A plant operated by E.ON Wasserkraft in Germany will be the first in the world to commercially apply high-temperature superconductor materials to the hydropower sector. Those involved say use of the materials offers significant output and efficiency benefits over conventional copper-based generators, particularly for small hydropower facilities.
By Andrew Tan
Refurbishment and upgrade of aging facilities are high on the agenda as owners of hydroelectric projects seek to maximize performance and output of existing assets. The use of technologies that can increase output and efficiency while minimizing project cost is of particular interest as a means of increasing the financial returns from energy investments.
One approach being pioneered commercially for the first time in the world involves the use of variable-speed generators based on high-temperature superconductor (HTS) materials to upgrade smaller run-of-river hydro plants. This technology can dramatically increase annual electricity output when used as an alternative to conventional copper-based generators.
Zenergy Power plc is one company involved in the development of HTS materials for hydroelectric facilities. As a result of a four-year collaboration, the 3.75-MW Hirschaid plant on the Regnitz River in Bavaria, Germany, will be the first plant in the world to demonstrate the commercial application of HTS technology to hydropower generation.
Choosing a superconductor for Hirschaid
E.ON Wasserkraft GmbH owns and operates Hirschaid. The plant was built in the 1920s and contains three 1.25-MW twin Francis turbine-generators. To increase the electrical efficiency of the generator and improve the economic return of the project, E.ON Wasserkraft decided to replace one of the three units with a 1.7-MW machine based on HTS rotating coils. This unit will be connected directly to the German grid as Hirschaid's new and ongoing baseload generator.
As well as an increased capacity of around 36 percent from the 1.2-MW original, the use of HTS technology offers significant size and weight advantages. For example, use of the HTS generator will allow a plant to be upgraded without a need for major construction work to accommodate much larger units. This keeps refurbishment costs to a minimum, reduces refurbishment downtime, and avoids issues surrounding permits and insurance.
Along with Zenergy Power, which developed the superconducting technology, E.ON Wasserkraft was a partner in a project called Hydrogenie, which the European Union funded to develop and test a compact HTS hydropower generator. Other partners in Hydrogenie are: Converteam Ltd., United Kingdom; KEMA Power Generation & Sustainables, The Netherlands; Stirling Cryogenics & Refrigeration BV, The Netherlands; Politechnika Slaska, Poland; and Vector Fields Ltd., United Kingdom.
Converteam designed and manufactured the generator, while KEMA provided power network analysis and testing. Stirling Cryogenics worked on cryocooler development, Politechnika Slaska carried out HTS materials analysis, and Vector Fields contributed software tools design.
Once operational at Hirschaid, the superconducting generator will be responsible for supplying 3,000 of E.ON's customers served by the facility, creating a landmark for the use of HTS technology in the hydro sector. Upgrade work at Hirschaid is under way now and is due to be complete by the end of 2010.
RWE also investigating HTS technology
E.ON is not the only hydropower operator investigating the potential of HTS technology. Zenergy Power has been commissioned by German utility RWE Power AG to prepare a study evaluating the potential increase in electrical output that could be achieved by the use of HTS variable-speed generators at nine of its run-of-river plants along the Mosel River.
RWE operates ten hydro plants totalling 180 MW on the Mosel River system, including two plants in France. Zenergy will analyze data from one selected plant on the river, taking into account water flow rates, turbine efficiencies, and turbine conditions. That study will be used to estimate the effect of Zenergy's superconducting technology on nine similar plants. Once RWE receives the study results, the utility will conduct an economic evaluation of the business case for replacing conventional generators with the new technology. This work is ongoing.
Technical background to HTS
Superconductors are materials that display zero resistance to the flow of electricity under certain low temperature conditions. As a result, they have two distinct characteristics: 100 percent efficiency and 100 times the current carrying capability of copper.
Recent developments have opened the way for new materials that exhibit superconducting properties at significantly higher temperatures than previously possible. These developments bring their operating temperature within the reach of commercially available cooling systems and make them practical for application in hydropower and other industries, such as offshore wind.
When applied to the production of generators, these materials have profound implications for performance. In the case of variable-speed hydropower, this results in compact generators with very high power take-off performances, enabling the efficient production of electricity under varying generator speeds and torques.
It is this very high power take-off characteristic that results in hydropower stations being able to produce more power, under low or varying water flow conditions, all year round.
Another advantage of HTS generators is their compact size. Superconductor wire can carry more than 100 times the electrical energy current of copper wires (see Figure 1), enabling builders to dramatically reduce the size of rotating machines.
|The generator room at Hirschaid before replacement of the baseload unit|
The machine developed through the Hydrogenie project features HTS rotor coils and a copper stator. The rotor's field coils are cooled to around 30 degrees Kelvin using high-pressure helium gas transferred from static cryocoolers via a custom-designed rotating coupling.
One further advantage to the use of zero-resistance rotor coils is an improvement in machine efficiency due to the reduction in internal heat losses. As a result, the machine being built for E.ON will be more than 98 percent efficient.
Opportunities for HTS in hydropower
Opportunities for HTS technology in hydropower are focused on small to medium run-of-river and storage locations, which have the lowest environmental and ecological effects. The technology is viable for application to hydropower above the 1 MW level.
According to the European Small Hydropower Association, 56 percent of Europe's installed hydropower assets are already in the "refurbishment cycle" (40+ years old). In fact, 32 percent of plants are 40 to 60 years old and 24 percent are older than 60 years.
Of this 130,000 MW of old installed hydropower, 55 percent is in an immediate addressable market considered to be micro (<1 MW), small (1 to 10 MW) or medium (10 to 100 MW).
Depending on site-specific conditions, the superconductor engineering approach can increase the annual power output of a given hydropower asset so that it generates additional electricity equivalent to a further 1,250 hours of full load operation annually, a 25 percent increase on conventional hydropower solutions.
For European installed assets greater than 40 years old (and therefore in the refurbishment cycle), that equates to a further potential annual electricity production of about 90,000 gigawatt-hours.
The focus of those developing HTS technology is on applying variable-speed turbine applications at existing stations that contain older, fixed-speed turbines. Adoption of a variable-speed setup has its greatest financial return when the power intensity of the water flow varies. Typically, locations with variable water flows tend to be equipped with smaller hydro stations operating in the lower range of heads (30 meters and below).
For this reason, there is a particular focus on the refurbishment of fixed-speed Kaplan turbines in vertical and horizontal bulb configurations. In many cases, these hydro stations are operating with low heads in water that is subject to a high degree of variability throughout the course of a year. In these instances, implementation of the HTS variable-speed turbine solution enables the conversion of a greater amount of water energy into electrical energy. This is made possible because variable-speed turbines are able to harness the lower power levels drawn from reduced water flows, which typically mean that fixed-speed turbines have to operate in part-load, where their efficiency levels drop dramatically.
In a typical arrangement of a fixed-speed Kaplan bulb turbine, the turbine drives a gearbox that is attached to the generator. These turbines are known as "double regulated," in reference to the fact that they need to operate two sets of hydraulic turbine blades to maintain a consistent speed of rotation in water flows with variable speeds and power.
This arrangement also requires investment in a hydraulic control system to control these turbine "regulation blades" and also in excitation machines for the copper rotor in the fixed-speed generator. These two ancillary systems, combined with energy losses in the gearbox, all contribute to an overall lower efficiency performance in converting the energy in the water flow delivered to the turbine into electricity.
In the case of the same bulb Kaplan turbine operating with a variable-speed solution, there is no gearbox requirement. Instead, a superconductor "direct-drive" generator is connected directly to the variable-speed turbine. HTS generators are more efficient than conventional copper-based machines and most importantly display very high power take-offs. This makes them ideal for operation in the direct-drive arrangement where often they will be rotating at lower speeds with less power being provided by the turbine operating in lower water flows. Despite these conditions, HTS generators still generate useable and saleable electrical power, and removal of the gearbox contributes to greater efficiencies and a higher power output under all operating conditions.
In addition, a variable-speed turbine has no requirement for hydraulic regulator blades. Thus, there is no need for ancillary hydraulic control systems that require upfront capital investment and ongoing monitoring and maintenance and draw energy from the turbine during operation.
A further benefit from the HTS machine is the eliminated requirement for investment in larger excitation machines. This results from the high-performance "zero resistance" rotor coils. Instead, the variable-speed configuration requires just the investment in a converter for grid connecting the variable-speed generator.
Increased power production
The real economic gain of this approach is the increased power generation achieved as a result of the refurbishment. This increase can be 10 to 25 percent annually, depending on site-specific conditions.
Figure 2 illustrates the efficiency with which a fixed-speed turbine is able to deliver power from a varying water flow to the generator compared to the greater efficiency achieved across the various water flow rates achieved when operating a variable-speed turbine.
The distinct superiority of the variable-speed arrangement means that, in all but perfect flow rate conditions, more energy is being drawn from the water source and delivered to the generator to produce electricity.
Summary of benefits to hydro
The key aspect of the HTS proposition for hydropower is the ability to significantly increase the rate of return on refurbishment projects by offering a high power output technology solution at a cost that is lower than traditional solutions. For example, assuming a 5-MW project, the equipment cost for a HTS installation is about 3 million euros (US$3.7 million) against 3.7 million euros (US$4.6 million) for a conventional system.
In this way, superconductors are opening up the world's installed hydropower asset base as a new means of investing in renewable power capacity. Unlike conventional investments in renewable power, these opportunities are characterized by, among other factors:
— Reduced upfront capital deployment and project risk;
— Minimal infrastructure investment and construction;
— No grid connectivity issues;
— Minimal civil engineering works and supply chain issues;
— Short project time frame (less than one year to revenue generation);
— Minimal regulatory interaction and licence application requirements; and
— Highly visible, predictable, and measurable long-term revenue generation.
Adding to the attractiveness of HTS technology is the high quality of the capacity being brought on line. This quality is characterized by the high number of full load hours offered by HTS hydro assets, their predictable nature (high capacity factors and low implied grid storage requirements), and their long-term generating potential (40 to 60 years).
Andrew Tan is head of corporate relations for Zenergy Power plc, a developer of high-temperature superconductor materials and devices. The company has subsidiaries in Germany, the United States, and Australia.