High Temperatures ^ High Pressures, 2000, volume 32, pages 143 ^ 158
15 ECTP Proceedings pages 167 ^ 182
DOI:10.1068/htwi5
Transparent thermal insulation materials and systems:
state of the art and potential for the future
Volker Wittwer, Werner Platzer
Fraunhofer Institute for Solar Energy Systems, Oltmannsstrasse 5, D-79100 Freiburg, Germany;
fax: +49 761 458 8132; email: [email protected]
Plenary lecture, 15th European Conference on Thermophysical Properties, Wu«rzburg, Germany,
5 ^ 9 September 1999
Abstract. Transparent or translucent insulation materials (TIM) represent a class of materials
with a high potential for increasing the efficiency of solar thermal systems. A large number of
materials and material combinations have been subjected to theoretical and experimental investigation. An overview of generic types, characterisation methods, simulation models, commercial
materials, and actual systems is given, and the future potential of transparent insulation is shown
as well as current market penetration.
Results of selected demonstration projects with active and passive systems are presented.
The most prominent application of transparent insulation was in the self-sufficient solar house
in Freiburg. A detailed energy balance of this house is given. In addition the use of TIM in
retrofitting as well as in low energy apartment blocks is discussed.
1 Introduction
From the physical point of view the effective use of solar energy in low-temperature
thermal systems is possible because of the shift in the spectral distribution of blackbody
radiation as a function of temperature. The solar radiation comes from the Sun's surface,
which has an approximate temperature of 5760 K, whereas our houses or collectors
emit with a surface temperature typically between 270 and 400 K. This causes the strong
shift in the wavelength of maximum emission from 0.5 mm to about 10 mm. This change
in wavelength allows the application of spectrally selective surfaces or materials, with
which the radiation exchange between the absorber and its surroundings can be dramatically reduced.
The simplest solar collector is a black absorber which gains energy by absorption of
solar radiation and which loses thermal energy by thermal radiation exchange with the
surroundings and by thermal conduction and convection. Under normal conditions, the
heat losses are dominated by the radiation losses.
For low working temperatures as needed in absorbers for swimming pools (30 8C),
the efficiency is around 50% under sunny conditions in summer. The first improvement
is to add a thin pane of glass, which has characteristic spectrally selective characteristics.
Glass is transparent for the incoming solar irradiation but highly absorbing for the
thermal radiation of the collector. Thus one glass pane reduces the radiation losses by a
factor of two. The efficiency, Z, of a collector is given by the following formula:
Zˆ
taG ÿ UDT
.
G
(1)
The first term, taG, is the product of the transmittance, t, of the cover glass, the
absorptance, a, of the absorber, and the solar irradiation, G, defining the maximum
possible solar gain. The second term, UDT, describes the losses as the product of the
heat loss coefficient, U, and the temperature difference between the absorber and the surroundings. In most applications, not the power but the energy is the most relevant quantity.
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Therefore, equation (1) has to be integrated over time. The energy efficiency, Zen , of a
system is:
„
„
taGdt ÿ UDTdt
„
Zen ˆ
.
(2)
Gdt
The first term now gives the energy gains over a certain period and the second
term the losses of the system. For a system in equilibrium, the gains and the losses should
be the same and the efficiency is zero. For the equilibrium state, equation (2) can be
rewritten in the following way:
„
DTdt
ta
.
(3)
ˆ „
U
Gdt
The ratio ta=U determines the maximum temperature difference, which can be maintained in a system without additional heat sources. This number is a characteristic of a
collector system for a certain solar input. From the physical point of view, it is now
very interesting to consider how this number can be optimised for a certain amount of
transparent material, like glass for example. The starting point is a collector with a black
absorber and a 5 mm thick glass pane mounted 10 cm away from the absorber as a
cover, as shown in the upper part of figure 1. Losses from the back of the collector are
ignored for simplicity. If conventional window glass and a painted absorber with
a ˆ 0:95 are used, the ta product is about 0.85 and the U value 6 W mÿ2 Kÿ1. The first
step to improve the U value is to split the single pane into two thin sheets. The infrared
(IR) radiation transport is further reduced as well as the influence of convection in the
air gap. On the other hand the losses due to reflection from the surfaces become greater,
but the ratio of ta to U is still increased. In principle this procedure can be continued
up to n very thin glass films. However, as the reflection losses become higher, they will
eventually outweigh any gains. With 3 or 4 layers, the convection could be suppressed
and only the conductivity of the air contributes significantly to thermal losses. A fundamental change occurs if the thin films are rotated by 908. Then the reflection losses
disappear but the thermal radiation transport and the convection are still suppressed.
These structures can be produced by capillary panels or so-called honeycombs. One
disadvantage is the solid-state conductivity in the glass, but with an appropriate wall
thickness, this is not a problem in practice.
A further dramatic improvement can be achieved with a homogeneous distribution of
the glass in air, as is possible in aerogel. Then the reflectance is low, at least theoretically.
In reality there is some scattering in most such materials. Both the thermal radiation
transport and the heat conduction by air are suppressed and the solid state conductivity
is very low. As shown in table 1, the U value of the system can be decreased by a factor
of 30 in comparison to the single-glazed collector without decreasing the transmittance
very much for the best systems. The equilibrium temperature difference is calculated for
the irradiation input of a cloudy winter day. These short, simplified calculations show
Table 1. Values of ta, U, and equilibrium values of DT for different collector covers made from
the same amount of glass for a daily irradiation input of 500 W h dÿ1 (cloudy winter day).
Structure
ta
U value=W mÿ2 Kÿ1
DT
1 pane
2 panes
3 panes
honeycomb
aerogel
0.85
6
3
0.77
3
5.3
0.71
2
7.4
0.95
0.90
22
0.77
0.20
80
Transparent thermal insulation materials
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15 ECTP Proceedings page 169
transparent insulation
absorber
5 mm
convection
radiative transfer
one pane
2.5 mm
2.5 mm
convection
radiative transfer
two panes
1.7 mm
three panes
n panes
capillaries
or
honeycombs
aerogel
1.7 mm
1.7 mm
thermal conductivity of air
radiative transfer
thermal conductivity of air
radiative transfer
thermal conductivity of air
thermal conductivity of solid
radiative transfer
decreased:
thermal conductivity of air
thermal conductivity of solid
radiative transfer
Figure 1. The principle of optimisation of a transparent cover. The main heat losses are given in
the right column.
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15 ECTP Proceedings page 170
the fundamental importance of material optimisation and the potential for utilising solar
energy in thermal systems with transparent insulation.
Experiments with a transparently insulated cube filled with 150 l water demonstrated
that the water temperature was far above the freezing point even in the strongest winter,
whereas the air temperature was below ÿ10 8C.
Geometrical variations of the glass or plastics result in a wide palette of materials,
which are classified as transparent insulation materials (TIM). Some of the materials
are also called translucent materials as they scatter light strongly, so are not optically
transparent.
2 Generic types of transparent insulation materials
Starting from figure 1, four generic types are proposed, which display different physical
properties and include most real TIM (Wittwer et al 1986). The first type is the wellknown absorber-parallel cover, with multiple glazing or plastic films, which may be
either transparent or translucent. High optical reflection losses prohibit the use of a large
number of layers. The glass panes or plastic films have defined temperatures (approximately constant), but because of convection in the intermediate gaps, they cannot be
described adequately by a one-dimensional temperature distribution.
The absorber-perpendicular structures include honeycomb or capillary materials
with different cross-sectional geometries, and lamellar structures (plastic films stretched
parallel across the collector) (Tabor 1969; Hollands and Wright 1983). As the incident
beam is reflected and transmitted by the structure walls towards the absorber, optical
losses are very small. Only some scattering and absorption within the films reduces the
overall transmittance. For clear films with low extinction, the transmission properties
are nearly independent of the material thickness, so very thick samples may be used. In
contrast to the absorber-parallel structures, convection is effectively suppressed, if the
aspect ratio is well chosen.
If the previous two types are combined, a cavity structure is obtained, which is
represented in reality by transparent multichannel duct plates or transparent foam with
bubble sizes of the order of millimetres. From the optical viewpoint, these materials have
approximately the same transmittance as an equivalent multiple film cover. Reflection is the
dominant loss mode. However, they have the advantage of effectively suppressing the
convection (Wittwer et al 1986).
Quasi-homogeneous materials are characterised by similar optical properties, but the
loss mechanisms are now scattering and absorption. Aerogel, a microporous `silicate foam',
belongs to this class. Because of pores with sizes of 10 ^ 50 nm, light is scattered within
the material, comparable to the Rayleigh scattering of blue sky. Glass fibre materials do
not have this macroscopic homogeneity, but they can be treated and analysed with similar
methods (Rubin and Lampert 1983; Fricke 1986).
For each of the four generic types, theoretical approaches exist which satisfactorily
describe the basic features of the materials. Of course, transitional materials exist which
cannot be strictly classified. Folded or V-corrugated films are one example. If the corrugation angle is small, the cover behaves essentially like an absorber-perpendicular
material; if the angle is large, it behaves almost like an absorber-parallel cover. Honeycomb structures with cells at a slant with respect to the absorber are intermediate
between the absorber-parallel and the cavity structure type. Nevertheless, the generic
types provide a useful approach to reality. Understanding these types is necessary before
the more complex materials can be analysed. In principle, all these materials can be
combined with selective coatings, gas fillings, and low pressure. Therefore a large variety
of system modifications is possible in practice (Pflu«ger et al 1988).
From the theoretical point of view, different models are necessary to describe different
TIM.With a theoretical description, the materials can be mathematically handled and their
Transparent thermal insulation materials
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15 ECTP Proceedings page 171
physical properties optimised. The choice of basic material, the geometrical structure of the
TIM, and fundamental physical boundary conditions influence heat transport processes
and solar transmittance. Therefore, these parameters have to be included in the models
(Hollands et al 1984; Pflu«ger 1988; Platzer 1988).
Models are bound to idealise and to describe physical reality only to a certain level
of sophistication. Theoretical calculations have the advantage of yielding continuous
functions of interesting parameters, whereas experiments only give single data points.
3 Experimental characterisation of TIM
Three fundamental experimental setups which give global information about the basic
physical quantities, heat transmittance coefficient U, solar transmittance t, and total
solar energy transmittance, TSET or g, are described in the following section. Other
experiments yielding spectral information, scattering phase functions, etc, are sometimes
necessary to understand fully materials like aerogel, for example.
3.1 Measurement of solar transmittance and reflectance
For solar energy applications, it is not necessary to see clearly through a collector cover.
The total transmitted solar radiation reaching the absorber is the relevant value, whether
it is scattered, reflected, or direct. Therefore, the transmittance, tb , of incident beam
radiation in dependence on the incident angles (polar, azimuth) is important. Very often
an azimuthal dependence on incident angle does not exist (eg for conventional glazing)
or is negligible (most honeycombs). The numerical integration of tb for isotropic irradiation results in the diffuse solar transmittance, tdif . A device for measuring the directhemispherical transmittance is an integrating sphere, where the light incident under
different angles contributes to the homogeneous radiance of the sphere wall (apart from
certain spots) because of multiple reflections inside the sphere. Small spheres for spectrometers commonly use double-beam geometry, whereas the large spheres for building
components are usually single-beam devices.
In principle, reflectance measurements can also be made with an integrating sphere,
if an additional sample port is at the back of the sphere. The light enters the front
entrance port and is reflected from a reference surface or from the sample into the
sphere. For each incident angle, however, a separate opening for the incoming light is
needed. As an alternative, a centre-mounted sample can be rotated.
A further problem is the scattered light in thick samples. Radiation which is scattered
or reflected back towards the sphere may not return through the sample port. As a
consequence, the measured transmittance or reflectance for thick scattering samples is
often smaller than the actual value (Symons 1982; Platzer 1987). Large sample ports
with a small beam diameter are usually necessary so that the fraction of radiation loss
is small. For transmittance measurements, large area irradiation of the sample with a
beam cross-section significantly larger than the aperture represents a better option.
3.2 Measurement of heat transport
For the measurement of the total heat transport within TIM, any hot-plate apparatus
for the determination of the U value can be used, as described in the standards. As the
materials are partially transparent to IR radiation, the surfaces should have a known
emissivity and be interchangeable.
To allow the convection properties of the samples to be determined for different
inclinations, the whole apparatus should be rotatable by 1808. The position 1808 (hot
plate above the horizontal material) can be used to determine the heat transport without
convection. The difference to this value for any other position gives the convective contribution to the heat transport and may be expressed in terms of the Nusselt number Nu.
For fast and exact measurements, a hot-plate apparatus with heat flux meters on both
plates is advantageous.
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15 ECTP Proceedings page 172
If the heat transport of a collector system is measured, convection may be present
not only within the materials but also in the air gaps. The main reason for introducing
air gaps into the system is to allow the use of low emissive surfaces, which reduce the
radiation ^ conduction coupling between the surface and the TIM. Effects on the heat
transport due to added air gaps can be determined for various inclinations. The whole
heat transport measurement chamber can be evacuated down to a pressure below 1 Pa,
for special systems even below 10ÿ4 Pa. Therefore the influence of different gas fillings
and pressures on convection can be tested in a very simple way.
Whereas measurements of the U value of the system are quite standard, measurement of the thermal conductivity of a certain TIM material is quite difficult, as most
of these materials are more or less transparent in the thermal radiation range. Therefore
the conductivity depends on the selectivity of the surface plates, on the orientation, if
there is some convection in the system, and on the thickness of the TIM.
In a simple model, which gives very good results for many of the TIM, a so-called
effective thermal conductivity is introduced, which depends on the thickness (Pflu«ger
1984). The heat transport is divided into radiative and conductive components. The IR
radiation component is characterised by an extinction coefficient, x, which is dependent
on the IR absorptivity and can be calculated from measurements. If this number is
known, the effective thermal conductivity, leff , can be calculated for different thicknesses
of the TIM:
leff ˆ
4sT 3
…s0 x†
ÿ1
‡ x ÿ1
‡ lair ,
(4)
with 1=s0 ˆ 1=ea ‡ 1=eb ÿ 1; ea and eb are the emissivities of the end plates.
Figure 2 shows the dependence of the effective thermal conductivity on thickness
and temperature for an acrylic foam.
0.20
0.12
0.08
100
lps = W mÿ1 Kÿ1
0.16
0.04
80
60
40
Temperature=8C
20
0
20
40
60
80
0.00
100
Thickness=mm
Figure 2. Dependence of the thermal conductivity of a capillary structure on thickness and
temperature.
3.3 Total solar energy transmittance
Because of some solar absorption, solar radiation will result in a temperature rise within
most TIM. This leads to superposition of a heat flux, qin , towards the absorber, which is
proportional to the irradiation, G, at least to a first approximation. Therefore the total
solar energy transmittance g [or (ta)e ] of a collector with plate absorptance, ap , a solar
Transparent thermal insulation materials
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15 ECTP Proceedings page 173
transmittance, t, and a backside reflection, rback , for various incident angles, f, defined by
g…f† ˆ …ta†e ‡
qin
,
G
(5)
tap
6ˆ tap ,
…ta†e ˆ
1 ÿ …1 ÿ ap †rback
is the relevant parameter for thermal systems. Of course, qin depends on the heat resistances from the TIM to the absorber and the environment, but also on the absorber
reflectivity and temperature levels. However, calculations show that the latter parameters
are of minor influence and the first ones may be corrected, if a measurement of g for
defined conditions is made.
Only a few direct measurement methods, which have to be calorimetric, are known.
Most instruments have an absorber consisting of heat flow meters mounted on a cooled
aluminium plate. The advantages of this device are a relatively fast response, good accuracy, and the ability to measure at different angles.
In principle, the measurement is analogous to the efficiency determination of a solar
collector. The ambient temperature, T0 , the absorber temperature, Ta , and the irradiation, G, are needed as well as the net gain, qnet , of the absorber:
qnet ˆ gG ÿ U…Ta ÿ T0 † .
(6)
This formula can be used to determine g, if the U value of the TIM cover is known.
This can be measured with the same apparatus without irradiation. The total energy
transmittance for the diffuse isotropic irradiation, gdif , can also be determined by numerical integration over the angular dependence, g(f).
Figure 3 shows typical characteristic data for commercially available TIM (g extrapolated to ap ˆ 1).
0.7
IV-2
IV-3
WSV-2 (Ar)
WSV-3 (Kr)
WSV-3 (Kr)
NEH PC-Kapillaren
PC-Waben
Helioran
StoTherm
Kapilux-H
gdif
0.6
0.5
0.4
0.3
0.0
0.5
1.0
1.5
2.0
U=W mÿ2 Kÿ1
2.5
3.0
Figure 3. Characteristic data (U and g) of TIM available on the market.
4 Advances and problems of transparent insulation on the market:
professionalisation, diversification, and demonstration
4.1 Introduction
For more than 15 years now, transparent insulation has been a subject of research and
development. In spite of promising experimental results and demonstration projects,
only a small market is currently visible. However, fully developed products from several
companies have been on the market for only two years. A high thermal resistance combined
with high solar gain factors characterise these different systems.
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Reflecting on the wide variety of past and expected future developments, the state
of the art including current problems will now be presented here.
4.2 State of the art and recent developments
A wide diversification has taken place amongst TI components. Although seemingly
contradictory, even non-transparent products are on the market, eg the SOLFAS module
of Schweizer with an integrated absorber plate. Most elements today are open and air-filled.
4.2.1 Plastic film TI. Absorber-perpendicular TI structures are still the most efficient
material group judged on the basis of thermal resistance and solar energy transmittance.
The usual extrusion or spinning procedures for fabrication are suboptimal with respect
to small production quantities and flexibility in material choice. Structures based on
commercial plastic films were investigated in an Austrian research project by Joanneum
Research, Leoben. Slat structures were discarded because of electrostatic and tension
problems. However, the prototype production of honeycomb-slat structures yielded
promising results. Hemispherical total solar energy transmittances between 55% and
60% with thermal resistance well above 1 m2 K Wÿ1 were achieved for PC, PMMA,
and some other polymer materials between two low-iron glass panes. Because of the
optimised structure, the mass density was below 3%.
4.2.2 HLB modular fac°ade elements. Within the development of modular TI for demonstration projects, construction improvements led to a decrease in cost. The small molecules of Holz- und Leichtmetallbau Leipzig may be mounted even on walls with lower
structural stability by a simple technique. Including the external shading device, the costs
were reduced from 1370 DM mÿ2 to 985 DM mÿ2 in a real project.
4.2.3 Semitransparent modules. Although traditional TI products emphasise the high
solar gains possible with this technology, a series of recent developments aim at very
low solar gains intended simply to offset the remaining thermal losses of opaque insulation. The motivation on the one hand is to present a cheaper system öprices range from
400 to 600 DM mÿ2 öand on the other hand, to have no need for a costly solar shading
element.
4.2.3.1 ESA. The cardboard TI by ESA was presented some years ago to the market.
The cardboard honeycomb fac°ade has a g value of 13% for diffuse radiation, due
almost entirely to radiation absorbed within the cardboard structure. Thus the system
approximately reaches a net energy balance of zeroöthe solar gains over the heating
season offset the thermal losses. This is true if the solid wall behind the cardboard
fac°ade is able to take up the solar gains. However, some projects have argued with
solar gains while having an insulating light-weight construction behind the system.
In this case, obviously the solar gains become negligible and the system is effectively a
conventional insulation system.
4.2.3.2 G ‡ H modules. Based on their mineral fibre insulation products, the companies
G ‡ H Montage, VEGLA, and G ‡ H Isover developed the so-called Optratec system
where a coloured mineral fibre mat is placed behind a vented safety glass pane. Experimental data show that a large share of the thermal losses is due to heat bridges in the
construction (around 30%). Solar radiation is absorbed in the outer portion of the fibre
insulation and reduces the thermal losses. However, a positive heat gain over the heating
period could not be observed.
4.2.3.3 Insulating glazing TI (Karlsruhe). In order to decrease both the construction
thickness and the cost, the University of Karlsruhe, Germany, is developing a TI
system based on insulating glass technology. Low e coated glazings filled with xenon
are coated with a black absorber (a black film). Thermal contact to the solid wall (in
order to prevent thermal breakage) is provided by a layer of metallic fins. The system
has to be evaluated in the near future (Maiwald 1998).
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4.3 Selected projects
4.3.1 The self-sufficient solar house, Freiburg
4.3.1.1 Introduction. The Fraunhofer Institute for Solar Energy Systems has built a selfsufficient solar house (SSSH) in Freiburg, Germany (figure 4). Its entire energy demand
for heating, domestic hot water, electricity, and cooking is supplied solely by solar
energy. The combination of state-of-the-art energy saving technologies with highly efficient solar systems minimises the mismatch between the solar radiation input and the
building energy demand in winter. The remaining seasonal energy storage is accomplished by electrolysis of water during summer with electricity from a photovoltaic
generator. Pressurised storage of hydrogen and oxygen allows reconversion to electricity
by a fuel cell as well as catalytic combustion of hydrogen for cooking and a back-up
for space heating. A schematic diagram of the energy supply system and the main
building parameters are provided in figure 5. A detailed description is given by Stahl
et al (1997). The SSSH was occupied by a family for three years.
Figure 4. The self-sufficient solar house,
Freiburg (view from south), architect
D Ho«lken.
4.3.1.2 Energy consumption. Common concepts for low energy houses mainly deal with
the reduction of the space heating demand, which dominates with typically 80%, the
total energy demand of residential buildings under Central European climatic conditions (3500 ^ 4000 degree days, yearly total of global radiation approximately
900 ^ 1100 kWh mÿ2 ). In the case of a fully independent solar energy supply, a more
general approach is needed, covering all sectors of demand. Therefore, a major effort
was made to limit the demand by using energy efficient household appliances, energyoptimised ventilation heat recovery, and transparent wall insulation for space heating.
The results of three years of operation are summarised in table 2. The comparison with
a present German standard house underlines the enormous drop in energy consumption.
Table 2. Annual energy consumption=kWh mÿ2 for three periods compared to the current German
standard. Energy figures are given related to the living area of 145 m2 and a 3-person household
Additional electricity was consumed for measurement and control purposes. Energy consumed
was delivered by the PV/hydrogen system. Domestic hot water is heated entirely with solar
collectors and the waste heat of the fuel cell.
German
standard
Household
Ventilation
Hot water
Heating
Total
20
20
70 ± 100
110 ± 140
Self-sufficient solar house
1993
1994
1994/1995
10.7
1.3
7.8
0.8
7.9
0.6
2.5
12.0
0.5
9.1
0.2
8.7
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daylighting
1
space heating
2
18
19
17
14
16
15
cooking
5
3
electricity
7
6
9
8
domestic
hot water
11
4
11
13
light
air
current
12
O2
10
hydrogen/oxygen
water
H2
heat transfer fluid
Figure 5. Schematic diagram of the energy supply, system, components, and parameters: 1, windows; 2, TI wall; 3, PV generator (4.2 kW); 4, thermal collector (14 m2 ); 5, control and data acquisition; 6, battery (20 kWh); 7, inverter (3 kW); 8, electrolyser (2 kW); 9, fuel cell (1 kW); 10, H2 and
O2 storage tanks (15=7:5 m3 ); 11, heat exchanger; 12, water storage tank (1 m3 ); 13, mains water;
14, ventilation heat recovery; 15, air heater; 16, earth-to-air heat exchanger; 17, ambient air;
18, exhaust air; 19, return air.
4.3.1.3 Thermal energy supply. According to table 2, space heating was almost zero
and only necessary in extreme winter periods. Consequently, seasonal storage of lowtemperature heat could be avoided. If necessary, back-up heat was supplied from
catalytic combustion of stored hydrogen via the central air supply system of the house.
No other heat distribution system (eg radiators) was installed. Hot water was heated
with a bifacially illuminated flat-plate collector; a back-up was provided by the waste
heat of the fuel cell released while reconverting hydrogen to electricity.
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Transparent insulated walls. Several demonstration projects have shown that TI not
only minimises heat transmission losses, but also converts the building fac°ade into a
source of heat gains, compensating for losses from other parts of the building. Combined
with a well-insulated building envelope (opaque 5 0:2 W mÿ2 Kÿ1 ), thermally improved
windows (U ˆ 0:6 W mÿ2 Kÿ1 ), and efficient ventilation heat recovery, it was thus
possible to reduce the space heating demand close to zero. The energy performance of
the TI wall is given with the Ueff diagram in figure 6. The Ueff value is defined by:
Ueff ˆ
q
ZG
ˆ Utot ÿ
,
Ti ÿ To
Ti ÿ To
(7)
where q is the heat flux, Ti and To are the indoor and outdoor temperatures, respectively,
G is the incident solar radiation, and Z the thermal conversion efficiency of the system.
Utot refers to the wall's overall U value. Negative Ueff values correspond to a heat flux
from the wall to the room and occur when the solar gains exceed the thermal losses.
Equation (7) gives a linear correlation of Ueff to a climate parameter with the solar ^
thermal conversion efficiency Z as the slope of the line. As shown in figure 6, the
efficiency was experimentally determined to be 0.47. By the use of blinds, the conversion
efficiency could be switched to less than 0.04 outside the heating season to prevent
overheating.
0.5
summ
er mo
de
ÿ0.5
ÿ1.0
win
ÿ1.5
ter
mo
Ueff =W mÿ2 Kÿ1
0.0
de
ÿ2.0
ÿ2.5
ÿ3.0
0
5
10
15
20
ÿ1
I …Ti ÿ To † =W mÿ2 Kÿ1
25
30
Figure 6. Ueff diagram of the TI wall. Data points are averaged over 28 days. The lines correspond to the calculated system performance [winter/summer mode, equation (6)].
Indoor temperatures and thermal comfort. The monthly average indoor air temperatures
are presented in figure 7. The difference in the temperature development during the
two winters relates to climatic differences as well as to improvements in some of the
components. Detailed measurements were carried out to determine the thermal comfort
in the building, especially at low air-temperature levels. Experience shows that thermal
comfort according to the ISO 7730 comfort calculation procedure is guaranteed down
to air temperatures of 19 8C. This is mainly due to the increase in the radiant temperature because of the overall high level of insulation and additionally the use of transparent insulation. In contrast to opaquely insulated walls, the surface temperature of
the TI wall always remains above the air temperature level.
In the case of back-up heating with hydrogen combustion, the low power (1.5 kW)
central heat supply proved to be ineffective in meeting the inhabitant's requirements.
A local heat source in the main living room may be a more suitable and convenient
approach for the future.
Domestic hot water. The hot water demand to be met includes the demand for the
washing machine and the dishwasher, thereby reducing their electricity consumption.
Because of the temporal coincidence of thermal and electrical back-up, waste heat
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15 ECTP Proceedings page 178
30
indoor
Temperature=8C
24
18
12
outdoor
6
0
ÿ6
OCT
1993
JAN
APR
1994
JUL
OCT
JAN
APR
1995
Figure 7. Monthly mean values of the indoor and outdoor temperatures. Indoor temperatures are
averaged over the living rooms. The bars indicate the range of the measured indoor temperatures.
recovery from the fuel cell in principle matches the auxiliary demand for domestic
hot water. Figure 8 shows the efficiency curve for the bifacially illuminated flat-plate
collector based on measurements under real operating conditions. The advantage of
decreased heat losses at high temperature differences outweighs the disadvantage of
increased optical losses due to the mirror optics and the TI material. The optical losses
were found to be higher than expected as a result of faults in the mirror geometry.
Theoretically stagnation temperatures of more than 250 8C can occur, at which plastic
TIM melt. Situations like this were counteracted by automatic shading with blinds
but nevertheless occurred. With new TIM made of glass, this type of collector can
withstand stagnation conditions. An alternative approach would be incorporation of a
thermotropic layer in the collector cover.
4.3.1.4 High-exergy energy supply and seasonal energy storage. The task of this part of
the energy supply system was to meet the demand for electricity and high temperature
heat (cooking). To overcome the seasonal mismatch between solar radiation and building
energy demand, long-term storage (1500 kWh) based on hydrogen was introduced in
combination with short-term electricity storage with lead-acid batteries (20 kWh). The
hydrogen was produced by an electrolyser powered by a 3.5 kW PV system.
Heat for cooking and back-up heating was generated with diffusion burners for
hydrogen gas. The catalyst does not contain platinum, so unintended ignition is avoided;
0.8
0.7
0.6
Z
0.5
0.4
Z ˆ Z0 ÿ …UDT=I †
U ˆ 1:63 W mÿ2 Kÿ1
Z0 ˆ 0:61
0.3
0.2
0.1
0.0
0.0
0.01
0.02
0.03
0.04
0.05
DTI ÿ1 =m2 K Wÿ1
0.06
0.07
0.08
Figure 8. Efficiency curve for the bifacially illuminated flat-plate collector derived from measurements of the hot water system in the SSSH. Data refer to the aperture area of the collector.
Transparent thermal insulation materials
155
15 ECTP Proceedings page 179
ignition is provided by a piezoelectric spark. The combustion is nearly free of NOx
emissions because of the limited temperature of 600 ^ 800 8C. This allows the burner to
be integrated directly into the supply air system without any heat exchange cycle. The
only reaction product is water vapour. Four burners of 1 to 2.5 kW power replace the
original burners of a conventional gas stove. For the kitchen an electric oven was preferred to a hydrogen one for safety reasons.
4.3.1.5 Summary of the results of the SSSH project. Energy self-sufficiency is generally
not the aim for a building power supply system. Besides the economic background,
the problem of long-term storage, especially of high-exergy energy, is the main difficulty. The SSSH project proves that, in principle, a technical solution exists with a
hydrogen-based approach. In the absence of a fossil fuel back-up and the public grid,
an outstanding example of an energy efficient building was created with a yearly total
energy consumption of less than 10 kWh mÿ2 , met by solar energy utilisation throughout the year. Three years of experience prove that it is practicable to construct residential buildings with almost no heat demand in the Central European climate. Besides
the data measured, the occupants' positive feedback stimulates further efforts in this
field (Stahl et al 1997).
4.3.2 Renovation of schools. In Eastern Germany many schools were built at the end
of the 1960s with a standardised prefabrication method. A high heating consumption
exceeding 200 kWh mÿ2 aÿ1 as well as outdated heating systems and a poor state of
repair led to a need for renovation. In addition to better insulation (0.2 W mÿ2 Kÿ1 for
the roof and 0.4 W mÿ2 K ÿ1 in the fac°ade) and low e windows (Ucg ˆ 1:4 W mÿ2 Kÿ1 ),
transparent insulation seemed to have a good application potential.
4.3.2.1 Paul-Robeson School, Leipzig. One of the first renovation projects was the
Paul-Robeson School in Leipzig. The building consists of four storeys in light concrete
slab construction with 60% window area in the south and north fac°ades. Before renovation, 225 kWh mÿ2 aÿ1 heating energy was needed. A local company developed
wooden TI modules filled with PMMA capillary modules of area 2.5 m61.24 m. Integrated roller blinds posed difficulties for the necessary maintenance. The heating energy
consumption in the years 1994/1996 was around 70 kWh mÿ2 aÿ1 . The cost of the TI
construction amounted to 1370 DM mÿ2.
4.3.2.2 Wurzen School. Improving the system design and the manual production process
led to greatly reduced costs (reduction 40%) in the case of a second project, the Wurzen
School, with a heating energy consumption of 300 kWh mÿ2 aÿ1 before renovation. This
time the shading devices were placed outside the modules. The area as well as the
thickness of the modules were decreased as a result. A simple mounting construction
became possible. The roller blinds may first shade the transparently insulated spandrel
and then the windows. The commercial control unit is capable of distinguishing
between two shading states (figure 9).
4.3.3 Apartment block in Gundelfingen. The apartment block in Gundelfingen is a
recently built demonstration project combining the different new technologies of water
heating with solar collectors, and air heat recovery with a heat pump, transparent
insulation, and advanced windows (figure 10). Individual flats are sold on the real
estate market, with solar energy featuring as a sales-promoting factor. The targeted
heating energy consumption is 30 kWh mÿ2 aÿ1.
4.4 Restrictions and problems
A number of reasons can be given to explain why TI has had relatively little economic
success up to now, although it is one of the most promising solar fac°ade types for new
architecture and building renovation. Some of them will be discussed here.
156
V Wittwer, W Platzer
15 ECTP Proceedings page 180
Figure 9. View of the TI fac°ade with
shading device (School of Wurzen).
Figure 10. View of the solar apartment block in Gundelfingen (south fac°ade) with transparent insulation.
4.4.1 Performance assessment. With a wider variety of products and system designs, a
simple method to compare and assess the energy performance is needed. Architects
and consultants should be able to assess the benefits and restrictions of a certain
project quickly, especially with respect to solar control. While it may be needed sometimes, it does not automatically cause additional costs, if achieved by partial coverage,
or natural or stationary shading measures.
4.4.2 Building regulations. This leads to the second aspect: in order to take account of the
solar gains from TI in the building codes, first well-defined product values are needed.
There must be a way to feed them into the current code. The situation there will improve
soon with the new European standard EN832 for heating requirements of dwellings
and subsequent national translations. In this standard, TI is considered in a generic
form. A precise and detailed methodology to deal with TI systems has been developed
and discussed with German bodies by the manufacturers' association FVTWD eV,
Gundelfingen, Germany, fitting into the framework of EN832 (FVTWD 1999).
An important problem still is the slow progress with building product certification.
Not being a regular building product, each variant of a TI system has to apply for
admission to the building market or ask for permission to use the product in every single
case. The product then should be in certifiable compliance with certain quality standards,
eg with respect to fire resistance or structural performance. However, these regulations have to be discussed first. This is a time-consuming and costly process. SMEs and
companies with steadily improving products have difficulties with this procedure.
4.4.3 Costs. Due to the small niche market the costs of TI systems in general are high.
Production and mounting is dominated by labour costs. The complexity of the systems
in combination with new individual applications creates a need for extensive work in
planning, consulting, and modifying. The prefabrication and preplanning potential is
still high although progress has certainly been made. Unnecessary caution related to
using an unknown new technology sometimes creates extra costs, eg for mechanical
solar control. Table 3 gives an overview of costs of available systems.
Transparent thermal insulation materials
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15 ECTP Proceedings page 181
Table 3. Cost estimation for different TI systems.
System
Solar control
Price range=ECU mÿ2
Transom-mullion system
Prefabricated TI modules
Transparent exterior insulating finish systems
Semitransparent TI
Industrial glazing with TI
mechanical
mechanical
none
none
none
450 ± 750
400 ± 500
200 ± 500
200 ± 300
150 ± 200
4.4.4 Information. Information on the different systems available is not widespread
amongst builders and planners. Only a small percentage has heard about this technology
and even fewer are able to integrate it into a building project. Planning tools and
planning information need to become widely available. The FVTWD has dedicated
itself to produce and provide information independent of individual manufacturers.
As a first step information material on products and companies is provided.
4.5 Market
With each demonstration project completed, the market is growing slowly but steadily
(figure 11). Subsidised projects provided the initial momentum for the technology. Subsequently a professionalisation and slow uptake by private initiative can be observed
without much change in installed area per year. This seems to be typical in a transformation phase for an innovative solar technology.
Installed area=m2
8000
estimated
survey
extrapolation
6000
4000
2000
19
87
19
88
19
8
19 9
90
19
91
19
92
19
9
19 3
94
19
9
19 5
96
19
97
19
98
0
Year
Figure 11. Accumulated installed area of TI systems (1997 and 1998; approximate numbers
obtained from interviews with manufacturers).
4.6 Outlook
Coming from ideas and concepts, TI systems have reached a state of commercialisation
and well-developed products. Although research and development should not be
neglected in the field of TI, the primary demands are in the fields of standardisation,
information dissemination to the planning professions, and marketing. The recently
founded manufacturers' association is aiming to meet these challenges. Cost reduction is
mainly a function of production quantity (Kerschberger et al 1998).
In the technical field new glass coatings may be strong competitors to conventional TI systems in the future. The combination of highly effective antireflective layers
(Gombert et al 1999) with IR selective coatings enable the construction of systems with
high solar transmittance and low U values in a parallel sheet technology. Thermal
stresses on the glass panes used and sealing problems for the inert gas conventionally
used might be a problem because of the temperature gradient and the thermal extension
in the system. It must be stated here that the development of TIM has stimulated a lot of
research work to improve conventional window systems. The first products are available
now on the market.
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V Wittwer, W Platzer
15 ECTP Proceedings page 182
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ß 2000 a Pion publication printed in Great Britain