The Transmission Value of Energy Storage and Fundamental Limitations

TL;DR

The paper tackles the problem of quantifying the transmission value of grid-scale energy storage and identifying fundamental limits between storage and transmission lines. It introduces a cumulative-energy perspective that couples storage SoC with line flows via a DC power-flow model, enabling a reformulation of the planning problem around line power flows $f_{ij}[t]$ and revealing tight, closed-form limits. The key contributions are explicit, tractable limits: (i) the minimum total storage needed when transmission capacity is abundant, $\sum_i S_i \ge \max_t \sum_i E_i[t] - \min_t \sum_i E_i[t]$, and (ii) the minimum per-link line capacity under sufficient storage, $C_{ij}+c_{ij} \ge (1/N)\sum_{s=1}^N |f'_{ij}[s]|$, with explicit expressions for storage, state-of-charge, and energy balances. The approach is demonstrated on a 2-bus system, modified RTS-24, RTS-GMLC, and a Texas synthetic grid, illustrating how storage can be used as a transmission asset to reduce required line capacity and informing future market design for storage-based transmission services.

Abstract

This study addresses the transmission value of energy storage in electric grids. The inherent connection between storage and transmission infrastructure is captured from a "cumulative energy" perspective, which enables the reformulating of the conventional optimization problem by employing line power flow as the decision variable. The study also establishes the theoretical limitations of both storage and transmission lines that can be replaced by each other, providing explicit closed-form expressions for the minimum capacity needed. As a key departure from conventional practice in which transmission lines are designed according to the peak power delivery needs, with sufficient storage capacity, the transmission line capacity can be designed based on the average power delivery needs. The models of this paper only rely on a few basic assumptions, paving the way for understanding future storage as a transmission asset market design. Numerical experiments based on 2-bus, modified RTS 24-bus, RTS-GMLC, and Texas synthetic power systems illustrate the results.

Figures (8)

• Figure 1: The time slots in a period
• Figure 2: The 2-bus system
• Figure 3: The optimal storage and transmission line capacity under different cost parameters
• Figure 4: The total optimal storage and transmission line capacity under different cost parameters
• Figure 5: The storage SoC behavior under minimum transmission line capacity (2-bus system)

Theorems & Definitions (14)

• Definition 1: Local Cumulative Energy
• Definition 2: Local Cumulative Net Energy
• Definition 3: Transferred Cumulative Energy
• Definition 4: Original Transferred Cumulative Energy
• Corollary 1: Equivalent SoC Balance
• proof
• Theorem 1: Minimum Storage Capacity Needed
• proof
• Corollary 2: Storage SoC
• Theorem 2: Total Minimum Storage Capacity Needed