Energy Explainer: Renewable Energy Sources and the Challenge of Intermittency
“A transition towards high shares of VRE [variable renewable energy] requires a re-thinking of the design, operation and planning of future power systems from a technical and economic point of view.” (IRENA, 2015)
Since the dawn of renewable energy development, intermittency has been cited as one of the main barriers to large scale deployment. Intermittency in this context describes the variability of the energy output from such sources, which may ultimately produce irregular or disrupted patterns of electrical output.
Fossil- and nuclear-based energy sources, such as gas, oil, coal, and uranium can provide energy continuously and on demand; whereas supply inputs of renewable energy generators, such as solar or wind energy, completely rely on the externality of weather patterns, and therefore produce power intermittently. Specifically, power generation of solar farms or panels on residential rooftops depends on the existence of sunlight and the absence of cloud cover. Similarly, wind turbines require a finite wind speed to produce electricity output. Weather patterns can be forecast to an extent, more accurately for sunlight, although less reliably for cloud cover and wind speed.
It is possible to compensate for intermittency by backing up the variable source. This way an alternative means of energy supply, such as battery storage or dispatchable renewable energy sources (i.e., plants that can control their output, such as geothermal, hydropower and biomass) can be relied on in the event the source cannot produce. However, when factoring out intermittency this way, other issues, including economic infeasibility and increased pollution arise simultaneously. Intermittency therefore remains an essential implication of the differences of utilising renewable and non-renewable energy sources, which leaves non-renewable energy sources with a key advantage over renewable ones.
The problem with integrating large amounts of intermittent energy onto the electricity grid is the inherent unpredictability of electricity output. Without accurately predicting the output, the system operator cannot sufficiently plan ahead to manage the grid to ensure a simultaneous match of supply and demand. Electricity supply must be available at the exact time when it is required for consumers. For example, when you plug in your vacuum cleaner, a power station at that moment must be producing electrical energy. This is because there isn’t sufficient storage on the system to allow electricity to be saved up for when it is needed in the future. Relying on large amounts of intermittent energy can leave the system being unable to supply. Say the penetration (the proportion of an energy source on a system) of solar energy was 100% on a grid: when you turned your vacuum cleaner on at midnight - when the sun wasn’t out - you wouldn’t be able to use it.
Another challenge of integrating a growing proportion of intermittent sources into the grid is that with fewer manageable sources of energy, the larger the physical constraint - this could lead to system failures such as blackouts. Again, let’s consider a grid that is powered entirely by solar panels. The sunniest hour of the day enables all solar panels to produce electricity at the same time. Say this hour is when the majority of office workers turn off their computers and go outside on their lunch break. In this extreme scenario, you have a large oversupply without the demand to satisfy it. In theory, this could cause what is known as an electrical trip (circuit breaks in the event of overload or short circuit).
Despite issues with their integration into the existing grid system, use of intermittent sources are increasing globally, at a higher rate than expected. This increase is attributed to supportive government policies, subsidies and industry learning which has led to technological and operational cost reductions over time. In 2004, the International Energy Agency (IEA) projected renewable capacity (apart from hydro) would increase from 2% in 2002 to 6% in 2030. In 2016, IEA had revised their projection to a 42% increase in global renewable electricity capacity from 2015-2021.