The summer solstice has come and gone, leaving our exploration of solar air conditioning still without a clear answer.  Is there no help to be had?  What is a green person (now slightly brown from the sun, or khaki in toto) to do?

The bottom line is no, Viriginia, there is no solar AC in the sense of something you can stick in a sunny window and expect to cool your room to any reasonable degree of satisfaction or payback.

There appear to be, however, some solar air conditioning solutions that, in the right circumstances, might be worth pursuing.  The smaller conventional air conditioner I considered running off solar panels in the previous post (with attendent batteries and other electronics to turn the direct current output of solar cells into the alternating current your house provides) would be rated 12,000 BTU/hr. capacity.  That's roughly enough to cool two fair-sized rooms.

It turns out that if you can run your air conditioner on direct current, avoiding the conversion to normal house power, it can be more than twice as efficient.  One company, Solar Panels Plus, for instance, offers a 17,000 BTU air conditioner that runs on direct current that requires only about 550 watts of electrical power instead of the 1200 watts we figured.  That's over 50% more cooling power (17,000 BTU vs. 12,000 BTU) for about half the electrical power.  That's about half a house worth of cooling for about $2500 in solar panels, or a payback period of only 50 years! Yee-hah!

OK, so your excitement threshold is not quite low enough to get excited over a 50-year payback.  Mine neither.  Even with tax credit of 30%, we're talking a 35-year payback.

If your electric company offers rebates and other incentives (our power company, for instance, offers a $1500 rebate on solar for homeowners), you might be able to trim that payback substantially down, to say, 20 years or less.

Various companies are exploring clever combinations of technology and intelligent controllers, but it will be a while before these are tried and true suggestions for homeowners.  For instance, removing humidity is a large part of air conditioning's function.  A drier home simply feels cooler and more comfortable.  Generally, dehumidification just happens anyway as you cool air, but it turns out that if you can remove humidity in some other way, you need less AC power.  One solar AC solution is to move air past a solid or liquid dessicant that absorbs the moisture, then circulate the dessicant to an area where the sun dries it out.  Another technology is evaporative cooling, known to your body principally in the form of sweating.  This involves evaporating water in the sun.  Generally, this works best in dry climates where ... water....is.....scarce........anyway......Hmmm.