Small-Scale Thermal Energy Harvester With Copper Foams and Thermal Energy Storage

2015 ◽  
Author(s):  
S. Shrestha ◽  
A. Hays ◽  
S. Thapa ◽  
D. Wood ◽  
D. Bailey ◽  
...  
Keyword(s):  
Thermal Conductivity ◽  
Heat Exchanger ◽  
Phase Change ◽  
Thermal Energy ◽  
Operating Temperature ◽  
Heat Pipes ◽  
Paraffin Wax ◽  
Working Fluid ◽  
Small Scale ◽  
Copper Tubing

This article investigates the use of advanced, high porosity thermally conductive foams and a thermal energy storage (TES) device for small scale thermal energy harvesting. In final application, it may be employed in various real world situations that include existing systems like thermoelectric generators (TEGs) and thermal scavenging systems that provide power output from freely available thermal sources. Experimental tests were conducted using various porosity metallic copper foams ranging from 85 % to 89 % porosity. Copper foams were selected to serve as the heat exchanger innards and examined for several key attributes. These included the ability of the foams to yield capillary action with working fluids like water or 3M™ HFE7200. Thermal energy absorption by the exchanger to the working fluid was also monitored. These results were compared to other exchangers based on capillary channel fabrication techniques as previously reported by the research team. Full characterization was based on operating temperature, measured thermal input, mass transfer rate, and heat transfer capability. Preliminary investigation of a matching, small-scale TES unit designed to integrate with the heat exchanger and a future thermoelectric for energy harvesting application was also conducted. Thermal storage was accomplished via solid-liquid phase change of a paraffin wax within the TES device forming a so-called “thermal battery.” In a final design, the TES includes what is defined by thermodynamics as heat pipes. The integrations of several heat pipes, made of copper tubing and filled with working fluid, mounted vertically and immersed in the wax medium will transfer heat to the wax by means of thermal conductivity and phase transition. This represents a first of its kind in this small-scale, thermal harvesting application. The specific tests performed in this initial work included one TES unit filled with a paraffin wax medium and a second that contained several copper vertically placed tubes surrounded by the paraffin wax. The overall thermal conductivity of the phase change medium (wax) was investigated for both constructions as was the ability of each to absorb thermal energy directly. Results indicated capillary action of the working fluid was possible via incorporation of copper foams within the heat exchanger. Maximum heat flux observed in exchanger tests was 0.27 kW/m2 given an operating temperature of 76.6 °C and 2.5W thermal input. Thermal storage tests indicated a maximum thermal capture rate of 0.91 W and phase change material thermal conductivity of 1.00 W/mK for the TES device constructed with copper tubing innards. This compared favorably to the baseline wax conductivity of approximately 0.32 W/mK. Future efforts will fully incorporate both the heat exchanger and matching TES device for a complete harvesting and thermal capture system. The ability of the exchanger to provide thermal energy for storage to the “thermal battery” will be monitored.

Optimization of a Phase Change Thermal Storage Unit

2010 ◽  
Author(s):  
Monica F. Bonadies ◽  
Mark Ricklick ◽  
J. S. Kapat
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Energy Storage ◽  
Heat Exchanger ◽  
Phase Change ◽  
Thermal Energy ◽  
Paraffin Wax ◽  
Heat Transfer Fluid ◽  
Storage Unit ◽  
Domestic Use

When collecting the energy of the sun for domestic use, several options exist, one being the use of evacuated tube collectors with internal heat pipes. This study proposes a system integrating these collectors with a storage unit using the phase change of paraffin wax to store energy. The storage unit makes use of a finned heat exchanger, with paraffin wax on the shell side and glycol on the tube side as the heat transfer fluid. The heat exchanger is embedded within the storage paraffin wax with a volume of 2 ft3. The heat exchanger also includes a separate loop for water to flow through and receive thermal energy from the melted wax. Although the wax has the benefit of being inexpensive and nontoxic, it has the problem of low thermal conductivity. Therefore, the heat exchanger has large copper fins brazed to it to extend areas of high thermal conductivity into the wax reservoir. The unit used in this study contains 14 fins. The use of fins will help to speed up the melting of the wax while solar energy is collected, since there is more heat transfer area. When most of the wax is melted, heat can be exchanged to water for domestic use. To determine the benefit of the fins, wax and working fluid temperature data will be taken from a constructed thermal energy storage unit, and then it is used to verify a finite-difference analytical model of the thermal operating characteristics. The maximum operating temperature of the glycol/water mix heat transfer fluid was approximately 65° C when the fluid flowed at 1 gallon per minute. The storage unit was able to store melted wax overnight with a 2–3°C temperature drop with the ambient temperature approximately at 30°C. City water at approximately 3 gpm was used to test the freezing side. The one dimensional model proved useful in predicting the heat storage mode of the system but had some error in predicting the heat release mode of the unit. The model also points to the fact that there are several considerations to be taken when simulating phase change energy storage processes.

Optimization of a Phase Change Thermal Storage Unit

2012 ◽  
Vol 4 (1) ◽  
Author(s):  
Monica F. Bonadies ◽  
Mark Ricklick ◽  
J. S. Kapat
Keyword(s):  
Thermal Conductivity ◽  
Heat Exchanger ◽  
Thermal Energy ◽  
Thermal Storage ◽  
Paraffin Wax ◽  
Heat Transfer Fluid ◽  
Working Fluid ◽  
Fluid Temperature ◽  
Storage Unit ◽  
One Dimensional

Several options exist to collect thermal energy from the sun for domestic use. This study examines a system integrating evacuated tube collectors with heat pipes with a storage unit using melted paraffin wax to store thermal energy. A shell-and-tube heat exchanger is embedded within the paraffin wax storage with a volume of 0.23 m3. The heat exchanger includes two loops: one for glycol to transfer heat to the paraffin and one for water to extract heat from the melted paraffin. Although the paraffin has the benefit of being inexpensive and nontoxic, it has low thermal conductivity. Therefore, the heat exchanger has large brazed copper fins to extend areas of high thermal conductivity into the wax reservoir. To determine the benefit of the fins, wax and working fluid temperature data are taken from a constructed thermal energy storage unit and then used to verify a finite-difference one-dimensional analytical model of the unit. The maximum operating temperature of the glycol/water mix heat transfer fluid was approximately 65 °C when the fluid flowed at 3.78 l/min. City water at approximately 11.34 l/min was used to test the water heating capabilities of the unit. The one dimensional model proved useful in predicting the heat storage mode of the system. Due to its form, which was specifically developed for the unit in the study, the model could be adjusted to calculate thermal performance of similarly constructed thermal storage units.

A Copper Microchannel Heat Exchanger for MEMS-Based Waste Heat Thermal Scavenging

2014 ◽  
Author(s):  
E. Borquist ◽  
A. Baniya ◽  
S. Thapa ◽  
D. Wood ◽  
L. Weiss
Keyword(s):  
Heat Exchanger ◽  
Thermal Energy ◽  
Waste Heat ◽  
Heated Surface ◽  
Copper Substrate ◽  
Working Fluid ◽  
Bottom Plate ◽  
Small Scale ◽  
Thermal Source ◽  
Micro Channels

The growing necessity for increased efficiency and sustainability in energy systems such as MEMS devices has driven research in waste heat scavenging. This approach uses thermal energy, which is typically rejected to the surrounding environment, transferred to a secondary device to produce useful power output. This paper investigates a MEMS-based micro-channel heat exchanger (MHE) designed to operate as part of a micro-scale thermal energy scavenging system. Fabrication and operation of the MHE is presented. MHE operation relies on capillary action which drives working fluid from surrounding reservoirs via micro-channels above a heated surface. Energy absorption by the MHE is increased through the use of a working fluid which undergoes phase change as a result of thermal input. In a real-world implementation, the efficiency at which the MHE operates contributes to the thermal efficiency of connected small-scale devices, such as those powered by thermoelectrics which require continual heat transfer. This full system can then more efficiently power MEMS-based sensors or other devices in diverse applications. In this work, the MHE and micro-channels are fabricated entirely of copper with 300μm width channels. Copper electro-deposition onto a copper substrate provides enhanced thermal conductivity when compared to other materials such as silicon or aluminum. The deposition process also increases the surface area of the channels due to porosity. Fabrication with copper produces a robust device, which is not limited to environments where fragility is a concern. The MHE operation has been designed for widespread use in varied environments. The exchanger working fluid is also non-specific, allowing for fluid flexibility for a range of temperatures, depending on the thermal source potential. In these tests, the exchanger shows approximately 8.7 kW/m2 of thermal absorption and 7.6 kW/m2 of thermal transfer for a dry MHE while the wetted MHE had an energy throughput of 8.3 kW/m2. The temperature gradient maintained across the MHE bottom plate and lid is approximately 30 °C for both the dry and wetted MHE tests though overall temperatures were lower for the wetted MHE.

Fabrication of Heat Pipes on an Acrylic Resin Board

2013 ◽  
Author(s):  
Yasushi Koito ◽  
Hiroyuki Maehara ◽  
Toshio Tomimura
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Phase Change ◽  
Heat Pipe ◽  
Heat Pipes ◽  
Acrylic Resin ◽  
Working Fluid ◽  
Liquid Volume ◽  
Wiring Board ◽  
Phase Change Heat Transfer

As a first step to develop an electronic wiring board in which micro or miniature heat pipes are internally fabricated, the experimental and analytical studies are performed on a wickless gravity-assisted heat pipe, namely thermosyphon, fabricated on a surface of an acrylic resin board. This proposal aims at performing a phase-change heat transfer inside an electronic wiring board having a low thermal conductivity. In experiments, the evaporator section of the heat pipe is heated by a heater while the condenser section is water-cooled by a heat sink. Water is used as a working fluid. Changing a heat input and a liquid volume ratio inside the heat pipe, the temperature distribution is measured by thermocouples and then compared to the case where the working fluid is not charged. Moreover, the simple model of the heat pipe is made based on a thermal resistance network, and the analysis is performed on a phase-change heat transfer and a conductive heat transfer inside the resin board having the heat pipe. The effective thermal conductivity of the heat pipe is evaluated. Although this study is an initial stage, the operational and the heat transfer characteristics of the resin board having the heat pipe are confirmed.

Small-Scale Thermal Energy Storage With Capillary Conductivity Enhancement

2016 ◽  
Author(s):  
A. Hays ◽  
E. Borquist ◽  
D. Bailey ◽  
D. Wood ◽  
L. Weiss
Keyword(s):  
Energy Storage ◽  
Heat Source ◽  
Phase Change ◽  
Thermal Energy ◽  
Thermal Energy Storage ◽  
Heat Sink ◽  
Large Scale ◽  
Heat Pipes ◽  
Small Scale ◽  
Conductivity Enhancement

Thermal 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 storage systems and phase change materials to harvest energy. However, on the smaller centimeter scale, there have been historically fewer investigations of these same techniques. The main goal of this paper is to investigate thermal energy storage (TES) as applied to a small scale system for thermal energy capture. Typical large-scale TES consists of a phase change material that usually employs a wax or oil medium held within a conductive container. The system stores the energy when the wax medium undergoes a phase change. In typical applications like buildings, the system absorbs and stores incoming thermal energy during the day, and releases it back to the surrounding environment as temperatures cool at night. This paper presents a new TES unit designed to integrate with a thermoelectric for energy harvesting application in small, cm-scale applications. In this manner, the TES serves as a thermal battery and source for the thermoelectric, even when originating power supply is interrupted. A unique feature of this TES is the inclusion of internal heat pipes. These heat pipes are fabricated from copper tubing and filled with working fluid, mounted vertically, and immersed in the wax medium of the TES. This transfers heat to the wax by means of thermal conductivity enhancement as an element of the heat pipe operation. This represents a first of its kind in this small-scale, thermal harvesting application. As tested, the TES rests atop a low temperature (60 °C) heat source with a heat sink as the final setup component. The heat sink serves to simulate thermal energy rejection to a future thermoelectric device. To measure the temperature change of the device, thermocouples are placed on either side of the TES, and a third placed on the heat source to ensure that the energy input is appropriate and constant. Heat flux sensors (HFS) are placed between the heat source and the TES and between the TES and heat sink to monitor heat transferred to and from the device. The TES is tested in a variety constructions as part of this effort. Basic design of the storage volume as well as fluid fill levels within the heat pipes are considered. Varying thermal energy inputs are also studied. Temperature and heat flux data are compared to show the thermal absorption capability and operating average thermal conductivities of the TES units. The baseline average thermal conductivity of the TES is approximately 0.5 W/mK. This represents the TES with wax alone filling the internal volume. Results indicate a fully functional, heat pipe TES capable of 8.23 W/mK.

Comparison of Thermal Performance of 3D Printed Shell and Tube Heat Exchanger With Commercial Plate Heat Exchanger for Thermal Energy Storage (TES) by Incorporating Phase Change Materials (PCM)

2020 ◽  
Author(s):  
Ashok Thyagarajan ◽  
Nandan Shettigar ◽  
Debjyoti Banerjee
Keyword(s):  
Heat Transfer ◽  
Energy Storage ◽  
Heat Exchanger ◽  
Phase Change ◽  
Thermal Energy ◽  
Thermal Energy Storage ◽  
Working Fluid ◽  
Temperature Differential ◽  
Shell And Tube ◽  
Dry Cooling

Abstract Wet cooling is predominantly used in thermoelectric power plants for condensing steam from the turbine owing to their low system cost and for their ability to render lower turbine back pressure (in comparison to dry cooling). However, the implementation of wet cooling in arid regions is costly while implementation of dry cooling in arid regions can degrade operational reliability (while also increasing both capital costs and operational costs). As a result, alternate technologies are needed to wean power plants from using fresh water resources while also enhancing the operational reliability of dry cooling. Air cooled heat exchangers installed in arid climates are inoperable on certain days during summer as the ambient air temperature can exceed the temperature of the steam at the turbine exhaust and may lead to power plant shutdown, in-turn, causing instability in the electric supply grid infrastructure (thus compromising reliability). In order to combat these shortcomings, supplemental cooling options may be needed. Thermal Energy Storage (TES) platforms can provide an attractive option for supplemental cooling. Phase Change Materials (PCM) are often used as viable options for Latent Heat Thermal Energy Storage Systems (LHTESS) as they have small footprint owing to the high latent heat values of PCM. The objective of this study is to analyze the performance of various LHTESS platforms by utilizing different configurations of the Heat Exchangers (HX) that are filled with PCM. The scope of this study is limited to using an organic Phase Change Material (PCM) and two different HX configurations are explored in this study: (a) a Shell and Tube Heat Exchanger that was fabricated using Advanced Manufacturing (AM) technique (i.e., “3D Printing”); and (b) a conventional Chevron Plate Heat Exchanger (PHX) that was procured commercially from a vendor. The thermal response and performance characteristics (e.g., power rating and HX effectiveness) of the two HX configurations are measured experimentally in order to ascertain their efficacy for melting and solidification of the PCM for different flow rates and inlet temperature values of the working fluid. The working fluid is called the Heat Transfer Fluid (HTF). The HTF used in this study is tap water. The PCM used in this study is PureTemp 29 (commercially procured from Pure Temp Inc., Minneapolis, MN). The propagation of the melt and the freeze fronts were monitored and tracked based on the nature of the transient temperature profiles recorded by an array of thermocouples. The array of thermocouples were strategically mounted at different locations within the HX containing the PCM. For the 3D-Printed HX (Shell and Tube HX), the thermocouples were located at different radial and axial locations within the shell containing the PCM. For the PHX, the thermocouples were located at different heights (for different plates containing the PCM). The transient values of the power and capacity ratings for the HX were estimated based on the time-history of the transient values of the temperature differential for the bulk temperature of the HTF flowing between inlet and outlet ports of the HX (and this was correlated with the transient profile and location as well as the propagation of the solid-liquid interface within the HX). The performance characteristics of both HX, analyzed from the experimental data, show that the average power rating for the melting-cycle is consistently higher than that of the solidification-cycle due to the dominance of free convection during melting (resulting in higher values of the effective heat transfer coefficients for the same temperature differential values); while the solidification process is dominated by transient conduction (resulting in lower values of the effective heat transfer coefficients for the same temperature differential values).

scholarly journals Thermophysical Properties of Multifunctional Syntactic Foams Containing Phase Change Microcapsules for Thermal Energy Storage

Polymers ◽  
2021 ◽  
Vol 13 (11) ◽  
pp. 1790
Author(s):  
Francesco Galvagnini ◽  
Andrea Dorigato ◽  
Luca Fambri ◽  
Giulia Fredi ◽  
Alessandro Pegoretti
Keyword(s):  
Thermal Conductivity ◽  
Energy Storage ◽  
Phase Change ◽  
Epoxy Resin ◽  
Storage Modulus ◽  
Thermal Energy ◽  
Thermal Energy Storage ◽  
Neat Epoxy ◽  
Low Density ◽  
Syntactic Foams

Syntactic foams (SFs) combining an epoxy resin and hollow glass microspheres (HGM) feature a unique combination of low density, high mechanical properties, and low thermal conductivity which can be tuned according to specific applications. In this work, the versatility of epoxy/HGM SFs was further expanded by adding a microencapsulated phase change material (PCM) providing thermal energy storage (TES) ability at a phase change temperature of 43 °C. At this aim, fifteen epoxy (HGM/PCM) compositions with a total filler content (HGM + PCM) of up to 40 vol% were prepared and characterized. The experimental results were fitted with statistical models, which resulted in ternary diagrams that visually represented the properties of the ternary systems and simplified trend identification. Dynamic rheological tests showed that the PCM increased the viscosity of the epoxy resin more than HGM due to the smaller average size (20 µm vs. 60 µm) and that the systems containing both HGM and PCM showed lower viscosity than those containing only one filler type, due to the higher packing efficiency of bimodal filler distributions. HGM strongly reduced the gravimetric density and the thermal insulation properties. In fact, the sample with 40 vol% of HGM showed a density of 0.735 g/cm3 (−35% than neat epoxy) and a thermal conductivity of 0.12 W/(m∙K) (−40% than neat epoxy). Moreover, the increase in the PCM content increased the specific phase change enthalpy, which was up to 68 J/g for the sample with 40 vol% of PCM, with a consequent improvement in the thermal management ability that was also evidenced by temperature profiling tests in transient heating and cooling regimes. Finally, dynamical mechanical thermal analysis (DMTA) showed that both fillers decreased the storage modulus but generally increased the storage modulus normalized by density (E′/ρ) up to 2440 MPa/(g/cm3) at 25 °C with 40 vol% of HGM (+48% than neat epoxy). These results confirmed that the main asset of these ternary multifunctional syntactic foams is their versatility, as the composition can be tuned to reach the property set that best matches the application requirements in terms of TES ability, thermal insulation, and low density.

scholarly journals Nano-Enhanced Phase Change Materials in Latent Heat Thermal Energy Storage Systems: A Review

Energies ◽  
2021 ◽  
Vol 14 (13) ◽  
pp. 3821
Author(s):  
Kassianne Tofani ◽  
Saeed Tiari
Keyword(s):  
Energy Storage ◽  
Phase Change ◽  
Latent Heat ◽  
Thermal Energy ◽  
Thermal Energy Storage ◽  
Phase Change Materials ◽  
Storage Systems ◽  
Heat Pipes ◽  
Energy Storage Systems ◽  
Thermal Energy Storage Systems

Latent heat thermal energy storage systems (LHTES) are useful for solar energy storage and many other applications, but there is an issue with phase change materials (PCMs) having low thermal conductivity. This can be enhanced with fins, metal foam, heat pipes, multiple PCMs, and nanoparticles (NPs). This paper reviews nano-enhanced PCM (NePCM) alone and with additional enhancements. Low, middle, and high temperature PCM are classified, and the achievements and limitations of works are assessed. The review is categorized based upon enhancements: solely NPs, NPs and fins, NPs and heat pipes, NPs with highly conductive porous materials, NPs and multiple PCMs, and nano-encapsulated PCMs. Both experimental and numerical methods are considered, focusing on how well NPs enhanced the system. Generally, NPs have been proven to enhance PCM, with some types more effective than others. Middle and high temperatures are lacking compared to low temperature, as well as combined enhancement studies. Al2O3, copper, and carbon are some of the most studied NP materials, and paraffin PCM is the most common by far. Some studies found NPs to be insignificant in comparison to other enhancements, but many others found them to be beneficial. This article also suggests future work for NePCM and LHTES systems.

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