The previous series of “Insights” briefs covered the problems of integrating large scale renewable power (notably solar and wind generation) to an existing power grid. The series concluded by highlighting the need for grid-scale storage to assist in the accommodation of renewable energy sources when they exceeded a certain percentage of the grid input.
Such large-scale storage is required in addition to other balancing mechanisms, such as averaging wind farm output across large areas to take account of local wind variability.
This series of Insights briefs looks at the main contenders to provide such grid-scale storage, namely: hydro pumped storage, batteries, underground storage whether or methane or compressed air. We start with hydro in this brief.
The viewpoint taken throughout this series of briefs is to evaluate the ease or difficulty of each of the storage options providing support for renewable energy integration on a power grid. We will not be considering other pros or cons, such as landscape alteration or population displacement in the case of hydro dam building and reservoir creation.
Hydro pumped storage
The diagram and picture below show the layout and operation of a typical hydro-electric power plant. Rainfall accumulates in the upper reservoir behind the dam over time. The reservoir is connected by pipes to a number of turbines near the base of the dam. These turbines are driven by the hydrostatic head of water from the reservoir when the power is needed, notably at hours of peak demand. Water, once used, is frequently stored in a lower reservoir so that in periods of low electricity demand, the water can be pumped back up to the higher reservoir to be recycled when required. This reverse pumping requires external power, provided typically by fossil fuel plants.
Tumut 3 power station and Talbingo dam, Snowy Mountains, NSW, Australia
In more conventional operation of power grids, load balancing of this sort takes place once or twice a day; releasing water from the upper reservoir during periods of high demand, for example in the morning and evening peaks in temperate zones, and pumping water back up the slope during periods of low demand, such as mid-day or at night.
So what are the pros and cons of pumped hydro storage from the point of view of grid balancing?
Scale. Existing hydro plants have been constructed on the hundreds of megawatt or gigawatt scale which would allow them to make a significant contribution to grid balancing on most regional or national grids.
Their adaptability to be used to fine-tune differences between demand and supply over the course of a 24-hour period, rather than just once or twice a day, as traditionally. They represent a reasonably fast response tool, as the pumps typically take only a matter of minutes to go from zero to maximum power.
When used in this more reactive way, pumped hydro storage can also provide valuable support to the grid operator in terms of frequency and voltage control.
Historically, major hydro plants have taken 20 – 30 years to design, plan and implement in democratic countries. They have been controversial not least because of their large-scale effects on river flow, land use, flora and fauna and sometimes the displacement of agriculture and rural communities. In considering the integration of renewable power to existing grids in the next 20 – 30 years, options will therefore realistically be confined to existing hydro plant and those projects already well advanced in the planning and construction process.
Their cost. The desirable scale of hydro goes hand-in-hand with a commensurate cost, meaning that their construction is limited to sponsorship or underpinning by governments and the larger energy/utility companies. This has hitherto required long term commitments to power and capacity charges on the part of the grid operator to support the necessary investments.
These hydro plants have to-date used fossil fuels to provide the power for top reservoir recharging during low demand periods. To replace this fossil fuel requirement with renewable energy will require the consistent availability of wind/solar in periods of low demand, a requirement which cannot of course be guaranteed and indeed represents the very issue the hydro scheme is trying to overcome. If pumped storage projects have made economic sense, it has been on the basis that the price of electricity in a market-driven system is lower during low demand periods and higher during peaks of demand, so although the plant may be a net consumer of electricity over a day, it can be a net generator of revenue over the same period due to the diurnal variations in power price.
The capacity of most pumped storage systems to cover shortfalls in renewable supply is measured in hours, or a day or two at most for the very large projects. To fully cover the intermittency of wind and solar, pumped storage would require substantially larger capacity, enough to cover 1 – 2 weeks of shortfall.
To be used as fine-tune demand-supply balancers, pumped hydro storage facilities should be as close to the centres of demand as possible, otherwise supply can be limited by electricity grid capacity constraints. The feasibility of this obviously varies between countries and regions. Distances in North America are typically far greater than those involved in European hydro projects for example.
The next part of this series of Insight briefs will consider the relatively recent developments in large scale battery storage.