GB/T 44140-2024Technical requirements for heliostats of solar power tower plant (English PDF)

3 Terms and definitions

3.1 Heliostat: a reflector that by mechanical drive reflects the direct normal solar radiation constantly in one direction; a note states that a heliostat comprises the mirror, the support and the drive. Taken from GB/T 12936-2007, 8.9, modified. 3.2 Transmission mechanism: the device that drives the heliostat so that it tracks the sun, generally comprising the transmission gear, the motor and the driver or controller.

3.3 Homing: a rotation of the heliostat that brings it to the reference position. 3.4 Tracking accuracy, also called tracking error: the angle subtended by the line joining the geometric centre of the pattern formed on the target surface by the sunlight reflected from the heliostat and the target point on that surface, seen from the centre of rotation of the heliostat; a note states that it is usually expressed in milliradians. 3.5 Tracking: automatic following of the sun by the heliostat so that direct solar radiation is thrown onto the stated position on the surface of the receiver.

3.6 Slope deviation: the angular difference between the actual normal vector and the ideal normal vector at a point on the reflecting surface of the heliostat; notes state that it is usually expressed in milliradians and refer to the calculation and to a diagram. 3.7 Windproof mode: the state of the heliostat when it is at the angle of its greatest wind resistance; a note states that in windproof mode the heliostat is often close to a flat attitude.

4 Normal service conditions

4.1 Ambient temperature: -40 °C to 55 °C for the low-temperature type and -10 °C to 55 °C for the normal-temperature type. 4.2 Relative humidity not greater than 95 %.

4.3 Altitude is graded at 1 000 m, 2 000 m, 3 000 m, 4 000 m and 5 000 m. 4.4 Operating wind speed is below 13 m/s, taken as the 10 min average at 10 m above the ground.

5 Appearance, dimensions and structure

5.1 to 5.3 The heliostat shows no obvious deformation, denting or breakage, the anti-corrosion coating does not peel, and the marking or nameplate is clear and complete.

5.4 to 5.7 For a single heliostat the mirror thickness deviation is not greater than 0.2 mm and the deviation of any other single dimension is not greater than 2 mm; the deviation between the actual and design mirror area is not greater than 0.2 %; where the outline is rectangular, the ratio of mirror width to mirror height is preferably 1.0 to 2.0; and the root mean square of the slope deviation of the mirror surface in the x and y directions is not greater than 2.5 mrad in either direction.

5.8 to 5.11 In the working state the heliostat meets the safety requirements at the maximum operating wind speed and in windproof mode it meets those at the local extreme wind speed with a return period of 50 years; its stiffness satisfies the tracking accuracy requirement; and the minimum distance between the mirror and the ground in any state is preferably not less than 0.5 m for a heliostat with a single reflecting area greater than 100 square metres and not less than 0.3 m for one with a single reflecting area of 100 square metres or less.

6 Functions and performance

6.1 The heliostat tracks the sun and reflects the direct normal radiation in real time; it locks its position when it stops rotating under any permitted condition; each rotation axis carries a zero-position sensor and has a homing function; it feeds its status back, the status covering at least homing, rotation angle and tracking; it can be controlled locally and remotely; it has a communication interface through which it receives control commands and reports its status, angle and fault diagnosis information; communication may be wired or wireless and may use RS485, industrial Ethernet including Ethernet and wifi, zigbee or Lora; a fault in one or some heliostats on a communication bus does not affect the normal running of the others on that bus; and the security of the data communication network meets GB/T 33009.1. Under safety protection, the heliostat enters windproof mode automatically when the wind speed exceeds the maximum operating wind speed, and it has a fault diagnosis function covering at least homing faults, abnormal rotation, which includes motor stalling and wrong direction of rotation, and communication faults.

6.2.1 Tracking accuracy satisfies three conditions at once: at a wind speed of 4 m/s it is preferably not lower than 2.5 mrad, at 8 m/s not lower than 4.0 mrad and at 11 m/s not lower than 6.0 mrad.

6.2.2 Mirror reflectance: for a glass mirror, not less than 94.5 % where the mirror thickness is greater than 0 mm and not greater than 2 mm, not less than 93.5 % where it is greater than 2 mm and not greater than 4 mm, and not less than 92.5 % where it is greater than 4 mm and not greater than 6 mm; for a highly reflective aluminium mirror, not less than 89 %; and for a silvered polymer or other type of mirror, not less than 91 %.

6.2.3 to 6.2.9 Homing accuracy is not lower than 0.5 mrad. The rotation angle accuracy of each axis is not lower than 2 mrad and the rotation speed of each axis is not lower than 0.01 r/min and satisfies the speed needed for emergency defocusing and emergency field shutdown, the time to reach windproof mode from any position being not greater than 10 min. Immunity reaches test level 3 of GB/T 17626.2, GB/T 17626.3, GB/T 17626.4 and GB/T 17626.5. The insulation resistance and insulation strength between the electronic parts and the metal structure meet GB/T 15479. The enclosure protection of the electrical housing reaches IP54, the protective coating of the mirror covers and protects the reflecting layer for the 25-year design life, and the heliostat withstands the local extreme temperature with a return period of 50 years without damage. The supply may be alternating or direct current and the permitted voltage deviation is +/- 10 %. The heliostat body has an earthing point, its metal parts and structures are bonded equipotentially with a connection resistance to that point not greater than 1 ohm, and the earthing resistance is not greater than 4 ohm where the supply is not at safety voltage, or is determined by the requirements of the electrical equipment inside where it is.

7 Tests and inspections

7.1 Table 1 lists the fifteen inspection items with, for each, the technical requirement clause and the clause of the test method under each of the three inspection classes, type test, factory test and site test, a dash marking a class in which the item is not tested. The items are appearance; dimensions, split into dimensional deviations and slope deviation; position locking function; status feedback function; communication; fault diagnosis function; tracking accuracy; mirror reflectance; homing accuracy; rotation angle, split into accuracy and speed; electromagnetic compatibility; insulation and withstand voltage; environmental adaptability, split into degree of protection and high and low temperature endurance; power supply; and earthing, split into connection resistance and earthing resistance.

7.2 A type test checks whether the heliostat meets the document and all its items have to pass; it is carried out on trial production of a new product, after a major change of material or process in series production that may affect quality, on resumption of production after a long stoppage, where the factory test result differs markedly from the last type test, and where the national market supervision body calls for a routine inspection. The factory test is carried out when the product leaves the works, may use sampling with not fewer than 10 units sampled from one batch, and all its items have to pass. The site test is carried out before delivery and acceptance, is run with the heliostat operating normally and free of faults unless otherwise stated, and all its items have to pass.

7.3 and 7.4 Appearance inspection uses a magnifier and a video camera, the heliostat being laid horizontally in a well-lit position; the marking or nameplate is checked for presence and completeness and the surface for deformation, denting, breakage and peeling of the anti-corrosion coating, a magnifier assisting where viewing conditions are poor and a camera or other photographic equipment being used where direct viewing is not possible. Dimensional deviation inspection uses an invar rule of accuracy not lower than 1 mm and a vernier caliper of accuracy not lower than 0.02 mm, the caliper measuring the mirror thickness and the rule the mirror width and height, from which the mirror area and, for a rectangular outline, the width to height ratio are calculated. Slope deviation inspection uses a ground camera of resolution not lower than 2456 by 2058, a measuring camera of resolution not lower than 3856 by 2764, a projector of resolution not lower than 1920 by 1080 and brightness not lower than 5 000 lm with keystone correction, and a total station with an angle accuracy better than +/- 1 arcsecond and a distance accuracy better than +/- 1 mm plus 1.5 parts per million, measuring to a prism.

7.4 (continued) The slope deviation inspection is carried out at night or in a dark room. A target with a uniform diffusely reflecting white flat or regularly curved surface is set in front of the heliostat, its length and width being more than twice those of the heliostat where a single measuring camera is used, or the product of the target area and the number of cameras being more than four times the heliostat area where several are used. The measuring camera is set so that its field covers the whole heliostat and so that the image of the target reflected in the heliostat covers the whole image of the mirror; the projector and the ground camera are aimed at the target centre so that the projection reflected in the heliostat covers the image of the heliostat in the measuring camera. Not fewer than four marker points are set on the target, generally the four corners of the projection, and not fewer than four around the heliostat, covering it; the total station measures the coordinates of the marker points and of the measuring camera and target and heliostat coordinate systems are set up. Not fewer than three groups of phase-shifted fringe images are prepared, each group generally having four phases, and for each group the vertical black and white fringes are projected at each phase in turn and photographed both on the target by the ground camera and in the mirror by the measuring camera, after which the fringes are turned to the horizontal and the sequence repeated.

7.9 Tracking accuracy inspection uses a camera of resolution not less than 640 by 480, a direct normal irradiance meter with a range of 0 to 4 000 W per square metre and a resolution not lower than 0.03 W per square metre, and an anemometer with a range of 0 m/s to 60 m/s and a resolution not lower than 0.8 m/s. The heliostat is mounted on its foundation, connected to the supply and to the field control system, and a target as described above is set in front of it. The inspection is carried out at a direct normal irradiance above 300 W per square metre: the field control system throws the spot of the heliostat under test onto the target point, the camera aperture and exposure are adjusted so that the spot image is bright without being overexposed, the spot image is captured and its geometric centre on the target calculated, and the wind speed and direct solar irradiance are recorded at the same time. The tracking accuracy of Formula (8) is the angle between the vector from the heliostat centre of rotation to the target point and the vector from that centre to the actual geometric centre of the spot, expressed in milliradians. The inspection covers three wind speed intervals, above 0 and up to 4 m/s, above 4 and up to 8 m/s, and from 8 to 12 m/s, and in each interval is carried out once within 2 h after sunrise, once within 1 h either side of true solar noon and once within 2 h before sunset; every result has to meet 6.2.1.

7.10 to 7.12 Mirror reflectance is measured over the wavelength range 300 nm to 2 500 nm following the test procedure of GB/T 33234. Homing accuracy inspection uses an angle sensor whose measuring accuracy is preferably not lower than +/- 0.005 °, fixed to the axis under test; the heliostat is connected to the supply and to the field control system or a hand-held operator, the axes are driven to their zero positions and the initial relative zero value of each is read, and the axis under test is then driven to one third of its maximum rotation angle and back to zero, the deviation from the initial relative zero being calculated, then to two thirds of that angle and back, and then to the maximum angle and back.

8 Packaging, storage and transport

8.1 The heliostat and its parts are packed after the factory inspection items have passed, with safety protection measures in place. Parts with special requirements are preferably removed, marked and packed separately according to those requirements. Where several cases are used, the case number is written as a fraction with the case number as numerator and the total number of cases as denominator, following GB/T 13384. Shipping marks and storage marks are applied by content, colour, position and size. Each case contains the manufacturer's quality inspection certificate, the packing list and the other technical documents and loose accessories where there are any. Electronic parts packed and transported separately use antistatic packaging and are protected against water, damp, dust, vibration and impact.

8.2 Electronic products of the heliostat are stored at a temperature of -40 °C to 60 °C and a relative humidity below 95 %, away from perishable, flammable and explosive dangerous goods and away from strong magnetic environments.

8.3 Verification, monitoring and improvement of the transport packaging follow GB/T 36911, and the heliostat and its parts are not subjected to impact or abrasion during transport.