Collector Efficiency

The efficiency of a solar collector is constantly changing throughout the day, factors such as ambient temperature, fluid temperature, insolation levels play a role in dictating how well a collector converts the sun’s energy into usable heat. Most certifications publish information that will allow you to determine a collectors performance.

Solar water heater performance is often presented as a graph, or set of three performance variables. Values may be provided based on gross area, aperture area or absorber area. In Europe, aperture or absorber is often used, in the US, gross area is often used. It doesn't really matter which value is used, as long as you use the correct value. ie. Don't use absorber area when using performance values based on gross area.

To adjust from one to the other, multiply by the size difference.  ie. If aperature area = 32.05 ft² & gross area = 44.76 ft² then (44.76/32.05) = 1.39), so multiply the performance factorsby 1.83 to convert from gross to absorber.

The three performance variables for the Apricus solar collector as provided by the Solar Rating & Certification Corporation (SRCC Certificate #2007033A) are as follows (for imperial calculations):

As well as the three performance variables show above, insolation level (G) in Btu/ft², ambient temperatures (Ta) and average manifold temperature (Tm) must be known.  These values give the value X, also known as the fluid parameter, used in the formula below. 

(other slightly different forms of this formula are used, but provide the same result) 

How to use the formula:

Based on the ambient temperature, average manifold temperature and insolation level, first calculate the value for X.

Eg. At midday; ambient temperature of 77oF (25oC); average water temp [(Tinlet+Texit)/2] of 122°F (50oC); insolation level of 252Btu/ft² (800Watts/m²).

x = (122-77)/252 = 0.1785

Now enter all the values into the formula:

h(x) = 0.634 - (0.33076*0.1785) - (0.00051*252*.1785²)



h(x) = 0.634 - 0.0590 - 0.0040 = 0.571

The solar conversion efficiency for that specific point in time and set of environmental conditions is 57.1% based on aperture area. That is: 57.1% of the energy provided by the sun is actually used to heat the water.



Below is a first order graph showing the performance curves for the Apricus solar collector based on the inlet fluid parameter.

Most domestic water heating applications occur in the range of 0.2-0.4 with higher temperature applications happening further to the right on the graph.