U.S. patent application number 11/576939 was filed with the patent office on 2007-10-25 for device and method for photovoltaic generation of hydrogen.
Invention is credited to Frank Dimroth.
Application Number | 20070246370 11/576939 |
Document ID | / |
Family ID | 35668809 |
Filed Date | 2007-10-25 |
United States Patent
Application |
20070246370 |
Kind Code |
A1 |
Dimroth; Frank |
October 25, 2007 |
Device and Method for Photovoltaic Generation of Hydrogen
Abstract
The invention relates to a device and a method for the
photo-voltaic generation of hydrogen from hydrogen-containing
compounds, sunlight being concentrated on solar cells by means of
an optical concentrator and the consequently generated voltage
being used directly for the electrolysis of a hydrogen-containing
compound, in particular deionised water, in order to generate
hydrogen.
Inventors: |
Dimroth; Frank; (Freiburg,
DE) |
Correspondence
Address: |
KAPLAN GILMAN GIBSON & DERNIER L.L.P.
900 ROUTE 9 NORTH
WOODBRIDGE
NJ
07095
US
|
Family ID: |
35668809 |
Appl. No.: |
11/576939 |
Filed: |
October 7, 2005 |
PCT Filed: |
October 7, 2005 |
PCT NO: |
PCT/EP05/10844 |
371 Date: |
May 30, 2007 |
Current U.S.
Class: |
205/628 |
Current CPC
Class: |
C25B 9/70 20210101; Y02E
10/50 20130101; Y02P 20/133 20151101; H01L 31/0521 20130101; Y02E
60/36 20130101; C25B 1/04 20130101 |
Class at
Publication: |
205/628 |
International
Class: |
C25B 1/00 20060101
C25B001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 18, 2004 |
DE |
102004050638.8 |
Claims
1. A device for the generation of hydrogen from hydrogen-containing
compounds, comprising. a plurality of units made of respectively at
least one optical concentrator for concentrating sunlight onto at
least one solar cell, and at least one solar cell which is not in
contact with the hydrogen-containing compounds and is electrically
connected to an electrolysis unit which has an anode and a cathode
in contact with the hydrogen-containing compounds, wherein the
units are disposed on a tracking system following the position of
the sun.
2. The device according to claim 1, wherein each unit has an
electrical power of less than 100 watts.
3. The device according to claim 1, wherein the electrolysis unit
has an operating temperature of -10.degree. C. to 200.degree. C.,
in particular of 30.degree. C. to 100.degree. C.
4. The device according to claim 1, wherein the optical
concentrator is a point-focusing lens, Fresnel lens or a
line-focusing lens.
5. The device according to claim 1, wherein the optical
concentrator is a parabolic mirror with a line focus.
6. The device according to claim 1, wherein the optical
concentrator is a dished mirror with a point focus.
7. The device according to claim 1, wherein the solar cell consists
of a plurality of layers of semiconductor materials which are
connected to each other in series and have respectively different
band gap energy.
8. The device according to claim 7, wherein the semiconductor
materials are selected from the group consisting of silicon,
germanium and ITT-V compounds of aluminium, gallium, indium,
nitrogen, phosphorus, arsenic and antimony.
9. The device according to claim 8, wherein the solar cell has an
np polarity.
10. The device according to claim 1, wherein the solar cell has a
pn polarity.
11. The device according to claim 1, wherein the solar cell has a
pn or np transition and a voltage of more than one of: 1.4 V, and
1.6 to 2.4 V.
12. The device according to claim 1, wherein the solar cell has a
plurality of series-connected pn or np transitions and has a
voltage in the range of 1.5 to 6 V.
13. The device according to claim 1, wherein the solar cell has an
area of 0.01 to 1 cm.sup.2.
14. The device according to claim 1, wherein the electrolysis unit
contains a proton-permeable polymer membrane (PEM) which is in
direct contact with the cathode and the anode.
15. The device according to claims 1, wherein the anode and the
cathode to includes at least one of: noble metals, taken from the
group consisting of: palladium and iridium, the compounds thereof,
iridium oxide, and metals coated with noble metals, taken from the
group consisting of: iron or copper.
16. The device according to claim 1, wherein a distribution
structure, having a metal grating, is disposed on the electrodes in
order to distribute the current.
17. The device according to claim 1, wherein at least one channel
or a gas-permeable material is disposed at the cathode in order to
discharge the generated hydrogen.
18. The device according to claim 17, wherein the channel and/or
the coating on a side orientated towards the light is
light-impermeable or metal-coated.
19. The device according to claim 1, wherein the
hydrogen-containing compound is of deionised water in substantial
parts.
20. The device according to claim 1, wherein the electrolysis unit
consists of a plurality of series-connected units consisting of
electrodes and proton-permeable membranes.
21. A method for the generation of hydrogen from
hydrogen-containing compounds, in which sunlight is concentrated on
at least one solar cell by means of an optical concentrator and,
with the photovoltaically generated voltage, the
hydrogen-containing compounds are electrolysed and the protons
formed by the electrolysis are conducted from the anode to the
cathode where they are reduced to form molecular hydrogen, a
plurality of units being used which track the position of the sun
and consist of at least one concentrator and at least one solar
cell which is not in contact with the hydrogen-containing compounds
and is contacted electrically with an electrolysis unit with a
cathode and an anode.
22. The method according to claim 21, wherein solar cells
consisting of a plurality of pn or np transitions which are
connected to each other in series and consist of semiconductor
materials which have respectively different band gap energy are
used.
23. The method according to claim 21, wherein the semiconductor
materials are selected from the group consisting of silicon,
germanium and ITT-V compounds of aluminium, gallium, indium,
nitrogen, phosphorus, arsenic and antimony.
24. The method according to claim 21, wherein the number of pn or
np transitions of the solar cell is chosen such that the solar cell
has a voltage in the range of 1.5 to 6 V.
25. The method according to claim 21, wherein the light is
concentrated by the factor of one of: 50 to 1500, and 300 to
1000.
26. The method according to claim 21, wherein the light is
concentrated with a point-focusing Fresnel lens, a point-focusing
dished mirror, a line-focusing optical lens or a parabolic
mirror.
27. The method according to claim 21, wherein a proton-permeable
polymer membrane is used as electrolysis unit.
28. The method according to claim 21, wherein the hydrogen from the
units is assembled and collected.
29. The method according to claim 21, wherein the generated
hydrogen is discharged via a channel system.
30. The method according to claim 21, wherein the
hydrogen-containing compounds are used for cooling in that the
hydrogen-containing compounds are made to flow along the solar
cells.
31. The method according to claim 21, wherein molecular oxygen is
generated as by-product.
32. The method according to claim 21, wherein the
hydrogen-containing compound consists of deionised water in
substantial parts.
Description
[0001] The invention relates to a device and a method for the
photo-voltaic generation of hydrogen from hydrogen-containing
compounds, sunlight being concentrated on solar cells by means of
an optical concentrator and the consequently generated voltage
being used directly for the electrolysis of a hydrogen-containing
compound, in particular of deionised water, in order to generate
hydrogen.
[0002] Solar hydrogen can be obtained with the help of biological
water splitting by bacteria, photoelectrochemical cells, from
biomass reforming or by solar thermal splitting of water at high
temperatures.
[0003] The electrolysis of hydrogen by means of photovoltaics has
been achieved to date generally by separate, successively connected
systems made of solar module and electrolysis unit. The same
applies to systems which use wind energy for the electrolysis.
Preliminary systems are described for example also in the review
paper by M. A. K. Lodhi "A Hybrid System of Solar Photovoltaic,
Thermal and Hydrogen: a Future Trend" Int. J. Hydrogen Energy, vol.
20, number 6, pp. 417-484 (1995). This publication also describes
the use of concentrating PV systems for current generation and
hybrid systems which both use solar-generated electricity and heat
for the electrolysis.
[0004] The level of effectiveness of the hydrogen generation is
relatively low for all these systems and the method is altogether
expensive. In the past, there were also ideas of using solar cells
directly for the electrolysis of water or an aqueous solution but
the voltage of most solar cells at the operating point is too low
to implement the electrolysis.
[0005] The electrolysis of water with the help of a semiconductor
electrode was described for the first time in July 1972 in Nature
vol. 238 "Electrochemical Photolysis of Water at a Semiconductor
Electrode" by Akira Fijishima and Kenichi Honda. This publication
shows how hydrogen can be separated from an aqueous solution with
the help of sunlight. Water is hereby brought into direct contact
with an n conducting semiconductor layer made of TiO.sub.2 and a
Pt-counter-electrode. In the case of TiO.sub.2, the potential
difference achieved in sunlight suffices for the splitting of
water. It is described how hydrogen and oxygen can be obtained from
water with this photoelectrochemical process. Because of the high
band gap energy of TiO.sub.2 however, only a very small part of the
solar spectrum is captured in the electrode and used for the
electrolysis. Hence this process is not efficient.
[0006] In a patent by D. I. Tchernev from 1975 (U.S. Pat. No.
3,925,212) "Device for Solar Energy Conversion by
Photo-Electrolytic Decomposition of Water", it is shown for the
first time that also separated p and n conducting semiconductor
layers can be used as electrodes with illumination for splitting
water. In this arrangement also there are semiconductor layers in
direct contact with the electrolyte.
[0007] A further patent from 1984 "Photolytic production of
hydrogen", U.S. Pat. No. 4,466,869 by A. Williams, describes for
the first time that the photoelectrode can also consist of a layer
structure of a plurality of semiconductor layers which are mounted
one above the other and have different band gap energy. Hence the
photoelectrode corresponds in principle to a cascade solar cell, as
is used also preferably in the invention described here. At the
National Renewable Energy Laboratory NREL, work has taken place
since 1998 on a system for the generation of hydrogen by means of
solar energy. O. Khaselev, J. A. Turner describe in Science; vol.
280, Apr. 17 (1998), p. 425-427 "A Monolithic
Photovoltaic-Photoelectrochemical Device for Hydrogen Production
via Water Splitting" a photoelectrochemical method for water
splitting. For the first time, a cascade solar cell made of III-V
semiconductors was hereby used as one of the photoelectrodes. The
counter-electrode consisted of platinum. In addition, the use of
concentrated sunlight was mentioned for the first time. The
semiconductor layers in all the mentioned arrangements are in
direct contact with the electrolyte and themselves represent one of
the electrodes for the water splitting. The photovoltaic energy
generation and the electrolysis of water are hence not spatially
separated.
[0008] It was therefore the object of the present invention to
provide a system for the photovoltaic generation of hydrogen which
has high efficiency in the hydrogen generation and thereby is
simultaneously economical in production.
[0009] This object is solved by the device and the method for
photo-voltaic generation of hydrogen according to claim 1 and 17.
The further dependent claims reveal advantageous developments.
[0010] According to the invention, a device for the photovoltaic
generation of hydrogen from hydrogen-containing compounds is
provided, which consists of a plurality of units which track the
position of the sun, which device has an optical concentrator for
concentrating sunlight onto a solar cell, at least one solar cell
which is not in contact with the hydrogen-containing compounds and
is electrically connected to an electrolysis unit which has an
anode and a cathode in contact with the hydrogen-containing
compounds, the units being disposed on a tracking system following
the position of the sun.
[0011] In comparison with systems known from prior art in which two
separate systems are used, on the one hand, for photovoltaic
current generation and, on the other hand, for electrolysis, the
system underlying the present invention is characterised by the
integration of solar power generation and hydrogen production in
one system and hence by a lower material and spatial requirement,
higher efficiency and potentially lower costs for the solar
hydrogen. Hence electrical losses which are normally produced by
the wiring of solar cells in a module are hence dispensed with.
Even if individual cells within a module do not function, the
functional capacity of the remaining units is not impaired. A
substantial advantage relative to photoelectro-chemical methods is
based on the fact that the photovoltaic cell is not in direct
contact with the electrolyte. This can otherwise lead to
significant problems, such as e.g. the oxidation of semi-conductor
layers or the removal or deposition of material by the
electrolysis. This extends the long term stability of such systems.
In addition, optical absorption losses of the sunlight in the
hydrogen-containing compound are avoided.
[0012] Preferably each individual unit of the device has an
electrical power of 1 to 100 W.
[0013] The electrolysis unit according to the invention preferably
has an operating temperature of -10.degree. C. to 200.degree. C.,
particularly preferred of 30.degree. C. to 100.degree. C.
[0014] A point-focusing lens, such as e.g. a Fresnel lens, is used
preferably as optical concentrator. Alternatively, a curved Fresnel
lens with a line focus, a parabolic mirror with a line focus or a
dished mirror with a point focus can be used.
[0015] The solar cell is preferably constructed from a plurality of
layers made of semiconductor materials which are connected to each
other in series and have respectively different band gap energy.
The semiconductor materials are thereby preferably selected from
the group consisting of silicon, germanium and the III-V compounds
of aluminium, gallium or indium with nitrogen, phosphorus, arsenic
or antimony.
[0016] The polarity of the solar cell is freely selectable so that
both an np polarity and a pn polarity is possible. The solar cell,
if merely a pn or np transition is present, can have a voltage of
more than 1.4 volts, particularly preferred of 1.6 to 2.4 volts. If
the solar cell has a plurality of series-connected pn or np
transitions, then a voltage in the range of 1.5 to 6 volts can be
achieved. The solar cell thereby preferably has an area of 0.01 to
1 cm squared.
[0017] Preferably a proton-permeable polymer membrane (PEM) with
two electrodes, the cathode and the anode is used as electrolysis
unit.
[0018] Preferably the anode and the cathode consist of noble
metals, in particular here platinum, palladium or iridium, the
compounds thereof, e.g. iridium oxide, or of metals coated with
noble metal, in particular here nickel, iron or copper. These
materials also serve as catalyst for the electrolysis. The
electrodes can preferably have in addition a distribution structure
which is disposed on the electrodes in order to distribute the
current. This is preferably a metal grating.
[0019] A further variant of the device according to the invention
provides that the anode is connected to a channel system through
which the hydrogen-containing compounds flow. The cathode is
likewise connected to a channel system or to a gas-permeable
material through which the generated hydrogen is discharged.
[0020] A further embodiment of the device according to the
invention provides that the electrolysis unit consists of two or
more units which are connected to each other in series and have a
correspondingly higher operating voltage.
[0021] According to the invention, a method for the generation of
hydrogen from hydrogen-containing compounds is also provided, in
which sunlight is concentrated on at least one solar cell by means
of an optical concentrator and, with the photovoltaically generated
voltage, the hydrogen-containing compounds are electrolysed at a
temperature preferably in the range of -10.degree. C. to
200.degree. C., particularly preferred of 30.degree. C. to
100.degree. C., the solar cell being contacted electrically with an
electrolysis unit with a cathode and/or an anode and the protons
formed by the electrolysis being conducted from the anode to the
cathode where they are reduced to form molecular hydrogen.
[0022] A preferred embodiment of the method according to the
invention provides that the hydrogen-containing compounds are also
used for cooling in that the hydrogen-containing compounds are made
to flow along the solar cell.
[0023] Preferably the hydrogen-containing compound contains
deionised water in substantial parts. In this case, it is then also
possible to generate also oxygen in addition to hydrogen.
[0024] The subject according to the invention is intended to be
explained in more detail with reference to the subsequent Figures,
without wishing to restrict said subject to the embodiments shown
herein.
[0025] FIG. 1 shows a schematic representation of the method for
generating hydrogen according to the invention.
[0026] FIG. 2 shows a first embodiment of the device according to
the invention.
[0027] FIG. 3 shows a device for photovoltaic generation of
hydrogen as an overall system according to the invention.
[0028] FIG. 4 shows schematically the principle of energy
conversion in the method according to the invention for generating
hydrogen.
[0029] FIG. 5 shows a second embodiment of the device according to
the invention.
[0030] FIG. 6 shows a third embodiment of the device according to
the invention.
[0031] FIG. 7 shows the schematic construction of a device
according to the invention in which an electrolysis unit is
combined with a plurality of solar cells.
[0032] In FIG. 1, the system is represented schematically, which
can generate hydrogen efficiently by the electrolysis of
hydrogen-containing compounds, e.g. aqueous solutions, such as
deionised water, with the help of photovoltaically generated
energy. This system consists of a concentrator 2 which concentrates
the sunlight 1 onto a solar cell 3. The concentration factor of the
sunlight can thereby be in the range of 50 and approx. 1500.
Preferably concentrations of sunlight here are in the range of 300
and 1000. A solar cell 3 which converts the sunlight into
electrical power is situated at the focal point of the concentrator
2. Voltages>1.4 volts, as are necessary for the electrolysis,
are hereby generated at the operating point of the solar cell. This
can be achieved by solar cells made of III-V semiconductors having
one or more pn or np transitions. As cascade solar cells, for
example those made of GaInP/GaInAs or AlGaInAs/Ge can be used. The
band gaps of the solar cells should hereby be chosen such that the
current-voltage characteristic line of the cell, with the
concentrated solar spectrum, achieves as high as possible an
efficiency for the electrolysis of the hydrogen-containing
compounds. The polarity of the solar cell can both be p to n and n
to p. The voltage applied to the solar cell 3 is used directly for
the electrolysis of the hydrogen-containing compounds 5. The p and
n conducting layers of the solar cells are connected directly to
the electrodes of the electrolysis unit 4. The thereby produced
hydrogen 6 is discharged and stored. If water is used for the
electrolysis, then oxygen can also be obtained as further gas. Each
individual solar cell in the system illustrated in FIG. 1 is
connected directly to an electrolysis unit. It is however also
possible that up to 4 solar cells or even more are connected
directly to a single electrolysis unit. Furthermore, it is possible
that the electrolysis unit consists of two electrolysis units which
are connected in series one behind the other, as a result of which
the operating voltage is doubled. The integration of a plurality of
separate concentrator-solar cells-electrolysis unit units in an
overall system is essential for the invention. These units then can
(but need not) be completely separated from each other
electrically. They are thereby disposed on a tracking unit and
track the sun.
[0033] A first embodiment of a device according to the invention
for the photovoltaic generation of hydrogen is illustrated in FIG.
2. This device consists of a Fresnel lens 2 which concentrates the
sunlight 1 by a factor 300 or more and directs it onto a cascade
solar cell 3 made of III-V semiconductors. The surface area of the
solar cell is thereby between 0.01 to 1 cm.sup.2. In the solar
cell, the concentrated sunlight is converted into electrical energy
with high efficiency of more than 30%. The voltage of the solar
cell at the operating point is thereby >1.4 volts.
[0034] The III-V materials have not been used for terrestrial
energy generation to date since they are too expensive. By using
concentrated light, the semiconductor surface is however
significantly reduced and use becomes economical. In future, this
is intended also to be used for solar power generation on earth.
The Fraunhofer ISE has been working in this context for some years
on the so-called FLATCON.TM. concentrator. This system likewise
uses cascade solar cells with concentrated sunlight for the
generation of electrical power.
[0035] In a cascade solar cell, a plurality of layers made of III-V
semi-conductors of different band gap energy are deposited one on
the other. These partial cells are monolithically, i.e. on the
substrate, connected in series to each other. As a result,
operating voltages between 1 volt for a single solar cell and
approx. 6 volts for a solar cell with 5-6 series-connected pn
transitions can be achieved. Solar cells with 3 pn transitions have
achieved efficiencies of up to 37% for the conversion of
concentrated sunlight into electrical energy (R. King et al.
"Metamorphic III-V Materials" Proc. of 19.sup.th European
Photovoltaic Solar Energy Conference Paris 2004). The combination
of the band gaps and materials for the application described here
must be reoptimised with respect to maximisation of the efficiency
for the electrolysis of water. Examples of possible material
combinations are for example GaInP/GaInAs, GaAs/Ge, AlGaInAs/Ge,
AlGaAs/Si, GaInP/GaInAs/Ge, AlGaInP/GaAs/GaInNAs/Ge or
AlGaInP/GaIn/AlGaInAs/GaInAsN/Ge. In addition to the lower
consumption of materials, a further advantage in the use of
concentrated light resides in the fact that the voltage of a solar
cell increases logarithmically with the concentration.
[0036] The front and rear contact of the solar cell is connected
directly via a metal grating 6 to electrodes (e.g. made of noble
metals, such as platinum, palladium, iridium or iridium oxide which
serve also as catalyst for the electrolysis, or made of nickel,
iron or copper electrodes which are coated with such noble metals)
on a proton-permeable polymer membrane (PEM) 4. The surface of the
PEM membrane can extend up to the total surface of incidence of the
sunlight (apart from the surface of the solar cell). The PEM
membrane can however also adopt only a much smaller surface area.
The membrane is on the positive side of the anode in direct contact
with the hydrogen-containing solution which consists of e.g.
deionised water 5. However other solutions can also be used which
also need not necessarily be transparent. The solution will firstly
flow through below the solar cells in one possible arrangement and
contributes there to the cooling. As a result, the efficiency of
the solar cells can be increased. Subsequently, the solution is
conducted through a channel system to the anode and is split there
into oxygen and hydrogen ions. The oxygen molecules produced on the
anode side rise within the liquid and can be collected there. The
H.sup.+ ions migrate through the PEM membrane to the negative
cathode where they react with respectively two electrons to form
molecular hydrogen. The cathode side is covered in turn with a
channel system through which the hydrogen-containing solution flows
or with a gas-permeable or porous material through which the
hydrogen can be conducted to the store.
[0037] FIG. 3 shows a device 1 according to the invention which is
assembled to form an overall system for photovoltaic generation of
hydrogen. The gases are collected here at the upper edge of the
individual modules and supplied to a store 3. This store can
consist of e.g. compressed gas cylinders. The inflow pipe to the
modules can be evacuated. The modules are mounted on a 2-axis
tracking unit 2 which follows the course of the sun. This is
necessary to retain the focus of the lens always precisely on the
solar cell. Since PEM electrolysis units achieve degrees of
efficiency of 80 to 90%, with the system described here made of
III-V cascade solar cells and PEM electrolysis unit, system degrees
of efficiency of 27% can be achieved for the generation of hydrogen
by means of sunlight.
[0038] The principle of energy conversion is represented
schematically in FIG. 4. A hydrogen-containing compound is guided
along the anode for example through a channel. The result hereby is
then splitting of water into oxygen and protons. The protons can in
turn pass through the proton-permeable polymer membrane (PEM) and
thus reach the cathode. The result here is reduction of the protons
to form molecular hydrogen. In the present example, the polymer
membrane is disposed adjacent to the solar cell. In addition, the
solar cell can be cooled from the rear by a channel through which
cooling water flows.
[0039] FIG. 5 shows a further embodiment of the invention in which
the PEM electrolysis unit 4 is disposed under the solar cell 3. The
water flows here directly through channels 5 below the solar cell
which are soldered on a Cu plate 6. The Cu plate can thereby be
separated electrically by an insulator from the water. Good thermal
contact between the water for the electrolysis and the solar cell
is produced. Hydrogen and oxygen are conveyed in this case as gas
bubbles in the liquid.
[0040] In a further embodiment of the invention described here, two
electrolysis units are connected in series. This is sensible if the
voltage of the concentrator solar cell at the operating point
achieves twice the voltage necessary for the electrolysis, i.e.
approx. 3 volts. Such high voltages can be achieved with a single
highly efficient cascade solar cell made of III-V semiconductors. A
possible construction for the series connection of two PEM
electrolysis units is shown in FIG. 6. The following meanings apply
in this Figure: [0041] 1 Sun [0042] 2 Point-focusing lens [0043] 3
III to V cascade solar cell [0044] 4 PEM membrane with electrodes
[0045] 5 Water or hydrogen-containing solution for the electrolysis
for cooling the solar cell [0046] 6 Metal plate made of Cu as
carrier for the solar cell [0047] 7 Structured, conductive
separator between the two PEM membranes with water channels, e.g.
made of titanium
[0048] In a further embodiment of the invention, respectively two
to four concentrator solar cells are connected to only one
electrolysis unit (see FIG. 7). This arrangement is suitable if the
current generated by one concentrator solar cell does not suffice
to operate the electrolysis unit efficiently. The arrangements of
FIG. 2 and FIG. 7 or FIG. 5 and FIG. 7 can also be combined
together.
* * * * *