Title of Invention	AN IMPROVED PHOTOVOLTAIC DEVICE.
Abstract	The invention relates to an improved Photovoltaic device (14) with a front side exposed to radiation, and an opposite backside for converting radiation energy into electric power, with a cooling unit, disposed between the front face of a photovoltaic module (10) and a radiation source (21), the cooling unit (13) having a liquid medium (11) which serves as selective filter allowing the passage of the radiation fraction used for the photovoltaic effect and converting the longer waved radiation into heat which is dissipated to substantially prevent heating of the photovoltaic module.

Title of Invention

AN IMPROVED PHOTOVOLTAIC DEVICE.

Abstract

The invention relates to an improved Photovoltaic device (14) with a front side exposed to radiation, and an opposite backside for converting radiation energy into electric power, with a cooling unit, disposed between the front face of a photovoltaic module (10) and a radiation source (21), the cooling unit (13) having a liquid medium (11) which serves as selective filter allowing the passage of the radiation fraction used for the photovoltaic effect and converting the longer waved radiation into heat which is dissipated to substantially prevent heating of the photovoltaic module.

Full Text	PHOTOVOLTAIC DEVICE The invention relates to a photovoltaic device with a front side exposed to a radiation and an opposite backside for convening radiation energy into electric energy with a coating unit Photovoltaic electric power generators are normally fixed and aligned to the direction of Incident primary sunlight. Occasionally, the systems have also been equipped w th a uniaxial or biaxial solar tracking unit, or sunlight concentrators have been used. In particular when using light concentrators, there is the problem that the attainable efficiency deteriorates at elevated temperatures of the photovoltaic device. The reason for that resides in that the electrons liberated by incident light photons are partially thermallv recombined so that the useful outer current flow of the photovoltaic module is reduced. This problem is solved in the prior art by providing on the backside of the photovoltaic modules, in like manner as in electric components, heat-conducting sheets to improve the heat emission. In the event of higher temperatures, the photovoltaic modules are activeiy cooled by conducting a coolant across the backside of the modules, The processes for active as well as passive heat dissipation are, however, structurally complex and thus only rarely applied. The invention is thus based on the object to so improve a photovoltaic device of this type as to exhibit a higher efficiency. The object is attained by providing the cooling unit with a liquid medium arranged between the front side and the radiation source. In order to prevent a reduction of radiation incident on the photovoltaic device, to date only cooling units have been proposed which are arranged on the backside of the photovoltaic device. The invention is based on the recognition that a cooling unit implemented with a liquid medium can also be arranged on the front side of the photovoltaic device. The liquid medium can hereby be so selected that the useful range of the solar spectrum for the photovoltaic effects is not, or only insignificantly absorbed by the liquid medium, while the radiation energy in the regions which are of secondary importance for the photovoltaic effects are absorbed by the liquid medium. The liquid medium thus permits passage of radiation energy useful for photovoltaic effects and absorbs the remaining radiation energy. It has been shown that liquids consisting substantially of water are particularly superior as liquid medium. Depending on the employed photovoltaic module, it is, however, also possible to use oils, alcohols or similar substances. These media are advantageously mixed with substances which optimize the filter characteristic in a solution or suspension. A simple photovoltaic device is attained by circulating the liquid medium between front side and radiation source as a result of differences in gravity between warm and cold medium. This arrangement known under the name thermosiphon includes a process water reservoir having a lower region with a cold water outlet. From here, cold water flows into the lower region of the photovoltaic device and rises within the photovoltaic device to its upper end from where the water flows back again to the reservoir. As warm water enters the reservoir at a higher location, a temperature gradient forms in the water reservoir, with cold water at the bottom and warmer water in the upper zone. Heated water can be withdrawn directly from the reservoir. Preferred however is the disposition of a process water heat exchanger in the reservoir to heat cold process water to the desired process water temperature. According to an advantageous embodiment, the cooling unit includes a pumpjpr the liquid medium. This permits a flow of the liquid medium through the cooling unit to thereby continuously dissipate heat. Advantageously, the cooling unit includes a thermostat for controlling the pump. This enables a combination of a sufficient cooling with an effective warm water recovery. The temperature adjusted at the thermostat and the pump output are determined by the required warm water temperature and the demanded cooling action. Particularly good cooling actions can be realized when conducting the liquid medium directly across photovoltaic elements. The efficiency can be increased when conducting the liquid medium first across the backside and then across the front side of the photovoltaic device. The still cold medium is thereby heated on the backside of the device and absorbs further heat energy on the front side of the device. In this way, an effective cooling of the photovoltaic device is realized on the one hand, and a liquid medium at relatively high temperature is made available for further utilization. Further increase of the efficiency are implemented through several cooling units in parallel or serial relationship. A preferred embodiment provides for the arrangement of a further cooling unit at a distance to the front side. While this distanced cooling unit serves mainly as selective filter, a direct placement of the cooling unit on the photovoltaic modules allows at the same time filter effects and a cooling of the modules. Particular filter characteristics can be realized with the selection of the cooling medium. It has been shown as advantageous to arrange between the liquid medium and the radiation source a selective radiation-transparent layer. On the one hand, this selective radiation-transparent layer serves the conduction of the fluid, and, on the other hand, the combination of radiation-transparent layer and liquid medium produces a filter characteristic which is suited to the solar spectrum to be exploited for photovoltaic effects. It has been shown as advantageous to preferably coat the radiation-transparent layer on the side facing the radiation source in selective areas radiation- transparent. The selection of different coating materials and processes, too, has an impact on the filter characteristic to attain an optimal characteristic curve of the filter in a positive manner. Extensive series of tests have shown that a plate or a film of a fluoropoiymer as radiation-transparent layer yields particularly good results. Especially, films of fluoropoiymer are good for manufacture and useful for conduction of liquid coolant and as radiation filter. Good results have also been realized with acryl, polycarbonate and glass because these materials offer a high transparency in the incident spectrum and are mechanically stable as well as weather-resistant and waterproof. This can be implemented in a cost-efficient manner, for example, with double webbed acryl (PMMA) plates and double webbed polycarbonate plates. Advantageously, the radiation-transparent layer forms an envelope which surrounds the liquid medium. This envelope thus represents a closed component which is usable as filter and can easily be exchanged. The various described embodiments are applicable for non-concentrated as well as for concentrated radiation. Explanations and exemplified embodiments of the described invention are illustrated in the drawing and will be explained in detail hereinafter. It is shown in Fig. 1 the relative intensity of the solar spectrum over the wavelength and the transparency of a water layer of a thickness of 5 cm and a fluoropolymer film of a thickness of 100 microns over the wavelength, Fig. 2 a single layer photovoltaic device, Fig. 3 a cutaway view of a photovoltaic module, Fig. 4 the temperature distribution over the layer thickness of the photovoltaic module shown in Fig. 3, Fig. 5 a double-layer photovoltaic device and Fig. 6 a double-layer photovoltaic device with concentrator and pre-cooler. Fig. 1 shows on the left ordinate 1 the relative intensity, on the right ordinate 2 the radiation transparency x in percent and on the abscise 3 the wavelength in nanometer. Plotted in this coordinate system are the solar spectrum 4 and the area 5 of this spectrum 4, useful for photovoltaic effects. The transmission of a water layer of a thickness of 5 cm is elucidated by line 6, and line 7 shows the transmission of a fluoropolymer film of a thickness of 100 microns. This illustration shows that the 5 cm thick water layer allows passage of almost the entire radiation of the spectral range useful for photovoltaic effects and absorbs only the longer wave radiation. The film allows almost unaltered passage of the radiation across the entire spectral region and absorbs a portion of the radiation only in the short-wave range. A photovoltaic module disposed beneath the water layer is thus exposed to nearly the entire radiation useful for photovoltaic effects whereas the longer wave radiation is absorbed by the water layer and leads to a heating of the water. This effect is utilized in the photovoltaic device shown in Fig. 2. Hereby, a water layer 11 flows across the photovoltaic module 10 which is thus cooled. The water layer 11 is enveloped by a transparent film 12 so that water is directed into this film 12. A pump 13 pumps the water from a reservoir (not shown) through the photovoltaic device 14 to a reservoir 15 from which water can be withdrawn from outlet 17 in controlled doses via the valve 16. The heating coil 18 permits a after- heating of the water, when the heating of water generated by the photovoltaic device is not sufficient. Disposed at the photovoltaic device 14 is a temperature probe 19 which controls the pump 13 in such a manner that heated fluid is pumped into the reservoir 15 and fresh, cool fluid flows into the arrangement, whenever the temperature probe 19 has reached a defined, adjustable limit temperature. The described filter 11, 12 is selective as it permits passage of radiation of only certain wavelength. It is, however, also recuperative because it recovers recuperatively the heat flow occurring on the surface of the photovoltaic module 10 by means of essentially two mechanisms. On the one hand, this is the heat exchange realized by the direct contact of the fluid with the hot surface of the module. On the other hand, the module surface radiates with a radiation sninea according to the Wien's displacement law in dependence on the temperature to the long wave range. This radiation is absorbed in accordance with the invention by the filter fluid, the water, and converted into heat. During cooling in accordance with the present invention, the long wave photons which cannot trigger a photo effect are converted into heat already before reaching the module, whereas in conventional photovoltaic devices they are absorbed in the module and the generated heat flux must be withdrawn through the module. The advantage of the photovoltaic device according to the invention thus resides in the particular intense exposure of the uppermost layer, i.e. the side facing the radiation, to a cooling action. This is especially relevant because in a photovoltaic module - as shown in Fig. 3 - the light quanta 20 of the radiation 21 are absorbed in the uppermost layer 22 of the photovoltaic module 23 of the layer thickness d so that a temperature gradient is created, as indicated in Fig. 4 by line 24. The line 24 shows the linear temperature profile between the bottom side 25 of the photovoltaic module 23 with the temperature Tu and the top side 26 of the photovoltaic module 23 with the temperature T0 This representation again clearly shows that the front side cooling according to the invention of the module is especially advantageous because it acts directly on the hottest surface of the module 23. Fig. 5 shows a further development of the photovoltaic device shown in Fig. 2. In this photovoltaic device 30, a first fluid layer 32 flows across the radiation-distal side of the photovoltaic module 31 and a second fluid layer 33 flows across the radiation-proximal side. A pump 34 transports a water stream 35 along the backside 36 of the photovoltaic module 31 in the first layer 32, and the water cools hereby the backside of the photovoltaic module 31. A deflection device 37 guides the water stream 35 at the lower end of the photovoltaic module 31 around the module to the upper side 38 where it flows in the second layer 33 along the upper side 38 upwards. Water further heated on the front side then flows in a reservoir 39 and from there via a valve 40 to the outlet 41. The water-carrying layer 32 on the backside may either be a coiled pipe of suitable geometry in good heat contact with the backside, or a whole-area plate heat exchanger. To minimize heat losses to the outside, the entire arrangement is supplemented by an opaque heat insulation 42 as well as second transparent covering 43. The second transparent covering 43 is arranged at a distance to the transparent fluoropolymer film 44. The control 45 permits a desired adjustment of the temperature increase of the cooling fluid. The temperature desired at outlet 17 depends on the application at hand, and may lie at 30 °C, for example for a swimming pool heater, while shower water requires about 40 °C. Both these typical types of use of solar-heated warm water find application in moderate latitudes in particular during the summer half-year. As the mean photovoltaic module temperature during this season lies above 50 °C, the system according to the invention provides not only for electric power and warm water but also increases at the same time the power efficiency. According to the invention, a surface element which to date was able to generate electric power at about 10% efficiency from the available radiation can be used to make a module which generates current and warm water at an overall efficiency of about 60%. Fig. 6 shows that the system according to the invention is basically applicable also for photovoltaic arrangements with sunlight concentration. Especially in cases of increased energy density on the surface of the module, the described, favorable selective and recuperative heat extraction mechanisms have an even stronger impact. When using a photovoltaic device, shown in Fig. 5, for concentrated radiation, the energy content of the long wave, photovoltaically non-useful part of the solar spectrum is used to heat water to a relative low temperature. When the coolant stream is heated to higher temperatures, the efficiency of the photovoltaic module is decreased. Thus, the photovoltaic device, shown in Fig. 6, is proposed for concentrated radiation. This photovoltaic device 50 includes essentially the concentrator lens 51, the pre-cooler 52 and the cooled photovoltaic element 53. The concentrator lens 51 may also be replaced by a different concentrator optics, such as, for example, a mirror system. The photovoltaic module 53 has a configuration which corresponds to the photovoltaic device 30 shown in Fig. 5. The pre-cooler 52 serves as pre-filter which includes in case of a linear concentrator a transparent cuboid of the dimension of the focal line at this location. In case of spot-like concentrator, a transparent flat hollow cylinder is used of the dimension of the focal sport at this location. A fluid flows through the hollow cuboids or hollow cylinders 54 and has besides the selectivity, illustrated in Fig. 1, a boiling point which is as high as possible so that the system pressure remains low. In the case at hand, water is used with respective additives. This water is heated in the hollow cuboid or hollow cylinder 54 by the radiation 55 concentrated by the lens 51 to temperatures in the range of about 100 °C. When the predetermined temperature has been reached, the temperature probe 56 acts on the pump 57 so that a new fluid stream can be pumped into the cavity 54. As a consequence, very high temperatures are generated in the focal spot of the lens 51 and used for heating of a selectively radiation-transparent fluid. The device 50 is thus capable to produce electric power 60, process water heat 61 and high-temperature process heat 62 from primary incident energy 59. The energy of the high-temperature heat may hereby be converted, for example via suitable thermodynamic machines, into mechanical work or additional electric power. The quantity of warm water produced at the photovoltaic devices 53 may, however, further heated in the pre-filter 52 by making a connection between the line end 61 and the pump 57. The described photovoltaic devices are based on the correct selection of the fluid and the transparent material for the sheathing. In the area of the liquid, there are many options ranging from water via oils, alcohols etc.. As photovoltaic modules of different configuration (e.g. silicon, GaAs, ZnS etc.) can be used, the selective filter must be matched to the respectively required photovoltaically active spectral region. This matching is, optionally, realized in a relative precise manner through filter characteristic curves of the transparent sheathing materials and/or liquids. The filter properties can be varied by selectively coating the enveloping materials themselves and admixing additives to the liquids. In the described exemplified applications, a commercially available fluoropolymer film of a thickness of 100 microns was used. This film is chemically inert, environmentally neutral and is flexible to process. Care should be taken, when using films, that a suitable mechanical support or channel-like subdivision of the water-carrying layer is used to prevent a pillow-shaped bulging in order to form a relatively even thickness of the water layer across the entire area. Photovoltaic elements are normally covered on their surface with a glass sheet or plastic sheet in order to protect the active photovoltaic area against mechanical impacts. When using a sheathing for conducting liquids, which bears upon the active photovoltaic area, the use of a further covering of the photovoltaic elements may be omitted as the water conducting elements assume the function of a protective surface. For example, so-called dual web plates can be used to guide the liquid upon the photovoltaic elements and to protect the photovoltaic elements at the same time. Of course, conventionally used glass plates may also be provided with liquid-pervious channels which extend in the plate plane, or may allow as double plate a liquid-guiding layer. In the described exemplified embodiments, water was used as selective fluid. Water is cheap and environmentally neutral. When using additives for water, the use of a heat exchanger is required for generating process water. The temperatures of the produced amount of warm water may, however, be so set that water without additives can be used. As a consequence, the use of a heat exchanger may be omitted. We Claim 1. An improved Photovoltaic device (14) with a front side exposed to radiation, and an opposite backside for converting radiation energy into electric power, with a cooling unit, disposed between the front face of a photovoltaic module (10) and a radiation source (21), characterized in that the cooling unit (13) having a liquid medium (11) which serves as selective filter allowing the passage of the radiation fraction used for the photovoltaic effect and converting the longer waved radiation into heat which is dissipated to substantially prevent heating of the photovoltaic module. 2. The Photovoltaic device as claimed in claim 1, wherein the liquid medium (11) is substantially water. 3: The Photovoltaic device as claimed in claims 1 or 2, wherein the liquid medium (11) flows between the front side and the radiation source as a result of gravity differentials between warm and cold medium. 4. The Photovoltaic device as claimed in one of the preceding claims, wherein the cooling unit comprises a pump (13) for the liquid medium (11). 5. The Photovoltaic device as claimed in claim 4, wherein the cooling unit comprises a thermostat (19) for controlling the pump (13). 6. The Photovoltaic device as claimed in one of the preceding claims, wherein the liquid medium (11) flows first across the backside (36) and then across the front side (38). 7. The Photovoltaic device as claimed in one of the preceding claims, wherein an additional cooling device (52) is disposed at a distance to the front side. 8. The Photovoltaic device as claimed in one of the preceding claims, wherein a selectively radiation-transparent layer (12) is arranged between the liquid medium and the radiation source. 9. The Photovoltaic device as claimed in claim 8, wherein the radiation- transparent layer (12) is coated, selectively radiation-transparent, on the side facing the radiation source. 10.The Photovoltaic device as claimed in one of the claims 8 or 9, wherein the radiation-transparent layer (12) comprises a fluoropoiymer film or fluoropolymer plate. 11.The Photovoltaic device as claimed Hi one of the claims 8 or 9, wherein the radiation-transparent layer (12) forms an envelope surrounding the liquid medium (11). DATED THIS 5th DAY OF FEBRUARY 2001. The invention relates to an improved Photovoltaic device (14) with a front side exposed to radiation, and an opposite backside for converting radiation energy into electric power, with a cooling unit, disposed between the front face of a photovoltaic module (10) and a radiation source (21), the cooling unit (13) having a liquid medium (11) which serves as selective filter allowing the passage of the radiation fraction used for the photovoltaic effect and converting the longer waved radiation into heat which is dissipated to substantially prevent heating of the photovoltaic module.

Full Text

PHOTOVOLTAIC DEVICE
The invention relates to a photovoltaic device with a front side exposed to a
radiation and an opposite backside for convening radiation energy into electric
energy with a coating unit
Photovoltaic electric power generators are normally fixed and aligned to the
direction of Incident primary sunlight. Occasionally, the systems have also been
equipped w th a uniaxial or biaxial solar tracking unit, or sunlight concentrators
have been used.
In particular when using light concentrators, there is the problem that the
attainable efficiency deteriorates at elevated temperatures of the photovoltaic
device. The reason for that resides in that the electrons liberated by incident light
photons are partially thermallv recombined so that the useful outer current flow of
the photovoltaic module is reduced.
This problem is solved in the prior art by providing on the backside of the
photovoltaic modules, in like manner as in electric components, heat-conducting
sheets to improve the heat emission.
In the event of higher temperatures, the photovoltaic modules are activeiy cooled
by conducting a coolant across the backside of the modules,
The processes for active as well as passive heat dissipation are, however,
structurally complex and thus only rarely applied.
The invention is thus based on the object to so improve a photovoltaic device of
this type as to exhibit a higher efficiency.
The object is attained by providing the cooling unit with a liquid medium arranged
between the front side and the radiation source.
In order to prevent a reduction of radiation incident on the photovoltaic device, to
date only cooling units have been proposed which are arranged on the backside
of the photovoltaic device. The invention is based on the recognition that a
cooling unit implemented with a liquid medium can also be arranged on the front
side of the photovoltaic device. The liquid medium can hereby be so selected
that the useful range of the solar spectrum for the photovoltaic effects is not, or
only insignificantly absorbed by the liquid medium, while the radiation energy in
the regions which are of secondary importance for the photovoltaic effects are
absorbed by the liquid medium. The liquid medium thus permits passage of
radiation energy useful for photovoltaic effects and absorbs the remaining
radiation energy.
It has been shown that liquids consisting substantially of water are particularly
superior as liquid medium. Depending on the employed photovoltaic module, it is,
however, also possible to use oils, alcohols or similar substances. These media
are advantageously mixed with substances which optimize the filter characteristic
in a solution or suspension.
A simple photovoltaic device is attained by circulating the liquid medium between
front side and radiation source as a result of differences in gravity between warm
and cold medium. This arrangement known under the name thermosiphon
includes a process water reservoir having a lower region with a cold water outlet.
From here, cold water flows into the lower region of the photovoltaic device and
rises within the photovoltaic device to its upper end from where the water flows
back again to the reservoir. As warm water enters the reservoir at a higher
location, a temperature gradient forms in the water reservoir, with cold water at
the bottom and warmer water in the upper zone. Heated water can be withdrawn
directly from the reservoir. Preferred however is the disposition of a process
water heat exchanger in the reservoir to heat cold process water to the desired
process water temperature.
According to an advantageous embodiment, the cooling unit includes a pumpjpr
the liquid medium. This permits a flow of the liquid medium through the cooling
unit to thereby continuously dissipate heat.
Advantageously, the cooling unit includes a thermostat for controlling the pump.
This enables a combination of a sufficient cooling with an effective warm water
recovery. The temperature adjusted at the thermostat and the pump output are
determined by the required warm water temperature and the demanded cooling
action.
Particularly good cooling actions can be realized when conducting the liquid
medium directly across photovoltaic elements. The efficiency can be increased
when conducting the liquid medium first across the backside and then across the
front side of the photovoltaic device. The still cold medium is thereby heated on
the backside of the device and absorbs further heat energy on the front side of
the device. In this way, an effective cooling of the photovoltaic device is realized
on the one hand, and a liquid medium at relatively high temperature is made
available for further utilization.
Further increase of the efficiency are implemented through several cooling units
in parallel or serial relationship. A preferred embodiment provides for the
arrangement of a further cooling unit at a distance to the front side. While this
distanced cooling unit serves mainly as selective filter, a direct placement of the
cooling unit on the photovoltaic modules allows at the same time filter effects and
a cooling of the modules.
Particular filter characteristics can be realized with the selection of the cooling
medium. It has been shown as advantageous to arrange between the liquid
medium and the radiation source a selective radiation-transparent layer. On the
one hand, this selective radiation-transparent layer serves the conduction of the
fluid, and, on the other hand, the combination of radiation-transparent layer and
liquid medium produces a filter characteristic which is suited to the solar
spectrum to be exploited for photovoltaic effects.
It has been shown as advantageous to preferably coat the radiation-transparent
layer on the side facing the radiation source in selective areas radiation-
transparent. The selection of different coating materials and processes, too, has
an impact on the filter characteristic to attain an optimal characteristic curve of
the filter in a positive manner.
Extensive series of tests have shown that a plate or a film of a fluoropoiymer as
radiation-transparent layer yields particularly good results. Especially, films of
fluoropoiymer are good for manufacture and useful for conduction of liquid
coolant and as radiation filter. Good results have also been realized with acryl,
polycarbonate and glass because these materials offer a high transparency in
the incident spectrum and are mechanically stable as well as weather-resistant
and waterproof. This can be implemented in a cost-efficient manner, for example,
with double webbed acryl (PMMA) plates and double webbed polycarbonate
plates.
Advantageously, the radiation-transparent layer forms an envelope which
surrounds the liquid medium. This envelope thus represents a closed component
which is usable as filter and can easily be exchanged.
The various described embodiments are applicable for non-concentrated as well
as for concentrated radiation.
Explanations and exemplified embodiments of the described invention are
illustrated in the drawing and will be explained in detail hereinafter.
It is shown in
Fig. 1 the relative intensity of the solar spectrum over the wavelength and the
transparency of a water layer of a thickness of 5 cm and a fluoropolymer
film of a thickness of 100 microns over the wavelength,
Fig. 2 a single layer photovoltaic device,
Fig. 3 a cutaway view of a photovoltaic module,
Fig. 4 the temperature distribution over the layer thickness of the photovoltaic
module shown in Fig. 3,
Fig. 5 a double-layer photovoltaic device and
Fig. 6 a double-layer photovoltaic device with concentrator and pre-cooler.
Fig. 1 shows on the left ordinate 1 the relative intensity, on the right ordinate 2
the radiation transparency x in percent and on the abscise 3 the wavelength in
nanometer. Plotted in this coordinate system are the solar spectrum 4 and the
area 5 of this spectrum 4, useful for photovoltaic effects. The transmission of a
water layer of a thickness of 5 cm is elucidated by line 6, and line 7 shows the
transmission of a fluoropolymer film of a thickness of 100 microns.
This illustration shows that the 5 cm thick water layer allows passage of almost
the entire radiation of the spectral range useful for photovoltaic effects and
absorbs only the longer wave radiation. The film allows almost unaltered
passage of the radiation across the entire spectral region and absorbs a portion
of the radiation only in the short-wave range. A photovoltaic module disposed
beneath the water layer is thus exposed to nearly the entire radiation useful for
photovoltaic effects whereas the longer wave radiation is absorbed by the water
layer and leads to a heating of the water.
This effect is utilized in the photovoltaic device shown in Fig. 2. Hereby, a water
layer 11 flows across the photovoltaic module 10 which is thus cooled. The water
layer 11 is enveloped by a transparent film 12 so that water is directed into this
film 12. A pump 13 pumps the water from a reservoir (not shown) through the
photovoltaic device 14 to a reservoir 15 from which water can be withdrawn from
outlet 17 in controlled doses via the valve 16. The heating coil 18 permits a after-
heating of the water, when the heating of water generated by the photovoltaic
device is not sufficient.
Disposed at the photovoltaic device 14 is a temperature probe 19 which controls
the pump 13 in such a manner that heated fluid is pumped into the reservoir 15
and fresh, cool fluid flows into the arrangement, whenever the temperature probe
19 has reached a defined, adjustable limit temperature.
The described filter 11, 12 is selective as it permits passage of radiation of only
certain wavelength. It is, however, also recuperative because it recovers
recuperatively the heat flow occurring on the surface of the photovoltaic
module 10 by means of essentially two mechanisms. On the one hand, this is the
heat exchange realized by the direct contact of the fluid with the hot surface of
the module. On the other hand, the module surface radiates with a radiation
sninea according to the Wien's displacement law in dependence on the
temperature to the long wave range. This radiation is absorbed in accordance
with the invention by the filter fluid, the water, and converted into heat.
During cooling in accordance with the present invention, the long wave photons
which cannot trigger a photo effect are converted into heat already before
reaching the module, whereas in conventional photovoltaic devices they are
absorbed in the module and the generated heat flux must be withdrawn through
the module. The advantage of the photovoltaic device according to the invention
thus resides in the particular intense exposure of the uppermost layer, i.e. the
side facing the radiation, to a cooling action. This is especially relevant because
in a photovoltaic module - as shown in Fig. 3 - the light quanta 20 of the
radiation 21 are absorbed in the uppermost layer 22 of the photovoltaic
module 23 of the layer thickness d so that a temperature gradient is created, as
indicated in Fig. 4 by line 24. The line 24 shows the linear temperature profile
between the bottom side 25 of the photovoltaic module 23 with the
temperature Tu and the top side 26 of the photovoltaic module 23 with the
temperature T0 This representation again clearly shows that the front side
cooling according to the invention of the module is especially advantageous
because it acts directly on the hottest surface of the module 23.
Fig. 5 shows a further development of the photovoltaic device shown in Fig. 2. In
this photovoltaic device 30, a first fluid layer 32 flows across the radiation-distal
side of the photovoltaic module 31 and a second fluid layer 33 flows across the
radiation-proximal side. A pump 34 transports a water stream 35 along the
backside 36 of the photovoltaic module 31 in the first layer 32, and the water
cools hereby the backside of the photovoltaic module 31. A deflection device 37
guides the water stream 35 at the lower end of the photovoltaic module 31
around the module to the upper side 38 where it flows in the second layer 33
along the upper side 38 upwards. Water further heated on the front side then
flows in a reservoir 39 and from there via a valve 40 to the outlet 41.
The water-carrying layer 32 on the backside may either be a coiled pipe of
suitable geometry in good heat contact with the backside, or a whole-area plate
heat exchanger. To minimize heat losses to the outside, the entire arrangement
is supplemented by an opaque heat insulation 42 as well as second transparent
covering 43. The second transparent covering 43 is arranged at a distance to the
transparent fluoropolymer film 44. The control 45 permits a desired
adjustment of the temperature increase of the cooling fluid. The temperature
desired at outlet 17 depends on the application at hand, and may lie at 30 °C, for
example for a swimming pool heater, while shower water requires about 40 °C.
Both these typical types of use of solar-heated warm water find application in
moderate latitudes in particular during the summer half-year. As the mean
photovoltaic module temperature during this season lies above 50 °C, the system
according to the invention provides not only for electric power and warm water
but also increases at the same time the power efficiency.
According to the invention, a surface element which to date was able to generate
electric power at about 10% efficiency from the available radiation can be used to
make a module which generates current and warm water at an overall efficiency
of about 60%.
Fig. 6 shows that the system according to the invention is basically applicable
also for photovoltaic arrangements with sunlight concentration. Especially in
cases of increased energy density on the surface of the module, the described,
favorable selective and recuperative heat extraction mechanisms have an even
stronger impact.
When using a photovoltaic device, shown in Fig. 5, for concentrated radiation,
the energy content of the long wave, photovoltaically non-useful part of the solar
spectrum is used to heat water to a relative low temperature. When the coolant
stream is heated to higher temperatures, the efficiency of the photovoltaic

module is decreased.
Thus, the photovoltaic device, shown in Fig. 6, is proposed for concentrated
radiation. This photovoltaic device 50 includes essentially the concentrator lens
51, the pre-cooler 52 and the cooled photovoltaic element 53. The concentrator
lens 51 may also be replaced by a different concentrator optics, such as, for
example, a mirror system. The photovoltaic module 53 has a configuration which
corresponds to the photovoltaic device 30 shown in Fig. 5. The pre-cooler 52
serves as pre-filter which includes in case of a linear concentrator a transparent
cuboid of the dimension of the focal line at this location. In case of spot-like
concentrator, a transparent flat hollow cylinder is used of the dimension of the
focal sport at this location.
A fluid flows through the hollow cuboids or hollow cylinders 54 and has besides
the selectivity, illustrated in Fig. 1, a boiling point which is as high as possible so
that the system pressure remains low. In the case at hand, water is used with
respective additives. This water is heated in the hollow cuboid or hollow cylinder
54 by the radiation 55 concentrated by the lens 51 to temperatures in the range
of about 100 °C. When the predetermined temperature has been reached, the
temperature probe 56 acts on the pump 57 so that a new fluid stream can be
pumped into the cavity 54. As a consequence, very high temperatures are
generated in the focal spot of the lens 51 and used for heating of a selectively
radiation-transparent fluid.
The device 50 is thus capable to produce electric power 60, process water heat
61 and high-temperature process heat 62 from primary incident energy 59. The
energy of the high-temperature heat may hereby be converted, for example via
suitable thermodynamic machines, into mechanical work or additional electric
power.
The quantity of warm water produced at the photovoltaic devices 53 may,
however, further heated in the pre-filter 52 by making a connection between the
line end 61 and the pump 57.
The described photovoltaic devices are based on the correct selection of the fluid
and the transparent material for the sheathing. In the area of the liquid, there are
many options ranging from water via oils, alcohols etc.. As photovoltaic modules
of different configuration (e.g. silicon, GaAs, ZnS etc.) can be used, the selective
filter must be matched to the respectively required photovoltaically active spectral
region. This matching is, optionally, realized in a relative precise manner through
filter characteristic curves of the transparent sheathing materials and/or liquids.
The filter properties can be varied by selectively coating the enveloping materials
themselves and admixing additives to the liquids.
In the described exemplified applications, a commercially available fluoropolymer
film of a thickness of 100 microns was used. This film is chemically inert,
environmentally neutral and is flexible to process. Care should be taken, when
using films, that a suitable mechanical support or channel-like subdivision of the
water-carrying layer is used to prevent a pillow-shaped bulging in order to form a
relatively even thickness of the water layer across the entire area.
Photovoltaic elements are normally covered on their surface with a glass sheet or
plastic sheet in order to protect the active photovoltaic area against mechanical
impacts. When using a sheathing for conducting liquids, which bears upon the
active photovoltaic area, the use of a further covering of the photovoltaic
elements may be omitted as the water conducting elements assume the function
of a protective surface. For example, so-called dual web plates can be used to
guide the liquid upon the photovoltaic elements and to protect the photovoltaic
elements at the same time. Of course, conventionally used glass plates may also
be provided with liquid-pervious channels which extend in the plate plane, or may
allow as double plate a liquid-guiding layer.
In the described exemplified embodiments, water was used as selective fluid.
Water is cheap and environmentally neutral. When using additives for water, the
use of a heat exchanger is required for generating process water. The
temperatures of the produced amount of warm water may, however, be so set
that water without additives can be used. As a consequence, the use of a heat
exchanger may be omitted.
We Claim
1. An improved Photovoltaic device (14) with a front side exposed to
radiation, and an opposite backside for converting radiation energy into
electric power, with a cooling unit, disposed between the front face of a
photovoltaic module (10) and a radiation source (21), characterized in
that the cooling unit (13) having a liquid medium (11) which serves as
selective filter allowing the passage of the radiation fraction used for the
photovoltaic effect and converting the longer waved radiation into heat
which is dissipated to substantially prevent heating of the photovoltaic
module.
2. The Photovoltaic device as claimed in claim 1, wherein the liquid medium
(11) is substantially water.
3: The Photovoltaic device as claimed in claims 1 or 2, wherein the liquid
medium (11) flows between the front side and the radiation source as a
result of gravity differentials between warm and cold medium.
4. The Photovoltaic device as claimed in one of the preceding claims,
wherein the cooling unit comprises a pump (13) for the liquid medium
(11).
5. The Photovoltaic device as claimed in claim 4, wherein the cooling unit
comprises a thermostat (19) for controlling the pump (13).
6. The Photovoltaic device as claimed in one of the preceding claims,
wherein the liquid medium (11) flows first across the backside (36) and
then across the front side (38).
7. The Photovoltaic device as claimed in one of the preceding claims,
wherein an additional cooling device (52) is disposed at a distance to the
front side.
8. The Photovoltaic device as claimed in one of the preceding claims,
wherein a selectively radiation-transparent layer (12) is arranged between
the liquid medium and the radiation source.
9. The Photovoltaic device as claimed in claim 8, wherein the radiation-
transparent layer (12) is coated, selectively radiation-transparent, on the
side facing the radiation source.
10.The Photovoltaic device as claimed in one of the claims 8 or 9, wherein
the radiation-transparent layer (12) comprises a fluoropoiymer film or
fluoropolymer plate.
11.The Photovoltaic device as claimed Hi one of the claims 8 or 9, wherein
the radiation-transparent layer (12) forms an envelope surrounding the
liquid medium (11).
DATED THIS 5th DAY OF FEBRUARY 2001.
The invention relates to an improved Photovoltaic device (14) with a front side
exposed to radiation, and an opposite backside for converting radiation energy
into electric power, with a cooling unit, disposed between the front face of a
photovoltaic module (10) and a radiation source (21), the cooling unit (13)
having a liquid medium (11) which serves as selective filter allowing the passage
of the radiation fraction used for the photovoltaic effect and converting the
longer waved radiation into heat which is dissipated to substantially prevent
heating of the photovoltaic module.

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Patent Number

224158

Indian Patent Application Number

IN/PCT/2001/0139/KOL

PG Journal Number

40/2008

Publication Date

03-Oct-2008

Grant Date

01-Oct-2008

Date of Filing

05-Feb-2001

Name of Patentee

POWERPULSE HOLDING AG

Applicant Address

GRAFENAUWEG 8, CH - 6304 ZUG

Inventors:

#	Inventor's Name	Inventor's Address
1	KLEINWACHTER, JURGEN	INDUSTRIESTRASSE 8, D-79541 LORRACH

PCT International Classification Number

H01L 31/00

PCT International Application Number

PCT/DE99/02366

PCT International Filing date

1999-08-05

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	198 35 304.9	1998-08-05	Germany
2	199 23 196.6	1999-05-20	Germany