Based on detailed theoretical preliminary research and a wide range of wind tunnel experiments Schlaich Bergermann has designed, constructed and operated an experimental plant with a peak output of 50 kW on a site made available by the Spanish utility Union Electrica Fenosa in Manzanares (about 150 km south of Madrid) in 1981/82 (see background figure), with funds provided by the German Ministry of Research and Technology (BMFT).
The aim of this research project was to verify, through field measurements, the performance projected from calculations based on theory, and to examine the influence of individual components on the plant’s output and efficiency under realistic engineering and meteorological conditions.
The table to the right presents the main parameters of our experimental plant.
The collector roof of the solar updraft tower shall not only be translucent but also durable and reasonably priced. A variety of plastic membranes, as well as glass, were selected in order to determine which was the best – and in the long term, most cost effective – material.
Glass resisted heavy storms for many years without harm and proved to be self-cleaning thanks to the occasional rain showers.
In Manzanares membranes became brittle with time and thus tended to tear. Material (temperature and UV stability) and design improvements (e.g. membrane domes) achieved in the last years will help to overcome this particular disadvantage.
The turbine is supported independently of the tower on a steel framework. It has four blades, which are adjustable according to the face velocity of the air in order to achieve an optimal pressure drop across the turbine blades. Vertical wind velocity is 2.5 m/s on start-up and can attain a maximum of 8 m/sec.
The tower comprises a guyed tube of trapezoidal sheets, gauge 1.25 mm, corrugation depth 150 mm. The tower tube stands on a supporting ring 10 m above ground level; this ring is carried by 8 thin tubular columns, so that the warm air can flow in practically unhindered at the base of the tower.
A pre-stressed membrane of plastic-coated fabric, shaped to provide good flow characteristics, forms the transition between the roof and the tower.
The tower is guyed at four levels, and in three directions, to foundations secured with rock anchors. The tower was erected at ground level, utilizing a specially developed incremental lifting method: First, the top section of the tower was installed on a lifting ring on the ground, and then it was raised onto the supporting ring by means of hydraulic jacks. Subsequently the other sections were assembled on the ground, connected to the already installed top tower section(s) and then the whole assembly was lifted. So the complete tower was built in 20 shots of 10m each.
In 1987 the plant was in operation for a total of 3197 hours, which corresponds to a mean daily operating time of 8.8 hours.
As soon as the air velocity in the tower exceeded a set value, typically 2.5m/s, the plant started up automatically and was automatically connected to the public grid. These results show that the system and its components are dependable and that the plant as a whole is capable of highly reliable operation. The advantages of the system are these: global solar radiation is exploited and the thermodynamic inertia is a characteristic feature of the system, continuous operation throughout the day is possible.
The figure shows a comparison between the measured and calculated average monthly energy outputs, showing that there is good agreement between the theoretical and measured values. Overall, it may be said that the optical and thermodynamic processes in a solar updraft tower are well understood and that models have attained a degree of maturity that accurately reproduces plant behavior under given meteorological conditions.
After the Manzanares proof-of-concept Solar Updraft Tower (‘solar chimney’) prototype had been dismantled, the German government sponsored a large “Transferability study” that investigated, if and how the results from Manzanares can be used to design and calculate large scale solar updraft power plants up to 200 MW.
The results were very encouraging.