DC powered ground source heat pump?

I'm interested in figuring out the numbers in order to build an 800 sq ft passive solar home and I was curious if anyone had heard of a DC powered geothermal heat pump before?  DC powered anything is kind of rare and I'm not sure why, being so much more efficient than AC it seems like there would be a huge market for these things with all the solar being installed these days.

So anyone ever heard or a DC powered GSHP or a really small AC powered one?

I bet you could achieve your goal of a comfortable passive solar home without any heat pump at all.  and when you do, I think you should spread the news far and wide and show it to as many people as you can.

which is to say: no, I have never heard of a heat pump like that.  but I'm not really any kind of authority on heat pumps.

Someone correct me if I am wrong, but my understanding is that DC motors are expensive in part because they don't make as many of them and in part because they require more copper wire internally and to feed them than higher voltage AC motors.

You are right though that there is a significant loss in converting DC current to AC. I think it runs roughly 10-15% with modern inverters.

tel jetson wrote:
I bet you could achieve your goal of a comfortable passive solar home without any heat pump at all.  and when you do, I think you should spread the news far and wide and show it to as many people as you can.

which is to say: no, I have never heard of a heat pump like that.  but I'm not really any kind of authority on heat pumps.

I have been thinking about it and I think I could live with making the home even smaller than 800 sq ft.  Most of the systems are for much larger homes anyway and if I can make the inside envelope incredibly tight with a very high r value roof then I think possibly just a radiant floor setup with passive solar gain from the south facing windows might be enough.  I was looking at http://www.warmboard.com/ and thinking that might be enough with a solar hot water heater and a propane back up?

ahh but then I don't know what to do about keeping it cool in the summer, it gets very hot in the summer... any ideas?

Well I think I found the solution to the problem, and that solution is a mini split ductless air sourced heat pump, bit of a mouthful eh?  Seems to be what many people use in Japan, is very small and very efficient and some of them perform fairly well in the winter months even.  It does draw quite a bit of power during the colder months because it become less efficient as it gets colder outside.

So I had an idea - why not build a box around the outside condenser unit that is half white and half black and when the seasons change from hot to cold you switch the direction of the color of the box to face the sun, obviously black in the winter and white in the summer?  Build the box out of sip panels, shouldn't cost much money for a tiny box like that.

Maybe no box in the summer months and maybe just a completely black box in the winter to raise the starting point of the condenser unit outside?

Thoughts?

AC motors tend to be more efficient.

I second the suggestion to avoid heat pumps if possible.

There are peltier heat pumps if you absolutely need to use DC, but they are inefficient.

There are also absorption/adsorption type heat pumps, which would be driven directly by solar thermal energy, and make no (or very minor) demands on any sort of electrical system. If you have enough sunlight to play with, the sun could build up the capacity of the system during the day, and the system could use this energy to pump heat at night.

I think the question re: to heat pump or not to heat pump depends on your local climate.  I live on the border between 5 and 6, and around here everyone I've talked to is very satisfied or enthusiastic about the performance of their ground-loop-based heat pumps. 

Brian wrote:

ahh but then I don't know what to do about keeping it cool in the summer, it gets very hot in the summer... any ideas?

If your summer is dry and hot, some swamp coolers can be very efficient.  This guy has  a simple design, though it is for a tiny house.  http://thefieldlab.blogspot.com/search/label/Pepino%20II

The principles are very simple just find ways to create edge between water & air. 

Of course, green plants are the best air conditioners on earth.  Ivy's, trellises, deciduous trees...blah, blah but it really is true.  There is this giant banyan tree in town near here - probably 150 feet tall with 300 foot circumference. It feels at least 20 degrees cooler under that tree than it does 30 feet away. 

When concentrating on DC I always look utilizing parts from an automobile.
They are mass produced in adequate numbers to drive down costs.

I made a prototype GSHP using a car radiator and a garden hose.
It worked well but I did not continue to develop it.
Basically I pumped well water through it while blowing a fan over it.
One could use such an idea for individual room AC units. 
There would need to be a condensation drain on each unit because water from the air would precipitate onto the radiator as it cools.  So the unit would need to sit in a tray and have a drip hose to an outside plant.

My Florida Heat Pump GSHP unit is basically set up like this.
You have a worthy idea and pursuit perhaps your determination can take you farther than mine did.

I have not come across one yet, in time, I think they will eventually exist.. for now, if you want to heat and cool 800sq ft and you have 48volts dc available and 30-51amps, a split cool dc heat pump might be your ticket. If you need cooling now, there are 12, 24 and 48vdc A/C's out there, they aren't dc connected to an inverter connected to an AC a/c.. all dc stuff in them..
If they don't look like they were made for use in a dwelling, they still work, and if it is 110f degrees outside do you care? they aren't ugly.. they can still do their thing..
telcom sheds use dc air conditioning  now and they still do their stuff quite well and they still have all DC everything inside.. don't imagine they are inexpensive, just very well built.. (dc air co.)
If u can find a small enough absorption chiller, it would require a heat source to keep it low in current draw, either propane burner (fossil fuel and pollution) or other means to provide it with hot water.
The electricity it uses is quite low, are they in dc only..? well, if anyone knows of one, I'd like to see it, one company did start a production of such a decade ago.. robur in conjunction with an AE company.. it didn't take.. I don't know what became of them since the only ones I have run into so far are 10ton and larger (via a different company)... a bit over kill for 800sq ft.
I would think an absorption chiller would be a bit easier to be built in dc only, since there is no compressor.
If you live in a dry enough area, evaporator cooling, requires some water of course, but solar chillers run on 12, 24 or 48vdc.. PV direct or on batteries.. In time, I have no doubt that a DC powered ground source heat pump will exist..  DC anything is not so rare anymore, but there still has to be a pretty large interest.. AC units are just cheap, mass produced, semi-traditional way to cool, but useless when u don't have the energy to power it.... but with AC not being so cheap anymore, and the thought of having A/C in a summer grid failure, brown out or black out, might make people wanting something more.. It seems unfair to discriminate against people with alternate energy powered dwellings.. but, I am pretty sure that is why all dc powered heat pumps and A/C's are becoming more available.. makes it easier to live in the middle of no where now..
Plus side is when an all DC heat pump will provide heating as well as cooling, consumes more current, but if you have the energy,... hey, pretty neat deal..
still solar heating is less expensive, possibly easier to do, but of course requires a lot of sun.. and duct work..
yes, I know, so does charging batteries if there's no wind or other means to charge them in winter.. that is when a heat stove comes in handy.. now then, adapt a way to charge batteries via the heat of your wood burning heat stove, you might be pretty set..
Back to 800Sq Ft.. if 17,000 -18,000 btu of cooling is enough..  a split cool dc heat pump unit might be for you, 2 piece unit, its not the gshp u want and the catch is.. it's production, I don't think it will be available till after the summer (2010)..
well, that was,
Just my 2 cents worth..

Taking a good look a CAT 950 Wheel Loader.
This thing has a huge radiator, shroud  and fan.

Why not run this, or one from a truck, outside of the house in an insulated box.

It could just recirculate house air and cool it by running water from a well, burried coil or deep pond through the radiator.  The fan would blow air over the radiator and back into the house.

We have a lot of humidity here so the sweat could drip outside onto som plants.

Have you looked at what's available to provide A/C for RVs?

Ground source heat pump (GSHP) systems use a heat pump to exchange heat with the earth which remains at a relatively constant temperature and are the only practical way to employ geothermal energy in many parts of the country.