Keeping the lights on when renewable power varies
A simulated future grid in North America shows how gas-fired power can back-up intermittent renewables
The sun doesn’t shine at night and wind constantly varies in strength. So how will demand be met when the power systems of the future contain high levels of renewable energy?
The highlights
The above graph, taken from research for the BP Technology Outlook 2018, shows how different forms of energy could contribute to power demand (in gigawatts) over five days in a future world if renewables (wind and solar) grow from around 6% of the power mix to 40%.
Taking a southern US state in the summer, where the climate is similar to Mediterranean countries and the Middle East, this simulation shows how solar peaks during the daylight hours of a typical working week.
When the sun stops shining, nuclear provides a base of power with wind also making a contribution. But more is needed, particularly in the evening and at night.
The graph shows how hydropower and gas can make up the remaining demand. They can be flexed up and down rapidly making them more suited to this role than nuclear power.
In more detail – intermittent renewables
When a large share of a power grid is served by intermittent renewables and more supply is at risk, grid operators need to hold higher reserves of energy and ensure they can be brought online quickly. This requirement creates ‘integration costs’ that rise with the share of renewables in the system.
Integration costs include the use of power storage, typically using batteries, shown by the line at the base of the chart. Where the line falls below zero, the batteries are charging up using excess renewable power, and when it rises above the line they are contributing to the supply.
Intermittency can also be mitigated using ‘demand-side response’ measures whereby industrial and business customers on special tariffs lower their power use when called upon and domestic customers use 'smart-enabled' appliances and electric vehicles whose charging can be phased to manage demand.
The dotted line shows the natural demand as it rises and falls during a typical working week. The solid black line shows actual demand once it is moderated by such demand-side response systems.
High levels of solar are costlier to accommodate than wind, because wind blows through the day and night whereas the sun declines in the early evening — just as power demand peaks. For a US power system with a 40% share of renewables, of which wind makes up four-fifths, the Outlook estimates it could cost less than $5 for each additional megawatt-hour (MWh) of wind power generated, but if solar made up four-fifths of the renewables, the extra cost would be around 20 $/MWh for each additional megawatt-hour (MWh) of solar power.
For the full analysis download the outlook
