Page 32 - Low Temperature Energy Systems with Applications of Renewable Energy

Principles and operation of refrigeration and heat pump systems 21

5.0

4.5
4.0
Ideal coefficient of performance 3.0 T , C
3.5

2.5
o
L
2.0
-5
-10
1.5
1.0
-50
0.5 -25
0.0
50 100 150 200 250 300 350
o
Heat source temperature, C

Fig. 1.15 Ideal COP for absorption refrigerators: T A ¼ 25 C.

heat. This type of system was commercialized in 1933 when many homes were con-
nected to municipal natural gas service. Figure 1.16 shows a simplified schematic of
such a machine. There are three working fluids: ammonia (the coolant), water (the
absorber) and hydrogen (a catalyst).
The heat input at the generator (3) is supplied by a gas burner. At that place, the
working fluid is a liquid mixture of ammonia and water. The ammonia having the
lower boiling point boils off and percolates up a tube, lifting the water-ammonia
mixture to the rectifier (4) where the hot vapor (nearly pure ammonia) is separated
and rises to the condenser (5). There it comes in thermal contact with the ambient tem-
perature room air, releases its latent heat and liquefies. It then passes through a trap
(throttle) and into coils surrounding the freezer section of the refrigerator (1). The
now cold ammonia receives heat from the freezer space and evaporates. However,
before the ammonia enters the freezer it mixes with hydrogen gas rising from the
absorber (2), creating a denser fluid than either of the fluids alone. This allows the
ammonia and hydrogen mixture to fall under its own weight to the absorber (2) where
the ammonia is absorbed into the water that had been separated at the rectifier.
Hydrogen cannot absorb into the water (water is saturated with hydrogen) and is
released to rise through the absorber coils where it passes through the descending water
and eventually rises to the freezer, completing a loop of travel. The hydrogen releases
heat at the coils, giving up the heat it had picked up in the freezer.
No mechanical work or electricity is needed to operate this RM, only heat from
burning natural or bottled gas, making these units popular for remote residences
such as hunting cabins. Of course, if one desires a light bulb inside the refrigerator