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ABSORPTION REFRIGERATION Chapter 13 Geothermal Direct Use Engineering and Design Guidebook CHAPTER 13
ABSORPTION
REFRIGERATION
Kevin D. Rafferty, P.E.
Geo-Heat Center
Klamath Falls, OR 97601
13.1 INTRODUCTION
The absorption cycle is a process by which refriger-
ation effect is produced through the use of two fluids and
some quantity of heat input, rather than electrical input as in
the more familiar vapor compression cycle. Both vapor
compression and absorption refrigeration cycles accomplish
the removal of heat through the evaporation of a refrigerant
at a low pressure and the rejection of heat through the
condensation of the refrigerant at a higher pressure. The
method of creating the pressure difference and circulating
the refrigerant is the primary difference between the two
cycles. The vapor compression cycle employs a mechanical
compressor to create the pressure differences necessary to
circulate the refrigerant. In the absorption system, a
secondary fluid or absorbent is used to circulate the
refrigerant. Because the temperature requirements for the
cycle fall into the low-to-moderate temperature range, and
there is significant potential for electrical energy savings,
absorption would seem to be a good prospect for geother-
mal application.
Absorption machines are commercially available today
in two basic configurations. For applications above 32
o
F
(primarily air conditioning), the cycle uses lithium bromide
as the absorbent and water as the refrigerant. For applica-
tions below 32
o
F, an ammonia/water cycle is employed with
ammonia as the refrigerant and water as the absorbent.
13.2 LITHIUM BROMIDE/WATER CYCLE
MACHINES
Figure 13.1 shows a diagram of a typical lithium
bromide/water machine (Li Br/H
2
O). The process occurs in
two vessels or shells. The upper shell contains the
generator and condenser; the lower shell, the absorber and
evaporator.
Heat supplied in the generator section is added to a
solution of Li Br/H
2
O. This heat causes the refrigerant, in
this case water, to be boiled out of the solution in a
distillation process. The water vapor that results passes into
the condenser section where a cooling medium is used to
condense the vapor back to a liquid state. The water then
flows down to the evaporator section where it passes over
tubes containing the fluid to be cooled. By maintaining a
Figure 13.1
Diagram of two-shell lithium bromide
cycle water chiller (ASHRAE, 1983).
very low pressure in the absorber-evaporator shell, the water
boils at a very low temperature. This boiling causes the
water to absorb heat from the medium to be cooled, thus,
lowering its temperature. Evaporated water then passes into
the absorber section where it is mixed with a Li Br/H
2
O
solution that is very low in water content. This strong
solution (strong in Li Br) tends to absorb the vapor from the
evaporator section to form a weaker solution. This is the
absorption process that gives the cycle its name. The weak
solution is then pumped to the generator section to repeat the
cycle.
As shown in Figure 13.1, there are three fluid circuits
that have external connections: a) generator heat input, b)
cooling water, and c) chilled water. Associated with each of
these circuits is a specific temperature at which the
machines are rated. For single-stage units, these tempera-
tures are : 12 psi steam (or equivalent hot water) entering
the generator, 85
o
F cooling water, and 44
o
F leaving chilled
water (ASHRAE, 1983). Under these conditions, a coeffic-
ient of performance (COP) of approximately 0.65 to 0.70
could be expected (ASHRAE, 1983). The COP can be
thought of as a sort of index of the efficiency of the machine.
It is calculated by dividing the cooling output by the
299 required heat input. For example, a 500-ton absorption
chiller operating at a COP of 0.70 would require: (500 x
12,000 Btu/h) divided by 0.70 = 8,571,429 Btu/h heat
input. This heat input suggests a flow of 9,022 lbs/h of 12
psi steam, or 1,008 gpm of 240
o
F water with a 17
o
F

T.
Two-stage machines with significantly higher COPs
are available (ASHRAE, 1983). However, temperature
requirements for these are well into the power generation
temperature range (350
o
F). As a result, two-stage machines
would probably not be applied to geothermal applications.
13.3 PERFORMANCE
Based on equations that have been developed
(Christen, 1977) to describe the performance of a single-
stage absorption machine, Figure 13.2 shows the effect on
COP and capacity (cooling output) versus input hot-water
temperature. Entering hot water temperatures of less than
220
o
F result in substantial reduction in equipment capacity.
The reason for the steep drop off in capacity with
temperature is related to the nature of the heat input to the
absorption cycle. In the generator, heat input causes boiling
to occur in the absorbent/refrigerant mixture. Because the
pressure is fairly constant in the generator, this fixes the
boiling temperature. As a result, a reduction in the en-
tering hot water temperature causes a reduction in the
temperature difference between the hot fluid and the boiling
mixture. Because heat transfer varies directly with temper-
ature difference, there is a nearly linear drop off in absorp-
tion refrigeration capacity with entering hot water tempera-
ture. In the past few years, one manufacturer (Yazaki,
undated) has modified small capacity units (2 to 10 ton) for
increased performance at lower inlet temperature. How-
ever, low-temperature modified machines are not yet avail-
able in large outputs, which would be applicable to
institutional- and industrial-type projects. Although COP
and capacity are also affected by other variables such as
condenser and chilled water temperatures and flow rates,
generator heat input conditions have the largest impact on
performance. This is a particularly important consideration
with regard to geothermal applications.
Because many geothermal resources in the 240
o
F and
above temperature range are being investigated for power
generation using organic Rankine cycle (ORC) schemes, it
is likely that space conditioning applications would see
temperatures below this value. As a result, chillers
operating in the 180 to 230
o
F range would (according to
Figure 13.2) have to be (depending on resource tempera-
ture) between 400 and 20% oversized respectively for a
particular application. This would tend to increase capital
cost and decrease payback when compared to a conven-
tional system.
An additional increase in capital cost would arise from
the larger cooling tower costs that result from the low COP
of absorption equipment. The COP of singe effect equip-
ment is approximately 0.7. The COP of a vapor compres-
sion machine under the same conditions may be 3.0 or
higher. As a result, for each unit of refrigeration, a vapor
compression system would have to reject 1.33 units of heat
at the cooling tower. For an absorption system, at a COP of
0.7, 2.43 units of heat must be rejected at the cooling tower.
This results in a significant cost penalty for the absorption
system with regard to the cooling tower and accessories.
Figure 13.2 Capacity of a lithium bromide absorption chiller (Christen, 1977).
300 0
100
200
300
400
500
600
Installed Cost in $ *1000
0
200
400
600
800
1000
Capacity in Tons
Abs chlr
Elec chlr
Abs twr
Elec twr
In order to maintain good heat transfer in the generator
section, only small Ts can be tolerated in the hot water
flow stream. This is a result of the fact that the machines
were originally designed for steam input to the generator.
Heat transfer from the condensing steam is a constant
temperature process. As a result, in order to have equal
performance, the entering hot water temperature would have
to be above the saturated temperature corresponding to the
inlet steam pressure at rated conditions. This is to allow for
some T in the hot water flow circuit. In boiler coupled
operation, this is of little consequence to operating cost.
However, because

T directly affects flow rate, and thus
pumping energy, this is a major consideration in geothermal
applications.
For example, assuming a COP of 0.54 and 15
o
F

T on
the geotherm