Vanadium flow batteries

VRB power systems have provided a 12 MWh energy storage system for
the Sorne Hill wind farm in Ireland, whose current capacity is “32 MW,”
increasing to “39 MW.” (VRB stands for vanadium redox battery.) This
storage system is a big “flow battery,” a redox regenerative fuel cell, with
a couple of tanks full of vanadium in different chemical states. This storage
system can smooth the output of its wind farm on a time-scale of minutes,
but the longest time for which it could deliver one third of the capacity
(during a lull in the wind) is one hour.

A 1.5 MWh vanadium system costing $480 000 occupies 70 m² with a
mass of 107 tons. The vanadium redox battery has a life of more than
10 000 cycles. It can be charged at the same rate that it is discharged (in
contrast to lead-acid batteries which must be charged 5 times as slowly).
Its efficiency is 70–75%, round-trip. The volume required is about 1 m³ of
2-molar vanadium in sulphuric acid to store 20 kWh. (That’s 20 Wh/kg.)

So to store 10 GWh would require 500 000 m³ (170 swimming pools) –
for example, tanks 2 m high covering a floor area of 500 m × 500 m.

Scaling up the vanadium technology to match a big pumped-storage
system – 10 GWh – might have a noticeable effect on the world vanadium
market, but there is no long-term shortage of vanadium. Current worldwide
production of vanadium is 40 000 tons per year. A 10 GWh system
would contain 36 000 tons of vanadium – about one year’s worth of current
production. Vanadium is currently produced as a by-product of other pro-
cesses, and the total world vanadium resource is estimated to be 63 million
tons.

“Economical” solutions

In the present world which doesn’t put any cost on carbon pollution, the
financial bar that a storage system must beat is an ugly alternative: storage
can be emulated by simply putting up an extra gas-fired power station to
meet extra demand, and shedding any excess electrical power by throwing
it away in heaters.

Seasonal fluctuations

The fluctuations of supply and demand that have the longest timescale are
seasonal. The most important fluctuation is that of building-heating, which
goes up every winter. Current UK natural gas demand varies throughout
the year, from a typical average of 36 kWh/d per person in July and August
to an average of 72 kWh/d per person in December to February, with
extremes of 30–80 kWh/d/p (figure 26.16).

Some renewables also have yearly fluctuations – solar power is stronger
in summer and wind power is weaker.

Figure 26.16. Gas demand (lower graph) and temperature (upper graph) in Britain during 2007.