Decarbonisation Technology August 2022 issue

duration storage – for example, three to four hours. As such, PV technology is largely limited to supplying a base load of power while the sun is shining. In comparison, CSP can generate thermal energy by concentrating the sunlight on a collection point. In principle, CSP can generate temperatures greater than 2,000°C. However, for practical applications, receiver temperatures operate in the range of 500°C to 1,000°C. That thermal energy is then commonly used in a CSP plant to generate electricity via technologies such as steam turbines, Organic Rankine cycles (ORC), or supercritical carbon dioxide (sCO₂) cycles. In general, higher operating temperatures are desirable because the overall conversion efficiency from light to heat to electricity is higher. While a CSP plant can easily provide on- demand base-load electricity while the sun is shining, to fully leverage the capacity of this technology, there are strong drivers to be able to store the thermal energy generated during daylight hours. The objective is to have enough thermal storage so electricity can be produced even when the sun is not shining over a period of days or longer. Today’s commercial CSP LD-TES plants commonly use molten salt to store the thermal energy collected from the sun. A key limitation of this configuration, though, is the overall conversion efficiency that can be realised due to the operating temperatures practically achieved with molten salts. In addition, there are safety, environmental, and operating constraints with molten salt that need to be considered. This is driving the development of CSP plants that use other types of media for storing thermal energy. Within this development arena, there is a growing preference for CSP LD-TES based on solid particles. This is primarily because solid particles can withstand temperatures greater than 1,000°C without decomposition. They are also inert, do not contain any unusual corrosion mechanism, and their erosion characteristics scale with temperature. In addition, in the event of a system leak, particles will not cause hazards beyond the initial transient high temperatures. As a result, the systems do not require hermetic seals

Solids in

Air (or gas) out

Air (or gas) in

Solids out

and are compatible with silo and storage technologies that exist already. While the particle-handling technologies must be designed to withstand high temperatures and minimal heat losses, these are manageable engineering challenges and not fundamental challenges, at least up to around 1,000°C. And in the case of a cool-down event, particles can still flow and will not freeze, as is the case with molten salt. These types of particle-based systems, when coupled with a CSP plant where the heliostat field heats up the particles directly as they fall into the hot storage silo, are an elegant solution for LD-TES that requires only a single MBHE to transfer the thermal energy from the hot solids to the working fluid used in the power generation block. MBHE design considerations Silica sand is emerging as a preferred working media to store renewable energy for extended Figure 2 If the working fluid is a gas, then a vertical tube heat exchanger is the preferred design