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Cooling Against the Night Sky

The world is buying airconditioners (also called HVAC), because the climate heat is being turned up by the use of fossil fuels (and airconditioners). We attended a congress on solar cooling a couple of years back and learned that european power plants are build expeciting the load of airconditioning. This is of course a large cashflow generator for banks. If that is the case you always need to wonder if the energy efficiency is optimal.

Efficiency of airconditioners is dimensioned in COP, or Coefficient of Performance. Airco’s are pumps, they move heat from one side (the hot one) of the system to the other (the cooler one) thereby cooling the hot side. They can also do the opposite, so take heat from a cold side, move it to the hot side, these are the new heatpumps used to heat homes. The COP can be 2 to 4. If its 2 you spend 1 kWh to get 2 kWh wordth (heat or cold) out, or if its 4 you spend 1 kWh to get 4 kWh (heat or cold) out.

Airco’s in the sun..

If you run an airco in 54 Celsius heat in Quatar you are really pushing the envelope, its COP will be very low or negative, simply because you are asking it to take heat from inside (cooling it) and moving it to the hot outside. To do that your radiator outside must be hotter than the outside, say 60 degrees, so that the heat can flow from 60 Celsius metal to 54 Celsius air. It never flows the other way.

How can you improve this situation? Its not difficult to imagine, it is just not selling fossil fuels (it makes no economic sense). Airco’s are sold to generate cashflow, to sell energy. That is the reason why we focus on the COP, not on the overal energy efficiency of the system. This is true for heatpumps as well as for airconditioners (if you choose different names for heating and cooling heatpumps). How is that? The answer is : Because of the possibililty to gain cooling or heating from the environment and thus start pumping from a much better heat.

In the case of the airco, they run during the day, they are usually exposed to the sun. Providing shading and the ability to radiate to a clear sky to your airco alone drops the temperature of the radiator 5 degrees, which means it can now drop the heat from inside in a cooler environement, this raises the COP instantly. Now what would happen if you run your airco at night? Not much use because you are not in the office then, but what if you cool water at night. Then the COP would be much better, because the airco cools against the night sky and air, which is much cooler than that of the sunny day. Then if you use that reservoir of cool water during the day to cool against, your COP will again be higher. We think this could save a lot of energy because in dry desert regions the nights can be stone cold.

Storing night cold in a water reservoir to help your airco cool during the day. Maybe you don’t need an airco at all!

When looking at heat pumps you can say the same. For the sake of ease of istallation they are used simply to heat ground water temperature water to whatever heat is needed, this is a bit moronic. I asked once what the max temp would be (and this varies from system to system) but it was 25 Celsius. So if you start on the ‘cold’ side of the heatpump with 25 Celsius, your heatpump has to do way less work to get to the temperature you need. Now it may seem futile but again a reservoir of water could help out, if during winter you have some solar collectors on your roof you can collect heat all day, and then use it to bump up the performance of your heat pump. No installer will talk about it, in general they hate solar thermal panels because they reduce energy consumption. No surprise there.