Dr. Norberto Flores - Centro de Investigación en Matemáticas A.C.
Roberto G. Galàn - Centro de Investigación en Matemáticas A.C.
Mexico is in an ideal position for implementing solar technologies because of its location in the Earth’s sunbelt. The countries average yearly insolation is greater than 5.5 kWh/m2 per day. This high-quality solar resource makes this area ideal for the implementing concentrating solar technologies (CST), either for electrical power generation or for the production of solar fuels as hydrogen.
To promote the development of CST in México, a high radiative flux solar furnace (HRFSF) was constructed in the Centro de Investigación en Energía (CIE, Energy Research Center). The HRFSF offers the possibility of using solar radiation for basic and applied research, as well as for the development of industrial processes. Our primary objective of HRFSF was to develop components for thermoelectric solar towers used in central tower electricity generation plants. Another objective was to process and manufacture advanced materials and perform characterization of thermo-physical, mechanical, and optical properties of materials exposed to sunlight.
We needed a control and data acquisition system to operate all the integrated components of the HRFSF. The industrial math department of the Centro de Investigación en Matemáticas A.C. (CIMAT, Mathematics Research Center) was assigned the task of developing the control system in collaboration with personnel from CIE.
Components of the HRFSF
The HRFSF is composed of three main components: a concentrator, a heliostat, and a shutter (Figure 1). The concentrator is the heart of the system, and its function is to concentrate solar radiation to high levels to reach high temperatures (up to 3000° K) in the focal zone. The concentrator is placed in a solar furnace and does not move; all the movement required for tracking the sun is carried out by the heliostat. This is done to have a static focal zone, which provides a more controlled environment for experiments. The performance of the furnace depends on the ability of the heliostat to track the sun accurately. The shutter is opened and closed partially to different degrees to control the amount of radiation that is allowed to enter the system. In particular, the HRFSF built consists of a heliostat with an area of 81 m2, a shutter of42.2 m2, and an optical concentrator consisting of 409 hexagonal first surface polished glass mirrors.
In addition to the above components, there is a moving platform that allows precise positioning of the experiments in different points of the focal region. Also, data acquisition systems are required for monitoring different experimental variables, such as temperature, pressure flow, solar radiation, and concentrated radiative flux distribution. A cooling system for the experiments was also required. Additionally, a weather monitoring station was integrated in the system. All furnace components, except for the heliostat, the cooling system, and some sensors for solar radiation and wind speed, were located inside the furnace building.
The Development Platform
We chose the NI platform for the control and data acquisition system because we can use it to develop all the control, data acquisition, and vision functions with one intuitive, versatile development environment.
With the inherent ruggedness, accuracy, expansion capabilities and network integration of the CompactRIO, Compact Field Point, and NI Compact Vision System platforms, we were able to develop a reliable, distributed application within the time constraints of the project.
Implementation of the Control and Data Acquisition System
The control system is integrated by 1 central PXI computer, four NI cRIO-9074 Integrated System Controllers, 1 cFP-2120 controller with a cFP-BP8 backplane and a CVS-1450 all connected in an Ethernet network. The distribution of the components in the furnace building is shown in Figure 2:
The heliostat is controlled by NI cRIO-9074 Integrated System Controllers which have two NI 9505 servo modules that control the two heliostat motors; one for the azimuth movement, and one for elevation movement. The heliostat position is determined using solar tracking equations which calculate the solar vector based on the latitude and longitude of the heliostat position. Knowing the solar vector we can determine the exact azimuth and elevation angles of the heliostat.
For feedback we have encoders of 2000 p/r which, when combined with the heliostat gearboxes, allows us to control the heliostat position. To detect the safety position of the heliostat we use limit switches and a NI 9421 Sourcing Digital Output Module. We also can move the heliostat to user-defined angles with manual operation.
The heliostat control system gets visual feedback from a 16-bit depth IEEE 1394 camera, which takes pictures of the focal zone. This determines the exact position of the solar spot and makes slight adjustments to the heliostat position. We acquire and process images using the NI CVS-1450.
We control the shutter using a cRIO-9074 controller with an NI 9505 module and an NI 9421 module. The NI 9505 module controls the shutter motor to regulate the opening area. This motor is also attached to a gearbox and has feedback from a 2,000 p/r encoder. The NI 9421 module is used to read the limit switches that determine the shutter’s home position. A similar system setup is used for the focal zone positioning platform. However, this system has three motors that control each of the three movement axes so we can precisely position the platform.
Our cooling system carries water to the experimental setups on the positioning platform. This system is controlled with another cRIO-9074 controller. The CompactRIO program activates the water pumps, monitors the water storage tank levels and adjusts water flow through a proportional valve operated with an NI 9265 analog output module. We also used NI 9472 and NI 9421 modules to control the cooling system. Because the pumps and tanks are located outside the furnace building, we communicate with the CompactRIO controller using two NI WAP-9071 wireless bridges. One is located inside the cooling system control enclosure and one is in the furnace building.
We communicate with the weather station via its integrated web server. We monitor several environmental variables, but care mostly about direct radiation and wind speed. The former indicates when it’s a good time to conduct an experiment in the furnace and the latter is critical because a high wind speed may damage the heliostat if it is not in a safety position.
We exchange data between each of the subsystems and the central control system using network published shared variables. The shared variable engine is hosted in the central computer. We use shared variables to develop quick and reliable communication without compromising the system security and the control loop speed.
The HRFSF is currently operating normally to perform several experiments. Next year, we are adding more equipment to the furnace. The HRFSF is a research tool that we hope will help us develop new materials and technologies for electrical power generation using clean, renewable energy from the sun.
Because the HRFSF is used for a variety of experiments, the data acquisition system needs to be flexible. We chose a variety of analog input modules from the Compact FieldPoint family to cover a broad range of input signals. We can adjust the input configuration of the modules from the central control system to fit the specific needs for any experiment.
All the subsystems are operated from a central control system with a single interface. Figure 3 shows some of the control user interfaces.
Roberto G. Galàn
Centro de Investigación en Matemáticas A.C.