The concept of dispatchable renewables seems almost contradictory: how can something generated from an inherently variable resource be dispatchable? The answer is storage.
When we aim to make variable renewables dispatchable or available whenever needed, the first consideration is exploring how well different wind and solar PV sites naturally work together to firm supply (i.e. how likely it is that dips in one source are filled by peaks in another). Once this is understood, we need to consider how much storage is required to manage residual variability. Storage is critical here as it provides flexibility to store excess or low-value energy for times when it is really in demand.
When patterns of renewable generation are highly correlated (in other words, the timing of generation is very similar), more storage is required. For example, if the east coast of Australia develops a very high proportion of solar PV generation capacity, then all of these will be generating within about two hours of each other during the day (because of similar sunrise and sunset times across this region), and not at night.
To fully utilise this energy, much of it would need to be made dispatchable by adding substantial storage for the night-time hours, or we would need to firm the supply using another generation source, such as a gas turbine. But with a suitable proportion of wind in the mix (and stronger interconnectors to solar generation from other regions), the same dispatchability can be achieved with a more moderate amount of storage. This example demonstrates the importance of achieving a mix of renewable generators to meet the goal of dispatchability.
Various studies of generation in the Australian National Electricity Market (NEM) over time have demonstrated that wind and solar generation are not highly correlated. These studies have shown that even with low to moderate correlation, when considered over a large geographical area, a combination of such generators reduces variability and increases reliability of supply.
Understanding this effect enables appropriate sizing of storage to create a dispatchable renewable portfolio with maximum value. There will always be some times when multiple generators produce near maximums, as well as some times when both wind and solar produce near minimums; these occasions are not common, but could have significant consequences. This is a risk that needs to be managed by the system.
The amount of firm capacity can be increased by over-installing generation, and curtailing its output when there is too much generation. However, there are still those infrequent periods when multiple generators are at their minimum and parts of the grid need extra support. Having this support available during these rare occasions will be critical to managing risk and maintaining reliable supply.
This indicates that the mixture of different renewables won’t take us all the way to the goal of achieving dispatchable renewables; storage remains a critical ingredient.
WHAT’S THE FUTURE FOR ENERGY STORAGE?
The media is awash with reports of new energy storage options. It is important to recognise, though, that different types of storage solutions vary widely in their ability to discharge power over different time frames. Therefore one type of storage will not necessarily deliver the same solution as another type of storage. Understanding this is critical to the concept of dispatchable renewables.
The power and duration of the storage are the two key variables in determining the most suitable solution. Low-power, short-term storage is currently more cost-effective using batteries, but longer periods and larger power requirements are likely to rely on bigger storage options, such as pumped hydro energy storage and traditional hydropower.
With individual wind and solar plants pushing 1 GW, pumped hydro and modified traditional hydropower solutions need to be considered. Smoothing out the daily variability in renewables can be achieved effectively through pumped hydro, but multi-day storage to supplement periods of extreme events of both low wind and low solar will require traditional hydropower with very large reservoirs.
In the long run, short-term storage will not be sufficient alone to achieve the aim of dispatchable renewables. Achieving full dispatchability of combined wind and solar PV power will depend on utilising pumped hydro storage and existing hydropower storages to their full potential.
To achieve all the elements of the energy trilemma (affordability, reliability and sustainability), the world will need dispatchable renewables in the energy mix. Storage is the key to achieving this transition, and the time to start planning for it is now.
