Page 99 - Sustainable On-Site CHP Systems Design, Construction, and Operations

Thermal Design for CHP 77

Once the desiccant material has adsorbed all the water vapor it can handle, it
needs to be regenerated before continuing the process. The adsorber had two “evapo-
rator/adsorber” chambers and operates as a batch system providing cooling in one
chamber while hot water is used to regenerate the desiccant in the other chamber. The
water vapor that is driven out of the desiccant during the regeneration process
is passed through a condenser section where it is condensed and returned to the
evaporator. Once the cooling cycle is complete the system interrupts the flows and
switches from cooling to regeneration in the first chamber and begins cooling in the
second chamber.
The adsorber is driven by hot water and has a significantly lower operating tem-
perature range than a single-stage LiBr absorber. The adsorber can use hot water at
down to 150°F but will have a significant capacity and efficiency derating at this
temperature. For adsorption the maximum temperature to the chiller is more restrictive
than absorption at 195°F. The adsorber COP ranges from 0.5 to 0.7 depending on hot water
temperature input and is rated at similar conditions to the ARI Standard 560 conditions
used for absorption.
While adsorbers have the advantage of being able to use relatively low temperature
hot water, they are considerably more expensive per ton and are larger than compa-
rably sized absorbers. Adsorbers require chilled water and hot water storage tanks,
since it is a batch process and also require significantly higher condenser water flow
rates of approximately 8 gpm/ton by comparison to absorbers which require typi-
cally 3.5 to 5 gpm/ton. It should be noted that single-stage hot water fired absorbers
do require increasing flows of condenser water when trying to operate on low tem-
perature hot water. The requirement for a hot water storage tank does provide for
stable operation through engine heat recovery variations and the adsorber itself will
continue to operate at lower part load on an engine beyond the capability of an
absorber. This is particularly useful for designs incorporating multiple engine modules
and a fixed flow hot water header. In this scenario, the adsorber will be able to main-
tain a higher cooling output at a lower power output from the CHP system than
absorber. However, as stated above, a typical CHP system generally should not have
significant drop in power output.
As the condenser water and hot water share the same tube bundle in the adsorber,
a heat exchanger with associated circulating pump will be required between the cooling
tower water and the adsorber. This adds to the project cost as well as the size of the
cooling tower.

Steam Turbine Chillers
Steam turbine–driven chillers are essentially the same as electric centrifugal vapor com-
pression chillers except that the electric motor is replaced with a steam turbine drive. In
an effort to maximize the efficiency of these units, steam turbine chillers are generally
designed with condensing turbines and are provided with a steam condenser as well as
a refrigerant condenser. These chillers are driven by high-pressure steam from 100 to
600 psig and are typically rated at 125 psig.
Steam turbine chillers have a full load COP of 1.2 at ARI conditions which is com-
parable to two-stage absorption. They differ from two-stage absorbers in being able to
use low condenser water temperatures even at full load for significant efficiency gains
at full load during off-design ambient conditions. This is a considerable advantage for
the steam turbine in CHP applications where the chiller is base loaded and the steam