Energy tower (downdraft)

However, power generation by the energy tower is affected by the weather: it slows down each time the ambient humidity increases (such as during a rainstorm), or the temperature falls.

Updraft towers do not pump water, which increases their efficiency, but do require large amounts of land for the collectors.

If the conversion efficiency turns out to be much lower, it is expected to have an adverse impact on projections made for cost of energy.

Projections made by Altmann[4] and by Czisch[5][6] about conversion efficiency and about cost of energy (cents/kWh) are based only on model calculations[7], no data on a working pilot plant have ever been collected.

Actual measurements on the 50 kW Manzanares pilot solar updraft tower found a conversion efficiency of 0.53%, although SBP believe that this could be increased to 1.3% in a large and improved 100 MW unit.

The usage of water as a working-medium dramatically increases the potential for thermal energy capture, and electrical generation, due to its specific heat capacity.

While the design may have its problems (see next section) and the stated efficiency claims has yet to be demonstrated, it would be an error to extrapolate performance from one to the other simply because of similarities in the name.