"Electricity from hydropower" had started a true triumphal progress and for a long time nobody tinkered with the idea of questioning the turbine as the heart of a power station. This was not necessary, since the turbine industry knew perfectly well how to account for customer needs and natural constraints and how to supply appropriate products.

Has this situation actually changed in the recent past? Let us look for this purpose at the dimensioning theory for turbines or hydropower appliances. The best suitable technology is determined by the two magnitudes "head" and "rated discharge", which allows for a practically unlimited number of combinations. Nevertheless, there are a few domains where turbines are simply not the optimal solution. Reasons for lacking "enthusiasm" are either technological "forced solutions" or excessive costs, which make it difficult for turbine producers to access these niches.

On the one hand, these are the domains of low heads of about 1 to 3 metres (irrespective of the rated discharge) and on the other, of small flow rates with low and medium heads, i.e. the marginal zones between Q and H in the well-known dimensioning diagrams.

It is understood that these zones did exist in former times, but with the difference that there were other attractive possibilities of hydropower use and that the “peanuts” in power generation were therefore generously left aside. However, it is known today that smaller, apparently inefficient potentials, too, can be reasonably activated by a new approach!

In this context, the “classical” water wheel, for example, has experienced a revival for several years. Most effectively used in low heads (several metres) and smaller rated discharge quantities (several hundred l/sec), water wheels are mostly not made of wood any more, but of metal.

The generators are driven via belts and/or by a gear unit. The manufacturing precision offered, the design tailored to hydraulic needs, and the best-possible bearing presently allow efficiency degrees of already up to 75 per cent!

Only recently has a technology found its way into hydropower use, which is already well known in water conveying and wastewater management. It is the classical Archimedes conveying screw, which was simply turned upside down and thus “mutated” to a screw turbine.

The advantages of this a thousand-times- proven technology are, for example, low-cost manufacturing, good efficiency even in partial load, a technically simple overall system, and also its suitability for fish migration, a quality that should not be underestimated in times of the implementation of the EU Water Framework Directive. Low revolution speeds between 20 and 80/min promise minimal wear.

The unavoidable soiling tendency of small turbines in particular is hardly a problem for screw turbines. If polluted wastewater can be conveyed upwards, then flotsam, too, can be transported downstream without any problem. In addition, the installation of fine screens and thus of a screening system is not necessary. Screw turbines are limited by a head of approx. 10 metres and a maximal rated discharge of 5 m3/sec.

This old/new technology might find a further field of application, which would abolish these limits. The key word is “reserved flow” which will become a welcome possibility for many a hydropower station operator of limiting damage in connection with the specifications of the Water Framework Directive. In special cases of application, the classical turbine has thus found its master and small hydropower as a whole has been well served!
(Source: aqua press Int. 2/2004, Univ.-Prof. Dr. Bernhard Pelikan)

Contact & Information:

Universität für Bodenkultur Wien
a.o. Univ.-Prof. Dr. Bernhard Pelikan
Tel.: +43/676/33 100 37