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UNDERSTANDING COLLECTOR EFFICIENCY

By Solarbair, LLC © February 12, 2010

 

The basics of solar thermal collector efficiency:
Collector efficiency is based on: Solar intensity or insolation, (P) or Ti-Ta also known as delta T and specific coefficients determined by testing each collector.


Solar intensity depends on weather, location, time of day, time of year and collector orientation.  For well oriented collectors on a sunny day 316 Btu/sq ft hr is a reasonable starting point.  To calculate the non-weather dependant insolation for your specifics see the RadOnCollector program at:
http://www.builditsolar.com/Tools/RadOnCol/radoncol.htm


(P) or Ti-Ta or delta T is the difference between temperature of the water going into the collectors (inlet temperature) and the temperature of the surrounding air (ambient temperature). 


To calculate the collector specific coefficients the SRCC tests the collector, compiles a set of data points and then approximates them with either a straight line ( 1st order approximation) or more accurately a curve (2nd order approximation).  The ratings for collectors are called the SRCC OG-100 and can be found at:  http://www.solar-rating.org.

Using the SRCC OG-100 collecotor ratings:
What I would like to explain is how to use the information on the rating.  Take the Sun Earth Empire EC-40 collector rating page which I have reproduced below.  Under COLLECTOR PERFORMANCE RATING we are given an idea what daily output to expect from the collector in question.  At the top of the chart the insolation value is indicated by the Clear, Mildly Cloudy and Cloudy day catagories which correspond to 2000, 1500 and 1000 Btu/sq ft *day respectively.  Ti-Ta values are indicated on the left side of each chart.  When comparing collectors using this chart keep in mind the size of the collector is already factored in to these numbers.  The larger outputs for larger collectors does not indicate more efficiency just more surface area.


COLLECTOR SPECIFICATIONS and COLLECTOR MATERIALS are self explanatory.


In the ratings sheet below under TECHNICAL SPECIFICATIONS in the IP UNITS row outlined in red lies the information needed to calculate and graph collector efficiency with the Collector Efficiency Calculator.

OG-100 Rating Page for Sun Earth EC-40 Collector

The calculator will produce a graph of collector efficiency like the following:

first and second order flat plate efficiency graph

To make a graph for your specific collectors or to compare collectors just input the information corresponding to your collectors and situation into the COLLECTOR EFFICIENCY CALCULATOR, an excel program available on this website.


Some things are clear from the physics.  More insolation or solar intensity will improve collector efficiency.  Greater Ti-Ta will decrease collector efficiency.  The above graph shows how the first order or linear approximation differs from the 2nd order or curved line approximation, with the linear graph showing higher efficiencies than will be achieved in actuality. 


The reason I am writing this is because I fell into this pitfall myself.  When designing the solar space heating system for my home, I used the linear approximation to calculate collector efficiency.  This led me to believe my system would produce 200˚F water on a 0˚F day at an efficiency of 27% instead of 14%.  This misinformation resulted in inaccurate payback calculations and unnecessary overheating concerns. The graph shows that even on a 100˚F day, under stagnation conditions the collectors will not get much above 325˚F with an insolation value of 300  Btu/sq ft hr.  Hot yes, but nothing near the 410˚F indicated by the linear approximation.


Consider the following graph showing the first and second order approximations for vacuum tube collectors compared to flat plate collectors.

flat plate vs tube collector efficiency graph

You can see that the second order approximation differs from the first order for the vacuum tube collectors as well.  Vacuum tube collectors are often cited as being better at collecting heat during low insolation levels.  The following graph indicates performance at 150 Btu/sq ft hr.  You can see that if the ambient temperature is 0˚F the flat plate will heat water over 125˚F, while the tube could heat water up to 250˚F at a similar efficiency.  The argument for tube collectors is clear.  The proponents of flat plates will point out that at these low insolation levels there is not much energy to be collected anyway.  10% of not much is very little.  I’m sure the debate continues.  

Graph of flat plate vs tube collector performance at 150 Btu insolation.jpg

If you want to do some collector comparisons of your own go to my COLLECTOR EFFICIENCY CALCULATOR. Be advised the calculator uses the Excel program.