Storage of Wind Energygreenready
Storage of Wind Energy
Wind is an intermittent form of energy. But unlike solar energy generation which can be predicted, future wind speeds cannot be accurately estimated and thus planning for the evacuation of the generated energy is problematic. Vagaries of the wind speeds destabilize the grid and adversely affect power transmission. This situation creates a need for storage of energy produced during periods of low energy demand and high wind speeds. The stored energy should then be released during peak energy consumption periods or low wind speeds. This is a paper on possible wind power storage solutions in Ireland.
Eirgrid, an electricity transmission company in Ireland, tabulates the data of forecasted wind generation against actual wind power generated. These two parameters usually do not coincide leading to load flow problems in the transmission lines. The generated power usually falls below or shoots above the forecasted values.
Figure 1: Curves of wind power generation against predicted values in Ireland in 5th April 2015. Source: Eirgrid, 2015
Figure 2: Graph of weekly forecasted and actual wind energy generation for the period 30th March to 5th April 2015. Source: Eirgrid, 2015
From the graphs above, it can be seen that wind energy generation fluctuates between zero and around 300MW (Eirgrid 2015). Therefore, an energy storage system with a capacity of over 300MW would ensure the turbines’ output never falls below the 300MW mark. The grid operator can then plan on a transmission schedule without the hindrance of sharp generation fluctuations because the storage system offers a minimum generation capacity reference point. The required storage capacity is 300MW per day. Thus the total energy storage capacity should be: 300MW*24h=7,200MWh. The proposed technologies below possess the efficacy to solve this storage problem.
An energy storage system would smoothen the energy generation curve regardless of the forecasted generation values. Energy produced at high wind speeds would not be fed to the grid, but instead would be directed to charging the storage systems, which would then feed a constant energy value to the grid. Doing so would avoid flicker and voltage variations caused by variations in wind farm output (Twidell & Weir 2006).
The wind energy produced in Ireland can be stored through two technologies, batteries and hydrogen energy storage. Battery energy storage for renewable energy projects is done through deep cycle type of batteries. These are batteries that can withstand repeated charging and discharge cycles without losing their efficiency or getting damaged (Institution of Mechanical Engineers 2000). The batteries absorb the short bursts of energy due to sudden wind speed spikes and store it in a chemical form. When the wind speed falls below a given point, the turbines’ inverters tap on available battery energy and use it to complement the turbines generation. The energy fed to the grid directly from the turbines is set at a given limit and any surplus directed to the batteries.
Also, wind energy can be stored in the form of hydrogen gas, which can then be used as a fuel. The surplus energy produced from the wind turbines can be used to electrolyze water to its constituent elements, water and hydrogen. A device called Proton Exchange Membrane (PEM) is used in the extraction of hydrogen from water. This technology already exists but its widespread and large scale application has not been adopted. Hydrogen is a potent fuel with a capacity of 120MJ/Kg, higher than any other fuel (Sørensen 2007). This means that a very small hydrogen volume can be used to store large amounts of energy. Hydrogen is also a stable element and therefore it can store energy for longer periods than any other element (Schiller 2014).
Energy storage is an advantageous undertaking for the power producers and grid operators. For example, the grid might not be able to absorb all the energy produced, which ends up being wasted. This reduces possible earnings for the power producers. Energy storage enables investors in the wind technology enjoy the extra profits that were not available before. Also energy storage enhances grid integration by ensuring producers feed constant energy to the grid, which helps the grid operators to seamlessly evacuate the energy produced. Finally, energy storage improves the turbines’ flexibility such that they can respond to sudden load changes due to unpredicted demand. Turbines can use stored energy to feed a demand spike, without straining the generation infrastructure connected to the whole grid.
Eirgrid, 2015. Wind Generation: All Island [online] (updated 5 April 2015) Available at: < http://smartgriddashboard.eirgrid.com/#all/wind>
Institution of Mechanical Engineers (Great Britain). (2000). Renewable energy storage its role in renewables and future electricity markets. Bury St Edmunds, Professional Engineering Pub. for the Institution of Mechanical Engineers. http://app.knovel.com/web/toc.v/cid:kpRESIRRFB.
Schiller, M., 2014. Hydrogen Energy Storage: A New Solution To the Renewable Energy Intermittency Problem. [Online] Available at: <http://www.renewableenergyworld.com/rea/news/article/2014/07/hydrogen-energy- storage-a-new-solution-to-the-renewable-energy-intermittency-problem>
Sørensen, B. (2007). Renewable energy conversion, transmission, and storage. Amsterdam, Elsevier/Academic Press. http://site.ebrary.com/id/10276464.
Twidell, J., & Weir, A. D. (2006). Renewable energy resources. London, Taylor & Francis.