congo40
There is a recent thread on this issue titled "Air Cooled Chillers vs Water cooled Chillers". You should read that. I am not sure where
imok2 got his figures (you should give us a link to this study) since it all depends on the refrigerant, suction pressure, suction superheat, discharge pressure, liquid subcooling, and compressor design. If you follow the refrigeration cycle on a mollier diagram then you know that it doesn't specify whether the system is air-cooled or water-cooled, only that the temperature at which the system rejects the heat. I AGREE THAT IN MOST HOT CLIMATES THAT THE TEMPERATURE OF THE WATER IS COOLER THAN THE AIR. We also must be clear by what we mean by water-cooled. I believe that both
congo40 and
imok2 mean water cooled systems where the water is recirculated and cooled by evaporative cooling. There are also one pass through water-cooled systems (sometimes used where there is a large source of surface water such as a river, lake, or ocean).
There is a lot more to achieving energy efficiency than lowering the temperature of the heat sink. Most (if not all) residential and commercial air conditioners are DX systems which use a thermostatic expansion valve. These TEVs require a minimum pressure drop across the valve in order for the valve to function properly. If the condensing pressure is too low then the unit will not function properly. In fact many air conditioning units artificially hold the condensing pressure high with features such as hot gas bypass (very energy inefficient), condenser flooding, fan cycling, or other methods. There is no use in lowering the heat sink temperature if they system cannot take advantage of the lower temperature. There are balanced port TEVs and electronic TEVs which can minimize this problem but they are not widely used.
In some areas of the country there is not much difference between the wet-bulb temperature and the dry-bulb temperature so there is not much advantage to evaporative cooled water condensing units. The other big issue is the cost of treating the water and the cost of pumping the water (both in dollars and in energy). A ground source heat pump (which uses the ground as a heat sink and does not evaporate any water) at leasts eliminates the treatment of the water and the use of water. The water needs to be pumped. Water usage in an evaporative cooled condenser is about twice the amount evaporated (you need to bleed the same amount as has been evaporated in order to prevent buildup of dissolved solids in the system). I have never calculated it based upon the rate of water flow through the system.
I don't believe that water usage is the main problem with water-cooled residential systems. The main problem is capital cost and maintenance. If you really want to save energy in places where water-cooled systems give the most advantage (like Las Vegas) then you should just use an evaporative cooler or use an evaporative pre-cooled air conditioner. They work well, in fact my house has both an evaporative cooler and an air conditioner.
If you want to actually calculate how much water (evaporative) cooled air conditioners will save you compared to air-cooled systems you can download the Carlyle (a division of Carrier) compressor rating software at
In this software there is a BIN analyzer which allows you to specify the degree of approach of the condensing temperature to the heat sink temperature. The heat sink is selectable as either dry-bulb (air) temperature or the wet-bulb temperature. You also put in the minimum condensing temperature (to take care of the problem of the TEV minimum condensing temperature). Select the location (or the location with a similar climate) and the system will go through the weather data hour by hour and calculate the energy used. The compressors that they have are more small commercial compressors but the COP's or EER's are basically the same as residential. After you do this then you need to add the additional water pumping energy, water treatment energy, and water usage energy to the energy used by a water-cooled air conditioner.
Don't get me wrong, I am a big believer in evaporative condensers (for industrial systems) and cooling towers (for large building systems) but there actually are some reasons why air-cooled systems are used. It is hard to get small and medium commercial air conditioning users to go to water-cooled systems let alone a residential user. Remember that whenever I have mentioned "water-cooled" in the above I actually mean "recycled evaporative cooled water for the condenser".