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Micro Heat Pumps, DIY  RSS feed

 
Marcos Buenijo
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See how easy it is to convert a small window a/c unit for water heating using the condenser: http://www.youtube.com/watch?v=GBlrewwpt8M. I considered this idea a while back. I'm glad I stumbled on this video as it's great to get a visual with videos like this. Now, note the energy recovered at 7500 btu/hour! This represents the energy removed from the air at the evaporator and most of the energy consumed by the compressor motor. That's a lot of energy at more than 2 KWh per hour (i.e. 2 KW for a roughly 500 watt compressor motor). This unit would have to be operated for only a few hundred hours to fully pay for itself with the electricity savings alone. Kinda seems absurd to consume energy in operating an a/c system while also consuming energy to heat water, doesn't it?

It should be possible to enclose the condenser with an insulated shroud, then pump water slowly through for heating. Also, while the evaporator doesn't look quite as accessible as the condenser, it may be possible to do the same for the evaporator. This latter configuration might make it practical to send solar heated water to the evaporator, and this can allow the unit to take the condenser temperature a little higher for efficient space heating during winter. Since solar heated water can be stored fairly easily at relatively low temperatures, then this might yield good results. Note that this seems to make sense only for quite cold regions where heat pumps don't do well due to the high temperature differentials. What I'm thinking is a water/glycol solution might be heated to a warm temperature that may not be useful for direct space heating, but can be used efficiently by a heat pump. Not having to take the fluid temperatures high with the solar system will reduce thermal losses a great deal, and this should allow a DIY solar water heating system to work well. However, for regions with very mild winters it makes a lot more sense to just install a window a/c unit in reverse for space heating.

Also note that water cooling the unit would allow the "window" unit to NOT have to be mounted in a window. This is great since these are rather unsightly.
 
Marcos Buenijo
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This is a continuation of the previous post. Note that since the fan is removed on the fan motor shaft on the water cooled side (the motor has a shaft penetrating both ends, one for the condenser fan and one for the evaporator fan), then perhaps it can be practical to install a small pulley on the freed shaft for driving a water pump. The cover for the unit might be replaced and one of the heat exchangers (condenser or evaporator) could be enclosed in an insulated enclosure that can be mounted to the outside of the unit. I was thinking that even without the heat recovery, then perhaps this kind of conversion would be worthwhile by allowing the unit more versatility with respect to mounting. The pump might also circulate water that is cooled (or heated) with a nearby water source or other ground source heat sink (i.e. geothermal). Basically, there are more options when water is the heat transfer medium, and installing a water pump integral with the unit could make for a practical system. Also, I believe water cooling the condenser, even assuming the water pump has the same work load as the fan it replaced, would increase efficiency (even w/o heat recovery).
 
Marcos Buenijo
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(COOLING) This is just another discussion of a means to devise a heat pump for cooling an off grid home. I have yet to see this done or discussed elsewhere, but it seems reasonable to me that a photovoltaic array can be used to power an automotive a/c compressor by using a dc motor. The system can be configured to freeze a mass of water (at least partially) for providing a chilled water system. I see many advantages to this configuration. For example, one would not have to worry about the massive current draw (especially starting current) on such a motor because it can be powered directly by a battery system vs. a large inverter. This would allow a modest battery system to be used since the system is operated only while the PV array is putting out, so little battery storage is required. BTW, what I considered was using "small" dc motors often used for EV conversions. Many of these are reasonably priced at a few hundred dollars and operate at 48 volts. Many also have a continuous rating compatible with common automotive a/c compressors, most of which can handle 3-5 hp at 2000-3000 rpm. The system could be configured to power the compressor at a rate roughly proportional to the output of the PV array to minimize battery discharge and charge rates. The simplest way to do this that I've considered is to start the motor with a continuous duty solenoid switch using a control circuit that prevents operation when battery voltage drops below a certain value (various ways to do this).

(HEATING) Example of a large DIY solar water heating system:
. Something like this might be used to provide heated water for use in air heating using the same "chilled" water system described in the paragraph above. The additional electricity provided by a large PV array required to power a chilled water system could be used during the winter months to power electric space heaters. Furthermore, a simple biomass furnace could be used to heat the water store via a thermosiphon heat exchange system when solar is insufficient (redundancy is nice). A simple rocket furnace could work well: http://www.youtube.com/watch?v=5IRLVCJ1olA .

Specs on common automotive a/c compressors: http://sanden.com/index.php?tag=U5H0AMX7Q
A source for dc motors and controllers: http://www.electricmotorsport.com/
(one of many sources)

 
Marcos Buenijo
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It's interesting to consider how a window a/c unit might be configured for different uses. Consider placing a window a/c unit in a window backwards for heating a room. Now, enclose part of the exterior wall of the home that includes the window in what is essentially a green house (or trombe wall). Place the unit on a thermostat to ensure it operates only when the air temperature is high enough for efficient operation. Heat pumps are not normally a good idea for cold climates, but enough solar gain here should make for efficient operation. If someone is already heating with electricity, then this approach seems reasonable.



 
William Bronson
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Found this thread while googling .

I often wait for water to come up to the tempature I desire,and waste the cooler water down the drain.
I started out noodling about a small (less than 3 gpm)electric point of use water heater as my only water heater.
We wash clothes in cold water only and our fixtures are low flow.

I thought we might be able to run it off of a small photovoltaic array,and looking into that,found a suggestion that this could be more effective than thermal solar water heating

The trick was using the PV to power a heat pump water heater,which are pricey.
This led me to DIY solutions, which led me here.
Somehow I'm not surprised.
😏
If one repurpused a RV air conditioner , it would presumably be DC to start with.
This is cool,but isn't solving my water wasting issue.

 
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