What is concentrated solar?
Concentrated solar describes a range of technologies that collect and concentrate sunlight in order to make use of its energy by converting it to heat. The idea of concentrated solar goes back at least to Archimedes, who may have (historians like to argue about this) used an array of mirrors to focus sunlight on approaching warships and setting fire to them during the Siege of Syracuse over two thousand years ago. Concentrated solar found some interest after the industrial revolution, and in 1866, Augustin Mouchot used a parabolic mirror to boil water and drive a steam engine for the first time.
More recently, since the 1980s, concentrated solar has been in use around the world for producing electricity, this is called Concentrated Solar Power (CSP). CSP has been implemented using a variety of different technologies. The two most common configurations are known as troughs and towers.
In a trough (or parabolic trough) plant, curved mirrors are arranged into long linear rows, and rotate to track the sun in a single axis. The curvature of the mirrors and the tracking motion work together to concentrate the sunlight onto a collecting element that absorbs the sunlight and transfers the energy as heat into a working fluid. That hot fluid is then used to boil water into steam, which in turn drives a steam turbine to produce electricity.
In a tower plant, flat (or more gently curved) mirrors are individually tracked relative to the sun, such that each mirror directs its reflection to a common focal point at the top of a large tower. Atop that tower is a solar receiver, which absorbs the sunlight and transfers it as heat into a working fluid. From that point on, the process is similar to that of a trough plant, with the hot fluid being used to boil water into steam and driving a steam turbine to produce electricity.
An interesting aspect of concentrated solar, which differentiates it from photovoltaic solar power, is that the heat generated by concentrated sunlight can potentially be used for purposes other than electricity generation. This is called Concentrated Solar Thermal (CST). Worldwide, 75% of energy consumed is heat from the burning of fuels like coal, diesel, and natural gas. CST can address much of this 75% by using concentrated solar to make heat from the sun. Using CST in this way, for industrial process heat applications, would significantly reduce CO2 emissions and greatly contribute to solving climate change. Unfortunately, there are relatively few examples of this being put into practice (one well-known example is a Frito-Lay installation in Modesto, California, which uses parabolic trough collectors to generate steam used in the production of SunChips).
Heliogen’s Technology
Heliogen is a clean energy company focused on eliminating the need for fossil fuels. By cost-effectively using heat from the sun to replace fossil fuels, Heliogen enables our customers to meet their sustainability goals and help mitigate climate change without hurting their bottom line.
The core of Heliogen’s technology is a tower-based concentrated solar thermal (CST) system that is made up of an array of computer-controlled mirrors called heliostats and a receiver on top of the tower that accepts the concentrated sunlight. Our system is constructed and controlled to achieve very high optical accuracy at low cost. High optical accuracy allows Heliogen to generate high temperatures (~1500ºC) efficiently, while the low cost, of course, makes the system commercially viable for our customers. We achieve this through the innovative design of both the heliostat hardware and the heliostat field control system.
Heliogen’s heliostats are small – around 1.5 square meters, or slightly smaller than a standard solar panel. This differs greatly from conventional heliostats, which may range anywhere from 20 to 150 square meters. Using small heliostats provides an advantage in the optical performance because larger mirrors suffer from optical aberrations – beam distortion caused whenever the sun isn’t perfectly aligned with the mirror (which is essentially all the time in a heliostat field). The drawbacks historically associated with small heliostats include manufacturing cost and calibration/control. Heliogen has minimized the manufacturing cost of our small heliostats by optimizing the design for high-volume, highly automated production techniques (such as injection molding plastics, die-casting aluminum alloys, etc.) and by minimizing the precision with which the heliostats need to be built and installed in the field. This allows for cost-effective production and fast installation, keeping costs low. And to address the calibration and control challenges of small heliostats, we’ve developed and patented an innovative computer vision closed-loop tracking control system. This allows heliostats to be installed and manufactured in a relatively imprecise manner because the control system can detect and correct their actual tracking position in real-time during operation. Together, these advancements in small heliostat manufacturing and control allow Heliogen to create carbon-free, ultra-high temperature heat, which we call HelioHeat™.
One of the factors limiting the adoption of CST in industrial process heat applications is the need for very high temperatures. With previous CST technologies limited to temperatures below 600 degrees Celsius, they haven’t been able to address the needs of many industries that require process temperatures of 700 to 1,500 degrees. With the superior optical accuracy of Heliogen’s heliostat field, we’re able to efficiently reach these very high temperatures to create HelioHeat. Using a proprietary solar receiver located atop the tower, we collect the concentrated sunlight and transfer the energy to a heat transfer fluid and deliver that fluid – our HelioHeat – to the industrial process. In this way, Heliogen is opening up a broad array of industrial processes like calcining, reforming, and ore roasting in industries such as cement, refining, and mining to the possibility of meeting their high-temperature thermal energy demands with clean, renewable solar energy.
One subset of high-temperature industrial process heat applications that is of particular interest is the production of synthetic fuels, which we call HelioFuel™. Fuel synthesis involves the production of chemical fuels (typically hydrogen, methane, or liquid hydrocarbons) from non-fuel feedstocks such as carbon dioxide and water. Synthetic fuel holds great promise in our energy future, as a means of enjoying the benefits and conveniences of fossil fuels, but without the fossil fuels and associated environmental impact. This is an energy-intensive process however, so producing clean fuels requires the use of a clean energy source to drive the thermochemical reactions. This is where Heliogen’s CST technology comes in. By efficiently delivering solar energy to thermochemical reactors operating at up to 1,500 degrees Celsius, Heliogen is working to make HelioFuel competitive with its fossil fuel counterparts.
Potential Impact
Because Heliogen’s advanced CST technology unlocks higher temperature applications than ever before, industrial plant operators now have a cost-effective option for integrating fossil fuel-free process heat to their system. This is an important capability in order to fulfill the ever more aggressive corporate sustainability goals becoming prevalent throughout the industry today. With Heliogen CST providing greenhouse gas-free thermal energy, customers in industries like cement manufacturing – which account for 7% of the world’s man-made carbon dioxide – can finally address their sizeable non-electricity energy consumption. Beyond process heat, the ability to synthesize green fuels using solar energy allows greater flexibility in how we address climate change. No longer is electrification the only answer; with green fuels, we can address the climate impact of air and ocean transportation, or other fuel demands that aren’t easily electrified.
In Conclusion
With Heliogen’s innovative technologies, CST is now able to provide high-temperature process heat and green fuels cost-effectively and at scale for the very first time. And by so doing, we will enable our customers to meaningfully address the climate impacts of humanity’s non-electricity energy consumption.
