Energy Institute

What types of power plant supply our electricity?

There are different types of plant which operate on different fuel types and in different scenarios; baseload, mid-merit and peak generators. The breakdown of 2013 output by generation type is shown in the diagram below (Fig. 5).

Figure 5. Electricity output in 2013 output by generation type

SEM Committee (2014) Generator Financial Performance in the Single Electricity Market SEM 14/111 Available Online

  • Baseload
  • Midmerit
  • Peak
  • Renewable
  • Price taker

Baseload generators meet some or all of Ireland’s continuous electricity demand, and produce at a constant rate, usually at a low cost. They shut down or reduce power only to perform maintenance or repair. These plants produce electricity at the lowest cost of any type of power plant, and so are most economically used at maximum capacity. The largest baseload plant in Ireland is Moneypoint, indicated by the coal share in the annual generation mix in light blue in the bar chart below (Fig. 6).

Figure 6. Fuel Mix in the Irish Electricity Market (2005-2014)

SEAI (2016) Energy in Ireland. Available Online

  • Coal
  • Peat
  • Oil
  • Natural Gas
  • Renewables
  • NR Wastes
  • Net Imports

Mid-merit generators, run on either coal or gas in Ireland, typically come on line when daily electricity demand picks up in the morning and they shut down when the demand drops off in the evening. A load-following power plant, also known as mid-merit, is a power plant that adjusts its power output as demand for electricity fluctuates throughout the day (See list in Table 1. below 1). Mid-merit power plants fill the gap between the peak load and base load. Load-following plants are typically in-between base load and peaking power plants in efficiency, speed of startup and shutdown, construction cost, cost of electricity and capacity factor.

Table 1. Types of power plants operating in Ireland in 2014

SEM Committee (2014) Generator Financial Performance in the Single Electricity Market. SEM/14/111. Available Online

Type Plant Name
Baseload Moneypoint Unit 1, Synergen
Mid Merit Tynagh, Coolkeragh, Moneypoint Unit 2 & Unit 3, Aghada CCGT, Huntstown 2, Whitegate, Kilroot 1 & 2
Peak Aghada (Units 1, CT Unit 1, CT Unit 2, CT Unit 4, Marina, Northwall Unit 5, Poolbeg Combined Cycle, Huntstown 1, Ballumford B Station, Ballylumford C Station, Rhode, Tawnaghmore Cushaling Power Ltd, Kilroot (KGT1-KGT4), Ballylumford (BGT 1 & 2), Tarbet, Great Island

As more and more wind is connected to the Irish electricity system the requirements for and the demands on load-following plant grow. This is because the natural variability of the wind increases the variability of the electricity demand (net of wind) 2. Thus there is more of a need to call on load-following plant and it has to respond faster.

Peaking power plants operate only during times of peak demand. In Ireland the demand for electricity peaks around the start and end of the working day. So, a typical peaking power plant will start up as demand ramps up to a peak and will shut down as the demand subsides. However, the duration of operation for peaking plants may vary from hours per day to less than a couple dozen hours per year. Peaking power plants include pumped storage hydro-electricity and open cycle gas turbine power plants.