In heat engine design, the usual objective is to maximize thermal efficiency. However, for heat engines applied to waste heat recovery, an appropriate objective is to maximize power production by converting as much of the waste heat as possible into work. An organic Rankine cycle (ORC) is particularly well-suited to waste heat recovery because of its compactness relative to a steam Rankine cycle at typical waste heat temperatures. For a single-phase (sensible) waste heat stream with a finite capacity, maximization of thermal efficiency does not result in maximum power production. Therefore, traditional approaches aimed at increasing cycle thermal efficiency are not helpful. Instead, it is necessary to find designs that properly balance heat extraction from the source and thermal efficiency of the heat engine. In this regard two alternative ORC configurations are studied and compared using a uniform modeling strategy. These configurations are the ORC with two-phase flash expansion and the ORC with zeotropic working fluid mixture (ZRC). There are two key elements of the modeling strategy. Pinch point temperature differences are used to characterize the heat exchangers, and the air-side condenser fan power requirements are estimated. Each cycle configuration is modeled and compared to a baseline ORC for a range of potential working fluids and source fluid temperatures.
Based on the model, the ORC with flash expansion shows the most consistent improvement over the baseline ORC. The highest increase in net power of 84% over the baseline is seen at the low source temperature of 80 °C with water as the working fluid. However, this cycle and working fluid present more costly challenges to implementation, particularly in the expander design. This is due to the high volume ratios needed to expand low-pressure, two-phase water. The ZRC gives some of the highest relative improvements, but only when condenser fan power consumption is high. For a 100 °C source temperature and a mixture of R134a and R245fa as the working fluid, an improvement of 92% over the baseline is seen if the required condenser fan power is 846 W/(m3/s). In addition, the ZRC has the benefit that it can utilize existing ORC expanders, giving it a potentially lower cost than the ORC with flash expansion. These results are valid in terms of their comparison of the thermodynamic potential of the different cycles and working fluids. However, a more detailed analysis incorporating the geometry and cost of each component is necessary to arrive at a final recommendation for a given application.
ORC, waste heat, rankine, organic
High Performance Buildings, Thermal Systems and Air Quality
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