From: Essential Information <ei@igc.apc.org>
Newsgroups: sci.energy
Date: 31 Dec 92 13:54 PST
Subject: Re: Energy Ideas - HVAC-2
Nf-ID: #R:cdp:1466300134:cdp:1466300137:000:6303
Nf-From: cdp.UUCP!ei    Dec 31 13:54:00 1992



HOW TO COOL WITH HEAT: ABSORPTION COOLING

Absorption cooling systems are as effective as conventional vapor
compression cooling systems, but require heat rather than
electricity. As a result, these systems can use a variety of
energy sources, including natural gas combustion, waste heat from
electricity generation or solar thermal power. More importantly,
these systems commonly use either ammonia or water as
refrigerants, neither of which contributes to the depletion of
the ozone layer or to global warming.

How They Work

Like vapor compression chillers (see "Electric Heat Pumps
Revisited," Energy Ideas, November, 1992, p. 6), absorption
chillers circulate a working fluid (the refrigerant) through an
expansion and condensation cycle. Heat from air in a building
causes a refrigerant to boil (see diagram below). Rather than
being compressed into a hot liquid, the gaseous refrigerant is
moved to a chamber where it is absorbed by another liquid
substance. The refrigerant-absorbent mixture is heated in a
boiler to separate the two fluids. Each is sent to its own
condensing chamber. The absorbent cools and condenses and is
returned to absorb more refrigerant. The refrigerant cools and
condenses and is returned to the evaporator where it will pick up
more heat from the indoor air. While earlier systems used ammonia
as the refrigerant and water as the absorbent, newer systems use
water as the refrigerant and a non-toxic lithium bromide salt
solution as the absorbent. The lithium bromide cycle occurs in a
vacuum, giving water a very low boiling temperature. 

Absorption cooling offers a few advantages over vapor-compression
systems. The first and foremost is the absence of a toxic and
ozone-depleting refrigerant. Since absorption cooling only
requires heat and small pumps for circulation of the water and
salt solution and the evacuation of the system, an absorption
chiller has very few moving parts. Consequently, these systems
are very reliable and easy to maintain. In addition, the systems
provide quiet, vibration-free operation. The greatest advantage
is the low cost of natural gas as compared to electricity costs
for large chillers (see life-cycle cost analysis, p. 7).

Site Efficiency and Source Efficiency

The greatest disadvantage of an absorption cooling system is its
low coefficient of performance (COP). The COP is the ratio of the
amount of energy that goes into a chiller to the amount of energy
removed by the chiller. Typically, an absorption chiller has a
site COP of 1; for every British thermal unit (Btu) of natural
gas burned on site, one Btu of energy is removed from the air.
Newer models are expected to attain a site COP of 1.5. Electric
vapor-compression chillers of intermediate size have a site COP
of 5, making them far more efficient. Since the generation of
electricity by fossil fuel combustion is only about 35 percent
efficient, the source, or overall, COP of an electric chiller is
only 1.75.

Cooling by Cogeneration

According to the Institute for Energy and Environmental Research
(IEER), of Takoma Park, Maryland, an absorption chiller-heater
which also produces electricity is the most efficient cooling and
heating system. The cogeneration of heat and electricity vastly
improves the energy efficiency of any combustion system by using
heat which would normally escape into the environment (see
"Cogeneration: Increased Efficiency without Increased Fuel,"
Energy Ideas, November, 1992, p. 4). In addition, the electricity
produced by a cogenerating chiller-heater could be sold to a
local electric utility pursuant to the Public Utility Regulatory
Policy Act (PURPA).

Developing such cogeneration units also contributes to smarter
energy planning for utilities. By installing cogeneration systems
to provide all energy - heating, cooling and electricity - to new
facilities, power generation increases only as demand increases.
This provides greater flexibility for electric utilities, which
usually forecast the long-term regional electricity demand and
build large central power stations, which may or may not be
needed in the future. In addition, the use of on-site natural gas
establishes a system which can be compatible with solar-produced
hydrogen fuel or biogas.

CASE STUDIES:

Absorption Chillers at Audubon Headquarters
When retrofitting their national headquarters in New York City,
the National Audubon Society installed rooftop absorption
chillers because these systems do not use CFCs, whereas
compression coolers do. The installation of a 180 ton natural
gas-fired chiller received a $72,000 rebate from the local
electric utility based on a $400 per ton rebate. (Contact: Dr.
Jan Beuea, Vice-President for Science, National Audubon Society,
700 Broadway, New York City, NY 10003, (212) 979-3000.)

Absorption Chiller at Junior High School
In 1987, the Northbrook, Illinois Junior High School installed a
200-ton absorption chiller-heater to provide air conditioning to
the 150,000-square-foot building. The school needed to maintain
its backup heating capability in addition to providing the air
conditioning. While an electric vapor compression chiller would
have required the expansion of the boiler room, the absorption
chiller-heater simply replaced one of the boilers and can provide
the necessary backup heat at any time. Although the
chiller-heater cost slightly more than an electric chiller, the
additional cost will be paid back. Natural gas is available at a
direct purchase price during the summer, only 17 cents per therm.
Electricity, at 7 cents per kilowatt hour, costs about $2.05 per
therm. 

While the system has operated reliably since its installation,
the building manager stressed the importance of the quality of
service received from the installer and the service company. They
took the time to fully train the building staff in the complete
operation of the chiller-heater and are readily available to
service the system. In addition, the school district was
impressed by the lack of ozone-depleting and toxic refrigerants
in the system. The building manager strongly suggests considering
these systems to anyone who is refitting a building. (Contact:
Tim Melting, Director of Buildings and Grounds, Northbrook School
District, 1475 Maple Avenue, Northbrook, IL 60062, (708)
498-7900.)


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