Sunday, 1 November 2015

Emlagh Wind Farm - Capacity Credit too low to make it worthwhile project

Kells - an important Irish town both culturally and historically

Emlagh Wind Farm is a 120MW wind farm, consisting of 47 turbines, planned for North County Meath. It is sited near the historic town of Kells, an area of unique cultural and historic heritage, that attracts many tourists each year. Kells is most famous of course for the Book of Kells which dates back to the 6th Century.

The benefits from the proposed wind farm would have to be very significant to justify it's imposition in such an area. A decision on the wind farm was due in late September but An Bord Pleanala have delayed this until November 26th.

Capacity Credit 

The best way to measure the value of a wind farm is by calculating it's capacity credit. This is the equivalent amount of conventional power plant that can be shutdown as a result of the new wind farm. It is used by Eirgrid to measure the contribution from a new generator to generation adequacy.

In Eirgrid's Generation Adequacy Report for 2009 – 2015 they noted :

“Although the expected large growth of installed renewable capacity will increase portfolio diversity, it will only offer a limited contribution to generation adequacy.”

The intermittent nature of wind means that the contribution of wind power to generation adequacy is significantly less than its installed capacity. When considering the generation adequacy of wind as an energy source, wind is given a lower “capacity credit” than conventional thermal generation, primarily because of its intermittent nature. 

The capacity credit curve for wind used by EirGrid in the Generation Adequacy Report 2009 – 2015 is presented in the below Figure:


Figure 1, the capacity credit attributable to wind declines with increasing installed wind capacity. The capacity credit for Wind Power Generation (WPG) was predicted by EirGrid to decrease from 19% in 2009 to 12% by 2015 as illustrated in Table below:

Table 1: Wind Capacity Forecast and Associated Capacity Credit

Installed Wind Capacity (MW)
Capacity Credit (MW)
Capacity Credit as
% of Installed

From Table 1 above, it can be seen that Emlagh Wind Farm will have a capacity credit of 12.5% in 2015 (total capacity has not reached predicted 2015 levels). At an installed capacity of 120MW, this then represents a contribution to generation adequacy of just 15MW (equivalent to just 3-4% of a conventional gas plant). This means that should there be a security of supply issue in Meath or the surrounding region, then new conventional plant will need to be built to keep the lights on, despite the large investment and environmental impact of this wind farm.

The following statement made by the Emlagh Wind Farm developer in their Environmental Impact Statement then, is completely at odds with the accepted method of accounting for wind farms in the energy industry:

“Should the wind farm not be developed, fossil fuel power stations will be the primary alternative to provide the required quantities of electricity. This will further contribute to greenhouse gas and other emissions, and hinder Ireland in its commitment to meet its target to increase electricity production from renewable sources and to reduce greenhouse gas emissions. Ireland currently imports up to 85% of our energy. Emlagh Wind Farm offers Ireland an indigenous form of electricity, and provides for security of supply against our dependence on imports.”

But when measured scientifically, the wind farm will not be an alternative to fossil fuel power stations. Instead, a duplicate system will be required, with a need for new transmission infrastructure running alongside existing lines to be built - one powered by fossil fuel, the other by wind (when available). 

A contribution of 15MW for 47 turbines is a very poor benefit given such significant environmental (and indeed economic) costs.  


  1. Very interesting, would the same apply to Maighne, the 47 turbine development planned for N Kildare and S Meath. How is this calculated or do we just depend on Eirgrids figures.

    1. Of course, it would apply to all wind farms currently in planning. You would need to refer to Table 1 above. Current installed wind farms in Ireland is around 2,600MW so a new wind farm would have a capacity credit of 12.5%. Whereas older wind farms built 6 years ago would have a higher CC of 19% (when they were built).

      Eirgrid's figures are sound. They come from 2009 when their job was still to keep the lights on and nothing more. Since then, the Renewables Directive was introduced and they have broadened their role to facilitate this Directive, even though it is often at odds with their original role.

      As more wind farms get built, the CC of the entire wind fleet declines and the marginal CC of new wind farms makes them less and less viable.

  2. Capacity credit is a long established method to measure the contribution of any generator to a system requiring a continuous supply of synchronised power.. It is defined as “the amount of other plant in the system which can be shut down and replaced by the one being measured without endangering supply”. With the advent of intermittent renewables the term “shut down” might be considered a little harsh and might possibly be replaced by “temporarily shut down” .

    The “temporarily shut down” definition has the shortcoming of requiring the shut down plant to be kept installed and available to dispatch when required. Staff and maintenance costs still apply. The effect is that no plant can be shut down and taken out permanently. This is why we see that intermittent renewables do not permanently decrease the amount on other plant anywhere they occur. In Ireland, we need 6,000 mw of thermal generating plant. We installed about 2,300 mw of wind up to last winter and 2,000 mw of fast acting gas plant to balance wind. Our capacity now is about 10,100 mw. We have 66% surplus capacity, with domestic charges @ 24.5 cent per kwh (245 euros per mwh).

    Only half of demand can be wind, viz: 2,500 mw at the time of winter peak @ 5,000 mw in total, dropping to 1,100 on a summer's night. Therefore any fast acting balancing plant above 2,500 mw cannot be used. That same report referred to above sees Eirgrid dreaming about 15,100 mw in total. This would by necessity comprise 6,000 mw of conventional thermal plant, 2,500 fast acting gas plant and 6,600 mw of wind. No thermal plant can be sold off and no staff can be laid off.

    If the above graph is applied to the Irish experience, a phenomenon described by the economist Colm McCarthy as “bazaar” becomes evident and is so weird that it surprised me too. Between 1990 and 2015, the penetration of wind increased from about 100 mw to about 2,300 mw. Total capacity rose from 6,000 mw to 10,100 mw in the same period. It is known that some thermal plant is shut down when the wind is strong, Talbert is regularly shut down. But this shutting down is on an inflated thermal capacity.

    Note in the above, no mention is made regarding the saving of fuel by wind. I cannot find any evidence or a saving. We are burning the same amount of fuel in 2015 as we were in 1990. Denmark and Germany show the same result. This indicates that wind never saves fuel and we were always better off with none at all. How could this be? I suspect the secret lies in the word “shut down” as a function of time. The traditional definition “capacity Credit” was based on the concept that no supplier would keep plant which was surplus to requirements. Surplus plant was sold off, as new plant took its place and the site was sold for other purposes.

    With the advent of subsidised intermittent renewables, there were many hidden costs and the state intervention not only covered the higher cost of sales, but turned a blind eye to the fact that conventional suppliers simply kept all their plant at the ready irrespective of how much renewables there was. Without the word “permanent” in the definition, the term “capacity credit” is an entirely different technical measurement. With the word “permanent” in it, wind has no capacity credit or indeed if one counts parasitic power consumed by wind farms, wind's contribution is negative and the more wind the more negative it becomes. The system is a machine, a big machine and like the operating principles of many physical phenomena, physics lecturers remind their students when a topic is counter intuitive.

    With a projected capacity for wind of 6,600 mw, duty comptrollers will have to decide which wind farms are allowed to dispatch. We could then see the term meaning how much of one wind farm is shut down an replaced by another wind farm like musical chairs. As Rita May Brown remarked “intuition is the suspension of logic due to impatience”