Image copyright Thinkstock

The smallest and most isolated of the Canary Islands, El Hierro, has a way of combining hydro and wind power that may allow it, one day, to get all its energy from renewable sources. In August it went for two hours without using its diesel power station at all - but this could be the start of a bigger green transformation.

For more than 30 years, El Hierro has been dreaming of becoming self-sufficient. And this year it took a big step forward. At the end of June its new hydro-wind facility, Gorona del Viento, came fully on stream and in July and August it provided roughly half of the island's energy needs.

That means the island's 10,000 inhabitants are suddenly less reliant on supplies of diesel arriving over unpredictable seas from Tenerife, 200km away.

In July, Gorona del Viento saved 300 tonnes of fossil fuels, but that is predicted to rise to 500 tonnes per month before long - the equivalent of saving 40,000 barrels of oil and 19,000 tonnes of emitted CO2 per year.

The system consists of five wind turbines with a total capacity of 11.5MW and two water reservoirs - one at 700m above sea level, the other down near the coast. The reservoirs are connected by two 3km-long pipes, and any water running from the upper to the lower reservoir passes through a series of water turbines, generating electricity.

Fresh water is used, rather than sea water, to ensure that the aquifers are not contaminated if there are any leaks.

What's unique about it is the way the wind part and the hydro part work together.

"When we get enough wind from the wind farm, we produce electricity and distribute it through the grid. Whatever is left, we use it to pump water from the lower reservoir to the higher one, and then, when the wind drops, we let that water fall through a set of hydraulic turbines and we generate electricity again for the population," says Juan Gil, chief engineer at Gorona del Viento.

Image copyright Getty Images

The key disadvantage of wind power - the unreliability of the supply - is made up for by the water in the upper reservoir which can be released "within milliseconds" according to one of the engineers involved, whenever the wind starts to blow less strongly. The system is continually switching between releasing water from the upper reservoir and pumping water back up, depending on the strength of the wind and the demand for electricity.

But the goal is to go further than the 50-50 mix of renewables and diesel generation achieved over the summer.

The station should already be able to cover 70% or 80% of total consumption, according to some of those involved.

Juan Pedro Sanchez, an industrial engineer who works as an adviser to Gorona del Viento, foresees steadily increasing the length of time the plant is used to cover 100% of the island's needs. This was done for two hours on 9 August, the next step will be to try it for 24 hours and ultimately it should be possible for weeks on end, he thinks.

"I think that in a year or so, the plant could supply all the electricity the island needs for about 200, 250 days," Sanchez says.

Image copyright EPA

He points out that there is a learning curve for those operating the plant, and that the energy company, Red Electrica, also needs to be convinced that diesel output can be safely reduced.

The dream, one day, is for Gorona del Viento to provide 100% of the island's electricity all year round. This can be achieved, according to Thomas Padron, ex-president of the local council and one of the founders of the project, but only with more investment.

"It's possible. But for this we have to enlarge the capacity of the water reservoirs, to have more hydro energy when the wind falls," he says.

The length of time Gorona del Viento can continue generating hydro power is determined by the volume of the smallest reservoir - in this case, the lower one, which is less than half the size of the upper one. As soon as the lower reservoir is full, hydro generation has to stop because fresh water, which is in short supply on El Hierro, cannot just be released into the sea.

Green credentials

Image copyright ALAMY Image caption Wind turbine parts are unloaded from the landing craft at the quayside, Eigg

El Hierro is not the only island to aim for renewable energy self-sufficiency.

Since 2008, residents on the Scottish island of Eigg have made a concerted drive to cut down on the use of fossil fuels, reduce waste and invest in sustainable resources. Nearly 100 people live on Eigg and their electricity is now supplied by hydro, wind and solar energy

Denmark's island of Samso, with a population of 4,000, produces all its electricity from renewable sources and aims to be fossil fuel free by 2030. The islanders own shares in the turbines, and have set up an Energy Academy to disseminate their expertise

If the wind were, hypothetically, to stop blowing and the lower reservoir just happened to be empty at the time, the island could survive with uninterrupted hydro power for about two days, at average levels of consumption, according to Juan Pedro Sanchez. But if the lower reservoir was enlarged to match the size of the upper reservoir, that would stretch to five or six days - a big difference.

Because the cost of generating electricity at Gorona del Viento is low, there is a tendency to regard all the diesel that might have been used, but wasn't, as a pure saving.

The current council president, Belen Allende, calculates that over 20 years, by not burning 6,000 tonnes of diesel per year, El Hierro will save the central government about 80m euros. By Spanish law, the cost of electricity has to be the same all over the country, so bills haven't gone down. But the island will benefit, Allende says, as the Spanish state is expected to give the island compensation of 5-7m euros per year.

There are some, however, who argue that the cost of building Gorona del Viento - 82m euros already - means that in reality the electricity is not cheap at all, but actually very expensive.

Image copyright Getty Images Image caption The pump room at the Gorona del Viento power station on El Hierro island

Even on the island there are those who think that the money could have been spent on other things, such as developing El Hierro's almost non-existent tourism business.

Many of the islanders, though, remember a time when the island's dependence on the outside world for energy caused recurrent problems.

"This island was abandoned, neglected and forgotten by the central authorities," says Padron.

"Until the beginning of the 1970s, El Hierro only had electricity from dusk to midnight, and only in the capital and two other towns."

If conditions at sea were bad, the island could be cut off from the rest of the world for days or weeks, putting even that limited energy supply at risk.

Another problem, historically, was lack of water. "El Hierro suffered many droughts and people were forced to emigrate to the bigger neighbouring islands, and to Cuba, Argentina and Venezuela," says Padron.

There are no rivers or lakes on El Hierro, so the only way to get water was from underground aquifers reached by deep shafts. Nowadays, there are also three desalination plants - running these, and pumping fresh water to villages at higher altitudes, consumes half of the island's electricity.

But this means, says Padron, that the more El Hierro controls its own electricity supply, the more it controls its water supply.

There are plenty of other green initiatives on the cards.

One is a plan to offer incentives to persuade residents to swap their 6,000 conventional petrol and diesel cars for electric ones over the next 10 years.

Another is to build a composting plant that would turn half the island's rubbish into agricultural fertiliser.

There are also proposals to improve the efficiency of the water distribution system, and to experiment with wave power - Allende wants the island to be a place where green technology is put to the test.

Large numbers of foreign scientists and policy makers have already been visiting El Hierro to learn about Gorona del Viento.

Image copyright Thinkstock

But it's actually a great island for anyone who likes nature, and solitude.

You can be down on the coast one minute, in an unusual volcanic landscape, and half an hour later in a forest of wild juniper trees bent into peculiar shapes by the constant battle with high winds.

Wait until darkness comes, and if there are no clouds, there is a stunning night sky entirely free of light pollution.

It's also a safe place - the kind where some people still leave the keys in the ignition of their car.

If you are thinking of visiting though, remember that it's windy 80% of the time - so if you have long hair, you may need a big bottle of hair spray.

The green island

Media playback is unsupported on your device

The BBC's Laura Plitt discovers how the islanders have turned natural challenges into opportunities.

This is part of the Island Stories series.

Subscribe to the BBC News Magazine's email newsletter to get articles sent to your inbox.