Solar chimneys

Solar chimneys have long been used both to aid in natural cooling of homes and for passive solar heating. In recent times, a device similar to the traditional solar chimney has been used together with a solar collector and wind turbine to create a means of generating electricity from solar energy.

Background

There are three different applications for the device called a solar chimney. In all three cases, the application is called a solar chimney. The use that has been around the longest is for enhanced ventilation and cooling of living space. A second use is a particular type of passive solar-heating system, also called a thermosyphon. The third application is for the generation of electricity from solar energy.

The operation of a chimney in general is based on the fact that heated air will rise because of its decreased density. A chimney is used to exhaust flue gases from combustion in a stove, furnace, or fireplace; the air in the chimney is heated by the combustion at the base of the chimney and thus rises, creating an updraft through the chimney. With a solar chimney, however, nothing burns at the base of the chimney; rather, the air in the chimney is heated by the Sun, causing it to rise up the chimney and create an updraft.

For enhanced ventilation and natural cooling, the updraft caused by solar-heated air rising up the chimney is used to draw air through the house, increasing the ventilation rate and ideally drawing cool air into the house. The hot air is exhausted out the top of the chimney. Solar chimneys for natural cooling were in use hundreds of years ago in Rome and the Middle East.

The device that is sometimes called a solar chimney for passive solar heating is simply a rather standard air-heating solar collector mounted vertically on a house wall, facing the equator (facing south if in the Northern Hemisphere, and facing north if in the Southern Hemisphere). Just as in any solar chimney, there will be an upflow of solar-heated air. For this application, the heated air is sent into the living space to provide heating, rather than being sent out the top of the “chimney.” Cool air is drawn into the bottom of the vertical collector from the living space.

In the late 1970s, use of a large solar chimney, together with an extensive solar collector at the base of the chimney and a wind turbine or turbines at the top to drive a generator, was proposed as another way to generate electricity from solar energy (in addition to photovoltaic cells and heating a fluid with solar energy for use in generating electricity). The term “solar chimney” has come to be used for this latter system, as well as for the traditional type used for enhanced cooling or for passive heating. This concept has been demonstrated, and large facilities of this type are in the planning stages.

Enhanced Ventilation

The solar chimney for this application could be as simple as a traditional chimney painted black on the outside to increase the heating of the air in the chimney by the Sun shining on it. A more effective design would be to use glass for the side of the chimney facing the Sun, with a black absorbing surface on the back wall of the chimney, so the air between the glass and the black absorber would be heated. There must be a vent into the living space at the bottom of the chimney, so that the airflow up the chimney will draw air from the living space into the bottom of the chimney to be heated. This will cause increased airflow or ventilation through the living space, making it more comfortable. Increased natural cooling can be accomplished if the air drawn into the living space is cool. In some cases, the air into the house is drawn through an underground tunnel to cool it before it enters the house.

Passive Solar Heating

For a solar-chimney-type passive solar heater, the collector consists of insulation against the house and then a rectangular enclosure with a black absorber at the back, glazing in the front facing out, and an airspace between. The air between the glazing and the black absorber is heated by the solar radiation and rises up the airspace. Instead of venting the heated air out the top, as is done for enhanced ventilation, the heated air is directed into the living space through a vent that passes through the building wall at the top of the solar chimney collector. Another vent through the building wall draws cool room air into the bottom of the collector. This type of device could also be used to increase summer ventilation through the living space, by closing the vent into the living space at the top of the collector and opening the top of the collector so that the hot air is exhausted out the top. For this use, the top of the collector should be near the top of the house, or else the heated air going out of the collector will heat the portion of the house above the collector. This type of passive solar heating system is also called a thermosiphon.

Electricity Generation

The type of solar chimney used to generate electricity is a combination of three established technologies: the greenhouse, the chimney, and the wind turbine. The chimney, which is a long tubular structure, is placed in the center of the circular greenhouse, and the wind turbine is mounted inside the chimney. The solar-to-electric energy conversion involves two intermediate stages. In the first stage, conversion of solar energy into thermal energy is accomplished in the greenhouse (also known as the collector). In the second stage, the chimney converts the generated thermal energy into kinetic energy and ultimately into electric energy by using a combination of a wind turbine and a generator.

In its simplest form, the collector is a glass or plastic film cover stretched horizontally and raised above the ground. This covering serves as a for reradiation from the ground. The ground under the cover is heated and, in turn, heats the air flowing radially above it. The height of the collector cover gradually increases toward the center of the collector, providing a smooth transition for the hot air flowing from the collector into the chimney. A flat collector of this kind can convert a significant amount of irradiated solar energy into heat. The soil surface under the collector cover is a convenient energy storage medium. During the day, a part of the incoming solar radiation is absorbed by the ground; it is later released during the night. The mechanism is capable of providing a continuous supply of power all year round.

The chimney itself is the actual thermal engine. The upthrust of the air heated in the collector is proportional to the rise in air temperature in the collector and the volume of the air flowing. The latter depends on the height of the chimney. Mechanical output in the form of rotational energy can be extracted from the vertical air current flowing in the chimney by using a suitable turbine (or turbines). This mechanical energy can be converted into electric energy by coupling the turbines to the generators.

Solar chimneys do not necessarily need direct sunlight. They can exploit a component of the diffused radiation when the sky is clouded. The system also has an advantage over traditional systems that use wind to provide power in that it does not depend on the natural occurrence of wind, which always fluctuates. Moreover, because the direction of air movement is fixed, the complicated tracking mechanism necessary for a horizontal-axis wind turbine is not needed. Solar chimneys are relatively reliable and simple to build, and they do not have adverse effects on the environment. The necessary materials are readily available, and maintenance costs are minimal.

Less than four years after the solar chimney was first proposed in the late 1970s, construction of a pilot plant began in Manzanares, Spain. A 36-kilowatt pilot plant was built; it produced electricity for seven years, thus proving the efficiency and reliability of the technology. The chimney tower was 194.6 meters high, and the collector had a radius of 122 meters.

Bibliography

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