As our oil resources are getting scarcer day by day, we are forced to look for new sources of energy for our energy hungry world. Enormous efforts are being made to harness solar energy efficiently and economically. The potential of solar energy can be estimated from the fact that we receive about 1000 watts per square meter and a total of 105 TW (terra watts) of energy from the sun every year. Less than 1 per cent of his energy is sufficient to meet electricity demands of the whole world. The most common way and known way to harness suns energy has been the solar cell which converts light energy from the sun directly to electrical energy based on the Photoelectric effect discovered by the famous Scientist Albert Einstein in 1905. Due to high costs involved in extracting silicon and converting it into a semiconductor, solar cell is still the most expensive method to produce power from the solar energy. An extensive research has been carried out in the last years to lower the costs and improve its efficiency which is about 15-18 per cent till todays state of the art. Secondly, the storage devices must be attached to a solar cell to ensure a continuous supply of power. Storage devices such as batteries simply enhance the initial as well as the running costs of the system as they degrade with time.
A solar updraft tower is, however, based on a very simple principle conceived by Isidoro Cabanyes in 1903. Most of us know that the hot air is lighter than the cold air and drifts upwards. Hot air balloons fly in the air using this principle. A solar updraft tower uses this simple principle to harness energy from the sun. As the name suggests it consists of a tower with openings at its base for the air to pass through and drift to the top. A wide area around the tower is covered with glass or plastic sheets, called collector, to trap the air. When the sun shines, the earth is heated and it starts radiating back. The air below the glass roof is heated and tends to rise upwards. This hot air gradually moves towards the tower and starts rising up due to density difference between the top and the bottom of the tower. Greater the height of the tower higher the speed of hot air. In this way a continuous stream of hot air is generated. A turbine mounted at the base of the tower is set into rotation by the stream of air to produce electricity.
In contrast to solar cells a solar updraft tower keeps operating in the night as the earth continues to radiate heat enabling the air to flow for a much longer time. This duration can significantly be increased by placing water bags on the earth. The heat capacity of water is higher than the earth and hence can store more energy. Additionally, there are no operating costs as there are no batteries to be replaced or repaired and no fuel to be poured in. However, the efficiency of an updraft tower is much lower than the solar cells. It ranges theoretically from 2-3 per cent depending upon the temperature and height of the tower. At first, updraft tower does not seem to be a practical solution due to its extremely low efficiency. However, solar efficiency must not be the decisive parameter for determining the viability as the hydral power plants work economically and nobody criticized them ever due to their lower solar efficiency. Very few are aware of the fact that sun is actually fueling hydle power plants to produce electricity by evaporation, cloud formation and finally blowing winds to transport water to the lakes and reservoirs where dams are built. Although less efficient but simple to manufacture and easy to maintain, a solar updraft power is fully capable and a feasible way of harnessing solar energy.