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Rotating prism wall as a passive heating element
Solar Energy Wol. 25, pp. 563-564 © Pergamon Press Ltd., 1980. Printed in Great Britain
0038-092X/80/1201-0563/S02.00/0
TECHNICAL
NOTE
Rotating prism wall as a passive heating elementt DAVID FAIMAN Ben-Gurion University of the Negev, The Institute for Desert Research, Sede Boqer and Department of Physics, Beersheva, Israel
(Received 31 January 1980; accepted 27 May 1980)
wall formed by the columns is capable of presenting surfaces of differing thermal properties in any desired direction. In order to make clear our ideas let us consider the case where the columns are right prisms on an equilateral triangular base. This example is possibly not the easiest to effect in practice but illustrates clearly all the salient ingredients. Each of the three faces of the columns is treated with a different finish. One face is highly absorbing: black for example. The second face is highly reflecting: perhaps being covered with aluminium foil. The finish of the third face is optional and can be chosen to suit ones personal sense of aesthetics. A plan view of four such neighbouring columns is sketched in Fig. 1.
1. I N T R O D U C T I O N
The so called "passive" approach to space heating aims at using the building itself as a solar collector and distribution system. This holds out a dual promise. On the one hand it minimizes the amount of special technology (e.g. collectors, ducting, pumps, storage volumes, etc.) that has come to be associated with the harnessing of solar energy. On the other hand, total costs are reduced since the actual building elements (windows, walls, etc.) themselves constitute the heating system. A number of methods have been discussed in the literature [1], all offering several advantages and a few disadvantages. These methods fall into three broad classes: direct gain systems; wall collectors and roof collectors. The direct gain method employs large south-facing (in the northern hemisphere) window areas to admit sunlight directly into the living area. It is the simplest to execute but domestic life can be disturbed in that severe limits are placed on the type and location of room furnishings, in order to allow the radiation to be absorbed by thermal storage masses. Roof collectors obviate this problem but they present a leak risk (in the case of water storage systems) and may require a costly support structure for a heavier than usual roof. They are also only useful for single-story buildings. Wall collectors are of interest in that they overcome all of these problems. One particularly celebrated example is the "Trombe Wall" [2] which consists of a darkened massive south-facing wall, glazed on the outside. Space heating is effected after a designed time lag of several hours by the heat that has built up in the massive wall. In addition to this delayed heating, the space between wall and glazing may be vented so as to allow solar heated air to enter the building by natural convection during sunny but cold periods. Such a structure clearly presents none of the above-mentioned problems. It does however present two draw-backs. First, southern light and view are denied users of such a building unless a small window is cut in the storage wall. More serious however is the fact that in regions having hot summers it is difficult to prevent over-heating. The purpose of the present paper is to present a modification of the Trombe scheme that does not suffer from these two problems and which exhibits several additional virtues.
3. SYSTEM OPERATION In winter during day-light hours the columns are oriented so as to present a uniform absorbing surface to the outside. If some day-light and view are required into the building the columns may be rotated by a few degrees in order to achieve the desired effect for as long as it is required. When no more useful heat is to be gained the columns are rotated through 120 ° so as to present a reflecting wall to the outside. There is a moot point here in that aluminum surfaces tend to have a hole in their reftectivity curves I-3] in the vicinity of 9/~m. This unfortunately coincides with the peak of such a column's black body emission spectrum! If however a coating could be found that exhibited high reflectivity here, the system would radiate preferentially inwards after sun-down. Pending the discovery of such an ideal reflective surface however, this face of the prism would be covered with insulating material and the latter given a reflective finish. During summer time a continuous reflective surface would be presented to the outside with once again the option of allowing slits of light to penetrate as desired. Such questions as double glazing and venting the air space between wall and window are of course relevant here but we do not discuss them since they have been adequately treated elsewhere [4, 5]. We do however point out that
2. COUPLED ROTATING PRISMS Our scheme involves replacing the continuous massive storage wall by a set of identical vertical columns each being rotatable on its axis in unison with the others. Various geometries are possible regarding the crosssections of the columns; the idea being that the resulting tResearch supported by the Center for Absorption in Science, The Ministry for Immigrant Absorption, Israel. 563
4 f i
i ~ i
11
11
1 1 1 1
11
~ 1 1
1 1 1 1 1 1 1 1 1 1 1 1
#
Fig. 1. Plan view of four neighbouring columns and the exterior glazing. By rotating the columns, each of whose faces has a different finish, walls of various desired absorbing qualities can be produced.
564
Technical Note
the columns would be mechanically coupled to each other so that their orientation is easily adjusted by hand. 4. SOME SAMPLE DIMENSIONS
The actual dimensions of the columns are dictated by the amount of thermal storage required and the choice of building material. Indeed the problem is actually highly constrained so depending upon one's precise requirements it may be necessary to change the geometry (to, for example, a curvilinear cross section). For definiteness however let us continue with our prisms of equilateral triangular cross section. Let the base of the triangle have length 2a.
(i) Concrete columns Trombe walls typically involve a concrete storage wall about 30 cm thick [2, 4]. Thus if unit area of glazing is to be backed by equivalent storage volume in the columns we must have: W/3 a 2 = 2a × 30cm
(1)
a -~ 35 cm.
(2)
thus
(ii) It reflects away unwanted summer radiation. This feature together with venting should remove the tendency to overheat. (iii) The anisotropic cooling down of the prisms effected either by reflection from some appropriately treated surface as discussed above or via the use of permanently installed stick-on insulation results in higher efficiency. (iv) This method of rotating the insulated surface out of the way when not required neatly solves a not insignificant storage problem that is often associated with the use of night insulation. (v) In the case of hollow columns (for liquid storage) the thermal mass can be varied at will by the introduction of various liquids. (vi) Of related advantage is the possibility of filling the prisms with materials which undergo a liquid-solid phase change at some appropriate temperature [6]. (vii) Finally by appropriate choice of colours for the dark absorbing face and the surface of optional finish any interior decoration scheme can be accomodated. As a last word we emphasize that equilateral triangles have only been discussed as one definite example among many possibilities. In particular the columns could be ovals, squares, over-lapping rectangles or any other convenient geometry.
For this situation the width of the air space between the glazing and the columns in their fully closed position is a/~/'3 or about 20 cm.
(ii) Water columns Equivalent thermal storage to the above considered concrete wall could be provided by a water wall of approx. 15 cm thickness. Thus if our prisms were filled with water all of the above dimensions would be reduced by a factor of two. 5. ADVANTAGES OF THE PRISM WALL
The rotating prism wall discussed above is a Trombe wall but has seven possible advantages over the latter. (i) It allows direct gain in the form of slits of light, continuously adjustable in width up to about 13 per cent of the glazing area.
REFERENCES
1. See for example, Passive Solar Heating and Cooling, Conf. and Workshop Proc., Albuquerque, NM, Los Alamos Scientific Laboratory of Univ. of California, Los Alamos, N M 87545, 18-19 May 1976. 2. F. Trombe, J. F. Robot, M. Cabanat and B. Sesolis, ibid., p. 201. 3. A. B. Meinel and M. P. Meinel, Applied Solar Energy, p. 253. Addison-Wesley, Reading, Mass. (1977). 4. J. D. Balcomb, J. C. Hedstrom and R. D. McFarland, Solar Energy 19, 277 (1977); P. Ohanessian and W. W. S. Charters, Solar Energy 20, 275 (1978). 5. H. Akbari and T. R. Borgers, Solar Energy 22, 165 (1979). 6. M. Telkes, AHSRAE GRP 170, VII-1 (1977).
0038-092X/80/1201-0563/S02.00/0
TECHNICAL
NOTE
Rotating prism wall as a passive heating elementt DAVID FAIMAN Ben-Gurion University of the Negev, The Institute for Desert Research, Sede Boqer and Department of Physics, Beersheva, Israel
(Received 31 January 1980; accepted 27 May 1980)
wall formed by the columns is capable of presenting surfaces of differing thermal properties in any desired direction. In order to make clear our ideas let us consider the case where the columns are right prisms on an equilateral triangular base. This example is possibly not the easiest to effect in practice but illustrates clearly all the salient ingredients. Each of the three faces of the columns is treated with a different finish. One face is highly absorbing: black for example. The second face is highly reflecting: perhaps being covered with aluminium foil. The finish of the third face is optional and can be chosen to suit ones personal sense of aesthetics. A plan view of four such neighbouring columns is sketched in Fig. 1.
1. I N T R O D U C T I O N
The so called "passive" approach to space heating aims at using the building itself as a solar collector and distribution system. This holds out a dual promise. On the one hand it minimizes the amount of special technology (e.g. collectors, ducting, pumps, storage volumes, etc.) that has come to be associated with the harnessing of solar energy. On the other hand, total costs are reduced since the actual building elements (windows, walls, etc.) themselves constitute the heating system. A number of methods have been discussed in the literature [1], all offering several advantages and a few disadvantages. These methods fall into three broad classes: direct gain systems; wall collectors and roof collectors. The direct gain method employs large south-facing (in the northern hemisphere) window areas to admit sunlight directly into the living area. It is the simplest to execute but domestic life can be disturbed in that severe limits are placed on the type and location of room furnishings, in order to allow the radiation to be absorbed by thermal storage masses. Roof collectors obviate this problem but they present a leak risk (in the case of water storage systems) and may require a costly support structure for a heavier than usual roof. They are also only useful for single-story buildings. Wall collectors are of interest in that they overcome all of these problems. One particularly celebrated example is the "Trombe Wall" [2] which consists of a darkened massive south-facing wall, glazed on the outside. Space heating is effected after a designed time lag of several hours by the heat that has built up in the massive wall. In addition to this delayed heating, the space between wall and glazing may be vented so as to allow solar heated air to enter the building by natural convection during sunny but cold periods. Such a structure clearly presents none of the above-mentioned problems. It does however present two draw-backs. First, southern light and view are denied users of such a building unless a small window is cut in the storage wall. More serious however is the fact that in regions having hot summers it is difficult to prevent over-heating. The purpose of the present paper is to present a modification of the Trombe scheme that does not suffer from these two problems and which exhibits several additional virtues.
3. SYSTEM OPERATION In winter during day-light hours the columns are oriented so as to present a uniform absorbing surface to the outside. If some day-light and view are required into the building the columns may be rotated by a few degrees in order to achieve the desired effect for as long as it is required. When no more useful heat is to be gained the columns are rotated through 120 ° so as to present a reflecting wall to the outside. There is a moot point here in that aluminum surfaces tend to have a hole in their reftectivity curves I-3] in the vicinity of 9/~m. This unfortunately coincides with the peak of such a column's black body emission spectrum! If however a coating could be found that exhibited high reflectivity here, the system would radiate preferentially inwards after sun-down. Pending the discovery of such an ideal reflective surface however, this face of the prism would be covered with insulating material and the latter given a reflective finish. During summer time a continuous reflective surface would be presented to the outside with once again the option of allowing slits of light to penetrate as desired. Such questions as double glazing and venting the air space between wall and window are of course relevant here but we do not discuss them since they have been adequately treated elsewhere [4, 5]. We do however point out that
2. COUPLED ROTATING PRISMS Our scheme involves replacing the continuous massive storage wall by a set of identical vertical columns each being rotatable on its axis in unison with the others. Various geometries are possible regarding the crosssections of the columns; the idea being that the resulting tResearch supported by the Center for Absorption in Science, The Ministry for Immigrant Absorption, Israel. 563
4 f i
i ~ i
11
11
1 1 1 1
11
~ 1 1
1 1 1 1 1 1 1 1 1 1 1 1
#
Fig. 1. Plan view of four neighbouring columns and the exterior glazing. By rotating the columns, each of whose faces has a different finish, walls of various desired absorbing qualities can be produced.
564
Technical Note
the columns would be mechanically coupled to each other so that their orientation is easily adjusted by hand. 4. SOME SAMPLE DIMENSIONS
The actual dimensions of the columns are dictated by the amount of thermal storage required and the choice of building material. Indeed the problem is actually highly constrained so depending upon one's precise requirements it may be necessary to change the geometry (to, for example, a curvilinear cross section). For definiteness however let us continue with our prisms of equilateral triangular cross section. Let the base of the triangle have length 2a.
(i) Concrete columns Trombe walls typically involve a concrete storage wall about 30 cm thick [2, 4]. Thus if unit area of glazing is to be backed by equivalent storage volume in the columns we must have: W/3 a 2 = 2a × 30cm
(1)
a -~ 35 cm.
(2)
thus
(ii) It reflects away unwanted summer radiation. This feature together with venting should remove the tendency to overheat. (iii) The anisotropic cooling down of the prisms effected either by reflection from some appropriately treated surface as discussed above or via the use of permanently installed stick-on insulation results in higher efficiency. (iv) This method of rotating the insulated surface out of the way when not required neatly solves a not insignificant storage problem that is often associated with the use of night insulation. (v) In the case of hollow columns (for liquid storage) the thermal mass can be varied at will by the introduction of various liquids. (vi) Of related advantage is the possibility of filling the prisms with materials which undergo a liquid-solid phase change at some appropriate temperature [6]. (vii) Finally by appropriate choice of colours for the dark absorbing face and the surface of optional finish any interior decoration scheme can be accomodated. As a last word we emphasize that equilateral triangles have only been discussed as one definite example among many possibilities. In particular the columns could be ovals, squares, over-lapping rectangles or any other convenient geometry.
For this situation the width of the air space between the glazing and the columns in their fully closed position is a/~/'3 or about 20 cm.
(ii) Water columns Equivalent thermal storage to the above considered concrete wall could be provided by a water wall of approx. 15 cm thickness. Thus if our prisms were filled with water all of the above dimensions would be reduced by a factor of two. 5. ADVANTAGES OF THE PRISM WALL
The rotating prism wall discussed above is a Trombe wall but has seven possible advantages over the latter. (i) It allows direct gain in the form of slits of light, continuously adjustable in width up to about 13 per cent of the glazing area.
REFERENCES
1. See for example, Passive Solar Heating and Cooling, Conf. and Workshop Proc., Albuquerque, NM, Los Alamos Scientific Laboratory of Univ. of California, Los Alamos, N M 87545, 18-19 May 1976. 2. F. Trombe, J. F. Robot, M. Cabanat and B. Sesolis, ibid., p. 201. 3. A. B. Meinel and M. P. Meinel, Applied Solar Energy, p. 253. Addison-Wesley, Reading, Mass. (1977). 4. J. D. Balcomb, J. C. Hedstrom and R. D. McFarland, Solar Energy 19, 277 (1977); P. Ohanessian and W. W. S. Charters, Solar Energy 20, 275 (1978). 5. H. Akbari and T. R. Borgers, Solar Energy 22, 165 (1979). 6. M. Telkes, AHSRAE GRP 170, VII-1 (1977).