufacturer. [19] 𝑚𝑚 ̇ �� = 0.00007994 ∙ 𝑃𝑃 ��� � − 0.01236 ∙ 𝑃𝑃 ��� � + 0.7599 ∙ 𝑃𝑃 ��� + 0.003202 (4) 𝑚𝑚 ̇ �� = 0.00002231 ∙ 𝑃𝑃 ��� � − 0.002278) ∙ 𝑃𝑃 ��� � + 0.1225 ∙ 𝑃𝑃 ��� + 0.001664 (5) 𝑃𝑃 ������� = 0.00006536 ∙ 𝑃𝑃 ��� � − 0.00707 ∙ 𝑃𝑃 ��� � + 0.3642 ∙ 𝑃𝑃 ��� + 0.003177 (6) where 𝑚𝑚 ̇ �� is HP steam mass flow from HRSG, 𝑚𝑚 ̇ �� is LP steam mass flow from HRSG and 𝑃𝑃 ������� is generated DH heat from HRSG. Now that the amount of generated HP steam and LP steam is known, the ST calculation unit can also calculate the power generated by ST generator: [20] 𝑃𝑃 ��� = 𝑚𝑚 ̇ �� ∙ (ℎ �� −ℎ ��� ) ∙ 0.9 (7) (2) where P HHV is the power of the natural gas taking into account HHV. P HHV is used in the economic calculation of natural gas consumption. The GT calculation unit calculates the power of the natu- ral gas taking into account lower calorific value (LHV) using the equation: [18] 6 Dušan Strušnik, Jurij Avsec JET Vol. 15 (2022) Issue 4 ---------- relationships between the input and the output target of a system. [15] The ANN approach is an evolutionary and fast calculation methodology that does not require complex mathematical equations to explain a non-linear and multi-dimension system. [16] The ANN that was used in the auxiliary ST calculation unit was selected using a validation process. However, the ANN architecture that gave the best results in the validation procedure was used in the auxiliary ST calculation unit. The calculation of the payback period of the investment is based on the calculation of the net present value of cash flows and depends on the net cash flow, something which in turn varies according to investment costs, maintenance costs, tax rate, quantity and market price of fuel consumed, quantity and market price of generated electricity and thermal energy, etc. The GT calculation model calculates natural gas consumption for GSCCP operation using an equation generated based on GT manufacturer data GSCCP operation using an equation generated based on GT manufacturer data [17]: 𝑚𝑚 ̇ �� = �� �,����� ∙� ��� � �� .����∙� ��� � �� .���� ∙� ��� �� .��� �� � ��� � � ∙� ��� �� .��� �/0.752� ∙ 3600 (1) where 𝑚𝑚 ̇ �� is natural gas flow and 𝑃𝑃 ��� is the power of GT generator. Now that the natural gas flow for GSCCP operation is known, the GT calculation unit calculates the power of the natural gas consumed in two different ways. The power of natural gas, taking into account higher calorific value (HHV), is calculated by the GT calculation unit using the equation: [18] 𝑃𝑃 ��� = 𝑚𝑚 ̇ �� ∙ 𝐻𝐻𝐻𝐻𝐻𝐻 = 𝑚𝑚 ̇ �� ∙ 0.011348 (2) where 𝑃𝑃 ��� is the power of the natural gas taking into account HHV. 𝑃𝑃 ��� is used in the economic calculation of natural gas consumption. The GT calculation unit calculates the power of the natural gas taking into account lower calorific value (LHV) using the equation: [18] 𝑃𝑃 ��� = 𝑚𝑚 ̇ �� ∙ 𝐿𝐿 𝐻𝐻𝐻𝐻 = 𝑚𝑚 ̇ �� ∙ 0.01028 (3) where 𝑃𝑃 ��� is the power of the natural gas taking into account LHV. 𝑃𝑃 ��� is used in all other process calculations, for example to calculate process useful efficiency, etc. The amount of generated HP steam, LP steam and generated heat for DH is calculated by the HRSG calculation unit using the generated equations below. calculations, for example to calculate process useful efficiency, etc. The amount of generated HP steam, LP steam and generated heat for DH is calculated by the HRSG calculation unit using the generated equations below. The equations are generated based on the data of the HRSG manufacturer. [19] 𝑚𝑚 ̇ �� = 0.00007994 ∙ 𝑃𝑃 ��� � − 0.01236 ∙ 𝑃𝑃 ��� � + 0.7599 ∙ 𝑃𝑃 ��� + 0.003202 (4) 𝑚𝑚 ̇ �� = 0.00002231 ∙ 𝑃𝑃 ��� � − 0.002278) ∙ 𝑃𝑃 ��� � + 0.1225 ∙ 𝑃𝑃 ��� + 0.001664 (5) 𝑃𝑃 ������� = 0.00006536 ∙ 𝑃𝑃 ��� � − 0.00707 ∙ 𝑃𝑃 ��� � + 0.3642 ∙ 𝑃𝑃 ��� + 0.003177 (6) where 𝑚𝑚 ̇ �� is HP steam mass flow from HRSG, 𝑚𝑚 ̇ �� is LP steam mass flow from HRSG and 𝑃𝑃 ������� is generated DH heat from HRSG. Now that the amount of generated HP steam and LP steam is known, the ST calculation unit can also calculate the power generated by ST generator: [20] 𝑃𝑃 ��� = 𝑚𝑚 ̇ �� ∙ (ℎ �� −ℎ ��� ) ∙ 0.9 (7) (3) where P LHV is the power of the natural gas taking into account LHV. P LHV is used in all other process calculations, for example to calculate process useful efficiency, etc. The amount of generated HP steam, LP steam and generated heat for DH is calculated by the HRSG calculation unit using the generated equations below. calculations, for example to calculate process useful efficiency, etc. The amount of generated HP steam, LP steam and generated heat for DH is calculated by the HRSG calculation unit using the generated equations below. The equations are generated based on the data of the HRSG manufacturer. [19] 6 Dušan Strušnik, Jurij Avsec JET Vol. 15 (2022) Issue 4 ---------- relationships between the input and the output target of a system. [15] The ANN approach is an evolutionary and fast calculation methodology that does not require complex mathematical equations to explain a non-linear and multi-dimension system. [16] The ANN that was used in the auxiliary ST calculation unit was selected using a validation process. However, the ANN architecture that gave the best results in the validation procedure was used in the auxiliary ST calculation unit. The calculation of the payback period of the investment is based on the calculation of the net present value of cash flows and depends on the net cash flow, something which in turn varies according to investment costs, maintenance costs, tax rate, quantity and market price of fuel consumed, quantity and market price of generated electricity and thermal energy, etc. The GT calculation model calculates natural gas consumption for GSCCP operation using an equation generated based on GT manufacturer data GSCCP operation using an equation generated based on GT manufacturer data [17]: 𝑚𝑚 ̇ �� = �� �,����� ∙� ��� � �� .����∙� ��� � �� .���� ∙� ��� �� .��� �� � ��� � � ∙� ��� �� .��� �/0.752� ∙ 3600 (1) where 𝑚𝑚 ̇ �� is natural gas flow and 𝑃𝑃 ��� is the power of GT generator. Now that the natural gas flow for GSCCP operation is known, the GT calculation unit calculates the power of the natural gas consumed in two different ways. The power of natural gas, taking into account higher calorific value (HHV), is calculated by the GT calculation unit using the equation: [18] 𝑃𝑃 ��� = 𝑚𝑚 ̇ �� ∙ 𝐻𝐻𝐻𝐻𝐻𝐻 = 𝑚𝑚 ̇ �� ∙ 0.011348 (2) where 𝑃𝑃 ��� is the power of the natural gas taking into account HHV. 𝑃𝑃 ��� is used in the economic calculation of natural gas consumption. The GT calculation unit calculates the power of the natural gas taking into account lower calorific value (LHV) using the equation: [18] 𝑃𝑃 ��� = 𝑚𝑚 ̇ �� ∙ 𝐿𝐿 𝐻𝐻𝐻𝐻 = 𝑚𝑚 ̇ �� ∙ 0.01028 (3) where 𝑃𝑃 ��� is the power of the natural gas taking into account LHV. 𝑃𝑃 ��� is used in all other process calculations, for example to calculate process useful efficiency, etc. The amount of generated HP steam, LP steam and generated heat for DH is calculated by the HRSG calculation unit using the generated equations below. calculations, for example to calculate process useful efficiency, etc. The amount of generated HP steam, LP steam and generated heat for DH is calculated by the HRSG calculation unit using the generated equations below. The equations are generated based on the data of the HRSG manufacturer. [19] 𝑚𝑚 ̇ �� = 0.00007994 ∙ 𝑃𝑃 ��� � − 0.01236 ∙ 𝑃𝑃 ��� � + 0.7599 ∙ 𝑃𝑃 ��� + 0.003202 (4) 𝑚𝑚 ̇ �� = 0.00002231 ∙ 𝑃𝑃 ��� � − 0.002278) ∙ 𝑃𝑃 ��� � + 0.1225 ∙ 𝑃𝑃 ��� + 0.001664 (5) 𝑃𝑃 ������� = 0.00006536 ∙ 𝑃𝑃 ��� � − 0.00707 ∙ 𝑃𝑃 ��� � + 0.3642 ∙ 𝑃𝑃 ��� + 0.003177 (6) where 𝑚𝑚 ̇ �� is HP steam mass flow from HRSG, 𝑚𝑚 ̇ �� is LP steam mass flow from HRSG and 𝑃𝑃 ������� is generated DH heat from HRSG. Now that the amount of generated HP steam and LP steam is known, the ST calculation unit can also calculate the power generated by ST generator: [20] 𝑃𝑃 ��� = 𝑚𝑚 ̇ �� ∙ (ℎ �� −ℎ ��� ) ∙ 0.9 (7) (4) 6 Dušan Strušnik, Jurij Avsec JET Vol. 15 (2022) Issue 4 ---------- relationships between the input and the output target of a system. [15] The ANN approach is an evolutionary and fast calculation methodology that does not require complex mathematical equations to explain a non-linear and multi-dimension system. [16] The ANN that was used in the auxiliary ST calculation unit was selected using a validation process. However, the ANN architecture that gave the best results in the validation procedure was used in the auxiliary ST calculation unit. The calculation of the payback period of the investment is based on the calculation of the net present value of cash flows and depends on the net cash flow, something which in turn varies according to investment costs, maintenance costs, tax rate, quantity and market price of fuel consumed, quantity and market price of generated electricity and thermal energy, etc. The GT calculation model calculates natural gas consumption for GSCCP operation using an equation generated based on GT manufacturer data GSCCP operation using an equation generated based on GT manufacturer data [17]: 𝑚𝑚 ̇ �� = �� �,����� ∙� ��� � �� .����∙� ��� � �� .���� ∙� ��� �� .��� �� � ��� � � ∙� ��� �� .��� �/0.752� ∙ 3600 (1) where 𝑚𝑚 ̇ �� is natural gas flow and 𝑃𝑃 ��� is the power of GT generator. Now that the natural gas flow for GSCCP operation is known, the GT calculation unit calculates the power of the natural gas consumed in two different ways. The power of natural gas, taking into account higher calorific value (HHV), is calculated by the GT calculation unit using the equation: [18] 𝑃𝑃 ��� = 𝑚𝑚 ̇ �� ∙ 𝐻𝐻𝐻𝐻𝐻𝐻 = 𝑚𝑚 ̇ �� ∙ 0.011348 (2) where 𝑃𝑃 ��� is the power of the natural gas taking into account HHV. 𝑃𝑃 ��� is used in the economic calculation of natural gas consumption. The GT calculation unit calculates the power of the natural gas taking into account lower calorific value (LHV) using the equation: [18] 𝑃𝑃 ��� = 𝑚𝑚 ̇ �� ∙ 𝐿𝐿 𝐻𝐻𝐻𝐻 = 𝑚𝑚 ̇ �� ∙ 0.01028 (3) where 𝑃𝑃 ��� is the power of the natural gas taking into account LHV. 𝑃𝑃 ��� is used in all other process calculations, for example to calculate process useful efficiency, etc. The amount of generated HP steam, LP steam and generated heat for DH is calculated by the HRSG calculation unit using the generated equations below. calculations, for example to calculate process useful efficiency, etc. The amount of generated HP steam, LP steam and generated heat for DH is calculated by the HRSG calculation unit using the generated equations below. The equations are generated based on the data of the HRSG manufacturer. [19] 𝑚𝑚 ̇ �� = 0.00007994 ∙ 𝑃𝑃 ��� � − 0.01236 ∙ 𝑃𝑃 ��� � + 0.7599 ∙ 𝑃𝑃 ��� + 0.003202 (4) 𝑚𝑚 ̇ �� = 0.00002231 ∙ 𝑃𝑃 ��� � − 0.002278) ∙ 𝑃𝑃 ��� � + 0.1225 ∙ 𝑃𝑃 ��� + 0.001664 (5) 𝑃𝑃 ������� = 0.00006536 ∙ 𝑃𝑃 ��� � − 0.00707 ∙ 𝑃𝑃 ��� � + 0.3642 ∙ 𝑃𝑃 ��� + 0.003177 (6) where 𝑚𝑚 ̇ �� is HP steam mass flow from HRSG, 𝑚𝑚 ̇ �� is LP steam mass flow from HRSG and 𝑃𝑃 ������� is generated DH heat from HRSG. Now that the amount of generated HP steam and LP steam is known, the ST calculation unit can also calculate the power generated by ST generator: [20] 𝑃𝑃 ��� = 𝑚𝑚 ̇ �� ∙ (ℎ �� −ℎ ��� ) ∙ 0.9 (7) (5) 6 Dušan Strušnik, Jurij Avsec JET Vol. 15 (2022) Issue 4 ---------- relationships between the input and the output target of a system. [15] The ANN approach is an evolutionary and fast calculation methodology that does not require complex mathematical equations to explain a non-linear and multi-dimension system. [16] The ANN that was used in the auxiliary ST calculation unit was selected using a validation process. However, the ANN architecture that gave the best results in the validation procedure was used in the auxiliary ST calculation unit. The calculation of the payback period of the investment is based on the calculation of the net present value of cash flows and depends on the net cash flow, something which in turn varies according to investment costs, maintenance costs, tax rate, quantity and market price of fuel consumed, quantity and market price of generated electricity and thermal energy, etc. The GT calculation model calculates natural gas consumption for GSCCP operation using an equation generated based on GT manufacturer data GSCCP operation using an equation generated based on GT manufacturer data [17]: 𝑚𝑚 ̇ �� = �� �,����� ∙� ��� � �� .����∙� ��� � �� .���� ∙� ��� �� .��� �� � ��� � � ∙� ��� �� .��� �/0.752� ∙ 3600 (1) where 𝑚𝑚 ̇ �� is natural gas flow and 𝑃𝑃 ��� is the power of GT generator. Now that the natural gas flow for GSCCP operation is known, the GT calculation unit calculates the power of the natural gas consumed in two different ways. The power of natural gas, taking into account higher calorific value (HHV), is calculated by the GT calculation unit using the equation: [18] 𝑃𝑃 ��� = 𝑚𝑚 ̇ �� ∙ 𝐻𝐻𝐻𝐻𝐻𝐻 = 𝑚𝑚 ̇ �� ∙ 0.011348 (2) where 𝑃𝑃 ��� is the power of the natural gas taking into account HHV. 𝑃𝑃 ��� is used in the economic calculation of natural gas consumption. The GT calculation unit calculates the power of the natural gas taking into account lower calorific value (LHV) using the equation: [18] 𝑃𝑃 ��� = 𝑚𝑚 ̇ �� ∙ 𝐿𝐿 𝐻𝐻𝐻𝐻 = 𝑚𝑚 ̇ �� ∙ 0.01028 (3) where 𝑃𝑃 ��� is the power of the natural gas taking into account LHV. 𝑃𝑃 ��� is used in all other process calculations, for example to calculate process useful efficiency, etc. The amount of generated HP steam, LP steam and generated heat for DH is calculated by the HRSG calculation unit using the generated equations below. calculations, for example to calculate process useful efficiency, etc. The amount of generated HP steam, LP steam and generated heat for DH is calculated by the HRSG calculation unit using the generated equations below. The equations are generated based on the data of the HRSG manufacturer. [19] 𝑚𝑚 ̇ �� = 0.00007994 ∙ 𝑃𝑃 ��� � − 0.01236 ∙ 𝑃𝑃 ��� � + 0.7599 ∙ 𝑃𝑃 ��� + 0.003202 (4) 𝑚𝑚 ̇ �� = 0.00002231 ∙ 𝑃𝑃 ��� � − 0.002278) ∙ 𝑃𝑃 ��� � + 0.1225 ∙ 𝑃𝑃 ��� + 0.001664 (5) 𝑃𝑃 ������� = 0.00006536 ∙ 𝑃𝑃 ��� � − 0.00707 ∙ 𝑃𝑃 ��� � + 0.3642 ∙ 𝑃𝑃 ��� + 0.003177 (6) where 𝑚𝑚 ̇ �� is HP steam mass flow from HRSG, 𝑚𝑚 ̇ �� is LP steam mass flow from HRSG and 𝑃𝑃 ������� is generated DH heat from HRSG. Now that the amount of generated HP steam and LP steam is known, the ST calculation unit can also calculate the power generated by ST generator: [20] 𝑃𝑃 ��� = 𝑚𝑚 ̇ �� ∙ (ℎ �� −ℎ ��� ) ∙ 0.9 (7) (6) where ṁ HP is HP steam mass flow from HRSG, ṁ LP is LP steam mass flow from HRSG and P DH-HRSG is generated DH heat from HRSG. Now that the amount of generated HP steam and LP steam is known, the ST calculation unit can also calculate the power generated by ST generator: [20] 6 Dušan Strušnik, Jurij Avsec JET Vol. 15 (2022) Issue 4 ---------- relationships between the input and the output target of a system. [15] The ANN approach is an evolutionary and fast calculation methodology that does not require complex mathematical equations to explain a non-linear and multi-dimension system. [16] The ANN that was used in the auxiliary ST calculation unit was selected using a validation process. However, the ANN architecture that gave the best results in the validation procedure was used in the auxiliary ST calculation unit. The calculation of the payback period of the investment is based on the calculation of the net present value of cash flows and depends on the net cash flow, something which in turn varies according to investment costs, maintenance costs, tax rate, quantity and market price of fuel consumed, quantity and market price of generated electricity and thermal energy, etc. The GT calculation model calculates natural gas consumption for GSCCP operation using an equation generated based on GT manufacturer data GSCCP operation using an equation generated based on GT manufacturer data [17]: 𝑚𝑚 ̇ �� = �� �,����� ∙� ��� � �� .����∙� ��� � �� .���� ∙� ��� �� .��� �� � ��� � � ∙� ��� �� .��� �/0.752� ∙ 3600 (1) where 𝑚𝑚 ̇ �� is natural gas flow and 𝑃𝑃 ��� is the power of GT generator. Now that the natural gas flow for GSCCP operation is known, the GT calculation unit calculates the power of the natural gas consumed in two different ways. The power of natural gas, taking into account higher calorific value (HHV), is calculated by the GT calculation unit using the equation: [18] 𝑃𝑃 ��� = 𝑚𝑚 ̇ �� ∙ 𝐻𝐻𝐻𝐻𝐻𝐻 = 𝑚𝑚 ̇ �� ∙ 0.011348 (2) where 𝑃𝑃 ��� is the power of the natural gas taking into account HHV. 𝑃𝑃 ��� is used in the economic calculation of natural gas consumption. The GT calculation unit calculates the power of the natural gas taking into account lower calorific value (LHV) using the equation: [18] 𝑃𝑃 ��� = 𝑚𝑚 ̇ �� ∙ 𝐿𝐿 𝐻𝐻𝐻𝐻 = 𝑚𝑚 ̇ �� ∙ 0.01028 (3) where 𝑃𝑃 ��� is the power of the natural gas taking into account LHV. 𝑃𝑃 ��� is used in all other process calculations, for example to calculate process useful efficiency, etc. The amount of generated HP steam, LP steam and generated heat for DH is calculated by the HRSG calculation unit using the generated equations below. calculations, for example to calculate process useful efficiency, etc. The amount of generated HP steam, LP steam and generated heat for DH is calculated by the HRSG calculation unit using the generated equations below. The equations are generated based on the data of the HRSG manufacturer. [19] 𝑚𝑚 ̇ �� = 0.00007994 ∙ 𝑃𝑃 ��� � − 0.01236 ∙ 𝑃𝑃 ��� � + 0.7599 ∙ 𝑃𝑃 ��� + 0.003202 (4) 𝑚𝑚 ̇ �� = 0.00002231 ∙ 𝑃𝑃 ��� � − 0.002278) ∙ 𝑃𝑃 ��� � + 0.1225 ∙ 𝑃𝑃 ��� + 0.001664 (5) 𝑃𝑃 ������� = 0.00006536 ∙ 𝑃𝑃 ��� � − 0.00707 ∙ 𝑃𝑃 ��� � + 0.3642 ∙ 𝑃𝑃 ��� + 0.003177 (6) where 𝑚𝑚 ̇ �� is HP steam mass flow from HRSG, 𝑚𝑚 ̇ �� is LP steam mass flow from HRSG and 𝑃𝑃 ������� is generated DH heat from HRSG. Now that the amount of generated HP steam and LP steam is known, the ST calculation unit can also calculate the power generated by ST generator: [20] 𝑃𝑃 ��� = 𝑚𝑚 ̇ �� ∙ (ℎ �� −ℎ ��� ) ∙ 0.9 (7) (7) where P STe is the power of the ST generator, h HP is specific enthalpy of HP steam and h OUT is specific enthalpy of steam from ST expansion cylinder. The thermal power of LP steam, which is used for industrial purposes and the heat generated from ST for DH is calculated by the mathematical model using the equations: [21] Energo-economics payback investment calculation modelling of recently built gas-steam combined cycle power plant 7 ---------- where 𝑃𝑃 �� � is the power of the ST generator, ℎ �� is specific enthalpy of HP steam and ℎ �� � is specific enthalpy of steam from ST expansion cylinder. The thermal power of LP steam, which is used for industrial purposes and the heat generated from ST for DH is calculated by the mathematical model using the equations: [21] 𝑃𝑃 �� = 𝑚𝑚 ̇ �� ∙ ( ℎ �� − 0 .1 2 6 ) (8) 𝑃𝑃 �� � � = 𝑚𝑚 ̇ �� ∙ ( ℎ �� � − 0. 251 ) (9) where 𝑃𝑃 �� is power of LP steam, ℎ �� is specific enthalpy of LP steam, 0.126 is specific enthalpy of water at 1 bar and 30 °C, 𝑃𝑃 �� ��� is the heat generated from ST for DH system and 0.251 is specific enthalpy of water at 1 bar and 60 °C. The GT useful efficiency and the total GSCCP useful efficiency is calculated by the mathematical model using the equations: [22] 𝜂𝜂 �� = � �� � � �� � ∙ 100 % (10) 𝜂𝜂 � � ��� = � � �� � �� ��� �� � �� �� �� �� �� �� � �� � �� � � ∙ 100 % (11) where 𝜂𝜂 �� is GT useful efficiency and 𝜂𝜂 � � ��� is GSCCG useful efficiency. In the calculation of the payback period of the investment, the payback calculation unit takes into account the remaining costs and carries out the calculation in several steps. The payback calculation unit calculates the economic eligibility of the investment, assesses the profit that the investment will yield and, based on the duration of the investment and the discount rate, determines whether the investment will be repaid or not. The payback calculation unit calculates profit by first estimating annual income and deducting energy costs, maintenance costs, operating costs, and depreciation: [23] 𝑃𝑃𝑃𝑃 𝑃𝑃 𝑃𝑃 = 𝐼𝐼𝐼𝐼 𝐼𝐼 − 𝐷𝐷 𝐷𝐷𝐷𝐷 − ∑ 𝐼𝐼𝑃𝑃𝑐𝑐 𝑐𝑐 𝑐𝑐 (12) where 𝑃𝑃𝑃𝑃𝑃𝑃 𝑃𝑃 is annual profit, 𝐼𝐼𝐼𝐼 𝐼𝐼 is annual income, 𝐷𝐷 𝐷𝐷𝐷𝐷 is the annual depreciation, and 𝐼𝐼𝑃𝑃𝑐𝑐 𝑐𝑐 𝑐𝑐 are annual costs. In the case of electricity and heat production, the annual income is the product of the annual production of energy products and the price of energy products: [23] 𝐼𝐼𝐼𝐼 𝐼𝐼 = ( 𝑃𝑃𝑃𝑃 � � � � �� ∙ 𝐶𝐶 �� ) + ( 𝑃𝑃𝑃𝑃 � �� � � � �� ∙ 𝐶𝐶 � � �� ) (13) where 𝑃𝑃𝑃𝑃 � � �� � is the annual production of electricity, 𝐶𝐶 �� is the price of electricity, 𝑃𝑃𝑃𝑃 � �� � � � �� is the annual production of heat and 𝐶𝐶 � � �� is the price of heat. The payback calculation unit adds to operating costs, financing costs, maintenance costs, etc. Depreciation costs are calculated by the payback calculation unit as the ratio of the value of the investment and the duration of the investment, linear depreciation: [1] 𝐷𝐷 𝐷𝐷𝐷𝐷 = � �� �� � (14) where 𝐼𝐼𝐼𝐼 𝐼𝐼 is investment value, and 𝐷𝐷𝐷𝐷𝑃𝑃 is investment duration. Profit is subject to state tax determined by the effective tax rate. After paying the tax, the net profit remains: [1] 𝑃𝑃𝑃𝑃 𝑃𝑃 𝑃𝑃 � �� = (1− 𝑐𝑐 𝑡𝑡𝑡𝑡 ) ∙ 𝑃𝑃𝑃𝑃𝑃𝑃 𝑃𝑃 (15) where 𝑃𝑃𝑃𝑃𝑃𝑃 𝑃𝑃 � �� is net profit, and 𝑐𝑐 𝑡𝑡𝑡𝑡 is the effective tax rate. The money coming from the investment is called the net cash flow and consists of the net profit that the investor can freely dispose of and the depreciation that they must allocate for new investments: [1] 𝑁𝑁𝐶𝐶𝑁𝑁 = 𝑁𝑁𝑃𝑃 𝑃𝑃𝑃𝑃 � �� + 𝐷𝐷 𝐷𝐷𝐷𝐷 (16) (8) (9) where P LP is power of LP steam, h LP is specific enthalpy of LP steam, 0.126 is specific enthalpy of water at 1 bar and 30 °C, P DH-ST is the heat generated from ST for DH system and 0.251 is specific enthalpy of water at 1 bar and 60 °C. The GT useful efficiency and the total GSCCP useful efficien- cy is calculated by the mathematical model using the equations: [22] Energo-economics payback investment calculation modelling of recently built gas-steam combined cycle power plant 7 ---------- where 𝑃𝑃 �� � is the power of the ST generator, ℎ �� is specific enthalpy of HP steam and ℎ �� � is specific enthalpy of steam from ST expansion cylinder. The thermal power of LP steam, which is used for industrial purposes and the heat generated from ST for DH is calculated by the mathematical model using the equations: [21] 𝑃𝑃 �� = 𝑚𝑚 ̇ �� ∙ ( ℎ �� − 0 .1 2 6 ) (8) 𝑃𝑃 �� � � = 𝑚𝑚 ̇ �� ∙ ( ℎ �� � − 0. 251 ) (9) where 𝑃𝑃 �� is power of LP steam, ℎ �� is specific enthalpy of LP steam, 0.126 is specific enthalpy of water at 1 bar and 30 °C, 𝑃𝑃 �� ��� is the heat generated from ST for DH system and 0.251 is specific enthalpy of water at 1 bar and 60 °C. The GT useful efficiency and the total GSCCP useful efficiency is calculated by the mathematical model using the equations: [22] 𝜂𝜂 �� = � �� � � �� � ∙ 100 % (10) 𝜂𝜂 � � ��� = � � �� � �� ��� �� � �� �� �� �� �� �� � �� � �� � � ∙ 100 % (11) where 𝜂𝜂 �� is GT useful efficiency and 𝜂𝜂 � � ��� is GSCCG useful efficiency. In the calculation of the payback period of the investment, the payback calculation unit takes into account the remaining costs and carries out the calculation in several steps. The payback calculation unit calculates the economic eligibility of the investment, assesses the profit that the investment will yield and, based on the duration of the investment and the discount rate, determines whether the investment will be repaid or not. The payback calculation unit calculates profit by first estimating annual income and deducting energy costs, maintenance costs, operating costs, and depreciation: [23] 𝑃𝑃𝑃𝑃 𝑃𝑃 𝑃𝑃 = 𝐼𝐼𝐼𝐼 𝐼𝐼 − 𝐷𝐷 𝐷𝐷𝐷𝐷 − ∑ 𝐼𝐼𝑃𝑃𝑐𝑐 𝑐𝑐 𝑐𝑐 (12) where 𝑃𝑃𝑃𝑃𝑃𝑃 𝑃𝑃 is annual profit, 𝐼𝐼𝐼𝐼 𝐼𝐼 is annual income, 𝐷𝐷 𝐷𝐷𝐷𝐷 is the annual depreciation, and 𝐼𝐼𝑃𝑃𝑐𝑐 𝑐𝑐 𝑐𝑐 are annual costs. In the case of electricity and heat production, the annual income is the product of the annual production of energy products and the price of energy products: [23] 𝐼𝐼𝐼𝐼 𝐼𝐼 = ( 𝑃𝑃𝑃𝑃 � � � � �� ∙ 𝐶𝐶 �� ) + ( 𝑃𝑃𝑃𝑃 � �� � � � �� ∙ 𝐶𝐶 � � �� ) (13) where 𝑃𝑃𝑃𝑃 � � �� � is the annual production of electricity, 𝐶𝐶 �� is the price of electricity, 𝑃𝑃𝑃𝑃 � �� � � � �� is the annual production of heat and 𝐶𝐶 � � �� is the price of heat. The payback calculation unit adds to operating costs, financing costs, maintenance costs, etc. Depreciation costs are calculated by the payback calculation unit as the ratio of the value of the investment and the duration of the investment, linear depreciation: [1] 𝐷𝐷 𝐷𝐷𝐷𝐷 = � �� �� � (14) where 𝐼𝐼𝐼𝐼 𝐼𝐼 is investment value, and 𝐷𝐷𝐷𝐷𝑃𝑃 is investment duration. Profit is subject to state tax determined by the effective tax rate. After paying the tax, the net profit remains: [1] 𝑃𝑃𝑃𝑃 𝑃𝑃 𝑃𝑃 � �� = (1− 𝑐𝑐 𝑡𝑡𝑡𝑡 ) ∙ 𝑃𝑃𝑃𝑃𝑃𝑃 𝑃𝑃 (15) where 𝑃𝑃𝑃𝑃𝑃𝑃 𝑃𝑃 � �� is net profit, and 𝑐𝑐 𝑡𝑡𝑡𝑡 is the effective tax rate. The money coming from the investment is called the net cash flow and consists of the net profit that the investor can freely dispose of and the depreciation that they must allocate for new investments: [1] 𝑁𝑁𝐶𝐶𝑁𝑁 = 𝑁𝑁𝑃𝑃 𝑃𝑃𝑃𝑃 � �� + 𝐷𝐷 𝐷𝐷𝐷𝐷 (16) (10) JET 17 Energo-economics payback investment calculation modelling of recently built gas-steam combined cycle power plant Energo-economics payback investment calculation modelling of recently built gas-steam combined cycle power plant 7 ---------- where 𝑃𝑃 �� � is the power of the ST generator, ℎ �� is specific enthalpy of HP steam and ℎ �� � is specific enthalpy of steam from ST expansion cylinder. The thermal power of LP steam, which is used for industrial purposes and the heat generated from ST for DH is calculated by the mathematical model using the equations: [21] 𝑃𝑃 �� = 𝑚𝑚 ̇ �� ∙ ( ℎ �� − 0 .1 2 6 ) (8) 𝑃𝑃 �� � � = 𝑚𝑚 ̇ �� ∙ ( ℎ �� � − 0. 251 ) (9) where 𝑃𝑃 �� is power of LP steam, ℎ �� is specific enthalpy of LP steam, 0.126 is specific enthalpy of water at 1 bar and 30 °C, 𝑃𝑃 �� ��� is the heat generated from ST for DH system and 0.251 is specific enthalpy of water at 1 bar and 60 °C. The GT useful efficiency and the total GSCCP useful efficiency is calculated by the mathematical model using the equations: [22] 𝜂𝜂 �� = � �� � � �� � ∙ 100 % (10) 𝜂𝜂 � � ��� = � � �� � �� ��� �� � �� �� �� �� �� �� � �� � �� � � ∙ 100 % (11) where 𝜂𝜂 �� is GT useful efficiency and 𝜂𝜂 � � ��� is GSCCG useful efficiency. In the calculation of the payback period of the investment, the payback calculation unit takes into account the remaining costs and carries out the calculation in several steps. The payback calculation unit calculates the economic eligibility of the investment, assesses the profit that the investment will yield and, based on the duration of the investment and the discount rate, determines whether the investment will be repaid or not. The payback calculation unit calculates profit by first estimating annual income and deducting energy costs, maintenance costs, operating costs, and depreciation: [23] 𝑃𝑃𝑃𝑃 𝑃𝑃 𝑃𝑃 = 𝐼𝐼𝐼𝐼 𝐼𝐼 − 𝐷𝐷 𝐷𝐷𝐷𝐷 − ∑ 𝐼𝐼𝑃𝑃𝑐𝑐 𝑐𝑐 𝑐𝑐 (12) where 𝑃𝑃𝑃𝑃𝑃𝑃 𝑃𝑃 is annual profit, 𝐼𝐼𝐼𝐼 𝐼𝐼 is annual income, 𝐷𝐷 𝐷𝐷𝐷𝐷 is the annual depreciation, and 𝐼𝐼𝑃𝑃𝑐𝑐 𝑐𝑐 𝑐𝑐 are annual costs. In the case of electricity and heat production, the annual income is the product of the annual production of energy products and the price of energy products: [23] 𝐼𝐼𝐼𝐼 𝐼𝐼 = ( 𝑃𝑃𝑃𝑃 � � � � �� ∙ 𝐶𝐶 �� ) + ( 𝑃𝑃𝑃𝑃 � �� � � � �� ∙ 𝐶𝐶 � � �� ) (13) where 𝑃𝑃𝑃𝑃 � � �� � is the annual production of electricity, 𝐶𝐶 �� is the price of electricity, 𝑃𝑃𝑃𝑃 � �� � � � �� is the annual production of heat and 𝐶𝐶 � � �� is the price of heat. The payback calculation unit adds to operating costs, financing costs, maintenance costs, etc. Depreciation costs are calculated by the payback calculation unit as the ratio of the value of the investment and the duration of the investment, linear depreciation: [1] 𝐷𝐷 𝐷𝐷𝐷𝐷 = � �� �� � (14) where 𝐼𝐼𝐼𝐼 𝐼𝐼 is investment value, and 𝐷𝐷𝐷𝐷𝑃𝑃 is investment duration. Profit is subject to state tax determined by the effective tax rate. After paying the tax, the net profit remains: [1] 𝑃𝑃𝑃𝑃 𝑃𝑃 𝑃𝑃 � �� = (1− 𝑐𝑐 𝑡𝑡𝑡𝑡 ) ∙ 𝑃𝑃𝑃𝑃𝑃𝑃 𝑃𝑃 (15) where 𝑃𝑃𝑃𝑃𝑃𝑃 𝑃𝑃 � �� is net profit, and 𝑐𝑐 𝑡𝑡𝑡𝑡 is the effective tax rate. The money coming from the investment is called the net cash flow and consists of the net profit that the investor can freely dispose of and the depreciation that they must allocate for new investments: [1] 𝑁𝑁𝐶𝐶𝑁𝑁 = 𝑁𝑁𝑃𝑃 𝑃𝑃𝑃𝑃 � �� + 𝐷𝐷 𝐷𝐷𝐷𝐷 (16) (11) where η GT is GT useful efficiency and η GSCCP is GSCCG useful efficiency. In the calculation of the payback period of the investment, the payback calculation unit takes into account the remaining costs and carries out the calculation in several steps. The payback calcula- tion unit calculates the economic eligibility of the investment, assesses the profit that the invest- ment will yield and, based on the duration of the investment and the discount rate, determines whether the investment will be repaid or not. The payback calculation unit calculates profit by first estimating annual income and deducting energy costs, maintenance costs, operating costs, and depreciation: [23] Energo-economics payback investment calculation modelling of recently built gas-steam combined cycle power plant 7 ---------- where 𝑃𝑃 �� � is the power of the ST generator, ℎ �� is specific enthalpy of HP steam and ℎ �� � is specific enthalpy of steam from ST expansion cylinder. The thermal power of LP steam, which is used for industrial purposes and the heat generated from ST for DH is calculated by the mathematical model using the equations: [21] 𝑃𝑃 �� = 𝑚𝑚 ̇ �� ∙ ( ℎ �� − 0 .1 2 6 ) (8) 𝑃𝑃 �� � � = 𝑚𝑚 ̇ �� ∙ ( ℎ �� � − 0. 251 ) (9) where 𝑃𝑃 �� is power of LP steam, ℎ �� is specific enthalpy of LP steam, 0.126 is specific enthalpy of water at 1 bar and 30 °C, 𝑃𝑃 �� ��� is the heat generated from ST for DH system and 0.251 is specific enthalpy of water at 1 bar and 60 °C. The GT useful efficiency and the total GSCCP useful efficiency is calculated by the mathematical model using the equations: [22] 𝜂𝜂 �� = � �� � � �� � ∙ 100 % (10) 𝜂𝜂 � � ��� = � � �� � �� ��� �� � �� �� �� �� �� �� � �� � �� � � ∙ 100 % (11) where 𝜂𝜂 �� is GT useful efficiency and 𝜂𝜂 � � ��� is GSCCG useful efficiency. In the calculation of the payback period of the investment, the payback calculation unit takes into account the remaining costs and carries out the calculation in several steps. The payback calculation unit calculates the economic eligibility of the investment, assesses the profit that the investment will yield and, based on the duration of the investment and the discount rate, determines whether the investment will be repaid or not. The payback calculation unit calculates profit by first estimating annual income and deducting energy costs, maintenance costs, operating costs, and depreciation: [23] 𝑃𝑃𝑃𝑃 𝑃𝑃 𝑃𝑃 = 𝐼𝐼𝐼𝐼 𝐼𝐼 − 𝐷𝐷 𝐷𝐷𝐷𝐷 − ∑ 𝐼𝐼𝑃𝑃𝑐𝑐 𝑐𝑐 𝑐𝑐 (12) where 𝑃𝑃𝑃𝑃𝑃𝑃 𝑃𝑃 is annual profit, 𝐼𝐼𝐼𝐼 𝐼𝐼 is annual income, 𝐷𝐷 𝐷𝐷𝐷𝐷 is the annual depreciation, and 𝐼𝐼𝑃𝑃𝑐𝑐 𝑐𝑐 𝑐𝑐 are annual costs. In the case of electricity and heat production, the annual income is the product of the annual production of energy products and the price of energy products: [23] 𝐼𝐼𝐼𝐼 𝐼𝐼 = ( 𝑃𝑃𝑃𝑃 � � � � �� ∙ 𝐶𝐶 �� ) + ( 𝑃𝑃𝑃𝑃 � �� � � � �� ∙ 𝐶𝐶 � � �� ) (13) where 𝑃𝑃𝑃𝑃 � � �� � is the annual production of electricity, 𝐶𝐶 �� is the price of electricity, 𝑃𝑃𝑃𝑃 � �� � � � �� is the annual production of heat and 𝐶𝐶 � � �� is the price of heat. The payback calculation unit adds to operating costs, financing costs, maintenance costs, etc. Depreciation costs are calculated by the payback calculation unit as the ratio of the value of the investment and the duration of the investment, linear depreciation: [1] 𝐷𝐷 𝐷𝐷𝐷𝐷 = � �� �� � (14) where 𝐼𝐼𝐼𝐼 𝐼𝐼 is investment value, and 𝐷𝐷𝐷𝐷𝑃𝑃 is investment duration. Profit is subject to state tax determined by the effective tax rate. After paying the tax, the net profit remains: [1] 𝑃𝑃𝑃𝑃 𝑃𝑃 𝑃𝑃 � �� = (1− 𝑐𝑐 𝑡𝑡𝑡𝑡 ) ∙ 𝑃𝑃𝑃𝑃𝑃𝑃 𝑃𝑃 (15) where 𝑃𝑃𝑃𝑃𝑃𝑃 𝑃𝑃 � �� is net profit, and 𝑐𝑐 𝑡𝑡𝑡𝑡 is the effective tax rate. The money coming from the investment is called the net cash flow and consists of the net profit that the investor can freely dispose of and the depreciation that they must allocate for new investments: [1] 𝑁𝑁𝐶𝐶𝑁𝑁 = 𝑁𝑁𝑃𝑃 𝑃𝑃𝑃𝑃 � �� + 𝐷𝐷 𝐷𝐷𝐷𝐷 (16) (12) where Prof is annual profit, Inc is annual income, Dep is the annual depreciation, and costs are annual costs. In the case of electricity and heat production, the annual income is the product of the annual production of energy products and the price of energy products: [23] Energo-economics payback investment calculation modelling of recently built gas-steam combined cycle power plant 7 ---------- where 𝑃𝑃 �� � is the power of the ST generator, ℎ �� is specific enthalpy of HP steam and ℎ �� � is specific enthalpy of steam from ST expansion cylinder. The thermal power of LP steam, which is used for industrial purposes and the heat generated from ST for DH is calculated by the mathematical model using the equations: [21] 𝑃𝑃 �� = 𝑚𝑚 ̇ �� ∙ ( ℎ �� − 0 .1 2 6 ) (8) 𝑃𝑃 �� � � = 𝑚𝑚 ̇ �� ∙ ( ℎ �� � − 0. 251 ) (9) where 𝑃𝑃 �� is power of LP steam, ℎ �� is specific enthalpy of LP steam, 0.126 is specific enthalpy of water at 1 bar and 30 °C, 𝑃𝑃 �� ��� is the heat generated from ST for DH system and 0.251 is specific enthalpy of water at 1 bar and 60 °C. The GT useful efficiency and the total GSCCP useful efficiency is calculated by the mathematical model using the equations: [22] 𝜂𝜂 �� = � �� � � �� � ∙ 100 % (10) 𝜂𝜂 � � ��� = � � �� � �� ��� �� � �� �� �� �� �� �� � �� � �� � � ∙ 100 % (11) where 𝜂𝜂 �� is GT useful efficiency and 𝜂𝜂 � � ��� is GSCCG useful efficiency. In the calculation of the payback period of the investment, the payback calculation unit takes into account the remaining costs and carries out the calculation in several steps. The payback calculation unit calculates the economic eligibility of the investment, assesses the profit that the investment will yield and, based on the duration of the investment and the discount rate, determines whether the investment will be repaid or not. The payback calculation unit calculates profit by first estimating annual income and deducting energy costs, maintenance costs, operating costs, and depreciation: [23] 𝑃𝑃𝑃𝑃 𝑃𝑃 𝑃𝑃 = 𝐼𝐼𝐼𝐼 𝐼𝐼 − 𝐷𝐷 𝐷𝐷𝐷𝐷 − ∑ 𝐼𝐼𝑃𝑃𝑐𝑐 𝑐𝑐 𝑐𝑐 (12) where 𝑃𝑃𝑃𝑃𝑃𝑃 𝑃𝑃 is annual profit, 𝐼𝐼𝐼𝐼 𝐼𝐼 is annual income, 𝐷𝐷 𝐷𝐷𝐷𝐷 is the annual depreciation, and 𝐼𝐼𝑃𝑃𝑐𝑐 𝑐𝑐 𝑐𝑐 are annual costs. In the case of electricity and heat production, the annual income is the product of the annual production of energy products and the price of energy products: [23] 𝐼𝐼𝐼𝐼 𝐼𝐼 = ( 𝑃𝑃𝑃𝑃 � � � � �� ∙ 𝐶𝐶 �� ) + ( 𝑃𝑃𝑃𝑃 � �� � � � �� ∙ 𝐶𝐶 � � �� ) (13) where 𝑃𝑃𝑃𝑃 � � �� � is the annual production of electricity, 𝐶𝐶 �� is the price of electricity, 𝑃𝑃𝑃𝑃 � �� � � � �� is the annual production of heat and 𝐶𝐶 � � �� is the price of heat. The payback calculation unit adds to operating costs, financing costs, maintenance costs, etc. Depreciation costs are calculated by the payback calculation unit as the ratio of the value of the investment and the duration of the investment, linear depreciation: [1] 𝐷𝐷 𝐷𝐷𝐷𝐷 = � �� �� � (14) where 𝐼𝐼𝐼𝐼 𝐼𝐼 is investment value, and 𝐷𝐷𝐷𝐷𝑃𝑃 is investment duration. Profit is subject to state tax determined by the effective tax rate. After paying the tax, the net profit remains: [1] 𝑃𝑃𝑃𝑃 𝑃𝑃 𝑃𝑃 � �� = (1− 𝑐𝑐 𝑡𝑡𝑡𝑡 ) ∙ 𝑃𝑃𝑃𝑃𝑃𝑃 𝑃𝑃 (15) where 𝑃𝑃𝑃𝑃𝑃𝑃 𝑃𝑃 � �� is net profit, and 𝑐𝑐 𝑡𝑡𝑡𝑡 is the effective tax rate. The money coming from the investment is called the net cash flow and consists of the net profit that the investor can freely dispose of and the depreciation that they must allocate for new investments: [1] 𝑁𝑁𝐶𝐶𝑁𝑁 = 𝑁𝑁𝑃𝑃 𝑃𝑃𝑃𝑃 � �� + 𝐷𝐷 𝐷𝐷𝐷𝐷 (16) (13) where Pr el-ann is the annual production of electricity, C el is the price of electricity, Pr ther-ann is the annual production of heat and C ther is the price of heat. The payback calculation unit adds to operating costs, financing costs, maintenance costs, etc. Depreciation costs are calculated by the payback calculation unit as the ratio of the value of the investment and the duration of the investment, linear depreciation: [1] Energo-economics payback investment calculation modelling of recently built gas-steam combined cycle power plant 7 ---------- where 𝑃𝑃 �� � is the power of the ST generator, ℎ �� is specific enthalpy of HP steam and ℎ �� � is specific enthalpy of steam from ST expansion cylinder. The thermal power of LP steam, which is used for industrial purposes and the heat generated from ST for DH is calculated by the mathematical model using the equations: [21] 𝑃𝑃 �� = 𝑚𝑚 ̇ �� ∙ ( ℎ �� − 0 .1 2 6 ) (8) 𝑃𝑃 �� � � = 𝑚𝑚 ̇ �� ∙ ( ℎ �� � − 0. 251 ) (9) where 𝑃𝑃 �� is power of LP steam, ℎ �� is specific enthalpy of LP steam, 0.126 is specific enthalpy of water at 1 bar and 30 °C, 𝑃𝑃 �� ��� is the heat generated from ST for DH system and 0.251 is specific enthalpy of water at 1 bar and 60 °C. The GT useful efficiency and the total GSCCP useful efficiency is calculated by the mathematical model using the equations: [22] 𝜂𝜂 �� = � �� � � �� � ∙ 100 % (10) 𝜂𝜂 � � ��� = � � �� � �� ��� �� � �� �� �� �� �� �� � �� � �� � � ∙ 100 % (11) where 𝜂𝜂 �� is GT useful efficiency and 𝜂𝜂 � � ��� is GSCCG useful efficiency. In the calculation of the payback period of the investment, the payback calculation unit takes into account the remaining costs and carries out the calculation in several steps. The payback calculation unit calculates the economic eligibility of the investment, assesses the profit that the investment will yield and, based on the duration of the investment and the discount rate, determines whether the investment will be repaid or not. The payback calculation unit calculates profit by first estimating annual income and deducting energy costs, maintenance costs, operating costs, and depreciation: [23] 𝑃𝑃𝑃𝑃 𝑃𝑃 𝑃𝑃 = 𝐼𝐼𝐼𝐼 𝐼𝐼 − 𝐷𝐷 𝐷𝐷𝐷𝐷 − ∑ 𝐼𝐼𝑃𝑃𝑐𝑐 𝑐𝑐 𝑐𝑐 (12) where 𝑃𝑃𝑃𝑃𝑃𝑃 𝑃𝑃 is annual profit, 𝐼𝐼𝐼𝐼 𝐼𝐼 is annual income, 𝐷𝐷 𝐷𝐷𝐷𝐷 is the annual depreciation, and 𝐼𝐼𝑃𝑃𝑐𝑐 𝑐𝑐 𝑐𝑐 are annual costs. In the case of electricity and heat production, the annual income is the product of the annual production of energy products and the price of energy products: [23] 𝐼𝐼𝐼𝐼 𝐼𝐼 = ( 𝑃𝑃𝑃𝑃 � � � � �� ∙ 𝐶𝐶 �� ) + ( 𝑃𝑃𝑃𝑃 � �� � � � �� ∙ 𝐶𝐶 � � �� ) (13) where 𝑃𝑃𝑃𝑃 � � �� � is the annual production of electricity, 𝐶𝐶 �� is the price of electricity, 𝑃𝑃𝑃𝑃 � �� � � � �� is the annual production of heat and 𝐶𝐶 � � �� is the price of heat. The payback calculation unit adds to operating costs, financing costs, maintenance costs, etc. Depreciation costs are calculated by the payback calculation unit as the ratio of the value of the investment and the duration of the investment, linear depreciation: [1] 𝐷𝐷 𝐷𝐷𝐷𝐷 = � �� �� � (14) where 𝐼𝐼𝐼𝐼 𝐼𝐼 is investment value, and 𝐷𝐷𝐷𝐷𝑃𝑃 is investment duration. Profit is subject to state tax determined by the effective tax rate. After paying the tax, the net profit remains: [1] 𝑃𝑃𝑃𝑃 𝑃𝑃 𝑃𝑃 � �� = (1− 𝑐𝑐 𝑡𝑡𝑡𝑡 ) ∙ 𝑃𝑃𝑃𝑃𝑃𝑃 𝑃𝑃 (15) where 𝑃𝑃𝑃𝑃𝑃𝑃 𝑃𝑃 � �� is net profit, and 𝑐𝑐 𝑡𝑡𝑡𝑡 is the effective tax rate. The money coming from the investment is called the net cash flow and consists of the net profit that the investor can freely dispose of and the depreciation that they must allocate for new investments: [1] 𝑁𝑁𝐶𝐶𝑁𝑁 = 𝑁𝑁𝑃𝑃 𝑃𝑃𝑃𝑃 � �� + 𝐷𝐷 𝐷𝐷𝐷𝐷 (16) (14) where Inv is investment value, Dur and is investment duration. Profit is subject to state tax deter- mined by the effective tax rate. After paying the tax, the net profit remains: [1] Energo-economics payback investment calculation modelling of recently built gas-steam combined cycle power plant 7 ---------- where 𝑃𝑃 �� � is the power of the ST generator, ℎ �� is specific enthalpy of HP steam and ℎ �� � is specific enthalpy of steam from ST expansion cylinder. The thermal power of LP steam, which is used for industrial purposes and the heat generated from ST for DH is calculated by the mathematical model using the equations: [21] 𝑃𝑃 �� = 𝑚𝑚 ̇ �� ∙ ( ℎ �� − 0 .1 2 6 ) (8) 𝑃𝑃 �� � � = 𝑚𝑚 ̇ �� ∙ ( ℎ �� � − 0. 251 ) (9) where 𝑃𝑃 �� is power of LP steam, ℎ �� is specific enthalpy of LP steam, 0.126 is specific enthalpy of water at 1 bar and 30 °C, 𝑃𝑃 �� ��� is the heat generated from ST for DH system and 0.251 is specific enthalpy of water at 1 bar and 60 °C. The GT useful efficiency and the total GSCCP useful efficiency is calculated by the mathematical model using the equations: [22] 𝜂𝜂 �� = � �� � � �� � ∙ 100 % (10) 𝜂𝜂 � � ��� = � � �� � �� ��� �� � �� �� �� �� �� �� � �� � �� � � ∙ 100 % (11) where 𝜂𝜂 �� is GT useful efficiency and 𝜂𝜂 � � ��� is GSCCG useful efficiency. In the calculation of the payback period of the investment, the payback calculation unit takes into account the remaining costs and carries out the calculation in several steps. The payback calculation unit calculates the economic eligibility of the investment, assesses the profit that the investment will yield and, based on the duration of the investment and the discount rate, determines whether the investment will be repaid or not. The payback calculation unit calculates profit by first estimating annual income and deducting energy costs, maintenance costs, operating costs, and depreciation: [23] 𝑃𝑃𝑃𝑃 𝑃𝑃 𝑃𝑃 = 𝐼𝐼𝐼𝐼 𝐼𝐼 − 𝐷𝐷 𝐷𝐷𝐷𝐷 − ∑ 𝐼𝐼𝑃𝑃𝑐𝑐 𝑐𝑐 𝑐𝑐 (12) where 𝑃𝑃𝑃𝑃𝑃𝑃 𝑃𝑃 is annual profit, 𝐼𝐼𝐼𝐼 𝐼𝐼 is annual income, 𝐷𝐷 𝐷𝐷𝐷𝐷 is the annual depreciation, and 𝐼𝐼𝑃𝑃𝑐𝑐 𝑐𝑐 𝑐𝑐 are annual costs. In the case of electricity and heat production, the annual income is the product of the annual production of energy products and the price of energy products: [23] 𝐼𝐼𝐼𝐼 𝐼𝐼 = ( 𝑃𝑃𝑃𝑃 � � � � �� ∙ 𝐶𝐶 �� ) + ( 𝑃𝑃𝑃𝑃 � �� � � � �� ∙ 𝐶𝐶 � � �� ) (13) where 𝑃𝑃𝑃𝑃 � � �� � is the annual production of electricity, 𝐶𝐶 �� is the price of electricity, 𝑃𝑃𝑃𝑃 � �� � � � �� is the annual production of heat and 𝐶𝐶 � � �� is the price of heat. The payback calculation unit adds to operating costs, financing costs, maintenance costs, etc. Depreciation costs are calculated by the payback calculation unit as the ratio of the value of the investment and the duration of the investment, linear depreciation: [1] 𝐷𝐷 𝐷𝐷𝐷𝐷 = � �� �� � (14) where 𝐼𝐼𝐼𝐼 𝐼𝐼 is investment value, and 𝐷𝐷𝐷𝐷𝑃𝑃 is investment duration. Profit is subject to state tax determined by the effective tax rate. After paying the tax, the net profit remains: [1] 𝑃𝑃𝑃𝑃 𝑃𝑃 𝑃𝑃 � �� = (1− 𝑐𝑐 𝑡𝑡𝑡𝑡 ) ∙ 𝑃𝑃𝑃𝑃𝑃𝑃 𝑃𝑃 (15) where 𝑃𝑃𝑃𝑃𝑃𝑃 𝑃𝑃 � �� is net profit, and 𝑐𝑐 𝑡𝑡𝑡𝑡 is the effective tax rate. The money coming from the investment is called the net cash flow and consists of the net profit that the investor can freely dispose of and the depreciation that they must allocate for new investments: [1] 𝑁𝑁𝐶𝐶𝑁𝑁 = 𝑁𝑁𝑃𝑃 𝑃𝑃𝑃𝑃 � �� + 𝐷𝐷 𝐷𝐷𝐷𝐷 (16) (15) where Prof net is net profit, and tax is the effective tax rate. The money coming from the invest- ment is called the net cash flow and consists of the net profit that the investor can freely dispose of and the depreciation that they must allocate for new investments: [1] Energo-economics payback investment calculation modelling of recently built gas-steam combined cycle power plant 7 ---------- where 𝑃𝑃 �� � is the power of the ST generator, ℎ �� is specific enthalpy of HP steam and ℎ �� � is specific enthalpy of steam from ST expansion cylinder. The thermal power of LP steam, which is used for industrial purposes and the heat generated from ST for DH is calculated by the mathematical model using the equations: [21] 𝑃𝑃 �� = 𝑚𝑚 ̇ �� ∙ ( ℎ �� − 0 .1 2 6 ) (8) 𝑃𝑃 �� � � = 𝑚𝑚 ̇ �� ∙ ( ℎ �� � − 0. 251 ) (9) where 𝑃𝑃 �� is power of LP steam, ℎ �� is specific enthalpy of LP steam, 0.126 is specific enthalpy of water at 1 bar and 30 °C, 𝑃𝑃 �� ��� is the heat generated from ST for DH system and 0.251 is specific enthalpy of water at 1 bar and 60 °C. The GT useful efficiency and the total GSCCP useful efficiency is calculated by the mathematical model using the equations: [22] 𝜂𝜂 �� = � �� � � �� � ∙ 100 % (10) 𝜂𝜂 � � ��� = � � �� � �� ��� �� � �� �� �� �� �� �� � �� � �� � � ∙ 100 % (11) where 𝜂𝜂 �� is GT useful efficiency and 𝜂𝜂 � � ��� is GSCCG useful efficiency. In the calculation of the payback period of the investment, the payback calculation unit takes into account the remaining costs and carries out the calculation in several steps. The payback calculation unit calculates the economic eligibility of the investment, assesses the profit that the investment will yield and, based on the duration of the investment and the discount rate, determines whether the investment will be repaid or not. The payback calculation unit calculates profit by first estimating annual income and deducting energy costs, maintenance costs, operating costs, and depreciation: [23] 𝑃𝑃𝑃𝑃 𝑃𝑃 𝑃𝑃 = 𝐼𝐼𝐼𝐼 𝐼𝐼 − 𝐷𝐷 𝐷𝐷𝐷𝐷 − ∑ 𝐼𝐼𝑃𝑃𝑐𝑐 𝑐𝑐 𝑐𝑐 (12) where 𝑃𝑃𝑃𝑃𝑃𝑃 𝑃𝑃 is annual profit, 𝐼𝐼𝐼𝐼 𝐼𝐼 is annual income, 𝐷𝐷 𝐷𝐷𝐷𝐷 is the annual depreciation, and 𝐼𝐼𝑃𝑃𝑐𝑐 𝑐𝑐 𝑐𝑐 are annual costs. In the case of electricity and heat production, the annual income is the product of the annual production of energy products and the price of energy products: [23] 𝐼𝐼𝐼𝐼 𝐼𝐼 = ( 𝑃𝑃𝑃𝑃 � � � � �� ∙ 𝐶𝐶 �� ) + ( 𝑃𝑃𝑃𝑃 � �� � � � �� ∙ 𝐶𝐶 � � �� ) (13) where 𝑃𝑃𝑃𝑃 � � �� � is the annual production of electricity, 𝐶𝐶 �� is the price of electricity, 𝑃𝑃𝑃𝑃 � �� � � � �� is the annual production of heat and 𝐶𝐶 � � �� is the price of heat. The payback calculation unit adds to operating costs, financing costs, maintenance costs, etc. Depreciation costs are calculated by the payback calculation unit as the ratio of the value of the investment and the duration of the investment, linear depreciation: [1] 𝐷𝐷 𝐷𝐷𝐷𝐷 = � �� �� � (14) where 𝐼𝐼𝐼𝐼 𝐼𝐼 is investment value, and 𝐷𝐷𝐷𝐷𝑃𝑃 is investment duration. Profit is subject to state tax determined by the effective tax rate. After paying the tax, the net profit remains: [1] 𝑃𝑃𝑃𝑃 𝑃𝑃 𝑃𝑃 � �� = (1− 𝑐𝑐 𝑡𝑡𝑡𝑡 ) ∙ 𝑃𝑃𝑃𝑃𝑃𝑃 𝑃𝑃 (15) where 𝑃𝑃𝑃𝑃𝑃𝑃 𝑃𝑃 � �� is net profit, and 𝑐𝑐 𝑡𝑡𝑡𝑡 is the effective tax rate. The money coming from the investment is called the net cash flow and consists of the net profit that the investor can freely dispose of and the depreciation that they must allocate for new investments: [1] 𝑁𝑁𝐶𝐶𝑁𝑁 = 𝑁𝑁𝑃𝑃 𝑃𝑃𝑃𝑃 � �� + 𝐷𝐷 𝐷𝐷𝐷𝐷 (16) (16) where NCF is net cash flow. The sum of all discounted values of net cash flow over the life of the investment gives the present value of revenue and, if the value of the investment is deducted from it, the payback calculation unit can calculate the net present value: [1] 8 Dušan Strušnik, Jurij Avsec JET Vol. 15 (2022) Issue 4 ---------- where 𝑁𝑁𝑁𝑁𝑁𝑁 is net cash flow. The sum of all discounted values of net cash flow over the life of the investment gives the present value of revenue and, if the value of the investment is deducted from it, the payback calculation unit can calculate the net present value: [1] 𝑁𝑁𝑁𝑁𝑁𝑁 𝑁 ∑ ��� ( � � ��� �� � � ) � − 𝐼𝐼𝐼𝐼 𝐼𝐼 � � � � (17) where 𝑁𝑁𝑁𝑁𝑁𝑁 is the net present value, which is a basic indicator of the cost-effectiveness of the investment. Only when 𝑁𝑁𝑁𝑁𝑁𝑁 is positive is the investment economical. When we compare two or more investments, the most economical is the one that reaches the highest 𝑁𝑁𝑁𝑁𝑁𝑁 . [1] 3 ACTUAL DATA SET FILTRATION AND MODEL VALIDATION Actual data set filtration and model validation is the process of determining whether the model accurately represents the behaviour of the actual system. However, it is important to consider the quality of the data, whether it truly represents the system, and if it is the best test of the model. [24] Before the validation procedure, all actual data set should be properly prepared. All data that do not belong to the actual data group, error data, are removed in the filtration process. Error data in an individual data actual group are caused by measurement errors, recording errors, or turbine trip and other measurement failures. An example of an unfiltered and filtered actual data set from the SCADA is shown in Fig. 4. Figure 4: (a) Unfiltered data from the SCADA system with error data and (b) filtered data used for training and validation of architectures applied in the ST calculation unit. Filtered data from the SCADA has been used in the ANN training and validation process to choose the ANN architecture that gave the best results which were applied in the auxiliary ST calculation unit. Training an ANN is an iterative process in which training data examples are presented to the network one by one, and the values of the weights are adjusted each time. [25] In the ST (17) where NPV is the net present value, which is a basic indicator of the cost-effectiveness of the investment. Only when NPV is positive is the investment economical. When we compare two or more investments, the most economical is the one that reaches the highest NPV. [1] 3 ACTUAL DATA SET FILTRATION AND MODEL VALIDATION Actual data set filtration and model validation is the process of determining whether the model accurately represents the behaviour of the actual system. However, it is important to consider 18 JET 18 JET Dušan Strušnik, Jurij Avsec JET Vol. 15 (2022) Issue 4 the quality of the data, whether it truly represents the system, and if it is the best test of the model. [24] Before the validation procedure, all actual data set should be properly prepared. All data that do not belong to the actual data group, error data, are removed in the filtration pro- cess. Error data in an individual data actual group are caused by measurement errors, recording errors, or turbine trip and other measurement failures. An example of an unfiltered and filtered actual data set from the SCADA is shown in Fig. 4. Figure 4: (a) Unfiltered data from the SCADA system with error data and (b) filtered data used for training and validation of architectures applied in the ST calculation unit. Filtered data from the SCADA has been used in the ANN training and validation process to choose the ANN architecture that gave the best results which were applied in the auxiliary ST calculation unit. Training an ANN is an iterative process in which training data examples are presented to the network one by one, and the values of the weights are adjusted each time. [25] In the ST calcula- tion unit development process, an input data set was used during the learning phase, expressed in the [3x5400] matrix form and an output data set equally expressed in the [2x5400] matrix form. For each input data set, which in our case are steam mass flow, steam temperature and steam pressure into the ST, there is a specific output data, which are exhaust steam temperature and exhaust steam pressure from the ST. The validation of ANN algorithm structures is carried out by means of the calculations of the error between the results provided by the ANN algorithm structure and the actual process data. The errors can be computed in several ways. The most useful way of error computation is called the mean square error (MSE) and is defined as: [26] Energo-economics payback investment calculation modelling of recently built gas-steam combined cycle power plant 9 ---------- calculation unit development process, an input data set was used during the learning phase, expressed in the [3x5400] matrix form and an output data set equally expressed in the [2x5400] matrix form. For each input data set, which in our case are steam mass flow, steam temperature and steam pressure into the ST, there is a specific output data, which are exhaust steam temperature and exhaust steam pressure from the ST. The validation of ANN algorithm structures is carried out by means of the calculations of the error between the results provided by the ANN algorithm structure and the actual process data. The errors can be computed in several ways. The most useful way of error computation is called the mean square error (MSE) and is defined as: [26] 𝑀𝑀 𝑀𝑀𝑀𝑀 = � � ∑ � 𝑡𝑡 � − 𝑜𝑜 � � � � � �� (18) whereas the root mean square (RMS) is defined as follows: 𝑅𝑅𝑀𝑀 𝑀𝑀 = � ( 1/ 𝑝𝑝 )∑ � 𝑡𝑡 � − 𝑜𝑜 � � � � � �� � � / � (19) The correlation coefficient (R 2 ) and mean absolute error (MAE) are respectively defined as: [27], [28] 𝑅𝑅 � =1− � ∑ � � � �� � � � � � �� ∑ � � � � � � ��� �(20) 𝑀𝑀 𝑀𝑀𝑀𝑀 = � � ∑ � 𝑡𝑡 � − 𝑜𝑜 � � � � �� (21) where tj is the target value, oj is the output value, and p is the pattern. The R 2 are normalised ranges between 0 and 1. A very good fit yields an R 2 value of 1, whereas a poor fit result in a value near 0. [27], [28] Using Eq. 18-21, the ANN structures of different architectures have been validated, where the number of hidden layers and the number of neurons in each hidden layer has been changed. The results of the validations of ANN structures of various architectures for the selection of the winning structure used in the auxiliary ST calculation unit are shown in Table 1. Table 1: Results of validations of ANN structures of various architectures for the selection of the winning ANN algorithm structure used in the auxiliary ST calculation unit. Algorithm Architecture Layers At Epochs Data Set Size MSE RMSE R 2 MAE ANN 20-18-25 5 120 5400 13.5643 3.6830 0.9994 1.5018 ANN 12-10-11 5 112 5400 10.7807 3.2834 0.9996 1.3753 ANN 5 164 5400 12.4752 3.5320 0.9995 1.5122 (18) whereas the root mean square (RMS) is defined as follows: Energo-economics payback investment calculation modelling of recently built gas-steam combined cycle power plant 9 ---------- calculation unit development process, an input data set was used during the learning phase, expressed in the [3x5400] matrix form and an output data set equally expressed in the [2x5400] matrix form. For each input data set, which in our case are steam mass flow, steam temperature and steam pressure into the ST, there is a specific output data, which are exhaust steam temperature and exhaust steam pressure from the ST. The validation of ANN algorithm structures is carried out by means of the calculations of the error between the results provided by the ANN algorithm structure and the actual process data. The errors can be computed in several ways. The most useful way of error computation is called the mean square error (MSE) and is defined as: [26] 𝑀𝑀 𝑀𝑀𝑀𝑀 = � � ∑ � 𝑡𝑡 � − 𝑜𝑜 � � � � � �� (18) whereas the root mean square (RMS) is defined as follows: 𝑅𝑅𝑀𝑀 𝑀𝑀 = � ( 1/ 𝑝𝑝 )∑ � 𝑡𝑡 � − 𝑜𝑜 � � � � � �� � � / � (19) The correlation coefficient (R 2 ) and mean absolute error (MAE) are respectively defined as: [27], [28] 𝑅𝑅 � =1− � ∑ � � � �� � � � � � �� ∑ � � � � � � ��� �(20) 𝑀𝑀 𝑀𝑀𝑀𝑀 = � � ∑ � 𝑡𝑡 � − 𝑜𝑜 � � � � �� (21) where tj is the target value, oj is the output value, and p is the pattern. The R 2 are normalised ranges between 0 and 1. A very good fit yields an R 2 value of 1, whereas a poor fit result in a value near 0. [27], [28] Using Eq. 18-21, the ANN structures of different architectures have been validated, where the number of hidden layers and the number of neurons in each hidden layer has been changed. The results of the validations of ANN structures of various architectures for the selection of the winning structure used in the auxiliary ST calculation unit are shown in Table 1. Table 1: Results of validations of ANN structures of various architectures for the selection of the winning ANN algorithm structure used in the auxiliary ST calculation unit. Algorithm Architecture Layers At Epochs Data Set Size MSE RMSE R 2 MAE ANN 20-18-25 5 120 5400 13.5643 3.6830 0.9994 1.5018 ANN 12-10-11 5 112 5400 10.7807 3.2834 0.9996 1.3753 ANN 5 164 5400 12.4752 3.5320 0.9995 1.5122 (19) The correlation coefficient (R 2 ) and mean absolute error (MAE) are respectively defined as: [27], [28] JET 19 Energo-economics payback investment calculation modelling of recently built gas-steam combined cycle power plant Energo-economics payback investment calculation modelling of recently built gas-steam combined cycle power plant 9 ---------- calculation unit development process, an input data set was used during the learning phase, expressed in the [3x5400] matrix form and an output data set equally expressed in the [2x5400] matrix form. For each input data set, which in our case are steam mass flow, steam temperature and steam pressure into the ST, there is a specific output data, which are exhaust steam temperature and exhaust steam pressure from the ST. The validation of ANN algorithm structures is carried out by means of the calculations of the error between the results provided by the ANN algorithm structure and the actual process data. The errors can be computed in several ways. The most useful way of error computation is called the mean square error (MSE) and is defined as: [26] 𝑀𝑀 𝑀𝑀𝑀𝑀 = � � ∑ � 𝑡𝑡 � − 𝑜𝑜 � � � � � �� (18) whereas the root mean square (RMS) is defined as follows: 𝑅𝑅𝑀𝑀 𝑀𝑀 = � ( 1/ 𝑝𝑝 )∑ � 𝑡𝑡 � − 𝑜𝑜 � � � � � �� � � / � (19) The correlation coefficient (R 2 ) and mean absolute error (MAE) are respectively defined as: [27], [28] 𝑅𝑅 � =1− � ∑ � � � �� � � � � � �� ∑ � � � � � � ��� �(20) 𝑀𝑀 𝑀𝑀𝑀𝑀 = � � ∑ � 𝑡𝑡 � − 𝑜𝑜 � � � � �� (21) where tj is the target value, oj is the output value, and p is the pattern. The R 2 are normalised ranges between 0 and 1. A very good fit yields an R 2 value of 1, whereas a poor fit result in a value near 0. [27], [28] Using Eq. 18-21, the ANN structures of different architectures have been validated, where the number of hidden layers and the number of neurons in each hidden layer has been changed. The results of the validations of ANN structures of various architectures for the selection of the winning structure used in the auxiliary ST calculation unit are shown in Table 1. Table 1: Results of validations of ANN structures of various architectures for the selection of the winning ANN algorithm structure used in the auxiliary ST calculation unit. Algorithm Architecture Layers At Epochs Data Set Size MSE RMSE R 2 MAE ANN 20-18-25 5 120 5400 13.5643 3.6830 0.9994 1.5018 ANN 12-10-11 5 112 5400 10.7807 3.2834 0.9996 1.3753 ANN 5 164 5400 12.4752 3.5320 0.9995 1.5122 (20) Energo-economics payback investment calculation modelling of recently built gas-steam combined cycle power plant 9 ---------- calculation unit development process, an input data set was used during the learning phase, expressed in the [3x5400] matrix form and an output data set equally expressed in the [2x5400] matrix form. For each input data set, which in our case are steam mass flow, steam temperature and steam pressure into the ST, there is a specific output data, which are exhaust steam temperature and exhaust steam pressure from the ST. The validation of ANN algorithm structures is carried out by means of the calculations of the error between the results provided by the ANN algorithm structure and the actual process data. The errors can be computed in several ways. The most useful way of error computation is called the mean square error (MSE) and is defined as: [26] 𝑀𝑀 𝑀𝑀𝑀𝑀 = � � ∑ � 𝑡𝑡 � − 𝑜𝑜 � � � � � �� (18) whereas the root mean square (RMS) is defined as follows: 𝑅𝑅𝑀𝑀 𝑀𝑀 = � ( 1/ 𝑝𝑝 )∑ � 𝑡𝑡 � − 𝑜𝑜 � � � � � �� � � / � (19) The correlation coefficient (R 2 ) and mean absolute error (MAE) are respectively defined as: [27], [28] 𝑅𝑅 � =1− � ∑ � � � �� � � � � � �� ∑ � � � � � � ��� �(20) 𝑀𝑀 𝑀𝑀𝑀𝑀 = � � ∑ � 𝑡𝑡 � − 𝑜𝑜 � � � � �� (21) where tj is the target value, oj is the output value, and p is the pattern. The R 2 are normalised ranges between 0 and 1. A very good fit yields an R 2 value of 1, whereas a poor fit result in a value near 0. [27], [28] Using Eq. 18-21, the ANN structures of different architectures have been validated, where the number of hidden layers and the number of neurons in each hidden layer has been changed. The results of the validations of ANN structures of various architectures for the selection of the winning structure used in the auxiliary ST calculation unit are shown in Table 1. Table 1: Results of validations of ANN structures of various architectures for the selection of the winning ANN algorithm structure used in the auxiliary ST calculation unit. Algorithm Architecture Layers At Epochs Data Set Size MSE RMSE R 2 MAE ANN 20-18-25 5 120 5400 13.5643 3.6830 0.9994 1.5018 ANN 12-10-11 5 112 5400 10.7807 3.2834 0.9996 1.3753 ANN 5 164 5400 12.4752 3.5320 0.9995 1.5122 (21) where t j is the target value, o j is the output value, and p is the pattern. The R 2 are normalised ranges between 0 and 1. A very good fit yields an R 2 value of 1, whereas a poor fit result in a value near 0. [27], [28] Using Eq. 18-21, the ANN structures of different architectures have been validated, where the number of hidden layers and the number of neurons in each hidden layer has been changed. The results of the validations of ANN structures of various architectures for the selection of the winning structure used in the auxiliary ST calculation unit are shown in Table 1. Table 1: Results of validations of ANN structures of various architectures for the selection of the winning ANN algorithm structure used in the auxiliary ST calculation unit. Algorithm Architecture Layers At Epochs Data Set Size MSE RMSE R 2 MAE ANN 20-18-25 5 120 5400 13.5643 3.6830 0.9994 1.5018 ANN 12-10-11 5 112 5400 10.7807 3.2834 0.9996 1.3753 ANN 9-7-6 5 164 5400 12.4752 3.5320 0.9995 1.5122 ANN 45-37 4 112 5400 12.6885 3.5621 0.9995 1.5140 ANN 22-21 4 149 5400 12.5996 3.5496 0.9995 1.5002 ANN 12-9 4 167 5400 11.5722 3.4018 0.995 1.4968 ANN 7-9 4 187 5400 13.2776 3.6438 0.9995 1.5565 ANN 60 3 276 5400 11.7001 3.4205 0.9995 1.4307 ANN 40 3 108 5400 14.7699 3.8432 0.9994 1.6032 ANN 20 3 317 5400 12.6844 3.5615 0.9995 1.5384 ANN 12 3 258 5400 13.1586 3.6275 0.9995 1.5341 ANN 5 3 187 5400 15.6925 3.9661 0.9994 1.6541 Table 1 shows that the winning ANN structure used in the auxiliary GT calculation unit is the structure with 12-10-11 architecture (written in bold), as it has the lowest error rate. The process of creation and the regression of the ANN structure, used in the auxiliary GT calculation unit, are shown in Fig. 5. 20 JET 20 JET Dušan Strušnik, Jurij Avsec JET Vol. 15 (2022) Issue 4 Figure 5: Process of creation and regression of the winning ANN non-linear structure used in the simulation model of the non-linear ANN unit. The creation of the ANN structure, used in the auxiliary ST calculation unit, was performed with 212 epochs. The best validation agreement of the MSE is 10.7807 and it was reached at the 112 th epoch, whereby regression is R 2 0.9996. 4 THE RESULTS OF THE ENERGO-ECONOMIC PAYBACK INVE- STMENT CALCULATION MODEL The results of the energo-economic payback investment calculation model are designed so that the amount and power of natural gas consumed and required for the operation of the GSCCP is presented first. Then, the quantities of HP steam, LP steam and thermal power generated by GSCCP are presented. Following this is a presentation of the useful efficiency of GT and GSCCP operation, energy flows and the amount of greenhouse CO 2 gas released into the atmosphere after 5400 hours of GSCCP operation. At the end of the chapter, the results of the calculations of the payback period of the investment depending on the price ratio of the fuel required for the operation of the GSCCP and the total generated electricity are presented. Fig. 6 shows the natural gas consumption for GSCCP operation as a function of generator pow- er GT. At the power of the GT generator of 5 MW, the natural gas consumption amounts to 4341.1 Nm 3 /h, a standard cubic metre per hour defined at a natural gas reference temperature of 0 °C and a natural gas reference pressure of 1.013 bar. At the power of the GT generator of 30 MW, the consumption of natural gas amounts to 8859.8 Nm 3 /h, and at a maximum load of 55 MW of the GT generator, the consumption of natural gas for GSCCP operation is as much as 13465 Nm 3 /h. JET 21 Energo-economics payback investment calculation modelling of recently built gas-steam combined cycle power plant Figure 6: The results of GT calculation unit; (a) natural gas consumption depending on the load of the GT generator; and (b) power of consumed natural gas taking into account HHV and LHV. Fig. 6(b) shows the power consumption of natural gas needed for the GSCCP operation. The power of natural gas considering HHV is taken into account in the analysis of the payback period of the investment, as the economic cost calculations of gas consumption take into account the HHV. The power of natural gas considering the LHV is used in all other technological calculations, such as the useful efficiency calculations, etc. It is evident from Fig. 6(b) that at the power of the GT generator of 5 MW, the power of the consumed fuel when considering HHV is 49.2 MW and when considering LHV the power of the consumed fuel is 44.6 MW. At a maximum load of the GT generator of 55 MW, the power of consumed fuel amounts to 152.8 MW, when taking into account HHV, and 138.4 MW when taking into account LHV. However, if the process useful efficiency calculations were based on the HHV power of the fuel consumed, they would be sig- nificantly lower. Fig. 7 shows the results of the HRSG and ST calculation unit. It is evident from Fig. 7(a) that at the power of the GT generator of 5 MW, the amount of generated HP steam is 3.5 kg/s, the amount of generated LP steam is 0.5 kg/s and the generated DH heat from HRSG is 1.6 MW. At the power of the GT generator of 30 MW, the amount of generated HP steam is 13.8 kg/s, the amount of generated LP steam is 2.2 kg/s and the generated DH heat from HRSG is 6.3 MW. At the maxi- mum power of the GT generator of 55 MW, the amount of generated HP steam is 17.7 kg/s, the amount of generated LP steam is 3.5 kg/s and the generated DH heat from HRSG is 9.5 MW. 22 JET 22 JET Dušan Strušnik, Jurij Avsec JET Vol. 15 (2022) Issue 4 Figure 7: The results of HRSG and ST calculation unit; (a) flows of generated HP steam, flows of generated LP steam, generated DH heat from HRSG and (b) generated DH heat from ST, electri- cal power of ST generator and power of LP steam for industrial use. It is evident from Fig. 7(b) that at the power of the GT generator of 5 MW, the generated DH heat from ST is 7.7 MW, the power of the ST generator is 1.8 MW and the power of LP steam is 1.4 MW. At the power of the GT generator of 30 MW, the generated DH heat from ST is 30.5 MW, the power of the ST generator is 7.4 MW and the power of LP steam is 5.8 MW. At the maximum power of the GT generator of 55 MW, the generated DH heat from ST is 39.1 MW, the power of the ST generator is 9.5 MW and the power of LP steam for industrial purposes is 9.3 MW. Figure 8: GT useful efficiencies and GSCCP useful efficiencies as a function of the power of the GT generator. JET 23 Energo-economics payback investment calculation modelling of recently built gas-steam combined cycle power plant Fig. 8 shows GT useful efficiencies and GSCCP useful efficiencies as a function of the power of the GT generator. At the power of the GT generator of 5 MW, the GT useful efficiency amounts to 10% and the GSCCP useful efficiency to 39%. At the power of the GT generator of 30 MW, the GT useful efficiency amounts to 34% and the GSCCP useful efficiency amounts to 87%. At the maximum power of the GT generator of 55 MW, the GT useful efficiency amounts to 40% and the GSCCP useful efficiency amounts to 88%. Fig. 9 shows the energy flows and the amount of greenhouse CO 2 gas released into the atmos- phere after 5400 hours of GSCCP operation, as the constant 50 MW power of the GT generator is taken into account. As much as 768,624 MWh of natural gas are required for 5400 hours of un- interrupted operation, taking into account HHV, while 696,475 MWh of natural gas are required when taking into account LHV. The GSCCP generates 319,793 MWh of electricity, 250,729 MWh thermal energy for DH and 45,549 MWh thermal energy of LP steam used for industrial purpos- es. At the same time, the GSCCP emits 140,660 tons of greenhouse CO 2 gas into the atmosphere. Figure 9: Energy flows and the amount of greenhouse CO 2 gas released into the atmosphere after 5400 hours of GSCCP operation. The results of the payback calculation unit depending on the ratio of the price of natural gas, taking into account HHV, and the price of electricity excluding and taking into account the cost of purchasing CO 2 carbon offsets is shown in Fig. 10. The calculations take into account that GSCCP operates 5400 hours per year, the investment costs amount to 75,000,000.00 monetary units, the discount rate is 7%, tax rate is 22%, maintenance costs are 2% of investment costs per year, the price of district heating heat is fixed and amounts to 70.00 monetary units per MWh, the purchase price of carbon offset is fixed and amounts to 70.00 monetary units per tonne of CO 2 emitted, and the power of the GT generator is fixed and amounts to 50 MW. In addition to this, the calculation does not take into account a possible subsidy for the production of high-efficiency electricity. The said subsidy can in fact be offset by the cost of purchasing CO 2 carbon offsets. The grey areas in Figure 10 represent the zero balance or payback period of the investment. This subsidy can in fact be offset by the cost of purchasing CO 2 carbon offsets. The grey area in Fig. 10 represents the zero balance or payback period of the investment. 24 JET 24 JET Dušan Strušnik, Jurij Avsec JET Vol. 15 (2022) Issue 4 Figure 10: The calculation of the payback period of the investment depends on the ratio of natu- ral gas prices at HHV value and electricity price; (a) excluding the cost of purchasing CO 2 carbon offsets and (b) taking into account the cost of purchasing CO 2 carbon offsets. It is evident from Fig. 10(a) that the payback period of the investment, excluding the cost of purchasing CO 2 carbon offsets, ranges from 4 years for the 0.35 fuel/electricity price ratio and up to 20 years for the 0.49 fuel/electricity price ratio. Without taking into account the cost of purchasing CO 2 carbon offsets at a fuel/electricity price ratio of 0.46, the payback period is 10 years. However, with fuel/electricity price ratios higher than 0.49, the investment does not pay off even across 20 years. It is evident from Fig. 10(b) that the payback period of the investment, taking into account the cost of purchasing CO 2 carbon offsets, ranges from 7 years for the 0.35 fuel/electricity price ra- tio and up to 20 years for the 0.42 fuel/electricity price ratio. Taking into account the costs of purchasing CO 2 carbon offsets, the payback period of the investment is 10 years at 0.39 fuel/ electricity price ratio. However, with fuel/electricity price ratios higher than 0.42, the investment does not pay off even across 20 years. With fuel/electricity price ratios higher than 0.49, taking into account the cost of buying CO 2 carbon offsets, the cash flow of net present value becomes negative, which means that we start to generate a negative return or loss. The essential impor- tance of the cost-effective operation of the GSCCP is therefore dictated by the market dynamics of fuel, electricity and thermal energy prices. 5 CONCLUSION This paper has presented both the energo-economic calculation modelling of GSCCP operation with the basic characteristic properties of the system behaviour in different operating regimes, and the energy flows generated by the uninterrupted 5400-hour operation of the GSCCP. The results of the calculation modelling show that the GSCCP can achieve a useful efficiency of up to 88% in the backpressure operation of a steam turbine. The useful efficiency of the gas turbine is JET 25 Energo-economics payback investment calculation modelling of recently built gas-steam combined cycle power plant up to 40.5%. In 5400 hours of continuous operation and 50 MW of uninterrupted constant pow- er, the GT generator requires as much as 768,628 MWh of natural gas at HHV value and 696,475 MWh of natural gas at LHV value. The GSCCP generates 319,793 MW of electricity, 250,729 MWh of thermal energy for DH, and 45,549 MWh of thermal energy for industrial purposes, while 140,660 tons of CO 2 greenhouse gas are emitted into the environment. The calculation of the payback period of the investment is based on the calculation of the net present value taking into the account the HHV value of natural gas. The results of the calculation of the payback period of the investment show that the payback period depends mainly on the market conditions of ener- gy products. 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Nomenclature Abbre viations ANN artificial neural network DH district heating GSCCP gas-steam combined cycle power plant GT gas turbine HHV higher heating calorific value HRSG heat recovery steam generator HP high pressure LHV lower heating calorific value LP low pressure 28 JET 28 JET Dušan Strušnik, Jurij Avsec JET Vol. 15 (2022) Issue 4 MAE mean absolute error MSE mean square error RMS root mean square R 2 correlation coefficient SCADA supervisory control and data acquisition ST steam turbine CO 2 carbon dioxide P arame t ers C el electricity price, monetary unit C ther thermal price, monetary unit c os ts annual coast, monetary unit Dep annual depreciation, monetary unit Dur investment duration, years h HP specific enthalpy of HP steam, MJ/kg h LP specific enthalpy of LP steam, MJ/kg h OUT specific enthalpy of steam from expansion cylinder of ST, MJ/kg Inc annual income, monetary unit I nv investment value, monetary unit NCF net cash flow, monetary unit NPV net present value, monetary unit P DH-HRSG DH generated heat from HRSG, MW P DH-ST DH generated heat from ST, MW P GTe GT generator power, MW P HHV power of natural gas taking into account HHV, MW P LHV power of natural gas taking into account LHV, MW Pr el-ann annual production of electricity, MWh Pr ther -ann annual production of heat, MWh