Renewable
energy is an important alternative to conventional burning of fossil fuels and
natural gases. Solar energy is one example of a renewable energy source. There
are two main methods for converting solar energy into more useful forms of
energy like electricity or heat: 1) photo-voltaic cells, and 2) solar thermal
systems. Photo-voltaic cells are widely used and have been heavily
investigated. Solar thermal systems are less common and have not been
investigated with the same detail as photo-voltaic systems.
Within the field of solar thermal systems, there are two primary
constructions: 1) flat plate collectors (FPC), and 2) evacuated tube collectors
(ETC), and each has its own strength. Flat plate collectors can collect both
direct and diffuse radiation, and also operate better at low temperatures. In
an effort to optimize the efficiency of both systems in a rainy climate, this
project combined the two designs into one small hybrid prototype collector
utilizing both ETC and FPC systems. While most solar collectors focus only on
one solar collection method, the small hybrid system uses a flat plate
collector in conjunction with five evacuated tubes to absorb the most energy
possible from both direct and diffuse solar radiation.
Data was collected over four months while the system operated at
different flow rates and with various levels of available insolation from the
sun to evaluate the performance of the solar collector. To understand the
relative contribution of the flat plate collector and the evacuated tubes,
temperature differences across each part of the system were measured.
The results indicate the average first law efficiency of the hybrid
system is 43.3%, significantly higher than the performance of the flat plate
alone. An exergy analysis was performed for this system to assess the
performance of the flat plate system by itself. Results of the second law
analysis were comparable to the exergetic efficiencies of other experimental
collectors, around 4%.
In conclusion, though the low efficiencies found in this analysis
are not unusual among other flat plate collectors with water as the working
fluid, they show that improvements should be made. Theoretically, flat plate
second law efficiencies can reach 60%. This leaves a great deal of room for
improvement for this system’s FPC. Recommended improvements focus on
transferring the energy from the sun to the working fluid.
Article by Amy
Lebar and Heather E. Dillon, from University of Portland, Portland, USA.
Full access: http://t.cn/EbXxBwI
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