This is the stuff of dreams.
During a dry year, hydropower generation is reduced by about 4000 GWh. Since the 1950s, power stations at Meremere and then Huntly, have maintained a coal stockpile sufficient to substitute for the hydropower shortfall. The government believes that the Onslow scheme will eliminate the need for the stockpile.
The scheme envisages building a high dam at Lake Onslow and a tunnel from the dam to the 1200 MW underground power station near Lake Roxburgh. The generating plant would comprise a number of pump/turbines that rotate in one direction to generate electricity and in the reverse direction to pump the water back up into the lake.
The scheme is based on operating Lakes Tekapo and Pukaki at a much lower level so that they can store floodwater instead of spilling it and then generate hard to bring the level down and provide surplus power to pump water into Lake Onslow. Pumping could also take place when there was surplus wind or solar power.
In a dry year the water would flow back down the tunnel and generate electricity before being discharged back into Lake Roxburgh.
Most pumped storage schemes operate in a much different mode. They have sufficient storage for only 6 to 10 hours of operation and pump when power is cheap and generate when prices are high. Pumped storage stations also operate in a system support mode responding rapidly if the system is suddenly short of power or suddenly loses a significant amount of demand. They can also stabilise system voltage during disturbances. Many of these stations change operating mode dozens of times a day.
There are two major problems with Lake Onslow’s mode of operation. The first is that, unlike a conventional pumped storage scheme, it will spend a lot of time waiting for a flood so that it can pump and waiting for a dry year so that it can generate. It might generate a substantial amount of power once every five years or so. This means that the utilisation factor of the plant will be very low and, when it does generate in a dry year, the return from generation needs to be very high.
The second problem is that because of the way our existing electricity market works the other generators would jack up the power price as soon as pumping started so depriving the pumped storage scheme of cheap power. This problem could only be fixed by a major overhaul of the electricity market and this might delay completion of the scheme by several years.
Evaporation from the lake depletes storage. Evaporation losses in Central Otago are about 1 m per year so that the lake would lose about ~700 GWh (15% of storage) to evaporation every year. When this loss is added to the pumping/generating efficiency of 75%, the overall efficiency drops to around 60%. For every 5000 GWh of pumping power, only 3000 GWh would be generated.
Filling the lake is likely to take several years depending on rainfall and the amount of renewable energy available to fill it. If we get two dry years in quick succession the lights may well go out in the second year.
It cannot be in service by 2030. Feasibility studies including extensive drilling along the line of the tunnel will take at least one year, environmental approval will take two years or more, capital raising another year, final design and tendering another two years, six years for construction and then three years to fill the lake. It might be ready in 2037.
Economically, it is a disaster. It is estimated to cost $4 billion but, after allowances for interest during construction, escalation and contingencies, it is likely to cost $5 billion or more.
Annual capital charges at 6% would be $300 million, operation and maintenance would be about $20m and transmission charges might be $5 million. Making up for evaporation loss at 5 cents/kWh could be as high as $35 million per year. Refilling the lake every five years at 5 cents/kWh would amount to $40 million per year. So the station has to earn an average of $400 million per year ($2 billion every five years) just to break even.
Assuming that it needs to generate 3000 GWh one year in five it needs a price of $0.70/kWh to bring in $2 billion.
Because of the way our electricity market works all the generators would be paid this $0.70 so each dry year would cost the consumer and the economy more than $20 billion and the domestic power price will treble. This would fall heaviest on poor people and decimate our economy.
Onslow would save an average of 300,000 tonnes of CO2 per year at a cost of at least $1300/tonne. The current CO2 price is $32. Keeping the aluminium smelter in operation could save 2,500,000 tons of worldwide emissions each year at a cost of about $100 million or $40/tonne and has the potential to significantly minimise the dry year problem.
It is easy to see why the Interim Committee on Climate Change recommended burning gas during dry years. Why was this advice ignored?
Bryan Leyland is a Consulting Engineer with wide
interests in modern technology. His columns expose the truth behind popular misconceptions.
4 comments:
I find it difficult to believe - in fact, I flat out do not believe - that the proponents of this hare-brained scheme are not aware of the facts stated here by Mr Leyland. The agenda of these people is 'get rid of fossil fuels at any cost', and it is literally at any cost! The economics of such a long-term scheme, as opposed to a daily pumping system as used elsewhere, are the stuff of fantasy. The advocates of this scheme will be happy to use your money to put it into operation, they would never risk their own.
TOBY..
Also consider the environmental consequences of vast areas of dry lakebed exposed in a windy environment during the dry years. Swampy areas and organic soils will be flooded. When dry there will be no stability in the sediment and it will be blown away.
Brian is bang-on. What he is stating is very simple. Pumped storage is generally added to a system where there is a need to cover short term peaks of a few hours and where there is a large amount of generation capacity available at other times.
It works really well when you have a large reserve of big coal or nuclear power, power that cannot be ramped up and down quickly. Through the night and at other off-peak times, the surplus generation pumps water into storage,and when a peak hits, be it from demand or failed main plant, the pumped storage bridges the gap for a few hours before the whole cycle starts over.
In NZ, where hydro can ramp up and down on demand, the only time at which there is geunuine surplus capacity is when the dams are spilling.
No amount of generation or pumped storage in the South Island will help address the basic problem that most of the generation is in the South Island and most of the demand is in the north. Pumped storage north of Auckland may help a little, but new generation, close to and preferably north of Auckland is the only way to reduce costs for the north island. Anything else may please the Geenies but is a waste of money, but most Jacinda's crew seem have post graduate degrees in that.
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