Pumped Water Storage

Pumped water systems store energy by increasing the potential energy of a mass of  water by pumping it from a lower reservoir to a higher one, then recovering that energy with a turbine when it flows back down again. The diagram below shows the  main components of a system which acts as a form of battery. During the "charge"  phase electric motors drive pumps which move water from the lower reservoir to the upper one. The energy is recovered when the water flows back to the lower reservoir and passes through turbines which drive "generators".  There are losses associated with the process, the figures quoted in Wikipedia suggest that typical efficiencies are in the range 65 - 85%.

The basic equation which describes the systems storage capacity is shown below.  The simplicity of the equation is in contrast to the construction of these of system which are often massive civil engineering projects.

The key term is the product of V and H.  For utility scale projects, the volume V is typically of the order of millions of cubic metres whilst the height, is tens or hundreds of metres.  The density of water rho is constant of at 1,000 kg/m3.  The acceleration due to gravity is also a constant at 9.81 m/s2 and the efficiency eta is a fraction less than one.  The efficiency of the Dinorwig plant in North Wales is thought to be around 75%. Q is the energy stored in Joules (1 kwh represents 3.6 MJ).

A typical urban water tower with a height of 30 metres and a storage volume of 1,000 m3 if used as a  pumped water storage system would have a capacity of approximately 50 kwh, assuming an efficiency of 60%.  However, major installations such as Dinorwig have sufficient capacity to provide some grid management capability by using electricity during off-peak periods to fill the upper reservoir and provide 1 to 3 GW of generating capacity during peak demand.

Pumped water is the principal grid scale storage technology.  The energy storage density is low, in the water tower example used above, the density is 20 tonnes/kwh. For small scale systems, the cost could exceed £1,000/kwh, these figures are high compared to lead/acid  batteries which might cost around £250/kwh for a similar sized system.  The main strength of pumped water is that very large quantities of energy that can be stored, much more than is possible with the various battery technologies.  Pumped storage systems are major civil engineering projects, some are based on disused quarries, others on large dams and disused mine workings are being considered for conversion. Existing systems have generating capacities similar to those of small to medium sized power stations, the buffer capacity for most grid systems is measured in hours rather
than days.

Links to other sources

• Wikipedia - Pumped Water Storage
• Wikipedia - Dinorwig Power Station
• Wikipedia - Raccoon Mountain
• Wikipedia - Guangdong