Researchers at Stanford University have developed a fluid-filled roof panel which can make air-conditioning significantly more efficient.

Conventional air-conditioning systems simply vent hot air from an outdoor fan unit. In the Stanford design, a heat exchanger transfers heat from the normal air-conditioning refrigerant into a fluid, which can be either water or glycol. This fluid then flows into rooftop cooling panels and the heat is dissipated into the sky.

In previous attempts to use this approach, on hot sunny days the sun made the fluid-filled panels heat up rather than cool down – as in a solar hot water system.

This problem has been solved by the design of super-reflective surfaces that can repel 97% of the incoming solar energy.

The best-performing surface is a shiny, flawless layer of silver – but that doesn’t remain shiny in outdoor conditions. Three research groups developed possible solutions, two involving plastic coverings for the silver and the third involving a complex layering of different oxide materials.

With these super-reflective surfaces, the rooftop will lose heat during the day in the same way that it does on a clear night. In testing, the panels cooled water up to 5°C below the ambient air temperature.

The Stanford researchers calculate that the panels could reduce air-conditioning costs by 21% for a typical two-storey building in the sunny climate such as Las Vegas in summer.

The panels can easily be retrofitted onto existing air-conditioning systems.

Cooling systems consume 15% of electricity generated globally and account for 10% of global greenhouse gas emissions. With increasing use of air-conditioning, the potential savings are enormous.