Solar Hot Water Part II: Background on Our System

I've been researching solar hot water systems for a number of years, waiting until the state of California instituted a subsidy. Unfortunately, a trial program in San Diego only netted double digit installations, indicating a general lack of interest in solar hot water (more on this in a later post). That and the sorry state of California's finances caused the state to indefinitely postpone any subsidy for solar hot water. For that, this year the Feds put in place a really nice 30% tax rebate (not a deduction, a rebate) on alternative energy. The rebate includes the cost of everything involved in installation, not just the equipment. So this spring we decided to take the plunge, and I began looking around for solar hot water installers.

One problem is that there are very few companies, even here in northern California, that specialize in solar hot water, in contrast to the case with solar PV electric installers which are really common. Most companies that do solar hot water include it as a sideline in addition to their mainstream solar PV business. The consequence of this is that there are few companies that have a lot of experience in installing solar hot water. As in any field, more experience leads to a more reliable outcome. The reason very few companies specialize in solar hot water is that very few people want it, and also, it is more difficult to install because it involves plumbing and tank installation. The amount of site specific customization that needs to be done for solar hot water is therefore much more than for solar PV, which leads to more costs and more possibility for errors.

After checking out another company that does solar hot water installations as a sideline to their main solar PV business, we decided on Sunwater Solar in Richmond. Sunwater only does solar hot water systems and only indirect systems.

Another question was what kind of system to go with (direct or indirect, thermosyphon or ICS or drainback or indirect). A friend had a thermosyphon-like system, which uses a non-water based heat transfer fluid, installed a couple of years ago for around $7K, but it involved putting a 40 gallon tank up on the roof. I favored an indirect system over a  thermosyphon system to reduce the potential aesthetic problems (looks less like a science project) and because of the lower efficiency in a thermosyphon system, and over a direct system because of the potential for freeze problems in a direct system (temperatures here in coastal northern California were around 24F at night a few days ago, and that's also the temperature my collector was at). Some research on the Internet indicated that flat plate collectors perform better in our climate, due to the overheating problem in summer. For that, they take up more roof space but all in all, flat plate seemed like the way to go. So we decided to go for an indirect system with flat plate collectors.

We also needed to decide what we were going to do for solar water storage. The cheapest alternative would have been to use our existing gas hot water tank for solar storage and install a heat exchanger between the cold water inlet and the tank. Silicon Solar makes such a heat exchange. The problem with this kind of system is that it is much less efficient than if the heat exchanger were in the tank itself because heat is lost to the air. In addition, the tank we have is 40 gallons which means we would be able to store less hot water for cloudy days. So that suggested instead installing a second tank for solar hot water storage.

I researched storage tanks a bit and found out that there are basically three types: glass (actually ceramic) lined, steel, and stainless steel. Glass lined tanks are much the same as the cheap electric and gas hot water tanks that one can find at Home Depot and elsewhere, and that most homes in the US have. Both glass lined tanks and steel tanks require a magnesium anode to prevent corrosion. A little known (at least, I didn't know it) fact about the cheap electric and gas hot water tanks is that they, too, need a magnesium anode. Over time, the anode dissolves until, after 5-10 years (depending on the ionic content of your water), the anode disappears. When it disappears, it needs to be replaced, something most homeowners probably never do. If it isn't replaced, the tank corrodes and starts leaking. The upshot of this was that I decided to go with a stainless steel tank, to reduce maintenance. Unfortunately, this added substantially to the cost of the system.

The final decision we needed to make about our system architecture was what we would use for backup. In the short term, it seemed to make sense to use our existing 40 gallon gas-fired hot water tank as backup, since it had originally been installed around 2001 and was therefore relatively new. In addition, I had vague plans to do some deeper energy remodeling in the future, and wanted to put off the ultimate decision about what to do for backup until then. I briefly considered getting a solar tank with an integrated gas heater, but rejected that because the cost was higher, and because I was hoping at some point to reduce our gas usage to a minimum. So we decided to keep the gas fired tank for now and use the solar tank as a preheater, at least in winter. In summer, we could turn off the gas tank (it has a pilot light, very wasteful) because there is enough sun and the tank gets hot enough to last for several days without sun, though we rarely get many cloudy days in summer.