Switching to Sun: Hybrid solar collectors may help with the transition to renewable energy

 

solar-house-croppedThe sun not only supports all life on earth, it also has the potential to meet the world’s energy needs. Solar energy is one of the most promising forms of renewable energy. Despite continuing improvements in solar technology, this resource is still scarcely used. The U.S. Department of Energy reported that only .07% of the energy used in the U.S., both for electricity and transportation, comes from solar power. With such a small portion of energy coming from solar sources, increasing the efficiency of solar systems is crucial. One way of improving solar efficiency is through hybrid collectors.

Hybrid solar systems combine the two main types of solar power: photovoltaic, which produces electricity, and solar thermal, which heats water. Solar water heating systems either heat water directly in the solar collectors, or indirectly with heating fluid that is transferred from the solar collector through water storage tanks. Solar power can heat the water to about two thirds of the way (US DOE). A conventional water heater would warm the water the rest of the way.
A photovoltaic (PV) cell in a solar panel absorbs sunlight to produce electricity (US DOE). Energy in the absorbed light transfers to the electrons in the atoms in the PV cell. The energy causes the electrons in the atoms move around, forming an electrical current. PV cells have two layers, one positive and one negative, that allow an electrical current to be carried through the panel. Photovoltaic cells can only absorb certain types of light. The radiation from the sun that the PV cell cannot absorb turns into heat. However, as temperature rises, PV cells produce less electricity.

Hybrid collectors address this issue by using the excess heat in a PV cell to heat water. Though not very common, hybrid systems have been around since the 1970s. There are different ways to arrange a hybrid system, but the basic idea is this: a photovoltaic cell rests on top of a thermal cell, made of a series of tubes. Sunlight hits the hybrid cell. The PV cell absorbs some light, producing electricity. The unabsorbed light turns into heat. The heat is transferred to the tubes in the thermal cell. The tubes, filled with water or heating fluid, become hot, allowing the water to be heated. Since the excess heat from the PV cell is used by the thermal cell, this heat does not cause the efficiency of the PV cell to drop.

One study compared the efficiency of a hybrid system to the performance of a conventional PV system. Researchers built a hybrid spiral flow collector: a PV module with a series of tubes beneath it. In this type of collector, the tubes wind around the module from the center, creating a compact spiral shape. The tubes carry water, heating it as it travels through the tubes. The researchers found that the ideal operating temperature for the hybrid system was 12 degrees C higher than the optimal temperature for a conventional PV system. So, the hybrid system allows more electricity to be produced at higher temperatures. In this study, maximum electrical efficiency (a comparison between sunlight absorbed and energy produced) was 11.4%. Compared to the conventional PV system in the study, the hybrid system produced about 20% more electricity. Thermal efficiency was much higher than electrical efficiency at about 53%. So, total efficiency of the hybrid system was 64.4%.

Hybrid systems seem very practical, but could solar energy really power all of America? We use so much electricity, it may seem like we might have to cover a good portion of the country in solar panels to meet our electricity demands. This is not the case, though. According to the U.S. Department of Energy, PV panels would have to cover only 7 % of the area of all U.S. cities (just cities) to produce all of our electricity from the sun. No natural habitats would need to be obscured by solar panels. No farmers would have to sacrifice their fields to produce energy. Panels could simply be placed on any sunny patch of developed land.

Compare this to coal, the number one source of electricity in the U.S. Coal mining destroys natural habitat, pollutes drinking water, and even blows mountains to pieces. While this energy may be cheaper than solar energy at first glance, the price of coal doesn’t include the hidden costs, such as the health problems caused by coal plant pollution. Considering the advancements in solar technology, such as the development of hybrid collectors, and the effects of coal mining, why wait any longer to make our electricity generation more sustainable?

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