Autonomous domestic heliostat with automatic calibration via accelerometer and solar aiming

Abstract

The invention relates to a device for reflecting daylight in a fixed direction chosen by the user, despite the sun's movement. The device is distinguished by its ability to be placed in any position and requires no calibration by the user. It positions itself in the Earth's frame of reference using an accelerometer coupled to a solar sighting system, which allows it to determine its position in the Earth's frame of reference and orient the mirror to maintain a constant direction of the reflected beam. The beam is unaffected by any cast shadows, and the system's accuracy is increased compared to compass-based devices. (Shorthand figure: Figure 1)

Description

Title of the invention: Autonomous domestic heliostat with automatic calibration by accelerometer and solar aiming. FIELD OF THE INVENTION

[0001] This invention falls within the field of solar technologies, and particularly within that of devices for directing sunlight in a desired direction. These devices make it possible to use solar energy (light and heat) where it does not occur naturally or with an undesirable direction. More specifically, the invention falls within the field of optimizing existing systems, providing significant improvements in the accuracy of the solar tracking system and its ease of use. This is achieved through a new positioning method using solar sighting, on the one hand, and by implementing a calibration technique that requires no human intervention, on the other. STATE OF THE ART

[0002] Heliostats have seen great progress since the mid-18th century, mainly to adapt them for energy production by solar concentration. This explains why several techniques already exist for directing sunlight onto a given target, but which are generally poorly suited to domestic use.

[0003] There are fixed mirrors of various shapes arranged so that a portion of their surface always reflects light in the correct direction. As the sun moves, another surface takes over, reflecting the rays back towards the target, and so on. For these systems to operate all day, a large mirror surface is required, as each mirror is only effective for a minor period within a single day. It is therefore a simple system, but with mediocre efficiency.

[0004] To use the mirror 100% of the time, it must rotate, but calculating its position over time, which depends on the sun's movement, is difficult. The simplest system for adequately controlling a rotating mirror, currently found in domestic applications, is therefore to use a feedback system based on the reflected ray (example patent FR3113311). The orientation of the ray reflected by the mirror is then used to adjust the mirror's position, bypassing the problem of calculating the sun's movement. This system eliminates the need for a microcontroller and avoids tedious calculations. Unfortunately, it has the disadvantage of having to block some of the reflected rays to control the feedback, and therefore generates a shadow on the image. reflected sunlight is unsightly and reduces efficiency. Furthermore, these systems often have the drawback of being able to "disengage." Indeed, if the sun does not illuminate the heliostat for a certain period (due to cloud cover or simply nightfall), the reflected ray, when it reappears, may have shifted sufficiently to be undetectable by the sensors controlling the feedback. The system then needs to be reset, which requires human intervention and is undesirable for everyday home use.

[0005] A device that does not cast a shadow on the reflected ray is the remote-controlled system. This is simply a mirror mounted on two axes, the position of which can be adjusted remotely by the user using a remote control. Therefore, the user must manually, and unfortunately frequently, direct the sun's rays to the desired location to compensate for the sun's movement. This solution thus requires far too much human intervention.

[0006] To solve the problems of shadowing and shading without human intervention, microcontroller-controlled systems have been developed, primarily in concentrated solar power plants. Thanks to precise calibration and meticulous adjustment by specialized technicians, these microcontroller-controlled systems can efficiently reflect sunlight to a fixed point. They contain equations that determine the sun's position at any given time and calculate the orientation of the mirror so that the ray is reflected in the correct direction. Unfortunately, these highly precise systems require a laborious adjustment process, as the position of their axes and mirror must be accurately determined in the Earth's frame of reference. This calibration step presents a significant challenge, as it cannot be performed by an untrained user.This lock makes the system incompatible with general public marketing.

[0007] To circumvent the previous problem, one solution is to equip the heliostat with the sensors necessary to perform this calibration automatically. To determine the heliostat's position relative to the horizontal, an accelerometer can be used, allowing the microcontroller to locate the zenith. To determine its position relative to the cardinal points, connecting the microcontroller to a compass seems ideal. With more complex calculations, this solution works, at least theoretically. However, searches for "heliostat compass," "heliostat compass," or "heliostat IMU" return no results in the INPI database (as of August 27, 2024), even though it is highly likely that systems based on this principle have already been tested.From our experience, the use of an electronic compass is a bad idea for three major reasons, which in our opinion explain why no convincing system has been developed so far: .

[0008] a) Compasses are sensitive to surrounding magnetic and paramagnetic objects; the information they provide can be distorted by the presence of metallic objects. To circumvent this problem, a dual calibration (soft iron / hard iron) must be performed, ideally at the site where it will be used. Unfortunately, the results obtained from this calibration are not always highly accurate.

[0009] b) Another problem with compasses is that they indicate magnetic north, not true north, which is necessary for the calculations performed by the microcontroller. The difference in angle between these two "norths" is called magnetic deflection, and it is not constant with respect to location or time. The magnetic masses moving within our planet affect the magnetic field and cause its orientation to vary, thus distorting the data provided by compasses. While some models exist to correct this defect, they must be implemented in the microcontroller, and even if this were done, there would still be local variations in the magnetic field that the models cannot predict. The resulting errors cannot be known in advance, making the commercialization of the system uncertain.

[0010] c) Compasses are sensitive to temperature. However, a heliostat left outdoors can be exposed to temperatures ranging from -20 to 50°C. While some adjustments are possible through prior calibration, there remains a risk of inaccuracy in determining North. Furthermore, we have observed that temperature gradients appear to pose a significant problem with the reliability of the information transmitted by magnetic compasses.

[0011] However, the expected accuracy of a good heliostat, to limit the movement of the reflected beam to a few centimeters when the device is located a few meters from the target, is on the order of a degree of angle. We were unable, with our carefully calibrated prototype, to obtain a compass-based system that met the reliability requirements for a large-scale commercialization project.

[0012] The idea presented in this patent addresses the shortcomings of the compass by providing a reliable reference point which, coupled with the accelerometer, gives the system the two vectors that allow it to locate itself in the Earth's frame of reference with the necessary precision. To achieve this, the device presented here adds a solar sighting system, which locates the center of the Sun and uses it to position itself without error, enabling precise operation of the heliostat even if it is not horizontal, without calibration by the user, without loss of alignment, and without casting a shadow. Summary of the invention

[0013] The invention relates to a device for reflecting daylight in a fixed direction chosen by the user, despite the sun's movement. The device is distinguished by its ability to be placed in any position and requires no calibration by the user. It positions itself in the Earth's frame of reference using an accelerometer coupled to a solar sighting system, which allows it to determine its position in the Earth's frame of reference and orient the mirror to maintain a constant direction of the reflected beam. The beam is unaffected by any cast shadows, and the system's accuracy is increased compared to compass-based devices. DETAILED TECHNICAL PRESENTATION

[0014] As shown in [Fig.1], the device consists of a mirror(1) mounted on a motorized system allowing its rotation around two axes(2), an accelerometer(3), a solar sighting system(4), a microcontroller(5), a GPS chip(6), an RTC chip(7), a motor control board(8), a battery(9) and a block of solar panels forming an angle between them(10).

[0015] The solar aiming system (4) is a device for determining the position of the Sun from the perspective of the heliostat, characterized in that it comprises a matte tube (not reflecting light) containing a photodiode at one end ([Fig. 2]). When this tube is oriented towards the Sun, the photodiode changes state, which can be detected by the microcontroller and indicates to it that the aiming system is currently pointed directly at the Sun.

[0016] The solar panel array consists of four solar panels ([Fig. 3]) mounted at an angle to each other, so that the difference in power received by each pair of panels can provide an approximate indication of the sun's position. In this configuration, when the element is not facing the sun, the panel receiving the most light generates more power, allowing the microcontroller to know which way to turn to face the sun. These panels can also be used to power the battery.

[0017] The device automatically locates itself on Earth by using several direct orthonormal reference frames and specific vectors for orientation ([Fig.4]): a. The Earth reference frame (Rf), whose three axes are: a. xr directed towards the South instead of using the heliostat, b. y, directed towards the East instead of using the heliostat, c. zT, directed towards the Zenith instead of the heliostat being used. b. The Accelerometer reference frame (R^), whose three axes are those of the electronic chip carrying the accelerometer. c. The fixed heliostat frame of reference in RT, whose three axes are: a. xH, perpendicular to and in the XT / zT plane, b. y, direct with xH and zH, c. zH, oriented along the main axis of the heliostat, from its base towards the mirror. a. The vector jÇf, oriented perpendicularly to the mirror, exits from the reflecting side. Since the accelerometer is mechanically fixed to the mirror, this vector is fixed in b. The y-vector, placed on the axis of the sighting system, in an arbitrary direction defined during design or calibration. Since the accelerometer is mechanically attached to the solar sighting system, this vector is fixed in Ra c. £, oriented towards the center of the sun from the location of the heliostat. d. p, which defines the desired orientation of the reflected ray, from the center of the heliostat towards the target.

[0018] The use of the solar sighting system is done in automatic mode in four stages ([Fig.5]): 1. The battery supply from the solar panels is temporarily interrupted, and the voltage generated by each panel is compared. The voltage difference measured across the four panels allows the microcontroller to roughly position all the solar panels towards the sun. Since the panels are constructed differently and have fluctuating efficiencies, this method is approximate and its orientation is only rough. 2. Knowing the position of the solar aiming system relative to the solar panels, the microcontroller orders the motor rotations necessary to roughly point the solar aiming system towards the Sun. 3. The solar tracking system moves in a spiral pattern until the photodiode detects the sun 4. The solar tracking system then makes several cross-shaped movements to precisely locate the center of the sun. First, one of the two motors, chosen arbitrarily, moves until the photodiode no longer detects light. The angle traversed is measured, and the motor positions itself on the bisector of this angle. The other motor makes the same measurement. In theory, it should already bisect the sun, but as a precaution, we repeat the procedure once more. After the final traverse, the bisector of the last measured angle indicates the center of the sun.

[0019] The use of the heliostat then takes place in 3 steps for the user and 6 steps for the microcontroller: 1. The user attaches the foot to a support, 2. He manually rotates the mirror so that the reflected ray is in the correct direction. 3. He presses the ON button.

[0020] The microcontroller then takes care of the rest: 1. It calculates the position of the sun in the Earth's frame of reference using pre-recorded astrophysics equations, the GPS chip(6) and the RTC(7), 2. He positions the solar sighting system towards the center of the sun using the procedure defined beforehand. 3. Knowing the zenith orientation from the accelerometer and that of the solar sighting system, which is then pointing towards the center of the sun, the microcontroller calculates the position of the Earth's frame of reference in the accelerometer's frame of reference ([Fig. 6]), and by transposition, its inverse. The position of the mirror, which is known from the factory in the accelerometer's frame of reference, can then be defined in the Earth's frame of reference. 4. The microcontroller then commands the main motor to rotate one full turn, taking regular measurements with the accelerometer. These measurements form a cone around the main axis of rotation of the heliostat. This axis being the zH axis in the A-frame, we will call it ([Fig.7]). This axis is measured in the accelerometer's frame of reference, and converted into the Earth's frame of reference using the equations found in point 3. The other axes of the Heliostat's frame of reference in the Earth's frame of reference are deduced by definition. 5. The position of the Mirror can then be calculated in the reference frame of the Heliostat, and the exact position of the angles that the motors make with the axes of the reference frame of the Heliostat can be deduced from this ([Fig.8]). 6. Having recorded all the movements made by the motors since the ON button was pressed, and now knowing the position of all the reference frames relative to each other, we can now determine the position of the mirror in the Earth's reference frame when the ON button was activated. Having recorded the position of the Sun in the Earth's reference frame at that moment, we can determine, using Snell's law applied to reflection, the position of the target in the Earth's reference frame ([Fig. 9]). 7. All the data necessary for the calibration phase now having Once the data has been found, the microcontroller can regularly recalculate the Sun's position in the Earth's frame of reference, as well as the position required for the mirror so that the reflected ray hits its target. This position is then transposed into the heliostat's frame of reference, where it is converted into position commands for each of the motors. By performing this operation in a loop, the reflected ray is maintained at the desired position. PRESENTATION OF A METHOD OF IMPLEMENTATION

[0021] A prototype was built based on what has been mentioned above:

[0022] For the structure, most of the parts were produced using 3D printing. The The system's rotational guidance elements are made from commercially available pipe fittings, inserted into bores machined into 3D-printed parts. The microcontroller is an Arduino chip. The solar panels, GPS(6), accelerometer, and RTC are standard components available from major retailers. The aiming system was also 3D-printed. The mirror is a laser-cut piece of polished stainless steel. The stepper motors were coupled to gearboxes with relatively high reduction ratios to ensure smooth movement of the reflected beam during motor adjustments and to prevent wind from causing the motors to skip steps. However, the reduction ratio was not chosen to be too high, so that the mirror could still be rotated manually. A 1:50 ratio on motors with 200 steps per revolution yielded good results. They are controlled by a dedicated motor driver board.To determine the sun's position relative to the Earth at the heliostat's location, a GPS chip (6) combined with an RTC (7) provides the microcontroller with the necessary information to calculate the sun's position using equations borrowed from astrophysics. A set of transistors allows the solar panels to be disconnected from the battery during the calibration phase and the voltage they generate individually to be measured.

[0023] The accelerometer and the solar sighting system are glued behind the mirror. The coordinates of the vector and the y vector were calibrated after the heliostat was built. We performed this calibration manually. To calibrate y, we aligned the solar sight vertically using a plumb line and assigned to y the value read at that instant on the accelerometer. To calibrate y, we shone a beam of light vertically downwards onto the mirror and assigned to y the value read by the accelerometer when the reflected beam returned to its source.

[0024] The device described here was tested at different angles in August 2024. It gave excellent results for optimal ease of use: no data to be entered by the user and no calibration to be done.

Claims

Demands

1. Heliostatic device characterized in that it comprises a mirror(1), a motorization system enabling the mirror to rotate(2), an accelerometer(3), a sighting system enabling it to locate the sun(4), a microcontroller(5), an electrical power supply system(6) and a system enabling it to determine the position of the Sun in the terrestrial reference frame(7).

2. Claims to be made upon bringing the prior patent application into compliance

Images