Off-design optimisation of organic Rankine cycle (ORC) engines with piston expanders for medium-scale combined heat and power applications

Abstract

Organic Rankine cycle (ORC) engines often operate under variable heat-source conditions, so maximising performance at both nominal and off-design operation is crucial for the wider adoption of this technology. In this work, an off-design optimisation tool is developed to predict the impact of varying heat-source conditions on ORC operation. Unlike previous efforts where the performance of ORC engine components is assumed fixed, here we consider explicitly the time-varying operational characteristics of these components. A bottoming ORC system is first optimised for maximum power output when recovering heat from the exhaust gases of an internal-combustion engine (ICE) running at full load. A double-pipe heat exchanger (HEX) model is used for sizing the ORC evaporator and condenser, and a piston expander model for sizing the expander. The ICE is then run at part-load, thus varying the temperature and mass flow rate of the exhaust gases. The tool predicts the new off-design heat transfer coefficients in the heat exchangers, and the new optimum expander operating points. Results reveal that the ORC engine power output is underestimated by up to 17% when the off-design operational characteristics of these components are not considered. In particular, the piston expander isentropic efficiency increases at off-design operation by 10-16%, due to the reduced pressure ratio and flow rate in the system, while the evaporator effectiveness improves by up to 15%, due to the higher temperature difference across the HEX and a higher proportion of heat transfer taking place in the two-phase evaporating zone. As the ICE operates further away its nominal point, the offdesign ORC engine power output reduces by a lesser extent than that of the ICE. At an ICE part-load operation of 60% (by power), the optimised ORC engine with fluids such as R1233zd operates at 77% of its nominal capacity. ORC off-design performance maps are generated, predicting system performance and can be used by ORC system designers, manufacturers and plant operators to identify optimum performance under real operating conditions.

Organic Rankine cycle systems for engine waste-heat recovery: Heat exchanger design in space-constrained applications

“…The net power output (Wnet) is used to indicate the ORC system's thermodynamic performance, while the net power output per unit heat exchanger area (Wnet/A) is used as an economic indicator. R245fa is considered here as the working fluid as it is a well-known fluid that has been extensively investigated in the literature and that is currently being used in many ORC systems for its good performance [16]. power while the pressure drop in the condenser causes higher backpressure in the expander and reduces power generation.…”

“…Investigations on ORC technology for converting recovered waste-heat from ICEs have been conducted both computationally and experimentally [9,10], including on configuration design [11][12][13] and system optimization [14][15][16], working fluid selection by traditional pre-selection (including pure fluids [17,18] and mixtures [19,20]) and by new computer-aided molecular design methods [21][22][23] integrated into cycle optimization [24,25], expander selection and design [26,27] (including piston [28][29][30], screw [31][32][33] and rotary-vane [34] expanders), and off-design performance [35,36]. These excellent studies have enabled significant progress with the performance of ORC technology towards through careful considerations of the working fluid, key components and system parameters.…”

“…Similarly, Astolfi et al [51] assumed that the pressure drop in the heat exchangers was either 2% of the inlet pressure or 50 kPa when comparing different ORC and CO2 cycle systems. Chatzopoulou et al [16] set a limit for the pressure drop through the heat exchanger of 10-60 kPa on the exhaust-gas side when sizing heat exchangers for an ORC system in an ICE WHR application. Tian et al [52] conducted a thermo-economic analysis of ORC systems and set the heat exchanger pressure drop limit to 10 kPa, 20 kPa and 30 kPa for the exhaust, the cooling water and the working fluid, respectively.…”

Experimental and Numerical Characterization of the Sliding Rotary Vane Expander Intake Pressure in Order to Develop a Novel Control-Diagnostic Procedure

“…Among them, rotary and reciprocating types have been extensively considered. Screw and scroll machines have good robustness to fluctuating conditions [20], while reciprocating ones, which are characterized by a higher built-in volume ratio [21], can achieve more easily a higher expansion ratio and demonstrated very good off-design behavior [22]. Vane machines suffers toughly the leakages [23] due to vane chatter [24], which implies leakages across the adjacent chamber due to the lack of contact between the blade tip and stator [25].…”

Dynamic Performance Comparison of CO2 Mixture Transcritical Power Cycle Systems with Variable Configurations for Engine Waste Heat Recovery

“…Xie et al [26] pointed out that the efficiency of the WHR system under driving cycles was less than half of that in design condition. Chatzopoulou [27] proposed an off-design optimization tool considering the component time-varying performance, and concluded that the ORC power output was underestimated by 17% when the ORC components operated in off-design conditions without optimizing the performance. Some control strategies are necessary to ensure the normal and satisfactory operation of the system over a broad range of engine operating conditions due to the transient heat source of the engine.…”