Photovoltaic Windows Have Been Heating Water in a Bucharest Apartment for Two Years

A two-year test in a Bucharest apartment offers a look at a different way to use solar power if you don't have roof access. Instead of a typical balcony solar panel setup that sends electricity into the apartment's wiring, this system uses semi-transparent solar glass built into windows to send power directly, as DC current, into a regular electric water heater. There's no battery and no inverter. The water tank itself acts as storage: solar power collected over several hours gets converted straight to heat and held as hot water for later use. When the sun isn't providing enough, the same heater switches over to normal grid power to finish heating.

Over two one-year test periods, the window system covered about 44-47% of the hot water needs for a household of four. The setup, with roughly 845 watts of panel capacity built into 11 window sections, worked well with the water heater's electrical demands in a way that a couple of standard balcony panels typically would not, according to the testers. Beyond the water heating, the same glass also cut down heat coming through the windows, which the study estimated could reduce summer air-conditioning use by several hundred kilowatt-hours. Producing hot water locally, right at the point of use, also cut down on the water wasted while waiting for it to run hot from a shared building system.

This is an early pilot project, not a widely available product yet, but it points to one option for apartment dwellers who want to tap solar power without roof access, using window or balcony glass instead.