Solar Photovoltaic vs. Solar Thermal Systems

With an MD degree from the New York Medical College, Peter “Pete” Killcommons is the CEO of Medweb, a San Francisco, California-based teleradiology and telemedicine service provider. Outside medicine, Peter Killcommons takes an avid interest in solar power for consumers, including solar photovoltaic (PV) and solar thermal systems.

Solar PV and solar thermal systems both provide sustainable energy solutions by using energy from the sun. The two, however, differ fundamentally in their purpose and mechanism.

A solar PV system turns sunlight into electricity. It typically includes one or more rooftop solar PV panels, an inverter, and other electrical and mechanical hardware, such as a charge controller and utility meter. The PV panels are made from two layers of semiconductors, most often silicon, which create an electric field. The sunlight that strikes this field generates a small voltage, a direct current (DC), which the panels transport to the inverter, which, in turn, converts it into alternating current (AC) used by the electrical grid and most home appliances.

Meanwhile, a solar thermal system turns sunlight into heat. Individual systems may have different configurations, but most share components like rooftop solar thermal collectors, heat-transfer fluid, a hot water tank, a pump station, and a controller. Solar thermal collectors are filled with heat-transfer fluid and come in two main types: flat-plate and evacuated tube. As its name suggests, the first consists of flat, dark plates encased in a thermally-insulated box, while the latter comprises a set of glass tubes.

The heated by the sun heat-transfer liquid warms the hot water tank via a copper coil. The pump moves the liquid around the system, while the controller prevents freezing liquid from cooling the tank in colder weather. Besides heating household hot water, the produced thermal energy can also find application in space heaters like radiant floor heating. In this case, homeowners must install pipes on the floor through which the heat-transfer fluid can flow.