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Energy Storage and the Grid
4 June 2015
When a renewable energy system has no connection to a mains supply, there is a clear need for energy storage, since the sun does not always shine, nor the wind blow, and the reservoir may run dry. A temporary electricity supply is then needed, and is worth paying for. The Island of Eigg is a well-known example of an off-grid community, using battery storage and diesel generators to supplement PV, wind and hydro sources (note 1). Storage may also be worth the expense where there is a connection to the grid, but the mains supply suffers frequent interruption.
When, however, a system has a reliable mains supply, why should energy storage be considered? If the output of the renewable source exceeds immediate need, the excess is exported to the grid, and when it fails to meet the local load, power is taken from the grid. The need for energy storage in such a situation can arise in at least two ways: first, the grid may be unable at times to accept excess power, and second, it may be more economical to store energy for later use than to buy it from the grid. In the first case, the power distribution organisation may insist on some mechanism to prevent unwanted input of power, or may even refuse a grid connection. In the second case, the economic argument for local energy storage can only be assessed when adequate information on energy generation and use is available, and feed-in-tariffs, cost of buying energy, and cost of investment are known. If storage is not employed, then excess output to the grid can only be avoided by undesirable or inconvenient means such as shutting down the system or disconnecting it from the grid at times, changing usage patterns or wasting excess power.
Western Power Distribution offers a variety of connection schemes intended to cope with the output from renewable energy installations, allowing curtailment of energy export at fixed times, curtailment only when grid conditions make it necessary, or active network management in complex situations. WPD has experience of battery storage installations in the range 4kW to 4GW (note 2)
and recommends “The Good Practice Guide to Electrical Energy Storage” published by the Energy Storage Operators’ Forum (note 3).
If storage is employed, there is a wide range of techniques, some of which follow.
Oxford Energy Futures (note 4) describes work primarily on grid-storage systems, in the following areas: new electrode and electrolyte materials and manufacturing technology for batteries, real-time management of battery systems, super-capacitors, thermal storage, production and storage of hydrogen, and conversion of carbon-dioxide to hydrocarbons.
Battery research for grid storage includes Lithium-ion and Sodium-ion types. (No details are given of supercapacitor research, but perhaps this includes graphene capacitors?).
Thermal storage involves work on phase-change materials as stores for Solar-Stirling generators. Domestic thermal storage (e.g. hot water systems) has potential to enable cost-effective smart grid activities and demand-side management. Chemical storage is researched using hydrogen and ammonia as storage systems, or hydrogen in the synthesis of hydrocarbons using carbon dioxide.
Some additional storage methods are listed by EnergyStorage (note 5).
Chemical methods: Liquid nitrogen, Oxy-hydrogen, hydrogen peroxide, vanadium pentoxide.
Electro-chemical methods: Fuel cell.
Electrical methods: Superconducting magnetic energy storage.
Thermal methods: hot water, hot ceramic (as in night-storage heaters), steam accumulator.
Mechanical methods: hydraulic accumulator, flywheel.
Two case studies using battery storage follow.
A 60kWh energy storage system integrated with solar PV using vanadium redox flow battery (VRFB) (note 6)
A system employing used electrical vehicle batteries to give 2MWh storage, and with an output of 2MW is being planned and constructed. The energy will be fed into the energy balancing market to balance out short-term fluctuations in the power grid. The entire system is compact enough to fit into a small building, and can supply 30 households for seven days. The storage unit should be put into operation by the end of 2015. Two application are envisaged: a power buffer for fast-charging EV-stations, and the storage of solar power to increase self-consumption. The aim is to help local battery storage systems to become established on the market. The used batteries come from the BMW car models Active E and i3. They still have a high storage capacity, namely 80% of their former 32 kWh and 18.8 kWh. As a result, they are still very valuable and can be used extremely efficiently as stationary buffer storage for many more years (note 7)
Secretary: Maggie Fyffe; firstname.lastname@example.org