Patent Application
20140251315
Various technologies have been explored, developed, and implemented for harnessing solar energy ranging from simple trapping of incident sunlight to the more advanced conversion of sunlight using photovoltaic and thermal means. Heliostats are one such device which is based on the reflective-collection principle. Heliostats are devices that include a plurality of plane or curved reflectors that focus incident sunlight onto a stationary target. Reflective surfaces of said reflectors have to be maintained to remain perpendicular to the bisector of the angle between the solar vector and the target vector as seen from the mirror.
Present day heliostats are generally controlled by computers. Heliostats are periodically oriented with respect to the moving sun and a fixed target such that the incident solar radiation is focused on the fixed target. Computer Programs are needed for automatically maneuvering reflector mirrors of the Heliostats. A heliostat control computer program aims the heliostats by periodically predicting the location of the sun in the sky. Predictions are based on the date, time, longitude and latitude. The position of each mirror of each heliostat is periodically determined, wherein each mirror of each heliostat is kept perpendicular to a bisector of an angle, wherein said angle is formed by a solar vector and a target vector.
Generally a conventional heliostat is mounted on a pedestal and is pivotally rotatable about altitudinal and azimuthal axes. Each axis is equipped with a motorized limb that can pivotally rotate the mirror of the heliostat to adjust its position.
The conventional heliostat control methods that are being used in the prior art are developed for the conventional heliostats. Such conventional heliostat control methods are of no use for orienting arrays of mechanically linked heliostats (ganged heliostats) positioned on rotatable shafts.
There is a need to develop a control method and an apparatus for periodically orienting mechanically linked heliostats mounted on rotatable shafts so as to accurately and continually concentrate incident sunlight on a stationary locus.
Ravindra Patwardhan and Rajeev Pandit in their U.S. patent application Ser. No. 12/798,847 titled ‘a solar central receiver system employing common positioning mechanism for heliostats’ describe a system of concentrating and harvesting solar energy, wherein such arrays of mechanically linked heliostats positioned on rotatable shafts are included. Said U.S. patent application Ser. No. 12/798,847 is incorporated herein in its entirety by reference. The present invention is specifically intended for periodic reorientation of such arrays of mechanically linked heliostats positioned on rotatable shafts so as to accurately and continually concentrate incident sunlight on a stationary locus.
In the said ‘solar central receiver system employing common positioning mechanism for heliostats’, arrays of heliostats mounted on rotatable shafts are required to be periodically oriented with respect to a stationary object and the sun such that incident solar radiation upon said heliostats is focused upon said stationary object and thereby deliver concentrated solar radiation upon said stationary object from dawn to dusk. Said solar central receiver system comprises: A heliostat field consisting of flat or curved light reflecting heliostats mounted on horizontal parallel arrays of rotatable shafts. The rotatable shafts are positioned about a stationary object. The stationary object is mounted at a predefined height above ground level. Each rotatable shaft provides a linear array of mounting means for mounting a linear array of heliostats. The mounting means are linked mechanically. Each mounting means is designed in a manner which permits individual pivotal movement of mounted heliostat with respect to related rotatable shaft about a first rotation axis. Each said rotatable shaft supports a rotation mechanism (drive means) for synchronously rotating the linear array of heliostats about the first rotation axis. Each said rotatable shaft is rotatable about its axis and is coupled with a drive means to rotate each said rotatable shaft about its axis such that when each said rotatable shaft is rotated, said linear array of heliostats rotates about a second rotation axis that is perpendicular to said first rotation axis.
The present invention is specifically intended for periodic orientation of mechanically linked (ganged) heliostats positioned on a rotatable shaft. For illustrative purpose, one of the embodiments as described by Patwardhan et al in ‘a solar central receiver system employing common positioning mechanism for heliostats’ is reviewed in
When the actuating (pusher) rod 111 is linearly moved in the eastern direction by the actuator 124, said link 115 moves said pin 122 towards eastern direction and this results in the clockwise rotation of the arm 121 along support 117 about pin 118. Said clockwise rotation is preferably up to 45° or more than 45° if needed depending on the movement of the actuating rod 111. Initially when the movement is zero, the said clockwise rotation is zero and the support 117 is in the horizontal plane. And said clockwise rotation increases as the said movement of actuating rod 111 in eastern direction increases. However, the ratio of the said clockwise rotation and the said linear motion may or may not be constant. Similarly, when the actuating rod 111 is linearly moved in the western direction, the arm 121 along with the support 117 rotate in anticlockwise direction. And said anticlockwise movement is preferably up to 45° or more than 45° if needed depending on the movement of said actuating rod 111.
Each rotatable shaft is coupled with a drive means to drive the rotatable shaft to rotate about its axis such that an array of heliostats mounted on each said rotatable shaft rotates tracking an apparent azimuthal motion of the sun in the sky. Referring to
An Array of heliostats is positioned on a linear array of mounting means on each rotatable shaft. Each heliostat is positioned with a defined angle of orientation and defined angle of inclination according to its location in a heliostat field similar to a facet of a Fresnel type reflector.
The angle of orientation at which a heliostat is positioned on a rotatable shaft depends on minimal distance between a central receiver and a longitudinal axis of said rotatable shaft. Said minimal distance is a side opposite an angle of orientation in an imaginary right angled triangle including the angle of orientation as an acute angle. A distance between said stationary receiver and a midpoint position on said heliostat is a hypotenuse in said imaginary right angled triangle. And the sine of said angle of orientation is equal to said length of a said side opposite divided by length of said hypotenuse. The angle of inclination at which a heliostat is fixedly positioned with respect to the rotatable shaft varies according to a location of said heliostat in said heliostat field. The angle of inclination at which a heliostat is fixedly positioned with respect to the rotatable shaft is angle ε/2. An angle ε depends on a distance between said central receiver to a midpoint of said heliostat in said heliostat field, which distance is a length of a side opposite said angle ε in an imaginary right angled triangle including the angle ε as an acute angle. The height of said stationary object from ground level, which height is a length of an adjacent side of said angle ε. The angle ε is calculated by finding tangent of said angle ε, wherein said tangent of said angle ε is equal to said length of said side opposite said angle ε divided by said length of said adjacent side.
The sun path refers to the apparent positional changes of the sun as the earth rotates and orbits around the sun. The sun apparently travels a curved path in the sky while moving from east to west. In the northern hemisphere, in winter, the sun rises south east and sets south west. In summer, the sun rises north east, crosses to the south of the east-west line during the day as it rises, and then crosses back to the north side of the east-west line as it sets and then sets north west. In the southern hemisphere, it is vice versa. Solar noon occurs at the middle of the arc shaped solar path across the horizon when the sun and meridian cross paths. To be more specific, the solar noon occurs when the sun azimuth is 180° or the sun azimuth is 360°. The solar noon is half way between sunrise and sunset and occurs when the sun reaches its highest point in the sky.
In the northern hemisphere, from dawn to solar noon, as the sun apparently moves from east to west, the azimuthal solar movement is always southwards. Whilst from solar noon to dusk, as the sun apparently moves from east to west, the azimuthal solar movement is always northwards. In the southern hemisphere, it is vice versa. Therefore, for each azimuthal tracking, it is necessary to determine whether the azimuth movement of the sun is southward or northward. Similarly, it is necessary to determine azimuthal displacement of the sun from the east west line and to introduce a calibrated compensation in heliostat orientation necessary to focus sunlight onto a central receiver. As the arrays of heliostats in the said solar central receiver system are in operational linkage to the related rotatable shafts, rotating the rotatable shafts in clockwise/anticlockwise direction would compensate the solar azimuthal movement.
When arrays of mechanically linked heliostats, mounted on horizontal parallel arrays of rotatable shafts, are to be periodically oriented such that the incident sunlight is to be focused on a stationary object, the conventional solar tracking method for heliostats will not work.
For identifying the problem and finding the solution for altitudinal and azimuthal tracking of arrays of mechanically linked heliostats mounted on rotatable shafts, an example of various values of solar altitude (elevation) and azimuth on 1 Jan. 2012 are considered. On this day, at 9:40 hours, the solar altitude is 30° and azimuth is 132°. At 9:43 hours the solar altitude is 30.5° and azimuth is 132.6°. At this juncture, both the altitude and azimuth have changed. But on the same day, at 12:26 hours, the solar altitude is 48.4° and azimuth is 176°. At 12:29 hours the solar altitude is 48.4° and azimuth is 177°. At 12:32 hours the solar altitude is 48.4° and azimuth is 178.1°. At 12:35 hours the solar altitude is 48.4° and azimuth is 179.1°. At 12:38 hours the solar altitude is 48.4° and azimuth is 180.2°. At 12:41 hours the solar altitude is 48.4° and azimuth is 181.2°. At 12:44 hours the solar altitude is 48.4° and azimuth is 182.2°. At 12:47 hours the solar altitude is 48.4° and azimuth is 183.3°. At 12:50 hours the solar altitude is 48.3° and azimuth is 184.3°.
From 12:26 to 12:47, the solar altitude remains 48.4° while solar azimuth changes from 176° to 183.3°. In this situation, if one merely tries to track the changing solar azimuth from 12:26 to 12:47, the mirrors of the ganged heliostats would be misaligned and the system would fail to focus the incident solar radiation on the fixed target. This is because from 12:26 to 12:47, the sun continues to travel westwards at a regular pace even if the sun elevation is unchanged. In said ‘solar central receiver system employing common positioning mechanism for heliostats’, in northern hemisphere, the southward rotation of the rotatable shaft tracks the changing solar azimuth from 90° to 180° and the northward rotation of the rotatable shaft tracks the changing solar azimuth from 180° to 270°. Therefore, the changing solar azimuth from 12:26 to 12:47 (solar azimuth 176° to 183.3°) could only be tracked by southward rotation of the rotatable shaft, if and only if, the heliostats also move altitudinally along with the sun! In said ‘solar central receiver system employing common positioning mechanism for heliostats’, the eastward actuation by the actuator results in synchronous westward rotation of the array of mounted heliostats tracking the westward altitudinal movement of the sun.
For explanation purpose, as an example, it is assumed that the arrays of mechanically linked ganged heliostats are positioned in east west direction, and the sun elevation is 30° (eastern) in the morning at the start of the operation and the mirror elevation is 45° (eastern), wherein the solar radiation is focused on the target. Let it be further assumed that the arrays of mechanically linked ganged heliostats function from sun elevation 30° (eastern) to sun elevation 30° (western). The solar path from sun azimuth 180° to dust is a mirror image of the solar path from dawn to sun azimuth 180°. Consequently, at the end of the operation, the sun elevation would be 30° (western) and the mirror elevation would be 45° (western). The altitudinal rotation of the heliostats from heliostat mirror elevation 45° (eastern) in the morning to heliostat mirror elevation 45° (western) in the evening should be regular and should take the same time as the time taken by the sun to move in the sky from 30° in the morning at the start of the operation to 30° in the evening at the end of the operation. This happens because all the installed heliostats in the said solar central receiver system behave as a dynamic flattened manifestation of a single parabolic reflector, which is initially positioned to focus the solar radiation on a stationary target for a fixed position of the sun (like the sun at the zenith) in the sky.
On that 1st day of January 2012, the time taken by the sun from sun elevation 30° in the morning to sun elevation 30° in the evening is 435 minutes. If the mirrors of the heliostats are to be rotated every 3 minutes for altitudinal tracking the sun, it is essential to regularly rotate the mirrors such that the rotation of the mirrors should take the same 435 minutes to change the position of heliostat mirrors from mirror elevation 45° (eastern) in the morning to mirror elevation 45° (western) in the evening.
The present inventors have examined heliostat control programs available in the market or on internet. The available heliostat control programs cannot be implemented in a system of reflectors wherein the reflectors (mirrors) are arranged in arrays and are mounted on rotatable shafts. Control of the arrays of heliostats mounted on rotatable shafts for periodic orientation for tracking an apparent movement of the sun in the sky requires a considerably different premise, thus preserving the necessity to invent for the present inventors.
What is needed, therefore, is an amalgamation of astronomical and trigonometrical calculations from astronomical theory to result in logic for maneuvering arrays of mechanically linked heliostats positioned on horizontal and parallel rotatable shafts. The present inventors, in cognizance of aforesaid needs, have undertaken focused research and come up with novel solutions to effectively address the same. The following description presents one way of performing the present invention. A principle object of the present invention is to provide a method and apparatus adapted for synchronously controlling a field of mechanically-linked (ganged) heliostats positioned on rotatable shafts to accurately concentrate incident sunlight on a stationary locus.
Yet another object of the present invention is to provide a method and apparatus adapted for synchronously controlling a field of mechanically-linked (ganged) heliostats characterized in having simple protocol of usage within skill level of ordinary person.
Yet another object of the present invention is to provide a method and apparatus adapted for synchronously controlling a field of mechanically-linked (ganged) heliostats characterized in having low costs of materials, assemblage, maintenance and operations.
These together with other objects, advantages and their attainment will become subsequently apparent in the details of products, their preparation and application as more fully hereinafter described and claimed hereinafter reference being had to the accompanying figures. Thus, for tracking an apparent movement of the sun in the sky, the altitudinal and azimuthal orientation of the arrays of heliostats mounted on rotatable shafts require a considerably different premise. The rationale of the present invention as described in detail in the detailed description is to provide many novel features that are not anticipated or implied by any of the prior art.
The present invention is specifically intended for periodic orientation of mechanically linked (ganged) heliostats positioned on rotatable shafts such that the incident sunlight is focused on a stationary object. Method for altitudinal orientation comprises of finding the total functional time 2×(T2−T1) in seconds that is needed for altitudinal rotation of the heliostats in a day to track the altitudinal movement of the sun, wherein the mirrors of the heliostats rotate from a defined mirror elevation (for example 45° eastern) to a defined mirror elevation (for example 45° western). T1 is the time of the day wherein the rotation of the heliostat starts at the said defined mirror elevation. T2 is the time when the sun azimuth is 180° or 360°. It is at the middle of the total functional time. In each altitudinal orientation a minuscule push (or pull) is given by an actuator. A calculated defined number (n) of altitudinal orientations (minuscule pushes) by an actuator would result in inducing the heliostats to rotate from the said defined mirror elevation (eastern) to said defined mirror elevation (western). The time interval between each altitudinal orientation would be 2×(T2−T1) divided by n.
For implementing the azimuthal orientation, at the scheduled starting time T1, the computation means determines the solar elevation and azimuth at that instant. When it is the start of the operation, the magnitude of rotation [(Y×Sin γ)/2 degrees] is determined. As per the position of the sun in the sky at that instant, the determined value in degrees is added/subtracted to angle θ to form angle θ′. A central/distal arm affixed with each rotatable shaft and coupled with a linear actuator (drive means) provides a capability to rotate the related rotatable shaft in clockwise or anticlockwise direction by extension/retraction of the length of the arm of the linear actuator. The length of the said arm of the linear actuator ‘Cx’ can be extended/retracted to ‘C1’ such that angle θ′ is achieved.
If it is not the start of the operation, then the formula 2*(T2−T1)/p gives total number of azimuthal orientations required in a total functional time of the day, where p is a user defined arbitrary time unit like say 30 minutes (1800 seconds). Let this value [2*(T2−T1)/p] be called as Δ. The length of the linear actuator C1 at T1 and the length of the linear actuator C2 at T2 is determined and the difference |C2−C1| millimeters is calculated and then divided by Δ/2. Let this value be called as Ω mm. At each interval of p seconds, subsequent to the primary azimuthal orientation at the defined mirror elevation (eastern) till T2, the rotatable shaft is to be rotated in southward direction such that the value of the length the arm of the linear actuator is varied by Ω mm. Similarly, at each interval of p seconds, from T2 to the defined mirror elevation (western), the rotatable shaft is to be rotated in northward direction such that the value of the length of the said arm of the linear actuator is varied by Ω millimeters.
Ravindra Patwardhan & Rajeev Pandit in their U.S. patent application Ser. No. 12/798,847 have substantially described a solar central receiver system titled a ‘solar central receiver system employing common positioning mechanism for heliostats’. Said solar central receiver system employing common positioning mechanism for heliostats is intended for periodically orienting arrays of heliostats positioned on rotatable shafts such that incident solar radiation upon said arrays of heliostats is focused upon a stationary object and thereby deliver concentrated solar radiation upon the stationary object. Said patent application is herein incorporated in its entirety with reference. The present invention, namely, ‘a method and apparatus for orienting arrays of mechanically linked heliostats for focusing the incident sunlight on a stationary object’ is specifically meant for a methodology and an apparatus for orienting arrays of mechanically linked heliostats (ganged heliostats) positioned on rotatable shafts as described by Ravindra Patwardhan & Rajeev Pandit in their U.S. patent application Ser. No. 12/798,847. Furthermore, Ravindra Patwardhan & Rajeev Pandit, in their application for registration of US copyrights titled ‘A method and apparatus for orienting arrays of mechanically linked heliostats for focusing the incident sunlight on a stationary object (The software for Solar fire)’, which is accepted at the US Copyright Office and allotted open case No. 1-1062961501, have substantially described a methodology and control means used to develop software and a control system for the purpose of orientation of ganged heliostats (mechanically linked heliostats) positioned on rotatable shafts to track the sun such that the incident sunlight is focused on a stationary object. Said application for registration of US copyrights (allotted open case No. 1-1062961501) is also herein incorporated in its entirety with reference.
The present invented method and the apparatus are to be employed for tracking the altitudinal and azimuthal movement of the sun, wherein arrays of ganged heliostats positioned on rotatable shafts are periodically oriented with respect to the moving sun and a fixed stationary object (receiver) such that incident solar radiation on the said arrays of ganged heliostats is focused upon the stationary object and thereby deliver concentrated solar radiation upon said stationary object.
For the purpose of explanation only, certain assumptions are made to explain the invented method for periodic orienting arrays of mechanically linked heliostats mounted on horizontal parallel arrays of rotatable shafts for focusing the incident sunlight on a stationary object.
Assumptions:
The horizontal, parallel rotatable shafts of the said solar central receiver system employing common positioning mechanism for heliostats are placed in east west direction. The rigidly fitted brackets of the mounting means on rotatable shafts are perpendicular to the horizontal ground at the start of the operation. While fixedly positioning the arrays of mirrors on the pivotally rotatable mounts of the rotatable shafts, the position of the mirror of each heliostat is such that rays perpendicular to the horizontal ground [like the rays from the sun at the zenith (solar azimuth 90° and solar elevation 90°)] incident on the said mirror would be reflected on the target.
It is assumed that at the start of the operation, the mirror elevation is 45° and the sun elevation is 30°.
The mirrors of the heliostats rotate from 45° mirror elevation (eastern) to 45° mirror elevation (western) while tracking the sun from sun elevation is 30° (eastern) to sun elevation is 30° (western).
A stepper motor ball screw linear actuator is connected with the actuating rod on the western aspect of the rotatable shaft & the clockwise rotation of the stepper motor achieves forward (eastward) movement of the actuating rod which in turn causes backward (westward) rotation of mounted heliostats and vice versa.
As substantially described in the U.S. patent application Ser. No. 12/798,847, all the installed heliostats in the said solar central receiver system behave as a flattened dynamic manifestation of a single parabolic reflector, which is initially positioned to focus the solar radiation on a stationary target for a fixed position of the sun in the sky. Hence the target elevation and target azimuth are not assumed.
A southward projecting arm is affixed at right angle with each rotatable shaft and positioned parallel to the ground and coupled with a linear actuator, wherein retraction/extension of the arm of the linear actuator would cause clockwise/anti-clockwise rotation of the related rotatable shaft respectively resulting in southward/northward rotation of the rotatable shaft respectively.
The invented heliostat control software is run in a place in northern hemisphere like Pune city, India.
Method for altitudinal orientation of arrays of mechanically linked heliostats mounted on arrays of rotatable shafts for continually focusing the incident sunlight on a stationary object:
The position of a celestial object like the sun can be defined by specifying its altitude (elevation) and its azimuth. The altitude of an object is equal to its angle in degrees above the horizon. The azimuth of an object is equal to its angle in the horizontal direction, with north at 0°, east at 90°, south at 180°, and west at 270°. As the Earth rotates, the sun appears to rise and set, constantly changing its altitude and azimuth. Sun's altitude and azimuth also varies according to an observer's location on Earth. The sun rises in the east and sets in the west everywhere on the Earth. The Earth's rotation tilts about 23.5 degrees on its pole-to-pole axis, relative to the plane of Earth's solar system orbit around our sun. The sun's declination at the spring equinox is 0°. It moves up to 23.5° in the summer, then drifts back down through 0° at fall equinox, and down to −23.5° in the winter. As the Earth orbits the sun, this creates the 47 degree peak solar azimuth angle difference and the hemisphere-specific difference between summer and winter. In the northern hemisphere, the winter sun rises in the southeast, peaks out at a low angle above the southern horizon, and then sets in the southwest. In the northern hemisphere in summer, the sun rises in the northeast, peaks out nearly straight overhead (depending on latitude), and then sets in the northwest.
In the present invention, the elevation angle and the azimuth angle of the sun are the two key angles that are used in the ‘method and apparatus for orienting arrays of mechanically linked heliostats for focusing the incident sunlight on a stationary object’. To calculate the sun's position throughout the day, excellent algorithms are available.
Based on the equations of the astronomical ephemeris, and the American ephemeris and nautical almanac, Pitman and Vant-Hull presented formulae for the calculation of the ecliptic sun coordinates, its declination and the equation of time. They discussed about various phenomena affecting the Sun's position, and estimated the errors resulting when these phenomena are disregarded. Similarly, a solar vector can also be calculated with the help of algorithms of Spencer, Walraven, and Michalsky.
“NOAA_Solar_Calculations_day.xls” file is available for downloading on interne. The calculations in the NOAA (National Oceanic & Atmospheric Administration) solar position calculators are based on equations from Astronomical Algorithms by Jean Meeus. The said program is available for downloading from the following websites:—http://www.esrl.noaa.gov/gmd/grad/solcalc/calcdetails.html http://www.srrb.noaa.gov/highlights/sunrise/azel.html
A computer-control program for heliostats by Mr. David Williams is available—http://www.iwilltry.org/b/projects/build-a-heliostat-for-solar-heating-and-lighting/Heliostat control programs/algorithms are also available: ‘Solar Position Algorithm for Solar Radiation Applications’—a technical report by Ibrahim Reda and Afshin Andreas of National Renewable Energy Laboratory, Department of Energy (USA)=http://www.nrel.gov/docs/fy08osti/34302.pdf
The computation means for synchronous orientation of ganged heliostats uses modified available software like NOAA_Solar_Calculations_day.xls, wherein the solar elevation and the solar azimuth are determined when the location specific date, time, longitude, latitude are inputted. To keep down the price, a low-cost microprocessor based controller is preferably used.
The computation means under its control program continuously keeping track of the real time in the RTC, checks for the occurrence of a scheduled starting time, say 8:00:00 AM. At the occurrence of the scheduled starting time, the computation means starts the operation. Corresponding to the occurrence of the 30° eastern sun elevation, the time T1 is determined and recorded as a first predefined event. Similarly, corresponding to the occurrence of the solar azimuth 180° or 360° (solar noon), the time T2 is determined and recorded as a second predefined event. In fact, the solar path from sun azimuth 180° (or 360°) to dust is a mirror image of the solar path from dawn to sun azimuth 180° (or 360°). The sun azimuth 180° or 360° occurs at the middle of the curved solar path in the sky. Therefore T2 occurs at the middle of the westward solar path from solar elevation 30° eastern to the solar elevation 30° western. Similarly, T2 occurs at the middle of the westward mirror rotation from mirror elevation 45° eastern to a mirror elevation 45°. Hence, the time 2×(T2−T1) in seconds/minutes is the total time in seconds/minutes that a heliostat is needed to regularly rotate to track the sun. While tracking the sun from solar elevation 30° eastern to the solar elevation 30° western, the arms of the supports of the mirrors of the heliostats rotate from an elevation 45° eastern to an elevation 45° western. This happens because all the installed heliostats in the said solar central receiver system behave as a dynamic flattened manifestation of a single parabolic reflector, which is initially positioned to focus the solar radiation on a stationary target for a fixed position of the sun (like the sun at the zenith) in the sky. The target elevation and azimuth is already considered when the reflectors are fixedly positioned like Fresnel type reflectors. The alignment of said single parabolic reflector receiving solar radiation from the sun at solar elevation 30° eastern and focusing the solar radiation on a fixed target would be mirror image of the said single parabolic reflector receiving solar radiation at solar elevation 30° western. Since each mirror of each heliostat of the said solar central receiver system behaves as a part of that said single parabolic reflector, mirror alignment receiving sunlight from the sun at solar elevation 30° eastern would be mirror image of the mirror alignment for the solar elevation 30° western.
It is assumed that the stepper motor ball screw actuator is connected on the western aspect of the pusher (actuator) rod & the clockwise rotation of the stepper motor achieves forward movement of the ball nut on the lead screw that impels forward movement of the pusher rod that causes westward rotation of the heliostats. Let it be further assumed that the realizable forward movement of the ball nut on the lead screw (5 mm pitch) is 200 mm for rotating the heliostats from say mirror elevation 45° (eastern) at the start of the operation to the sun azimuth 180° (or 360° azimuth). This means that, for rotating the heliostats from mirror elevation 45° (eastern) at the start of the operation to mirror elevation 45° (western) at the end of the operation, a total forward movement of the ball nut on the lead screw (5 mm pitch) would be 400 mm in each day. Let it be further assumed that at each altitudinal orientation, the stepper motor is rotated 50 steps, wherein each step is 1.8° (that is, the stepper motor is rotated ¼th of a circle). The total rotations of a ball nut for traveling on a 400 mm length lead screw having 5 mm pitch would be 80. For each altitudinal orientation, the stepper motor is rotated by ¼th of a circle each time. Hence, there would be 320 altitudinal orientations in a day. This means that the total number (n) of altitudinal orientations in a day are fixed and depend on the length of the lead screw that a ball nut travels for actuating the pusher rod, pitch of the lead screw, and the number of steps (like 50 steps in the present example) that a stepper motor rotates in each altitudinal orientation. Hence, the time interval between each altitudinal orientation would be 2×(T2−T1) seconds/minutes divided by n. Consequently, from the occurrence of the time of the day T1 [mirror elevation 45° (eastern)], n number of regular successive altitudinal orientations (320 in the present case)—like 50 steps in the present example—at interval of ‘2×(T2−T1)/n’ seconds/minutes would achieve 45° mirror elevation (western) of the heliostat.
The said CPU 301 under its control program executes or initiates execution of routines for solar tracking utilizing stored application software and/or various sensor inputs. Said CPU, a memory, and application software including CPU-executable code loaded in said memory predict location of the sun in the sky. Each prediction is based on date, time, longitude and latitude related to location of said heliostats. The CPU receives feedback from sensory means that comprise rotary encoders or optical sensors or position-sensing detectors or optoelectric sensors or feedbacks from drive motors. Said CPU, said memory, and said application software including CPU-executable code loaded in said memory periodically calculate required azimuth and/or altitude axis rotation for heliostats using predicted location of the sun in the sky, and feedbacks from sensory means. Said CPU generates controlling commands for a single geared motor drive unit or a plurality of gear motors drive units for rotating related rotatable shaft/shafts to rotate about a first rotation axis for tracking an apparent motion of the sun in the sky. Said CPU generates controlling commands for a single actuator or a plurality of actuators for synchronously rotating pivotably rotatable mounts of said heliostats located on related rotatable shaft about a second rotation axes that are perpendicular to said first rotation axis for tracking an apparent motion of the sun in the sky. Said CPU includes a set of logic programs for converting commands into electronic drive signals.
As an example, one of the ways of creating application software including CPU-executable code:—
An updated/modified version of the original “NOAA_Solar_Calculations_day.xls” file is available for downloading. The said program is available for downloading from the following websites:—http://www.esrl.noaa.gov/gmd/grad/solcalc/calcdetails.html http://www.srrb.noaa.gov/highlights/sunrise/azel.html
The said excel worksheet (“NOAA_Solar_Calculations_day.xls” file) is updated and modified to suit the requirement of the present invention. In this ‘solar position calculation program/software’, the data in the said updated and modified excel work sheet changes as per the user inputs and is updated continuously as per the position of the sun. The data that is required to be entered in the solar position calculation program/software is latitude, longitude, target azimuth, target elevation, time zone, time, date, sun elevation to check (for example 30° sun elevation), sun azimuth to check (for example, 180° azimuth), arm ‘A’ length, arm ‘B’ length, length ‘C’ at start position, start position angle θ, value of n, for azimuthal tracking value of ‘p’ etc.
Method for azimuthal orientation of arrays of mechanically linked heliostats mounted on arrays of rotatable shafts for continually focusing the incident sunlight on a stationary object:
The sun rises in the east and sets in the west everywhere on the Earth. The Earth's rotation tilts about 23.5 degrees on its pole-to-pole axis, relative to the plane of Earth's solar system orbit around our sun. The sun's declination at the spring equinox is 0°. It moves up to 23.5° in the summer, then drifts back down through 0° at fall equinox, and down to −23.5° in the winter. As the Earth orbits the sun, this creates the 47 degree peak solar azimuth angle difference and the hemisphere-specific difference between summer and winter. In the northern hemisphere, the winter sun rises in the southeast, peaks out at a low angle above the southern horizon, and then sets in the southwest. In the northern hemisphere in summer, in a city like Pune (India), the sun rises in the northeast, peaks out nearly straight overhead (depending on latitude), and then sets in the northwest. The sun travels a curved path while apparently moving from east to west. Therefore, for each azimuthal tracking, it is necessary to determine whether the azimuth movement of the sun is southward or northward. For this purpose, as depicted in
1) From 0° azimuth to 90° azimuth=quadrant 504, henceforth named as quadrant A.
2) From 90.01° azimuth to 180° azimuth=quadrant 505, henceforth named as quadrant B.
3) From 180.01° azimuth to 270° azimuth=quadrant 506, henceforth named as quadrant C.
4) From 270.01° azimuth to 0° azimuth (or 360° azimuth)=quadrant 507, henceforth named as quadrant D.
Let the center of the sky at the cross section of the above said imaginary lines be called as β. Furthermore, here, it is necessary to determine an angle γ, which is less than or equal to 90°, and denotes the displacement of the sun from the east west line. As depicted in
Determination of the angle γ:—1) If the sun lies in quadrant A, then angle γ=90°−azimuth value of the sun. 2) If the sun lies in quadrant B, then angle γ=azimuth value of the sun−90°. 3) If the sun lies in quadrant C, then angle γ=270°−azimuth value of the sun. 4) If the sun lies in quadrant D, then angle γ=azimuth value of the sun−270°.
To introduce a calibrated compensation in heliostat azimuth orientation according to the apparent azimuthal displacement of the sun, wherein each east west directional rotatable shaft 207 (
For a given longitude, latitude, time, and date, the software determines the sun elevation and the sun azimuth values, for that time of day. As an example, let it be assumed that at T1 the value of the sun elevation is 30° and the sun azimuth is 110°. From the identified solar azimuth, the quadrant in which the sun lies at that instant can be determined. From the determined solar azimuth and the quadrant in which the sun lies, angle γ can be determined as described hereinbefore. It is necessary to determine angle γ for determining the required rotation of the rotatable shaft for tracking azimuthal movement of the sun.
The said software also yields concurrent solar elevation at the instant of the determination of angle γ. Now the distance of the sun in degrees from the center P can be ascertained as 90°−concurrent solar elevation=Y degrees. Y is the length of the hypotenuse of the triangle formed with respect to the position of the sun like R, F, G, and H. Here, Sin γ=length of the side opposite divided by the length of the hypotenuse=length of the side opposite/Y. Therefore the ‘length of the side opposite’ in degrees=Y×Sin γ. For illustrative purpose, it is assumed that the angle of elevation of the sun at position R is 10°, at position F is 30°, at position G is 48°, and at position H is 62°. It is further assumed that at the position R of the sun, it is the start of the actual operation (time T1). The distance of the sun from the center P can be defined as ‘90°−concurrent sun elevation’=Y. Here, {dot over (Y)} is the hypotenuse in the triangle formed with respect to the position of the sun. The distance RP is the hypotenuse in the triangle RPL and represents the distance of the sun from the center P. Hypotenuse RP can be defined as 90°−concurrent sun elevation=90°−10°=80°=Y1. Similarly, hypotenuse FP=Y2=90°−30°=60°. Similarly, hypotenuse GP=Y3=90°−48°=42°. Similarly, hypotenuse HP=Y4=90°−62°=28°. Therefore, RL=Y1×Sin γ1, FM=Y2×Sin γ2, GU=Y3×Sin γ3, and HO=Y4×Sin γ4. For tracking the apparent azimuthal position of the sun, the rotatable shaft should be rotated such that the apparent lateral displacement of the sun, like RL, from the east west midline is compensated. For the position of the sun at R, the apparent displacement RL should be compensated. As per the law of reflection, the rotatable shaft should be rotated such that the shafts should rotate RL/2 degrees. In other words, for the position of the sun at R, for tracking the apparent azimuthal displacement RL, the rotatable shaft should be rotated by (Y1×Sin γ1)/2 degrees. Now RL/2=(Y1×Sin γ1)/2 degrees=(80°×Sin γ1)/2 degrees. If it is assumed that solar azimuth at position R is 110°, then as the sun lies in quadrant B, and γ1=110°−90°=20°. Therefore, RL/2=(80°×0.342)/2=13.7°. Thus, at the start of the operation, the determined magnitude of rotation of the rotatable shaft at the instant wherein the sun elevation is 10° and sun azimuth 110° is =13.7°
To determine the direction of rotation of the rotatable shaft for tracking the azimuthal movement of the sun:
If it is the start of the operation:
If it is the start of the operation and if the sun lies in the quadrant B, then for tracking the azimuthal movement of the sun, rotate the (east west directional) shaft in southward direction. If it is the start of the operation and if the sun lies in the quadrant A, then for tracking the azimuthal movement of the sun, rotate the shaft in northward direction.
If it is not the start of the operation:
To determine the direction of rotation of the rotatable shaft (east west directional) for each successive tracking of the azimuthal movement of the sun: If it is not the start of the operation and if the sun azimuth is ≦180°, then rotate the shaft in southward direction. If it is not the start of the operation and if the sun azimuth is >180°, then rotate the shaft in northward direction.
To determine the magnitude of the required rotation of the rotatable shaft for tracking the azimuthal movement of the sun:
If it is the start of the operation: At the actual start of the operation at T1, when suppose the sun elevation is 10°, the sun azimuth 110°, and the mirror elevation is 45° (eastern), the determined magnitude of rotation of the rotatable shaft for tracking the solar azimuth status will be (Y×Sin γ)/2 degrees, where the angle γ is formed between the east west line and the solar vector at that instant and Y is the hypotenuse as described hereinbefore. At the start of the operation at T1 it is necessary to rotate the rotatable shaft for the entire magnitude of rotation=(Y×Sin γ)/2 degrees towards the determined northern/southern direction as per the sun's position in quadrant A or B.
For illustration, as per the example, at the start of the operation at T1, the sun azimuth is 110° and hence the sun lies in the quadrant B. Therefore, the direction of rotation should be southward. Inputting the values (as per the example) of Y and γ in (Y×Sin γ)/2 degrees, the magnitude of rotation will be (80°×0.342)/2=13.7°. The rotatable shafts are positioned in east west direction and the arm 201 is projecting in southward direction. As the sun lies in quadrant B, The rotatable shaft is to be rotated in southward direction. The retraction of the arm 202 would rotate the rotatable shaft in southward direction, and would decrease the angle θ. To track for the determined solar azimuth 110°, the rotatable shaft would have to be rotated by 13.7° towards south so that the mounted array heliostats rotate by 13.7° towards south. To achieve the 13.7° rotation of the rotatable shaft towards south, the arm 201 would have to be moved in a clockwise manner in turn rotating the rotatable shaft by 13.7°, wherein the arm 202 of the linear actuator 203 would have to be retracted to such an extent that the angle θ would become from 90° to 76.3°.
For azimuthal tracking:—
If it is not the start of the operation:
Determine the value of (T2−T1) seconds, and 2×(T2−T1) seconds.
Decide an arbitrary time interval p for azimuthal tracking like say a time interval of 1800 seconds.
Determination of the required extension/retraction of the arm 202 for the required rotation of the rotatable shaft for tracking the azimuthal movement of the sun:
As already described, each east west directional rotatable shaft is rotatable about its axis and is coupled with a drive means to achieve clockwise/anti-clockwise rotation to track the azimuthal movement of the sun. As depicted in
In trigonometry, the law of cosines (cosine formula or cosine rule) relates to the lengths of the sides of a plane triangle to the cosine of one of its angles. Using notations with reference to
(C2=A2+B2−2AB cos θ equation 1)
Inputting the known unchanging values of lengths A and B and the determined angle θ in the equation 1, C2 can be determined. With determination of C2, value of C can be determined. Now to determine the needed value of requisite C′, from the original C, wherein the angle θ would be changed from 90° to 76:3°, one would have to input the value 76.3°, which is henceforth called the angle θ′ along with the known values of A and B in the equation 1. Here, in this cosine rule C′2=A2+B2−2AB cos θ′, wherein the values of A2+B2−2AB are constant and only the value of cos θ′ would be changed. Therefore, as per the example, by inputting the value 76.3° (θ′) in the equation 1, the value C′2 can be found. With the determination of C′2, the value of requisite C′ can be found. The decrease of the angle θ means, as per the assumption, there would be a southward rotation, wherein the arm of the linear actuator would be retracted. In the present case, by finding the difference |C−C′| millimeters, one can find the required length of the needed retraction of the arm of the linear actuator.
The length of the ‘C’ is known, which is the length of the linear actuator with its arm at the start of the operation when angle θ is 90°. The hereinbove description exemplifies and elucidates the methodology of determination of the length of the ‘C1’, which is the length of the linear actuator with its arm at the time T1, and the length, of the ‘C2’, which is the length of the linear actuator with its arm at the time T2.
Determine the length of the ‘C1’ at T1 and the length of the ‘C2’ at T2, wherein, the length of the linear actuator 203 with its arm 202 at time T1 is C1, and the length of the linear actuator 203 with its arm 202 at time T2 is C2. Once the values of C, C1 and C2 are known, one can find |C−C1| mm and |C2−C1| mm.
Find |C2−C1| millimeters.
Determine the value of (T2−T1) seconds divided by p seconds. Let this value be called as Δ′. Here, Δ′ value denotes the number of times one needs to perform the azimuthal tracking from T1 to T2. By using the ‘floor or ceiling function’ on the value of Δ′ find the nearest positive integer number, which is henceforth called as Δ. Here, the minor mistake, which happens due to floor or ceiling function can be easily compensated by the software.
Divide |C2−C1| mm by Δ. Let this value be called as Ω mm.
At each interval of p seconds, from the morning at T1 when the mirror elevation is 45° (eastern) till T2, the rotatable shaft is to be rotated in southward direction, wherein the length of the linear actuator with its extensible/retractable arm is varied by Ω mm.
Similarly, at each interval of p seconds, from T2 to the evening till the mirror elevation is 45° (western), the rotatable shaft is to be rotated in northward direction, wherein the length of the linear actuator with its extensible/retractable arm is varied by Ω mm. The length of the linear actuator with its extensible/retractable arm is varied such that if the sun azimuth is ≦180°, then the rotatable shaft is rotated in southward direction, and if the sun azimuth is >180°, then the rotatable shaft is rotated in northward direction. (It is assumed that the rotatable shafts are positioned in east west direction.).
To overcome the accumulated mechanical error due to conditions like mechanical backlash, in one of the embodiments, the altitudinal and azimuthal status is re-adjusted at the interval of say 90 or 120 minutes. Accumulated mechanical error is negated by readjusting the altitudinal and azimuthal status whereupon functionality extremes of the heliostat orientation means are force-achieved via control check-loop. Using the sensors like rotary encoder or a proximity switch, the starting position for the shaft is achieved by instructing the DC motor linear actuator to rotate till the position of the actuator arm extension is at its original position, wherein the arm 201 (
The hereinabove mentioned embodiments of the method for orienting arrays of mechanically linked heliostats for focusing the incident sunlight on a stationary object are described as an example only and those experts in the art can very well recognize that numerous variations are possible without departing from the main theme.
The present invention may, of course, be carried out in other specific ways than those herein set forth without departing from the spirit and essential characteristics of the invention. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the detailed description are intended to be embraced therein.
| Number | Date | Country | Kind |
|---|---|---|---|
| 588/MUM/2012 | Mar 2013 | IN | national |
Priority is claimed for US provisional patent application: An application claiming the benefit under 35 USC 119(e) US Provisional patent application number: U.S. 61/850,810 Filing date: Feb. 25, 2013 Priority claimed: Yes Priority is also claimed for two Indian patent applications (foreign patent applications): 1) Provisional Patent Application number: 588/MUM/2012 Filing date: 7 Mar. 2012 Priority claimed: Yes 2) Nonprovisional Patent Application number: 588/MUM/2012 Filing date: 6 Mar. 2013 Priority claimed: Yes All of the aforementioned patent applications are incorporated herein in their entirety by reference.
