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150 V. Minea / Applied Thermal Engineering 69 (2014) 143e154 3.1.3.2. Exergetic conversion efficiency. Irreversibility expresses the net availability (exergy) destruction of the control mass and sur- roundings, which is proportional to the net entropy increase. The less the irreversibility associated with a given change of state, the greater the amount of work that will be done (or the smaller the amount of work that will be required). The greater the irrevers- ibility we have in all our processes, the greater the decrease will be in our available reserves. It is desirable to accomplish a given objective with the smallest irreversibility. Work costs money, and in many cases a given objective can be accomplished at less cost when irreversibility is lower. Like the energetic heat-to-electricity net conversion efficiency, the exergetic net conversion efficiency can be defined as Eavailable mwasteewaste Q_ 1 totinput where Wnet is the net power output (kW), E_ available e the exergy flux available in the waste heat source at the pre-heater/evaporator inlet (kW); m_ waste e the waste heat flow rate (kg/s); eIN e the waste pump speed at maximum 37 Hz, it was found that, with waste heat inlet temperatures higher than 105 C, this maximum speed became insufficient to keep the evaporator superheating at opti- mum values (i.e. 4e5 C). In other words, with waste heat entering the ORC-50 machine at temperatures lower than 105 C, the speed of the feed pump varied between 0 and 37 Hz and was able to provide optimum superheating amounts for all cooling fluid inlet temperatures provided. However, with waste heat inlet tempera- tures higher than 105 C, excessive superheating amounts have been provide because the maximum speed of the feed pump was fixed at 37 Hz. It can be noted that this maximum current frequency wasn’t arbitrary chosen, but it was determined after many experi- mental trials and associated data analysis. First, it can be seen that with waste heat inlet temperatures above 105 C and feed pump speed fixed at maximum 37 Hz, the pressure of the superheated vapour at the expander inlet port dropped steeply (Fig. 8a), while the expander inlet temperatures began to increase (Fig. 8b). In order to better illustrate these phe- nomena, in all the running tests represented in Fig. 8a and b, the Wnet Wnet Wnet hex1⁄4_ 1⁄4_ IN 1⁄4 (7) mass exergy of the waste heat carrier fluid entering the cycle (kJ/ kg); ð1 ðTa=TIN ÞÞ e the Carnot factor, i.e. the maximum amount waste heat inlet temperatures varied from 85 C to 115 C by 5 C increments. waste of waste heat source which can be transformed into mechanical Second, by setting the feed pump speed at maximum 37 Hz, the organic fluid flow rate sharply dropped at waste heat inlet tem- peratures above 105 C for all cooling fluid inlet temperatures (Fig. 9a). On the other hand, the evaporator superheating reached values as high as 14e19 C and 23e25 C with waste heat inlet temperatures of 110 C and 115 C, respectively (Fig. 9b). As a direct consequence of reducing the organic fluid flow rate and of exces- sively increasing the evaporator vapour superheat, the net power output (Fig. 10a) as well as the heat-to-electricity energetic net conversion efficiency rate (Fig. 10b) both stopped increasing at waste heat inlet temperatures above 105 C, for all cooling fluid inlet temperatures. 4. Cycle improvements More or less practical and/or theoretical improvements could be made to ORC machines to increase energy performance and reli- ability. First of all, by appropriately selecting the working (organic) fluids, based on the machine’s actual operating conditions, as well as by using an advanced design and selecting the size of the expander, the net conversion efficiency rate may be increased. The working fluids, such as HFC-134a, HFC-245fa, n-pentane and silicon oils, have relatively high critical points and achieve optimal per- formance in term of cycle efficiency. Among several alternatives, screw expanders, developed for relatively low-scale ORC machines (<250 kWe), have fixed built-in volume ratios and can use wet fluids with limited superheat at the inlet supply. There are also small screw expanders which can manually or automatically vary the volume ratio within a nearby work; Ta e the ambient absolute temperature (expressed in K) and TIN e the waste heat inlet absolute temperature (expressed in K). waste By ignoring the potential and kinetic energies, the maximum reversible work per unit mass flow, equal to the decrease in flow availability plus the reversible work that can be extracted from an ORC cycle operating between the waste heat inlet absolute tem- perature ðTIN waste defined as e 1⁄4 ðhhaÞTaðssaÞ Þ and the ambient absolute temperature (T ), is a The a subscript refers to the dead-state, usually the environment temperature, but here, Ta is the absolute temperature (K) of the cooling fluid entering the condenser of the ORC-50 machine. Table 3 summarizes both the energetic and exergetic net version efficiency rates as a function of the actual waste heat and outlet temperatures (at a constant flow rate of 11.6 kg/s) reference ambient temperature of 20 C equal to the cooling temperature entering the condenser of the ORC-50 machine. 3.2. Impact of superheating con- inlet for a fluid The impact of evaporator superheating on the ORC-50 chine’s operating parameters and energy performance was exper- imentally investigated by setting the feed pump speed to vary between 0 and maximum 37 Hz, while varying the cooling fluid inlet temperatures between 15 C and 30 C. By fixing the feed 1400 1200 1000 800 600 400 200 ma- ORC-50 120 100 80 60 40 20 0 ORC-50 T T IN a waste (8) cooling fluid inlet temperature was kept constant at 20 C while the Variable feed pump speed: 0-37 Hz Cooling fluid temperature = const = 20°C 0 80 85 90 95 100 105 110 115 120 Waste heat inlet temperature, °C (a) 80 85 90 95 100 105 110 115 120 Waste heat inlet temperature, °C (b) Fig. 8. Expander inlet pressure (a) and temperature (b) as functions of waste heat inlet temperatures, at variable feed pump speed (0e37 Hz) and fixed cooling fluid inlet tem- perature (20 C). Variable feed pump speed: 0-37 Hz Cooling inlet temperature = const = 20°C Expander inlet pressure, kPa,r Expander inlet temperature, °CPDF Image | Power generation with ORC machines low-grade waste heat
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