What does Energy Storage mean?
Energy storage is a mechanism to contain useful energy which can then be used at some later time in the future. It is a naturally occurring process, for example, fossil fuels are naturally created stores of the suns energy from millions of years ago. It is also a process that has been exploited by humans since they have existed – think of people gathering wood to burn to produce heat for cooking or heating.
Energy can be stored in many forms, but some are easier to contain than others. For example, it is difficult to store large amounts of energy as electricity, it is comparatively easier to an equivalent amount of chemical potential energy or even gravitational potential energy.
When the term ‘Energy Storage’ is used today it generally excludes fossil fuels, referring to a process by which potentially useful energy is absorbed at an earlier time and then released on demand at some useful time in the future without using traditional fuels like coal, gas, uranium etc. However it is important to realise that fuels (including traditional fuels like fossil fuels and uranium for nuclear fission as well as non-traditional fuels like hydrogen etc) are all forms of energy storage. The energy storage aspect is what makes them a fuel! At present the most important form of energy storage in most energy systems is fossil fuels – these are unmatched in terms of energy density, accessibility, low-cost, ease of handling and conversion and transportability.
The last paragraph was semantics really, and our focus on energy storage will now switch the general definition that excludes traditional fuels, however by considering fuels as energy stores, we have illuminated the crucial nature of energy storage. Indeed, we have also shed light on the size of the task that would be required if we were to try and switch from fossil fuels to some other energy source, which didn’t include the intrinsic storage associated with fossil fuels…
So what is the big deal about energy storage?
Recent interest in energy storage has boomed due to the widespread uptake of renewable energy sources, which aim to reduce our dependency on fossil fuel generation. The motivation for this switch comes from the compelling evidence around climate change (the reality of which is, as I always feel like I should point out, entirely accepted by the scientific community despite how it is portrayed in mainstream media) and its dangers to our species. As the electricity industry is the biggest source of global GHG emissions, renewable energy is an important method of mitigating climate change.
There is certainly enough renewable energy to power our global energy needs, especially if we engage in a significant amount of energy conservation and efficiency measures, however the biggest problem with renewable energy lies with its associated intermittency. That is to say that the wind does not blow all the time, nor does the sun shine all the time, nor even do the tides flow all the time and unfortunately we cannot store the wind as wind or the sunlight and sunlight (we can in a way store the tide as potential energy in water – tidal barrages do this – however this is probably a small contribution to our energy needs and id not without its drawbacks)! Further still, much of this intermittency is largely unpredictable; of the three examples listed only one renewable energy source is largely predictable – the tidal energy resource. Wind and solar can be predicted fairly accurately a few days before (with increasing accuracy as the forecast window to real time shortens) but beyond that we cannot tell when these energy resources will be available.
Energy storage is one way to tackle this intermittency issue head on. The idea is that by storing energy extracted from these renewable resources it can be available to use at a later more convenient point in time. Of course, for an entirely satisfactory solution it would be necessary to know that if we stored say a weeks’ worth of energy the renewable resources would be available again sometime within that week to re-charge the energy storage before we ran out. It would also require that the energy storage had the power to charge and discharge at the necessary rates, and the capacity to store all the energy that we needed. That is the ideal – affordable large scale energy storage would be the “holy grail” for an entirely renewable based electricity system.
As well as facilitating widespread deployment of renewable energy generation, energy storage also offers many other benefits to energy systems which include:
- facilitating increased deployment of inflexible nuclear generation – traditionally nuclear energy has been regarded as an inflexible technology which is well suited to providing cheap baseload power. Historically Pumped Hydro was often used alongside nuclear to cover peak electricity demand.
- increasing reliability for end-users – energy storage can allow the lights to remain on during blackouts. This is common in many developing countries where rolling blackouts are relatively common. Energy storage used in this way is often known as Uninterruptable Power Supply (UPS). Energy storage is also used to deliver very high quality power for applications in precision manufacturing by smoothing out the frequency and voltage fluctuations associated with electricity from the grid.
- reducing the volatility of electricity prices – although most end-user do not see time-varying electricity prices, there is some evidence to suggest that high price volatility pushes up electricity prices for consumers.
- increasing system reliability and flexibility – energy storage is a useful resource for electricity network reserves, as energy storage technologies usually have high ramp rates and can respond quickly to instructions from grid operators.
- reducing the need for transmission upgrades/new transmission infrastructure – this is known as asset enhancement. Energy storage can defer of replace the need to upgrade transmission or distribution infrastructure, potentially resulting in a cost saving.
- reducing overall pollutant emissions – depending on how energy storage is used, it may be able to reduce the overall emissions associated with the operation of an electricity network. This is particularly true if storage increases production from low-carbon energy sources while decreases generation from more polluting fomr os electricity generation, like lignite or brown coal.
Is energy storage the only way to tackle the intermittency associated with renewable energies?
No. There are a couple of different approaches to dealing with intermittency. One way that was heavily favoured over energy storage until recently was interconnection – the idea being essentially that we could rely on the geographical spread of renewable energy generators and compensate for reduced generation in one area by increased generation in another. Say that if it wasn’t windy in the South of England it might be windy in the North of Scotland, or it may be sunny in the south of Wales (although South Wales residents may think this unlikely), so the energy from the area with favourable conditions would provide the whole system with renewable energy. This approach is very useful, but has two fundamental limitations. Firstly the geographical spread has to be very large and be very well connected via energy infrastructure and as it turns out, renewable resources are usually most energy dense where these is a sparse population and hence a weak grid infrastructure. Secondly this requires an impractical amount of over-capacity in many geographical locations.
Another approach is demand-side management. That is, instead of changing supply to match demand, demand can be reduced when supply is scarce. This is a great approach as it is often a very cheap solution (the cost of not using electricity at a particular time is often relatively low), however it requires an incentive for users to perform certain tasks at certain times, by making the price of electrical units (kWh) more expensive at certain times. For example, cheaper electricity at night encourages people to use electricity when the demand for electricity is usually lower. There are several existing schemes that already use this technique, and many electricity suppliers offer a two-tariff payment option meaning that electricity at night is cheaper than during the day. This is especially useful for technologies like storage-heating systems. The biggest problem with this approach is that in order to be effective consumers have to see real-time electricity prices, and be able to respond to them. There is significant resistance from legislators and the industry around this, and political concerns that some vulnerable consumers may suffer from very high bills due to price spikes. Crucially too, demand side management can only go so far, as most would consider it unacceptable if they could only turn on their television when the wind was blowing! Demand-side management can also be used to avoid asset enhancement.
It seems sensible to thinks that the best way forward is through a combination of energy storage, interconnection and demand side management. These perhaps shouldn’t be regarded as competing options either, but complimentary approaches to aiding the transition to a sustainable energy system.
So what’s the problem, why don’t we just get on with it and start connecting energy storage devices to the grid?
The big hurdle with energy storage currently is that at present we do not have an effective, affordable and proven method of large-scale energy storage. This is partly to do with the development of the technology, and indeed if an effective very-low-cost technology option was suddenly developed it is likely that it would be widely adopted, but there are several other factors that prevent the development and uptake of the current energy storage options. The main factor is prototype devices of the current energy storage technologies have large capital costs and a low anticipated return on investment. Many of these options have potential and deserve further development, and it may be that manufacturing costs could be significantly reduced for non-prototype systems. However due to the low anticipated return on investment for storage devices it is difficult to persuade would-be investors. This begs the question of whether the market structure around energy storage should be reformed to encourage the development of our existing energy storage technologies and this will be further discussed in this website!