AU731495B2 - Apparatus for separating solar radiation into longer and shorter wavelength components - Google Patents

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Applicant(s): JOHN BEAVIS LASICH Invention Title: APPARATUS FOR SEPARATING SOLAR RADIATION INTO LONGER AND SHORTER WAVELENGTH COMPONENTS 0* *000 0 0 The following statement is a full description of this invention, including the best method of performing it known to me/us: 1 2 APPARATUS FOR SEPARATING SOLAR RADIATION INTO LONGER AND SHORTER WAVELENGTH COMPONENTS The present invention relates to an apparatus for separating longer and shorter wavelength solar radiation so that the separated components of the solar radiation spectrum can be used as required in selected end-use applications, such as the production of hydrogen.

The present invention has been divided from Australian application 55539/94 for a standard patent and the disclosure in the patent specification of that patent application is incorporated herein by cross-reference.

The use of hydrogen as a carrier of energy, particularly in the context as a fuel, has the following significant technical advantages over other energy sources.

1. Supply side considerations hydrogen is inexhaustible, storable, transportable, and has a high energy density compared with other chemical fuels.

i 25 2. Demand side considerations hydrogen is non-polluting, more versatile than electricity, more efficient than petrol, and convertible directly to heat and electricity for both mobile and stationary applications.

By way of particular comparison, the large scale use of solar energy as an energy source has been limited for technical reasons and cost by a lack of a suitable short and long term storage medium for solar energy.

\\melb01l\homeS\Narelle\Keep\speci\55539 94 DIV LASICH.doc 18/06/98 29/01 '01 MON 12:20 FAX 81 3 9243 8333 GRIFFITH HACK 1 005 -3- However# notwithstanding the above technical advantages of hydrogen as; an energy source, the Cost of production of hydrogen has been too high hitherto for widespread use as a fuel.

in the case of the production of hydrogen by electrolysis of water, a major factor in the high Cost Of production has been the cost of electricity to operate electrolysis cells.- In the gpecif ic case of solar radiation-generated electricity, the high cost of electricity is due in large part to the relativrely low efficiency of photovoltaic (or thermal) conversion of solar energy into electricity which means that a relatively large number of photovoltaic cells (or, in the case of thermal conversion, a large collection area) is required to generate a unit output of electricity.

An object of the preaent invention is to provide *20 an apparatus for separating longer and shorter wavelength components of the solar radiation spectrum such that the separated components can be uoed efficiently in a solar radiation based method and apparatus for producinig hydrogen in an electrolysis cell- According to the present invention there is provided an apparatus for separating solar radiation into a longer wavelength component and a shorter wavelength component, the apparatus comprising: a means for concentrating solar radiation, a mirror for selectively **reflecting either the longer wavelength component or the 0: shorter wavelength component of the solar radiation spectrum, the mirror being positioned in the light path of the solar radiation from the concentrating means, the mirror comprising a spectrally selective filter to make the mirror transparent to the non-reflected component of the R-1 solar radiation spectrum to allow the non-ref lected 29/01 '01 MON 12:20 [TX/RX NO 7826] 29/01 '01 MON 12:20 FAX 81 3 9243 8333 GRIFFITH HACK__ 00L~~6 -3acomponent to pass throughL the mirror to a first receiver, and the mirror being appropriately curved ii order to selectively concentrate and direct the longer Or Shorter wavelength component towards a receiver that is excternial to the apparatus.

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4 4* *4 \13)rt \19 3896-9101101o 29/01 '01 MON 12:20 [TX/RX NO 7826] 4 It is preferred that the mirror be appropriately curved so that it can concentrate and direct the reflected longer wavelength component or the shorter wavelength component to a distant point for collection by a receiver.

It is preferred that the apparatus further comprises, a non-imaging concentrator for concentrating the reflected longer or shorter wavelength component.

It is preferred that the apparatus further comprises, an optical fibre of light guide for transferring the concentrated reflected longer or shorter wavelength component for use in an end use application.

15 It is preferred particularly that the end use application be the generation of hydrogen by electrolysis of water. In this end use application the longer wavelength component is suitable for use as a source of thermal energy and the shorter wavelength component is suitable for use as a source of electrical energy. In pparticular in this end use application there is provided: an electrolysis cell having an inlet for steam and outlets for hydrogen, oxygen, and excess 25 steam; the above-described apparatus for separating solar radiation into a longer wavelength component and a shorter wavelength component; a means for separately converting the longer wavelength component into thermal energy and the shorter wavelength component into electrical energy arranged in series or in parallel relationship for providing the energy required for converting water into steam and/or heating steam for operating the electrolysis cell to \\melbOl\homeS\Narelle\Keep\speci\55539 94 DIV LASICH.doc 18/06/98 5 decompose the steam into hydrogen and oxygen at high temperatures of at least 700 0 C, more preferably at least 1000 0

C.

The present invention is described further by way of example with reference to the accompanying drawings, in which: Figure 1 illustrates schematically an apparatus for producing hydrogen which incorporates a conventional array of solar cells and thermal energy receiver; Figure 2 illustrates schematically another apparatus for producing hydrogen which incorporates an 15 embodiment of an apparatus for separating solar radiation into longer and shorter wavelength components in accordance with the present invention; Figure 3 illustrates schematically an apparatus for producing hydrogen which incorporates another embodiment of the apparatus in accordance with the present S. invention; Figure 4 illustrates schematically an apparatus 25 for producing hydrogen which incorporates another embodiment of the apparatus in accordance with the present invention; Figure 5 is a diagram which shows the major components of an experimental test rig based on the apparatus shown in Figure 1; and Figure 6 is a detailed view of the electrolysis cell of the experimental test rig shown in Figure 4.

As is indicated above, the present invention has been divided from application 55539/94. The invention \\ielbOl\home$\Narelle\Keep\speci\55539 94 DIV LASICH.doc 18/06/98 6 disclosed and claimed in application 55539/94 relates to using solar energy to provide the total energy requirements, in the form of a thermal energy component and an electrical energy component, to form hydrogen and oxygen by the electrolysis of water. In this connection, the applicant found that the combined effect of solar-generated thermal energy and solar-generated electrical energy results in a significant improvement in the efficiency of the electrolysis of water in terms of energy utilisation, particularly when the thermal component is provided as a by-product of solar-generated electricity production.

The following description of the present invention is in the context of the production of hydrogen.

The apparatus shown schematically in Figure 1 comprises, a suitable form of solar concentrator 3 which .'.focuses a part of the incident solar radiation onto an array of solar cells 5 for generating electricity and the remainder of the incident solar radiation onto a suitable form of receiver 7 for generating thermal energy.

The electricity and the thermal energy generated by the incident solar radiation are transferred to a 25 suitable form of electrolysis cell 9 so that: a part of the thermal energy converts an inlet stream of water for the electrolysis cell 9 into steam and heats the steam to a temperature of about 1000 0 C; and the electrical energy and the remainder of the thermal energy operate the electrolysis cell 9 to decompose the high temperature steam into hydrogen and oxygen.

The hydrogen is transferred from the electrolysis \\melbOl\home$\Narelle\Keep\speci\55539 94 DIV LASICH.doc 18/06/98 7 cell 9 into a suitable form of storage tank 11.

The receiver 7 may be any suitable form of apparatus, such as a heat exchanger, which allows solar radiation to be converted into thermal energy.

The apparatus shown in Figure 1 further comprises a heat exchanger means (not shown) for extracting thermal energy from the hydrogen and oxygen (and any exhaust steam) produced in the electrolysis cell 9 and thereafter using the recovered thermal energy in the step of converting the inlet stream of water into steam for consumption in the electrolysis cell 9. It is noted that the recovered thermal energy is at a relatively lower temperature than the thermal energy generated by solar radiation. As a consequence, preferably, the recovered thermal energy is used to preheat the inlet water, and the solar radiation generated thermal energy is used to provide the balance of the heat component required to convert the feed water or steam to steam at 1000 0 C and to contribute to the operation of the electrolysis cell 9.

It is noted that the component of the thermal energy which is used endothermically at high temperature in 25 the electrolysis cell 9 is consumed at nearly 100% efficiency. This high thermal energy utilisation is a major factor in the high overall efficiency of the system.

It is also noted that high temperatures are required to achieve the high thermal energy efficiency and as a consequence only systems which can collect and deliver thermal energy at high temperatures (700C+) can achieve the high efficiency.

The apparatus shown in Figure 1 is an example of a parallel arrangement of solar cells 5 and thermal energy receiver 7. The apparatus shown schematically in Figures 2 to 4 are examples of series arrangements.

\\melbOl\ho.eS\Narelle\Keep\speci\55539 94 DIV LASICH.doc 18/06/98 29/01 '01 MON 12:20 FAX 61 3 9243 8333 GRIFFITH RACK i007 In addition, the apparatus shown schematically in Figures 2 to 4 incorporate examples of apparatus in accordance with the present invent ion for Separating solar radiation into longer and shorter wavelength components.

The apparatus shown sematically in Figures 2 to 4 tak~e advantage of the fact that solar cells selectively absorb shorter wavelengths and may be transparent to longer wavelengths of the solar radiation spectrumZ. In this connection, the threshold is in the order of 1.1 micron for silicon solar calls arnd 0.89 micron for GaAs cells leaving to 35% of the incoming energy of the solar radiation, which is normally wasted, for use as thermal energy.

In each cage, the apparatus shown in Figures 2 to 4 are arranged so that, in use, solar radiation is reflected from a solar concentrator 3 onto a first receiver in the form of a solar cell 15 to generate electricity from shorter wavelength component of the solar radiation and solar radiation that is not used for electricity generation, i.e. the longer wavelength component, is :directed to a second receiver in the form of thermal energy receiver (not shown) of an electrolysis cell 1.7 to convert the solar radiation into thermal energy. more particularly, in each case the apparatus shown in Figures 2 to 4 comprises a means which, in use, separates the long"er and shorter wavelength components of the solar radiation spectrum so that the components can be used separately for.

thermal energy and electricity generation, respectively.

The solar radiation separating means comprises a mirror 27 (not shown in Figure 2 but shown in Figures 3 and 4) positioned in front of or behind the solar cells in situations where the mirror 27 is positioned in front of the solar cells 15. as shown in Figures 3 and 4, the mirror 27 comprises an interference filter or edge R9 A 4 1O 29/01 '01 MON 12:20 [TX/RX NO 7826] 29/01 '01 MON 12:21 FAX 81 3 9243 8333 GRIFFITH HACK Ii008 -9f ilter (not shown) which makeS the mirror 21 transparent to the shorter wavelength component of the solar radiation spec truim.

S The mirror 27 may be of any suitable shape to reflect and selectively direct the longer wavelength component of the solar radiation spectrum. For example, in situations where the mirror 27 is positioned in front of the solar cells 15 and the focal poinlt of the solar concentrator 3, an shown in Figures 3 and 4, the-mirror 21 may take the form of a Cassigranian mirror, and in situations where the mirror 27 is positioned behind the focal point of the solar concentrator 3, the mirror may take the form of a Gregorian mirror- The longer wavelength radiation reflected by the mirror 27 may be transferred to the electrolysis cell 17 by any suitable transfer means 21 such as a heat pipe (not shown) or an optical fibre (or light guide), as shown in Figures 2 and 4, or directly as radiation,. as shown in Figure 3.

With particular regard to the apparatus shown in Figure 4, the electrolysis cell 17 is positioned remote from the solar cells 1S, and the apparatus further comprises a non-imaging concentrator 33 for cozieentratifig the reflected longer wavelength component of the polar radiation prior to transferring the concentrated component to the optical fibre or light guide 21.

It is also noted that the present invention is *,not limited to use of the reflected longer wavelength component of the solar radiation spectrum to provide thermal energy to an electrolyois cell anid may be used to provide thermal energy in any end use application.

The electrolysis cells 9,17 shown in the figures R-BrtKe\pc~l3-9dg 29/0l/Ot 29/01 '01 MON 12:20 [TX/RX NO 7826] 10 may be of any suitable configuration. Typically, the electrolysis cells 9,17 are formed from a material, such as yttria stabilised zirconia (YSZ), which is porous to oxygen and impermeable to other gases, and the accessories, such as membranes and electrodes (not shown), are formed from materials, such as alloys and cermets.

The apparatus described above take advantage of the facts that: the electrical potential and the electrical energy necessary to produce hydrogen in an electrolysis cell decreases as the temperature increases and the balance of the energy requirements to operate the electrolysis cell can be provided in the form of thermal energy; the efficiency of generation of thermal energy from solar radiation is significantly higher (in the order of 3 to 4 times) than the efficiency of generation of electricity from solar radiation; and the efficiency of consumption of the thermal 25 energy endothermically in the electrolysis cell approaches 100%.

It is noted that it is believed by the applicant that the use of the by-product thermal energy can only be practically executed by the means described herein since other currently known methods are not capable of transferring energy to produce a temperature in excess of 1000 0

C.

In other words, a particular advantage of the present invention is that, as a consequence of being able to separate the longer and shorter wavelength components of \\melbOl\homeS\Narelle\Keep\apeci\55539 94 DIV LASICH.doc 18/06/98 11 the solar radiation spectrum, it is possible to recover and convey and use that longer wavelength component in high temperature applications where otherwise that longer wavelength component would have been converted into low temperature heat (typically less than 45 0 C) and being unusable.

Further advantages of the present invention in the context of hydrogen production are as follows: 1. The efficiency of hydrogen production is greater than any other known method of solar 2 radiation generated hydrogen production.

2. The present invention increases the overall efficiency of the system, i.e. the efficiency of producing hydrogen by this method is greater than the efficiency of just producing electricity.

3. The present invention provides a medium, namely hydrogen, for the efficient storage of solar energy hitherto not available economically and thus overcomes the major 25 technological restriction to large scale use of solar energy.

It should be noted that the performance of the present invention in the context of hydrogen production is expected to exceed 50% efficiency. The theoretical performance is in the order of 60%, whereas the existing technology is not expected to practically exceed 14% efficiency and has a threshold limit of 18%.

In order to illustrate the performance of the invention disclosed and claimed in application 55539/94 the applicant carried out experimental work, as described \\melbOl\homeS\Narelle\Keep\speci\55539 94 DIV LASICH.doc 18/06/98 12 below, on an experimental test rig shown in Figures 5 and 6 which is based on the embodiment of the apparatus shown in Figure 1.

With reference to Figures 5 and 6, the experimental test rig comprised a 1.5m diameter paraboloidal solar concentrating dish 29 arranged to track in two axes and capable of producing a solar radiation flux of approximately 1160 suns and a maximum temperature of approximately 2600 0 C. It is noted that less than the full capacity of power and concentration of the concentrating dish 29 was necessary for the experimental work and thus the receiving components (not shown) were appropriately positioned in relation to the focal plane and/or shielded to produce the desired temperatures and power densities.

The experimental rig further comprised, at the focal zone of the solar concentrating dish 29, an assembly of an electrolysis cell 31, a tubular heat shield/distributor 45 enclosing the electrolysis cell 31, a solar cell 51, and a length of tubing 41 coiled around the heat shield/distributor 45 with one end extending into the electrolysis cell 31 and the other end connected to a source of water.

The solar cell 51 comprised a GaAs photovoltaic (19.6mm active area) concentrator cell for converting solar radiation deflected from the concentrator dish 31 into electrical energy. The GaAs photovoltaic cell was selected because of a high conversion efficiency (up to 29% at present) and a capacity to handle high flux density (1160 suns) at elevated temperatures (100 0 In addition, the output voltage of approximately 1 to 1.1 volts at maximum power point made an ideal match for direct connection to the electrolysis cell 33 for operation at 1000 0

C.

With particular reference to Figure 6, the \\melb01l\home$\Narelle\Keep\speci\55539 94 DIV LASICH.doc 18/06/98 13 electrolysis cell 31 was in the form of a 5.8cm long by 0.68cm diameter YSz closed end tube 33 coated inside and outside with platinum electrodes 35, 37 that formed cathodes and anodes, respectively, of the electrolysis cell 31 having an external surface area of 8.3cm 2 and an internal surface area of 7.6cm 2 The metal tube 45 was positioned around the electrolysis cell 31 to reduce, average and transfer the solar flux over the surface of the exterior surface of the electrolysis cell 31.

The experimental text rig further comprised, thermocouples 47 (Figure 5) connected to the cathode 35 and the anode 37 to continually measure the temperatures inside and outside, respectively, the electrolysis cell 31, a 1mm 2 platinum wire 32 connecting the cathode 35 to the solar cell 51, a voltage drop resistor (0.01U) (not shown) in the circuit connecting the cathode 35 and the solar cell 51 to measure the current in the circuit, and a Yokogawa HR- 1300 Data Logger (not shown).

The experimental test rig was operated with the electrolysis cell 31 above 1000 0 C for approximately two and 25 a half hours with an excess of steam applied to the electrolysis cell 31. The output stream of unreacted steam and the hydrogen generated in the electrolysis cell 31 was bubbled through water and the hydrogen was collected and measured in a gas jar.

When a steady state was reached, readings of temperature, voltage, current and gas production were recorded and the results are summarised in Table 1 below.

\\melb01\homeS\Narelle\Keep\speci\55539 94 DIV LASICH.doc 18/06/98 14 Time Electrolysis Electrolysis Electrolysis Gas Cell Cell Current Cell Production Voltage Temperature V Amps OC ml 2.22 1.03 .67 1020 0 2.39 1.03 .67 1020 net 17 net minutes On the basis of the measured electrolysis cell voltage of 1.03 V recorded in Table 1 and a determined thermoneutral voltage of 1.47, the electrical efficiency of the electrolysis cell 31, calculated as the ratio of the thermoneutral and measured voltages, was 1.47 1.43 1.03 In terms of the solar cell efficiency, with the solar cell 31 positioned to receive a concentration ratio of 230 suns and assuming: an output voltage 1.03 (=voltage across electrolysis cell and allows for connection losses); *5 S S a current of 0.67 Amps; direct solar input is 800 w/m' 2; and an active solar cell area 19.6 x 10- 6 m 2 the efficiency of the solar cell 51 (ipv) was Tpv output 1.03 x .67 .69 .19 input 19.6 x230x800 3.6 106 With a spectral reflectivity of 0.9 for the \\melbO\homeS\Narelle\Keep\speci\55539 94 DIV LASICH.doc 18/06/98 15 mirror surface of the solar concentrating dish 29, the efficiency of the solar concentrator dish 29 was 0.85.

Thus, the total system efficiency of the solar cell 51 and the electrolysis cell 31 and optics (ritotal) was Ttotal 0.85x.19x1.43=.22 (22%) The above figures of 22% is approximately twice the best previous proposed systems and more than three times the best recorded figure for a working plant.

The results of the experimental work on the experimental test rig establish that: it is possible to produce hydrogen by high temperature electrolysis of water driven totally by solar radiation, the efficiency of production is greatly improved over known systems, and a significant portion of the heat of solar radiation can be used directly in the 25 electrolysis reaction thus reducing greatly expensive electrical input by almost half.

Many modifications may be made to the preferred embodiments of the present invention as described above without departing from the spirit and scope of the present invention.

By way of example, whilst the preferred embodiments describe that the present invention separates the longer and shorter wavelength components of the solar \\melbOl\homeS\Narelle\Keep\speci\55539 94 DIV LASICH.doc 18/06/98 16 radiation spectrum by reflecting the longer wavelength component, it can readily be appreciated that the present invention is not limited to such an arrangement and. extends to arrangements in which the shorter wavelength component is reflected.

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Claims (5)

1. An apparatus for separating solar radiation into a longer wavelength Component'and a shorter wavelength component, the apparatus comprising: a means for concentrating solar radiationj a mirror for selectively reflecting either the longer wavelength component or the shorter wavelength component of the solar radiation spectrilm, the mirror being positioned in the light path of the solar radiation from the cocnrtn means, the mirror comprising~ a spectrally selective filter to make the mirror transparent to the non-reflected component of the solar radiation spectrum to allorw the non-reflected component to pass through the mirror to a first receiver. and the mirror being appropriately curved in order to selectively concentrate arnd direct the longer or shorter wavelength component towards a receiver that is external to the apparatus.

2. The apparatus defined in claim wherein the :spectrally selective filter compriseo an interference or edge filter. The apparatus defined claim 1 or claim 2 further comprising, a non-imaging concentrator for further concentrating the reflected longer or shorter wavelength -component from the mirror.

4. The apparatus defined in claim 3 further .30 comprising, a means for conveying the concentrated reflected longer or shorter wavelength component for use in an end use application. The apparatus defined in claim 4, wherein the conveying means is an optical fibre or a light guide. I The apparatus defined in claim 4 or claim 291o1/ot 29/01 '01 MON 12:20 [TX/RX NO 7826] 29/01 '01 MON 12:21 FAX 61 3 9243 8333 GRIFFITH HACK__ 18 wherein, the end use application comprises any one of the following, the generation of hydrogen by electrolysis of water, the generation of electricity for shaft power by the use of a Stirling engine, a steam heater, or a super heater.

7. The apparatus defined in any one of the preceding claims wherein the second receiver is remote from the first receiver.

8. A apparatus for separating solar radiation into a longer wavelength component with a shorter wavelength component substantially as hereinbefore described with reference to the accompanying drawings. im 010 Dated this 29th day of January 2001 JOHN BEAVIS LASICH By their Patent Attorneys GRIFFITH HACK 20 Fellows institute of Patent Attorneys of Australia .0.00. 0*0* 0* *0 H. Drat \KcepN.,i \7J19-98tl .do c 29/0101/ 29/01 '01 MON 12:20 [TX/RX NO 7826]