Page 65 - Sustainable On-Site CHP Systems Design, Construction, and Operations

44 CHP B a s i c s

The total amount of waste heat from an engine is the total amount of fuel energy
input less the energy value of the rotary power produced. Not all the waste energy
produced can be usefully recovered. As an example, engine heat loss by radiation to the
space is not ordinarily recovered. Also, most CHP systems do not recover heat past the
point that the water vapor produced as part of the combustion process (due to oxida-
tion of hydrogen in the fuel) is condensed. Unless the exhaust is specifically designed
for condensation, condensation in the exhaust is avoided because, due to the formation
of carbonic acid (H CO ), condensate is corrosive and can damage the exhaust system.
2 3
The latent heat in the water vapor is a significant part of the heat of combustion (around
10 percent in the case of natural gas) and depends on the type of fuel burned. For exam-
ple, fuel oil derives more energy from carbon, which creates less water vapor than does
natural gas. Because the portion of the latent heat of vaporization for water in the fuel
heat of combustion is a function of the fuel type and its chemistry, and because most
processes do not recover the water vapor energy (i.e., the latent heat of vaporization for
water), most engine manufacturers rate their engines in a fuel’s lower heating value
(LHV). The LHV does not include the latent heat of vaporization for water in the fuel
heat of combustion, compared to the higher heating value (HHV), which does include
the latent heat of vaporization for water in the fuel heat of combustion. While engine
performance may be rated based on the LHV, fuel purchases are typically based on the
HHV, and owners, operators and engineers must take these differences into consider-
ation in their calculations.
The amount of waste heat which can be recovered from the IC engine depends on
the type of engine, the temperature at which the heat recovery occurs, and on the type
and capacity of the heat recovery equipment. In general, a turbocharged engine has
more of its waste heat in the exhaust gases than a naturally aspirated engine. The higher
the temperature at which beneficial heat recovery must occur, the less energy that can
be recovered.
The typical distribution of input fuel energy for a reciprocating engine operating at
rated load can be broken down as follows:

1. Shaft power 32%
2. Convection and radiation 3%
3. Rejected in jacket water 32%
4. Rejected into the exhaust 30%
5. Lube oil cooling 3%
The latent heat of vaporization for the water vapor created by combustion of hydro-
gen is lost in the exhaust gases unless the gases are cooled to a point where the water
vapor condenses. Condensing systems can be highly efficient and improve CHP sus-
tainability, but, as noted, the exhaust system must be designed for the corrosive con-
densate (e.g., constructed of stainless steel). Most of the heat in jacket water and lube oil
cooling can be recovered and used. Usual heat recovery practices can recover some 60
to 80 percent or higher of the heat in the exhaust gases depending on various factors
including the thermal output temperature, with the highest efficiency achieved when
the exhaust gases are cooled to near ambient temperatures.
With respect to the fuel distribution percentages discussed above, it should be noted
that the percentages vary with manufacturer, model as well as with engine load.