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Creating a Hybrid Hydro-Wind System on a Spanish Island

When a hybrid hydro-wind plant is developed on the island of El Hierro, nearly 65% of total electricity demand will be met using these renewable resources. Currently, all of the island's demand is being supplied using expensive, environmentally unfriendly diesel.

By Miguel Á. Fernández Centeno, Agustín Marrero Quevedo, Juan Manuel Quintero Gutiérrez, Rafael Caballero Nueda and John Hart

The island of El Hierro, a part of the Canary Islands chain off the coast of Spain, is implementing a hybrid hydroelectric and wind plant to meet its energy needs using renewable sources. This facility integrates a pumped-storage plant with 11.32 MW of generating capacity and 6 MW of pumping capacity with an 11.5 MW wind farm. Working together, the two technologies will help create reliable, efficient and stable power supply for the 10,960 persons residing on the island.

This project, being developed by Gorona del Viento El Hierro S.A., is under construction. The facility is expected to be completed by the end of 2012 and operating by March 2013.

Understanding the situation

El Hierro, the smallest and most southwestern of the Canary Islands, is 250 km off the African coast in the Atlantic Ocean. Peak power demand on the island is 7.56 MW, and electricity is supplied by an 11.36 MW diesel plant. Annual energy demand on the island is about 35 GWh, and the growth rate has been 8% a year. This is expected to stabilize in the next three to five years at an annual rate of 4%.

Results of this situation are:

— Dependence on foreign energy sources, through buying 100% of the fossil fuel consumed;

— Increasing greenhouse gas emissions as energy consumption grows above the amounts Spain agreed to in the Kyoto Protocol; and

— High cost of electricity generation on El Hierro of €0.242/kWh (US$0.318/kWh).

El Hierro has a total available wind energy resource of 49.6 GWh, which could completely supplant diesel generation while reducing the undesirable consequences mentioned above. Wind energy costs on average €0.072/kWh ($0.095/kWh).

Moreover, the island was declared a Biosphere Reserve through the Man and the Biosphere Programme of UNESCO in January 2000. This seal was awarded to El Hierro for the special conservation of its environmental and cultural richness, as well as for its efforts toward the progress and development of its people. This means any action aimed at reducing the anthropic pressure on its habitats through the self-supply of electricity via renewable means is considered of outstanding importance.

Within this framework, three organizations — the Cabildo of El Hierro (island government), Endesa S.A. and the Canary Islands Technological Institute (ITC) — formed Gorona del Viento El Hierro in 2004 to develop a project called the El Hierro Hydro-Wind Plant. The company is a partnership of the council (60%), Endesa (30%) and ITC (10%). The goal of this work was to make the island fully sustainable with its own renewable energy sources.

This possibility was included in the El Hierro Sustainability Plan, approved in November 1997, and the El Hierro Management Island Plan, which was approved in June 2002. Both plans helped move the project forward.

In March 2007, the general director of the Institute for Diversification and Energy Saving (IDAE) and the council president, representing Gorona del Viento El Hierro, signed an agreement governing the mechanisms for the provision of public funds, as well as control and monitoring of the actions of this initiative. This guaranteed the majority of the financing necessary to perform the relevant works. IDAE is providing its experience to this project by performing monitoring, inspection and control tasks during design, supply, assembly, start-up and operational testing, ensuring the correct application of budget funds.

The initial project was developed by Endesa and ITC (D. Juan Manuel Buil Sanz and D. Ramón Rodríguez Tomás) before Gorona del Viento El Hierro was formed. This project has been the basis of the entire development.

The partnership selected IDOM S.A. in March 2008 to provide engineering and consulting services. IDOM's responsibilities for this project include:

— Consulting engineering services for hydro-wind plant design;

— Basic and detailed design of the hydroelectric plant;

— Basic and detailed design of the electrical, protection and control systems; simulation and analysis of the power system behavior; and design of system operation;

— Procurement, site supervision and commissioning support;

— Analysis of production electricity scenarios according to hydro-wind plant configuration alternatives and operation strategy definition; and

— Definition of a feed-in tariff, which includes developing economic and financial models and hypotheses, tariff calculation and sensitivity analysis.

Project concept

The basic concept is to inject as much wind energy into the grid as possible to reduce diesel consumption. Wind energy presents two aspects that make this objective a challenge: variability of the resource vis-a-vis demand and uncertainty of supply. El Hierro demand increases the difficulty of the problem due to its variability. Peak demand is about 7.5 MW, and the lowest demand is about 3 MW.

With no relation between wind power and demand, four scenarios may occur:

— Wind power is above or below energy demand;

— Wind power and energy demand are similar;

— Sudden decrease or increase in wind power; and

— Wind generator trip.

Because energy production and demand must match in a continuous way, an additional element is required to absorb excess wind energy generated over demand, store it and supply it back to the system when wind generation is below demand. This element must also ensure stability of the grid regarding sudden variations in generation or demand. Thus, this additional element must have sufficient storage capacity to use as much wind energy as possible and capacity to respond to sudden surges to keep the system stable.

A system capable of covering these two requirements with proven technologies is hydroelectric pumped storage.

A preliminary configuration was designed based on a strategy in which 10% to 20% of the available wind power was added directly to the grid, with the rest being dispatched to the pumped-storage station in such a way that the demand not covered by wind energy was supplied by the hydro turbines. Dynamic response of El Hierro's electric grid to the sudden imbalance between generation and demand, due to connection or disconnection of pumps and generating units failing, was modeled to check viability of the strategy.

This initial configuration was:

— Fourteen 500 kW pumps powered by induction motors, and adjustment power factor by means of capacitors;

— Two 1.5 MW pumps connected by a variable speed drive with power factor compensation;

— Two 12 MVA transformers with 20/6 kV transformer ratio, magnetized through the auxiliary services;

— Four 2.83 MW Pelton turbines coupled to alternators of 3.3 MVA with 3 seconds inertia; and

— Four compensation capacitors of 350 kVAr connected to 6 kV bars.

The upper reservoir location was chosen to take advantage of an old volcanic cone that offered both high elevation and sufficient storage capacity. An initial capacity of 500,000 m3 was estimated for this element. Water level at full capacity would be 714.5 metres above sea level.

The lower reservoir location suffered more restrictions due to environmental reasons and was preliminary designed with only 200,000 m3 of storage capacity and a maximum water elevation of 56 metres above sea level. This elevation difference provides a total head of 658 metres.

The wind plant will feature five turbines, and it will be located about 2.5 km from the pumped-storage plant.

Developing the concept

Reconsidering the proposed solution, IDOM observed that system efficiency could be improved. With this strategy, 80% to 90% of energy demand was supplied by the hydraulic system. This meant that before being added to the grid, most of the energy generated at the wind farm would pass through: the pumps and their motors, the hydraulic circuit on the way to the upper reservoir, the penstock on the way down and the turbines and generators. Every step of this path meant a loss in efficiency (total efficiency loss of about 40%), so a better method would be to add all the wind energy directly to the grid, managing only the imbalance in demand by means of the hydraulic system.

The lower reservoir of the pumped-storage plant being built on the island of El Hierro is lined with a high-density polyethylene geomembrane for waterproofing.
The lower reservoir of the pumped-storage plant being built on the island of El Hierro is lined with a high-density polyethylene geomembrane for waterproofing.

This new strategy required hydro turbines to: offer enough spinning reserve to respond to wind fluctuations or generator trip; and keep minimum hydropower dispatched to maintain the necessary spinning reserve as low as possible to maximize the wind power that could be directly added to the grid.

The solution arrived at was to use the Pelton turbines as synchronous compensators. This method keeps the needles closed, leaving the rotor spinning at synchronous speed, consuming a small amount of energy because of friction in the bearings and energy loss in the generator.

Operating this way, all the energy demand could be supplied directly by the wind farm (when sufficient wind resource is available), and the hydro system would be ready to cover wind power fluctuations and generator trips. To ensure grid stability, the process of assuming the energy demand by the hydro system must be quick enough to avoid frequency variations above or below accepted values. The low short-circuit power and low inertia also must be considered because the power installed in the hydro system is in the same range as the total power demand of the island.

How fast the turbines can assume the energy demand depends on the hydraulic/mechanic capabilities of the penstock. The maximum power given per second by the hydraulic system is limited by sub-atmospheric pressures and mechanical capabilities of the pipe. A dynamic analysis was performed to determine the maximum power per second the hydraulic circuit was capable of providing. A rate of 2 MW/sec was obtained.

However, if the hydraulic system has more inertia, the needed velocity in response required is lower. These factors were studied together to get the optimal combination. A dynamic model of the generation system was created. Different combinations of inertia and opening time of the needles were evaluated. Finally, inertia of 6 seconds and a minimum opening time of the needles of 5 seconds were selected. For the El Hierro project, the minimum frequency value was established at 48.5 Hz.

The penstocks for the pumped-storage plant on El Hierro carry water to and from the turbines and pumps.
The penstocks for the pumped-storage plant on El Hierro carry water to and from the turbines and pumps.

Notice that the maximum power is about 1 MW/sec, less than the mechanical/hydraulic capability of the penstock.

These values conditioned the final design of the hydro turbines and generation set. Each hydro generation group is equipped with a flywheel that provides the desired inertia.

Demand loss scenarios were also studied, but they were not so restrictive for the machinery design. To avoid excessive waterhammer in turbine shutdown operation due to quick needle closure, the deflector actuates, reducing the power delivered while the needle closes properly. Deflector construction also allows a better frequency regulation capacity as it deflects the flow toward the turbine axis rather than deflecting it away.

The stability study is conservative in that it neither considered this regulation capability nor other aspects that will provide better plant performance. One example is the capability of the 1.5 MW pumps to regulate the power consumed through frequency regulators up to 500 kW each or wind turbine-generator frequency regulation capability.

Later studies of the frequency regulation capability of wind generators showed better behavior when a generator trip occurs, as the frequency drop was limited to 49.6 Hz in spite of the 48.5 Hz obtained simulating the wind generator as a simple negative charge. In this way, the wind generator was requested to change its power (see Figure 1 ). Plimited is the active power setting to the wind generator, and Pavailable is the available power of the wind generator at that precise moment.

Figure 1 Wind Power and Generator Trip

An energy production study was carried out with the configuration established after reconsidering the initial concept to determine the amount of energy demand provided by the hydro-wind system. A reduction of the total pump power installed was decided because the maximum power consumed by the pumping station exceeds 6 MW only a few hours a year. This makes sense as the wind energy is to be directly fed to the grid and only imbalances between wind energy production and grid demand will be managed by the hydro system.

Figure 2 Generation Technologies to Meet Demand

Figure 2, shows production by various combinations of plants compared with demand.

The hydro plant features six 500 kW pumps that adjust demand due to wind generation fluctuations in case of wind energy excess. Another two 1.5 MW pumps driven by 1,500 to 500 kW power regulators will be used to do the fine adjustment.

The upper reservoir was intended to store more than 500,000 m3. However, final capacity was reduced to 379,634 m3 because geotechnical surveying revealed extremely deformable materials in the subsoil that could not be compacted as needed. Thus, a reduction in the maximum level of water was established to limit deformations that could put at risk the integrity of the membrane used to make the reservoir impervious.

The lower reservoir has always been considered the weakest part of the system, as its storage capacity is only the half that of the upper one. During engineering development of the project, efforts were made to increase or even maintain this storage capacity. However, environmental restrictions, unfavorable topography and geology resulted in a reduction of the final storage capacity for the lower reservoir to 150,000 m3.

Final configuration

The final configuration of the hydro-wind plant is as follows:

— Upper reservoir: Maximum capacity of 379,634 m3, 12 metres of maximum water level with maximum elevation of 709.5 metres above sea level, 2 mm high-density polyethylene (HDPE) geomembrane for waterproofing.

— Lower reservoir: Storage capacity of 150,000 m3, rockfill dam with maximum height of 23 metres, 15 metres of maximum water level with a maximum elevation of 56 metres above sea level, 2 mm HDPE geomembrane for waterproofing.

— Penstocks: Two, one 800 mm in diameter and 3,015 metres long for the pumping system and the other 1 metre diameter and 2,350 metres long for generation; both made of S355NL steel and running in parallel through a 530 metre-long gallery; pumping system suctions from the lower reservoir through a S355NL steel penstock encased in concrete that is 1 metre in diameter and 188 metres long.

— Pumping plant: Contains two 1.5 MW pump sets driven by 1,500/500 kW power regulators and six 500 kW pump sets.

— Hydro plant: Four Pelton turbine-generator groups of 2.83 MW of power each, with maximum flow during generation of 2 m3/sec and a total head of 654 metres.

— Wind farm: Five 2.3 MW wind turbine generators.

— Electrical substation: Interconnects the hydraulic plant, pumping plant and wind farm and is in an area adjacent to the T.C. Llanos Blancos Substation.

Production

The production strategy for the hydro-wind plant is based on the following principles:

— Maximize wind generation capacity to supply electricity demand, thus minimizing primary energy losses;

— If the wind resource is higher than the expected demand, the excess will be used for pumping;

— If the wind resource is lower than the expected demand, the difference will be covered by hydraulic production;

— Under high reservoir scenarios, the combined plant will cover up to 100% of electricity demand; and

— Under low reservoir scenarios, the combined plant will cover part of the electricity demand.

Following this strategy, a production study has been carried out for the hybrid hydro-wind plant. The results obtained are that total demand on the island is 47.4 GWh. Available wind energy is 49.6 GWh. Wind energy that can reliably be produced during periods of demand is 25 GWh, with 9.2 GWh for pumping and 1.8 GWh for synchronous compensation. Hydroelectric production is expected to be 5.6 GWh, and in the end the hydro-wind plant is expected to provide 30.6 GWh during periods of demand, for a total of 64.56% of total energy needed for the island.

Main suppliers for the project are:

— Civil Works: Acciona Infraestruc-turas and Constructora Herreña Fronpeca

— Electromechanical Systems and Equipment: Elecnor

— Penstocks: Montajes Rotelu and Comercial Dipehi

— Wind Farm: Enercon and Comer-cial Dipehi

Construction work for this project began in June 2009 and is due to be completed by the end of 2012. The plant will be operating by March 2013.

Conclusion

The low penetration of wind energy in an island grid can be highly increased by combining wind farms with hydroelectric systems capable of regulating energy produced to supply it when demand requires. Adding wind energy to the grid, an increase of demand supplied by renewable energy is achieved. Most of the energy supplied is not penalized by hydro system efficiency. The more wind energy is supplied to the grid, the less primary energy is lost.

In the Canary Islands, the coverage provided by wind energy is only 4% to 5%. Wind power penetration in the Canary Islands is limited to 30% in terms of power. But the system designed for El Hierro allows the possibility of covering 100% of the total power demanded by the island. Realistically, however, about 65% of island's total annual energy demand will be covered by the hybrid hydro-wind plant.

Special characteristics of the Pelton turbines were needed for this scheme because they will act as synchronous compensators. The opening time of the needles has been set at 5 seconds, and the deflector has been requested to close in such a way that it adds regulation capacity to the system. Additional inertia has been coupled to the generators to accomplish stability requirements. And the wind farm has been designed with power frequency regulation capacity.

This combination of hydro and wind generation will allow significant reduction in the consumption of fossil fuels on the island. The result is a lower dependence on fuel prices and fuel supply, as well as reduction of about 22,000 tons of CO2 emissions per year.

Miguel Fernandez Centeno is a civil engineer and Augustin Marrero Quevedo is an industrial engineer with IDOM Internacional. Juan Manuel Quintero Gutiérrez is the head of Gorona del Viento El Hierro S.A. Rafael Caballero Nueda is an industrial engineer with IDOM Internacional. John Hart is vice president of business development with AEC Engineering Inc., an IDOM Group company.


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