The Value of Large-Scale Offshore Energy Storage in the Future Electricity Market

Electricity is a real-time product. Supply and demand, or generation and consumption, have to match exactly at any instance to support the stable operation of the power system. This results from the fact that electricity is not economically storable on a large scale. While this is challenged by techno-economic developments leading to increased and improved storage capacities, storage levels remain well below that of other energy commodities. In addition, in light of the growing importance of sustainability, there is an ongoing transition towards variable renewable energy sources. Their limited controllability and predictability result in an increasing need for flexibility, which is the ability to provide power adjustments to compensate for temporary imbalances between generation and consumption. At the same time, the flexibility offered by the generation side is threatened by closure of conventional power plants that are currently experiencing decreasing profitability. While flexibility can also be provided by flexible supply, flexible demand, and the electric grid, electricity storage is expected to play an important role to fill the flexibility gap.

This thesis studies the participation and modeling, and role and value, of electricity storage in short-term electricity markets, including day-ahead and intra-day energy markets, and real-time balancing markets. These markets are important tools to deal with the variability in the system, in which the need for flexibility is expressed and its provision is valorized. As such, they are becoming increasingly important with the ongoing integration of variable renewables. The geographical scope includes Belgium and the Central Western European region, including the French, German, and Dutch market zones next to that of Belgium.

First, the concept of electricity storage is discussed, along with a quantitative study on the role, value, and benefits of storage in the transition to, and operation of, highly renewable power systems. The former includes a discussion on the definition of electricity storage, applications for which storage systems can be used, techno-economic parameters by which storage systems can be characterized, and storage technologies that are often considered for grid integration. The latter includes the presentation of a system-wide generation expansion planning model that decides on the cost-minimizing generation mix and scheduling to meet the demand for energy and frequency control, subject to detailed operating requirements and constraints. This model is applied to a test system to derive system-independent and broadly-applicable insights on the role and value of storage, and the interdependency of flexibility sources.

Second, since understanding short-term markets is essential for analyses related to flexibility, their design rules are studied in detail along with the implications for flexibility. This is done for the four market zones of the Central Western European region, and provides insight in whether flexibility is treated consistently and appropriately among the different markets, both in time and in space. Where appropriate, desirable future market reforms are indicated. 

Third, the storage participation, including its trading and operation, in short-term markets is studied. In a first study, employing storage systems for a single application is considered, namely day-ahead market arbitrage. A single-player storage operator perspective is assumed, resulting in a price-based unit commitment formulation. Detailed operating constraints are considered, and a new methodology to study the price-effect of storage actions is introduced based on so-called market resilience functions. This price-effect states that storage generally reduces price spreads by increasing low prices and decreasing high prices. In addition, a stepwise approximation to the piecewise linear market resilience functions is proposed, offering the capability to reduce computation time while providing accurate lower and upper bounds. The developed models are applied to Belgian market data to quantify the arbitrage value and price-effect. Since determining the true value of storage requires the aggregation of applications, and the co-optimization of these applications to avoid conflicting uses, in a second study the day-ahead market arbitrage models are extended to allow for the aggregation of different arbitrage opportunities in the three short-term markets. In addition, the price-effect is studied for the intra-day and real-time markets as well. These models are used to analyze the opportunities for storage in the three short-term markets and four market zones, while differences in storage value are traced back to differences in market design. 

Fourth, the aggregation of applications can also be achieved through the co-operation and sharing of storage resources by different players. New markets, or market products within existing markets, to enable such storage uses, and thus the decoupling of ownership from operation, can be valuable levers to capture the true value of storage. The concept of physical storage rights is introduced, which can be auctioned to different players and entitle the holders to the right to use storage resources. Based on a case study with Belgian data, the storage value in a range of fixed a priori allocations is compared to that of allocations resulting from the proposed auction-based mechanism to show its merits.