We’re still seeing inefficient 1970s-circa solar designs.
Today’s solar thermal designs are more efficient and less expensive.
So, what’s wrong with this solar hot water system?
Heck, it’s got an engineering drawing laying it out: three 30-tube solar collectors, preheating water for two 119-gallon storage tanks and a large water heater providing backup.
Well, I received this drawing in early 2012, but it would be perfect for a solar system circa 1979.
Forty years of technology improvements have made solar much cheaper and more efficient. But I still regularly see engineering schematics like this – something that would have made Pres. Carter proud to install on the White House.
Engineers, I recommend a refresher course on solar hot water technology.
First of all, let’s look at what’s out-of-date on this drawing:
- The goal is to have solar heat the storage tanks, and then have this warmed water preheat the water heater. This would only work (semi-efficiently) if someone was using water constantly. Unless someone turns on the tap, there’s no way for the solar-heated water to get to the water heater; at the same time, the water heater is burning gas constantly to maintain its own hot water. So the nice hot solar water just sits there while the water heater runs 24-7 at full speed.
- It uses an external heat exchanger, as well as an additional pump and control.
How should this system be designed today? Modern technology allows the storage and backup water heater to be in one unit, so the gas only fires when there’s not enough solar hot water. Today’s technology also features direct, submerged heat exchangers. In 2012, solar is consolidated and streamlined, reducing the amount of equipment you need to buy.
Notice in the updated drawing I’ve done for this engineer (and blog post), there are two HTP Solar Phoenixes, which easily match the firepower of the large water heater, and also contain their own storage. So, you’ve reduced the footprint to two tank units and eliminated the external heat exchanger, as well as its own dedicated pump and controller. Plus, now the solar is the primary heating source and gas is secondary; if solar doesn’t meet the load, the Phoenix’s modulation will kick in and heat only the difference the solar falls short.
The design is simplified, easier to install and saves significantly more energy.
Another issue is that only a portion of this 1970s-style system is eligible for today’s solar tax credits – the water heater won’t qualify. Units such as the Solar Phoenix, which combine storage and heating capability, are SRCC OG-300-certified, the measuring stick used by most government and municipal solar tax credits.
Considering our political and economic situation, there may not be that many things that are actually better in 2012. But solar is one of them.
(See related blog post, “When You’re Just Waiting for Someone to Turn on the Tap: Designing Solar for Commercial Applications,” here.)