There are lots of ways to store energy, and lots of criteria by which storage solutions are judged. Figure 26.13 shows three of the most important criteria: energy density (how much energy is stored per kilogram of storage system); efficiency (how much energy you get back per unit energy put in); and lifetime (how many cycles of energy storage can be delivered before the system needs refurbishing). Other important criteria are: the maximum rate at which energy can be pumped into or out of the storage system, often expressed as a power per kg; the duration for which energy stays stored in the system; and of course the cost and safety of the system.
Figure 26.15 shows a monster flywheel used to supply brief bursts of power of up to 0.4GW to power an experimental facility. It weighs 8001. Spinning at 225 revolutions per minute, it can store 1000 kWh, and its energy density is about 1 Wh per kg.
hydrogen propane petrol ethanol, methanol firewood li alka line hyd oal hium ion
ogen fuel cell
vana air supercapacitor
lead acid perc air/oil ed storag
flywheel dium pumped storage
1Q 1QQ 1QQQ 1QQQQ1QQQQQ Cycles
hydrogen fuel ell
supercap ai orage
Q.4 Q.6 Efficiency
Q.4 Q.6 Efficiency
Figure 26.13. Some properties of storage systems and fuels. (a) Energy density (on a logarithmic scale) versus lifetime (number of cycles). (b) Energy density versus efficiency. The energy densities don't include the masses of the energy systems' containers, except in the case of "air" (compressed air storage). Taking into account the weight of a cryogenic tank for holding hydrogen, the energy density of hydrogen is reduced 39 000 Wh/kg to roughly 2400 Wh/kg.
fuel calorific value
diesel oil (DERV) 12.7
heating oil 12.8
methanol 5.5 bioethanol coal 8.0
natural gas 14.85
Table 26.14. (a) Calorific values (energy densities, per kg and per litre) of some fuels (in kWh per kg and MJ per litre).
(b) Energy density of some batteries (in Wh per kg). 1 kWh = 1000 Wh.
energy density lifetime (Wh/kg) (cycles)
nickel-cadmium 45-80 1500
NiMH 60-120 300-500
lead-acid 30-50 200-300
lithium-ion 110-160 300-500
lithium-ion-polymer 100-130 300-500
reusable alkaline 80 50
A flywheel system designed for energy storage in a racing car can store 400 kJ (0.1 kWh) of energy and weighs 24 kg (p126). That's an energy density of 4.6 Wh per kg.
High-speed flywheels made of composite materials have energy densities up to 100 Wh/kg.
Supercapacitors are used to store small amounts of electrical energy (up to 1 kWh) where many cycles of operation are required, and charging must be completed quickly. For example, supercapacitors are favoured over batteries for regenerative braking in vehicles that do many stops and starts. You can buy supercapacitors with an energy density of 6 Wh/kg.
A US company, EEStor, claims to be able to make much better supercapacitors, using barium titanate, with an energy density of 280 Wh/kg.
VRB power systems have provided a 12MWh 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 $480000 occupies 70 m2 with a mass of 107 tons. The vanadium redox battery has a life of more than 10000 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 10GWh would require 500000 m3 (170 swimming pools) -for example, tanks 2 m high covering a floor area of 500 m x 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 40000 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 processes, and the total world vanadium resource is estimated to be 63 million tons.
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Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.