Objective Against the backdrop of flexible operation and deep peak shaving, improving the energy efficiency of thermal power units under low-load conditions is a critical issue to be addressed.

Method The study, based on the exergy analysis method, investigates the exergy efficiency and exergy loss of the heat regenerative system in a supercritical secondary reheating unit under six load conditions ranging from 20% to 100%.

Results The results indicated that under the 100% load condition, the exergy efficiencies of all high-pressure heaters were above 92%, with #3 high-pressure heater having the lowest (92%). In contrast, the exergy efficiencies of all low-pressure heaters were below 89%, with #11 low-pressure heater being the lowest (65.3%). As the load decreased from 100% to 20%, the total exergy efficiency of the heat regenerative system dropped by 4.3%. With the exception of #11 low-pressure heater, the exergy efficiencies of the other heaters showed a downward trend as the load decreases, with #10 low-pressure heater exhibiting the largest decline (11.2%). Conversely, the exergy loss rates of the #5 deaerator, #6–#9 low-pressure heaters, and the Out1 and Out2 external steam coolers increased, with Out2 showing the most significant rise (7.6%). This suggested that under low-load conditions, these seven components were the primary sources of increased system exergy loss. Across various loads, the #1 and #3 high-pressure heaters, #6 low-pressure heater, and Out1 external steam cooler consistently exhibited high exergy loss rates, indicating substantial potential for energy savings.

Conclusion The research provides a theoretical basis and data support for the energy-saving optimization of thermal power units operating under low-load conditions in the context of flexible peak shaving.