Phase Change Materials for Thermal Storage: An Economic and Sustainable Analysis

TL;DR

Phase Change Materials (PCMs) deliver higher energy densities than water at low temperature deltas but carry higher costs per unit stored energy and much higher embodied footprints. Salt hydrates are promising solutions for high energy density applications where water is not dense enough while waxes (especially paraffin) remain wholly uncompetitive.

We need competitive thermal storage

Thermal storage is a key technology for balancing energy grids and maximizing the use of intermittent renewables like solar and wind. By storing heat (or cold), we can shift energy use from peak times or low renewable production to times when it's needed. While many thermal storage materials exist, significant research continues into finding the most effective solutions, especially for low-temperature applications like building heating and hot water.

This post dives into Phase Change Materials (PCMs) – substances that store and release heat during their phase transition (e.g., solid to liquid) rather than just through temperature change. PCMs offer potentially high energy storage density with small temperature differences. We'll compare two common low-temperature PCM types, salt hydrates (specifically Calcium Chloride Hexahydrate, CaCl2⋅6H2O) and paraffin waxes, against the benchmark: simple water storage.

We'll focus on three key questions:

1. Are these PCMs economical?
2. Are they sustainable?
3. How realistic is their large-scale deployment?

(Note: We are setting aside important technical challenges like low thermal conductivity, supercooling, safety concerns like corrosion or flammability, and the need for encapsulation and indirect heat transfer via heat exchangers for this analysis.)

Are these PCMs economical: Material vs. System Cost

Let's start with the raw material cost per unit of thermal energy stored (kWhT).

• Water: Using typical US residential rates ($2/ton in US) and a 20°C temperature difference (yielding 84 kJ/kg energy density), the cost is less than $0.10/kWhT. For high volume users, water costs are a few cents per ton reducing material costs by almost two orders of magnitude, ~$0.001/kWhT.
• CaCl2⋅6H2O: At $250/ton for the salt (assuming water content is free) and an energy density of 180 kJ/kg, the cost is roughly $2.50/kWhT.
• Paraffin Wax: At $1000/ton and 210 kJ/kg energy density, the cost jumps to about $17/kWhT.

Clearly, water is vastly cheaper on a material basis. Paraffin is particularly expensive, almost 7 times the cost of the salt hydrate.

However, PCMs boast higher energy density per volume. For a hypothetical 1 MWhT storage system:

• Water requires ~43 m3.
• CaCl2⋅6H2O needs ~13.3 m3.
• Paraffin wax needs ~19 m3.

The relative performances are summarized in Table 1 below.

Table 1: Summary of material costs per kWh, the volume required for a 1MWhT thermal storage and carbon footprint for the different thermal storage materials.

Water needs 2-3 times more space. Does this smaller volume make PCMs cheaper overall when container costs are included?

We can calculate the maximum container cost (per m3) where the PCM system (material + container) becomes cheaper than an equivalent water system (material + container, assuming identical container /m3 cost):

• CaCl2⋅6H2O breaks even with water if container reaches ~$80/m3.
• Paraffin breaks even with water if container reaches ~$710/m3.

This suggests CaCl2⋅6H2O could be economical for smaller systems where container costs per volume are higher. However, large utility-scale storage often targets much lower tank costs (~$35/m3), keeping water cheaper.

This comparison assumes storage for only heating or cooling. Most buildings need both. Water handles both within the same tank. PCMs are typically tuned for one temperature range, requiring separate systems (and costs) for heating and cooling. If we double the PCM material and container costs to account for needing both heating and cooling capacity:

• CaCl2⋅6H2O only becomes cheaper than water if container costs exceed ~$300/m3.
• Paraffin requires container costs over ~$7000/m3 to compete.

Adding the costs and complexities of heat exchangers needed for PCMs (which water avoids by often being the direct working fluid) further tips the scales towards water. Furthermore, water can utilize its own phase transition via ice storage for cooling, boosting its energy density significantly (often exceeding PCMs) at a very low material cost. When considering ice storage for cooling, neither PCM appears competitive (see last column in Table 1).

(Note: Most waxes and salt hydrates need additional materials for enhancing conductivity, reducing phase separation and reducing supercooling, all of which increase the costs above, sometimes by a factor of 2 or more. Water doesn't require any of these additional costs or complexities.)

Are they sustainable: The Footprint

For those prioritizing environmental impact, the material footprint matters (also embodied carbon). Using estimated carbon footprints (also summarized in Table 1):

• Water: 0.008 kgCO2e/kg yields 0.35 kgCO2e/kWhT (potentially much lower).
• CaCl2 salt: 1.53 kgCO2e/kg yields 15.3 kgCO2e/kWhT (for the CaCl2⋅6H2O hydrate).
• Paraffin Wax: 3.75 kgCO2e/kg yields 64 kgCO2e/kWhT.

Water's carbon footprint per unit of stored energy on the high end is roughly two orders of magnitude lower than for PCMs. For our 1 MWhT example, the embodied carbon in the storage material alone would be around 15 tons for the salt hydrate and 64 tons for paraffin, compared to less than 1 ton (potentially as low as 10 kg) for water. These PCM figures double if separate heating and cooling systems are needed. Using ice storage or water with an average carbon footprint improves water's footprint further to ~0.07 kgCO2e/kWhT, which is about three orders of magnitude lower than for PCMs.

How realistic is their large-scale deployment: Challenges

Despite decades of research, PCM thermal batteries are only starting to see niche commercialized use (e.g., Sunamp). Their overall system costs can approach that of electrical batteries (like LFP), raising questions about competitiveness.

While PCMs offer 2-3x higher volumetric energy density than sensible water storage, this advantage may be marginal for typical residential applications needing only 1-2 m3 of storage.

Integrating PCMs into building materials (like drywall or concrete) is another proposed application, aiming to add thermal mass and stabilize temperatures. This could save on dedicated container and heat exchanger costs. However, challenges remain:

• Comfort: The fixed phase transition temperature might not align with occupant preferences, creating thermal inertia that's hard to overcome.
• Safety: Incorporating large amounts of flammable paraffin wax into building structures faces significant hurdles with fire codes. Many salt hydrates are corrosive or potentially toxic, limiting their integration potential.

Conclusion: Water Remains the Benchmark

Based on this analysis of economics, sustainability, and deployment factors for low-temperature thermal storage:

• Salt hydrates (CaCl2⋅6H2O) might offer an economic edge in niche, heating-only applications with higher container costs or extreme space constraints.
• Paraffin waxes appear uncompetitive due to high material costs and significant sustainability and safety concerns.
• Water generally remains the superior thermal storage medium due to its extremely low cost, negligible environmental footprint, versatility (heating, cooling, direct use), and simplicity.

While the higher energy density of PCMs is appealing, the practical advantages of water often outweigh the volume savings, especially when considering total system cost, sustainability goals, and safety. Before opting for PCMs solely based on density, it's crucial to evaluate if the space constraints truly justify the economic, environmental, and technical trade-offs compared to water.