Parametric Investigation and Off-Design Simulation of Low Temperature Organic Rankine Cycle for Residential Applications

Abstract

The energy demand across the world is increasing rapidly as a result of massive urbanization and industrialization. This is coupled with scarcity of traditional energy sources and severe environmental issues such as global warming and climate change. In order to counter this problem, there is a sense of urgency to explore alternative sources of energy. In this regard, harnessing the renewable energies and waste heat recovery are considered as potential solutions that can effectively address these issues. The organic Rankine cycle is proved to be reliable technology that can efficiently convert these low to medium-grade heat sources into useful power. The ORC power block consists of a pump which is used for pumping the working fluid to the desired pressure. This pressurized fluid is then passed on to the evaporator where heat addition takes place. The pressurized vapor passes through the expander, where the actual expansion of the working fluid takes place and the pressure drops. Finally, the vapor condenses in the condenser to complete the cycle. In ORC systems, the enthalpy drop across the expanders is less. Net work output per unit mass of the working fluid is small in ORC plants. In order to achieve higher power output (greater than 10 kWe), mass flow rate of working fluids has to increase. This will increase the size and cost of the ORC system. Therefore, it is reasonable to adopt small capacity ORC systems (1-5 kWe). The present study focussed in detail the overall system performance in terms of thermal and exergy efficiencies. Moreover, major components constituting the ORC system such as evaporator and expander are assessed. A thermodynamic model for ORC system was developed based on laws of mass and energy conservation. Using this model, ORC thermal and exergy efficiencies were evaluated for four different working fluids; R245fa, R123, Isobutane and R134a. Sensitivity analysis was performed using key thermodynamic parameters including expander inlet temperature, expander inlet pressure, condensation temperature and pinch point temperature difference (PPTD) to study its effect on net work output, mass flow rate, thermal and exergy efficiencies. Optimization of the system was also performed using genetic algorithm. The system was optimized to maximize cycle exergy efficiency. Parametric analysis was carried out to investigate the impact of evaporator pressure, condensation pressure, superheat, dead state temperature and PPTD on the system performance. Thermo-hydraulic model of plate heat ii exchanger evaporator was used to study the effect of evaporator pressure, PPTD and superheat on evaporator area. The effect of expansion ratio, shaft speed and expander inlet temperature on mass flow rate, work output and efficiency of open-drive scroll expander was studied using validated semi-empirical model. Finally, cost analysis and exergoeconomic optimization of 1 kWe driven solar ORC system was performed to compare the cost of solar ORC with solar PV in India and to determine the minimum cost of electricity respectively. Optimization results showed that the highest thermal efficiency (7.1%) and exergy efficiency (45.53%) at lowest expander inlet pressure (3.66 bar) was attained with R123. This was followed by R245fa with thermal efficiency of 7.04% and exergy efficiency of 44.98% at expander inlet pressure of 6.07 bar. R245fa was preferred for this study as it is a zero ODP fluid and also has a lower specific volume compared to R123. Sensitivity analysis showed that, expander inlet pressure showed the highest degree of sensitiveness for all working fluids. Detailed exergy analysis of ORC components was performed to identify the location and to assess the magnitude of exergy losses occurring within the ORC system. Exergy analysis of 1 kWe ORC system showed that, evaporator accounted for the maximum exergy loss. 41% of the total exergy loss occurred in the evaporator. It was also observed that evaporator pressure had significant effect on both energy and exergy efficiencies of the ORC. Significant reduction in evaporator area (75.87%) and cost (63.59%) was observed when evaporator pressure was increased from 4 to 10 bar. Heat exchanger area decreased by 89.65% and evaporator cost was reduced by 74.86% when PPTD was increased from 2 to 14 ºC. It was also observed from the model that the cost increases whereas the pressure drop decreases with increase in plate width and plate spacing. The trade off point for plate width was at 0.0065 m, where the evaporator cost was found to be 1166 USD (Rs 87,450) and frictional pressure drop was 2.03 kPa. In case of plate spacing, it was at 0.003 m, where the evaporator cost was 1210 USD (Rs 90,750) and frictional pressure drop was 1.27 kPa. Parametric investigation of scroll expander showed that the scroll expander should be operated in a range close to its adapted expansion ratio to achieve maximum efficiency. It was also revealed that increasing expander inlet temperature led to increase in thermal energy dissipation. This leads to the deterioration in efficiency of the expander. The economic analysis showed that the capital cost of small scale ORC systems is very high (Rs 7,42,500) compared to equivalent solar PV system (Rs 85,000) in India. Exergoeconomic optimization showed that minimum electricity cost of 3.9 Rs/kWh could be attained at maximum evaporator pressure of 13.9 bar.