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Goethermal vs. solar water heater efficiency

Posted by jj_qqqqq (My Page) on
Fri, Nov 20, 09 at 7:46

Hi - I live in Maryland and had a geothermal heat pump system installed in an. 2009 (without a desuperheater). I'm considering a geothermal water "add-on" or a high-efficiency Schuco solar water heater. The stumbling block in the decision process is comparing the efficiencies between geothermal & solar since the geo community uses "Coefficient of Performance (COP)" while the solar guys use"solar energy factor (SEF)". Near as I can tell, they both are ratios between the energy input and the energy output. I suspect that the COP & SEF numbers are not directly comparable -- do you happen to know? The geothermal unit boasts a COP of 6.0 while the solar unit has an SEF of 2.8. If COP & SEF are comparable, then the geothermal unit is twice as efficient - but, if true, is puzzling since the solar unit energy inputs seem minimal since the only electricity needed is for driving a small pump to circulate the water while the geothermal unit has pumps and a compressor. The only thing that I can think of for the relative 'inefficiency' of the solar unit is that it can only heat the water to a point during winter & then relies on the inefficient conventional resistance heater to heat it up to the desired temperature.

Follow-Up Postings:

RE: Goethermal vs. solar water heater efficiency

I wouldnt get too wrapped up in the numbers relating to efficiency. The fact is you already have a geothermal heat pump. I would opt for the desuperheater over the solar system, as the compressor does not really figure into the equation in that the desuperheater only works while the compressor is running anyway. The only real additional power consumed is the circulating pump; rather nominal in this case.

Since you have a GSHP, why would you opt for yet another mechanical system with all the associated outdoor plumbing? What I do recommend is the installation of a 40-gallon buffer tank between the desuperheater and the DHW tank for maximum efficiency.

In the A/C, when your HP is rejecting heat, it will be rejected into your DHW tank, essentially making your hot water free; hard to beat free.

Besides, only the desuperheater can generate hot water at night!


RE: Goethermal vs. solar water heater efficiency

Hi Gary,

Im not so sure that the pump energy is not factored into the COP in some fashion, where closed loops may be concerned. I would suggest doing a Google search of ARI/ISO/ASHRAE 13256-1.

I could not find the appropriate link to include, so here are the pertinent excerpts:

About ARI/ISO/ASHRAE 13256-1

The performance standard ARI/ASHRAE/ISO 13256-1 became effective January 1, 2000 and replaces ARI Standards 320, 325, and 330. This new standard has three major categories: Water Loop (comparable to ARI 320), Ground Water (ARI 325), and Ground Loop (ARI 330). Although these standards are similar there are some differences:

Pump Power Correction
Within each model, only one water flow rate is specified for all three groups and pumping Watts are calculated using the following formula. This additional power is added onto the existing power consumption. Pump power correction = (gpm x 0.0631) x (Press Drop x 2990) / 300
Where gpm is waterflow in gpm and Press Drop is the pressure drop through the unit heat exchanger at rated water flow in feet of head.

ISO Capacity and Efficiency Equations
The following equations illustrate cooling calculations: ISO Cooling Capacity = Cooling Capacity (Btuh) + (Fan Power Correction (Watts) x 3.412) ISO EER Efficiency (W/W) = ISO Cooling Capacity (Btuh) x 3.412 /
[Power Input (Watts) - Fan Power Correction (Watts) + Pump Power Correction (Watt)]
The following equations illustrate heating calculations: ISO Heating Capacity = Heating Capacity (Btuh) - (Fan Power Correction (Watts) x 3.412) ISO COP Efficiency (W/W) = ISO Heating Capacity (Btuh) x 3.412 /
[Power Input (Watts) - Fan Power Correction (Watts) + Pump Power Correction (Watt)]

I would second the rest of your commentary.


RE: Goethermal vs. solar water heater efficiency

I guess one more thing to consider is how much of your water heating a desuperheater would actually provide. I re-read the paper in the attached link and the homes they did the field measuremnts on that had desuperheaters did not provide a very high fraction of water heating at all.
Maybe these were isolated and bad cases, but this is certainly something I would try to get some hard numbers on. I'd hate to pay for a probably not so cheap desuperheater, and have it only provide 10% of my hot water.


Here is a link that might be useful: Field measure performance of GSHPs

RE: Goethermal vs. solar water heater efficiency

I am also curious about the pricing of the geo water heater. Our installer says they cost nearly $2,000 for a two-tank system installed. Is this is price others are getting?

RE: Goethermal vs. solar water heater efficiency

By the way,You can make a solar water heater with a COP of near infinity by just using enough collector area to provide year round water heating and PV power pump.

Here is a link that might be useful: Appliance Repair in Los Angeles

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