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MIL-HDBK-1003/13A
Concentrating or evacuated tube collectors are usually used in these
applications.  If concentrating collectors are used (see Section 2.1.10.2),
the associated higher costs and potentially increased maintenance for the
tracking mechanism must be considered.  In general, concentrating collectors
operate at higher efficiency at these higher temperatures.  However, the
higher temperatures are usually not required to operate the space heating
system.  Therefore, the relative importance of the two thermal loads must be
considered when selecting a system.
The second consideration involves the means of delivery of the heated fluid
to the absorption cooler.  Since, in many climates, the cooling load is
simultaneous with and often proportional to the solar insolation, it may be
desirable to allow the heated collector fluid to bypass the storage unit.
Other climates may require a hot storage unit but one of considerably smaller
size than the one used for heating purposes.  The important requirement is
that high temperatures be available during periods of heavy cooling load.
A third consideration deals with the problem of reduced efficiency of the
absorption cooler under start up and transient conditions.  Typical
absorption coolers do not reach operating efficiency until after an hour or
more of operation time.  A machine which is cycled on and off regularly will
have a drastically reduced average coefficient of performance when compared
to a machine in steady state performance.  This problem has been overcome in
at least one installation by the use of a cold storage unit (Beckman, 1977).
The cold storage unit permits continuous operation of the absorption cooler
and thus allows some reduction in the system and cooler size.
A fourth consideration is the need for some means of cooling the absorber and
the condenser.  A cooling tower or some other low temperature cooling system
must be used to obtain reasonable performance.  All of the commercially
available units require a cooling tower which is another maintenance item.
Current research is underway to develop units that do not have a separate
cooling tower.
2.7.2 Rankine cycle heat engine cooling.  Rankine cooling systems are still
in development with only a few in operation (Anderson 1979; Barber 1975).
In these systems the shaft power produced by a heat engine drives the
compressor in a conventional vapor compression-type cooling machine.  The
thermal energy input to the heat engine can be from a solar collector or from
a solar collector and a fossil fuel combustor.  The fossil fuel can
supplement solar energy, or it can be used alone as the auxiliary energy
supply when no solar energy is available.  Alternatively, electricity can be
used as the auxiliary energy supply by coupling an electric motor directly to
the compressor shaft.  Another option is a motor-generator using a heat
engine for generating electricity when solar energy is available and there is
little or no cooling load.
From state-of-the-art considerations, two types of fluid heat engines are
primarily feasible in solar cooling units.  In one type of engine, the
working fluid cyclically changes phase from liquid to gas and back to liquid.
The most widely used engine of this type operates on the Rankine cycle.
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