Can an A/C system be designed to effectively use waste heat for other purposes?

[Loren Booda]

Artman, what you're referring to, I believe, is a comparatively inefficient electronic solid state device (which however has no moving parts). It puts out a lot of heat, from what I recall. If only it could be improved. Does anyone know the physics behind it?

 

[russ_watters]

I think that's a "Peltier" device. I have no idea about efficiency, but some people use them to cool computer cpus.

 

[Artman]

Originally posted by russ_watters
I think that's a "Peltier" device. I have no idea about efficiency, but some people use them to cool computer cpus.

Yes that was it. I'm not sure about the efficiency either.

 

[ray b]

I had a water pre-heater on my central A/C at my last house about 5 years ago
I put it in myself, with the help of an A/C teck to do the connections
it was a regular comercial unit that took the output freon from the compressor and extracted some heat before the freon ran thru the normal cooling finned coils and dumped the heated water into the home water heater
it was a small box, with a small water pump and coils for the water and A/c freon
mounted outside near the A/c unit

the freon hook up part was way eazyer then the water pipes routing under the house to this unit and back to the water heater

saving were minimal and hard to see as the new central A/C was the big incresse in power use but it allso lowered the heat load on the a/c unit

 

[zoobyshoe]

ray b,

It would have been nice if you
could have gotten about a years
worth of bills without the water
preheater in use to compare with
a years worth of bills after you
got it hooked up. Then we could
have a more concrete idea of what
you were actually saving by using
the AC unit to preheat water.

When I was thinking about this I
was working off the assumption
there would be a way to use the
water pressure itself to circulate
it through the system. Adding a
pump cuts down on the benefits.

-Zooby

 

[Artman]

What those of you who think that there can be no benefit in using waste heat from AC units are failing to account for is when the system that benefits is required to run anyway. Then the work can be eliminated from the equation to find the savings.

Take my balanced water source heat pump system, water is already being circulated to both the units requiring heat and those required to cool. The savings is found in not using a boiler to add heat to the loop or not using a cooling tower to remove heat from the loop.

The loop temperature raises slightly leaving the cooling units, but lowers again after leaving the heating units, so the cooling units are not placing their heat into a hotter heat sink, they are seeing the same cooler temperature as before.

Also, the work losses can actually be reduced by bypassing the cooling tower and the boiler to eliminate head losses in the piping through these.

So, where are the increased losses you are referring to?

Remember the cost to move the heat from one unit to the other in this system already exists, the water is moving constantly. You need to compare this to two separate systems one doing heating and one doing cooling. Then you need to add the boiler losses and the cooling tower losses, including pumping the water through these units, then you can see your savings.

 

[russ_watters]

Originally posted by Artman
...cooling tower...
So, where are the increased losses you are referring to?

Cooling tower? What cooling tower? The system as I understood someone had actually done was for a home and it simply piped the water to the water heater via a heat exchanger in the air conditioner's condenser coils. The problem is that the water isn't always circulating. So if its not circulating its not pulling away heat and you are left with a somewhat more cumbersome and therefore less efficient air cooled heatsink.

In a large building using water source heat pumps and a cooling tower (your example), you might be able to take your hot water from the loop before it gets to the cooling tower. In that case you both gain effiency in the air conditioner and lower the load on the water heater.

 

[Artman]

Originally posted by russ_watters
Cooling tower? What cooling tower? The system as I understood someone had actually done was for a home and it simply piped the water to the water heater via a heat exchanger in the air conditioner's condenser coils. The problem is that the water isn't always circulating. So if its not circulating its not pulling away heat and you are left with a somewhat more cumbersome and therefore less efficient air cooled heatsink.

Correct. However, one of the best uses of this technology is in Geothermal systems where the ground absorbs the heat. A preheater on the domestic water reduces the amount of heat required to be absorbed and dispersed by the ground. During the cooling season most of a residence's domestic water can be heated using this waste heat.

Originally posted by russ_watters

In a large building using water source heat pumps and a cooling tower (your example), you might be able to take your hot water from the loop before it gets to the cooling tower. In that case you both gain effiency in the air conditioner and lower the load on the water heater.

My example makes up heat lost at the perimeter of the building by heat gained from the interior spaces, to provide a balanced loop temperature. Three-way control valves can be employed to bypass the cooling tower or boiler during balanced operation. You can also add water-to-water heat pumps to heat water off this loop, if desired. The water temperature in this piping doesn't get very high so a heat exchanger will not provide much benefit as a domestic water pre-heater.

 

[wimms]

Doesn't AC efficiency depend mainly on how hot the compressed fluid becomes in exchanger, the temp diff defining how fast heat exchange happens? If the exchanger is cooled 'too well', then compressed fluid cannot reach optimal temperature for fastest heat exchange. This means that AC looses its efficiency. I'm not sure how compressor 'feels' fluid that doesn't get hot - will it behave as uncompressable/unrarifiable, simply circulating? AC's don't work very well in cold, as I understand it, because efficiency of heat transfer drops.

Unless AC is designed to heat water by exchanger, it seems adding such feature interferes with AC functioning instead of using its heat for good. It would pump less heat.
If that's not issue, then AC is simple heat pump, pumping heat from one place to another, no differently than water is pumped around in house.

 

[Artman]

Originally posted by wimms
Doesn't AC efficiency depend mainly on how hot the compressed fluid becomes in exchanger, the temp diff defining how fast heat exchange happens? If the exchanger is cooled 'too well', then compressed fluid cannot reach optimal temperature for fastest heat exchange. This means that AC looses its efficiency. I'm not sure how compressor 'feels' fluid that doesn't get hot - will it behave as uncompressable/unrarifiable, simply circulating? AC's don't work very well in cold, as I understand it, because efficiency of heat transfer drops.

Unless AC is designed to heat water by exchanger, it seems adding such feature interferes with AC functioning instead of using its heat for good. It would pump less heat.
If that's not issue, then AC is simple heat pump, pumping heat from one place to another, no differently than water is pumped around in house.

The shorter the lift (difference between source and sink) the higher the possible efficiency. Air-to-air have a high lift and poor efficiency. Water-to-air have lower lift and greater efficiency.

There are other concerns with running an AC unit in cold temperatures: flooded condenser and evaporator coils, short cycling compressor, possiblity of liquid forming in the compressor, which could damage it. Vapor freezing on the outdoor coils (not as big a concern with AC as with Heat pump, but heat pumps are usually designed to compensate for it).

 

[russ_watters]

Originally posted by wimms
Doesn't AC efficiency depend mainly on...

I think you're talking about the efficiency of the heat exchanger itself, not the entire air conditioner. The efficiency goes down as the delta-T goes up but the overall quantity of heat exchanged still goes up as well. At some point though you do start messing with the cycle so there are limits as Artman pointed out.
HERE is a condensing unit. Page 3 lists capacity vs delta-T. Interestingly, the capacity:delta-T ratio is constant over the whole range, but it only goes up to 30F delta-T. It could be it doesn't start to lose efficiency until its more than that.

 

[ray b]

the pump was there to move the water slowly out if it reached to high a temp when the hot water is not flowing, or more of a safty then a allways running device

the A/C teck thought the extra cooling of the freon by water would improve the efficiancy by lowring the heat load on the air cooling coils and so both cooling more and lessining the runing time of the unit
in addition to pre-heating the water

 

[wimms]

Originally posted by russ_watters
I think you're talking about the efficiency of the heat exchanger itself, not the entire air conditioner. The efficiency goes down as the delta-T goes up but the overall quantity of heat exchanged still goes up as well. At some point though you do start messing with the cycle so there are limits as Artman pointed out.

Yes, but I also thought that efficiency of heat exchanger is main limiter of AC efficiency. Heat cannot be forced to flow, its freely flowing. The only thing we can do is create temp difference.

I assume you meant that creating higher temp difference consumes more energy, thus efficiency goes down. But lower temp difference means slower heat transfer, and same amount of heat transfer needs more work time from AC compressors. As this is really the only forced action, this is also the only point of losses, which seems to imply more losses for same amount of heat transferred.

Given same AC system, how many KWh of energy is spent to pump same amount of KWh of heat for cases where delta-T is high and delta-T is low? I assume that for high-delta AC would need to apply more energy for shorter time, and low-delta AC would need to apply less energy, but for longer time.

Heat transfer from exchanger to air is limited as air is good insulator. So I guess delta-T at exchanger contact with air is quite low. Cooling exchanger with water increases delta-T at this point and helps increase heat transfer. This is positive side. I just wondered what effect could it have if freon in exchanger wouldn't have chance to rise to its normal temp (and delta-T) due to better heat conduction with water, what kind of change in working cycle of AC would this make?

 

[Artman]

Originally posted by wimms
Yes, but I also thought that efficiency of heat exchanger is main limiter of AC efficiency. Heat cannot be forced to flow, its freely flowing. The only thing we can do is create temp difference.

I assume you meant that creating higher temp difference consumes more energy, thus efficiency goes down. But lower temp difference means slower heat transfer, and same amount of heat transfer needs more work time from AC compressors. As this is really the only forced action, this is also the only point of losses, which seems to imply more losses for same amount of heat transferred.

Given same AC system, how many KWh of energy is spent to pump same amount of KWh of heat for cases where delta-T is high and delta-T is low? I assume that for high-delta AC would need to apply more energy for shorter time, and low-delta AC would need to apply less energy, but for longer time.

Heat transfer from exchanger to air is limited as air is good insulator. So I guess delta-T at exchanger contact with air is quite low. Cooling exchanger with water increases delta-T at this point and helps increase heat transfer. This is positive side. I just wondered what effect could it have if freon in exchanger wouldn't have chance to rise to its normal temp (and delta-T) due to better heat conduction with water, what kind of change in working cycle of AC would this make?

The unit can't tell if the high pressure refrigerant gas is liquified by the condenser coil or a domestic water pre-heater, so why not use the pre-heater and put to use the waste heat?

The main concern is the pressure in the upstream condenser (the condenser after the pre-heater). As long is this remains within acceptable limits then there is no problem with using this waste heat. Mutiport refrigerant valves can be used to insure that the pressure limits of the condenser are within the safety factor of the system.

The initial stages of the condensing cycle are the best for heat recovery, this is where the largest amount of heat is wasted. The final stage of the cycle, after the refrigerant turns to liquid the subcooling of the liquid, is the poorest cycle for heat recovery and can be given to the condenser for heat rejection.