Decarbonisation Technology August 2022 issue

periods because it is abundant, affordable, stable, non-toxic, and can be heated up to high temperatures without degradation. In addition, the hot sand can be easily stored in silos at high temperatures. Other materials also considered suitable for these applications include alumina, bauxite, magnetite, and engineered ceramics. MBHEs are used to recover thermal energy from the hot sand and transfer it to the Figure 3 Model of a diffusion- bonded heat exchanger core that is designed for a particulate material on the solid side and CO₂ on the fluid side

example, air – a vertical tube heat exchanger is the preferred design (see Figure 2 ). In situations where sCO₂ is the working media, a diffusion- bonded heat exchanger core is preferred because of its ability to accommodate the pressure and temperature needed for high heat exchange efficiency (see Figure 3 ). Operating temperatures of MBHEs for CSP plants are limited by the thermal stability of the materials of construction, which today is mainly high-temperature- resistant steel such as 316 stainless steel. Nickel-based alloys can also be used, but come with increased capital cost. To extend the operating boundaries beyond what is practical to achieve with today’s technology and materials in the

working media of the CSP power generation block – for example, sCO₂. These MBHEs are compact, efficient, and simple to operate as gravity is used to flow the sand through the heat exchanger. With the low velocity of the particles in these MBHEs, the natural abrasion rates on the heat exchanger surfaces are minimal, resulting in high availability and a long lifetime. Depending on the working media, the MBHE configuration and design will vary. If the working media is a liquid, the most common option involves the use of vertically oriented pillow plates. If the working fluid is a gas – for

drive to improve the efficiency of CSP plants, MBHEs constructed from ceramic materials are also being developed. LD-TES in action Canadian-headquartered Solex Thermal Science, which specialises in thermal and bulk materials engineering, is seeing increased applications of its MBHE technology within a number of CSP LD- TES project developments around the world. Together with US-based research organisation Sandia National Laboratories and Vacuum Process Engineering (VPE) of Sacramento, Calif.,

HT solar receiver

Hot storage

Solid particle loop

Heat exchanger

Generator

Solar eld

Turbine

Cold storage

Figure 4 Process diagram of a concentrated solar plant with solid particle energy storage