Performance of a 250 kW Organic Rankine Cycle System for Off-Design Heat Source Conditions

Abstract

An organic Rankine cycle system with a preheater, evaporator, condenser, turbine, generator, and pump was used to study its off-design performance and operational control strategy. R245fa was used as a working fluid. The net power output is 243 kW and the system thermal efficiency is 9.5% under the design conditions. For an off-design heat source flow rate (m W ), the operating pressure was controlled to meet the condition that the R245fa reached the saturation liquid and vapor states at the outlet of the preheater and evaporator, respectively. The analytical results demonstrated that the operating pressure increased with increasing m W ; a higher m W yielded better heat transfer performance of the preheater and required a smaller evaporator heat capacity; and the net power output and system thermal efficiency increased with increasing m W . The net power output increased by 64.0% while the total heat transfer rate increased by only 9.2% for the studied range of m W . To conclude, an off-design operation is studied for a heat source flow rate which varied from -39.0% to +78.0% from the designed rate, resulting in -29.2% to +16.0% and -25.3% to +12.6% variations in the net power output and system thermal efficiency, respectively.

A Flow Rate Control Approach on Off-Design Analysis of an Organic Rankine Cycle System

“…In previous studies [6,7], TW,out increased rapidly from 85.1 to 112.3 °C (or from 72.9 to 110.9 °C) for the same range Figure 2b. In previous studies [6,7], T W,out increased rapidly from 85.1 to 112.3 • C (or from 72.9 to 110.9 • C) for the same range of T W,in (or m W ) but under the pressure control approach, indicating that temperature distributions differed under different control strategies and off-design conditions. These results also indicate that increasing trends of T W,mid and T W,out for off-design temperature and flow rate significantly differed in the present study.…”

“…In the present analysis, the TW,in and mW values ranged from 123.6 °C to 153.7 °C and from 9.39 to 27.39 kg/s, respectively. Mathematical models for each component and system performance as well as the assumptions for the analysis have been described in previous studies [6,7]. For the present study, the operating flow rate of R245fa was controlled to ensure reaching saturation liquid and vapor states of R245fa at the outlets of the preheater and evaporator, respectively, under off-design heat source conditions.…”

“…In addition, for the flooded and shell-and-tube type evaporators, the heat transfer coefficient of the tube side was also calculated using the Gnielinski correlation and that of the shell side was estimated using the classical Gorenflo correlation [13]. The detailed equations for the evaluation of the evaporator performance are as follows: Mathematical models for each component and system performance as well as the assumptions for the analysis have been described in previous studies [6,7]. For the present study, the operating flow rate of R245fa was controlled to ensure reaching saturation liquid and vapor states of R245fa at the outlets of the preheater and evaporator, respectively, under off-design heat source conditions.…”

“…Fu et al [6,7] adopted the pressure control approach, which entails using a new preheater (and evaporator) operating pressure to meet vital requirements at off-design heat source temperatures (TW,in) and flow rates (mW), to study off-design operations. They determined that a higher TW,in or mW resulted in higher preheater performance and lower requirement for the evaporator's heat capacity.…”

“…In addition, increasing T W,in or m W considerably improved the system power output and thermal efficiency. On the basis of previous studies [6,7], this work uses a different control approach under off-design conditions to analytically examine the effect of off-design heat source temperature or flow rate on the heat transfer characteristics (particularly in the preheater and evaporator) and system performance of a 250-kW ORC system by using the flow rate control approach. Figure 1 depicts the studied ORC system, indicating that it comprises pump, preheater, evaporator, turbine, generator, condenser, hot water (serviced as a heat source), and cooling water circulation systems.…”