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5.2.4 Absorption Cooling
Technical Information An absorption cooling cycle is similar to
a vapor-com-pression cycle in that it relies on the same three
basicprinciples (1) when a liquid is heated it boils (vapor-izes),
and when a gas is cooled it condenses; (2) lower-ing the pressure
above a liquid reduces its boiling point;and (3) heat flows from
warmer to cooler surfaces. In-stead of mechanically compressing a
gas (as occurs witha vapor-compression refrigeration cycle),
absorptioncooling relies on a thermochemical compressor.
Twodifferent fluids are used, a refrigerant and an absor-bent, that
have high affinity for each other (one dis-solves easily in the
other). The refrigerant (usuallywater) can change phase easily
between liquid andvapor and circulates through the system. Heat
fromnatural gas combustion or a waste-heat source drives
the process. The high affinity of the refrigerant for
theabsorbent (usually lithium bromide or ammonia)causes the
refrigerant to boil at a lower temperatureand pressure than it
normally would and transfers heatfrom one place to another.
Absorption chillers can be direct-fired or indirect-fired,and
they can be single-effect or double-effect (explana-tion of these
differences is beyond the scope of this dis-cussion). Double-effect
absorption cycles capture someinternal heat to provide part of the
energy required inthe generator or desorber to create the
high-pressurerefrigerant vapor. Using the heat of absorption
reducesthe steam or natural gas requirements and boosts sys-tem
efficiency.
Absorption cooling equipment on the market rangesin capacity
from less than 10 tons to over 1,500 tons(35 to 5,300 kW).
Coefficients of performance (COPs)range from about 0.7 to 1.2, and
electricity use rangesfrom 0.004 to 0.04 kW/ton of cooling. Though
an elec-tric pump is usually used (the principal exceptionsbeing
the small hotel and recreational vehicle [RV] re-frigerators), pump
energy requirements are relativelysmall because pumping a liquid to
the high-side pres-sure requires much less electricity than does
compress-ing a gas to the same pressure.
High-efficiency, double-effect absorption chillers aremore
expensive than electric-driven chillers. They re-quire larger heat
exchangers because of higher heat-rejection loads; this translates
directly into higher
On the surface, the idea of using an open flame or steamto
generate cooling might appear contradictory, but theidea is
actually very elegant. And it has been aroundfor quite a whilethe
first patent for absorption cool-ing was issued in 1859 and the
first system built in1860. Absorption cooling is more common today
thanmost people realize. Large, high-efficiency, double-ef-fect
absorption chillers using water as the refrigerantdominate the
Japanese commercial air-conditioningmarket. While less common in
the U.S., interest inabsorption cooling is growing, largely as a
result of deregulation in the electric power industry. The
tech-nology is even finding widespread use in hotels thatuse small
built-in absorption refrigerators (because of their virtually
silent operation) and for refrigeratorsin recreational vehicles
(because they do not requireelectricity).
Opportunities Absorption cooling is most frequently used to
air-con-dition large commercial buildings. Because there areno
simplifying rules of thumb to help determine whenabsorption
chillers should be used, a life-cycle costanalysis should be
performed on a case-by-case basisto determine whether this is an
appropriate technol-ogy. Absorption chillers may make sense in the
follow-ing situations: where there are high electric demandcharges,
where electricity use rates are high, wheresummertime natural gas
prices are favorable, or where
utility and manufacturer rebates exist. Absorptionchillers can
be teamed with electric chillers in hybridcentral plants to provide
cooling at the lowest energycostsin this case, the absorption
chillers are used dur-ing the summer to avoid high electric demand
charges,and the electric chillers are used during the winterwhen
they are more economical. Because absorptionchillers can make use
of waste heat, they can essen-tially provide free cooling in
certain facilities.
Absorption cooling systems can most easily be incor-porated into
new construction, though they can alsobe used as replacements for
conventional electric chill-ers. A good time to consider absorption
cooling is when
an old electric chiller is due for replacement.
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costs. Non-energy operating and maintenance costs forelectric
and absorption chillers are comparable. Sig-nificant developments
in controls and operating prac-tice have led the current generation
of double-effectabsorption chillers to be praised by end-users for
theirlow maintenance requirements.
The potential of absorption cooling systems to use wasteheat can
greatly improve their economics. Indirect-firedchillers use steam
or hot water as their primary en-ergy source, and they lend
themselves to integrationwith on-site power generation or heat
recovery fromincinerators, industrial furnaces, or
manufacturingequipment. Indirect-fired, double-effect
absorption
chillers require steam at around 370 F and 115 psig(190 C and
900 kPa), while the less efficient (but alsoless expensive)
single-effect chillers require hot wateror steam at only 167270 F
(75132 C). Triple-effectchillers are also available.
ReferencesCler, Gerald, et al., Commercial Space Cooling and
Air
Handling Technology Atlas , E Source, Inc., Boulder,CO, 1997;
(303) 440-8500; www.esource.com.
ASHRAE Handbook: 1997 Fundamentals , pp. 1-20, American Society
of Heating, Refrigerating, and Air-Conditioning Engineers, Atlanta,
GA; www.ashrae.org.
Wood, Bernard D., Application of Thermodynamics , Addison-Wesley
Publishing Company, Reading, MA,1982, pp. 238-257.
ContactsDistributed Energy Resources Program, Office of
PowerTechnologies, EERE, U.S. Department of Energy,
1000Independence Avenue, SW, Washington, DC 20585.
Building Equipment Research Program, Energy Divi-sion, Oak Ridge
National Laboratory, Oak Ridge, TN37831; (865) 574-2694.
American Gas Cooling Center, 400 N. Capitol Street,
NW, Washington, DC 20001; (202) 824-7141, www.agcc.org.
This York Millennium Direct-Fired Double-Effect Ab- sorption
Chiller/Heater replaces an electric chiller and
boiler, reducing the floor-space requirement by up to 40%.
Source: American Gas Cooling Center
TYPICAL INSTALLED COSTS OFVARIOUS TYPES OF CHILLERS ($/TON)
SMALL MEDIUM LARGECHILLER TYPE (
