One of the drawbacks of generating electricity using solar panels or wind turbines is intermittency: output can fluctuate greatly during the course of just a few minutes, as the sun dips behind clouds or the wind dies down. But recent developments in battery technology and control systems have led to devices that can even out those fluctuations by storing surplus electricity, which can power appliances at any time, instead of drawing electricity from the grid.
A range of products are already on the market, with more to follow, but they are still relatively expensive compared to the achievable savings. However, 'smart' control systems mean that in some cases cheaper off-peak power can be used to charge the batteries, and hence save money, even if your panels are having a 'dull day'!
Powervault G200 energy store is a sealed maintenance-free unit that contains either lead-acid or lithium-ion batteries. Performance is monitored remotely via the internet. Image courtesy of Powervault.
The UK company Powervault has a range of storage systems based on either lead-acid or lithium-ion batteries, with storage capacities from 2 to 6 kilowatt hours (kWh). These are designed to integrate seamlessly into the home circuitry, drawing charge from the alternating current (AC) side of the inverter for the solar panels and giving AC output of 230V and 50 Hz (see diagram) just like the mains power supply.
Units containing lead-acid batteries are the cheaper option and are claimed to last for 1200 charge/discharge cycles, giving a life expectancy of 5-7 years. Typical installed cost varies from around £2500 for the 3kWh capacity model to around £3000 for the 4kWh version. The lead can be reclaimed and recycled at the end of the battery's life.
Newer lithium-ion technology is used in three further options, with storage capacities of 2, 4 and 6 kWh. These batteries are reckoned to last for more than 4000 charge cycles, giving an operating life of 17 to 20 years.
Because of its AC connection, the Powervault store is compatible with all existing solar PV installations. The unit contains batteries, inverters, chargers, management systems and control board. Image courtesy of Powervault
Both types of unit are packaged in floor-standing steel cabinets, measuring 500 x 580 x 820 mm, weighing from 85 kg up to 245 kg for the heaviest lead-acid battery unit. They can be wheeled around on castors and installed anywhere within 25 metres of the household consumer unit/mains entry, for example under the kitchen work surface or in a utility room.
A WiFi connection allows their performance to be monitored remotely, and customers are able to log onto the Powervault portal to check on their own unit. All units have an emergency power socket for running appliances during power outages.
Powervault claim savings on electric bills of up to 20% from storing solar energy, and a further 15% by drawing off-peak cheap electricity and storing it for use at peak times. Use of Powervault in the 'average UK home' is estimated to save 0.3 tonnes of CO2 per year, and the installation in no way affects how your feed-in tariff is calculated.
Tesla, the US maker of electric cars, has ventured into the energy storage market with its Powerwall system, again designed primarily to store surplus electrical charge from solar panels and release it on demand. Based on a lithium-ion battery, the Powerwall has a 6.4 kWh storage capacity and 3.3 kW maximum power output, sufficient to power most homes during the evening. The cabinet can be wall mounted, indoors or outdoors.
Unlike the Powervault, the Powerwall charges using DC current directly from the solar panels, and thus requires a suitable inverter to change this to AC current for powering most household appliances. Cost is about £5000-5500 including installation charges.
Maslow from Moixa Technology
Another product based on lithium-ion technology is the maslow integrated smart battery and power delivery system from Moixa Technology. This charges using AC, and delivers the stored electricity via low-voltage DC circuits to power LED lights and electronic devices such as routers, laptops and monitors plugged in with smart USB sockets. The wall-mounted units are rated at 2 or 3 kWh capacity, and the batteries have a projected life of 20 years. The cost is around £2000 plus VAT plus installation.
Maslow connects to a web-based interface so that it can be monitored via a computer or smart phone. Because of its DC output, maslow can be configured to store charge directly from solar panels without the need for an inverter, thus enabling the creation of small stand-alone DC solar power systems for LED lighting or small DC devices.
Electric vehicles as energy stores
Some of the most effective and flexible storages devices are electric cars and other electric vehicles (EVs). Nissan is piloting a vehicle-to-grid (V2G) scheme in the UK whereby bidirectional charging enables EV users to charge vehicles using off-peak electricity and sell it back to the grid at peak times! Of course, any EV can serve to store electricity generated by solar panels, and subsequently discharge to the grid. This all helps to balance production and demand in the national grid. Announcing the launch of the scheme in May 2016, Paul Wilcox, chair of Nissan Europe, observed that 'If all [Nissan LEAFs in the UK] were connected they would have the capacity to store 180 MW of energy - equivalent to two power plants.' See 'Time to switch on to electric vehicles?'
Nissan have also introduced their xStorage system, using 'retired' Nissan LEAF batteries, again to store electricity from on-site generation in households. The cost is expected to be around £3000.
Key to a flexible, decentralised grid
In the long run, localized energy storage is seen as key to a more flexible and decentralised power grid, evening out peaks and troughs created by increased use of renewables. This should mean that there is less need for large baseload power stations, enabling us to reduce carbon emissions while 'keeping the lights on'.