by Willie Smit, Stellenbosch University, South Africa

The PreMa project aims to demonstrate that CO₂ emissions from Manganese production can be reduced by preheating the ore with sustainable energy sources. The project is considering three sources of sustainable energy, namely: industrial off-gas, bio-carbon and solar energy.

Many areas of the world receive direct sun, where sunlight casts a shadow, for the larger part of the year. In these areas, mirrors or heliostats can be used to reflect the solar rays and concentrate it unto a single point where the temperatures can reach 1000°C.

This technology, called concentrated solar power or CSP, is being used at large scale to generate electricity. The common size of the typically built plants varies between 50 MW and 100 MW. Such plants will have tens of thousands of mirrors and a central tower that is 150 m or higher. At the moment, it is too expensive to consider the use of CSP on a small scale to generate electricity. However, a small scale CSP is an ideal technology to generate process heat, as the solar radiation is effectively converted to heat. There are not yet many commercial small scale CSP plants in operation. This situation will hopefully change as the technology matures and as mass production drives costs down

Figure 1: The 100 kW thermal heliostat field and central receiver tower at Mariendahl, just outside Stellenbosch.

Stellenbosch University (SU) has a small 100 kW thermal CSP plant. Over the last few years, they have conducted several studies into making such plants more portable and autonomous. As part of the PreMa project, they have in particular looked at reducing the costs of the heliostat field by using wireless heliostats. These heliostats use small PV panels to get electricity and communicate with the central controller via Wi-Fi.

A heliostat field can be a harsh and demanding environment for Wi-Fi communication: there are steel structures that reflect and distort radio waves; the Wi-Fi devices are densely placed in a heliostat field; and emergency messages should quickly and reliably be delivered to heliostats.

SU completed a study to determine if off-the-shelve Wi-Fi technology and hardware can meet the requirements of heliostat communication in CSP plants. The study performed various experiments with 50 Raspberry Pi Zeros. The Raspberry Pi Zero is a budget, entry-level computer that has sufficient computational power to control several heliostats. The study used the version that comes standard with Wi-Fi capability. Various performance measures were assessed for each test. Two of the most important measures were latency and throughput. Latency is the delay between the time a message is sent and the time a unit successfully receives it. Throughput is a measurement of the rate at which information can be transmitted. If the signals are distorted and reflected, then messages take longer to be successfully received, therefore the latency increases and the throughput decreases. The study found than the latency and throughput are well within the minimum requirements of communication in a heliostat field, even with densely distributed controllers.

SU is also looking into heliostats that are self-calibrating. That means that a heliostat can be placed anywhere in a heliostat field and it will figure out where it is placed, what its orientation is and within a few hours be able to track the sun to sub-degree accuracy.