Has anyone seen a solar thermal system that includes an insulated cistern for large thermal storage in the winter and then shifted to water use in the summer?
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| Posted in Mechanicals on
It seems to me that cisterns are most valuable in the summer for irrigation purposes and in the winter are not used. thermal storage can be used more in winter for space heating as well as domestic water, especially with radiant floors. Insulating the cistern and having a heat exchanger that could be disconnected in the summer and the solar go directly to a Solar water heater for domestic use would make the tank twice as useful.
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This is a test: I had attempted to post and answer to this question earlier and the post failed to appear.
I'll attempt to recreate my initial answer:
While solar thermal water storage can be an effective diurnal strategy or, with phase-change salts, perhaps offer a week of storage capacity, seasonal thermal storage would not be either cost- or space-effective.
There are strategies, however, to use dry earth for long-term seasonal storage, using either passive or active technologies, but they require very dry deep soil (very low water table and well-draining soils) and sophisticated engineering to make them function as intended.
The Passive Annual Heat Storage approach uses the insulated ground around and under an uninsulated - usually concrete - sub-soil building envelope but can be problematic in maintaining comfortable and consistent temperatures.
The Annualized Geo-Solar approach uses isolated-gain collectors and active distribution systems to store summer heat in large areas of insulated earth, which can be retrieved with a geo-thermal heat pump for winter use.
These systems are not appropriate in many geological areas and can be rather expensive to build and maintain, particularly the latter.
A discussion of the difference between the two can be found at: http://www.greenershelter.org/index.php?pg=2 (but be advised that it was written by the developer of the Geo-Solar system).
I was thinking of this as an extended thermal storage for a domestic water heating system that would be an active system with solar collectors that would add a more significant amount to the space heating that is radiant floor. Instead of a normal sized solar system with 120 gallon storage tank a beefed up collector system to heat a 1000 gallon insulated cistern. the idea being that this could be heated on sunny days and even if it was only at 100 degrees that is a lot of btus that could be used in the radiant floor that wants 105 degrees. In the summer the solar loop could be cut off from the cistern and go directly to the water heater for plenty of domestic hot water. It would seem that this would make the tank twice as useful as both a solar thermal storage tank and a rainwater catchment tank. ?
Boone,
While it is possible to do what you suggest, it doesn't make much sense.
1. First of all, it will only work with drainback solar collectors, not a glycol system. A glycol system needs to have a large active solar tank during the summer, to avoid overheating the glycol.
2. Okay, let's say you uncouple the solar collectors from the tank at the end of May. In June, July, and August, you drain the water tank, using it to irrigate your vegetable garden and fruit trees. In September, it's time to start charging up your solar tank again, to take advantage of the last sunny days of the late summer and early fall, before the dark days of winter. But your tank is empty. How do you fill your 1,000-gallon cistern, during the driest time of the year? Pray for rain? I guess you just use municipal water or a deep well — by that kind of undermines the whole idea of a cistern.
good points. thanks. you would need an alternate heat sink during the summer and to fill the tank if it was completely empty would take a little over an inch of rain off of 2000 sq ft of roof. IN our area the average rainfall in aug is 4.7 and Sept is 3.6. So though it has risks it seems like on average it would work out as far as water. I guess it would be a question of integrating the heat sink and it would probably be too complex a system to really be practical. Thanks for the feedback.
One last option- you could easily shade the amount of collector surface area to right size the system in the summer to avoid the overheating the glycol. Still a complex system and probably the dollars spent could be better put into a more efficient envelope .
I see what you're getting at. But remember that "a lot of BTUs" are not recoverable unless there is a significant delta-T to allow heat transfer to the returning water in the radiant floor heating system. And, though the 100°F cistern water could be used to preheat the return flow before entering the boiler, the smaller delta-T in the boiler makes the heat transfer there less efficient.
In fact, if you're using a very-high-efficiency condensing boiler, the return water temperature needs to be less than 135°F for any flue-gas condensation to occur and maximum efficiencies are achieved with return water at 85°F.
I am interested in geo thermal and was thinking of collecting rain water in a large cistern (3000-5000+ gallons) underground so the temp would be tempered by soil temp, then use air piped through the water for a heat pump for a small superinsulated passive solar house. some water could be used to water the garden in the summer. Would this be cheaper than deep wells and water/glycol systems?
Jane,
I've never heard of an air-source heat pump configured this way.
I have seen (on the net) pond-source heat pumps, both open and closed loop. I can't quantify how this works like others might be able to but it seems like you need a very large water supply in a not-so-cold climate for the earth to have the desired tempering effect.
My thinking may be askew here but I don't think a 5000 gallon "heat battery" would recharge fast enough to provide the amount of energy you would need to heat even a small well insulated house of the course of a winter.
Drilling is expensive, in my area it ranges from $35-$50/per foot.
I put a 1200 gallon cistern below my garage floor in a small, insulated utility room. I insulated it with foamboard to approx R60 on the sides and bottom. The ceiling is the concrete of the garage floor so the floor temperature will be largely based on the temperature of the cistern water. There are thermal breaks in the concrete floor above the cistern that line up with the walls of the utlility room. This room is thermally separated from the garage proper. The second floor living space is a 13 ton concrete/brick mass floor with radiant. . I'll report back when I hook up the cistern to the solar hot water panels and pump it through the floor. I live near Ottawa Ontario Canada.