Optimum condensing temperature at part-load operation of air-cooled chillers for energy efficiency

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Optimum condensing temperature at part-load operation of air-cooled chillers for energy efficiency

 

Author: Yu, Fu-wing
Title: Optimum condensing temperature at part-load operation of air-cooled chillers for energy efficiency
Degree: Ph.D.
Year: 2004
Subject: Hong Kong Polytechnic University -- Dissertations
Air conditioning -- Equipment and supplies -- Energy consumption
Buildings -- Environmental engineering
Buildings -- Energy conservation
Department: Dept. of Building Services Engineering
Pages: xix, 258 p. : ill. ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1781120
URI: http://theses.lib.polyu.edu.hk/handle/200/3146
Abstract: Air-cooled chillers have long been used to provide comfort cooling, and the operation of these chillers accounts for significant electricity consumption in local commercial buildings. Traditionally, air-cooled chillers operate under head pressure control where the condensing temperature is kept high subsequent to the peak design outdoor temperature. This results in low chiller efficiency at part load. This research is aimed at developing a workable method of floating condensing temperature control to lower the condensing pressure and reduce compressor power, and formulating strategies for staging multiple chillers at an optimum range of efficiencies for energy saving. The objectives are achieved through a combination of field investigation, experimentation and system simulation. Four existing chilled water plants have been investigated and evaluated in this study in order to identify their operating characteristics and energy performance. This field investigation reveals that chillers are often improperly staged, and the chiller efficiency drops considerably when the chiller load or outdoor temperature decreases. An experiment has been carried out on an air-cooled reciprocating chiller in a test chamber to verify the workability of condensing temperature control with electronic expansion valves and to identify the extent of decrease in the condensing temperature. Using the operating characteristics of existing chillers, the parameters identified by regression of experimental findings and mechanistic relations between chiller components and their operating balance, an iterative static model of air-cooled reciprocating chillers is developed for plant simulation using TRNSYS. This chiller model is validated against the experimental results of the chiller test. An algorithm is introduced to compute the number of staged condenser fans by a set point of condensing temperature to simulate the chiller operation under condensing temperature control and head pressure control. The implementation of condensing temperature control involves using electronic expansion valves and putting a new algorithm into the microprocessor of air-cooled chillers to adjust the set point of condensing temperature in response to changes of outdoor temperature. An evaporative pre-cooler to pre-cool the air stream before entering the condenser is modelled using empirical equations, which will complement condensing temperature control to further enhance chiller efficiency. Using the chiller model developed and using TRNSYS to model the building and air-conditioning system, simulations are carried out to establish the load-frequency and weather-load profiles of representative office and hotel buildings. The matching between building cooling load and chiller part-load ratio in multiple-chiller systems is studied. The strategy for optimizing the efficiency of chilled water plants by proper design and operation of the systems is developed. Accordingly, the implication of this improvement in chiller performance on the growth of electricity demand by the commercial building sector is analysed. The main contribution to knowledge of this work is the development of a workable method of floating condensing temperature control applicable to make air-cooled chillers more efficient and sustainable. Building services engineers will benefit from the information disseminated on how to stage multiple chillers and pumps in the most efficient manner in the design and operation of chilled water plants. This information should help engineers evaluate different chiller plant design options for energy efficiency, and support chiller plant operators in setting up proper operating schemes for their chilled water plants to cut down electricity bills. On the legislative aspect, a scheme is proposed for setting requirements on the chiller efficiency in relation to the part-load ratio, outdoor temperature and limit of condensing temperature. The results of this study will also help chiller manufacturers apply condensing temperature control in their search for the most efficient chiller products.

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