Session: 11-10-01 Single/ Two-Phase Heat Transfer in Active and Passive Systems

Paper Number: 72389

Start Time: Tuesday, 07:30 PM

72389 - Theoretical Analysis of a Single-Stage Gas-Fired Ejector Heat Pump Water Heater 

The ejector driven system is able to be powered by low grade thermal energy, significantly reducing our fossil fuel consumption known to have negative effects on the environment. Similarly, they do not require high ozone potential refrigerants to operate, rather, steam can be utilized as the working fluid.  In addition to the environmental concerns, ejector driven systems pose an economic advantage as they require low maintenance due to their lack of moving parts. The ejector driven system can be used to generate refrigeration or pumping heat as a heat pump. Extensive research has been done to advance these systems. However, the system’s low efficiency limits its application. In particular, the critical back pressure limits the condenser temperature, directly restricting its applications.  

Ejector driven systems have the ability to operate at high efficiencies, utilizing recycled thermal energy as a power source. For a typical ejector heat pump system, the increase of the condenser temperature reduces the system COP. In addition, if the condenser temperature is higher than the critical temperature, the ejector may not function. In this situation, the condenser temperature must be reduced, and an additional heater will be utilized to heat the production water from the condenser temperature to the desired temperature. In this investigation a single-stage gas-fired ejector heat pump (EHP) is investigated and thermodynamically modeled in order to optimize the coefficient of performance (COP) for the purpose of heating water by utilizing the thermal energy from the ambient air. Utilizing the constant pressure model, a new theoretical model was developed to investigate the effects of back pressure and LTE temperature on the COP of an EHP system. Additionally, the working conditions corresponding to an optimized COP for the entire system were determined. The EHP COP was found to increase at nozzle exit pressures beyond critical conditions, however, experimental studies have shown the ejector ceases to function properly when operated under these conditions.  The effects of the high-temperature evaporator (HTE) and low-temperature evaporator (LTE) temperatures on the ejector critical back pressure and on the EHP system performance are examined for a HTE temperature range of 120-180°C and LTE temperatures of 15.5, 17.5 and 19.5°C. Results show that an optimized COP of the EHP system exist which depends on HTE and LTE temperatures, primary nozzle throat diameters In addition, it is found that the EHP COP is be independent of the ejector COP. From this investigation a maximum EHP COP of 1.53 is able to be achieved for a HTE temperature of 160°C and a LTE temperature of 19.5°C with a total heat capacity of 10,813W.

Presenting Author: Jeremy Spitzenberger University of Missouri

Authors:

Jeremy Spitzenberger University of Missouri
Pengtao Wang University of Missouri
Laith Ismael University of Missouri
Hongbin Ma University Of Missouri
Ahmad Abuheiba Oak Ridge National Laboratory
Kashif Nawaz Oak Ridge National Laboratory