The following post on solar powered differential temperature controllers for solar thermal hot water heating systems is written by Guy Marsden. I was so impressed with his sustainable living habits that I wrote a blog on him back in October. Guy runs the website Art Tec, and along with providing excellent information on sustainable living, he also sells solar powered differential temperature controllers. His controllers are the only ones designed for solar powered applications that are made in the U.S.! Unlike many of the shysters out there trying to scam people out of their hard-earned money, Guy walks the walk and talks the talk. We need more people out there who know what they are talking about selling well-made products that enhance people's efforts to live sustainably!
By: Guy Marsden
There's something wrong with most solar heating systems.
If you are not familiar with how solar heating systems work you may not get the point, so first a brief primer. If you want to use solar energy to heat up the water in your home, you put solar collectors up on the roof. The sun heats the fluid in the collector and the heated fluid gets pumped down into a storage tank through a heat exchanger that separates the antifreeze fluid from the potable domestic water (see the solar thermal schematic diagram at the top of the post). Pretty simple huh? But the devil lies in the details.
People that typically install solar heating systems are plumbing professionals. Makes sense because most of the work is plumbing, plus some roofing and a bit of carpentry. So these guys work from within their paradigm and when they want to pump water they use an AC powered pump. Sounds fine until you look at the specs of that pump which uses 50 to 70 Watts to operate. These pumps run whenever the sun is shining - so an average of 5 hours a day -- more in the southern states. That's 300 Watt Hours a day = 9 kWh a month, that's costing you about buck or so a month to run the pump to get your FREE solar heat!
Now the pump needs to know when to run. You don't want it running at night, you just want it to run when the collectors are hotter than the solar storage tank. So you need a black box called a "solar differential temperature controller" to sense this temperature difference and switch the pump on at the right time. Now of course this controller is plugged in all day 24/7 which adds another costly electrical load to your "free" solar energy system.
So what's the alternative? Well a lot of solar dealers who were not originally trained as plumbers often use a 12 Volt DC pump that can run directly from a 12 volt solar panel. Now you are getting your solar water heated for free! The cost of the pumps are about the same, and the only additional cost is the solar panel and that represents a token percent of the system cost, so the return on investment is short.
So that's all you need in theory. But actually a solar panel can have enough power to run a pump very early in the solar day - even on days when it's well below freezing outside. Same is true in the afternoon on a cold day. Now you have gotten your solar storage tank nice and hot, but if it's really cold outside as the sun goes down, you need to shut off the pump or it will be pumping the heat from the tank out to the collectors. Not a good idea. So now you are back to needing a differential controller. It turns out that there is a controller made for just this application by ART TEC Solar. It uses the power from the solar panel rather than from an electric outlet. So now your solar heating system is totally off the grid and there is another benefit.
Say the utility power goes out in the middle of a hot summer day. Well if your system is utility powered, then that pump stops dead. The antifreeze in the collectors sits there stagnant and starts to cook. If it gets over 350F - which it can, then the chemicals in the antifreeze go through a cascade failure and turn acidic and now your fluid will start eating through your copper pipes. Eventually the pipes or collectors spring a leak and have to be replaced, this can get really expensive. But long before that happens the fluid gets so hot that the pressure valve opens and dumps boiling fluid all over your utility room, this depressurizes your system causing it to fail completely. You really don't want to go there, it's another pricey service call to the solar installers.
So if you are getting interested in a solar water heating system, be sure to tell the folks that you get estimates from that you want an off-grid system!