Balancing. Matching electricity supply precisely to consumer demand on a second-by-second basis is the overriding principle of operation of any large-scale electricity network, the report notes. Failure to do so within tight limits will result in shutdown of the network. To achieve this, grid operators look to balancing services.

There are three main types of balancing services for the grid, covering both demand and generation: frequency response, fast reserve, and short term operating reserve (STOR). Balancing services are essentially divided into two main types: those that can be applied quickly and automatically, termed “response” services; and those that can be dispatched and sustained for longer periods, termed “reserve” services.

Reserve services deal with variations in electricity demand and actual out-turn of variable generation output against forecasts, as well as to cover unplanned events such as generating plant failures.

Of the reserve services, Fast Reserve acts as a conduit between Frequency Response and STOR. This can be required in order to aid system recovery or to cover predictable but dramatic changes in demand. Balancing services must be made available and delivered in specified minimum quantities and timescales.

Supply and demand balancing. Source: Bucks for Balancing. Click to enlarge.

Plug-in vehicles for grid balancing. With appropriate control of a large pool of plug-in vehicles, the report says, the net power flow within the grid can be influenced with a similar effect to that of conventional balancing measures. For plug-in vehicles, there are two possible modes of operation:

• Demand-Side Management (DSM). Demand-Side Management (DSM) in the context of plug-in vehicles is simply the interruption or reduction of recharging when required to ease grid imbalances, and the resumption and completion of recharging at a later time.

  Once fully charged each vehicle can no longer participate in the balancing service as it has no further load available for disconnection from the grid. Similarly, once a vehicle with a depleted battery comes within the time period required for recharging prior to its next road use, it too can no longer provide a demand management balancing service as it needs to remain connected to fulfil its primary purpose as a road going vehicle.

  This mode of balancing provision requires no extra investment from the vehicle owner, but is clearly limited in the extent of service that can be provided to the grid.

  An alternative DSM approach (not explored in the report) is for vehicle charging to take place at a power level below the charger’s maximum, allowing some headroom to increase as well as decrease power in response to dynamic high and low system frequency grid balancing needs.

• Vehicle-To-Grid (V2G). V2G enables a transfer of energy from the vehicle battery to the grid. Subject to the availability of the hardware required to allow bi-directional power flows, and appropriate control mechanisms, the vehicle’s battery could be made available while connected to the grid as an energy buffer for balancing services, rather in the same manner as pumped storage hydro.

National Grid provided detailed technical and economic data for the research, including a potential generating mix scenario for the year 2020 that would enable the UK to meet its climate change targets based on, for example, a large shift towards renewable power provided in the main from wind farms. In addition to this the company also provided data on the expected daily and seasonal patterns of demand for the various types of grid balancing services it expects to require at this time, together with their commercial value.

Using these data and a range of the potential market penetrations of electric plug-in vehicles in the UK market by 2020 together with typical vehicle usage and driver behaviour patterns, Ricardo created a grid simulation model which was designed to model the impact of electric vehicle charging based on a five-minute time step throughout the day.

Modelling assessed the two possible modes of balancing interaction with the grid in Great Britain in the year 2020: DSM and V2G. Ricardo performed two distinct modelling activities in order to analyse the opportunity for grid balancing from two different viewpoints:

• That of a grid operator or service provider, covering the macro-level effects of a large number of vehicles in use with an assumed variation in recharging patterns across the entire plug-in vehicle parc: the grid-level model.

• That of an individual vehicle user or owner, covering specific recharging patterns in order to investigate how revenue could be maximized: the individual vehicle model.

Key findings of the research include:

• Using demand side management alone, the projected fleet of plug-in electric vehicles in 2020 would be able to provide an average of 6% of daily GB network balancing service requirements. This rises to a maximum of 10% in the evening and overnight.

• Demand side management would provide a modest annual financial return to the individual vehicle owner of approximately £50 (US$82) for zero investment (effectively the equivalent of an 18% saving on recharging costs).

• Vehicle-To-Grid (V2G) based grid balancing was shown to provide significantly greater revenue on an individual vehicle basis—ranging from approximately £600 (US$980) per year for a 3 kW system to in the region of £8,000 (US$13,000) per year for a 50 kW three phase installation. However, the very significant capital cost of a vehicle based bi-directional power interface and the balancing market size limitations that would restrict the value of the service if implemented fleet wide, would serve to render the fleet scale roll-out of the V2G balancing service uneconomic.

• V2G operation may however be attractive for owners of captive vehicle fleets such as industrial or local delivery vehicles, battery exchange depots or aggregated batches of life expired vehicle batteries, where interface costs might be shared across multiple vehicles or battery packs.

• With the increased requirement for grid balancing services arising from the changing dynamics of the generation mix, plug-in vehicles could be made to work in synergy with the electricity market to help balance supply and demand, so reducing the reliance on conventional generation for the provision of these services; hence this has the potential to reduce CO₂ emissions.

It is clear from this study that there may be an opportunity for the plug-in vehicle parc to provide a new source of balancing services. PIVs may be able to work in synergy with the electricity market to smooth the daily demand profile by providing services in a manner that is not currently available to the system operator, and thereby reducing the need to meet additional balancing requirements by simply running more ‘conventional’ generation and potentially incurring additional CO2 emissions.

...The realization of the opportunities presented by wide-scale use of vehicle charging demand management would require the implementation of smart metering technology as well as mechanisms for the effective fleet level aggregation of plug-in vehicles...A key enabler for the success of V2G-based grid balancing is the development of an efficient charging infrastructure, appropriate to the needs of individual or fleet-based vehicles in order to maximize vehicles’ availability for grid balancing throughout the day. Also worthy of further investigation are opportunities for lower cost inverter systems for bi-directional power flow, which could considerably improve the economic case for V2G-based grid balancing.

Finally, while the limited size of the grid balancing market may make development of mainstream V2G-capable battery management systems difficult for OEMs to justify, the very significant potential revenue streams available from this mode of operation are likely to offer incentives for implementation on a smaller scale in collaboration with captive fleets or vehicle aggregators.

—Bucks for balancing

Resources

• Bucks for balancing: Can plug-in vehicles of the future extract cash – and carbon – from the power grid?