Blog: Max Carcas clarifies Pentland Firth power potential
Last month the Royal Society issued a report called “The available power from tidal stream turbines in the Pentland Firth”. It was claimed that this paper was the first to make a robust estimate of the maximum power that could be generated from the Pentland Firth after taking into account the effect multiple arrays of turbines might have on the extractable resource. According to the Royal Society’s press release “up to 1.9GW could be generated (on average) — almost half of Scotland’s electrical consumption”.
There is enough potential in the Pentland Firth to provide almost half of Scotland’s electricity from tidal power!
This of course is a very impressive figure – and generally it was reported as such, but some media viewed it more negatively as it was claimed that this was significantly lower than had been suggested previously.
In fact a number of estimates have been made of the UK and Scottish marine renewable resources over the years and when one takes into account the approach taken the results have been reasonably consistent.
However as with any statistic one must be careful not to compare apples with pears as there are many pitfalls for the unwary! Apart from confusing wave with tidal energy it is very common for people to get confused by the following parameters (which are often used in the electricity sector when describing various statistics):
- Annual average power measured in TW (Terrawatts) GW (Gigawatts) MW (Megawatts) or kW (Kilowatts)
- Annual energy production measured in TWh/year (Terrawatt hours per year), GWh/year, MWh/year or kWh/year
- Installed generating capacity typically measured in GW (Gigawatts) MW (Megawatts) or kW (Kilowatts)
NB kilowatt = 1000 Watts; Megawatt = 1000kW; Gigawatt = 1000MW and TW = 1000GW
Power is a measure of the rate of energy transfer whereas energy is power (the rate) multiplied by time. For example the UK’s annual electricity demand, ie the energy we consume over a year, is around 350 Terrawatt-hours per year. In simple terms to meet this demand would require an annual average power generation of 40GW (40,000MW). A constant generation of 40GW over the course of a year would yield an energy production of 40 x 8760 (the number of hours in the year) = 350,400GWh/year or 350TWh/year, sufficient to meet this demand.
However the installed generating capacity actually required to deliver this in practice is an altogether different matter – historically the UK has had around 80GW of generating capacity, primarily to cope with fluctuations in demand, but also to provide backup for when plant is unavailable due to maintenance or unplanned outages, or in the event of disruption to fuel supplies.
The capacity of a generator, otherwise known as the plant’s ‘rating’ defines the maximum power that can be fed to the grid. With renewables such as wind, wave and tidal energy, where the fuel is ‘free’ but variable, it is most economically efficient to design generators to maximise the annual energy yield per unit cost – this generally translates into having a maximum output or rating significantly higher than the average. The ratio of the average output to the maximum or rating is called the capacity factor*.
Generally it has been assumed that tidal turbines will have similar capacity factors to wind turbines for the reasons mentioned above. Hence to produce an annual average output of 1.9GW, as the Royal Society report suggests, could require an installed capacity of 6.3GW, assuming a 30% capacity factor (ie 6.3GW multiplied by a 30% capacity factor gives an annual average power output of 1.9GW, equivalent to an annual energy production of 16.5TWh per year).
The installed capacity figure of 6.3GW is similar to a 2001 study carried out by Garrad Hassan for the Scottish Executive which suggested that a 7.5GW capacity of tidal turbines could be installed across Scotland (as opposed to the Pentland Firth alone). It also suggested that 14GW of wave energy converters could be installed in Scottish Waters. (To put it in perspective that’s 7,500 and 14,000 x 1MW rated machines respectively!)
A more recent report, “UK Tidal Current Resource & Economics” was published by the Carbon Trust / Black & Veatch in 2011, which included an estimate for annual energy production for the Pentland Firth being around 11.5TWh/year, but with potential to increase by another 7TWh/year with a different economic constraint criteria applied – in other words a result broadly similar to the Royal Society study.
The key point remains, whilst resource estimation is not a precise science, particularly at this early stage of development, there is a lot of tidal energy ‘out there’. There will be differences of opinion on overall scale, but it is important to put this in context. The figures suggest that the installed capacity of tidal turbines required to harness just the Pentland Firth resource to its fullest extent could be larger than all the onshore wind turbines that have been built in the UK over the past 25 years!
This scale of endeavour will not only make a sizable contribution to the Scottish and UK electricity supply system, but more importantly has enormous potential for economic benefit in the form of jobs, investment and carbon saving. To get the most out of that we need to continue to focus all our efforts to make R&D and industrial development happen here first. The Olympics showed what can be done with the right prioritisation and focus; with similar ambitions this could be our next big win.
*Capacity Factor
Whilst on the topic it is worth mentioning another poorly understood fact, that capacity factor is not necessarily a measure of “efficiency”! To illustrate this: one could increase the capacity factor of a 2MW wind turbine by artificially ‘de-rating’ or limiting it to a 1MW output, so that for a greater proportion of the year the turbine was at, or closer to its maximum rated output. Despite the capacity factor being substantially higher, clearly this would be economically inefficient since a lot of energy would be needlessly ‘thrown away’ and the overall annual energy produced would be substantially lower (and yet the cost would remain the same). Wind turbine manufacturers compete to produce products which provide electricity at the lowest possible cost to their owners – hence the optimisation between energy yield/capacity factor and rating/cost. The same considerations also apply in marine renewables.
The Royal Society report is available to read here: The available power from tidal stream turbines in the Pentland Firth
