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146 V. Minea / Applied Thermal Engineering 69 (2014) 143e154 Average temperature lift = waste heat inlet - cooling fluid inlet 110 100 90 80 70 60 50 40 30 20 10 0 Fig. 2. Average difference (or cycle temperature lift) between the waste heat and cooling fluid inlet temperatures. ORC-50 Cooling fluid inlet temperature = 15°C 20°C 25°C 30°C pressure. Only a relatively small portion of the power transferred is due to the dynamic effects associated with fluid motion. The expander can run in wet conditions, which means that the inlet refrigerant does not have to be 100% superheated vapour. The presence of liquid, together with the vapour or gas being expanded, has therefore little effect on its operation and efficiency. Moreover, since the liquid seals the gaps between the rotors and the casing, the lubrication process is enhanced [13,14,24,25]. The main lubri- cation system consists in dispersing up to approximately 5% oil by mass in the organic fluid [26]. The oil is transported by the liquid organic fluid through the evaporator and then through the expander, where it lubricates the rotors as the liquid working fluid dries during the expansion process. Also, as previously noted, some of the liquid organic fluid leaving the feed pump, prior to entering the evaporator, are distributed to the expander bearings where the frictional heat will evaporate it, leaving sufficient oil to lubricate them. This arrangement eliminates the need for any oil separator, storage tank, filters, cooler and circulating pump, usually required for conventional oil-flooded expander lubrication systems. This integrated lubrication method yields lower maintenance costs and reduces the total cost of the expander by roughly 80% compared to standard steam power plants [14,27]. 3. Experimental results First, this section presents part of the experimental results ob- tained with the organic fluid feed pump running at variable speeds (theoretically, with current frequency varying between 0 Hz and 60 Hz) in order to get small amounts of superheat at the evaporator outlet. Second, in order to illustrate the importance of this control strategy, a number of representative results obtained with the feed pump operating at a constant speed (i.e. 37 Hz), are provided. 3.1. Full variable feed pump speed Experimental tests of the organic fluid feed pump running at variable speed over whole theoretical range (i.e., between 0 and maximum 60 Hz) were conducted with the waste heat thermal carrier (water) and cooling fluid (glycol/water 50% brine) entering the ORC-50 machine at temperatures of 85e105 C and 15e30 C, respectively. Under these thermal boundary conditions, the ORC-50 machine’s average temperature increases (i.e. the difference be- while the mass flow rate of the cooling fluid varied with the use of a motorized mixing valve MV (see Fig. 1) between 10 kg/s and 14.8 kg/s (155e228 US GPM), depending on the actual ambient air temperature and the preset inlet temperature of the cooling fluid. At waste heat inlet temperatures above 105 C, the operating pa- rameters and cycle performance were predicted based on the experimental empirical linear equations (see Table 1) as well as on simulation models developed by using the EES software. Under all these boundary conditions, the ORC-50 machine ran continuously for more than 3000 h, proving its long term mechanical reliability and endurance. 3.1.1. Cycle validation The main parameters (pressures, temperatures, flow rates and electrical power) that were measured helped validate each ther- modynamic cycle by using the actual thermo-physical properties of the organic fluid. As can be seen in Fig. 3a and b, both the pressure and temperature of the superheated vapour entering the expander at state 5 increased with the waste heat inlet temperature ac- cording to the empirical linear correlations shown in Table 1 for waste heat inlet temperatures between 85 C and 115 C, at a constant cooling fluid inlet temperature (20 C) in this example. The expander operated with expansion ratios of around 8 (see Table 2). Fig. 4a shows the profile of the organic fluid flow rate for another typical test at a constant cooling fluid inlet temperature (15 C), the feed pump running at variable speeds. It can be seen that, by controlling the feed pump speed, the organic fluid flow rate increased with the waste heat inlet temperature according to the empirical linear correlation shown in Table 1. As a result, the vapour superheat at the evaporator outlet remained below 4e5 C (Fig. 4b) allowing the dry organic fluid to be further superheated during the expansion process without affecting the cycle thermodynamic performance. Even without any evaporator superheating (i.e. with Table 1 Experimental correlations with feed pump running at variable speeds (0e60 Hz). 80 85 90 95 100 105 110 115 120 Waste heat inlet temperature, °C tween the waste heat and cooling fluid inlet temperatures) varied waste Parameter Expander inlet pressure (kPa, r) Expander inlet temperature (C) Organic fluid mass flow rate (kg/s) Net power output (kW) Net conversion efficiency rate (%) Empirical linear correlationa p T 1⁄4 16:369TIN waste 1⁄4 0:7773TIN waste 810:76 þ 12:574 m_ 1⁄4 0:0278TIN waste 1:0643 5 5 OF net Þ between 85 C and 116 Wnet 1⁄4 0:7801T IN waste h 43:577 1⁄4 0:0745TIN þ 0:0114 between 55 C and 100 C (Fig. 2). The mass flow rate of the waste heat thermal carrier was kept constant at 11.2 kg/s (173 GPM), ðT IN waste C, at a a For waste heat inlet temperatures constant cooling fluid inlet temperature (20 C). Temperature lift, °CPDF Image | Power generation with ORC machines low-grade waste heat
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