Page 29 - Low Temperature Energy Systems with Applications of Renewable Energy

18 Low-Temperature Energy Systems with Applications of Renewable Energy

1.5.4 Thermodynamic efficiency of sorption refrigerating
machines
Sorption RMs and HPs are divided into two types: (1) adsorption and (2) absorption.
Adsorption refrigerators are based on the use of a solid adsorber and a refrigerating
working fluid. The refrigerant is adsorbed onto the surface of the solid material. Ab-
sorption refrigerators are based on the use of a liquid sorbent and a refrigerating work-
ing fluid (or refrigerant), where the two fluids physically mix together. Water-ammonia
RMs are commercially available, whereas modern absorption-diffusion RMs and HPs,
proposed earlier [17] are being developed.
Absorption refrigeration systems (AR) are a well-known alternative to vapor-
compression systems to achieve a cold space, especially where electricity prices are
high. Indeed, some AR systems completely eliminate mechanical work input and
use gravity to circulate the working fluid. This requires a 3-component working fluid
such as water-ammonia-hydrogen, first devised in 1922 by von Platen and Munters in
Sweden and described in Section 1.5.5.
For AR systems, the primary motive force is heat, not work, thus eliminating the
need for a pump and an electrical supply. The heat can be obtained from steam if a
boiler or other supply of steam is available. In many industrial plant applications,
excess (“waste”) steam is sent to the AR. However, if the steam has to be raised by
burning fossil fuel specifically for use in the AR, the question arises as to whether
that is the best means to obtain the heat. Renewable sources are to be preferred.
One steady renewable source of heat is hot geothermal water found in many fields
around the world. Another renewable, but intermittent, source is solar thermal energy
collected in reflecting parabolic troughs. One effective use of AR systems is in build-
ings with high peak loads on the power supply system.
Historically, the first absorption refrigerating machine was created in France by Fer-
dinand Carré in 1858, using as the working fluid a mixture of water and sulfuric acid.
The first two-stage absorption refrigerating machine was created in 1950. A three-stage
AR with three condensers and three generators (1985), with double condensers (1993)
exceeds the efficiency of the two-stage system by 30e50%. ARs of both direct and
indirect heating are in wide use.
AR systems may be used, for example, in ice-making or in air conditioning. Ice-
making must achieve sub-freezing temperatures so the working fluid normally is a
water-ammonia mixture where ammonia is the refrigerant and water is the carrier.
Air conditioning does not require such low temperatures, and a lithium bromide-
water mixture may be used where water is the refrigerant and the Li-Br is the carrier.
Since the working fluid is a binary-mixture as opposed to a simple pure substance,
there is an additional degree of freedom when determining the thermodynamic
properties.
Figure 1.14 shows a block diagram for a generic ideal AR system in which the main
components, namely, the vapor generator, condenser, absorber and evaporator, are
within the boundary of the refrigerator proper. The AR is in thermal contact with three
heat reservoirs, i.e., the source of the motive heat at T H , the created cold space at T L ,
and the surroundings at the ambient temperature T A . Both the condensation and the