Air conditioning in cars

[Blue Peter]

Due to conjunctivitis, I heard a brief snippet of "Simon Mayo" on radio 5 yesterday, which dealt with the subject of energy prices in the UK and air conditioning. There was a couple of quotes, which I thought that I'd check with the collective energy consciuousness here:

1. Using a/c in a car uses less extra fuel (around 10%) than driving with the windows open;

2. Not transport related, but the spokesman for HEVAC (a/c lobby) said that a/c used extra energy in the summer, but in the winter it could be used as a heating system and was far more efficient than other options, presumably gas or electric boilers.


Neither seemed intuitively obvious to me. Anyone like to comment?


Peter.

[enso]

Using a/c in a car uses less extra fuel (around 10%) than driving with the windows open;

No straightforward answer to this apparently.

http://www.newscientist.com/article/mg18925432.200.html

[Blue Peter]

Using a/c in a car uses less extra fuel (around 10%) than driving with the windows open;

No straightforward answer to this apparently.

http://www.newscientist.com/article/mg18925432.200.html

Cheers,


Peter.

[mikepepler]

1. Using a/c in a car uses less extra fuel (around 10%) than driving with the windows open;

I've heard that when moving at low speeds (<30mph?) it's more efficient to have the windows down, but at motorway speeds it's better to have the aircon on.

However, my car doesn't have aircon, as I view it as just another thing to go wrong, and the fact is (from past experience) if it's there you'll use it, no matter when it might be more or less efficient to. For example, it's great at demisting the windows in winter or wet weather, so you end up having it one when it's cold too...

[bigjim]

My car's got aircon. It was in the car when I bought it. I hardly ever use it- I prefer winding the windows down.

[snow hope]

My car has air-con too. I rarely use it as I know it reduces my mpg, but when it is very hot, it is brilliant!

[Max]

Air conditioning (or any cooling device like a fridge) can act as a cooling device or a heating device - depending upon which "end" is considered the useful end.

Ever noticed how a fridge belches out heat from behind - well thats the energy it is removing from the cold inside. Air conditioning units remove heat from the inside of a building and emit it outdoors, if an air0-con unit was installed back to front (not quite literally) it would heat the building interior and cool the outside environment. Ground heat-pumps work on this principle, they are effectively removing heat from the ground (like a fridge removes heat from your food) and dumping it in your house.

Heat pumps are supposedly more efficient that regular heating systems because of the relationships between heat transfer coefficients and temperature (I'd need to recheck my thermodynamics!!)

on a side - moving your fridge away from the cooker, away from underneath counter tops and keeping a space clear behind it will increase it effeciency and reduce your power bill.