In the ever-expanding market for waste-heat recovery methods, different approaches have been established in order to combat the latest environmental restrictions while achieving more attractive power plant efficiencies. As gas turbine cycles continue to expand within the energy market, one particular technology has seen a significant upsurge due to a number of its beneficial contributions. Supercritical CO2 (S-CO2) bottoming cycles have allowed low power units to utilize waste heat recovery economically. For many years, the standard for increasing the efficiency level of a GTU (Gas Turbine Unit) was to set up a steam turbine Rankine cycle to recycle the gas turbine exhaust heat. However, the scalability constraints of the steam system restrict its application to only units above 120MW.
HRSGs (Heat Recovery Steam Generators) are water-to-steam boilers which capture the waste heat exhaust of GTUs and convert this heat into energy in the form of high-pressure, high-temperature steam. These systems can exist in a single or modular fashion depending on the scope of the project. Modular HRSGs consist of any number of low pressure, intermediate pressure, and high pressure sections. Each section allows for the extraction of gas turbine exhaust heat using separate steam drum and evaporator sections. Even in a single pressure HRSG combined cycle, the immense amount of auxiliary equipment, the high installation costs, and the frequent maintenance necessary for such a system prevent them from providing viable heat recovery for low power GTUs.
With the introduction of a different fluid, gas turbines of small and medium size are able to utilize waste heat recovery. Unlike steam, a supercritical CO2 system is designed to lie in the simply in the gaseous phase. This single-phase fluid design removes the boiling process necessary for a steam system and therefore results in higher fluid temperatures and cycle efficiencies. As well, the high energy density reduces the system component’s size and cost, and offers higher system efficiencies, reduced footprints, and significantly easier installation methods. While all these advantages do exist within a supercritical CO2 system, working with a relatively new fluid presents different challenges that have not had the time and exposure with engineering experts as steam and gas systems have. In particular, developing a turbine that will most efficiently run under this new fluid presents perhaps the tallest demand within the supercritical cycle. The task becomes to embrace these challenges for the benefit of higher efficiencies, lower O&M costs, and reduced greenhouse emissions.