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182 S. Quoilin et al. / Renewable and Sustainable Energy Reviews 22 (2013) 168–186 Fig. 14. Schematic view of the turboden turbine–recuperator–condenser assembly. depends on the pressure head (i.e. the difference between evaporating and condensing pressure). 8.2. Efficiency In traditional steam Rankine cycles, the pump consumption is very low compared to the output power. However, in an ORC cycle, the pump irreversibilities can substantially decrease the cycle over- all efficiency. The ratio between pump electrical consumption and expander output power is called Back Work Ratio (BWR): BWR1⁄4 W_pp ð7Þ W_ exp Fig. 15 shows the BWR for a few typical fluids as a function of the evaporating temperature. Two conclusions can be drawn from this figure: (1) The higher the critical temperature of the working fluid, the lower the BWR. (2) BWR increases with Tev, and gets significantly high when operating the cycle close to the critical point. Therefore, the pump efficiency is a crucial parameter in low temperature cycles and in transcritical cycles. Few pump efficien- cies are reported in the literature, and they are usually quite low for low capacity units: An overall isentropic efficiency of 25% has been reported by some of the authors in a 2 kWe ORC unit [89]. Reid [90] reports a pump efficiency of 7% on kW-scale ORC cycle using HFE-7000; Quoilin [91] obtained a 22% efficiency on a diaphragm pump using RHFC-245fa. Bala et al. [92] studied the influence of different working fluids on the overall efficiency of sliding-vane refrigerant pumps; the highest reported efficiency was about 20%. Melotte [88] performed an experimental study on a centri- fugal pump working with solkatherm, and obtained an effi- ciency varying between 10% and 20%. Note that these efficiencies are all electrical efficiencies, i.e. incorporate the motor efficiency, which can be low for small units or if the motor is oversized. Fig. 15. BWR as a function of evaporation temperature for different fluids. According to manufacturer data, centrifugal pumps (usually used in larger-scale units) should exhibit efficiencies higher than 60%, and diaphragm pump should operate over 40–50%. However, no actual data is available in the literature to confirm these figures. 8.3. Tightness Organic fluids are expensive and can be flammable, toxic and have high GWP or ODP values. Hence, it is important to ensure full tightness of the cycle. This explains why diaphragm pumps are usually preferred to piston pumps. Note that diaphragm pumps generate a pulsed flow rate, which can, in some cases, constitute a drawback (e.g. because of fluctuations in pressure and flow rate measurements). When using centrifugal pumps, tightness is ensured by a shaft seal. 8.4. Low net pressure suction head (NPSH) This parameter is critical for the design of the ORC. Strategies must be set up to avoid pump cavitation, which can lead to damages in the pump, to a reduction of the working fluid flow rate, and to the necessity to shut down the cycle. The most common strategies are briefly described in the next sections.PDF Image | Techno-economic survey of Organic Rankine Cycle (ORC) systems
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