Comparative Study On The Thermo-economic Performance Of ORC Under Different Heat Source Conditions And Utilizing Patterns

This project mainly focused on the external heat source conditions for subcritical organic Rankine cycle(sub-ORC), and compared the thermo-economic performance between the sub-ORC and transcritical organic Rankine cycle(trans-ORC) or Li Br absorption chiller system under different heat source conditions.For sub-ORC, the variations of performance indicators: net work(Wnet), exergy efficiency(ηex) and levelized energy cost(LEC) versus Tgo were discussed from the perspective of thermo-economics. Considering the corrosion of low temperature flue gas, the necessity and reasonability of limiting Tgo at its minimum allowed discharge temperature were analyzed. Moreover, the critical heat source conditions available for different working fluids in sub-ORC were discussed based on the thermodynamic constraint that the evaporator pressure of working fluid must be lower than its critical pressure and the economic constraint that a reasonable ORC system should have a non-negative net benefit. The critical heat source conditions include the maximum heat source temperature(Tgi,max), the minimum heat source temperature(Tgi,min) and the minimum heat source mass flow rate(mg,min). Then, the coupling relationships between heat source conditions(non-dimensional inlet temperature αTgi and non-dimensional mass flow rate of flue gas αmg) and performance indicators of sub-ORC including non-dimensional net power output(αWnet), ηex and annualized net benefit per unit investment(NBI) were conducted. Results show that there exist optimal outlet temperature of flue gas(Tgo,opt) for Wnet and LEC, while Tgo,opt for ηex does not appear under the investigated range of Tgo. For the waste heat recovery of low temperature flue gas, it is reasonable to fix Tgo at its minimum allowed discharge temperature if Wnet or LEC is selected as primary performance indicator under the pinch point temperature difference of evaporator below 20 K. What’s more, no matter for pure working fluid or the zeotropic mixture working fluid, Tgi,min available for the working fluid increases with the increasing of its critical temperature(Tcr). For the pure working fluid, Tgi,min is not sensitive to the changing of Tcr. For the zeotropic mixture working fluid, Tgi,min available for the working fluid grows up first and then drops down versus the augment of Tcr. No matter for pure working fluid or the zeotropic mixture working fluid, with the increase in mass flow rate of heat source(mg), the region of the available heat source temperature obviously expands, and mg,min reduces sharply with the increasing of heat source temperature. The αWnet, ηex and NBI increase in function of quadratic polynomial, power and linearity along with the increasing of αTgi, respectively. And NBI follows discipline of power function with the increasing of αmg.For the comparative between sub-ORC and trans-ORC, the Wnet, ηex and LEC were also selected as performance indicators. Firstly, the turbine inlet temperature and pressure meeting the requirement of pinch point temperature difference of evaporator in trans-ORC were determined based on performance optimization. Subsequently, the thermo-economic performance of a sub-ORC using R601 as working fluid and a trans-ORC using R134 a as working fluid have been compared under different heat source temperatures and a fixed Tgo. Results show that, for trans-ORC, when the pinch point temperature difference of evaporator lies between the inlet and outlet of evaporator, a lower inlet pressure of turbine is favorable; when the pinch point temperature difference of evaporator is located at the outlet of evaporator, there exists an optimal inlet pressure of turbine. Either for sub-ORC or trans-ORC, Wnet increases and LEC decreases with the increase in heat source temperature(Tgi). For sub-ORC, ηex increases monotonously with Tgi, while, for trans-ORC, ηex grows up firstly and then reduces(or keeps constant) with the increasing of Tgi. Moreover, for Wnet and ηex, there exists a range of heat source temperatures making trans-ORC better than sub-ORC, and the heat source temperature region extends with the increase in pinch point temperature difference of evaporator. For LEC, the sub-ORC is always superior to trans-ORC.Lastly, the comparison between sub-ORC and Li Br absorption chiller system was conducted combined with an engineering project from the perspective of economics. Results show that, at any Tgi, mg and local electricity or cooling price(Se or Sc), refrigeration cycle and sub-ORC using zeotropic mixture working fluid are better than that of sub-ORC using pure working fluid. There exists critical Tgi(Tgi,cr), critical mg(mg,cr) and critical Se(Se,cr) making the economic benefit of sub-ORC using zeotropic mixture working fluid and refrigeration cycle to be equal. If Tgi(or mg or Se) is lower than Tgi,cr(or mg,cr or Se,cr), the refrigeration cycle is prior, on the contrary, the sub-ORC using zeotropic mixture working fluid is preferential. The Tgi,cr, mg,cr and Se,cr are all increase with the augment of the ratio of Sc to Se(α).