Saturday, 30 January 2016

Lesson 13: How do air conditioners work?

I made a bit of a miscalculation. There were three lessons left and three student presentations, one of which was on low-energy buildings in hot climates, and another on generating your own power. I had planned to do a lesson on cooling, and had some leftover material on solar electricity, and should really have had these presentations on three different days, then filled in the rest of the class with my information, assuming that the students had not covered it in their presentations. Or if they had, then continue the lesson with discussions on the relevant topics. Unfortunately they both ended up in week 14, so week 13 became empty, and the only material ready for it was the left-overs on cooling and solar electricity. 

If I'd been doing a full lesson on cooling, I would have started with a brainstorm of different ways to stay cool, with my non-exclusive list ready in the wings: windows, insulation, thermal mass, trees, heat-exchange ventilation, fans, air conditioners, de-humidifiers, and ice—preferably large blocks. Another thing that was not on my explicit list, which probably should be, was shading. 

Not wanting to take away too many options for the following week's presentation, and keen to teach some science, I skipped this and went straight into an exposition of the workings of air conditioners. 

Before getting into the details, we needed to understand four concepts relating to gases: volume, pressure, temperature and heat. These are all interrelated. Other things being equal, if a fixed amount of gas is in a smaller volume, it will have a higher pressure. Other things being equal, if the amount of heat in a fixed amount of gas goes up, the temperature will rise. If you don't add any heat to some gas, but squash it in to a smaller volume, then the amount of heat won't change, but the temperature will go up. And if you expand it, the temperature goes down. 

A heat pump sends a fluid in a circuit through a hot area and then a cold area. The Fluid is compressed as it goes into the hot area, which will increase the temperature and allow it to transfer heat to the hotter area. It is then allowed to expand when it goes into the colder area so the temperature will drop and heat well flow from the cold area into the fluid. 

That's the Carnot cycle. Heat pumps are basically trying to defy the second law of thermodynamics, by getting heat from a colder place to a hotter place. We use them in our fridges and air conditioners, and increasingly they are used for space heating and water heating. 

The coefficient of performance is used to measure the efficiency of a heat pump, and it measures the amount of heat that is transferred divided by the amount of energy that goes in. Typical domestic heat pumps have average COPs of 3 to 5, but precise numbers are very difficult to find. 

There are limits to the COP, and as the temperature difference goes up, the COP will go down. If a heat pump is used for generating hot water in the winter, using cheap night-time electricity, the COP can get very low, and the heat pump is not performing much better than an electrical heating element.