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Thursday, October 15, 2009

Yet another DIY solar hot water project



I've been meaning to build a solar hot water system for some time now. I will post on my progress periodically, including details for anyone wanting to learn from my mistakes. Although I've read everything I could find on the Internet about solar hot water, I'm sure there is a lot I don't know but will soon find out.

By doing it myself I'll be able to pass on what I learn. One critique of solar is the expense. Telling people "I then paid $9,000 to have solar hot water installed" does not make for interesting reading. It irritates me that something this simple can cost as much as an economy car. I also like the idea of recycling materials when practical. The Internet, and Craigslist in particular, may save us all!

I recently got the ball rolling by picking up two glass panels that were being given away on Craigslist. They each measure 46 x 76 inches. I doubt if they are low iron (for less heat loss) and have no idea if they are tempered.

I also ordered two solar absorbers off the Internet that will fit under that glass. Building my own absorbers was clearly going to be time consuming, difficult, and expensive. I picked them up a few days ago at the trucking terminal. The crate had been smashed in on one side and I found out after unpacking them that both absorber panels had a damaged pipe and broken welds at the manifold. If you look closely you can make out the bent pipe on the left side.

Lesson number one, pay with Visa so you stand a chance of being compensated if your panels arrive with damage.

Although I'm not ready for one, I also checked Craigslist for new or like-new electric hot water heaters and found several available, some for free, some for a hundred bucks or so. I will remove the heating elements and use it to store water that was heated by passing it through copper heat exchangers in the solar panel drain down tanks with a low-powered circulation pump.

I spent a lot of time deliberating over the system design. I was very tempted to use the simplest design that provided the most energy per dollar invested using the minimum number of parts. It would run the clean water supply pump, one tank in addition to my existing hot water tank, no anti-freeze fluid, and no heat exchangers. You lose a lot of heat with heat exchangers and anti-freeze fluid.

However, because of the other engineering tradeoffs involved, I finally decided against that system. First, the above design is vulnerable to freeze damage. Because panels radiate heat up at the sky, the water in the tubes can freeze on a clear night even when the air temperature is above freezing. We have all seen this in action on our car front windows that slope back like a panel does. Several days a year you will notice that only your front window has frost on it. Decades of experience with designs that tried to prevent these panels from freezing by draining them when it gets cold with a complicated and unreliable valve system have proven that given time, they will one day freeze anyway when one of these valves fail. I tried to convince myself that I could manually drain the panels on cold nights but reality finally sunk in. Best not to deny reality just because you don't like it.

The next reason is overheat damage. This happens on hot summer days when you have all the hot water you need but the sun keeps heating the water trapped in the panels creating high temperatures and pressures. Repeatedly overheating and pressurizing your system will fatigue joints, melt rubber and plastic parts and eventually leading to failure or possibly to a hot water tank pressure relief valve opening. My panels will have to be able to handle continuous dry stagnation.

I chose a design that automatically (and in a fail-safe manner) drains the water out of the panels into holding tanks every time the pump stops running, which happens when the sun isn't shining because the pump is powered by a solar panel. If you live where the water is hard you will have to use special solar panel anti-freeze for the liquid pumped through the panels to prevent mineral scaling from building up. You can also use distilled water and be prepared to replace any that evaporates. Here in Seattle the water is very low on minerals. I suspect I can safely use tap water for a decade or two.

The pump will also stop running when the water has reached its max temperature and trips a heat sensor, which is also backed up by an overheat sensor for redundancy and safety. This solves the problem of overheating the system because there is no liquid in the panels to cause problems and because the system is open to atmospheric pressure (unpressurized).

The downsides are that I need another tank, two pumps, a heat exchanger, and about 20% more solar panel to make up for the losses associated with having to use exchangers.

Because my roof is not only two stories up, has a steep incline, and also happens to face the wrong way, I will put the panels on the ground right in my front yard, which faces south. This will make maintenance easy. I'll also have to make sure they look nice with no pipes showing.

Wish me luck and stay tuned. Feel free to chip in with advice to help me up the learning curve.

4 comments:

  1. Anonymous10:11 AM

    You need most sun collection on the equinox, so place the panels vertical. This minimizes, but doesn't eliminate, summer gain and night radiating.

    Place the panels on the face of an insulated box with the storage tank located horizontally above the panels, and inside the insulated box.

    The warmth from the tank, even with the integral tank insulation should prevent the panels from freezing, and the thermosiphon effect of the tank above the panels will eliminate one, or maybe all, of the pumps and controls.

    I haven't tried this yet, but I plan to soon.

    Good luck.

    Brian Lovell

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  2. I did not want a design that reduced the chance of freezing. I wanted one that would eliminate the chance.

    Other than using anti-freeze, this design is the only one that will make sure the pipes never freeze. It also avoids overheat problems. The only compromise is the need for a main pump and a small circulation pump. The main pump will use solar DC.

    Try posting again to see if this blog setting eliminates your comment problems.

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  3. Understand the desire to eliminate freeze risk - before I put a freeze sensor on my differential temp controller, I had to repair split tubes in the absorber twice. But never quite trust the dtc to work every time.
    Only other suggestion is to use high temp pipe insulation around the absorber - the black or grey foam stuff from HD melts.

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  4. I don't normally comment on blogs, however I have to say that I rather enjoyed yours as it was indepth. Keep up the good work.

    ReplyDelete

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