Friday, 13 January 2017

Swansea Bay Tidal Lagoon - do we really need it?

I have been reading the Hendry report on Tidal Lagoon technology [1]. Should we go ahead with a 'pathfinder' project like Swansea Bay or not? I say yes - but not because it is good value for money or has the potential to meet a large portion of our electricity needs - in fact the potential is quite limited. What swings it for me is that tidal lagoon technology has the potential to provide useful energy storage.

The overall UK potential for power generation from tidal lagoons is quite small. Hendry puts the likely limit at about 18 GW capacity providing a total 30 TWh per year. This compares to 14 GW capacity of onshore and offshore wind installed now - providing 40 TWh in 2015 [2]. So the maximum generation from all the tidal lagoons we are ever likely to build (in the UK) will supply less energy than the wind turbines we already have. On the other hand, we only have 2.4 GW pumped storage and tidal lagoons could, theoretically improve on this substantially.

Swansea Bay is expensive. A larger lagoon would be better value for money if we could raise the capital.
Tidal Lagoons do not require ground breaking technology. We already know how to make marine turbines and we know how to build sea walls. However, no-one has actually operated a tidal lagoon before. (We do have tidal barrages but not as big as this.) Also, lagoons are expensive to build simply because of their size. The plans for Swansea bay require 9.5 km of sea wall and this is a small one, only 320 MW. In fact the larger lagoons are proportionately cheaper to build because you get a better ratio of water stored per km wall. The ratio enclosed area/wall length for Cardiff is nearly three times as good (3.4) as that for Swansea Bay (1.2). Cardiff would be better value for money - except the prospect of raising £9.7 billion for unproven technology is unimaginable. That is why we need something like Swansea Bay, at £1.3 billion, as a pathfinder.

Tidal lagoon power looks cheap if you ignore the lengthy contract term
Hendry calculates that the cost of tidal lagoon power is less than offshore wind if you apply a fair comparison. However comparisons are very difficult. The table below shows the estimated CfD strike price for lagoons, offshore wind and nuclear from his report. Lagoon power at £113.10/MWh looks like the most expensive of the three technologies. However the equivalent cost if the price were fully linked to inflation like the others would be only £70.40 which is the lowest. The equivalence is based on net present value. Hendry thinks this is a fair comparison but I'm not so sure because of the difference in contract terms. Hendry has proposed a lengthy 90 year contract, nearly 3 times longer than that for nuclear power and 6 times as long as the contracts for offshore wind. This increases the commitment for the energy companies (and hence us consumers) while it reduces the risk for investors, because they have a guaranteed return for longer. Well it is not quite guaranteed. They only get paid while the installation keeps running.

Offshore windNuclear (Hinkley Point C)Tidal lagoon power
Contract term/
expected lifetime (years)
Indexation for inflationindexedindexedpartially indexed
CfD strike price £/MWh105 (cap for the next auction)92.5113.1 (Cardiff)
 70.4 (Cardiff, indexed equivalent)

Lagoon power has an advantage over wind power because it is reliable,
In the table above the cheapest technology is nuclear power but there are question marks over that (which I don't want to go into here). The next one is wind power but tidal lagoon power has a big advantage over wind because it is reliable. We can predict accurately when and how high the tides will be, albeit with some adjustment for level depending on the wind at the time. This means we will be able to forecast generation from tides days and weeks in advance with high confidence. However, just because we can predict it doesn't mean we can control it.

If you run the lagoon for maximum generation it runs for less than half the time.
The most efficient way to run a standard tidal lagoon is one-way generation on the ebb tide. This means you let the lagoon fill on the incoming tide. Then as the tide goes out you wait until there is a good height difference before you start letting the water flow out again, generating power as you go. You are generating for less than half the time.

Having more lagoons in different places at different tide times help to fill the gaps
Ramping power on and off twice a day is not nice for the grid. Other generators have to compensate. If there were several lagoons around the coast they would be on and off in different phases because they have different tide times. This helps a lot.

Tidal lagoons can be more flexible about timing and even provide a storage service.
However, in principle the lagoon power is at least partly controllable. Turning on the turbines a little earlier or later than optimal would generate less power overall but could be beneficial for the grid. Also, in principle you can use spare power on the flood tide to pump more water into the lagoon - a form of energy storage. If you make the walls higher you can pump water in at low head near high tide and get more power out on the ebb. If you subdivide the lagoon into separate areas and install pumps between them you can be even more flexible. The trouble is, the way the CfD price guarantee works there is little incentive for the lagoon operators to do anything other than maximise their generation at minimum capital cost.

Tidal lagoons could triple our pumped storage capacity.
Pumped storage in freshwater lakes has proven incredibly useful but we only have a total of 2.4 GW capacity and the potential is limited by the shape of our mountains. Tidal lagoons could provide a storage service perhaps a third of the time - if 18 GW lagoon power gives 6 GW of storage we could more than triple our current capacity. Swansea Bay Tidal Lagoon would be the first step on the way.

For more about the potential for flexible operation of tidal lagoons, see this discussion by David MacKay.

[1] The Role of Tidal Lagoons (Charles Hendry) Dec 2016
[2] Digest of UK Energy Statistics 2016 ( Sep 2016

1 comment:

  1. Nicola, you do know that the Swansea design is bidirectional so there are 4 peaks/day, right? Slight loss of efficiency compared to a unidirectional twice/day turbine, but nearly twice as much energy and twice as much of the time. It's still _very_ lumpy.

    You say that CfD gives no incentive to optimise for smooth output over max output, but turbine manufacturers are all now optimising for more continuous operation, rather than max power as they used to. Apparently the incentives line up in that case, for reasons I don't fully understand.