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Solar Thermal (Hot Water) – The Other Type Of Solar

Posted on Friday, December 16th, 2011 at 11:30 am by Solar Energy USA

As one of Georgia’s top residential solar installers, we get calls daily from homeowners with questions about solar power. When most people envision a solar energy system they picture a roof filled with rectangular solar panels designed to produce electricity. And while this image is correct – that type of solar is referred to as photovoltaics or PV – today we want to discuss another type of solar known as solar thermal (solar systems designed to produce hot water).

In the cooler months, your home’s air conditioning unit is not running which means your electricity bills are dramatically lower (and cheaper). The unfortunate downside to this scenario is that most homes are heated with gas, including water heating, and people like longer, warmer showers when its cold outside. While electric bills are nice and low in the fall and winter, gas bills are much higher during these seasons, and if you pay a variable rate your gas bills can be a lot higher than your highest electric bills. Do not let this frustrate you, however, because there is a quick and easy solution – an answer so obvious you might even say it’s blinding.

Did you know that heating a home’s water is the second largest energy hog for most households, higher than lighting, refrigeration, electronics, and computer energy usage combined??

According to ENERGY STAR, it takes 356 kWh per month to heat the average homeowner’s 80 gallon hot water tank. At a rate of 12.5 cents per kWh that is a cost of $44 dollars per month and over $500 dollars a year! And if you have gas water heating the energy cost can be even higher!

Let us introduce you to the quick and easy solution – solar thermal. There are two popular versions of solar thermal systems. Each has a different look, but they work in a very similar way.

The first main version of a solar thermal system involves what is called a “flat-plate” or “framed panel” collector. Think of a framed panel collector as a big panel that sits on your roof and absorbs sunlight, much like a photovoltaic solar panel, but instead of creating electricity like a PV panel, a framed panel solar thermal collector absorbs sunlight and creates heat. Inside a framed panel collector is a series of tubes, usually copper or some other metal. These tubes are filled with a special liquid that gets very, very hot when exposed to sunlight (temperatures can reach over 400 degrees). The liquid is called propylene glycol and its a lot like antifreeze only its nontoxic and would not harm anyone exposed to it. Through a series of tubing this propylene glycol is cycled into your water tank where it keeps that water tank at a constantly hot temperature. The propylene glycol is contained inside a big heat coil called a heat exchanger. Water inside your water tank never comes in contact with the propylene glycol, it just absorbs the heat that is given off by the heat exchanger.

Flat-plate collector version of a solar thermal system:



The other main type of solar thermal uses what is called an “evacuated tube” collector. This version of solar thermal is very similar to a framed panel collector but the tubes are not concealed in a box or panel, they are entirely exposed to sunlight. A glass or metal tube contains the propylene glycol and is surrounded by a larger glass tube. This design creates a vacuum so that very little heat is lost. Evacuated tube solar thermal systems are more efficient at absorbing sunlight because of their design and their cylindrical shape which allows them to absorb multiple sunlight angles including sun rays that hit a roof and bounce back up. Because of this unique characteristic, northern states with colder climates that have fewer hours of sunlight during the winter, like Vermont, typically prefer evacuated tube solar thermal systems over framed panel solar thermal systems.

Evacuated tube collector version of a solar thermal system:



Solar thermal systems can be sized exactly for your household hot water needs depending on the number of people in your home, how much hot water you use, and the size of your current water tank. Framed panel collectors typically come in sets of two with the option to add more framed panels for more hot water, and evacuate tube collectors typically come in a series of 30 tubes with the option of adding 10 extra tubes or 30 extra tubes for additional hot water.

Just like PV solar panels, solar thermal systems are tax credit eligible on both state and federal levels. And, just as there are utility rebates for installing a new, more energy efficient water tank, most power companies also have special incentives for installing solar thermal, as high as $450 dollars per solar thermal installation!