Page 101 - Sustainable On-Site CHP Systems Design, Construction, and Operations

Thermal Design for CHP 79

resultant “strong” solution is returned to the air stream to collect more moisture. Solid
desiccants generally use a wheel impregnated with an adsorbent material such as silica
gel to remove the moisture from an air stream. As the wheel becomes saturated it is
rotated into the regenerator section where thermal energy is used to desorb the material
and prepare it for further moisture removal.
Desiccants are used in humid regions for fresh air treatment where they remove the
latent load and work in conjunction with chillers to meet the building total cooling
needs. They are also applicable in buildings requiring low dew points such as refriger-
ated warehouses and supermarkets, where large internal humidity loads need to be
removed such as natatoriums. Desiccants are generally available in relatively small
sizes compared to thermally driven chillers and are therefore suitable for application
with small CHP systems.
For desiccants, the COP varies from 0.5 to 0.7 based on the enthalpy of the water
removed divided by the energy input with newer versions of the liquid desiccant
system having the higher COP. Desiccants are air side systems and as such are also
easier to apply in smaller applications where hydronic thermal distribution system are
not available. Hot water–driven liquid desiccant systems generally require lower acti-
vation temperatures at approximately 180°F whereas hot water–driven solid desiccants
require up to 240°F for regeneration. In CHP applications using exhaust heat recovery,
the desiccant system can be used as a bottoming-cycle using the exhaust after the
primary thermal conversion steam boiler or chiller. Where hot water is recovered from
the primary thermal conversion device, the exhaust temperature will generally be too
low to regenerate the desiccant.
Liquid desiccant units require a cooling tower to remove the heat of absorption
from the system. Solid desiccants are also exothermic devices but do not require a cooling
tower and instead pass the process latent to sensible heat gain into the space. Generally
this sensible heat gain is removed using a downstream sensible cooling coil as the desic-
cant system typically works in conjunction with a chiller to handle both latent and
sensible loads. Desiccant systems are denominated in terms of cubic feet of air that pass
through the conditioning section, but when denominated in tons at maximum moisture
removal capacity are comparable to absorption on a cost-per-ton basis.

Technology Comparison
When designing a CHP system, the selection of a thermal conversion device will depend
on the type and size of addressable facility loads and the type of prime mover to be
employed. As discussed above, the thermal-electric ratio or T/E ratio is the primary
characteristic used to ensure high load factor when matching a facility with a CHP con-
figuration. CHP design does not limit the selection of a prime mover when the thermal
output has been selected as each prime mover will have multiple T/E ratios depending
on the heat recovery equipment and the thermal technology selected. From this per-
spective Table 4-1 outlines the different prime movers available together with typical
T/E ratios when coupled with various thermally driven technologies.
The T/E ratios given in Table 4-1 represent the an average value for each of
the CHP configurations described and can vary depending on the specific charac-
teristics of the prime mover selected. T/E ratios for turbines can be doubled or
tripled with the addition of supplemental firing providing additional flexibility for
these configurations.