Recycling A/C water?

The other day I found out to my dismay that the balcony underneath my air conditioner at home leaks water into the living room below it. Instead of sealing the balcony (as I should…) I temporarily solved the problem by placing a baby bath tub underneath. Yep, I’m lazy.

Two days later, I noticed that the baby bath tub is almost full. So, I took it down and watered the lawn with it. It made me think whether the total water condensed in Israel’s A/C’s is sufficient to be of any real use. If you live in Houston, you probably noticed that the A/C’s condense a lot of water, however, since it rains profusely there, this water is not going to make any difference. Israel, on the other hand, is using all its available fresh water and has began desalination. Watering gardens with condensed water could help. The question is by how much.

Living near Jerusalem, I actually don’t need too much A/C, perhaps two weeks over the summer. Namely, 7 baby bathtubs worth of water, or about 150 kg, or 0.15 of a cubic meter. Since I pay my municipality about 1$ per cubic meter I could save about 15 cents by recycling the water over the summer. Not much, to say the least.

What about someone living along the coast, such as in Tel-Aviv? Over there, the high humidity implies two things. First, it is unpleasant all summer, so the A/C is constantly on for a few months. In addition, more water is condensed by the A/C per unit time, because of the higher water content. Let’s suppose that during the day, the typical outdoor conditions are 30°C with 70% humidity, while the indoor A/C conditions are about 23 and 60% humidity. Using a simple water content calculator, one finds that we’re cooling the temperature by 7 degs and taking out of it about 10 g of water per kg of air. Using the heat capacity of air, and the latent heat of water, implies that we need about (1kg)(7°K)(1000J kg^-1°K^-1) = 7,000J to cool a kg of air and 10gr*2272J/g=22,000J to condense the water in the kg of air. In other words, for every unit of energy used by the A/C, 25% are used to cool the air, and 75% to dehumidify it, at least, if you’re cooling fresh outdoor air. After the initial dehumidifcation of the closed area, the A/C would start spending more of its energy on just cooling. In desert areas, on the other hand, there is little water to begin with and a smaller fraction would be used for dehumidification also when cooling fresh outdoor air.

If you’re a private person in a humid place like Tel-Aviv or Houston, you would be operating an A/C. For every kWhr of electricity, you typically get 3.5 times as much “cooling energy” because of the large efficiency of A/C which are heat engines. Thus, 1kWhr, which would be used by a typical one room A/C of say 12000 BTU/hr, operating for an hr, would give you 36001000J. If half this energy is going to dehumidify the air, then you would condense about 0.53600*1000J/(2272 J/gr) = 800 gr of water. Of course, the ratio between the cost of electricity and water is different from place to place. In Israel, a kWhr would cost you somewhat more than 10 cents, while a cubic meter, costs about 1$. This implies that by recycling, you could save just less than 1$ per summer on A/C-ing, you would save a few bucks, enough to buy a simple dripper line. After a few years, you would save enough money to buy yourself a simple dinner.

What about the national level? Over summer, Israel spends about 150,000 kWhr per year on A/C (e.g., this document). If half the energy is used for dehumidification, Israel condenses 0.5 * 150,000,000 Whr * 3,600,000 J/Whr / (2272 J/gr) / (1,000,000 gr/m³) = 0.1 million m³ of water a year. For comparison, Israel annually uses 1,800 million m³, i.e., A/C condensed water is literally a drop in the ocean. Using condensed water on a large scale is simply not practical. Oh well, another idea down the drain…