Constructing year-round energy-efficient fresh air handling unit with exhaust air heat recovery using multi-condition design

Traditional fresh air handling units with exhaust air heat recovery insufficiently utilize exhaust air waste energy, and are designed under the design-load condition, which are unable to efficiently adapt to the varying fresh air conditions for both the energy consumptions of compressors and fans. To promote energy efficiency, a fresh air handling unit with exhaust air heat recovery is constructed in this study by grade matching of loads and waste and natural energies under different typical fresh air parameter conditions, and the improved year-round efficient system configuration is determined by combining the configurations obtained under each typical condition. In addition, numerical models are adopted to calculate the energy and economic performances of the system in an office building in Nanjing, China, and a traditional system is also analyzed for performance comparison. The main conclusions are as follows: 1) The improved system configuration is determined to be a heat-recovery device plus a two-stage heat pump that can operate close to a separately designed configuration under various cases, thereby ensuring efficient operation throughout the year; 2) Compared with a traditional fresh air handling unit for fresh air load treatment with a variable refrigerant flow system in Nanjing, China, the annual saving rate of the proposed system is 14.4 %; 3) Compared with the traditional system, the static payback period is 1.1 years.