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Energy storage - Part 2

As more renewables come on the grid, it is helpful to have more energy storage capacity. However, as I said in my previous Blog, storing electricity is not easy or cheap and there are other ways to balance grids. http://environmentalresearchweb.org/blog/2011/10/energy-storage—-part-1.html

For countries like the UK, where the renewable contribution is still relatively small, it’s easier and cheaper just to providing balancing power from the already existing gas turbines. But in countries with larger renewable inputs and access to large hydro projects, hydro reservoirs provide an opportunity for pumped storage, and there are some interesting new developments underway, making use if hydro facilities that already exist.

For example in the Voralberg area in Austria a set of relatively small reservoirs at different levels in the Alps have been linked up, with several being modified so that surplus power from the nearby Bavarian wind turbines can be used at off-peak periods to pump water back up to higher reservoirs, so that it can be used to generate power when needed. www.univie.ac.at/crm/ew06/Schmoeller.ppt

Also, in the Harz Mountains in Germany, old mine workings are being looked at as possible sites for new underground pumped storage facilities. That’s not a new idea: Dinorwig in Wales already does this- storing off peak excess power.

For a good global overview of pumped storage, see www.renewableenergyworld.com/rea/news/article/2011/08/balancing-the-grid-with-pumped-storage?cmpid=WNL-Friday-August12-2011

You don’t have to have large mountain ranges, and it can be done on a relatively small scale. For example, El Hierro, the smallest of the Canary islands (278 sq km) is to generate power for its 10,700 residents and its many tourists from a five turbine 11.5MW wind farm. Some of the electricity will go into the mains network, some will be used for desalination, and any excess will be used to pump water from a 150,000 cubic metre reservoir near the harbour up to a much larger reservoir (550,000 cu metres) at an elevation of 700 metres, housed in a volcanic caldera. 3 km of pipes will connect the two reservoirs. If the wind drops, water is released at the top to drive six hydraulic turbines (11.3MW).

Reporting on this project, the Guardian noted that there was nothing very revolutionary about pumped-storage hydro of this type. Morocco has already built one facility and is about to launch another project near Agadir. The Grand Maison dam in the French Alps, operates along similar lines. But El Hierro is the first big scheme not to use conventional power.

The idea is spreading and developing. Copenhagen-based architect firm Gottlieb Paludan has proposed an innovative Green Power Island idea using seawater pumped into a lagoon-like reservoir built into an artificial island. When demand is low, pumps driven by wind turbines empty the reservoir. At peak demand periods, water is allowed to flow back into the reservoir, via turbines generating electricity to meet the demand.

Green Power Islands have been proposed for Copenhagen, Bahrain, Jiangsu, and Tampa, Florida. Although primarily using wind , the islands can also include PV and concentrated solar, and even biomass. The developers says that Some could also have recreation trails, beaches, harbours, and even residential and business units. www.greenpowerisland.dk/

However the hunt is also on for other forms of storage not reliant of geographical features. Some look to various types of advanced flow batteries, others to compressed air storage or flywheels. Another choice is to convert electricity produced by wind turbines or PV arrays to hydrogen , which can be stored as a compressed gas or a cryogenic liquid and then used to generate power when needed, either by direct combustion in an engine or turbine, or in a fuel cell.

There are several projects around the UK testing this out, including the Hydrogen Office in Fife, Scotland, using surplus energy from a 750 KW wind turbine stored as hydrogen for use in a fuel cell, which can generate power when needed. www.hydrogenoffice.com/

The system was developed by the Pure Energy Centre who have pioneered this approach. www.pure.shetland.co.uk/

Some cleaver new ideas are also emerging for new types of hydrogen storage media. I mentioned the Highview cryogenic storage system in my previous Blog. But another, perhaps more advanced option was outlined on the Clean Tech Blog recently, in an interesting article on hydrogen storage of wind energy by David Anthony and Ken Brown, based on Safe Hydrogens system with which they are involved. As normal, they suggest using the electrolysis of water to produce hydrogen, noting that with an 80% efficient electrolysis unit, it takes about 50 kWh of electricity to create 1 kg of hydrogen. But then they say that ‘To avoid the high cost of compressing hydrogen or of cooling and liquefying hydrogen, a good alternative is to store the gas in a metal hydride slurry’.

Basically, hydrogen is chemi-absorbed as metal hydride in a molten mix, which can then be made to release it’s hydrogen, when required, in a managed reaction with water, producing hydrogen and heat. That process converts the metal hydride to a metal hydroxide, which can be recycled back to a metal hydride.

The article explain that ‘Safe Hydrogen uses magnesium as the metal and mineral oil as the liquefying agent. With the use of small particles and a suitable dispersant, the particles will stay in suspension almost indefinitely. Using a hydriding reactor, hydrogen is absorbed by the Magnesium Slurry with suitable pressure and temperature that ensures rapid reaction. The Magnesium Hydride Slurry that is created in this reactor then can be stored in large quantities at ambient conditions. The hydriding reaction to create the magnesium hydride slurry creates heat. This heat is about 30% of the heating value of the hydrogen gas. About 10 percentage points of this heat, or one-third of the heat, can be used to perform useful work such as generating more electricity. The rest of the heat can be used for space heating or to produce hot water. Thus the hydriding step in the process can be from 110-130% efficient’.

The hydrogen can be used to power a gas turbine-generator, so the wind farm owner can either sell power either directly or at later stage- and the hydrogen slurry can thus be used as a store to compensate for the variabilty of the wind power. The article says ‘typically, only 15% of a wind farm’s output can be counted on as reliable capacity-likely to be available in any given time period’. e.g. 7MW out of a 50MW wind farm. But with storage, this can be doubled. Clever stuff. See: www.safehydrogen.com

Posted by Dave Elliott on October 15, 2011 12:47 PM | Permalink