WO2005120172A2

WO2005120172A2 - Modular solar concentrating dish

Info

Publication number: WO2005120172A2
Application number: PCT/IN2005/000149
Authority: WO
Inventors: Krishna Ramchandra Datye
Original assignee: Krishna Ramchandra Datye
Priority date: 2004-05-06
Filing date: 2005-05-06
Publication date: 2005-12-22
Also published as: WO2005120172A3

Abstract

The invention consists of a point focus solar radiation thermal receiver-collector two axis (PA, SA) tracking system. In the invention, a virtual paraboloid geometry can be created for reflector elements (RC) assembly, mounted on flat top modular space frame (MF), using suitable adjustments for varying angle of reflector element (RC) with respect to flat top surface of space frame (MF). As the system is modular, it can be easily transported and erected on rooftops and remote locations without requiring special equipments for erection. The system can be easily fabricated in small workshops or manufacturing units using local materials & simple tools. This system can be optimally designed for a given application or a particular load demand and hybridization with cogeneration owing to the modular structure.

Description

ODUALR SOLAR CONCENTRATING DISH

This invention relates to a point focus solar thermal radiation collector - receiver system comprising an assembly of reflector elements mounted on a hinge supported flat top modular space frame, rotating about the hinge axis for precise tracking of the sun on daily basis by a programmed or light sensor actuated motor with a safe braking arrangement. The invention allows the adjustment of inclination of the reflector elements relative to the flat top frame so as to create a virtual paraboloid. This invention also provides for a second axis adjustment of reflector elements with respect to a receiver to achieve the focus at all times of the year. Further the invention has an arrangement for inducing curvature in the initially flat reflector elements so as to concentrate the reflected radiation at a point focus. The invention optionally provides for use of the assembly of reflector elements mounted on the frame as heliostats with the receiver placed on a tower.

Background of the Invention

There are three generic types of solar reflector element concentrators; heliostats or central receiver type, parabola troughs with line focus type, and paraboloid dishes or point focus type. Heliostats are substantially flat reflectors concentrating sunlight onto distant towers. The disadvantages of heliostats include critical reflector element contour requirements and the expensi /e, tall towers supportin remote receivers. Troughs are curved parabolic reflectors concentrating sunlight onto long receiver pipes spanning the full length of the reflectors. The disadvantages of troughs include low maximum solar concentration, high receiver heat ioss, and high receiver cost. Both heliostats and troughs have reduced performances known as cosine losses as they do not directly face at the sun. Solar dishes are compound-curve paraboloidal reflectoi s concentrating sunlight onto small receivers supported near the centers of cish aoertures. Dishes achieve the highest solar concentrations, high efficiency, and face directly at the sun. The disadvantages of dishes include the cost of compound and complex reflector curves and expensive reflector element substrates. Both heliostats and dishes require accurate optical reflector element contours and accurate optical dual axis tracking. These optical accuracy requirements have been significant cost barriers in the prior art.

Point-focusing dish solar concentrators provide the highest possible optical and operating performance, high temperature capability, and minimum land use. Accordingly, such dish concentrator systems are versatile and adaptable markets for solar thermal applications steam operated engine/turbine-generator, as well as, providing industrial process heat, for application in the production of high value chemicals, renewable fuels (hydrogen) and destroying toxic wastes.

In a typical prior art, a concentrating point focus solar collector system comprises of a plurality of reflector elements & a receiver for collecting or absorbing the concentrated solar radiation along with support structures for the reflector elements. The solar collector system is provided with suitable tracking device so that the reflector elements are made to follow the apparent motion of the sun on a ds-ily & on a seasonal basis. The point focus solar collector system is arranged ar.d constructed in a manner that the sun's rays falling on the reflective surface are focused onto the receiver, which can be used to heat any circulating fluids for diverse applications.

A stow locking system or braking arrangement is usually required to prevent damage under storm conditions like typhoons or hurricanes.

The prior art is replete with a multitude of designs of point focus solar collector systems, support and tracking structures. It is well established, that the equatorial mounted, polar tracking solar collector systems are desirable since they have the advantage of a constant tracking rate at the earth's rotational speed (15 degrees per hour) and a slow seasonal movement about the declination axis. However, the advantage of this simple tracking has been offset by several disadvantages.

US Patent 5,325,844 (Power Kinetics Inc) uses a curved rim & structural means (which could be either rectangular or elliptical or of any desired shape etc) to support the reflector elements arranged in Fresnel configuration. Mutual stabilization is achieved by interconnecting the curved rim and structural means to form an integral, distributed force, tension-compression unit. Tension members are used to connect the curved rim & structural means for stabilization However both curved ring & structural means have to be made very stiff & large number of tension members are required to withstand the irregular wind forces. This system becomes heavy & remains prone to failure under storm conditions & therefore involves high cost.

Prior art system as illustrated in figure 1 in US Patent 5325844 comprises of two non planar structural means carrying reflector elements segments joined with central structural beam, which is suspended between two hinged supports. All the forces acting on the reflector element assembly are transmitted through the framework to central beam & then to bearings located at both ends of the framework and finally to the hinged supports resting on the foundation.

As the system rotates about the hinge supports, under high wind condition various forces of large magnitude are induced in the framework & its components producing high thrust loading and moments. In addition to these forces the weight of the receiver & the reflector elements distort the concentrator frame as the collector rotates about its axis. In this prior art system, it becomes imperative to use heavy & expensive structural members for the framework in order to maintain the desired geometry to achieve a high concentration ratio. Further, flexible members such as guy ropes are required to ensure stability of the system of reflector elements and frames. Use of tension flexible force transmitting element s and flexible connections may result in loss of stiffness, thereby adversely affecting the geometry and making the system susceptible to failure particularly under storm conditions.

Owing to the small depth of the supporting framework, the braking arrangement cannot avail of the stiffness of the frame thereby requires use of a circular guide ring of high strength & expensive material for applying braking force resulting in a system unsuitable for roof top applications & windy regions.

Further, it becomes an arduous task to design an accurate, low power, and inexpensive tracking mechanism for such heavy prior art point focus solar collector systems of large areas of 50-100 sq. m and more.

When the structural frame of solar concentrating system is large, costs increase, as heavy sections become essential to overcome buckling hazards in long members used to impart stiffness.

These problems are further compounded by difficulties faced in erection of such prior art systems prefabricated in the workshops. They usually require heavy equipment, such as cranes, for installation at trie site making such prior art systems unattractive for dispersed installations & rooftops.

In summary, though prior art dynamic Fresnel type concentrator solar collector systems have performed satisfactorily with respect to their ability to concentrate and collect 'solar energy, they continue to be expensiv9 & unstable due to sub optimal design of structural components & mechanical fixtures for hinge supports, braking etc.

For a solar radiation concentrating collector system to be economically viable several requirement should be simultaneously satisfied

> The framework should be of modular design so that system can be optimized to suit the application, size & site conditions.

> The framework should be constituted of lightweight members, which can be prefabricated in the workshops, transported and finally assembled at the site.

> The system should be amenable to manufacture by small fabricators and easy to maintain by locally available technicians.

> The system should employ a support structure, which is stiff, lightweight, and capable of withstanding loads due to wind and gravity forces while supporting large collector assembly, and receivers placed at appropriate focal distance from the reflector elements.

> System should be able to sustain harsh weather conditions, including severe winds, hail, ice, snow etc. to ensure extended service life.

> Mirror mounts and curvature-imparting means should be able to achieve designed concentration for rigid, wear resistant reflector element materials, which restricts curvature of reflector elements.

> The system should have a precise, lov/ cost and low power consuming drive and tracking sy'stems using readily available components.

Summary of the invention

It is an object of the invention to provide a modular, stit rotating space frame to support an array of reflector elements to accurately track the apparent movement of the sun at varying wind conditions including high wind velocities. It is further object of the invention to provide a rigid rotating framework capable of maintaining geometry of reflector element assembly to obtain high concentration ratio even in diverse weather conditions under high wind/cyclonic condition. It is yet another object of the invention to provide robust supports capable of transmitting the forces acting on reflector elements & hinged rotating framework to the ground without affecting the geometry of reflector elements assembly.

Another object of the invention is to provide optimum concentrator size to facilitate its integration or hybridization with super heater. It is another object of this invention to provide reflector element supporting tracking structures for solar radiation collector systems capable of facile integration with a standard, rigid & low power consuming drive for accurately following the sun.

It is further object of the invention to provide design options to achieve onsite construction of the reflector element assembly with the desired curvature without use of additional turnbuckles & adhesives. It is another object of the invention to use reflector element assembly & frame as heliostat.

It is further object of the invention to provide inexpensive braking arrangement to ensure stability under cyclonic conditions.

, It is yet another object of the invention to reduce convection losses at the receiver in order to obtain high solar collector efficiency.

It is a further object of this invention to provide an easily maintainable & repairable tracking structure for a solar collector system. It is another object of the invention to construct cost effective framework & supports by using wood-bamboo-steel-F.R.P composites & associated joinery

Brief Description of the Drawings

Fig 1 illustrates the schematic of general arrangement of the invention of solar concentrator system depicting the main components of the system showing flat top main frame (MF), receiver (R), reflector elements (RC), hinges and shaft (HS), supports (CLS, CLN), braking arrangement with balancing weight (bW) {shown on one side only}, primary & secondary axes (PA &SA respectively) etc. {Drive is not shown. Pyramidal and lower frame members are not visible in the projection depicted}

Fig. 2a, 2.b & 2.c are the plan, end view & elevation of the flat top space frame depicting upper frame (U), lower frame (L) along with cross members (C11-C14)& shaft connections (D1 , D2, D3).

Fig 3.a & 3.b illustrates the hinge supports at two ends comprising bearings with housing (H1 , H2 ) column (CL, & CL_S), stays (S1 to S6) & bases (B1 to B6).

Fig.4 illustrates the first drive-tracking mechanism for daily tracking consisting of motor (M), worm gear (G), flexible force transmi ting element (w), puileys (P, P2, P3), and counterweight (CW).

Fig 5.illustrates the mountings for the reflector elements with hinged supports (hs) and fork and screw adjustment means with prop (P) to control inclination of the reflector elements in relation to the flat top frame. It also illustrates screws (Sc) supported by the diagonal members (Md) for imparting curvature to the reflector element. Further it illustrates the hinge suppor ed ( MH), longitudinal members (LM), running perpendicular to the primary hinge axis, supporting the reflector mountings with adjustment means for second axis tracking.

Fig.6 illustrates braking arrangement comprising flexible force transmitting elements (T1T2, S1 S2), guide rings (G1R1 , G2R2), pulleys (P1 , P2), balancing weights (bW1, bW2) and guiding shafts (GS1, GS2) with clamps (C1 , C2) for braking.

Fig 7a illustrates receiver & details e.g. spiral tubing, insulation of spiral tubing. Fig 7.b illustrates air suction creating fan.

Detailed description of Embodiments:

The invention comprises of reflector element assembly, tracking framework supporting the reflector elements, hinged supports buttressing the framework, drive for tracking, brakes, receiver and piping.

As illustrated in fig 1 & 2a,b, c the supporting & tracking structure for reflector element assembly is a essentially a space frame comprising upper frame U & lower frame L constituted of square segments. The nodes of each upper frame segments are connected to a corner / node of lower frame by a set of members arranged in a pyramidal form. In an embodiment as illustrated in figure 2a,b, c the upper frame U has 9 square segments while lower frame L has four square segments. In a typical pyramidal segment as illustrated in Fig. 2 a, b, c the four nodes A, B, C, D of upper segment US1 (A, B, C, D) are connected to the node a of lower frame LS1 by four cross members C11 , C12, C13, C 14. The frame geometry chosen results in modularization with equal lengths of members & standardization of connections & joinery. As illustrated fig 2,b & 2,c three members (D1 , D2, D3) connect ends of central array of pyramids to a short shaft (sh) supported by bearings placed inside a housing (Hs1 ) at one end. Symmetrical and triangular arrangement of these members (D1 , D2, D3) as illustrated in figure 2,b, and c transfer the irregular forces due to wind acting on reflector element assembly & framework to the supports.

As illustrated in fig 3, the housing (Hs1) in turn is fixed on top of column (CL_N) this is additionally supported, by stays (S1 , S2, S3) & base (B1 , B2). The shaft, bearing, housing, column together form support structure NS at one end of the rotational axis (PA). Similar support structure SS comprising CL_S stays (S4, S5, S6), base (B3, B4) exists at the other end of the rotational axis (PA).

In an embodiment, balancing weights are attached to ensure that system's centre of gravity is close to the hinge support.

The geometry & substantial depth of the space frame makes the system stable & capable of limiting the distortions to achieve the desired optical performance.

In another embodiment, optimization is achieved by using wood - bamboo-steel composite materials with appropriate joinery, whereby long service life and resistance to buckling are achieved obviating the use of flexible connections or tension wires

As the supporting framework is modular, it can be optimized to suit diverse applications.

After assembly of the modules, the frame functions =s a rigid body. The distortions in frame are thus small enough to maintain desired level of concentration achieving the designed peak performance of 350 W / sq. m of net area or higher and temperatures 200°C & above. m As illustrated in fig 4.the drive comprises of motor M with its shaft coupled with worm gear (G) mounted on support (NS). Number of turns (section xy) of flexible force transmitting element (vw) is wound on the shaft of worm gear (G). The end x is tied rigidly with the upper frame at point S while other end y passes over three pulleys viz. First pulley P1 on support (SS) (refer fig.1 ), second pulley (P2) at the end of the frame & third pulley (P3) near centre of the frame at (axes of both pulleys are parallel to rotational axis of the frame) & carries a counter weight (W). A rotation of shaft of the motor will get transmitted through the mechanical arrangement resulting into rotation of mainframe about its shaft.

Large torque sufficient to rotate the whole space frame can be produced from a small input torque of the motor (M) through the mechanical advantage derived by using gears (G), substantial lever arm & pulleys (P1 &P2).

By controlling the input to the motor (M) the whole frame can be made to follow the daily movement of the sun i.e. first axis tracking is achieved.

The counterbalancing weight (W) maintains the flexible force transmitting element (vw) taut under varying conditions and enables accurate tracking by preventing vibration & flutter of the framework.

One directional rotational tracking of the frame is sufficient to follow the motion of the sun & rotational axis or primary axis (PA) tracking is illustrated in figure 1.

In another embodiment, the drive motor is preprog.ammed or controlled by light sensor and actuator. The 2^nd axis or secondary axis (SA) tracking can be programmed manually or electromechanically taking into account the seasonal variation & is illustrated in figure 1.

11 In an embodiment as illustrated in figure 5, the reflector element mountings (M) are fixed on the longitudinal members (Lm1.Lm2) running throughout the length of frame, which are orthogonal to rotational axis (PA).

By means of a screw (Sc) supported by the diagonal member (Md), force is applied to impart the desired curvature to the reflector element to achieve the desired concentration ratio at the receiver. The diagonal members (Md1 , Md2), are attached rigidly to the reflector mountings.

The main frame has a flat' top & uses reflector elements with mountings and secondary axis adjustments in combination with screw and fork arrangement (FS) with a prop (P) to create a virtual paraboloid assembly of reflector elements with focus at receiver at all times of the year.

As illustrated in fig 6, in a preferred embodiment the braking system comprises of two flexible force transmitting elements (T1 , T2& S1S2) wound around two guide rings (G1 R1 & G2R2) in the form of circular segment. End T1 & S1 are tied to the two ends of the frame while other ends T2 & S2 pass over pulleys (P1 & P2)& weights (bW2 & bwl ) respectively are attached to tfie other ends

The weights (bW1 & bW2) move along guide shafts GS1 & GS2 respectively. Braking is achieved by locking the weight with the guide shafts (GS1, GS2) rod using electrically operated clamp actuated by wind sensor.

By choosing compatible geometry of guide ring with respect to framework & optimal member dimensions, stability & rigidity of the framework under braking condition are achieved.

12 In addition, due to the substantial depth of the space frame, the braking guide rings can avail of the stiffness of the space frame making the frame capable of withstanding uneven distribution of wind loads, irregular forces of earthquake, gravity & resulting moments and effect of local eddies on reflector element assemblies mounted on it under storm condition.

As illustrated in fig 1 & 7,the receiver (R) is located above the focal point of the reflector element assembly above the frame & is supported by twin tubes originating from vertices of frame & connected to the receiver framework (6). The outer tube (2) forms a structural support while the inner tube (1) high-pressure small diameter tube (spirally wound) is used for the circulation of high temp fluid. The annular space between the outer & inner tube is used for low temperature fluid circulation. In an example depicted in figure 7.a, fluids are fed through feed line inlet (4) & heated fluids come out of feed line outlet (5)

The introduction of twin tube for receiver enables to modify the relative proportion of heat energy collected by the inner and outer tubing to suit the application requirements where the hot air may be used for one set of applications such as drying while the oil, water or steam could be used for process heat or driving the heat engines

Receiver is constituted of twin tubes (1&2) bent in spiral with variable radius. The over all geometry of tubing takes form of two frustums of cone. The lower frustum (10) of cone has large diameter to intercept the radiation reflected by the collector, which deviate from ideal focal point while the upper frustum (11 ) of cone having small radius concentrates to the extent required. The spiral tubing has tube cover (9) & also has insulated airline cover (7), which is housed in receiver outer housing (8).

13 As illustrated in fig 7.b several layers of wire in the form circular disc are placed at the top of the upper frustum of cone to collect the residual radiation not absorbed by spiral tubing. By air suction, the heat absorbed by wire mesh (WM) is recovered, reducing the convection loss & increasing collector efficiency.

In preferred embodiment air suction is achieved by using fan (F) as illustrated in figure 7.b

In another embodiment, oil or pressurized water at high temperature or steam is circulated through the inner tube.

In another embodiment, the reflector element assemblies mounted on framework function as heliostats focusing at the focal point at a stationary receiver located on the top of the tower. All the structural system including tower are space frames of modular design.

The distinct advantages of the present invention over the prior art systems:

> An economical drive is created & accurate tracking is achieved by a system consisting low power motor, worm gear, pulleys, flexible force transmitting element , guiding ring & counterbalancing weights. The system facilitates incorporation of standard, readily available drive & tracking mechanisms (either preprogrammed or sensor actuated) driven by battery).

> The cost effective modular design of the system facilitates fabrication in various sizes from 10 to 100 sq. m or larger by optimising the member length, number of ^'joints as per the materials & methods of fabrication. Owing to the modularity, large assemblies with solar collecting areas 10-100 sq. m & larger can be easily transported to remote locations & rooftops &

14 assembled at the site. The erection does not require special equipments such as crane making it suitable for dispersed locations & rooftop applications. The system can be easily fabricated in small workshops or manufacturing units for light frames, doors, and windows using local materials & simple tools since fabrication involves repetitive.production of modular components of frames.

> System can be optimally designed for a given application or a particular load demand & hybridization with cogeneration owing to the modular structure.

> Modular design for the space frames with identical end connection features suitable for use of wood-bamboo-steel composite materials to reduce cost. Compatibility of space frame modules and modules for mirror mounting with associated longitudinal members and hinged supports for the mountings The components of braking system lie below the plane of upper frame ensuring that no shadow falls on the reflector elements thus minimizing the collection losses.

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Claims

We claim:

1. A point focus solar thermal radiation collector-receiver system comprising: a flat top modular space frame, rotating about a primary hinge axis passing through its center; single or plurality of reflector element secured in single or plurality of mounting means provided with adjustment means for varying angle of inclination of said reflector elements with respect to flat surface of the said space frame and curvature imparting means for the said reflector elements ; supporting means for carrying said reflector element mounting means, connected to said space frame, capable of being rotated about a secondary axis orthogonal to said primary hinged axis for adjusting to seasonal movement of the sun for achieving the focus at all times of the year; a receiver means located at ooint of focus of assembly of said reflector elements; ground support means housing the primary axis hinges supporting said space frame at the ends of said primary hinge axis and admitting rotation of said space frame ; first drive means to impart ro'ationai movement said space frame carrying the reflector element assembly to result in desired tracking of daily motion of the sun and associated first drive control means;

16 a secondary axis adjustment means for said reflector mounting means to adjust the setting of said reflector elements to match the seasonal movement of the sun for achieving the focus at all times of the year;

5 braking means for braking the frame under-storm condition and associated braking control means; wherein combination of said adjustment means and said curvature imparting means to is used to create a virtual paraboloid geometry with focus at receiverC for the assembly of said reflector elements carried by said flat top space frame.

2. A point focus solar thermal radiation collector receiver system as claimed in claim 1 wherein said space frame comprises a hinged flat top modular space frame rotating around said primary hinged axis passing through its5 center and capable of resisting the wind forces & transferring them though the hinges to said ground support means

3. A point focus solar thermal radiation collector receiver system as claimed in claims 1-2 wherein said supporting means are members running0 perpendicular to said primary hinge axis and supporting said reflector element mounting means in a distributed manner between sides of said space frame, capable of being rotated around said secondary axis for adjusting to seasonal movement of the sun to achieve focus at all times of year.5

4 A point focus solar thermal radiation collector-receiver system as claimed in claims 1-3 wherein said reflector element mounting means comprises:

17 i. one or plurality of hinge supported reflector element mounting means with each carrying reflector elements, connected to said support means means; ii. said adjustment means comprising fork and screw arrangement for each of the said hinged reflector element mounting means for varying the angle of each reflector element with reference to flat surface of space frame; whereby the said secondary axis adjustment and fork and screw arranaged are used in combination to create a parabloid geometry for assembly of reflector elements with focus at receiver at all times of the year.

5. A point focus solar thermal radiation collector-receiver system as claimed in claims 1-4 wherein said curvature imparting means comprises a pyramidal support connected to reflector element mounting means housing a nut and screw for imparting desired curvature to the said reflector element fixed in it for achieving desired concentration at receiver

6. A point focus solar thermal radiation collector-receiver system as claimed in claims 1-4 wherein braking means comprises: i. flexible force transmitting element with one end attached to the said space frame while the other end carrying counterbalancing weight; ii. circular segmental ring attached to the said space frame for guiding flexible force transmitting element; iii. pulleys for changing direction of flexible force transmitting element; iv. clamping device for the flexible force transmitting element operated by control means

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7. A point focus solar thermal radiation collector-receiver system as claimed in claims 1-5 wherein first drive means comprises: i. worm gear drive; ii. flexible force transmitting element wound around shaft of gear with one end attached to one end of space frame while other end supporting counterweight; iii. guide pulleys for the flexible force-transmitting element.

8. A point focus solar thermal radiation collector-receiver system as claimed in claims 1-6 wherein said ground support means comprises a structural system of vertical, horizontal and inclined members for transferring the forces resulting from wind and gravity loads to the ground and resist uplift forces on said space frame and said assembly of reflector elements and said receiver means .

9. A point focus solar thermal collector receiver system as claimed in claims 1-6 wherein receiver means comprises- i) hat shaped type receiver formed by joining two frustums of cone of different radii with upper frustum closed at upper end and having lesser radius than the lower frustum located at focus of said solar radiation reflector element: ii) Tubes wound on inner surface hat type receiver inside inner for recovering energy as concentrated solar radiation in form of heat by passing fluid through the tubing.

10. A point focus solar thermal collector receiver system as claimed in claims 1 -7 wherein said reflector elements are arranged to form a paraboloidal reflecting surface and said receiver means is rigidly connected with said space frame at the point of focus

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11. A point focus solar thermal collector receiver system as claimed in claims 1 -7 wherein said reflector element means are reflector elements arranged to form a paraboloidal reflecting surface and said receiver means is fixed at the point of focϋs of said and optionally placed at a location independent of said space frame.

12. A point focus solar thermal collector receiver system as claimed in claims 1-9 wherein receiver means comprise of more or plurality of conduits for carrying fluids, for reducing convection losses.

13. A' point focus solar thermal collector receiver system as claimed in claims 1-9 wherein optional use of wire-mesh in the form of circular plates places in the centre of top of upper frustum for collecting residual radiation not absorbed by spiral tubing in addition to air suction for reducing convection losses.

14. A point focus solar thermal collector receiver system as claimed in claims 1 -9 wherein second drive means optional use of drive wheels & linkages for individual reflectors for the said secondary axis adjustment.

15. A point focus solar thermal collector receiver system as claimed in claims 1-9 wherein optional use of light sensor controlled motor or preprogrammed motor for daily tracking of the sun.

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