Smart Cooling for the 21st Century

Delta Thermal Apparel

Technology

 
Photomicrograph of MicroPCM Powder of 10-50 micron diameter particles.  These particles have walls that are 1-2 microns thick, which can be slurried in two-component coolants, embedded in textile fibers, foams and composites, and used in microPCM paints and coatings.  The dimpled walls of the particles indicate that the PCM is in its solid state.  As the core of the PCM changes phase, the microPCMs expanded by 12-15% into spherical particles.

 

 

 

 

 

 

Stack of 2-4mm Macroencapsulated Phase Change Material (MacroPCM™) Particles on a Dime.  COOLBEADS™ are rugged, safe and highly fire-resistant due to their unique encapsulation coating.  The air spaces between the particles permit heat and moisture to escape between them.  A layer of the spherical COOLBEADS™ becomes a “packed-bed heat exchanger,” which facilitates heat transmission to the phase change material remote from the immediate surface as well as the recharging of the macroPCM™ thermal media.  Like the 10-50 micron diameter microPCM™ developed earlier at TRDC, the latent solid/liquid phase change of the core material can provide enhanced thermal storage for coolants, composites, textile fibers, foams and coatings, but the larger macroPCM™ particles can hold significantly more heat, facilitate thermal recharging, and also provide superior fire protection.   

 

 

 

 

PCM – 101

ENCAPSULATED PHASE CHANGE MATERIALS

David P. Colvin, Ph.D.

Thermal energy or heat can be stored as either sensible or latent heat.  Sensible heat storage depends upon a change in temperature and occurs with all materials and temperatures.  Latent heat storage, however, depends upon a change in phase of a material, such as a solid to a liquid or a liquid to a gas, and occurs at specific temperatures for all materials.  When a phase change occurs in a material, it requires an unusually high quantity of energy to be transferred in order to move from one physical state to another.  For example, when ice changes to water, it must absorb approximately 80 calories/gram.  When water changes back to ice, it must also give up that much thermal energy.  In a latent phase change material (PCM), the energy storage is based upon the heat absorbed or released when a material passes through a reversible phase transition.  Compared to sensible heat storage systems, latent heat storage systems have a higher storage capacity per unit weight or volume, greater system efficiency, and require a simpler control system. 

In selecting a PCM for a specific application, the operating temperature of the heating or cooling cycle must be matched to the phase transition temperature of the material.  In this regard, PCMs are less versatile than sensible heat storage materials.  However, if a PCM can be found that operates at the desired temperature, it can be used to store large quantities of thermal energy or heat at that temperature.  For example, water has a sensible heat storage of 1 calorie/gram, but a latent energy storage that is 80 times larger.  The problem for water is that the solid/liquid phase change only occurs at 0°C or 32°F, which greatly limits its application as a PCM.  Other PCMs exist, however, that change phase at different temperatures.  For example, the phase change temperature of some pure paraffins occur at temperatures ranging from sub-ambient to greater than 60°C, depending on the length of their carbon chain.  The temperature that a paraffin, such as octadecane changes its phase from a solid to a liquid, is about 26-28°C, depending upon its purity.  Furthermore, octadecane can store up to 58 calories/gram depending upon its purity.  Other paraffins and materials exhibit different phase change temperatures, but can exhibit similar quantities of heat storage, albeit not all associated with the phase change.  If the number of carbons in the chain is odd and /or the chain length is greater than 20 carbons, a portion of the latent heat is associated with secondary transitions that occur in the solid state.

Microencapsulation is a process that separates a selected material from its surroundings or the media in which it is placed.  The microencapsulation of materials is not new.  Timed-release drugs, inks for copy papers, and scratch & sniff samples are examples of the microencapsulation of a variety of materials.  However, all of these examples use a capsule wall that is designed to fail in order for the system to be successful.  In contrast, the microencapsulation of PCMs is new and relies upon the use of a capsule wall that is designed to last.  Since 1983, the application of this technology has been pioneered by investigators at Triangle Research and Development Corporation (TRDC) through more than 28 Small Business Innovation Research (SBIR) programs for DOD (USAF, NAVY, SDIO, SOCOM, and USMC), NSF, NASA, HHS and the USDA.  When added to selected materials, microencapsulated phase change materials or microPCMs have been shown to significantly enhance the thermal storage and performance of coolants, textile fibers and textiles, foams, composites and coatings.  Microencapsulation also prevents the selected PCM from mixing with the surrounding media when it melts.  Figure 1 shows the cross-section of a microPCM in which the core PCM is surrounded by a polymer shell.  Diameters of microPCMs can range from 0.5 to 1,000 microns.  For use within textile fibers, small particles under 1.0 micron are common; while in foams, larger particles ranging from 10 to 50 microns are often used.  Figure 2 illustrates a scanning electron micrograph of a microPCM powder.  The dimpled surface of the particles indicates that the core material is in its contracted or solid state.  The phase change temperatures are also affected by the microencapsulation process and the size of the particles.  The larger the particles, the closer the phase change temperature is to that for the bulk material.  The smaller the particles, the greater is the difference between the melting and freezing temperatures for the PCM. 

 

  Figure 1. Cross-Section of MicroPCM Particle             Figure 2. SEM of MicroPCM Powder

The application of microPCMs can be divided into two general areas: active coolant circulation and passive thermal storage.  For active systems, microPCMs can be added to conventional coolants to produce two-component slurries with 10-50 micron diameter particles suspended within them.  The base or conventional coolant can be either aqueous or non-aqueous; i.e., water or an oil.  When a microPCM coolant is circulated in an optimal manner (i.e., when practically all of the heat is transferred in the latent phase change of the particles rather than in the sensible heat transfer of the coolant), the enhancement in its effective thermal capacitance has been shown to be up to 40x greater (up to 4,000%) than the same coolant without microPCMs.  Furthermore, the optimal use of microPCMs can further improve heat transfer across the heat exchanger boundary wall significantly, sometimes up to 3x or 300%.  Numerous programs at TRDC since 1983 with NSF, NASA, USAF, NAVY, Lockheed and General Motors have demonstrated the effectiveness of these new microPCM coolants to transport more heat than any other conventional coolant. When added to a machine coolant, for example, microPCMs can reduce tool wear, temperature excursions and coolant evaporation, thus resulting in a safer and more environmentally-friendly material.  Ten comprehensive patents have now been awarded to TRDC for different applications involving microPCMs. 

For passive systems such as apparel, microPCMs can also be added to a variety of conventional materials.  For example, when microPCMs are added to fibers and foams, their presence can enhance the effective thermal capacitance of these materials by as much as 10x or 1,000%.  MicroPCMs added to composites such as molded electronic heat sinks can result in lighter, but more capable thermal management of electronic components.  The addition of microPCMs to liquid sprays has also been demonstrated to significantly affect the microclimate on the surfaces of plants.  For example, the use of a microPCM spray has been shown to enhance the protection of plants to both heat and cold stress.  When added to selected mycoherbicides, microPCMs and microH20 capsules can significantly enhance their effectiveness as an environmentally-friendly biocontrol agent in agriculture.

SBIR Programs at TRDC for the USAF, NAVY and NSF have repeatedly demonstrated that microPCMs can be either spun into the textile fibers or coated onto them.  They can also be embedded in a variety of foams.  The presence of microPCMs in these materials can store latent heat at a specific comfort temperature, thus preventing large temperature excursions.  TRDC licensed both Outlast Technologies in Boulder, Colorado in 1991 the use of microPCMs either into or onto textile fibers.  A wide variety of microPCM products, such as clothing and bedding from manufactures using a sublicense from Outlast Technologies, are currently sold in stores nationwide under the trade name Outlastä.  MicroPCM foams and apparel were also licensed to Frisby Technologies from 1995 to 2002 for gloves, clothing, hand warmers and boots, but that license was terminated in March of 2002. 

TRDC is currently developing other products that will utilize significantly larger capsules or macroPCMs for enhanced apparel cooling.  These applications have now been awarded three patents, and more are pending.  TRDC and its associated companies, Delta Thermal Systems and Delta Thermal Apparel, continues to explore new ways to use micro and macroPCMs.   PECS™ and COOLTECH™ microclimate cooling apparel for the military and civilians are an example of a macroPCMÔ garment that will be used beneath NBC protective clothing to provide superior cooling for 1-3 hours under unusually high heat stress conditions.  Ongoing programs at TRDC for NSF are currently the development of new capsule wall materials that can sustain temperatures over 300°C, which makes them suitable for use in melt-spun fibers such as polypropylenes and nylons.   Research is also underway at TRDC to develop new microPCM spinning technologies as well as more economical means for producing both micro- and macroPCM particles.  Essentially, PCM apparel can acts as a thermal spring; i.e., its can absorb heat at a selected temperature during physical exercise, and release it later during quiet periods of rest.  MicroPCM textile fibers and foams also result in more comfortable as well as thinner or less bulky apparel.

 

Contracts

  1. Colvin, D.P. and J.C. Mulligan: “Investigation and Development of a Phase Change Material Energy Storage System Using Microencapsulated Phase Change Materials.’ SBIR Phase I Final Report, NASA Contract No. NAS8-35840, May 1984.
  1. Colvin, D.P. and J.C. Mulligan: “Spacecraft Heat Rejection Methods: Active and Passive Heat Transfer for Electronic Systems.” SBIR Phase I Final Report, USAF Contract No. F33615-85-C-3420, July 1986.
  1. Colvin, D.P. and J.C. Mulligan: “Investigation and Development of a Phase Change Thermal Energy Storage System Using Microencapsulated Phase Change Materials – Phase II Final Report.” NASA Contract No. NAS8-35840, 1987.
  1. Bryant, Y.G.: “Enhanced Thermal Energy Storage in Clothing with Impregnated Microencapsulated PCM.” SBIR Phase I Final Report, USAF Contract No. F33657-87-C-2138, February 1988.
  1. Colvin, D.P. and J.C. Mulligan: “Enhanced Heat Transfer and Storage materials for Strategic Defense Systems.” SBIR Phase I Final Report, SDIO Contract No. F33615-87-C-2746, May 1988.
  1. Colvin, D.P.: “Space Suit Thermal Control Using Non-Toxic, Two-Phase PCM Fluid.” SBIR Phase I Final Report, NASA Contract No. NAS9-17952, August 1988.
  1. Colvin, D.P.: “Spacecraft Heat Rejection Methods: Active and Passive Heat Transfer for Electronic Systems.” SBIR Phase II Final Report, USAF Contract No. F33615-86-C-3430, July 1989.
  1. Bryant, Y.G.: “Spacesuit Glove-Liner with Enhanced Thermal Properties for Improved Comfort.” SBIR Phase I Final Report, NASA Contract No. NAS9-18110, August 1989.
  1. Colvin, D.P. and J.C. Mulligan: “Microencapsulated Phase Change Material Heat Transfer Systems.” SBIR Phase II Final Report, Contract No. F33615-86-3072, Wright Research and Development Center, August 1989.
  1. Colvin, D.P.: “Two-Component MicroPCM Cutting and Coolant Fluids for High-Speed Machining.” SBIR Phase I Final Report, NSF Contract No. ISI-8961499, January 1991.
  1. Colvin, D.P.: “Passive PCM Microclimate Body Undergarment Thermal Management System.” SBIR Phase I Final Report, USMC Contract No. N60921-92-C-A332, February 1993.
  1. Colvin, D.P and J. Bailey: “Two-Component Lubricant/Coolant Fluids for High-Speed Machining.” SBIR Phase II Final Report, NSF Contract No. ISI-8941499, November 1994.
  1. Bryant, Y.G.: “Enhanced Thermal Energy Storage in Clothing with Impregnated, Microencapsulated PCM.” SBIR Phase II Final Report, Navy Contract No. N00600-93C-3785, November 1994.
  1. Colvin, D.P.: “Using Diamond Film Heat Spreaders and Microencapsulated Phase Change Materials for Electronics Cooling.” SBIR Phase I Final Report, Navy Contract, June 1996.
  1. Bryant, Y.G.: “Materials for Extending Endurance and Productivity of Underwater Divers in Cold Waters.” SBIR Phase I Final Report, NSF Grant, August 1996.
  1. Colvin, D.P.: “Lightweight, Regenerative MicroPCM Portable Environmental Control Systems (PECS) Garments.” SBIR Phase I Final Report, USMC Contract, June 1997.
  1.  Bryant, Y.G.: “Investigation of Phase Change Material Microcapsules for High Temperature Applications.” SBIR Phase I Final Report, NSF Grant, 1998.
  1. Colvin, D.P. and D.K. Cartwright:  “Stabilization of Microenvironmental Temperatures on Plant Surfaces with MicroPCMs to Prevent Frost/Freeze Damage.” SBIR Phase I Final Report, NSF Contract, 1998.
  1. Colvin, D.P.: “Reduced Mist Generation with MicroPCM Machining Coolants.” SBIR Phase I Final Report to HHS/NIOSH/CDC, June 1999.
  1. Colvin, D.P.: “Phyllosphere Microclimate Regulation with MicroPCMs to Improve Mycoherbicides.” SBIR Phase I Final Report, USDA Contract, January 1999.
  1. Cartwright, D.K.: “Reduction of Heat-Induced Symptoms in Plants with MicroPCMs.” SBIR Phase I Final Report to NSF, 1999.
  1. Colvin, D.P.: “Lightweight, Regenerative MacroPCM Environmental Control Systems (PECS) Garments.” SBIR Phase II Final Report, USMC/NAVY Contract No. N00140-98-C-1461, December 2000.

  Patents

  1. Yvonne G. Bryant and David P. Colvin: “Fiber with Reversible Enhanced Thermal Storage Properties and Fabrics Made Therefrom.”  U.S. Patent No. 4,756,958, filed on Aug. 31, 1987, issued on July 12, 1988.  Assigned to Triangle Research and Development Corporation.

  2. David P. Colvin and James C. Mulligan: “Thermal Energy Storage Apparatus Using Encapsulated Phase Change Materials.”  U.S. Patent No. 4,807,696, filed on Dec. 10, 1987, issued on Feb. 28, 1989.  Assigned to Triangle Research and Development Corporation.

  3. David P. Colvin and James C. Mulligan: “Method of Using a PCM Slurry to Enhance Heat Transfer in Liquids.” U.S. Patent No. 4,911,232, filed on Apr. 27, 1989, issued March 27, 1990.  Assigned to Triangle Research and Development Corporation.

  4.  Raymond A. Whitney, Virginia S. Colvin, David P. Colvin, and James C. Mulligan.  “Two-Component Cutting/Cooling Fluids for High Speed Machining.” U.S. Patent No. 5,141,079, filed on Jul.26, 1991, issued on Aug. 25, 1992.  Assigned to Triangle Research and Development Corporation.

  5. David P. Colvin and James C. Mulligan: “Quick-Disconnect Thermal Coupler.” U.S. Patent No. 5,148,861, filed Jul. 31, 1991, issued Sept. 22, 1992.  Assigned to Triangle Research and Development Corporation.

  6. David P. Colvin, Yvonne G. Bryant and James C. Mulligan: “Method of Using ThermalEnergy Absorbing and Conducting Potting Materials.”   U.S. Patent No. 5,224,356, filed Sept. 30, 1991, issued on July 6, 1993.  Assigned to Triangle Research and Development Corporation.

  7. David P. Colvin, Yvonne G. Bryant and James C. Mulligan: “Heat Shield.” U.S. Patent No. 5,290,904, filed on Jul. 31, 1991, issued on March 1, 1994.  Assigned to Triangle Research and Development Corporation.

  8. Yvonne G. Bryant and David P. Colvin: “Fabric with Reversible Enhanced Thermal Properties.” U.S. Patent No. 5,366,801, filed on May 29, 1992, issued on Nov. 22, 1994.  Assigned to Triangle Research and Development Corporation.

  9. David P. Colvin and Yvonne G. Bryant: “Micro-Climate Cooling Garment.” U.S. Patent No. 5,415,222, filed on Nov. 19, 1993, issued on May 16, 1995.  Assigned to Triangle Research and Development Corporation.

  10. Yvonne G. Bryant and David P. Colvin: “Moldable Foam Insole with Reversible Enhanced Thermal Storage Properties.” U.S. Patent No. 5,499,460, filed on Jul. 13, 1994, issued on Mar. 19, 1996.  Assigned to Triangle Research and Development Corporation.

  11. David P. Colvin and Yvonne G. Bryant: “Thermally Enhanced Foam Insulation.” U.S.   Patent No. 5,637,389, filed on Mar. 19, 1996, issued on June 10, 1997.  Assigned to Triangle Research and Development Corporation.

  12. David P. Colvin, Yvonne G. Bryant, John C. Driscoll and James C. Mulligan: “Thermal Insulating Coating Employing Microencapsulated Phase Change Material and Method.” U.S. Patent No. 5,804,297, filed on Jul. 5, 1995, issued on Sept. 8, 1998.  Assigned to Triangle Research and Development Corporation.

  13. David P. Colvin and Donald K. Cartwright: “Microclimate Environmental Control on Vegetation and Seeds Employing Microencapsulated Water and Phase Change Materials and Method.” U.S. Patent No. 6,057,266, filed on Aug. 4, 1998, issued on May 2, 2000.  Assigned to Delta Thermal Systems, Inc.

  14. Virginia S. Colvin and David P. Colvin: “Microclimate Temperature Regulating Pad and Products Made Therefrom.  U.S. Patent No. 6,298,907 B1, filed on Apr. 26, 2000, issued on Oct. 9, 2001.  Assigned to Delta Thermal Systems, Inc.

 Many of the above patents are also issued in foreign countries.

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