Renewables such as wind and solar are being given priority in many national energy grids as a result of either financial incentives or, more simply, by being defined as ‘free spill’ or ‘must-run’ technologies which the grid must accept as a priority over energy produced from fossil fuel.

At the moment, renewable energy systems, especially wind-based systems, are not entirely predictable and they currently supply electricity in an intermittent and non-dispatchable manner. The remainder of the energy demand is then met from the cheapest possible options from available and dispatchable sources such as gas, coal and, to a lesser extent, nuclear plants.

The order in which these dispatchable systems are called upon is determined by economic merit – that is, cost. Since many old subcritical coal plants have relatively low running costs, they are often called upon to supply energy to meet demand at times when renewable energy is insufficient. These coal plants, which were designed to run in a steady, baseload state, are increasingly being asked to ramp and cycle, altering their output up and down, sometimes at very short notice. This leads to issues with increased wear and tear and more frequent enforced outages.

The IEA report looks at the potential strains being put on older coal plants through increased cycling and the associated costs to the plant. In some situations, costs can increase by orders of magnitude.

The graph below, taken from the report, which shows shares for various energy sources by both capacity and production for Germany during December 2013, highlights the important difference between available capacity and actual electricity supplied at any time. Although wind and solar power amounted to almost 40% of the available capacity, conditions were such that these sources produced less than half of the electricity during the study period.

During this situation, and others similar, dispatchable systems such as gas, coal and lignite plants are called upon to supply electricity when the renewable output is limited. This puts the fossil fuel plants under stress, as they work to produce increased output within short periods.

Plants under stress

Various issues arise when combining intermittent energy sources with subcritical coal plants that were designed to run at baseload, such as:

  • Grid managers must predict and co- ordinate the energy input from both renewable and baseload sources with fluctuating demand from end users. Older, subcritical coal plants have relatively low running costs and can provide electricity at relatively short notice. But this puts strain on plant systems.
  • Changing plant performance changes plant economics. Plants running to provide power during peak periods will often receive higher income for doing so but this income is offset by increased costs for plant repair and maintenance for systems running outside design parameters. Plants sitting idle as back-up may well do so at a risk of not covering idling costs.
  • Individual countries have committed to different levels of renewable energy. Those that are attempting to achieve high renewable energy contributions within a short period, such as Germany and the UK, are finding that this comes at a cost. Consumers are facing increased domestic electricity charges and industries are being asked to adjust demand in order to balance national grids.

With respect to subcritical coal plants, the focus of the IEA report, the main issue with ramping and cycling is the stress upon power plant equipment.

As coal plants cycle, temperatures fluctuate. This causes metal parts to expand and contract and changes in the chemistry of flue gases will result in more condensation and corrosion. As a result, ramping and cycling a coal plant quickly ages the plant. Damage and breakages can be avoided by investing in monitoring and preventative operation and maintenance, which can be costly. But, even with increased vigilance and investment, cycling plants are more prone to enforced outages, when accidents or breakages occur that require the plant to come offline at very short notice for costly repairs. Cold starts (plants starting up after having been offline for 2 days or more) can cost tens of thousands of pounds. Plants moving from baseload to cycling can find their cycling costs increasing several times over.

New coal-fired plants, supercritical and ultrasupercritical, are designed with flexibility in mind and can work well in a grid with high renewable input. However, countries such as the UK and Germany are still calling on older coal-fired units to meet demand at short notice to make up gaps in demand caused by fluctuating output from intermittent sources. And so, for the next few decades the desire for more renewable energy is likely to lead to issues with the back-up capacity being provided by older coal units. And, until sufficient, reliable, renewable energy is available (through increased capacity and/or energy storage solutions), coal plants will require further investment to ensure that they can provide the electricity required to support expanding and modernising national grid systems.