Geothermal & Solar

jp_2

but with larger storage, you need a large collectors to match.

imagine one little solar collector and a 1000 gal tank?

a cup of boiling water into a cold bathtub does little to increase tubs temp.

Mark A. Utz

Geothermal & Solar

I have been installing geothermal systems, forced air and radiant for about twelve years. I am now looking at solar because I think that the two may be a good fit together when offering home owners energy efficient systems. I'am curious to know if anyone has tried to supplement their geothermal loops with solar in order to improve the efficiency of the system during the winter time operation. I have some ideas as to how to do this using a solar collector supplying hot water to a storage tank and running my geothermal loop through a heat exchanger in the storage tank thus delivering higher water temperature to the geothermal unit. In the summer time the loop side would by pass the storage tank by means of a bypass valve. The hot water that is generated in the summer could then be used to heat a swimming pool in this particular scenario. If anyone has done this or has any information that could steer me in the right direction, I would really appreciate it.

Thanks
Mark

Unknown

Has anyone considered using heat pumps directly with solar.

Sending chilled fluid to a collector could theoretically boost it's efficiency. Since the highest heating loads are at night this idea would obviously have some practicality problems, but is it theoretically sound? Wondering if anyone has considered this possibility?

jp_2

easy answer

its all in the numbers!

your loads plus the amount you can collect during the day, no magic here.

if you can collect 100,000 btu's a day but require 50,000 per hour, you aren't going to get far.

Unknown

That makes sense, since collector can't operate above %100 efficiency , even with chilled water running to it. Only so much radiation available at any given moment, and that's what it comes down to. Weak thinking on my part. Heat pumps can make the definition of an energy source kind of mysterious. It's easy to understand how a chilled fluid can pick-up energy from a 55 deg. mass. It harder to "get" how we can use this (with compressors and refrigerant) to move heat from a colder space to a warmer space. To me Geo- exchange has the feel of a "free lunch", of course nothing is free, discounted perhaps, but never free.

Wayco Wayne_2

If you had large storage

I have a customer who has a large solar system with an 800 gallon storage tank. I was considering installing a heat pump that got it's heat from the solar water since you can triple the output if the COP is 3 or even more depending. I didnt do it because than how do you switch to cooling? Mark, I'm still working on your question up in my sub concious. We should meet for breakfast and draw it out on napkins, as many great ideas have been done that way. WW

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GW

understand?

Do you understand the refrigeration process?


gary

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Mark A. Utz

That sounds good. I don't think my idea is all that different then producing solar hot water for radiant floor heat. The difference being that I would be putting the BTUs in the closed loop, ground loop which would increase the COP of the geothermal units. As you know the loop temperatures drop as the Winter progresses and as the loop temperatures drop so do the cop. Supplementing the loop with solar increases efficiency of the geo system. Dumping the hot water to a pool in the summer seams like a good way to use the solar in the summer. I was hoping one of the solar guys could steer me in the right direction.

Thanks
Mark

Unknown

Yes,I know how it works, but I still have some trouble with the counterintuitive part about using a 55 deg mass as a heat source. This solar question points to some gaps in my full understanding of this.

Could a heat-pump significantly boost the performance of a solar collector?
Would the sky, once the solar radiation part had been extracted, be able to supply additional heat? wouldn't any available heat at this point be coming from the air? and inefficiently because of the glazing?

Wayco Wayne_2

What about

supplying solar to the ground loop? During heating it could supplement the heat coming from the ground loop. When it's sunny outside it you could continue to run the solar and store heat out in the ground. Check drawing.

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Unknown

Jp, I don't think the C.O.P of 3 would apply to a solar collector. A glazed collector is a lot different from the earth or even the air. Wouldn't the amount of heat recoverable esentialy be limited to the amount available in the form of solar radiation? Since a collector under good conditions can operate at 70% efficiency, how much more energy(heat) could we theoretically pull. If it's not coming from the sun where is it coming from? How could we possibly triple the output of a collector if it is already operating at 50-75% efficiency?

Unknown

have you seen this? ; http://www.dlsc.ca/borehole.htm

Unknown

geo doesn't produce heat; it just moves it around and "concentrates" it.

the cop assumes that you have a virtually unlimited amount of heat (in the ground) to concentrate to a usable heat level.

You might get a cop of a lot more if you hook a geo unit up to a solar system, but you are still limited by the amount of BTUs you have available to move, and that is limited by your solar collection ability.

jp_2

problem I see

i am not really sure you are going to be able to retrieve all the heat you just collected form the soalr panel you've just dumped into the ground?

I think a better solution is to store that collected heat in a well insulated tank.

seems like over kill? you should be able to pull solar heat directly from panels to source.

jp_2

surely can not

pull more heat from what a collector can collect. its all surface area, plan and simple.

Wayco Wayne_2

Why wouldnt you

be able to collect it. Where's it going to go? I like the idea of storing the heat in the loop because the differential of the water temperature between the stored water in the ground and the temp at the solar panel with be wider and more heat transfer can take place. The down side is when you switch over to cooling the ground will not have stored the cold as much from the heat cycle, depending on the size of the solar collector. You could switch the solar panel pipes over to a tank wt HX with 3 way valves in the Summer. WW

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jp_2

my thinking wayne,

couple reasons wayne.

first off, think back to my bathtub & boiling cup of water statement. sure, when you drop that cup in the tub, you've kepth those btu's. but it takes energy to get back to a cup of boiling water you once had.

once you've collected heated water, why not use it right away?

i'd inject that collector water directly into the heat pump output.

hot_rod

use it or lose it

is my motto with solar. As soon as you try to shove it around, or store it for future use, mother nature starts slowly grabbing it back.

Storing large quantities of solar for future use gets to be a big project and investment. As seen in this solar storage tank for 95% solar heat fraction.

I'm not convinced, Wayne that putting it into the ground and pulling it back would pencil out that well considering the costs to move and convert. Run some numbers to see how it looks.

hr

Unknown

What about this:

Under winter conditions (when first line dhw pre-heat conditions are met.) Alter Wayne's schematic, such that his solar "injection" takes place downstream of the geo-field. This would still send very cool(efficiency boosting) water to the collectors. Relax about comfort a bit, and consider permitting room temps to swing a bit. Leverage the storage of a high mass emitter(thick slab) and the structure it self. I don't see why we can't allow room temps to swing a bit day to night in solar houses. What's wrong with letting a house (that has good thermal mass) warm up a bit beyond it's comfort target, and then loose this heat over the course of the night. Consider source heat (solid fuel) for morning comfort. Hot coffee and a warm stove can make almost any thing bearable. By designing mass into the living space we give up the ability to control the release of this energy, but also avoid the pitfalls (to some extent) of storage losses.

I love the idea of sending cool water to collector. But What are the parameters for return water temps on a heat pump? would the effect of sending warmer than ground temperature water to the heat pump be efficient? I would imagin that these units are designed to operate in a limited range. Any thoughts?

I do think that forward looking heating design needs to balance comfort and efficiency. The 24/7 69deg. evenly distributed heat we expect, may be more than many of us can afford to run at some point. What is the point of -20 design temps if you can't afford to achieve this. The reality is that heating bills and all the other inflationary (food and fuel) trends are going to redefine "comfortable", for a lot of people.

Solar is back, I hope. Unfortunately the glut of poorly oriented moderately insulated suburban sprawl we are stuck with, will not have as many options.

Wayco Wayne_2

Scott

Try to explain a little more please. Keep in mind I am not an engineer and have learned things from the ground up instead of from the concept down. My thinking on sending heat into the ground comes from 2 sources. 1. When I went to class for geo thermal HP I was told to install a back up elec duct heater in case the Winter ran long. The reason was that by the end of the Winter there is a 1 foot diameter ball of ice around the tube in the ground that impedes the transfer of heat into the tube. My thinking was to shring that ball of ice with the solar during the day when the heat was off and the solar was running. This would result in better efficiency for the HP. 2. Way back in the 70's I took a solar class and we learned about Trombe Walls and passive storage. I remember getting headaches at the equations to figure out how to manage the thermal mass. (Ack! My brain's on fire) Ground storage at the low temps of a geo HP makes a certain amount of sense to me although I have no idea of the actual engineering benefits. We could be looking at the cup of hot water in a bathtub of cold water. (around here we call it, "a fart in a windstorm") but I bet there's some benefit somewhere. WW

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hot_rod

At Cedar Mountain Solar

they use the mass of the floors to store heat as you mention, Scott. Allowing some moderate over shoot. Even with large heating arrays they rarely use more than 120 gallon of storage.

They also use steep panel angle or vertical mounts to limit summer over heating.

I learning more and more "control" options for solar as I meet more and more long time installers. I don't know we could ever describe the "perfect' control for solar heating. They really become a site and job specific design. More so than radiant it appears.

hr

Craig R Bergman

Rich L.

solar-geo-radiant

I have geo water to water in my house now and have been seriously considering adding solar to the equation.

Here's how I envision doing it: 5 ton water to water heat pump with 50 gallon buffer tank (existing). Add a storage tank with 2 heat exchanger coils internal to it, such as the Superstor Solar.

http://www.htproducts.com/literature/lp-196.pdf

One coil tied to the buffer tank as an injection loop. Add heat as needed (W1) when tank is warm enough, to meet ODR buffer tank settings. Have a summer shut down means (WWSD) to keep from adding hot water to the buffer in cooling mode. Have menual isolation valves to "lock out" heat in the summer.

If buffer tank is satisfied and heat is still coming from solar, dump in preheat DHW tank by way of the second tank loop heat exchanger.

The solar itself would be hooked up to the Superstor Solar tank with a glycol solution. Controls should be interesting!

Please pick it apart! it's much easier to change while still on paper than after the project's under way.

Thanks for the help, Rich L.