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Solar cooling plants: some characteristic system arrangements
International Journal of Low Carbon Technologies 2/4
Solar cooling plants: some characteristic system arrangements
Renato M. Lazzarin 0
0 Dipartimento di Tecnica e Gestione dei Sistemi industriali, DTG Università di Padova , Italy
Some schemes of operating plants are reported analysing the role of the storage and the sizing of the solar section and absorption chiller together with recorded results. After considering some schemes described in the literature, a hot and cold storage tank system designed by the author is presented. First of all layout and control logic are detailed, then experimental data recorded on the plant are illustrated concerning the seasonal performance both of the solar section and of the absorption chiller.
solar cooling; absorption; hot storage; cold storage
Nomenclature
Introduction
A previous paper highlighted the importance of a correct arrangement of the various
parts of a solar cooling plant to determine its seasonal performance [
1
]. A high
performance absorption chiller and very efficient solar collectors are not enough to give
an efficient solar cooling system. How the chiller is controlled or how the solar
energy is stored can definitely influence system performance. The role of the storage
is of paramount importance in this application. Here the storage not only has the
obvious function of accumulating excess solar energy to use later, but it also aids
the functioning of the absorption chiller as its capacity is not easy to be modulated.
The risk is that the seasonal COP is very low, even lower than 0.2 due to the
ON-OFF operation of the absorption chiller [
2
]. This is the main reason why a cold
storage should be provided and its operation must be properly planned. Three
different examples of solar cooling absorption plants are reported here, equipped with
hot storage only or both cold and hot storage tanks in order to better illustrate the
principles described in the paper [
1
]. The last example refers to my personal
experience and reports not only the details of the system but also seasonal performance
data of the absorption chiller and solar collectors.
Two different solar cooling system schemes
An example of a solar cooling plant equipped with only a hot storage is at the
Arizona State University (Phoenix, Arizona) [
3
]. The plant is comprised of solar
collectors, hot pressurised storage, absorption chiller, sanitary water circuit and
refrigerating circuit. One can notice in Fig. 1 pumps, heat exchangers and auxiliary
boilers.
Solar collectors cannot directly drive the absorption chiller. With regard to the
control of solar collectors: collectors and storage pumps are turned on when a proper
temperature difference is reached between the solar collectors and storage. Of course
the connections can take advantage of stratification. A room thermostat turns on the
refrigerating and hot water pumps.
An important component is the regulator DT-4 that determines whether the
absorption chiller can be driven by the storage. When the water temperature at the top of
the tank is over a minimum value (say 80°C) and is higher than the hot water at the
outlet of the generator, valve V-2 activates the circuit toward the storage. Control
capacity is obtained through the three-way valve V-1 from which a fraction of the
heating fluid is bypassed toward the storage. The valve V-1 is commanded by the
refrigerating water temperature. If this increases too much, the auxiliary boiler is
switched on; the hot water is soon over the storage temperature and the valve V-2
activates the boiler circuit. Control capacity is always obtained by means of valve
V-1.
It is worth noticing that control capacity is realised in this plant only by flow rate
variation whereas the hot water temperature is at the level achieved by the storage.
In principle the auxiliary boiler could operate as a booster, raising the hot water
temperature from the storage, modulating through valve V-2. The control system
does not allow this operating method, probably owing to the modest attainable
advantages compared to higher complexity.
Another interesting scheme is derived from a plant that serves the Mount Cotton
Solar House in Australia [
4
], which gave useful suggestions for the design of the
plant I myself designed. It is equipped with both hot and cold storage. As usual the
connections between the tanks take advantage of stratification.
Some reference values must be set. The highest acceptable temperature to the fan
coils is 14°C whereas the lowest temperature produced by the chiller is 8°C. The
minimum temperature to drive the chiller is 76°C.
The solar collector’s circuit is controlled independently of the rest of the system
by a differential controller. The system can produce heating or cooling according to
a main switch. Heating operation is not considered here while summer cooling is
detailed. Consider the scheme of the plant represented in Fig. 2 where the activated
cooling mode connectio (...truncated)