WO2011101676A1

WO2011101676A1 - Hydrogen generation system

Info

Publication number: WO2011101676A1
Application number: PCT/GB2011/050311
Authority: WO
Inventors: Nicholas Simon Walker; Brian Holcroft
Original assignee: Microsharp Corporation Limited; Stored Solar Limited
Priority date: 2010-02-18
Filing date: 2011-02-17
Publication date: 2011-08-25
Also published as: GB201002807D0

Abstract

A system (1) for generating hydrogen using solar energy. The system comprises an electrolyser (14) which comprises a photocatalyst arranged to generate hydrogen from an electrolyte (e.g. water) on illumination with solar radiation (15). The system also comprises a photovoltaic cell (6) arranged to generate a bias voltage on illumination with solar radiation (7) and means for applying the bias voltage to the electrolyser (14). The system further comprises at least one solar concentrator (2) arranged to concentrate incident solar radiation (3) onto the electrolyser (14) and the photovoltaic cell (6).

Description

Hydrogen generation system

This invention relates to an improved system for generating hydrogen by electrolysis. It relates in particular to such a system in which hydrogen is generated using solar energy.

Hydrogen is an important future energy source and energy storage medium. The use of solar cells has become a focus in the area of hydrogen production as they use a non-polluting renewable energy source. The hydrogen which is produced can either be used locally to produce energy (e.g. using a fuel cell to generate electricity or a burner to produce heat), or remotely once transported to a desired location, e.g. by gas pipelines or tanker ships.

In a solar catalysed electrolytic process, solar energy may be used directly to split water to generate hydrogen (and oxygen). In this process a photoactive material ("photocatalyst"), typically a semiconductor such as a metal oxide, is illuminated whereby to generate a voltage of a magnitude which is capable of cleaving an electrolyte. The reaction involves the generation of positive charge carriers (i.e. holes) in the valence band of the semiconductor on excitation by light whose energy exceeds the band gap energy of the semiconductor. However, charge mobility in such materials is often poor with the result that additional energy in the form of a bias voltage is often needed to enable them to split water or to increase the volume of hydrogen produced. In addition many photons in the solar spectrum deliver energy below that necessary to release oxygen. This additional voltage bias may be provided by a photovoltaic (PV) cell which is capable of converting solar energy into electrical energy.

A common method used to generate hydrogen from solar energy thus involves the use of a photovoltaic system to generate electricity which can then be applied, either directly or indirectly, as a DC voltage to an electrolyser which splits water into hydrogen and oxygen. The overall efficiency of this method is dependent on several factors, including the conversion efficiency of the photovoltaic cells, the means of collecting power from them and producing the required drive voltage for the electrolyser, and the efficiency of the electrolyser. The cost for hydrogen production in turn depends on the cost of the various components of the system, especially the photovoltaic cells, and other factors such as the intensity of the sunlight.

Given the abundant supply of solar energy, photoelectrochemical hydrogen generation can become a viable technology. However, for this to happen, costs must be reduced and efficiencies improved. It is therefore desirable for alternative hydrogen production systems to be developed which are more efficient and which enable the use of less complex and less costly components. One aspect of the invention comprises a hydrogen generation system which incorporates a photocatalysed electrolyser, a photovoltaic cell and at least one solar concentrator. Integration of a solar concentrator can produce a system whose performance exceeds that of standard hydrogen generation systems in terms of the costs of the production of hydrogen.

In a first aspect the present invention thus provides a system for generating hydrogen using solar energy, said system comprising:

an electrolyser which comprises a photocatalyst arranged to generate hydrogen from an electrolyte (e.g. water) on illumination with solar radiation;

a photovoltaic cell arranged to generate a bias voltage on illumination with solar radiation;

means for applying said bias voltage to the electrolyser; and

at least one solar concentrator arranged to concentrate incident solar radiation onto the electrolyser and the photovoltaic cell.

Methods for the generation of hydrogen using such a system form a further aspect of the invention. In an alternative aspect the invention thus provides a method for generating hydrogen using solar energy, said method comprising:

capturing solar radiation with at least one solar concentrator;

directing a first part of the captured solar radiation to an electrolyser which comprises a photocatalyst whereby to generate hydrogen from an electrolyte (e.g. water); and

directing a second part of the captured solar radiation to a photovoltaic cell whereby to generate a bias voltage, said bias voltage being selectively applied to the electrolyser. ln the systems and methods herein described the voltage bias is necessary since most solar radiation is insufficiently energetic to directly split water. Therefore electricity generated by the photovoltaic cell is used to provide a voltage bias to the electrolyser which, operating at this bias and illuminated by solar radiation, is able to split water the electrolyte (e.g. water) to produce hydrogen.

Thus it can be seen that the present invention provides a hydrogen generation system which, by using a solar concentrator, allows solar radiation from a relatively wide area to be focused onto a reduced area of higher light intensity in which the photovoltaic cells and electrolyser are located. This enables fewer cells or a smaller area of cells to be used in order to produce the same amount of hydrogen. This in turn reduces the overall cost of the system which is typically dominated by the cost of the photovoltaic cells and the electrolyser.

As the solar concentrator focuses the incident solar radiation onto a reduced area, this further allows the redundant area to be beneficially used to house additional components within the system, e.g. tubing to remove gases from the electrolyser, wires to transfer power from the photovoltaic cells, etc. These components can be positioned in those areas of the system which are not illuminated by the focused light.

Preferably the system in accordance with the invention further comprises a wavelength splitter, i.e. a device capable of partitioning incident solar radiation according to its wavelength. Partitioning of the solar radiation into discrete wavelength ranges which are optimised for the electrolyser and photovoltaic units further increases the efficiency of hydrogen production.

Typically, the wavelength splitter will be a device capable of splitting the incident radiation into a first part having a first range of wavelengths and a second part having a second range of wavelengths. The splitter may be arranged to direct the first part of the solar radiation spectrum to the electrolyser (which is responsive to illumination of the first range of wavelengths) and the second part of the solar radiation spectrum to the photovoltaic cell (which is responsive to the second range of wavelengths). The first range of wavelengths may be distinct from the second range of wavelengths, or there may be a degree of overlap between the two ranges.

The system of the first aspect may be used with a wavelength splitter in the absence of a solar concentrator, and such an arrangement is considered to form a further aspect of the invention.

Therefore, viewed from a second aspect the invention provides a system for generating hydrogen using solar energy, said system comprising:

means for applying said bias voltage to the electrolyser; and

a wavelength splitter arranged to direct a first part of the incident solar radiation to the electrolyser and a second part of the incident solar radiation to the photovoltaic cell.

Methods for generating hydrogen using this system form a further aspect of the invention. In a further aspect the invention thus provides a method for generating hydrogen using solar energy, said method comprising:

partitioning solar radiation with a wavelength splitter into a first part having a first set of wavelengths and a second part having a second set of wavelengths; directing said first part of said radiation to an electrolyser which comprises a photocatalyst whereby to generate hydrogen from an electrolyte (e.g. water); and directing said second part of said radiation to a photovoltaic cell whereby to generate a bias voltage, said bias voltage being selectively applied to the electrolyser. It will be appreciated by those skilled in the art that this alternative aspect of the invention provides a hydrogen generation system which beneficially uses all of the solar radiation spectrum. The part of the solar radiation spectrum to which the photovoltaic cell is responsive is directed to it, whereas the remaining portion of the solar radiation spectrum to which the photocatalyst present in the electrolyser is responsive is directed towards the electrolyser. Use of a wavelength splitter further introduces an element of flexibility into the system in that the positioning of the different components, e.g. the photovoltaic cell and electrolyser, need not be fixed; the different parts of the solar radiation can be directed to these separate components as required.

Whilst the invention is generally described with reference to a single photovoltaic cell and a single electrolyser, it will be appreciated by those skilled in the art that a plurality of each of these units may be used. The invention thus also extends to an array of electrolysers and photovoltaic cells according to any aspect of the present invention so that they can collectively produce large amounts of hydrogen.

Conveniently the concentrator systems herein described can be arranged to operate at a number of different light intensities, e.g. a low light concentration (5 to 25 suns), a medium light concentration (25 to 100 suns) or at a high light concentration (100 to >1 ,000 suns) where one sun represents the light intensity of normal direct sunlight.

A solar concentrator is an optical device capable of concentrating (i.e. focusing) solar radiation. Solar concentrators are generally known and used in the field of photovoltaics and any known device may be employed in the invention. In operation, the concentrator is generally kept aligned normal to the direction of incident direct solar radiation in order to capture the maximum amount of sunlight.

The optical design of concentrators can be either reflective or refractive such that these concentrate the solar energy collected from the sunlight. Typically, these may achieve a concentration of up to 10,000 suns. In some embodiments the solar concentrator may comprise a reflector. In other embodiments, it may be refractive, i.e. it comprises a lens, preferably a linear refractive lens, e.g. a curved linear refractive lens. A suitable example of a refractive lens is a linear Fresnel lens concentrator. Alternatively, the solar concentrator may be provided in the form of one of the applicant's solar concentrators which are disclosed in detail in WO 2009/001059 and WO 2009/106798, the contents of which are hereby incorporated by reference. Preferably the solar concentrator comprises a tracking solar concentrator, i.e. one which tracks the motion of the sun such that this always points towards the sun when the sun is visible. Tracking the sun in this way maximises the total flux of solar radiation incident on the system and therefore maximises the production of hydrogen under any given set of illumination conditions.

In the systems and methods herein described, one or more solar concentrators may be used depending on the precise arrangement of the electrolyser and the photovoltaic cell.

Where the electrolyser and photovoltaic cell are arranged side by side, a single solar concentrator can be arranged to focus the full solar spectrum of light onto both units, i.e. the photovoltaic cell and the electrolyser share the same incident solar radiation. Alternatively, two or more concentrators may be used which

independently direct the full solar spectrum to each of the units.

If a single solar concentrator is used, either the photovoltaic cell and the

electrolyser can be arranged sufficiently close together or, alternatively, a beam splitter can be used to direct different parts of the solar radiation to each of the units. The use of a wavelength splitter (e.g. a wavelength selective mirror) configured to split the incident light from a single concentrator and direct this to each of the units forms a particularly preferred aspect of the invention.

In one embodiment of the invention the photovoltaic cell and electrolyser may be arranged in a tandem manner (i.e. the units are arranged one behind the other). In this arrangement a single concentrator may be used such that the incident radiation passes through one of the photovoltaic cell and the electrolyser before being incident on the other. This is particularly beneficial in the case where the electrolyser and the photovoltaic cell are chosen so that they are responsive to different parts of the solar radiation spectrum. When arranged in tandem it is preferred that the electrolyser and the photovoltaic cell are arranged so that the incident solar radiation first passes through the electrolyser before illuminating the photovoltaic cell. If the electrolyser and photovoltaic cell are adjacent to each other, and particularly if they are responsive to different parts of the solar radiation spectrum, a wavelength splitter arranged in the same manner as in the second aspect of the invention can be used. For aspects of the invention where the system comprises a solar concentrator and a wavelength splitter, preferably the solar concentrator is arranged to focus incident solar radiation onto the wavelength splitter, e.g. in such a way as to optimally illuminate both the electrolyser and photovoltaic cell.

Preferred for use in the invention is a wavelength splitter which is capable of directing light of a particular wavelength range to the electrolyser, which range substantially matches the absorption spectrum of the photocatalyst used in the electrolyser.

The desired partitioning of wavelengths will be dependent on the selected components of the system, i.e. the chosen photovoltaic cell and electrolyser, more specifically the photoactive elements used in these units. A suitable wavelength splitter may readily be chosen based on the known absorption spectra for the photoactive elements. Typically, the wavelength splitter will be one which is configured to direct the shorter wavelengths of the solar radiation spectrum (i.e. in those in the UV and blue visible range) to the electrolyser and the longer wavelengths (i.e. those in the red visible light and IR) to the photovoltaic cell. This is because the absorption spectra of typical photocatalysts is generally towards the ultraviolet end of the solar radiation spectrum. On the other hand, certain photovoltaic cells preferentially absorb light towards the infrared end of the solar radiation spectrum. For example, Fe₂0₃ (which may be used as a photocatalyst) absorbs radiation having a wavelength of between 300 nm and 600 nm, whereas silicon (often used as the semiconducting material in a PV cell) absorbs radiation having a wavelength of between 500 nm and 1000 nm. Different photocatalysts and different photovoltaic cells have different absorption profiles and so the choice of wavelength at which the wavelength splitter will partition the incident solar radiation (the "wavelength split") will be chosen according to the photocatalyst and photovoltaic cell used. In the particular example illustrated above, a wavelength split in the range between about 500 and 600 nm, e.g. at about 550 nm, would be suitable. Although an approximate value for the wavelength split is given, wavelength splitters generally change from being transmissive to reflective over a certain range of wavelengths.

The wavelength splitter may be selected from a number of known devices, e.g. a grating, a lens, a mirror or any combination thereof, but in a preferred embodiment it may comprise a wavelength selective mirror. This type of mirror reflects radiation in a certain range of wavelengths and transmits all other wavelengths and can thus be used to direct one range of wavelengths to the electrolyser and the other to the photovoltaic cell. Preferably, the wavelength selective mirror reflects shorter wavelengths to the electrolyser and allows longer wavelengths to pass through to the photovoltaic cell.

Wavelength selective mirrors are generally known in the art and may be selected for use in the invention based on their known spectral profiles. In one embodiment the wavelength selective mirror comprises a glass or plastic element having one or more selective spectrum reflective coatings. The coating could, for example, be a dichroic liquid crystal material with two layers for each wavelength reflection type and light polarisation, or it could be an inorganic deposition of multiple layers of different refractive indices (a dichroic or hot mirror). In another embodiment the wavelength selective mirror comprises a plastic element, wherein the plastic element comprises multiple layers having different refractive indices. Examples of such materials include the plastic films manufactured by 3M. Preferably, the wavelength selective mirror comprises a coated glass or plastic element, typically in which the coating comprises a printed dichroic liquid crystal.

As has been discussed, the choice of wavelength split depends on the

photocatalyst and the photovoltaic cell used. In one set of embodiments the wavelength split is between 500 nm and 600 nm, e.g. about 550 nm. In one set of embodiments the wavelength split is chosen to match the absorption spectrum of the photocatalyst. In one set of embodiments the wavelength splitter is arranged to direct ultraviolet and blue visible light to the electrolyser and to direct red visible and infrared light to the photovoltaic cell. These different sets of embodiments are not necessarily mutually exclusive, they just illustrate different ways of configuring the wavelength splitter. Electrolysers for use in the electrolytic conversion of an aqueous electrolyte into hydrogen are widely known in the art. These generally comprise an electrolyte, an anode, a cathode, a photocatalyst and a hydrogen separation membrane.

Typically, the photocatalyst forms part of the anode, e.g. a surface coating or photocatalytic layer on the anode. The electrolyte is in direct contact with the photocatalyst and is positioned to receive incident sunlight. The photocatalyst produces oxygen from the electrolyte at the anode from where it is removed from the cell. Simultaneously hydrogen is produced at the cathode from where it removed from the cell by any suitable means, e.g. by pipes to a gas storage system. The hydrogen separation membrane is gas impermeable and functions to separate the hydrogen produced at the photocatalytic layer from other components (e.g. oxygen).

Preferably, one or both of the anode and cathode consist of noble metals, e.g. platinum or palladium, or metals coated with noble metals, for example coated nickel, iron or copper. The cathode will typically comprise a platinum or platinum- coated material.

The photocatalyst in the electrolyser is arranged to generate a potential from the incident radiation to drive the electrolyser and will typically be provided in the form of a coating on the anode. The photocatalyst can be one of any number of suitable substances which are photoactive, but will generally comprise a photoactive semiconductor, e.g. a metal oxide. Examples of suitable metal oxides include iron (II) oxide, iron (III) oxide, titanium dioxide, tungsten trioxide, etc. These examples preferentially absorb light towards the ultraviolet end of the solar radiation spectrum. This is also advantageous because the shorter wavelengths are energetically more favourable for splitting water to produce hydrogen. Some catalysts, for example, titanium dioxide, are able to split water using solely ultraviolet radiation with no voltage bias, though the quantity of these wavelengths in solar radiation is small.

The electrolyte may be any suitable substance for producing hydrogen by electrolysis, but preferably the electrolyte comprises an aqueous electrolyte such as a strong acid or alkali. Solar catalysed electrolysis takes place in the electrolyser whereby water is split into hydrogen and oxygen using a photocatalyst. Usually only the hydrogen is collected, e.g. for use as fuel. As will be appreciated, other electrolytes may be used which result in the generation of hydrogen and which produce gasses other than oxygen. For the purposes of the invention, the term "electrolyte" is considered to encompass any electrolytic solution which enables hydrogen to be evolved on operation of the electrolyser, i.e. during electrolysis. The electrolyte in the electrolyser can be flowed in order to keep the cell cool and remove gas from the active surfaces.

The photocatalyst will often have an illumination intensity dependent effect on the voltage required to generate a particular quantity of gas. An example is iron (II) oxide which reduces the voltage bias required to split water when it is illuminated with solar radiation.

As will be apparent, whilst the invention is primarily described with reference to a single electrolyser cell, the scope of the invention is not limited to this, but includes any number of electrolyser cells connected to one another. Such cells may be arranged in series or parallel electrical connection.

Photovoltaic cells capable of generating electricity from solar energy are well known in the art and any of these may be used in the invention. Such cells comprise a p-n heterojunction. The photovoltaic cell may comprise any one of monocrystalline silicon, polycrystalline silicon, amorphous silicon, nano or microcrystalline silicon. Other suitable semiconductor materials include the compounds of aluminium, gallium or indium with nitrogen, phosphorus, arsenic or antimony. A high efficiency multiple junction gallium arsenide cell is particularly suitable. Other suitable PV cells include dye sensitised solar cells (DSCs) (e.g. those incorporating

mesoporous Ti0₂) and organic photovoltaic cells. So-called CIGS cells

(Cu(ln,Ga)Se₂) may also be employed. Simple photovoltaic cells such as those based on silicon are much less expensive, have higher efficiency and can operate under concentrated sunlight and more readily available than relatively complex dye sensitised mesoporous cells and for this reason are generally preferred.

As has been previously discussed, in one set of embodiments the photovoltaic cell comprises a material which absorbs wavelengths towards the infrared end of the solar radiation spectrum. In this way the longer wavelengths of the solar radiation spectrum can be used to generate a bias voltage for the electrolyser which uses the shorter wavelengths of the spectrum for photocatalysis.

As will be apparent, whilst the invention is primarily described with reference to a single photovoltaic cell, the scope of the invention is not limited to this, but includes any number of photovoltaic cells connected to one another, e.g. an array of photovoltaic cells. Where a plurality of photovoltaic cells are employed these are generally connected together in series since this reduces the power losses through parasitic resistance.

The electricity produced by the photovoltaic cell may be directly applied to the electrolyser to provide the required voltage bias. In one aspect of the invention the photovoltaic cell is thus configured to produce a voltage which directly biases the electrolyser.

Alternatively, the electricity generated by the photovoltaic cell is collected by a power management means, e.g. a maximum power point tracking (MPPT) unit. The power management means is able to continually change the load on the photovoltaic cells to maximise their power output. The power management means, either directly or through a DC/DC converter, provides the voltage bias to the electrolyser. The DC/DC converter is arranged to supply the required voltage bias to the electrolyser.

In one set of embodiments the system comprises a battery or other means for storing charge, e.g. supercapacitors. The power management means can also be arranged to charge the battery, however the battery can be charged directly by the photovoltaic cell, i.e. the battery may be arranged to be charged by the electricity generated from the photovoltaic cell. This enables the voltage bias to the electrolyser to be provided either by the power management means directly or through the DC/DC converter, from the battery, or from a combination of these components. The power management means, battery or DC/DC converter, or any combination thereof, can also be used to drive an external load or export electricity to the national grid, either if an excess amount of electricity is generated from the photovoltaic cell or if there is a surplus charge on the battery. As will be appreciated, this gives flexibility in terms of applying the required voltage bias according to the set up of the system and the available solar radiation. First, connecting the power management means to the electrolyser via a battery enables an electrolyser with lower capacity (maximum voltage bias applied) than the peak output of the photovoltaic cell to be used, thereby reducing the cost of the electrolyser. The photovoltaic cell can be used to run the electrolyser at full capacity as long as the solar radiation is sufficient to do so. If the output from the photovoltaic cell exceeds the electrolyser capacity the excess power can be used to recharge the battery (when the battery is fully charged the excess power can be used elsewhere or dumped into a passive load). When the power output of the photovoltaic cell is below the maximum capacity of the electrolyser, power from the battery can be used to continue applying the maximum voltage bias to the electrolyser. The choice of the optimal relative capacities of the photovoltaic cell, the battery and the electrolyser depends on their relative costs, efficiencies and solar radiation illumination conditions.

As well as using different sources to provide the voltage bias, the voltage bias can be varied depending on the ability of the electrolyser to drive the electrolysis reaction, i.e. depending on the amount of solar radiation available. For example, when it is dark no solar radiation is available to drive the electrolyser via the photocatalyst and so the voltage bias, from the battery if it is suitably charged, can be maximised in order to provide all of the power needed to drive the electrolyser. When the maximum amount of solar radiation is available, it might not be necessary to provide all (or any) of the voltage bias as there may be sufficient available to run the electrolyser solely using the photocatalyst. In the intermediate situations where some solar radiation is available, the voltage bias can be adjusted as required to provide the necessary voltage bias to the electrolyser.

Therefore preferably the system comprises control means arranged to control the voltage bias applied to the electrolyser. This can control the voltage bias dependent on the solar radiation conditions and it can also be optimised, e.g. for the required production of electricity and hydrogen. This can be done, for example, according to prices and demand based on the performance of the system at any given level of solar radiation intensity. ln one set of embodiments the control means is connected to the electrolyser and the photovoltaic cell, either directly or through the power management means, battery or DC/DC converter (where any of these components are present). In one set of embodiments the control means is arranged to detect the potential produced at the anode of the electrolyser, i.e. by the photocatalyst. This therefore allows the control means to determine the voltage bias needed to be applied to the

electrolyser. In the set of embodiments which comprise a battery, preferably the control means is connected to the battery and arranged to determine its charge level. This allows, in one set of embodiments, the control means to be arranged to selectively determine which one or more of the photovoltaic cell, (directly or through the power management means) or the battery (directly or through the DC/DC converter) should be used to provide the required voltage bias.

The control means may also be connected to an external load and be able to detect the demand of this external load, i.e. the voltage required by it. Alternatively a 'smart' grid is present which can send data to the control means to indicate that power is or is not required. Therefore when there is any surplus electricity, e.g. in times of peak solar radiation intensity or when the battery is charged to capacity, this surplus could be used to drive such an external load.

This arrangement of the control means gives the system the full flexibility to run at many different intensities of incident solar radiation, battery charge levels and external load requirements. The control means is able to detect the requirements and capabilities of each of these and control the voltage bias to the electrolyser and any voltage to an external load accordingly.

In some embodiments the bias voltage is between 1 .4 and 2 volts. In other embodiments the bias voltage is between 2 and 3 volts. Preferably the systems comprise an electrical system arranged to collect and distribute the electricity generated by the photovoltaic cell, e.g. to the power management means, the battery, the DC/DC converter and the electrolyser.

Preferably the systems comprises an electrolyte circulation system for keeping the electrolyte in the electrolyser topped up. Preferably the systems comprises a gas collection system arranged to remove gas from the electrolyser and store the gas in a storage means. The means for storing the gas can comprise gas storage tanks. The gas can be stored initially at low pressure before being compressed by a compressor to be stored at high pressure thus reducing the necessary storage volume. To avoid the gas storage reaching its full capacity, outlet pipes may be provided to transport the gas to a remote location. Typically the gases produced by the system will be hydrogen and oxygen. The gas storage can store both of these gases or it may be chosen only to store hydrogen with the oxygen being released into the atmosphere.

As has been discussed earlier, in the embodiments which comprise a solar concentrator, the electrical system and the gas storage system can beneficially be positioned in the dead space onto which the solar radiation is not focused around the photovoltaic cells and electrolyser.

The generated hydrogen can be stored in a gas system as previously described or can be used directly. The hydrogen could be used, for example, as a source of fuel in a fuel cell to generate electricity, or as a source of fuel in a combustion unit to generate heat.

Methods of generating hydrogen using any of the systems herein described form a further aspect of the invention. Certain preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Fig. 1 shows a schematic diagram of a system for generating hydrogen in accordance with an embodiment of the invention; and

Fig. 2 shows a schematic diagram of an electrolyser which may be used in a hydrogen generation system in accordance with the invention.

Fig. 1 shows a schematic representation of a hydrogen production system 1 in accordance with an embodiment of the invention. The system 1 comprises a solar concentrator lens 2, e.g. a linear refractive Fresnel lens, which is arranged for solar radiation 3 to be incident upon it. A wavelength selective mirror 4 is arranged in the path of the focused light and has a cut-off wavelength of about 550 nm, below which it reflects incident radiation and above which it transmits. An array of photovoltaic cells 6 is arranged such that radiation 7 passing through the mirror 4 is incident upon it. The type photovoltaic cell is chosen such that it operates efficiently at the appropriate light concentration and at the wavelength of light which is incident upon it. For example, an array of monocrystalline silicon cells, particularly laser grooved surface contact silicon cells such as those supplied by Narec (New and Renewable Energy Centre, UK), are suitable for medium levels of solar concentration, as are copper indium gallium selenide (CIGS) cells, e.g. those developed by the National Renewable Energy Laboratory (NREL), USA. A high efficiency single or dual junction gallium arsenide cell could also be used. The array of photovoltaic cells 6 is connected to a maximum power point tracking unit (MPPT) 8 which functions to optimise the power output from the photovoltaic cells 6. Connected to and arranged to be charged by the MPPT unit 8 is a battery 10. The MPPT unit 8 is arranged to provide a measure of the charge status of the battery 10.

A high efficiency switchable DC/DC converter 12, arranged to supply the required voltage bias to an electrolyser 10, is connected to the MPPT unit 8. The DC/DC converter is capable of switching between a number of voltages that are optimised for different sun conditions, e.g. for producing hydrogen under high or low sun illumination, or for producing hydrogen in the dark.

The system further comprises an electrolyser 14, i.e. an electrochemical cell, onto which radiation 15 which has been reflected by mirror 4 is incident. Connected to the electrolyser 14 is a gas collection and storage system 20 for the hydrogen and oxygen produced in the electrolyser 14. The collection and storage system 20 comprises a gas collector, means for cleaning and storage of the gas, as well as an electrolyte circulation system (e.g. a pump - not shown). The electrolyte circulation system can flow electrolyte through the electrolyser 14 to rapidly remove bubbles from the system thereby facilitating the production of gas. The temperature of the electrolyte may also be measured. The hydrogen produced at the cathode of the electrolyser 14 is collected for subsequent use and can be transported elsewhere, if desired. For example, a compressor pump (not shown) may be employed to pump the hydrogen to a larger, higher pressure facility. The oxygen gas evolved at the anode may simply be vented to the atmosphere or, alternatively, may be collected in addition to the hydrogen.

A control system 22 is connected to both the DC/DC converter 12 and the collection and storage system 20 to control their operations and thus optimise the production of hydrogen gas under any given set of illumination conditions.

Fig. 2 shows a schematic diagram of the electrolyser 14 of Fig. 1 in more detail. The electrolyser comprises an anode 16 on which radiation 15 is incident. Anode 16 comprises a metal base (e.g. a steel plate) on which a high efficiency

photocatalytic material such as nanostructured iron oxide is deposited. Cathode 18 is made from a conducting material, e.g. nickel, and includes a high efficiency nanostructured surface catalyst such as platinum or ruthenium. The anode 16 is separated from the cathode 18 by a gas separation membrane 24. The electrolyser 14 is filled with water 26 and an electrolyte to provide the medium for electrolysis.

Operation of the embodiment shown in Figs. 1 and 2 will now be described. The hydrogen generation system 1 is placed in a location so that solar radiation 3 is incident upon the solar concentrator lens 2. The lens 2 focuses the light towards the wavelength splitting mirror 4 which has its split at 550 nm. Radiation 7 above this wavelength is transmitted through the mirror 4 and radiation 15 below this wavelength is reflected by the mirror 4. The transmitted radiation 7 is incident upon the array of photovoltaic cells 6 where the energy is used to generate electricity. The generated electricity is fed to the MPPT unit 8 which maximises the power output from the photovoltaic cells 6.

Electricity from the MPPT unit 8 can be fed to either or both of the battery 10 and the DC/DC converter 12, depending on the operating conditions of the system. Once charged, the battery 10 can also supply electricity to the DC/DC converter 12. The DC/DC converter 12, controlled by the control system 22, applies a voltage bias as necessary to the electrolyser 14 to provide the required potential to drive the electrolysis reaction. This voltage is supplied in addition to any which is generated directly by the electrolyser 14 from incident radiation 15. The reflected radiation 15 is incident upon the anode 16 of the electrolyser 14 where, aided by the photocatalyst deposited on the anode 16, it creates a potential which drives the electrolysis of the water. If necessary the potential is increased by a voltage bias supplied from the DC/DC converter 12 as previously described. The control system 22 monitors the potential created by the photocatalysis, and the electrical current produced (a measure of the gas production) in order to control the voltage bias applied from the DC/DC converter 12.

An oxidation reaction takes place at the anode 16 which produces oxygen gas and a reduction reaction takes place at the cathode 18 producing hydrogen gas. The gas produced by electrolysis is removed from the electrolyser 14 by the gas collection and storage system 20 for storage and subsequent use.

It will be appreciated that the system of the present invention can operate under a number of different conditions, e.g. high or low intensity solar radiation, or even darkness if the electrolyser 14 is driven solely by a voltage from the battery 10. The flexibility of the system to do this is due to the provision of the battery 10, the DC/DC converter 12 and the control system 22 which is able to vary the voltage bias output from the DC/DC converter dependent on the potential required to operate the electrolyser 14. As has previously been explained, surplus electricity, either from the photovoltaic cells 6 or the battery 10, can be used to power an external load or feed electricity into the national grid.

Table 1 outlines a number of different conditions under which the system is able to operate and therefore from where the voltage bias is applied.

Solar External Battery Bias voltage

radiation load charge

intensity level

High High High or Low: Use minimal most efficient voltage

Low bias for electrolyser and export surplus electricity

High Low High Medium: Maximise hydrogen generation in electrolyser

High Low Low Low: Use minimal most efficient voltage bias for electrolyser and recharge battery

Low or Dark High High Nil : Use battery to power external load

Low High Low Nil : Use photovoltaic cells to power external load

Low or Dark Low High Medium or high: Use battery to provide bias voltage for electrolyser

Low Low Low Low: Use photovoltaic cells to provide bias voltage for electrolyser to generate hydrogen

Dark High Low Nil : No power output possible

Table 1 : Different operating characteristics of the hydrogen generation system.

It can therefore be seen that the system is very flexible in being able to adapt to both the solar radiation conditions, the battery charge level and the requirements from external loads (if connected). Obviously there are certain circumstances in which not all the requirements of the system can be fulfilled, e.g. if it is dark, the battery charge level is low and the external load is high (last row in Table 1 ), but for most circumstances the system, controlled by the control system 22 is able to maximise the usefulness (probably measured in terms of the monetary value) of the overall system in terms of the amount of hydrogen produced in the electrolyser 14 as well as charging the battery and supplying electricity to an external load if required. A nil (zero) bias voltage results in no hydrogen production in the electrolyser 14, a low bias voltage is optimally efficient when the solar radiation is most intense (high solar illumination), a medium bias voltage gives increased hydrogen production at all solar radiation intensities (all illumination levels), and a high bias voltage enables the electrolyser 14 to produce hydrogen in the dark.

In addition the control system 22 is arranged to run the electrolyte pump at a rate which matches the rate of gas production, e.g. an increased rate of gas production results in increased rate of pumping. The rate of gas production can be determined from the current produced in the electrolyser circuit.

It will be appreciated by those skilled in the art that only a small number of possible embodiments have been described and that many variations and modifications are possible within the scope of the invention. For example the system 1 need not include a wavelength splitting mirror 4, the solar radiation 3, after passing through the solar concentrator lens 2, could be incident on a single device in which the electrolyser 14 and the photovoltaic cells 6 are combined, e.g. the radiation 3 passes through the electrolyser 14 first before being incident on the photovoltaic cells 6. Neither is it necessary to have a solar concentrator lens 2, the solar radiation 3 could be incident upon the wavelength splitting mirror 4 without first being focused.

Claims

Claims:

1. A system for generating hydrogen using solar energy, said system comprising:

means for applying said bias voltage to the electrolyser; and

2. A system as claimed in claim 1 wherein the solar concentrator comprises a lens, preferably a linear refractive lens, preferably a curved linear refractive lens.

3. A system as claimed in claim 1 or 2 wherein the solar concentrator comprises a tracking solar concentrator.

4. A system as claimed in claim 1 , 2 or 3 wherein the photovoltaic cell and the electrolyser are arranged in tandem with respect to the incident solar radiation.

5. A system as claimed in claim 4 wherein the photovoltaic cell and the electrolyser are arranged so that the incident solar radiation first passes through the electrolyser before illuminating the photovoltaic cell.

6. A system as claimed in any preceding claim comprising a wavelength splitter arranged to direct a first part of the solar radiation spectrum to the electrolyser and a second part of the solar radiation spectrum to the photovoltaic cell.

7. A system as claimed in claim 6 wherein the solar concentrator is arranged to direct incident solar radiation onto the wavelength splitter.

8. A system for generating hydrogen using solar energy, said system comprising: an electrolyser which comprises a photocatalyst arranged to generate hydrogen from an electrolyte (e.g. water) on illumination with solar radiation;

means for applying said bias voltage to the electrolyser; and

9. A system as claimed in any of claims 6 to 8 wherein the wavelength splitter directs the shorter wavelengths of the solar radiation spectrum to the electrolyser and the longer wavelengths to the photovoltaic cell.

10. A system as claimed in claim 9 wherein the wavelength split is about 550nm.

1 1 . A system as claimed in any one of claims 6 to 10 wherein the wavelength splitter comprises a wavelength selective mirror.

12. A system as claimed in claim 1 1 wherein the wavelength selective mirror comprises a coated glass or plastic element.

13. A system as claimed in any preceding claim wherein the photovoltaic cell and the electrolyser are arranged adjacent each other.

14. A system as claimed in any preceding claim wherein the photocatalyst comprises a metal oxide.

15. A system as claimed in any preceding claim wherein the electrolyser comprises an aqueous electrolyte.

16. A system as claimed in any preceding claim wherein the photovoltaic cell comprises a plurality of photovoltaic cells coupled together.

17. An apparatus as claimed in any preceding claim comprising a control means arranged to control the voltage bias applied to the electrolyser.

18. A system as claimed in any preceding claim comprising a maximum power point tracking unit arranged to collect the electricity generated by the photovoltaic cell.

19. A system as claimed in claim 18 comprising a battery arranged to be charged by the electricity generated by the photovoltaic cell.

20. A system as claimed in claim 18 or 19 comprising a DC/DC converter arranged to supply the required voltage bias to the electrolyser.