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Old 08-28-16, 12:14 PM   #1
Geo NR Gee
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Default Help needed incorporating Solar collectors

For years I have been wanting to use the suns energy to heat and power my home. I have read countless stories of people here and elsewhere adding energy wise equipment to improve their home and workshop comfort in the winter heating and summer cooling times with great success and money savings as well.

Recently that desire was made front and center when I got a call from our very own, AC_Hacker with an offer for some free used flat plate solar collectors. After a long drive and some great conversations with AC and his son, we loaded them up for my journey home. Thank you AC, you are very generous and I am very grateful for our friendship!

There is a total of 8 collectors. 6 which are very similar. The other two are smaller and have curved plexi glass like material over the flat plate collectors. I cannot find company name on them, just a serial number and date code which are not legible. I haven't measured them yet.

They have low iron content in the glass and consist of flat plate copper collectors. I tested one by hooking it up to a garden hose and was amazed to see the water come out steaming hot. Too hot to touch with your fingers!

The question is how to incorporate this gift into my house and the heating and cooling system?

I have been planning on drilling 6 - 200ft. 4" bore holes into my back yard for the geothermal heating and cooling. I have a used geothermal unit, 6 - 210 ft rolls of down and up geo pipe (including the u-bends), fusion tools, and fittings along with the drill rig almost ready to go.

Installed in the house, I have a 3 ton heat pump with a backup high efficiency gas furnace in line. We use the gas furnace part a couple of weeks in the coldest time of winter.

Without knowing the specs on these panels, it would seem that 6 panels would put out a lot of hot water.

My gas-electric bill lately is about $110-$120 a month with an small increase in the winter time. I have a 2012 all electric Nissan Leaf that I use as a daily driver too. I recently installed a slightly used heat pump water heater for the hot water and noticed a significant drop on the monthly bill.

The house is a two story 1720 sq. ft. made of mostly 2x4 construction and built in 1998. It was labeled as a "Good Sense Home", however, I have reservations about that. The attic was recently super insulated and a huge improvement was noticed. The windows are vinyl and are not of high quality, but are double pane.

A blower door test was conducted and air intrusions were eliminated for the most part. The home would benefit from additional exterior wall insulation which is not in the budget at the moment.

The underfloor crawl space insulation is subpar and an additional layer of insulation would increase the floor temperature comfort as the winter time the first floor is cold on the feet.

Incorporating the solar thermal heating into the existing house heating and cooling system is my current dilemma. Do I drill the boreholes, dump the heat generated from the solar collectors in the ground and use the heat for the winter?

Do I use the collectors for heated floors on the first floor where I spend a majority of the day? The bedrooms are on the upper level and everyone likes the rooms cold when they sleep anyway.

There is room for 3 or 4 collectors on the side of my house or all of them on my roof. However, I am not educated at this moment on the optimal angle.

Talking to Randen about this, he suggested to read about the Drakes Landing solar community Drake Landing Solar Community. Thank you Randen for that. The community is using solar thermal energy to heat their home and cover about 95% of their heat load!

Maybe I could use this set up in connection with my solar collectors? One for the solar flat plate panels and the other for the geothermal heat pump?
Geothermal Sure Clip LLC - Double Ubends

Not sure where to go from here?

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Old 08-28-16, 06:00 PM   #2
Steve Hull
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Great that you have "new" solar hot water panels. Now what to do.

I have had hot water solar panels on a few of my homes since the late 1980's. All used a drain back system (simple, inexpensive and little danger of leaks).

I rebuilt mine and then added extra high temp foam on the back. Pressure tested each BEFORE rebuilding and putting them on roof.

Orientation is not critical. A due south exposure is "best", but even a SE or SW exposure will work. Most homes have a 3/12 roof pitch (14 degrees) and that is OK. In your northern area, a higher pitch of 25-30 degrees is ideal, but then this complicates the install if your roof pitch is less. If you have a southerly roof with a decent pitch, then just put them there.

I am in sunny and warm central Oklahoma. You are in cool, wet and damp Seattle. Quite a difference in performance.

I feel the best "bang" for your efforts is to create a preheat solar storage water tank. In my case, I preheat a 100 gallon tank with roof top solar water. The output of this goes through a tempering valve to a tankless water heater.

The tempering valve is set to 125 F so that I never get scalding water through the system. The tankless is set to about 120 F. By the time it gets to faucets/showers, the max water temp is about 115 F or so.

My cold water inlet temp is about 60 F. For domestic hot water, I need a rise of about another 60 degrees to get to 120 F. For all spring/summer and much of the fall I get almost all "free" solar water as the tankless unit never fires up (three 4 x 8 foot panels).

In the winter, the storage tank gets typically gets to about 80-90 F or so. Call this a rise from ground temp of 30 degrees. My tankless unit now has to supply the other 30 degrees to get it up to 125 or so. But I have cut my winter hot water bill by about 50%.

Your inlet water temp is a LOT colder - probably in the upper 40s or so. You need a rise of about 80 F to get to a good hot water temp - expensive! With panels, even on a cloudy day, you will boost this cold water inlet temp to about 70-80 F. Solar based water heating is most efficient with this large difference in inlet and output temperatures. In other words, it is easy to heat up cold water - harder to get it to a higher temperature. This makes your solar hot water system more efficient than mine even though you are further north.

Some ideas. Look for a tankless water heater that heats to a specific output water temp - NOT one that just turns off and on.

In my area, the cost of propane per BTU and the cost of electric resistance heating ($/BTU) are almost the same. I bet your electricity costs are low due to hydroelectric.

The cost of an electric tankless water heater unit is very low ($250-$400). The cost of a gas (natural gas or propane) tankless is much higher ($700-1000). Consider the ease of installation of an electric booster tankless heater.

Make sure to use energy efficient circulation pumps (you will need two) for your drain back system. One is a tiny 1/20 HP as it just recirculates water through the heat exchanger into the storage tank only overcoming water pipe frictional resistance. The other one needs to pump water up some height to your roof. Depending on the height, you may need a 1/5 HP water pump.

Lots of information is on the web on constructing drain back systems. No glycol in water loop and simple to make.

Have fun!

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

Last edited by stevehull; 08-28-16 at 07:09 PM..
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Old 08-28-16, 11:27 PM   #3
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Some good advice from stevehull, but there is one thing I would question. He says:

With panels, even on a cloudy day, you will boost this cold water inlet temp to about 70-80 F.
Maybe the cloud cover here is different from Steve's, but I don't find that. I have 100 evacuated tubes and for three months at the end of the year, when there is typically heavy cloud cover here, they produce basically nothing. I don't know what the cloud cover is like in Seattle but don't count on a solar contribution year-round.

Another comment I would make is you might consider linking your solar and geothermal efforts. I have a GSHP and when that was installed the installer talked me out of linking it to my solar panels. I now think that was a mistake. The relatively cool water coming from the ground loop would be a good way of harvesting heat from the solar panels on days when they are too cool to contribute to heating or hot water. This may not be a huge gain but since you are installing both I suggest you allow for this in your design.
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Old 08-29-16, 06:51 AM   #4
Steve Hull
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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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Old 08-31-16, 09:31 AM   #5
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Originally Posted by SDMCF View Post
...I have 100 evacuated tubes and for three months at the end of the year, when there is typically heavy cloud cover here, they produce basically nothing. I don't know what the cloud cover is like in Seattle but don't count on a solar contribution year-round...
Good advice. I have noticed here in Portland, OR, which has very similar weather pattern as Seattle, that even in overcast flat panels can hit 80F in winter over cast. 80F is not a lot, but it is a contribution to water preheat. I must add, that while I did see a panel deliver 80F, I did not have any information about the rate of energy falling on the panel.

The panels will be big producers all during the summer, gradually less during the shoulder seasons.

If storage is possible that would definitely be a winning strategy. If you can get your GSHP holes drilled, dumping your summer heat into the ground would be a real winner. The problems I foresee with that is water flow through your loop field would wash away your heat.

You could dedicate 2 or 3 panels to heating DHW for the house through the summer, and use the DHW heating method of choice for the non-sun months. Likewise, you could dedicate the other 2 or 3 for DHW in your shop.

Some kind of control will be called for, because there are many intermittently sunny days even during the winter. Shoulder seasons, even more intermittent.


I'm not an HVAC technician. In fact, I'm barely even a hacker...
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Old 09-04-16, 01:37 AM   #6
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I guess it depends a bit on how much you intend to spend on incorporating the panels.

By far the most efficient heating is under floor but there is a lot of labour and cost involved, however you do say your floor insulation is sub-par, maybe a good time to improve it?

Under floor heating only needs the water to be warm, not hot. The water in mine is usually between 26C and 32C.

The system I have is a real hybrid, 3 solar panels connected to a 100L tank with a heat exchanger. The tank is the first point of heat generation for the heating system. The control system monitors the tank and panel temperature and reduces the flow through the panels depending on the temperature difference, this extracts maximum energy.

Next the now warm water goes through a plate heat exchanger connected to a home-hacked heat pump, this boosts the temperature (if needed). On occasions (when it's very cold which isn't very often!) the heat pump can't satisfy the heating requirements so the water then goes through a conventional oil boiler.

This combination works incredibly well. It's difficult to accurately say what difference this has made but it is substantial. Previously I had just the heat pump and boiler and was heating around 120M2, now I'm heating around 240M2. Adding in the solar panels has reduced my electric consumption from 43Kw/day to 30Kw/day, and at the same time I've doubled the heated area.

The great thing with under floor heating is that in the summer you can use it for cooling. I have a set of valves to isolate the 100L tank in summer and ceiling fans to circulate the air in the rooms to minimise dew forming on the floors.

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Old 10-14-16, 01:04 AM   #7
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I wanted to test three of the solar collectors. I connected them to my old natural gas water heater. I ran the cold line out the bottom of the water heater through a grundfos pump and into the bottom of the first panel. The heated return line at the top goes down to the top of the water heater.

The water at the beginning of the test was 54F (12C) and then 20 minutes later was over 120F (49C). Another 30 minutes it was too hot and I had to shut it down. It got to over 240F (116C). The sun was pretty bright though. Just amazed how much heat the sun produces and that some of us are not utilizing!

The next day I installed a small Dayton Hydronic heater that I found on Craigslist and learned that the air being blown across the coils was pretty warm. It's not hot like a electric heater, but for sure usable heat.

Now that I know it works, its time to take them apart and clean the cobwebs off of the selective material and backside of the glass. I spoke to a couple of solar flat plate mfg. today and they said to be careful on the cleaning, but that the black chrome is pretty durable. Not sure exactly what selective material is on there, but I will be careful.

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