Energy Storage

Electrical Energy Storage at Grid Scale

There is a fundamental problem with grid-level electricity. Whatever electricity is produced this second must be consumed this second. The requirements (consumption/load) varies over the course of a day, with the peak load being in the evenings. The generation from solar varies during the day – with the peak in the afternoon, and zero being produced at night. Wind energy varies depending when the wind blows. Balancing these issues is a significant challenge.

Massive growth in renewable energy generation is driving the expected global energy storage market. It is expected to increase several-fold over next few years. Efficient and flexible energy storage means renewable generation would need to target average loads rather than peak.

Because of this, there are now many companies developing different battery technologies. This appears to be an active market in working technology, rather than just at the research-paper stage; this is really positive progress. However, many of the battery technologies are being developed by small private companies, and some which appeared to have promising ideas, have been forced out of business over the past few years.

Governments and those purchasing storage systems to back up renewable generation schemes, need to make wise choices from the potential systems available, and to support those companies whose technologies enable a greater contribution of renewables to the world energy supply.

On average the UK uses about 900GWh of electricity every day or about 13 KWh per person (ie 13 units of energy per person per day)

For energy storage, at a grid scale, we need to store multiple GWh amounts of energy, and need to be able to charge and discharge again in multiple GW quantities with the ability to change from charging to discharging as quickly as the wind might change in strength. The storage needs to match the level of renewable production.

If you can smooth out the renewable energy production by installing storage systems which will respond rapidly to the levels of supply and demand, the renewable power bank becomes much more useful to the electricity grid. The combination of renewables and efficient rapidly responding storage can present more like a constant power supply, rather than a variable one.

To store energy, generally the energy is changed to some other form for storage. Any change in the form of energy will lose energy in the change, so no energy storage system will be 100% efficient. You will never extract as much energy out of a system as you put in, initially.

We are thinking here about storing Electrical energy.

For the storage of electrical energy, you want as high an efficiency as possible, but you also want the storage mechanism to be able to reproduce electricity or absorb spare electricity easily on-demand, with a very rapid response time – ideally less than half a second. It also needs to be able to spread the production over whatever time period you might expect to need it. For Solar PV – this means storing excess during the day when the sun might be shining, and releasing it when it is dark or on days when it is so dull that very little PV can be produced. For Wind generation this means storing energy on windy days or at windy times of the day, and letting it out when it is not windy. For Tidal power, the cycle would be more predictable – two storage and release cycles every 25 hours.

Most countries have more Solar PV and Wind renewable sources, so we certainly need energy storage overnight, but ideally over a couple of days.

Varying demand to match production

Load shedding

The idea is to encourage people NOT to use electricity when it is scarce.

Some proposals involving smart meters would encourage people to change their patterns to help here by choosing to turn off devices, but it is not likely to have much effect.

Certain industries already do this – at least a bit. One example is the water industry which pumps water only at certain times of the day. Another example is cold storage facilities where chilled and frozen goods are stored. These receive their electricity on a special tariff at a reduced rate – because they have agreed with the grid that they can be effectively switched off for a short time. The grid does this if there is a sudden shortage of electricity – because of a drop in wind power, or failure of some other system or a surge in demand for some reason – eg everyone putting on their kettles during the break in the world cup final(!)

Load shifting

The idea is to encourage people both to USE electricity when it is abundant, but NOT when it is scarce.

In the bad old days when we heated houses using electric radiators, these were usually tied to an overnight tariff called Economy-7 that gave cheaper power in the night – when there was surplus electricity. This is another form of “load shifting”. The New potential “load shifting” – is Smart domestic refrigeration.

If this sort of thing could be done in all fridges and air-con units in a country, it might be another way of coping with the variable level of production from renewables. It is another form of load-shifting.

If the wind was blowing significantly more than expected, and any fridge/freezer was not at the coldest point in its cycle and could take some energy, it could switch on its compressor. Or if the wind suddenly dropped, and the fridge/freezer was not already at the warmest point in its cycle, it could switch off its compressor.

Similarly with Air-con units and many other thermostatically controlled devices which generally tend to use energy in cycles.  That would be a big change throughout any country – perhaps time for the introduction of a smart plug of some sort? This needs to happen automatically, without anyone having to physically switch things on or off.

You (or at least an electrical device) can tell if there is too much or too little electricity being produced, compared with the demand, by looking at small changes in the frequency of the electricity supply. If more electricity is being produced than is actually needed, the frequency goes up very slightly. The frequency goes down a tiny bit when the demand is greater than the supply. 

In the UK – population about 68 million, perhaps that is about 17 million households, and perhaps about 15million fridge/freezers. If each one takes about 200W and they normally run for about a quarter of the time then at any time you would expect nearly 4 million of them to be running, using about 750 million watts or 750MW.

For comparison a typical UK power station contains two identical generators each capable of producing 600MW.

So that is quite a big variable load if it could be switched on or off to use up or release electricity to follow fluctuations in renewable production. Each automatic switch could have built in a random amount of time for the change in frequency to have been apparent before actually switching on or off, so that the response would be suitably in line with any change in production, and to prevent them all switching on or off simultaneously which would in itself cause a shock to the grid.

Other potential new ideas for Load shifting

There are other industrial processes eg de-salination of water in some countries, or electrolysis of water to produce Hydrogen, which could be switched on or off depending on the level of renewable production, that would also help.

Similarly building intelligent chargers for Electric vehicles could be very helpful – this could be picking overnight times when there was very little domestic demand, but a lot of wind production. Perhaps the car owner could indicate when the next planned use of the vehicle was going to be, to allow the intelligent charger to pick the best charging slot before then.

Grid resilience services

To provide a reliable level of service, the National Grids needs to be able to “keep the lights on” even when things go wrong – as they will inevitably do.

For these reasons, in the UK, the Grid keeps the ability to absorb a shock event of about 1.2GW – equivalent to the largest power station shutting down unexpectedly – or falling off the grid – perhaps due to a transmission line failure. This “reserve power” is a mixture of:

  • the fast reserve eg Hydro which can switch on in < 2 minutes, and supply power for 15 minutes
  • fast start stations which can switch on in 5 minutes and run for 4 hours – ie typically keeping some power stations running on idle
  • and load shedding.

It works so well that people only have their power disrupted if the rare occasions when multiple faults happen simultaneously.

However for a carbon free future we need to buffer more than just a few hours of energy for the nation, and this requires bulk storage…

Methods of bulk energy storage

We have all become used to batteries in our smart phones/tablet. However providing a a battery storage big enough for a nation is an entirely different task, and almost certainly requires other technologies than simply a mountain of laptop batteries!

Two basic types of energy storage are Physical and Chemical. 

Physical storage:

  • Pumped Hydro-Electric Power – but there are only limited places where the geography works
  • Heat (eg Solar-thermal)
  • Compressed air – some companies have tried this, but none successfully so far. Systems only seem to be about 50% efficient.
  • Pumping water or weights within old mines effectively creating sites for more small scale pumped Hydro-Electric Power – has been suggested, but not aware of anywhere that it has actually ever been tried.

Chemical storage:

  • Creating some energy-dense material with the energy needing to be stored, as a way of using excess electricity – eg liquid fuels 
  • Rechargeable Grid-scale Batteries (see below)

Note, for electric vehicles, it is important for the batteries to be light weight and low volume, so that the vehicle does not waste energy dragging a heavy battery around with it, and the battery does not take up too much space in the vehicle – i.e. for Electric vehicles the most important factor is the energy density of the battery.

However, for grid-level storage, the weight of the battery system really does not matter. The volume of the system, matters very little too, provided it can plausibly be transported to where it needs to be installed or can be easily constructed where it is needed. For grid-level storage you do want a battery system which will work for many years without problems, and ideally is very stable, safe and does not need a lot of active maintenance.

There are two aspects to consider for grid-level storage: battery capacity, i.e. the total electrical energy able to be stored in the battery in MegaWatt hours (MWh), and the maximum Power, i.e. the maximum rate of charge/discharge of the battery in MegaWatts (MW).

Battery technologies

A broad summary of different rechargable battery technologies – more details in individual pages. Most of these are already commercial propositions or realities. 

A few notes on how batteries work and common limitations

Liquid metal / Molten salt batteries

Initial heating is needed to make these work, but thereafter they maintain their own heat. No moving parts so low maintenance. No complicated or fragile internal membranes so will not degrade over time. Cannot over-heat. Non flammable. High capacity and responsivity. These seem like a sensible choice for grid-level storage.

Flow-batteries

Active pumping is required to make these work. The systems store the chemicals which react inside the battery, in external tanks and pump them through the battery cell when charging or discharging. There are several different options for the internal chemical processes involved. The relationship between Capacity (MWh) and Power (MW) is configurable by the particular physical design of the system. Some designs and choices of chemicals are non flammable. Some designs seem like a sensible choice for grid-level storage. There are several companies making these.

Zinc-Air batteries

Very low maintenance, cheap components, no moving parts. Capacity of the battery and its maximum Power is configurable by the design of the system, but they are not usually particularly high power. Non flammable and not heat-sensitive. Good for remote local storage of renewables where mains electricity is not available, particularly in hot countries. There appears to be only one company producing these. Others have tried, but failed to make them work.

Lithium – ion batteries

Everyone has heard of these – they are light weight and so are good for Electric vehicles and portable domestic goods like phones and laptops. However, they have potential problems with “Thermal runaway” – they can get hot and catch fire! Because of this they have only a limited safe operating temperature range. They also degrade over time.

Large, high power lithium ion batteries usually need active cooling and there have been several fires involving lithium ion batteries in places where they have been used for grid-level storage. There are a lot of companies making these – mostly targeting the handheld devices and the car industry. Unfortunately, given that everyone has heard of them, those with political and purchasing power are more likely to approve them for grid level storage, even if they are a really bad choice for that purpose.

Also remember that lithium is a scarce resource, so we should be using this for small movable batteries (phones, laptops cars etc) not for factory sized unmoving batteries. Lithium batteries also need cobalt which is also in very short supply.

Note: Lithium battery producing companies claim that the global lithium resources are not a problem. Technically there is plenty of lithium in the earth’s crust, however practically, it is very expensive and difficult to extract it because it is not collected in dense deposits. In the subterranean brine lakes and old lake beds where it can be found it is only there as a tiny proportion of the material – “a few hundred to 7000 parts per million” – in other words you might extract between about 3g and 70g of lithium from a ton of material processed, so a huge volume of material has to be extracted and refined in order to extract the lithium that is wanted.

Zinc hybrid cathode batteries

These are designed for grid level storage and a 1GW installation was announced in 2020. They are only produced by one company at the moment.

Salt-water batteries

The company who was making these went bankrupt in 2017. At that stage they had some small scale installations so the technology was demonstrable.

How much total grid storage will be needed

Quick calculation to look at scale of energy storage needed

Between 2010 and 2017 the global total annual energy usage went up from about 100,000 TWh to 113,000 TWh (according to Wikipedia). This tries to count all forms of energy, but if we want to become carbon-free, then transport has to be powered by electricity or liquid fuels possibly made using electrical energy. If energy usage continues to increase at the same rate as it did on average year, by year over that time, then by 2050 we will be using about 200,000 TWh of energy per year. One or two days worth of storage, if this was all electrical energy, would will be about 1,000 TWh – worldwide. That is 1,000,000,000 MWh

Looking at any storage mechanisms, we need to work out what raw materials that would require and what will be possible.

The critical ingredient in Liquid Metal / Molten Salt batteries is antimony. Antimony is about 10 times more common than silver, and is used in many things. China appears to have been buying up supplies of it recently. However the Liquid Metal / Moten-Salt, battery design is intrinsically safe, has no moving parts and is low maintenance. It feels like a very good option for grid-level storage. Other chemistries which work with this basic concept may well emerge. The technology needs to be positively encouraged.

The biggest problem with the raw materials in lithium ion batteries is with the supply of cobalt which almost all of them use. Lithium ion batteries also degrade over time, suffer from potential thermal runaway issues in high power versions of the batteries, and need active cooling. They do not seem like a sensible option for grid-level storage.

Flow batteries based on vanadium – again the global supply of vanadium is only sufficient for a tiny percentage of the total energy storage requirements, and it is mostly used in steel making in any case. Not many countries have enough vanadium for it to make sense outside those countries (China, Russia and South Africa).

Hydrogen/Bromine Flow batteries – there is plenty of bromine and hydrogen to supply all the global storage required. The down-side is that the energy storage, when the battery is charged, is in high pressure tanks of hydrogen.

References / external websites:

Wikipedia articles

https://en.wikipedia.org/wiki/World_energy_consumption

https://en.wikipedia.org/wiki/Electricity_sector_in_the_United_Kingdom

Bell, Terence. “An Overview of Commercial Lithium Production.” ThoughtCo, Aug. 26, 2020

https://www.thoughtco.com/lithium-production-2340123#