04-10-2012, 12:11 PM
Experiment Analysis of Flat Plate Collector and Comparison of Performance with Tracking Collector
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Abstract
Application of solar energy for domestic and industrial heating purposes has been
become very popular. However the effectiveness of presently used fixed flat plate
collectors is low due to the moving nature of the energy source. In the present work, an
attempt has been made to compare the performance of fixed flat plate water heater with that
of heater with tracking by conducting experiments.
A flat plate water heater, which is commercially available with a capacity of 100
liters/day is instrumented and developed into a test-rig to conduct the experimental work.
Experiments were conducted for a week during which the atmospheric conditions were
almost uniform and data was collected both for fixed and tracked conditions of the flat
plate collector. The results show that there is an average increase of 4oC in the outlet
temperature. The efficiency of both the conditions was calculated and the comparison
shows that there is an increase of about 21% in the percentage of efficiency.
Introduction
In the solar-energy industry great emphasis has been placed on the development of "passive" solar
energy systems, which involve the integration of several subsystems: Flat Plate collectors, heat-storage
containers, fluid transport and distribution systems, and control systems. The major component unique
to passive systems is the Flat plate collector. This device absorbs the incoming solar radiation,
converting it into heat at the absorbing surface, and transfers this heat to a fluid (water) flowing
through the Flat plate collector. The warmed fluid carries the heat either directly to the hot water or to a
storage subsystem from which can be drawn for use at night and on cloudy days.
Flat Plate Collectors
Of the many solar collector concepts presently being developed, the relatively simple flat plate solar
collector has found the widest application so far. Its characteristics are known, and compared with
other collector types, it is the easiest and least expensive to fabricate, install, and maintain. Moreover,
it is capable of using both the diffuse and the direct beam solar radiation. For residential and
commercial use, flat plate collectors can produce heat at sufficiently high temperatures to heat
swimming pools, domestic hot water, and buildings; they also can operate a cooling unit, particularly if
the incident sunlight is increased by the use of a reflector. Flat plate collectors easily attain
temperatures of 40 to 70ºC. With very careful engineering using special surfaces, reflectors to increase
the incident radiation, and heat-resistant materials, higher operating temperatures are feasible.
Absorber Plates
The primary function of the absorber plate is to absorb as much as possible of the radiation reaching
through the glazing, to loose as little heat as possible upward to the atmosphere and downward through
the back of the container, and to transfer the retained heat to the circulating fluid. In general, absorption
of solar energy impinging on an absorber plate should be as high as possible, but re-emission (loss)
outward from the collector should be minimized.
In hydronic collectors the absorber is usually made of copper, aluminum or steel. Factors that
determine the choice of absorber material are its thermal conductivity, its durability and ease of
handling, its availability and cost, and the energy required to produce it.
Absorber plates are usually given a surface coating (which may be a black paint) that increases
the fraction of available solar radiation absorbed by the plate (its absorptance α). Black paints, for
which α = 0.92 to 0.98, are usually applied by spraying and are then heat-treated to evaporate solvents
and improve adherence. These surfaces must be able to withstand repeated and prolonged exposure to
high temperatures without appreciable deterioration or out gassing.
It is well known that a black body is a perfect absorber of radiant energy and is a perfect
radiator; that is, it has an absorptance α and an emittance ε, each equal to unity (emittance is the ratio
of the amount of radiation emitted by the surface to the amount of "blackbody" or perfect radiator
would emit at the same temperature). Actual surfaces do not behave like perfect absorbers or perfect
radiators and have absorptance and emittance less than unity. Parenthetically, it may be pointed out
that a black body is not necessarily non-luminous but may be as bright as the sun (which is not quite a
black body). The term merely indicates a surface that is a perfect radiator and a perfect absorber.
According to Kirchoff's law, at thermal equilibrium the absorptance and emittance of a body are the
same.
Selective Absorber
A surface that has a high absorptance and is a good absorber of solar radiation usually has a high
infrared emittance as well and is a good radiator of heat. A flat-black paint that absorbs 96% of the
incoming solar energy will also reradiate much of the energy as heat, the exact amount depending on
the temperature of the absorber plate and the glazing. Ideally, one would like a surface to be selective,
absorbing all the solar wavelengths and emitting none of the heat wavelengths, so that more heat could
be transferred to the working fluid; for such a surface, α = 1 and ε = 0. Selective absorbers can be
manufactured that approach this ideal, and several are available commercially (Table 1.3).
Selective absorbers often consist of a very thin black metallic oxide on a bright metal base. The
oxide coating is thick enough to act as a good absorber, with α = 0.95, but it is essentially transparent
to longer wavelength heat radiation, neither absorbing nor emitting much of the 3 to 30 micron
radiation. On the other hand, the bright metal base of the absorber surface has a low infrared emittance
and radiates very little heat. The combination, in effect, gives a surface that is a good absorber but a
poor radiator. As a result, the efficiency of the collector is greater when this type of surface is used.
Conclusion
From the above results, it has been found that the system provided with manually tracking has got
higher efficiency than the fixed flat plate collector by 21%. Hence Flat plate collector with tracking
method utilizes maximum beam radiation and gives high efficiency when compared to fixed flat plate
collector.