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convert waste energy to power from low-grade heat sources. Lemort et al. [14] studied ORC system performance with a scroll expander instead of a turbine with R123 as the working fluid. Dai et al. [15] adopted a genetic algorithm to optimize the working conditions of ORC and compared 10 working fluids for low-grade waste heat recovery. Angelino and Colonna di Paliano [16] evaluated organic- fluid mixtures as working media for Rankine power cycles. Chen et al. [17] compared a carbon dioxide, trans-critical power cycle with an organic Rankine cycle with R123 as the working fluid. The engine of a vehicle does not work in a steady state when it is driven. Determining how to recover the exhaust gas waste heat of a vehicle engine using an ORC system is still a tough task for the automotive industry because there are several obstacles that must be overcome. First, a proper expander needs to be developed that can adapt to the various working conditions of a vehicle engine with a high efficiency. Second, a compact evaporator that can withstand the high temperatures of engine exhaust gas for a long duration must be created. Third, developing of an appropriate working fluid that can accommodate well to the hardware and provide the highest output power is also a hard task to realize. Last, keeping the entire ORC system safety and offering a durable life when operating within the vehicle engine environment, is also a significant challenge. Diego et al. [18] studied three configurations of Rankine cycles for waste heat recovery in a hybrid vehicle. A supercritical, reciprocating Rankine engine for waste heat recovery of diesel engines was proposed by Teng et al. [19,20]. Water and ethanol as working fluids were considered for two kinds of ORC for waste heat recovery of gasoline engines by Ringler et al. [21]. Atan [22] studied heat recovery equipment (i.e., a generator) for an absorption air-conditioning system when the working fluid was a combination of water and lithium bromide. If the ORC system structure and the working conditions are setup properly, the selection of the working fluid has a significant impact on system performance. Because the engine exhaust gas states vary widely when a vehicle is running, it is important to define the operating conditions of ORC that achieve the maximum utilization of the waste heat. In this paper the operating conditions of an ORC system using a single screw expander for recovering the vehicle engine waste heat were analyzed. The thermodynamic models of nine various organic working fluids under these con- strained situations were fabricated and calculated using a Matlab application with REFPROP. The performances of each fluid were compared in a feasible pre-defined region based on the evaporating pressure and condensing temperature. To accommodate the tran- sient, operational characteristics of a vehicle engine, the control strategy of an ORC system is discussed based on the calculation results. 2. Thermodynamic modeling of ORC The schematic of a simple organic Rankine cycle is depicted in Fig. 1. In this paper, this ORC structure is named A type, where the working fluid is pumped from the reservoir to the high pressure pipe line. The waste heat is absorbed in the evaporator under constant pressure and then shifts into a saturated gas state. The high enthalpy saturated gas is then expanded in the single screw expander. At the same time, power is generated and output to the generator. A prototype of the expander assembly examined is shown in Fig. 2. (The single screw expander was invented by Beijing University of Technology, China. Its rated power is 10 kW [23].) Various working fluids usually can be classified into three categories according to the slope of the saturation vapor line in a Tes diagram. A dry fluid has a positive slope; a wet fluid has a negative slope, whereas an isentropic fluid has infinitely large slopes. All the working fluids investigated in this study are dry or Fig. 1. Schematic of A type of ORC. isentropic fluids, which translate into superheated gas states after expansion. Subsequently, the low pressure superheated gas is cooled down to saturated liquid in the condenser. The thermal efficiency of an ORC system can be augmented by adding an internal heat exchanger (IHE). The schematic of an ORC with an IHE is delineated in Fig. 3 and is denoted as B type. The high temperature working fluid exhausted from the screw expander is transported to the inlet of low pressure side of IHE. The low- temperature working fluid exported from the pump is conveyed to the inlet of high pressure side of IHE. The heat is transferred from the low pressure side to high pressure side in IHE. The Tes diagrams of these two ORCs are shown in Fig. 4. The pressure and heat loss of the working fluid in the pipes were neglected. At state point 1, the working fluid condition is at a saturated liquid state. At state point 3 it locates at the saturated vapor line. The pump process is from 1 to 2, where 1e2s is the corresponding isentropic compression process. The state point at the high pressure side outlet of the IHE is 2a. The evaporation process is represented as 2e3 for the A type of ORC, and 2ae3 for the B type, respectively. The expansion process in the single screw expander is 3e4. If the IHE is added, process 4e4a describes what occurs in the low pressure side. Process 3e4s is the relevant isen- tropic expansion process. The mathematical model of the A type of ORC is expressed by equations (1)e(13). The work consumed by the pump W_ p is listed in equation (1): W_p 1⁄4m_ðh2h1Þ 1⁄4m_ðh2sh1Þ (1) hp Fig. 2. Prototype of the single screw expander. E.H. Wang et al. / Energy 36 (2011) 3406e3418 3407PDF Image | Study of working fluid selection of organic Rankine cycle (ORC) for engine waste heat recovery
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