Last year I came across the story of Dutch company Kema and their energy island idea – basically a variant on the usual pumped hydro energy storage concept where water is pumped out of a space below sea level then allowed to flow back in, generating power as it does. The “island” uses wind power to pump water out of the enclosed area. An obvious extension to this idea would be to harness ocean energy as well – letting wave and/or tidal power supplement the output of the wind turbines. An attraction of this concept is that it potentially allows a large amount of new energy storage to be brought online – and this storage would be along the world’s coastlines, where most of the population lives.

Another form of energy island has been in the news recently, this one a substantially more ambitious proposal which envisions artificial islands to collect wind, wave, ocean current and solar power in the tropics, along with a more unusual energy source – harnessing the difference in water temperatures between the warm surface and the cold depths using a technique called OTEC (Ocean Thermal Energy Conversion).

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These islands are being proposed by architects Dominic Michaelis and his son Alex Michaelin as a response to Richard Branson’s Virgin Earth Challenge, which offers $25 million in prizes for innovative solutions for combating global warming.

While the practicality of these particular proposals has yet to be put to the test, the various forms of ocean power are probably the most overlooked of the big 6 renewable energy sources (along with solar, wind, geothermal, biomass and hydro).

Other forms of renewable energy are sometimes criticised for being more intermittent and less predictable than traditional power generation, however ocean energy is much more reliable – steady ocean currents could provide good baseload power, as could OTEC, tidal power is diurnal and highly predictable and waves are predictable days in advance.

In this post I’ll have a look at the amount of energy that could potentially be harvested from these sources and the various projects underway to try and make this a reality.

Tidal and Ocean Current Power

Tidal power stations usually take the form of a dam (or barrage) built across a narrow bay or river mouth. As the tide flows in or out, it creates uneven water levels on either side of the barrier. The water flows through the barrier, turning turbines to generate electricity.

Benefits of tidal barrage power generation include :

* Predictable source of clean energy
* No dependence on foreign fuel sources
* Flood protection
* Transport links for road and/or rail
* Better shipping and boating conditions behind the barrier

Disadvantages include :

* The timing of the tides doesn’t often correlate with peak demand times (less of a problem if there are good energy storage options available)
* Existing ecosystems behind the barrage tend to be heavily altered
* Likely to stimulate silting in some areas and coastal erosion in others
* Enhance flood risk on the seaward side
* Shipping would have to navigate locks
* Industrial discharges behind the barrage are less likely to be dispersed out to sea

Variations on this theme include offshore tidal lagoons, which use a water impoundment structure and low-head hydroelectric generating equipment on shallow tidal flats, and tidal fences, which are composed of a number of individual vertical axis turbines mounted within the fence structure, known as a caisson.

Underwater turbines can also be used to harness both tidal power and ocean current power. The turbines (sometimes called aquanators) are similar to wind turbines. In water moving between 6 and 9 km per hour, a 15 m diameter water turbine could generate as much energy as a 60 m diameter wind turbine. Given the smaller amount of infrastructure required and the larger range of possible sites that this technology could be deployed to, it seems likely that underwater turbines will become much more widespread than tidal barrage style generation.

World tidal energy resources have been estimated at around 3000 GW, however less than 3% of this is located in areas considered suitable for power generation (these figures probably don’t include ocean current power, which doesn’t seem to be well studied).

A 240 MW tidal-barrage power plant has been operating at La Rance in Brittany since 1966. Other operational barrage sites are at Annapolis Royal in Nova Scotia (18 MW), the Bay of Kislaya near Murmansk and at Jangxia Creek in the East China Sea.

The largest tides in the world are found in Canada’s Bay of Fundy, which has been earmarked to become a 4-berth test site for tidal power generation next year.

On the west coast of Canada, Marine Current Turbine and BC Tidal Energy Corporation plan to install at least three 1.2 MW tidal energy turbines in Vancouver Island’s Campbell River by 2009. This the first step in a plan to develop larger tidal farms off British Columbia’s coast, which the company says have a tidal energy potential of up to 4,000 MW.

In the United States, at the southern end of the Bay of Fundy, lies Passamaquoddy Bay, which has long been a target for a tidal power development – first initiated in 1935 by the Public Works Administration under the Roosevelt administration, then halted by Congress a year later. John F Kennedy revived the 550 MW project in 1963, however the plan died with him (spawning one of the stranger JFK assassination conspiracy theories I have come across).

Further south, in the Martha’s Vineyard area, two underwater turbine projects are trying to get started – one a 300 MW proposal from Oceana Energy Company and the other from Natural Currents Energy Services. Other projects are being considered in the Cape Cod and New Bedford areas – part of a “gold rush” for good tidal power sites (the most desirable ones usually have hourglass figures, to get maximum force in the incoming tide) which has seen the FERC issue 47 preliminary permits for ocean energy projects (and generated mainstream news coverage on the NBC network).

New York’s East River is the location of one of the more high profile tidal power experiments currently underway, with Verdant Power experimenting with underwater turbines there. The first attempt eventually ended in failure, with the strong tides breaking the devices.

The Gulf Stream has also caught the eye of hopeful ocean energy companies, particularly in Florida, with the 30 mile wide current pushing 8.5 billion gallons of water along per second and prompting some observers to consider the prospect of “Infinite Underwater Energy“.

Californian utility PG&E is also investigating tapping tidal power in San Francsico Bay, with some observers talking about a plant of up to 400 MW in size.

Another bay famous for its tides is the Severn river estuary in Britain, with a tidal range of 14 metres. Plans for damming the Severn estuary or Bristol channel have existed since the 19th century (with tidal power generation being just one proposed application). The UK government recently proposed a new barrage design, which could produce 5% of the UK’s electricity requirements, with a peak rate of 8.6 GW. A feasibility study is expected to be complete by 2010. An alternative proposal, by Tidal Electric, involves a series of lagoons, the first of which would be built in Swansea Bay. Some observers have noted underwater turbines may be more appropriate than a barrage.

Pentland Firth in Scotland is another UK location that is considered to have a large amount of tidal power potential – a DTI study in 1993 indicated that if all potential sites were developed, the total UK tidal stream resource could be about 60 TWh. Of this, almost half (28 TWh) could come from the Pentland Firth. The water depth is 60m or more, making potential energy capture huge but technically difficult – 63% of the tidal stream resource is estimated to be in waters deeper than 40m.

Marine Current Turbines launched the world’s first underwater turbine project off north Devon in 2003. MCT also began installing a 1.2 MW “SeaGen” tidal current turbine in Northern Ireland’s Strangford Lough in 2007, with the company planning to scale up to build a 10MW tidal power farm off Anglesey in North Wales, and to have 500MW of tidal capacity by 2015. Also in Wales, Lunar Energy and Eon are hoping to build an underwater tidal project off Pembrokeshire.

Another UK tidal power proposal is part of a plan by Metrotidal to build a tunnel under the Thames, currently under fire from environmental groups. There is also talk about regions like the Isle Of Wight and the Humber estuary harnessing tidal power as part of initiatives to become energy self-sufficient (like other “Transition Towns”).

Norway has also begun investigating the use of tidal power, with an experimental facility opening in Hammerfest in 2003. The company that developed that technology, Hammerfest Strøm, is working with Scottish Power to develop a project near the Orkney Islands (the islands have also been a test site for another venture by Lunar Energy and Rotech).

There has been no tidal power development in Australia thus far, though the Kimberly region has long been a target for would be developers of tidal power projects, due to its enormous potential (a tidal range of 11 metres). Thus far all of the proposed projects have been stymied by the remoteness of the location from the Western Australian and national electricity grids and by environmental concerns. A number of possible sites have been identified, including Secure Bay, Walcott Inlet, George Water and St. George’s Basin.

Liberal backbencher Wilson “Ironbar” Tuckey has been the most vocal supporter of a Kimberly tidal project, pointing out if a link was built to the eastern states grid it would obviate the need for any consideration of nuclear power. Some Kimberly tidal power advocates have also tried to base the idea of a “hydrogen economy” on the resource, though this seems a lot more far-fetched than a grid link (the grid link could also potentially include large scale CSP solar in the western australian deserts, which are one of the best solar resources in the world) .

The Bass Strait area is also considered to have significant potential for tidal / ocean current power generation (one estimate claiming there is potential for 3000 MW of generation in the channel between King Island and Cape Otway).

New Zealand is another country with large tidal resources but without any existing tidal energy generation. According to TVNZ, there are at least 24 wave and tidal power projects currently under development. Trying to get a handle on who might be behind these projects isn’t easy – there is an NZ wave and tidal power association, but it doesn’t list members or projects – according to their latest newsletter they have 59 members. Crest Energy seems to be the most prominent local company, with a plan for a 200 MW plant in Kaipara Harbour using underwater turbines. Other potential locations include Manukau and Hokianga Harbours, and Tory Strait and French Pass in the Marlborough Sounds. The harbours produce 5 to 6-knot currents and tidal flows of 100,000 cu m a second from the flood and ebb tides, with tidal volumes 12 times greater than the flow in the largest local rivers.

The Phillipines is another potential location for tidal power, with a 2.2GW tidal fence proposed for the Dalupiri Passage using the Davis turbine, from the Blue Energy company and an estimated cost of $US 2.8 Billion is unfortunately on hold due to political instability.

South Korea also has ambitions to generate power from ocean currents, with pilot underwater turbines being installed at Uldolmok, in the country’s south-west. Researchers at the Korea Ocean Research and Development Institute (KORDI) chose the site because it has flows up to 12 knots, believed to be among the fastest in Asia. The strong currents have resulted in a number of accidents, hampering progress. KORDI is also trying to improve the efficiency of more conventional barrage-type tidal power plants. The primary project involves building a power plant with a capacity of 250 MW at Lake Sihwa, with another plant up to 520 MW being considered for Garolim Bay.

Taiwan is another Asian nation considering the the possibility of large-scale ocean current power generation. There have been discussions about using the strong Kuroshio current off the east coast of Taiwan to generate up to 1.68 trillion kilowatt-hours per year (compared to Taiwan’s current annual demand of electricity of around 98 billion kilowatt-hours).

Wave Power

Surface waves and pressure variations below the ocean’s surface can be used by floating buoys or submerged platforms to generate intermittent power. Wave energy sources are widely available, are relatively consistent and predictable and (According to analysts Frost and Sullivan) have the highest energy density among all renewable energy sources. The best resource is found between 40-60 degrees of latitude where the available resource is 30 to 70 kW/m, with peaks of 100 kW/m. The potential global wave power potential has been estimated to be around 8,000-80,000TWh/y (1-10TW), which is the same order of magnitude as world electrical energy consumption.

The UK, for example, is estimated to possess the capacity to generate approximately 87 TWh of wave power per year – equivalent to almost 25 per cent of current UK demand. There are two main research centres in Europe focusing on the development and commercialisation of ocean energy technologies. The first is the European Marine Energy Centre located in Orkney, Scotland, which provides developers with sites to test their prototypes. The other is the Wave Energy Centre in Portugal.

Wave energy ideas are plentiful but real world examples are still rare – there are around 1000 patents for wave energy converters currently on the market and no consensus has emerged yet on which technologies will succeed.

Australian company Oceanlinx (previously known as Energetech) has had a 450 kilowatt wave power unit running at Port Kembla in NSW for a number of years, and plans to connect to the commercial power grid in early 2008. Oceanlinx is also at the advanced permitting stage for a project in Portland, Victoria which would deploy eighteen 1.5MW units for a total capacity of 27MW, which the company claims will be the largest wave energy project in the world.

The company has other projects planned in Rhode Island, Hawaii and Namibia, and intends to participate in the South West of England Regional Development Agency’s “Cornwall Wave Hub” in the UK.

The Cornwall Wave Hub aims to create the world’s first large scale wave energy farm by constructing a wave hub, or “socket”, on the seabed. Oceanlinx is participating along with Ocean Power Technologies, Fred Olsen Renewables and WestWave. Ireland is looking to build a similar grid connected test facility on the Mullet Peninsula in Ireland’s County Mayo. While the marine renewables industry in the UK seems to be quite vibrant, government programs to fund the sector have been criticised for not spending the money they have been allocated.

Another Australian company, Carnegie Corp has installed a small array of its CETO II units off Fremantle in WA, and is looking to set up a 50 MW facility in South Australia to desalinate seawater for the Adelaide market and the mining industry. The CETO technology was devised in the 1970s by Carnegie’s chairman Alan Burns, a well-known Perth oil man who also founded Hardman Resources. It operates mostly underwater rather than on the surface like many buoy based alternatives, which the company believes will result in a much lower likelihood of damage from storms and rough conditions.

Another Australian company exploring wave (and tidal) power is Sydney based BioPowerSystems, which is trying to is commercialise “biomimetic ocean energy conversion technologies” (an example of “biomimicry”, which I’ll be doing a post on at a later date). BioPower has been awarded a $5 million grant under the Australian Government’s AusIndustry Renewable Energy Development Initiative to test prototypes of the wave energy device (most likely at King Island) and the tidal energy device (at Flinders Island), with each generating around 250 kW.

Pelamis Wave power is a Scottish company that is constructing a 3 MW wave farm off the coast of the Orkney Islands. The company is also involved in the construction of a 2.25 MW plant in Portugal at Aguçadoura, which will soon be expanded to 20 MW, and is providing the technology for the WestWave project in Cornwall. The Pelamis design is a distinctive device resembling a 150m long red snake.

The Scottish government is considering building a connection linking the north and west coasts of Scotland with England, Norway, Germany and the Netherlands by 2020 which could be connected to the proposed European Supergrid, with the aim of harvesting up to 10 GW of wind and wave power.

Spain is also dipping a toe into the waters of wave generation, with a 300 kW “breakwater wave energy plant” being constructed on the north coast, using Wavegen (now owned by Siemens) equipment.

In the US, the wave energy company getting the most attention has been Finavera, which has received preliminary approval to build a 100 MW facility off northern California (and has signed a power purchase agreement with PG&E for part of this). At hasn’t all been plain sailing for Finavera however, with a test AquaBuoy device sinking off Oregon late last year.

The Electric Power Research Institute (EPRI) estimated that waves off the Washington, Oregon and California coasts could produce from 250 to 500 terawatt-hours per year – around 12% of US energy demand. Finavera also has approval for a project in Washington state, along with others in South Africa and Canada.

Another US based company is Ocean Power Technologies, which is looking at developing projects in Hawaii, New Jersey and Spain.

OTEC

Ocean Thermal Energy Conversion is not a new idea, it has been around for more than a century. OTEC uses the temperature difference between warm surface water and cold deep water to drive a power-producing cycle. For this to be practical, the temperature difference needs to be at least 20 degrees C, which tends to limit potential application to the tropics. The potential of this energy source has been estimated to be about 10 TW, according to some experts.

The economics of energy production today have delayed the financing of a permanent, continuously operating OTEC plant. However, OTEC is promising as an alternative energy resource for tropical island communities that rely heavily on imported fuel. OTEC plants in these markets could provide islands with power and desalinated water. Other applications that have been considered are aquaculture and mineral extraction.

OTEC plants have been trialled in Nauru and India (along with extensive research in Hawaii). There are also plans to build plants for the US military base on Diego Garcia, and in the Marianas Islands.

One unusual apparent application of this energy source that I came across recently is a robotic “thermal glider” which, at the least, seems like a very interesting tool for environmental monitoring.

Regular news updates on OTEC can be found at OTEC News.

Energy Island Ideas

The thinking behind harnessing ocean power has traditionally focussed on systems built on or near the shoreline. The amount of power available is large, however we are still at the very early stages of learning to harness it, and it is unlikely that ocean power will provide a significant proportion of our energy needs in the next decade or two.

The Energy Island concepts that I began the post with show that people are now beginning to consider harnessing ocean power out at sea as well, which vastly increases the amount of energy that could be tapped.

(The term “energy island” is an overloaded one unfortunately – the Danish island of Samso, for example, calls itself Energy Island as it is completely self-sufficient. There is also a “solar island” being developed off Dubai known as Ras Al Khaima.)

Dominic Michaelis’ energy islands are by far the most ambitious plan I’ve seen for harnessing ocean power in the open seas. These hexagonal islands, are designed to generate electricity using wave, ocean current, OTEC, wind and solar sources. The group estimates that each island complex could produce around 250 MW of power. 50,000 energy islands could meet the world’s energy requirements – ands provide two tons of fresh water per person per day for the entire world population as a byproduct of the OTEC process.

The island design also supports farming seafood in small pens below deck and growing vegetables in shaded areas on the platform. The group is planning to conduct a pilot in the waters off the British Virgin Islands or in the Indian Ocean over the coming year.

Most observers consider the likelihood of energy islands appearing in the near term as remote, however the ideas are thought provoking and put into context just how much energy could be obtained out at sea.

One of the main issues with generating power offshore is how to store or transfer the energy (assuming that the islands don’t simply become mobile aquatic arcologies of the sort science fiction writers used to dream about). One possible way of storing the energy would be to produce hydrogen, and to use the islands as refuelling stations for ships that use hydrogen fuel cells. Alternatively, the energy could be used to process raw materials, or to produce materials like ammonia.

Crossposted from Peak Energy.

The Canberra Times recently published an article, rather misleadingly entitled “Generating solar energy in the dark“, which looked at the use of purified graphite for thermal energy storage.

The company developing the technology is called Lloyd Energy Systems, and they are prototyping solar energy storage, a wind-to-heat plant and a small-scale plant that combines water treatment, energy storage and steam turbine generation.

The company has received a $5 million Federal Government grant as part of its advanced energy storage technology program in the western NSW town of Lake Cargelligo, with Country Energy agreeing to purchase the power generated. Lloyd Energy also has an agreement with Ergon Energy in Queensland to build a $30million plant at Cloncurry in Queensland, partially funded by the Queensland state government, which the Sydney Morning Herald reported on last year.

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[Lloyd Energy CEO] Mr Hollis said large amounts of coal-fired energy were lost during long transmission to remote areas. As power loads built up over time, mainly because of demand for air-conditioners, the grid could no longer cope in peak periods. Towns at the end of the line suffered the most from power shortages.

“We’re putting environmentally friendly generation out at the end of the branches of the tree if you like, so it can pump energy back in when the branches are in trouble,” he said. “It actually serves three purposes. Firstly, it is a renewable energy replacement for coal. Secondly, it avoids the country energy authorities having to upgrade their transmission lines so they can get more power out in the peak.” The third benefit was having an energy source at the end of the line that could return power into the grid.

Sixteen full-scale models would go to Lake Cargelligo and 54 to Cloncurry. The system’s mobility and flexibility had caught the attention of key Australian mining companies, which use diesel and gas generators.

Mr Hollis said making renewable energy available when it was required added to the system’s value. … “You can store thermal energy in a lot of things, but high-purity graphite is an extremely efficient way of storing it it doesn’t have any losses. You can move the heat in and out very quickly.”

Graphite based storage does not seem to have been used anywhere else in the world thus far. Storage for renewable energy has usually been limited to compressing air underground (Compress Air Energy Storage or CAES), where it can later be released under pressure, or pumped hydro, where the power is used to pump water back up into dams that can generate hydro-electricity. While both techniques are effective, they require suitable locations and complex infrastructure to be put in place.

The Queensland project will make Cloncurry the first town in Australia to rely exclusively on solar power, produced by a concentrated solar power (CSP) system. The system contains almost 7200 mirrors, which will guide the sun’s rays into holes in the bottoms of 54 elevated graphite cubes, heating them to 1800 degrees (C). The stored heat is then used to generate steam for turbines on demand. The company claims the turbine will use less water than falls in an average year on the power station’s roof.

Wind to Heat on King Island

A third system using the graphite system is being planned by CBD Energy, which has licensed the Lloyd technology and will build a wind-powered version of the system on King Island. The island, in the Bass Strait north-west of Tasmania, currently relies primarily on diesel to generate power for its 1800 residents.

The joint venture with Hydro Tasmania is not expected to make the island wholly powered by renewable energy, but it will eliminate the need for 1.25 megalitres of diesel fuel a year, says CBD’s chief engineer, John Giannasca.

CBD plans to install two megawatts of wind turbines to supplement existing systems along with six graphite blocks. The blocks are each the size of a standard shipping container, and will be heated to 800 degrees (C).

Some solar panels will be also be installed for periods when the island is without wind, and there are ongoing investigations into harnessing ocean current and tidal energy in the region.

CBD Energy is run by ex-Impulse Airlines chief Gerry McGowan, with the company partly owned by German clean energy company Solon. CBD is also looking to develop solar energy projects in Australia, with plans for the first operation to be set up in the northern NSW town of Moree.

Graphite energy storage in context

King Island received a lot of press attention for an earlier project to store energy using Vanadium Redox flow batteries that began in 2003.

The company involved in that initiative, Pinnacle VRB, has since changed name to Cougar Energy and doesn’t seem to have any active VRB projects going.

Another Australian company developing a slightly different form of Vanadium based batteries (Vanadium Bromide) is VFuel, though there hasn’t been much news from them in some time either. Both VFuel and Pinnacle/Cougar are using technology pioneered at the University of NSW.

What will happen to the flow battery installation isn’t clear, though a visiting parliamentarian (pdf) reported in 2004 that “The vanadium batteries would appear to best suit the ironing out of the wind fluctuations rather than holding larger quantities of power. The battery is expensive and takes up considerable space” and that graphite was being considered as an alternative.

TreeHugger noted last year that the advantage the Lloyd Energy graphite system has is that they have apparently managed to figure out how to refine low grade graphite into high quality crystalline graphite, and the storage capacity “ranges from around 300kWh (thermal) per tonne at a storage temperature of 750°C to around 1000kWh (thermal) per tonne at 1800°C.”.

The Australian Greenhouse Office has a review paper on Energy Storage Technologies (pdf), published in 2005, which includes a brief look at graphite in a section on thermal energy storage.

Thermal energy storage systems use material that can be kept at high temperatures in insulated containments. Heat recovered can then be applied for electricity generation using steam Rankine cycle or other heat engine cycles. Energy input can, in principle, be provided by electrical resistance heating but the overall round trip efficiency will be low. However, as with thermochemical energy storage, thermal systems have considerable advantages when integrated with Concentrating Solar Power (CSP) technologies (ie parabolic troughs or dishes, central receiver/heliostat systems and Linear Fresnel systems).

Integration of thermal storage for several full load hours, together with new storage materials and advanced charging/discharging concepts, would allow for increased solar thermal electricity production without changing the power block size (ECOSTAR, Nov 2004). Provided that the storage is sufficiently inexpensive, this would lower the levelised energy cost, and additionally increase the dispatchability of the electricity generation.

The kind of storage system used for solar energy storage depends on the Concentrating Solar Power (CSP) technology, the heat transfer medium used and the required temperature of operation. In general, high-temperature thermal storage development will need several scale-up steps over an extended development time before market acceptance will be achieved.

Storage systems for thermal energy storage need to:
• be efficient in terms of energy loss and temperature drops
• have low cost
• have a long service life
• have low parasitic power requirements.

The development of storage systems for high pressure steam and pressurized, high temperature air, is especially challenging. If or when developed, such storage systems would lead to a significant drop in CSP electricity costs. The high-temperature thermal storage technologies utilised or under development now are (ECOSTAR, Nov 2004):

Molten salt storage and Room Temperature Ionic Liquids (RTILs)

• State of the art is the 2-tank molten salt storage tested in the “Solar Two” Central Receiver Solar Power Plant demonstration project in California, combined with using molten salt as heat transfer fluid. The use of new, so called Room Temperature Ionic Liquids (RTILs) has recently been proposed. RTILs are organic salts with negligible vapour pressure in the relevant temperature range and a melting temperature below 25°C. Room temperature ionic liquids are new materials that have the potential to be stored at temperatures of many 100s of degrees without decomposing. It is not yet clear whether they are stable up to the temperature level required for CSP and also whether they may be produced at reasonable costs.

Concrete Storage

• The concept of using concrete or castable ceramics to store energy at high temperatures for parabolic trough power plants with synthetic oil as the heat transfer fluid (HTF) has been investigated in European projects. The implementation of a concrete storage system is claimed (ECOSTAR, Nov 2004) to be able to be realised within less than 5 years.

Phase Change Materials (PCM)

• Phase change materials are materials selected to have a phase change (usually solid to liquid) at a temperature matching the thermal input source. The high “latent heat” in a phase change offers the potential for higher energy storage densities than storage of non phase change high temperature materials. Because a solid/liquid phase change is involved, a heat transfer fluid is needed to move heat from source to PCM. At present, two principle approaches are being investigated:
- encapsulation of small amounts of PCM
- embedding of PCM in a matrix made of another solid material with high heat conduction.
• The first measure is based on the reduction of distances inside the PCM and the second one uses the enhancement of heat conduction by other materials (e.g. graphite). Storages based on PCM are in an early stage of development but the cost target is to stay below A$34/kWh based on the thermal capacity. Although the uncertainties and risks of the PCM storage technology are in a medium range, the technology time required for full development and commercial implementation is likely to be more than 10 years (ECOSTAR, Nov 2004).

Storage for air receivers using solid materials

• Storage types using solid material for sensible heat are normally used together with volumetric atmospheric or pressurized air systems. The heat has to be transferred to another medium, which may be any kind of solid with high density and heat capacity. Another innovation is to develop for pressurised closed-air receivers a storage container that has to be pressure resistant up to about 16-20 bar depending on the gas turbine pressure ratio.
• For both cases the time for development and implementation is considered to be between 5 to 10 years and the risks and uncertainties are in the medium range (ECOSTAR, Nov 2004).

Storage for saturated water/steam

• The steam drum, which is a common part in many steam generators, is often used to provide process heat storage in industry. The main problem is the size of the steam vessel for larger storage capacity and the degradation of steam quality during discharging. However, this storage type is ideal as buffer storage for short time periods of several minutes, to compensate shading of the solar field by fast moving small clouds. Using appropriate encapsulated PCM inside the storage could enhance the storage capacity because the latent heat content can be used to slow down the temperature and pressure decrease and enable smaller storage vessels for the same thermal capacity.
• Recently, underground thermal energy storage has been proposed again as a lowcost solution to high-temperature, low-loss thermal storage for CSP systems (Mills et al, Nov 2004). It involves storage of water under pressure in deep metal lined caverns where the pressure is contained by the surrounding rock and the overburden weight.

High-purity graphite.

• This readily available material has the attractive property of increasing its heat storage capacity as the temperature of storage rises. However, the relatively low temperatures of solar thermal systems are not optimal for this storage medium unless the graphite storage blocks could be positioned at the very high temperature focus of a concentrating solar collector.

For another good description of a range of energy storage technologies, try Richard Baxter’s book “Energy Storage: A Nontechnical Guide“.

One obvious advantage for graphite is that carbon is extremely common, unlike some of the minerals used in various battery technologies and so there will be no meaningful material “limits” to the creation of these. Perhaps one day we’ll see CO2 being sequestered in the form of graphite blocks, ready to be installed into CSP power stations.

On semi-related news, energy storage has also been getting some attention in The Economist lately, courtesy of EEStor’s ultracapacitor technology.

(Crossposted from Peak Energy).