H.Grassmann, M.L.Ganis, Dipartimento di Fisica, Univ.di Udine, Italy

The importance of software systems like STAR-CD for engineering applications is well known, and as we show in this article, is an important tool for understanding the underlying flow physicists.

Our objective is to reduce costs and increase the power output of wind turbines. The principle behind our studies is to use the effect of static wing or sail structures, which convert energy more efficiently, to increase the efficiency of turbines. Many attempts have already been made during the last decades to use external shrouded systems, but with success only in wind tunnel studies, not in ambient air. The reasons become clear from our use of STAR-CD.

Based on a patent of the Grumman Corporation, a private company built a prototype at considerable expense, which failed to meet the expected success. Our STAR-CD studies of wind turbines with and without shrouds immediately showed the relationship between the force exerted by the flow on the turbine and transfer of both energy and linear momentum. Given a certain force, the energy transfer does not depend on the velocity of the flow, but the momentum transfer does. As a consequence, it is not possible to increase the power of a conventionally shrouded wind turbine beyond the theoretical limit for the same turbine without shroud (the so called Betz limit). With this realization, millions of dollars could have been saved before the prototype stage, with obvious benefits to the project profitability and overall success.

But the success did not stop there. STAR-CD was able to assist in finding a solution. Past shrouded systems closely fitted the propeller to minimize tip-vortex drag. If instead, one leaves a larger space between the propeller tips and the shroud, it has a beneficial effect over a wider radius of the propeller. Figure 1 shows one of our wind turbine models, surrounded by a shroud, which is curved like a sail. The surface area of the shroud is about 3 times larger than the area covered by the rotating propeller. Figure 2 shows the velocity in a cross section through the model in the flow direction. Contrary to the conventional system, the air accelerates as it approaches the turbine, and the static shroud plays an active part in the energy extraction of the system, hence the name “partially
static turbine”.

Figure 3 compares the mean total pressure in the flow tube, which passes through the propeller for the bare wind turbine and the shrouded one. The large pressure drop for the shrouded turbine could in principle also be achieved in an unshrouded system, but only for small wind velocities. In the shrouded system this large pressure drop occurs while the air is moving through the propeller at a mean axial velocity of 7.2 m/sec (while the ambient wind has a velocity of 5 m/sec only) – in an unshrouded system, or in a shrouded system, which does not interact with an additional flow of air, this situation would constitute a severe violation of energy and momentum conservation.

Figure 4 compares the power of the shrouded wind turbine compared to the unshrouded design. The increase in peak power is a factor of 4.
The same principle can also be applied to water. For a given flow rate, one can significantly reduce size of a Kaplan turbine. Or for a given turbine size, one can produce the same power at a lower flow rate. We expect this not only to reduce the price of hydro-power, but it should also open new applications, since the partially static turbine allows for hydro power construction in places where large dams are not feasible.

STAR-CD has taught us a lot about partially static systems. Still more can be learnt in the optimization of shrouded designs and prototype builds. We are actively searching for partners and collaborators in industry and other research institutes to take these studies to the next stage.

REFERENCES:
Bet F. and Grassmann H., ‘Upgrading conventional wind turbines’, Renewable Energy, January 2003, Elsevier Press, www.elsevier.com/locate/renene

Grassmann H., Bet F., Cabras G. Ceschia M>, Cobai D> and DelPapa C. ‘A partially static turbine – first experimental results’, Renewable Energy, to be published, ElsevierPress, www.elsevier.com/locate/renene

Ganis M., “CFD analysis of the characteristics of a shrouded turbine” www.diplom.de