Solar water heater efficiency?

4 Posts
Oct 8, 2012 09:27 pm
Solar water heater efficiency?

Okay, PV panels are really inefficient -- around 15%. With 1 KW of solar energy falling on every square meter of the earth's surface at noon, I can expect up to 150 watts of electrical power from a typical PV panel.

What can I expect from a typical water heater panel?

Each hour, that 1 KW of solar energy is equivalent to 3,412 BTU, so a 30 sq.ft. (2.79 sq. meters) water panel will receive 2.79 KW or 9,519 BTU per hour.

How much of this heat is transferred to the water?

Asked another way, if water at 60F enters a 30 sq.ft. panel in full sun, what will be its temperature when it exits?

Thanks for any definitive information.,

--- Mike
462 Posts
Oct 20, 2012 12:35 pm
Re: Solar water heater efficiency?

Well Mike, it's hard to make a calculation with the limited information you gave. It take 1 BTU to raise a lb or water by one degree so first you have to determine the amount of water in your panel. Second you have to have the material properties of what your panel is made from. Third, you have to state how much absorber area is dedicated to each riser in the panel or the ratio of surface area to the tube area. Fourth,  the amount of insulation value from the glazing and the box.  Also include surrounding ambient temperature, fluid flow rate etc...
May 5, 2014 09:38 pm
Re: Solar water heater efficiency?

I really appreciate it, thanks for the information.
1 Posts
Jun 16, 2015 07:05 am
Re: Solar water heater efficiency?

Your guidance was very helpful to all for water heater efficiency, but if I want immediate result, so, is there any query to check the solar water heater efficiency level.?� Is there any other short procedure..?
« Last Edit: Jun 20, 2015 06:33 am by Robert Morris »
462 Posts
Jun 27, 2015 11:51 am
Re: Solar water heater efficiency?

Robert, you could fill the collector with a known temperature water. Set a time, then drain the water and take the temperature.  Once you have the delta T, the time, then you can figure out how many BTU's that have been gained  q= kA(T2-T1),  where q will be the heat gain (BTU/hr), k is the conductivity of the material (BTU/hr*ft^2*F), A is the area of the collector (ft^2), T2 and T1(F). Then multiply by the time in the collector to determine BTU's gained.

or hook up a small pump to a known temperature water supply in a bucket, circulate it through the panel for a given time and then take final temperature reading, then calculate q again

Also realize that the amount of solar radiation throughout the year changes so your gain will also change over the course of the year. With solar hot water, it's make what you can when you can and use it wisely.
« Last Edit: Jun 27, 2015 11:58 am by Tom Mayrand »

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