Tuesday, May 29, 2007

Low Tech Solar Heat and Cooling

I've been thinking for quite a while about solar heat and cooling, and have written a bit about these in this space. The article about cooling a house using heat from the attic, in particular, drew some interest. And, it seems I've become something of a lightning rod for my belief that the more popular photovoltaics are not the best way to use solar energy.

I've thought for years about working up some prototypes, but a few things have kept me from pursuing the interest. First, I'm residing in a temporary rental. But even when I owned my home, I have to admit that the fact that I would need to cut a hole in the roof and install a conspicuous chimney kept me from trying some of the ideas.

So, I decided to do some lower key experimentation. This initially took the form of evaluating the desiccants needed for the process. After looking at some of the commercial alternatives, I decided to experiment with some lower tech ideas. Ultimately I settled on popcorn for the desiccant. Using some clear plastic bottles and a thermometer and humidistat, I found that popcorn could reduce the humidity of room temperature air to about 10% humidity. Even better, I found that the popcorn could be repeatedly regenerated (dehydrated) at temperatures between 120-200 degrees fahrenheit. It seemed that popcorn could act as the dessicant, and could be regenerated quickly by simple, low tech solar collection.

Next step, to build a prototype for experimentation. I built a 32 sq ft solar collector from materials bought at Lowe's for about $50. I was pleased to find temperatures attained were typically 70-80 degrees above ambient. Based on what I could see, it looks like the collector could save about $20/month on my electric hot water bill. Of course, it would take some additional work to tie into the house, which I won't do, since I only expect to be in the house about 3 more months. But, it seems reasonable to expect that a similar system could provide most of my hot water needs with a return on investment of more than 100% per year. A similar system could provide home heating, although with about one fifth(20%) the return, since the heat pump used to heat the house is more efficient than the electric resistance water heater. Also, the collector would turn out less heat in the winter and would not be used about half the year.

So, overall, a great return for replacing my electric hot water heater. For the home heating, the return would be marginal, without a cooling component to better utilize the system during the summer months. That's where the desiccant or adsorption type cooling would come in.

I built this prototype to experiment with these systems. Essentially, the air rises through the left column through the popcorn dessicant for drying, them moves to the right through a heat exchanger cooled by evaporation. The the air exits down through the right hand column with a second stage of direct evaporation. The popcorn is regenerated by the air exiting the solar collector on the far left and then is rotated into the left column to keep the desiccant dry.

So far, the results are disappointing. The water/air mixture exiting the system is running about 72 degrees when the ambient was 94 degrees and 61% relative humidity. Later in the day, I measured about 70 degrees when the ambient was 78 degrees and 59% relative humidity. I was expecting to attain about 10 degrees cooler temperatures.

There are several possible problems:
1. I'm using natural circulation related to the different temperatures. I may need to have taller columns to get enough circulation, or I may get better results by adding a fan.
2. The popcorn may absorb the moisture too slowly. A different desiccant or increasing the amount of desiccant may be needed.
3. I'm using irrigation type misters, but the droplets are fairly large. Better atomization might help.
4. I'm using direct tap water from my faucet, rather than recirculating pumps I would normally use.

I also plan to try an ammonia-water absorbtion system and bought the parts for about $60, again at Lowe's. Unfortunately, I'm out of town for the next couple of weeks, so I won't be able to tinker further until I return. Meanwhile, I'm open to any ideas for solutions or other possibilities.

1 comment:

kimberly said...

Solar energy is the best natural resource that we have this time even more that fuel is too expensive. In fact i want to approach costa rica investment opportunities and look all the alternative this country can have because it climate. We must to find the way to save our planet and to use solar energy could be the first step.