Solar heat collector roofs

A solar heat collector roof comprises an absorber plate (1) for incident radiation energy and a heat exchange plate (2) placed directly thereunder. The heat exchange plate is corrugated in such a manner that it comprises channels extending down along the roof. The heat exchange plate (2) is moreover in direct contact with the absorber plate (1) thereabove. A heat carrying or heat removing liquid medium (3), e.g. water, flows in the channels. The liquid medium (3), which removes heat from the heat exchange plate (2), is supplied at such a rate that the flow rate down along the roof in the channels lies below the rate at which the surface tension of the heat carrying medium (3) is broken, so that a winding and travelling flow pattern is imparted thereby to the liquid medium (3), and up along the channel walls (4).

BACKGROUND OF THE INVENTION 
The present invention relates to an improvement in solar heat collector 
roofs comprising a heat carrying or heat removing medium, e.g. water, 
flowing down said roof in channels formed by corrugations in a heat 
exchange plate, and said solar heat collector roof preferably also 
comprises a translucent and insulating cover plate system as an upper and 
outer layer. 
In addition to the characteristics of this roof in connection with 
collecting solar energy, the solar collector roof in accordance with the 
invention constitutes a "complete" roof construction in the sense that 
there is no need of other structures under the roof, than rafters/laths 
upon which said roof shall be laid and fastened. The roof thereby becomes 
watertight, without any need of further layers for reinforcement/sealing. 
DESCRIPTION OF THE PRIOR ART 
There are previously known solar heat collector roofs of a type which 
comprises means for supplying water at the roof summit and for collecting 
this water at the lower roof edge after heating the water by passage 
thereof between a plate for absorption of incident radiation energy and a 
translucent and insulating outer cover plate. 
Several experiments have been made regarding solar energy houses. One of 
these experiments is the Soltun project which is situated in Jelly near 
Moss in Norway. The housing estate consists of seven houses built around a 
country courtyard. In this case two layers of corrugated aluminum roof 
plates have been used as a solar collector, water flowing between these 
two layers. The plates are covered by a transparent double polycarbonate 
layer of the type ordinarily used in greenhouses. The waterflow system 
consists of distribution and collection channels, and is constructed 
specially for this project. 
From European patent publication no. 69103 there is known a solution where 
cooling liquid flows as a film on the underside of a radiation absorber 
plate, utilizing the surface tension of the cooling liquid as well as the 
cohesion forces between the liquid and the absorber plate. Thereby good 
thermal contact is obtained between the absorber and the liquid, and 
simultaneously the absorber plate prevents the liquid vapour from reaching 
the cover plate situated above. However, it is obvious that such a 
solution needs a separate roof on the underside, i.e. this solution cannot 
in itself constitute a complete roof. 
Further there is known from European patent publication no. 5701 a device 
where cooling liquid flows on top of an absorber plate in channels 
constituted by profiled channels in the absorber, and with a translucent 
cover plate laid closely down upon the absorber as an upper boundary. 
Thus, the radiation will in this case pass through the liquid before it is 
absorbed in the absorber plate. In this case a very good thermal contact 
is achieved between cooling liquid and absorber. 
For a comparison, the present invention is based inter alia upon the fact 
that water shall not fill the channels in the heat exchange plate 
mentioned in the introduction. In order that heat shall be transferred to 
the water, it must therefore be transferred first from a specially 
provided absorber plate to the heat exchange plate, in the places where 
these plates are in metallic contact with each other, and thereafter the 
heat is conducted in the walls of the heat exchange plate channels before 
it can be transferred to the water flowing in these channels. The two 
patent publications mentioned above show solutions with good heat 
transfer, because the liquid is in direct physical contact with the 
absorber plate. However, with the solution in accordance with the present 
invention heat transfer from the absorber to the cooling liquid is 
somewhat more problematic. The solution of this problem is the basis of 
the present invention. 
Of course it is natural to pose the question regarding why one should 
select suggesting a solution where heat exchange is a problem, when there 
are previously known solutions of this problem. However, it should be 
noted that an indirect heat transfer of the type appearing in the present 
invention, has constructional advantages regarding the solution when 
viewed in its entirety, and this makes the solar collector principle of 
the present invention more effective and favourable as a total solution, 
than the solutions of the two publications mentioned above. 
It should also be noted that the two mentioned publications represent 
solutions which still imply problems in relation to a technical/economical 
utilization of solar heat in the form of solar collectors in roofs. 
Thus, the patent publication EP 69103 describes only part of the complete 
roof solution, as mentioned in the introduction. When this principle is 
used in solar heat collectors on a roof, there must be a further roof 
below the solar collector, said further roof preventing liquid drops and 
vapour from penetrating down into the house. Further, the translucent 
cover plate in the construction in accordance with EP 69103 must bear the 
mechanical strains to which a roof normally can be exposed, in the form of 
wind and snow loads, since the liquid film would be broken up by the 
mechanical supports which in the alternative case would have to be erected 
from an under-roof to the cover plate. 
Besides, experiments conducted by the inventors show that the solution 
mentioned above is very critical as to requirements regarding the nature 
of the metal surface and the liquid for maintaining such a liquid film. It 
must therefore be supposed that the construction mentioned above is not 
well suited to be able to maintain a stable operation during many years. 
European patent publication no. 5701 describes a construction where liquid 
flows on top of the absorber plate, and where cover plate, absorber and/or 
liquid have been treated so as to reduce or avoid evaporation and 
condensation. A disadvantage of such a solution is that the cover plate 
provides poor heat insulation when it lies tightly upon the absorber. 
Besides, the transparent cover plate will imply that the liquid flow will 
be visible from the outside. This is an aesthetical problem which has not 
been solved in the construction of EP 5701, and which limits the 
usefulness of this solution. 
Also, in the solution mentioned above a cooling liquid must be used which 
does not evaporate and condense. In other words, clean water cannot be 
used in this solar collector, which entails that the heat must be 
transferred via a heat exchange unit to the rest of the heating system. An 
extra heat exchange unit has economical as well as operational 
consequences. 
SUMMARY OF THE INVENTION 
The problems mentioned above are eliminated, and at the same time a simpler 
construction is provided of a solar heat collector which is easily 
adaptable to any roof without special constructions, by putting into use 
an improvement of the kind mentioned in the introduction, and with the 
particular features stated in the characterizing part of the enclosed 
patent claim 1. 
Thus, in accordance with the present invention the cooling liquid flows on 
the underside of the absorber plate, and therefore is not visible from the 
top/front. The absorber plate prevents water vapour in reaching the cover 
plate, so that the condensation problem is avoided. The cover plate can be 
mounted with an optimum spacing from the absorber plate regarding 
insulation, and the solar collector has mechanical characteristics 
providing a distribution and transfer of loads on the cover plate, to a 
rafter layer thereunder. 
The remaining problem, which is clearly specific to the present 
construction, and not to the publications mentioned above, is the ability 
to provide an effective heat transfer from the absorber plate via the heat 
exchange plate to the cooling liquid (the water). 
Thus, the object of the present invention is to bring the water as close as 
possible to the absorber plate, however in such a manner that it flows in 
a stable manner upon the heat exchange plate. To achieve this, one 
utilizes the per se well known characteristic that water in small 
quantities will not flow rectilinearly in a string, but due to contact 
tensions, friction and surface tensions tends to flow in windings, i.e. 
meander fashion. 
It should also be noted that when the roof, and thereby the channels in 
which water flows, has an angle of inclination, the water will maintain 
this meander pattern while flowing down the channel. When a suitable shape 
(cross section) of the channel is selected, the water will partly flow in 
the channel bottom, partly on the channel sides. When water flows on the 
channel sides, the distance between water and absorber becomes smaller, 
and conduction resistance to heat becomes less. In other words, here the 
heat is transferred easier than if the water stayed along the bottom of 
the channel. 
When the improvement in accordance with the invention merely consists of a 
cover plate, an absorber, a corrugated heat exchange plate as well as an 
insulation plate thereunder, the construction is cheaper than the 
previously mentioned ones regarding material consumption. The channels for 
guiding the heat carrying medium is quite simply constituted by the 
corrugations, which are connected to the water supply and water collecting 
channels respectively along the summit of and at the lower edge of the 
roof. The fact that the absorber plate is flat, makes it simpler to 
support the cover plate, and therefore an improved insulation situation is 
achieved on the outside of the absorber plate. Moreover, the water courses 
become simple, and the risk of stopping up due to extraneous matter will 
be less, since the water courses are relatively large in cross section. As 
a consequence of the fact that the water quantity supplied is controlled 
in such a manner that the surface tension keeps the water from breaking 
up, a winding will be created which will bring the water further into 
contact with parts of the corrugated plate, and thus the water absorbs 
additional heat quantities in relation to the situation where the water 
runs straight along the channel bottom. 
The mechanism forming the basis of the above mentioned winding phenomenon, 
is supposed to be the friction between the water and the channel surface, 
and between the water layers and the water surface (the surface tension). 
The friction between water and channel surface is the larger one, 
therefore the water velocity is slowed down on the underside. Water above 
the lower layer will flow somewhat faster, but the surface tension will 
maintain the water as an aggregate. The result hereof is a winding, and as 
soon as such a winding is developed, the mechanism mentioned above will 
seek to enhance the winding phenomenon. This flow pattern will be 
maintained as long as the flow rate is sufficiently small not to break the 
surface tension. 
When the water creeps up the channel wall, one component of the water 
weight, seeking to counteract the winding, will increase. Therefore the 
beam of water will swing back and over toward the opposite channel wall. 
The total wave pattern will move down the channel, in such a manner that 
over time a large part of the channel bottom and walls will come into 
direct contact with the water. 
Further characteristics and features of the present invention will appear 
from the rest of the apparatus claims, and from claims regarding a method 
of removing heat from a solar heat collector roof of the type here in 
question.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1 is shown an embodiment of the solar roof construction, where a 
corrugated plate 2 of e.g. aluminum is provided upon an insulation layer 
7, and upon the corrugated plate 2 there is placed an absorber plate 1. 
Further there is a cover plate 6 above the absorber plate. Said cover 
plate 6 may e.g. consist of translucent polycarbonate material. The 
absorber plate 1 may consist of aluminum, and it can be coated by black 
colour in order to absorb heat as well as possible. The corrugated 
aluminum plate 2 may be equipped with a relatively smooth surface, formed 
by slight anodizing and painting of the aluminum plate. In the embodiment 
shown, the corrugations form channels with a bottom 5 and side walls 4 
constituting a trapezium with a cross section as indicated in FIG. 2. The 
channels have their supply of water via a distribution channel 8 at the 
roof top, and the water flowing down the channels due to gravity, is 
collected in a collecting pipe 9. The collecting pipe 9 may e.g. lead to a 
tank, from which the water is pumped up again to the distribution channel 
8. 
The construction shown in FIG. 1 provides a solar roof construction which 
is rigid and easily handled. The height H of the channels should as a 
starting point have been as small as possible, in order to give the water 
contact with the absorber plate 1 itself, however this is not possible 
because the mechanical strength of the roof is reduced with a decrease in 
height H. Thus, the height H is dependent on the requirements set 
regarding roof strength. Therefore, in order to utilize maximally the heat 
absorbed by absorber 1, it should be necessary that the water fills the 
whole channel. However, this would result in a roof which would become 
very heavy, and therefore rather unsuitable. In order to avoid such a 
situation it has been found that it is possible to make the water passing 
through the channel undertake a winding motion up along the channel walls 
4, so that the water can draw heat from areas situated rather near the 
absorber plate, even when there is a relatively low water flow rate in 
each channel. This winding mechanism is as previously described dependent 
on the friction between water and channel surface, and of the surface 
tension. 
The side surface and the channel bottom must also have such a constitution 
that the water wets these surfaces only to a small degree. It is possible 
to measure separately the degree of wetting of a surface by a certain 
liquid, by immersing a plate 12 of a corresponding material as the one to 
be used, in our case as a corrugated plate, in a vessel 10 containing 
liquid 11, see FIG. 4. In our case the liquid is water. Plate 12 is placed 
perpendicularly in relation to the water surface in the vessel. The water 
will pull itself up along plate 12, depending on surface treatment. The 
rim angle .beta. between the tangent 13 to the water surface up along 
plate 12, and plate 12, is a measure of wetting. The deciding parameter 
regarding the value of this angle .beta. is, as previously mentioned, the 
surface treatment of the plate. In order to achieve the desired winding, 
the corrugated heat exchange plate 2 should have a surface treated so that 
the wetting rim angle .beta. is larger than 60.degree.. 
Besides, the windings can be regulated by means of water flow rate. 
However, there will be different inclinations for different roof 
constructions, and the flow rate will take on different values depending 
on roof angle. In order to achieve the same velocity or flow rate at 
different roof angles, and in order that the flow rate shall not exceed 
the rate which implies that the surface tension is broken, the water 
quantity supplied must be adapted to the roof angle. However, the windings 
are also dependent on the friction between the water and the channel 
bottom 5, and therefore it is necessary to adapt the water quantity 
supplied in distribution channel 8 also with due consideration to the 
surface treatment of the channel. This surface treatment can be e.g. a 
slight anodizing of the aluminum plate, and it may be provided with paint. 
It has turned out that the width of bottom 5 advantageously can be in the 
range 1-10 cm. It has also turned out that the flow rate of the water 
preferably can be in the range 0,1-1 l/min. for each channel. The water 
temperature may then be between 10.degree. and 80.degree. C. The roof 
inclination angle may vary within wide limits, i.e. between 20.degree. and 
85.degree.. 
Alternatively, each channel may have a cross section with approximately 
half cylinder shape as shown in FIG. 6. Its diameter is in a range of 1-10 
cm. 
From the above it should appear clearly that the flow rate necessary to 
make the desired winding pattern indicated in FIG. 3 appear, depends on 
several parameters, and these parameters are difficult to calculate for 
each particular example. Thus, the simplest method is to adjust the flow 
rate after mounting the roof by adjusting the pumped quantity of water in 
such a manner that the flow rate approaches the maximum allowed value in 
the particular case. 
In FIG. 5a is shown a particular embodiment of the cover plate system, made 
possible by the special construction thereunder, with absorber plate 1 
upon a heat exchange plate 2 as previously mentioned. In order to achieve 
as good an insulation as possible against heat "leaking out" toward the 
roof top side from absorber plate 1, spacer elements or fixing profiles 14 
are formed of the same insulating material as the other parts of the cover 
plates 6, and integrated with the cover plates 6, i.e. as a unity 
therewith. Simultaneously there is provided an optimum distance up to 
cover plate 6 regarding heat conduction by convection. This optimum 
distance depends inter alia on roof inclination, and lies in a range about 
10 mm. Cover plates with such integrated fixing profiles 14 of e.g. 
polycarbonate material can be manufactured directly and simply by 
extruding. 
FIG. 5b shows in detail a fixing profile 14 integrated with a cover plate 
6, where said fixing profile 14 is hollow. FIG. 5c shows a joint between 
two such cover plates 6, where a fixing profile 14 is placed on the edge 
of a cover plate.