Page 94 - Sustainable On-Site CHP Systems Design, Construction, and Operations

72 CHP B a s i c s

210 300

205 250

200
200
Temperature (°F) 195 150 Engine output (kW)

190
100
185 50
Coolant outlet Coolant return Engine output
180 0
10:10:00 AM 10:20:00 AM 10:30:00 AM 10:40:00 AM 10:50:00 AM 11:00:00 AM 11:10:00 AM 11:20:00 AM 11:30:00 AM 11:40:00 AM 11:50:00 AM 12:00:00 PM 12:10:00 PM 12:20:00 PM 12:30:00 PM 12:40:00 PM 12:50:00 PM 1:00:00 PM

FIGURE 4-5 Coolant temperature versus engine output. (Courtesy of Integrated CHP Systems
Corporation.)

temperature is reduced, the size and cost of the heat exchanger increase significantly
as more surface area is required to transfer the same amount of heat with a smaller
temperature differential.
For exhaust-based systems that produce hot water or steam, heat recovery devices
include water tube or fire tube boilers (often called heat recovery steam generators or
HRSG when used to produce steam) and coil type water heaters. Exhaust heat can also
be recovered through gas-to-air heat exchangers or in some cases used directly in
drying or heating processes. Boilers can be used to produce hot water or various steam
pressures to meet the facility heating needs as well as supply energy to thermally
driven chillers.
In some cases, the heat recovery system can be built into the thermal conversion
device as in the case of hot water absorbers where the heat recovery loop can be con-
nected to the absorber without the need for an intermediary heat exchanger. Exhaust-
fired absorbers have recently become available where the high temperature exhaust
from an engine can be used directly by the chiller without the need first to convert the
exhaust energy to hot water or steam. In these cases, care needs to be taken to ensure
precise control of the exhaust inlet flow to avoid crystallization in the absorber, espe-
cially when operating at varying load.
In the case of combustion turbines, the exhaust gases contain sufficient oxygen to
allow the efficient combustion of natural gas. If a high T/E ratio is desired, the turbine
heat recovery boiler may be fitted with a supplemental burner or duct burner to allow
more thermal energy to be added to the system at very little additional cost. These
supplemental burners operate at very high efficiencies and provide additional flexibility
in meeting facility thermal needs.