Page 96 - Sustainable On-Site CHP Systems Design, Construction, and Operations

74 CHP B a s i c s

strengthen the solution, which is then returned to the absorber section of the chiller to
continue the absorption process. The water vapor that is driven off in the generator is
directed to the condenser section where it is condensed and returned to the evaporator
to continue the cooling process.
The absorption principle was discovered in the late eighteenth century and was
commercialized early the following century using ammonia (NH ) and water as the
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refrigerant/absorbent pair. These devices were used in the American Civil War for food
storage related refrigeration and were applied for space conditioning early in the twentieth
century. In the later twentieth century two-stage absorbers were developed and water
was used as the refrigerant with lithium bromide (LiBr) being used as the absorbent as
an alternate to the ammonia/water pair. Absorbers encompass a variety of forms based
on thermal energy sources as follows:
• Single- and two-stage hot water fired LiBr
• Single- and two-stage steam fired LiBr
• Two-stage exhaust-fired LiBr
• Single- and two-stage hot water- or steam-fired NH
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• Two-stage exhaust-fired NH
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• Hybrid single-/two-stage LiBr and NH
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While LiBr absorbers now form the mainstream choice for space conditioning,
ammonia absorbers are still the choice for low temperature applications as well as having
the ability to be configured as thermally driven heat pumps that can significantly
increase the volume of hot water generated from a CHP system. LiBr systems operate
under high vacuum, while ammonia systems operate at high pressure. The choice of
chiller will generally depend on the particular application requirements with ammonia
absorption units being the only option for refrigeration. Absorption efficiency is
expressed as a coefficient of performance (COP) which is calculated as the cooling output
divided by the energy input in consistent units.
Single-stage absorbers generally have a full load COP of approximately 0.7 when
fired with 15-psig steam or 240°F hot water at ARI conditions. Two-stage absorbers
have a full load COP of approximately 1.2 when fired with 120-psig steam or 350°F
hot water at ARI conditions. ARI conditions refer to the American Refrigeration Insti-
tute Standard 560 for absorption chillers, which measures performance based on pro-
viding 44°F chilled water supply based on 54°F chilled water return using 4 gallons
per minute (gpm) of 85°F condenser water per ton of cooling. This essentially means
that for every 1,000,000 Btu/h of heat input under the above conditions, a single-stage
absorber will generate 58.3 tons and a two-stage chiller will generate 100 tons, based
on a COP of 0.7 and 1.2, respectively. Since CHP systems are generally designed to
address the base load and therefore operate at or close to full load through most oper-
ating hours, we are not so concerned with part load efficiencies. The part load COP
can be significantly higher due mainly to the lower condenser water temperatures
associated with the lower ambient temperatures concurrent with lower loads. The full
load COP can be increased by lowering the condenser water temperature but there is
limited capacity to achieve this due to the danger of crystallization of the LiBr
solution. This is particularly true for two-stage absorbers which operate at high solution