The energy revolution has begun. Almost daily reports trumpet price breakthroughs in solar and wind electricity while fossil-fuel generation facilities are disparaged as stranded assets.
Engineering advances have seen renewable electricity generation costs plummet so fast they are competitive with oil and gas, and giving coal a nudge. Yet they still represent less than 4% of global electricity production.
The biggest roadblock to the large-scale uptake of solar and wind electricity is their intermittency. We need storage systems for them to be useful. Storage that can supply whole countries in winter when – inevitably – for perhaps 10 days in a row the wind is still and the sun doesn’t shine.
Developing large-scale storage systems is the key to converting our electricity production to zero-emission sources.
One possibility is pumped hydroelectric storage. In this approach, excess electricity is used to pump water uphill from a lower reservoir. When electricity is in short supply the pipe gates in the upper reservoir are opened, allowing a torrent of water to spin turbines and generate electricity.
The potential is huge. The world’s biggest ‘battery’ is the pumped storage facility at Bath County, Virginia, with thecapacity to store 30,000 megawatt-hours of energy, enough to supply Los Angeles for 12 hours. This method far exceeds the global electricity storage capacity of all other storage forms combined.
But change is on the way. Tesla Motors Corporation has been developing battery packs with ever greater storage to give their electric cars a competitive advantage. They are now committed to applying their battery know-how to store intermittent renewable energy. The recently announced Tesla Gigafactory is projected to produce 50,000 megawatt-hours of storage across five million, 0.01 megawatt hour lithium- ion battery packs annually in 2020.
Encouragingly, Tesla is not alone. Mercedes Benz is already selling home storage solutions and the Chinese automobile company BYD (Build Your Dreams) has announced plans for 34,000 megawatt-hours of annual battery pack production by 2020.
Not to be outdone, traditional battery manufacturers such as LG, Samsung, Panasonic and Toshiba are developing and supplying battery backup systems of up to 40 megawatt-hours each. Manufacturing efficiency has kept pace and pushed battery prices down fourfold in less than a decade, from $1.3 million per megawatt-hour in 2006 to $350,000 per megawatt-hour this year. This trend is likely to continue.
Around the world, in private and publicly funded labs, extensive research is being done to develop batteries based on magnesium, zinc or aluminium instead of lithium. These approaches might be even cheaper, and will relieve the pressure on lithium mining.
Storage technologies must not only compete on cost but also on their ‘round-trip’ efficiency. That is: start with electricity, store it, convert it back to electricity. For batteries, the round- trip efficiency is 85%. For pumped hydroelectric storage it’s more than 80%. These values are good enough to be extremely practical.
Hydrogen storage, in which electricity is used to split water into hydrogen and is then converted back to electricity, has the potential to be used at large scale in a turbine generator, or for a car’s fuel cell.
But most hydrogen storage systems operate at 35% round-trip efficiency. A more respectable 50% round-trip efficiency can be expected in coming years.
We need storage systems for solar and wind electricty to be useful.
Compressed air energy storage systems, in which excess electricity is used to compress large volumes of air that is subsequently released to drive an electricity generator, have occasionally been installed, but their round-trip efficiency remains less than 50%.
There is still a long way to go. To store the world’s electricity requirements for 10 days would require 500 million megawatt hours – 10,000 times the projected annual production capacity of the Tesla Gigafactory in full swing. To get there we have to think big.
The message for governments is clear. Governments already use subsidies to encourage renewable electricity generation. It is time now to shift their investments to improving storage.