Phase Change Materials

Phase Change Material Principles

Phase change materials (PCM) store and release thermal energy through solid-liquid phase transitions occurring at specific temperatures, leveraging latent heat of fusion without temperature change during the phase transition. This isothermal energy storage maintains precise temperature control while providing energy density intermediate between sensible water storage (1 BTU/lb-°F) and ice storage (144 BTU/lb). Organic PCMs including paraffin waxes provide 60-90 BTU/lb latent heat with phase change temperatures ranging from 40-80°F, while inorganic salt hydrates achieve 100-150 BTU/lb but face reliability challenges.

The narrow temperature range of phase transition (typically 2-4°F) differentiates PCM storage from sensible systems requiring 15-25°F temperature swings for equivalent capacity. This characteristic enables precise temperature maintenance during discharge, suiting applications requiring strict temperature control. The phase change temperature selection determines system applicability, with materials matched to specific applications including building cooling (72-78°F), refrigeration (32-50°F), and solar thermal storage (120-160°F).

PCM Classification and Selection

Organic PCMs including paraffin waxes, fatty acids, and polyethylene glycols offer reliable long-term performance with minimal degradation over thousands of freeze-thaw cycles. Paraffins provide phase change temperatures from 40-70°F with latent heat of 60-75 BTU/lb and good chemical stability, though flammability requires containment in fire-rated enclosures. Fatty acids and their mixtures achieve similar performance with higher cost but non-flammable properties. Bio-based PCMs from renewable sources are emerging but face cost and availability limitations.

Inorganic PCMs including salt hydrates (calcium chloride hexahydrate, sodium sulfate decahydrate) provide higher energy density of 100-150 BTU/lb and lower cost per BTU stored compared to organics. The challenges include supercooling where material cools below phase change temperature without solidifying, phase segregation after repeated cycles reducing effective storage capacity, and corrosion of metal containers requiring specialized coatings. Nucleating agents and thickening additives mitigate some problems but add complexity.

Eutectic mixtures combining multiple components achieve specific phase change temperatures not available from pure materials, enabling precise temperature matching to application requirements. The eutectic composition exhibits single-temperature phase change rather than temperature range of non-eutectic mixtures. Commercial PCM products typically use proprietary eutectic formulations optimized for target phase change temperatures with additives enhancing reliability and thermal conductivity.

Encapsulation Methods

Encapsulation contains PCM in sealed packages providing mechanical strength, preventing leakage during liquid phase, and enabling heat transfer with surrounding air or water. Macroencapsulation in containers 1-6 inches diameter (tubes, panels, pouches, canisters) suits building-scale applications where large thermal storage capacity and modest surface-to-volume ratio are acceptable. The encapsulation wall material (typically HDPE or metal) must withstand thermal cycling, accommodate volume expansion during melting (typically 10-15%), and provide adequate heat transfer.

Microencapsulation creates PCM droplets 1-1000 micrometers diameter surrounded by protective polymer shells, producing free-flowing powder or slurry that integrates into building materials, textiles, or heat transfer fluids. The high surface area enables rapid heat transfer and complete phase change during short thermal cycles. Applications include PCM-enhanced drywall, ceiling tiles, concrete, and insulation where distributed thermal mass moderates temperature swings. The microencapsulated PCM typically comprises 20-30% by mass of the composite material.

Shape-stabilized PCMs use porous support structures (expanded graphite, diatomaceous earth, polymer foams) to absorb and retain liquid PCM through capillary forces, eliminating need for impermeable encapsulation. The composite maintains solid shape during phase transition even though PCM portion liquefies. The support structure also enhances effective thermal conductivity, addressing the low conductivity (0.1-0.3 W/m-K) limiting heat transfer rates in pure PCMs. Graphite composites achieve 1-5 W/m-K effective conductivity enabling rapid charging and discharging.

Building Integration Strategies

PCM-enhanced building materials including wallboard, ceiling tiles, and floor systems provide passive thermal storage without mechanical systems. The PCM with melting point near indoor temperature (72-76°F) absorbs heat as space temperature rises during day, solidifying at night as space cools. This diurnal cycle reduces temperature swings and peak cooling loads by 20-40% in applications with significant thermal swing. The strategy suits buildings with night ventilation or setback enabling PCM discharge, and climates with sufficient diurnal temperature range.

Underfloor heating systems incorporating PCM-enhanced concrete provide thermal storage enabling off-peak charging with on-peak discharge. The floor slab with embedded hydronic tubing and PCM maintains comfortable radiant temperatures throughout day after nighttime charging. Storage capacity of 15-20 BTU/ft² per °F of PCM enables load shifting from peak to off-peak electric rates. The radiant heating with thermal storage reduces equipment capacity requirements compared to conventional systems.

Ceiling-mounted PCM panels absorb heat from lights, occupants, and equipment, reducing space cooling loads. The panels contain macroencapsulated PCM in aluminum or plastic containers mounted in suspended ceiling grid. During occupied hours, PCM melts absorbing heat before it enters space air. Night ventilation or setback solidifies PCM for next-day cycle. Capacity of 30-40 BTU/ft² per cycle enables 20-30% peak cooling reduction in perimeter zones with good night cooling potential.

Active PCM Storage Systems

Active PCM thermal storage integrates encapsulated materials in thermal storage tanks with air or water flowing through void spaces between containers. The system operates like ice storage with charging (PCM solidification) during off-peak hours and discharge (melting) during peak loads. PCM with higher melting point than ice (45-55°F) enables higher chiller efficiency during charging compared to ice making, while providing 3-4 times greater energy density than chilled water at equivalent temperature range.

Air-based PCM systems circulate ventilation or recirculation air through packed beds of PCM containers, cooling air through direct contact heat transfer. Supply duct integration enables peak load shifting by precooling ventilation air before entering occupied spaces. The low thermal conductivity of air limits heat transfer rates, requiring large container surface area and low face velocities (200-400 fpm) for effective heat transfer. Applications include packaged rooftop unit retrofit and residential night ventilation pre-cooling.

Water-based systems circulate chilled water through storage tanks containing encapsulated PCM, operating similarly to conventional ice storage but at higher temperatures. The PCM acts as quasi-isothermal storage maintaining supply water temperature as PCM melts, eliminating temperature drift of sensible chilled water storage. Discharge capacity remains constant until PCM completely melts, then rapidly decreases compared to gradual capacity reduction in stratified water storage.

Thermal Conductivity Enhancement

Pure PCMs exhibit low thermal conductivity (0.15-0.3 W/m-K for paraffins, 0.4-0.6 W/m-K for salt hydrates) limiting heat transfer rates during charging and discharging. This low conductivity creates thermal resistance requiring large temperature differences between heat transfer fluid and PCM for adequate power density. Extended surfaces including fins, honeycomb structures, and metal foams increase effective surface area by factors of 5-20, improving heat transfer rates proportionally.

Thermal conductivity additives including graphite particles, carbon nanotubes, and metal particles dispersed in PCM increase bulk conductivity to 1-10 W/m-K depending on additive concentration and dispersion quality. Typical graphite loading of 5-10% by mass doubles effective conductivity while reducing latent heat capacity proportionally. Optimized composites balance enhanced conductivity against capacity reduction, achieving 2-3 times faster charging with 10-15% capacity reduction.

Matrix materials including metal foams, graphite foams, and honeycomb provide continuous high-conductivity paths through PCM volume, achieving effective conductivity of 5-20 W/m-K with 80-90% PCM fraction by volume. The matrix structure provides mechanical support eliminating need for separate container walls while dramatically improving heat transfer. These enhanced materials enable compact storage with power densities approaching ice storage while maintaining intermediate temperature operation and higher chiller efficiency.

Performance and Economics

PCM system economics depend on energy density advantage versus cost premium over conventional storage. Paraffin-based systems typically cost $15-30 per kWh thermal storage capacity (4-8 times chilled water, 1.5-2.5 times ice storage), justified by space savings where floor space costs exceed $100-200 per square foot. The compact volume suits urban retrofit applications with limited space for conventional storage. Long-term performance requires stable phase change characteristics over thousands of cycles, achievable with properly formulated and encapsulated organic PCMs.

Performance degradation mechanisms include encapsulation failure allowing leakage, thermal cycling fatigue of encapsulation materials, and inorganic PCM phase segregation or supercooling reducing effective storage capacity. Accelerated cycle testing (500-1000 cycles at 1-4 hour cycle times) validates 20-30 year projected life with less than 10% capacity degradation. Field monitoring of early PCM installations shows 85-95% of design capacity remaining after 5-10 years operation when properly designed and commissioned.

Emerging applications include solar thermal storage, waste heat recovery, battery thermal management, and transport refrigeration where PCM’s isothermal discharge and intermediate energy density fill gaps between conventional storage technologies. Research focuses on improving thermal conductivity, reducing cost through bio-based materials, and developing high-temperature PCMs (above 200°F) for industrial process heat storage. The technology transition from passive building integration to active thermal storage systems requires continued cost reduction and demonstrated long-term reliability.