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This invention relates to a reflector for use in a solar heat boiler and a
solar heat boiler unit which utilizes solar heat energy.
BACKGROUND OF THE INVENTION
The total amount of solar energy striking the entire surface of the earth
is enormous. However, the energy per unit of area is fairly small and
calculated by the following equation, due consideration being given to the
revolution of the earth and spherical shape of the earth's surface: solar
constant of 2 Cal/cm.sup.2.min .times. 1/4 (efficiency of incidence)
.times. 2/3 (the remaining 1/3 is reflected from the surface) .times. 2/3
(the remaining 1/3 is absorbed in the atmosphere). The amount of energy
thus calculated comes to approximately 133.3 Kcal/m.sup.2.h.
Consequently, in order to utilize the heat energy radiated from the sun as
a heat source, it is necessary to concentrate solar light rays to increase
the energy density thereof.
However, in calculating the incident solar energy of a solar boiler, it is
not required to take into account the above-mentioned efficiency of
incidence (1/4) and the efficiency drop caused by reflection from the
surface of the earth (2/3), provided that the average energy per unit of
area radiated on the entire surface of the earth is always received at a
right angle to the reflector surface. Thus, the incident energy received
per unit area of the reflector amounts to 799 Kcal/m.sup.2.h (2
Cal/cm.sup.2.min .times. 2/3). A concave mirror is efficient as a
condensing means. However, a large concave mirror is difficult to
manufacture and is costly.
In order to overcome this difficulty, a method can be considered in which
small flat mirrors are placed on a large concave surface to concentrate
solar rays received by the mirrors. However, this method is not efficient
as an energy concentrating means.
A method can also be considered which utilizes concave spherical mirrors or
curved mirrors in place of the small flat mirrors in order to concentrate
solar rays on a focal point to increase the energy density. This method is
also deficient because generally a concave mirror, regardless of whether
it is spherical or parabolic, has a tendency to scatter incident rays
projected at an angle to the optical axis.
To gain a better understanding of the scattered focal length, reference is
made to FIG. 1 in which an incident ray projected at an angle to the
optical axis connecting the center of curvature A and focal point B of a
curved mirror reflects from the surface of the curved mirror and does not
focus at a point on the optical axis, but is scattered to points outside
of the length thereof.
Accordingly, even if a heating surface screen of a solar boiler is
positioned at the focal point of such a mirror, the solar rays which are
not parallel to the optical axis reach the surface of the boiler before or
after they are focused, thus making it difficult to concentrate the solar
energy.
As can be understood from the unit Cal/cm. sec. C.degree. of heat
conductivity in a heated metal body, the larger the temperature difference
between the two opposite surfaces, the more efficient the heat transfer.
Therefore, the more concentrated the solar energy and the higher the
temperature of the solar rays received at the heating surface, the better
the solar boiler efficiency, and the scattered rays impair the function of
the solar boiler.
OBJECT AND BRIEF SUMMARY
The objects of this invention are to remedy the above-mentioned deficiency
and further to provide an economical solar boiler.
These objects are achieved by a solar furnace according to the invention
which consists of a bottom part having a turning body supported on a base
frame for rotation around a horizontal axis, a reflector mounting frame
supported on an upper part of said turning body rotatable about an axis
perpendicular to the said horizontal axis, and turning mechanisms provided
for the turning body and reflector mounting frame, respectively. A large
number of curved mirrors are arranged on the reflector mounting frame in a
concave spherical configuration as a reflector body. A boiler body
comprising a water jet screen boiler is provided which has the
heat-receiving section located at the focal point of the reflector body.
The optical axis of the reflector body is directed toward and made to
follow the sun, thus making use of the heat energy radiated from the sun
for heating boiler water.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in detail in connection with the
accompanying drawings, in which:
FIG. 1 is a side view of a curved mirror showing schematically focussed
solar rays projected thereon at an angle to the optical axis of the curved
mirror;
FIG. 2 is a perspective view of a curved mirror showing an outline of the
focused image of solar rays projected thereon parallel to the optical axis
of the curved mirror;
FIG. 3 is a section on line III--III of FIG. 2;
FIG. 4 is a perspective view of the parts of a mirror supporting rack for
shaping a mirror plate into a curved mirror.
FIGS. 5 and 6 are diagrams showing the focus deviation of solar rays
directed onto a curved mirror;
FIG. 7 is a partial sectional side view of a solar boiler showing
concentrated rays;
FIG. 8 is a partial plan view showing an arrangement of curved mirrors;
FIG. 9 is a schematic side elevation view of a solar heat boiler;
FIG. 10 is a vertical section of a boiler unit thereof;
FIG. 11 is a front exploded view of the rack of FIG. 4;
FIG. 12 is an enlarged elevation view of a nozzle jetting water and forming
a water screen; and
FIG. 13 is a vertical section view, partially broken away, of a modified
boiler unit similar to that of FIG. 10.
DETAILED DESCRIPTION OF THE INVENTION
With reference now to the drawing and more particularly to FIG. 2 thereof,
a curved mirror 1 is provided as an element for a reflector, the mirror
having the shape of a sector of an annulus in plan view and a concave
circular section in longitudinal section. The said curved mirror is
mounted on a mirror supporting rack 2 having supporting legs 3.
In order to facilitate holding of said curved mirror, the arrangement as
shown in FIGS. 4 and 11 is used. A mirror supporting assembly or rack
comprises upper member 7 having an inwardly extending flange 7' at the top
and a lower member 6 having an upright flange 5 thereon. The lower member
6 is filled within upper member 7, flange 7' corresponding with the top
edge of flange 5 and being vertically arched very gently or slightly in
the shape of a circle, as shown in FIG. 11, while the assembly itself in
the plan view has the shape of a sector of an annulus as shown in FIG. 4.
A flexible mirror plate 4 corresponding in the plan view to mirror 1 and
having a vapor deposited coating on its top surface, and having a shape
generally corresponding to the interior of upper frame 7, is placed on the
top edge of flange 5 and then covered with upper member 7 under pressure,
so that flange 7' presses the mirror plate 4 against the flange 5 to form
a curved mirror, and thereafter the upper and lower members 6 and 7 are
fastened together by nuts or screws 8. For this purpose, a mirror plate 4
a little longer than the inside length dimension of the upper member 7 is
necessary.
Since the mirror plate is flexible and can be formed separately from the
supporting rack, the tin vapor coating can be placed on the plate surface
by a mass-production operation by vacuum vapor coating. Such tin coating
preferably uses bronze containing 40% of tin, instead of tin alone.
A parabolic curve is most preferable for the curvature of the curved
reflector because of its image focussing effect.
However, it is quite difficult to form a mirror in the shape of a parabolic
curve.
For example, when forming the mirror by pressing, it is necessary to
compensate for dimensional deviations between the curves of the original
mold and those of the pressed mirror caused by spring back. According to
the present invention, a simple circular section can be used.
In order to calculate the aberration of the curvature at the end of the
curved surface, let the radius of the curvature be .alpha. = 2.3m and the
width .beta. = 0.3m, as illustrated in FIGS. 5 and 6. An angle .theta.
between the optical axis and a line connecting the outside end and the
center of the curvature is calculated as .theta. = 3.degree.43' from
equation
##EQU1##
as seen from FIG. 5. Since .theta. is considerably small, X can be
calculated by solving
##EQU2##
from the equation in FIG. 6. Then
##EQU3##
An incident solar ray along the optical axis of the curved reflector
reflects at an angle .theta. of 0.degree., so that X = .alpha./2 holds
true for this case.
Likewise, incident solar rays near the optical axis, which are less
affected by the aberration of the curved surface, focus their images at X
= 1.150m, i.e. half the length of the radius of curvature. On the other
hand, incident solar rays reflected near the outside ends of the curved
surface focus their images at X = 1.152m due to the aberration of the
curved surface.
The deviation y amounts to 2mm along the optical axis, and the width or
amplitude z of the non-focussed image is
Z = y tan 2.theta. = 2 tan 7.degree.26' .apprxeq. 0.26mm.
As compared to the focussing ability of a telescope, in which the resolving
power of an image is important, a solar boiler reflector designed chiefly
for concentrating the solar energy is unaffected by such a small
deviation.
Consequently, the curvature of the mirror surface need not necessarily be
parabolic, and only a sector of a simple circle is sufficient.
Since the curved mirror described in this invention is a part of the
cylindrical surface as shown in FIG. 2, the centers of curvature lie along
a line a'b', and the focal points also lie along a line c d. The optical
axes define an optical axial plane abdc, each of the lengths a'b' and c d
equaling the width a b of the curved mirror.
Accordingly, a shown in FIG. 3, incident solar rays S S parallel to the
optical axial plane abcd will focus on a line ef on an extension of line
cd, even if the rays are projected at some angle to line ab.
Making a practical application of this principle, as shown in FIGS. 7 and
8, small curved mirrors 1 assembled in a supporting assembly or rack, as
shown in FIGS. 4 and 11, are so arranged that the sector shapes lie on
circular lines concentric with the optical axis of the concave spherical
surface of the reflector unit, and their optical axial planes are directed
or oriented to the optical axis or generally to a supporting post 9 of a
large concave spherical surface, and at the same time the small curved
mirrors are so arranged at some angles on the spherical surface in
respective places that the incident solar rays SS parallel to the optical
axis of the large concave spherical surface are focussed and overlapped or
concentrated on the same conical surface after reflecting from the small
curved mirrors 1. Thus, radial focal images are obtainable on a relatively
narrow area, such as a boiler heating surface 10, to increase the energy
density.
The efficiency of this concentrating device will be slightly inferior to
that of a small convex lens having a focal point concentration. However,
an extremely large energy density is obtainable from the above-described
concentrating means as compared with other concentrating means using a
large spherical surface. Even with actual dimensions of 0.1 m for the
width of the small curved mirror surfaces, and 5m outer diameter of the
concave spherical surface, a load of approximately 20,000 Kcal/m.sup.2.h
is obtainable at the heating surface provided at the focal part thereof.
Since the energy thus obtained is utilized as the heat source of the
boiler, the concentrating means in accordance with this invention provides
a solar boiler which has an extremely simple construction, which has
reduced manufacturing cost, and which is further very durable for its
service life.
In order to utilize the solar heat which can be obtained by concentrating
solar rays by the above-described sort of reflector means most
efficiently, it is necessary to carry out efficient heat exchange and
further to change the position of the optical axis of the reflector body
so that it is directed toward and follows the sun.
According to one embodiment of the invention, the apparatus has, as seen in
FIGS. 9 and 10, a base frame 11 with a bearing mount 12 provided on the
base frame. The lower part of a turning body 13 is freely rotatably
supported on a supporting shaft 14 on bearing mount 12.
A reflector mounting frame 15 is freely rotatably mounted on a supporting
shaft 16 on the upper part of the turning body 13, and the supporting
shaft 16 is at right angles to said turning body supporting shaft 14, thus
permitting the reflector mounting frame 15 to turn perpendicularly to the
turning body 13.
A turning mechanism 17 mounted on the bearing mount 12 is coupled to the
turning body 13, and a turning mechanism 18 mounted on turning body 13 is
coupled to the reflector mounting frame 15, and both turning mechanisms
are driven by chains 21 which are fixed at both ends thereof to
symmetrical points on the turning body 13 and reflector mounting frame 15,
and guided over several chain sprockets 20 to the respective turning
mechanisms driven by separate motors 19. Thus independent turning motions
of the turning body 13 and reflector mounting frame 15 are possible.
The reflector body 22, as described above, comprises a large number of
curved mirrors 1 arranged on the reflector mounting frame 15 in a concave
spherical configuration, the curved mirrors 1 being so arranged that their
optical axial planes are all oriented to the optical axis of the reflector
body 22, whereby incident solar rays parallel to the optical axis of the
reflector body 22 reflect from each curved mirror 1 and are focussed in a
linear image f and concentrate on the same conical surface.
A boiler unit 23 has a heating wall 10 at the position f which faces toward
the reflector body 22. The boilder body 23 is in the shape of a thin disk
and is designed in such a way that an injection nozzle 25 connected to a
feed water pipe 24 is provided at the center of a heating space within the
body and causes ejected water to spread radially outwardly in the heating
space in a screen flowing over the heating surface 10, and heated water
collector path 26 is provided for around the periphery of the heating
space, and is connected to a return pipe 27.
The other end of said feed water pipe 24 is connected through a circulating
pump 28 driven by a motor 31 to a water supply (not shown), and the other
end of the return pipe 27 is connected through a service tank 29 to the
circulating pump 28 and a steam pipe 39.
A supporting post 9 holds the boiler body 23 in place, and insulating
material 32 covers the feed water pipe 24 and the return pipe 27.
In operation, the optical axis of the reflector 15 is directed toward the
sun by the action of the turning mechanisms 17 and 18 on turning body 13
and reflector body 22, and the solar rays reflect from each curved mirror
1 forming the reflector body 22 onto the heating wall 10 of the boiler
body 23.
Feed water from the feed water pipe 24 is ejected from the injection nozzle
25 of the boiler body 23 against the inside surface of the heating wall
10, and the water forms a water screen on said surface while spreading out
radially, being heated as it flows across said inside surface into the
heated water collector path 26.
The heated water is further circulated through the return pipe 27, service
tank 29, and circulating pump 28 to the boiler body 23 in order to heat it
up to a specified temperature, so that it can be utilized as steam or
heated water.
Analyzing the heat transfer at the boiler heating wall using an
experimental equation for heat transfer to a laminar flow across a flat
plate which closely resembles the fluid motion in the water screen in the
present invention, the average heat transfer rate (.alpha..sup.cm)
(Kcal/m.sup.2.h..degree. C) can be calculated by the following
experimental equation:
##EQU4##
where 1 = length of flat plate in m;
.lambda. = coefficient of thermal conductivity of fluid in
Kcal/m.h..degree. C;
.nu. = coefficient of kinematic viscosity in m.sup.2 /S;
a = coefficient of thermal diffusivity of fluid in m.sup.2 /S; and
u.sub.o = flow speed in m/S.
Supposing that 1, .lambda., .nu., and a for boiler water which flows over a
heating surface of some fixed dimensions are constant, the heat transfer
rate .alpha..sup.cm becomes proportional to the flow speed u.sub.o.
The above equation is quoted from A Handbook of Mechanical Engineering,
published Apr. 15, 1968 by Nihon Kikai Gakkai, Tokyo, chapter 11, "Heat
and Thermal Dynamics," section 1, 9-3-b "Laminar Flow Heat Transfer in
Flat Plate," equation (49).
Further, a shown in FIG. 12, the flow speed of the fluid in the water
screen, which is formed when water ejected from the nozzle 25 collides
with the flat plate, is the same as the speed of a water column ejected
from a nozzle (u.sub.o = v.sub.o) neglecting a friction effect between
them.
A water screen boiler utilizing this arrangement provides the highest fluid
speed applicant is aware of among the circulating systems in which boiler
water flows over a boiler heating surface.
As can be seen from the above two reasons, the water screen boiler has an
extremely good performance and a thermal efficiency as high as 97% can
actually be obtained at present in the laboratory.
In a solar boiler, solar rays reflected from a reflector should necessarily
be concentrated on a surface of a body of rotation such as a conical or a
spherical surface, which is as small as possible. This is due to two
reasons. One is to decrease to a minimum the loss caused by heat
radiation, and the other is to provide a heat exchanging section which is
as light-weight and compact as possible. Moreover, since this solar energy
absorbing surface is subjected to exposure to a high temperature and
further to heat radiation, it must be composed of a boiler heating surface
having an extremely high sensitivity and heat exchange performance. For
this reason, the water screen formed by a jet nozzle is the most suitable
mechanism for use with the above-described heating surface.
The water screen type heating wall illustrated in FIG. 10 has a small
capacity, uses only one nozzle, and requires a sufficient quantity of
circulating water and circulating water pressure so as to form a
continuous water screen, even when the boiler unit swings approximately
120.degree. with the spherical frame, so as to flow over the curved
surface of the wall 10 of the solar boiler 23 of FIG. 10. Therefore, a
relatively large amount of power is required to operate this embodiment.
The heating wall shown in FIG. 13 is a modification for a larger boiler
system. Water is supplied through a pipe 24a to nozzles 25a mounted on a
semi-spherical member 40 around a post 9a. Reflected solar rays are
concentrated on a concave spherical surface side of wall 10a. Water
ejected from nozzles 25a positioned at intervals over the surface 10a
collides with the convex surface of wall 10a of the spherical heat
exchanging section, thus producing the water screen. A relatively small
amount of operating power is required, and a high heat transfer efficiency
is obtainable with this mechanism.
In order to keep the optical axis of the reflector body 22 directed toward
the sun, the turning mechanism 17 and turning mechanism 18 for the turning
body 13 and reflector mounting frame 15 are periodically and
intermittently operated. The operation is automatically performed by a
control mechanism in accordance with a predetermined program based on the
solar movement time table, such as is done with modern radio telescopes.
According to the invention as described above, the solar rays are reflected
on a large number of curved mirrors forming the reflector body, linear
images of each ray are radially focused on a small area, and the boiler
body of the water screen boiler is positioned at this focal position with
the heating wall opposed to the reflector body, so that an extremely high
heat exchange efficiency is obtained. Further, the directing of the
optical axis of the reflector body toward the sun in accordance with the
solar movement is automatically performed by the intermittent actions of
the turning body supported on the base frame and the reflector mounting
frame supported on said turning body, which are mounted to turn
perpendicularly to each other, so that the solar heat is always fully
utilized to heat boiler water at the best efficiency. Thus this invention
provides a useful technique for making good use of solar energy.
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