High pressure and high-temperature gas handling is a critical challenge in both Underground Coal Gasification (UCG) and modern solar thermal energy systems. In UCG, synthesis gas is produced deep underground under extreme thermal and pressure conditions, requiring efficient and reliable cooling before surface utilization. Similarly, solar thermal systems such as concentrated solar power (CSP) plants generate high temperature working fluids that demand advanced heat exchanger technologies for energy recovery, storage, and power generation. Conventional heat exchangers often show limited thermal performance and reduced reliability under such severe operating conditions. This review presents a comprehensive assessment of augmented and membrane based heat exchanger technologies developed for high pressure gas applications, with a focus on their use in underground coal gasification and their potential extension to solar thermal systems. The review discusses heat transfer enhancement mechanisms, geometric configurations such as membrane helical coils and serpentine tubes, numerical and experimental studies, and material considerations. Common challenges and design requirements in both UCG and solar thermal applications are highlighted. Finally, key research gaps and future opportunities are identified to support the development of efficient, reliable, and sustainable heat exchanger systems for combined fossil and renewable energy applications.

Underground coal gasification, solar thermal energy, high pressure gas, membrane heat exchanger, heat transfer enhancement