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Old 08-29-16, 07:51 AM   #4
Steve Hull
Join Date: Dec 2012
Location: hilly, tree covered Arcadia, OK USA
Posts: 829
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Thanks for your comments/ideas.

I really like your idea of coupling the output of the solar water heaters to the geothermal unit. Will be thinking of that for my new home designs.

What is your inlet water temperature (deep ground water temp)? Here is is 60 F and I use an open loop system. The COP of my GSHP is ~ 3.5 to 4 (with water pumping included). But if I increase the incoming GSHP water, by just a few degrees (via solar panels), then the COP goes way up. VERY interesting.

There are several reasons why I feel that winter conditions (even cloudy) can result in significant water heating via solar with flat plate collectors.

First, - it works! In January and February, our Oklahoma daytime temps are rarely above 35 to 40 F, but are typically cloudy. Water is clearly warm in the preheat tanks. On some days, even up to about 100 F.

I believe this is because of the drain back feature and differential controllers. The circulation pump will only turn on if the roof top solar collector plate temperature is at least 10 degrees F higher than the storage tank. My observation were (and are) that the system works in pulses for brief periods of time in the middle of the day. Each time, water in the storage tank gets a bit warmer.

My second design had a variable pump speed and I used this very effectively in the winter to keep water flow down and to heat water up a bit more. The slower water flow through the flat plate collectors did not "wash out" the sun induced temperature (little pulsing) - - just low flow, but warm water coming out. It also decreased pumping electricity costs a lot. In summer, the variable speed water pump prevents water boiling in the collectors.

I believe Seattle also has warmer winter temps (similar to mine in Oklahoma) and warmer inlet ground water temps than you have. Rarely does Seattle go below 32 F for extended winter periods. Having been stationed there, I know it was not terribly cold. Note sure where in Finland you are and indeed there are also vast differences in climate (and deep water temperature) north to south.

Lastly, flat plate collectors are very efficient at increasing inlet water temp by 10-30 degrees at low water temperatures. Not so at high temps at all where significant IR effects limit performance. Your evacuated glass tubes are superb at high temp efficiencies. So I was also giving specifics for flat plate type (as the OP has) considering his likely inlet water temps of ~ 50 F and the relatively mild Seattle winter temp (even if cloudy).

The Seattle summer is absolutely gorgeous with large number of superb solar days (no clouds, temps in the 80s F and little wind. I think we would agree that the likelihood of large water boosted temps are guaranteed.

Again, good constructive comments!

consulting on geothermal heating/cooling & rational energy use since 1990
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