Seasonal storage is defined as being able to accommodate yearly cycles in electricity demand and generation from variable renewable sources.
Typically required properties for seasonal storage are low losses and costs, large storage volume, and an acceptable cycle efficiency — criteria that can be met by synthetic fuels. DNV GL focuses on a renewable electricity-to-electricity solution.
The Promise of Seasonal Storage report calculates the levelised cost of energy (LCoE) for different seasonal storage options in a VRES scenario in 2050, based on the Netherlands. This scenario comprises 42GW of wind, 39GW solar PV and 40GW centralised dispatchable generation. The seasonal storage typically charges in summer, when there is a surplus of solar generation, and discharges in winter, when electricity demand is higher. To calculate the LCoE of seasonal storage options, the study assumes three months each for charging and discharging.
While seasonal storage can facilitate power systems with 100% VRES, other flexibility options, such as batteries, compete for the use of cheap surplus electricity generated at times of lower demand, narrowing the price spread between summer and winter and reducing volumes of available surplus electricity. DNV GL’s study takes this into consideration to arrive at a theoretical maximum need for seasonal storage capacity on a yearly basis.
Seasonal storage option
The chart shows the different LCoE calculations for the seven different seasonal storage options evaluated.
The first is a benchmark, where electricity production is based on carbon-taxed natural gas, which occurs today.
The second fuel switch option, between synthetic and natural gas, also relies on the intrinsic storage capacity provided by the natural gas market. In summer, when electricity prices are low, electricity is converted into hydrogen, then methane, which is injected into the natural gas grid. During the winter, natural gas is used to produce electricity.
The remaining options rely on the production and storage of synthetic fuels; hydrogen, ammonia and synthetic natural gas.
Options 3 to 5 use liquefied synthetic fuels. In each case a dedicated large-scale solar farm at a high irradiance location, somewhere in Asia, generates renewable electricity, which is converted into a synthetic fuel — hydrogen (option 3), ammonia (option 4) or methane (option 5) — then liquefied and shipped globally. After storage, the fuel is re-gasified and, for ammonia, converted into hydrogen, which is turned into electricity in fuels cells, assumed to be the technology of choice in 2050.
Options 6 to 8 are based on regional synthetic fuels produced using low-priced locally generated electricity, such as from solar PV in summer. The fuels are fed into existing national or regional infrastructure, negating long-distance transportation. These require a seasonal storage facility, such as a cavern, and depend on volatile electricity market prices.
The results of the LCoE calculation for the different options vary considerably, with electricity production based on carbon taxed natural gas (option 1) the cheapest, followed by compressed-hydrogen seasonal storage (option 6).
The chart shows how additional conversion steps, or high transportation losses in the case of liquefied hydrogen (option 3), increase LCoE.
The price gap between the marginal cost-based electricity price and the LCoE from seasonal storage indicates a challenge for seasonal storage. However hydrogen, and other synthetic fuels, can be used in markets other than the power wholesale market, for example, for mobility or as industrial feedstock. Price will determine uptake.
The study concludes that the idea of seasonal storage of electricity has great appeal as it can potentially use surplus electricity, typically in summer, that might be curtailed otherwise and can decarbonise electricity generation when demand is high.
There are caveats, however. Short-term storage and demand response will address a significant proportion of variability in electricity load and consumption, decreasing the average summer-winter electricity price spread. This means the most viable option for seasonal storage with the lowest LCoE is compressed hydrogen combined with subsurface storage.