Performance and Optimization of a Small Hybrid Solar-Thermal Collector

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.

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