Revisiting the Marine Current Turbines capacity factor

Some while ago, in this blog post we calculated an apparent capacity factor for Marine Current Turbine's SeaGen in Strangford Narrows of 38%.

We've been rooting around in OFGEM's ROC database to see if there's further information available on SeaGen's performance.

Looking at the ROC register, which reports the number of MWh generated for ROC-accredited projects, it seems that the SeaGen achieved monthly average capacity factors of up to 58% in its best years.

This analysis is based on SeaGen having a capacity of 1.2 MW and is consistent with another blog analysis we did of the capacity factor of the technology.



This analysis begs some other questions though - what went wrong in 2011?  and has the technology been abandoned since 2014? It also seems that the capacity factor was pretty variable, pointing to a lack of reliability - fair enough in a prototype technology.

We'll be using this approach  to look at other technologies too, in the upcoming Redfield review of tidal technologies and its companion volume - Redfield review of wave technologies.  Contact us for details: info(at)redfieldconsulting.co.uk

Hours on the clock

Marine Current Turbines have now racked up 1000 hours of export to the National Grid, as reported here.

MCT claims an average capacity factor of 66% for the period of operation. We have calculated a capacity factor, based on ROC register figures, which shows a steady(ish) increase to around 25% in November 2009. This calculated CF does not make any allowance for downtime, availability of marine mammal onservers, or (crucially for MCT) the limitations imposed by daylight-only working.

Since daylight in November is less than 50% of the time, and the trend is upwards, we can see that a claimed 66% capacity factor in December could be realistic.

If true, it's excellent news for the company and the technology, as it's always all about cost per MWhr, and more MWhrs means a better metric.

MCT power curve - analysis

MCT has just published a power curve for its Seagen device in Strangford Narrows (see figure above and link here). The curve shows a period of output at the design capacity of 1.2 MW, in what is described as a “medium tide”. This tide appears to have peaked at 3.1 m/s, which may be medium for Strangford, but is pretty impressive for most sites.

We realised that it’s possible to drill into this curve to come up with some (very) theoretical ideas of the capacity factor which might be achieved by the technology. First we constructed a velocity lookup table by taking a ruler to the graph, which shows the power output (kW) at various stream speeds (m/s).

m/s - Power
0 - 0
1 - 20
1.25 - 100
1.5 - 180
1.75 - 400
2 - 600
2.25 - 900
2.4 - 1200

We then constructed a model which characterises a simplified tidal environment, with stream speed varying according to a diurnal cycle (sinusoidal variation over 24 hours, in 2 flood, 2 ebb tides) and a 28 day lunar cycle (again simple sinusoidal variation).

We entered a maximum stream speed (peak rate achieved at spring tide) and a minimum stream speed (peak rate achieved at neap tide) and constructed a lookup on an hour by hour basis to estimate the power output over a month.

Based on a maximum stream speed of 3.2 m/s and a minimum stream speed of 1.6 m/s (ie neap maximum is half as fast as spring maximum), we find that the average theoretical power output (assuming no outages) to be 450 kW, making the capacity factor 38%.

The model shows that output is sensitive to both maximum stream speed and the ratio between spring and neap peak rates. The table below shows the relationship between capacity factor and the maximum stream speed) assuming that neaps are limited to 50% of the maximum stream speed in springs. The month-average capacity factor for various maximum spring stream speeds is:

m/s - CF (%)
2.8 - 28%
3 - 34%
3.2 - 38%
3.5 - 45%
4 - 53%
The table below shows how the capacity factor is influenced by the ratio between the maximum neap speed and the maximum spring speed based on a maximum spring stream speed of 3.2 m/s. The table shows maximum neap speed and month-average capacity factor.

m/s - CF (%)
0.8 - 31%
1.2 - 33%
1.6 - 38%
2.1 - 45%
2.4 - 48%

All of these power output estimates are wildly theoretical – and should be treated with extreme caution. Next we’re going to combine this power curve with some actual tidal data from tidal diamonds on charts to see how that looks.

It's all go in Strangford Lough

Oceanflow Energy has just deployed a 1:10 scale "Evopod" device in Strangford Narrows, presumably a hop and skip away from MCT's SeaGen device. The Evopod is a semi-submersible, moored energy converter which floats and yaws to optimise turbine alignment with tidal flow.

This company's been below the radar a bit, but seems to be making good technical progress. It's leading light is Graham Mackie, last seen as Technical Director at Wavegen, and founder of renewable energy consultancy Overberg (which has now morphed into Oceanflow.

Looking at Oceanflow's website, it appears that the plan is to develop a 1.5 MW evopod design.

Newsflash - MCT installs in Strangford Lough

Congratulations to MCT for getting into Strangford Lough. It's well known that the installation has been the victim of a number of spanners in the works, so a big hand is due. We'll be keenly interested to see how commissioning goes, and hope that MCT is successfully powering those thousand local homes in short order.

The Guardian has covered the story pretty thoroughly here.