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Old 03-21-18, 10:29 AM   #1
Tpm
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Default Solar assisted heat pump

Newbie here looking for an answer I can't find on the web. If I bolted a water-to-air heat exchanger in front of the fan on an air-to-water heat pump, could I amp up my 24 tube solar thermal system enough to make an impact on central heat?

I'm looking to replace an electric tankless backup in a radiant-floor house
that is otherwise heated by passive solar, a wood stove boiler and 280 gallons of storage. The immediate reason to replace the tankless is the surcharge it places on winter vacations. But I am also looking to get a month off from stove duty on either end of the heating season plus a few sunny days off in between. (South central PA is not passive solar heaven.) The control is me. I consult Weather Underground every morning to gauge how soon I need to fire the stove. In optimal conditions for a heat pump, I'd flip a switch on a motorized three-way valve and divert solar from DHW to heat pump. If I can get storage to 140 degrees in daylight hours I'm good to go.

Stray facts. The solar system has a heat dump radiator. AC is a pair of air to air mini splits. A drain back system connects vented main storage with an 80-gallon dual hx solar tank so that the former backs up DHW.

A second question. I can buy a two-ton ATW heat pump on eBay in Britain for about $2,300 including shipping. Trouble is, it's 50 cycle rather than 60. I'd want time of day and set point control only. And a reliable compressor needless to say.

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Old 03-21-18, 12:44 PM   #2
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Ok so if you can't heat the place with the solar thermal system now, what do you hope to see happen? Heat is heat. The heat pump is going to make up the difference no matter which what way or do how you go about it. Rig and reroute away if you feel froggy. Put in enough capacity and everything will be fine.

Trying to freeze a cistern or cesspool is almost a better way to do what you desire than to try to get reliability out of the sun.
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Old 03-21-18, 02:22 PM   #3
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I don't need reliability from the sun in a heating mode I select (vs wood stove boiler) because I've looked outside and seen that bright yellow ball. And it appears right along in the shoulder seasons. With passive solar keeping things toasty in the afternoon and evening, and heat loads in the upper teens of kbtus/ hour overnight, solar assist could make a difference.

Towit: by heating the entry air with solar thermal does not the heat pump operate at a higher COP, in effect multiplying solar output by some fraction of it?

Yes, the heat pump at two tons will handle the whole load. That would be the definition of backup. The question I asked is best and highest use, as measured first in fewer trips to the woodshed, and second in electric bills that don't appear horrific next to my normal central heat use of about 400w/hr - intermittent -- plus chainsaw gas.
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Old 03-22-18, 12:05 AM   #4
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Going from water to air to air to water is going to need some COP math to see whats going on
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Old 03-22-18, 12:54 AM   #5
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To the to wit question: no. In this configuration, all the heat pump is going to do is offset the temperature gradient outdoors from indoors. The fraction of heat grabbed outdoors from the solar preheating circuit is transported (at electrical power cost) indoors. The increase in COP is the useful energy gleaned from the preheating rig.
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Old 03-23-18, 08:17 AM   #6
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Quote:
Originally Posted by jeff5may View Post
To the to wit question: no. In this configuration, all the heat pump is going to do is offset the temperature gradient outdoors from indoors. The fraction of heat grabbed outdoors from the solar preheating circuit is transported (at electrical power cost) indoors. The increase in COP is the useful energy gleaned from the preheating rig.
Well, no to your 'no.' (The rest of your post is largely incomprehensible.) If I add 5 degrees to entry air in 40 degree weather then the cold side temp is effectively 45 rather than 40, the Delta T with the hot side shrinks and output temp and COP move higher than can be explained by addition alone.

Hx efficiency? I invite you to consider 100 degree water moving at 0.6 gpm through a radiator cooled by 800 cfm of 40 degree air. Solar collector efficiency shoots up as well.

Potential? A two ton heat pump can be expected to move 48,000 cf of air per hour. At 40 degrees, that volume of dry air weighs 800 pounds, give or take. My solar thermal is good for 3,600 btu/hr on a bright day with much hotter entry water. And, please, do not make me define the word 'potential.'

No thoughts on 50 cycle vs 60?
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Old 03-24-18, 12:33 PM   #7
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Ok, so unicorns exist. You're moving your solar heat back and forth between heat exchangers and States of matter for free. Your sustained energy savings is attainable at low cost. A third of a ton of heat capacity is going to make a world of difference. Have at it big bang scientist. Don't forget to send lots of pics and test figures.

Not trying to pick a fight here. You just repeated part of what I said. The rig you propose will do something. How much of something depends on materials and execution. Time and temperature, combined multiple hx chain effectiveness, and outdoor thermal storage and insulation come to mind. I would definitely not run any extra pumps or blowers that consume electric power unless they are also solar.

Considering between the lines here. You hint at a tubular collector. Is it an evacuated tube collector? If so, the advantage of this type is high grade output. This means hot water that can burn you with indirect sunlight. Concentrated energy should be kept that way and only spent at the other end of the chain. If the solar panels can boost the heat pump indoors, either or however, it would certainly be more efficient.

Last edited by jeff5may; 03-24-18 at 01:21 PM.. Reason: Soelling and grammer
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Old 03-24-18, 01:37 PM   #8
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Quote:
Originally Posted by jeff5may View Post
Ok, so unicorns exist. You're moving your solar heat back and forth between heat exchangers and States of matter for free. Your sustained energy savings is attainable at low cost. Have at it big bang scientist. Don't forget to send lots of pics and test figures.

Thanks for looking, ecorenovators. I'll take the absence of credible criticism as support in some measure for a $200 gamble. Sorry if I sounded peevish.

The best I can find on the frequency question is advice by expats, to people who are heading overseas, to leave their US refrigeration equipment behind. 'Nuff said. On the US front, ATW heat pumps are godawful expensive compared to the rest of the world, and one would hesitate to void the warranty right out of the box. (Attn, jeff5may: A legitimate knock on my scheme!)

Happily, I found a guy on line who hacked a Daikin mini split outside unit to a water heater tank and flatplate collector cum radiator. A two-ton no-name mini costs as little as $900 with a surplus indoor head to sell. The price is right even though this hack probably won't get to the 140-degree storage I need for overnight in the dead of winter. It can handle radiant floors directly, which work just fine with 90-degree water.

I hope I haven't worn out my welcome. This is terra incognita for me.
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Old 03-24-18, 01:48 PM   #9
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To be clear, the guy hacked a Daikin air to air mini split to make it ATW. He didn't like the idea that DHW and CH was an either/or proposition on Daikin's Altherma ATW offering.

Air to air minis are dirt cheap thanks to China. Think of them as preassembled junk.
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Old 03-24-18, 09:41 PM   #10
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I think this has merit - the warmer the source for the heat pump, the better. I've wondered about a central water tank to pool heat into. If our refrigerator could sink heat easily, into the water, and solar heat could be added in the winter, that would make a heat pump more efficient.

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