Fine Homebuilding - solar-collector-pumped HP with geothermal storage

[jesup]

If anyone saw the latest Fine Homebuilding, they have a "zero-energy" house article (Oregon coast area). Green roof, solar panels, insulated, passive solar, etc, etc. One unusual aspect was a solar-collector-pumped heat pump.

They had a battery of hot-water solar collectors on the slope below the house. These fed (via a water-to-water heat pump apparently) a set of storage tanks. When they're up to 120F, extra heat is directed down a borehole in a closed-loop system (and I assume it's reversed to retrieve the heat when needed).

Anyone know anything more about this system?

[heatpumpguru]

We did a home with a ZERO impact in Kennett Square that was designed by the EPA,it had something like this installed.

[gmcd]

These "zero energy" buildings are a bit of a sham- the "zero energy" is used to mean that they self-generate their own power/energy and don't use anything off the grid. The problem is that many of these "zero energy" buildings are not very energy efficient at all, and by simply adding more PV panels and solar water panels, to generate their own loads doesn't make them "more efficient". Now, how many homeowners and commercial building maintenance types could even start to trouble shoot a system like that, let alone keep it operating well for more than 10 years? The real answer lies in passive building design- design the building envelope properly to take advantage of the local, natural climate and minimize the energy needs in the first place. Windows, walls and roofs don't need a lot of techno-maintenance to function, and they last a heck of a lot longer than other techno-solutions like PV panels, solar water panels, heat pumps and all the complications that go along with them.

[jesup]

So what did you do for the Kennett Square home?

I'm still interested in the idea of solar-pumped heatpump; you avoid one major bugaboo of solar cells (low efficiency) and a major inefficiency of air-to-air heatpumps (low outside temps hurting efficiency), and even improve on the heating efficiency of geothermal possibly (due to water from the solar collectors/tanks/borehole being warmer than the 50-55ish of bedrock).

As for passive being preferred - sure, and they did incorporate a fair bit of passive in this particular design (clerestory windows and interior design to let light in to minimize use of lights during the day; south-facing siting and windows, green roof on the north side, high insulation values, etc). In most climates, though, it's hard-to-impossible to be fully passive, or if it's possible, it requires major tradeoffs and/or major cost.

Even many of the things they did in this house probably aren't truly cost-effective even under pretty pessimistic energy-cost assumptions, let alone now, though that depends. I've heard some Californians pay up to $0.45ish a KWHr peak over a certain usage; that can change tradeoffs a lot. (The house in question here was in Oregon.) Locally, we have 'high' electric prices, and that means $0.13-0.14ish. (Winter all-electric heating rate for PECO/Exelon here is much lower; around $0.065/KWHr.)

[Alf]

You cannot heat the earth with solar. The earth is nothing but a large heat sink. The higher the delta T between the solar and the earth, the quicker it will disappear. You are ahead to put the heat from the solar directly into the structure. The structure is insulated to keep the heat from escaping.

[key]

Alf:

good point, a real good point, however if you look at the 1st law of thermodynamics where heat moves to lack of heat, running the solar tubes next to the geo tubes would make sense.

[sheltermonkey]

Sounds like what they are trying to do is store heat from the summer in the ground for all those days when it will be cloudy.
I know the ground is a big heat sink but it does take time for that heat to move through the ground so that area would tend to heat up over the sumer.
Does a regular geothermal system not tend to use up some of the heat energy as the winter goes on-not as efficient at the end of the winter as it is at the start.

Ive been trying to learn a little about geothermal and i saw an example of a system that uses solar panels to heat the pool and then the pool water is run through the heatpump in the winter(solar heated reservoir system).

Does the COP get raised to what 3.5 from 3?
Sounds like a lot of equiptment for the thing to work though.

Those big windows on passive solar houses don't make any sense unless you live in the southwest. The windows loose one third of the energy taken in during the eight hours the sun shines and then loose the other two thirds at night. What makes sense is glazing seperated from the house by insulation either as a sun space or as a solar thermal collector. But, the windows do sell.

[jesup]

They were (I believe) basically using a heatpump to move heat from solar-water panels (during the day) into large insulated water storage tanks. Those are used to provide heat via a water-to-air exchanger after the air handler. When the tanks reach a preset limit (120? 140?) instead of dumping heat into the tanks, it's dumped into a borehole. Later that day/week/etc when it's colder (night), I assume* it starts pulling heat from the borehole to heat the water tanks. The earth is used as a really big (though rather leaky) storage tank.

*The assumption above is because they don't explain the details; that's what I was looking for here.

This is a solar-water heated house. It's only incidentally geothermal I assume. They're using the ground to avoid needing a huge amount of storage tanks. They lose on the leakage, but only the energy to run the pump for the water. I'm guessing the heatpump is used to transfer heat from the pipes (including the ones going into the borehole) to the storage tanks, when needed. I also guess that they might shut off the pumps if the water coming out of the hole is close to the temp of the water coming from the collectors for more than N minutes.

[gto boy]

I agree that the home is kind of a shame,it will never take it self off the grid due to demand.He found money and got alot help from GREEN GUYS,It looks like the house from BeetleJuice,grass roof cisturns for rain water. And those cells. It just does not blend here in Horse Conutry.

[gto boy]

Jesup,did you do your house with 2 speed heat pumps awhile back?

[cem-bsee]

what is the life- cycle cost?

eg: one can always spend $$$$$ & get lower monthly operating costs -- it takes $ to make $ --

I investigated solar in late 1970s & concluded in Indiana to use warm air collecting solar panels disguised as window shutters = passive. because solar panels at the "proper" angle had 2" of snow in Jan 84! but then got laid- off, so wound up in South --

it will take BIG tanks to store enuf water, say in Ft Wayne IN!

[jesup]

Yes, I installed 2-stage air-2-air HPs for 2/3's of my house a couple of years ago. Working well; balance temp is pretty low since I've done a LOT of sealing and insulation since then. No problems with humidity in the summer; it tends to run a lot on the low stage.

I agree - like I said, quite a few things aren't truely cost-effective, at least on a purely monetary basis.

Off-grid isn't important to most people; that's more of a religious issue unless you're talking about a cabin out in the way outback.

Oregon coast isn't a high heating-degree-day area, I believe, so that makes it easier to use solar water heat collection.

Solar HW heat is relatively simple technically and relatively low-cost/low-tech compared to things like solar cells, etc, and I imagine fairly efficient compared to solar cells (anyone know how efficient it is?). The biggest trick is how to store more than you can use immediately for when you need it (i.e. at night). Sounds like this house is an interesting attempt to do that; I wonder how well that works. Even in areas where a reasonably-sized solar HW array can't provide all the heat, with an auxiliary/secondary heat source it could make a big difference in total costs - use this, and if it can't keep up kick in a regular (though smaller than normal) heatpump that's also used for AC in the summer. (In many northern areas, sizing for heat would mean oversizing for cooling.)

[heatpumpguru]

We have seen almost all the panels that were installed when solar was HOT,going in the Trash with the old shingles when they redo the roof.I have found that the payback is long,and just when you get postive return your stuff is shot or obsolete.I have done more GEO because people do not want outdoor units than people who are GREEN!

[fpopjr]

i am just a homeowner doing research on geothermal for a new home and had no intention of posting but...whats with the solar bashing here? i live in an earth sheltered, passive solar house with no furnace built in 1980 and it works fine. we put in domestic shw in 1982; still works great no problems- although i have gone thru 2 tanks which is about normal here. 3 high quality locally made panels ground mounted on heavy duty aluminum angle frame about 15' from the house. we shut off the backup electric coil in the tank in the summer unless we get 3 heavily overcast days. nothing very complicated and it has paid for itself several times. basic solar works just fine.