THERMODYNAMIC ANALYSIS OF SOLAR INTEGRATED THERMAL POWER PLANT USING ORC FOR WASTE HEAT RECOVERY

Abstract

Solar power tower (SPT) and parabolic trough solar collector (PTSC) system is really a promising option to harness the solar energy with the purpose of solar thermal electricity generation via power cycles. These days, combined cycles especially based on supercritical carbon dioxide (sCO2) cycle are very much popular. Performance of different configurations of sCO2 cycles driven with SPT/PTSC have been further investigated in this thesis work. Apart from this, organic Rankine cycle (ORC), parallel double evaporator ORC (PDORC), supercritical ORC (SORC) were used as bottoming waste heat recovery cycle, also a short analysis have been performed with SORC combined with vapor absorption refrigeration (VAR) cycle, vapor compression cycle (VCC) for cooling and power generation. Simultaneously, effects of the working fluids on the system performance were investigated. At firstly, the output of the SPT-driven combined pre-compression sCO2 cycle-ORC for waste heat recovery was investigated. The use of ORC increased thermal efficiency and net power output of the pre-compression cycle by 4.52% and 4.51%, respectively. Solar irradiation improved the highest power output, exergy efficiency and thermal efficiency of the combined cycle, with highest values of 278.5kW, 74.06 and 51.83% at 1000 W/m2 of direct normal irradiation (DNI) using R227ea. Waste heat recovery ratio (WHRR) was improved with heat exchanger effectiveness. Based on R227ea, highest value reached to 0.5673 at effectiveness of 0.95. Also this system further examined using the low GWP HFO fluids such as R1234ze(Z), R1224yd(Z), R1225ye(Z), R1233zd(E), R1234yf, R1243zf, R1234ze(E), and R1336mzz(Z) and compared with the R134a fluid. It was concluded from the results that HFO working performed better than the R134a. R1336mzz(Z) gave highest power output, exergy efficiency and thermal efficiency for the combined cycle by 298.5kW, 59.60%, and 55.02% at 950 W/m2 of DNI respectively. At end, it was found that the R1336mzz(Z) and R1234ze(Z) estimated the lowest and highest specific investment cost i.e. 2234 and 2290 €/kWe respectively. It was also concluded from the results that if fluid is best for thermal performance, it is not necessary it would be best for the economic point of view. Secondly, for the performance evaluation of the SPT driven recompression with main compressor intercooling (RMCIC) sCO2 cycle incorporating the PDORC as bottoming cycle using eight ultra-low GWP HFO working fluids such as R1234ze(Z), R1224yd(Z), R1225ye(Z), R1233zd(E), R1234yf, R1243zf, R1234ze(E), and R1336mzz(Z) were therefore considered for the PDORC analysis to reduce the global warming and ozone depletion. Using the PDORC instead of the basic ORC, waste heat from exhaust and from the intercooler cycle vi was recovered simultaneously to enhance performance of the standalone RMCIC cycle. Exergy, thermal efficiency, efficiency improvement and waste recovery ratio were considered as performance parameters. It has been found that by the incorporation of the PDORC thermal efficiency was improved by 7-8% at reference conditions. Maximum combined cycle’s thermal and exergy efficiency were found 54.42% and 80.39% respectively of 0.95 kW/m2 of solar irradiation based on R1243zf working fluid. Among the results it was also found that maximum waste heat was recovered by the R1243zf about 54.22 % at 0.95 effectiveness of low temperature recuperator. Thirdly, parametric analysis of the SPT integrated combined cascade sCO2 (CSCO2) cycle and ORC. Effects of topping cycle parameters on combined cycle and ORC performance were investigated. Results show that application of the ORC to the topping cycle improved the thermal performance by approximately 6-6.5 %. Highest thermal, exergy efficiency and net output work of combined cycle were obtained by 45.35, 66.99% and 204.9kW respectively at 1000W/m2. In order to examine the utility of the basic ORC and PDORC with the standalone intercooled CSCO2 cycle, two configurations were considered, such as the intercooled CSCO2 cycle/ORC (configuration-1) and the intercooled CSCO2 cycle/PDORC (configuration-2). The effects of SPT design parameters, high temperature turbine inlet temperature, compressors inlet temperature and main compressor’s inlet pressure on the system performance were investigated. It was concluded that by incorporating the basic ORC and PDORC to standalone intercooled CSCO2 cycle, thermal efficiency improved by 2.26 and 6.66% respectively. Therefore, for recovering the waste heat PDORC (configuration-2) performed better than basic ORC (configuration-1) and waste heat recovery ratio for basic ORC and PDORC were obtained by 0.1197 and 0.1775 respectively at LTR effectiveness of 0.95. Finally, the performance of the combined cycles can be further enhanced by reducing solar emittance and improving the concentration ratio. Apart from sCO2cycles, also SPT system incorporated with combined SORC and VAR system, further performance analysis was performed. Thermal and exergy efficiency of the combined system increased with DNI. Maximum thermal and exergy efficiency were obtained 46.60% and 68.25% respectively at 950 W/m2 while maximum exergy destruction was obtained 7589.46 kW at 500W/m2. The maximum COP for heating and cooling were found 1.4452 and 0.4448 respectively at 90℃ of generator temperature. Apart from the SPT system, performance of the PTSC system also examined incorporating with sCO2 cycle and SORC and VCC systems in further study. The parametric evaluation of the partial heating sCO2 (PSCO2) cycle combined with ORC was performed in this thesis vii considering six working fluids such as R1224yd(Z), R1234ze(Z), R1234yf, R1234ze(E), R1233zd(E) and R1243zf for low temperature application. The basic PSCO2 cycle was then compared to previous studies that had not included partial heating. The PSCO2 system was found to be 1-3% more thermally efficient than the without partial heating cycle. Moreover, it was discovered that incorporating ORC into the current PSCO2 cycle increased thermal efficiency by 4.47% over the standard PSCO2 cycle. The impact of the PTSC on combined cycle efficiency was investigated further. Without taking into consideration the performance of PTSCs, the combined cycle using R1233zd(E) got the highest exergy and thermal efficiency of 83.26% and 48.61%, respectively, at 950 W/m2 of DNI. When taking into consideration the performance of PTSCs, the combined cycle achieved exergy efficiency of 42.31% because PTSCs alone accounted for 62.93% of the total exergy. Also the high solar incidence angle was responsible for the system's poor performance. Furthermore, thermodynamic analysis PTSC driven SORC coupled with VCC system simultaneously for cooling and power production were carried out. Influences of the PTSC design parameters, turbine inlet pressure, and condenser and evaporator temperature on system performance were discussed. Furthermore, the performance of the cogeneration system was also compared with and without PTSCs. It was concluded that it is necessary to design the PTSCs carefully in order to achieve better cogeneration performance. At 0.95 kW/m2 of DNI based on R227ea, the highest exergy destruction, thermal efficiency and exergy efficiency of the cogeneration system were 1437 kW, 51.13% and 92.9% respectively, but the highest coefficient of performance was found to be 2.278 on the basis of R134a. At last, study examined the effect of SPT design parameters on SPT integrated combined CSCO2-ORC using ultra GWP HFO fluids. Exergy efficiency, thermal efficiency and net output power were considered as performance parameters. It was investigated that thermal and exergy efficiencies of the standalone (SPT+ CSCO2) cycle improved by 2.36% and 2.41% respectively by the incorporation of the ORC as bottoming cycle. Highest exergy efficiency, thermal efficiency and net output power were increased with DNI, concentration ratio, HTF velocity while decreased with solar emittance. Highest performance were found with R1224yd(E) while lowest with R1234yf among other considered low GWP fluids at current input conditions. Finally, comparison analysis was carried out. Combined cycle RMCIC sCO2-PDORC was considered best performing cycle for power generation among all considered models. While SORC-VCC system was selected as best system for simultaneously cooling and power generation.