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Power system transformation and flexibility |
production during a number of hours, charging can be adjusted to occur during that time. Similar applications are becoming available for large consumers.
Dynamic load curtailment. In contrast to dynamic load shifting, dynamic load curtailment reduces power demand during critical hours, without recovering this demand later. One example would be a temporary halt of production during times of very high electricity prices. This can be profitable for a consumer if the cost of electricity exceeds the value added of the process for several hours.
Structural reductions in electricity demand via energy efficiency. This can be achieved, for example, by replacement of less efficient lighting in residential buildings with lightemitting diodes (LEDs), which leads to a structural load reduction that is concentrated in hours of low or no sunlight. Hence, such measures naturally improve the match of load with the output profile of solar PV.
Structural increases in electricity demand via electrification. As introduced in the section on phases of system integration, at Phase 5 and beyond structural surpluses of VRE emerge. These are concentrated during the middle of the day for solar PV and during windy weather periods for wind power. In the absence of electricity demand during such periods, VRE output will need to be curtailed and the market price and value of VRE will be very low. Hence, increasing demand through electrification of industry, heating or transport can reduce curtailment, although Power-to-X can also provide viable options for system integration.
After reviewing these general changes in the roles of system resources, the next sections take a more in-depth look, differentiating centralised and distributed resources.
Implications for centralised system resources
The previous sections discussed the main drivers of change in the power system and identified flexibility as a critical enabler for power system transformation. The following sections now take a complementary perspective, highlighting in each case how traditional and new system resources can contribute to system flexibility from a technical perspective, and also identifying the underlying challenges and potential policy options.
Operational regime shifts for thermal assets
With increasing penetration of VRE and a broader range of system resources, conventional power plants may experience changes in their operational conditions. With their very low cost of operation, it is generally most economic for the power system to accept all VRE output when available, while turning off conventional plants with higher running costs and utilising available (and cost-effective) flexible resources such as grid infrastructure, demand response and storage resources. However, the remaining power plants that are still running need to be able to accommodate wind and solar PV generation to maintain the reliability of the system. In order to do this cost-effectively and reliably, conventional power plants need to possess the ability to operate flexibly, while also being available to cover periods of low VRE availability. Under these new operating conditions, it is possible that the operating hours and energy output of these conventional power plants may be reduced. The flexibility of power plants is determined by four main parameters:
Stable operating range: This refers to possible generation levels that can be chosen, given a long lead time. The minimum stable output of the power plant is the lower bound of the
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China Power System Transformation |
Power system transformation and flexibility |
stable operating range, while the maximum output is the upward constraint.25 The larger the stable operating range, the more operationally flexible the power plant.
Ramp rate: This is the speed at which output can be adjusted upwards or downwards within the stable operating range. Upward and downward ramping rates vary depending on the plant’s technical characteristics and the technical attributes of the control system.
Minimum up and down times: These are the time constraints within which generating units have to a) remain on line once they have been synchronised onto the system (minimum up time) or b) remain off line once they have been decommitted (minimum down time). These constraints are due to the technical limits as well as economic factors of conventional thermal generation technologies.
Start-up time: This is the required advance notice to make generation available, i.e. the time necessary to reach the minimum stable output level. The start-up time can be further categorised into cold, warm and hot start-up, which is based on the operational status of power plants at the time. These relate to the temperature of the plant and depend on the time between operation cycles (a thermal plant will remain hot for several hours after shutdown). Cold start-ups take the most time, while hot start-ups can be significantly shorter. During the start-up period a plant is not available to provide services.
There are numerous options for extracting additional flexibility from existing power plants. These range from simple operational improvements to more comprehensive investments in retrofitting. Updating operational guidelines and providing appropriate price signals are important to strike the right balance with additional costs resulting from increased wear-and tear or reinvestment. A number of operational improvements and retrofit options for conventional power plants are discussed in IEA (2018a). One example of a lignite-fired plant that has been upgraded to provide increased flexibility in Germany is highlighted below (Box 9).
Box 9. Improving flexibility parameters in legacy coal plants
RWE’s Neurath power plant in Grevenbroich, Germany, was designed as a baseload lignite-fuelled plant, with five units totalling a gross capacity of 2.2 GW commissioned between 1972 and 1975. Retrofits to its fourth and fifth units were completed in August 2012 by Siemens to fulfil the requirement for increased flexibility coming from higher shares of VRE in Germany and Europe more widely. The retrofit aimed to increase their (very) short-term flexibility characteristics (primary and secondary frequency control). This was implemented via advanced monitoring and control techniques (so-called advanced state-space unit control) and other technical interventions (condensate throttling, partial deactivation of heat pump preheaters and optimisation of the feedwater, aid and fuel controls). The plant’s technical capabilities have been significantly improved since the retrofit.
Source: Case study information provided by Siemens, March 2018.
25 Depending on the plant type and operation regime, there may be further types of minimum stable output, such as economic, emergency and environmental minimum outputs.
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Power system transformation and flexibility |
Table 6. Performance parameters per unit
Parameters
Minimum stable load
Maximum ramp-up
Maximum ramp-down
Primary frequency control (within 30 s)
Secondary frequency control (within 5 min)
Notes: min = minute; s = second; MW = megawatt.
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Performance |
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Performance after |
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before |
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intervention |
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intervention |
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|
|
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|
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MW |
|
440 |
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270 |
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MW/min |
5 |
15 |
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||
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||
MW/min |
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5 |
|
15 |
|
MW |
18 |
45 |
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||
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|
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||
MW |
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0 |
|
100 |
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|
|
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Matching VRE to system requirements
The output of VRE plants is limited according to the instantaneous availability of their primary resource, wind or sunlight. Consequently, it may appear illogical to consider VRE as a source of flexibility. However, within the boundaries of available wind and sunlight, modern VRE plants can be remarkably flexible. They can be started and stopped almost instantaneously because they do not require heat to be built up for their operation. They can be ramped over a wide range of outputs (up to the limit imposed at any moment by how much wind and sun is available) and this ramping can be carried out very rapidly (IEA, 2016a; REserviceS, 2013a, 2013b). Additionally, in some cases continuous low-magnitude curtailment can be considered as an additional flexibility option, allowing VRE plants to maintain both headroom and foot room in output. This can allow VRE plants to provide operating reserves, which ultimately raises the amount of VRE generation that can be integrated in the system.
However, these capabilities are currently not commonly used by system operators to obtain flexibility, for three main reasons. First, such operation requires direct and automated controllability of the VRE plants, which is not always implemented (yet). Second, VRE forecasting systems need to be established and very well tested in order to make sure that generation levels can be forecasted with sufficient accuracy. Third, regulatory and market arrangements need to allow for VRE participation and this usually requires changes to market design. For example, while it is generally not possible to forecast wind output sufficiently accurately a few days in advance, forecasts are very reliable for a few hours or even shorter horizons. This means that the system operators need to procure flexibility more frequently and closer to real time to unlock the contribution from VRE. Advances in digitalisation are further likely to improve forecasting accuracy.
A growing number of demonstration projects and field tests are validating the feasibility of large-scale VRE plants providing flexibility. Indeed, international experience demonstrates how a more optimised use of VRE plants can help to save costs in the power system (e.g. avoided fuel consumption from redispatching thermal plants) while reducing VRE curtailment (Box 10).
Compared to previous generations of VRE technology, system-friendly VRE can be used to reduce the need for more expensive flexibility assets. For this to happen, it may be necessary to review existing support mechanisms to provide the right incentives. Additionally, ancillary service prequalification requirements and dispatch parameters may have to be amended to allow for fair remuneration of VRE flexibility. The following chapter presents a more detailed discussion of the options available to increase technological diversity in the provision of flexibility.
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Power system transformation and flexibility |
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Box 10. Flexible operation of solar PV
The development of advanced control technologies and operational strategies for VRE technologies is improving the perception amongst utilities that VRE may indeed be part of the solution for system integration. A recent study commissioned by Tampa Electric Company (TECO), sponsored by First Solar and carried out by Energy and Environmental Economics, shows that the operation of PV plants in fully flexible mode can yield substantial operational cost savings, even at annual penetration levels as high as 28%.
The study evaluated four modes of operation:
Must-take: solar PV generates all of its potential output, with the grid operator meeting the resulting net load through dispatchable resources.
Curtailable: the option for unscheduled curtailment.
Downward dispatch: the option for curtailment scheduled in advance.
Full flexibility: both curtailment and upward dispatch are an option; this is enabled by maintaining constant headroom based on real-time maximum output estimation.
In short, the report shows that enabling a controlled level of de facto curtailment, to ensure headroom, can in fact increase the total average PV share of generation, along with having a positive effect on system savings. An additional finding is that PV in fully flexible mode reduces the operational value of storage (if they are both providing frequency control services). However, there is still a value for storage at high penetrations of PV due to increased balancing requirements, rising solar curtailment and system capacity value of storage.
Average generation and headroom per scenario
Average Generation and Headroom (% of Annual TECO Demand)
100%
90%
80%
70% Headroom
60%
50%
40%
Generation
30%
20%
10%
0%
Full Flexibility
Downward Dispatch
Curtailable
Gas Combined |
Gas Combustion |
Gas Steam |
Coal |
Solar |
Cycle |
Turbine |
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Source: Energy and Environmental Economics, Inc. (2018), Investigating the Economic Value of Flexible Solar Power Plant Operation.
From the policy maker’s perspective, there are two important considerations: 1) establishing clear guidelines on benchmarking; and 2) the PV operator’s incentive to switch to fully flexible mode from a fixed tariff with priority dispatch.
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