# Heatpump



## TX_Dj (Jul 25, 2008)

Build your own with the vehicle's existing AC system?


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## RoughRider (Aug 14, 2008)

you can run a heatpump in the RAV4 as a heater and as a cooler and you need less than 1kWh...

a existing ac-system cant be run as a heater and it needs more energy...

thats why i am looking for a heatpump...


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## TX_Dj (Jul 25, 2008)

A heat pump is an air conditioner that can reverse the flow such that the evaporator becomes the condenser and the condenser becomes the evaporator, thus an ac system can be run as a heater. I think 1 kW isn't a bad guestimate, most folks seem to concur that a modern vehicle's compressor requires somewhere between 750-1500w to operate effectively.

Another alternative (for heat only) would be to use peltier-effect thermoelectric devices. Yes, they can cool, but they're only 5-10% efficient as a cooler (compared to 40-60% for an AC compressor), but they are more efficient than the equivalent wattage ceramic heater, because not only are you generating heat from the energy put in, but they also extract heat from the cold side. Problem of course is that unless you happen to have a handy supply of modules, they're pretty cost prohibitive.


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## HDS (Aug 11, 2008)

TX Dj
As you mentioned, a heat pump is an air conditioner that reverses the role of the evaporator and the condenser coils. This involves reversing solenoids in the refrigerant lines that "toggle" the direction of flow of the hot gas from the compressor and the return to the compressor. 

My question is, has anyone ever tried to "toggle" the air flow instead. Let the a/c run in one direction and, by motorized dampers (like the one that switches on the "VENT" in most cars) "toggle" the interior air flow across one coil or the other, and the outside air across one or the other. In theory it should work.

HDS


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## bblocher (Jul 30, 2008)

I've been debating trying something with thermoelectronics for an alternative a/c heating solution but didn't know how effective it would be for cost. If anybody finds any information on somebody who has done this it would be most helpful.


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## Schmism (Aug 16, 2008)

HDS said:


> TX Dj
> As you mentioned, a heat pump is an air conditioner that reverses the role of the evaporator and the condenser coils.


 to reverse the air flow.... IF the evaporator and condenser coils were identical in design, (tubing spacing, fin spacing, physical size of coils etc) i would say it would work in reverse if you did some fancy ducting of the air flow. but my guess is one coil is designed for a much higher air flow being in the front of the car, than the air flow across the coil that lives in the car powered by an electric fan. u also have a physical space issue, the coil for the condenser in the front of a standard AC system is quite large (but thin) in surface area. The coil for the evaporator is significantly smaller but thicker. Are there surface area's the same? could be? (thermodynamics isnt my thing)


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## piotrsko (Dec 9, 2007)

used to be back in the DARK ages of my youth, you could buy JK thermocouple wire pairs and make your own thermo's by just twisting the two wires into a junction. JK junctions will heat and cool, {too old to remember the peltier effect theory} but you will have to make about 100 to 150 junctions to get a noticable effect. Wire can be had from an aircraft or racing supply place anymore.


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## TX_Dj (Jul 25, 2008)

Exactly piotrsko... 

I actually have a small surplus of 2" square thermoelectric transistor arrays and each module has probably 100 or so individual devices in the "sandwich".

They're great, if you have a way to pipe the heat off them, and aren't worried about energy consumption or overall efficiency.

I got them free from a guy who got a huge box full for a ridiculously low price because he went to auction and nobody knew what they were.

bblocher- the problem is that using any of these types of devices for cooling are at best 5-10% efficient vs. a gas refrigerant system w/ compressor (40-60%) so if a traditional AC is going to chew up your range, thermoelectric modules will chew up even more at even less BTU capacity.

The one thing they are great for, though (which I mentioned) is as a heater. You put the cold side out in the cold, and keep the hot side where you want things warm. You get heat from the electricity being put into the devices, plus you get additional heat (extracted from the cold side) due to the peltier effect. The amount additional is negligable, but measurable. I'm not exactly sure, however, what the efficiency in that configuration is compared to a ceramic heater element, but I've been told it is slightly more efficient. As such, I may end up building my heater from these modules if I can get enough heat from them at a reasonable power consumption, but I will not be using them for their cooling ability. (They put off far more heat than they provide in cooling.)


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## dimitri (May 16, 2008)

Is this device similar to what you are discussing?

http://www.surpluscenter.com/item.asp?UID=2008082714484124&item=1-2335-A&catname=electric

Seems reasonably small, light, inexpensive and only takes 3.5 Amps. I wonder how many you would need for a small car to get decent HVAC results.


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## bblocher (Jul 30, 2008)

Yeah I was hoping something like that could be used to do hvac more efficiently but I guess it's not very efficient.


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## TX_Dj (Jul 25, 2008)

same here.. that's why I pilfered some of these modules from my friend when he offered them to me years ago. Recently when I was struggling with ideas for AC, I thought I'd play with them again and read up on them... and that's when I found they were so inefficient as coolers... really burst my bubble, because at the time I got them, I thought they'd be the perfect solution, since you can reverse them and get your hot side cold and the cold side hot... what a letdown.


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## piotrsko (Dec 9, 2007)

actually: if you want to be kinda bizarre, extra heat on the motor could be fed into the hot side junctions for power output and then extract the cabin heat via the cold side being powered by the motor cooling side.

sounds almost like perpetual motion????


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## HDS (Aug 11, 2008)

Schmism
Thanks for the input. You are right. I forgot about the difference in air flow across the respective coils. It is a big factor in the equation.

Novell Concepts, LLC is a manufacturer of these units. They have one that boasts a temperature change of 50 degrees (C). It draws 15 amps at 38 VDC (as I recall). I emailed them to ask about condensation and icing up problems and they replied that gas a/c systems are far more efficient and that these units are not used for this application.


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## bblocher (Jul 30, 2008)

Good to know HDS, thanks. I guess that's why cars use them  It's not just because we like to waste gas apparently which was my initial thought


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