Rapidly rising global temperatures and the intensification of the urban heat island (UHI) effect necessitate new, energy-efficient solutions to mitigate heat stress in cities. Passive radiative cooling (PRC) offers a highly promising, low-energy pathway to achieve sub-ambient temperatures by reflecting incoming solar radiation while emitting long-wave infrared radiation through the atmospheric infrared window. This review summarizes key aspects of PRC and its role in reducing UHI impacts. Furthermore the fundamental physics of heat transfer and radiative heat exchange, including the materials properties such as solar reflectance and thermal emissivity which are correlated with the figures of merit, temperature drop below ambient temperature and cooling power. A comprehensive classification of current PRC materials is presented based on both structural architectures and physical effects. Additionally an overview on measurement techniques are employed to determine the performance of PRC materials, focusing on the key performance indicators. For this purpose in-field as wells as laboratory measurement techniques are introduced and opportunities in standardizing testing protocols are highlighted. Finally, future research directions are outlined, focusing on novel material development, theoretical advancements, scalable fabrication processes, and integration strategies within urban infrastructures. These innovations are important for enhancing building energy efficiency, reducing urban heat stress, and promoting sustainable urban development in the face of climate change.

The guarded-hot-plate (GHP) method is a method used world-wide for the determination of the thermal conductivity of solid materials, especially low-conducting insulation materials, based on the direct application of Fourier’s law. It is an absolute method, which means that no reference material is needed, though regular performance checks using reference materials are highly encouraged. All measured quantities are traceable to SI-units. Applying the GHP method in accordance with existing standards, relative uncertainties in the determined values of the thermal conductivity below 2.5% are achievable. This work describes the basic theory of the measurement and is focused on difficulties and possible stumbling blocks in daily measuring practice and laboratory routine when applying the GHP method on specimens which are not or only partly covered by standards. The work explains in which cases the definition of a thermal conductivity value is not possible for a specimen and only system-related values as the thermal resistance can be attributed to a specimen. The aim of this work is to improve the measuring quality of GHP measurements for the determination of the thermal conductivity in daily laboratory operation.

Measurements of the thermal conductivity were performed as a function of gas pressure from 10–1 hPa up to 105 hPa on several bimodal silica xerogels. The xerogels exhibit a mesopore and a macropore phase. The measurements were done using a hot-wire apparatus, which can do automated, gas pressure-dependent measurements of the thermal conductivity from 10–3 up to 105 hPa. Results were fitted with a bimodal gas pressure-dependent thermal conductivity model to gain information on the thermal conductivity of the materials, its various contributions and on structural parameters such as the two main pore sizes, the macro- and mesoporosities. The pore sizes and porosities were compared to values gained from mercury porosimetry and nitrogen adsorption measurements. The porosities from the thermal conductivity measurements are in very good agreement to the other measuring methods. The macropore sizes from the thermal conductivity measurements are mostly in agreement within the given uncertainty range and the mesopore sizes show a good estimate of the order of magnitude of the pores.

Aerogels are an exciting class of materials with record-breaking properties including, in some cases, ultra-low thermal conductivities. The last decade has seen a veritable explosion in aerogel research and industry R&D, leading to the synthesis of aerogels from a variety of materials for a rapidly expanding range of applications. However, both from the research side, and certainly from a market perspective, thermal insulation remains the dominant application. Unfortunately, continued progress in this area suffers from the proliferation of incorrect thermal conductivity data, with values that often are far outside of what is possible within the physical limitations. This loss of credibility in reported thermal conductivity data poses difficulties in comparing the thermal performance of different types of aerogels and other thermal superinsulators, may set back further scientific progress, and hinder technology transfer to industry and society. Here, we have compiled 519 thermal conductivity results from 87 research papers, encompassing silica, other inorganic, biopolymer and synthetic polymer aerogels, to highlight the extent of the problem. Thermal conductivity data outside of what is physically possible are common, even in high profile journals and from the world’s best universities and institutes. Both steady-state and transient methods can provide accurate thermal conductivity data with proper instrumentation, suitable sample materials and experienced users, but nearly all implausible data derive from transient methods, and hot disk measurements in particular, indicating that under unfavorable circumstances, and in the context of aerogel research, transient methods are more prone to return unreliable data. Guidelines on how to acquire reliable thermal conductivity data are provided. This paper is a call to authors, reviewers, editors and readers to exercise caution and skepticism when they report, publish or interpret thermal conductivity data. Graphical Abstract

This chapter provides an insight into the different aspects of heat transfer in aerogels and their thermal properties. In this context, the principle heat transfer mechanisms are discussed and illustrated by exemplary experimental results. Additionally, various experimental methods are presented allowing the analyses of the different aspects of heat transfer. Typical thermal conductivity values and radiative properties as well as their dependency on external conditions such as temperature or atmosphere are discussed for different classes of aerogels. This chapter concludes with a brief discussion about the specific heat of aerogels.

High-performance thermal insulations (HPI) have outstanding thermal properties minimizing heat transfer for a particular application. The term "high performance" is a relative assessment: whether an insulating property for a material or material system, i.e. the thermal conductivity or thermal transmittance, is significantly lower than for conventional insulating materials or systems depends on the use case and the associated conditions. Fields of applications for thermal insulations include energy, construction, industry and transport sector, as well as space and aviation industries. The total effective thermal conductivity of evacuated HPI depends on the contributions of the heat transfer via the solid skeleton of the porous insulation material and radiative heat exchange. At a given operating temperature, the effective thermal conductivity results in a minimum that depends on the density and the infrared-optically extinction properties of the insulation material. Examples for HPI with outstanding low thermal conductivity values are presented and compared with the theoretical predictions based on a percolation model for the solid thermal conductivity of the skeleton and on a diffusion model for the radiative heat transfer.

A vacuum super insulation for temperatures up to 1073 K has been developed using an opacified powder mixture with high-density (rho= 180 kg m(-3)) expanded perlite as base material. To analyze radiative transfer, the massspecific extinction coefficient e of expanded perlite and various opacifiers (SiC, B4C, FeTiO3, TiO2) has been determined by FTIR spectrometry. For reduction of radiative transfer, a SiC powder with mean grain diameter of 2 mu m-4 mu m and bulk density of 870 kg m(-3) has been identified as most suitable opacifier, also under economic aspects. Subsequently, six homogeneous powder mixtures with weight fraction w of SiC between 0% and 60% have been prepared. In order to determine the optimum w, guarded hot plate (GHP) measurements under high vacuum conditions (p < 0.1 hPa) and at 673 K mean temperature have been performed on these mixtures, yielding a minimum effective thermal conductivity of lambda(e) = 13.10(-3) W m(-1) K-1 for w = 40% and p = 263 kg m(-3). This mean temperature corresponds to an insulation application with 950 K hot-side and ambient temperature cold-side. For the optimum mixture with w = 40%, e has likewise been determined with FTIR spectrometry. Furthermore, lambda(e) has been measured with the GHP method also at mean temperatures between 373 K and 873 K, where excellent values between 4.10 -3 W m(-1) K-1 and 20.10(-3) W m(-1) K-1 have been obtained. For the evaluation of these measurements, two different methods have been applied in order to separate radiative thermal conductivity. lambda(r) and solid thermal conductivity lambda(s): first the common plot of lambda(e) versus T-3, and second an improved method, which includes the temperature dependencies of e and lambda(s). Expanded perlite, opacified with SiC, has proven as a highly efficient and economic vacuum super insulation material for industrial applications up to 1073 K.

An improved apparatus for measuring the spectral directional emissivity in the wavelength range between 1 µm and 20 µm at temperatures up to 2400 K is presented in this paper. As a heating unit an inductor is used to warm up the specimen, as well as the blackbody reference to the specified temperatures. The heating unit is placed in a double-walled vacuum vessel. A defined temperature, as well as a homogenous temperature distribution of the whole surrounding is ensured by a heat transfer fluid flowing through the gap of the double-walled vessel. Additionally, the surrounding is coated with a high-emitting paint and serves as blackbody-like surrounding to ensure defined boundary conditions. For measuring the spectral directional emissivity at different emission angles, a movable mirror is installed in front of the specimen, which can be adjusted by a rotatable arrangement guiding the emitted radiation into the attached FTIR-spectrometer. The setup of the emissivity measurement apparatus (EMMA) and the measurement procedure are introduced, and the derived measurement results are presented. For evaluating the apparatus, measurements were performed on different materials. The determined emissivities agree well with values published in literature within the derived relative uncertainties below 4% for most wavelengths.

Due to their special combination of nanostructure and high porosity, aerogels are key materials for high performance thermal insulation. However, measuring reliable thermal conductivity values, which are essential for material’s optimization and as product parameter, is a challenging task in the case of aerogels. Experimentally derived thermal conductivity values for aerogels are so far more or less influenced by the experimental set-up and the experimental conditions and have to be carefully discussed. Here we present results of an intercomparison on thermal conductivity measurements performed for a polyurethane aerogel produced on a pilot scale as stiff panels by BASF. Prior to the intercomparison, the material was checked for reproducibility in production and homogeneity. The dependence of thermal conductivity on atmospheric pressure and temperature was also determined. We discuss the results submitted by 12 participants with respect to the different experimental techniques applied and identify experimental parameters with severe impact on the resulting thermal conductivities. The derived mean values for the thermal conductivity at 20 °C, 40 °C and 60 °C were related with relative uncertainties in the range from 0.6 % to 0.9 %. The dependence of the derived thermal conductivity values on the geographical location of the participating laboratory and the atmospheric weather conditions could be clearly observed and the precision of the results could be significantly improved by correcting for these effects. The values had to be partly corrected up to 2.5 %.

It has been shown that structural properties of open porous solids, such as the mean pore size, can be derived from gas pressure-dependent thermal conductivity data. However, a reliable prediction of the total thermal conductivity of a porous sample with complex backbone structure from structural data is not possible, because the degree of coupling of gaseous and solid thermal conduction is hard to estimate. To explore the impact of structural effects, the thermal performance of different model structures, generally characteristic for porous solids (necks, dead ends, tortuosity), was theoretically evaluated by means of finite-difference calculations. As a result, we find that dead ends cause the highest amount of thermal coupling. On the other hand, independent experimental investigations were performed to support the theoretical findings. That means, the gas pressure-dependent thermal conductivities of two sample systems in a nitrogen atmosphere were analyzed: At first, thermal conductivity data for three organic, resorcinol–formaldehyde based aerogels with different structural properties were received from hot-wire measurements. Secondly, the regular cell structure of melamine resin foam was systematically changed by uniaxial compression within a guarded hot plate apparatus prior to determining the resulting thermal conductivity in the direction of compression. Overall, the measured gas pressure-dependent thermal conductivities of both systems indicate that the connectivity of the solid network significantly affects the solid–gas coupling term in porous solids. Both the experimental and theoretical results show that the coupling term decreases with increasing connectivity of the backbone material of a porous solid.

We measured the total thermal conductivity λ of four resorcinol-formalde-hyde (RF-) aerogel tiles in a guarded hot plate apparatus and a hot-wire device. The temperature was varied between 20 and 80°C, the gas pressure (air) from 1000 mbar down to 1 • 10−4 mbar. All samples with bulk densities of ϱ = 158, 180, 205 and 236 kg/m 3 have been prepared using the same molar ratio of resorcinol to catalyst (R/C = 200). The measured thermal conductivities are between 5 and 8 • 10−3 Wm −l K −l in the evacuated state and in the range of 11 to 13 • 10−3 Wm −l K −1 in air at room temperature. The thermal conductivity data derived from the hot plate and the hot-wire device agree within 10−3 Wm −l K −l or 10 to 20 % of the absolute value, respectively. From the thermal conductivity measurements as a function of air pressure a typical pore size in the aerogels between 20 nm for ϱ = 236 kg/m 3 and 30 nm for ϱ = 158 kg/m 3 was derived. From additional infrared-optical transmission and reflection measurements the mass specific extinction e and the temperature dependent radiative conductivity could be calculated. The solid conductivity was separated by subtracting the infrared-optically derived radiative conductivity from the total conductivity of the evacuated samples. The solid thermal conductivity was found to increase by about 20 % in the temperature range from 20°C to 80°C. The total thermal conductivity in air as a function of the aerogel density shows a broad minimum. The optimal densitity for minimized thermal conductivity at room temperature was found to be about 180kg/m 3. To our knowledge the measured conductivities of the resorcinol-formaldehyde aerogels are within the lowest thermal conductivities ever measured for any solid body in air.

The effective thermal conductivity of two evacuated expanded perlite powders has been measured at temperatures between 295 K and 1073 K. Since conduction via the gas phase is suppressed, thermal transport occurs only via solid conduction and thermal radiation. Due to thermal contact resistances between the powder particles, solid conduction is very small and radiative heat transport dominates, especially at high temperatures. Applying the guarded-hot-plate (GHP) method to optically thick specimens, the true effective thermal conductivity λeff, which is the sum of solid thermal conductivity λs and radiative thermal conductivity λr from the diffusion model, has been measured. After plotting λeff as a function of third power of absolute temperature and calculating the regression line, λs is obtained from the intercept of the line, and the extinction coefficient for thermal radiation E is determined from its slope. The resulting values are λs = (6.4 ± 1.5) ∙ 10−3 W m−1 K−1 and E = (1600 ± 40) m−1 for the first perlite powder and λs = (3.1 ± 1.3) ∙ 10−3 W m−1 K−1 and E = (5700 ± 350) m−1 for the second. With an effective thermal conductivity below 0.01 W m−1 K−1 up to a mean sample temperature of 473 K, the second material is suitable to realize an economic evacuated powder insulation for medium-temperature applications up to approximately 673 K at the hot side. Both materials have also been investigated with the transient-hot-wire (THW) method. This technique has the advantage of shorter measurement time, but underestimates the effective thermal conductivity according to numerical calculations from literature, especially for samples with 1000 m−1 ≤E≤ 10 000 m−1. This leads to an apparent extinction coefficient Eapp>E from the λeff vs. T3 plot. Using the same procedure as above, the THW measurements deliver λs = (3.9 ± 2.7) ∙ 10−3 W m−1 K−1 for the first perlite powder and λs = (2.2 ± 1.4) ∙ 10−3 W m−1 K−1 for the second, which agrees with the values obtained from the GHP method. The apparent extinction coefficients are Eapp = (2170 ± 110) m−1 and Eapp = (6730 ± 370) m−1, which corresponds to an overestimation by 35.6% and 18.1%, respectively. Both results are in good agreement with the numerical calculations from literature, which have now been verified experimentally for the first time. Because such calculations can in principle be used to correct experimental THW data, it is possible to extend the applicability of the THW method to materials with 1000 m−1 ≤E≤ 10 000 m−1, i.e. near the limit of radiation diffusion.

Stationary gas turbines are still an important part of today's power supply. With increasing temperature of the hot combustion gas inside a gas turbine, the efficiency factor of the turbine increases. For this reason, it is intended to operate turbines at the highest possible gas temperature. Therefore, in the combustion chamber and especially at the position of the first stage guide vanes the gas temperature needs to be measured reliably. To determine the gas temperature, one promising approach is the application of a non-contact measurement method using a radiation thermometer. A radiation thermometer can measure the gas temperature remotely from outside of the harsh environment. At ZAE Bayern, a high temperature and high pressure gas cell has been developed for this purpose in order to investigate gases and gas mixtures under defined conditions at high pressures and high temperatures. This gas cell can be placed in a FTIR-spectrometer in order to characterize the infrared-optical properties of the gases. In this work the measurement setup is introduced and gas mixtures, which are relevant for gas turbine applications are analyzed thoroughly. The derived results are presented and discussed in detail. To identify suitable wavelength regions for non-contact gas temperature measurements, first tests have been performed. Based on these tests, an appropriate wavelength region could be chosen, where future gas temperature measurements can be carried out.

Although in the case of fluids there are several reference correlations of low uncertainty for thermal conductivity, for solids there are very few and in a restricted temperature range. The available experimental data for the thermal conductivity of five widely used solids, BK7, PMMA, Pyrex 7740, Pyroceram 9606, and SS304, have been critically examined with the intention of establishing thermal conductivity reference correlations. All experimental data have been categorized into primary and secondary data according to the quality of measurement specified by a series of criteria. A new reference correlation is proposed for BK7, and improved ones for PMMA, Pyrex 7740, Pyroceram 9606 and SS304.

The German Thermophysics Working Group (AKT) within the Society for Thermal Analysis (GEFTA) initiated and conducted an intercomparison with the objective of the determination of the thermophysical properties of pure iron, an interstitial-free steel, and a multi-phase steel. The values of heat capacity, thermal diffusivity, thermal expansion, and thermal conductivity were measured from 20 °C up to 1000 °C. In all cases, a mean value could be derived. In the case of pure iron, the mean values are in good agreement with the literature values. For the values of the thermal expansion coefficient and thermal diffusivity, the relative uncertainties are below 4 %. The relative uncertainties for the specific heat are between 4 % and 5 % up to 600 °C. Above this temperature, the uncertainty is in the range from 6 % to 8 %. The relative uncertainty of the thermal conductivity values is about 6 % below 600 °C and up to 9 % above.

While research on using the latent heat of so called phase change materials (PCMs) for thermal energy storage has gained increasing interest in the last decade, the measurement of its thermal properties are still subject to research. The T-History method has been frequently used by researchers to measure the enthalpy temperature curve of PCMs but the factors influencing its accuracy and precision have rarely been discussed. This work provides a systematic experimental study of an organic PCM based on different insulated sample holders. It is first shown that the data evaluation method has to be adjusted against noise to improve both accuracy and precision for all experimental setups. The results moreover show that neglecting the insulation thermal mass in the experimental setup leads to systematic errors in the enthalpy results due to oversimplification of the mathematical model. This confirms a previous numerical study by the authors. It is recommended that either the mathematical model or the experimental setup are adjusted in future work to decrease this error. Until then it is generally recommended to use sample holders with a high ratio between the thermal mass of the PCM to the insulated sample holder. This is further supported by a measurement uncertainty analysis via Monte Carlo simulations.

Thermal energy storage with phase change materials (PCM) provides high storage capacities in small temperature intervals. For the design of storage systems, the enthalpy curve of the used PCM has to be known with high precision. The T-History method has evolved to a widely used method for the measurement of the enthalpy as a function of temperature of PCM because of its simplicity and the advantage to be able to investigate larger sample volumes than typically used for differential scanning calorimeters. In order to ensure isothermal specimen during the measurement a thermal insulation is often mounted around the sample holder, but this insulation material is not considered in the evaluation model. In this work a new evaluation model for insulated T-History setups is developed by an analytical heat balance. This model is validated by numerical simulations of insulated T-History measurements and experimentally. By the use of the new model for the evaluation of the enthalpy a significant increase of accuracy can be achieved.

Zusammenfassung Das Ziel dieser Arbeit bestand in der Entwicklung eines langwelligen Strahlungsthermometers zur berührungslosen Messung von Oberflächentemperaturen in stationären Gasturbinen während des Betriebs der Turbinen innerhalb des EU-geförderten Projektes „Sensors Towards Advanced Monitoring and Control of Gas Turbine Engines (Acronym STARGATE)“. Im Rahmen der Arbeit wurden die infrarot-optischen Eigenschaften der Wärmedämmschichten und der vorhandenen Brenngase am ZAE Bayern bei hohen Temperaturen bis 1600 K und Drücken bis 13 bar bestimmt. Mit Hilfe dieser experimentellen Charakterisierungen konnte ein geeigneter Spektralbereich um 10 μm für das langwellige Strahlungsthermometer identifiziert werden. Entsprechend dieser Erkenntnisse wurde zunächst ein Laboraufbau mit geeigneten optischen Bauteilen (Filter, IR-Wellenleiter, etc.) realisiert und verifiziert. Anschließend wurde ein Prototyp für Messungen in Gasturbinen während des Betriebs der Turbinen entwickelt und in einem Turbinenteststand der Firma Siemens AG in Berlin erfolgreich getestet. Abschließend wurde eine Unsicherheitsanalyse durchgeführt, die eine erweiterte Messunsicherheit der gemessenen Temperaturen von etwa ± 30 K ergab.