Passive Solar Walls

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Passive Solar Walls

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Passive solar walls
1 Classification and principles
Opaque walls, and not only glazing, can collect solar energy. Passive solar walls complement direct gains through glazing by delaying the energy transmitted thanks to their thermal inertia. This energy can be distributed into the heated room by simple heat transfer from the wall or also by air circulation. The following figure presents the various possible alternatives for passive solar walls.

A) NO AIR CIRCULATION

C1) EXTERNAL AIR CIRCULATION

B) INTERNAL AIR CIRCULATION

C2) EXTERNAL AIR CIRCULATION, SUMMER POSITION

1 : TRANSPARENT COVER, 2 : ABSORBING SURFACE, 3 : THERMAL MASS, 4 : AIR GAP, 5 : INSULATION LAYER, 6 : LOUVRE, 7 : SOLAR PROTECTION (E.G. ROLLER BLIND)

Case A) is the simplest : there is no air circulation. The energy collected through the transparent cover (1) is absorbed at the masonry wall surface (2), painted black or at least dark. The
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heat produced is stored in the masonry (3) and emitted with a time lag towards the room by radiation and convection. An external shading device (7) prevents from overheating. In case B), the objective is to avoid external solar protection in order to simplify the maintenance and to reduce costs. Control of solar gains is provided by a switchable air circulation. In the winter position, the two louvres (6) are open and let air, heated in the air gap (4), flow into the room. In summer, the louvres are closed and the opaque insulation layer (5) reduces the heat transfer. Alternative C) is interesting because no opaque insulation layer is needed, which reduces the wall thickness and improves heat transfer efficiency. But external air circulation may deposit dust on the transparent cover (1). This cover must thus be accessible for cleaning. Solar protection may either be provided by an external shading device (fig. C1), or by circulating air to cool the wall (fig. C2). In this last case, a supplementary louvre (6) must be integrated in the facade. It should be closed in winter.

2 Transparently insulated walls
2.1 Principle The simpliest Transparent Insulation Material (TIM) we know is a pane of glass covering the surface of an absorber to reduce the heat losses by convection. To enhance the greenhouse effect, a number of new materials and layers has been developed with more or less benefits. The first possibility to reduce the heat transfer coefficient (U-value) is to increase the number of layers of glass or foils. But this also reduces the transmittance for solar light, because each layer reflects part of the sun light. Using low emitting coatings on glass panes also reduce the U-value drastically, but reduce the solar transmittance, too. Therefore, the most promising materials for thermal conversion systems are windows with fillings of honeycomb or capillary materials /2/, which are oriented perpendicular to the absorber area. In this case the reflection of sun light from the filling material is towards the absorber. Similar results can be realised by using aerogel granules as filling. This is a quasihomogeneous material produced from silica-gels/3/. In table 1 the most significant material data are given for different transparent insulation systems. To distinguish between typical glazing and these high performance materials for applications in solar thermal conversion systems the expression "Transparent Insulation (TI)" is only used for these special class of new materials. A U-value below 1 W/(m2K) and an energy transmittance greater than 60% characterise the best TI-materials. With these physical properties TI materials outperform all conventional glazing systems and further increase the efficiency of thermal conversion of solar radiation. Low winter radiation levels can be used effectively. Experiments, simulation calculations and results from demonstration projects show the space heating potential of transparently insulated walls. A properly built passive TI element on a south orientated massive building facade can save heating energy by up to 150 kWh per square meter each heating period.

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Thermal and optical properties of transparent systems Materials d [mm] 4-16-4 4-16-4 U [W/(m2K)] 2.9 1.3 τ dif [-] 0.63 0.42 0.28 0.32 0.45 0.65 0.57 g dif [-] 0.67 0.53 0.35 0.46 0.50 0.67 0.64

2-pane window with air filling 2-pane window with low-e coating and argon-gas filling 3-pane window with low-e coating 4-8-4-8-4 0.7 and crypton-gas filling 2-pane window (low iron) with 6-36-6 0.6 aerogel-granules filling 4-24-4 0.8 2-pane-window (low iron) filled 4-50-4 1.3-1.4 with PC-honeycombs 4-100-4 0.8-0.9 d = thickness of glass panes and gaps U = heat transfer coefficient (_ m = 10° C) τ dif = diffuse solar light transmittance (whole spectrum) gdif = diffuse energy transmittance

To reduce the heat losses of a thermal conversion system Transparent Insulation Materials influence all three heat loss factors: • they reduce the convection losses by suppression of air movement. In the honeycomb and capillary structures, but also in aerogels the air gaps are too small so that the air cannot move inside. • they reduce the radiation losses. In honeycomb and capillary structures the material used is Polycarbonate (PC) or Polymethylmethacrylate (PMMA), which are opaque for the infrared radiation. In aerogels the gaps of the structure are smaller than the wavelength of the infrared radiation. • they do not rise the heat conduction losses significantly to due very low material content. More than 95% of the filling is still air. 2.2 Performances The energy balance of a conventional wall is calculated with the heat transfer coefficient of the wall. This heat transfer coefficient is constant; it is determined by the physical properties of the wall materials. Meteorological influences by solar radiation and wind are negligible. A transparently insulated wall is designed for solar energy utilisation and therefore the energy balance cannot be calculated by heat transfer coefficients alone; it is a sum of conduction losses and solar energy gains. The basic principles of transparently insulated walls were first described by Goetzberger1. In a steady-state approximation, the heat flux through the wall is given by qw = {UTIUW/(UTI+UW)}(Ti-Ta) - _ 0G (1) The solar conversion efficiency _ 0 of the transparently insulated wall can be defined as2

_ 0 = gd_ Uw/(Uw+UTI) (2) The single glazing used by Trombe led to efficiencies in the range of 10 to 15%. The step to 30% solar efficiency is the basis for the benefits of direct solar space heating with transparently insulated walls. These efficiencies surpass those of solar space heating systems with seasonal storage. The passive design of the TI wall is an important advantage in view of the long term operation. No auxiliary energy is needed to operate a TI wall. An effective U-value for the transparently insulated wall can be defined as :
1

A. Goetzberger, J. Schmid, V. Wittwer, ´Transparent Insulation Systems for Passive Solar Energy Utilization in Buildings´, Int. J. Solar Energy, Vol. 2, pp. 289-308, 1984. 2 P.O. Braun, A. Goetzberger, J. Schmid, W. Stahl, ´Transparent Insulation of Building Facades-Steps from Research to Commercial Applications´, Solar Energy Vol. 49, No. 5, pp.413-427, 1992.
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Ueff = qw/(Ta-Ti) (3) Due to the solar radiation G, the value Ueff is dependent on time and the orientation and location of the TI wall. Ueff is negative, if heat gains prevail. Effective U-values do not characterise the thermal behaviour of a material and it is not a material constant anymore. The energy gains of a TI wall are utilised to offset heat losses of other building components or ventilation losses. If the gains approach the order of magnitude of the remaining total heat load, excess heat will be produced, which may not be needed. This is taken into account adding the utilisation factor N in eq. 1 to the solar conversion efficiency. By multiplication with the appropriate temperature differences and integration over time, eq. (3) can be written in terms of energy : Qeff = Qw - N Qsol (4) The thermal conductivity and the heat capacity of the wall material and the thickness of the wall determine the time delay between solar radiation being absorbed and thermal energy reaching the inside surface of the wall. Therefore, direct energy gain through windows during the daytime can be combined with the delayed energy gain through transparently insulated walls during the evening and the night. The orientation of the wall and the usage of the room behind have to be considered, when the walls material and thickness is selected. Furthermore, all building components relevant to the heating demand have to be matched to each other. Only in this way the maximal efficiency and best solar utilisation factors can be achieved to minimise the auxiliary heating demand. For detailed analysis, dynamic calculations are necessary. Therefore, a type for the TRNSYS simulation package was written, which is available at the FhG-ISE. With such analyses, the thermal behaviour of a passive solar building can be estimated before realisation. E.g. for a certain building the next figure shows the energy balance of a TI-wall system per year. As can be seen the heat losses are 10 times lower than that of comparable high insulated walls, but on the other hand the building will be heated by TI with heat gains of about 100 kWh/m2a if oriented to the south. Small deviations from that optimum do not influence that gains too much, but real east or west orientations will reduce the gains to the half, whereas the losses will rise to nonnegligible values. East and west orientations also increase the risk of overheating in summer. This could even affect the durability of materials.

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Energy balance of a transparently insulated lime stone wall in dependence of the walls orientation. The values are sums for the heating period of low energy houses (Oct.March) for southern German climate (Freiburg) per square meter of TI-aperture.

2.3 Architectural integration The general construction principle of a TI element is shown in next figure. It consists of an outer glass cover for weather protection, a shading device, the TIM and an inner plastic film or thin glass cover. In a modular construction, these components are held together by a frame. The frame is mounted onto the black painted wall. The outer glass cover can be of non-shattering glass. A rough inner glass surface reduces direct reflection, while a low iron content is advantageous to increase solar transmittance. In such a configuration without air circulation, a shading device is necessary to prevent from overheating and to regulate energy gains during the transition periods. Roller blinds have been used very often for this purpose; venetian blinds are being tested in some of the new projects. Plisséstores or curtains are further possibilities today, whereas thermochromic or electrochromic coatings can be used, if available in future. For the highly efficient TI elements only shading devices with a preferable 100% reflection of solar radiation can provide efficient protection against overheating in summer. A closed shading device can also reduce heat losses through the TI element by night, if its surface is able to reflect infrared light. The roller blinds often used are made from an aluminised textile. The low emissivity of the aluminised surfaces reduces the radiative exchange between the glass cover and the TIM, which reduces the U-value of the TI element with the roller blind closed. The inner plastic film or glass cover touches the TIM surface directly. This is necessary to avoid convective air exchange from the absorber air gap to the glass cover air gap. The temperature difference between the glass cover and the absorber results in a buoyant force which may be larger than the convective current resistance of the TIM. The walls have been painted up to now with a watersoluble black paint. Selective absorbers based on metal foils are being discussed for a further increase of the solar conversion efficiency. The frame of the TI element has to be mounted onto the wall with an air tight seal to prevent air movement by external wind and by air convection between the individual elements. The frame material should have a low thermal conductivity. To maximise the solar aperture, the frame thickness should be as small as possible.

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General Construction of a TI Element in Passive, Indirect Systems

A different type of TI-system is being under development by the German company STO, which call their product a „Transparent Exterior Insulation and Finish System (TEIFS)“ with a stucco-like look. It is directly connectable to standard insulation systems, which will form the frame of the TEIFS system. The system has a self-regulating overheating mechanism due to lower light transmission values with a rising irradiation angle, but nevertheless it should not be installed in large areas connected to the same room. The performance is slightly lower (about 70 to 80%) than for standard TI-systems. On the other hand, the stucco-like design of the system is more attractive for standard residential buildings, so that the architectural integration is quite easier. An example of the integration of that system in buildings is shown in § 2.6.3. 2.4 Directions of use The use and maintenance of TI walls is very easy if all components are well selected and installed. Solar protection should be provided in summer to avoid uncomfortable high room temperatures. This protection, e.g. shading device might be controlled by the user itself if the principle of the system is well explained. Of great architectural interest is the change of the building's appearance due to open or closed shading devices. The building looks dark for solar absorption in winter time; it looks essentially white for solar reflection in summer. Just as people choose to wear a shirt or a pullover, depending on the outdoor temperature, the building changes colour like a chameleon. Architectural constraints to reduce heating demand by minimising the ratio of the building surface to building volume (compacity) are removed. New aspects are relevant for the shape of low-energy buildings with TI walls. Increasing the south facing TI wall area decreases the heating demand. 2.5 Example realisations In 1983, the German Ministry for Research and Technology started a TI research program, which was extended in 1986. The program led to fruitful cooperation between research institutes, and also initiated and gave financial support for cooperation with companies interested in producing and marketing TI systems. The activities in testing and demonstrating TI applications have increased significantly in recent years. Meanwhile, about 15.000 m2 transparently insulated walls are installed world wide; within half of this the Fraunhofer-Institute for Solar Energy Systems has been involved for consulting, planning, construction advice, simulation or data analysis; a selection of those projects is given in next table.

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Selection of massive wall TI-projects with participation of the FhG-ISE
Coun- Location try Building type Year TIM Type Area (m2) 1-Family House Semi-Detached House Bungalow Office Building 8-Family-House Terrace Houses Student Residential Semi-Detached House Semi-Detached House Bungalow Residential Building Self-Sufficient Solar House Crew-Training centre School Building Residential Building 1983 1988 1988 1988 1989 1989 1990 1990 1991 1991 1991 1992 1992 1993 1994 capillary aerogel honeycomb honeycomb honeycomb honeycomb honeycomb honeycomb aerogel honeycomb capillary capillary capillary capillary capillary passive passive passive passive passive passive passive passive passive passive passive passive wall wall wall wall wall wall wall wall wall wall wall wall 12 120 40 31 120 4*22 1000 70 70 40 150 80 900 330 100

D CH D D D D GB D D D D D D D

Freiburg-Merzhausen Ardon München-Grasweg Sauter-Freiburg FreiburgSonnenäckerweg Düsseldorf-Hellerhof Glasgow, Strathclyde Freiburg-Tiengen I Freiburg-Tiengen II Shanghai, SERI Kloster Windberg Freiburg-Christaweg DLR, Köln-Porz Leipzig, Wahren Freiburg-Villa Tannheim

passive wall passive wall passive wall

2.6 Demonstration Projects 2.6.1 Residential Building Sonnenäckerweg, retrofitted with TI As part of a modernisation program by the city of Freiburg, several identical blocks of flats built in 1957 were renovated. The housing company Freiburger Stadtbau GmbH (FSB) and the institute cooperated to retrofit one of these buildings with TIM. The two-storey building consists of eight apartments and has a total heated living area of 400 m2. The renovation, realized in 1989, included the following measures: 120 m2 TIM elements with 10 cm thick honeycomb material on the south-east and south-west walls with roller blinds for shading (as shown in previous figure) • double glazed windows with photovoltaically powered roller blinds for thermal and solar protection • opaque insulation of the north-east and north-west walls with 10 cm hard polyurethane foam • opaque insulation of the basement ceiling and the attic floor. • The south-east wall of the building is constructed of 30 cm thick light hollow masonry; the south-west wall is made of 30 cm brickwork. The next figure is a picture of the transparently insulated south-east facade of the retrofitted building. An outstanding feature of this TI application is the direct comparison with the renovation of the neighbouring building with opaque insulation according to low-energy building standards. The conventional measures applied to this identical building were the following: • opaque insulation of all the walls with 10 cm hard polyurethane foam • opaque insulation of the basement ceiling and the attic floor • installation of low-e insulating glazing • forced air ventilation with heat recovery system.

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View to the Transparently Insulated Southeast Facade, Sonnenäckerweg The mean U-values of the two buildings for the total outer surfaces after renovation are: • TI building U = 0.4 W/m2K • opaque insulated building U = 0.3 W/m2K An example of measured temperature levels outside, through the wall and inside is shown in the next figure.

Measured temperature levels of the ambient, the transparently insulated southeast facade and the room temperature behind and the accordingly measured irradiation values on this facade.

It was a period of 3 bright days in January 1990 with a peak level of irradiation of 600 W/m2, followed by 2 foggy days with less than 100 W/m2 peak. As the outside temperature of +2 to -8°C
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indicates, this was part of the heating period. The outside wall temperature was rising to up to 60°C with a little delay to the maximum of irradiation and was falling back to the level of room temperature by night. With a delay of some 6 to 8 hours the inside temperature of the wall was also rising to a peak temperature of 27°C and was falling back to room temperature level, too. Room temperature was always around 20°C, but also a little bit less on the foggy days, which was still comfortable due to the high surface temperatures all around! As long as the inside wall temperature is higher than the room temperature there is a heat flux from outside to the room, so there are no transmission heat losses at all from these walls to the ambient. Although on the foggy days the irradiation is quite low, the very little peaks of temperature rising indicate the high performance of the TI wall. In total, the resulting calculated (before retrofit) and measured net heating energy of the buildings for the year 1990 are given in the table below. Properties of the applied Insulation Systems and the Heating Energy Demand for 1990, Sonnenäckerweg, Freiburg Building Standard Insulation Technology Type/Um-value (W/m2K) Orientation
south none/1.0 opaque/0.4 opaque/0.25 transparent north none/1.0 opaque/0.4 opaque/0.25 opaque/0.25

Heating Energy Demand kWh/m2a
simulation 225 100 77 45 measured 50 62 +11 40

Comments

before retrofit standard retrofit low-energy TI-insulated

not measured before user influence! +11=el. energy for fans decentralized heating devices

2.6.2 Self Sufficient Solar House, Freiburg (Germany) The concept of the Self Sufficient Solar House (SSSH) was developed after the first measurements had shown the high potential of TI elements to meet the space heating demand. The project was started in 1989 and the building was finished in 1992. Without fossil fuels and without connection to the public grid, all energy needs in the single-family house have to be covered by solar radiation falling onto the building's outer surface. Under German climatic conditions the heating energy demand in residential buildings dominate the total amount of energy needs: 80% of the energy is for space heating, another 10% will cover the DHW demand, the rest is for electricity. Since seasonal storage of a large amount of energy today is still both technically and financially prohibitive, it was decided to reduce the energy demand by all available energy saving technologies. For the heating energy TI was the key to reduce the energy demand drastically3. With conventional opaque insulation, a small surface to volume ratio should be achieved to reduce heat conduction losses through the walls. The constraints on the architecture of low-energy buildings are noticeable, most of them having a cubic shape. Buildings with TI walls must comply with other conditions to minimise the space heating demand (SHD). Because of the energy gains of the TI walls, a very long building with a TI south wall would have the lowest SHD, although the surface to volume ratio is very high. Calculations with the simulation programme TRNSYS optimised both, the shape of the facade of the building and the dimensions of the technical equipment, which is necessary to guarantee comfortable conditions for the occupants of the house all over the year. After the completion of the building in 1992 a detailed monitoring programme has been started to evaluate the performance of the building. The intention of the project is to show the technical potential of solar energy applications to replace all environmentally damaging energy carriers in dwellings. Only a few of them are feasible under today's economic conditions or in near future.

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W. Stahl, K. Voss, A. Goetzberger, ´The Self-Sufficient Solar House in Freiburg´, Solar Energy Vol. 52, No. 1, pp. 111-125, 1994
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View of the transparently insulated self-sufficient house, Freiburg Due to an excellent opaque insulation of the whole building to the ground, to the roof and to the north (U<0.2 W/m2K), the well insulated windows (U<0.6 W/m2K) and the installation of 70 m2 of TI on the south oriented facades, the net heating energy demand of the building is approx. 300 kWh per year. A photovoltaic powered H2/O2-system supplies this auxiliary heat and all electricity. 2.6.3 Villa Tannheim - the new Headquarters of ISES, Freiburg (Germany) In autumn 1994 the International Solar Energy Society (ISES) decided to move its headquarters from Melbourne, Australia to Freiburg, Germany. Before the old historically significant building could be used as the new international headquarters, a thorough and particularly energysaving renovation was required. The aim of the renovation was to reconstruct the outer appearance of the patrician building using renewable energy sources, without destroying the character of the building. The outer walls were insulated with an 8 to 10 cm thick exterior insulation and finishing system; the stone embrasures and other distinctive details of the facade were re-modelled. Large areas of the buildings western facade were covered with transparent exterior insulation and finishing system, which is especially suitable for the renovation of residential buildings. Glazing with a U-value of approximately 0.4 W/m2K was installed by using triple glazing with a new low-e coating and a special inert gas filling. Finished in January 1995, the building has been monitored and analysed for a period of two years as part of a research project of the FhG-ISE for the International Energy Agency (IEA-SHCP, Task 20). By the simulations made with TRNSYS it can be expected that the heating energy demand will be reduced to 30% of the status before, which means, that with about 70 kWh/m2a the building will come up to a low-energy building standard.

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View to the west and north facade of the renovated Villa Tannheim. The Transparent Exterior Insulation and Finishing System (TEIFS) can be seen on the right hand (WestFacade).

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3 Trombe walls4, external air circulation
3.1 Principle a) winter Solar radiation, transmitted through the transparent cover (1), is absorbed on the black or dark surface (2) of the wall and transformed into heat. This heat is transmitted : - by conduction in the masonry (3), then it is stored and provided to the room with a time lag ; - by convection to the air situated in the air gap (4) ; this air is heated and thus lighter, which makes it rise by thermosiphon effect and circulate into the room through the louvres (6), which are open in winter. If the transparent cover is not a transparent insulation material but a simple or double glazing, the temperature in the air gap might fall at night or during cloudy days. In this case, the air could be cooled in this space and a reverse thermosiphon circulation might occur : being cooled, the air is heavier and circulates downward in the air gap, thus cooling the room. To prevent this, the occupant should close one louvre. Of course this imposes to close one louvre every evening in winter, and to open it every sunny morning. Transparent insulation is recommended in order to avoid this constraint and to achieve a better efficiency (cf next §). An alternate possibility is to place a light plastic foil on one louvre : when reverse thermosiphon begins, this foil is blown against the louvre and air circulation is automatically blocked. Winter configuration

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The natural convection effect is rapid. On the other hand, conductive transfer introduces a time lag due to the thermal inertia of the masonry. The external circulation Trombe wall (Patent TrombeMichel) combines thus instantaneous gains and time delayed ones. b) summer The upper louvre (6) is closed in summer. There are two possibilities for solar protection. Either the wall is equipped with a shading device (roller blind,...) or air is circulated to cool the wall (cf next figure). Another louvre must then be integrated in the facade. The chimney effect draws air from the room, allowing external air to be introduced by pressure difference, preferably from the north (and presumably cooler) side of the building.

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Patent Trombe-Michel, 1971. This patent being more than 20 years old, the invention is now in public domain.
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Summer configuration without shading device

North

3.2 Performances The performance of a solar component depends on the building site and its climate, on the functional use of the building (housing, offices, school,...) and also on the rest of the envelope : if the envelope is made of masonry walls, it will be able to store more energy and the utilisability of solar gains will thus be higher compared with a very light structure (e.g. wooden frame). At last, the performance by square meter depends on the area of component related to the area of the building : the first square meters of solar collector are the most efficient ; the higher the area of component is, the lower is the performance by square meter. It is thus very difficult to provide a general figure characterising only the component studied. We prefer here to explain the principles of performance evaluation and to present a few sensitivity studies. The thermal benefit from a component should be evaluated by balancing the supplementary gains and losses provided by this component. Such a balance, obtained by thermal simulation or measurement, provides the net gains. The productivity is obtained by comparing the net gains to the heat losses of a reference component. In the case of a wall, this reference component should be a standard opaque insulated wall, according to the thermal regulation corresponding to the building site planed. To summarise, we have : net gains = gross solar gains - heat losses productivity = net gains + reference heat losses. Example - according to the meteorological data for the site considered, the incident solar radiation on a vertical south plane and over the heating season is 540 kWh/m2/year and the degree days5 amount to 2,700 (base 18) ; - the transparent cover of the solar wall transmits around 60% of this radiation and its heat loss coefficient is around 0.7 W/m2/K (corresponding e.g. to 10 cm of transparent insulation); the absorbing surface of the wall is painted black matt ; the net useful solar energy, obtained by simulation accounting for the thermal inertia of the building (16 cm masonry walls and slab) and its occupation pattern (housing, constant occupation), is 145 kWh/m2/year ; this corresponds to 10 m2 of component for a 100 m2 living area (the net useful energy provided by one square meter of solar wall would be lower with a larger area of solar wall) ; - the reference heat losses, considering a reference wall with a heat loss coefficient of 0.45 W/m2/K, amount to 0.45 x 2,700 (degree days) x 24 (hours/day) = 30 kWh/m2/year ; - the productivity of the solar wall is 145 + 30 = 175 kWh/m2/year. The following table presents productivities in three climates : Paris, Mediterranean and dry, Mediterranean and coastal.

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The degree days are calculated by adding over a heating season the daily mean differences between the base temperature (usually 18°C in dwelling) and the external ambient temperature.
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LOCATION PRODUCTIVITY (KWH/M2/YEAR)

PARIS CARPENTRAS (SOUTH OF FRANCE) 175 235

NICE (MEDITERRANEA) 180

Because there is no opaque insulation layer in these walls, the quality of the transparent cover is very important. A transparent insulation material with an appropriate thickness is thus needed. Simple or double glazing would lead to high solar gains, but very high heat losses so that the net gains would be very small, or even negative in most European climates. Solar walls should be oriented between south-east and south-west in northern hemisphere (north-east and north-west in southern hemisphere) in order to receive the highest possible radiation during the heating season. The optimal thickness of the masonry wall amounts between 15 and 35 cm. Experience shows that for a wall of 35 cm, it is possible to obtain an air circulation until the early hours of the morning. The air gap (4) should be 8 cm thick in order to allow sufficient air circulation by thermosiphon. The performance of a component should not be restricted to energy saving : thermal comfort is also essential. A global study is necessary to evaluate the risk of overheating in terms of the thermal inertia of the building, the ventilation pattern, the possible shading devices protecting glazing and solar components. East or west orientations are unsuitable for solar walls, because solar gains would be high in spring and summer, which may lead to overheating. 3.3 Architectural integration The first solar walls have been criticised a lot because of the dark aspect of the buildings. The new translucent insulation cover now hides the black surface of the absorber, and this makes architectural integration of such components much easier. The masonry wall provides energy storage with practically no over-cost since it is amortised in construction by its role as a load bearing wall. 3.4 Directions of use The use and maintenance of solar walls is very easy. For this particular type of wall (external air circulation), the transparent cover might need regular cleaning (e.g. once a year). Solar protection should be provided in summer, either by closing a shading device or by opening the external louvre to circulate air in the component (thus closing the internal upper louvre). Even if these actions are very simple, we advise to explain the principle of solar walls so that users clearly understand what they should do and why. 3.5 Example realisations - Odeillo solar houses The figure below shows the block of three solar houses built in 1974 on a rocky peak situated in the Pyrenean mountains (latitude : 42°, altitude : 1,550 m). The climate is sunny (2,500 hours of sunshine annually), the global radiation on horizontal being 1,600 kWh/m2/year. The mean annual temperature is 7.2°C, which corresponds to 3,940 degree-days (base 18°C).

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Odeillo solar houses, Architect Jacques Michel In this project, the architecture takes the environment, climatic conditions and orientation into account. The facades incorporating solar collectors face south, east and west. The openings form one unit with the solar collectors. The thermal system is based on the Trombe wall system (external circulation) with a 0.37 m thick concrete wall. The three dwellings are owned by three researchers of the Odeillo CNRS (National research centre) Institute. The living area of the central dwelling is 150 m2 and the heated volume 360 m3. Customised architecture, taking account of three different briefs, demonstrates the flexibility of application of vertical collectors in a free plan. The houses are built using traditional materials (walls of concrete blocks, wooden frame roof covered with asphalt shingle). The absorbing surface is coated with brown vinyl painting, and covered with a double glazing. Each collector is 2.5 meter high and 1.55 meter broad. The height between the vents is 2.2 m. The dimensions of the vents are 0.84 x 0.095 (0.08 m2). The collectors are integrated together with glazing in the facades (20 m2 of south oriented double glazed windows). The insulation is made of polystyrene, with a U-value of 0.36 in walls, 0.24 in the roof and 0.35 in the floor. The total heat loss of one house is around 20,400 kWh/year, and the heating load is only 10,000 kWh (65 kWh/m2/year) : the solar fraction is thus around 50%.

4 Trombe walls6, internal air circulation
4.1 Principle Solar radiation is transmitted through the external transparent cover (1) and absorbed at the surface (2) of the masonry wall, preferably painted dark. Energy is stored in the masonry (3) and transmitted by conduction with a time lag until the air gap (4). Heated air is circulated by thermosiphon through louvers (6).

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Patent Trombe-Michel
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ROOM

Like in the case of external air insulation (cf. previous §) if the transparent cover is not a transparent insulation material but a simple or double glazing, the temperature in the air gap might fall at night or during cloudy winter days. In this case, the air is cooled in this space and a reverse thermosiphon circulation occurs : being cooled, the air is heavier and circulates downward in the air gap, thus cooling the room. To prevent this, the occupant should close one louvre. Of course this imposes to close one louvre every evening in winter, and to open it every sunny morning. Transparent insulation is recommended in order to avoid this constraint and to achieve a better efficiency (cf. next §). An alternate possibility is to place a light plastic foil on one louvre : when reverse thermosiphon begins, this foil is blown against the louvre and air circulation is automatically blocked. In summer, one louvre should be closed because solar heating is no more needed. The opaque insulation layer (5) reduces the heat flow from the wall. This avoids the need of an external shading device. 4.2 Performances The definitions and principles are similar to those presented in the case of external air circulation. The example considered here is the same, with two differences : the air gap is situated at the inner side of the masonry wall, and is separated from the room by 10 cm of opaque insulation (encapsulated in plasterboard or wood). The following table gives the productivity for three climates.
LOCATION PARIS CARPENTRAS (SOUTH OF FRANCE) PRODUCTIVITY (KWH/M2/YEAR) 150 195 NICE (MEDITERRANEA) 160

The productivity figures are lower than the corresponding values for an external air circulation. This is due to the opaque insulation layer, which stops radiative transfer from the masonry wall to the room. But the constraints for the occupants are much lighter : the transparent cover does not need cleaning. Also, an external shading device is not necessary, which simplifies maintenance and reduce cost. In order to study the influence of design choices on the performance, we performed sensitivity studies concerning the characteristics of the component itself, but also the use of the building (cf. table below). In the case of intermittent heating (e.g. offices, school), the productivity is lower because the solar gains stored in the masonry wall are useless at night. The references are indicated between parenthesis, and correspond to the 0% vertical axis. Variations from the reference are indicated on the bar graphs. For example, the reference transmission factor of the transparent cover is 0.78 and this factor was varied between 0.70 and 0.86. The corresponding productivity varied between -10% (compared to the reference) for 0.70 transmission and +10% for the highest transmission factor.

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Sensit ivit y in % TIM t r ansmission (r ef 0.78) TIM U-value (r ef . 1 W/ (m2.K)) absor pt ivit y (r ef . 0.9) t hickness of t he masonr y wall (r ef . 16 cm) mat er ial (r ef . concr et e) or ient at ion (r ef . due sout h) slope (r ef ver t ical) ar ea of t im wall (r ef . 10 m2) occupancy (r ef . housing)

- 40%

-25%

-10%
0.70 1.1

0 %

+10%
0.86 0.9 0.95

0.6 50 cm aer at ed br ick w est sout h w est = lat it ude 15 m2 of f ice 5 cm

magnesium br ick

5 m2 housing

The parameters having the largest influence on the performance are the orientation of the wall (e.g. west orientation reduces the productivity by 35% compared to due south), the absorptivity of the absorbing wall surface (e.g. a light red colour reduces the productivity by 35% compared to a black painting). The properties of the transparent cover are also important. In the table below, only transparent insulation material (TIM) is considered and the variation of the properties is limited. In an experimental study, two types of cover were compared : a transparent insulation layer (10 cm thick) and a polycarbonate plate (roughly equivalent to a double glazing). The productivity with TIM was twice the productivity with polycarbonate plate. The following table presents results for various types of transparent covers (the figures are specific to the building and the climate considered and were obtained using the simulation tool COMFIE). type of transparent cover single glazing double glazing low emissivity double glazing or three panes polycarbonate plate 5 cm thick transparent insulation 10 cm thick transparent insulation U (W/m2/K) 5.5 3.3 2 1.5 0.9 productivity (kWh/m2/year) 51 70 82 111 165

The thickness of masonry has a negligible effect : a thicker layer provides a better energy storage but on the other hand, it constitutes a larger thermal resistance to heat transmission. This is why the performance is lower with a 50 cm thickness than with the reference 16 cm. Too thin walls (e.g. 5 cm thickness) do not provide enough energy storage and their performance is also lower. The optimal interval is between 10 and 30 cm. As an example, the following figure presents a sensitivity analysis. The annual heating load of a building is given in terms of the thickness of a masonry wall (made of concrete). The energy consumption figures (obtained by simulation) are of course specific to the building studied and the climate considered, but the trend is general.

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energy c onsump t ion (kWh/ y e ar )

7800 7600 7400 7200 7000 0 + + + + 25 + 50 75

+

+

100 cm t hickness of wall

Dense materials (e.g. magnesium brick) are preferable to light ones (e.g. aerated brick), though concrete is a good economical compromise. Water walls (where the masonry wall is replaced by a water tank) do not have a much higher performance but could be interesting in industrialised construction. A sloped component (the slope being equal to the latitude) would have a higher productivity (+15% in this example) but thermal comfort should be precisely checked. The air gap (4) should be 8 cm thick in order to allow sufficient air circulation by thermosiphon. On the other hand, the air gap between the transparent cover and the masonry wall should be thin enough (e.g. 1 to 2 cm) in order to avoid air circulation (which would increase heat losses). If the solar wall covers a high facade, the thin air layer should be compartimented at each storey (approx. 3 meters high) in order to avoid chimney effect that would cause air circulation (even in a thin layer). As mentioned above, the productivity of one square meter decreases if the area of component increases. The area of component is rather sized by economical and esthetical reasons than by thermal performance objectives. An area of 10 to 20% of the floor area of the heated zone seems sensible. Solar walls complement solar collection through windows. Solar walls can be only oriented from south-east to south-west, because other orientations lead to poor performances. This limits the area of these components. The following table presents example simulation results (productivity and solar fraction) according to the area and the orientation of solar walls (the window area does not vary in all designs). The quantities are specific to the building studied and the climate considered, but the trend is general. If all walls are solar, the solar fraction is very high (63%) but the productivity is poor : the supplementary 24 m2 oriented north increase the solar fraction by only 8% (63 - 55). A good compromise between high solar fraction and productivity is the south-east and south-west orientations : the solar fraction is 44% and the productivity is correct. In the first case (20 m2 due south), the solar fraction is lower (29%) but the productivity is high. orientation and area of solar walls 20 m2 south 10 m2 south 20 m2 south-east and 20 m2 south-west 5 m2 south-east and 5 m2 south-west 20 m2 south, 20 m2 east and 20 m2 west 84 m2 (all walls are solar) solar fraction 29 16 44 14 55 63 productivity (kWh/m2/year 145 160 112 145 92 76

Concerning thermal comfort, experimental measurements showed that in a mid European climate (north-east of France), a 10 cm thick opaque insulation layer is sufficient to prevent from overheating in summer, provided that air circulation is stopped (one louvre should be closed). The temperature difference between the room adjacent to the solar wall and a north oriented room was less than 1K.
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4.3 Architectural integration The example described here concerns the integration of a Trombe wall system in a wooden frame construction. The internal insulation layer is encapsulated between plaster boards. This wall is part of the construction modules, prefabricated in the factory, including the complete envelope (walls and windows…) and even building finishes (wallpaper, carpeting) and various equipment (heating, sanitary,...).

Aurore solar estate in Ardennes - Architect Jacques Michel On the other hand, the masonry wall is constructed on site and wedged in the wooden structure. According to section a/b (see next figure), this wall is built on the clamping concrete. It is braced horizontally by the structure of the high wooden floor, and by the wooden beam of the roof edge. If needed (hot climates), a summer ventilation louvre can be planned on top of the wall. A facade board stands at the basis of the wall and in a medium position (at the level of the high wooden floor). It is insulated, ventilated and is bound to the transparent cover (either a polycarbonate plate or a transparent insulation panel). The windows or glazed doors are situated in the same plan as the transparent cover of the solar walls, avoiding thermal bridges. Watertight dilatation joints are used. The internal air gap is 8 cm thick. The encapsulated insulation is mounted on the wooden structure, and includes two louvres made of slatted aluminium (manual operation). A side framework closes the air gap. It is insulated along the windows. The air gaps of the lower and upper solar walls are separated in order to avoid chimney effects.

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The next figure gives details of the basis and foundation of the solar wall. The masonry wall is built on the clamping concrete, on which lays the lower floor. The insulated facade element stands on the external side. The transparent cover is mounted on the upper frame of this element.

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The internal insulation layer stands on the wooden frame of the floor. It is part of the modules, prefabricated in a factory. These modules, 3m80 wide and 8m15 long, are assembled on site to form the ground floor. The upper floor is constructed on site by assembling prefabricated pannels. This system is fast, and economical if several houses of the same type are built. It provides a good quality concerning the reduction of thermal bridges, air and water tightness of facades and roof. Global rigidity is obtained by the balloon frame forming the envelope (external walls, floors and roof). This envelope is insulated and ventilated using a vapour barrier avoiding condensation (see details on next figure). The external skin is made of navy plywood with an external building finish and internal plaster board. The floor can be covered with carpeting or glued tiling. The vertical section (next figure) shows details of the gable, with the thermal insulation of walls (10 cm of fiber glass) and upper ceiling (20 cm). The external building finish is made of varnished wood in the upper part and navy plywood on the lower part.

4.4 Directions of use
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The functioning of this type of wall is very simple. One louvre should be closed in summer to stop air circulation, and both louvres should be open during the heating season. In very cold regions, it might be necessary to close one louvre at night and during cloudy days if the transparent cover is not thick enough to maintain a mild temperature in the air gap. Despite of this simplicity, a one page advice and explanation text with a clear scheme of the component should be distributed to occupants. 4.5 Example realisations - Aurore solar estate in Mouzon, passive houses - Architect Jacques Michel Description Four houses were already built some years ago on the site by Jacques Michel (1984), with a traditional masonry construction. This solar housing estate called "Aurore", situated in Mouzon, acts as a regional demonstration project for solar heating. The owner of the present project is the social housing company ESPACE HABITAT in Charleville Mézières. The manufacturer is HOUOT (wooden frame buildings). Four houses are equipped with passive systems, two with active air collectors. Transparent insulation and polycarbonate plates were used each in three houses (two passive and one active). The transparent insulation being considered is produced by the German firm OKALUX, after the research works of the Fraunhofer-Institute Freiburg by A. Goetzberger et al. (1984 and 1992) and Leslie Jesh (1986). It consists of a capillary structure where the capillaries are mounted perpendicular to an absorber surface, and encapsulated between two glazing. This advanced technology will be compared to less expensive transparent covers produced by the French firm CELAIR. These are extruded polycarbonate plates having the form of a triple glazing with connection joints. The corresponding physical properties are given in Table below. The solar transmission values were measured by CSTB on the OPTORA test facility as explained by Soler and Chevalier (1993). Both materials improve the efficiency of Trombe walls, covered until now with simple or double glazing. Physical values for the two types of transparent covers considered material 10 cm encapsulated capillary structure 16 mm polycarbonate plate heat loss factor W.m-2.K-1 0.8 2.4 transmission of solar radiation (diffuse-hemispherical) 0.67 0.64

The passive system is made of "Trombe-Michel" walls (Fig. below) which are mounted on the south façade of the houses. They comprise 4 layers : a transparent cover, a masonry wall, an air gap and an opaque insulation. The external transparent cover transmits solar radiation in but holds back heat. The brick wall is 11 cm thick and painted black at its outer surface to act as an absorber. It stores heat from the day and releases it with a time delay to the air layer between this wall and the opaque insulation. The air is heated in contact with the brick wall, rises and circulates towards the room (the louvers are open in winter during good weather periods).

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Passive system, Trombe wall

In summer, the louvers are closed by the inhabitants. The internal opaque insulation layer prevents from overheating, as the air circulation is stopped when the louvers are closed. This is an alternative to sophisticated shading devices. All houses are wooden framed, with a good insulation (20 cm of glass wool in the loft and 10 cm in the walls). The cost of a reference house is 67000 $ (for two 50 m2 storeys). The solar overcost ranges from 5,000$ (CELAIR cover) to 12,000 $ (OKALUX components), see table below. The houses are constituted by modules, produced in a fabric and transported by truck : the wallpaper, sanitary equipment, convectors, etc... are already in when the modules arrive on the building site. Cost for each system in $ 1992 (*) configuration (2 x 50 m2 living area) Trombe wall, polycarbonate Trombe wall, 10 cm OKALUX solar over cost 5,000 12,000 total cost 72,000 79,000 cost per m2 of living area 720 790

(*) 1$ 1992 was 6 FF and around 0.9 Euro The social housing company ESPACE HABITAT has been supported by ADEME which financed 50% of the over cost. But the cheapest solar houses, even without financial support, can be built within the cost limit corresponding to social housing, they cost 720$ per m2 of living area (see table above).

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First predictive simulation results The simulation tool COMFIE developed by Peuportier and Blanc Sommereux (1988) has been used during the design in order to compare various possibilities during the design : - transparent cover (single or double glazing, polycarbonate plate, 5 or 10 cm capillaries); - thickness of masonry wall (concrete, brickwork of 5, 11, 16 cm,...); - wall colour (light or dark brick, black); - control of the active system, air flow-rate. Simulations were performed over a heating season, the climate considered is the Short Reference Year (Lund, 1985) of Nancy (SRY) : 8 typical weeks, 2 per season. According to these predictive simulation results, the annual heating consumption is reduced and the solar fraction can reach 30 to 45% according to the system (cf table below). This avoids to reject 2 tons of CO2 per year and house (electric heating with fuel or coal power plant during the peak hours). The performance of the passive system is partly reduced because the radiative heat transfer is stopped by the internal double wall, but the advantage is that overheating can be avoided without shading device. Also, the very low inertia of the houses reduces the utilisability of solar gains. Masonry walls, particularly in the active system, could improve the thermal performance, concerning both the heating season (increased utilisability of solar gains) and the summer (damping of overheating). But thermal inertia is difficult to achieve with industrial wooden construction. Comparative simulation results for the different systems configuration (2 x 50 m2 living area) Trombe wall, polycarbonate Trombe wall, 10 cm OKALUX heating load (kWh/a) 7,700 7,000 load per m2 (kWh/m2/a ) 64 58 solar fraction 31 35

The simulation showed also a risk of discomfort in summer. But this risk is due to the very low inertia of these wooden houses and not to the solar heating itself. Inhabitants can solve this problem by a proper management of venetian blinds and night ventilation. Monitoring The experimental follow-up allows to check the performance predicted and to study the acceptability and management of the systems by inhabitants. The data acquisition was connected to the phone, and the measurements were accessible by Ecole des Mines and the Technological University Institute of Longwy. CSTB provided a technical assistance. The monitoring system has been working from the end of May 1992 until the beginning of June 1993. The collected data has been used for an evaluation of the summer comfort. We also studied a week in autumn and one during winter. Finally, a global performance assessment was obtained by the analysis of the whole heating season. Summer Results The comfort temperature limit is often reached during normal hot periods (30°C outside, 27°C to 28°C inside). This is due to the very light construction (wooden walls, wooden floors), and not to the solar systems: the temperature difference between the north and south rooms is lower than 1 K. The comfort can not longer be guaranteed during exceptional days (with outside temperatures above 30°C). On the other hand, the advantage of this very low inertia is that the houses can be quickly refreshed after sun-set. Attention was paid to the highest absorber temperature occurring in the solar systems to be sure that the limit admissible for the polycarbonate (130°C) will not be reached. The measurements showed the highest absorber temperature to be 80°C in solar walls, and the polycarbonate temperature is lower. The polycarbonate materials are guaranteed by manufacturers for 10 years under proper Education of Architects in Solar Energy and Environment page 29
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conditions. Previous projects using such materials as transparent cover have shown a satisfying durability, and the quality of products is improving, particularly concerning U.V. protection. Previous experience with Trombe walls shows that the brick wall can also stand the temperature fluctuations without damage. Autumn Results We studied in detail the period from October 25th to November 1st 1992 (Fig. below). First, there were three cloudy days, followed by 4 sunny days and one overcast day at the end. Therefore, this period is a good representation of the possible weather situations during mid-season. The ambient outside temperatures at the beginning of the period ranged from 5°C (night minimum) to 12°C (day maximum). They then dropped to -2°C and +3°C during the last day. Autumn climate conditions in Mouzon, 25/10/92 - 01/11/92

The temperature at the absorbing surface of the collector can reach 47°C and even 64°C with transparent insulation components (Fig. below). The heat is stored in the brickwork and therefore the temperature in the air gap remains temperate, between 15°C and 25°C. Energy savings are thus achieved, as the heating losses are reduced. When the temperature is higher in the Trombe wall than in the room, a natural air circulation is established (if the louvers are open). When the louvers are closed, the maximum temperature in the inner air gap of the Trombe wall was 45°C in summer, and the opaque insulation protected the dwelling from a high heat flux.

Temperatures in the Trombe wall of the passive system, 25/10/92 - 01/11/92

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Results during the coldest period We analysed the week from 31 December 31 to 7 January 1993. The first four days of this period were sunny and the temperature dropped to -12°C. The next two days were unsettled and the last two were very cloudy but with increased temperatures (between 5°C and 7°C). The temperature between the brick wall and the transparent cover (at the level of the black painted surface) reaches at the same period 55°C using the capillary structure and 40°C using the polycarbonate plate. An example heating load profile is given in Figure below. Reduction of the heating load during a cold but sunny winter day (02/01/1993)

Comparison between measurements and predictions The heating loads, measured by electricity meters, have been integrated over a whole heating season. They are compared in table below with the predictions obtained by simulation.

Measured and predicted energy consumption on a heating season

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house

measured consumption (kWh) 3,560 (1) 8,218 7,037 5,051

predicted actual mean actual degree consumption (19°C) temperature hours 7,000 7,700 7,000 7,700 16.32 18.97 18.75 16.74 52,496 65,787 65,255 55,107

3 (passive, TIM) 3 bis (passive, PC) 7 (passive, TIM) 9 (passive, PC)

(1) The mechanical ventilation was stopped in this house, it would add a 3,500 kWh heating load. The discrepancies between measures and predictions are mainly caused by the occupants behaviour. First the simulations were performed assuming a constant 19°C thermostat set point and a SRY (Short Reference Year, see § 2.2) for Nancy (66,540 degree hours). The actual climate was slightly different (65,880 degree hours). In the reality, some people stop the heating while they are working during the day and lower the set point at night and in the bedrooms. This is shown by the actual degree hours column. Some occupants have stopped the mechanical ventilation, which would have provided a constant 0.6 ach air renewal. The corresponding load is 3,500 kWh per year (for a constant 19°C inside temperature). The management of shading devices or Trombe wall louvres as well as the internal gains for lighting etc. may also explain certain discrepancies. Finally, the physical assumptions of the model and the uncertainty on parameters produce supplementary errors. In order to compare the different systems, it is necessary to eliminate the effect of different users behaviour. Simulation allows to perform such an analysis, provided that some parameters (e.g. thermal bridges) are identified or corrected. The methodology adopted is the following. We decided to study in detail the coldest week, during which it is assumed that the occupants do not open windows or doors for a long time, air being renewed by the mechanical system. Thermal bridges through collector frames were identified during the cloudy period of this week, when solar gains are negligible. The product of the transmission factors of transparent covers by the absorption factor of the absorber was slightly modified in order to meet the measured temperature profile during sunny days. There remains a temperature difference during clear nights due to insufficient modelling of radiation towards the sky. This parameter identification was then checked during another week, in mid-season. The agreement was rather good, except for one day during which we suppose that condensation was particularly important. Using this corrected model, we simulated all systems using the same occupancy pattern as in the predictive calculation : a constant 19°C set point temperature, a constant 0.6 ach air renewal and constant 400 W internal gains. The heating loads obtained on a typical year (SRY) were quite similar to the predicted values. Measured heating consumption was also corrected in terms of the measured degree days, assuming a linear dependence. Also, the load corresponding to mechanical ventilation was added in the houses where occupants had stopped this system. Comparative results are presented in fig. below. According to the social housing company, the mean heating load in the region for such detached houses is typically 10,000 to 11,000 kWh.

Comparison between measured and calculated heating consumption after correction

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Both calculations and measurements give an advantage to the active systems (houses 4 and 5), for which the solar fraction reaches 40%. But these systems need a maintenance twice a year. In may, the collectors are to be naturally ventilated from outside and the inside air circulation must be stopped. In october, the system must be set in winter position, and the control system must be checked. On the contrary, passive systems need no maintenance. The thermal performance could be improved by increasing the inertia of the houses, but the wooden frame concept would then have to be replaced by a concrete masonry. This could also reduce the overheating problems, which are not due to the solar systems but to the low inertia of the houses. On the other hand, wood is a renewable material which corresponds to the sustainable development approach. The present industrial process allows to produce low cost solar houses with a good thermal performance : in the rather unpromising climate considered, the annual heating load ranges from 45 to 65 kWh.a-1.m-2 living area. This project has shown that solar energy is also accessible to low income families. New technologies allow to achieve high solar fractions (30 to 45%) within the cost limit of social housing.

5 Curtain wall solutions
5.1 Main interesting general characteristics of curtain walls The curtain wall solutions are more often used in offices and commercial buildings for their aspects and the quickness of installation, facilitated by the repetitive arrangement of the plans (quickly the building is out of air and water). 1Curtain walls components are prefabricated in factories or workshop and are systematic « mecano » construction. The components are more often modular and of exact size. The weight is between 30 and 50 kg/m2 - the third than a masonry wall. 23The aesthetic aspects are due to the quality of materials (brightness, reflection) and the variety of material such as glass, aluminium, steel and many other new components. As a curtain wall solution is built of thin components (5 to 10 centimeters deep), the surface saving compared with a standard wall is about 20 to 25 centimeters on the whole perimeter. That means about 8 square meter for thousand square meter (the price of a heating system or of « a new car » !). In fact, the use of thin walls and high efficient insulation materials, save a useful living space which can be selled by the property developer.

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4-

Curtain walls solutions are sometimes, or can be a part of HVAC7 system (heating, air filter, solar panel,...). Curtain walls, used more often in offices building or other commercial buildings are associated with light thermal mass (even concrete structure) because partition are light, floors are covered of fitted carpet and or a double foor which is used as technical space for fluid distribution. This situation (light inertia), and the occupation mode (intermittent use) require a heating system (and cooling if necessary) which have the following characteristics : - light inertia, that means quick response, - regulation by zone (a zone can be 1 office until a complete floor....) - high efficient control (centralised by meteorological data and individual adjustment).

5.2 Thermal Insulation - Summer Comfort 5.2.1 Thermal Insulation As curtain walls solution are made of light inertia components, the building shell has to be a high efficient thermal filter : - Neither allow the heat to get out - Neither allow the solar gains to get in. • The glazing surfaces have to be high insulated as much as possible, that means : - For north orientation : low emissivity, double to triple glazing, and for the future : components as « Aerogels ». - For East / west and south facing walls : double glazing. • The opaque parts of the components or modules have also to be of efficient insulation. These high efficient insulation materials have a role as well for summer and winter conditions : a barrier for heat. Super insulation materials are available on the building market which for the same deepness have a double efficiency. Unfortunately some of these products were made with a CFC process. That means that we have to take care of the insulation material choice, mineral wool, or any other row material. • As the main structure of the building is most often a steel frame, attention should be paid to thermal bridges. Technical details of components installing and linkage have to be carefully studied to ensure the insulation continuity. As steel is a perfect heat conductive, no steel part must cross the component (from outside to inside). • As curtain wall solutions are made of light materials, there is no thermal mass. It is an additional reason to take a great care of the high efficiency of the global air - water tightness of the façade, to have a completely controlled internal climate.

5.2.2 Curtain walls and Summer comfort At the opposite of the conventional and traditional buildings, curtain wall solutions have to be treated with another « philosophy », as described above the building shell must keep the solar gains to enter the rooms when temperature raise a fixed level. Heat can be evacuated to the top (through an efficient solar shading). - For South orientation : double glazing and solar protection - Preferably it would be external shading but as Curtain walls and structural glazing need, of an Architectural point of view, a total bright and smooth facade, internal blinds can be used. - For East and West orientations : any efficient glazing can be used associated with high efficient solar protection especially for west facing facades. Reflective surfaces can be a solution also but this sort of materials cut out solar winter gains. Solar protections as venetian blinds can have a reflective surface. The aim of the shading system would be also to provide a sufficient day-lighting.
7 Heating, Ventilation, Air conditionning system

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• In the future (for economical reasons), electrochronic glazing will be an interesting solution with the possibility to vary the glass properties depending of the climatic conditions. • In any case, the objective of the design would be to avoid the use of an electric cooling system. All design efforts would be to minimise cooling needs. • If components are thick, the position of the glazing frame is important for summer comfort : the shading made by structural components to the glazing can reduce solar gains up to 20 % depending of the thickness. 5.3 Solar wall for industrial or commercial buildings • The canadian « Solarwall », a curtain wall for air solar preheating solution, is an evolution of the Trombe wall system. The solar wall is preferably south facing (south west to south east). These walls or part of walls are used as solar collectors to preheat fresh air for ventilation. In Ontario (Canada, The Ford factory), a facade of 1872 square meter has a thermal output of 1,6 GJ/m2/year. The calculated payback period was 4 years. In fact, it was less than two years. Another company « Bombardier de Valcourt » with a 1100 m2 Solar wall saved 33 000 Canadian $. After this success, Ford company decided to install solar walls on the other factories in USA. More than 32,000 m2 of solar wall are installed in North American, the largest is in Chicago with 5,575 m2. There are also some solar walls in Europe (Germany, Italy...). This cheap technique of solar walls can be an interesting product to retrofit Eastern Europe commercial and industrial buildings. • The solar wall system : It is a simple system using directly solar irradiation for ventilation preheating. Hot air is efficiently and uniformly distributed using one or several fans and distribution pipes. The solar collection and air heating is obtained with the perforated metallic wall. The air passes through the wall, absorb the heat on the external surface and hot air is taken out from a cornice box and distributed into the building by perforated pipes. The ventilation unit is equipped with thermostat to control new air and back mixing. In summer, hot air is evacuated through the cornice by mechanical valves. With such a solar wall system the temperature difference between floor and ceiling is only about 2°C due to air mixing. The wall, for a better exchange, is made with a dark micro-perforated sheet of Aluminium or iron. The aesthetic of the building can be increased with this materials. The efficiency of the system is more than 70 % and the solar collection output reach 50 % The solar wall system can also be used in retrofitting buildings. Micro perforated metal sheet of the Solarwall system

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1 - Solar wall 2- Air feeded cavity 3 - In draught air 4 - Fan 5 - Hot air production (Source Systèmes solaires )

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References [1] A. Goetzberger, J. Schmid and V. Wittwer, Transparent insulation system for passive solar utilisation in buildings, 1st E.C. Conference on solar heating, Amsterdam, 1984. [2] A. Goetzberger, Special issue on transparent insulation, Solar Energy vol. 49 number 5, 1992. H. Lund, Short Reference Years and Test Reference Years for EEC countries, EEC contract ESF029-DK, 1985. [3] L. Jesh, TI1...TI6 "Transparent Insulation Workshop", Birmingham and Freiburg, 1986...1993. [4] J. Michel, Patent ANVAR TROMBE MICHEL BF 7123778 (France 29/06/1971) and addition: Patent "Stockage thermique" MICHEL DIAMANT DURAFOUR 75-106-13, Paris, 1971. [5] B. Peuportier and I. Blanc Sommereux,Simulation tool with its expert interface for the thermal design of multizone buildings, Int. Journal of Solar Energy, 8/1990 (received in 1988). [6] S. Soler, M. Gery and J.L. Chevalier, Theoretical and experimental study of the behaviour of multi-wall ribbed materials under solar radiation, Transparent Insulation Workshop TI 6, Birmingham, 1993.

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