From the time electricity was first discovered we have tried to find ways to store it. By 1800 Alessandro Volta (for whom the volt is named) had already developed the first usable battery and we are still working on that same basic principal. But batteries are not the only option and we now have not less than four potential, practical ways of storing electricity.

Lead-acid batteries: Well-known, well-developed technology that has been used for many years. Batteries from milk floats and even submarines have been used by intrepid renewable energy uses. The problems are cost, a relatively short life-span (they have a limited number of charge/discharge cycles) and disposal.

Solid state batteries: The batteries used in mobile phones, laptop computers, cordless power tools, etc. are solid state batteries of one type or another. These are usually lithium-ion (Li-ion) or nickel-cadmium (Ni-Cd) and can be three to four times more efficient than lead-acid. They have low power density (i.e. not so good for things that want a lot of power all at once – like a submarine) but good energy density (i.e. they can store a lot of energy in a small space).

Thermal: There are a number of products available that will enable electricity from renewable energy sources (PV, wind turbines, etc.) to run an immersion heater in a hot water cylinder. Storing electricity as heat is not a new idea and it means that the electricity (usually – at a domestic level) cannot be converted back to electricity.

The grid: The default option for most renewable energy system owners is to ‘store’ surplus electricity production on the grid. That is, to sell it (at around 4.7p/kWh) to an energy supplier and then buy it back (at around 13p/kWh) when they need it.

There are more esoteric options for storing electricity, that are being actively developed including flywheels, compressed air and hydro pumps. So far these are all big and expensive and aimed at the industrial market rather than for domestic applications but one day, maybe…

Cost/Benefit Analysis

As with all these things considering the cost against the benefit can be a complex process and has to start with some idea of how much electricity is to be stored. Perhaps obviously storing a small amount of electricity is cheaper than storing a large amount.

We take the grid as the default option, as it has no capital cost, irrespective of how much electricity we want to store, but has a ‘running cost’ being the difference between the sale and purchase prices – currently about 8p/kWh. It is then reasonable to suggest that any storage system needs to be cheaper than that to justify a capital investment.

Until recently lead-acid batteries were the only viable option and a typical domestic-scale system could cost upwards of £10,000 – including racking, a shed to house it and disposal costs. If we assume that the batteries are storing surplus from a 4kWp PV array and that they will last 10 years, that equates to a ‘running cost’ of perhaps 52p/kWh – making the grid look quite cheap.

Solid state battery technology has advanced in leaps and bounds in recent years and the much advertised Tesla Powerwall is a result of that development. It will store up to 10kWh in a weekly cycle, which is a reasonable amount for a domestic user. A 4kWp PV array (i.e. typical domestic scale) will, in spring and summer, generate around 12kWh per day and the Powerwall is designed to store the midday peak production for evening demand.

We don’t know how long the Powerwall will last but it comes with a 10 year warranty. At a capital cost of around £2,500 (installed) that equates to a ‘running cost’ of about 13p/kWh – so the grid is still looking cheap.

Thermal storage systems are lead by the Immersun unit – this will enable the electricity produced by the PV array to operate an immersion heater in a hot water cylinder. All of these options have an efficiency factor and in the case of thermal storage we also need to consider losses from the hot water cylinder. This sort of conversion unit will cost around £500 installed and if it similarly last about 10 years will have a ‘running cost’ of about 3p/kWh – cheaper than the grid, but now energy that is only usable as heat.

Conclusion

Electricity storage is not a purely hardware problem. The Tesla Powerwall also contains software that determines when to store electricity and when to use it; something the grid cannot do. It optimises its performance to provide the best result for the house, so the pure ‘running cost’ calculation might not be wholly valid.

If renewable energy has a future in the domestic market then battery storage will have its place. Be it as a principal energy source, as back-up or merely for those wishing to safeguard themselves against an apocalypse. If you want the surplus electricity to be usable as electricity then storing on the grid is still the cheapest and most convenient option. But that may not always be the case. History tells us that the Powerwall will be copied and improved, and the price will fall. What the Powerwall has done is open a market and start a debate and that has to be good for all domestic scale renewable energy producers.

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