Page 66 - Sustainable On-Site CHP Systems Design, Construction, and Operations

Power Equipment and Systems 45

As noted, the quantity and quality of the heat that can be collected from reciprocat-
ing engines per kilowatt of power produced (thermal-electric ratio) is lower than that
can be obtained from CTGs which have higher thermal-electric ratios. First, less of
the fuel energy is converted to shaft horsepower in a typical CTG versus a typical IC
engine meaning more waste heat is available with a CTG. Second, with a CTG, nearly
all the waste heat is in the exhaust air stream versus an IC engine. And third, the exhaust
gas temperatures are much higher with a CTG than most of the waste heat from a recip-
rocating engine. Most engine jacket cooling systems operate at around 200°F and offer
good opportunity to recover heat in the form of hot water. For many applications, the
exhaust heat can be recovered into the coolant loop using an exhaust-to-liquid heat
exchanger to provide a single form of heat recovery. As noted above, a few internal
combustion engines permit coolant to reach 250°F at above atmospheric pressure and
then allow the coolant to flash into low-pressure steam (15 psig) after leaving the engine
jacket in an ebullient cooling system.
Heat Rate and Electrical Efficiency
As noted in Chap. 1, heat rate is defined as the amount of input energy required by the
prime mover to produce 1 unit of power. CHP heat rate for natural gas–fueled spark
ignition engines can range between about 10,000 to almost 14,000 Btu/kWh, while the
heat rate for diesel engines can be as low as 7000 Btu/kWh. As shown in Fig. 3-1, the
heat rate for spark ignition engines tends to decrease (i.e., the engines become more
efficient at producing power) as the rated power output increases (i.e., the engines
become more efficient with increased size). Note that a lower heat rate means that less
energy is required per kilowatthour produced.
As shown in Fig. 3-2, spark ignition engine electric efficiencies (electric power
output divided by fuel input in consistent units) generally increase as the engine’s rated
power increases. Typical spark ignition engines between 100 and 900 kW have observed
electric efficiencies between about 25 and 30 percent based on the HHV. Larger spark

16000

14000
12000
Heat rate (Btu/kWh) 8000
10000

6000
4000

2000
0
0 1000 2000 3000 4000 5000 6000 7000
Rated power (kW)

FIGURE 3-1 Heat rate (HHV) of spark ignition engine. [Source: 2008 ASHRAE Handbook: HVAC
Systems and Equipment (Ref. 1).]