(simplified explanation, read a better one here) We even found a site with plans for a build-your-own unit (shown above with the hot end, an old propane cylinder, cooking on a coleman stove). Not being able to open envelopes without suffering injury we note the warning: It needs to be emphasized that ammonia gas is extremely toxic and flammable. One deep breath of pure ammonia gas will KILL a human being. If you are not a mechanically inclined person who pays close attention to details, do not attempt to build one of these units. That counts us out, but that's why we are an architect, not a carpenter. It is a short leap from fridge to AC on the cold end- a small fan and perhaps a duct and you are in business.
2) replace the propane with solar heat. Comments on our last post suggested hot water as a medium, with evacuated tubes as the generator. Justin did an overview of these here. These give the highest grade heat of any hot water system but are expensive, but are coming down in cost quickly, having been offshored to China. We don't have the science to know if hot water is actually hot enough- one site says "the whole [absorption] unit operates by the heat applied to the boiler system and it is of paramount importance that this heat is kept within the necessary limits and is properly applied." and, um, we don't know what that temperature is.
3) There may be a need for storage- it may get cool in Arizona at night but it doesn't necessarily do so in Florida. Should there be a water storage tank or should there be a phase changing storage medium to keep the thing running?
4) There may be even smarter solutions than this complex ammonia based concoction. Last year Justin wrote about two teenagers who invented a Peltier effect based system. This might be cheaper and easier.
We wish we were like the X-Prize guys and could offer $10 million to the person who does this; it is an important, if less spectacular challenge. We will think of something.