Indonesia targets 23% renewable energy by 2025, but integrating variable sources like solar and wind presents significant grid challenges. As the Oliver Wyman study notes, neither Indonesia’s grid nor its storage infrastructure is currently ready to absorb significantly more renewables.

Long-Duration Energy Storage (LDES) is crucial for balancing supply and demand over days and seasons, enabling a reliable supply of Indonesia renewable energy. In fact, experts warn that massive overnight storage will be needed to back up Indonesia’s expected solar-led system.

CIIC 2025’s Energy Transition track is on the lookout for pioneering storage solutions that can speed up Indonesia’s journey to a low-emission energy landscape. This report compares two promising LDES families – gravity-based storage (e.g. pumped hydro and lifting-weight systems) and thermal-based storage (heat retention systems) – to determine which is most promising for Indonesia’s energy transition.

Gravity-Based Long-Duration Storage

Gravity-based LDES uses gravitational potential energy, most commonly via pumped-hydro or novel weight-lifting systems. Pumped-storage hydropower (PSH) dominates the global energy storage landscape, making up approximately 99% of the total installed storage capacity across power grids worldwide.

In this well-established method, excess electricity, such as midday solar output, is used to pump water uphill into a storage reservoir, which is then released through turbines to generate power during periods of high demand. Major advantages include proven reliability, high round-trip efficiency (typically 70–85%), very long lifetimes (often >50 years), and firm capacity that stabilizes the grid.

Given Indonesia’s abundant highland terrain and waterways, ample locations exist for development; studies highlight the country’s significant opportunity to deploy cost-effective off-river pumped hydro systems with limited ecological disruption.

Implementation

Indonesia’s first large pumped-hydro project (Upper Cisokan, 1,040 MW in Java-Bali) has secured financing. Another 4.2 GW of pumped-hydro storage is already slated for development, and projections indicate that Southeast Asia’s PSH capacity could climb nearly eightfold to roughly 18 GW by 2033, driven primarily by Indonesia, Vietnam, and Thailand. This strong regional pipeline, backed by government and international support, demonstrates high readiness.

Challenges

Pumped hydro requires specific geography (two reservoirs at different elevations), land use, and civil works. Siting and environmental permitting can be slow. Projects are capital-intensive, though World Bank financing indicates robust public support. Furthermore, PSH is geographically limited; not every island or locale can host a plant.

Emerging gravity systems

Beyond water, companies are developing gravity battery concepts (e.g. lifting heavy blocks or concrete masses). These promise modular LDES without water or dams. Emerging gravity battery concepts, like Energy Vault's block-lifting system (80% round-trip efficiency), offer modular LDES without water or dams.

Though early-stage and currently higher in capital cost ($643/kWh), they are environmentally friendly, durable, and location-flexible. While no large projects exist in Indonesia, regional interest is growing.

Thermal-Based Long-Duration Storage

Thermal energy storage (TES) stores energy in the form of heat (or cold). Common approaches include molten-salt tanks, phase-change materials, or hot water/steam reservoirs. Typically, excess electricity powers heaters or boilers to store heat, which can later drive steam turbines or provide heating.

Thermal storage systems are often paired with CSP installations; for example, the Noor plant in Morocco stores daytime solar heat in molten salt and later uses it to generate electricity once the sun has set.

The key advantage of TES is cost-effectiveness: BloombergNEF finds that thermal LDES has among the lowest capital costs of all LDES technologies ($~232 per kWh installed globally). By contrast, gravity-based LDES averaged ~$643/kWh in the same survey.

Applications

Thermal storage can also store industrial waste heat or enable power-to-heat-to-power cycles (so-called pumped heat). It excels where high-grade heat is needed.

In theory, a large TES plant could be charged by abundant daytime solar PV and then run a steam turbine at night. The Jiangsu University paper notes CSP+molten salt “allows [solar power] to continue to produce electricity even though it is dark”.

Finland’s “Sand Battery” pilot, storing solar heat in hot sand for district heating, also demonstrates long-duration heat storage. However, converting stored heat back to electricity incurs energy losses and requires turbines or engines; round-trip efficiency is typically lower than for pumped hydro or batteries.

Suitability for Indonesia

Despite high solar potential, CSP with molten salt isn't yet scaled in Indonesia. Tropical weather, humidity, and land costs challenge large solar thermal plants. Furthermore, TES requires generator integration, which Indonesia's current utility sector lacks (no utility-scale CSP or geothermal for this purpose).

Thus, most LDES innovation in Indonesia has not focused on grid-scale thermal storage. Thermal TES may find niches in industry (e.g. waste heat capture or green hydrogen production) rather than as a primary electricity storage.

Challenges

Despite low capex, TES is less mature in the Indonesian energy sector. Cost data is primarily from projects in China or the Middle East, and local expertise and supply chains for large-scale TES remain limited.

Additionally, TES systems require materials that withstand high temperatures over decades, which poses technical risk. Lastly, unlike pumped hydro, TES does not inherently provide grid inertia or frequency support.

Comparative Analysis

When comparing gravity-based LDES (mainly pumped hydro) with thermal-based LDES in the Indonesian context, several factors emerge:

Scale and Capacity

Pumped hydro offers gigawatt-scale, multi-hour capacity on a single site (e.g., 1 GW for 8+ hours), significantly larger than gravity batteries or thermal plants (tens to hundreds of MW). Given the extensive scale of Indonesia’s power network, most notably on Java-Bali, pumped hydro is ideally suited to meet its large-capacity requirements.

Technical Maturity

Pumped hydro is a proven, commercially mature technology (TRL 9), with most Indonesian projects funded and underway. Thermal storage (TRL 7–8 for CSP molten salt) and gravity batteries (TRL ~6–7) are less established, implying lower near-term risk and faster deployment for PSH.

Economic Viability

While global analyses suggest thermal LDES has lower upfront capex per kWh, pumped hydro benefits from its very long lifespan and low O&M costs. EnergyVault's gravity battery, despite advertised low lifetime costs, currently has high capital expenditure.

New Indonesian PSH, especially with international financing, could be lifetime-competitive with conventional peaker plants. Though billions in investment are needed, private participation is increasing.

Geography and Impact

Indonesia’s landscape is ideal for pumped hydro, and utilizing off-river reservoirs helps minimize ecological disruption. While land use is significant, careful design can reduce it.

Gravity batteries offer a small footprint, zero water use or emissions, and high sustainability. Thermal storage plants, however, require large surface areas and may use scarce materials.

Grid Integration

Pumped hydro offers immediate response and enhances grid inertia, aiding stability. Thermal storage, especially with thermal generators, provides fewer ancillary services. Gravity batteries offer fast response and pose no chemical fire risk, improving safety.

In summary, gravity-based LDES, particularly pumped hydro, is highly promising for Indonesia's near-term energy transition, given its current implementation, vast potential, and direct role in enabling higher renewable penetration. While thermal LDES holds economic promise for specific uses, it is currently secondary in Indonesia due to technical and environmental factors.

However, as renewable capacity expands, all sustainable storage technologies will be needed, and innovations in TES could complement gravity storage with strategic investment from Indonesian industry and research.

For Indonesia’s ambitious energy transition, gravity-based LDES (pumped hydro and related systems) appears most ready to scale. It aligns with the country’s renewable energy geography, enjoys policy support, and can be deployed in timeframes compatible with CIIC 2025 goals.

Thermal-based LDES remains an important innovation frontier, especially for future 24/7 renewables and industrial decarbonization, but it will require more R&D and pilot projects before matching pumped hydro’s impact in Indonesia.

In the context of CIIC 2025’s Energy Transition track, prioritizing proven gravity-storage projects while continuing to explore thermal storage pilots offers the best balance. By harnessing robust gravity storage and researching sustainable storage technologies, Indonesia can accelerate its transition to a clean, reliable power system.