Integration of flexible consumers in the ancillary service markets
Introduction
The renewable energy sector is the fastest growing power generation sector and is expected to keep growing over the coming years [1], [2]: the global share of non-hydro renewables has grown from 2% in 2006 to 4% in 2011 and is predicted to reach 8% in 2018 [2]. Many actions have been taken all over the world to increase the penetration of renewables: in the US, almost all states have renewable portfolio standards or goals that ensure a certain percentage of renewables [3]; similarly, the commission of the European Community has set a target of 20% renewables by 2020 [4].
A number of challenges arise as the penetration of renewables increases. Many renewable sources are characterized by highly fluctuating power generation and can suddenly increase or decrease production depending on weather conditions. A recent example of this phenomenon took place Denmark on October 28, 2013 where a large number of wind turbines were shut down because of a storm. This caused a decrease from a level where more than 100% of the Danish electricity consumption was covered by wind to a level less than 45% in just 2 h,1 see Fig. 1. Such rapid production changes can imply severe consequences for grid stability due to the difficulty of accurately predicting the timing of the events [6].
Further, as more renewables are installed, the conventional generators are phased out: in Denmark, the increase of renewables during the last years has caused a petition for shutting down 8 central power plants [7]. This, however, causes another major challenge because the central power plants currently are the providers of system stabilizing ancillary services. As the conventional power plants are replaced with renewables, the ability to provide ancillary services in the classical sense is lost as the renewables usually do not possess the ability to provide such system stabilizing reserves: First of all, keeping renewables in reserve will entail that free energy is wasted making this a very expensive solution. Second, the highly fluctuating nature of the renewables caused by weather conditions can make it difficult to deliver a well-defined power response.
It is therefore evident that alternative sources of ancillary services must be established as renewables replace conventional generation. One approach to obtain ancillary services is to purchase reserves in neighboring countries; however, this requires that transmission line capacity is reserved for the reserve markets which will limit the capacity in the day-ahead spot markets and thereby possibly cause higher electricity prices [7]. Further, the ENTSO-E (European network of transmission system operators for electricity) grid code sets limits on the amount of reserves it is allowed to exchange internationally [8].
An alternative approach to obtain alternative ancillary services is the smart grid concept, where local generation and demand-side devices with flexible power consumption take part in the balancing effort [9], [10]. The basic idea is to let an aggregator control a portfolio of flexible devices such as thermal devices, batteries, pumping systems etc. Hereby, the aggregator can utilize the accumulated flexibility in the unbundled electricity markets for primary, secondary, and tertiary reserves, on equal terms with conventional generators [11], [12].
In this work, we identify the difficulties of including flexible consumption devices in the existing ancillary service markets and propose a method for better integration of this type of devices.
Section snippets
Scope and structure of the article
The increase of renewables and shutdown of central power plants call for alternative sources of primary, secondary, and tertiary reserves. This work proposes a method for making better conditions for flexible consumption devices to deliver these services. The method is valid for both the primary and secondary reserve, but not for the tertiary reserve, as will be come evident later. For the following reasons, we still believe the method is most relevant.
The first reason is that flexible
Architecture
For many consumption devices, the flexibility is too small to make isolated bids into the electricity markets; for example, the threshold for primary frequency control reserves is 300 kW in Western Denmark [19] while the capacity of a domestic flexible consumption device is in the magnitude of a few kW at most. Only certain very large consumers such as large pumping facilities, heating elements for combined heat and power plants, etc. will be able to reach the minimum threshold. For this
Flexible consumption devices and storage devices
In this section, we present a model that describes a portfolio of flexible consumption devices managed by an aggregator. The model is very simple but captures characteristics in focus in this work: power and energy limitations and inaccurate knowledge of the consumption baseline.
Ancillary service markets
We limit our focus to the active power ancillary services although other ancillary services exist. The active power services are denoted primary, secondary, and tertiary reserve as previously mentioned. In ENTSO-E's network code on load-frequency control and reserves, the terminology used for these services are frequency containment reserve, frequency restoration reserve, and replacement reserves [8], [36]. These terms describe the functionality of the reserves in case the system frequency
Ancillary services by flexible consumers
The limiting factors for conventional generators to provide ancillary services are their power limitations, the startup time, and ramping limitations. Generally, the energy capacity of a conventional generator is a non-issue: the generator will be able to continuously produce both minimum and maximum power simply by using more or less fuel.
For flexible consumers the situation is completely different. Consumption devices will typically hardly have any rampling limitations and have a very low
Proposal of market interaction
In the previous section we have illustrated three major issues of using a portfolio of flexible consumption devices as providers of ancillary services. The first two issues deal with the energy limitation and the uncertain baseline. The third issue illustrates how combined deliveries can lead to suboptimal market operation.
In this work we propose the following approach to improve the possibility for flexible consumers to participate in the fast ancillary service markets.
Proposal. Operational
Numerical results
In this section, we present a number of numerical results that illustrate the benefit of allowing continuous operational schedule adjustments according to the proposal in Sec. 7. First, we illustrate the overall concept; following, we illustrate how the method is able to handle both energy limitations and inaccurate baseline predictions.
Conclusion
In this paper we considered an aggregator that provided ancillary services based on a portfolio of flexible consumption devices. We proposed a method where the aggregator was allowed to continuously adjust its operational schedule and hereby restore the energy level of the flexible consumption devices. This made it possible to utilize flexible consumption devices with energy limitations and with inaccurate baseline predictions to participate in the ancillary service markets to a much larger
Acknowledgments
The work is completed as a part the iPower project which is supported by the Danish government via the DSR-SPIR program 10-095378.
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