Patent Application
20180230972
The present invention generally relates to the use of phase change materials (PCMs) to delay icing or to cause de-icing. The use of materials of this type is particularly contemplated for this purpose in wind-driven power generators. Furthermore, the method for delaying icing or causing de-icing by means of incorporating the PCMs into different wind-driven power generator components is contemplated.
Wind-driven power generator blades installed in cold climates and at high altitudes are exposed to icing and problems of ice growth and build-up of ice. This phenomenon affects the design of a wind-driven power generator in different ways: icing will have serious effects on aerodynamics and will also affect the structural behavior of the turbine. The effects of temperature, and particularly the formation of ice masses in the structure, can change the natural frequencies of wind-driven power generator blades, bringing about dynamic problems in the entire turbine, and therefore reducing the annual energy production (AEP) and negatively affecting the power curve. In addition, if the power curve is too low, ice adhesion can even generate unscheduled shutdowns, which severely affects energy production. Furthermore, the control system can be affected and control instruments can even freeze or ice over, providing incorrect information to the turbine control system.
The structural integrity of the turbine can be affected by the significant imbalance caused by asymmetrical icing due to resonances caused by changes in natural frequencies of the components, exceeding designed fatigue loads and giving rise to unscheduled shutdowns, with the corresponding increase in operational expenditures (OPEX). Furthermore, the safety of the wind turbine and of its surrounding area will also be affected by icing or, generally, by operating in a cold climate. Detached ice fragments, or even large pieces of ice that may fall from the rotor, can injure people or animals, or cause material damage. There are even laws in some countries that require shutting down wind-driven power generators in the presence of ice given the safety issues mentioned above, with the subsequent loss of revenue.
Various anti-icing and de-icing methods have been developed in the state of the art, such as those methods based on nanocoatings and other nanostructured surfaces or active de-icing systems based on heatable fabrics, for example.
There are certain strategies based on biomimetics (Tak Sing Wong et al. “Bioinspired self-repairing slippery surfaces with pressure-stable omniphobicity” (2011) Nature 477, 443-447) that deal with different aspects such as, for example, the development of omniphobic surfaces that are so slippery that they prevent icing, inspired by carnivorous plants, or the development of superhydrophobic surfaces, combining surface roughness with low surface energy and a micro- and nanoscale fractal dimension, such as the lotus flower (Jianyong Lv et al. “Bio-inspired strategies for anti-icing” (2014) ACS Nano 8 (4), pp. 3152-3169; Michael J. Kreder et al. “Design of anti-icing surfaces: smooth, textured or slippery?” (2016) Nature Reviews Materials, 1; Kshitij C. Jha et al. “On modulating interfacial structure towards improved anti-icing performance” (2016) Coating 6 (1), 3).
Phase change materials (PCMs) are used as materials for thermal energy storage in different fields of application, depending on the working range of the selected PCM. PCMs are substances with high heats of fusion that melt and solidify at a specific temperature, storing and releasing large amounts of thermal energy. When the material freezes, it releases energy in the form of latent heat of crystallization, and when it melts, it stores heat in the form of latent heat of fusion. This phenomenon is due to the fact that a phase change involves large amounts of heat without the temperature of the material changing. Depending on their composition, they are classified as follows:
In recent years, PCMs have raised a lot of interest on the energy market because unlike conventional storage materials, such as sensible heat storage materials, PCMs adsorb and release heat at a virtually constant temperature. Furthermore, they are capable of storing between 5-14 times more heat per unit of volume.
PCMs are used in different industrial sectors as attemperators, such as in construction, for example. However, the only references found referring to the use of PCMs for anti-icing are the following:
Despite the needs in the state of the art, the use of PCMs as an anti-freeze solution in wind-driven power generators has not been described up until now.
Here in the present invention, the authors contemplate for the first time the use of PCM materials of this type as energy storage means in wind-driven power generator materials, mainly in the blades, to delay icing or to cause de-icing.
The authors of the present invention have been able to demonstrate based on important research work that incorporating PCMs into the coating or structural material of the different wind-driven power generator components, under specific conditions, delays icing, therefore increasing the AEP.
To that end, they have developed a method that allows incorporating PCMs into different wind-driven power generator components for the purpose of optimally delaying icing or causing de-icing.
In the developed method, PCMs that are used can either be confined in capsules (organic or inorganic) or organic supports can be impregnated with same in order to prevent the PCM from exuding during the phase change. The possibility that the PCMs, whether confined or not, can be anchored to the material of different wind-driven power generator components through reversible or irreversible covalent bonds is also contemplated. The PCMs can be incorporated into the paint or coating covering different wind-driven power generator elements; into the material forming the internal structure of different wind-driven power generator elements; into the putty (material for shaping defects identified once the blade is taken out of the mold) or as a thin layer deposited on the surface by means of spraying.
In a first aspect, the present invention contemplates the use of PCMs to delay icing or to cause de-icing in different wind-driven power generator elements.
In a second aspect, the method for delaying icing or causing de-icing in different wind-driven power generator elements based on the use of phase change materials (PCMs) is contemplated.
Based on the icing problems existing in the state of the art, especially in connection with wind-driven power generators, and particularly in the blades thereof, the authors of the present invention have developed a method for delaying icing or causing de-icing in different wind-driven power generator elements based on the use of phase change materials (PCMs).
Therefore, in a main embodiment of the present invention the use of both synthesized and commercial PCMs to delay icing or to cause de-icing in different wind-driven power generator elements is contemplated for the first time.
The PCMs used in the present invention must comply with the following requirements:
There are PCMs commercially available on the market today; however, synthesized PCMs (paraffins) can also be used in the present invention provided that the aforementioned requirements are met. The synthesized or commercial PCMs are selected from organic paraffins, esters, alcohols, acids, eutectic mixtures or hydrated inorganic salts.
In another main aspect of the invention, the method for delaying icing or causing de-icing in different wind-driven power generator components based on the use of phase change materials (PCMs) (hereinafter method of the invention) is contemplated.
The method of the invention comprises the following steps:
As mentioned above, the obtained PCMs may be confined or not. If they are not confined, the incorporation (b) of the PCMs is carried out by means of chemically anchoring them to the material of different wind-driven power generator elements through reversible or irreversible covalent bonds. In the case of confined PCMs, chemical anchoring is optional.
In a particular embodiment of the method of the invention, the obtained PCMs are confined. Preferably, the confinement of the PCMs can be carried out by means of:
In an even more preferred embodiment of the method of the invention, the confinement of the PCMs is carried out by means of nano-microencapsulation thereof. More preferably, said encapsulation of PCMs is based on the use of inorganic nano-microcapsules, with crystallization starting in the range −10° C. to 10° C.
The PCM is encapsulated in nano/micrometric capsules prepared by means of inert materials such as silica, titanium or zirconium (or mixtures thereof). These capsules have suitable shapes and sizes customized for the needs of the PCMs to be encapsulated in order to thereby introduce them into desired working systems, avoiding leakages. Furthermore, use of materials of this type improves the thermal efficiency of the working area due to its small size, having an associated increase in the contact surface and, therefore, improving thermal working.
The confinement of the PCMs in inorganic nano/microcapsules comprises the following steps:
In detail, to carry out the encapsulation of PCMs (e.g. organic paraffins, esters, alcohols, acids or eutectic mixtures or hydrated inorganic salts), according to the described method, it is necessary to create an emulsion or microemulsion, such as oil in water (O/W), water in oil (W/O) or W/O/W or O/W/O double emulsion type emulsions, for example, depending on the nature of the PCM. To form the emulsion, it is necessary to add anionic, cationic, or non-ionic surface active or surfactant agents, such as Triton X, Span 80, Span 60, Tween 20, Tween 80, PVP (polyvinylpyrrolidone), AOT (dioctyl sodium sulfosuccinate), for example, or a mixture of surfactants which are selected depending on the PCM to be encapsulated and which aid in forming the emulsion and stabilizing same, since they reduce the surface tension of the liquid obtaining the desired drop size. The drop size can therefore be modulated and is related to the capsule size obtained, and this size will be defined depending on the matrix in which the capsule is to be dispersed. The temperature can also help in stabilizing the emulsion or microemulsion, so it is advisable to work at temperatures between 25-200° C. The size of the drops formed in the emulsion or microemulsion largely depends on the surfactant-PCM/water ratio, the percentage by weight of the surfactant in the mixture being between 1-30% and the percentage of PCMs in the mixture between 1-50%.
In addition to the surfactants and surfactant-PCM/water ratios, the agitation used to develop the emulsion/microemulsion is also important. To that end, as mentioned above, in this particular embodiment of the method of the present invention both mechanical agitation and ultrasonic agitation are contemplated. Depending on the desired drop size, more or fewer revolutions will be used during agitation, mechanical agitation preferably being comprised between 3000 and 24000 rpm and ultrasonic agitation preferably being comprised between 20 and 70 W, until obtaining the emulsion consisting of drops of PCM in water.
Once the emulsion is developed with the selected phase change material and the optimal surfactants, the next step of the process is to add the inorganic precursor dropwise in the emulsion/microemulsion created with the desired PCM. The precursor can be of different inert materials, such as silica, titanium or zirconium.
In preferred embodiments, silicon precursors such as TEOS (tetraethyl orthosilicate), TMOS (tetramethyl orthosilicate), SiCl4 (silicon tetrachloride), GPTMS (3-glycidoxypropyl)methyldiethoxysilane), APTMS ((3-aminopropyl)-diethoxymethylsilane), MPTMS (3-methacryloxy-propyl-trimethoxy-silane), MTMS (methyltrimethoxysilane), HDTMS (hexadecyl-trimethoxy-silane), among others, are used.
Before being added to the emulsion, the precursor may or may not be hydrolyzed by adding water and a catalyst (e.g., hydrochloric acid, acetic acid, nitric acid) at an optimal concentration that acidifies the solution until reaching a pH between 1 and 4 so that said hydrolysis can be carried out.
Once the precursor is hydrolyzed, it is added to the emulsion dropwise so that it gradually surrounds the drops of PCM created. The hydrolyzed precursor surrounds the drops of PCM of the emulsion, and as a result of the surfactant hydrogen bonds are created, which bring about the formation of a capsule around each drop. Once the condensation reaction has ended, the capsules formed are cleaned with the suitable solvent (which will be selected based on the nature of the PCM and it must be soluble in said solvent), such as water, ethanol, propanol, ether, acetone, among others, to remove the PCM residues that have not been encapsulated, and it is left to dry in an oven between 4-24 hours at a temperature between 25-300° C.
To study the drop size of the emulsions, a light scattering system that allows, by means of laser, studying the drop size obtained in each emulsion, such as Masterziser 2000, can be used. Therefore, capsules of the desired size could be synthesized by means of controlling drop size.
To characterize the capsules obtained, primarily a differential scanning calorimeter (DSC) (e.g., Mettler Toledo HP DSC827) is used to study the storage energies and temperatures of fusion-crystallization, as well as the stability of the materials developed over several consecutive cooling-heating cycles.
This particular embodiment of the method of the invention allows obtaining synthesized or commercial encapsulated PCMs having the highest possible optimal enthalpies of crystallization (ΔHc) and fusion (ΔHm) so that a large amount of heat is released during the phase change, and suitable temperatures of crystallization (Tc) and fusion (Tm) so as to allow the development of paints working in the suitable temperature range, as described above. These PCMs allow reducing icing in different wind-driven power generator elements, particularly in wind-driven power generator blades, delaying the build-up of ice by about 15-30 minutes, depending on the type of PCMs and the technique used in the measurement, increasing the AEP.
Encapsulation of phase change materials (PCMs) based on the described method allows obtaining capsules comprised between 30 nm and 30 μm by means of combining the sol-gel technology with emulsion/microemulsion techniques, such as, for example, oil in water (O/W), water in oil (W/O) or W/O/W or O/W/O double emulsion type emulsions. The size and shape of the capsules to be synthesized can be controlled by means of this technique, which allows preparing customized capsules.
In step b) of the method of the present invention, the confined PCMs can be incorporated 1) into the paint or coating covering different wind-driven power generator elements; 2) into the material forming the internal structure of different wind-driven power generator elements (resin or fibers used in manufacturing the composites); 3) into the putty; or 4) as a thin layer on the surface thereof by means of spraying.
In the case of the paint or coating, when confined PCMs (organic paraffins, esters, alcohols, acids, eutectic mixtures or hydrated inorganic salts) are used, the confinement size must be less than 30 microns. In the case of the resin, used in the internal structure of different wind-driven power generator components, or the putty, when the PCMs (inorganic or organic) are confined, the allowed confinement size could be larger, even being capsules measuring 1 mm.
In the case of the putty, paint or coating of different wind-driven power generator components, the incorporation of the confined PCMs is carried out by means of the following steps:
To reduce viscosity of the dispersion, a solvent (e.g. N-butylacetate, butanediol, etc.) compatible with the base of the paint and/or a thixotropic agent can furthermore be added and mixed with the solution for a time comprised between 2-15 min.
In the case of the resin, the incorporation is carried out by means of dispersing the confined PCMs by means of mechanical or ultrasonic methods. The resins will be used to prepare composite materials forming the inside of different wind-driven power generator elements by means of infusion methods, the use of pre-impregnated materials, hand lay up molding, resin transfer molding, pultrusion, in situ polymerization, etc.
In the case of the fibers used for manufacturing composites forming the inside of different wind-driven power generator elements, the incorporation is carried out by means of impregnating said fibers with a dispersion of confined PCMs or by means of immersing said fibers in a dispersion of confined PCMs.
In preferred embodiments of the method of the invention, the PCMs (confined or by means of chemical anchoring) are used in wind-driven power generator blades. In this case, in the event of ice being generated, the PCM is melted and the temperature starts to drop, approaching the beginning of crystallization of the PCM between −10° C. and 10° C. The drop in temperature of the blade with the PCM added thereto is reduced since the material releases its latent heat in order to become solid, maintaining the temperature in the range of its phase change (less pronounced slope compared with the blade “without the PCM added thereto”, see
The present invention is additionally illustrated by means of the following examples, which do not seek to be limiting of the scope of the invention.
To develop the PCM capsules, between 0.1 and 2 g of high molecular weight polyvinyl alcohol (PVA) were dissolved in water at a temperature comprised between 25° C. and 100° C. In addition, between 1 and 20 g of PCM (two types of PCMs, hexadecane and octadecane, were used), which were mixed at a temperature between 25° C. and 100° C. with a percentage of surfactant (Tween 80) between 1% and 50% with respect to the amount of PCM added (preferably between 20% and 40%), were weighed. Once the PVA solution was dissolved, it was added to the PCM solution dropwise and kept under agitation at 15000 rpm for a time between 1 and 4 h to form the PCM-water emulsion. While waiting for the indicated time, TEOS, GPTMS and acetic acid were mixed at a ratio of 1:0.5:0.05 and were left to agitate for a time between 1 and 3 h at a temperature between 25° C. and 100° C. for hydrolysis of the precursors to commence. After the indicated time, the reaction was cooled and was left to agitate. The solution was centrifuged, obtaining the PCM-filled silica microcapsules. Said capsules were washed several times with ethanol to remove the surfactant residues and PCM residues that had not been encapsulated. Finally, the solid obtained was dried in a vacuum oven between 8-24 hours at a temperature between 50-200° C.
This test consisted of forming a water droplet on a panel painted with the reference coating without PCMs and with a coating with PCM capsules (5, 15 and 25% Paraffin Type 1 capsules and 10% Paraffin Type 2 capsules) (see Table 1).
The temperature was dropped in 5° C. increments, remaining isothermal for 2-10 min until reaching −20° C. The measurement parameters were:
According to the obtained results, the following was concluded:
Tests were also conducted in an environmental chamber in order to check functionality according to the following methodology:
Similarly to the preceding case, it was observed that the freezing of the water droplet was delayed and that the temperature on the surface was higher due to the release of heat from the PCM, the following order of improvement being established: the paint with the best behavior in the environmental chamber was the one that contained a higher percentage of Paraffin Type 2 capsules, followed by paints with 15% Paraffin Type 2 capsules and 10% Paraffin Type 1 capsules, all having a very similar behavior, then the paint with 5% Paraffin Type 1 capsules, and finally, the one showing the worst behavior is the reference paint without any PCMs added thereto.
| Number | Date | Country | Kind |
|---|---|---|---|
| 201700079 | Feb 2017 | ES | national |
