ADDITIVE MANUFACTURING OF PHASE CHANGE MATERIALS AND PHOTOCURABLE RESIN

A thermal energy storage heat exchanger comprising can include a core defining a plurality of airflow passages to receive an airstream therethrough, the core can include microencapsulated phase change material suspended in a photocurable resin. The phase change material can be configured to change phases to store energy from and deliver stored energy to the airstream when the airflow passes through the core.

CLAIM OF PRIORITY

This patent application claims the benefit of priority, under 35 U.S.C. Section 119 (e), to Boetcher et. al., U.S. patent application Ser. No. 18/480,746, entitled “ADDITIVE MANUFACTURING OF PHASE CHANGE MATERIALS AND PHOTOCURABLE RESIN,” filed on Oct. 4, 2023, (Attorney Docket No. 4568.018PRV) which is hereby incorporated by reference herein in its entirety.

BACKGROUND

Phase change materials (PCMs) are materials that can change phases, such as between a liquid and a solid, and are able to store or expend latent heat in the process. PCMs such as hydrocarbons can be useful for energy storage for heating or cooling applications, such as in heating ventilation and air conditioning (HVAC). For example, PCMs are often positioned in containment devices to interact with a heat exchange medium (such as water, air, or refrigerant) when the medium flows through the containment device.

DETAILED DESCRIPTION

PCMs are useful for exchanging energy or heat and have been used in many heat exchanger and energy storage applications. For example, PCMs can be used to store energy during off-peak electrical times for use in comfort cooling. PCMs can also be used within heat exchangers for HVAC or battery cooling. PCMs can also be used in building thermal management, perishables refrigeration, and biomedical applications. PCMs can absorb large amounts of energy in the form of heat as they undergo a phase change, remaining at a nearly constant temperature. PCMs have become increasingly popular in thermal energy storage systems due to their ability to remain at a near constant temperature while they absorb and release large amounts of heat during a solid-liquid phase change; however, these materials can require a shape-stabilizing agent or a containment technique that allows them to be used in thermal applications.

The devices and methods of this disclosure help to address these issues by directly suspending microencapsulated phase-change material (MEPCM) into photocurable resin for the purpose of additively manufacturing heat exchangers capable of storing latent heat for thermal energy storage applications. By using MEPCM, leaking can be limited while still taking advantage of the heat-transfer-enhancing benefits associated with polymer heat exchangers, such as their light weight, complex geometries, and non-fouling behavior. As PCMs undergo thermal cycling in their respective applications, a material continuously cycles between a solid and liquid phase. Using a microencapsulated form of PCM can mitigate this issue and can allow for higher PCM retention and improving system longevity.

Additive manufacturing can enhance the fabrication of PCM-based thermal energy storage (TES) systems by allowing for complex geometries that were previously unobtainable through traditional methods, such as injection molding and casting. The complex geometries can provide high surface-area-to-volume ratios, maximized PCM content, and thinner achievable walls, which can actively enhance the heat transfer performance of the system. The use of these techniques can also allow for the use of materials that were seen as unconventional for thermal applications in previous years. Contrary to their metal counterparts, polymer heat exchangers can be low in weight and cost, antifouling, and anticorrosive. Another advantage to polymers is that they can possess low melting points and can be processed at lower temperatures, meaning that they can be manufactured with less energy than metal heat exchangers. By introducing thinner walls or thermal-conductivity-enhancing additives, such as expanded graphite, graphene, carbon nanotubes, and ceramic fillers, some of the drawbacks to polymers, such as their low thermal conductivity, can be mitigated.

In TES applications, a PCM should have a high latent heat, a high thermal conductivity, chemical stability, a low cost, and be non-flammable. Organic materials can be a common type of PCM used in TES applications because of a wide range of transition temperatures and the chemical stability, thermal stability, and low toxicity, but can also be flammable and possess low thermal conductivity.

The present disclosure includes use of a liquid crystal display (LCD) printer to print functional composites with differing mass fractions of MEPCM and resin. The LCD printing methods discussed herein can provide lower-maintenance printing in shorter print times than other PCM-polymer printing techniques such as fused-filament-fabrication (FFF). The composites can be fabricated in short durations, using low-cost equipment and simple preparation methods. The enhanced geometrical features achievable through additive manufacturing can include a high surface-area-to-volume ratio, thin fins or walls, and uniform PCM suspension across the geometry.

Other additive manufacturing techniques for the printing of PCM composites can include fused-filament-fabrication (FFF), also referred to as fused filament deposition (FDM). FFF and FDM are filament-extrusion-based and can use a continuous filament of a thermoplastic material for printing. FFF and FDM are popular in the field due to a low cost involved with the materials and associated equipment, a wide array of material options, and an overall ability to scale. However, FFF and FDM can be difficult to achieve consistently because FFF and FDM typically involve fabricating a custom composite filament. Additionally, the elevated temperatures involved in the filament extrusion process can compromise the integrity of the encapsulation shells or result in low PCM retention during fabrication. Filament-dependent techniques can usually result in large air gaps in the three-dimensional (3D) printed parts, which can increase the thermal resistance that is found between the print layers and can decrease the effective thermal conductivity of the component.

Other additive manufacturing techniques for the printing of PCM composites can include direct-ink-writing (DIW), which relies on the depositing of an ink through a layer-by-layer extrusion similar to that of FDM or FFF. Other resin-based processes such as stereolithography (SLA), direct light projection (DLP), can also be used to fabricate PCM-based structures. These manufacturing processes can expose a layer of photocurable resin to an ultraviolet (UV) light source or laser. The resultant 3D printed geometries can have an improved resolution in shorter time frames compared to other techniques.

Custom PCM composite filaments can be created using high-density polyethylene for 3D printing using FFF. The thermal properties of the resulting composite can include 40% PCM42 by mass, which can be analyzed to determine the TES capability and thermal conductivity using differential scanning calorimetry (DSC) and transient plane source (TPS), respectively. The composite can have an effective PCM content corresponding to 31.8%, indicating that there can be significant PCM loss during production. This loss can be limited by using MEPCM (micro-encapsulated phase change material). A content of 40% MEPCM by mass can be optimal for printing while maximizing the effects of the PCM, such as by limiting loss of MEPCM reflected as degradation of the latent heat of fusion for a duration of over 50 DSC cycles, alluding to the successful containment of PCM during continuous thermal cycling.

MEPCM can be suspended in a photocurable resin for LCD 3D printing of composites possessing the capability of TES. The composites can be fabricated in short durations, using low-cost equipment and simple preparation methods. The addition of MEPCM to resin can result in a general increase in thermal conductivity when compared to that of pure resin. With the use of encapsulated materials, leaking and PCM retention issues can be greatly reduced. Furthermore, by suspending the MEPCM in photocurable resin, an additional method of containment or encapsulation can be achieved.

The above discussion is intended to provide an overview of subject matter of the present patent application. It is not intended to provide an exclusive or exhaustive explanation of the invention. The description below is included to provide further information about the present patent application.

FIG.1illustrates a schematic view showing a mixture manufacturing method100, in accordance with at least one example of this disclosure. The method100can be a method of manufacturing a resin-MEPCM composite disk114. More specific examples of method100are discussed below. The steps or operations of the method100are illustrated in a particular order for convenience and clarity; many of the discussed operations can be performed in a different sequence or in parallel without materially impacting other operations. The method100, as discussed, includes operations performed by multiple different actors, devices, and/or systems. It is understood that subsets of the operations discussed in the method100can be attributable to a single actor, device, or system and could be considered a separate standalone process or method.

At step106, an MEPCM102and a resin104can be mixed to create a resin-MEPCM mixture108. At step110, the resin-MEPCM mixture108can be loaded to a resin printer109. At step112, the resin-MEPCM composite disk114can be printed using the resin printer109. The resin-MEPCM composite disk can have a diameter of 10-50 mm, 10-30 mm, 30-50 mm, 20-40 mm, 25 mm-35 mm, or about 30 mm. The resin-MEPCM composite disk can have a thickness of 2-10 mm, 2-6 mm, 6-10 mm, 3-7 mm, or about 5 mm. The resin-MEPCM mixture108can have different mass ratios of MEPCM and resin, which can be used in the step106above. The method100can also include any of the steps discussed below.

As shown in Table 1 above, different mixtures of resin and MEPCM can be created by combining the two materials to form specific mass ratios, such as using the method discussed above. Mixing can be done manually with low intensities and speeds to prevent shell fractures as a result from stirring. Since varying the amount of MEPCM in the composite directly correlates to the TES capacity, a greater ratio of MEPCM within the composite can lead to a greater TES capacity; however, 3D printing methods that use photocurable resin as a printing medium can be particularly sensitive to variations in the resin properties such as its viscosity and depth of penetration. Introducing additives such as the MEPCM can alter the viscosity and depth of penetration, and thus can affect the resin curability and cause the printing process to fail. To determine an amount of MEPCM that can result in a resin suitable for LCD printing, the mass content can be increased in 10%, 5%, 2%, or 1% increments. A mixture of 37% MEPCM by mass can have a high PCM content that can also allow for consistent printed samples and consistent quality.

Mixtures with higher MEPCM contents can be prepared, but it can be found that increasing the MEPCM content, along with a corresponding increase in mixture viscosity, can directly correlate with the occurrence of an unsuccessful print. In some examples, the resin-MEPCM mixture108can be screened for consistency or viscosity prior to adding the resin-MEPCM mixture108into the resin printer109.

In some examples, the resin104can be a photocurable resin, such as a High Tensile UV Photopolymer supplied by Photocentric (Avondale, AZ). The resin104can have a heat deflection temperature of 45-75° C., 50-70° C., 55-65° C., 60-70° C., or about 63° C., can have a density of 0.8-1.5 g/cm3, 0.9-1.2 g/cm3, 1.1-1.5 g/cm3, 1-1.2 g/cm3, or about 1.16 g/cm3, and can have a viscosity of 350-650 cPs, 400-550 cPs, 500-650 cPs, 475-525 cPs, or about 510 cPs. The MEPCM102can be raw or a pure phase change material, such as EnFinit PCM 28RPS-T supplied by Encapsys (Applton, WI). The MEPCM can be a paraffin wax, such as hexatriacontane, dotriacontane, triacontane, or any other paraffin wax PCM. The MEPCM can also be other types of PCM's, such as a non-paraffin organic, a hydrated salt, or a metallic PCM. The MEPCM can have a phase change temperature of 20-40° C., 25-35° C., 20-30° C., 25-30° C., or about 28° C.

In some examples, the resin104can be chosen from a diglycidyl ether of bisphenol F, a low epoxy equivalent weight diglycidyl ether of bisphenol A, a liquid epoxy novolac, a liquid aliphatic epoxy, a liquid cycloaliphatic epoxy, a 1,4-cyclohexandimethanoldiglycidylether, 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate, tetraglycidylmethylenedianiline, N,N,N′,N′-tetraglycidyl-4,4′-methylenebisbenzenamine, a triglycidyl of para-aminophenol, N,N,N′,N′-tetraglycidyl-m-xylenediamine, or a mixture thereof. In some examples, the acrylate resin can be chosen from a methacrylate, a methyl acrylate, a ethyl acrylate, a 2-chloroethyl vinyl ether, a 2-ethylhexyl acrylate, a hydroxyethyl methacrylate, a butyl acrylate, a butyl methacrylate, or a mixture thereof.

In some examples, the resin-MEPCM composite disk114can be sliced for printing by a slicing software, such as Chitubox software. The resin printer can be a desktop printer, such as an Elegoo Mars 3.

FIG.2illustrates a schematic view of a printing configuration200. The printing configuration200can include a build plate202, a support structure204, and a sample224. The support structure204can include a support structure base206, a support structure shaft208, a support structure neck210, a support structure contact212, a support structure shaft diameter214, a support structure neck diameter216, a support structure contact diameter218, a support structure contact depth220, and a sample224.

The build plate202can be a sandblasted aluminum alloy and can act as a foundation for a printed structure. The support structure204can be a resin structure that can adhere to the build plate202and the sample224. The support structure base206can be printed directly on the build plate202. The support structure shaft208can be printed on the support structure base206, can extend from the support structure base, and can have a support structure shaft diameter214. The support structure neck210can be printed on the support structure shaft208and can have a support structure neck diameter216. The neck210can extend from the shaft208at an angle, such as between 30 degrees and 60 degrees. The support structure contact212can be printed on the support structure shaft208and can have a support structure contact diameter218. The support structure contact212can be spherical, round, or the like. The contact212can contact the sample224and the support structure contact212can support the sample224. The support structure contact212can have a support structure contact diameter218and a support structure contact depth220. The sample224can be a mixture of resin and MEPCM.

The support structure204can provide support to the sample224while printing unstable portions, such as an overhang, and can prevent the sample224from moving while the sample224is being printed. The build plate202can be a surface that the support structure base206can adhere to. The support structure base206can have a longer exposure time to help with adhesion to the build plate. The support structure shaft208can act as a lifting structure for the printing configuration200. The support structure shaft208can also act as a support to prevent buckling from the sample224and can lift the sample224to prevent the printing of the sample224from being interrupted by the build plate202. The support structure neck210can act as a junction between the sample224and the support structure shaft208and can reduce the area of contact between the sample224and the support structure204. The support structure contact212can adhere directly to the sample224to attach the support structure204to the sample224. The support structure shaft diameter214can be modified to increase the strength of the support structure204. The support structure neck diameter216can also be modified to increase the strength of the support structure204and can be tapered to provide for a brittle point of contact between the sample224and the support structure204for easier removal of the support structure204. The support structure contact diameter218can be adjusted to increase adhesion between the support structure contact212and the sample224. The support structure contact depth220can be adjusted to increase the adhesion area between the support structure204and the sample224. The sample224can be printed using the support structure204, with different mixtures of resin and MEPCM altering the printing configuration200components.

Because the printing configuration200uses a relatively simple support structure204, the printing configuration200can be used to provide an accessible method for printing a mixture of resin and MEPCM. The printing configuration200can also be used to print a mixture of resin and MEPCM with higher concentrations of MEPCM without the occurrence of an unsuccessful print.

The printing configuration200can use different print settings for different MEPCM and resin ratios.

As shown in Table 2 above, resin-MEPCM composite disks can be 3D printed with different print settings. When printing these mixtures, adhesion to the build plate can be challenging due to the increasing viscosity and altered resin properties. Print settings such as exposure time, lift speed, and bottom exposure time can be modified across the different mixtures to account for varying light penetration depth and viscosity. The support structure204can include different settings at the time of printing.

TABLE 3Support settings applied to each sampleratio at the time of printing.SettingUnit0%10%20%30%35%37%Contact diametermm0.800.802.002.002.002.20Upper diametermm0.400.401.001.001.001.00Lower diametermm1.201.201.501.501.501.50Contact depthmm0.400.400.600.600.600.60

As shown in Table 3 above, different settings can be used for the support structure shaft diameter214(lower diameter), the support structure neck diameter216(upper diameter), the support structure contact diameter218(contact diameter), and the support structure contact depth220(contact depth). The supports applied to samples 20%, 30%, 35%, and 37% can feature increased contact diameter, upper diameter, lower diameter, and contact depth. The supports can be carefully removed post-printing, and the sample surface can be thoroughly cleaned using a solvent, such as isopropyl alcohol, methanol, acetone, or the like. The surface can then be wiped to remove any excess uncured resin. The samples can then be cured with a UV light for 30-210 minutes, 60-180 minutes, 75-135 minutes, 105-165 minutes, 90-150 minutes, 110-130 minutes, or about 120 minutes using a UV curing station, such as by using Form Cure by Formlabs (Sommerville, Massachusetts).

FIG.3illustrates an example DSC graph300showing heatflow versus temperature of MEPCM suspended in a resin matrix obtained from differential scanning calorimetry (DSC). The example DSC graph300can include a baseline301, a first axis302, a second axis304, and a sample curve305. The sample curve305can include an onset temperature306, a peak temperature308, an endset temperature310, a sample curve area312, a leading edge313, a leading edge tangent line314, a trailing edge315, a trailing edge tangent line316, and a melting range318.

The baseline301can be a horizontal line on the example DSC graph300. The first axis302can be one of the axis represented on the example DSC graph300. The second axis304can also be one of the axis represented on the example DSC graph300. The sample curve305can be the output produced by DSC for a sample224. The onset temperature306, peak temperature308, endset temperature310, sample curve area312, and melting range318are each values that can be found by analyzing the sample curve305. The leading edge313can be the first edge of the sample curve305, and the trailing edge315can be the second edge of the sample curve305. The leading edge tangent line314and the trailing edge tangent line316can be found by creating a line tangent to the leading edge313and the trailing edge315respectively.

The example DSC graph300can be produced by a differential scanning calorimeter, such as a DSC 3 STARe, from Mettler Toledo. The baseline301can be the signal produced when there are no thermal events occurring and can be used as a reference for comparison to the sample curve305. The first axis302can represent heatflow in the example DSC graph300as a rate of heat transfer per unit mass and can be represented as watts per gram (W/g). The second axis304can represent the temperature and can be represented as degrees Celsius (C). The sample curve305can be produced by DSC. The onset temperature306can be the temperature at which the sample224begins melting, while the endset temperature310can be the temperature at which the sample stops melting. The onset temperature306can also be defined graphically as the intersection between the leading edge tangent lines314and the baseline301, while the endset temperature310can be defined graphically as the intersection between the trailing edge tangent line316and the baseline301. The peak temperature308can represent where, on the temperature axis, the heatflow peaks for the sample224. The melting range318can be determined as the range between the onset temperature306and the endset temperature310. The sample curve area312can be calculated as the area between the baseline301and the sample curve305and can represent the latent heat of fusion. By measuring the latent heat of fusion, the effective amount of MEPCM can be determined in the sample224through the ratio of the measured latent heat of fusion of the resin-MEPCM mixture and the measured latent heat of fusion of pure MEPCM.

In DSC, a sample with a mass of 7-30 mg, 10-25 mg, or 12-18 mg can be placed in an aluminum crucible with a volume of 20-160 μL, 25-120 μL, 30-100 μL, 35-80 μL, or about 40 μL. The sample mass can be measured using an analytical balance, such as an XS105DU from Mettler Toledo. The sample contained in the crucibles can be held at an initial temperature of 0° C. for a duration of 1-20 minutes, or 3-15 minutes, or 8-12 minutes, or about 10 minutes. Then, the temperature can be increased from 0° C. to 60° C. using a heating rate of 0.5-10° C./min, 1-8° C./min, 1.5-5° C./min, or about 2° C./min. The sample temperature of 60° C. can be maintained for 1-20 minutes, 3-18 minutes, 5-15 minutes, 8-12 minutes, or about 10 minutes before being cooled to 0° C. at 0.5-10° C./min, 1-8° C./min, 1.5-5° C./min, or about 2° C./min. Temperature and heatflow can be recorded every one second. These cycles can be performed a single time, or multiple times to help a sample melt and to facilitate thermal contact with the bottom of the crucible before measurement.

FIG.4illustrates a perspective view of a printed sample400, with a first location402and a second location404marked. A sample can be taken from multiple different locations of a printed sample400, such as the first location402and the second location404, to be DSC-tested. By testing samples from multiple different locations, as shown inFIG.4, it can be verified that the MEPCM is evenly distributed across the printed samples.

Samples can be collected from the two regions illustrated inFIG.4to verify the general material distribution as a result of the mixture preparation and printing process. Throughout the sample preparation process, it can be observed that the viscosity of the prepared MEPCM-resin mixture can allow for consistent suspension of the MEPCM even after periods of minimal mixing. It can be found that there is less than 1% latent heat of fusion variance across the tested samples, suggesting a uniform material distribution across the radius of the samples.

FIG.5illustrates a sample DSC graph500showing heatflow versus temperature of MEPCM suspended in a resin matrix obtained from DSC. The sample graph can include a sample curve for a first mixture502, a second mixture504, and a third mixture506.

The first mixture502can have a mixture MEPCM percentage of 10%. The second mixture504can have a mixture MEPCM percentage of 20%. The third mixture506can have a mixture MEPCM percentage of 30%. The different mixture MEPCM percentages can be analyzed to determine trends between values, such as the peak temperature, onset temperature, endset temperature, and latent heat of fusion.

The sample DSC graph500can be used to find the peak temperature, onset temperature, endset temperature, latent heat of fusion, and effective amount of MEPCM for the different resin-MEPCM mixtures from their respective sample curves.

FIG.5can display that a higher MEPCM percentages can provide a higher latent heat of fusion. A higher latent heat of fusion can reduce the size of a heat exchanger and can also increase the efficiency of the heat exchanger.

As shown in Table 4 above, the peak temperature (Tpeak), onset temperature (Tonset), endset temperature (Tendset), latent heat of fusion (hsl), and effective amount of MEPCM (Effective PCM %) can be determined for different resin-MEPCM mixtures from their respective sample curves. It can be shown that as the mixture MEPCM percentage increases, the peak temperature, the onset temperature, and the endset temperature remain similar. It can be shown that the samples have an average peak melting temperature of 30.61° C., an average onset temperature of 27.80° C., and an average endset temperature of 32.89° C.

It can be shown that the latent heat of fusion is generally proportional to the amount of resin in the sample. It can be shown that as the MEPCM content doubles from 10% to 20%, the latent heat of fusion can increase by a factor of 1.79. It can also be shown that as the MEPCM content triples from 10% to 30%, the latent heat of fusion can increase by a factor of 2.81. The factors can suggest an unaccounted loss, which can be attributed to ruptured encapsulation shells that occur during the mixture manufacturing method100, chemical interactions between materials and solvents used in rinsing, or a combination of both.

FIG.6illustrates a perspective view of a thermal content analyzer configuration600. The thermal content analyzer configuration600can include a pair of printed samples400, and a Kapton sensor602. The Kapton sensor602can include a temperature sensor604, and a resistance heater606.

The printed samples400can be manufactured through the process outlined inFIG.1. The Kapton sensor602can have a 4-mm diameter and can be a C7577 by Hot Disk (Gothenburg, Sweden). The Kapton sensor602can be sandwiched between a pair of printed samples400, creating contact between the printed samples400and both the temperature sensor604and the resistance heater606.

The resistance heater606can apply joule heating to the printed samples400. The temperature response of the temperature sensor604can then be measured to determine the thermal conductivity of the samples through a mathematical model. A thermal constants analyzer, such as a Transient Plane Source 2500S by Hot Disk (Gothenburg, Sweden) can take the thermal conductivity measurements. A power of 5 mW and a sampling time of 10 seconds can be used during testing.

LCD printing can have a lack of air gaps, which can result in identical thermal conductivity of a bulk material compared to that of a 3D printed sample that is solid in geometry. LCD printing can produce smooth surfaces, which can be ideal for transient plane source testing because it can ensure good thermal contact with the resistance heater606as shown inFIG.6. Poor contact with the resistance heater606can result in a lower measured thermal conductivity, which can skew results.

FIG.7illustrates a graph of thermal conductivity for samples with different MEPCM content.FIG.7can include error bars to represent standard deviation.

The samples can be measured in accordance with the thermal content analyzer configuration600inFIG.6. A sample can be measured 1-50 times, 2-20 times, 5-15 times, 8-12 times, or about 10 times to determine an average effective thermal conductivity (k) and standard deviation (St. D) for the different MEPCM/resin mixture ratios. The resulting thermal conductivity measurements can be documented.

As shown in Table 5, it can be seen that the addition of MEPCM can result in an increase in thermal conductivity. It can be shown that pure resin can have an average thermal conductivity of 0.200 W/m-K, while an MEPCM content of 10% has an average thermal conductivity of 0.234 W/m-k. The mixture containing an MEPCM content of 37% can have an average thermal conductivity of 0.2655 W/m-K. The thermal conductivity can be further improved by using conductivity-enhancing additives and adjusting the geometry of the mixture. Heat transfer performance can be improved with thinner design walls and high surface-area-to-volume ratios.

FIG.8illustrates an enlarged cross-sectional view of a resin-MEPCM mixture sample. The microstructures of the differing sample ratios can be observed using a scanning electron microscope, such as the Quanta 650 from ThermoFisher (Hillsboro, Oregon). The surface and cross-sectional topography of the samples can be visualized, which can verify the homogeneity of the composite as well as the structural integrity of the capsules post-printing and post-curing. An accelerating voltage of 1-10 kV, 3-8 kV, or about 5 kV can be used. A vacuum pressure between 0.75 and 4.5×10 (−6) Torr, 1.25 and 4×10(−6) Torr, or 1.75 and 3.50×10(−6) Torr can be used. A thin gold coating can be applied using a gold sputter, such as a Sputter Coater 108 from Cressington Scientific Instruments (Watford, United Kingdom), to improve the conductivity of the samples and prevent surface charging.

The cross-sectional view can show a generally homogenous mixture, including a generally even distribution of MEPCM particle802. The cross-sectional view can include areas lacking MEPCM particles802, such as surface804. The cross-sectional view can also include areas with a surplus of MEPCM particles802, such as at MEPCM accumulation806. The different distributions of MEPCM particles802can occur due to material clumping, poor mixing in the mixture manufacturing method100, or a combination of the two. Some of the MEPCM particles802can be hollow, such as in hollow particle808. The hollow particle808can form from rupturing during any of mixing in the mixture manufacturing method100, printing in accordance withFIG.2, or when fracturing the resin-MEPCM mixture to obtain the enlarged cross-sectional view. The hollow particle808can contribute to the MEPCM loss reflected in Table 2 and can result in leaking.

FIG.8shows an image that can be used to verify the general material distribution across the thickness of the samples post-fracture.FIG.9Aillustrates an enlarged perspective view of a surface of resin post-processing.FIG.9Billustrates an enlarged perspective view of a 37% MEPCM content mixture sample post-processing.FIGS.9A and9Bwill be discussed together below.

As shown inFIGS.9A and9B, the addition of MEPCM to a pure resin sample can result in a surface finish that is less smooth than that of pure resin. The pure resin sample inFIG.9Acan have a primarily smooth resin surface finish900, with minimal accumulations902. The MEPCM-resin mixture inFIG.9B, has a mixture surface finish904that can be less smooth from the MEPCM capsules906.

The change in surface finish can be considered for applications involving an internal or an external flow where the walls are made of a MEPCM-resin composite mixture. A sample224can be sanded to result in a smoother surface, however sanding the surface can rupture the particles and can lead to additional hollow particles808as seen inFIG.8. A sample224can follow a post processing cycle of a solvent bath, a wipe down, and UV curing, such as the process outlined inFIG.2, which can result in non-ruptured capsules, such as the MEPCM capsules906inFIG.9B.

FIG.10illustrates a heat exchanger1000that can be formed, at least in part, including MEPCM that is manufactured using the manufacturing processes discussed above. The heat exchanger1000can include a housing1002and a core1004.FIG.11shows the core1004, a first fluid1102, and a second fluid1104.FIGS.10and11are discussed together below.

The heat exchanger1000or the core1004can be manufactured using the process discussed above (the core1004can be manufactured similar to the resin-MEPCM composite disk114) and can use the resin printer109as a three-dimension printer to print a composite of the resin-MEPCM mixture108using the three-dimensional printer to form a three dimensional printed object. The core1004can be located at least partially within the housing1002. The heat exchanger1000can be a cross-flow type heat exchanger often used for exchanging heat between an exhaust air stream (e.g., first fluid1102) and an outdoor air or fresh air intake airstream (e.g., second fluid1104). In operation, energy from the exhaust air stream can interact with the core1004to exchange energy with the core1004and the PCM therein.

The heat exchanger1000can be a thermal energy storage heat exchanger, and can comprise the housing1002and the core1004. The core1004can be located at least partially within the housing1002. The core1004can define a plurality of airflow passages to receive an airstream therethrough (e.g., a first fluid1102). The core1004can comprise a composite of MEPCM (e.g., MEPCM102inFIG.1) suspended in a photocurable resin (e.g., resin104inFIG.1). The MEPCM102can be configured to change phases to store energy from, and deliver stored energy to, the airstream when airflow passes through the core1004.

The composite of the core1004can optionally comprise at least thirty-five percent or thirty-seven percent MEPCM by mass. The composite of the core1004can optionally have an effective MEPCM mass content of at least thirty percent, as discussed relating to Table 4.

The photocurable resin104of the heat exchanger1000can optionally be a high-tensile ultraviolet photopolymer. The MEPCM102of the heat exchanger1000can optionally have a phase change temperature of between twenty-five and thirty-five degrees Celsius.

FIG.12illustrates a perspective view of a portion of layer of fins1200for a reference heat exchanger. The layer of fins1200can also be a fin, row of fins, or the like, whereFIG.12shows a fin having or defining a fin thickness t, a fin height H, with fin spacing S, and fin length L.

FIG.13illustrates a heat exchanger core1304that can be produced using the manufacturing processes discussed herein. The heat exchanger core1304can include a first layer1306, a second layer1308, and a third layer1310. Any of the layers can include a fin or row of fins similar to the fin ofFIG.12. The first layer1306can be made of a first composite fabricated from a first resin-MEPCM mixture. The second layer1308can be made of a second composite fabricated from a second resin-MEPCM mixture that is different than (e.g., a higher MEPCM content by mass) the first resin-MEPCM mixture. The third layer1310can be made of a third composite fabricated from a third resin-MEPCM mixture that is different than (e.g., a higher MEPCM content by mass) the first and second resin-MEPCM mixtures. For example, the first layer1306can be optimized for rapid thermal cycling (and can therefore have the lowest latent heat of fusion to use less energy to change phases), the third layer1310can be optimized as a thermal buffer (and can therefore have the highest latent heat of fusion to absorb excess heat), and the second layer1308can be optimized for rapid thermal cycling, as a thermal buffer, or a latent heat of fusion in between to perform sub-optimally but effectively in both rapid thermal cycling and as a thermal buffer.

NOTES AND EXAMPLES

Example 1 is a thermal energy storage heat exchanger comprising: a core defining a plurality of airflow passages to receive an airstream therethrough, the core comprising microencapsulated phase change material suspended in a photocurable resin, the phase change material configured to change phases to store energy from and deliver stored energy to the airstream when the airflow passes through the core.

In Example 2, the subject matter of Example 1 optionally includes wherein the composite of the core comprises at least thirty five percent phase change material by mass.

In Example 3, the subject matter of any one or more of Examples 1-2 optionally include wherein the photocurable resin is a high tensile ultraviolet photopolymer.

In Example 4, the subject matter of any one or more of Examples 1-3 optionally include wherein the phase change material has a phase change temperature of 28 degrees Celsius.

Example 5 is a heat exchanger as shown and described herein.

Example 6 is a method for fabricating a heat exchanger or heat exchanger core as shown and described herein.

Example 7 is a system for conditioning an airstream as shown and described herein.

Example 8 is a thermal energy storage method as shown and described herein.

Example 9 is a thermal energy storage heat exchanger comprising: a housing; a core located at least partially within the housing, the core defining a plurality of airflow passages to receive an airstream therethrough, the core comprising a composite of microencapsulated phase change material (MEPCM) suspended in a photocurable resin, the MEPCM configured to change phases to store energy from and deliver stored energy to the airstream when airflow passes through the core.

In Example 10, the subject matter of Example 9 optionally includes wherein the composite of the core comprises at least thirty-five percent MEPCM by mass.

In Example 11, the subject matter of any one or more of Examples 9-10 optionally include wherein the composite of the core comprises thirty-seven percent MEPCM by mass.

In Example 12, the subject matter of any one or more of Examples 9-11 optionally include wherein the photocurable resin is a high-tensile ultraviolet photopolymer.

In Example 13, the subject matter of any one or more of Examples 9-12 optionally include wherein the MEPCM has a phase change temperature of between twenty-five and thirty-five degrees Celsius.

In Example 14, the subject matter of any one or more of Examples 9-13 optionally include wherein the thermal energy storage heat exchanger is a cross-flow heat exchanger.

In Example 15, the subject matter of any one or more of Examples 9-14 optionally include wherein the composite has an effective MEPCM mass content of at least thirty percent.

Example 16 is a method for fabricating a heat exchanger or heat exchanger core, the method comprising: providing a polymer resin; providing a microencapsulated phase change material (MEPCM); mixing the polymer resin and the MEPCM to form a mixture of the polymer resin and the MEPCM; loading a mixture of the polymer resin and the MEPCM into a three-dimensional printer (3D printer); printing a composite of the mixture of the polymer resin and the MEPCM using the 3D printer to form a 3D printed object.

In Example 17, the subject matter of Example 16 optionally includes wherein the polymer resin is a photocurable resin.

In Example 18, the subject matter of Example 17 optionally includes wherein the photocurable resin is a high tensile ultraviolet photopolymer.

In Example 19, the subject matter of any one or more of Examples 17-18 optionally include wherein mixing the polymer resin and the MEPCM is performed at a speed where fracture of the MEPCM is limited.

In Example 20, the subject matter of any one or more of Examples 16-19 optionally include wherein the composite comprises at least thirty-five percent MEPCM by mass.

In Example 21, the subject matter of any one or more of Examples 16-20 optionally include wherein the composite comprises thirty-seven percent MEPCM by mass.

In Example 22, the subject matter of any one or more of Examples 16-21 optionally include wherein the composite has an effective MEPCM mass content of at least thirty percent.

In Example 23, the subject matter of any one or more of Examples 16-22 optionally include wherein the 3D printed object is a core of a heat exchanger.

In Example 24, the subject matter of any one or more of Examples 16-23 optionally include wherein the MEPCM has a phase change temperature of between twenty-five and thirty-five degrees Celsius.

Example 25 is a method for encapsulating a phase change material for use in thermal applications, the method comprising; providing a photocurable resin; providing a microencapsulated phase change material (MEPCM); providing a conductivity-enhancing additive; suspending the MEPCM and the conductivity-enhancing additive in the photocurable resin to form a suspension; loading the suspension into a three-dimensional printer (3D printer); printing, using the 3D printer, a 3D-printed object from the suspension.

In Example 26, the subject matter of Example 25 optionally includes loading a UV curing station with the 3D-printed object; curing, with the UV curing station, the 3D-printed object for a period of time between 90 minutes and 150 minutes.

In Example 27, the subject matter of any one or more of Examples 25-26 optionally include wherein the MEPCM has a phase change temperature of between twenty-five and thirty-five degrees Celsius.

In Example 28, the subject matter of any one or more of Examples 25-27 optionally include wherein the suspension has an effective MEPCM mass content of at least thirty percent.

In Example 29, the apparatuses or method of any one or any combination of Examples 1-28 can optionally be configured such that all elements or options recited are available to use or select from.

In the event of inconsistent usages between this document and any documents so incorporated by reference, the usage in this document controls. In this document, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Also, in the following claims, the terms “including” and “comprising” are open-ended, that is, a system, device, article, composition, formulation, or process that includes elements in addition to those listed after such a term in a claim are still deemed to fall within the scope of that claim.