Working together

Janet Wood investigates hybrid systems, where hydro power and pumped storage is used to help other renewable energy sources operate more efficiently

ELECTRICITY demand in the Greek islands is growing by an average 8% annually, and peak load is growing even faster – by up to 10% a year. In the Canary Islands the situation is similar. The question of how to provide new generation sources to meet this demand is becoming more urgent.

Stepping up electricity supply on small island systems is very often a question of installing new diesel-fuelled power plants, usually simply adding another unit to existing plants. But adding more and more plants is increasingly seen as running counter to principles of sustainable development: it does not make use of local resources and it leaves consumers vulnerable to changes in fuel prices and irregular fuel deliveries. What is more, it makes a significant contribution to emissions of greenhouse gases and other pollutants.

Many islands have good wind resources but the weakness of wind generation is highlighted on an isolated island system. If the wind is not available there is no interconnection with other grids that can provide power. That is why so many small isolated systems are considering hybrid generation to provide a sustainable power source.

Diesel generators are quick and cheap to install – specialist companies can provide systems in the tens of megawatts range within weeks. Tailor-made systems that use the island’s resources are more complex and almost always more expensive – at least on a capital cost basis. In El Hierro, for example, one of the smaller of the Canary Islands, utility Endesa-Unelco and the Instituto Tecnologico de Canarias are developing a project called Central Hidreolica de El Hierro. That project would replace diesel generation with a mixed wind, hydro and diesel system. It would use wind and hydro for generation and would include a pumped storage plant with the pumps operated by wind and diesel. The scheme is far more complex than simply adding new diesel: it requires a wind park, a hydro power station, a pumping tation and two water storage facilities.

At Greece’s Laboratory of Soft Energy Applications & Environmental Protection (Sealab), J Kaldellis and K Kavadias have been exploring the costs and benefits of using renewable energy schemes in the Greek islands.

In a paper presented to the Global Windpower conference, held in Paris in April 2002, the researchers said that demand was barely met by the existing capacity on most islands and new generating capacity was required urgently. ‘All these islands possess outstanding wind potential,’ the researchers agreed, but the characteristics of the wind were not consistent with the demand pattern. As a result, they said, ‘creating a combined wind-hydro energy production station…is a solution for all these islands, providing maximum energy autonomy and limited installation cost’.

But the researchers note that while for local people maximum energy independence is the aim, for private investors a wind-hydro configuration ‘will be selected on pure medium/long term financial criteria’.

The group undertook a numerical analysis to calculate the optimum configuration for a hybrid system and applied it to the islands of Karpathos and Ikaria.

The system analysed provided more than power: it took account of the islanders’ needs for water as well. It included:

• One or more wind farms.

• A small hydroelectric power plant with at least two reversible turbines.

• A water pumping station able to absorb the system’s wind power supplies, with the reversible turbines working as water pumps.

• At least two reservoirs working in closed circuit.

• The existing power generation capability.

• A desalination plant able to use excess energy from the wind farm to produce clean water from sea water.

The separate parts of the system work together for maximum efficiency. When the wind farm production exceeds demand the surplus is used to pump water to the upper reservoir. When the upper reservoir is full, continuing surplus is used to operate the desalination plant. When demand is greater than the wind farm can supply, the hydro turbines cover the power deficit. At the bottom of the ranking order, the diesel generators are used when the wind farm cannot meet demand and the upper reservoir is empty.

The group considered the most economic configuration for the system in the two islands, both of which have mountainous terrain and wind speeds averaging 9m/sec. Karpathos, in the Dodekanessa complex, has a peak load (in summer) of around 6.5MW and can make use of a natural reservoir holding some 2M m3. It currently has a thermal station that operates at some 43.3% efficiency.

Ikaria is a smaller island in the Aegean Sea with a peak load of 5MW. It has a natural reservoir, at el 700m, that holds some 800,000m3. The efficiency of its thermal power station is 38.2%.

In their analysis, the group considered the capital cost of the system and its operation and maintenance cost over the lifetime of the investment. It compared the cost of the system in three different ways: using the current marginal electricity production cost in Greece (around 0.09/kWh); compared to the marginal energy cost of the power station on the island; and savings in O&M and fuel costs that would accrue if the power station was not operating. Other factors that were taken into account were the number and size of wind turbines used, the optimum size of the reservoir and the number of full power days it represented for the islanders. They also considered the penetration of renewable energy into the island’s system, using wind and weather data to calculate capacity factors etc for the turbines and the likely fill rate of the reservoirs, and examine how much of demand would be met by the renewable components of the system. Cost-benefit conclusions were produced for various configurations and compared.

In the case of Ikaria, the group found that limited storage capacity was favoured. For storage representing 1.5 days or more, the cost-benefit ratio was maximised with 17 wind turbines rated at 300kW. The optimum solution combined 17 turbines with storage for two days.

The group found that this would produce a 150% return on capital invested after tax, after 20 years of operation. That ratio was lower when calculated on the basis of fuel and O&M savings, but reached 700% when calculated on the basis of marginal production costs.

In Karpathos, the number of wind turbines was a more important characteristic of the system in calculating the penetration of renewable energy than the reservoir capacity. On a cost-benefit basis the most effective application had a water storage capacity of one day’s usage, and 17 300kW wind turbines. This configuration would allow some 80% of the island’s energy to come from the wind and water part of the system.

The analysis focused on cost-benefit ratios. It did not consider that a minimum renewable energy penetration was required, nor did it incorporate the effect of CO2 reduction measures if they were introduced. The EU plans to introduce a premium price for renewable energy and to tax CO2 production. If that plan becomes reality, the economic case for using hybrid systems will become still more attractive.