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Journal of Thermal Analysis and Calorimetry

, Volume 133, Issue 1, pp 321–327 | Cite as

Effect of the placement of aerogel insulation in the heat transfer properties

  • Ákos Lakatos
Article
  • 288 Downloads

Abstract

Nowadays, the thermal insulation both of existing and new buildings is one of the most important actions for reducing the energy loss of buildings and to reduce the emission of green house gases. In the European Union, buildings account for about 20–40% of the total final energy consumption. Examination of the thermal properties (e.g., effective thermal conductivity) of building materials and structures are very important both for the manufacturers and for the consumers. Several possibilities are available for measuring this parameter of materials. The mainly used thermal insulating materials are the plastic foamy and mineral wool materials; moreover, the nanotechnological insulators (e.g., aerogel, hollow nanospheres) are requiring spaces for themselves also. One promising them for the future is the silica aerogel-based slabs. Aerogels are nanoporous lightweight materials that were discovered more than 70 years ago. Nowadays, their applications are truly widespread. Firstly, in this article thermal transmittance measurements of wall structures will be presented with calibrated chamber method. These measurements were accomplished through an inbuilt plaster/brick/plaster wall construction individually. After that, it was covered with a 0.013-m-thick aerogel layer at first in a cold and then in the warm side. Comparison of the heat fluxes, insulation capabilities and effective thermal conductivities measured by the above-mentioned method will be presented. The change in the retardation time and in the surface temperatures will be also discussed. Secondly, in order to investigate the conductive effect, thermal conductivity measurements with Holometrix lambda 2000 apparatus were accomplished too.

Keywords

Heat transfer in building walls Aerogel Internal insulation External insulation 

Introduction

In the European Commission besides transport sector, buildings account for the second largest extent from the total energy consumption, approximately about 20–40%. The 23% of carbon dioxide emissions generated by building construction must be reduced. Reduction in thermal resistance, especially via improved insulation, is the most basic factor for decreasing energy consumption [1, 2, 3]. One of the most efficient methods for reducing energy consumption in buildings is the reduction in heat loss mainly by using surface coatings. In this article, the comparison of the effect of the placement (on the different sides of the wall) in the thermal properties of aerogel insulation material is investigated. In Ref. [4, 5, 6] the comparison of the internal and external use of the aerogel as a thermal insulating barrier is presented. Aerogel is said to be a state-of-the-art thermal insulation solution and looks to be the most promising with the highest potential. Aerogel blankets/panels have started to be used to insulate building walls, grounds, attics, etc. However, commercially available state-of-the-art aerogels have been reported to have thermal conductivities between 0.014 and 0.025 W m−1 K−1 at ambient pressure. However, its high production cost goes against its applications [7, 8, 9]. Aerogels have relatively high compression strength, but they are very fragile due to their very low tensile strength; moreover, the fibrous ones are very dusty. A very interesting aspect of aerogels is that they can be produced as either opaque (in a blanket), translucent or transparent materials, thus enabling a wide range of possible building applications. For aerogels to become a widespread thermal insulation material for opaque applications, the costs have to be lowered substantially. However, their thermal properties are much lower than those of the transparent ones. These blankets were developed as an insulation material based on silica aerogels. The heat transfer mechanisms for silica aerogel and its associated composites include solid conduction, gas conduction and thermal radiation; all of which have been extensively investigated in recent years by the authors. Aerogel is an open-cell nanoporous super insulation material made by a sol–gel process and supercritical drying technology and has excellent insulation performance for its nanoporous structure and very small solid grain size (2–5 nm). It has a skeleton density of 2.200 kg m−3, but the high porosity results in a bulk density as low as 3 kg m−3, e.g., compare with the density of air of 1.2 kg m−3. Its perfect adiabatic capability means that aerogel has very high application potential in the heat thermal protection fields of space shuttles, nuclear reactors and even ordinary steam pipes as high-insulation materials [10, 11]. In our previous papers, besides the measurements of the perfect insulation capability of the aerogel samples, its hygric properties were introduced also [12, 13, 14, 15]. The laboratory measurements of the individual building materials, as well as the building structures, are so important, from the point of view of giving the correct declared values of the thermal properties, moreover from indoor air quality point of view [16, 17, 18, 19, 20, 21, 22]. The novelty and the objective of this paper is to present an experimental investigation of the heat transfer mechanisms in silica aerogel blanket, carried out with in situ heat flux measurements (calibrated chamber/hot box method) as inbuilt structure both at the warm (as internal insulation) and the cold side (as external insulation) of the wall. Aerogel insulation can be either installed on the external or on the internal side of the wall. Also, in special cases, such as historic, traditional or cultural heritage buildings, external insulation is often not allowed to be installed, due to the resulted changes in the facade of the building. Thermal conductivity measurements were carried out too, with Holometrix lambda 2000 heat flow meter, at first on a pure solid brick sample with 30 cm × 30 cm × 7 cm geometries. Furthermore, this sample was combined once at the cold side, and then on the warm side with a 1.3-cm-thick aerogel sample with the same 30 cm × 30 cm base area.

The contrast between the external and internal use is that the configuration of the internal use does not interfere with the facade and exhibits significantly lower installation costs. Furthermore, internal insulations provide a solution for adjacent buildings, where no space for external thermal insulation is available, or for certain parts of a high-rise building [23, 24].

Based on the reached measurement results, calculations were carried out to define effect of the placement in the thermal properties of the aerogel. In this article, based on previous theoretical models regarding thermal performance parameters of wall structures, e.g., decrement factor, initial stage retardation and thermal resistance presented in Ref. [12, 13, 14, 19] will be given too.

Materials and methods

Theoretical background

In our previous papers (see Ref. [12, 13, 14, 19]), both the measurement and the theoretical methods used for the calculations are presented, now downer only a short revision from them will be presented.

Thermal resistance of a material

The thermal resistance of a material is the ability of a material to resist heat flow. Designated as R (R value, m2 K W−1), thermal resistance (R) indicates how effective any material is as an insulator. R is measured in seconds needed for 1 J to flow through 1 m2 of a given thickness of a material when the temperature difference is 1 °K.
$$R = \frac{d}{\lambda }$$
(1)

Decrement factor

The decrement factor (ν) of a building structure is the ratio of the amplitudes of the periodical internal surface temperature (A is) (defended side) and the amplitude of the periodical external surface temperature (A es) (attacked side).
$$\nu = \frac{{A_{\text{is}} }}{{A_{\text{es}} }}$$
(2)

Steady-state thermal transmittance measurements

This type of measurement is clearly presented in our previous works; however, this paper will introduce the method in brief [12, 13, 14, 19]. In order to measure the thermal resistance of inbuilt layer structures, an isolated chamber is available. The chamber is surrounded as well as divided into two rooms (cold and warm) with 2.2 m × 3.4 m areas each by a 0.5-m-thick EPS 200 insulation system. In the middle of the EPS, dividing-wall at 0.35 m parapet height (over the ground), a brick wall window with 0.25 m thickness and 1.44 m2 surface area can be found. This brick wall is mortared with 0.015-m plaster (this is a conventional-type mortar) both at the warm and the cold side. Furthermore, the brick wall was covered using chemical fixation with 0.013-m-thick aerogel insulation blanket both at the cold and the warm side. Here, has to be mentioned that mechanical fixings were not used.

Measurements with calibrated chamber method

The determination of steady-state thermal resistance and thermal insulation of wall structures by calibrated chamber (CC) should be executed according to the MSZ EN ISO: 8990 standard. The measurement setup can be seen in Ref. [12, 13, 14, 19]. The calibrated hot box is surrounded by air with fixed temperature parallel to its own, so that zero heat transfer can be expected through the wall of the box. The temperatures both of the air and on the wall surfaces at both sides are measured by Pt-100-type thermocouples. The surface temperature of the walls was measured at 16 points arranged in equal distances from each other (like a 4 × 4 matrix), and the air temperatures were measured with 4–4 pieces of Pt-100-type thermocouples at both sides; moreover, the results were stored at data storage. The average value of surface temperatures was calculated both at the warm and the cold sides from the measured data. Inside the box, a small fan was used to circulate air and it was heated by two bulbs with 40-W electric power each. The electric power of both the fan and the bulbs was measured outside the box with two calibrated electronic meters separately. At the cold side, one fan as well as two air baffles was used in order to reach a good air temperature homogenization. From the measured surface and air temperatures of the wall, their difference can be calculated (ΔT wall CC , ΔT air CC in K) after reaching the equilibrium (in steady-state stage). From the measured electric power and the operating time (t cc in h), an average power (P CC in W) can be calculated. At first, to achieve the thermal resistance of the layer structure, without the surface heat transfer coefficients, with A CC = 1.44 m2 surface area, the following equation can be used:
$$R_{\text{wall}}^{\text{cc}} = \frac{{\Delta T_{\text{wall}}^{\text{cc}} \times A_{\text{wall}}^{\text{cc}} }}{{P_{\text{wall}}^{\text{cc}} }} = \frac{{\Delta T_{\text{wall}}^{\text{cc}} }}{{\varPhi^{\text{cc}} }}$$
(3)
where Φ CC is the heat flux.
Moreover, to achieve the thermal resistance of the layer structure, with the surface heat transfer coefficients, with A CC = 1.44 m2 surface area, the following equation can be used:
$$R_{\text{air}}^{\text{cc}} = \frac{{\Delta T_{\text{air}}^{\text{cc}} \times A_{\text{air}}^{\text{cc}} }}{{P_{\text{air}}^{\text{cc}} }} = \frac{{\Delta T_{\text{air}}^{\text{cc}} }}{{\varPhi^{\text{cc}} }}$$
(4)
where R AIR CC is the total R value.
The difference between the Eqs. 3 and 4 will result in R s CC which is the surface heat transfer resistance:
$$R_{\text{s}}^{\text{cc}} = R_{\text{air}}^{\text{CC}} - R_{\text{wall}}^{\text{CC}}$$
(5)
$$R_{\text{s}}^{\text{cc}} = \frac{1}{{\alpha_{\text{is}}^{\text{CC}} }} + \frac{1}{{\alpha_{\text{es}}^{\text{CC}} }}$$
(6)
where α is CC , α es CC are the internal and external surface heat transfer coefficients carried out by the CC method.

Measurements with Holometrix apparatus

For the measurements of the effective thermal conductivity of the materials, a Holometrix lambda 2000 heat flow meter was used. These thermal conductivity measurements were carried out after drying the samples in a VentiCell drying instrument to changeless weight. With this device, materials can be dried by setting different air temperatures (up to 523 K). It works with hot air circulation using an inbuilt ventilator. The mechanism of the Holometrix equipment is clearly described in our previous articles. Five thermal conductivity measurements were carried out on each sample. Besides the average values, the estimated errors are also given [12, 14].

Results and discussions

Retardation time measurements before the equilibrium state

As previously presented in Ref. [12], before reaching the thermal equilibrium (steady-state) stage by using the climatic chamber method focusing on the beginning of the cooling down will give an opportunity to find the retardation time of the wall structures. By comparing the retardation time of the brick wall covered with aerogel first at the cold and then at the warm side, one can find the first thermal effect of placement (position). In Fig. 1a, b, we can see the estimation of the change in the retardation time, during cooling down the walls covered with aerogel both at the warm and at the cold side one after the other. In Fig. 1a, b, the internal surface temperature was indicated with T is, moreover its maximum value was signed with T is, max. From the CC method, about 21.7 and 23.25 h can be measured for the retardation times of the walls covered at the internal (warm) side and at the external (cold) side of the wall, respectively.
Fig. 1

Initial stage retardation of the aerogel as a internal insulation, b external insulation

Thermal resistance measurement with calibrated chamber method

From Tables 1 and 2, one can see the measurement results and the calculated values using Eqs. 16 reached by the measurement with the calibrated chamber method. As previously mentioned, thermal resistivities and surface heat transfer coefficients were found by using Eqs. 36. These values were reached both for the plaster/brick/plaster (PBP) (uncovered) and for this wall covered with aerogel both at the warm and at the cold side. From the measurement results, 0.025 and 0.023 W m−1 K−1 were found for the effective thermal conductivity of the aerogel placed on the warm and on the cold side; for the surface heat transfer coefficients, 5.36 and 5.487 W m−2 K−1 were calculated from the measured values. Here, it should be mentioned that the estimated errors are also presented in Tables 1 and 2. In these tables, the errors represent the uncertainties of the measurements: the average values of the deviances. The above-mentioned values should be explained by the results indicated in Table 1. From this table, one can see the measured temperature differences on the surfaces of the wall with about 33 K and about 32 K values for the internal and external insulations, respectively. It should be noticed that where the greater temperature difference is, the greater insulation capability should be there too. But one can see well that the measured value of the heat flux is much lower in the case of the external insulation 44 < 48 W m−2.
Table 1

Measured values from the calibrated chamber method

 

ΔT air/K

Er./K

ΔT wall/K

Er./K

Heat flux/W m−2

Brick wall

38.83

1.4

28.15

0.676

87

Inside

39.166

1.371

32.93

0.765

48.125

Outside

37.950

1.359

32.280

1.233

44.800

Table 2

Calculated values from the measured ones

 

R air/m2 K W−1

Er./m2 K W−1

R wall/m2 K W−1

Er./m2 K W−1

eff. Alfa/W m−2 K−1

Er./W m−2 K−1

Effective thermal conductivity of aerogel/W m−1 K−1

Brick wall

0.643

0.023

0.466

0.011

5.657

0.201

 

Inside

1.172

0.041

0.985

0.023

5.359

0.188

0.025

Outside

1.220

0.044

1.038

0.040

5.487

0.196

0.023

Measurement of the decrement of the walls

From the measurements with the CC method, if one changes the temperature of the cold air as a periodical function (T e) (Eq. 7) and keeps the temperature of the warm air fixed (T i), it will result in periodical surface temperatures both at the cold and the warm sides of the wall. Their amplitude will be A es and A is, respectively.
$$T_{\text{e}} = T_{\text{o}} + A_{\text{e}} \times \sin \left[ {\frac{{\pi \times (t - t_{\text{c}} )}}{w}} \right]$$
(7)

It is feasible to estimate the decrement factor by fitting the wall surface temperatures with sinusoidal functions. The measurement methods are clearly written in Ref. [12, 13, 19].

From Fig. 2a, b, one can see the next effect of the position of the aerogel in the surface temperature profiles. In the above-mentioned references, we previously presented that if one fits these surface temperature profiles with sinusoidal functions (Eq. 7) and takes the ratio of the amplitudes as presented in Eq. 2 the decrement factor for this case (for the current measurement order) can be found. Here, has to be mentioned that this method has been improved; furthermore, a modified, better form will be presented in this article. In Fig. 3a, b, one can see the box chart analysis of the values presented in Fig. 2a, b, with the mean values and with the upper and lower quartiles. This statistical method gives a good opportunity, with greater precision to estimate the amplitude (variance) of the temperature values. By taking the ratio of the mentioned amplitudes from the box chart method, for the decrement factors 0.24 and 0.124 can be found. The smaller value represents better decrement, better insulation capability, caused by the reduction in the greater amplitude of the wave of the external temperature.
Fig. 2

Temperature waves of the wall a insulated inside, b insulated outside

Fig. 3

Reached decrements from hot box method with a internal insulation, b external insulation

Measurement results with Holometrix apparatus

The measurement results with the heat flow meter are presented in Table 3. At first, results are carried out on a pure brick, and then the brick in combination with the aerogel sample is presented. The aerogel sample was placed at first over the brick and then under the brick representing the insulation on the cold and the warm side, respectively. One can observe that both combinations are reducing the thermal conductivity, since increasing the thermal resistance. It is also observable that while the resistance in the first case is jumping up from 0.25 to about 0.5 m2 K W−1, in the second case it goes up to about 0.61 m2 K W−1. It should be mentioned that this difference might happen by the pressing of the aerogel sample by the brick. Here, has to be mentioned that in Table 3 the errors of the measurements by averaging the deviances from the mean values can be found too. Let me notice here that this experiment was only a rough estimation in order to investigate the effect in the placement of the aerogel. Due to the measurement limit in the measuring thickness of the equipment (max 10 cm), only a 7-cm-thick solid brick was used. In the upper section, a conventional brick size was used and measured. By using greater brick thicknesses, other R values can be measured.
Table 3

Measurement results with Holometrix application

 

Thickness/cm

Density/kg m−3

Thermal conductivity/W m−1 K−1

1

2

3

4

5

Average

Resistance/m2 K W−1

Brick

7

1962.86

0.2877

0.2784

0.2800

0.2730

0.2795

0.2797

0.2503

Error

  

0.0080

0.0013

0.0003

0.0067

0.0003

0.0017

 

Aerogel with brick (outside)

8.3

1670.09

0.1670

0.1695

0.1700

0.1660

0.1658

0.1677

0.4950

Error

  

0.0007

0.0018

0.0023

0.0017

0.0019

0.0008

 

Brick with aerogel (inside)

8.3

1670.09

0.1347

0.1350

0.1349

0.1351

0.1347

0.1349

0.6153

Error

  

0.0002

0.0001

0.0000

0.0002

0.0002

0.0001

 

Conclusions

In this article, the investigations of key thermal transport properties of aerogel insulation material are presented. The role of insulation materials in the building energy and moisture balance is more significant compared to the other materials of the building structures. The estimations based on the laboratory measurements of these values of the insulating materials are very important either for the manufacturers or the contractors, planners and designers. In this research report, besides the presentation of a comprehensive comparison of the effect of the placement in the thermal properties of aerogel insulation, an improvement in the theory of the estimation of the decrement is presented too. The following differences for the external and internal applications were found:
  • The measured effective thermal conductivity of the aerogel as external insulation is lower than the values measured as an internal insulation.

  • Lower heat flux through the wall and higher resistance for the external application were showed.

  • Significant differences in the surface heat transfer coefficients were not observed.

  • Longer retardation time for the cooling down was detected for the external application.

  • Better decrement capability for the application of the external use was also manifested.

  • Based on the measurements with the Holometrix type, heat flow meter difference between the resistances was shown, with little benefit for the internal insulation.

  • From the cost point of view, the internal insulation can be cheaper by neglecting the price of the scaffolding.

As a result, one can conclude that the better choice is to use aerogel as insulation at the external side; however, if there is a case where is no place for the external use, aerogel as an internal coating can be applied also but only with limitations.

Notes

Acknowledgements

This paper was supported by the János Bolyai Research Scholarship of the Hungarian Academy of Sciences.

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Copyright information

© Akadémiai Kiadó, Budapest, Hungary 2017

Authors and Affiliations

  1. 1.Department of Building Services and Building Engineering, Faculty of EngineeringUniversity of DebrecenDebrecenHungary

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