Theoretical Optimization On Cycles And Experimental Investigation On Organic Rankine Cycle Performances Of Different Working Fluids For Low-temperature Geothermal Power

There are abundant geothermal resources in China, most of which are low-temperature (<100°C) water-dominated heat source. Because of the low-gradecharacteristics and the lack of economic with conventional power generationtechnologies, these sources are mainly used for direct heating. However, the heatdemand near the heat source is usually less than the source can provide. A lot ofresources are not developed. In recent years, the governments put out many renewableenergy encouragement policies, which promote the development of the renewableenergy power generation technology. With the improvement of some novellow-temperature power generation technology, such as Organic Rankine Cycle(ORC),Organic Trans-critical Cycle(OTC) and Kalina Cycle etc., the profitable temperaturefor power generation is gradually reduced. So the potential and economic prospects oflow-temperature geothermal power generation become more and more attracting. Ifthe low-temperature geothermal is developed with ORC, OTC or Kalina cycle, thecost of fossil fuel will be saved and the corresponding pollutant will be decreased.The investigations on power generation systems were conducted to improve theeconomic performances of low-temperature geothermal (80-100oC) power plant. Theeconomic objective function was determined by parameter analysis. The parameteroptimization and performance comparison of ORC, OTC and Kalina cycle wereperformed and the optimal cycle and parameters were put up. Based on the optimizedresults, the experimental apparatus was retrofitted and the performance of theexpander was tested. The experimental performances were studied for parameteroptimization and fluid screening.The ultimate goal was to decreasing the cost of geothermal power plant. Bycomparing the parameters optimized by the specific net work and the cost, the specificnet work was established as the objective function.Numerical simulation models of the ORC, OTC and Kalina Cycle weredeveloped based on methodology of pinch point analysis with the specific net work asthe objective function. The theoretical optimization was conducted by varying themany influencing factors. The results were shown as follows: With the specified cooling and heating resource conditions, the main influencing factors on specific network were fluids, parameters and component efficiencies. There existed the optimalparameters that could maximize the specific net work. The optimized working fluidswere MA6and MB8for ORC, MM and MH for OTC, and NH3/H2O (98/2mol%) forKalina. Due to the disadvantages of OTC and Kalina Cycle, such as the highBPR(back to power ratio) value, high pressure, complicated system, and the problemof toxic flammable working fluids, the technical economic aspect of non-azeotropicmixtures ORC was better than OTC and Kalina Cycle.The scroll expander was derived by retrofitting an automotive air-conditioningscroll compressor. In order to optimizing the performance of the expander, expandingthe temperature range of the heat source and the pressure range of working fluids,improving the indicator of the ORC system, the ORC experimental apparatus wasimproved by changing several devices. The performance of the expander and the ORCsystem were investigated with R245fa on the improved experimental system. Theresults were shown as follows. The rotation speed of the expander, the match of fluidsexpansion ratio and the build-in expansion ratio of expander, the leakage of the fluidsbetween the expansion chambers were main factors which affected the isentropicefficiency of the expander. The variation of isentropic efficiency with expansion ratioand rotation speed were obtained experimentally and optimal rotation speed was800rpm and optimal expand ratio was about2.There were two unfair factors when the performances of different working fluidswere compared in the same test device.(1)The optimize match of different workingfluids in the same test device was not identical.(2)The efficiencies of the componentsaffected the performance of different ORC systems. The evaluation methods wereinvestigated to rectify the unfair factors of the comparison results. The unfair factorswere analyzed and a novel experimental fair comparative method was put up. Theheat flux differences of heat exchangers and the low net power output due to the overlow efficiencies of working fluid pump were rectified. The experimental performancecomparative investigations were conducted on ORC system with the pure fluids(R245fa, R123, R227ea, R134a) and the mixtures (MA6, MA3, MD1, MC1, MB8).The results of corrected experimental results showed that the specific net power wasgreatly affected by the cooling and heating conditions. There existed the optimalparameters that could maximize the specific net work. The optimal evaporatingtemperature decreased with the decreasing of condensing temperature. When the inlet temperature of the heat source was90°C and the condensing temperature was30°C,MB8and MA3provided the the highest specific net work, which were10.58kJ/kg and10.79kJ/kg respectively. While the optimum evaporation temperature was59.2°C、62°C, respectively. When the condensation temperature was45°C, MC1and MB8exhibited the highest specific net work of5.96kJ/kg and5.82kJ/kg. The optimumevaporating temperature was71.2°C and71°C,respectively.