What does all this mean?
- Gross area:
- Gross area collector efficiency represents the percentage of sunlight
converted to heat, based upon the total area of the collector. This represents the true power density of the collector.
- Aperture area:
- Aperture area collector efficiency represents the percentage of sunlight converted to heat, based upon the aperture area.
- Absorber area:
- Many thermal collector distributors use absorber
area to describe efficiency. The absorber area specified for CPCs
needs a more detailed explanation. Due to the fact that the CPC surface
behind each vacuum tube directs solar radiation onto the vacuum tube
absorber surface the collector efficiency could become greater than
100% if the physical absorber plate area within each vacuum tube is
taken as absorber area reference. However, it is general practice
in the solar community to use the area of the entire concentrating
(CPC) surface as reference. This makes the maximum possible efficiency
of the CPC 100% if all components are ideal ( i.e. all radiation is
concentrated onto the absorber with no losses). Therefore, if we were
to compare absorber area of the CPC we would actually be comparing CPC
aperture area with smaller absorber areas of other non CPC types.
- G, 800W/m2:
- Represents the solar irradiation incident upon the collector.
- Many factors affect solar irradiation such as
season, clouds, time of day, orientation of the collector, and
potential obstructions of the collector (building, tree, hill).
- tm:
- tm represents the mean temperature of the fluid that is traveling through the collector.
- tm=(tinlet+toutlet)/2
- ta:
- ta represents the ambient temperature that the collector is placed in (i.e. how hot or cold it is outside).
- (tm-ta)/G:
- Collector efficiency drops as solar irradiation
decreases, and the temperature difference of the collector and the
ambient increases.
- Example values to consider:
- Domestic hot water:
- tm=48C
- ta=0C
- G=450W/m2
- (tm-ta)/G=.11
- Radiant floor heating:
- tm=30C
- ta=0C
- G=450W/m2
- (tm-ta)/G=.07
- Air handler heating:
- tm=65C
- ta=0C
- G=450W/m2
- (tm-ta)/G=.14
- Absorption chiller:
- tm=87.5C
- ta=30C
- G=700W/m2
- (tm-ta)/G=.08
What should I look for in a solar collector?
- The higher the gross collector efficiency, the less
space you will need to install thermal collectors and still satisfy
your thermal loads.
- Low cost/area/efficiency:
- We have developed a spreadsheet to assist you in comparing the cost of solar thermal collectors.
- Collectors compared above:
- Mazdon MS-20 - TMO 500, Thermomax, http://www.thermomax.com/
- Solamax-20 - TDS 300, Thermomax, http://www.thermomax.com/
- Apricus AP-20, Apricus, http://www.apricus.com/
- VitoSol 100 S2.5, Viessmann, http://www.viessmann-us.com/
- Vitosol 300, Viessmann, http://www.viessmann-us.com/
- Seido 1-16, Sun Spot Solar/Beijing Sunda Solar Energy Technology, http://www.sssolar.com/
- Seido 10-20, Sun Spot Solar/Beijing Sunda Solar Energy Technology, http://www.sssolar.com/
- Performance data for solar collectors obtained from SPF Solartechnik and Universität
Stuttgart Institut für Thermodynamik
und Wärmetechnik
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