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Old 06-05-13, 11:18 AM   #1
Zennlasurfer
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Default Convert freezer to space heater?

Greetings. I have a small insulated room, about 150 square feet, that I would like to heat cheaply and efficiently. I was thinking an air-source heat pump would be good, but they are expensive and small ones are hard to find. So I thought maybe a cheap freezer could be converted to heat this space, by basically using the vapor compression components from the freezer as heat pump components for the room. I would like this to operate down to 0F.

I have two main concerns:
1) How would I prevent frost buildup on the evaporator outside? Are there built-in defrost controls in freezers that would still work with this application?
2) If the compressor is forced to run at low lift for an extended period of time, for instance, when the outside air is 50F and the inside is at 65F, would this cause damage to the compressor or prevent the cycle from working?

Any thoughts or other concerns with this application would be appreciated. Thanks!

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Old 06-05-13, 10:40 PM   #2
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If you expect the evaporator (cold) side to stay frost free, you will have to use a limit thermostat (cheap on fee-bay) of some sort to keep it above 32F. If outside air is your heat source, perhaps you can emulate features of mini-split heat pumps, but the efficiency will plummet. The features your unit came with probably won't work-they are designed for the limited amount of moisture in a closed freezer and usually involve shutting off the compressor, then using an electric heating element to melt frost, then restart the system.

You could hack the system and create a GSHP using antifreeze in the liquid loop, also. Lots of good info, especially in AC Hacker's "Heat Pump Manifesto".
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Old 06-05-13, 10:44 PM   #3
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Low lifts will improve both efficiency and capacity, as long as the system is set up properly. Google the BTU specs from the manufacturers of various compressors at different temperatures. I recommend a thermostatic expansion valve, TXV, instead of the cap tube your donor unit probably uses. Cap tubes are great under a specific set of circumstances, but TXV's compensate for varying conditions and can be adjusted for optimal operation without tearing the system open. Fee-bay and Surplus City Liquidators are great sources for parts.

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Old 06-06-13, 12:04 AM   #4
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How about a cheap window air conditioner instead?
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Old 06-06-13, 02:01 AM   #5
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Quote:
Originally Posted by Zennlasurfer View Post
I was thinking an air-source heat pump would be good, but they are expensive and small ones are hard to find.
I agree with Ryland, a small air conditioner or a dehumidifier would be easy to find.

Quote:
Originally Posted by Zennlasurfer View Post
1) How would I prevent frost buildup on the evaporator outside? Are there built-in defrost controls in freezers that would still work with this application?
Frost will happen. If you get a unit with a reversing valve, use that. De-humidifiers have this problem... in fact that is how they de-humidify. So, what you need is something to sense when frosting has happened. de-humidifiers have this, too.

You may need to 'roll your own' controller. Do you know how to use an Arduino?

Quote:
Originally Posted by Zennlasurfer View Post
2) If the compressor is forced to run at low lift for an extended period of time, for instance, when the outside air is 50F and the inside is at 65F, would this cause damage to the compressor or prevent the cycle from working?
No damage from low lift. Only happy compressor.

-AC
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Old 06-06-13, 08:36 AM   #6
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How well insulated is the room?
You might be able to almost heat it with a handful of Peltier Junctions, I have 8 of them in a shoebox size box that I made as a heater for my electric car, 300 watt input for 800 watts worth of heat output, cost was about $80 and the only moving parts are the fans.
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Old 06-06-13, 01:01 PM   #7
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Quote:
Originally Posted by Ryland View Post
... 300 watt input for 800 watts worth of heat output...
800/300 x 100 = 267%

This is absolutely the first time I have heard of a Peltier junction efficiency achieving anything remotely close to 267% as you stated. The efficiency I have usually seen quoted is in the neighborhood of 17%.

Did you do the tests yourself? Do you know of any studies that match, or even vaguely approximate your results?

I would be very interested to know, as most ASHPs in operation today are about as efficient as you are stating.

Also...

Quote:
Originally Posted by Ryland View Post
...800 watts worth of heat output, cost was about $80...
This would mean that 800 watts out = 800 x 3.412 = 2,729.6 BTU/hr

So, in HVAC terms, where 1 Ton of AC = 12000 BTU...

You would require (12,000/2,729.6) x $80 = $343.35 worth of Peltier Junctions per Ton of HVAC equivalent heating. This would make it at a very minimum, 2x more cost efficient to buy than almost any existing vapor-compression machine.

Are you completely sure of your numbers?

If this is true, a revolution in heating and cooling should now be underway.

Personally, I suspect that there is an error in your numbers.

-AC
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Old 06-06-13, 05:06 PM   #8
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Zennlasurfer,

If you need the unit to operate down to 0 degF, that's a tall order for a first project. Unless you are going with a ground loop (not affected much by outdoor temp swings), you will need to devise a defrost control scheme to heat below about 40 degF. Otherwise, the evaporator will freeze up and the unit will quit heating until it thaws out.

With small units, you have 3 options:

1. Electric resistance melter (not efficient)
2. Hot gas defrost (suction accumulator and defrost valve needed)
3. Reverse cycle defrost (reversing valve needed)

By far the easiest to accomplish with a first project is option 1. But it is the least energy efficient. Good for a proof of concept that your unit will operate in heating mode, but not a long-term solution.

Both options 2 and 3 require you to break into the refrigerant plumbing and add new components. Option 2 is somewhat simpler, but the unit will only operate in heating or cooling mode (not both). Option 3 will allow the unit to work in heating and cooling mode, since defrost is just reverse-cycle heating. So you can use it year-round if need be.

Another thing you want to consider is this: As the outdoor temps drop below freezing, your heat pump output will drop in direct proportion. Unfortunately, this is the time you will want the most heat. Also, the smaller units I hacked all had undersized evaporators, which compounds this problem. At a certain outdoor temperature, the unit ceases to be useful. The unit will run constantly to maintain an indoor temp it cannot sustain. Below this temperature, you will need a backup heat source.

With my hacked 12k btu window a/c unit, I found that it would provide adequate useful heat down to about 25 degF. Below 20, it uses more energy than it heats, mainly due to the evaporator not having enough surface area. I believe if the evaporator was wider and/or taller and not so thick (2 layers of tube instead of 4), it would hold its own at 20 degF. But I doubt it would do much good at zero degrees. For my dirt cheap, recycled, beat down old window shaker project, working like it should for 350 days of the year is a better outcome than I had expected.

If you study the really high efficiency or sub-zero heat pumps, this is exactly how they are built. The outdoor unit has a heat exchanger like the back of a vintage refrigerator: lots of surface area and not very thick. The cube-shaped split units are twice as tall as the less efficient, older units of comparable capacity. The two ton mini-split units (24k btu capacity) have a heat exchanger in them the size and shape of a 42 inch flat screen tv! The indoor unit, in contrast, resembles a large space heater. The major manufacturers have figured this one out for us already.

Hint: automobile a/c condensers are cheap at salvage yards everywhere...

Last edited by jeff5may; 06-06-13 at 07:09 PM.. Reason: insight
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Old 06-06-13, 09:06 PM   #9
Ryland
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Quote:
Originally Posted by AC_Hacker View Post
800/300 x 100 = 267%

This is absolutely the first time I have heard of a Peltier junction efficiency achieving anything remotely close to 267% as you stated. The efficiency I have usually seen quoted is in the neighborhood of 17%.

Did you do the tests yourself? Do you know of any studies that match, or even vaguely approximate your results?
I haven't worked on that project in over a year and those figures were all off the top of my head, so they might be a bit off, but the peltier junctions are MUCH better heat heating then cooling because they are fighting them selves, their efficiency drops quickly as the temp difference of the two sides increases, so it really depends on your environment, clearly not perfect for a lot of applications, but a choose this for my little electric car because my old heater was pulling slightly warm air off the motor and it took a long time to heat up, this heats up much quicker and adds a boost by pulling the heat out of that slightly warm air from the motor.

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