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Solar PV and energy storage

By Dave Elliott

Some say that the vision of households and businesses moving largely off-grid by storing solar power generated during the day for use overnight is close to becoming a reality. The prospects for moving entirely off grid may be limited – most projects will still need grid links to allow for top-ups when solar input is low for long periods and the stores are exhausted. However, that still leaves a significant potential for self-generation and storage.                                                                                       

In its report on ‘Solar +storage opportunities’, the UK Solar Trade Association says that it is ‘excited about the value that solar and storage can together add to the energy system, leading to a more efficient, cleaner and more secure future network’. For example, it argues that ‘Energy storage means that normal appliances don’t necessarily need to “smartly” respond automatically to when the solar panels are generating. Further, homeowners need not change their behaviour by turning washing machines and dishwashers on during the day to get best use from their solar. Storage allows clean solar energy to be used by a consumer whenever they want to use it and by minimising their draw from the grid, solar power with storage makes a consumer less dependent on the grid: further reducing energy bills’.

Self-consumption by domestic generators is not the only option. The report notes that ‘Peer to peer trading of energy at a local level could provide value for a domestic customer’, although it states that, ‘other models adopt an aggregator-owned approach. In this case, a battery is installed in a home but remains owned by an aggregator, who provides balancing services to the grid through utilising a large number of small domestic batteries’. It also notes that ‘the synergies between solar and electric vehicles (EVs) could be significant. Electric vehicles are a readily-available source of energy storage, with many times more storage capacity than a typical home energy storage solution’.

In addition to domestic PV and storage, the association says ‘solar farms and energy storage seem intuitively to be a perfect match. Grid connections are typically underutilised due to the variable nature of solar generation (and lack of solar export at night), space is typically available and planning permission is either already available or relatively simple to obtain’.

However, it goes on to note that its overall solar optimism ‘is tempered by realism that energy storage systems are not commercially economic yet for all customers, and that more work needs to be done by industry, government and regulators to support the continuing cost reductions.’ It also adds ‘we are concerned about the potential for exaggerating the impact storage can have now’, and also the potential for mis-selling, which must be guarded against.

More generally it says ‘a need for storage is at times being used as a negative for solar, rather than a missing part of the overall puzzle. For example, there is some perception of storage as the solution required before increasing solar penetration, rather than a technology that can allow both industries to flourish far beyond even the large volume of solar that could be accommodated cost-effectively in our electricity system’.                  

Nevertheless, if problems like this can be overcome or avoided, the report is very enthusiastic about the future of storage: ‘there is a move towards variable renewables and more decentralised and flexible energy systems globally, which have benefits in terms of costs, carbon and certainty (security of supply). Storage can play an increasingly important role in this transition, which provides the potential for a vast market in the long term’.       

What happens next? Individual domestic projects are likely to flourish as battery costs fall, and some may be able to reach high levels of autarchy, with only limited grid inputs.

But community-level roll-out has its attractions: there are economies of scale in purchase and deployment terms, as well as in terms of maintenance and system management. There certainly are some big community solar projects emerging around the UK. We are also likely to see larger grid generation schemes with storage, especially in rural locations where there is room for large PV arrays, as witness this PV + storage project in Somerset.

Solar farms like this will no doubt continue to be deployed, in parallel with PV being retrofitted to roof tops, with storage added. Moreover, despite the closure of the Zero Carbon House programme, as PV costs fall, PV may increasingly be integrated into new build low/zero-carbon house projects. Some of them are already low cost. Adding storage may be the next logical step. Some even look to using PV for heating – via an immersion heater in the central heating boiler. There are also some clever new hybrid solar-thermal (PV-T) systems emerging e.g.

PV solar is clearly on a roll in its various applications in the UK, as elsewhere, with over 12GW now installed in the UK. That’s roughly equally split between domestic roof-top projects, backed by the Feed in Tariff, and larger arrays, including solar farms, backed by the Renewables Obligation and its replacement, the CfD system.  Germany has much more PV capacity (over 40GW) and storage is expanding rapidly there: around 41% of all new domestic PV projects installed in 2015 had battery storage. Something like that may well happen now in the UK.

Grid links will still be needed for most of the PV schemes, large and small, including those with storage, but it is all still about decentralising power generation. Indeed, some say we should re-plan the whole energy system from the bottom up. That may be hard. But it is clear that the system that is emerging, as distributed energy systems spread, will be different from the top-down centralised system we have now, and storage is part of that.

The STA has also looked at the impact of storage on balancing cost, as part of a wider study by Aurora on the costs of intermittency. That concludes that for the 11GW of solar currently on the system, dealing with intermittency adds about £1.3/MWh to its cost. With 40GW of  PV by 2030, that would increase to around £6.8/MWh, but with 45GW of wind on the system it would reduce to £5.1/MWh (since wind has a much higher load factor) and in a scenario with no further nuclear, it would fall to £3.1/MWh, ‘due to the more flexible alternative generation technologies we would expect to see emerge from the capacity market in the absence of new nuclear’.

Moreover, batteries would cut this cost further. The STA suggests that, by making balancing for the whole system easier, batteries could provide a net system benefit which might be larger than the cost of PV intermittency – a £3.7/MWh negative cost.  That takes a bit of swallowing. But the STA says thatthe existence of batteries on the system smooths out prices in the energy and balancing markets, thus reducing the impact that the timeliness of delivery has on intermittency cost’ and ‘batteries play a useful role in the capacity market, reducing capacity prices and therefore the cost of backup for renewables’. So that ‘with a large amount of batteries on the system, solar has a ‘negative cost’ of intermittency, meaning that the generation profile of solar is actually more desirable for a battery-enabled system than a baseload-equivalent output profile. Batteries and solar are a complementary combination, with batteries improving the capture prices of solar, and solar creating a generation profile whereby batteries can profitably store and then deliver to the market as needed’. The association really does like batteries!

Though not everyone in the solar field does – some think storage is still risky.



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