Waste Heat Conversion

A little more information about your fridges would be helpful. In some instances, refrigerant-to-air-to-water maybe the way to go because getting water in intimate contact with the refrigerant can be challenging because of packaging issues. If you have the ability to 'break into' your refrigerant loops, then a brazed plate heat exchanger is one way get excellent heat transfer from the refrigerant to the water.

http://www.globalspec.com/search/all?query=brazed+plate+heat+exchanger&newSearch=new

Tube and shell is another approach.

http://www.globalspec.com/search/all?query=tube+and+shell+heat+exchanger&newSearch=new

If the pre-heating of water will be for personal consumption then local codes may dictate a double-walled solution.

You can also look up desuperheater

http://standards.globalspec.com/standards/detail?docid=1599149&familyid=UKSGHAAAAAAAAAAA

https://www.google.com/?gws_rd=ssl#q=desuperheater

Good luck with your quest.

Thank you.

I had one of these systems

http://techcrunch.com/2011/06/12/air-conditioning-water-heater/

installed when I lived in Florida. It provided essentially 'free' hot water year-round by using the waste heat from my heat-pump HVAC system.

In northern cities a geothermal heat-pump would be needed for wintertime heating, but I imagine this type of waste heat system could still be used most of the year.

Try converting or modifying your condenser using the below(shell & tube), this will be a convenience, have someone to compute the right size specs for you --

Thx for the modification drawing. I think i am going for the solar conversion.

Epic fail. Review ideal carnot cycle, the reverse carnot cycle, you see cop will drastically improve minimizing the difference between hot and cold reservoir. Practical example. Take for instance your car air conditioning unit, when do you think is it operating smoothly? Night time or noon time?

I agree with Solar Eagle that this is not a good idea. If you are indeed wanting to pre-heat water, presumably above ambient temperature, you will increase the condensing temperature (and pressure). This will a) increase the power consumed by the compressor, and b) decrease the cooling duty of the unit. Depending on your alternate source of energy used to heat water, the first result might be advantageous. But, since you stated that your capacity is already limited, the second result would worsen the problem.

On the other hand, if you have a good supply of cool (below ambient temperature) water which could be used to reduce the condensing temperature, then that would reduce your power consumption and increase the cooling duty. However, if your fridges are ordinary domestic refrigerators, I don't think this is very viable. Your best bet would be to, as others have mentioned, keep the condensers clean, the evaporators defrosted, air circulating around the units, gaskets sealed, and minimize the opening of refrigerator doors (frequency and duration).

They are all domestic fridges. You mentioned i should keep air ciculating around the units. Could i then use an extractor fan to remove all hot air ( and have that air go over the pipes / through the water, i want to pre heat ) and a polystyrene product to seal around and between all fridges so only the hot air goes out, not the (air conditioned) air in the room?

Based on your statement that your problem is worse in the summer, I was under the impression that your refrigerators were in un-conditioned space. If you exhaust the hot air, it has to be made up from somewhere. If you make up from outside, you will have a higher temperature than you have now. As Rixter has said, you would get better performance with air conditioning capable of maintaining the desired room temperature.

There is another system which has not been discussed - heat pump or hybrid water heaters. These use a vapor-compression cycle to pump heat from the room air into water in the heater tank. They have the side effect of cooling the room air. But they only heat the water to about 120 - 135 degF. And they are expensive. Without knowing what your alternative fuel (to electricity) is and the relative cost, and your hot water needs, it's impossible to select the optimum system.

In addition to cleaning dust away from condenser coils and adding fans to circulate air, you might try adding air conditioning to the room where the fridges are located to lower the ambient air temperature.

I suspect that any attempt to capture the heat will be counterproductive. It would be difficult to make the condenser coils more efficient without a complete redesign.

The heat exchanger tubing should be copper so that electrolysis is minimized and heat exchange is maximized.

Using a heat exchanger on the high pressure side of the compressor to supplement heating water is possible however the amount of refrigerant is small and will severely limit the process efficiency.

Adding a heat exchanger on the return (low pressure) side to supplement cooling of the area will disrupt the operation of the compressor and may cause poor operation of the compressor and/or failure.

As far as running the temperature setting of the refrigerator at a lower setting when the ambient temperature is high during summer months:

If the refrigerator cannot maintain whatever setting is stamped on the dial then lowering the temperature setting will not make the machine reach a lower temperature.

In other words the temperature setting should be set and never tampered with summer or winter.

By lowering the temperature setting to an unattainable temperature the frost line will most likely travel back from the evaporator to the compressor which will cause worse compressor performance and will not lower the temperature.

Adding cooling on the high pressure discharge piping will somewhat improve the situation but it is not a long term solution.

As others have already stated;

The condenser(s) need to be kept clean and adding a forced air fan to cool the condenser(s) would be the most cost effective method of improving the cooling capacity.

Lowering the area (room) ambient temperature with HVAC will also improve the refrigerator performance.

There are "severe duty" refrigerators available in the marketplace but they are rather expensive. You must know the ambient temperature range in order to select a properly designed unit.

How are the seals on the doors to the refrigerators? When the doors are closed can you feel cool air near the edge of the door?

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As others have stated, clean the coils and fans. Make sure the path for airflow is as unobstructed as possible. Also look at the interior surfaces, make sure cooling surfaces are clean, are not blocked, and are not covered in ice.

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How do the bodies of the refrigerators look? Is there any indication of damage to the shell or to the underlying insulation? Could the insulation have become soaked with water or other liquids?

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How often are the refrigerator doors opened? How long do they stay open when they are opened? How full are the refrigerators? Are hot items regularly placed in the refrigerator before being allowed to cool? Is the building infested with one of more teenagers?

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The answers to the above questions may help identify the reason for the poor refrigerator performance. If the problem can be corrected or mitigated, not only will you refrigerator work better, but less heat will be introduced into your kitchen.