Using waste heat from photovoltaic panels to generate residential hot water – PV International Magazine

Scientists in the United States have developed a new thermal photovoltaic system design that uses parallel water tubes as a cooling system to reduce the operating temperature of photovoltaic panels. The waste heat generated from this process is then used to generate domestic hot water.

December 12, 2023

Researchers at the Multiphysics Interaction Laboratory (MiLab) in the US have developed a new design for a photovoltaic thermal (PVT) system that uses waste heat from PV panels to generate residential hot water systems.

The system relies on parallel water pipes that are attached to the back of the solar panels and lower their operating temperatures. “Our study presents a simple and practical cooling method that enhances the electrical efficiency of PV panels while at the same time offering a sustainable solution to residential hot water needs,” the scientists said.

The experimental system is based on a 250 W polycrystalline photovoltaic panel oriented towards the south with a temperature coefficient of -0.45%/m and an inclination angle of 30°. Copper cooling tubes are connected through the upper and lower headers and are covered with an aluminum cap to attach to the back side of the PV panel.

The experimental setup includes a hot water tank and an 11 W centrifugal pump that maintains a constant flow rate of 3 L/min.

Experimental setup

Image: Multiphysics Interaction Laboratory (MiLab), International Journal of Thermal Fluids, CC BY 4.0

“The water flow rate is monitored using a flow meter,” the scientists explained. “To collect accurate data, an array of thermocouples was strategically placed to simultaneously measure various temperatures, including the water temperatures entering and exiting the PV/T system, the surface temperature of the PV panel, and the ambient air temperature.”

The system also uses a multimeter to measure the PV module output power and a pyranometer to evaluate solar radiation.

The academics compared the performance of cooled PV panels to a reference panel without cooling during May at the University of Notre Dame Louaize campus, located in Zouk Mosbeh, Lebanon.

Illustration showing the different parts of a PVT system

Image: Multiphysics Interaction Laboratory (MiLab), International Journal of Thermal Fluids, CC BY 4.0

Their analysis showed that a PVT panel can generate 4% more power than a PV module, thanks to the cooling effect of the copper tubes. However, they also warn that the saturation effect associated with not using the hot water stored in the tank effectively, may limit the ability to cool the PV modules optimally at some point.

The researchers also explained that “this decrease is likely due to the absence of residential water demand in the waste heat recovery system.” “This absence causes the temperature of the cooling water in the hot water tank to gradually increase, which subsequently reduces the rate of heat removal from the PVT system.”

Tests also showed that photovoltaic panels achieved an average electrical efficiency of 11.5% while photovoltaic panels without cooling achieved an average efficiency of 10%. The scientists also revealed that the overall efficiency of the PVT system was about 75%, and that the reference PV system was only about 10%.

They also stated that “the average thermal efficiency, which represents the ratio of recovered waste heat to solar energy absorbed by PV panels, was about 60% in the cooled PV/T system.” “The chilled PV/T system not only generated a larger fraction of electrical energy, but also showed an approximately 40% higher rate of heat loss from the back side of the PV module, indicating efficient waste heat recovery.”

The system is presented in the paper”“Experimental analysis of hybrid photovoltaic thermal system through parallel water pipe integration,” published in the International Journal of Thermal Fluids.

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