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A kinetic wall for winter space heating
Energy and Buildings, 4 ( 1 9 8 2 ) 191 - 194
191
A Kinetic Wall for Winter Space Heating * DAVID FAIMAN
Applied Solar Calculations Unit, The Blaustein Institute for Desert Research, Ben-Gurion University of the Negev, Sede Boqer Campus (Israel)
A passive solar heating device is described which operates on the principle o f the Trombe Wall but which overcomes the main performance problems associated with the latter. In addition to the virtues o f the Trombe Wall, our structure exhibits the following useful features: (1) Direct radiant heating begins soon after sun-rise; (2) lighting and view are continuously adjustable as desired up to about 15% of the glazing area; (3) unwanted summer radiation is reflected away from the southern wall o f the house; (4) night insulation is automatically incorporated; (5) the problem o f where to store night insulation during the day is most elegantly solved; (6) various construction materials are possible including water or phase-change substances; (7) the wall has an aesthetic degree o f freedom useful for interior design considerations.
measures are taken to prevent such undesirable cooling. The same r o o f in summer will similarly create undesirable heating during the day. Fortunately, however, there is a time delay associated with these conflicting requirements, i.e., day/night or summer/winter. Thus, if the passive heating element is ingeniously enough designed it can be made to be successively a good absorber and then a p o o r radiator at the required times. One of the most successful passive heating elements is the Trombe Wall (Fig. 1). This consists of a massive storage wall, glazed so as to enable it to act as a solar collector. It has three great virtues that contribute to its success relative to competing devices.
1. I N T R O D U C T I O N
The fundamental practical problem associated with passive solar design is that the same living unit has to be alternately heated by, and protected from, radiation. Such a requirement seems to violate the basic law of physics that a good absorber is a good radiator. For example, a highly absorbing r o o f element will heat a building during a winter day b u t cool it o f f rapidly at night unless special * I n v i t e d talk a t the International S y m p o s i u m o n t h e I m p a c t o f C l i m a t e o n Planning and Building, Herzlia, Israel, N o v e m b e r 4 - 7, 1 9 8 0 . 0378-7788/82/0000-0000/$02.75
Py Fig. 1. S c h e m a t i c p l a n view o f classic T r o m b e Wall. Solar r a d i a t i o n passes t h r o u g h glazing and is a b s o r b e d in d a r k c o l o u r e d massive wall. H e a t i n g o f i n t e r i o r o c c u r s a f t e r a t i m e delay governed by thickness o f t h e wall. A l t e r n a t i v e l y , h o t air between glazing and wall ,, can be a d m i t t e d t o t h e i n t e r i o r via a vent. © Elsevier Sequoia/Printed in t h e N e t h e r l a n d s
192 First, it is part of the building structure itself: thus it is inherently a low-cost device. Secondly, being a wall heater it does n o t limit the height of the building to that of a single story: Trombe elements can be designed into all the storeys of a high-rise building. Thirdly, by skillfully optimizing the wall thickness and the a m o u n t of internal venting allowed, the wall automatically acts as a convective heater by day and as a radiant heater by night without entailing the expense of heat storage and retrieval mechanisms. In addition to the three great virtues just mentioned, the Trombe Wall principle is fairly adaptable to different building techniques and climates. For example, the wall itself can be made from a variety of materials including water, moderately sized windows can be included to allow for natural lighting, and removable insulated panels may be incorporated to cut down on heat losses during winter evenings and to prevent over-heating during the summer. There is, however, usually a price to pay in terms of inconvenience or thermal efficiency if any of the above types of modifications are made to Trombe's original design. The purpose of m y talk t o d a y is to discuss a novel way of adapting the Trombe Wall concept to a wide variety of climatic conditions, but in a manner that hopefully produces far more advantages than disadvantages. I call it the "Rotating Prism Wall".
I!I
I!I k-y~4
Fig. 2. Schematic plan view of rotating prism wall. Trombe's continuous massive storage .wall has been replaced by coupled rotatable columns. One face of each column is insulated, one is dark coloured for high absorption and the third face serves a purely decorative purpose.
Regarding the a m o u n t of storage mass per unit area of glazing, simple geometry indicates that triangular columns with width: W -
-
-
3 would contain the same storage mass as a continuous wall of thickness t. The air gap needed between wall and glazing in order to allow the columns to rotate is then 2
2. THE ROTATING PRISM WALL
g=--t 3
My idea is to replace the monolithic heat storage part of the Trombe Wall by a set of rotatable segments. In this way the absorptive properties of the wall may be changed at will. In my original paper on the device I discussed, for simplicity, a wall of coupled rotating columns, each one having an equilateral triangular cross-section {Fig. 2). One face of each column is painted black for winter, daytime absorption. A second face is insulated and finished with a reflective coating. This serves the dual purpose of reducing heat losses on winter evenings and unwanted heat gains on summer days. The third face has no thermal function and may be used for decorative purposes.
Numerically, a 30 cm thick continuous concrete Trombe Wall could be replaced by 70 cm wide triangular columns with a 20 cm air gap between wall (i.e., columns in their fully closed orientation) and glazing. Alternatively, a 15 cm thick water wall would be replaced by triangular " t a n k s " of width 35 cm with a 10 cm wide air gap.
3. A D V A N T A G E S
OF ROTATING
PRISMS
One can enumerate a fairly large number of immediately apparent advantages exhibited by our kinetic wall relative to its static mono-
193 lithic ancestor. Chief among these are the following: (i) Thermal insulation is automatically rotated into place or out of the way, at will. In m a n y passively heated buildings, finding space to store the night insulation is usually a major problem. Moreover, extensive structural alterations are usually necessary in areas such as deserts which have h o t summers. {ii) Because of the varying thickness of the resulting wall, venting for day-time heating is unnecessary. In particular, small strips of wall, being infinitesimally thick, heat up very quickly and begin to radiate into the room. As the day proceeds these hot strips broaden and at the same time the columns fill with thermal energy. When no further useful solar radiation is available the hot surfaces of the columns are rotated inwards. The resulting larger temperature difference between the room air and the h o t inner surface of the wall then combines with the exterior insulation to produce a highly effective and efficient space heater. (iii) The rotating prism wall also exhibits a couple of aesthetic virtues. On the one hand, the wall can be opened up at will in order to allow some natural lighting into the room either in winter while the exterior surface is being used to absorb radiation or in summer when the wall acts as a reflector. In particular, on a summer evening before sun-set it is possible to adjust the slits of light to their maximum width in order to admit exterior view to the extent of 13% of the glazed area. On the other hand, the third surface (in the case of triangular columns) serves no thermal purpose and always faces inwards. It may be used for interior decoration purposes. -
-
4. OTHER GEOMETRIES In the interests of clarity I have thus far restricted m y discussion to columns having triangular cross-sections. Clearly this is an unnecessary restriction and, indeed, rotating prisms of other geometries also offer advantages worthy of consideration. The following are three examples o f interest.
(a) Shaped triangles At Sede Boqer we are incorporating a rotating prism wall into an experimental adobe
house, hence, not unnaturally, the columns will be made of adobe. We found t h a t equilateral triangles have two undesirable features. First, the stick-on insulation also has to be infinitesimally thin at the line where adjacent columns meet. This produces an undesirable heat bridge. Secondly, the finite thickness of the stick-on insulation {typically 5 cm) means that the centre-of-gravity of each column does not lie on its axis of rotation. This leads to unnecessary mechanical stress on the bearings. One can, however, by skilfully shaping one vertex of the triangle, arrive at a geometry which allows stick-on insulation of constant thickness and which restores the centre of gravity of the columns to their respective rotation axes.
(b ) Fixed-axis triangles For high-rise buildings it is desirable that the columns themselves be load-bearing. Otherwise, unnecessarily large expense must go into designing a strong enough frame for the kinetic wall. One way to achieve this is to have the centre of each column non-rotatable: the axle's dual function being to hold up the building and to act as thermal mass. Figure 3 shows schematically what we have in mind.
A .
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.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
•
f
•
•
Fig. 3. Rotating prism wall with fixed (load-bearing) axes B. Face A is insulated, face C is absorbing and the remaining face is decorative. " B " is a fixed load-bearing column that contains much of the thermal mass. " A " is an insulated section of a rotating sleeve. " C " is a heat-absorbing section of the sleeve.
(c) Variation on a theme Once again, the above example has been given a triangular finish for purposes of clarity
Fig. 4. Example of another geometry, in which only the insulation A rotates. B is a fixed, circular crosssection column with absorptive surface C.
194 rather than for practicability. A circular finish, shown schematically in Fig. 4, is probably more desirable. Here the columns " B " are fixed and finished with an absorbing/radiating surface, and semi-circular insulated sleeves " A " rotate from one side to the other. For this geometry, columns of diameter 47 cm separated by 10 cm spaces would provide thermal storage equivalent to a continuous 30
cm thick wall. The columns would be 5 cm from the glazing at their point of closest approach. At the risk of ending on a flippant note, let me invite you all to while away some dull m o m e n t by designing a rotating prism wall of your own favorite geometry! Or did you already do so while I was talking?
191
A Kinetic Wall for Winter Space Heating * DAVID FAIMAN
Applied Solar Calculations Unit, The Blaustein Institute for Desert Research, Ben-Gurion University of the Negev, Sede Boqer Campus (Israel)
A passive solar heating device is described which operates on the principle o f the Trombe Wall but which overcomes the main performance problems associated with the latter. In addition to the virtues o f the Trombe Wall, our structure exhibits the following useful features: (1) Direct radiant heating begins soon after sun-rise; (2) lighting and view are continuously adjustable as desired up to about 15% of the glazing area; (3) unwanted summer radiation is reflected away from the southern wall o f the house; (4) night insulation is automatically incorporated; (5) the problem o f where to store night insulation during the day is most elegantly solved; (6) various construction materials are possible including water or phase-change substances; (7) the wall has an aesthetic degree o f freedom useful for interior design considerations.
measures are taken to prevent such undesirable cooling. The same r o o f in summer will similarly create undesirable heating during the day. Fortunately, however, there is a time delay associated with these conflicting requirements, i.e., day/night or summer/winter. Thus, if the passive heating element is ingeniously enough designed it can be made to be successively a good absorber and then a p o o r radiator at the required times. One of the most successful passive heating elements is the Trombe Wall (Fig. 1). This consists of a massive storage wall, glazed so as to enable it to act as a solar collector. It has three great virtues that contribute to its success relative to competing devices.
1. I N T R O D U C T I O N
The fundamental practical problem associated with passive solar design is that the same living unit has to be alternately heated by, and protected from, radiation. Such a requirement seems to violate the basic law of physics that a good absorber is a good radiator. For example, a highly absorbing r o o f element will heat a building during a winter day b u t cool it o f f rapidly at night unless special * I n v i t e d talk a t the International S y m p o s i u m o n t h e I m p a c t o f C l i m a t e o n Planning and Building, Herzlia, Israel, N o v e m b e r 4 - 7, 1 9 8 0 . 0378-7788/82/0000-0000/$02.75
Py Fig. 1. S c h e m a t i c p l a n view o f classic T r o m b e Wall. Solar r a d i a t i o n passes t h r o u g h glazing and is a b s o r b e d in d a r k c o l o u r e d massive wall. H e a t i n g o f i n t e r i o r o c c u r s a f t e r a t i m e delay governed by thickness o f t h e wall. A l t e r n a t i v e l y , h o t air between glazing and wall ,, can be a d m i t t e d t o t h e i n t e r i o r via a vent. © Elsevier Sequoia/Printed in t h e N e t h e r l a n d s
192 First, it is part of the building structure itself: thus it is inherently a low-cost device. Secondly, being a wall heater it does n o t limit the height of the building to that of a single story: Trombe elements can be designed into all the storeys of a high-rise building. Thirdly, by skillfully optimizing the wall thickness and the a m o u n t of internal venting allowed, the wall automatically acts as a convective heater by day and as a radiant heater by night without entailing the expense of heat storage and retrieval mechanisms. In addition to the three great virtues just mentioned, the Trombe Wall principle is fairly adaptable to different building techniques and climates. For example, the wall itself can be made from a variety of materials including water, moderately sized windows can be included to allow for natural lighting, and removable insulated panels may be incorporated to cut down on heat losses during winter evenings and to prevent over-heating during the summer. There is, however, usually a price to pay in terms of inconvenience or thermal efficiency if any of the above types of modifications are made to Trombe's original design. The purpose of m y talk t o d a y is to discuss a novel way of adapting the Trombe Wall concept to a wide variety of climatic conditions, but in a manner that hopefully produces far more advantages than disadvantages. I call it the "Rotating Prism Wall".
I!I
I!I k-y~4
Fig. 2. Schematic plan view of rotating prism wall. Trombe's continuous massive storage .wall has been replaced by coupled rotatable columns. One face of each column is insulated, one is dark coloured for high absorption and the third face serves a purely decorative purpose.
Regarding the a m o u n t of storage mass per unit area of glazing, simple geometry indicates that triangular columns with width: W -
-
-
3 would contain the same storage mass as a continuous wall of thickness t. The air gap needed between wall and glazing in order to allow the columns to rotate is then 2
2. THE ROTATING PRISM WALL
g=--t 3
My idea is to replace the monolithic heat storage part of the Trombe Wall by a set of rotatable segments. In this way the absorptive properties of the wall may be changed at will. In my original paper on the device I discussed, for simplicity, a wall of coupled rotating columns, each one having an equilateral triangular cross-section {Fig. 2). One face of each column is painted black for winter, daytime absorption. A second face is insulated and finished with a reflective coating. This serves the dual purpose of reducing heat losses on winter evenings and unwanted heat gains on summer days. The third face has no thermal function and may be used for decorative purposes.
Numerically, a 30 cm thick continuous concrete Trombe Wall could be replaced by 70 cm wide triangular columns with a 20 cm air gap between wall (i.e., columns in their fully closed orientation) and glazing. Alternatively, a 15 cm thick water wall would be replaced by triangular " t a n k s " of width 35 cm with a 10 cm wide air gap.
3. A D V A N T A G E S
OF ROTATING
PRISMS
One can enumerate a fairly large number of immediately apparent advantages exhibited by our kinetic wall relative to its static mono-
193 lithic ancestor. Chief among these are the following: (i) Thermal insulation is automatically rotated into place or out of the way, at will. In m a n y passively heated buildings, finding space to store the night insulation is usually a major problem. Moreover, extensive structural alterations are usually necessary in areas such as deserts which have h o t summers. {ii) Because of the varying thickness of the resulting wall, venting for day-time heating is unnecessary. In particular, small strips of wall, being infinitesimally thick, heat up very quickly and begin to radiate into the room. As the day proceeds these hot strips broaden and at the same time the columns fill with thermal energy. When no further useful solar radiation is available the hot surfaces of the columns are rotated inwards. The resulting larger temperature difference between the room air and the h o t inner surface of the wall then combines with the exterior insulation to produce a highly effective and efficient space heater. (iii) The rotating prism wall also exhibits a couple of aesthetic virtues. On the one hand, the wall can be opened up at will in order to allow some natural lighting into the room either in winter while the exterior surface is being used to absorb radiation or in summer when the wall acts as a reflector. In particular, on a summer evening before sun-set it is possible to adjust the slits of light to their maximum width in order to admit exterior view to the extent of 13% of the glazed area. On the other hand, the third surface (in the case of triangular columns) serves no thermal purpose and always faces inwards. It may be used for interior decoration purposes. -
-
4. OTHER GEOMETRIES In the interests of clarity I have thus far restricted m y discussion to columns having triangular cross-sections. Clearly this is an unnecessary restriction and, indeed, rotating prisms of other geometries also offer advantages worthy of consideration. The following are three examples o f interest.
(a) Shaped triangles At Sede Boqer we are incorporating a rotating prism wall into an experimental adobe
house, hence, not unnaturally, the columns will be made of adobe. We found t h a t equilateral triangles have two undesirable features. First, the stick-on insulation also has to be infinitesimally thin at the line where adjacent columns meet. This produces an undesirable heat bridge. Secondly, the finite thickness of the stick-on insulation {typically 5 cm) means that the centre-of-gravity of each column does not lie on its axis of rotation. This leads to unnecessary mechanical stress on the bearings. One can, however, by skilfully shaping one vertex of the triangle, arrive at a geometry which allows stick-on insulation of constant thickness and which restores the centre of gravity of the columns to their respective rotation axes.
(b ) Fixed-axis triangles For high-rise buildings it is desirable that the columns themselves be load-bearing. Otherwise, unnecessarily large expense must go into designing a strong enough frame for the kinetic wall. One way to achieve this is to have the centre of each column non-rotatable: the axle's dual function being to hold up the building and to act as thermal mass. Figure 3 shows schematically what we have in mind.
A .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
•
f
•
•
Fig. 3. Rotating prism wall with fixed (load-bearing) axes B. Face A is insulated, face C is absorbing and the remaining face is decorative. " B " is a fixed load-bearing column that contains much of the thermal mass. " A " is an insulated section of a rotating sleeve. " C " is a heat-absorbing section of the sleeve.
(c) Variation on a theme Once again, the above example has been given a triangular finish for purposes of clarity
Fig. 4. Example of another geometry, in which only the insulation A rotates. B is a fixed, circular crosssection column with absorptive surface C.
194 rather than for practicability. A circular finish, shown schematically in Fig. 4, is probably more desirable. Here the columns " B " are fixed and finished with an absorbing/radiating surface, and semi-circular insulated sleeves " A " rotate from one side to the other. For this geometry, columns of diameter 47 cm separated by 10 cm spaces would provide thermal storage equivalent to a continuous 30
cm thick wall. The columns would be 5 cm from the glazing at their point of closest approach. At the risk of ending on a flippant note, let me invite you all to while away some dull m o m e n t by designing a rotating prism wall of your own favorite geometry! Or did you already do so while I was talking?