Solar powered air conditioning
If you are trying to use the heat from the sun, the more you can concentrate it the more useful it is, the problem is that optically the more you concentrate the sun the more directional it is and the more accurately you have to point it at the sun. This means that you have to keep pointing your concentrator at the sun, which means lots of complicated motors, which is a bit of a pain, but the real problem comes when it is cloudy, because the clouds take the light which was originally all coming from the sun and bounce it around so it is coming from every direction this means that the more you concentrate the light the less of the sky you can collect light from.
Roland Winston at the University of California, Merced has been optimising this problem without trying to form an image, just get as much light as possible concentrated. For a start the sun is never below your feet so you don't need to collect any light from there which means that you can concentrate even diffuse light by a factor of two. Plus by putting the collector in something with a high refractive index, because light moves towards the normal as it enters it appears to be coming from a smaller range of directions, you can actually concentrate it by a factor in total of 4-4.5.
This enables them to heat water up to about 200 Celsius, even on a cloudy day, which is very useful for heating your house. But now the team have gone a step further and used this heat to drive an adsorption-based air conditioning system, which works by evaporating water to cool the air. The water is then absorbed into a material with a high surface area. The material is then dried using the heat from the sun, and the process is repeated.
The researchers are also investigating attaching their optics to solar cells, reducing the area of photovoltaic material needed by a factor of 4 and helping to cut the cost and weight.