The island of Ireland functions as a single electricity grid linked to the British mainland by two interconnectors with a combined capacity of 1 GW. The Republic of Ireland in the south has set a goal to have 40% of electricity generated from renewables, mainly onshore wind, by 2020. Variable intermittency will be balanced using frame type combined cycle gas turbines (CCGTs). As the level of wind penetration grows the CCGTs need to work harder ramping up and down to compensate for variable wind. This causes increased wear and tear on the CCGT plant and also significantly reduces the energy efficiency of the CCGTs raising their specific CO2 production. During 2014 and 2015, average wind penetration was 22%, the CCGTs produced 575 Kg of CO2 per MWh and the average fuel efficiency was 32% compared with a design specification of 55%.
Guest post by Maria Tsagkaraki and Riccardo Carollo, Incoteco (Denmark) ApS. Data Source: EirGrid
The island of Ireland, divided politically between the Republic of Ireland and Northern Ireland, is an example of an electricity island, having limited interconnections with Great Britain only. Those are HVDC submarine cables, the Moyle Interconnector and the East West Interconnector, with a rated power of 500 MW each. A new interconnection between Ireland and France, the Celtic Interconnector, has been discussed by the TSOs of both countries involved, EirGrid and Rté respectively, but the project is still under development and its installation is unlikely to take place before 2025 (EirGrid, 2014).
Ireland has the ambitious target to increase the use of renewable sources in the electricity sector, in order to decrease the dependence from imported fossil fuels and reduce CO2 emissions. For the year 2020 both the Republic of Ireland and Northern Ireland have set the target for 40% of renewable electricity in the electricity sector (EirGrid, 2014), (SEAI, 2014), almost exclusively (37%) coming from wind power (Deaneb, Leahy, & Garriglea, 2013). In 2015, Ireland generated just over 23% of its electricity demand from renewables, the vast majority of which came from onshore wind power (Burke-Kennedy, 2015). This represents one of the highest shares of wind power in the world and the highest for an electricity island (Burke-Kennedy, 2015).
Because of the limited interconnections, as already mentioned above, the integration of a large fraction of renewable energy sources is expected to bring new challenges for the transmission system operator and the electricity customers.
All modern power grids require demand and supply to be constantly in balance, so that the grid frequency can be held within the limits of 49.9 and 50.1 Hz (Xu, Østergaard, & Togeby, 2011). The complexity of this task increases when higher shares of intermittent power sources, such as wind power, are integrated into the network. The SNSP (System Non-Synchronous Penetration) issue, being addressed by EirGrid, is to ensure system stability during periods of high wind penetration (Dudurych, 2014). The SNSP limit is being imposed by the TSO in order to prevent excess percentages of total generation by any non-synchronous sources at any time (Deaneb, Leahy, & Garriglea, 2013).
To synchronize production and demand, part of the excess generated energy can be accumulated with the use of an energy storage technology and re-injected into the grid when needed. The island of Ireland has almost no electricity storage s, except for the 292 MW Turlough Hill Pumped Storage Generator. We found that this installation has a poor average efficiency (around 54%), when compared to modern pumped hydro installations or contending storage technologies.
From the analysis of the charge/discharge patterns of the last years, (pumping at night, discharge during late afternoons) it appears that this is being used mainly to trade electricity exploiting the day/night price difference in Ireland electricity market, rather than to absorb wind power fluctuations. However, we were told by an EirGrid spokesman that it is being mainly used to supply primary and secondary reserve.
At present, the Irish Combined Cycle Gas Turbine (CCGT) fleet is used to fill in the gaps left by the intermittent operation of wind power plants (Clancy, Gaffney, Deane, Curtis, & Ó Gallachóir, 2015). The CCGT fleet was originally designed to achieve high efficiencies with a nearly constant output. Because of the variable operation of these power plants, the average efficiency of the CCGT fleet is dramatically lower than the nameplate 55% of the individual new power plants. The frequent start up and shut down operations required to match the system demand with high wind power share in the network affects the plant’s efficiency and increases the wear out of the plants components. The efficiency is also affected by operating the plants below 80% of their rated capacity; a critical limit below which the efficiency drops dramatically.
A direct result of this way of operating the CCGT fleet is an increase of the CO2 emissions for each MWh generated by the gas turbines. While the increased wind penetration successfully reduces Ireland’s whole system specific CO2 emissions (kg/MWh), the values of the specific CO2 emissions of the CCGT fleet analyzed in this work are over 500 kg/MWh.
The main aim of this study is to establish how the specific CO2 emissions of Ireland’s fleet of frame-type CCGTs, and therefore the efficiency of the fleet, responds to the increased ramping, starting and stopping required by the fleet to balance wind power. The results throw light on the expected performance of incumbent, frame-type CCGTs in Great Britain (GB) where wind penetration, by TWh, is expected to be greater than 20% by 2020, up from 10% in 2015. GB’s fuel mix is sufficiently similar and its dependence upon CCGTs to balance wind is identical to that of Ireland.
This study analyzes the specific CO2 emission of the incumbent CCGT fleet as wind penetration varies from month to month in the Irish system. The analyzed years are 2014 and 2015 (23 months, from January 2014 until November 2015).
The EirGrid database was used for gathering all the relevant data for the analysis (http://www.eirgridgroup.com & http://smartgriddashboard.eirgrid.com). EirGrid group has been operating the national high voltage electricity grid in Ireland since 2006. The primary goal of EirGrid group is to deliver reliable and secure supply of electricity in the system (EirGrid). It is consists of a group of organizations (EirGrid, SONI and SEMO) in order to achieve better results.
The SEMO database was also used for further pricing data (http://www.sem-o.com). SEMO is a Single Electricity Market Operator for the island of Ireland and runs the whole electricity market.
2.2 SNSP (System Non-Synchronous Penetration)
In order to calculate the SNSP limit the following equation was used (Dudurych, 2014):
The SNSP limit was 50% until October 2015 and afterwards increased to 55%.
2.3 Calculation of CCGT efficiency
In order to calculate the CCGT efficiency for the Irish fleet, the heat of combustion and the CO2 emission factor for natural gas have been used. Natural gas used for power generation is very rarely pure methane (CH4). Since 95% of the Irish gas consumption comes from the UK through the UK-Irish interconnector, the UK heat of combustion values and the official UK CO2 Emission Factor (CEF) have been selected for this work (World Nuclear Association, 2016). The average heat of combustion for natural gas results then to be 39.5 MJ/m3, while the CEF accounts for 51 g/MJ. Based on these values, a CCGT unit with a 55% nameplate efficiency µ would have CO2 emissions E equal to 335 kg/MWh. Based on the measured CO2 emissions, it is easy to calculate the CCGT efficiency by using the following equation:
As a numerical example, 500 kg/MWh average CO2 emissions would indicate a CCGT plant with an average efficiency of (0.55*335)/(500) = 37%.
3.1 Main Findings
From the analyzed years of the Irish system, the main and important outcome of this study is that the increased wind penetration can successfully reduce the specific CO2 emissions of the whole system. Figure 3.1 presents the rates of CO2 emissions in Ireland compared to the wind penetration for years 2014 and 2015. It is obvious the sharp decrease of the specific CO2 emissions when the wind output increases.
Figure 3.1 CO2 emission per MWh, whole system, wind penetration 22%, 2014-2015.
As already mentioned, the Irish system relies on CCGTs for the grid stability of the system due to the increased use of wind power. The CCGT technology was originally designed to run with an efficiency of 55% or more, which corresponds to around 335 kg/MWh CO2 emissions. Table 3.1 shows the huge differences in the specific CO2 emissions of the CCGT fleet on a monthly range through the years 2014-2015. The frequent starting and stopping of CCGT plants affects their overall efficiency, resulting in increased CO2 emissions.
Table 3.1 Monthly results of the CCGT fleet’s CO2 emissions and wind penetration, years 2014-2015.
In March 2014 the specific emissions of the fleet reached 693 kg/MWh CO2 emissions with a HHV fleet efficiency of 27%. In April 2014, the CCGT specific emissions of the fleet were even higher, 773 kg/MWh CO2 emissions, which corresponds to a HHV fleet efficiency of only 24%. These extreme values can be explained due to a temporary loss of the most efficient CCGT plants during that period. Aghada CCGT (commissioned in 2010), Huntstown CCGT (commissioned 2007) and Coolkeeragh CCGT (commissioned in 2005) are the most efficient Irish CCGT plants and were offline for the entire month of April, as it can be seen in Figure 3.2.
On the other hand, Dublin Bay (commissioned in 2002) and Whitegate CCGTs have been operated at approximately the same capacity factor during both months, 90% and 60% respectively. In March 2014, Dublin Bay provided 37% of the entire CCGT fleet energy production, while this value rises to 46% for April 2014. Poolbeg and Tynagh CCGTs showed a decrease in the capacity factor from March to April 2014, while the Great Island CCGT still had to be commissioned (commissioned in the middle of April 2014). Figure 3.2 can better explain the extreme values of CCGT CO2 emission fleet in March and April 2014.
Figure 3.2 Capacity Factor of the Irish CCGTs plants, March and April 2014.
Figure 3.3 CCGT Fleet, CO2 Emission and Wind Penetration, for years 2014-2015
For the analyzed years 2014 and 2015, the average CCGT fleet CO2 emission was 575 kg/MWh with an average wind penetration of 22%. This mean that the average HHV fuel efficiency was 32%, as it can be seen from Figure 3.4. In 2015, the average CCGT fleet CO2 emission was 540 kg/MWh and the average wind penetration was 23%, which corresponds to 34% average HHV fuel efficiency. The results for 2014 appeared even higher CCGT CO2 emissions and lower HHV fuel efficiency. The CCGT fleet CO2 emission was 607 kg/MWh, the HHV fuel efficiency was 30% and the average wind penetration was 20%.
The results are better being presented in Figure 3.3. Figure 3.4 illustrates the CCGT HHV fuel efficiency compared to specific CO2 emissions, which appears a surprising low HHV efficiency of CCGT plants when they reach 600 kg/MWh CO2 emissions.
Figure 3.4 CCGT fuel HHV Efficiency vs Specific CO2 Emissions, kg/MWh.
As it can be seen from Figure 3.5 the CCGT fleet output decreases with increased wind penetration, whereas the CCGT fleet specific CO2 emissions rise. On the other hand, with almost no wind power, the CCGT CO2 emission fleet is close to 400 kg/MWh, which corresponds to 46% HHV efficiency.
Figure 3.5 CCGT CO2 Emission Fleet, CCGT and Wind Output, November 2015.
Figure 3.6 illustrates the connection between the specific CCGT CO2 emission fleet and wind penetration. An example month, November 2015, can verify that the specific CCGT CO2 emissions rise as wind penetration increases. The same results appear during all analyzed months of years 2014 – 2015, especially those months with high wind penetration.
Figure 3.6 CCGT fleet, CO2 Emission and Wind Penetration, November 2015
Figure 3.7 (example November 2015) shows that system emissions decline as wind penetration increases. However, the specific emissions of the non-wind generation rise as wind penetration increases, as it can be seen in Figure 3.8.
Figure 3.7 Specific CO2 emission per MWh, whole system demand, t/MWh, wind penetration 30%, November 2015.
Figure 3.8 Specific CO2 emission per MWh of non-wind generation, t/MWh, wind penetration 30%, November 2015.
3.2 Side Findings
The SNSP (System Non-Synchronous Penetration) issue, being addressed by EirGrid, was also analyzed for the years 2014-2015 in order to check the system stability during periods of high wind penetration. The increased SNSP is being achieved due to curtailing wind power and altering the ROCOF settings of rotating generators. An example of SNSP rate is being presented in Figure 3.9. As a reminder, the SNSP was raised to 55% in October.
Figure 3.9 System Non-Synchronous Penetration (SNSP), Wind penetration 30%, November 2015
In Ireland there is one storage system, the Turlough Hill Pumped Storage Generator. It has a capacity of 292 MW and we found that its efficiency is around 54%. Turlough Hill is supplying primary and secondary operating reserve for the Irish system. Figure 3.10 illustrates the pumping and generation of Turlough Hill for years 2014 and 2015, while Figure 3.11 presents an example month (September 2014) for a more clear picture of Turlough Hill’s operation.
Figure 3.10 Turlough Hill Pumped Storage Pumping and Generation, years 2014-2015
Figure 3.11 Turlough Hill Pumped Storage Pumping and Generation, September 2014
Ireland has the highest wind penetration of any electricity generation system operated as an electricity island in the world, with a target of 40% by 2020. Ireland uses CCGTs plant for balancing the system. This means frequent, stochastic starts, stops and ramping of CCGTs power plants, which results in increased specific CO2 emissions and wear and tear costs of the plants.
For the analyzed years 2014 and 2015, the average CCGT fleet CO2 emission was 575 kg/MWh with an average wind penetration of 22% and an average HHV fuel efficiency of 32%. In 2015, the average CCGT CO2 emission was 540 kg/MWh (average fuel efficiency 34% HHV) with an average wind penetration of 23%. The CCGT CO2 emission in 2014 was 607 kg/MWh with an average wind penetration of 20% and a HHV fuel efficiency of 30%).
The main conclusion of this study is that wind balancing and infill power generation is far more costly than is generally believed at high wind penetration. The results throw light on the expected performance of incumbent, frame-type CCGTs in Great Britain (GB) where wind penetration, by TWh, is expected to be greater than 20% by 2020, up from 10% in 2015. GB’s fuel mix and weather conditions are sufficiently similar and its dependence upon CCGTs to balance wind is identical to that of Ireland.
The findings of this study should be also relevant to other “high wind” electricity islands that rely mostly on CCGTs for system balancing and provide some guidance for planning alternative and more economic technologies for infill and balancing power. Alternative methods for providing infill and balancing power need to be implemented, including more flexible thermal units and electricity storage. Incoteco is working with these alternative solutions.
A novel, thermally efficient CCGT has been developed by Incoteco and is available commercially with the main characteristics as follows:
47 % HHV (51.2% LHV) efficiency achieved 15 minutes after cold start up
- Hot starts and stops in 10 minutes (synchronous power in 5 minutes)
- Robust to multiple starts and stops
- Low wear and tear costs
- No water consumption
- Fully field proven components
- Can be an operated unmanned
- Lower capex than a ”flexible” frame-type
- Two years from Financial Investment Decision to commissioning (Austell, Hanstock, & Sharman, 2015).
Another solution could be low cost, robust “power” battery power storage for fast frequency reserve, which is becoming commercially available. At the end of 2016, low cost robust “electricity storage” battery will be also commercially available in MW quantities (by Q4 2016).
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