Mixed-Fluid Organic Rankine Cycles for Low Temperature Power Conversion

Abstract

The purpose of the thesis is to investigate and optimize refrigerant fluid mixtures for applications in low-temperatures ORC in order to maximize brine effectiveness (b ) rate for geothermal energy conversion in organic Rankine cycles. An in-house developed Matlab program interfaced with the SimulisTM thermodynamic database software was used to ascertain the performance of cycles having various mixture compositions. Brine effectiveness (the ratio of net work to geofluid mass flow rate) was used as the main metric to measure the performance of the Organic Rankine Cycle according to mixture composition, turbine inlet temperature and pressure. The optimum mixture concentrations have been found to depend on the plant design. There is an optimal pressure at different composition. The results highlight that it is not possible to obtain the maximum brine effectiveness, the maximum thermal efficiency, the smallest exergy destruction in the condenser and minimum turbine size factor simultaneously. For a geothermal plant, it is significant to maximize the power generated per unit flow of the geothermal fluid. The first-law efficiency is of secondary importance. These results demonstrate the importance of using brine effectiveness as the main performance metric rather than the thermal efficiency, which may not always describe the cycle that makes optimal use of the available energy. Similar optimization goals are presented in waste heat recovery, and the methods and findings of this thesis may be of interest in waste heat recovery applications.Based on the results obtained so far, selection of working fluids should depend on the specific conditions for each application, most notably the temperature of the resource. Working fluid critical temperature is very important in selecting the optimal working fluid. It is better to use a fluid with a critical temperature lower or a little bit higher than the heat resource temperature for cycles without superheating. The fluidrs critical temperature then tends to determine the optimal cycle pressure. Mixed fluids have some advantages over pure working fluids. Most importantly, temperature glide (non-isothermal phase change) improves the performance in power cycle heat exchangers, especially the condenser, improving the performance of the overall power cycle. According to the fluid selection criteria, this thesis will focus on isentropic and dry fluids, which have higher thermal efficiencies because the turbiners expansion process terminates in the superheated region. 17 pure fluids were identified for low-moderate temperature power conversion and the performances of their mixtures were simulated at various ratios at 90dC, 110dC and 130dC, respectively.Experimental Vapor-Liquid equilibrium (VLE) data (R134a+R227ea at isotherms of 324.15, 349.15 and 368.15 K and R134a+R245cb at isotherms of 324.15, 349.15 and 373.15 K) were collected to evaluate the VLE results obtained with a predictive thermodynamic model (SRK-MHV2-UNIFAC) and for the optimization of the binary interactions parameters of the Non-Random-Two-Liquid (NRTL) model used for the calculation of the Gibbs Free Energy of Excess inside of the Michelsen-Huron-Vidal-second-order mixing rule (MHV2) used in combination with the Soave-Redlich-Kwong (SRK) equation of state. At the very least that the VLE data allowednvalidation of the use of the equations of state for these fluids, and a good indication that they should be generally applicable to all the fluids which were studied in this thesis.The results obtained with the two thermodynamic models were also compared in order to evaluate the predictive methodrs reliability. The two EoS/GE thermodynamic models were also used, to discuss and validate results of the simulation.Experiments on a laboratory scale ORC, using a mixture of a 50/50 mass ratio of R134a/R245fa as working fluid, were carried out for verifying the simulation results according to the geo-fluid mass flow rate values. Error between experimental heating fluid flow-rate and predictions from the cycle model were consistently around 2%, indicating that the model is accurate and well-suited to the addressing the purpose of the thesis of optimizing cycle working fluid selection. The approaches and outcomes of this research are applicable to a more efficient utilization of low temperature geothermal resources and could also be applied in the area of low temperature waste heat recovery.