Conference Agenda

Heat pump water heater products are attracting increasing attentions due to energy-saving feature and continuous improvement in energy efficiency standards. Control logic is a key factor affecting the energy efficiency of heat pump water heater, besides refrigeration cycle design, compressor choice, refrigerant selection, etc. The traditional development method of control logic requires repeated experiments and consumes a lot of time and labor resources. In order to rapidly and efficiently develop control logic for heat pump water heater, a model based control development method is proposed in this paper. A dynamic simulation model of heat pump water heater is established and validated first, which includes a simplified model of the heat pump and a stratified model of the water tank. The heat pump water heater model is used to predict the power, energy consumption, sensor temperature of the water tank, and outlet water temperature during the operation of the heat pump water heater. An energy-saving control logic and a traditional control logic are then compared based upon the proposed dynamic simulation model of heat pump water heater. Simulation results showed a 15.7% reduction in energy consumption compared to traditional control logic. The dynamic simulation model completed a performance test of 7 days through 10 minutes of calculation, which significantly improves the efficiency of control development for heat pump water heater.

A 120 V heat pump water heater (HPWH) is a direct plug-in option to replace gas water heater (WH) without needing expensive electric panel upgrade to 220 V. To enable the smooth transition, the HPWH should provide comparable water heating capacity as the gas WH. WH capacities are rated in the form of first hour rating (FHR). Typical home gas WHs have FHRs > 65 gallon with a 40-gallon water tank. It imposes a major challenge on 120V HPWHs. Most 120V circuits in US can provide 1,800 to 2,400W, not adequate to drive electric resistance heat to boost FHRs. Thus, all the heat needs to come from the heat pump with its top power below 1500 Watts, which is constraint by the installation footprint.

This study uses a hardware based, HPWH design model, i.e. the DOE/ORNL Heat Pump Design Model to design a 120V unit with a brazed plate condenser, fin-and-tube evaporator and an adequately sized compressor. To maximize the FHR, multiple strategies were simulated, including use of a mixing valve, overheating the tank temperature to 140F, a new sensing method for quicker response, and an innovative water circulation path. We also simulated 24-hour unform energy factors (UEF) to show the tradeoff between the capacity and operation efficiency.

Transition to lower GWP refrigerants in the US is expected to accelerate driven by the largest CO₂ ­allowance reduction of 30% in 2024 and the finalized technology transition rule. This trend is also true for heat pump water heaters and clothing dryers with their expected growth in adoption due to decarbonization efforts as well as DOE’s proposed minimum efficiency regulation for water heaters. To that end, three low GWP alternatives, R-456A, R-444A, and R-485A, will be discussed and compared against incumbent HFCs. Refrigerant properties, cycle performance, and lubricant selection results will be presented. Furthermore, the benefit of utilizing zeotropic blends with glide and its positive impact on system efficiency will also be explored.

Water heating constitutes approximately 18% of energy consumption and is the second largest energy expense in United States homes. Heat pump water heaters (HPWHs) are energy efficient technologies with lower carbon footprints as compared to conventional water heating technologies, such as gas and electrical resistance heaters. The performance of water-source HPWHs can be improved by recovering heat from blackwater using drain heat recovery heat exchangers. Depending on water draw patterns of single family and multifamily residences, the operation and heat transfer performance of these heat exchangers are highly transient. This paper examines the thermo-hydraulic performance of a drain heat recovery heat exchanger during its transient and steady-state operations. The heat recovery performance of the exchanger was evaluated at different inlet temperatures and flow rates ranging from 9°C to 36°C and 0.03 kg/s to 0.28 kg/s, respectively. The obtained effectiveness varies from 45%–85% depending on the operating conditions. The test facility, steady-state and transient experimental procedures are reported in detail. Further, the effect of operating conditions on the effectiveness is discussed. The experimental approach and results of the study will provide insights into the transient operation of drain recovery heat exchangers as well as, guide sizing for various steady-state conditions.

Growing awareness of the potential environmental impacts of various refrigerants has led to the phasedown of hydrofluorocarbon (HFC) refrigerants and initiatives replacing HFCs with hydrocarbons or other environmentally friendlier fluids. This study evaluated the performance of R290 (propane) as a substitute for R134a (an HFC) for heat pump water heating (HPWH). Comprehensive experimentation is performed to predict the performance of propane as a refrigerant for a domestic HPWH. Key performance parameters such as unified energy factor, first-hour rating, condenser discharge temperature, thermal stratification in the water tank, and total refrigerant charge have been established and compared to the baseline system. A drop-in-replacement study indicates that the system can provide reasonable performance with substantially reduced refrigerant charge. To further reduce the refrigerant charge in the system, a comprehensive work plan has been developed which focuses on individual components including the evaporator, compressor, and condenser. The study aims to develop a roadmap for next-generation heat pump water heaters compatible with hydrocarbons as refrigerants and can be deployed safely for domestic applications.