Effect of Working-fluid Mixtures on Organic Rankine Cycle System Performance: Heat Transfer and Cost Analysis

A thermodynamic limitation of single-component working fluids in organic Rankine cycles (ORCs) is the large exergy destruction (and, consequently, useful power loss) associated with evaporation and condensation. Due to their non-isothermal phasechange behaviour, non-azeotropic working-fluid mixtures have shown reduced exergy losses, leading to improved cycle efficiencies and power outputs. These benefits are exclusively observed from a thermodynamic perspective. The present paper considers the effects of selecting such working-fluid mixtures on heat transfer performance, component sizing and system costs compared with those of pure fluids; a mixture of n-pentane and nhexane is selected. While the fluid-mixture cycles do indeed allow higher efficiencies and the generation of higher power outputs, they require larger evaporators, condensers and expanders; thus, the resulting ORC systems are more expensive than those based on the pure fluids. While a working-fluid mixture (60% npentane + 40% n-hexane) leads to the thermodynamically optimal cycle, a pure n-pentane ORC system has reduced costs of 37% per unit power output over the thermodynamic optimum. INTRODUCTION Recently, the selection of working fluids for organic Rankine cycles (ORCs) has received close attention, including a particular interest in multi-component fluid mixtures, due to the opportunities they offer in improving the thermodynamic performance of ORC systems. Various authors have carried out investigations to demonstrate and quantify these benefits, which have shown that working-fluid mixtures can exhibit an improved thermal match with the heat source compared to the isothermal evaporation profile of (isobaric) single-component fluids, resulting in reduced exergy destruction [1,2], and increased thermal and exergy efficiencies [3,4]. Various other authors have carried out both experimental and theoretical investigations into the benefits of employing refrigerant [5-10], hydrocarbon [9-13], and siloxane [1,14], working-fluid mixtures. Compared to pure fluids, suitable binary mixtures were shown increased power outputs by about 30% and thermal efficiencies by over 15%. Additionally, fluid mixtures can be used to adjust environmental and safety-related properties of the working fluid or to improve design parameters of cycle components [2]. However, some exceptions to these general trends have also been identified [15,16]. At the same time, a few investigators have begun to develop and employ advanced computer-aided molecular design (CAMD) methodologies [16-19] to identify optimal mixtures for ORCs. While these efforts have demonstrated the potential advantages of working-fluid mixtures, notably in terms of power output and efficiency, many of the associated conclusions have been derived strictly based on thermodynamic cycle analyses that do not fully consider the expected heat transfer performance between the heat source/sink and working-fluid streams in the heat exchangers of ORC engines. In particular, the heat transfer and cost implications of using fluid mixtures have not been properly addressed. Refrigerant mixtures are known to exhibit reduced heat-transfer coefficients compared to their pure counterparts [20,21]. Specifically, heat-transfer coefficients for refrigerants mixtures are usually lower than the ideal values interpolated between the mixture components. This may invariably lead to larger and more expensive heat exchangers in an ORC system. Therefore, although working-fluid mixtures may allow a thermodynamic advantage over single-component working fluids, they may also lead to higher system costs owing to deterioration in their thermal performance. This work aims to explore the effects of working-fluid mixtures on the heat transfer processes in ORC engines, which is important in understanding the role that these fluids play on the overall system performance and cost. A simple ORC engine model is presented that incorporates a heat transfer description of the heat exchangers used for the heat addition and heat rejection processes. The heat exchangers are discretized along their lengths into segments (accounting for phase-change and single-phase regions), with suitable estimates of the heat-transfer coefficients for the different segments. Overall heat-transfer coefficients and heat-transfer areas are then evaluated, and simple cost models are used to estimate the relative costs of the components, and by extension of the entire engine. Using a selection of alkane working-fluid mixtures, the heat transfer characteristics and ORC-system equipment/component costs are thus investigated. ORC THERMODYNAMIC OPTIMIZATION Following an approach similar to previous studies, we carry out a simple thermodynamic optimization of an ORC cycle with a set of working-fluid mixtures. In particular, we study straightchained alkane mixtures of n-hexane and n-pentane. Pentane is presently being used in actual installations especially in geothermal ORC setups. Furthermore various authors [11-13] have shown that mixtures of these particular fluids can provide significant thermodynamic benefits to an ORC system. ORC Model A non-regenerative ORC, similarly to a steam-Rankine cycle, consists of four processes (pumping, heat addition, expansion and heat rejection), carried out by an organic working fluid. A typical such ORC is presented in the T-s diagram in Figure 1. The power required to pump the working fluid from State 1 to State 2 is: ?̇?pm = ?̇?wf(h2 − h1) = ?̇?wf(h2s − h1)/ηis,pump . [1] The heat extracted from the heat source is transferred to the working fluid assuming no heat losses in the evaporator. This is assumed to be an isobaric process. The working fluid exits the evaporator as a saturated vapour as depicted in Figure 1. The working fluid is not superheated, since superheating has been shown to be detrimental to ORC performance [16]. Thus the rate of heat input from the heat source is given by: ?̇?in = ?̇?wf(h3 − h2) = ?̇?hscp,hs(Ths,in − Ths,out) . [2] The power generated as the working fluid is expanded is: ?̇?exp = ?̇?wf(h3 − h4) = ηis,exp?̇?wf(h3 − h4s) . [3] 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics