Why have storage?
Why do we need storage? Wind generated electricity is intermittent and difficult to predict so some form of backup generation is required for the periods when there is little or no wind to produce electricity. Many contemporary reports into wind power cite having a 'spinning reserve' of thermal generation using natural gas as a primary fuel source. This reserve is ready to be used at very short notice and is most likely the least expensive form of backup generation. The only problem with this form of backup is that, to a certain extent, it defeats the purpose of having an indefinitely renewable, non-polluting power source. Cost aside, the most ideal form of backup would itself be renewable and environmentally friendly. If there was a way to store the excess electricity generated by wind (e.g. at night when demand is low) and use it when it was required then there would be no need for thermal backup. There are many forms of power storage and it was not possible to examine them all in great detail.
This project concluded, based on a number of assumptions that with higher levels of wind penetration-above 20%, storage of energy is required. The assumptions are that conventional (mainly thermal) generation will not be able to provide sufficient flexible capacity, and that the likely hood of having no wind across entire Scotland for seven days is probable.
Because electricity is an instantaneous phenomenon it must be created at when it is required. Electricity is not easily stored, as it exists; therefore, an intermediate form of storage is introduced to the problem. These are known as Energy Vectors and they are means by which electricity can be converted and stored into more manageable and accessible energy forms (kinetic, potential, electro-chemical, etc). Electricity can then be generated from these energy vectors, when required.
What storage (energy vectors) technologies are available?
It was necessary to Identify what storage technology was most suitable to manage the scale of energy and peak power required by an interconnected power system that contained large percentages of wind power.
Critical analyses and comparison of current storage technologies were carried out. The criteria on which they were compared were their: Power and Energy Density, Capital Cost, Number of Cycles, Storage Time, Discharge Time, and Efficiency. They were also evaluated on their present status, for example were they still in their research and development stage or were they well established technologies.
Flywheels are essentially kinetic energy storage devices; they store energy by means of a rotating mass (rotor). The stored energy (inertia) is accumulated by application of an accelerating torque. For electrical applications the flywheel is connected to an electrical machine (motor/generator) and a power converter. There are broadly two types of flywheel--low speed and high speed.
Electrochemical capacitors store electrical energy in the two series capacitors of the electric double layer, which is formed between each of the electrodes and the electrolyte ions. Compared to lead-acid batteries, EC capacitors have lower energy density but they can be cycled tens of thousands of times and are much more powerful than batteries (fast charge and discharge capability).
From the many various types of battery available, the PSB battery stood out as having the highest power and energy capabilities.
Polysulfide Bromide battery (PSB) is a regenerative fuel cell technology that provides a reversible electrochemical reaction between two salt solution electrolytes. The cells are electrically connected in series and parallel to obtain the desired voltage and current levels. The battery has a net efficiency of about 75% and operates at room temperature.Regenesys Technologies was due to build a 120 MWh, 15MW energy storage plant at Innogy's Little Barford Power Station in the UK, in 2003. However, financial problems have terminated the project
Pumped Hydro is a gravitational potential energy storage system. The energy capacity is related to the volume and head of water stored in the upper reservoir. Usually the turbines used to turn the generator can be reversed--acting as pumps, to recharge the reservoir.
The energy and power capabilities of this technology are huge in comparison to the other technologies mentioned.
Why did we choose pumped-hydro?
The following link is to a number of slides (produced by..) that show each storage technologies specific individual characterists such as energy density, discharge time, etc. The slides/charts allow direct comparison between technologies.