Comments on: Floating tidal turbine off Orkney Islands smashes generation records

By: Phil B

Phil B — Wed, 14 Jun 2017 20:40:47 +0000

Oops! My attempt at a table screwed up. The figures should read :

“Hour Number”, followed by “Flow rate” (or rise of tide) and then “Total accumulated flow” (or total rise of tide).

[Isegoria: Fixed!]

By: Phil B

Phil B — Wed, 14 Jun 2017 20:38:21 +0000

There is also the intermittent power delivery where the tidal stream velocity is sinusoidal in nature and at peak stream may indeed produce the quoted power but away from the hour or so of peak stream velocity, less and less. At slack tide (both at low and high tide, twice a day for each) there will be zero flow so zero electricity generated. The rule of twelfths applies — in other words, from slack low tide, the flow goes like this:

Hours after low tide	Flow volume (as proportion of total volume)	Total flow
1	1/12	1/12
2	2/12	3/12
3	3/12	6/12
4	3/12	9/12
5	2/12	11/12
6	1/12	12/12

This is now at high tide slack water The flow then reverses in the same way until low tide slack water is reached. Plot this out as a graph and the shape is a sine wave. You can see that peak flow occurs approximately 3¹⁄2 hours after high or low tide and this will be the peak power generation quoted. Away from these times you will get somewhere between zero and "a bit". As any electrical grid power engineer will tell you, the problem with solar and wind generated power is the fluctuating surges which seriously screw up the system. Nice concept but unless the politicians, greenies and other dreamers can rewrite the laws of Physics and can overcome the fluctuating power surges, it can't be connected to the national grid successfully.

By: Scipio Americanus

Scipio Americanus — Wed, 14 Jun 2017 18:42:27 +0000

Its maximum power-to-weight ratio of 0.04 KW/tonne is comparable to that of large marine diesels like those used on container ships.

Of course, as Redan points out it’s not producing anywhere close to that in test operation and, as Borepatch notes, no costs were mentioned. With the specs given above I could see having a cost per ton roughly equivalent to that of small commercial ships, which would certainly make it far cheaper than the wind turbines it can supposedly replace.

Let’s take Scotland as an example. To cover, say, 50% of current peak demand (somewhat less than 6.8 GW) you’d need at minimum 1,600 of these tidal generators (assuming they’re all running at full capacity). Using their test results, you’d need over 4,000 of them.

Maybe they should look at a much larger version.

By: Redan

Redan — Wed, 14 Jun 2017 15:22:53 +0000

18 MWh generated in 24 hours = 0.75 MW average, therefor 0.75 MW from a 2 MW rating = 0.375 ratio or 37.5% of rated capacity.

By: Borepatch

Borepatch — Wed, 14 Jun 2017 14:35:10 +0000

Numbers not shown:

Cost of ship

Cost of crew

Annual maintenance cost

Expected lifetime of ship until it sinks in a storm

Of course, including these costs would allow you to calculate a cost per kWHr which might take away from the “gee whiz” focus on the technology.

The other number not shown was 5 cents/kWHr which is the cost of US coal fired baseload generation.