It’s amazing how many types of air-conditioning or cooling systems exist.
The one most of us know is compression-based: A liquid with a low boiling point is allowed to evaporate, absorbing heat from the room or building. The gas is then taken to an outdoor unit, where it’s compressed back to a liquid. This releases the heat that was absorbed, which is then vented to the atmosphere. This takes up a lot of energy.
A better option is solar air-conditioning, which uses the heat from the sun to cool the house. As before, a liquid with a low boiling point is allowed to evaporate, cooling the room. The gas is removed by dissolving it in a second liquid, allowing more of the refrigerant to evaporate. Finally, the dissolved solution is separated by subjecting it to the heat of the sun. This consumes far less energy, and works more effectively in hot places, which are the ones that need the most air-conditioning.
A third option is evaporative cooling. Here, water is allowed to evaporate, which cools the room. The water vapour is eventually vented into the atmosphere . This is similar to old-fashioned air coolers, but can also be installed in a centralised system, and costs far less than an air-conditioner, while consuming far less energy, but producing only a limited amount of cooling . But evaporative cooling can be used to increase the efficiency of another type of air-conditioner, making it less power-hungry. Alternatively, it would be an option for people who can’t afford anything better — let them at least have some comfort. And there are probably billions of such people in the world.
A fourth option uses water, but in a different way. Instead of evaporating it, it’s merely circulated through a heat exchanger to remove heat from the building, and dumped back in the lake or sea it came from. This makes sense for a couple of reasons: first, water in a lake or sea is almost always colder than the temperature on land. Here’s a map of the surface temperature of the sea:
In India, for example, where it frequently reaches 40 C in the summer, note that the water is only around 30 C or cooler. And this is at the surface. If you go deeper, the water gets even colder. This will again make a huge difference in comfort levels.
A fifth method of air-conditioning relies on the fact that while air temperature fluctuates with the seasons, the temperature underground varies less, and is therefore cooler than the surface in summer. So, air, water, or another fluid is circulated to cool the house. This is called geothermal cooling.
It’s amazing that there are so many alternatives to conventional, power-hungry air-conditioning: solar air-conditioning, evaporative air-conditioning, seawater-based air-conditioning, and geothermal air-conditioning. No matter where you live — hot, temperate or cold, and humid or not — at least one of these technologies should work, saving energy and money. Now we just need to deploy these systems widely, and multiple of them, so that the best option can be used in each place, and to drive a virtuous cycle of competition that will hopefully improve all these systems further.
 This consumes water, while saving energy. Besides, every unit of power generated at a power plant consumes a certain amount of water, so this may not be as bad a tradeoff as it seems. Sure, water is scarce, but so is energy. Market pricing for both energy and water is needed here, so that people will make the most environmentally friendly decision on what the right tradeoff is.
 Except in very dry places.