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2,5 2 1,5 1 0,5 0 30 25 20 15 10 5 ORC-50 range to improve efficiency. According to Leibowitz et al. [13], by taking full advantage of the potential of screw expanders, it is possible to produce ORC units for heat recovery from low- temperature heat sources with outputs as low as 50 kW at economically viable costs. However, any risks of oil and/or working fluid leakage through the shaft seal must be limited or avoided. To improve the net energetic or exergetic conversion efficiency rate of ORC machines, various methods have been proposed, such as using cascade evaporators with high and low-pressure ex- panders, or two-cycle concepts with fluids having different prop- erties. However, such improvements may significantly increase the initial costs of the systems. In the case of dry organic fluids entering the expander close to their saturated state (i.e. with small superheat amounts), the cycle can eventually be improved by integrating a regenerator, a counter- current heat exchanger installed between the expander and the condenser. In this cycle, since the fluid does not reach a two-phase state at the end of the expansion process, its temperature is higher than the condensing temperature and it can thus be used to preheat the liquid before it enters the evaporator. Such a process is intended to reduce the thermal power required from the waste heat source to generate the same electrical power, while lowering the overall irreversibility and increasing the conversion efficiency. By using low-temperature heat sources giving at low expander inlet pres- sures, the amount of waste heat required could be 7.5% lower and the second-law efficiencies of regenerative ORC machines may be approximately 12% higher than those of basic ORC cycles. However, a significant factor in the decision to use or not a regenerator should be the impact of the additional irreversibility in form of pressure drop introduced before the pre-heater and after the expander. To improve the efficiency and reliability of ORC machines, a number of practical, simple measures have to be carefully applied. First, if high-temperature refrigerants (i.e. refrigerants having relatively high critical temperatures compared to those of con- ventional ones) are employed as working fluid (e.g. HFC-245fa, of which the critical temperature is 154.05 C), inlet temperatures of 80 85 90 95 100 105 110 115 120 Waste heat inlet temperature, °C (a) the cooling fluid below certain values (in this example, bellow about 10 C) must be banned in order to avoid condensing pres- sures below the atmospheric pressure (vacuum). This situation may eventually allow incondensable gases from the ambient air to enter the system and increase the pressure of the working fluid above its saturation values corresponding to the actual process temperature, whether the machine is running or not. The incondensable gases may also increase the liquid sub-cooling at the condenser outlet as well as the expander outlet pressure and thus reduce the net power output. At lowest waste heat source inlet temperatures (e.g. at 85 C), the impact may be even higher. By bleeding the liquid receiver prior to each operating period, the overpressures caused by incondensable gases can be drastically reduced and/or eliminated. Second, before starting the ORC machine, it would be useful to control both the waste (hot) and cooling (cold) fluid flows through the pre-heater/evaporator and condenser, respectively. This pre- caution may help manage the organic fluid migration within the heat exchangers and other components (expander, liquid receiver, etc.) in order to provide adequate starting sequences. Third, the feed pump variable speed as well as the expander vapour by-pass and liquid injection have to be carefully controlled during the starting sequences in order to ensure optimum super- heating at the evaporator exit and avoid undesirable on/off cycles. For example, during the starting periods it may be helpful to allow the feed pump to run at a given, fixed speed during a certain period of time prior beginning operating at variable speed. Fourth, in actual industrial applications, the parasitic power consumptions have to be analysed carefully and, if possible, elim- inated or substantially reduced. This approach is sometimes possible because many industrial sites are already equipped with waste heat and cooling fluid circulating pumps, as well as with cooling towers or other similar devices. Finally, for air-cooled ORC systems, the cooling fluid inlet tem- perature depends on the ambient temperature and cannot be adjusted easily. The challenge consists in finding combined cooling 50 Variable feed pump speed: 0-37 Hz ORC-50 ORC-50 80 85 90 95 100 105 110 115 120 Waste heat inlet temperature, °C (b) V. Minea / Applied Thermal Engineering 69 (2014) 143e154 151 Variable feed pump speed: 0-37 Hz ORC-50 30°C Coolingfluid inlet temperature = 15°C Variable feed pump speed: 0-37 Hz 30°C Cooling fluid inlet temperature = 15°C 80 85 90 95 100 105 110 115 120 Waste heat inlet temperature, °C (a) 0 80 85 90 95 100 105 110 115 120 Waste heat inlet temperature, °C (b) Fig. 9. Organic fluid flow rate (a) and evaporator superheat (b) as a function of waste heat inlet temperatures, at variable feed pump speed (0e37 Hz) and fixed cooling fluid inlet temperature (15 C). 10 45 9 40 8 35 7 30 6 25 5 20 4 15 3 10 2 51 00 Variable feed pump speed: 0-37 Hz Cooling fluid inlet temperature = 15°C 20°C 25°C 30°C Cooling fluid inlet temperature = 15 C 20 C 25 C 30°C Fig. 10. Net power output (a) and net conversion efficiency rate (b) as a function of waste heat inlet temperatures, at a variable feed pump speed (0e37 Hz) and cooling fluid inlet temperatures at between 15 and 30 C. Net output power, kW Net conversion efficiency, % Evaporator superheating, °C Organic fluid flow rate, kg/sPDF Image | Power generation with ORC machines low-grade waste heat
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