The UK electricity transmission system was built to allow spare capacity to be shared rather than replicated within each local network. This allowed output from the lowest cost generation to be maximised. It also meant larger, more efficient power stations could be built further away from electricity demand.
But this transmission system requires very careful ‘balancing’ to provide customers a stable supply of 50Hz at the appropriate voltage.
If demand is greater than generation, the frequency falls. If generation is greater than demand, the frequency rises. The network cannot store energy, so to maintain a stable frequency supply and demand must be matched at all times. This requires highly flexible supply.
Historically, fossil fuel power stations have generated the majority of the nation’s power, supplemented by a number of nuclear generators designed to operate continuously at maximum output. These traditional fossil fuel and nuclear stations produce electricity using synchronous generators, so called because the frequency of power output is directly linked to the speed that the rotors turn. This meant that it used to be relatively simple to keep system frequency stable.
But that stability is now changing. Targets for renewable energy to contribute 15% of all energy (or 30% of all electricity) by 2020, plus a reduction in CO2 emissions by 80% before 2050 has led to significant investment in wind and solar generation. Given weather dependencies, output from these forms of generation is both intermittent and uncertain. This volatility of supply makes it much more challenging to ensure consistency of system frequency across the power supply as a whole.
To date, flexibility to deal with generation losses or errors in demand forecasting has been provided by pumped-storage hydroelectricity power plants, or by running gas or coal plants at less than full capacity (known as part-loading) with the ability to ramp up output at short notice. However, the availability of sites for additional pumped-storage are extremely limited, whilst the use of part-loaded plant lowers the efficiency of the plant affected.
As a result, the UK transmission network needs more flexible assets to provide frequency response services.
One of the most flexible ways of addressing these needs is through investments in Reserve Power projects. These are typically installations of 8-10 small containerised 2MW diesel-fired generating units. When aggregated into 16-20MW projects, these generators are quick to build, provide material capacity with short response times, low fixed costs, and the ability to significantly reduce the need for additional infrastructure build by being located close to existing substation and transmission assets.
In most cases, the generating equipment has dual-fuel capability for both diesel and gas. This allows the generator to benefit from both quick start-up (from diesel, full power within 22 seconds) and also lower emissions (by switching to gas once the generator is at full power).
By providing a solution to the problem of volatile renewable energy output, Reserve Power generation can actually help build the case for developing more renewable energy installations. Although Reserve Power uses fossil fuels, when used in combination with renewables it is far less carbon intensive than traditional sources of power. A coal-fired power station produces a 870g per KwH, compared to an average of 165g per KwH from a combined solar and Reserve Power solution – a carbon reduction of 80%.
As a result, investments in Reserve Power generation position the UK power market to both meet commitments to CO2 reduction, and also to ensure power supplies are available to meet demand and maintain stable system frequencies. As such, these investments form an essential part of the UK power infrastructure network.