Up-Draught Solar Chimney and Down-Draught Energy Tower - A Comparison

Two power plant types that utilize a convective flow for electricity generation are discussed: The so-called 'Solar Chimney', and a down-draught power plant named 'Energy Tower'. Both power plants utilize a convective flow caused by the density difference between the air column inside a large chimney - open at the bottom and the top - and the surrounding atmosphere: - Inside a Solar Chimney the air is less dense than the adjacent atmospheric air outside the chimney. This is accomplished by combining a large air collector with the central chimney. Hot air is produced by solar radiation and flows up the chimney, driving turbines installed at the chimney base. - The air inside an Energy Tower is denser than the adjacent air outside the chimney due to water being sprayed into the chimney at the chimney top. The water evaporates, thus cooling the air inside the chimney below ambient temperature. The air therefore flows down the chimney. The thermodynamic basics required to model both power plants are given. Results obtained with the simple models are compared to each other, to the values found in literature, and – in the case of the Solar Chimney – to measured values of a Solar Chimney prototype. In the case of the Energy Tower a comparison to measured values was not possible, as no test results from a prototype have been published yet. In general, there is good agreement between the values found using the simple models and the values from literature. In the case of the Energy Tower this is true with the restriction that agreement is only found as long as very optimistic meteorological conditions are taken as given. Otherwise the calculated electric output is significantly lower than the numbers claimed by the proponent. It is found that under real world meteorological conditions the electric power that can be generated with an Energy Tower is in the range of one fifth up to a maximum of approximately one third of the electric power of a Solar Chimney of the same (chimney) dimensions. When comparing the two power plant types, it must be kept in mind that the Solar Chimney requires a large air collector surrounding the chimney, whereas the Energy Tower does not.