Solar heat collector

A solar heat collector, in which an absorber plate is located in a box member provided at one surface thereof with a transparent sheet, being of such an arrangement that the absorber plate has been subjected at the surface thereof to a selective absorption surface treatment and a transparent heat trap formed of a fluoroplastics film and having a height of about 1/2 of the interval across the transparent sheet and the absorber plate is disposed between the transparent sheet and the absorber plate in contact with the transparent sheet. With this arrangement, the heat losses caused by convection, radiation and conduction can be suppressed at the same time.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a solar heat collector for a hot water supply, 
solar heat air conditioning systems and the like. 
2. Description of the Prior Art 
Heretofore, there have been proposed various types of solar heat collectors 
for the hot water supply, solar heat air conditioning systems and the 
like. 
FIG. 1 is an explanatory view illustrating a solar heat collector 1 widely 
used. This solar heat collector 1 comprises: an absorber plate 2 for 
converting the solar rays into thermal energy to transmit the thermal 
energy to a heat transfer medium such as water; a heat transfer medium 
flow path 3 formed in the absorber plate 2; a transparent sheet 4 made of 
glass or the like for preventing the convection heat loss from the 
absorber plate 2 and protecting the absorber plate 2 against the 
contamination and damages caused by the external factors; a heat 
insulating material 5 for preventing the heat loss through the rear 
surface of the solar heat collector; and an outer box 6 for protecting the 
absorber plate 2 and the insulating material 5, totally covering the solar 
heat collector in cooperation with the transparent sheet 4. 
The collector performance of the abovedescribed solar heat collector is 
improved by suppressing the convection radiation and conduction losses 
from the absorber. As the methods of suppressing the heat losses described 
above, heretofore, there have been adopted such methods that the surface 
of the absorber plate 2 is subjected to a selective absorption surface 
treatment for suppressing the radiation heat loss, a convection preventive 
structure such as a honeycomb transparent heat trap for suppressing the 
convection heat loss is provided between the absorber plate 2 and the 
transparent sheet 4. 
FIGS. 2 and 3 are explanatory views showing the conventional solar heat 
collectors 10 and 20 having convection preventive structures, 
respectively. In the solar heat collector 10 shown in FIG. 2, the 
honeycomb heat trap 11 is provided between an absorber plate 2 and a 
transparent sheet 4 in a manner to contact both the absorber plate 2 and 
the transparent sheet 4. Furthermore, in the solar heat collector 20 shown 
in FIG. 3, a honeycomb heat trap 21 is provided between the absorber plate 
2 and the transparent sheet 4 in a manner to contact the absorber plate 2, 
but not to contact the transparent sheet 4. 
However, in the abovedescribed conventional solar heat collectors 10 and 
20, the honeycomb heat traps 11 and 21 are provided in a manner to contact 
the absorber plate 2, whereby the heat traps are heated to radiate large 
quantities of infrared rays, with the result that the radiation heat 
losses from the honeycomb heat traps 11 and 21 to the transparent sheets 4 
increase to a high extent irrespective of that the absorber plates 2 have 
been subjected to the selective absorption surface treatment for 
suppressing the radiation heat losses on the surfaces of the absorber 
plates 2. Furthermore, in the conventional solar heat collector 10 and 20, 
the honeycomb heat traps 11 and 21, being disposed in contact with the 
absorber plates heated to a high temperature, are required to have a 
durability for temperature up to200.degree.-250.degree. C. Additionally, 
as shown in FIG. 2, in the solar heat collector 10, in the case of the 
honeycomb heat trap 11 being located in close contact with the absorber 
plate 2 and the transparent sheet 4, it presents a disadvantage that the 
honeycomb heat trap 11 may be broken down when the transparent sheet 4 is 
deformed due to an external force such as wind pressure, snow load or the 
like. 
SUMMARY OF THE INVENTION 
The present invention has been developed to obviate the abovedescribed 
disadvantages of the prior art and has as its object the provision of a 
solar heat collector wherein the maximum service temperature required for 
a heat trap is low, the heat trap is set at a state where possibilities of 
break-down is eliminated, and heat losses of three forms due to 
convection, radiation and conduction can be suppressed. 
To achieve the abovedescribed object, the present invention contemplates 
that a solar heat collector, wherein an absorber plate is located in a box 
member provided at one of the outer surface thereof with a transparent 
sheet, is of such an arrangement that the surface of the absorber plate is 
subjected to selective absorption surface treatment, a transparent heat 
trap is interposed between the transparent sheet and the absorber plate in 
such a manner that the height of the heat trap is substantially one half 
of the interval across the transparent sheet and the absorber plate and 
the heat trap is disposed in contact with the transparent sheet.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Description will hereunder be given of the results of experiments conducted 
by the inventors, which have led to the development of the present 
invention. 
FIG. 4 is a chart showing the influences rendered to the quantities of 
radiated heat by the positions where the transparent heat trap 7A was 
provided between the transparent sheet and the absorber plate as shown in 
FIG. 5. In this experiment as shown in FIG. 5, a heat trap 7A having a 
height l (10 mm) and being formed of collectively assembled square pillars 
each being hollow and having openings of H.times.H (5 mm.times.5 mm) was 
moved from a position contacting the transparent sheet 4 to a position 
contacting the absorber plate 2 within an interval L (25 mm) across the 
transparent sheet 4 and the absorber plate 2. During that movement, the 
rates of heat losses were measured at respective positions where the heat 
trap 7A is provided. Here, in FIG. 5, the provision of the heat trap 7A is 
shown at a position where the center of the heat trap 7A is spaced apart a 
distance x from the transparent sheet 4. Additionally, in this experiment, 
an absorber plate subjected at the surface thereof to a black color 
coating treatment of non selective absorption (BP) and another absorber 
plate subjected to at the surface thereof to a selective absorption 
surface treatment (SS) were used, and the above-described rates of heat 
losses were measured under the use of the respective absorber plates. 
More specifically, in FIG. 4, there are shown a convection heat loss BP1, a 
radiation heat loss BP2 and a sum in value of convection and radiation 
heat losses BP3 under the use of the black color coating treated absorber 
plate 2 and a convection heat loss SS1, a radiation heat loss SS2 and a 
sum of convection and radiation heat losses SS3 under the use of the 
selective absorption surface treated absorber plate 2 respectively in 
correspondence to the position where the heat trap 7A are provided. 
According to the results of this experiment, it was ascertained that the 
rate of heat losses in the solar heat collector reached the minimum value 
when the absorber plate 2 had been subjected at the surface thereof to the 
selective absorption surface treatment and the transparent heat trap 7A 
was disposed in contact with the transparent sheet 4. 
FIG. 6 is a chart showing the influences rendered to the rate of heat 
losses from the solar heat collector by the height of the transparent heat 
trap disposed in contact with the transparent sheet. In this experiment, 
as shown in FIG. 7, a heat trap 7B formed of collectively assembled square 
pillars each being hollow and having openings of H.times.H (5 mm.times.5 
mm) was disposed in contact with the transparent sheet 4 within the 
interval L (25 mm) across the absorber plate 2 and the transparent sheet 
4, the height 1 of the heat trap 7B is varied, and then, the rate of heat 
losses in accordance with the varied heights of the heat trap 7B were 
measured. Furthermore, in this experiment, an absorber plate subjected at 
the surface thereof to a black color coating treatment of non-selective 
absorption and another absorber plate subjected at the surface thereof to 
a selective absorption surface treatment were used, and the abovedescribed 
rate of heat losses were measured under the use of the respective absorber 
plates. 
More specifically, in FIG. 6, there are shown a convection heat loss BP1, a 
radiation heat loss BP2 and a sum of convection and radiation heat losses 
BP3 under the use of the black color coating treated absorber plate 2 and 
a convection heat loss SS1, a radiation heat loss SS2 and a sum of 
convection and radiation heat loss SS3 under tee use of the selective 
absorption surface treated absorber plate 2 respectively in correspondence 
to the height of the heat trap 7A. According to the results of the 
experiment shown in FIG. 6, it was ascertained that the rate of heat 
losses from heat collector reached the minimum value when the absorber 
plate 2 was subjected at the surface thereof to the selective absorption 
surface treatment and the height of the heat trap disposed in contact with 
the transparent sheet was set at a value substantially equal to 
1/2.times.L. 
In addition, in the respective embodiments described above, the structure 
of collectively assembled square pillars each being hollow and having the 
openings of H.times.H (5 mm.times.5 mm) was used as an example of the heat 
trap, however, it was ascertained that the substantially same results can 
be obtained even when the heat traps differing from in cross section from 
one having the form of the collectively assembled square pillars as 
described above are adopted. Additionally, in the present invention it is 
desirable that the wall thickness of the heat trap is thin and its 
emittance is low. 
Description will now be given of one embodiment of the present invention 
with reference to the accompanying drawings. 
FIG. 8 is an explanatory view showing a solar heat collector 30 as an 
embodiment of the present invention. Similarly to the solar heat 
collectors of the prior art, the solar heat collector 30 comprises: an 
absorber plate 2 for converting the solar rays into thermal energy and 
transmit it to a heat transfer medium; a heat transfer medium flow path 3 
forming part of the absorber plate 2; a transparent sheet 4 made of glass 
or the like for preventing the convection heat loss from the absorber 
plate 2 and protecting the absorber plate 2 against the contamination and 
damages caused by the external factors; a heat insulating material 5 for 
preventing the heat loss through the rear surface of the solar heat 
collector; and an outer box 6 for totally housing the solar heat 
collector. The aforesaid absorber plate 2 is formed in such a manner that 
a multiplicity of tubes being flattened in cross section are connected to 
a flat plate at regular intervals or two corrugated sheets are bonded to 
each other to form heat medium flow paths. The absorber plate 2 is 
subjected at the surface thereof to a selective absorption surface 
treatment. This absorber plate 2 is detachably mounted on opposing 
brackets secured to the inner walls 6a of the aforesaid outer box 6 by 
suitable means. 
A honeycomb heat trap 31 formed of a transparent member formed of an 
ethylene fluoride resin film or the like as a heat trap is disposed 
between the absorber plate 2 and the transparent sheet 4. The honeycomb 
heat trap 31 is in contact at the upper end thereof with the aforesaid 
transparent sheet 4 and supported at the lower end thereof by thin lines 9 
such as piano wires stretched across the opposing brackets 8. The height 
of the honeycomb heat trap 31 is determined to be about 1/2 of the 
interval across the absorber plate 2 and the transparent sheet 4. It is 
suitable to use a fluoroplastics film as the material of the transparent 
heat trap, for example, a copolymer FEP obtained from 4-fluoroethylene and 
6-fluoropropylene, a copolymer PFA obtained from 4-fluoroethylene and 
perfluoroalkylvinylether and a copolymer ETFE obtained from fluoroethylene 
and ethylene. 
Description will hereunder be given of the heat collecting characteristics 
and the structural characteristics, referring to the following table, as 
compared with the heat collecting characteristics and the structural 
characteristics of the solar heat collectors 10 and 30 of the prior art 
and the solar heat collector 40 wherein, as shown in FIG. 10, the 
honeycomb heat trap 41 is interposed between the absorber plate 2 and the 
transparent sheet 4 in a manner to contact neither the absorber plate 2 
nor the transparent sheet 4. In addition, the effects of the respective 
characteristics are qualitatively indicated, and further, qualitatively 
indicated by use of the points of effects attached thereto. 
TABLE 
______________________________________ 
No. of Solar heat collectors 
30 10 20 40 
______________________________________ 
Effect of Medium High Medium Low 
Convection .circle.2 
.circle.3 
.circle.2 
.circle.1 
Heat Loss 
Prevention 
Defect of High Low Low Medium 
Radiation .circle.3 
.circle.1 
.circle.1 
.circle.2 
Heat Loss 
Prevention 
Conduction Low High Medium Medium 
Heat Loss .circle.3 
.circle.1 
.circle.2 
.circle.2 
Required Low High High Medium 
Maximum Service 
.circle.3 
.circle.1 
.circle.1 
.circle.2 
Temperature 
Destructivity 
Non- Destruc- Non- Non- 
destruc- table destruc- 
destruc- 
table .circle.1 
table table 
.circle.3 .circle.3 
.circle.3 
Points of .circle.14 
.circle.7 
.circle.9 
.circle.10 
Total Effects 
______________________________________ 
As with the solar heat collector 10 shown in FIG. 2, the effect of 
convection heat loss prevention is high when spaces formed by the 
honeycomb heat trap 11 between the absorber plate 2 and the transparent 4 
are completely partitioned, while, as with the solar heat collector 40 
shown in FIG. 10, the effect of convection heat loss prevention is low 
when the honeycomb heat trap 41 is open at both the upper and lower ends. 
As with the solar heat collector 30 shown in the embodiment of the present 
invention, the effect of the convection heat loss prevention is medium 
when the honeycomb heat trap 31 is at the lower end only. 
The effect of radiation heat loss prevention becomes high by the selective 
absorption surface treatment applied to the surface of the absorbed plate 
2. In the solar heat collectors 10 and 20 in which the honeycomb heat 
traps 11 and 21 as shown in FIGS. 2 and 3, respectively, are provided in 
contact with the absorber plates 2, the honeycomb heat traps 11 and 21 are 
heated to radiate large quantities of infrared rays, and in spite of the 
selective absorption surface treatment applied to the surfaces of the 
absorber plates 2, a high effect of radiation heat loss prevention cannot 
be achieved. In contrast thereto, in the solar heat collector 30 shown in 
the above-described embodiment of the present invention, the honeycomb 
heat trap 31 is disposed between the absorber plate 2 and the transparent 
sheet 4 in a manner to be spaced apart from the absorber plate 2 as far as 
possible, whereby the honeycomb heat trap 31 itself is suppressed in 
temperature rise, so that a satisfactory effect of radiation heat loss 
prevention can be achieved. 
In the solar hear collector 30 shown in the above-described embodiment of 
the present invention, wherein the honeycomb heat trap 31 is not in 
contact with the absorber plate 2 and the temperature of the honeycomb 
heat trap 31 is low, whereby the thermal conductivity becomes low, thereby 
enabling to minimize the conduction heat loss. 
As with the solar heat collectors 10 and 20 shown in FIGS. 2 and 3 in which 
the honeycomb heat traps 11 and 21 are in contact with the absorber plates 
2, the maximum service temperature required becomes high, whereas, with 
the solar heat collector 30 shown in the abovedescribed embodiment of the 
present invention, in which the honeycomb heat trap 31 is not in contact 
with the absorber plate 2 and spaced apart from the surface of the 
absorber plate 2 to a considerable extent, a temperature as low as 
150.degree. to 160.degree. C. suffices for a service temperature. 
The destructibility of the honeycomb heat trap is such that, when the 
honeycomb heat trap 11 is immovably confined between the absorber plate 2 
and the transparent sheet 4 as in the solar heat collector 10 shown in 
FIG. 2, the destruction occurs, that, whereas, when the honeycomb heat 
trap 31 is movable toward the absorber plate 2 as the transparent sheet 4 
is deformed as in the solar heat collector 30 shown in the abovedescribed 
embodiment of the present invention, no destruction occurs. 
FIG. 11 is a chart showing the collector efficience .eta.vs .DELTA.T/J, 
wherein .DELTA.T is a temperature difference (.degree. C.) between the 
ambient temperature and the mean temperature of the heat transfer medium, 
J is an amount of solar radiation per unit of absorbing area (Kcal 
m.sup.-2 h.sup.-1), whereby the solar heat collector 30 shown in the 
abovedescribed embodiment of the present invention is compared in 
collector efficiency with the aforesaid solar heat collectors 1, 10, 20 
and 40 of the prior art. FIG. 11 shows that, in the normal heat collecting 
zone of the solar heat collector excluding the low heat collecting zone, 
the solar heat collector 30 shown in the above-described embodiment of the 
present invention is outstanding in the collector efficiency. 
In the solar heat collector 30 shown in the abovedescribed embodiment of 
the present invention, description has been given of the case where the 
honeycomb heat trap 31 is applied as the trap, however, this honeycomb 
heat trap 31 may be replaced with the transparent heat trap 32 formed of 
collectively assembled square pillars as shown in FIG. 12 or the heat trap 
33 formed of collectively assembled cylinders as shown in FIG. 13. 
Further, the honeycomb heat trap 31 of the solar heat collector 30 shown in 
the abovedescribed embodiment may be replaced with a continuous heat trap 
34 being bent into continuous V-shapes in cross section at substantially a 
regular pitch and formed of a transparent member similar to the honeycomb 
heat trap 31 as shown in FIG. 14 or a heat trap 35 formed of arranged 
cones as shown in FIG. 15. The V-shaped heat trap 34 divides an 
intermediate air space layer between the absorber plate 2 and the 
transparent sheet 4 into two groups of spaces and the area thereof being 
in contact with the transparent sheet 4 becomes small, whereby the 
convection suppressing effect is enhanced, so that the conduction heat 
loss can be suppressed. 
As has been described hereinabove, according to the present invention, in 
the solar heat collector having the absorber plate in its box member which 
is provided at the surface thereof with the transparent sheet, the 
absorber plate is subjected at the surface thereof to the selective 
absorption surface treatment, there is the transparent heat trap having a 
height of about 1/2 of the interval across the transparent sheet and the 
absorber plate between the transparent sheet and the absorber plate in a 
manner to be in contact with the transparent sheet, so that such 
advantages can be achieved that the maximum service temperature of the 
heat trap material becomes low, the heat the is not destructed by an 
external force and it is possible to suppress the heat losses in three 
forms due to the convection, radiation and conduction.