I've been doing some calculations on tidal power. There are two locations of immediate interest ATM, Swansea Tidal Lagoon and the Pentland Firth Tidal Stream. The former envisages a horseshoe-shaped barrier built out from the shoreline at Swansea and enclosing a large body of water. As tides ebb and flow, turbines are driven by the flow of water out of and into the lagoon
The Swansea scheme has a much-publicised maximum capacity of 320MW. It also claims to be able to supply power for 14 hours a day, and to be able to supply 155,000 homes. But what does that boil down to? The fourteen hours a day is the sum of four 3.5 hour periods of generation, interspersed with 2.5 hour periods of no generation at slack water while the tides turn. So very On-Off-On-Off. The Grid operators will love that! Although Swansea doesn't give the average power over a year, it's easy enough to calculate from the house numbers: it works out at about 58MW*. With an installed capacity of
320MW, that gives a capacity factor of about 18%. The cost of Swansea is going to be about £1bn, so about £17 million per MW of output, compared to Hinkley Point C at £6.2 million per MWIn the Pentland Firth, the complete Atlantis Resources/Meygen scheme proposes a total of 269 turbines, each of 1.5MW capacity, and having a total capacity of about 400MW, to be installed in the channel between John o' Groats and Stroma island. It too will be intermittent, just like Swansea: On-Off-On-Off. They claim it will power 175,000 homes. Using the same calculation as before, this equates to an output of about 66MW averaged over a year**, and a capacity factor of nearly
17%, not far removed from that of the Swansea scheme. To match Hinkley C, you'd need about 11,700 of such turbines (2880/66x269), assuming a capacity factor of 90% for Hinkley!Oh, and I nearly forgot, the strike price for tidal stream electricity is a whopping £305/MWh!
The take-away from all that is to be very wary of the headline publicity put about by the promoters of these and other renewable schemes. They quote maximum capacities and the number of homes supplied. Both are large numbers, purposely designed to impress and mislead the average Joe Public. Squillions of kilowatt-hours sounds much more impressive than a few terawatt-hours (even assuming JP understands either)! They deliberately avoid average figures, simply because the reality is unimpressive, and if this was realised by TPTB, it's much less likely that their schemes would get the go-ahead. IMO such presentations are immoral at best and plain dishonest at worst. The promoters are much more interested in getting on to the renewables gravy-train than they are in saving the planet, and they pull the wool over the eyes of gullible and easily persuaded environmentalist members of the public.
*Calculated from the number of homes they claim to be able to supply, and OFGEM's figure of 3,300 kWh for the average household annual electricity consumption.**As the above calculation, but using 175,000 homes and with a maximum capacity of 398MW gives an actual output of 65.9MW and capacity factor of 16.6%.