Performance Study On System Integration And Evaporation Process For Organic Rankine Cycle

There are abundant heat sources with low or intermediate temperature, including that of solar energy, biomass and geothermal, as well as industrial waste heat. But steam Rankine cycle cannot be utilized for using these heat because of the low quality. Organic Rankine cycle (ORC) has been proposed to generate electricity by recycling low and medium heat for the reason that the evaporating process of it can match well with the heat source. This work developed the evaporator model and thermodynamic model by using software Matlab 2011 together with Refprop 8.0, analysis the performance of ORC system using pure and mixture working fluid in a large scale, summarized the law of selecting working fluid; applied the ORC system to geothermal and power plant waste heat; established flow boiling experiment rig and investigated the flow boiling of pure and mixture working fluid in horizontal circular tube.This work developed the temperature distribution model in evaporator under subcritical and supercritical working conditions, and revealed the matching coefficient between heat source and working fluid. Based on which, performances of subcritical and supercritical ORCs are investigated, including the influence of the type of working fluid, the critical temperature of working fluid, the ratio of components of mixture and the temperature profile of working fluid. A method for forming a new mixture is introduced and performance of different working fluids under different heat source and heat sink is investigated. The results show that in the case of a high heat source inlet temperature and a low temperature gradient, the performance of pure working fluids can be better than those of mixtures. When the heat source inlet temperature becomes lower, the heat source temperature gradient and heat sink temperature gradient become greater, the mixtures perform better. Working fluids with high critical temperatures are suitable for heat sources with small temperature gradients, and working fluids with low critical temperatures are suitable for heat sources with large temperature gradients.Performances of ORC under temperature range of no more than 90℃ were simulated. For this heat source, supercritical ORC has the largest work output when changing heat source outlet temperature, but with poor working stability at the condition of maximum work output. The ORC for mixture has the second highest work output and that for subcritical ORC of pure working fluid has the lowest. But these two types cycle have the lower optimal pressure than that of supercritical cycle, which means more system safety. Every type cycle have a minimum available temperature, the value of which for supercritical cycle is higher than the other two types of cycle.ORC system utilizing heat source of the exhaust flue gas of boiler in a 240 MW pulverized coal-fired power plant were simulated.The analytical results show that the mixture that matches with heat sink exhibits the best perfoRrmance among the three working fluids with the thermal efficiency as the objective function. Its thermal efficiencies are up to 6.01% higher than that of mixtures that matches with heat source, and up to 8.81% higher than that of pure working fluid. The mixture that matches with heat source has the lowest superheat degree and the pure working fluid has the highest superheat degree. Thus the use of mixture that matches with heat source is beneficial to reduce the total heat exchange area for the evaporator. The rate of heat exchanged in recuperator to that in evaporator has a maximum value with evaporating pressure, which has to be paid attention when selecting a recuperator with sufficient capacity. The evaluation of the costs/benefits ratio indicates that the expenditure of the proposed ORC plant can be recovered within around 4.5 years.A flow boiling experiment rig of 3 mm horizontal circular tube under pressure of 0.2-0.84 MPa is established in this study. Influence caused by mass, heat flux, evaporating pressure for R245fa and a mixture (R134a/R245fa,0.82/0.18 in mass) is investigated. A visualization study is conducted to explore the observed flow pattern. An empirical correlation of flow boiling heat transfer of the mixture is developed.