Timeshifting. The ability to store something for consumption at a more convenient time. Great for watching your favourite television shows. Ideal for solar PV, too, but sadly we are not at a stage where solar energy is widely timeshifted via a programmable box. At least, not yet.

Battery manufacturer Saft is actively working towards such a solution. The French firm is participating in a number of projects aimed at merging solar PV with energy storage systems (ESS) from feasibility in the laboratory to feasibility on an industrial scale.

One such project is a Franco-German affair named Sol-ion, which also involves solar PV manufacturer Tenesol, string inverter specialist Voltwerk, German utility E.ON and a number of academic institutions including Zentrum für Sonnenenergie- und Wasserstoff-Forschung (Centre for Solar Energy and Hydrogen Research, ZSW).

The project, which has financial backing from the French and German governments, was initially conceived with the German federal government’s decision to augment its national feed-in tariff regime with a ‘self-consumption’ tariff from 2009. This incentive was axed during the latest amendments to the Renewable Energy Sources Act (EEG), but the timeshifting concept remains pertinent as many nations move towards a ‘post-incentive’ solar world, where exporting power to the grid becomes decreasingly lucrative and self-consumption makes more economic sense.

The project will see the installation of a total 75 solar PV + lithium-ion battery ESS units in France and Germany, which makes it the largest such R&D project in Europe. Ten of the units have already been installed in the various institutes and partners participating in the project, while the remaining 65 will be deployed in individual households. In Germany, they will be mostly installed in the Bavarian town of Schwandorf, where there is a high penetration of solar power. In France, they are mostly going to the islands of Guadeloupe, as well as mainland France.

The Sol-ion systems comprise four of Saft’s Synerion 48E lithium-ion modules, rated at 48 V and with a capacity of 2.2 kWh; a 5 kWp string inverter and a battery converter. The Sol-ion unit’s EMS (energy management system) controls the overall system state and chooses the mode of operation: to use PV for self-consumption, to recharge the battery on each unit, for storage or for export to the grid.

Promising Early Results

The French systems are configured to prioritise backup power, as Guadeloupe suffers from frequent power outages. Only when the batteries are at least 70% fully charged is any solar energy consumed by the household.

The systems deployed in Germany, however, are optimised for self-consumption and are therefore configured to prioritise household loads. Surplus energy is sent to the batteries until fully charged, and only then is any excess power fed to the grid.

Initial test results from one installation at ZSW’s site in Widderstall, Germany, showed a fairly consistent boost in self-consumption of approximately 40% during 11 days of testing in November. On 18 November, for example, solar PV self-consumption with storage was 85%, of which 45% was delayed use. By comparison, the site self-consumed just 45% of solar PV output with an identical set-up minus the ESS on the same day.

These early results are in line with projections, according to Michael Lippert, head of Saft Energy Storage. ‘In our calculations and simulations we expect to shift self-consumption by 30-40% to 70% overall over an entire year,’ he says. ‘This includes what we call natural self-consumption — when production coincides with consumption — which is around 30%.’

Self-consumption will naturally vary in different months of the year. ‘You would probably have 100% self-consumption in January due to shorter days. If you produce just 2 kWh a day in January, most of this will be used.

‘On other days, the capacity of the energy storage system will be inadequate and the battery system will be fully discharged and recharged in a day, while other days may see only a 10% discharge. Over the course of a year, the batteries will cycle at 60% per day on average. This is why we think lithium-ion is the best technology for energy storage batteries, as it can cope with this high variability of discharge.’

Of course, they say the same thing about lithium-ion for electric cars. And even with billions of dollars in government funds, electric cars have struggled to find a market, mostly because of the high cost of lithium-ion batteries.

Lippert says costs will come down once the systems are mass produced. Saft’s recently commissioned manufacturing plant in Jacksonville, Florida will make up to three million cells a year, but it will be some time before the cost will come down to the company’s target price of ‚Ǩ400/kWh for ESS lithium-ion batteries.

Saft is not the only company planning to bring to market time-shifted solar power via lithium-ion batteries. In July, solar PV manufacturer Kyocera began shipping its solar PV + ESS system to households in Japan. The package, which features a 4.03 kW PV array with a 7.2kWh lithium-ion ESS made by Samsung retails for the tidy sum of ¥4,926,000 ($60,825).

Distribution and Investment

Lippert acknowledges that the uptake for ESS from solar PV producers will be limited in the short and medium terms due to the high cost. Furthermore, large scale solar power producers have so far shown little interest in storage, as due to feed-in tariffs (FiTs) they are incentivised to export as much power to the grid as possible.

So while the Sol-ion test results have shown some promise for residential users for self-consumption, the primary takeaway from the project for Lippert is the drastic reduction in power fed into the grid, which on some days fell to zero.

Grid stability is key to the adoption of energy storage for large-scale solar and other renewables, and is the reason why leading power equipment manufacturers are developing their own grid-scale ESS.

GE Goes Big on Storage

Energy storage batteries do not start and end with lithium-ion. GE has placed a large bet on sodium nickel chloride being the winner in the race to provide cost-effective batteries for the global energy storage market, which it estimates could be worth $65 billion by 2020.

The US firm has built the largest non-lead acid battery manufacturing plant in its home nation in Schenectady, New York, to manufacture its Durathon system. GE’s global research centre looked at all the available battery technologies and decided on sodium nickel chloride as the most viable.