The most effective thermal energy storage (TES) mediums for Compressed Air Energy Storage (CAES) plants, especially adiabatic CAES systems, are those that can efficiently store and release the heat generated during air compression for later use during expansion. This thermal management is critical to improving the round-trip efficiency of CAES plants.
Key Thermal Energy Storage Mediums for CAES
1. Sensible Heat Storage in Solids and Liquids
- Solid Media: Materials such as concrete or stone are effective for storing sensible heat. They have high heat capacity and can maintain temperature consistency over typical CAES discharge cycles (4–12 hours). Packed bed thermal energy storage, which uses solid granular materials, has been numerically simulated with system efficiencies around 70%.
- Liquid Media: Hot oils (up to ~300°C) and molten salt solutions (up to ~600°C) are commonly used liquid storage mediums. These liquids have a high heat capacity and can be contained in thermal storage tanks, supporting efficient heat transfer and retention. Hot water can also store thermal energy, but with somewhat lower efficiency (~65%).
2. Phase Change Materials (PCMs)
Phase change materials were considered but largely discounted for CAES because no single PCM can cover the wide temperature range (50 to 650°C) required for efficient thermal management in adiabatic systems. The use of PCMs is limited when extensive temperature variation is needed.
3. Hybrid Systems
Some CAES designs use hybrid TES systems combining salts and oils to cover the full temperature range required for effective heat storage (50 to 650°C), balancing cost, complexity, and performance. This dual-medium approach allows leveraging the strengths of each material in different temperature bands.
4. Pressurized vs. Non-Pressurized Containment
Thermal storage mediums may be stored in pressurized containers or non-pressurized tanks with heat exchangers. Liquid systems often use heat exchangers to avoid large pressurized tanks but at the cost of added complexity and expense.
Summary Table
| TES Medium | Temperature Range (°C) | Key Attributes | Suitability for CAES |
|---|---|---|---|
| Concrete / Stone | ~50 to ~650 | High heat capacity, inexpensive | Good for sensible heat storage |
| Hot Oil | Up to ~300 | Good heat transfer, liquid medium | Useful for moderate temp sensible heat storage |
| Molten Salt | Up to ~600 | High temperature capacity | Excellent for high temp sensible heat storage |
| Hot Water | Up to ~100 | Lower temp, less efficient | Limited application, moderate efficiency |
| Phase Change Materials | Limited temp range | High latent heat, but narrow range | Generally unsuitable for broad CAES temp ranges |
| Hybrid (Salt + Oil) | ~50 to ~650 | Covers broad temp range, more complex | Effective in covering full temperature span |
Efficiency and Performance
Adiabatic CAES systems that utilize these thermal storage technologies can achieve round-trip efficiencies exceeding 70%, a significant improvement over diabatic systems that lose heat to the environment during compression and require fuel for reheating during expansion. The choice of TES medium affects energy loss rates, cost, operational complexity, and thermal extraction rates crucial for CAES performance.
In conclusion, the most effective thermal energy storage mediums for CAES plants involve sensible heat storage via solids (such as concrete or packed beds) and liquids (hot oil and molten salts), with hybrid systems combining these media to cover the required temperature range optimally. Phase change materials are generally not favored due to their limited temperature range. The integration of these TES mediums with efficient heat exchangers and containment strategies is essential to maximize CAES efficiency.
Original article by NenPower, If reposted, please credit the source: https://nenpower.com/blog/what-are-the-most-effective-thermal-energy-storage-mediums-for-caes-plants/
