What if the same pane of solar hardware could favor electricity in July and warmth in January without a motor, sensor, or control system? Scientists at Harvard University have developed a solar system that is based on this concept. They have developed a solar system that can change the direction of sunlight using a phase change in water. It is a concept that solves a common problem that is experienced in most buildings. In most cases, solar electricity is most useful at times when solar heat is most needed. However, solar heat is most useful at times when solar electricity is not most useful. Instead of using a solar system that can provide one of these two solutions at a time, this system can provide two solutions at a time.
The solar system is made up of a Fresnel lens that is placed above a closed box containing water. There is a solar cell placed at the bottom of this box. When the temperature is above the dew point, water is in a state of vapor. In this state, the lens is able to focus sunlight directly to the solar cell. However, at times when the temperature is below the dew point, water changes state and turns to a liquid. In this state, the lens is not able to focus sunlight directly to the solar cell. As a result, sunlight is able to pass through the solar cell and provide heat.
The lead author, Raphael Kay, described this process in the following way: “The switching capacity is calibrated to seasonal building needs, which are temperature-dependent.” In the Harvard team’s tests, the dew point, which was close to 15 degrees Celsius, set the switch. In a Boston-style climate profile, electricity production would dominate in late spring through early fall, and the heat mode would dominate the rest of the time.
This seasonal self-selection is significant because buildings require many times as much as the electricity needed to power the devices plugged into the outlets. In fact, water heating, space heating, and space cooling account for a significant portion of the total household energy use in the US, according to data on household energy use in the US. Conventional solar panels satisfy only one of the equations needed to satisfy this requirement. Solar thermal systems satisfy the other equation. Hybrid solar photovoltaic/thermal systems have attempted to satisfy both equations for many years. Recent reviews of PV/T systems indicate that such systems can provide significant improvements in overall efficiency compared to conventional solar PV systems. The Harvard system is unique in the fact that it is not using the heat generated by the cooling process to provide the heat needed by the building. Rather, it is using the sun’s rays themselves.
When in heat mode, the system reportedly uses 90 percent of the sun’s rays to provide heat for the building’s interior. Kay estimated this to be approximately five times the amount of solar heating available using a conventional solar panel in combination with electric resistance heat.
The current limitation is geometry. The fixed nature of the device also means that its electrical mode can only be optimally utilized when the sun angle coincides with the lens. When the sun shines at an angle, the concentration is less effective, leading to a shift toward the thermal effect. The team is also working on ways to extend these periods, while maintaining the attractiveness of the simple design. Joanna Aizenberg summarized the architectural context well, saying, “It’s a component that can be laminated into skylights or façades and that naturally biases toward electricity during hot spells.” The significance of solar equipment in buildings, greenhouses, or even vehicles lies in its potential to be more than just a singular panel; rather, it can be more like a part of the building envelope.