Long Duration Energy Storage (LDES) systems work by capturing heat during times of excess energy production—typically midday when solar generation peaks—and storing this energy for later use. This stored energy can be converted back to electricity or used directly for heating purposes during the evening or on cloudy days when solar output is low. The beauty of LDES lies in its simplicity and efficiency. Materials such as sand (silicon dioxide) maintain high temperatures for extended periods and can be used to store vast amounts of energy with minimal losses over time.

The case for LDES in California and Texas is particularly strong due to the unique utility grid electricity profile for both states. Solar energy production peaks around midday, but the highest demand for electricity occurs in the early evening. This misalignment leads to what is known as the "Duck Curve", a phenomenon whereby there is a rapid increase in the demand for electricity just as solar production is waning. LDES can flatten the belly of the curve by shifting the solar energy collected during the day to the evening, thus smoothing out the supply-demand mismatch.

Moreover, LDES can enhance grid stability and reliability. By providing a steady, dispatchable source of energy, LDES helps mitigate the intermittency issues associated with solar power. This capability is crucial as California and Texas aim to increase their renewable energy share and reduce reliance on fossil fuel-powered peaker plants, which are often called upon during times of high demand but are costly and emit large amounts of greenhouse gases.

The Texas grid (ERCOT) and California’s grid (CAISO) rely heavily on solar and wind energy, which are intermittent and pose challenges for balancing supply and demand. Surplus solar and wind power, especially during certain times of the day or year, can lead to inefficiencies. Consequently, ERCOT’s demand for energy storage is expected to grow from 6.3 GW to 17.7 GW by mid-2025 and CAISO with over10 GW of battery storage currently in operation is planning to add 12.1GW this year.

Research highlights the benefits of using sand for LDES systems: it is low-cost, easily accessible, thermally stable, and non-toxic. Sand's dual role as a heat transfer and insulating material is influenced by its porosity, granularity, moisture content, and mineralogy. Sand with high quartz content, large grains, minimal porosity, and significant moisture content is preferred for heat transfer, while dry sand with lower quartz content and higher porosity is better suited for thermal insulation. These characteristics open doors for innovative LDES designs aimed at enhancing efficiencies.

Phase Change Materials (PCMs) further enhance LDES systems by significantly improving their thermal management capabilities. PCMs absorb and release latent heat during phase transitions, which allows for the efficient storage and release of energy over extended periods. This property is particularly valuable in stabilizing temperatures within LDES systems, minimizing thermal losses, and maintaining a consistent energy output. The integration of PCMs with sand-based systems leverages the advantages of both materials: sand’s excellent thermal stability and insulation properties complement the latent heat storage capacity of PCMs.

The combination of sand and PCMs, enhanced by new modeling techniques, has the potential to revolutionize the LDES industry. Advanced simulations and modeling tools enable precise optimization of sand and PCM mixtures, tailoring the thermal properties to specific energy storage needs. By accurately predicting the behavior of these materials under various conditions, researchers and engineers can design systems that maximize energy retention and minimize losses. This innovation not only improves the overall efficiency and cost-effectiveness of LDES systems but also broadens the scope of their applications, paving the way for more sustainable and resilient energy infrastructure.