NL2007048C2

NL2007048C2 - Solar power installation.

Info

Publication number: NL2007048C2
Application number: NL2007048A
Authority: NL
Inventors: Johannes Krieg; Sidi Mohamed Ezzahiri; Carlos Alberto Ferreira
Original assignee: Solfence Holding B V
Priority date: 2011-07-05
Filing date: 2011-07-05
Publication date: 2013-01-08
Also published as: WO2013006054A1; EP2729740A1

Description

Solar power installation BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a solar power installation for collecting and converting 5 solar radiation into useful forms of energy and more particularly to a system that is arranged to track the movement of the sun in order to maximize the amount of radiation collected. The invention also relates to a method of control for such a device for following the sun in two different planes.

2. Description of the Related Art 10 Many devices have been proposed and are presently in operation for collecting energy from the sun. These range from static devices such as roof mounted domestic solar water heaters to photovoltaic panels mounted on orbiting satellites.

In general, the amount of energy available to collect is determined by the overall area of the collector and its orientation with respect to the incident radiation. Other factors 15 such as the waveband of the captured radiation will also be decisive in determining the amount of energy actually collected. Since the position of the sun changes throughout the day, for a given collector, orienting it towards the sun can significantly increase the amount of energy collected. Nevertheless, the mechanism required to move the collector adds significant complexity to the design and the energy required for 20 movement reduces the net output. The choice of a mobile or static mounting is therefore a design trade-off.

During a day, the rotation of the earth causes the sun to follow a single path which can be effectively followed by movement in a single plane. For the sake of the following this will be referred to as the azimuth plane. From day to day however, the relative 25 positions of the sun and earth change causing the sun to appear higher or lower in the sky. In order to fully track the sun, a collector must also be able to move in a direction perpendicular to the azimuth plane, which we shall refer to as the elevation plane. A collector may need to track 360o to follow the sun in the azimuth plane. In the elevation plane, variations of 23.5o on either side of the equinox position are sufficient. 30 For this reason, some collectors compromise by only tracking in a single plane.

Once the suns energy has been received by the collector, it must be converted into a suitable form that may be used further. The above-mentioned solar water heaters operate by absorption of the sun’s heat by a suitable surface and the transfer of the heat 2 by conduction to a circulating fluid. Such devices are of relatively low cost and are rather effective at providing warm water for domestic purposes. Photo-voltaic panels convert the sun’s rays into an electric current usually through the use of photo-sensitive semiconductor materials. Their efficiency is relatively low and at present they are 5 relatively costly. Tracking of such devices is therefore a useful way of increasing the efficiency thereby reducing pay-back time. An advantage in cost may also be achieved by using a concentrator to focus the suns energy from a larger collector onto a relatively smaller photo-voltaic cell.

Another form of solar converter that may operate with a concentrator is the thermal 10 cycle engine. Such devices absorb the sun’s energy as heat which is then converted into mechanical energy in a motor. The most favoured motor for this purpose is the Stirling engine. In operation, the concentrated rays of the sun are focussed on a thermal input area of the motor, causing the working fluid to be heated to a temperature sufficiently high for it to generate useful work. Typical temperatures required in such devices range 15 from 250-700 °C, depending upon the power output required.

An example of a concentrating solar collector is described in US 2004031517, the contents of which are incorporated herewith in their entirety, which uses a parabolic reflector to focus the sun’s rays onto a reception surface of a conversion module. The reception surface may either be located on the concave side of the reflector close to the 20 focal point or a secondary reflector may be provided and the reception surface may be located behind the parabolic receptor. The reception surface may comprise an array of at least one photovoltaic solar cell or may be coupled to a thermal cycle engine with the mechanical output of the thermal cycle engine driving an electric generator. In each case, the conversion module is arranged to move together with the parabolic reflector to 25 track the sun. In particular, for the case of a thermal cycle engine, the combined mass of the engine and reflector is relatively high leading to increased force and energy consumption in order to move the engine and collector together. Additionally, the fixed relationship of the reflector and engine is inconvenient when retraction of the reflector is required e.g. for storm protection or during assembly. Other arrangements are known 30 from WO2010/045269, DE 29606687, US5735123, US 4821516 and WO2009/158177, all of which suggest that the engine and collector should be connected for movement together.

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BRIEF SUMMARY OF THE INVENTION

According to the invention there is provided a solar power installation comprising a collector, a converter and a support, wherein the collector and the converter are arranged to rotate together about a substantially fixed panning axis with respect to the 5 support and the collector is arranged to be adjustable independently of the converter about an elevation axis with respect to the support. As a result of the independent motion of the collector, a significant simplification in the design may be achieved and there may also be a reduction in energy required to operate the tracking system. The collector and converter together may be balanced for rotation about the panning axis 10 with minimal power requirement. As the skilled person will understand, this motion will generally take place continuously as the sun’s position changes from sunrise to sunset. Movement of the collector about the elevation axis need only be a slight adjustment on a daily basis to compensate for the changing elevation of the sun from winter to summer.

15 The invention is primarily directed to installations wherein the collector comprises a generally parabolic shaped reflector. A most preferable form of collector is the Cassegrain configuration whereby a primary parabolic reflector has a secondary reflector arranged to reflect the light through an aperture in the primary reflector to a focal point behind the primary reflector. The secondary reflector is usually of 20 hyperbolic shape. Nevertheless, the skilled person will understand that the principles may also apply to other dish forms and to Fresnel arrangements. In this context, the term collector is intended to be used in its broadest possible context to mean any reflector or lens arrangement capable of collecting and focussing the energy of the sun. In general, the converter will be located at or adjacent to a focal point of the collector. 25 According to a preferred embodiment of the invention, the converter comprises a

Stirling cycle engine. The Stirling cycle engine may have its heat receptor located at a focal point of the collector. Such devices are well known to be suitable for converting heat into mechanical energy which can be subsequently converted by dynamo or electrical generator into electrical energy. Waste heat from the heat sink may also be 30 used for low grade heating purposes such as domestic hot water. The skilled person will be well aware of other forms of converter that may also be used including photovoltaic devices that can convert the sun’s energy directly into electrical energy and chemical cells such as solar driven high temperature chemical conversion processes which may 4 use energy to produce hydrogen or methane. Nevertheless, the invention is particularly applicable to Stirling engines and other relatively massive devices, since their relative weight compared to that of the collector imposes additional considerations when designing a tracking system that is energy efficient.

5 According to a further preferred embodiment, the converter is located at a side of the collector distanced from the sun. This is preferably achieved through use of the above mentioned Cassegrain configuration. In the case that a Stirling engine is used as the converter, only the hot side of the engine need be exposed to the sun with the remaining parts of the engine located in relative shadow. This facilitates operation of the Stirling 10 engine since cooling of the cold side of the engine is better achieved in the shadow.

In one preferred arrangement the converter is connected to the support for rotation about the panning axis. It will be understood that the connection need not be a direct connection and that the converter may be part of a converter assembly or construction that moves together with the assembly around the panning axis. The collector may be 15 connected to the converter for rotation about the elevation axis. In the case that the converter is a part of a larger converter assembly, the collector may be connected to the converter assembly. In this context, the skilled person will understand that the important feature is that the collector is connected to the support through the medium of the converter or converter assembly and cannot rotate around the panning axis 20 independently of the converter.

According to a further aspect of the invention, the elevation axis is arranged to be coincident with the focal point of the collector at which point the active region of the converter is located. In this manner, the collector may rotate up and down about the elevation axis while maintaining focus at the converter.

25 In a preferred embodiment, the installation further comprises a drive arrangement for movement of the collector with respect to the support. The drive arrangement may comprise a panning drive and an elevation drive each controlling motion about the respective axis. Such drives are generally known and may comprise a brushless motor and gear construction or a direct drive form such as a stepper motor. Both drives may 30 be the same but preferably, due to the different characteristics of the respective motions, each drive can be optimised to its purpose. In particular, the panning drive may be optimised for continuous motion while the elevation drive may be optimised for periodic operation. Should it be required, the collector may also be provided with a 5 counter-weight to provide a mass balanced system whereby the requirements of the elevation drive may be further reduced.

Preferably the installation further comprises a controller for controlling the drives and hence the position of the collector in order for it to track the motion of the sun. Such 5 controllers and their algorithms are generally known and may be based on feedback of the sun position or may be pre-programmed based on geographical data. A Global Positioning System may be provided to generate the location, height and time information which is required for initial calibration of the installation. This may be part of the controller or coupled thereto for set-up purposes.

10 According to a still further aspect of the invention the collector has a retracted position in which it is rotated about the elevation axis to a generally downward facing position. This allows the collector to assume a position of least wind resistance e.g. when high winds are expected. A typical collector may have a diameter of up to 2 meters and high winds may cause significant damage to the collector, support and drive arrangement.

15 By rotating the collector to a substantially horizontal position, wind damage may be reduced. For dish shaped collectors, the concave side will preferably be oriented downwards in the retracted position. Nevertheless, for certain configurations the collector may have a retracted or protective position with the collector facing upwards. Appropriate arrangements may then be made for preventing collection of snow or 20 rainwater.

Preferably, the support comprises a hollow tubular structure which may be of steel, aluminium or composite materials. For convenience, conduits may be passed through the tubular structure for the transport of power and/or cooling fluids. The installation may be arranged to be used at a predetermined latitude with respect to the earth’s 25 equator. For this purpose, the support may comprise a base anchored to the ground or other appropriate structure and a generally inclined arm arranged at an angle with respect to the vertical, wherein the angle corresponds approximately to said latitude. In this way, the arm may be angled in a direction corresponding approximately to the equinoxial zenith of the sun i.e. the position of the sun at its highest point on 21 30 March/September. This denotes a preferred mid-point from which the sun will deviate upwards in the summer and downwards in the winter, to the east in the morning and to the west in the afternoon. The panning axis will preferably be inclined perpendicularly 6 upwards with respect to the arm. Although this represents one preferred configuration, it will be understood that other alternative orientations of the arm may also be used.

The invention also relates to a method of operation of a solar power installation as described above involving controlling the collector and the converter to rotate together 5 about a substantially fixed panning axis with respect to the support in order to follow the path of the sun during the day and arranging the collector to be adjustable independently of the converter about an elevation axis with respect to the support in order to adjust the elevation of the collector to compensate for changes in the elevation of the sun. At certain times of year, the movement of the collector about the panning 10 axis may correspond directly to the path of the sun and no compensation for elevation during the day may be required. At other times, elevation motion may take place during the day. This may be incremental or continuous.

The above described invention is best adapted to relatively small installations having a collector area of 1 m2 to 10 m2. Nevertheless, it will be understood the principles may 15 also be adapted to larger installations.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the invention will be appreciated upon reference to the following drawings of a number of exemplary embodiments, in which:

Figure 1 shows a representation of a solar installation according to the prior art; 20 Figure 2 shows a side view of first embodiment of the present invention;

Figure 3 shows a perspective view of the embodiment of Figure 2;

Figure 4 shows a rear view of the embodiment of Figure 2;

Figure 5 shows a side view of the installation of Figure 2 in a retracted position;

Figure 6 shows a perspective view of an alternative embodiment of the invention; 25 Figure 7 shows the installation of Figure 7 in a morning orientation; and

Figure 8 shows an installation according to another embodiment of the invention. DESCRIPTION OF IFFUSTRATIVE EMBODIMENTS

Figure 1 shows a representation of a conventional solar installation 1 comprising a collector dish 2, a Stirling cycle engine 3, a dynamo 4 and a support 5. A cable 6 30 connects the dynamo 4 to a frequency/voltage regulator 7 which supplies power to the electrical grid. The dish 2 and the engine 3 are connected to one another by a rigid arm 8 which is pivoted to the support 5 for rotation about pivot 9.

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The engine 3 is located in front of the dish 2 and at its focal point whereby the sun’s rays are focussed onto its heat receptor. Further details of the operation of the Stirling cycle engine 3 and dynamo 4 will not be discussed as they may be otherwise conventional and are not themselves the subject of the present invention. As the 5 position of the sun changes throughout the day, the dish 2 and engine 3 move as a single unit about the pivot 9 to keep the dish 2 pointing towards the sun. This movement takes place both from east to west and also in an upwards and downwards direction.

At present, solar installations of this type are not beneficial when scaled down to the 10 range of 0.25 to 3 kW electrical power output. There are several system energy losses such as tracking energy losses due to the powered drive motors used for this purpose. Additionally, collector dish surface cut-outs are sometimes required to facilitate certain positions of the collector dish e.g. safety positions. These systems thus lose valuable collector surface which must be compensated by increasing the collector diameter, 15 hence increasing cost and reducing effectiveness. Due to tracking movements of the total system, electrical, control and/or liquid tubes and connections have to be designed in a particularly reliable way to allow motion flexibility of the tubing and avoiding maintenance and/or failures of the installation. Positioning of the Stirling engine within the path of the solar energy also requires more cooling energy to keep the heat sink of 20 the Stirling engine at a desired low temperature, leading to lower conversion efficiencies.

Figure 2 shows a side view of a solar installation 10 according to a first embodiment of the present invention. The solar installation 10 comprises a collector 12 formed of a parabolic dish 13 and a hyperbolic reflector 14 located close to the focal point of the 25 dish 13 in a Cassegrain configuration. To the rear of the collector 12 is a Stirling cycle engine 16 having its heat receptor 18 positioned at the focal point of the collector 12. Such a Stirling engine may be as described in US 4821516, the contents of which are incorporated herein by reference in their entirety. To the rear of the engine 16 is located a dynamo 20. The engine 16 is mounted at a first bearing 22 to a support 24. The 30 bearing 22 allows the engine 16 to rotate about a panning axis P. The dish 13 is attached to the engine 16 by a bracket 26 mounted to rotate about an elevation axis E coinciding with the heat receptor 18. The reflector 14 is in turn rigidly connected to the 8 dish 13 by struts 28. As in the conventional device of Figure 1, the dynamo is connected via a cable 30 and frequency/voltage regulator 32 to the grid at 34.

Figure 3 shows a perspective view of the device of Figure 2 from the opposite side, indicating the angles and motion of the various elements. According to Figure 3, the 5 installation is located at a position corresponding to the zenith or highest point of the sun at noon at the equinox (spring or autumn). At this moment in time, the sun is perceived at an elevation of 30° with respect to the horizontal corresponding to a location having a latitude of around 60°. The support 24 comprises a base 36 and an angled arm 38. The support 24 is specifically adapted for installation at this latitude in 10 that the arm 38 is also angled at 30° to the horizontal. The collector 12 is pointed directly towards the sun. During the day, the engine 16 is driven to follow the movement of the sun by a pan drive 40 which causes the engine 16 to rotate about the panning axis P. On subsequent days, the sun will appear higher or lower above the horizon. In order to direct the collector 12 towards the sun, an elevation drive 42 is 15 provided to tilt the collector 12 about the elevation axis E. Since the heat receptor 18 (see Fig. 2) is coincident with the elevation axis E, the collector 12 remains accurately focussed on the heat receptor 18 despite changes in elevation. Figure 3 also shows a controller 50 operatively attached to the elevation drive 42 and the pan drive 40. Controller 50 provides the necessary signals to control the movement of the collector 20 12 as described above.

Figure 4 shows a rear view of the installation of Figure 3 taken in the direction R. In this view, the aperture 46 through the collector 12 can be noted, as can the manner in which the bracket 26 connects the collector 12 to the engine 16 at the elevation drive. As can be seen, the bracket 26 is a single arm asymmetrically mounted on one side of 25 the aperture 46. The skilled person will understand that a symmetrical arrangement with two arms may also be provided for greater stability.

Figure 5 shows a similar view to that of Figure 2 with the collector 12 in a retracted position. In this position, the collector 12 is rotated downwards to the maximum possible extent about the elevation axis E. In this position, wind forces on the collector 30 12 may be minimised and snow cannot build up within the dish 13. Because of the manner in which the collector 12 pivots with respect to the engine 16, the latter can remain in its operative position with respect to the support 24. The collector 12 can rotate downwards to a position in which the dish 13 approaches the support 24. This 9 arrangement allows such downwards rotation without requiring cut-outs or the like in the dish 13 to accommodate the support 24. Such cut-outs not only reduce the available area for energy reception but also weaken the structure of the dish 13. Although not preferred, the installation 10 may also have a retracted or protective position in which 5 the collector 12 is rotated about the elevation axis E to a position in which the dish 13 is facing upwards. Precipitation collecting in the dish may escape through opening 46 and dynamo 20 may be better protected in this manner.

In the embodiments of Figures 2 to 5, the support 24 is a generally hollow structure formed as a variable profile box section. The interior of the support 24 may carry pipes, 10 conduits, cables and the like to and from the dynamo 20, engine 16 and drives 40,42. Figure 6 shows a perspective view of a slightly different embodiment in which the support 24 comprises an alternative tubular structure formed of a round hollow tube of constant section. This support may also be used in a similar manner to carry the necessary conduits and cables. In the position of Figure 6, the collector 12 is directed 15 towards the sun in the noon position and the elevation drive is slightly raised representing a summer orientation where the sun is above its equinoxial position.

In Figure 7 there is shown the installation 10 of Figure 6 in which the collector is rotated eastwards about the panning axis P to a morning position.

Figure 8 shows an installation 100 according to another embodiment of the invention. 20 The installation 100 of Figure 8 comprises a collector 112 in the form of a parabola. In this embodiment, a Stirling engine 116 is placed in front of the collector 112 with its heat receptor 118 located at the focal point. A dynamo 120 is provided on the rear side of the engine 116. The dynamo 120 is mounted to a support 124 by a first bearing 122 and provided with a pan drive 140. The collector 112 is connected by struts 128 to an 25 elevation drive 142 carried by the engine 116 for rotation about elevation axis E.

The shape of the support 124 allows the collector 112 to be rotated downwards about axis E from an equinoxial position to a retracted position in which it is adjacent to the support 124. This position may be assumed automatically for wind speeds above a given value. It will be understood that the installation 100 may also have a retracted 30 position with the collector facing vertically upwards. In that case a suitable provision for removing precipitation may be required. An advantage of the design of Figure 8 is that reflection losses due to the second reflector required in a Cassegrain configuration are eliminated. Nevertheless, a portion of the collector 112 lies in the shadow of the 10 engine 116 and support 124. For a Stirling cycle engine, efficiency may also be reduced since its heat sink will be located in the sun. For other forms of converter, this configuration may be more advantageous.

Thus, the invention has been described by reference to certain embodiments discussed 5 above. It will be recognized that these embodiments are susceptible to various modifications and alternative forms well known to those of skill in the art. In particular, although the arrangements shown use a Stirling cycle engine for the purpose of converting the sun’s energy, other converters may be used as appropriate. Further design aspects may also be distinct from the schematically illustrated designs.

10 Many modifications in addition to those described above may be made to the structures and techniques described herein without departing from the spirit and scope of the invention. Accordingly, although specific embodiments have been described, these are examples only and are not limiting upon the scope of the invention.

Claims

A solar energy installation comprising: a collector; 5 a converter, and a support piece; wherein the collector and the transducer are adapted to rotate together about a substantially immobile axis of rotation relative to the support member, and the collector is adapted to be adjustable about an elevation axis 10 relative to the support member independently of the converter.

Installation according to claim 1, wherein the collector comprises a generally parabolic reflector, preferably of a Cassegrain configuration.

Installation according to claim 1 or 2, wherein the converter comprises a Stirling cycle motor.

Installation according to one of the preceding claims, wherein during use the converter is located on a side of the collector remote from the sun. 20

Installation according to one of the preceding claims, wherein the transducer is connected to the support piece for rotation about the axis of rotation.

6. Installation as claimed in any of the foregoing claims, wherein the collector 25 is connected to the transducer for rotation about the elevation axis.

Installation according to any of the preceding claims, wherein the elevation axis is arranged to substantially coincide with a focal point of the reflector.

Installation as claimed in any of the foregoing claims, further comprising a drive device for moving the collector relative to the support piece.

Installation according to claim 8, wherein the drive device comprises a rotary drive for jointly driving the collector and converter about the rotary axis.

10. Installation as claimed in claim 8 or 9, wherein the drive direction comprises an elevation drive for driving the collector about the elevation axis.

Installation according to one of the preceding claims, further comprising a control device for controlling the position of the collector.

Installation according to any of the preceding claims, wherein the collector has a retracted position in which it is rotated about the elevation axis to a generally downwardly directed position.

13. Installation as claimed in any of the foregoing claims, wherein the support part comprises a hollow tube structure, with lines within the tube structure for the transport of feed and / or cooling liquids.

14. Installation as claimed in any of the foregoing claims for use at a predetermined geographical width, wherein the support part comprises a base and a generally inclined arm adapted to be fixed at an angle with respect to the vertical, wherein the angle at approach corresponds to the geographic width.

15. A method for operating a solar energy installation according to any one of the preceding claims, wherein the method comprises: controlling the collector and the converter to rotate together about a substantially immobile axis of rotation relative to the support piece follow the path of the sun during the day, and arrange the collector to be adjustable about an elevation axis independently of the transducer relative to the base, to adjust the elevation of the collector to compensate for elevation changes of the sun.