Award Date

5-2009

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering

Department

Mechanical Engineering

First Committee Member

Robert Boehm, Chair

Second Committee Member

Woosoon Yim

Third Committee Member

Samir F. Moujaes

Fourth Committee Member

Daniel Cook

Graduate Faculty Representative

Eugene McGaugh

Number of Pages

180

Abstract

Recently, the Gas Engine-Driven Heat Pump (GHP) System has become an economic choice and more attractive climate control system than the conventional air conditioner due to its advantage in reducing fossil fuel consumption and environmental pollution. The GHP is a new type of heat pump in which the compressor (the core part) is driven by a gas engine. The GHP typically uses the work produced by the engine to drive a vapor-compression heat pump. At the same time, the waste heat rejected by the engine is used for heating purposes.

To improve the system performance of the GHP, a numerical and experimental study has been made by using suction-liquid line heat exchangers in cooling operation (particularly in high ambient operations) and suction line waste heat recovery to augment heating capacity. Detail experimental and modeling of a GHP in high ambient operating conditions using R410A as a refrigerant is firstly included in this study.

Computational fluid dynamics (CFD) techniques have been used to study the design of the heat exchangers to improve system performance during heating and cooling operations with refrigerant R-410a and ethylene glycol as the working fluids. Seven cases were investigated to obtain the optimal operating mode. For the first four cases, the operating fluids in the tube side (vapor refrigerant R410 A) and the shell side (aqueous ethylene glycol) were kept the same while the inlet temperature and mass flow rate for the tube and shell sides are changed in different cases. For the last three cases, the operating fluids on the shell side are changed to liquid refrigerant R410A. The numerical results show that although the effectiveness of the shell tube exchanger is small due to the small thermal conductivity of vapor refrigerant R410A, the goal of this numerical study still has been reached and over 30,000 Btu/hr heat exchange has been obtained with the current heat exchanger configuration. The output from the CFD analysis, total heat transferred and pressure drop, are used as an input to the overall GHP modeling.

The performance of overall GHP system has been simulated by using ORNL Modulating Heat Pump Design Software, MODCON, which is used to predict steady-state heating and cooling performance of variable-speed vapor compression air-to-air heat pumps for a wide range of system configuration and operational variables. The modeling includes: (1) GHP cycle without any performance improvements (suction liquid heat exchange and heat recovery) as a baseline (both in cooling and heating mode), (2) the GHP cycle in cooling mode with the suction to the liquid heat exchanger incorporated, (3) GHP cycle in heating mode with heat recovery (recovered heat from engine). According to the system simulation results, a performance gain by using suction liquid line heat exchanger is obtained especially at higher ambient conditions. The waste heat of the gas engine can take about 20-25%, of the total heating capacity in rated operating condition. The ambient temperature affects the performance of the heat pump but has little influence on the engine efficiency in the constant engine speed model. Because of the limitation of speed, the GHP still needs extra equipment to back up the heating in extremely low ambient temperatures.

The modulating heat pump model was compared to experimental trends with respect to compressor speed and the basis of coefficient of performances (COPs) and capacities. The experiment was conducted with use of a psychrometric test facility at Oak Ridge National Heat pump Laboratory. The trends in COP and capacity were generally well predicted. The results of the absolute comparisons over a range of speeds and ambient conditions indicated that best model agreement was obtained at lower speeds in both the heating and cooling modes, with increasing performance over predictions (to maximums of about 10% in both COP and capacity) occurring at higher speeds.

Finally, a comparison of applications of GHP with its most common counterparts, an electrical DX heat pump, in a 5000 ft2 office building was made in two typical locations (Las Vegas and Chicago) with using thermal simulation software. According to the comparisons, a primary energy saving (10.6% for the Las Vegas simulation and 22.6% for the Chicago simulation less than its nearest alternative) as well as much less CO 2 emissions (26% for the Las Vegas simulation, and 59.9% for the Chicago simulation less than its nearest alternative) for a GHP system were found.

Keywords

Air conditioning--Efficiency; Air conditioning--Equipment and supplies; Computational fluid dynamics; Energy conservation; Heating and ventilation industry

Disciplines

Applied Mechanics | Heat Transfer, Combustion | Mechanical Engineering

Language

English

Comments

Signatures have been redacted for privacy and security measures.


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