A comparison of Francis and Deriaz pump-turbine units for low-head applications indicates both can fulfill the wide operational range and power regulation requirements, but they differ in terms of implementation of the mechanical and electrical aspects of the technology.
By Ales Skotak and Petr Stegner
|This article has been evaluated and edited in accordance with reviews conducted by two or more professionals who have relevant expertise. These peer reviewers judge manuscripts for technical accuracy, usefulness, and overall importance within the hydroelectric industry.|
Energy production from renewable sources worldwide is highly dependent on weather conditions. Effective regulation of energy production is impossible for solar and wind facilities, and it is limited in the case of run-of-river hydropower plants. Thus, it is not practicable to adjust production from such sources to meet the requirements of consumers and the energy market.
Such a situation leads to the necessity of energy storage. There are many ways to store energy, but reasonable solutions are scarcer when discussing large storage with a capacity of hundreds of megawatt-hours. The most popular solution for large-scale energy storage involves the installation of pumped-storage hydro facilities. The most profitable solutions for this technology are usually expected at heads (water level difference) of about 300 m or more. However, for countries that are relatively flat, the only option is to build low-head pumped-storage plants, usually at heads of less than 100 m. Another reason low-head pumped-storage plants may be built is to situate energy storage facilities close to wind or solar farms, which are usually installed in relatively flat areas. The benefit is to shorten transmission lines from the alternative energy sources to the hydro storage facility, thus minimizing grid overloading due to energy transfer across a country.
Concept of low-head pumped storage
In building a pumped-storage plant, it is important to establish the useful water storage capacity of the site. Usually, pumped-storage plants are designed to supply electricity, when needed, for about four to six hours. Especially in the case of low-head pumped storage, such a requirement leads to a large variation in head, usually +/- 30% of the rated head. In addition, it is profitable to extend the ability to regulate power input and output in the pumping and generating modes up to 30% of rated power to allow effective stabilization of the electric grid. No less important is the ability to start pumping or generating in a very short period time or the ability to black start, which allows recovery of the electrical grid after a blackout.
As compared with high-head pumped-storage plants, low-head plants do not impose any special landscape requirements. Suitable locations for low-head pumped storage can be found on a river or at existing low-head dams. Artificial hills are not excluded. The dam is usually designed as a gravity dam structure. Because of the low loads this dam will experience, it is usually designed as an earth structure. The powerhouse is typically located outside the dam but is occasionally underground in a cavern. The headrace tunnel can lead under the dam or be placed in the open air as a tube-type penstock. Due to the short length of the headrace tunnel, it is not necessary to equip the hydraulic system with a surge tank. The main inlet valve, usually of a butterfly type, is able to close to the full discharge.
Selection of pump-turbine technology
The proposed solutions focus on a two-machine configuration, i.e. one impeller/runner connected to one motor/generator. The mostly used standard pump-turbines of the Francis type must be furnished with special electrical equipment that allows variable speed in order to cover the required operating range in both modes of operation. This should be realized by means of a doubly-fed asynchronous generator or a full frequency converter connected to a synchronous generator. This is a modern and progressive solution for units with a capacity up to 100 MW. The alternative solution is to install diagonal pump-turbines of the Deriaz type frequently installed in the 1960s. These types of machines are able to cover a wide operational range thanks to their adjustable runner blades without requiring special electrical equipment.
These solutions are comparable in both costs and hydraulic parameters. Figure 1 compares the main dimensions of the hydraulic outlines for an 85 MW pump-turbine. Other possible solutions are not discussed because their operating ranges are limited or too costly.
In the case of the Deriaz turbine, the advantage is that this is a purely mechanical solution. The adjustable runner blades require a more complicated design, including a regulating mechanism in the runner hub, a hollow shaft with a regulating rod, and a regulating oil head (see Figure 2 on page 30). A similar solution is well-known in Kaplan turbines.
Development of pump-turbines
The hydraulic design of the pump-turbine is a sophisticated process. The standard intuitive engineering approach is applied, supported by optimization methods and a numerical flow analysis (computational fluid dynamics). In addition to the requirement for efficiency and safe operation of every pump-turbine, it is necessary to design the machine with stable characteristics in the pumping mode of operation. Pumping mode instability is indicated by a fall in the pumping head accompanied by huge pressure pulsations. Analysis of the CFD results shows a possibility of predicting the margins of stable operation for the pump-turbine.
After the hydraulic design, pump-turbine models of the Francis and Deriaz type were constructed and tested at the hydraulic laboratory of CKD Blansko Engineering (a member of Litostroj Power Group) on a universal test rig. Tests were focused on the performance, efficiency and cavitation measurements in the pumping and turbine modes. The results were applied to determine final prototype dimensions and to evaluate the margins of operation due to cavitation, as well as instabilities or pressure pulsations. The Deriaz unit was tested with eight and 10 runner blades.
Operational range of pump-turbines
Connection of the Francis pump-turbine with a synchronous generator with full frequency converter allows for changing the actual machine speed, which provides one more parameter for regulation.
In the case of the Deriaz pump-turbine, the parameter for regulation is naturally the runner blade angular setting.
The operational diagrams of both types of units in the pump-turbine mode are considerably different (see Figure 3). The hydraulic efficiency level of both the Deriaz and Francis units is comparable at the maximum head, but the Francis pump-turbine with variable speed is more efficient at lower heads in both operational modes.
Submergence and cavitation features
The operating range of a pump-turbine is based mainly on the cavitation and stability limitations in pumping mode or pressure pulsations in both modes. Using model measurement in the laboratory, the influence of cavitation and pressure pulsations on the thoma cavitation number was evaluated and then the required submergence of the prototype turbine was determined. For the case of the pump-turbine submergence setting, the behavior in pumping mode is most important, especially due to leading edge cavitation. In the case of low-head pump-turbines, some amount of leading edge cavitation can be accepted without risk of cavitation erosion occurring. Due to its higher specific speed, the Deriaz pump-turbine has worse cavitation features, which results in higher submergence in comparison with the Francis type solution of about 6 m.
Starting up in pumping mode
Pump-turbines are generally required to start very quickly (less than 2 minutes) to react to grid requirements or to restore the electrical grid. The standard Francis pump-turbine with a synchronous generator needs runner aeration to start up in pumping mode, for example by means of a pony motor or starting frequency converter. The situation is different in case the unit is combined with a full frequency converter. This allows the unit to be started up in pumping mode with a water-filled runner space without any air pressure device or pony motor. The frequency converter is able to manage the pumping operation start-up by frequency adjustment with adequate power input without any bump to the electrical grid.
Deriaz pump-turbines connected to a synchronous generator can also be started up in the pumping mode with a water-filled runner space. This is done with closed runner blades by means of a starting up frequency converter dimensioned at about 1/10 of the pump-turbine rated power.
For low-head pumped-storage plants, both Francis and Deriaz pump-turbines fulfill the requirements for wide head range operation and power input/output regulation. In the case of a Francis pump-turbine, the additional parameter for power input regulation is the turbine variable speed regulated by a full frequency converter. The additional cost of this converter is compensated by a simpler mechanical solution of the runner and lower cost of excavation works for the lower submergence. The advantage of a Deriaz pump-turbine operating at synchronous speed is purely mechanical and therefore a robust solution for the wide operational range with the ability to regulate power input/output without any necessity of additional electrical equipment, which results in less opportunity of failure.
Ales Skotak is director of the research and development department of hydraulic machines and Petr Stegner is a senior design engineer responsible for mechanical design with CKD Blansko Engineering a.s. – Litostroj Power in the Czech Republic.