Overview of flexibility services in the GB power system

Flexibility is the ability of the power system to modify electricity supply and/or demand patterns to keep them in balance. Flexibility considers the variability and uncertainty in both electricity supply and demand. Historically, dispatchable (or controllable) power plants like coal, gas, and hydro provided the flexibility needed in the system. However, with the phase-out of fossil-based power plants and the increasing adoption of renewable energy generation such as wind and solar, the landscape is changing. These renewable sources introduce additional variability and uncertainty into the system, necessitating alternative sources of flexibility, particularly from the demand side.

Figure 1. Changes in the need and sources of flexibility

As the share of variable renewable generation increases, more effort and resources are required to maintain the operability of the power system. This includes, for instance, procuring more flexibility, proposing new flexibility services, and amending market rules to allow the procurement of flexibility in a more cost-effective manner. Meeting the growing need for flexibility via a Business-as-Usual approach (e.g. installing larger capacity of fast-responding back-up generators) is expected to lead to a significant rise in the cost of balancing the power system.

While flexibility ensures secure system operation at a lower cost compared to building new infrastructure, the cost of balancing the power system continues to rise, driven by the increasing share of variable renewable generation. Figure 2 shows the annual cost of balancing services procured by the GB electricity system operator. It illustrates three main categories: Response, which includes services aimed at providing frequency support to the power system; Reserve, which encompasses various types of reserve services including Short Term Operating Reserve (STOR), Negative Reserve, Fast Reserve; and Constraints which accounts for services to support reactive power and voltage, as well as transmission congestion management.

Figure 2. Cost of balancing the GB electricity system. The figure was created using data published by National Grid Electricity System Operator[1]. For each year the cost covers the period of April to March in the following year.

The total balancing cost has increased fourfold over five years, with particular increases in the cost of reserve and constraint.  Because of this increase in costs, there is a growing need for cost-effective alternative sources of flexibility. Energy storage and demand response initiatives can support the transition to a low-carbon energy system. For example, the integration of the electricity and heating sectors through optimal operation of “power-to-heat” technologies and thermal storage (in the form of hot water tanks, and also as thermal storage using the thermal inertia of district heating networks and buildings) enables greater flexibility in electricity demand for heating.

In order to address different system needs, the National Grid Electricity System Operator (NGESO) procures a range of different flexibility services. Some of the key flexibility services procured by NGESO are listed in Table 1.

Key technical and commercial information for the below flexibility services are summarised in the form of factsheets. These factsheets aim to help those researchers and stakeholders in the energy sector, who are not experts in power systems, to gain a slightly deeper understanding of the flexibility services and their intended purposes. The factsheets are also useful for potential future flexibility service providers.

EDRC aims to keep these factsheets up to date so that they reflect the latest developments.

Table 1. Summary of key flexibility services