Experimental and analytical evaluation of exhaust air heat pumps in ventilation-based heating systems

Abstract

The application of exhaust air heat pumps as an energy efficient heating technology to cover the heating and ventilation demand of highly efficient residential buildings is becoming popular. Available studies on exhaust air heat pumps tend mostly to focus only on comparing different technologies in efficient buildings. Most of the existing studies ignore the usual presence of the electrical heaters as backup. Moreover, the impact of varying boundary conditions on the heat pump's performance is often not considered in depth. In this sense, there is still a need for discussion on the influence of different buildings' standards and control strategies on heating performance. The present paper aims to consider the energy efficiency of exhaust air heat pumps under different operating conditions. In this study, the results of a long-term field monitoring are utilized to model the dynamic behavior of an exhaust air heat pump in MATLAB/Simulink using black box modeling approach. The impact of different boundary conditions such as outdoor and exhaust air conditions on the heat pump's efficiency is studied and additionally compared to other studies. Moreover, a physical approach to model the defrosting process of the evaporator was developed and integrated into the black-box model; to use annual simulations of ventilation-based heating systems in three different building standards. Finally, the influence of five different control strategies on system performance is investigated. The core findings of this study reveal that: (1) Due to the limited exhaust air volume flow rate the impact of the exhaust air moisture content with its condensation enthalpy on the HP performance is significant. (2) In designing ventilation-based heating systems with exhaust air heat pumps due to their limited power (e.g. 6 Wh/m³ at A-2), the control strategy of backup electrical heaters must be selected carefully. (3) Conventional heating control methods such as Nighttime Temperature Reduction (NTR) following a reference room are hardly suitable in ventilation-based heating systems. (4) However, employing the deployed control strategy, the electrical energy consumption of the system could be reduced up to 40% compared to the conventional heating control methods.