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Electricity storage: a smart move?

Powervault 3 energy store is a sealed maintenance-free unit that contains lithium-ion batteries.
Performance is monitored remotely via the internet. Image courtesy of Powervault. Powervault 3
  • Generating electricity using solar panels or wind turbines fluctuates as the sun dips behind clouds or the wind dies down.

  • Surplus electricity, whether from renewables or from the grid at off-peak periods, can be stored in batteries to even out those fluctuations and maximize what you can use in the home.

  • This stored electricity can power appliances at any time, or be supplied back to the grid at times of peak demand to help balance the grid. This cuts your energy bills and carbon emissions.

  • 'Smart' control systems can use cheaper off-peak power to charge the batteries, and hence save money, even if your panels are having a 'dull day' - provided you are on an off-peak tariff.

  • A range of products are available, the technology is evolving rapidly, and costs are falling, although payback times can still be lengthy.

  • Some companies offer customers incentive payments for the right to transfer electricity between your storage system and the grid.

 

Is battery storage for you?

Answer is yes...if:

You have solar panels or a wind turbine that produce more energy than you normally use. If you have solar panels and use little electricity during the day because you are out of the house, batteries will store your electricity for when you need it during the evening.

You have a cheap off-peak tariff. Batteries enable you to take cheap electricity from the grid overnight, and store it for use during the day, or to charge an electric vehicle.

You are not connected to the grid. Battery storage is essential if you rely on your own sources of renewable electricity generation. But, you will probably need a bespoke storage system designed to suit your particular situation and needs.

Your generation output is sufficient to warrant the expense of a storage system. For example, an array of solar panels rated at 4kW and working at maximum output could potentially charge a 4kWh battery in under 2 hours (depending on maximum charge rate). But this would apply only on the sunniest days of high summer if no appliances were working. At other times the charge rate would be lower, especially in darkest winter. Small arrays will struggle to fully charge the batteries.

Types of storage systems

DC-connected systems

These capture direct current (DC) electricity as soon as it is generated (e.g. by solar panels or wind turbine) before it is inverted to alternating current (AC) for use in the building.

  • Theoretically more efficient than an AC system at storing electricity, but not as efficient during discharge.
  • May reduce the metered generation output, and hence your revenue from the feed-in tariff.
  • Existing inverter may be incompatible, so best for new build.
  • Cannot store off-peak electricity from the grid.

AC-connected battery systems

These are connected after the electricity generation meter, and so do not affect the metered generation output or feed-in tariff revenue.

  • Incorporate an inverter to convert AC current back to DC to charge the battery, and then DC to AC during discharge.
  • Typically more expensive than DC systems.
  • Will generally retrofit easily with existing solar panels or turbine.

Battery technology

Lithium-ion or lithium polymer

  • Lightweight and compact and but more expensive compared with traditional lead-acid batteries.
  • Some companies offer a discount for 'second-life' batteries that have been 'retired' from electric vehicles.
  • Longer life - so extended warranties available, typically 10 years.
  • Contain less toxic material, although lithium is volatile and potentially flammable.

Lead-acid

  • Well-established, cheaper technology. Suitable for large storage systems where space is not limited - e.g. off-grid locations.
  • Relative low energy density and limited lifetime, so will need replacing typically every 5 years or so.
  • Lead will require specialist handling and recyling at end of life.

Installation and operation

The systems are enclosed in a single casing, varying in size from a shoebox to a fridge. The smaller ones can be wall mounted if desired. To simplify wiring, they are best located close to the mains input, ideally in a garage or utility area. Some systems are modular so additional batteries can be added in future if required.

They generate some noise, similar to a fridge-freezer, so best to avoid placing them in a living room. If installed outdoors, the unit will require a waterproof cabinet, protected against frost.

Systems are connected to the internet via Wi-fi and are supplied with an app that allows users to monitor and control the system remotely, and also provides access for the supplier. Operation can be synchronized with smart tariffs to maximize efficiency and savings.

The Renewable Energy Consumer Code gives technical guidance for installers and consumers.

A wide range of products is now available, with prices ranging from around £1700 to over £5000, depending on capacity. Just three of the main contenders are considered here,

Powervault

The UK company Powervault has a range of AC-connected storage systems based on lithium-polymer batteries, with storage capacities from 4.1 to 20.5 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.installation diagram

Batteries are reckoned to last for more than 6000 charge cycles, giving an operating life of 17 to 20 years. Warranty is for 10 years. Prices range from £3550 for the 4.1 kWh unit to £10,750 for the top-end 20.5 kWh unit, excluding VAT and installation. 'Second-life' lithium-manganese batteries from electric vehicles are also available at lower cost but with only a 3-year warranty.

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

A WiFi connection allows the unit's 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. Other features include a 'machine learning' capability, smartSTOR, claimed to optimize operation according to energy costs, household behaviour patterns, and the weather.

Benefits

Powervault reckon 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.

Powervault customers have the option of joining the GridFLEX platform. In return for a payment of £20/month, the company remotely manages the customer's battery to help balance supply and demand in the grid. Powervault can control multiple battery installations and other assets remotely via the internet to deliver this service to the grid, acting as a so-called aggregator.

Tesla's Powerwall

Tesla, the US maker of electric cars, has established itself in 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 2 has a usable 13.5 kWh storage capacity and 3.68 kW maximum power output, sufficient to power most homes during the evening. The cabinet can be floor or wall mounted, indoors or outdoors.

Powerwall 2 is AC connected and includes a built-in battery inverter to change the DC to AC current for powering most household appliances or export to the grid. Cost for a single Powerwall 2 unit is around £6300 plus the cost of installation, estimated at a further £1000 to 2500.

You can receive a free in-depth head-to-head guide comparing Powervault 3 with Powerwall 2 from Spirit solar PV consultants.

Moixa Smart Battery Technology

Another product based on lithium-ion technology is the Moixa Smart Battery from Moixa Technology. Manufactured in the Uk, this has its own in-built inverter and charges using 'spare' electricity generated by solar panels or off-peak electricity from the grid. The wall-mounted units are rated at 2, 3 or 4 kWh capacity, and the batteries have a projected life of 10,000 charge cycles, equivalent to 27 years if discharged once per day. It is guaranteed for 10 years. The cost is from around £3000 plus VAT plus installation.

The Moixa battery system connects to a web-based interface, the Moixa dashboard, so that it can be monitored via a computer or smart phone. Customers who sign up to the the GridShare platform receive £50 cashback annually in return for allowing Moixa to manage their batteries to balance the energy demands of the national grid, just like Powervault's GridFLEX platform.

Electric vehicles as energy stores

Some of the most effective and flexible storages devices are electric cars and other electric vehicles (EVs). Backed by government funding, Nissan and OVO Energy are 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?'

In collaboration with US power management company Eaton, Nissan have also introduced their xStorage system, using 'retired' Nissan LEAF batteries, again to store electricity from on-site generation or cheaper off-peak electricity in households. The cost is around £4600 excluding installation and VAT.

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'.

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