7 Greenhouse gas savings from energy storage
Energy storage technologies applied in electricity networks can be used, for example, to:
- Reduce or remove the requirement for the spinning reserve usually provided by conventional fossil fuel fired power stations that is necessary to operate the electricity grid in a stable manner;
- Directly link to intermittent or variable renewable energy power generation plant and increase the value of the output, by allowing it to be sold when the value is highest rather than when it is generated;
- Directly reduce the output - and hence the emissions - of fossil fuel fired power stations through storing surplus electricity at times of low demand and subsequently providing output at times of high demand, avoiding the use of peaking plant that is often fossil-fuelled.
These options lead to different greenhouse gas savings. For example the greenhouse gas reduction potential of an electricity storage device embedded within an electricity distribution network would be dependent on the carbon intensity of the generators connected to grid. This could vary with time, the store replacing output from a coal plant or alternatively a gas turbine.
Taking the case of pumped storage storing electricity this is likely to take place when the output from wind generation is high. The carbon intensity of the electricity stored will be that of the grid at the time the pumping occurs. In early years this is likely to be more carbon intense than later in the period under study once wind, other renewables and possibly clean coal, have achieved higher penetrations. When the pumped storage is discharging i.e. generating, the converse is likely to be true; wind generation will be at relatively low levels within the mix. As a result the calculation of actual carbon savings is complex and will change over the period up to 2030. Hence assessment of potential carbon savings can only be indicative.
The main case for storage must be built around allowing renewable generation to generate when it otherwise might be constrained off and in allowing the system operator to maintain grid stability in the presence of significant percentages of both intermittent and base load plant.
If we assume a storage device that is being charged with wind energy alone (so is storing zero carbon electricity) and that it is removing the requirement for a coal fired plant to be operated in spinning reserve mode, the carbon displaced could be as high as 0.25 t CO 2/ MWh. If the stored electricity, again from wind alone, is assumed to displace carbon at the average carbon intensity of the UK grid in recent years the carbon saving would be around 0.12 t CO 2/ MWh. If the storage is displacing modern gas fired combined cycle plant the displaced carbon would fall to of the order of 0.05 t CO 2/ MWh.
Table 7.1.1: UK CO2 emissions per kWh for electricity production (Source DECC, 2009)
These are all optimistic savings because a storage device embedded in the network will be charged by the generation mix operating at the time and hence will have a carbon content that reflects that mix. This carbon intensity of the charging electricity must be subtracted from that of the mix on discharge (and also allow for the efficiency of the process) to get a true overall figure.
As renewable generation increases and coal fired generation decreases the average carbon intensity in the UK will fall. As part of the analysis for the Low Carbon Transition Plan DECC developed projections of UK generation capacity and output. Part of this was to project future carbon intensity of electricity generation.
Figure 7.1.1 Projections of Carbon Intensity of Electricity to 2020 ( DECC 2009b)
This suggests a fall from 2007 levels of 0.54kgCO 2/kWh to just over 0.3kgCO 2/kWh in 2020.
Given the argument in the first paragraph above, that the store will charge at times of high wind and discharge at times of low wind, there will generally be a carbon saving but it will reflect the difference in carbon intensities of the grid mix during charging and discharging which in many circumstances will be relatively small. In addition, as the grid is progressively decarbonised the potential for carbon savings will decrease, however the requirement for storage will become progressively greater for the reasons given above.
These arguments will apply to all electrical energy storage devices. For example, EDF are using a small store with an advanced network voltage control system to enable the connection of additional wind farms to an existing MV network ( ENSG, 2009). The system fine-tunes the substation source voltage according to the output of the generators, so preventing voltage-rise issues. A Lithium Ion battery storage system mitigates the intermittency of the wind farm output and enable further distributed generation to be connected. The carbon savings in this example will be related to allowing more renewable generation to be connected however the electricity this will displace is unknown and will vary with time. Until recently it was likely to be open cycle gas turbines or coal fired spinning reserve however for the reasons given above this is less likely to be the case as we move closer to 2020 and beyond.
The conclusion from the above assessment is that the carbon savings from the utilisation of storage on electricity networks will be positive but the calculation of precise carbon savings is both difficult and open to debate. In addition, as the overall network is increasingly decarbonised the carbon savings from operating storage will decrease; however the marginal savings i.e. the potential for storage to displace the most carbon intense plant remaining on the network (fossil fuelled peaking plant) will increase, unless a low or zero carbon peaking plant, such as biomass, is introduced. Overall, we do not believe the case for electricity energy storage should be built around carbon savings.
In conclusion, the greenhouse savings from storage will potentially be high if:
1) Significant levels of high carbon generation remain connected and in operation - i.e. non CCS coal
2) The average carbon intensity of electricity generation is significantly lower than the carbon intensity of 1) - as this will provide carbon savings.
3) There are significant periods when zero carbon electricity is available and requires storage - this will provide greater savings than 2).
These conditions hold true in all three of the Scenarios produced for this study. Hence energy storage offers potential carbon savings across the period to 2030.