Chapter 28 Vacuum Super Insulated Thermal Storage Systems for Buildings and Industrial Applications Thomas Beikircher, Thomas BeikircherSearch for more papers by this authorMatthias Rottmann, Matthias RottmannSearch for more papers by this author Thomas Beikircher, Thomas BeikircherSearch for more papers by this authorMatthias Rottmann, Matthias RottmannSearch for more papers by this author Book Editor(s):Andreas Hauer, Andreas Hauer ZAE-Bayern, Garching, GermanySearch for more papers by this author First published: 25 April 2022 https://doi.org/10.1002/9781119239390.ch28 AboutPDFPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShareShare a linkShare onFacebookTwitterLinked InRedditWechat Abstract Vacuum super insulation (VSI) allows highly-efficient heat storage due to exceptionally low thermal conductivities of insulating storage envelope. Using expanded perlite powder, VSI has been applied for many years to store liquefied gases at −200°C. Recently, the technology has been transferred to thermal storages up to 160°C. Here, thermal conductivities between 0.007 and 0.01 W m −1 K −1 are realized, which is an improvement by a factor of 3 to 5 compared to conventional insulation materials. Expanded perlite is highly porous, non-flammable, non-toxic and low-cost. Due to micro-pores, vacuum pressures around 0.1 mbar are sufficient for practically complete suppression of gas conduction. Low bulk density and point-like grain contacts greatly diminish also solid conduction. Radiative heat transfer is small due to extinction in the perlite itself and can be further reduced by admixture of opacifiers that increase absorption and scattering. Currently, the VSI technology is under development for high-temperature applications, where it has great technical, economical and environmental potential. With 40 weight−% added SiC opacifier, high-density expanded perlite experimentally showed ultra-low thermal conductivity of 0.013 W m −1 K −1 at 400°C mean temperature, corresponding to the insulation of ca. 700°C storage temperature against ambient temperature. Apart from industrial high-temperature thermal storages, VSI can also be applied to furnaces, pipings, or transport containers for molten metals. An alternative VSI powder is fumed silica, which is commonly used in vacuum insulation panels (VIPs). Advances in Energy Storage: Latest Developments from R&D to the Market RelatedInformation

The effective thermal conductivity of pure and opacified expanded perlite is investigated at gas pressures between 10-4 hPa and 103 hPa and temperatures between 293 K and 1073 K by guarded hot plate and transient hot wire measurements. The examined materials are a low-density (p = 56 kg m-3) and a high-density (p = 180 kg m-3) commercial expanded perlite powder, denoted as P1.5 and P0.14, respectively, as well as a mixture of P0.14 with 40 weight-% SiC as opacifier, denoted as P0.14_O (p = 263 kg m-3). To obtain the gas pressure dependent contribution of the measured total effective thermal conductivity, the thermal conductivity in the fully evacuated state is determined experimentally and subtracted. A model for the gas pressure dependent contribution is developed, which takes into account all temperature dependencies of the occurring gas-kinetic parameters and successfully reproduces all measurement data within the experimental uncertainty (ca. 5%). The model includes four structural parameters determined by fitting, which are the effective pore diameters both inside and between the grains as well as the respective porosities. The obtained values agree well with the results of structural analysis by SEM and optical microscopy. Furthermore, the model contains an improved description of the coupling effect between solid-body and gas heat conduction by means of a single novel coupling parameter, which expresses the exceedance of the gas pressure dependent contribution of the effective thermal conductivity relative to the thermal conductivity of the gas inside the pores. At ambient pressure and 673 K mean temperature, the effective thermal conductivity of the opacified powder mixture amounts to 0.087 W m(-1 )Ksks, thus being competitive with commercial high temperature insulation materials like calcium silicate, but at a lower price (ca. 2-3 euro kgsks). Experiments with the transient hot wire method at ambient pressure yield a temperature dependent overestimation of the effective thermal conductivity by 4-22% compared to the guarded hot plate method, which is traced back to the pressure dependent contribution and discussed in appropriate terms of the difference between the coupling parameters obtained from both methods.

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.

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.

Accurate determination of direct normal irradiance (DNI) is essential for evaluating and monitoring of concentrating solar power (CSP) and solar process heat (SPH) plants. Currently, a bi-axially tracked Pyrheliometer (PHM) is the most accurate measuring device, but requires high maintenance effort and is cost-intensive. As an alternative, the cost-effective pyranometer SPN1 was investigated by comparing highly resolved minute mean DNI measurements of two SPN1 devices to a reference PHM at the Institute for Solar Technology SPF in Rapperswil (Switzerland). To reduce systematic errors, a linear and a quadratic correction function have been applied to four years of measurements. They reduced the root mean square error RMSE from 48.8 W/m(2) (11.5% of mean PHM DNI = 426 W/m(2)) to 37.9 W/m(2) (8.9%) and 29.6 W/m(2) (6.9%), respectively. It is also shown that even recalibrations based on only one month of data already reduced the RMSE to maximum 33.4 W/m(2) (7.8%), if conducted between April and September. Considering the impact of solar elevation and DNI range on the SPN1 accuracy by bifurcating the correction functions only resulted in a negligibly small improvement of the RMSE. Finally, the influence of SPN1's DNI measurement accuracy on the performance analysis of a real SPH plant in Switzerland was evaluated. After recalibration and correction of DNI values, the evaluation of monitoring data yielded annual efficiencies that were 15% and 12% higher in 2014 and 2015, respectively. It is recommended to recalibrate SPN1 devices over a minimum of one month against a PHM according to the methods presented in this paper.

Process heat represents a major share of final energy consumption in the industrial sector and can partly be provided by solar thermal systems. To date, there has been little experience with solar heat plants for industrial processes operating at medium temperature levels (100–250 °C). This paper focuses on the analysis of reduced solar gains by heating-up processes (capacitive thermal losses) in a parabolic trough collector field with an aperture area of 627 m2 providing solar heat for a Swiss dairy at 120 °C. Heating-up thermal masses is experimentally quantified by a new method using existing temperature sensors. The unused solar thermal gains of heating-up periods amount to 18% of possible useful solar gains in 2014. In winter months, this share can reach 50%. Preserving the hot fluid content in an ideally insulated storage in the evening could avoid heating-up in the morning and reduce capacitive thermal losses by 38%. With properly installed insulation thermal losses of the piping system during operation are theoretically proven to be below 3% of useful solar gains. The analyses are based on the evaluation of highly time-resolved measurements of one year.

Nearly 70% of final energy consumption in European industry is required for process heat. Part of this energy could be provided by thermal collectors in order to reduce fossil fuel consumption and CO2 emissions. Though, to date there is little experience with solar process heat plants (SPHP), especially at temperatures between 100 °C and 250 °C, where thermal losses of system components during stand-still periods play a major role compared to low temperature applications. After periods of non-operation, solar energy is required to heat up the components to operating temperature before solar heat can be delivered to the processes.

A new flat plate collector is under development for economic process heat of temperatures between 80° and 150 °C. A vacuum super insulation (VSI) with micro-porous powders (perlite or fumed silica) in an evacuated (0.1 mbar) stainless steel envelope has been developed for the rear side of the collector which replaces the mineral wool insulation of standard collectors. CFD simulations of the equivalent thermal conductivity of the VSI including losses via the powder and edge losses have been performed depending on geometry. Applying a 100 μm thin VA foil as VSI cover, the collector losses can be reduced by about 0.6 W/m2K. Between glass cover and absorber the collector will be equipped with a transparent ETFE foil reducing the loss coefficient further by about 1.2 W/m2K. Additionally, a full surface aluminium absorber with a very high collector efficiency factor F' of 0.97 is used, reaching maximum collector efficiencies of about 0.9. Up to a temperature difference of 110 K, the new collector is estimated to have higher efficiencies than standard flat plate and vacuum tube collectors. In the temperature range of 80° and 150 °C the efficiency is expected to lie between 65% and 35%. Four different and successively improved prototypes are under test and construction, respectively. The first prototype showed the principal feasibility of the concept and was the first VSI collector operated under outdoor conditions at temperatures up to 120 K over ambient.

Vacuum super insulation (VSI) with expanded perlite powder is commonly used at cryogenic temperatures, but principally can also be adapted to applications at higher temperatures, such as the long-term storage of hot water in solar thermal systems. Due to the lack of experimental data in the respective temperature range, especially without external load, thermal conductivity measurements have been performed with commercial perlite powder up to 150°C mean sample temperature, corresponding to storage temperatures of around 300°C. Two different experimental geometries have been used: a guarded hot plate (GHP) setup and a cut-off concentric cylinder (CCC) apparatus. Furthermore, the radiative heat transport has been determined separately by extinction measurements using Fourier transform infrared (FTIR) spectroscopy. In addition to the laboratory experiments, a real-size prototype of a solar VSI-storage tank with 16.4 m3 water storage volume has been constructed, and the effective thermal conductivity of the perlite insulation has been determined from a heat loss measurement. The heat transport in evacuated perlite has also been treated theoretically using common models and approaches for gas heat conduction, solid-body conduction and heat transfer by thermal radiation. For the coupling between solid-body and gas conduction which occurs in the intergranular spaces of a powder material, a simple model has been developed. The total effective thermal conductivity λeff of a vacuum super insulation with dry, evacuated perlite powder (p≤0.01 mbar,ρ≈60 kg/m3) amounts to 0.007–0.016 W/mK for mean sample temperatures between 50°C and 150°C, compared to 0.003–0.005 W/mK at cryogenic temperatures. For the real-size storage prototype, the value λeff=0.009 W/mK has been obtained at T=90°C (storage temperature), p = 0.08 mbar and ρ=92.4 kg/m3, which compares to 0.03–0.06 W/mK for dry conventional storage insulations. With the applied theoretical models and approaches, the effective thermal conductivity of evacuated perlite and its individual contributions can successfully be described at different densities (55-95 kg/m3), compression methods, vacuum pressures (10-3-1000 mbar) and filling gases (air, Ar, Kr) up to mean sample temperatures of T=150°C. With regard to practical purposes, it has shown that vacuum super insulation with perlite is a suitable and economic method to achieve low thermal conductivities also at medium storage temperatures.

For the economical supply of solar process heat at temperatures between 120 and 150 degrees C a new non-tracking, fiat, low-concentrating collector has been developed. The new collector is an edge ray collector with a concentration of 1 8 and Inert gas filling, existing of parallel mounted absorber-reflector units, aligned in east west direction The basic concept is the integration of an absorber tube and reflectors inside a low pressure enclosure. Asymmetrical reflectors below the headers with a concentration of 0 6X provide extra radiation and prevent longitudinal radiation losses To suppress heat losses due to gas-convection inside, air or inert gas like krypton at a pressure below 10 mbar is usedA prototype, with an aperture area of 20 m(2), was tested in Munich and showed efficiencies of about 50% for krypton at 001 bar at a temperature of 150 degrees C with a radiation of 1000 W/m(2) (900 W/m(2) direct, ambient temperature 20 degrees C) (C) 2010 Elsevier Ltd All rights reserved

A flat-plate solar collector for process steam production was developed. The operating temperatures are in the range between 100 and 150 degrees C. The boiling collector can be used for process heat supply in the industry and for solar cooling applications as well. It operates as a system with controlled influx of liquid. instabilities of the two-phase Bow in the internal evaporator have been successfully suppressed and the design of the system has been investigated. We constructed a prototype collector based on a commercially available evacuated flat-plate collector. To realise high thermal efficiencies at temperatures up to 150 degrees C, the thermal losses of the absorber have been drastically reduced using an ultra low emissive Selective absorber, a low pressure krypton filling (50 hPa) in the collector casing, and a highly reflecting aluminium foil between absorber and rear side. The prototype collector was dynamically tested at our outdoor test facility and showed very high efficiencies of more than 60% at 100 degrees C steam temperature and of 45% at 150 degrees C steam temperature (T-amb = 15 degrees C). The operation behaviour of the prototype was always stable and the steam mass quality showed excellent values of nearly 100%. (C) 1999 Elsevier Science Ltd. All rights reserved.