If you are serious about the forced air heat-exchanger you might be better off getting a 10W pool pump and a car radiator with fan, both activated when the temperatures are suitable (for heating the water or cooling the air I would kick the heat-exchanger in at about 28C, for heating the air at about 15C). A low difference in temperature is very inefficient for any heat-exchanger and the three things to optimise are surface area, surface conductivity and counterflow length (unintuitively for low differences the counterflow length term dominates practical designs so good designs are very long). Air bubbles have good surface conductivity but are hard to make a large surface area and counterflow length is very short. Better is a shower heat and fan combination (called bong-coolers in computer circles), and presumably a car radiator is very good too. It's probably a bit hard for most people as you really want a two set-point differential thermostat, something you'd have to build yourself (though it can probably be done with about 10 components). Alternatively you could use the air-exchanger for heat storing/air cooling, and under bench piping for plant heating. This would simplify the logic at the cost of two separate loops. I'd head in this direction if you are serious about thermal mass. You also want lots of water - gbrendemuehl's calculations are right on the money - we're talking up in the thousands of gallons. But you were all going to collect rainwater anyway, weren't you!

ok, if the 6 buck 30 gallon trash cans with lids are good, would they be any better with some type of material wrapped around them? Yes, I plan to recycle the water into the garden as dry periods dictate. Experiments? ok, I'll try it as soon as I can find enough wax for the right price. I guess a block of it could go into a 10 gallon glass aquarium that I have. That would be easy to move about into the right places with light. I'll put a thermometer on top and measure it at different times. The thickness would come to bear so if I can find the right supply place, I'll test different sizes and shapes. Anybody have more ideas for this unscientific experiment? thanks, ~Tom

SalmanBah- In a climate like yours, the main issue is killing the plants from too much heat. I live in a climate where the summer, outdoor temperatures only get to about 25C, but, even with venting, the greenhouse can easily get to 45C...which is a threat to many plants. In your country, you could get to much higher temperatures if the greenhouse was not well-designed. I don't have firsthand knowledge of greenhouses in Bahrain, but any greenhouse you would build there should have: 1) automatic roof vent openers, 2) one or more large volume fans operating on a thermostat, 3) a misting system on an automatic timer; and 4) shade cloth on the roof. What size greenhouse are you talking about, and what would you want to grow in it?

I'm in the same boat as you Dave. I certainly don't need the heat and I'd rather have the space. I was wondering if they might help keep things cooler during the heat of the day, but it doesn't seem like it would be all that effective. So I've been emptying them over the last couple weeks, whenever I need some extra water for the lawn.