Energy Storage

 

Energy storage has come to the fore as a major issue over the last year, in part because of concerns about the increasing use of variable renewables, but also since it is claimed cheaper energy storage systems have emerged- batteries in particular. For example it been suggested that consumer could have cheap battery stores to back up output from their own roof top PV arrays. Some see Tesla’s new 7/10kWh Powerwall as a breakthrough: www.teslamotors.com/powerwall  Meanwhile a variety of large-scale storage options are available, or emerging, to help balance the grid when renewables like wind are low. The most obvious is to use hydro reservoirs for pumped storage: http://www.consultkirby.com/files/NREL-CP-5500-58655_Role_of_Pumped_Storage.pdf  

Problem solved!

Sadly, it is not as simple as it may seem. For example, there may be limits to the amount  pumped storage capacity than can be developed, and, although costs are falling, batteries are still pricey. Moreover, given lucrative Feed in Tariff export payments, there is no incentive for domestic PV ‘prosumers’ to store, rather than sell, any of the excess power they generate: www.renewableenergyworld.com/rea/news/article/2014/10/viable-battery-backup-for-rooftop-solar-still-years-away.

It is also important to establish exactly what we are talking about here: renewables like wind and solar PV, have short-term variability, which can be dealt with by the normal grid balancing system, with for example, gas-fired plants being run up and down in rapid, short-term, response. Smart-grid demand-management systems can also help, delaying demand peaks. Energy storage systems, large and small, can also play a role, though at present they are mostly much more expensive than using back-up plants. But that may change, in some circumstances. For example see http://cleantechnica.com/2014/11/16/where-battery-storage-will-provide-most-peaking-capacity/. 

However, that is only part of the story. Renewable inputs can also vary over longer periods- e.g. there can be long lulls in wind availability, over days or even weeks.  If there are several different types of renewable on the grid, this may not matter much, since the lulls may not coincide: it’s often sunny when its not windy and vice versa, waves are in effect stored wind and tidal variations are unrelated to solar or wind. But if there is, for example, a large amount of wind capacity on the grid, as is likely in the future, there will be times when extra input will be needed. In theory, some storage systems can help with this, depending on the scale of the shortfall and its duration.  Unfortunately though, conventional batteries can’t help for more than a few hours and even large pumped hydro reservoirs are unlikely to be able to store more than around a few days worth of energy. Similarly for large underground compressed air storage systems: www.gaelectric.ie/index.php/energy-storage. And also possibly for ‘liquid air’ cryogenic systems: www.highview-power.com/.

This though is not a big problem, since the already existing gas-plants can be run up to fill the gap- and gas can be stored for any length of time.  Increasingly these plants, or replacements, can use green gas, biogas or gas produced by electrolysis using surplus wind derived power, rather than fossil gas, so that emissions can be avoided. Biomass and biogas can be easily stored for long periods. Some other renewables can also help during wind and/or solar lulls. Like biomass, geothermal and hydro are ‘firm’ continuous energy sources. In addition, heat can be stored and used to balance energy supply and demand. For example, if coupled with a heat store, a Combined Heat and Power plant  system, running on gas (perhaps soon green gas) can be used very flexibly for grid balancing. If there is too much wind generation and low demand, the CHP unit can switch to producing heat, for storage if it is not needed immediately. If wind is low and demand for electricity high, the power output from the CHP plant can be ramped up and heat supplied, if needed, from the store.  Heat can be stored for a relatively long time with low loses – there are even interseasonal heat stores, storing summer solar heat for winter use : ww.icax.co.uk/interseasonal_heat_transfer.html 

Heat storage makes most sense on a large scale, since the surface area to volume ratio decreases  with size, thus reducing energy losses. But there are some small scale options which, although less efficient, might catch on. For example home heat storage using immersion heaters run off domestic PV solar: www.immersun.co.uk There is an interesting debate over whether that makes sense compared, for example, to battery storage or direct solar heating!

So what is the bottom line?  Which approach will win out? Costs will shape the choice, (£200/kWh is often seen as the bench mark), but so will convenience and impacts.  We are happy to pay vastly over the odds (in £/kWh terms) for small portable energy systems- torch or laptop batteries for example.  At the other end of the scale, some cheaper energy storage systems take up a lot of room and can only be located in specific places- pumped hydro for example. In addition, the choice of system depends crucially on the end-use requirement, whether its for fast short-term power back-up, or for longer term reserve capacity.  

Assessments vary, and there are may new ideas emerging, for example for flow batteries: http://pubs.rsc.org/en/Content/ArticleLanding/2014/EE/C4EE02158D  And even ‘outlier’ ideas like mass/gravity storage: www.gravitypower.net/  But a review by the UK Energy Technologies Institute seemed to suggest that, overall for medium term grid balancing, district heating stores had the best prospects. In one of their scenarios it dominated by 2050, offering much more than anything else, including pumped hydro, the currently dominant option, presumably because there isn’t enough room in the UK for a lot more pumped hydro capacity: www.eti.co.uk/wp-content/uploads/2014/05/All-Energy-Storage-May-14-Phil-Proctor.pdf and www.eti.co.uk/wp-content/uploads/2014/06/Energy-Storage-The-Engineer-Conference-v3-CLEAN.pdf

The debate over storage can get heated at times- unsurprisingly since potentially there is a lot of money involved. One estimate is that the global market could have a net worth of $10.8 bn by 2018. There are also major strategic issues and disagreements. Some of them came to the surface in Germany last year when the Agora consultancy claimed that, over the next 10 to 20 years, the use of energy storage was not likely to be a more effective for allowing renewables to be integrated into the grid than a mixture of other options giving energy systems more flexibility. The chair of the German DENA energy agency responded, a little bitterly, ‘electricity storage facilities are essential for the energy turnaround. Anyone who alleges otherwise is damaging the energy turnaround and, in the end, is risking the supply security in Germany’.   http://www.dena.de/en/press-releases/pressemitteilungen/dena-fordert-stromspeicher-muessen-zuegig-ausgebaut-werden.html

For what it’s worth, the International Energy Agency, says, in its Energy Technology Perspectives 2014, ‘Electricity storage is expected to play multiple roles in future energy systems, but it is unlikely to be a transformative force itself. At current costs and performance levels, particularly for high-power and high-energy applications, it falls short of delivering the conceptual flexibility potential when compared with competing options’.  But they could be wrong!

For more on storage see www.electricitystorage.org/  and www.theiet.org/factfiles/energy/energy-storage-page.cfm  along with POST’s new briefing note: www.parliament.uk/briefing-papers/POST-PN-492/energy-storage