Two common sources of renewable energy – wind and solar – are non-despatchable. That is, they do not necessarily provide a useful output when it is needed (on the other hand, they can be productive when the electricity is not needed). This is not necessarily the case for all renewables; biomass is an exception, but its use on a large scale is controversial and it has limited potential. Hydroelectricity falls somewhere between the two. In the right circumstances, it is fully despatchable, but only while there is sufficient water left in the reservoir to drive the turbines.
Two other sources of potentially useful energy come from the sea: wave and tidal power. Despite the best effort of groups of enthusiasts, the challenge of engineering devices capable of surviving storms while still generating electricity under much gentler conditions has kept this very much at the experimental stage. Tidal energy, however, is conceptually simpler in engineering terms and this week received a possible boost in the UK.
Charles Hendry, formerly a junior minister at the late lamented Department of Energy and Climate Change, and who stood down from parliament in 2015, was commissioned by the government to lead a study on the possible future for tidal lagoons, an alternative concept to the tidal barrages that have been built in a few places. The report was delivered in December and published this week (The role of tidal lagoons).
Tidal energy’s strong point is its predictability; it can be considered, if this is not a contradiction in terms, semi-despatchable. The main drawbacks are that it can only be exploited in relatively few locations where the tidal range is sufficiently high, and that it is expensive. Despite the engineering being relatively straightforward, each project is a massive building exercise.
Given this, the expectation among many was that this would be another opportunity to kick any potential project into the long grass, despite strong lobbying for certain schemes, particularly in Swansea bay. That Mr Hendry came out with a positive view was therefore something of a surprise. In his words “The costs of a pathfinder project would be about 30p per household per year over the first 30 years. A large scale project would be less than 50p over the first 60 years. The benefits of that investment could be huge, especially in South Wales, but also in many other parts of the country. Having looked at all the evidence, spoken to many of the key players, on both sides of this debate, it is my view that we should seize the opportunity to move this technology forward now.”
The cost sounds very reasonable put like that, but we have to consider what it would buy. First, there are 27.1 million households in the UK, according to the Office for National Statistics. Assuming no growth in the numbers of households, the total cost comes to around £8 million a year for 30 years.
The pathfinder Swansea bay project is set to cost £1.3bn to build. It is rated at 320 megawatts and is promoted as ‘generating enough electricity to power 155,000 homes’. The generally quoted average household consumption is just under 4,000 kWh (4 MWh). This will vary with the severity of winters and is expected to show a gradual decline as houses and appliances become more energy-efficient, but it’s a good rule of thumb. 155,000 homes would therefore need 620,000 MWh of electricity.
A 320 MW generating station is theoretically capable of producing 2.8 million MWh of electricity (2.8 TWh). Supplying 155,000 homes translates to a capacity factor of 22%, lower even than onshore wind. What this means in practice is that, twice a day, the water which has accumulated in the lagoon at high tide can be released through turbines as the tide recedes, to generate electricity. This can happen during the six-hour window of an ebb tide but, with output varying because of the changing head of water, there is little control possible.
The Hendry report comes to its positive conclusion by arguing that lagoon projects could be funded on favourable terms over a long time period, because the infrastructure could be in place and productive for perhaps 120 years, double the life even of new nuclear power stations. But Tidal Lagoon Power, the outfit behind the Swansea bay proposal and other, significantly larger projects in South Wales, Somerset and Cumbria, is looking for a guaranteed price for 90 years, starting at £123 per MWh. Although this would slowly reduce, it is £30 higher than the agreed strike price for electricity from Hinkley Point C, heavily criticised for being too expensive.
Perhaps not surprisingly, then, the government is less enthusiastic. ‘Whitehall sources’ are already reported to consider that the scheme is too expensive (Tide turns against £1.3bn lagoon). The Hendry report recommends that a subsidy regime is set for 60 years (the Hinkley C agreement would run for 30 years). And none of these comparisons, of course, takes account of the extra costs of backup incurred by any intermittent generating plant.
One of the report’s arguments is that larger schemes would be more economic to build. That may be true to a small extent, but there are no economies of scale from ramping up production of standard plants. Each scheme would be bespoke because it would be designed for a unique geography. This is something tidal lagoons have in common with hydroelectricity schemes, but most of these are economic because of the much greater head of water involved and hence greater generating capacity for a given volume of water.
The Hendry report has been welcomed by Greenpeace (Greenpeace reaction to Charles Hendry’s tidal lagoon report), but not all environmentalist groups are happy (Tidal lagoon plans face challenge from RSPB). Given this and the cool response from Westminster, tidal lagoons are unlikely to be part of the UK’s energy mix. Doubtless schemes such as this will be raised again and there will be further tinkering with wave energy projects. What is really needed, though, is a well thought-out expansion of nuclear generating capacity, greater use of domestic gas supplies and development of viable large-scale energy storage options.