Numerical Analysis of an Organic Rankine Cycle with Adjustable Working Fluid Composition, a Volumetric Expander and a Recuperator

Abstract

Conventional Organic Rankine Cycles (ORCs) using ambient air as their coolant cannot fully utilize the greater temperature differential available to them during the colder months. However, changing the working fluid composition so its boiling temperature matches the ambient temperature as it changes has been shown to have potential to increase year-round electricity generation. Previous research has assumed that the cycle pressure ratio is able to vary without a major loss in the isentropic efficiency of the turbine. This paper investigates if small scale ORC systems that normally use positive-displacement expanders with fixed expansion ratios could also benefit from this new concept. A numerical model was firstly established, based on which a comprehensive analysis was then conducted. The results showed that it can be applied to systems with positive-displacement expanders and improve their year-round electricity generation. However, such an improvement is less than that of the systems using turbine expanders with variable expansion ratios. Furthermore, such an improvement relies on heat recovery via the recuperator. This is because expanders with a fixed expansion ratio have a relatively constant pressure ratio between their inlet and outlet. The increase of pressure ratio between the evaporator and condenser by tuning the condensing temperature to match colder ambient condition in winter cannot be utilised by such expanders. However, with the recuperator in place, the higher discharging temperature of the expander could increase the heat recovery and consequently reduce the heat input at the evaporator, increasing the thermal efficiency and the specific power. The higher the amount of heat energy transferred in the recuperator, the higher the efficiency improvement.

Experimental Investigation of a Small-Scale ORC Power Plant Using a Positive Displacement Expander with and without a Regenerator

“…Figure 2 shows the T-s diagram of the highest efficiency case with the regenerative portion of the cycle highlighted in green. Figure 3 shows a comparison between the highest efficiency experimental case (i.e., a regenerative cycle arrangement) and the theoretical cycle predicted by the thermodynamic model [32]. Several things can be observed from this figure.…”

“…The case with the highest thermal efficiency was achieved using a regenerative cycle with a heat source temperature of 90 • C. The results of the case with the maximum efficiency are given in Table 3, and also as a T-s diagram, as shown in Figure 2. Then, they are compared with the theoretical performance of a cycle calculated using the authors' thermodynamic model [32] as shown in Figure 3. As shown in Table 3, the heat source and heat sink temperatures, the key control parameters, are respectively set as 90 • C and 7 • C in both experiments and simulations.…”

Study on Binary Cycle Power Plant Optimization of PLTP Tulehu Ambon with Aspen HYSYS

“…Therefore, the working fluid phase entering the turbine is conditioned at a fraction of 100% with a maximum temperature limit below of geothermal temperature (100 o C). Besides that, the critical temperature also has a significant effect on the pressure [22]. By using the calculations in the REFTROP, the range of allowable TIP values for each working fluid can be seen in FIGURE 4.…”

Low-temperature waste heat recovery from internal combustion engines and power output improvement through dual-expander organic Rankine cycle technology

“…Similar findings were reported in the previous studies. [38][39][40] Thus, constant isentropic efficiency is not a good assumption for heat recovery application from a dynamic operating machine such as ICEs, having varying heat availability depending upon the size, speed and load.…”