Collector Efficiency
The purpose of this page is to provide a basic understanding of the many ways efficiency data is presented. Armed with this knowledge you will be able to determine for yourself the quality of a collector based on efficiency data, and determine which collectors are suitable for your application.
While our collectors are the most robust (over 25 years on the market) and efficient, they are also one of the most misrepresented technologies presented in North American performance literature.
The Correct Collector Comparison Graph
The following graph is based on independent test results obtained from the European Solar Collector testing facility SPF (see bottom of page for links to source data). SPF correctly evaluates collectors based on collector absorber area as well as aperture and gross areas. It is the absorber area that collects solar energy, and so evaluation of the efficiency based on the % of solar thermal radiation hitting the absorber and transferred to the fluid loops is more correct.
How to Read the Efficiency Graph
The graph represents collector performance when the ambient temperature (the outside air temperature) is 0°C. To determine efficiency (the percentage of solar radiation hitting the collector that is directly transferred to heat water) chose the desired water temperature.
The slopes of the lines in the graph represent their heat loss factors. The steeper the slope, the higher the collector loses heat. A line with a small slope represents a collector that loses very little heat to its surroundings, and is very efficient at heating.
For example, if you wanted to use the collector to heat water for your home you would look at the ‘60’ Temp value along the horizontal axis and follow the line up to see the efficiency of the collectors. This means that if it is a moderately clear day, and the outside temperature is zero degrees, the Thermomax collector would be able to convert about 70% of the available solar thermal radiation to heat your water tank to 60 degrees.
Gross Area Mistake
The most widely quoted efficiency performance results found in North America are the results from the SRCC (Solar Rating and Certification Corporation). The SRCC evaluates Evacuated Tube Collectors (ETC) as if they were flat plate collectors, which they are not. The resulting efficiency graphs released by SRCC undervalue actual performance of evacuated tubes by over 30%.
GROSS AREA = the gross area refers to the external mass of the collector; the area actually necessary for installation, that is simply the length times the width of the collector.
Instead of including the spaces between tubes, the manifold, the lateral glass wall thicknesses, in the gross area calculation, it would be much more accurate to base performance of collectors on APERTURE AREA = the area through which solar energy enters the collector.
Corrected SRCC Graph
Fortunately there is a simple solution for converting efficiency parameters from expression in terms of collector gross area (total footprint of collector) to expression in terms of aperture area (area of collector that gathers sunlight):
This formula was obtained from the NRC and ASHRAE standards, and is commonly used by the European Solar Testing group SPF. The SRCC uses it to convert from aperture or absorber area to gross area. We simply un-did the conversion. The result is a much more accurate graph:
How to Read the Graph
The slopes of the lines in the graph represent their heat loss factors. The steeper the slope, the more the collector loses heat as temperature increases. A line with a small slope represents a collector that loses very little heat to its surroundings, and is very efficient at heating.
The highest efficiency of each collector is when Ti-Ta=0, when the fluid entering the collector is the same temperature as the ambient environment (outside). This is also known as the optical efficiency.