Renewable energy may help reduce greenhouse gas emissions, but it presents a problem for electricity suppliers: how to meet peak demand levels with increasing reliance on intermittent energy sources.

A new analysis explores options for storage that would facilitate use of renewable energy, either in isolated locations or as part of the electricity grid. Electricity currently accounts for only 12 per cent of total worldwide energy production. However, this is predicted to rise to 34 per cent by 2025.

Currently, large amounts of electricity are generated using coal fired power plants, but it is anticipated that the shift to electricity will come with an increasing reliance on renewable energy sources, as countries seek to meet targets for reductions in greenhouse gas emissions.

Renewable energy sources, such as solar and wind power, fluctuate and so does demand from consumers. This presents challenges for energy suppliers who must find ways to stabilise the power network to balance supply and demand. One solution would be to store energy in times of excess production for use when demand outstrips supply. Until recently, little effort was invested in developing cost-effective storage solutions and today the capacity for electricity storage is only about 2.6 per cent of production.

An analysis of the patterns of use of electricity identified four scenarios which require different storage solutions:

1. Low power application - suited to isolated areas, essentially to feed transducers and energy terminals.

2. Medium power application - for isolated areas where individual electricity or town supplies are needed.

3. Network connection application - to provide peak levelling (e.g. stored energy is released during peak demand, thereby reducing the total capacity required in the system).

4. Power quality control application - short term use of stored energy to ensure quality of power supply. The researchers explored the energy storage options that would be suitable for each scenario.

The first two are small-scale systems, where energy could be stored as kinetic energy (flywheel), chemical energy, compressed air, hydrogen (hydrogen fuel cell) or in super capacitors, an electrical component used to store a charge temporarily, or superconductors, which transmit electricity without energy loss and can be used to store electricity.

Scenarios 3 and 4 are large-scale applications where energy would need to be stored as gravitational energy (hydraulic systems, e.g. pumped hydropower), thermal energy (e.g. heating a solid such as sodium or relying on energy released during phase transition), chemical energy (accumulators, such as nickel-cadmium or lead acid, or flow batteries, such as zinc-bromide) or compressed air (possibly coupled with liquid or natural gas storage).

When costs and performance are taken into account, for low power applications (a few kWh) relying on renewable energy sources, lead batteries provided the best storage solution when both cost and performance are considered. Lithium batteries showed overall better performance but this was outweighed by the cost of the batteries. For larger systems (100 kWh), lead batteries are still the most cost effective storage solution. Other systems, such as compressed air or hydrogen fuel cells, are either too expensive or less efficient.

For network applications (requiring many MWh), compressed air and flow batteries provided the best balance between performance and cost (compressed air is the least expensive of the two). In the final category, where energy release and cycling capacity are key, flywheels and supercapacitors are the most cost effective technologies. However, these technologies need to be improved in the short to medium term to meet the needs of delocalised production systems. Lithium-ion batteries performed well but development is required to make them more cost effective. Lead batteries have a short life expectancy and present issues in relation to disposal.

For network applications, the main technologies (flow batteries, compressed air, supercapacitors and flywheels) are fairly mature technologies, but could be made more cost-effective, more reliable and more efficient.

Source:Ibrahim, H., Illinca, A. Perron, J.(2008). Energy storage systems – characteristics and comparisons. Renewable and Sustainable Energy Reviews. 12(5): 1221-1250. Contact: Hussein.ibrahim@uqar.qc.ca