Solar Chimneys: Passive solar ventilation and electricity generation.
Posted by Alex Barrett on 7 June 2017 at 3:15 pm
The solar chimney is designed to harness the fact that hot air rises, providing passive ventilation for a building. A solar chimney can be used to cool a house during warm weather, and generate heat during cold times of the year. There have also been efforts to build industrial scale solar chimneys in order to generate clean electricity from hot air.
How does it work?
The density of air is strongly related to its temperature. As air heats up it becomes less dense than the colder air surrounding it. This means that the hot air will rise while the colder air sinks. This is why a hot air balloon floats when the air inside it is heated up, and why the hot, smoke filled air from a fire is drawn up the chimney, rather than spreading out to fill the room.
A solar chimney consists of a tall glass chamber. This is generally positioned on the equator facing side of the building, so that it will capture the most heat from the sun during the warmest part of the day. Like a greenhouse or conservatory this glass structure traps heat from the sun, warming up the air inside. This hot, buoyant air will rise to the top of the chimney, where it escapes through vents to the outside. This draws in cooler air from the bottom of the chimney to replace the warm air.
This system can be used to control the temperature in a house in a variety of ways. During warm weather the upper vents can be opened and cool air from outside can be drawn through the house by the action of the updraft in the chimney. It functions as a passive air conditioning system, with no need for electricity to run. During cold weather the vents can be shut, allowing the chimney to trap warm air. This can be used to supplement space heating provided the house’s insulation is good enough.
Another type of solar electricity
Larger, industrial scale, chimneys can also be used to generate renewable electricity. Unfortunately very few of these Solar Updraft Towers have ever been built, as they are very expensive to construct. A large area of desert, typically several kilometres square, is covered by glass, essentially creating a massive greenhouse. Rising air from across the greenhouse is channelled into a central chimney, called the updraft tower. It rises through the chimney where it turns turbines to generate electricity.
Updraft towers have always been very expensive to build, as a very large area has to be covered with glass or fabric. This can get broken, or covered with dust, and so requires regular maintenance. A test updraft tower was built at Manzanares in Spain in the early 1980s. It was 195 metres tall and 10 metres in diameter. The collection area covered 46 hectares. This tower produced 50 kW of power, and was operated for eight years.
A new version of the updraft tower is currently being developed, which replaces glass with light weight fabrics of the sort used in ballooning. This would have a massive advantage since the tower could be deflated during strong winds or bad weather. This could prevent it from suffering damage during storms, which is what ultimately destroyed the Spanish updraft tower.
There are plans to construct an updraft tower of this sort at the ALMA observatory in Chile’s Atacama Desert. This research facility is very energy intensive, but its remote location means that it is entirely off grid. All electricity is currently generated by onsite generators, burning fossil fuels. Replacing this with clean, solar energy would be a great way to capitalise on the readily available solar energy in the Atacama Desert.
It remains to be seen whether the newer fabric designs will prove more economically feasible than their predecessors. In a meantime a domestic solar chimney might not be large enough to produce electricity, but it is a great way to provide passive ventilation, making a house more energy efficient.
Solar Innovations: Solar Chimneys
DesigningBuildings.co.uk: Solar Chimneys
Wikipedia: Solar Updraft Towers
National Geographic: History of the Solar Updraft tower
Image Credit: “Solar Chimney prototype at Manzanares, Spain. Tower seen through the polyester roof” by Widakora CC BY-SA 3.0 via Wikimedia Commons
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