Modeling and Control of a Multi-Mode Heat Pump

Modern heat pumps have evolved significantly in the last 50 years to provide energy efficient cooling and heating in a broad range of conditions. However, one consequence of improved building codes and higher energy efficiency standards is that heat pumps can struggle to maintain comfortable levels of indoor humidity in some conditions and applications, especially in humid climates. This paper considers modeling and control of a residential heat pump system that can operate in several modes, including a conventional cooling mode and a reheat mode. In the reheat mode, condensed (warm) refrigerant is passed through an indoor heat exchanger to reheat the conditioned air. The configuration and action of feedback reduces the evaporator temperature, resulting in an increased rate of water condensation and reduced indoor relative humidity level. A control algorithm is proposed that coordinates the actions of the variable speed compressor, electronically actuated expansion valves and variable speed fans to achieve both indoor air temperature regulation and indoor humidity regulation. The algorithm includes hybrid logic to switch among operating modes. Simulations of the multi-mode heat pump coupled coupled with a dynamic model of a typical residential building located in a humid climate zone demonstrate both temperature and humidity regulation.