**1. Introduction**

Today, ensuring the energy delivery that societies need for productivity, economic growth and well-being is crucial. Since industrialization, the world is experiencing an increase in human population and energy demand per capita, and this has led to a rapid increase in carbon dioxide (CO2) concentration in the atmosphere from 280 ppm (parts per million) to over 400 ppm. [1]. Curbing a further increase in carbon emissions is one of the major challenges of this century as discussed in the negotiations culminating in The Paris Agreement in 2015 [2]. This agreement to address climate change was the first that was signed by 195 countries, and its overall aim is to keep the global temperature below 2 ◦C above the pre-industrialization level, encouraging additional efforts to limit global warming to below 1.5 ◦C. As a consequence, all parties of the United Nation Framework Convention for Climate Change (UNFCCC) act to reduce greenhouse gas (GHG) emissions to the atmosphere though a range of measures, among which decarbonizing electric grid systems plays a prominent role.

Worldwide, increasing and joint efforts have been put to analyse the possible energy transition pathways towards renewable energy sources, assessing their technical feasibility, environmental impacts, and energy implications, dating back to the mid-1970s [3–9].

Assessing the full environmental impacts and the energy burdens of key electricity generation technologies such as solar photovoltaics, wind and nuclear is crucial because although they are almost "zero carbon" at their use-phase, there are still impacts associated to their manufacturing, which should also be taken into the account. A fundamental framework that addresses the cradle-to-grave impacts of human-dominated systems and services is the Life Cycle Assessment (LCA) methodology, which characterises and quantifies all the life-cycle stages from raw material extraction to processing, distribution, manufacturing, operation and decommissioning.

It is also worth noting that it is important to assess each electricity grid as a whole, including all the electricity generation, transmission and storage technologies, and estimate the associated overall environmental impacts and energy implications, as discussed in some recent studies [10–17].

That is because the respective impact of each electricity grid depends on specific conditions such its composition, location, as well as on the local demand profile, and on the required amount (and type) of energy storage.

The state of California in the U.S.A. has established one of the most ambitious plans to bring about an energy transition from fossil fuel generation technologies towards renewable energies, with an intent to generate 60% of its electricity demand using renewable energies by 2030, and 100% thereof using a mix of "zero carbon" sources–including renewables and nuclear—by 2045 [18]. This plan also aims to reach a 40% reduction in GHG emissions below 1990 levels by 2030, and an 80% reduction by 2045.

Such rapid increase of renewable energy penetration in the California electricity grid is expected to require energy storage systems because of the intrinsic intermittency of renewable generation profiles. Specifically, at high penetration, increased photovoltaic (PV) installation is synergistic with storage technologies, which play a critical role in deep decarbonization scenarios, as discussed in recent publications [19–22].

According to a study prepared by the National Renewable Energy Laboratory [23], even with optimal grid improvements, California would still need an estimated 15 GW of additional storage just to reach 50% solar generation by 2030, which is more than 11 times the amount of storage currently mandated in California, and 66 times the total storage power deployed in the U.S. in 2016. This implies that energy storage will continue to be a main ingredient in the mix of strategies to balance supply and demand, support the California Independent System Operator (CAISO) in maintaining grid stability, avoid voltage and frequency imbalances, and support the state's transition to a renewables-centric energy infrastructure [24,25].

For instance, the technical feasibility of utility PV systems plus battery energy storage as an alternative to gas peakers in California is assessed in Roy et al. [26]. Their findings show that a 50 MWAC PV system with 60 MW/240 MWh battery storage can provide more than 98% capacity factor over the target 7:00–10:00 p.m. period, with lower lifetime cost of operation (LCOO) than a conventional combustion turbine natural gas power plant. LCOO includes installed costs, fixed and variable operation & maintenance (O&M), fuel costs as well as other policy factors such as tax credits/incentives.

As highlighted in the recent literature, there is also a need to assess the environmental impacts associated with the increasing energy storage technologies in combination with renewables in the electricity grids in order to better understand and mitigate them [27].

In a recent publication, Raugei et al. [28] estimate the incremental energy and environmental impacts of adding lithium-ion battery (LIB) storage capacity to photovoltaics. Such analysis shows that the energy payback time (EPBT) and life-cycle global warming potential (GWP) increase by 7–30% (depending on storage duration scenarios), with respect to those of PV without storage, and thus the benefits of PV when displacing conventional thermal electricity (in terms of carbon emissions and energy renewability) appear to be only marginally affected by the addition of energy storage. However, a generalized grid mix was considered in that study, and curtailment and storage figures were assumed without the support of specific historical data. The actual energy and environmental impacts of energy storage in real-world application scenarios will also depend on the specific storage quantities, types and use strategies [29].

In large renewable energy penetration scenarios, there is also a need for analyses at the whole grid level—taking into the account the specific electricity grid mix composition—with an accurate quantification of storage demand and curtailment, which should be informed by detailed hourly generation profiles. It is crucial to identify each hourly mismatch between the demand profile and actual generation provided, especially during peak hours. The importance of such mismatch was also highlighted in 2013 by CAISO in their published chart [30], famously nicknamed "the duck curve", which has since become part of common terminology for describing the effects of large-scale deployment of solar photovoltaic power into the electric grids. The curve shows the difference in electricity demand and the amount of available solar energy throughout, considering a 24-h period in California during springtime.

In light of all of the above, the decision was made here to collect full hourly electricity generation and demand data for California [31], and then use such data as the basis for modelling the amount of energy storage that will be required to minimize the reliance on natural gas and imports when larger quantities of renewables are deployed into the grid. Specifically, the aim of this study is to quantify the life-cycle environmental and energy burdens associated with the current (2018) composition of the electricity grid in California (in terms of greenhouse gas emissions, total and non-renewable cumulative energy demand, and energy return on investment), and compare them to those for a prospective grid mix in 2030, defined so as to achieve 80% of domestic renewable electricity generation, with a suitable amount of storage informed by the detailed hourly generation and demand model.
