The location of heat transfer pinch point in evaporator is the base of determining operating parameters of organic Rankine cycle (ORC). The physical mathematical model seeking the location of pinch point is established, by which, the temperature variations both of heat source and working fluid with UA can be obtained. Taking heat source with inlet temperature of 160 °C as example, the matching potentials between heat source and working fluid are revealed for subcritical and supercritical cycles with the determined temperature difference of pinch point. Thermal efficiency, exergy efficiency, work output per unit area and maximum work outputs are compared and analyzed based on the locations of heat transfer pinch point either. The results indicate that supercritical ORC has a better performance in thermal efficiency, exergy efficiency and work output while outlet temperature of heat source is low. Otherwise, subcritical performs better. Small heat transfer coefficient results in low value of work output per unit area for supercritical ORC. Introduction of IHX may reduce the optimal evaporating pressure, which has a great influence on heat source outlet temperature and superheat degree. The analysis may benefit the selection of operating parameters and control strategy of ORC.
There are abundant heat sources with low or intermediate temperature, including that of solar energy, biomass and geothermal, as well as industrial waste heat [1], [2] and [3]. A large number of solutions have been proposed to generate electricity by recycling waste heat, such as Organic Rankine cycle, Stirling cycle, Kalina cycle, et al. Among these ways, Organic Rankine Cycle (ORC) is preferred to be developed because of its high efficiency, reliability, flexibility and low requirement for maintenance [4] and [5]. Recently, some new techniques, buoyancy organic Rankine cycle [6] and pumpless Rankine-type cycle [7] and [8], for examples, have also been put forward for ORC.
Numerous studies have been carried on selecting a working fluid matching well with heat source, which is one of the most important steps in building organic Rankine cycle [9], [10] and [11]. However, no single pure fluid has been identified as optimal for the ORC, which is mainly due to the strong interdependence between the optimal working fluid, the working conditions and the cycle architecture [12] and [13]. Different inlet and outlet temperature of heat source together with diverse working fluids make a variety of combinations. Matching rules therein will contribute to the selection of appropriate working fluid for a given heat source. It was also found that using zeotropic mixtures as working fluids have some advantages. First, the temperature glide at phase change could provide a good match of temperature profiles in the evaporator and condenser. Second, equipment costs could be reduced using mixtures for their smaller volume flow rates [14], [15] and [16]. Third, using mixtures could find a balance point between flammability and environmental friendliness [17] and [18].
Research about heat exchangers in ORC includes the measurement of heat transfer coefficient and design of new types of the heat exchanger. There are numerous researches regarding the phase change process for organic working fluid for heat transfer coefficient of subcritical pressure [19], [20] and [21]. However, few researches can be reviewed for supercritical pressure, for the reason that heat transfer mechanisms about supercritical cycle around the critical point is still less known [22].
Pinch point has the minimum temperature difference in evaporator. Determination of the location of pinch point makes notable impact on the optimum pressure selection of the ORC system, and benefits the optimal design. Several researches introduced the methods of determining pinch point for subcritical ORC [23], [24] and [25]. For supercritical ORC, there is no isothermal boiling section in the evaporator. A few paper have been published stating that the location of pinch point may exist in the heating process in supercritical condition [26] and [27], but few researches present specific method for pinch point determination.
In the present study, mathematical and thermodynamic models are built and solved in Matlab together with REFPROP. A new method is introduced for finding pinch point in evaporator of ORC for pure working fluids under both subcritical and supercritical pressures. In further, operating parameter selections were discussed for the two types of ORC combining a typical practical case. Some conclusions are drawn for the selection of cycle styles, working fluids and operating parameters for low grade heat sources utilization.
Determining the location of heat transfer pinch point in evaporator is the foundation of choice of operating parameters, and will contribute to reducing resistance loss in designing ORC heat exchangers. An approach for seeking the location of heat transfer pinch point in evaporator is introduced, on basis of which, the effects on ORC performance under different operating parameters and cycle types are evaluated.
(1)
When the outlet temperature of heat source is low, heating curve of working fluid of supercritical ORC matches better with heat source than that of subcritical ORC. So the thermal efficiency, exergy efficiency and work output of supercritical ORC are higher than that of subcritical ORC. With rising outlet temperature of heat source, matching coefficient of subcritical cycle increases, as well as its cycle efficiency. Higher outlet temperature of heat source can reduce the advantages of supercritical ORC.
(2)
For the reason that heat transfer coefficient under supercritical pressure is low, the heat exchange area for supercritical ORC is much larger than that of subcritical cycle. Thus the work output per unit area is much lower. The selection of cycle type, supercritical or subcritical cycle, i.e., is a compromise of investment in earlier stage and productivity in later stage.
(3)
There is maximum work output for subcritical ORC. The top of its curve becomes more flat by introducing IHX to cycle. Optimal pressure for supercritical and subcritical cycle goes down after IHX is brought to cycle as well. When there is no IHX in the cycle, it has a constant optimal degree of superheat for subcritical ORC. Yet optimal degree of superheat is varying after IHX is introduced.
