The UK had splendid weather in April. With high pressure over the North Sea we had 8 days of splendid weather at the beginning of the month (2nd to 9th of April) and 9 days of splendid sunshine during the second half (15th to 24th April) (Figure 1). This of course left our massive fleet of wind power stations idling. 12 GW of installed capacity produced less than 1 GW for much of that time and less than 0.2GW for some of the time. This affords the opportunity to put some numbers on the energy storage requirements to survive lulls such as these.
Our standard unit of currency is going to be the proposed Coire Glas (CG) pumped hydro storage scheme  which has capacity to store 30 GWh and to release that energy over a two day period. As described below, the first lull requires about 347 GWh or 12 CGs. The second requires 339 GWh or 11 CGs but there is insuficient surplus wind between the lulls to recharge the magazines (Figure 1). A single Coire Glas is environmentally sensitive. Contemplating storage as a solution to intermittent wind must therefore be an exercise in vanity. Not quite! Enter the Strath Dearn (SD) Pumped Hydro Storage concept  beside Loch Ness in the Scottish Highlands. The SD will be our second unit of storage where 1SD = 227 CGs. With a capacity to store 6800 GWh it could easily have spanned the April 2015 lulls with room to spare. But as always there is more than one catch.
Figure 1 UK electricity demand and wind production as recorded by BM reports and reported by Gridwatch . Demand left hand scale, wind generation right hand scale. The storage required to span lulls is based on a 3 GW target for median wind output. In other words, when over 3 GW is being generated the surplus is used to pump water. Click chart for large version.
Before going on to look at the details of Strath Dearn I want to put some more numbers in place on UK wind, pumped hydro storage and power generation and the construction of Figure 1.
The UK currently has 4 operational pumped storage hydro schemes :
The combined capacity of existing = 26.7 GWh which is just below 1CG.
- The UK has about 12 GW installed wind capacity . BM only meter about 8 GW hence the wind reported by Gridwatch is grossed up by a factor of 1.46 .
- Peak wind generation (adjusted) in April was just below 9 GW but on many occasions was below 0.2 GW (Figure 1).
- Peak UK winter demand, 6pm in January or February is over 55 GW. Minimum summer demand at 6am on a Sunday morning in July is less than 22 GW.
The April Wind Lulls
I have estimated the size of the April lulls by assuming an average 25% capacity factor giving 3 GW median wind generation and deducting actual generation during the lulls from that median value and summing across time (Figure 1).
The first lull requires approximately 347 GWh of storage backup, the second around 339 GWh of storage backup. Between the lulls is an opportunity to recharge the magazine with gross surplus of 267 GWh. This however needs to be reduced by 20% to account for the energy efficiency of the system. Hence, the net recharge is reduced to 214 GWh. The total storage required to span both lulls is of the order 472 GWh.
With a capacity of 6800 GWh, Strath Dearn could easily cope with the wind system as it is currently configured. But as discussed below we will see that when scaled to a 100% renewable system it gets too close to call.
The Loch Ness Monster of Energy Storage
The Strath Dearn pumped storage hydro concept is proposed by new energy blogger “Scottish Scientist” . Scottish Scientist is hiding behind anonymity which lowers credibility and given the nature of the concept proposed many may wonder why I am giving this air time. The reasons for doing so are this: 1) the concept illustrates the enormous scale of the storage challenge 2) the blog post appears to contain carefully considered engineering calculations 3) David MacKay showed up in the comments to the post and 4) it illustrates how renewables enthusiasts see no boundaries to the environmental destruction they are prepared to create in pursuit of their ideology.
The proposal is to build a truly gigantic pumped storage hydro scheme in the Monadhliath Mountains, just south of Inverness (population 72,000) (Figure 2). This area in the upper reaches of the River Findhorn is remote, wild and beautiful (Figure 3).
Figure 2 The location of the worlds biggest proposed pumped storage hydro scheme in relation to Loch Ness and Inverness .
Figure 3 Strath Dearn, a remote wilderness not worth preserving?
The scheme proposes to pump seawater from a location on the Moray Firth just east of Inverness to an elevation of about 300 m above sea level from where the water will flow south along a canal to the base of dam at an elevation of about 350 m** where it is pumped into a reservoir with maximum surface elevation of 650 m. At one level, this is a standard pumped hydro storage scheme employing the sea as the lower reservoir. The scheme would have two pumping and generating stations, one by the sea and the other at the base of the dam. It is the awesome scale of this proposal that makes it stand out.
** In email correspondence Scottish Scientist explained how the shallow gradient of the terrain may be overcome:
The pumping station does not have to be immediately at the foot of the dam but could be some 100s of metres “down river”.
Also 350 elevation at the foot of the dam is including the fluvial sediment which all must be removed to build the dam on the bedrock. So we don’t know what the surface elevation of the bedrock there is going to be.
But just assume for a minute that 350 elevation had to be coped with at the dam end.
There are other options but simply making the canal 50 metres deeper and 100 metres wider near the dam where the surface elevation is now at 350 metres would do the job.
Do likewise in proportion as the surface elevation varies from 350 to 300 metres
So if the surface elevation is 325 metres then make the canal 25 metres deeper and 50 metres wider at that point.
Not that I am even absolutely committed to 300 metres as the canal’s water surface head. If it works out cheaper to design a canal height of, say, 310 metres meaning that the canal sides need embankments where the surface elevation is now 300 metres then that sort of thing is a possibility too.
One other possibility would be to divide the canal in 2 at some point with powered flow between the canal halves through another turbo-pump station but that’s a complication too far for me to detail at this stage since it is too early to rule out the simpler undivided canal with appropriate building works to cope with the varying surface elevation.
- Storage capacity = 6800 GWh
- Surface area of reservoir = 40 km^2, volume = 4.4 billion m^3 of water.
- Flow rate through pipes and canal = 51,000 m^3 per second (equivalent to the discharge of the Congo River)
- Generating capacity between 132 GW and 264 GW (2 to 5 times UK peak demand)
- Canal 30 km long, 170 m wide (minimum), depth at centre 85 m, flow velocity 10 to 11 ms^-1 (water flowing at this rate can move 10 tonne boulders).
- Dam crest length = 1860 m, top height 300 m, dam volume 80+ million m^3 (of concrete?)
- Cost: unknown
- Hoover Dam: crest length = 379m; height 221m; dam volume 2.48 million m^3, generating capacity 2.1 GW 
- Three Gorges Dam: crest length = 2335 m; height 181 m; generating capacity 22.5 GW 
- Loch Ness: area = 56.4 km^2; volume = 7.5 billion m^3.
In summary, the area of the Strath Dearn reservoir is comparable to Loch Ness and the volume is about half of Loch Ness. The dam would dwarf the Hoover Dam and is of comparable size to Three Gorges in China. Strath Dearn generating capacity of 132 to 264 GW dwarfs both Hoover and Three Gorges. That is because the reservoir may be emptied and filled regularly, it has a huge head of 650 m and flow is not restricted to the flow of a natural river that has been dammed.
So what is there not to like about this proposal beyond the obliteration of Strath Dearn, the River Findhorn and causing environmental chaos in the Moray Firth? At the outset it is important to note that The Energiewende is being conducted in the name of environmental protection. Its advocates seem to have totally forgotten that the fossil fuel age bought a future for whales and forests.
Catch 1 As described above, storage of the order 472 GWh would be required to span both April lulls for the wind system as it is currently configured with a median output of 3 GW. Scaling this to a 100% wind-pumped-storage system would increase that requirement so that median output from a gigantic wind carpet would be of the order 50 GW. The storage requirement for the 100% renewables system therefore grows to 50/3*472GWh = 7867 GWh. Strath Dearn is not large enough to guarantee supply.
Catch 2 The whole point of The International Energiewende is to get away from centralised power generation and to embrace distributed power. Strath Dearn is a pole away from that goal and would represent the greatest concentration of power generation anywhere in the world with a generating capacity 10 times that of the enormous three gorges dam in China.
Catch 3 follows on from Catch 2. 264 GW of generating capacity needs 264 GW of power lines to bring power into the pumps and to export power from the generators. The tentacles of environmental destruction would spread out across the whole of Scotland from Inverness. The controversial Beauly – Denny line that is nearing completion is rated at 0.4 GW. So we’d need 660 of those.
[Note added 27 May. Beauly – Denny is actually rated at about 2.5 GW, hence the number of power lines is reduced to 106]
Catch 4 lies in currently unquantified concern I would have about seawater ingress into the groundwaters that underlie and spread out from this high land location. The Caledonian Schists of this area are likely to be rather impermeable themselves. But groundwater storage is via fractures and joints and I would anticipate fairly rapid ingress of seawater into the fracture networks that would show up in springs and wells surrounding this locality. Loch Ness may become salty. Nessie would not like it.
Catch 5 lies in tectonic stresses caused by loading and unloading the site with 4.4 billion tonnes of water on a regular basis. Whilst I don’t know for sure, it seems possible that this could cause earth tremors which in turn could raise concern about dam failure.
I could probably go on to find another 17 catches but in interest of brevity will leave it at 5. However, Catch 22 may turn out to be that Strath Dearn is the lowest cost per GWh installed capacity of any energy storage scheme on Earth but that it is impossible to fund because of the sheer scale of the aggregate investment involved.
For wind power to work as a mature non-parasitic generating technology requires grid-scale, energy efficient storage and for the time being pumped storage hydro is the only show in town by a long, long way, a situation that I anticipate will persist. In my opinion, monster schemes like Strath Dearn are totally unworkable and head off in the wrong direction.
Scotland has acquired 8GW of wind power whether we like it or not and is likely to acquire a lot more in the years ahead. The Scottish Government has recently held a consultation on how to solve the mess they have created. The assumption has always been that energy storage would be used to store the peaks and release that energy into the troughs of wind generation. Mission impossible?
For the foreseeable future, it will make much more sense to balance wind against gas or coal plants as happens now. The wind energy offsetting the amount of coal and gas we need to burn which is good for UK and European energy security. Grid scale storage will most definitely have an increasing role to play but in the first instance that should target the diurnal demand cycle.
 The Coire Glas pumped storage scheme – a massive but puny beast Euan Mearns, Energy Matters.
 World’s biggest-ever pumped-storage hydro-scheme, for Scotland? Scottish Scientist
 Gridwatch Leo Smith.
 Sustainable Energy – without the hot air David MacKay
 Untangling UK Wind power production Clive Best.
 Hoover Dam Wikipedia.
 Three Gorges Dam Wikipedia.