Book - School of Science and Technology

CHP systems ± criteria for selection 697Table 24.1 Duties, etc., of available packaged plant (2000)Nominalelectricalrating (kW)Electricaloutput(kW)Heatoutput(kW)Fuelinput(kW)Electricalefficiency(%)Heat topowerratioOverallefficiency(%)80 88 139 284 31 1.58 8090 90 140 302 30 1.56 76110 112 177 379 30 1.58 76160 166 260 519 32 1.57 82167 170 258 545 31 1.52 79200 210 319 636 33 1.52 83210 210 317 687 31 1.51 77300 310 435 993 31 1.40 75315 318 442 935 34 1.39 81300 330 388 893 37 1.18 80400 400 609 1319 30 1.52 76500 526 667 1385 38 1.27 86500 532 678 1565 34 1.27 77600 600 873 1947 31 1.46 76600 608 720 1737 35 1.18 76600 625 786 1615 39 1.26 87800L a 770 954 2139 36 1.24 81800 800 1234 2679 30 1.54 76800 836 1047 2154 39 1.25 871000L a 979 1314 3053 32 1.34 751000L a 1018 1208 2751 37 1.19 811000 1048 1310 2692 39 1.25 881500 1461 1536 3529 41 1.05 851500 1527 1744 4510 34 1.14 732000 1944 2048 4706 41 1.05 853000 2717 2763 6481 42 1.02 85SourcesData are from Clarke Energy Ltd,Nedalo Combined Heat and Power Systems,Peter Brotherhood Aircogen.a L refers to low temperature.vis-aÁ-vis the building loads,electrical and thermal,to which it is to be connected. Generallyspeaking,the output of the equipment or system should be such that use can bemade of the heat recovered throughout the day during the summer months and for a largeproportion of all hours during the winter months.It is generally agreed that if the plant cannot run for a minimum of 4000 hours perannum (an overall load factor of 45%) it is unlikely to be viable: an annual load factor ofbetween 50% and 70% (4500±6000 hours per annum) would provide a simple pay-backperiod of three to four years. In consequence of these limitations,a plant running time ofabout 10 hours per day in summer and 18±24 hours per day in winter would be necessary.Taking account of these minimum recorded running hours,it must be remembered thatrepeated recycling of the prime mover will shorten its working life very considerably dueto additional wear during the first few minutes after start-up,particularly with coldlubricating oil. The control arrangements should limit multiple starts to less than sixper hour. Similarly,in the interests of maintaining an optimum efficiency,modulation ofdemand on the prime mover should be avoided.In consequence of these problems,it would seem that,for the generalised case,it is thethermal rather than the electrical load which is the determinant of plant size. If it beproposed that the size of the CHP plant be increased so that the alternator is large enoughto be used as a standby generator,it is almost certain that the machine would be too large