In our previous TerraBlog post, How CCAs can model and deploy DERs, we shared the process and results for modeling energy storage systems for CCA customers with existing solar systems under the NEM tariff. Part of this process included studying the impact of batteries on greenhouse gas (GHG) emissions of a CCA. In this blog post we will share how we modeled and assessed this impact.
Calculating the GHG emissions can get complicated as the electricity supply mix varies depending on the time of the day, geographical location and the demand on the grid.
One approach to calculating GHG emissions from electricity purchases is to use a constant value associated with electricity purchases from unspecified sources provided by the California Air Resources Board. The constant value, also referred to as the grid GHG content value, represents the emissions associated with power purchases outside a CCA’s contracted emission free sources. For the analysis, we used the GHG content for PG&E territory of 0.435 tCO2/MWh.
Another approach to calculating GHG emissions is to use the proposed Clean Net Short (CNS) method suggested by the California Public Utilities Commission. The CNS method utilizes standard reference tables with multipliers for every hour of a month to account for the grid GHG content. The reference tables provide an estimate of the grid GHG content for any location in the state of California. The CNS method is different from the current method because the tables display data using the 288-hourly format. The 288 hourly format is derived from the product of typical 24-hourly values in any given day for each month of the year. For example, the average grid GHG content on any day at 2pm in the month of January is estimated to be 0.2 tCO2/MWh per the following reference table for the year 2022.
TerraVerde has utilized both methods within the BEO software to compare the emissions impact of batteries for CCAs. It was found that using the current method, the GHG emissions increase after modeling the energy storage systems. Addition of an energy storage system to a building increases the electricity usage of the building due to the round-trip losses in the energy storage systems. A typical energy system has a round-trip efficiency of 86% which means 14% of electricity is lost during a charging and discharging cycle. Thus, when using the current GHG emissions accounting method, there is an increase in GHG emissions as only the annual electricity usage is taken into account. However, when we include the time of electricity usage, using the CNS method, the GHG emissions are reduced as a result of pairing the energy storage systems with existing solar systems. The GHG emissions reductions is achieved as a result of charging the batteries using solar electricity exported to the grid during the day when the grid GHG content is low and discharging the batteries in the evening when the grid GHG content is high.
As seen from the results of the BEO software, batteries for CCA customers with existing solar systems can provide both economic and GHG reduction benefits. In our next post we will share the potential impacts of batteries to Resource Adequacy costs for CCAs.
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