Harvard's solar device changes with weather, offers both power and heat

Harvard researchers have developed an innovative “contrarian” solar harvester that dynamically switches between generating electricity and providing heat based on ambient temperature, addressing a key limitation of traditional solar technologies. The device uses a simple phase change—water evaporating and condensing within a sealed cavity—to act as an optical switch. When warm, water vapor creates a refractive index mismatch that focuses sunlight onto a photovoltaic (PV) cell for electricity generation. When cold, condensed water reduces this mismatch, diffusing light to provide direct indoor heating instead. This passive, sensor-free system aligns energy output with seasonal building needs, supplying electricity for cooling in summer and heat in winter. Constructed as a layered assembly including a Fresnel lens, water cavity, and PV cell, the device demonstrated effective self-regulation in lab tests simulating Boston’s climate, switching modes around a dew point of 15°C (59°F). It achieves about 90% conversion of sunlight to heat in heating mode, roughly five times more efficient