Whether it is to extend the range and power of electrical vehicles, to smooth out variations in power output from intermittent renewable energy sources, or provide backup during peak demand, storage is becoming an increasingly important consideration in energy scenarios. A new report just published by the European Energy Research Alliance (EERA) joint programme on Smart Grids looks at the state of the art of electrical storage technologies and the various challenges that need to be overcome, in particular in terms of potential applications within Smart Grids. The report only looks at technologies that are mature enough to be deployed on an industrial scale by 2020.
Top of the list of challenges are energy efficiency, lifetime, safety and environmental considerations, as well as costs, says the report. At present, the costs of investment in new technologies outweigh the immediate potential for returns on a commercial scale. But, adds the report, with increased levels of research funding becoming available, “... the improvements in storage capacity and economy will allow present-day storage technologies to be more useful in Smart Grid applications and will make energy storage more competitive compared to alternative technologies.”
There is a wide range of technologies for storing electricity, ranging from mature technologies such as pumped hydropower, which has been around for over 150 years, and compressed air storage, to cutting edge supercapacitors, thermochemical storage, superconductive magnetic energy storage (SMES) and new, more cost effective lithium-ion and non-conventional batteries, such as ZEBRA cells and flow batteries. Storage technologies are often specific to certain applications and even, such as hydropower, precise geographical and geological locations. This, explains the report, makes it difficult to compare their respective strengths and disadvantages.
On the whole, cost considerations aside, the value of a given technology depends on how much energy it can store, how quickly this can be made available and what its discharge duration is. Uninterruptible power supplies (UPS), for example, need almost instant availability of back-up energy for a matter of seconds until, for example, diesel generators can come on-stream. This makes mature, low-cost, conventional battery technologies particularly attractive. At the other extreme, hydro-pumped storage (PHS) facilities can provide hundreds of megawatts within seconds, for many hours at a time, making them valuable for power regulation, peak power generation and to provide secondary and tertiary reserves of power.
For other applications, such as storing energy generated by intermittent renewable energy sources, storage and discharge requirements can be measured in hours, or even days. In these cases, slow response technologies, such as thermal energy storage, may be more appropriate. Molten salt storage is already being used to store electricity from solar towers for distribution on overcast days or at night, while thermal storage is used to store energy generated by wind turbines during off-peak times for use at peak times, when it can be sold at higher prices.
Meanwhile, new battery technologies, such as the ZEBRA cell (which has a central positive electrode) and flow batteries (where both the electro-active materials are dissolved in the electrolytes) are being used both for conventional large-scale power utility applications as back-up systems, and for renewables, such as large photovoltaic fields and wind farms.
While mature technologies, such as hydropower – which accounts for over 95 GW worldwide, with over 300 plants – compressed air and conventional lead-acid (PbA) batteries are widely used, little research effort is being devoted to developing them any further. “The technical potential of new pumped hydro plants in Europe is very low,” says the report, “due to the high potential impact on the environment and the necessity of an adequate altitude profile and geology.” About 75% of the total potential for hydropower in Europe has already been developed.
At present, says the report, the most intense research effort is being directed at improving the performance of relatively recent battery technologies, particularly lithium-ion (Li-ion) batteries and reducing their cost. First introduced in 1991 by the Sony Corporation (Japan), these batteries “represent the state-of-the art in small rechargeable batteries”. They are now widely used in consumer electronic devices, such as computers, digital cameras and cell phones, as well as military, space and electric vehicles. “Efficient cells could be put into packs for large-scale uses,” says the report, “but the cost is still too high.” Priority is therefore being given to lowering the cost of these batteries (currently over 420 EUR / kWh). In 2003, the production of small Li-ion batteries reached 800 million units and roughly doubled between 2003 and 2007, and is growing at 20% - 25% per year.
Although the automotive industry is currently driving much of the development in Li-ion battery technology, their fast charging, light weight and high energy density properties make them strong candidates for use for energy storage in grids and, says the report, “they are becoming the energy storage of choice for future electric mobility applications.” Supply of the raw materials is not expected to run out for a long time, either. Identified lithium resources are estimated to be about 39 Mt, whereas, says the report, “even the highest demand scenarios do not exceed 20 Mt for the period 2010 to 2100.” Research cited by the report anticipates that the market for Li-ion batteries will jump from $2 billion annually in 2011 to $14.6 billion by 2017 However, concludes the report, “despite their interesting perspectives, the electrochemical storage systems are poorly used in smart grid applications (<0.1 %), with lithium-ion batteries still covering a small share of this battery market.” To penetrate further, the technology has to evolve and costs come down dramatically.
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