Air Cooled or Water Cooled? An Owner-Driven Choice. Part 4.
We left off at the end of part three by stating that the extra maintenance work can be worthwhile with water cooled systems. How worthwhile?
A general rule of thumb is that the typical water cooled system is 20-40% more efficient than the typical air cooled system. That is true as far as it goes, and the wide variance between 20% and 40% covers lots of details that can impact the difference in your specific situation, such as operating schedule and the size and type of air conditioning system. But just looking at the efficiency difference misses an important point. The difference in power consumption is least at night (when it is cool outside and air cooled equipment works the best) and is greatest in the afternoon, when it is hottest outside. Of course, the afternoon is also when our electric utilities are straining to provide enough power for all of their customers. That is why utilities charge extra penalties, referred to as “Demand Charges”, for power that is consumed during their peak demand period. Water cooled systems tend to have significantly smaller demand charges than air cooled systems. So not only do water cooled systems save you money in terms of energy use; they also save you money in terms of demand charges. All together, it adds up to a lot of money. More than enough to pay for the additional maintenance that is required and still save a lot of money every year.
Early in this series I mentioned that water cooled systems were hard to justify financially in buildings with under 100 tons of cooling, but that they were almost an automatic decision over 500 tons. In between, some analysis is a good idea. In small systems the first cost premium takes a while to overcome. Oddly enough, water cost is so low compared to electricity that water cost seldom enters into the decision in a significant way. How long the California Energy Code continues to push efficiencies higher (and thereby increases the use of water cooled systems) will be interesting to follow if water remains in short supply. One thing seems certain, though. The drive for higher energy efficiency means that we will be seeing more, not less, water cooled systems in the future.
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