Alternative proposals to maximise the potential of wind energy have recently been prominent in the news. The people of El Hierro propose to use excess power from wind energy to run a hydro project which will in turn generate electricity when there is no wind energy available.
Xcel Energy is the biggest purchaser of wind generated electricity in the US and has chosen to investigate the possibilities of energy storage. It has installed a 1 MW sodium-sulphur battery within the Minwind wind farm in Minnesota with the aim of managing excess energy more flexibly. The experiment seeks to establish whether it is more effective to combine variable energy supply (i.e. wind energy) with stored power through the use of batteries instead of conventional reserve power generation.
A key feature of a wind turbine is that electricity is only generated when the wind speed reaches a threshold level. The objective of wind energy operators and grid network providers is to maximise the use of electricity generated so that as little as possible is ‘wasted’. The usual way of dealing with intermittent wind energy generation is to combine it with “reserve energy generation” already connected to the grid system in the form of gas storage, hydro-electric power, coal etc. and to ensure wind energy projects are located in geographically diverse locations.
Frank Novachek, Xcel’s Director of Corporate Planning, said storing wind energy will help the system run more efficiently. He said: “It would smooth out the ups and downs that wind has on our system …there are often times when Xcel doesn’t use all of the energy generated by a wind farm in a given day, and without a way to store it, that energy is lost. The largest barrier to the project is the high cost of the batteries“.
Novachek said that the battery is not intended to expand the integration of wind energy immediately but the project is being set up for when wind penetration on Xcel’s system is at its maximum thereby resulting in a real need for the storage. Xcel will be required to supply 30% of its electricity from renewable sources by 2020 and 25% of that is likely to be wind energy generated.
Testing of charge and discharge cycles has been carried out over the last few months and Xcel say that the system is working as envisaged. A fully charged battery will release a steady megawatt of power output over seven hours and on a busy grid network a one megawatt battery can potentially allow more wind energy generated megawatts on-line. The battery however has its down-sides, as 25% of the stored power is lost and it costs a whopping US $3 million (£2 million).
Haruchai
October 10th, 2010
The 75% efficiency is for AC current over the predicted system life – which is about 10-15 years. DC is as much as 85% over the same time span.
So, the short-term efficiency, say the first few years, is typically 87%-92%, depending on the application.