System integration and optimisation
As we move towards a low-carbon energy system, the electricity sector has a vital role to play, but faces two key challenges:
- Ensuring system adequacy (enough energy is available to users)
- Maintaining grid resilience and security (energy is able to flow to users)
An example of how big a challenge this might be can be seen in the blackout of August 2019 across the UK, when a Combined Cycle Gas Turbines (CCGT) power station and an offshore wind farm went offline unexpectedly, resulting in a ‘grid trip’ that cut power to transport networks and nearly 900,000 people across the country. The large pumped storage facilities at Dinorwig and Festiniog immediately came online, followed by a rapid ramping up of other CCGTs.
Systems that depend highly on renewables need to balance this intermittency of energy source in several ways. These are:
- Demand-side response (effectively compensating customers for reduced supply) which brings few technical issues in the UK.
- Importing from interconnectors, which are well established in the UK.
- Smart-grid technologies, which bring the opportunity to achieve higher degrees of flexibility, but may also introduce a risk to system balancing if not properly coordinated.
- Large-scale energy storage, which presents considerable uncertainty.
Frequently commentators suggest that the response to the problem of intermittency in renewable generation is energy storage. However, there is no storage system that can address grid-scale shortages of power. For the avoidance of doubt: there is no battery, nor is there the prospect of such a battery, that could compensate for a prolonged (few hours) simultaneous drop in intermittent renewable generation at one or two major offshore wind farms.
Large amounts of storage might be achieved by aggregating a very large number of small storage volumes (electric vehicle batteries and user-owned batteries), but to do this, we need to implement a very sophisticated Smart Grid and development of a ‘market’ mechanism to compensate owners.
A system that depends on large energy storage to maintain a consistent power supply has a high degree of technical risk. The Net Zero scenario and reports put to the government do not fully understand the storage requirements of the system. Proven energy storage systems, such as pumped storage hydro, require suitable sites, which are not easy to find. Other large-scale physical systems (flywheels, dropping weights and compressed air caverns) could potentially be used as storage, but they haven’t been fully developed yet. When we produce hydrogen from electrolysis, this can act as an energy store, but its efficiency as power-to-power round trip is low.
As we said above, no currently available battery technology is capable of grid-scale storage, and neither do we have the prospect of such a battery.
Balancing the interdependencies between power generation, heat, transport, and industry, in a wider integrated system needs a guiding mind and coordinating body, otherwise known as an Energy System Architect.
Various energy-system models look to the optimal system make-up that delivers energy at the lowest cost possible. The Net Zero model is no different, and looks to identify ways with the least costs incurred to achieve the carbon-reduction goals. The modelling therefore relies on assumptions of costs of every element in the system.
Modelling must take account of ‘whole-system’ costs and, in order to achieve an optimal balance between different forms of generation, system costs should, wherever possible, be allocated to the technology that requires their support. Furthermore, the sensitivity of the results to changes in input assumptions must be clearly stated.
Take, for example, the Net Zero scenario, where intermittent renewables contribute 58% of electricity in 2050. The remaining 42% comprises mostly CCGT and bio-energy (both with CCS) and nuclear. The system-wide requirements and costs of supporting high levels of intermittent generation are the subject of much academic debate. Net Zero suggests that up to 40% intermittent power will incur system-integration costs of about £10/MWhr, at 50% penetration the cost may be £20/MWhr or more and this will rise as the proportion of intermittent generation increases.
System architecture needs to be developed on the basis of whole-system costs and designed to minimise the cost of energy delivered to the customer.