First French Israeli Conference on Renewable Energies
Energy Storage, a way to secure renewable energies in the grid Pierre ODRU
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IFP Energies nouvelles Agence Nationale de la Recherche
In 2007 (IEA source) fossil fuels still represent 80% of total world energy consumption. Fossil fuels are very efficient, still abundant and cheap, they are easy to produce, use, transport and store. However they are not renewable and their availability will decrease. And they produce CO2, responsible for global warming.
In 2007 world electricity was generated at 68% by fossil fuels, 13,8 % nuclear, 15,6% hydro, and 2,6% by modern renewables. However modern renewables (windpower, solar...) are growing fast and will represent 20% of global electricity generation in the EC countries in 2020.
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AIE Source
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Although modern renewable energies are still in small minority in the electricity production, their share is not negligeable in some european countries: Denmark (19%), Spain (11%), Germany (7%)...
Windpower production
Electricity production from renewable is intermittent and may be out of phase with demand. This will be a more and more important source of instability for the grid. Severe accidents may result, and large improvements will be needed to accomodate.
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Electricity storage may be part of the future smart grid solution.
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Electricity demand
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Electricity cannot be stored directly. It needs to be converted in a potential (gravitational, chemical, mechanical...) and converted back in electricity on demand
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Pumped hydro is from far the most applied technology for electricity stationary storage. It consists in pumping water from a lower basin to an upper one in case of excess of electricity production; and to produce electricity through water fall and turbines when needed.
The technology is well mastered, with a global efficiency around 75%. The global installed power in the world is more than 130 GW, and still increasing. Large investments are needed but the long term return is good.
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Drawbacks are large footprint, long construction time, lack of practical sites, and public acceptability.
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The GrandMaisons complex (France): 1000 m water fall, upper basin 140 millions m3, maximum discharge rate of 1690 MW during 20 hours. The pumped hydro storage capacities in Europe are in a ratio of 6% with the total electricity production capacities. The biggest pumped hydro complexes are located in US and Japan and reach a 2700 MW capacity.
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Concepts of small pumped hydro systems, or of completely artificial ones, at sea shore for instance, are emerging. Their economical viability has to be established. They could be used in connection with local renewable energies.
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The CAES technology consists in compressing air in an underground cavern during electricity excess production, to recover energy through a turbine in peak demand period. Efficiency (40 to 50%) is better than using a gas turbine (35%), but lower than Pumped Hydro, heat of compression being lost.
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Needs use of natural gas during turbine air expansion.
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Two pilots in the world (Germany, USA) for a global power of 480 MW. Very large capacities may be developed (a project of more than 2 GW exists in USA), but it needs adequate geological sites. Investment is identical or lower to pumped hydro, but functional costs are higher.
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May be complementary in term of available sites.
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Huntorf CAES, Germany
Adiabatic CAES: storage of heat during compression and return to the air during expansion. 70% efficiency expected, no more use of natural gas. Important R&D challenge.
Use of different types of geological sites: salt mining, cavern, aquifer...
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Isothermal compression of air in pipes can provide 70% global efficiency, but with low power capacities.
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Secondary batteries use reversible electrochemical reactions to store electricity. Common batteries are Lead acid, NiMH, Lithium ion. Their specific energies are high, but power and life cycles are limited.
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Lead acid is the cheapest battery. However life duration is short when deeply cycled. In an autonomous photovoltaic system including lead acid batteries, the storage is the most expensive item after 20 years.
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Special technologies of electrochemical batteries aimed at large stationary applications have been developed.
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Hot NaS batteries, developed by Tepco in Japan. More than 200 MW installed in the world, strong growth.
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Redox Flow technology uses circulation of electrolyte. Power and Energy can be managed separately. Many pilots around the world.
Na S batteries associated to a windfarm, United States
27 MW NiCd batteries installed in Alaska
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Large batteries are going to be used in isolated aereas such as islands, not connected to a strong grid, with a high level of renewable energies. The case of Israel is probably close to these situations.
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Hydrogen may be used to store electricity provided by fluctuating renewable sources. Fluctuating excess source
Electrolyser
H2 Storage
Although 3,6 time weaker than the one of natural gas, volumic energy density of hydrogen is far more important than other energy storage technologies.
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It can be used at all scales.
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Fuel Cell
Consumer
Most of to day hydrogen production is through Natural Gas reforming, but it produces CO2 and NG is not renewable. Electrolyse has an efficiency of 60 – 65%, is expensive, and is not well suited to fluctuating sources. Conversion to electricity is made trough fuel cells, which are expensive too, with an efficiency around 50%, may be 70% in the future. Life duration is not guaranted.
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Global efficiency is only to day of 30%. However Hydrogen can be used for itself, as an energy vector or a raw material.
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Hydrogen storage Storage may be the strong point of hydrogen. It can be done through pressurization or liquefaction. Although both are energy consuming, they could allow to store high energy quantities in small contents.
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Storage of hydrogen under pressure can be done on a large basis and a long period (interseasonal for instance) in underground reservoirs with no losses.
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Solar Electricity Solar energy has from far the most important potential of all renewables. It is abundant especially in desertic locations. .
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However it works only during sunshine periods. And consumers needs for energy is all day long, especially evenings.
Heat storage associated to CSP
Concentrated solar power plants produce high level heat which is used to produce electricity.
The heat generated can be stored in different media and reused to produce electricity to meet the consumption of the evening peak load and of the night.
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Thermal storage media may be pressurized water, concrete, molten salts, phase change materials, sodium...
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Molten Salts Molten salts are liquid media at the temperature required for CSP, non toxic, non flamable, low cost, low viscosity; their thermal conductivity is however low. Graphite may be added to improve this last property. Molten salts used are 60% sodium nitrate and 40% potassium nitrate.
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The experimental Solar 2 plant in California used 27500 T of molten salt heat storage system made of two main exchangers, two reservoirs (high and low temperature 290 – 380 C), for a storage capacity of 1000 MWh (7,5 hr availability).
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The Andasol CSP in Spain is the first commercial application of molten salts heat storage (Flagsol).
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Conclusions Electricity energy storages are full complement technologies for wide introduction of intermittent renewables. Technologies available for stationary energy storage are numerous. • Pumped Hydro is from far the most widely used around the world. However it cannot be applied everywhere and needs sites and public acceptability. • On a smaller scale, stationary batteries are also widely used in isolated sites, or in small grids, and are growing fast. • CAES and Hydrogen need improvements to become fully competitive.
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• Heat Storage may improve competitiveness of Concentrated Solar Plants.
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Discussion
Will energy storage technologies emerged in the future as the solution to intermittence of large production of electricity from renewable energies, out of phase with demand? At what level? With what technologies? What economical model? Centralized or not?
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Will the grid still be the solution, even smart?
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