Residents of apartment buildings face challenges accessing solar energy compared to homeowners with dedicated roof space. A pilot project in Bucharest has demonstrated an alternative approach using semi-transparent photovoltaic windows installed on a south-facing glazed balcony to directly power water heating in direct current, without intermediate battery storage or solar inverters.
The system consists of approximately 11 photovoltaic windows with a nominal capacity of roughly 845 Wp that supply hot water to a four-person household. Rather than feeding electricity into the apartment’s main electrical system, the photovoltaic glazing connects directly to a standard electric water heater, which functions as thermal storage. When solar generation is insufficient, typically after sunset, the controller switches the same heater to the standard alternating current grid to reach desired temperature settings.
Monitoring across two consecutive annual periods showed the photovoltaic windows supplied between 446 and 462 kilowatt-hours to the water heater annually, meeting approximately 44 to 47 percent of the household’s domestic hot water energy needs. The system remained operational throughout regular daily use, including approximately one shower per person daily. The measurements effectively represented about 11 months of actual household consumption in each monitored period, accounting for monthly absences.
A key advantage of this approach involves matching solar generation timing with usable demand. Unlike conventional balcony photovoltaic systems that must align instantaneous generation with immediate electrical consumption, a water heater can absorb varying power levels over extended periods and store the resulting thermal energy for later use. This eliminates the mismatch problem where generated electricity exceeds current apartment demand. Additionally, photovoltaic glazing designed for direct-DC water heating can achieve better electrical compatibility with heating elements than conventional module configurations, enabling more efficient power transfer to the thermal load.
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