Page 198 - Sustainable On-Site CHP Systems Design, Construction, and Operations

The Engineering Pr ocess 171

degradation as it is with CTGs, all of the same heat exchanger considerations discussed
in the HRSG discussion above also apply to IC engines exhaust heat recovery with respect
to the trade-offs between the amount of heat recovered, the pressure drop through the
heat exchanger, the size of the heat recovery unit, and the capital costs required.
A hot water heat recovery unit (HW HRU), which is a gas-to-water heat exchanger,
can be arranged such that the jacket water supply from the engine is fed to the HW
HRU in order to increase the JW temperature by approximately an additional 10 to 15°F.
Increasing the JW temperature may be helpful if a hot water–fired absorption chiller
will part of the CHP plant design. This method can result in maximum JW delta-T of
about 25 to 30°F. Attention should be paid to the flow rate of the engine cooling water.
If the flow rate is too high, it can result in a lower than desired water temperature
leaving the HW HRU. Depending on the use and relative demands of the recovered
heat, some systems will split the jacket water cooling and the exhaust heat recovery into
separate systems, providing, for example, the lower quality jacket water heat to a water
heating system and the higher quality exhaust heat to an absorption chiller.
Lube oil heat represents about 5 percent of the fuel energy and is typically rejected
via an engine thermostat at about 130°F. A 130°F hot water may be used for various low
temperature uses including domestic water heating (or preheating), space heating, and
swimming pool heating.
Other heat recovery uses and options are available including using the prime mover
exhaust heat directly to fire an absorption chiller capable of providing simultaneous
chilled and hot water, or directly to drive a solid or liquid desiccant system, or to heat
air in an exhaust-to-air heat exchanger. As discussed in Chap. 24, another heat recovery
option with CTGs that may eliminate the need for full-time steam plant operators and
reduce many of the challenges associated with HRSGs in practice, is to use a nontoxic,
nonflammable high temperature heat transfer fluid in a cascade fashion that maximizes
heat transfer fluid delta-T.
Each prime mover should have its own heat recovery system, as well as a method
for operating at full electric load output and still being able to reject all heat if required
(e.g., during start-up and testing, or during emergency operations). For example, a steam
condenser can be used for CHP systems with a HRSG to reject heat and an air-cooled
radiator can be used for IC engine JW heat.

Alternative Heat Recovery Options
As discussed in Chap. 24, alternative heat recovery options are possible such as the
hot-oil circuit that can maximize the log mean temperature difference (LMTD), and
reduce backpressure losses. Claimed advantages include smaller thermal mass of
hybrid steam generator which permits quick response to varying loads, low-pressure
operation of high temperature heat transfer fluid recirculation loop which can eliminate
the need for 24/7 stationary engineer code requirement, reduced CTG exhaust extrac-
tion coil pressure drop which improves CTG power performance, lower overall life-
cycle cost, reduced installation time and operation complexity, reduced CHP system
downtime, and reduced overall footprint.

Fuel Systems
While reciprocating engines may be fueled from a variety of gas and liquid fuels including:
No. 2 diesel, natural gas, propane, landfill gas, digester gas (from wastewater treatment),
and biofuels including biodiesel, as discussed in Chap. 2, almost 90 percent of the CHP