Apparatus for separating solar radiation into longer and shorter wavelength components

Abstract: The apparatus comprises a mirror (27) including a spectrally selective filter, such as an interference, or edge filter, the mirror being curved to concentrate a selected radiation component for conveyance to external apparatus.

Description: 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 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. 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. 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 specific case of solar radiation-generated electricity, the high cost of electricity is due in large part to the relatively 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. EP-A-0,019,016 discloses a solar energy collector which uses a selective filter to separate the radiation into thermal and light radiation. The energy is collected by a paraboloidal mirror above which is mounted the filter. This transmits radiation below a certain wavelength and reflects that above. The reflected thermal energy is absorbed and utilised by a pipe through which a heat transfer medium flows, onto which the mirror is focused via the reflector. The transmitted energy is also focused onto an absorber which is a solar cell. The arrangement allows complete absorption of the energy without the need for cooling the solar 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 positioned in the light path of the solar radiation from the concentrating means, for selectively reflecting either the longer wavelength component or the shorter wavelength component of the solar radiation spectrum, the mirror comprising a spectrally selective filter to make the mirror transparent to the non-reflected component of the solar radiation spectrum to allow the non-reflected component to pass through the mirror to a first receiver, and the mirror being appropriately curved in order selectively to concentrate and direct the longer or shorter wavelength component towards a second receiver that is external to the apparatus. It is preferred that the apparatus further comprises, an optical fibre or light guide for transferring the concentrated reflected longer or shorter wavelength component for use in an end use application. It is preferred particularly that the end use application be the generation of hydrogen by electrolysis of water. The present invention is described further by way of example with reference to the accompanying drawings, in which: Figure 1 illustrates schematically one embodiment of an apparatus for producing hydrogen which is not in accordance with the present invention; Figure 2 illustrates schematically another embodiment of an apparatus for producing hydrogen which is not in accordance with the present invention; Figure 3 illustrates schematically a further embodiment of an apparatus for producing hydrogen in accordance with the present invention; Figure 4 illustrates schematically a further embodiment of an apparatus for producing hydrogen in accordance with the present invention; Figure 5 is a diagram which shows the major components of an experimental test rig based on the embodiment of 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. 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 suitable form of electrolysis cell 9 so that: (a) 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°C; and (b) 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 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°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 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 (700°C+) 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. The apparatus shown schematically in Figures 2 to 4 are examples of series arrangements. The apparatus shown schematically in Figures 2 to 4 take advantage of the fact that solar cells selectively absorb shorter wavelengths and may be transparent to loner wavelengths of the solar radiation spectrum. In this connection, the threshold is in the order of 1.1 µm for silicon solar cells and 0.89 µm for GaAs cells leaving 25% to 35% of the incoming energy of the solar radiation, which is normally wasted, for use as thermal energy. The apparatus shown in Figures 2 to 4, in each case, is arranged so that, in use, solar radiation is reflected from a solar concentrator 3 onto a solar cell 15 to generate electricity from the shorter wavelength component of the solar radiation and the solar radiation that is not used for electricity generation, i.e. the longer wavelength component, is directed to a thermal energy receiver (not shown) of an electrolysis cell 17 to convert the solar radiation into thermal energy. The apparatus shown in Figures 2 to 4, in each case, comprise a means which, in use, separates the longer 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 15. 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 filter (not shown) which makes the mirror 27 transparent to the shorter wavelength component of the solar radiation spectrum. 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 point of the solar concentrator 3, as shown in Figures 3 and 4, the mirror 27 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 solar cells 15 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 15, and the apparatus further comprises a non-imaging concentrator 33 for concentrating the reflected longer wavelength component of the solar radiation prior to transferring the concentrated component to the optical fibre or light guide 21. The electrolysis cells 9,17 shown in the figures 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 membranes and electrodes (not shown), are formed from materials, such as alloys and cermets. The apparatus as described above take advantage of the facts that: (a) 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; (b) 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 (c) the efficiency of consumption of the thermal 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°C. In other words, a particular advantage is that, as a consequence of being able to separate the longer and shorter wavelength components of 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°C) and being unusable. Further advantages are as follows: 1. The efficiency of hydrogen production is greater than any other known method of solar 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 technological restriction to large scale use of solar energy. It should be noted that the performance 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%. The applicant carried out experimental work, as described 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°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 29 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°C). 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 31 for operation at 1000°C. With particular reference to Figure 6, the 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 6) 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 (0.01Ω) (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°C for approximately two and 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. Time Electrolysis Cell Voltage Electrolysis Cell Current Electrolysis Cell Temperature Gas Production V Amps °C ml 2.22 1.03 - 67 1020 0 2.39 1.03 - 67 1020 80 net 17 minutes net 80ml 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 / 1.03 = 1.43 In terms of the solar cell efficiency, with the solar cell 31 positioned to receive a concentration ratio of 230 suns and assuming: (a) an output voltage = 1.03 (=voltage across electrolysis cell and allows for connection losses); (b) a current of 0.67 Amps; (c) direct solar input is 800 w/m i2 ; and (d) an active solar cell area = 19.6 x 10 -6 m 2 . the efficiency of the solar cell 51 (η pv ) was ηpv = output input = 1.03 x .67 19.6 10 6 x230x800 = .69 3.6 = .19 With a spectral reflectivity of 0.9 for the 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 (η total ) was η total = 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: (a) it is possible to produce hydrogen by high temperature electrolysis of water driven totally by solar radiation, (b) the efficiency of production is greatly improved over known systems, and (c) a significant portion of the heat of solar radiation can be used directly in the electrolysis reaction thus reducing greatly expensive electrical input by almost half. Whilst the preferred embodiments describe the use of solar cells to convert solar energy into electricity, it can readily be appreciated that the present invention is not so limited and extends to any suitable solar radiation to electricity converters. Furthermore, whilst the preferred embodiments describe that the present invention separates the longer and shorter wavelength components of the solar 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.

Claims: An apparatus for separating solar radiation into a longer wavelength component and a shorter wavelength component, the apparatus comprising: a means (3) for concentrating solar radiation; a mirror (27) positioned in the light path of the solar radiation from the concentrating means (3), for selectively reflecting either the longer wavelength component or the shorter wavelength component of the solar radiation spectrum, the mirror (27) comprising a spectrally selective filter to make the mirror (27) transparent to the non-reflected component of the solar radiation spectrum to allow the non-reflected component to pass through the mirror (27) to a first receiver (15), characterised in that the mirror (27) is appropriately curved in order to selectively concentrate and direct the longer or shorter wavelength component towards a second receiver (17; 31) that is external to the apparatus. The apparatus defined in Claim 1, wherein the spectrally selective filter comprises an interference or edge filter. The apparatus defined in Claim 1 or 2, further comprising a non-imaging concentrator (33) for further concentrating the reflected longer or shorter wavelength component from the mirror (27). The apparatus defined in Claim 3 further comprising a means (21) 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 (21) is an optical fibre or a light guide. The apparatus of claim 4 or 5 in combination with end use application equipment comprising an electrolytic generator for generating hydrogen from water, a Stirling engine for generating electricity for shaft power, a steam heater, or a super heater. The apparatus defined in any one of the preceding claims, wherein the second receiver (17; 31) is positioned at a location that is remote from the first receiver (15).

Inventors: LASICH JOHN BEAVIS, LASICH, JOHN BEAVIS
Applicants: SOLAR SYSTEMS PTY LTD
Application year: 1993
Main classification: F24J2
IPC codes: F24J2, H01L31, C25B1