HVAC Systems Encyclopedia

A comprehensive encyclopedia of heating, ventilation, and air conditioning systems

Phase Change Materials Advanced

Phase change materials represent an advanced thermal energy storage technology that exploits latent heat of fusion to store and release substantial quantities of thermal energy at nearly constant temperature. PCMs undergo phase transitions—typically solid-liquid—absorbing heat during melting and releasing heat during solidification, providing thermal storage densities significantly higher than sensible heat storage.

Organic Phase Change Materials

Organic PCMs consist primarily of paraffin waxes and fatty acids, offering congruent melting, chemical stability, and predictable phase change behavior across multiple thermal cycles.

Paraffin-Based PCMs

Paraffin waxes (CnH2n+2) provide melting temperatures ranging from -10°C to 70°C depending on carbon chain length. N-octadecane (C18H38) melts at 28°C with latent heat of 244 kJ/kg, suitable for building comfort applications. N-eicosane (C20H42) at 37°C targets domestic hot water preheating. Paraffins exhibit low vapor pressure, minimal supercooling (typically 1-2°C), and excellent thermal cycling stability exceeding 10,000 cycles without degradation.

Fatty Acid PCMs

Fatty acids including lauric acid (44°C, 178 kJ/kg), myristic acid (54°C, 187 kJ/kg), and palmitic acid (63°C, 203 kJ/kg) offer bio-renewable alternatives. These materials demonstrate sharp melting points and high heat of fusion but cost 2-3 times more than commercial paraffins. Fatty acid esters provide intermediate melting ranges through molecular structure modification.

Inorganic Phase Change Materials

Inorganic PCMs, predominantly salt hydrates, deliver higher latent heat storage density and thermal conductivity compared to organics but present challenges in phase separation and supercooling.

Salt Hydrate Systems

Salt hydrates (salt·nH2O) store energy through hydration-dehydration transitions. Calcium chloride hexahydrate (CaCl2·6H2O) melts at 29°C with 191 kJ/kg latent heat. Sodium sulfate decahydrate (Glauber’s salt, Na2SO4·10H2O) at 32°C provides 254 kJ/kg but exhibits 10-15°C supercooling and incongruent melting requiring nucleating agents and thickeners.

Magnesium nitrate hexahydrate (Mg(NO3)2·6H2O, 89°C, 163 kJ/kg) targets industrial process heat recovery. Thermal conductivity of salt hydrates (0.5-1.0 W/m·K) exceeds organic PCMs (0.2-0.3 W/m·K) by factor of 2-4, improving charge-discharge rates.

Eutectic Salt Mixtures

Binary and ternary eutectic mixtures eliminate incongruent melting through compositional optimization. A 61.5% CaCl2·6H2O and 38.5% MgCl2·6H2O eutectic melts at 25°C with 127 kJ/kg storage capacity. Eutectic formulations provide sharp melting points and minimize supercooling to 2-5°C.

Bio-Based Phase Change Materials

Renewable PCMs derived from plant oils, animal fats, and agricultural byproducts address sustainability requirements while maintaining thermal performance comparable to petroleum-based materials.

Soy wax (melting range 47-54°C, latent heat 120-140 kJ/kg) and coconut oil (24-26°C, 105 kJ/kg) serve low-temperature applications. Palm oil derivatives span 35-45°C range. Bio-PCMs demonstrate biodegradability and carbon neutrality but require stabilization against oxidation through antioxidant additives. Cost competitiveness improves as agricultural feedstock processing scales.

Encapsulation Technologies

Encapsulation contains PCM within protective shells, preventing leakage in liquid phase, increasing heat transfer surface area, and enabling integration into conventional building materials.

Microencapsulation Methods

Microencapsulation produces 1-1000 μm diameter capsules through interfacial polymerization, in-situ polymerization, or spray drying. Melamine-formaldehyde shells encapsulate paraffin droplets, creating free-flowing powders dispersible in gypsum wallboard, concrete, or plaster at 20-30% by weight loading.

Core-shell mass ratio typically reaches 85:15 to 90:10. Shell materials must withstand thermal cycling, maintain impermeability, and resist mechanical stress during construction handling. Properly encapsulated PCM retains 95% latent heat capacity over 5000+ cycles.

Macroencapsulation Systems

Macroencapsulation (>1 cm containers) uses panels, tubes, or pouches fabricated from polymers or metals. High-density polyethylene (HDPE) panels containing 1-5 kg PCM install in ceiling plenum spaces. Aluminum flat panels (300 x 600 mm, 10-15 mm thick) integrate into suspended ceiling systems providing 100-200 Wh/m2 storage capacity.

Tubular encapsulation embeds PCM in HVAC air streams or hydronic loops. Heat transfer enhancement through internal fins or external turbulence promoters compensates for low PCM thermal conductivity. Panel systems must accommodate 10-15% volume expansion during phase transition through flexible containers or void space allocation.

Shape-Stabilized Composites

Shape-stabilized PCMs combine phase change material with porous support matrices maintaining solid geometry throughout phase transition, eliminating encapsulation requirements.

Expanded graphite matrices (bulk density 30-150 kg/m3) absorb paraffin through capillary forces, achieving 85-95% PCM loading while maintaining structural integrity. Thermal conductivity increases from 0.2 W/m·K (pure paraffin) to 2-10 W/m·K depending on graphite content and orientation.

HDPE/paraffin blends (60-80% paraffin) create form-stable composites processing through conventional extrusion or injection molding. Crosslinked polymer networks physically entrap PCM molecules preventing leakage. These composites directly integrate into building envelope components without additional containment.

Thermal Conductivity Enhancement

Low intrinsic thermal conductivity of PCMs (0.2-0.5 W/m·K) limits charge-discharge rates. Enhancement strategies incorporate high-conductivity additives or structures.

Metal foams (aluminum, copper) with 90-95% porosity and 10-40 pores per inch infiltrated with PCM increase effective thermal conductivity to 5-20 W/m·K. Carbon-based additives including expanded graphite (3-10% by weight), carbon nanotubes (1-5%), or graphene nanoplatelets (2-8%) improve conductivity while maintaining high latent heat fraction.

Fin arrays, heat pipes, or embedded heat exchanger coils provide structured enhancement. Design optimization balances thermal performance against material cost, weight addition, and latent heat dilution.

Building Integration Applications

PCM integration into building envelopes shifts peak thermal loads, reduces HVAC capacity requirements, and dampens indoor temperature fluctuations.

Wallboard and Ceiling Panels

Gypsum wallboard impregnated with microencapsulated PCM (melting point 23-26°C) at 25-30% concentration stores 150-200 kJ/m2 (40-55 Wh/m2) per 12.7 mm thickness. This thermal mass equivalent matches 100-150 mm concrete while adding minimal weight (1-2 kg/m2 increase).

PCM ceiling panels in commercial buildings absorb daytime heat gains, releasing stored energy during unoccupied night periods. Properly sized systems reduce peak cooling loads 15-30% and shift electrical demand 2-4 hours.

Floor and Roof Integration

Underfloor heating systems with PCM-enhanced concrete (5-10% microencapsulated PCM by volume) extend thermal release periods from 4-6 hours (conventional) to 8-12 hours. Roof applications using PCM layers beneath membrane systems reduce summer heat gains and improve insulation effectiveness.

Ventilation Air Preconditioning

PCM heat exchangers in ventilation air streams pre-cool incoming air using night-time solidification. Compact units containing 20-50 kg PCM handle 500-1000 CFM, reducing ventilation cooling loads 30-50% in moderate climates. Round-trip efficiency reaches 70-85% depending on charge-discharge temperature differentials and heat exchanger effectiveness.

Performance Characteristics

Critical PCM selection criteria include phase change temperature, latent heat of fusion, thermal conductivity, cycling stability, volume change, cost, and safety.

Phase change temperature must align with application requirements: 18-28°C for passive building conditioning, 30-40°C for domestic water heating, 40-60°C for solar thermal storage. Latent heat exceeding 150 kJ/kg ensures practical storage density. Supercooling below 3-5°C prevents incomplete solidification.

Long-term thermal cycling stability requires testing over 1000+ melt-freeze cycles with less than 5% latent heat degradation. Compatible encapsulation materials must resist corrosion, maintain mechanical properties across temperature range, and accommodate volumetric expansion (typically 10-15% for paraffins, 5-10% for salt hydrates).

Economic viability demands material costs below $2-3/kg for building applications, $5-10/kg for specialized HVAC systems. Life-cycle analysis must account for embodied energy, maintenance requirements, and end-of-life recyclability.

Components

  • Microencapsulated Pcm Slurries
  • Shape Stabilized Pcm
  • Pcm Composites Enhanced Conductivity
  • Metal Foam Pcm Composite
  • Graphite Enhanced Pcm
  • Form Stable Pcm Hdpe Matrix
  • Pcm Nanotechnology Integration
  • Bio Based Pcm Materials
  • Salt Hydrate Pcm Advanced
  • Organic Inorganic Eutectic Pcm