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Design and Fabrication of Thermochromic Energy-Harvesting Windows

National Renewable Energy Laboratory

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Technology Marketing Summary

Smart windows, such as electrochromic or thermochromic windows, can intelligently regulate their transparency, allowing for control of transmitted light or heat into building interiors.  In turn, control of transmitted light and heat allows for improved energy efficiency and occupant comfort.  While electrochromic windows are user-controlled and change from a resting transparent state to a light-absorbing state when an external voltage is applied, thermochromic windows avoid the added expense of electrical switching by changing from a transparent state to a tinted state when heated by sunlight. As such, thermochromic technologies are projected to be both more energy efficient and cost-effective.

Today’s conceptual thermochromic technologies utilize a ligand exchange process to effectuate the phase change of vanadium dioxide. However, these technologies must inconveniently avoid moisture during manufacturing, maintain inert conditions for continued functionality, and are unable to convert sunlight into energy. To improve upon these limitations, NREL scientists have developed a novel design and fabrication of a window insert that combines the functionality of a thermochromic window with a photovoltaic (PV) cell.


This novel thermochromic window insert is processed from aqueous solutions and is composed of non-toxic materials for scalable solution processing. When heated by incident sunlight, the device switches from its resting, transparent state to a tinted, light-absorbing state by a phase-change process wherein a volatile ligand bonded to perovskite film undergoes a reversible phase-change into a gaseous phase, allowing the perovskite film to adopt a tinted crystalline structure that is photovoltaically active. When in this tinted state, the device can convert incident light into electricity through leveraging the known properties of perovskite films. Not only does this allow for cooling of a building interior by absorption of incident sunlight and heat, power is also generated that may be funneled back into the building to power air conditioning or similar units.

This window insert does not require a fixed temperature for phase changes, allowing this design to be tunable for any environment, is easily incorporated into existing window fabrications, and has demonstrated switchable absorption in the near infrared and visible spectrum. In addition, the insert does not require an inert environment, which can ultimately reduce the cost of production.

  • First example of a “switchable” energy-harvesting window
  • Reduces production costs by allowing manufacturing in atmospheric conditions
  • Can be inserted into existing window fabrications
  • Switching temperature is tunable for any climate
  • Switchable absorption in the near infrared and visible spectrum
Applications and Industries
  • Windows
  • Automobile Industry
  • Commercial and Residential Buildings
Technology Status
Technology IDDevelopment StageAvailabilityPublishedLast Updated
ROI 15-93 and ROI 15-108PrototypeAvailable10/05/201610/05/2016

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To: Bill Hadley<>