small scale thermal energy storage with capillary

SMALL-SCALE THERMAL ENERGY STORAGE WITH CAPILLARY CONDUCTIVITYENHANCEMENT

A. Hays, E. Borquist, D. Bailey, D. Wood, L. Weiss ∗

Institute for MicromanufacturingDepartment of Mechanical Engineering

Louisiana Tech UniversityRuston, LA 71272

Email: [email protected]

ABSTRACTThermal energy is a leading topic of discussion in energy

conservation and environmental fields. Specifically for large-scale applications solar energy and concentrated solar power(CSP) systems use techniques that include thermal energy stor-age systems and phase change materials to harvest energy. How-ever, on the smaller centimeter scale, there have been historicallyfewer investigations of these same techniques. The main goal ofthis paper is to investigate thermal energy storage (TES) as ap-plied to a small scale system for thermal energy capture. Typicallarge-scale TES consists of a phase change material that usuallyemploys a wax or oil medium held within a conductive container.The system stores the energy when the wax medium undergoes aphase change. In typical applications like buildings, the systemabsorbs and stores incoming thermal energy during the day, andreleases it back to the surrounding environment as temperaturescool at night.

This paper presents a new TES unit designed to integratewith a thermoelectric for energy harvesting application in small,cm-scale applications. In this manner, the TES serves as a ther-mal battery and source for the thermoelectric, even when origi-nating power supply is interrupted. A unique feature of this TESis the inclusion of internal heat pipes. These heat pipes are fabri-cated from copper tubing and filled with working fluid, mountedvertically, and immersed in the wax medium of the TES. Thistransfers heat to the wax by means of thermal conductivity en-

∗Address all correspondence to this author.

hancement as an element of the heat pipe operation. This rep-resents a first of its kind in this small-scale, thermal harvestingapplication.

As tested, the TES rests atop a low temperature (60 ◦C) heatsource with a heat sink as the final setup component. The heatsink serves to simulate thermal energy rejection to a future ther-moelectric device. To measure the temperature change of thedevice, thermocouples are placed on either side of the TES, anda third placed on the heat source to ensure that the energy inputis appropriate and constant. Heat flux sensors (HFS) are placedbetween the heat source and the TES and between the TES andheat sink to monitor heat transferred to and from the device.

The TES is tested in a variety constructions as part of thiseffort. Basic design of the storage volume as well as fluid filllevels within the heat pipes are considered. Varying thermal en-ergy inputs are also studied. Temperature and heat flux data arecompared to show the thermal absorption capability and operat-ing average thermal conductivities of the TES units. The base-line average thermal conductivity of the TES is approximately0.5 W/mK. This represents the TES with wax alone filling the in-ternal volume. Results indicate a fully functional, heat pipe TEScapable of 8.23 W/mK.

INTRODUCTIONThermal energy storage (TES) has many applications, but is

more commonly known for large-scale applications. These in-clude concentrated solar power systems and structures that have

Proceedings of the ASME 2016 10th International Conference on Energy Sustainability ES2016

June 26-30, 2016, Charlotte, North Carolina

ES2016-59582

1 Copyright © 2016 by ASME

Figure 1. WAX FILLED POROUS COPPER FOAM

energy storage capabilities. The first known application of phasechange materials (PCM) was for heating and cooling of build-ings [1], and this area of application continues to have signif-icant research by various groups [2, 3]. On the smaller scale,some unique TES applications include heating and cooling sys-tems in clothing for athletes to better monitor body temperature,and assisting the transport of temperature sensitive medicationsand foods [4, 5].

General TES technology is based on phase change material(PCM) concepts. These concepts are attractive due to the highenergy storage capability of the solid-liquid phase transition [6].The ability to absorb, store, and subsequently release the thermalenergy through the phase change process, similar to the work of atraditional battery, allows the TES presented here to be describedas a so-called “thermal battery.” The amount of heat that is storedwithin the media is dependent on several factors including thespecific heat of the medium, temperature gradient across the TESdevice, and the amount of storage material [6].

Most PCMs have low thermal conductivity and thus requireconductivity enhancements to increase the energy charging anddischarging rates [7]. Prior work at Louisiana Tech has fabri-cated several different enhancements for TES systems that maybe utilized alongside heat exchangers as either thermal batteriesor heat sinks. These systems have included copper inserts such ascopper foams or matrices as shown in Figure 1. Prior work hasalso considered the use of unique halloysite clay nanocompos-ites to maintain form and structure of the thermal storage mediaeven when undergoing solid-liquid phase change [8,9]. Prior ex-periments tested the limits of thermal conductivity enhancementon thermoelectric operation [10]. Results indicated an 8 fold in-crease in thermal conductivity from base line wax, and a systemcapable of extending thermoelectric operation by several minutesusing a copper foam enhancement [10].

In this present work, the motivation is to create a final de-vice that packages a TES with a MHE (Micro-Channel Heat Ex-

Figure 2. SMALL SCALE POWER SYSTEM SCHEMATIC

changer) and TEG (Thermoelectric Generator) in order to collectand transfer thermal energy into electrical energy. This small,cm-scale system will be able to provide a standalone power sup-ply for autonomous sensor application. The MHE is designedto operate from low-temperature ambient sources that includesolar or waste heat, and has been detailed in prior publications[11–13]. The full system setup is shown in Figure 2 where theTES role in this final setup acts as a thermal battery.

Figure 2 also indicates the flow of energy in the fully con-structed system. Energy is captured by the MHE from an ambi-ent source and flows into the TES where excess is stored beforepowering the TEG. The TES can continue to supply energy tothe TEG even after the original source has terminated supply.The duration of this additional energy supply and the effects onTEG performance are currently under investigation in our labs.The TES also provides a continuous temperature value to the hotside of the TEG, which helps provide the TEG with the stable en-vironment that it needs to produce reliable electricity. The workpresented in this paper specifically concentrates on design andcharacterization of the TES component itself.

Device Operating Principle and OverviewThe basic TES design in this effort included a lower surface

for thermal energy input, a volume filled with paraffin wax forphase change thermal energy storage, and a top that provided aseal to the inner phase change material. There were two princi-ple challenges in TES design that were the focus for this presentwork. First, increasing the thermal conductivity within the phasechange volume. Second, the containment of the phase changemedia itself as it transitioned from solid to liquid.

To address the thermal conductivity challenge, the new TESutilized copper tubing which was inserted in a matrix patternwithin the wax volume. The tubing was filled with a workingfluid that allowed the tubing to function as a useful, internal heatpipe. Figure 3 shows the vertical capillary tubing risers withinthe TES as part of larger scavenging system with an attachedheat exchanger (HX). This represented a departure from the priorreported efforts focussed on metallic foam inserts.

The device was able to store the thermal energy absorbedin the paraffin wax as it transitioned phases from a solid to aliquid as heat was applied. The challenge of containing the phasechange media was addressed by investigating the containmentdesign via 3D printed polystyrene siding. The volume of the


Figure 3. CAPILLARY TUBING RISERS WITHIN PCM ATTACHED TOHEAT EXCHANGER (HX)

Figure 4. POLYSTYRENE 3D FABRICATED TES SIDING

phase change material was also varied to determine the optimalvolume. Finally, various epoxies and gaskets were considered tocreate an effective seal between the polystyrene siding and thecopper plates that completed the exterior of the device.

EXPERIMENTAL METHODSFabrication

This section explains the techniques used in the fabricationof the TES device as well as the different constructions testedin this effort. The efficiency of operation of the TES was ofprimary consideration resulting in two distinct thrusts: overalldevice conductivity enhancement through internal capillary heatpipes (Figure 3) as well as sealing and encapsulating volumestudies that served to achieve the basic goal of this work.

Independent of device geometry or specific design dimen-sions, several general fabrication steps were necessary to createthe TES. First, the siding of the device was fabricated using a3D printer to create a layer-by-layer structure from HIPS (HighImpact Polystyrene). Figure 4 shows the basic siding and PCMstorage volume for a TES unit.

3D additive manufacturing provided highly controllable de-sign parameters that were easily modified for different designconsiderations. The material utilized, HIPS, further maintainedlow energy loss to surroundings due to its low thermal conductiv-ity properties. TES sidings were designed using SolidWorks andprinter settings were controlled with included Lulzbot 3D printer

software, Cura.Following the printing of the basic TES siding, capillary

copper tubing was prepared for insertion within the fabricatedTES volume. These internal heat pipes consisted of 25 coppertubes, or risers, of approximately 9.5 ±0.3 mm tall, 2 mm outerdiameter and 1.3 mm inner diameter. This gave an internal riservolume of about 13.0 µL. The risers were spaced approximately5 mm apart. The risers were set atop one copper plate that was700 µm thick. This copper plate was subsequently utilized as theTES bottom surface. Capillary tubes were epoxied in place us-ing JB Weld. This two-part epoxy had thermal conductivity of0.5 W/mK.

Copper tubes were then filled with 3MT M HFE 7200 work-ing fluid to form the working capillary tube. HFE 7200 is highlyevaporative and boils at a temperature of 76 ◦C. This fluid hasalso shown hydrophilic properties with previously studied fab-ricated nickel and copper channels [11, 14]. This made it wellsuited to the low-temperature thermal scavenging orientation ofthe present TES design as well as the melt point of the wax itself.After filling, each tube top side was immediately sealed with JBWeld epoxy.

The lower copper plate, with attached capillary tubes, wasthen sealed to the bottom side of the fabricated siding. Followingthe investigation of multiple techniques and epoxies, a combina-tion of Red RTV high temperature gasket maker and Elmer’s su-per fast epoxy cement was ultimately used to permanently bondand seal the siding to the copper plate surface. Other materialsconsidered ultimately proved less successful in terms of seal in-tegrity. These included film gasket sheets, semiconductor tape,PC-7 high temperature epoxy, and JB Weld.

Following sealing, eicosane wax (paraffin) was melted andpoured around the tubes to fill the TES interior volume and serveas the PCM. The wax had a melting point of 52 ◦C. A 700 µmthick copper plate was sealed to the top of the TES to fully com-plete the device and ready it for testing. Figure 5 shows the as-sembled capillaries and PCM within a partially completed TES.

Several distinct designs and assemblies were studied inthis effort to determine the effects of capillary tubing operationwithin the PCM as well as TES geometry on sealing integrity.Baseline constructions utilized a square TES design with the in-terior dimensions of 21 x 21 x 10 mm with 4 mm thick walls(Figure 5). This basic design was utilized for all specific capil-lary tubing tests described in the subsequent section. Capillarytubing tests considered a range of working fluid within the capil-lary tubing. Individual TES devices were fabricated with 0, 12.5,25, and 50 % working fluid fills by volume within the tubing.

Seal integrity tests required modified devices with differentwax fill amounts and siding corner geometries versus the base-line design. This was due to the slight expansion of the paraffinfrom solid to liquid state during operation. This increased theinterior TES pressure such that top and bottom seals could becompromised. A range of paraffin loading quantities were inves-


Figure 5. PARTIALLY FABRICATED TES WITH INTERNAL CAPILLARYRISERS AND PCM

Figure 6. PARTIALLY FABRICATED TES WITH 12 SIDED VOLUMEAND CAPILLARY RISERS

tigated to yield the preferred, final wax (PCM) mass for the TESas detailed in the Test Setup section.

Different TES siding geometries were also considered to ad-dress sealing and leakage challenges during operation. Theseincluded the baseline design with 90 degree corners (square vol-ume) as well as a design that increased the angle of the cornerssuch that the TES itself adopted a 12-sided geometry. Figure 6illustrates this assembly modification. This subsequent designmaintained similar interior volumes and cross sectional areas tothe initial, square baseline design. This allowed the amount ofuseful phase change material to remain constant across designsas well as the number of vertical tubing risers.

Test SetupThe general test setup for these experiments consisted of

an assembled TES device, thermal input provided via resistanceheater, heat flux sensor (HFS) that monitored energy input to theTES directly, and thermocouples located at the lower and topsurfaces of the TES. Thermal input was simulated by electricalresistance heater connected to a power supply. The basic datacollection setup for these tests is illustrated in Figure 7.

Figure 7. BASIC TEST SETUP SCHEMATIC

The TC utilized were sourced from Omega Engineering andwere k-type bare wire TCs. These were placed on the top andbottom of the TES plates to measure the temperature gradient.These TCs had reported experiment error of ±0.5 ◦C. The heaterused was a model KHLV-101/10-P, available from Omega Engi-neering. Dimensions were 22mm by 22mm. This was connectedto a BK Precision 1621 DC power supply to produce heat inputto the TES. An Omega HFS-4 thin film heat flux sensor (HFS)was placed between the heater and the device as noted in Figure7 to measure the thermal energy transfer. The HFS reported TESabsorption in kW/m2 which was used in all TES performancecalculations. HFS measurement error was ± 10%.

Power input to the heater was 2.1, 2.4 and 2.9 W for eachdevice tested. These inputs produced phase change within theTES and allowed the internal risers to operate across multipleheat rates and temperatures. Depending on riser operating effec-tiveness, thermal absorption to the TES varied and was recordedby the HFS.

All tests were operated at steady state, as determined fromthe recorded temperature data. The TES was allowed to fullyheat until operating temperatures of the device no longer varied.Data from the HFS, TCs, and electrical input was collected us-ing a PC connected to a Labview data acquisition system andsoftware for further analysis as detailed in this section.

Tests considered the appropriate fill-level for the TES andvaried the amount of wax within the sealed TES volume. Fol-lowing these tests, efforts focussed on thermal operation of theheat pipes in the baseline, square device design (Figure 5). Fi-nally, modified designs with 12-sided volumes were studied toimprove sealing reliability and integrity.

Paraffin loading quantity was investigated to yield the pre-ferred mass for the baseline TES design. A TES was fabricatedwith approximately 4 grams of paraffin and then tested using athermal input of about 2W, or enough to fully melt the internalwax as indicated by the recorded TC temperature readings. Asthe sample melted, seal integrity was observed. Subsequent testswere conducted that reduced the paraffin loading until a controlvolume was achieved that no longer expanded outside the boundsof the interior volume. As noted in the Results Section, the pre-ferred loading was no more than 3g for the baseline design. Thisrepresented a 91% fill of the useful internal volume.

Testing of the capillary heat pipes was conducted based on


thermal inputs to the TES and volumetric fill percentage of thecapillary tubes. The resistance heater was utilized to provide var-ied inputs of 2.1, 2.4, and 2.9 W for each TES fabricated. TheHFS recorded the actual thermal energy absorbed by the TESdevice during operation. Fill percentage was varied between 0,12.5, 25, and 50% by volume.

The recorded thermal absorption of the TES and the temper-ature gradient reported by the TCs were used to determine theequivalent thermal conductivity (keq) of the TES. This value rep-resented contributions from the TES walls, internal wax (PCM),and capillary risers. As the function of the capillary risers withinthe wax volume varied in operation, the overall thermal conduc-tivity of the TES was altered as reflected both in absorbed ther-mal energy and resulting top/bottom surface temperature gradi-ent.

Equivalent thermal conductivity, keq, was calculated usingEquation 1. q was the HFS measured thermal input per area,∆z was the space between temperature measurements (top andbottom surface of the TES), A was the heat input area, and ∆Twas the measured temperature difference across the device.

keq =q/A

∆T/∆z(1)

Following the baseline TES testing, modifications weremade to alleviate stress concentrating 90 degree angles, whilemaintaining the same interior volume of the device itself. A 12-sided geometry was selected for the large internal corner angle.Thermal testing of this modified TES was conducted using thesame thermal inputs and data acquisition established by the base-line TES. Capillary fill percentage was based on the observed be-havior of the baseline device, and selected to be 25% by volumefor this test. Sealing behavior was observed for this new designand utilized for ongoing, subsequent work.

RESULTSThis section presents results of the TES operation at differ-

ent thermal inputs, capillary fill percentages, and geometry de-signs as detailed in the Test Setup section. The temperature andheat flux data taken from these experiments was recorded andconverted into graphical form. All experiments were run untilthe temperature reached an equilibrium which occurred at differ-ent times based on the different operating conditions and deviceconstructions.

Heat pipe operation and overall device thermal conductivityresults were determined based on varied thermal inputs of 2.1,2.4, and 2.9W with varying heat pipe fill percentages. Typicaltemperature profile of the TES during operation is shown in Fig-ure 8, operating at 2.1W thermal input and 25% capillary fill.

As shown in Figure 8, heat pipe testing revealed impor-tant characteristics within the temperature profile graphical data.Recorded temperature slope changed between 45 ◦C and 55 ◦C.This represented the full wax phase change process. Following

Figure 8. TES TYPICAL OPERATING TEMPERATURE PROFILES

Table 1. TES PERFORMANCE FOR VARIED POWER INPUT ANDCAPILLARY HEAT PIPE FILL PERCENTAGE TESTSInput

(kWm2

)% Cap. Fill Absorbed q

(kWm2

)∆T (◦C) keq

( Wm·K

)4.34 50 % 2.28 12.3 2.04

4.96 50 % 2.61 14.0 2.05

5.99 50 % 3.19 18.1 1.94

4.34 25 % 4.11 5.5 8.23

4.96 25 % 4.71 10.7 4.85

5.99 25 % 2.20 18.5 1.31

4.34 12.5 % 2.25 22.1 1.12

4.96 12.5 % 2.14 15.4 1.53

5.99 12.5 % 3.60 18.9 2.10

4.34 0 % 2.15 15.8 1.49

this transition, temperature trends continue to rise until the equi-librium point with temperature gradient of 5.5 ◦C across the TES.

Results of the full range of thermal inputs and fill percent-ages indicated that at high thermal input and low capillary fillpercentage, the working fluid within the heat pipes completelyevaporated and was not able to function as a capillary tube. Inthese operating modes, the applied thermal input was too greatfor the heat pipe and limited thermal absorption of the TES.

Table 1 shows the cumulative results of these thermal tests.Heater input is presented in W/m2. Also recorded are the cap-illary riser tube fill percentages ranging from 0 to 50%. Therecorded TES thermal absorption and temperature gradient arealso presented along with the final equivalent conductivity (keq).

The best achieved equivalent thermal conductivity was cal-culated to be 8.23 W/mK using a TES with a 25% capillary fill.The input thermal energy at this operating point was 4.34 W/m2.However, irrespective of total capillary fill percentage, TES per-


formance decreased for all devices as thermal loading increased.This was the result of reduced performance of the capillary ris-ers. When all liquid within the risers was evaporated at increasedthermal loadings, the natural heat-pipe operation was stopped.

Results further indicated there existed an optimal fill per-centage for the risers for a given thermal loading. This balancebetween optimal fill percentage and thermal input would needto be established for any intended real-world application and ex-pected thermal input. It is worth noting that these thermal con-ductivity results show significant improvement over previouslyreported 3.8 W/mK achieved using copper foams for enhance-ment [10].

HIPS was the preferred material for TES fabrication in thesides of the device. In general, the 3D printing technique uti-lizing HIPS material reduced fabrication time and time to testbecause of fast throughput and immediate application to the cop-per top and bottom plates with assembled tubing in place. Newgeometries like 12-sided polygons were fabricated and tested todetermine if a larger interior corner angle would relieve pressurebuildup within the media. These new TES devices presented azero percent sealing failure rate during testing. This has leadto this modified corner/siding design in all subsequent ongoingwork in our group.

CONCLUSIONSA small scale Thermal Energy Storage (TES) device was

fabricated and characterized in this work. The TES was designedto interface with low temperature thermal scavenging operationsand provide a thermal storage media, similar in idea to a thermalbattery. In application, the TES would store energy for use by anattached thermoelectric generator when the originating source isunavailable.

Two studies were of particular focus in this work. First, theoverall device thermal conductivity was increased through theinclusion of internal capillary tubes, immersed within the phasechange material. Second, device geometry design was consid-ered to alleviate stress points (and increase seal integrity) at theTES volume corners. Best results were obtained using capillarytubing filled with 25% by volume HFE 7200 working fluid, ex-posed to 4.34 W/m2 thermal input. Design and sealing enhance-ment was observed through the use of a 12-sided geometry thatmaintained the same basic internal volumes as more traditional,square TES designs.

Thermal conductivity of 8.23 W/mK was measured for thisbest-case design, an improvement of approximately two timesover prior work with copper foam inserts utilized to increase TESthermal conductivity [10]. Testing also revealed the application-dependent nature of the TES. For example, as thermal input load-ing was increased, heat pipe functionality diminished. This wasdue to complete working fluid evaporation within the pipes.

This work is presently being advanced through integration

with real thermoelectric devices and an advanced heat exchanger,designed to scavenge low temperature thermal inputs from real-world sources.

ACKNOWLEDGEMENTSThe authors gratefully acknowledge the support of the NSF

via Grant ECCS-1053729 and NASA.

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