Patent Application
20140197355
This invention relates to thermal energy storage in phase change materials. Specifically, the invention relates to metal oxide encapsulated phase change materials.
According to the World Bank, U.S. per capita electrical consumption has exceeded 13,000 kW. Throughout the world electricity consumption was over 20,000,000 GWh/yr in 2009. Traditional electrical generation, through use of coal and other fossil fuels, has increased, due in part to the dwindling supply of resources. Low-efficiency peaking power plants, such as gas turbines, are still employed for meeting high peak energy demands. However, the electrical generation industry has begun providing lower cost, off-peak rates to change consumer usage to times where electricity may be produced at much lower marginal cost.
Ecologically friendly electrical generation has become more common, as a means of reducing carbon emissions and reducing reliance on nonrenewable resources. Photovoltaic cells and concentrating Solar Power (CSP) are competing solar power generation technologies. CSP systems use lenses or mirrors to focus a large area of sunlight into a small beam, which is used as a heat source for a conventional power plant. A working fluid is heated by the concentrated sunlight, and is then used for power generation or energy storage.
Thermal energy storage technology saves energy, provides economic benefits, and makes possible the use of periodic energy sources, such as solar energy. Solar energy is an inexhaustible source of future energy needs with a minimum of adverse environmental consequences (Goswami, et al., 2000, Principles of Solar Engineering, Taylor and Francis Group, New York). The energy storage is one of the best possible solutions to reduce the mismatch between supply and demand. Most current solar heating systems have storage for a few hours to a day's worth of energy collected, by using thermal energy storage materials (TESMs). Thermal energy storage (TES) helps overcome the intermittency of the solar resource for concentrating solar power (CSP) plants (Dincer and Dost, 1996, “A perspective on thermal energy storage systems for solar energy applications,” International Journal of Energy Research, 20(6), pp. 547-557). TES systems contains a thermal storage mass, and can be based on latent, sensible and thermo-chemical energy storage (Sharma, et al., 2009, “Review on thermal energy storage with phase change materials and applications,” Renewable and Sustainable Energy Reviews, 13(2), pp. 318-345; Gil, et al., 2010, “State of the art on high temperature thermal energy storage for power generation. Part 1—Concepts, materials and modellization,” Renewable and Sustainable Energy Reviews, 14(1), pp. 31-55).
Among these energy storage types, latent heat storage in phase change materials (PCMs) is very attractive because of high storage capacity and charging/discharging heat at a nearly constant temperature, require little maintenance, can be produced at a range of temperatures, and are easy to integrate into a power plant (Abhat, “Performance studies of a finned heat pipe latent thermal energy storage system,” Proc. Mankind's future source of energy; Proceedings of the International Solar Energy Congress, Pergamon Press, Inc., New Delhi, India, pp. 541-546). There are several studies on storing solar thermal energy using PCMs (Zalba, et al., 2003, “Review on thermal energy storage with phase change: materials, heat transfer analysis and applications,” Applied Thermal Engineering, 23(3), pp. 251-283; Al-Jandal and Sayigh, 1994, “Thermal performance characteristics of STC system with Phase Change Storage,” Renewable Energy, 5(1-4), pp. 390-399; Baran and Sari, 2003, “Phase change and heat transfer characteristics of a eutectic mixture of palmitic and stearic acids as PCM in a latent heat storage system,” Energy Conversion and Management, 44(20), pp. 3227-3246; Fouda, et al., 1984, “Solar storage systems using salt hydrate latent heat and direct contact heat exchange—II Characteristics of pilot system operating with sodium sulphate solution,” Solar Energy, 32(1), pp. 57-65; Morrison and Abdel-Khalik, 1978, “Effects of phase-change energy storage on the performance of air-based and liquid-based solar heating systems,” Solar Energy, 20(1), pp. 57-67; Rabin, et al., 1995, “Integrated solar collector storage system based on a salt-hydrate phase-change material,” Solar Energy, 55(6), pp. 435-444; Velraj, et al., 1999, “Heat Transfer Enhancement in a Latent Heat Storage System,” Solar Energy, 65(3), pp. 171-180; Medrano, et al., 2010, “State of the art on high-temperature thermal energy storage for power generation. Part 2—Case studies,” Renewable and Sustainable Energy Reviews, 14(1), pp. 56-72; Jotshi, et al., 1992, “Solar thermal energy storage in phase change materials,” SOLAR '92: American Solar Energy Society (ASES) Annual Conference Cocoa Beach, Fla., pp. 174-179). Stored solar thermal energy has the potential to provide cheaper peak-demand power than any other energy source. In addition, there are also a number of seasonal thermal stores, which store summer energy for space heating during winter.
PCMs store heat using solid-solid, solid-liquid, solid-gas and liquid-gas phase change, though only solid-liquid change is used in PCMs for electrical generation and thermal energy storage. Solid-liquid PCMs increase in temperature as they absorb heat, until PCM reaches the phase change temperature (melting temperature). At the phase change, the PCM absorbs large amounts of heat with minimal temperature change, until the material has transformed phase. When the ambient temperature around a liquid material falls, the PCM solidifies, releasing its considerable amount of latent energy. PCMs are widely used in the art because of the high energy storage density associated with the change of phase. PCM storage systems also have an advantage of being capable to operate with small temperature differences between charging and discharging.
In a latent heat energy storage system, selection of the PCM is a very important task. Most investigations are focused on salt hydrates, paraffin's, non-paraffin's, inorganic acids, clathrates and eutectic organic and inorganic compounds (Lane, 1986, Solar Heat Storage: Latent Heat Materials, CRC Press, Inc, Boca Raton, Fla.). For example, Lane (Lane, 1986, Solar Heat Storage: Latent Heat Materials, CRC Press, Inc, Boca Raton, Fla.) has compiled a list of the common PCMs for various applications. Inorganic compounds, in general, have a higher latent heat per unit volume and are non-flammable, and have lower costs in comparison to organic compounds (Tyagi, et al., 2011, “Development of phase change materials based microencapsulated technology for buildings: A review,” Renewable and Sustainable Energy Reviews, 15(2), pp. 1373-1391; Agyenim, et al., 2010, “A review of materials, heat transfer and phase change problem formulation for latent heat thermal energy storage systems (LHTESS),” Renewable and Sustainable Energy Reviews, 14(2), pp. 615-628).
Many PCM systems rely on molten salt to retain the temperature. Molten salt can be employed as a thermal energy storage method to retain thermal energy collected by a solar tower or solar trough so that it can be used to generate electricity in bad weather or at night. With concentrated solar power, the PCMs have been successfully utilized in electricity generation, to allow solar power to provide electricity on a continuous basis, such as Solar One, Two, and Tres. These molten salts, like potassium nitrate, calcium nitrate, sodium nitrate, lithium nitrate, absorb and store the heat energy that is later released when electricity is needed by pumping the hot salt to a conventional steam-generator to produce superheated steam for a turbine/generator.
However, there are some problems associated with PCMs. Salt based PCM solutions must be encapsulated to prevent water evaporation or uptake. One of the most significant drawbacks is that these materials have very low heat transfer characteristics, and tend to solidify at the edges of the encapsulating container and preventing effective heat transfer. Moreover, inorganic PCMs, in general, have low thermal conductivity (0.1-0.6 W/m. K), leading to low heat transfer rates and oxidation on exposure to heat transport medium (air or heat transfer fluids like oils). In order to overcome such problems, heat transfer enhancement techniques, such as use of extended surfaces and dispersion of high conductivity materials, have been identified and applied (Jegadheeswaran and Pohekar, 2009, “Performance enhancement in latent heat thermal storage system: A review,” Renewable and Sustainable Energy Reviews, 13(9), pp. 2225-2244). Another technique to overcome the low heat transfer is to encapsulate the PCM within a secondary supporting structure, and use of these capsules in a packed/fluidized bed heat exchanger (Hawlader, and Zhu, 2000, “Preparation and Evaluation of a Novel Solar Storage Material: Microencapsulated Paraffin,” International Journal of Solar Energy, 20(4), pp. 227-238).
No commercially available heat storage products possess heat transfer enhancement capabilities which would improve performance of low temperature devices. Enhancement of heat transfer in heat storage devices with a PCM typically use fins and Lessing rings from various materials and carbon fibers (Kenisarin and Mahkamov, 2007, “Solar energy storage using phase change materials,” Renewable and Sustainable Energy Reviews, 11(9), pp. 1913-1965). The disadvantage of their heat exchanger development is increasing the cost and complexity of thermal energy storage devices. In order to solve these problems, both material investigation and heat exchanger development were performed. Based on the investigation of studies focused on both the cases, it was observed and reported that one of the effective techniques is to encapsulate the PCM within a secondary supporting structure, and apply the packed/fluidized bed heat exchanger for better heat transfer enhancement (Hawlader and Zhu, 2000, “Preparation and Evaluation of a Novel Solar Storage Material: Microencapsulated Paraffin,” International Journal of Solar Energy, 20(4), pp. 227-238). Since the progress of latent heat storage systems mainly depends on assuring a high effective heat transfer rate to allow rapid charging and discharging, the required heat transfer surfaces should be large to maintain a low temperature gradient during these processes. This can be achieved efficiently through the macroencapsulation.
Macroencapsulated PCMs refer to capsules larger than 1 mm (Li, et al., 2012, “Fabrication and morphological characterization of microencapsulated phase change materials (MicroPCMs) and macrocapsules containing MicroPCMs for thermal energy storage,” Energy, 38(1), pp. 249-254). PCM macrocapsules are generally made by filling the PCM in preformed shells such as tubes, pouches, spheres, panels or other receptacles and sealing them (Cabeza, et al., 2011, “Materials used as PCM in thermal energy storage in buildings: A review,” Renewable and Sustainable Energy Reviews, 15(3), pp. 1675-1695). The most cost-effective containers are plastic bottles (high density and low density polyethylene and polypropylene bottles for low temperature storage), tin-plated metal cans and mild steel cans (Regin, et al., 2008, “Heat transfer characteristics of thermal energy storage system using PCM capsules: A review,” Renewable and Sustainable Energy Reviews, 12(9), pp. 2438-2458; Bauer, et al., 2012, “Characterization of Sodium Nitrate as Phase Change Material,” International Journal of Thermophysics, 33(1), pp. 91-104; Farid, et al., 2004, “A review on phase change energy storage: materials and applications,” Energy Conversion and Management, 45(9-10), pp. 1597-1615; Chou, T. P., Chandrasekaran, C., Limmer, S., Nguyen, C., and Cao, G. Z., 2002, “Organic-inorganic sol-gel coating for corrosion protection of stainless steel,” Journal of Materials Science Letters, 21(3), pp. 251-255).
While these problem may be overcome by microencapsulation or molecular-encapsulation of PCMs, the encapsulation process may be expensive and difficult. Therefore, what is needed is a thermal energy storage material composition which permits effective heat transfer, without unyielding encapsulation.
Low cost thermal energy storage is essential for solar power to become viable in large capacities. The following are important features of a good thermal storage system: high energy density, good heat transfer, a mechanically and chemically stable storage medium, and minimum thermal losses. PCMs create an opportunity for storing large amounts of thermal energy in a small amount of material, thus significantly reducing the costs. As such, a thermal energy storage material, which comprises an inorganic phase changing salt pellet, such as a PCM listed in Table 1, encapsulated in a metal oxide. NaNO3 and other PCMs for use in a CSP application in the temperature range from 300-400° C. may be used. The phase changing salt pellet is optionally cylindrical, spherical or oblong. However, other shapes would be known to one skilled in the art, with the advantages and disadvantages of different geometries of PCM encapsulation with different materials and their compatibility are discussed in the published literature (Zalba, et al., 2003, “Review on thermal energy storage with phase change: materials, heat transfer analysis and applications,” Applied Thermal Engineering, 23(3), pp. 251-283; Farid, et al., 2004, “A review on phase change energy storage: materials and applications,” Energy Conversion and Management, 45(9-10), pp. 1597-1615). In some variations the cylindrical pellet has a porosity of 1% to 5%, and the spherical pellet has a porosity of 22 to 29%. The macroencapsulated PCMs provides a self-supporting structure, and protects it from heat transfer fluids.
#monohydrate variant of composition
†dipotassium variant
The phase changing salt pellet is optionally surface functionalized, through hexane and silane functionalization.
The phase changing salt pellet optionally includes a void space within the phase changing salt pellet, permitting the phase change material additional space during melting. For example, the void space is optionally between about 15 to about 35% of the size of the phase changing salt pellet, and can optionally be evacuated of ambient air.
Because the PCMs melt during heat absorption, a high temperature polymer, such as a polyimide stable at temperatures of over 500° C., is coated over the exterior surface of the phase changing salt pellet. In some variations, the polyimide coating also includes an additive, such as a nickel, silicon carbide, or carbon. A metal oxide or graphene-metal oxide composite encapsulant is disposed on the exterior of the polyimide coating.
Metal oxide encapsulants include silicon dioxide, titanium dioxide, zinc oxide, calcium oxide, barium oxide, titanium dioxide-silicon dioxide composite, cerium dioxide, iron (III) trioxide, aluminum (III) oxide, magnesium oxide, lithium cobalt dioxide, lithium nickel dioxide, zinc oxide, zirconium dioxide, lithium, titanium oxide, lithium aluminum manganese oxide, lithium nickel manganese oxide, lithium dimanganese tetraoxide, indium tin oxide, or combinations thereof. The encapsulant optionally includes grapheme in the metal oxide as a composite. Graphene is a one-atom-thick sheet of sp2-bonded carbon atoms in a honeycomb crystal lattice, which is a current focus of materials science and condensed matter physics research (Wu, et al. Graphene/metal oxide composite electrode materials for energy storage. Nano energy. 2012(1); 107-131). It is the thinnest known material in the world and conceptually a basic build block for constructing many other carbon materials. It can be rolled into one-dimensional carbon nanotubes (CNTs), and stacked into three-dimensional (3D) graphite. With the addition of pentagons it can be wrapped into a spherical fullerene. As a consequence of its crystal structure, any low-energy quasiparticles in the material obey a linear dispersion relation. Graphene has the fastest electron mobility of ˜15,000 cm2 V−1 cm−1 or 106 Ωcm, a super high mobility of temperature-independent charge, and superior thermal conductivity of 5000 W m−1 K−1.
The present invention is a novel, nonobvious, and useful method of encapsulating widely-available low-cost PCMs on a level scalable for mass manufacturing. The invention provides a cost-effective and reliable solution for thermal energy storage. The thermal energy storage material is manufactured using an inorganic phase changing salt pellet, such as a PCM or other phase changing salt noted above or below. The phase changing salt is optionally formed into a pellet using power press a pelletizer, or wet granulation. In wet granulation the pellet is formed through agglomeration. A liquid solution or a granulating fluid was added to powders resulting in the massing of a mix of dry primary powder particles to form large sized particles. The fluid contains a solvent which must be non-toxic yet volatile so that it can be removed by drying. Typical liquids include water, ethanol and isopropanol either alone or in combination. In the power press method, which is similar to the briquetting process, dry powder is pressed between dies to produce pellets of required shape and size. Alternatively, the briquetting method may be used to form the pellet. One advantage of this method is that it does not require binders and hence is suitable for PCMs that are hygroscopic. Optional formed shapes include cylindrical, spherical, and oblong, though other shapes are envisioned. Optionally, a void space was formed in the phase changing salt pellet prior to polyimide coating, such as by drilling, briquetting, die casting the pellet with a void space, or any other means known in the art. Where a void space was formed, the void space was optionally evacuated of ambient air or pressure in the void space reduced by inserting a metal wire into the phase changing salt pellet during fabrication of the pellet, heating the phase changing salt pellet, removing the metal wire from the phase changing salt pellet, permitting heated gases to escape from the void space, and applying the polyimide coating to the phase changing salt pellet to seal the void space. The surface of the phase changing salt pellet was optionally functionalized by soaking the phase changing salt pellet in a solution of hexane and silane for 24 hours prior to applying the polyimide coating to the phase changing salt pellet.
The exterior of the phase changing salt pellet was coated in a high temperature polymer, like polyimide. In some variations, the polyimide was mixed with N-methyl 2-pyrrolidinone and a plasticizer organic solvent and applied in a thin layer onto the phase changing salt pellet by dip coating, spray coating or brushing. The polyimide optionally includes additives, such as nickel, silicon carbide, or carbon, and the polyimide cured, for example at up to 250° C. for 2 hours.
The exterior of the polyimide coating was then encapsulated with a metal oxide or a graphene-metal oxide composite. Exemplary metal oxide encapsulants include silicon dioxide, titanium dioxide, zinc oxide, calcium oxide, barium oxide, titanium dioxide-silicon dioxide composite, cerium dioxide, iron (III) oxide, aluminum (III) oxide, magnesium oxide, lithium cobalt dioxide, lithium nickel dioxide, zinc oxide, zirconium dioxide, lithium, titanium oxide, lithium aluminum manganese oxide, lithium nickel manganese oxide, lithium dimanganese tetraoxide, indium tin oxide, or combinations thereof. In some variations, a metal oxide precursor, such as a sol-gel silicon dioxide precursor like tetraethyl orthosilicate or tetraethyl orthosilicate containing grapheme, is obtained and mixed with 3-aminopropyltriethoxy silane to form a silicate precursor, and polyimide-coated phase changing salt pellets placed into the silicate precursor. The silicate precursor is heated, ethanol and hydrochloric acid added to the silicate precursor, the silicate precursor neutralized with sodium hydroxide, and the sol-gel silicon dioxide precursor encapsulated on the polyimide coated phase changing salt pellet. The metal oxide coating is optionally cured at various temperatures ranging up to 250° C., such as 250° C. at a rate of 4° C./minute for two hours, thereby forming a SiO2 shell on the pellet through self-assembly, hydrolysis, and simultaneous chemical oxidation at various temperatures.
For a fuller understanding of the invention, reference should be made to the following detailed description, taken in connection with the accompanying drawings, in which:
a) and (b) are illustration of phase change materials pellets containing voids. The voids provide for volumetric expansion space.
a) and (b) are illustrations of phase change material pellets as spheres, containing void space. (A) shows a spherical hemisphere, prior to fusion of two hemispheres, whereas (B) shows a pin-hole in the void, spherical phase change material pellets containing voids.
a) and (b) are graphs of FTIR results for PI-coated KBr pellets measured at room temperature.
a) and (b) are graphs of FTIR results on reaction of PI with NaNO3 pellets measured at room temperature.
a)-(c) are graphs of TGA Results for NaNO3 with PI coating. Results were obtained using a SDT Q600 (TA Instruments Inc., New Castle, Del.). (A) and (b) the dashed line indicates heat flow, whereas the solid line indicates weight.
a)-(f) are images showing scanning electron microscope (SEM) Analysis of the phase change materials with coatings showing the presence of (a) carbon; (b) sodium; (c) nickel; (d) nitrogen; (e) oxygen; and (f) silicon.
Provided herein is a thermal energy storage system with tunable operating range, such as 300° C.-450° C. for NaNO3, based on encapsulated phase change materials (PCM) that can meet the utility-scale base-load concentrated solar power plant requirements at much lower system costs compared to the existing thermal energy storage (TES) concepts. Encapsulation of PCMs with a metal oxide contributes in achieving high temperature performance (300° C.-500° C.) with cyclic performance capability. Suitable encapsulation methods were developed for existing low-cost phase change materials that provide a cost-effective and reliable solution for thermal energy storage to be integrated in solar thermal power plants. The novel technique of macro encapsulation provides a self-supporting structure to the phase change material, and separates it from heat transfer fluids and also increases the heat transfer rate.
As used herein, “about” means approximately or nearly and in the context of a numerical value or range set forth means±15% of the numerical.
As used herein, “complementary” means the elements are sized and shaped to allow at least one element from one complementary half to engage at least one element from the opposing complementary half.
As used herein, “evacuated of ambient air” means a pressure below the ambient, atmospheric pressure. Exemplary pressures may be low, medium, or high vacuum, as seen in Table 2.
As used herein, “graphene: is a one-atom-thick planar sheet of sp-bonded carbon oriented in a honeycomb crystal lattice.
As used herein, “oblong” means an elongated cylindrical or spherical shape. A nonlimiting example is an ovoid shape.
As used herein, “phase changing salt” is an inorganic salt, salt hydrate, or eutectic having a high heat of fusion at its melting temperature.
As used herein, “porosity” means the spaces in a material that is not occupied by the material. Such void space is typically occupied by other materials, such as ambient air or other gases. The porosity is a percentage of the volume of void space over the total volume.
As used herein, “surface functionalized” means a surface modified from its natural state through treatment.
NaNO3 was used because of its low cost and significantly high latent heat compared to other PCMs that melt at similar temperatures of between 300 and 400° C. (Bauer, et al., 2012, “Characterization of Sodium Nitrate as Phase Change Material,” International Journal of Thermophysics, 33(1), pp. 91-104). NaNO3 was formed into cylindrical TESM pellets using appropriate dies in a bench top power press (ranging from 13 to 30 mm diameter). Porosity of the cylindrical pellets varied with the pressure applied in the press and the diameter of the die, from 1 to 5% depending on the size and pressure applied. All the measurements of porosity were performed with the aid of Sartorius YDK01MS apparatus.
Encapsulation of PCMs with a metal oxide (SiO2) contributes in achieving high temperature performance (300 to 500° C.) with cyclic performance capability. A technique was developed to self-assemble a SiO2 layer over the pellets (Chou, et al., 2002, “Organic-inorganic sol-gel coating for corrosion protection of stainless steel,” Journal of Materials Science Letters, 21(3), pp. 251-255). This layer encapsulates the PCM, which separates the PCM from the heat transfer fluid, and provides the required strength to the capsules. However, the self-assembly process involves different solvents, which may affect the pellet characteristics.
As such, a precursor is coated initially on the pellet to protect the salt from dissolving. The reactivity of the solution with the molten NaNO3 plays an important role, keeping in mind the sustainability of the NaNO3 capsules over a large number of heating and cooling cycles. An intermediate PI coating renders the NaNO3 pellets insoluble in water and other solvents like ethanol, isopropyl alcohol, ether, acetonitrile, acetone etc. A high temperature polymer polyimide (PI) was coated onto the NaNO3 pellets, and cured so that the pellet becomes insoluble in water as well as in several organic solvents. An issue arises with the physical characteristics of PI and NaNO3, because NaNO3 is usually hydrophilic, whereas the PI is hydrophobic. Hence the interaction between these two oppositely characterized substances deteriorates the quality of the coatings. To avoid this problem, the NaNO3 pellets were soaked in a solution of hexane and silane at a ratio of 1:10, 1:20, or 1:30 ratio for 24 hours, for example at 1:10. This process of silanization not only turns the NaNO3 substrate hydrophobic but also helps to cover the pores over the NaNO3 pellets. This increases the ability of spreading of PI over the NaNO3 substrates and provides a more uniform PI coating, as the surface pores of the pellet were then covered prior to PI coating. The PI was then either dip coated or applied with brush over the pellets. Owing to the high viscosity, PI cannot be sprayed. The curing process involved air drying for 1 hour, followed by heating the pellet to 250° C. at a rate of 4° C./minute and dwelling for 2 hours at 250° C., thereby forming an intermediate layer of PI as seen in
A technique was developed to self-assemble a SiO2 layer over the PI coated pellets (Chou, T. P., Chandrasekaran, C., Limmer, S., Nguyen, C., and Cao, G. Z., 2002, “Organic-inorganic sol-gel coating for corrosion protection of stainless steel,” Journal of Materials Science Letters, 21(3), pp. 251-255), providing the additional strength required for encapsulation. Tetraethyl orthosilicate (TEOS), silane, ethanol, hydrochloric acid (HCl, 0.25 M), sodium hydroxide (NaOH, 1M) were used in developing the SiO2 coating. Initially, TEOS was added to silane, and mixed thoroughly. The ethanol, sodium hydroxide and hydrochloric acid were kept separately in different beakers. Due to the presence of silane, the TEOS self-assembled on the NaNO3 pellets using secondary coatings of PI or Ni nanoparticles+PI. The NaNO3 pellets, in TEOS-silane solution, were heated at 200° C. on a hot plate, which wetted the pellets. The TEOS and silane mixture was immediately treated by alcohol combined with HCl, which hydrolyzed the TEOS immediately at high temperature. After 5 to 10 minutes, the pellets were treated with NaOH, and immediately transferred to a hot environment (250° C.) using ambient atmosphere. A continuous reaction occurred at the molecular level between NaOH, alcohol and TEOS; as a result of which Si(OH)4, and later SiO2 were formed. The surface reactions occurring during the formation of SiO2 in the presence of silane are seen by
NaNO3 is used as a PCM, and was formed into TESM pellets. Spherical pellets of different diameters (2 mm to 10 mm) were formed using the Mars Minerals disc pelletizer. The porosity of the spherical pellets obtained from the pelletizer varied from 22 to 29%. All the measurements of porosity were performed with the aid of Sartorius YDK01MS apparatus.
Spherical pellets were alternatively made by agglomeration in a rotating drum. Water was sprayed at spaced intervals to help the powder form into balls as the drum rotated. However, little control over porosity and size of the pellet was provided by this method.
As disclosed in Example 1, above, a high temperature polymer polyimide (PI) was coated onto the NaNO3 spherical pellets by dip coating or brush application. The PI was cured by air drying for 1 hour, followed by heating the pellet to 250° C. at a rate of 4° C./minute and dwelling for 2 hours at 250° C., as disclosed above. The SiO2 layer was added to the pellets for encapsulation, using tetraethyl orthosilicate (TEOS), silane, ethanol, hydrochloric acid (HCl, 0.25 M), sodium hydroxide (NaOH, 1M). A mix of TEOS and silane was added to the NaNO3 pellets, and heated to 200° C. on a hot plate. Alcohol was added to the TEOS and silane mixture, hydrolyzing the TEOS, and the pellets treated with NaOH. The spherical pellets were immediately transferred to 250° C., oxidizing the later to form SiO2.
Uniform coating of the phase change material pellet directly controls the stability of the pellet during cyclic heating. The SiO2 coatings initially produced were occasionally highly non-uniform. Highly viscous liquids, like PI, create difficulty in uniform application over NaNO3 pellets. Dip coating the precursor solution was found to product extra solution over the NaNO3 pellets, which would protrude from the pellet after curing. During the curing process, the viscosity of the solution reduces as the temperature increases and excess solution drips down the NaNO3 pellets. Hence a thorough application of the right amount of precursor solution is required.
Another concern which is identified to play an important role in determining the uniformity of SiO2 coating role is surface roughness of the substrates. More rough surfaces are preferred in providing the uniform coatings. As the smooth PI coatings gives rise to non-uniform coatings, silicon carbide (SiC) is mixed with PI as an additive and the coating applied over the NaNO3 pellets to roughen the surface. At the lab scale the PI was applied over NaNO3 pellets by smearing with hand or brush. In order to ensure PI doesn't adhere to the substrate during the curing process, Teflon substrate is used for curing. Also, during the curing process, i.e., heating to the pellets at 250° C. at the rate of 4° C. per min, the NaNO3 pellet was rolled over the Teflon sheets so that the PI doesn't stick on the substrates and it gets coated uniformly. Instead of rolling the NaNO3 pellet over Teflon sheets another method considered was to give multiple coatings of PI over the NaNO3 pellets. Increasing the surface roughness of the substrates through use of SiC aided in providing more uniform coatings. The SiC also provided an extra advantage by giving additional strength to the encapsulating shell. Hence, SiC (20 to 100% wt. of PI) is used as an additive in PI.
The effectiveness of the phase change material encapsulation is the void space, which provides for thermal expansion of PCM. Because NaNO3 pellets obtained from the power press possess porosity of only 1 to 5% (depending on the force applied and the dimensions of the pellet), the volume change of sodium nitrate from the solid phase to the liquid phase is reported to be about 10.7% (Janz, G. J., Allen, C. B., Bansal, N. P., Murphy, R. M., and Tomkins, R. P. T., 1979, “Physical properties data compilations relevant to energy storage. II. Molten salts: data on single and multi-component salt systems,” No. NSRDS-NBS-61(Pt.2) 420p). Therefore, it is necessary to prepare the NaNO3 pellets with enough void space to take care of the expected volume change.
To address the volumetric expansion during the phase change, an extra void space of between 15 to 35%, was created by drilling holes in cylindrical pellets using endmills or a briquetting process. This provided an extra space to allow the expansion of air during melting. As seen in
The phase change material pellets were encapsulated by depositing a primary layer of polyimide (PI) as discussed above, which was followed by using self-assembly reaction for depositing silicon dioxide (SiO2) layer on top of the polyimide layer. The encapsulated pellets prepared using electroless deposition was tested for high temperature stability by heating them above the melting point of the PCM. The capsules were tested for cyclic stability by heating from room temperature to 350° C. at heating rate of 4° C./min, a dwell time of 2 hours at 350° C. and then cooled to 280° C., thus completing the first cycle. It was found that the PCM leaked from the capsule after 3rd cycle for the capsules with PI/silicon dioxide coating, which is assumed to be caused by non-uniform coating of PI/silicon dioxide layer as the coating was performed manually. Therefore a mixture of PI and SiC (silicon carbide) was prepared and manually coated on the PCM pellet, and followed by silicon dioxide coating. The mixture of PI/SiC provided a rough surface for proper deposition of silicon oxide. The pellet was cured in the oven at 250° C. for 30 minutes. The capsule prepared in this manner survived for four cycles before leakage.
Experiments were also performed to reduce the air pressure in the pellets by inserting a metal wire during fabrication of the pellet followed by coating of the pellet with PI/SiC and SiO2. Phase change material pellets were formed, as disclosed in Example 5, with the exception of a metal wire inserted into void base 6 of each pellet. The void space was sealed and the phase change material pellets heated to 200° C. The wire was immediately removed and the pellet was sealed with PI/SiC. The pellets were then cured as disclosed above and coated in metal oxide.
The cylindrical pellets eventually failed, possibly due to stress concentration at the sharp edges. Therefore, spherical pellets were prepared using hemispherical pellet halves having a void in the center by pressing the sodium nitrate powder in a die. Two spherical pellet hemispheres 1a were formed with a diameter of 26 mm. Void walls 3 define void 2, having a diameter of 16 mm, as shown in
The reactivity of the PI with the molten NaNO3 plays an important role especially keeping in mind the sustainability of the NaNO3 capsules over a number of heating and cooling cycles. The reactivity of the PI-metal oxide coating with the NaNO3 is studied by characterizing the coatings using FTIR and SEM on different samples.
Fourier Transform Infra-Red Spectroscopy (FTIR)
Pellets of potassium bromide (KBr) are used as KBr is transparent to IR radiation. A sample of KBr pellet was coated with the precursor solution and cured by heating to 250° C. and 300° C. The FTIR analysis indicated the absence of reaction between the KBr and the coating, up to temperatures of 300° C. The test was repeated with NaNO3 and the coating material on a glass slide which confirmed that there is no reaction between the coating solutions and NaNO3.
The readings for potassium bromide (KBr) were taken as it is transparent to IR radiation. A KBr sample was formed, coated with PI, and cured by heating to 250° C. and 300° C. The results obtained in the FTIR analysis as shown in
To test the reactivity of NaNO3 with PI, another set of FTIR studies are performed. The samples were made by coating a glass slide with PI solution, curing it, and then melting the NaNO3 powder (refined grade) on the PI coated glass slide. The change in the surface characteristics of the sample was compared with initial PI characteristics. From the
Thermogravimetric Analysis (TGA)
The NaNO3 prills (refined grade) and the small particles of the compressed pellet were coated with PI and cured by heating to 250° C. at the rate of 4° C. per minute. The TGA was performed. The sample was loaded onto a balance and heated in a furnace to between 350° C. and 400° C. The results are presented in
Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray (EDX) Analysis
The coating of SiC and PI (weight ratio 0.2:1) was applied to the spherical pellets prepared with the void space of about 34%. Nickel powder was used to seal the void and the NaNO3 pellets coated with multiple layers of PI and SiC. The pellets were cured by heating the samples to 250° C. at 4° C. per minute. After the samples are prepared they are heated to 350° C. and EDX and SEM results obtained.
The pellets were bombarded with an electron beam inside the scanning electron microscope, ejecting some of electrons from the pellet sample, forming a vacated electron position that is occupied by an outer shell electron. The emitted X-ray from the high-energy electron was measured, thereby identifying atoms of the pellet. EDX results are shown in
SEM imagery showed the elemental composition at the NaNO3-coating (PI, SiC and Ni) interface. The interface showed a clean differentiation between the phase change material and the coating, seen in
While it is noted that these tests indicate the coating solutions and the salt do not react during the test period, the materials must be studies under extended cycling. However, based on the tests performed, it is anticipated that the coating solutions will not react with the salt.
Thermal Cycling
The performance and efficiency of the encapsulation coatings was verified through heating and cooling cycles in a furnace, as shown in
A novel encapsulation procedure for NaNO3, with a metal oxide, such as SiO2, for use in a thermal energy storage system with an operating temperature range of 300° C. to 500° C. has been developed. The NaNO3 pellets were coated in polymer such as, PI (stable up to 500° C.) and cured, so that the NaNO3 pellets become insoluble in water as well as several organic solvents. The metal oxide is coated over such NaNO3 pellets with PI coating using self-assembly, hydrolysis, and simultaneous chemical oxidation at various temperatures. The NaNO3 capsules were characterized using FTIR, TGA, and SEM. These capsules were able to withstand up to 7 heating and cooling cycles. Upon examination, the reason for failure was determined to be non-uniformity of the coating. When incorporated in a TES system, the encapsulated PCMs can result in a substantially lower cost storage system.
NaNO3 pellets were made either by using power press or by using a pelletizer, with the final pellets having cylindrical, spherical, oblong, or other geometrical shapes.
A polyimide (PI 2525 or PI 2611) coating was applied onto NaNO3 pellets. The polyimide was mixed with varying amounts of N-methyl 2-pyrrolidinone (NMP), which is a plasticizer organic solvent, for one hour. A thin layer of the NMP and polyimide mixture was coated on the NaNO3 pellets by dip coating, spray coating, or brushing, and cured at temperatures of 100 to 120° C. Later, the pellets were heated to a temperature of 250° C. for two hours at a rate of 4° C. per minute, thereby making the pellets insoluble in water and many organic solvents, such as methanol, isopropyl alcohol, ether, acetonitrile, acetone etc. The insolubility of the salt is important for encapsulating the metal oxide in a liquid environment.
A SiO2 layer was then self-assembled over the surface of the PCM salt, using the following chemicals: tetraethyl orthosilicate (TEOS), 3-aminopropyltriethoxy silane, ethanol, hydrochloric acid, sodium hydroxide. Five to 10 ml of TEOS was added to 1 to 2 ml of silane solution, and mixed thoroughly, while 20 ml of ethanol, 1M sodium hydroxide, and 0.02M hydrochloric acid are kept separately in different containers. Due to the presence of silane, TEOS was self-assembled on the polyimide coating. The polyimide coated pellets were heated to 200° C. on a hot plate. The pellets were wetted by the TEOS and silane mixture. The pellets were then immediately treated by alcohol combined with 0.02M hydrochloric acid.
TEOS hydrolyzed immediately at a high temperature. The pellets were allowed to cool and then were treated with NaOH, followed by immediately transferring the pellet to a hot environment with a temperature of 250° C. There is a continuous reaction at the molecular level with NaOH, alcohol, and TEOS formed Si(OH)4 and later SiO2. A thin layer of SiO2 having white color eventually developed on the pellets. This procedure was repeated several times for obtaining the optimum thickness over the PCM pellets, as seen in Table 3.
The method disclosed above may also involve a step of taking the polyimide-coated pellets and further coating them with nickel powder by rolling the NaNO3 pellets over a nickel powder. The SiO2 coating was then applied over the nickel-coated polyimide NaNO3 pellets, as discussed above.
Alternatively, the SiO2 layer may be replaced with a graphene-SiO2 sol-gel suitable to coat the NaNO3 pellets. Graphene was added to the SiO2 precursor, such as TEOS, and the polyimide-coated NaNO3 pellet was dip coated at room temperature into the sol-gel. The sol-gel coated pellets were allowed to dry at room temperature and later heated to 250° C. at a rate of 4° C. per minute. Upon reaching the curing temperature of 250° C., the polyimide was cured over the pellets for two hours.
Alternative exterior coatings may replace the SiO2 layer over NaNO3, such as TiO2, ZnO, CaO, BaO, TiO2—SiO2, CeO2, Fe2O3, Al2O3, MgO, etc., as coatings, and various PCMs, such as NaCl, NaOH, KOH, CaCO3, NaCO3, KCl, NaNO3, etc., as the pellets.
In the preceding specification, all documents, acts, or information disclosed does not constitute an admission that the document, act, or information of any combination thereof was publicly available, known to the public, part of the general knowledge in the art, or was known to be relevant to solve any problem at the time of priority.
The disclosures of all publications cited above are expressly incorporated herein by reference, each in its entirety, to the same extent as if each were incorporated by reference individually.
While there has been described and illustrated specific embodiments of a thermal energy storage material, it will be apparent to those skilled in the art that variations and modifications are possible without deviating from the broad spirit and principle of the present invention. It is intended that all matters contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.
This nonprovisional application is a continuation of currently pending PCT application No. PCT/US2012/047165 filed Jul. 18, 2012, which claims priority to provisional application No. 61/508,896, entitled “Method of Encapsulating a Phase Change Material with a Metal Oxide”, filed on Jul. 18, 2011, the contents of which are herein incorporated by reference.
This invention was made with Government support under Grant No. DE-EE0003590 awarded by the Department of Energy. The Government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 61508896 | Jul 2011 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/US2012/047165 | Jul 2012 | US |
| Child | 14159874 | US |
