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1. INTRODUCTION
1.5 Heat driven cooling
The most common principle of producing cooling is the vapor compression
cycle (reverse Rankine cycle) which is driven by mechanical work
(normally an electric motor).
An alternative is to produce the cooling by a sorption cycle. ”Sorption”
covers absorption and adsorption cycles which are mainly operated by heat
as energy source [18].
The sorption cycle is in principle like the vapor compression cycle
with a condenser, an expansion valve and an evaporator. The main
difference is how the refrigerant is pressurized after the evaporator:
for the vapor compression cycle it is a mechanical compression, e.g.
a reciprocation or a screw compressor, which compresses evaporated
refrigerant. In the sorption cycle it is a ”heat driven compressor” 7 . How
this work will be explained in the next section.
The heat ratio, COP ABS , of a sorption cycle is the ratio between cooling
load and driving heat 8 .
COP ABS =
˙Q cooling load
˙Q dr i ving heat
(1.1)
This COP is not comparable to the COP of the vapor compression
cycle because the inputs have different value (heat at the relatively low
temperature has a lower exergy than mechanical or electric work) [18].
The medium in sorption cycles is a pair consisting of a refrigerant and
a ”sorbent”. For the absorption cycle the refrigerant is absorbed by the
absorbent (which is either a liquid or a dissolved salt), i.e. a solution,
whereas for the adsorption cycle the refrigerant is adsorbed on the surface
of the adsorbent. The two cycles can be divided into subcategories which
are described in following sections.
7 Actually the physical pressurization is done by a mechanical pump.
8 The small amount of electric or mechanical energy used for driving pumps in some
variants is not included. For systems larger than 1 MW of cooling the consumption of
electrical energy of the pump is about one per thousand of cooling power
8