A new hybrid energy harvester developed by researchers at the Karlsruhe Institute of Technology (KIT) has demonstrated that one surface can generate electricity, deliver useful heat, and cool itself below the surrounding air temperature at the same time. The prototype combines photovoltaic and solar-thermal technologies with passive daytime radiative cooling, a process that releases heat directly into the coldness of outer space. During outdoor testing, the system reached a cooling effect of up to 6.5 degrees Celsius below ambient temperature, produced an electrical power density of 60.6 watts per square metre, and generated temperatures as high as 110.8 degrees Celsius for heating applications. The results point toward a new class of multifunctional roofs and façades that could produce several forms of energy without requiring separate systems for every building need.
The concept addresses a growing problem in modern buildings and infrastructure. Conventional photovoltaic panels convert sunlight into electricity, while solar-thermal collectors capture solar radiation as heat. Cooling, however, is generally provided by electrically powered air-conditioning equipment, which increases demand precisely when temperatures and solar loads are highest. These technologies also compete for limited roof and façade space. The KIT system seeks to resolve that conflict by using a single optical and thermal architecture to direct different parts of the available energy toward different purposes. Electricity is generated from concentrated sunlight, heat is extracted from the solar collector, and unwanted thermal energy is emitted through the atmosphere into space.
At the centre of the device is a transparent radiative-cooling emitter made from a silica substrate coated with polydimethylsiloxane, or PDMS, a silicone-based polymer with carefully selected optical properties. The layer is designed to transmit incoming sunlight while emitting thermal radiation in the infrared. Its most important operating range is the so-called atmospheric window, a band of wavelengths, approximately between 8 and 13 micrometres, in which Earth’s atmosphere absorbs comparatively little radiation. Thermal energy emitted within this window can pass through the atmosphere and escape toward the extremely cold environment of outer space. Because the process does not require compressors, fans, or electrical input, it is known as passive daytime radiative cooling.
The challenge is that radiative cooling and solar-energy collection appear to demand opposite conditions. A solar collector must absorb as much sunlight as possible to generate heat and electricity, whereas a radiative cooler must avoid absorbing solar energy and remain as cold as possible. If the same material were forced to perform both functions in the same location, solar heating could overwhelm the cooling effect. The KIT design separates these thermal roles physically and optically. Sunlight first passes through the transparent cooling layer, while the cooling surface continues to radiate infrared energy upward. The transmitted sunlight then reaches a Fresnel lens, a lightweight optical element made of concentric grooves that can focus light much like a conventional thick lens.
The Fresnel lens concentrates the available sunlight onto a compact photovoltaic-thermal collector positioned beneath the cooling layer. Concentrating the sunlight increases the intensity reaching the active solar component while reducing the area of the collector required to absorb it. The collector can then produce electricity through photovoltaic conversion and capture heat through its thermal structure. At the same time, the concentrated solar receiver is kept physically apart from the transparent radiative cooler, preventing the hot component from directly heating the surface that must remain cold. This arrangement effectively creates a thermal boundary between the system’s “fire” and “ice”: the lower section harvests solar energy, while the upper section sends heat away.
In outdoor experiments, the prototype demonstrated all three functions simultaneously rather than switching between operating modes. Its cooling layer reached temperatures up to 6.5 degrees Celsius below the surrounding air, a meaningful result because radiative cooling must overcome solar illumination, atmospheric conditions, convection, and heat conducted from nearby components. The system also achieved an electrical output of 60.6 watts per square metre. Although this power density is lower than the peak output of many conventional photovoltaic installations, the comparison does not capture the device’s additional thermal and cooling functions. The same surface was also able to reach 110.8 degrees Celsius, illustrating its potential to provide high-temperature heat for selected industrial, building, or water-heating applications.
The researchers describe the device as a way of co-harvesting energy from two very different parts of the sky. Sunlight supplies concentrated visible and near-infrared energy, while the clear night-like radiative pathway above the atmosphere acts as a heat sink even during the day. The phrase “cold universe” refers not to the extraction of material from space, but to the radiative exchange between a terrestrial surface and the extremely low effective temperature of outer space. When the atmospheric window is unobstructed, a properly engineered surface can emit more thermal radiation than it receives from the sky and surrounding environment, producing net cooling without consuming electricity. Clouds, humidity, and atmospheric water vapour can reduce this effect, making optical design and local weather conditions important to future performance.
The potential applications extend beyond ordinary buildings. Roofs and façades equipped with multifunctional energy surfaces could generate electricity while supplying domestic hot water or process heat and reducing the cooling load of the building beneath them. The technology may be especially relevant to facilities that require power and cooling simultaneously. Data centres, including those supporting artificial-intelligence systems, consume substantial electricity and release large amounts of heat through densely packed computing hardware. A surface that produces additional power while passively rejecting heat could help reduce the burden on mechanical cooling systems, although practical deployment would depend on scale, climate, reliability, cost, and integration with existing thermal-management infrastructure.
The work builds on an earlier KIT development involving a transparent material that could cool buildings while allowing daylight to pass through. The new prototype advances that idea by adding a solar-energy harvesting pathway beneath the transparent cooling layer. Researchers say the next stages will focus on improving the optical design, refining thermal management, and developing more effective photovoltaic cells and system configurations. These improvements could increase electrical output, preserve the cooling benefit under more demanding weather conditions, and make the technology easier to manufacture over large areas. Published in Cell Reports Physical Science, the study presents a striking change in how energy surfaces may be designed: rather than treating sunlight only as a source of heat and electricity, future systems could use the sky itself as both an energy source and a thermal escape route.
Subject of Research: Hybrid passive daytime radiative cooling and solar-energy harvesting for simultaneous generation of cooling, electricity, and heating.
Article Title: Co-harvesting universe coldness and solar energy for tri-generation of cooling, electricity, and heating
News Publication Date: 13-Aug-2026
Web References: https://doi.org/10.1016/j.xcrp.2026.103492; https://www.kit.edu/kit/english/pi_2024_037_innovative-material-for-sustainable-building.php
References: Iván Alberto Cruz García, Botho Lehmann, Shanhui Fan, Gan Huang, “Co-harvesting universe coldness and solar energy for tri-generation of cooling, electricity and heating,” Cell Reports Physical Science, 2026. DOI: 10.1016/j.xcrp.2026.103492
Image Credits: Gan Huang, KIT
Keywords
Passive daytime radiative cooling, solar energy, photovoltaic-thermal systems, hybrid energy harvesting, solar heating, electricity generation, cooling technology, atmospheric window, Fresnel lens, sustainable buildings, energy-efficient façades, data-centre cooling, Karlsruhe Institute of Technology, KIT
