Temporary storage of energy is a topic of today. This is because of the tremendous growth rates of solar and wind power. Challenge is, that sun is providing it's power, when it is least needed. Northern Europe has the lowest electricity consumption during summer, when the solar generation peaks. Wind power in turn, often fails to deliver during the peak consumption periods.
Thus comes the need to store energy on a temporary basis. A lot of research is going on in this area. One of the areas is concerned about using large superconducting coils to store energy. When the high temperature superconductors were invented, the cost of cooling down the coils were reduced considerably and the technology became more feasible.
Let us investigate the limits of the Superconductive Magnetic Energy Storage (SMES) for use in large scale applications. The energy stored in a long solenoidal coils is :
Trial values (seeking high value of energy) and descriptions of the parameters of the above formulas are:
=permeability of air, 4π x 10-7 T m/A
N=number of turns in a solenoid, 300
A=cross sectional area of the solenoid, 10 000 m2
l=length of the solenoid, 10m
With these assumptions we get, that the inductance (L) is: 113 H.
From /1/ we know, that a current of 100kA can be achieved with high temperature superconducting wires. Using this value for I we get the energy of the coil to be 156 MWh. Let us take Finland as an example. We see that wind power capacity is 147 MW in 2009 /2/.
Thus,this hypothethical SMES described above, can store the energy of all wind power plants for one hour. This is a decent solution for a short-term load following power plant, as the power output can be controlled very rapidly. This would not, however, serve the needs for a longer-term energy storage.
The energy of the coil increases proportional to the square of the current. From /3/ it is found, that the critical current of some low temperature superconductors can be 1 MA / mm3. If it were possible to construct windings of the coil supporting 1 MA with such superconducting material, then the energy of the coil would increase to a level of 15 GWh. This would serve the electiricty needs for Helsinki city area for an average of 30 hours /4/. This would make large scale SMES feasible. But, the challenges are great:
- Achieving the high currents in the superconducting coil
- DC/AC conversions to connect SMES to grid
- potential side effects created by the high magnetic fields
- high forces inside the coil caused by the high currents
/1/ http://www.smartgridnews.com/artman/uploads/1/new_epri.pdf
/2/ http://www.tuulivoimatieto.fi/tuulivoima_suomi
/4/ http://www.tekniikkatalous.fi/energia/article362518.ece
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