Although solar panels are active while the sun is shining, they typically don't do much once the sun goes down. This is why some systems incorporate water tanks. The water is heated during the day via the panels, then that stored thermal energy (heat) is used to warm the home at night. While the tanks are effective, they also take up a lot of space, making them difficult to fit into peoples' homes. A newly-developed device, however, stores just as much heat in half the space – using paraffin instead of water.
The prototype was designed by researchers from Spain's Universidad del Pais Vasco and Euskal Herriko Uniberstitatea. It contains a commercial type of paraffin, sealed within thin aluminum plates. Water is still involved, although it serves to transport the heat instead of storing it.
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Here's how the system works ...
Water is heated by rooftop solar thermal panels and then flows into the house to the thermal energy storage unit. Upon reaching the device, it circulates through channels between the plates. As it does so, the heat in the water is absorbed by the aluminum and passes through to the paraffin – which is in a solid state at the time.
Upon reaching a temperature of about 60º C (140º F), however, the paraffin melts to store the heat energy. When heat is needed later, cool water is circulated through the channels, lowering the temperature of the paraffin. Once it drops below 60º C and re-solidifies, it releases the stored heat. The use of the thin plates allows the thermal energy to be released from the paraffin relatively quickly. Normally, such phase-change materials take a long time to release energy, which has limited their usefulness in thermal energy storage devices.
Along with being able to do the work of a water tank of twice the volume, the storage units could also be built in a variety of shapes – not just cylinders, as is the case with tanks. This means they could be tucked into corners, or even built into false ceilings.
The researchers are continuing to develop the technology, and are looking into replacing the paraffin with other phase-change materials that are capable of storing even more energy, such as fatty acids. They're also building a full-scale prototype, which will be tested via real-world use in a government office.
[Ed. note: This article initially suggested that the stored thermal energy would also be used to generate electricity, but has since been amended.]