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By Bill in Seattle, WA on 12/26/2006


Hello, I was considering a GSHP system with hydronic for new construction, but recently windstorms knocked out power here for several days. I'd just not been able to get used to the idea that GSHP uses electricity almost constantly (cycling ~5x per hour for 5 minutes), and the power outages just tore it. Now thinking about a near-zero energy ICF house. I seem to be the first one to ever put these two ideas together, as I can't find any info on it, but I have in mind putting up a large number of evac-tube solar water heating elements, which heat water in say, a 400 gallon underground water reservoir, in which is a coil from the hydronic system. Solar heats the reservoir water up over 100 during the day, which heat is expended through the night through the in-floor hydronic system. Also there'd be a coil in the reservoir to pre-heat domestic hot-water. Maybe free hot water? In summer, the reservoir hot water gets hot enough to drive an absorption-cycle water chiller for hydronic. Has anyone heard of this? Any actual experience with a system such as this?
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By Dale in Richland, AZ on 12/26/2006


Here are some basic questions:

1) how deep into the ground do you need to go to get a good temperature?
2) if doing a net-zero house what is your alternate electric supply?
3) is summer cooling an issue?
4) is natural gas available?
5) how much are you willing to push the envelope of technology and/or cost?

Ideally a GSHP works best when soil temperatures are near the desired indoor comfort zone. But humidity and air circulation become a factors, so how you address those items affects the rest of the possible solutions. Heat pumps in general are most efficient when operating near full-time, not cycling on-and-off. So a continuous power supply becomes almost mandatory.

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By Bill in Seattle, WA on 12/27/2006


>>1) how deep into the ground do you need to go to get a good temperature?

Idea is to get GSHP lines below the frost-line, which in Seattle is basically zero. I was planning slinky lines, in six parallel trenches of four feet each. Slinky coils set in vertically, backfilled, and soil compacted. Such trenches would be under the home's bottom slab, and so would not be subject to frost in any case. Very cheap, for GSHP. But I had in mind copper tubing, rather than polyethylene. Copper would be much more thermally-efficient, although I was cautioned that it would corrode underground. I doubt this though, unless the soil has a high sulfur content.

But remember, I am talking about solar rather than GSHP. No more GSHP, but maybe a small condensing boiler as backup, and standard hot-water heater in the circuit after the reservoir coil, as backup.

>> 2) if doing a net-zero house what is your alternate electric supply?

PV. But the plan is a near-zero house. As PV panels are so hard to get, they may be unobtainable. (I am suspicious that Big Oil is constricting the supply of raw silicon)

>> 3) is summer cooling an issue?

I think so. This is my first year in Seattle, and everyone says A/C is not necessary, but I think it is. So I have in mind some sort of absorption water chiller, driven by the high temps the solar evac-tube water can make, especially in Summer. These will be ICF houses, so needless to say, an ERV.

4) is natural gas available?

Yep.

5) how much are you willing to push the envelope of technology and/or cost?

To a reasonable economic balance, as I am a developer, and these are spec.


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By Dale in Richland, AZ on 12/27/2006


Is there enough solar available to actually provide heating? or cost effective PV economics? People in Portland they told me "yes/maybe/sometimes".

Actually the computer industry (and mr. bill's Vista) are tying up the  raw materials, which is why so many other technologies are being developed for PV panels.

Soil chemistry and PH will affect copper piping, we have to encase it in plastic here in the desert because of alkalinity issues.

Since  the GSHP is  gone, how about combing the solar collectors for space heating and domestic hot water. Run a closed loop with two heat exchangers. I think that is a standard off the shelf set-up. Provide a tankless for auxiliary hot water and an auxiliary boiler for  the hydronics. Alter-air makes a solar powered water chiller, not sure if it will work in your humid environment, for summer cooling..

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By Bill in Seattle, WA on 12/28/2006


I think there is enough sun, as this guy on the National Solar Tour had good success with his smaller system. Seems like a matter of scaling it up. We have an average of 3.8 hours of good sun per day here; not alot, but it's something.

We are told that electronics are what's consuming silicon, and that China is the main consumer draining the market. But I don't buy that. Silica is among the most abundant compounds on the planet, and it's not like making silicon is New ScienceĀ® or exotic technology. No, there's a constriction in supply somewhere. Notice Big Oil has bought most alternative-energy technologies? (GM-->Ovonics' NiMH batteries, Shell & BP-->PV)

Copper piping for swimming pools has been used for 90 years across much of the nation. I just think that people these days are used to PVC. It would be nice to know the relative thermal properties of copper vs HDPE.

Problem with just a closed-loop heat exchanger is there's no storage for nighttime. This is why I have in mind the reservoir. It would be say, 400gal of water and have two coils: one for hydronic, and one for domestic hot water. A differential thermostat would turn on the collector water when the panels are hotter than the water, and when off the water drains back by gravity for freeze-protection. The domestic coil pre-heats hot water, and since the reservoir water should reach or exceed 140, the in-line hot water heater should not normally be needed for backup except on cloudy days. The hydronic system should not run higher than 85 degrees, so its coil could have a mechanical mixing valve which automatically mixes cold return water with reservoir heated water.

But no one has set up this kind of system nor even thought of it before, as far as I can tell. There is no info. I did find two modeling apps though, which may apply for system design: Polysun, and T*Sol ... need to study them further.
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By Dale in Richland, AZ on 12/29/2006


I have seen closed loop systems where the heat storage was 200 gallons. And the collector output was at 180 degrees.  I'm sure that a larger thermal mass could be operated at lower temperatures. Some heating and cooling people will tell you that for hydronics ideal water temperature is 3-4 degrees above desired thermostat setting. So you pump warm water at near room temps....... Some of it depends on how much mass you have in your floor, its thickness, and the location of your lines.

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By Bill in Seattle, WA on 12/29/2006


Floors will be ~4" of concrete, so alot of mass and worrisome hysteresis there. In any case, the reservoir water will need to be above typical domestic hot water, so it will serve that system as well.

On another forum, I got a rather discouraging calculation: Let's suppose that your tank is totally insulated so that there isn't any heat loss there. Your indoor temp should be 68 degrees and for any kind of straight heat transfer the hot part has to be ten degrees hotter than the area heated. That makes the tank lowest useful temp 78 degrees. Now let's suppose that your collectors build the tank temperature to 138 degrees. The BTU's packed into the 400 gallons of water figures like this. 400 gallons times 8 pounds per gallon gives 3200 pounds of water heated 40 degrees. This figures out to 128,000 BTU's. If your house is really heat efficient and only needs 50,000 BTU's per hour to keep it warm, you'll have about 2-1/2 hours of heat stored up.

Maybe it would be worth buying one of the solar modeling apps, if it could handle this concept.
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