Thermotropic Materials Research Articles

Building integrated concentrating photovoltaic window coupling luminescent solar concentrator and thermotropic material

Building integrated concentrating photovoltaic window (BICPVW) has been proven to be used in sustainable building transparent envelopes. Unfortunately, the balance between optical and electrical properties is difficult. This paper proposes a building integrated concentrating photovoltaic window coupling luminescent solar concentrator and thermotropic material (LT-BICPVW). The luminescent solar concentrator and thermotropic layer were fabricated by bulk polymerization and glass sandwich structure, respectively. Optical models are built based on the optical properties of optical waveguide and thermotropic layer, and optical simulations are performed with Monte Carlo ray tracing technology. Window prototypes are characterized under controlled indoor experimental conditions. The experiment found that as the temperature of the thermotropic film increased from 30 °C to 42 °C, the solar transmittance of LT-BICPVW decreased by 43.3%, and the maximum output power increased by 25%. Compared with, building integrated concentrating photovoltaic window coupled with thermotropic material (T-BICPVW), the power conversion efficiency was improved by 59% at the termination temperature. The LT-BICPVW system can effectively reduce solar heat gain and avoid glare hazards, while using renewable energy to significantly increase power output. A series of predictions are made using validated optical model and tracking technique to better guide the design of PV windows in actual buildings.

A Comparative Study of Cellulose Ethers as Thermotropic Materials for Self-Tracking Solar Concentrators

The continuous growth in energy demand requires researchers to find new solutions to enlarge and diversify the possible ways of exploiting renewable energy sources. Our idea is the development of a solar concentrator based on trapping the luminous radiation with a smart window. This system is able to direct light towards the photovoltaic cells placed on window borders and produce electricity, without any movable part and without changing its transparency. Herein, we report a detailed study of cellulose ethers, a class of materials of natural origin capable of changing their state, from transparent aqueous solution to scattering hydrogel, in response to a temperature change. Cellulose thermotropism can be used to produce a scattering spot in a window filled with the thermotropic fluid to create a new kind of self-tracking solar concentrator. We demonstrate that the properties of the thermotropic fluid can be finely tuned by selecting the cellulose functionalization, the co-dissolved salt, and by regulating their dosage. Lastly, the results of our investigation are tested in a proof-of-concept demonstration of solar concentration achieved by thermotropism-based light trapping.

Optical evaluation of a smart transparent insulation material for window application

Thermotropic (TT) materials can provide dynamic regulation of solar energy and daylight, thus achieving building energy saving and indoor environment improvement. Parallel Slats Transparent Insulation Materials (PS-TIM) when integrated into the cavity of a double-glazed window can increase the thermal resistance of the window system, thus reducing building heating energy consumption. In this study, these two advanced technologies are combined to form a novel TT PS-TIM window system for adaptive daylighting control and enhanced thermal insulation. To achieve this, thermotropic hydrogels such as hydroxypropyl cellulose (HPC) and poly(N-isopropylacrylamide) (PNIPAm) are proposed for integration within polymethyl methacrylate (PMMA) slats between double glass panes. The optical properties of the PMMA-TT slats have been firstly investigated by optical simulations and experiments. Windows integrated with the PMMA-TT slats have been developed and predicted for their daylight performance under different solar incident angles, slat tilt angles and concentrations of HPC/PNIPAm polymer within hydrogel. It was found that the TT PS-TIM window can effectively reduce visible and solar transmittance when the TT material switches from clear (at low temperatures) to translucent (at high temperatures). In addition, the translucent TT PS-TIM window allows less light to be transmitted into the interior space when the slat tilt angle is increased from horizontal to inclined. More uniform daylight distribution in the indoor space can be achieved by increasing the slat tilt angle. The optical investigation results of this research will provide guidance for the further development of this system.

Use the LiquidCrystals and it as a heat sensor by using semicondactor laser beams

In this research, a practical study was conducted to measure the temperature of objects remotely using a semiconductor laser and a heat-sensitive liquid crystals thermotropic material at different temperature values, starting from 30 ℃ to 70 ℃, at each value of the temperature the temperature is known To the bodies by reflected laser beams loaded with a portion of the measured body heat.
 The final results showed that the best measurement of body temperature can be obtained at large distances such as 1, 1.5, 2 meters, as this is due to the fact that the scattered laser beams have less impact on the detector, and this is what was observed in the study when taking distances 1, 1.5 And 2 meters, where the graphs at these distances show good stability of the reflected laser beam loaded with a part of the heat to that point which reflex of it and whose temperature is to be known, which radiates heat perhaps more or less than other parts of the body.
 As for less distances, the results obtained from this study lack accuracy, and this is what was shown by the graphs at a distance of 0.5 meter, This is due to the interference caused by the reflection and scattering of the laser light falling on the polarized plates and returning it to the detector

Numerical investigation of a smart window system with thermotropic Parallel Slat Transparent Insulation Material for building energy conservation and daylight autonomy

Smart window designs have emerged as a means of providing dynamic regulation of solar energy and daylight, enhancing indoor comfort, and achieving building energy conservation. We evaluated a novel window design that integrated a thermotropic (TT) material and Transparent Insulation Material (TIM) and present the investigation in this paper. The Parallel Slat TIM (PS-TIM) structure contained within the window unit provides extra thermal resistance and helps to redirect daylight. The TT material, which is applied to the slats, provides automatic daylight and solar adjustment. Firstly, the TT PS-TIM window system has been characterised thermally and optically. Then, a comprehensive approach including both building energy and daylight simulation packages was used to predict building performance. The effects of geometry (i.e. slat spacing and slat tilt angle) and thermotropic features (i.e. transition temperature and optical properties) on building performance were investigated. The simulation results show that use of TT PS-TIM window system with carefully selected features can simultaneously improve building energy efficiency (up to 22% saving when compared with a conventional double-glazed (DG) window) and attain homogenous daylight distribution with an average Useful Daylight Illuminance, UDI 500–2000 lux, of 52.2%. It was also found that both the geometric configurations and thermotropic features of a TT PS-TIM have significant influence on energy and daylight performance. TT PS-TIM with horizontally placed slats performs better than the unit with tilted slats, in terms of balance between energy efficiency and daylight availability. This research provides design guidance and material development suggestions for integration of this novel window system in buildings.

Energy and daylight performance of a smart window: Window integrated with thermotropic parallel slat-transparent insulation material

With the increasing awareness of building energy efficiency, indoor environment quality for human wellbeing and working efficiency, efforts have intensified in to inventing intelligent building components. This paper provides a first step in developing a novel multi-effect smart window system, which achieves enhanced energy efficiency and an improved indoor luminous environment by integrating a Transparent Insulation Material (TIM) structure incorporating a Thermotropic material. This system automatically regulates the admittance of solar heat and natural light into the building by responding to a changing environment while taking advantage of the increased thermal resistance and scattered daylight of window integrated TIM. A comprehensive workflow via EnergyPlus and RADIANCE was used to accurately predict the luminous and energy performance of applying the smart window system on a typical south-facing office under selected climates (London, Stockholm, Rome and Singapore). The effect of the optical properties and transition temperature of thermotropic material on building performance was explored in detail. Annual simulation results predict that, with a careful selection of the Thermotropic material properties, installing the TT PS-TIM window system is able to yield up to a 27.1% energy saving when compared with a conventional double glazed window, under the modelled Rome climate. TT PS-TIM windows also provide dynamic daylight control, resulting in increased daylight availability with the percentage of working hours that fall into the UDI500–2000 lx range increasing to 62.3%. The results of this research provide guidance for the next step of the material design and development that seek to balance energy efficiency and solar and daylight control through the use of thermotropic materials.

Monte-Carlo optical model coupled with Inverse Adding-Doubling for Building Integrated Photovoltaic smart window design and characterisation

Building Integrated Photovoltaic (BIPV) glazings are promising technologies with the benefits of electricity generation, solar shading and building energy savings. A new approach is to integrate BIPV windows with thermotropic materials such as Hydroxypropyl Cellulose (HPC) hydrogel, which offers adaptive advantages for the systems to respond to time-varying weather conditions but also enhances the PV electricity generation. To design such systems, knowledge of the temperature-dependent scattering properties of the selected thermotropic materials is of significance. In this study, a Building Integrated Photovoltaic (BIPV) smart window system consisting of a thermotropic membrane synthesised using HPC and gellan gum gelling agent for electricity generation and also solar control has been designed and investigated. An advanced optical model, which combines a Monte-Carlo ray-tracing technique with an Inverse Adding-Doubling (IAD) method, has been developed for characterising the thermotropic membrane in terms of angular scattering distribution under various membrane temperatures and HPC concentrations. Then the developed optical model has been validated by comparison with experimental measurements. Subsequently, the validated optical model has been used to design and optimise the proposed BIPV smart window. The effects of HPC concentration, geometric concentration ratio, thermotropic membrane thickness and glass refractive index on PV power outputs have been evaluated. Finally, a prototype of the BIPV smart window with a 6 wt% HPC membrane has been manufactured and tested under indoor conditions. From the experimental tests, it was found that the total transmittance of the double-pane glass sample with a 6 wt% HPC membrane layer decreases from approximately 90%to 14%, when the membrane temperature increases from 27 °C to 56 °C. The measured short-circuit current for the prototype BIPV smart window is up to 1.15 times higher than that of its counterpart system with a similar PV area but no membrane.

Pressure-tensor method evaluation of the interfacial tension between Gay-Berne isotropic fluid and a smooth repulsive wall.

The interfacial properties of a confined thermotropic liquid crystalline material are investigated using a molecular dynamics simulation technique. The pairwise interaction among the soft ellipsoidal particles is modeled by the Gay-Berne (GB) potential. The GB ellipsoids are confined by two soft, smooth, repulsive walls defined by the Weeks-Chandler-Andersen (WCA) potential. The aperiodic confinement due to walls makes the system mechanically anisotropic. Hence using the pressure-tensor method, the interfacial tension of an interface between the bulk isotropic (I) phase and WCA wall at various number densities (ρ) is calculated. From the pressure tensor and orientational order profiles, the arrangement of ellipsoids in the bulk and the vicinity of the wall is determined. The effect of system size and the wall-particle interaction strength (εW) on is also analyzed by varying the system size and εW.

Encapsulation of stearic acid with different PMMA-hybrid shell materials for thermotropic materials

In this study, we synthesized and characterized particles with a stearic acid (SA) core encapsulated in poly(methyl methacrylate) (PMMA) and four PMMA-hybrid shell materials. The objective was to create spherical particles with diameters of several hundred nanometers for use as the dispersed phase in thermotropic materials for overheat protection of polymer solar absorbers. Encapsulation was accomplished by emulsion polymerization. The composition, morphology and thermal properties of the encapsulated particles were characterized by scanning and transmission microscopy, Fourier transform infrared spectral analysis, digital scanning calorimetry and thermogravimetry. There were no chemical interactions between core and shell materials. The mean diameter and the thickness of the encapsulating shell varied over relatively narrow ranges of 110–360 nm, and 17–60 nm, respectively, depending on the functional groups of the shell. Based on published recommendations for size and shell thickness for thermotropic materials for overheat protection of polymer solar absorbers, particles encapsulated with polymethylmethacrylate-co-2-hydroxy-ethylacrylate (PMMA-HEA) shell and with polymethylmethacrylate-styrene (PMMA-S) shell are good candidates for further evaluation.

Transmittance and Reflectance Studies of Thermotropic Material for a Novel Building Integrated Concentrating Photovoltaic (BICPV) ‘Smart Window’ System

A novel Building Integrated Concentrating Photovoltaic (BICPV) Smart Window has been designed and developed as a next generation intelligent window system. In response to climatic conditions, the smart window varies solar light transmission into the building for provision of light and heat with the reflection of light to the photovoltaic (PV) for electricity generation. This unique function is realised using an integrated thermotropic layer in conjunction with embedded PVs. As commercial PVs are readily available, the success of this novel BICPV design depends solely on the performance of the thermotropic material. This study aimed to develop a suitable reflective thermotropic layer for the proposed smart Concentrating Photovoltaic (CPV) system. A Hydroxypropyl cellulose (HPC) polymer was tested for its applicability as a potential reflective thermotropic material for this purpose. HPC concentration was systematically varied from 1 wt. % to 6 wt. % in aqueous solution so as to provide insight into the relationship between transmittance/reflectance properties, the concentration of the thermotropic material and their dependence upon the environmental temperature. The degree of hysteresis of light transmittance upon subjecting HPC to heating and cooling cycles was also investigated. Specifically, for the HPC liquid samples the measured threshold temperature/transition temperature (Ts) was observed to be approximately 40 °C for 6 wt. % HPC, increasing to approximately 44 °C for 1 wt. % HPC. No hysteresis was observed upon heating and cooling HPC samples. Reflectance below the Ts was recorded at ~10%, increasing up to ~70% above the Ts for 6 wt. % HPC. Finally, a HPC-based hydrogel membrane sample was developed and exhibited good thermotropic activity therefore demonstrating its suitability for use within the BICPV smart window. This study corroborates that HPC is a suitable thermotropic material in the application of next generation BICPV smart window systems.

A review of self-organising 2,5- and 2,4-disubstituted 1,3-thiazole-containing materials: synthesis, mechanisms and tactics

ABSTRACT2,5- and 2,4-disubstituted 1,3-thiazoles have been incorporated as core units in thermotropic liquid crystalline materials. However, synthetic approaches to these systems have been somewhat limited, and this is reflected in the relatively few reports of these mesogenic systems. This paper highlights both recent and well-established synthetic approaches to these systems using ring closure (Gabriel- and Hantzsch-type approaches) methodology and modifications of an intact 1,3-thiazole ring. The scope and limitations of each of these approaches are discussed.

Thermotropic overheating protection for façade-integrated solar thermal collectors

The capability of thermotropic glazing to reduce stagnation temperatures in solar thermal collectors is evaluated in in-situ testing (south-façade of an office building). Therefore, a thermotropic formulation which is curable by sunlight and which exhibits both, excellent light-shielding characteristics (transmittance reduction by ~20%) and long-term stability is specifically designed. Application relevant overheating protection efficiency is given: absorber temperatures are reduced by up to 29K in collector stagnation. Performance characteristics are not changed during the six month period of in-situ testing. The application demonstration emphasizes the high innovative capacity of thermotropic materials.

Energy efficient hydrogel based smart windows with low cost transparent conducting electrodes

Indoor light control using thermotropic materials is an active area of research. While active materials are available to switch transmittance with temperature, large area heaters with desired transparency are not easily affordable. In this paper, the fabrication of thermochromic devices using inexpensive Sn mesh electrodes (5Ω/□, transmission, 80%) produced by crackle lithography with hydroxypropyl methyl cellulose (HPMC) as active material is reported. When laminated and coated on the inner surface of a PET window (8×8cm2), the mesh served as a transparent heater to cause gelation in HPMC at ~40°C to switch from water-clear transparency to paper-white opaqueness with 1mm thickness of the active layer. The power consumption was only 0.2W/cm2. Few drops of a color ink produced interesting effects in this smart window prototype.

Role of n-alkoxy terminal end groups on mesomorphism

ABSTRACTA homologous series RO-C6H4-CH:CH-COO-C6H4-CO-CH:CH-C6H4-OC6H13 (n) of novel liquid crystals (LC) has been synthesized and studied with a view to understanding and establishing the relation between molecular structure of a substance and the LC properties or behavior of thermotropic LC materials. Novel series consist of 11 homologues. C1 and C2 homologue derivatives are non-LCs and the rest of homologues (C3 to C16) are nematogenic without exhibition of smectic property. Transition temperatures were determined by an optical polarizing microscopy equipped with a heating stage. Transition curve Cr-N/I of a phase diagram behaved in normal manner. N/I transition curve partly deviated at C6 and C10 homologues from normal descending behavior, and exhibited very narrow and sharp odd-even effects. Textures of nematic phase are threaded or Schlieren. Thermal, analytical, and spectral data confirmed the molecular structures of novel chalconyl ester derivatives. Thermal stability for nematic is 95.77°C and the upper and lower mesophase lengths vary from 35 to 16°C at C5 and C8 homologues respectively. Group efficiency order derived for nematic on the basis thermal stability is -OC10H21 (n) > -OC8H17(n) > -OC6H13(n).

Molecular structural flexibility dependence of mesomorphism through ortho-substituted bromo group

ABSTRACTA novel series of liquid crystal (LC) materials of Schiff's bases viz. RO-C6H4‒CH˭CH‒COO‒C6H4‒N˭CH‒C6H4‒Br (ortho) is synthesized and studied with a views to understanding and establishing the effect of molecular structure on liquid crystal properties and to provide novel thermotropic LC material to the scientific and technological community of research interest. The novel series consists of thirteen members (C1 to C8, C10, C12, C14, C16 & C18). All the members of a series are enantiotropic liquid crystals. Smectic (C10‒C18) and nematic (C1‒C18) property commence from C10 and C1 homologue respectively. Transition temperatures and the textures of homologues were determined using a polarizing optical microscope (POM) equipped with a heating stage. Transition curves Cr-M, Sm-N and N-I of a phase diagram behaved in normal manner. Odd–even effect is absent for Sm-N transition curve, but it is present for N-I curve. Sm-N transition curve is extrapolated to C8 and C6 nonsmectogenic homologues to determine and predict their latent ability for exhibition of smectic property. Analytical and spectral data confirms the molecular structures of homologues. Thermal stabilities of smectic and nematic are 85.4°C and 130.1°C °C whose upper and lower total mesophase length vary from 58.0°C to 06.0°C at the C10 and C1 = C5 = C6 homologue, respectively. Textures of nematic phase are threaded or Schlieren and smectic phase are of type A or C. LC properties of present novel series are compared with structurally series known series. Thus, present novel series is predominantly nematogenic and partly smectogenic with middle ordered melting type and considerable total mesophase length range.

Numerical Evaluation of the Optical Properties of Encapsulated Phase Change Particles for Thermotropic Materials

Phase change thermotropic materials have been proposed as a low cost method to provide passive overheat protection for polymer solar thermal absorbers. One challenge to their development is control of the size of the phase change particles dispersed within the matrix. Here we explore encapsulation as a means to resolve this challenge with a focus on the selection of materials, including the encapsulating shell, to achieve desirable optical behavior. Hydroxystearic acid (HSA) particles in a matrix of poly(methyl methacrylate) (PMMA) is down selected from candidate materials based on its optical properties and the melt temperature of the dispersed phase. The optical properties (normal-hemispherical transmittance, reflectance, and absorptance) as a function of the properties of the encapsulation shell and the particle volume fraction are predicted at a wavelength of 589 nm using a Monte Carlo ray tracing model. A range of shell relative refractive indices, from 0.95 to 1, and thicknesses, up to 35 nm, can be employed to achieve greater than 80% transmittance in the clear state and greater than 50% reflectance in the translucent state.

Synthesis and characterisation of star-shaped liquid crystalline polymer with a POSS core

The star-shaped POSS-graft-LCP with POSS as the core and liquid crystal polymer, poly{6-(4ʹ-octyloxyphenyl-4″-benzoyl)hexyl acrylate}, as arms was prepared by atom transfer radical polymerisation technique using octa(3-chloropropyl) polyhedral oligomeric silsesquioxane [POSS-(CH2CH2CH2Cl)8] as initiator. For comparison, the linear liquid crystal polymer, poly{6-(4ʹ-octyloxyphenyl-4″-benzoyl)hexyl acrylate} (LLCP), was obtained by conventional radical polymerisation. Both liquid crystal polymers were characterised by FT-IR, 1H NMR, 13C NMR, gel permeation chromatography, thermogravimetric analysis, differential scanning calorimetry, polarised optical microscopy and X-ray diffraction analysis. The liquid crystal phase behaviour research demonstrated that both liquid crystal polymers were reversible thermotropic nematic liquid crystal materials. The number of polymerisation degree of every arm attached on POSS in POSS-graft-LCP impacted greatly on the liquid crystal properties and only a small one was necessary for it to exhibit a broad liquid crystal range. Results further demonstrated that the special star-shaped topology of POSS and the eight arms attached helped POSS-graft-LCP form and stabilise liquid crystal phase easily. This research may further expand the way to star-shaped LCPs by employing a variety of (meth)acrylate and other vinyl liquid crystalline monomers.

The Effect of a Thermotropic Material on the Optical Efficiency and Stagnation Temperature of a Polymer Flat Plate Solar Collector

Solar hot water and space heating systems constructed of commodity polymers have the potential to reduce the initial cost of solar thermal systems. However, a polymer absorber must be prevented from exceeding its maximum service temperature during stagnation. Here, the addition of a thermotropic material to the surface of the absorber is considered. The thermotropic layer provides passive overheat protection by switching from high transmittance during normal operation to high reflectance if the temperature of the absorber becomes too high. A one dimensional model of a glazed, flat-plate collector with a polymer absorber and thermotropic material is used to determine the effects of the optical properties of the thermotropic material on the optical efficiency and the stagnation temperature of a collector. A key result is identification of the reflectance in the translucent state required to provide overheat protection for potential polymer absorber materials. For example, a thermotropic material in its translucent state should have a solar-weighted reflectance greater than or equal to 52% to protect a polypropylene absorber which has a maximum service temperature of 115 °C.

A Parametric Numerical Study of Optical Behavior of Thermotropic Materials for Solar Thermal Collectors

A Monte Carlo model is applied to determinate the steady state, solar-weighted optical properties of potential thermotropic composite materials for overheat protection of polymer solar absorbers. The key results are dimensionless plots of normal-hemispherical transmittance, reflectance and absorptance as a function of particle size parameter, scattering albedo, and overall optical thickness. The optical behavior of thermotropic materials at different temperatures is represented by a change in the relative refractive index which affects the scattering albedo and optical thickness. At low temperatures where overheat protection is not required, referred to as the clear state, the overall optical thickness should be less than 0.3 to ensure high transmittance for the preferred particle size parameter of 2. At higher temperatures where overheat protection is required, referred to as the translucent state, the overall optical thickness should be greater than 10 and the scattering albedo should be greater than 0.995 to achieve 50% reflectance. A case study of low molecular weighted polyethylene in poly(methyl methacrylate) is presented to illustrate use of the results to guide the design of thermotropic materials.

Solar Collector Cover with Temperature-controlled Solar Light Transmittance

Our aim is the development of a solar collector cover with temperature-controlled solar light transmittance in order to protect plastic solar collectors against overheating and to prevent collector damage during stagnation. The temperature-dependent reduction of solar transmittance is based on an increase of backscattering of the incident solar radiation (thermotropism). The thermotropic materials consist of two components: 1) a thermotropic additive, namely submicron-sized core-shell particles containing a phase-change material, and 2) an appropriate transparent matrix polymer. Thermotropic samples based on three different matrix polymers (UV-curable cast resin, EVA and PVB) were prepared as sandwich laminates according to industrially relevant processes. Temperature-dependent measurements of the total solar transmittance reveal absolute differences of up to 28% between OFF and ON state.