Technically it should be called a heat exchanger, we lovingly call it the water cooler, but it has been providing us with free cold air for more than 20 years now so it definitely does work.
Shortly after moving here (Montana) I noticed that the water from our well is really cold, under 50 degrees. I built this heat exchanger to take advantage of that cold source for use in the house in the summer and as a byproduct it heats up the water going to the garden a bit before it goes on the plants since the plants didn't seem to care to much for the super cold water. Good benefits all the way around and since I would be pumping the water to water outside anyway the only actual cost is the power to run the box fan that moves the air through the copper piping.
How well does it work? We hit a high temperature in July of 112F, hottest that I can remember. The temperature inside was 76F with the cooler running all the time. I almost didn't want to go out to move the sprinkler.
The disadvantages? Well, you have to move the sprinkler a lot, but it does keep the grass green. Also if the humidity gets high, water will condense on the exchanger the same as on a glass of cold water. So I keep towels underneath it to soak up the moisture. If it gets really muggy I have to change the towels several times a day. I just hang the wet set outside in the heat to dry and rotate the dry ones back under the cooler.
It will take some skill to put it together but once done it is maintenance free, except for the time I left it outside before putting it away for the winter and it froze some leftover water in a pipe and broke it. Make sure to get the water out of it and store it where its above freezing.
Another disadvantage, it isn't pretty, but it is unique and truly "green".
Step 1: check your water temp
For this to work at all you need to have cold incoming water. The colder the better. Get a standard waterproof thermometer and run your outside water for a while and find out what temp it is. This probably will not work in places where you have a municipal water supply unless its from a cold source. Also you need to have enough ground to water so you don't water log your garden and lawn. It will probably work best in rural areas.
With the fan turned off and the water running outside the temp of my copper pipe is 48F (Infrared non contact thermometer reading)