Effect of humidity on the performance of rooftop solar chimney

When the air gets heated up, it expands and hence, becomes lighter. The less-dense warm air rises up against the gravity, allowing cold, heavy air to sink. This is the fundamental mechanism by which heat gets redistributed with the fluid medium following the natural convection mode.

Natural convection, well, happens naturally. It means no external source of power is used to push the air around. Take a flat black plate on a rooftop and expose it to sunlight. You will notice that after a while, the plate gets hotter. Natural convection heat transfer will then take place between the hot plate and the surrounding air. If we expose the surface to solar radiation for a longer duration, its temperature will further increase. But air can only gain a certain amount of heat. So, in long exposures, the plate loses heat by the radiation mode of heat transfer as well.

Concept diagram of a solar chimney

In solar thermal applications, heat transfer by natural convection is almost always coupled with the radiation mode. Take a solar chimney, which is a device that consists of an absorber plate (to absorb sunlight) and a glass cover. The daylight first passes through the glass cover before it strikes the absorber plate. The glass cover is added to trap the long-wavelength radiation emitted by the absorber plate as it gets heated up. The chimney, with its bottom end open, is usually mounted on the rooftop of a room. 

The air present in the space between the absorber plate and glass cover will get lighter and rise upward. This creates a vacuum inside the room. Fresh cool air through windows or open ventilation inside the room will move towards the chimney, inducing air circulation.

While the air inside the chimney predominantly picks up the heat by convective mode of heat transfer, the fundamental mechanism changes when air carries some moisture with it. The thermal properties of water vapour are different from dry air and can potentially affect the natural convection inside the chimney. Also, water vapour can absorb long-wavelength emitted by the absorber plate.

Graduate students under my guidance had investigated the effect of humidity (amount of water vapour present in air) on the performance of a rooftop solar chimney. Our study [1] shows that water vapour present in the air can improve the overall performance of the solar chimney (measured in terms of air change per hour inside the room) by 10%.

[1] Himanshu Dahire, Srinivasa Ramanujam Kannan, and Sunil Kumar Saw, 2021, Effect of humidity on the performance of rooftop solar chimney, Thermal Science and Engineering Progress, (available online).