Page 35 - Sustainable On-Site CHP Systems Design, Construction, and Operations

14 CHP B a s i c s

for equipment in which its temperature glide can be matched better, such as a heating
system or an absorption chiller. Use of hot oil could be a better approach than use of
steam, hot water, or direct exhaust firing of absorption chillers in CHP systems. Each of
these more conventional approaches has its own drawbacks: use of steam reduces total
potential recovered heat due to the pinch points, hot water at high temperature requires
high pressures for double-effect chillers, and direct exhaust firing involves very large
ducts to transport the exhaust gases and generally involves greater backpressure on
turbines, which can reduce electric generator output.
In addition to eliminating space to accommodate HRSG footprint, the above alter-
native facilitates use of prefabricated steam generators, associated heat exchangers, and
pumping systems employing low-pressure, nonvolatile, recirculating heat-transfer
fluids capable of direct extraction of turbine exhaust gas waste heat to generate steam
and allow cascading the remaining captured waste heat to drive absorption chiller(s).
The heat transfer fluid can also be used for space and domestic hot water systems
enabling greater utilization of available heat reclamation potentials in satisfying highly
variable annual building power, heating, and cooling load demands. This is achieved
through maintaining favorable log-mean-temperature-differences (LMTDs) at the turbine
gas extraction coil also resulting in a lower exhaust gas temperature discharge to ambient
(see case study 6).

Types of Thermally Activated Technologies
In addition to using recovered waste heat for space heating, for example, waste heat, as
noted, can also be used for cooling. Specifically, instead of electric motor power to rotate
a refrigerant compressor, cooling can be generated in an absorption or adsorption
process. As discussed in Chap. 4, one method is to use an absorption chiller, which
typically uses the water/ammonia cycle to transfer and reject heat. Absorption chillers
can either be single-stage, double-stage, or triple-effect, and can provide simultaneous
heating and cooling. Absorption chillers are typically limited to a chilled water supply
temperature of 42°F, although advanced control of solution concentrations can report-
edly “lower the bar” a couple of degrees. As noted, steam can be produced in a HRSG,
and that steam can be used to run a steam-turbine-driven centrifugal chiller, which can
produce chilled water at a much lower temperature than 42°F, if needed.
In humid climates, waste heat can be used to remove moisture from thermally
powered solid or liquid desiccant dryers and offers an excellent opportunity for sus-
tainable energy savings versus electric-powered refrigerated dryers.

Understanding and Matching Facility Load Requirements
In an ideal case, the amount of recoverable heat from the prime mover tracks the power
load; however, in reality, perfectly matched power and thermal requirements are not
always possible. In brief, the following methods can be used to match the required on-site
power and thermal energy:

• Match the thermal-electric ratio (see Chap. 4) of the prime mover to that of the
user’s hourly load profile.
• Store excess power as chilled water or ice when the thermal demand exceeds
the coincident power demand.