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In heating mode, the unit turns roughly 90% of the sunlight hitting it into warmth for the room.
Researchers at Harvard have developed a “contrarian” solar harvester that solves one of renewable energy’s most persistent headaches: getting the right kind of energy at the right time.
The new device uses a simple phase change, which is water evaporating and condensing, to act as an optical switch. 
This allows the hardware to passively toggle between generating electricity during summer heat and providing direct warmth during winter chills, all without a single sensor, motor, or computer chip.
Traditional solar technology is considered rigid. Photovoltaic (PV) panels generate electricity regardless of whether a building needs power for AC or is already struggling to stay warm. Conversely, solar-thermal collectors keep pumping out heat even in the middle of a heatwave.
“The switching capacity is calibrated to seasonal building needs, which are temperature dependent,” said Raphael Kay, lead author of the study. 
By making the hardware responsive to its environment, the team has created a “dual-mode” harvester that aligns with human needs. It provides electricity for cooling in the summer and direct heat in the winter.
The device is an elegant sandwich of simple materials, which included a Fresnel lens (a thin, ridged lens), a sealed cavity of water, and a PV cell. The system operates based on the state of the water within the cavity. 
When conditions are warm or hot, the water remains in a vapor state. This creates a high refractive-index mismatch that allows the lens to focus sunlight directly onto the PV cell to produce electricity.
When the environment turns cold, the water reaches its dew point and turns into liquid condensation. This physical change reduces the refractive mismatch and blunts the focusing power of the lens. Instead of hitting the solar cell, the light bypasses it and enters the building as indoor heat.
In laboratory tests simulating a climate like Boston’s, the device was able to “self-correct” its output based on a dew point of 15°C (59°F). From May to October, the device prioritized electricity. From November to April, it automatically shifted to heating mode.
The performance jump is significant. In its heating mode, the system converts roughly 90% of incident sunlight into indoor heat. 
According to Kay, that is approximately five times the solar-heating yield of a traditional PV panel paired with an electric heater.
“A component that can be laminated into skylights or façades and that naturally biases toward electricity during hot spells could be compelling as demand for cooling rises on a hotter planet,” concluded Joanna Aizenberg, Amy Smith Berylson Professor of Materials Science at SEAS, in a press release.
Despite the breakthrough, the “sun angle” remains a hurdle. Because the units are currently fixed in place, they focus light most efficiently only during specific hours. 
When the sun is at an off-angle, the device defaults to heating mode.
The Harvard team is currently working on strategies to expand the “active hours” for both modes, with the goal of creating scalable, cheap components that can be integrated into greenhouses, vehicles, and the very glass of our office buildings.
An active and versatile journalist and news editor. He has covered regular and breaking news for several leading publications and news media, including The Hindu, Economic Times, Tomorrow Makers, and many more. Aman holds expertise in politics, travel, and tech news, especially in AI, advanced algorithms, and blockchain, with a strong curiosity about all things that fall under science and tech.
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