Work

Water heater storage heat pump cycle for higher operating range

Year: 2024

Type: article

Abstract: Heat pump water heater (HPWH) systems offer higher energy efficiency as compared to traditional methods like electric resistance heating and are better suited to reach decarbonization goals than natural gas fired systems. However, vapor compression HPWH systems are restricted to operate within a certain range of air (heat source) and water (heat sink) temperatures. These temperature limits are specified to protect the compressor from reaching an excessive compression ratio and discharge temperature that can damage the equipment. State of the art HPWH systems aimed at improving the operating range implement two-stage compression with relatively complex system architecture or use less efficient back-up electric heating elements. In this work, we propose a new storage heat pump concept for improving the operating temperature range by the means of energy storage at an intermediate temperature level. Performance of an R134a HPWH system with a wrap-around condenser coil operating on the storage heat pump cycle is determined using an experimentally validated system model which combines a vapor compression cycle model with a water tank model. The system operates in two modes to achieve a higher temperature lift when compared to a conventional heat pump system. Mode I operates as a typical air source HPWH while Mode II splits the wrap-around coil to use the top portion as the condenser and bottom portion as the evaporator. In Mode II, intermediate temperature water from the lower tank region is used as heat source for the evaporator. The storage heat pump system could achieve an additional increase of 12°C beyond the maximum possible water temperature of 46°C for the baseline conventional heat pump system. The system showed a 20% higher average heating capacity and a 15% lower average pressure ratio in Mode II than a conventional system operating at 27°C ambient temperature. The system performance is also compared to a conventional system that uses back-up electric heating elements. The storage heat pump system takes a longer time while showing 50% lower energy consumption than usage of electric heating elements for achieving the same increase in water temperature.

Source: Applied Thermal Engineering

DOI: 10.1016/j.applthermaleng.2024.124623

Authors: Purav Patel, Nenad Miljkovic, Stefan Elbel

Institutions: University of Illinois Urbana-Champaign, Kyushu University, Creative Thermal Solutions, Technische Universität Berlin

Language: English

Topic: Refrigeration and Air Conditioning Technologies

Subfield: Mechanical Engineering

Field: Engineering

Domain: Physical Sciences

SDGs: Affordable and clean energy, Climate action

Open Access status: hybrid

Estimated APC paid: $3,790