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10 9 ORC-50 V. Minea / Applied Thermal Engineering 69 (2014) 143e154 Variable feed pump speed: 0 - 60 Hz ORC-50 10 149 Waste heat inlet temperature = 105°C Waste heat inlet temperature = 85°C Net efficiency average decrease = 15.3 - 15.7% Cooling fluid inlet temperature, °C 25°C 30°C 88 7 66 5 44 3 22 1 0 80 85 90 95 100 105 110 115 120 Waste heat inlet temperature, °C (a) 0 10 15 20 25 30 35 Cooling fluid inlet temperature, °C (b) Fig. 7. Energetic net conversion efficiency as a function of waste heat thermal carrier (a) and cooling fluid (b) inlet temperatures with the organic fluid feed pump running at variable speed (0e60 Hz). overall thermal efficiency increases. However, raising system pressure is not always feasible for economic reasons (e.g. higher capital costs and system complexity, materials selection, etc.). 3.1.3.1. Energetic conversion efficiency. The net heat-to-electricity energetic conversion efficiency rate (hnet) of ORC machines is a dimensionless number defined as the ratio of the net electrical power output (Wgross-exp WOFP) to the sum of the pre-heater ðQ_ preheat Þ and evaporator ðQ_ evap Þ thermal power inputs. It repre- sents the fraction of thermal energy entering the ORC cycle that is converted into useful work. As the electrical power required by the feed pump was less than 3% of the expander power output, it has been ignored: fluid temperature increased from 15 C to 30 C. Table 3 shows the variation of the energetic net conversion efficiency with the waste heat inlet temperature and cycle temperature increase at a constant cooling fluid inlet temperature (20 C). It can be seen that the cy- cle’s energetic efficiency increases as the waste heat inlet temper- atures and temperature increases move incrementally higher. On the other hand, the ORC cycle’s energetic net conversion efficiency decreased linearly with the environmental weather conditions. Consequently, an ORC machine operating in an area with lower ambient temperature would have better conversion efficiency. The lowest energetic net conversion efficiency (5.5%) was achieved with waste heat inlet temperature of 85 C and cooling fluid inlet temperature of 30 C (Fig. 7b). For the same waste heat inlet temperature (85 C), but with a lower (15 C) cooling fluid inlet temperature, the net energetic conversion efficiency increased to 6.5%. Thus, with the waste heat inlet temperature remaining constant at 85 C and 105 C, the average net conversion efficiency decreased by 15.3% and 14.6%, respectively, when the cooling fluid inlet temperature increased from 15 C (typical cold climate winter conditions) to 30 C (summer conditions). In practice, there are some indirect, but practical, ways to in- crease the heat-to-power conversion efficiency, even though in practice they will eventually cost more than the value of the additional output. As already shown, one of them consists in increasing the difference between the waste heat water and cooling fluid inlet temperatures, i.e. increasing the cycle temperature lift. Usually, it is difficult to reduce the inlet temperature of the cooling fluid, as this is driven by environmental factors, such as ambient air or water temperatures. On the other hand, there are several tech- niques to increase the temperature of waste heat coming, for example, from engine exhaust gases by changing the radiator thermostat, or by operating a hot water boiler under pressure with insulated pipes. Also, in the case of ORC machines operating with waste heat at higher temperatures (e.g. >150 C) by reusing the condensing heat rejected by ORC machines at higher temperatures (up to 49 C) may provide free waste heat that can generate savings in heating costs and increase overall system efficiency [27]. Table 3 Energetic and exergetic net conversion efficiency as a function of waste heat inlet temperatures and cycle temperature increases.a W WgrossexpW ðh h Þhh hnet 1⁄4 net 1⁄4 $ $ OFP 1⁄4 5 6s s m (5) Q_ totinput Q preheat þ Q evap ðh5 h2s Þ where Wnet represents the net electrical power output, i.e. the gross power produced by the expander ðW_ grossexpÞ less the feed pump electrical power input (Wofp); hs e the isentropic expansion effi- ciency; hm e the mechanical expansion efficiency. The heat added to the cycle can be expressed as Q_ totinput 1⁄4 Q_ þ Q_ preheat hOUT waste waste 1⁄4m_ c tIN tOUT 1⁄4 m_ evap waste p;waste waste waste waste It can be seen in equation (5) that any parasitic electrical energy (or power) consumption, such as that of the waste heat (water) and cooling fluid (water/glycol) circulating pumps and of the air-cooled cooler fans, was taken in consideration. However, on the experi- mental bench, the total parasitic charges were about 18 kWe (w10 kW for the liquid air-cooled cooler fans, w3 kW for the hot heat source circulating pump and w5 kW for the cooling fluid (water/glycol) circulating pump). However, in actual industrial applications, such parasitic power has to be analysed carefully and, if possible, eliminated or substantially reduced. This approach is sometimes possible in practice because many industrial sites are already equipped with waste heat (hot water) and cooling fluids circulating pumps, as well as with cooling towers or other similar cooling devices. Fig. 7a shows the impact of the waste heat inlet temperature on the net conversion efficiency with cooling fluid inlet temperatures varying from 15 C to 30 C. It can be seen that e for example e with the cooling fluid inlet temperature remaining constant at 25 C, the net conversion efficiency rate increased from 5.1% to 7.5% when the waste heat inlet temperature increased from 85 C to 115 C. With waste heat entering the ORC-50 machine at 100 C, the net con- version efficiency decreased from 7.8% to 4.7% while the cooling Test TIN waste AD-1 85.8 AD-3 90.8 AD-8 95.6 AD-9 100.3 AD-14 105.5 (C) TIN TOUT waste waste 7.7 8.6 9.8 10.8 11.8 (C) Net conversion efficiency rate hIN (6) Energetic hen (%) 6.62 6.98 7.20 7.38 7.57 Exergetic hex (%) 3.95 4.11 4.29 4.38 4.43 a For variable feed pump speeds at a constant cooling inlet temperature (20 C). Energetic net conversion efficiency, % Net conversion efficiency, %PDF Image | Power generation with ORC machines low-grade waste heat
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