247 Solar plant uses gas, air, turbines

Materials World magazine
,
28 Oct 2019

ANB Mining Ltd. Director and Principal Mining Engineer, Andy Birtles FIMMM, and ROST Inernational Trading Limited Director, Stuart Whitelock, discuss a popup solar plant designed for remote locations.

Mining and processing operations are often located in remote areas, far from established electricity infrastructure. Generally they have significant power requirements in the order of tens of megawatts and millions of kilowatt-hours per site, per annum. This is particularly true for mining operations, which have underground energy requirements and metallurgical processing facilities. Bearing this in mind, the International Energy Agency (IEA) has estimated that more than 10% of total world energy consumption can be attributed to the mining sector.

Many countries are reliant on fossil fuel-generated power, and remote mines in such areas depend on local power utilities or governmental agencies to install electricity infrastructure at a tariff.  In addition, particularly in sub-Saharan Africa, grid supplied power is unreliable and backup generators are required, which can be expensive and are located at the end of long generator fuel supply chains. Energy can represent 20-40% of mining operational costs, and forecasts indicate this will rise further as traditional, reliable suppliers are no longer able to guarantee uninterrupted electricity.

Concentrated solar power

Energy from renewable sources including wind and solar can provide partial supply solutions, but cannot deliver fully dispatchable power, nor can they operate continuously, although new energy storage solutions are being developed which may improve their performance in future. Concentrated solar power (CSP) technologies have advanced considerably in recent years and can be used to generate 100% of the power requirements of mining operations on a 24/7 basis, even in areas of high direct normal irradiation (DNI). Among the areas where solar plants are efficient are the west coast USA, the Sahara Desert, Southern Africa, Central and West Australia, the Middle East, Chile, Peru and Mongolia.

The IEA has projected that by 2050, CSP will contribute 10% of the global energy power requirements and will be the world’s largest source of electricity.

CSP uses lenses or mirrors and tracking systems to concentrate sunlight, transferring the resulting heat in a fluid medium that is then used to spin a turbine producing electricity. A thermal storage system powers the turbine after sunset, or during periods when the sun is obscured by cloud cover. A wide range of concentrating and tracking technologies are used to track the sun and focus the light.

The problem for the mining industry is that conventional CSP requires large volumes of water and works most efficiently at power sizes greater than 50MW.

The turnkey 247 Solar power plant uses gas or air turbines instead of steam, reducing water requirements by 90%. The design is a pre-engineered, standardised system that can be deployed as a single unit (400kW) or as tens of megawatts using multiple units. It operates at atmospheric pressure, has few moving parts, and can adjust to the varying power demands of a mining operation.

Technology

The principal breakthrough technology enabling a low-pressure concept was the introduction of a high-temperature heat exchanger into the standard Brayton cycle power tower system configuration. Brayton is a thermodynamic cycle that uses gas instead of steam to spin a turbine. The heat exchanger transfers the heat from the exit air of the solar receiver to the turbine’s compressed air, avoiding high-pressure air being passed through the receiver. With this approach, near atmospheric pressure air from the turbine’s exhaust passes through the receiver.

Another advantage of a low-pressure system is that it allows the window diameter – the aperture – of the solar receiver to be designed significantly larger than conventional Brayton receivers. A 2m-diameter aperture is currently used on the 400kW low-pressure unit but may be increased in future following further research.

The large diameter aperture allows the generation of temperatures up to 970°C, 50% higher than conventional CSP systems. The 247 Solar power plant can power up to 400kW during the day while simultaneously storing enough heat to power the turbine for 10-15 hours at night for continuous operation. The low-pressure receiver has no moving parts. The main components are a tower, solar thermal storage system, light from heliostats, a conventional turbine, and off-the-shelf heliostats. About 20,000m2 of heliostats track the sun and focus 1,500 suns’ worth of energy onto a receiver that is mounted on a readily available 35m-tall truss tower, which holds the ducting carrying the air to and from the receiver. Toward the bottom of the tower, some of the hot air goes to an off-the-shelf 400kW turbine, equipped with generator and power electronics for quick grid connection and high reliability. The turbines use compressed hot air rather than steam, which require just 4–6 hours per year of maintenance, and have overhaul schedules of 50,000 hours. The package also includes the heat exchanger, which transfers the solar heat from the low-pressure air to the compressed air from the turbine’s compressor.

The rest of the low-pressure hot air from the solar receiver goes to a thermal storage system powering the turbine when the sun is not shining. The plant’s hot air heats inexpensive dry storage media, such as firebrick or small pieces of ceramic, rather than molten salts, which is typical of other CSP systems. If needed for backup, the turbine burns conventional liquid or gaseous fuels, or biofuels when the thermal storage system is depleted. This multi-fuel capability may be of use in remote areas where an uninterrupted power supply is critical, such as metallurgical processes in mining or for continuously operating ventilation fans.

The only water required is for cleaning the heliostats, and the system can be set up with less levelling than other CSPs. Some differences in elevation between various parts of the field are acceptable. The land may be untouched except for removing larger shrubbery, tower and storage construction, and access roads for service and maintenance.

Commercial demonstrator

The Kingdom of Morocco stated in 2009 that energy availability, security of supply and environmental protection was its top priority. Today, Morocco has 20 solar power projects and trials. Ouarzazate, a city in the country’s east, has been set aside to test and trial numerous CSP designs in partnership with the Moroccan Agency for Sustainable Energy (MASEN). 247 Solar’s first commercial demonstration plant is being constructed in that city.

The project in Morocco is a single, standardised 400kW system overseen by a team of international CSP and turbomachinery experts. The demonstrator will be a prototype of the standardised, pre-engineered modular plant that will be replicated on customers’ sites as many times as necessary to achieve the required power output.

The model will be used to provide incontrovertible, documentable, and independently verifiable evidence as to the performance of the technology so that customers and financiers can be confident it will perform to the designated specification. Data will be compared with economic modelling, which will be continually updated to ensure accurate, predictable models for the future. In addition, the demonstrator’s performance will be monitored against other CSP technologies to provide accurate cost or benefit analysis. The demonstrator is expected to be operational by mid-2020.

Mining industry solutions

Some mining companies are choosing to move to renewable energy sources as a backup when local fossil fuel-generated power is unobtainable or unreliable. Some of these cases are demonstrated in the report Renewable power of the mine, by Columbia Center for Sustainable Investment, published in 2018. When unable to offer a sustainable, uninterrupted supply, the alternative is to develop and install a source of renewable energy, backed up by appropriately sized diesel or heavy fuel oil generators, at or near a mining operation to power all their activities.

With the high energy costs currently experienced by mining projects, reducing electricity expenditure is now a major operational and strategic goal. Integrated renewable solutions enable mining companies to meet sustainability goals while also securing a reliable energy supply and a reduced long-term energy price. Using renewable sources reduces the carbon footprint of the mining company, as well as contributing to emission reductions to help countries meet the targets set in 2015 at the Paris Agreement of preventing global temperatures from rising more than 1.5-2oC above pre-Industrial levels.

Despite these advances, renewable energy formats currently provide only a small fraction of power at mines. The costs of solar, wind and battery storage systems have been declining at an unprecedented rate, which has encouraged an increasing number of mining companies to test renewable technologies at their sites.