Author:Charlie Tennessen From:Sunflower Solar Post time:11/04/2011 View:3625
My plan for world domination through Solar water heater
I have been interested in solar heating and solar power since I was a little boy. But I didn’t get serious about it until building a solar greenhouse onto my last house. Based on that experience I decided that my next project would be a pure solar heating project.
I started the project by ordering 3 vacuum-tube solar water heaters from Sunflower Solar in China. This was fun as I did all the importing myself. It was also a little dicey as I had to prepay $1100 to China by wire transfer without really knowing if I was going to get anything for it. But the crate showed up at the bonded warehouse in Milwaukee about 6 weeks later just fine. I paid my customs fees and loaded it onto our tiny pickup truck!
If I had purchased a similar type of system from a US solar provider, it would have cost around $3000. What I really want to do is purchase an entire container’s worth of collectors. The factory has promised me a much better price if I buy in quantity!
Evacuated Tube Solar Collectors are a weird beast. Pay attention: you have a long tube that has the air pumped out, with a flat aluminum or copper heat absorber inside. Inside the heat absorber is a phase-change heat transfer medium, usually alcohol or freon. The heat transfer medium will boil when it is exposed to the sun, the gas will rise to the copper heat exchanger at the top (beyond the evacuated portion of the tube), where it will condense and give up it’s heat. The copper end is shoved inside a heat-transfer manifold with water running through it, thus heating the water.
Here is the business end of an evacuated tube solar heat collector. The tubes are about 3″ in diameter and 5 feet long. Although they are expensive when compared to flat plate collectors, the extreme insulation of the vacuum allows them to collect solar energy very efficiently, even on hazy or cloudy days. On one day in November the collectors burned off the foam insulation on the copper pipes running into the manifold, and I have seen water temperatures as high as 190 degrees being pumped down into the basement. Wow!
Here you can see one manifold with the tubes installed, while the manifold next to it does not have the tubes installed. The roof installation was easy, I just assemebled the manifolds with frames on the ground and then screwed them onto the roof. I did all the piping, and then, when everything was ready, I brought the fragile tubes up onto the roof and installed them. Everything was lightweight and easy to work with. (OK the chinese translation was a little shaky in places. But the factory was very easy to reach by email and sent all kinds of pictures to demonstrate.) The important part is to seal up the roof as well as possible and to get the plumbing done right. I used 1/2 copper all around. The chinese manifolds had 22mm outlets, which match up perfectly to US 3/4″ copper. Soldering on the roof was a little frustrating, even on the calmest day there was enough wind to blow out the torch. But I finally got it done and it does not leak at all.
SO….the contoller at left monitors the temperature in the solar collectors and in the solar storage tank in the basement. When the temperature on the roof exceeds the temperature in the basement storage tank, the inline pump pictured at right is activated and hot solar fluid is circulated in the system, heating water in the basement.
Here is the view in the basement. The solar storage tank is our old 50 gallon electic water heater. Cold water that is going to be heated for domestic use goes into this tank first, where it is heated by the solar fluid from the roof. Water exits the solar storage tank and goes into the regular domestic water heater. If it’s been a cloudy day, this heater will serve as a backup and do all the water heating required. If it’s been sunny, water will be heated by solar energy alone.
At right you can see a bit of the homemade heat exchanger I made for the solar storage tank. I removed both upper and lower heating elements from the tank and inserted as much 1/2″ copper tubing as I could. I was able to use copper compression fittings to make a leakproof seal. The solar fluid (a mixture of water and propylene glycol–a food safe antifreeze) is pumped through this coil of copper tubing inside the tank and transfers it’s heat to the water. This is a temporary solution, I will eventually get a larger storage tank and probably switch to a dual-pumped flat-plate heat exchanger.
It’s been only a few days since the system has been fully functional, with very little sun. So I don’t have much performance data to share. Solar water heating in our climate can provide up to 75% of water heating needs on an annual basis. But most of that heating is in the Spring, Summer, and Fall. The sun is simply too low in the sky and out for too short of a time in Winter to be much use at our latitude.
November 27 data (clear sun, 30+ mph wind, high 20′s Farenheit):
|Time||tank||from collectors||to collectors|
|8:50 am||55 F||90 F||75 F|
|1:00 pm||114 F||126 F||110 F|
|3:35 pm||105 F|
I ran the dishwasher at about 1:00, I think that accounts for the temperature drop. There is some shading from a pine tree after 1:00 pm right now, which may also explain why the temperature did not recover. Heating 50 gallons of water 50 degrees Farenheit (it did at least this much work) represents 20,000 btu’s of heating, if my math is correct. (1 btu = 1 pound of water raised 1 degree Farenheit). I am thinking that more pipe insulation is in order–I actually fried some of the foam insulation on the roof when the pump wasn’t working one day. I have no idea how my homemade heat exchanger compares to others, but I think it leaves a lot to be desired. A factory-made solar storage tank with internal heat exchanger has about 120 feet of copper pipe in it for heat exchanging, whereas my tank has only 20 feet of copper pipe in it.
I really want to install a flat-plat external heat exchanger next. This would require a pump to take water out of the tank, through the heat exchanger, and back to the tank. But these heat exchangers are said to be super-efficient.
Overall, I think this system is close to what I want. All the advice I have read says you do not want to oversize the system. If it’s too big, the summertime heating is a problem–what do you do with all the extra heat? You are supposed to size it to do 100% of your heating needs in the summer. It seems like this system would be about right for a summer day, which has several times more energy available than a winter’s day.