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20 July . 2010

Solar Thermal vs. Solar PV: What’s the Difference??

By Graham Alexander, Southern Energy Management


We are extremely pleased to hear from Graham Alexander of Southern Energy Management today. You may not be aware, but SEM is the third party testing and certification company out here at Briar Chapel. Graham is going to talk to us about solar energy and incorporating it into your home - new or used.

A lot of people get excited about the idea of going solar, but aren’t completely sure what their options may be. Today, we’re going to break down the key differences between two of the most common solar technologies:

1. solar thermal systems, which use sunlight to heat water;
2. and solar photovoltaic (PV) systems, which convert the sun’s energy into electricity.

These two solar technologies are not only different in how they use the sun’s energy and the output they produce (hot water vs. electricity), but they also differ in appearance, cost, payback, and installation.

The science behind solar thermal isn’t exactly complicated: if you put something up on a dark-colored roof, it gets hot. The job of a solar thermal system is to absorb that heat and transfer it to your home or building’s water supply to reduce or even eliminate the use of an electric or gas water heater. Solar thermal systems come in a variety of types (flat panels, racks of glass tubes, sheets of black plastic, etc.), but the most common are either panels or tubes. Solar thermal systems typically use normal copper plumbing pipes to run a mixture of water and glycol through the solar panels and down to a dedicated water tank, which we call the solar storage tank. As the hot mixture from the panels moves through the closed pipe system in the solar storage tank, it transfers heat to the potable water coming in from the city or county water line, providing hot water that is available for use in your home or building. Solar storage tanks are usually bigger than your normal hot water tank, so they can store the hot water longer (that way you still get to take a hot shower provided by the sun even if it’s been cloudy for a couple of days). Even if it rains for a week, the vast majority of systems are set up with a gas or electric heating element in place as a backup, and people will never notice a difference after going solar. In most cases, thermal systems pay for themselves after 5-8 years for a family of four. The payback time is even less time for commercial solar thermal systems, which are ideal for facilities that require significant amounts of hot water for laundry, showers or serving meals.

Solar PV is an entirely different type of technology, and it’s a little more complex. PV panels are covered with semiconductors (usually made of silicon) that absorb the sun’s rays; their reaction to that light results in a flowing current of electrons. Metal contacts in the panel allow us to collect that current and transmit it as electricity. There are all kinds of ways that the power can be handled once you’ve reached that point, and it can get confusing fast. In North Carolina, most systems are set up so the power starts by going through an inverter that converts it from DC to AC electricity. After that conversion, the current is sent through two meters: the first measures how much power you’re generating, while the second one combines it with electricity coming from traditional power lines to light up your home. Again, if it’s cloudy out for an extended period, the lights aren’t going to go dim, since most buildings are still connected to the grid, which serves as a full-time backup. PV systems generally have a higher cost up front, and the payback time can be anywhere from 8-13 years for residential systems and as short as 5 years for larger, commercial-scale arrays.

I should also briefly mention a related misconception about solar – both PV and thermal – which is that your roof must be facing directly South in order for solar to actually work. That’s simply not true. On the contrary, it’s widely accepted that panels facing up to 30 degrees off of South don’t show any drop-off in performance. In fact, you can have an East or West facing roof and still be 85 percent as efficient as a South facing roof, depending on the vertical angle of the panels.

In the Briar Chapel neighborhood, all homes are built to high-performing green program standards – creating an ideal energy efficient foundation for the integration of a solar system. Southern Energy Management partners with Briar Chapel builders to verify that green standards are met for each home, and has already installed solar thermal and PV systems in the neighborhood.  

Hopefully this post gives a helpful overview of the two main types of solar technology available today. If you’re interested in learning more about your solar options, feel free to contact Southern Energy Management at 919.836.0330 or at graham@southern-energy.com.


Originally published on The Smart Energy Exchange. Graham Alexander is the Solar Design and Project Manager at Southern Energy Management.  Read more about him here.

tags: Helpful Tips

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