Energy modelling of district cooling system for new urban development

Abstract

District cooling technology is advantageous in warm and hot climatic regions, in that chilled water from a central refrigeration plant is delivered through a distribution network to groups of buildings. The technology is most suitable for new urban developments where system design and construction receive much freedom. With a focus on the energy use, this paper outlines an energy modelling methodology and decision approach to derive the most desirable scheme for a given project. The process involves a series of building design load computation, dynamic simulation, and plant energy consumption analyses for different phases of development. A proposed scheme for the South East Kowloon Development Project in Hong Kong is quoted as an example to illustrate the approach.

Introduction

District cooling refers to coolant circulation through a distribution network between a central cooling plant and a district comprising multiple buildings. At each connection point of the distribution mains, cooling energy is delivered to the terminal devices at the user premises to meet their space/process cooling requirements. District cooling system (DCS) offers massive and collective cooling energy production, which is higher in efficiency than the conventional plants at individual premises, and allows users to utilise building space more effectively. In order to maximise its benefits to a new urban development, an optimised cooling scheme should be used. Thus, the final schemes for different urban developments may vary. Fig. 1 shows an indicative DCS flow diagram suitable for coastal cities at the tropical/subtropical regions. In these cities, like Hong Kong as one example, space cooling is required district-wide round the year. Space heating demand is comparatively negligible, even during winter time. It is the land use allocation that determines the overall DCS design capacity, and the phases of urban development that determine the growth of the DCS central plant and distribution network.

DCS comprises four basic components: the central chiller plant, the heat rejection system, the distribution network, and the consumer substation. The chiller combination aims at maximising operating efficiency (the coefficient of performance), which generally increases with the chiller capacity. This increase is up to 1500 TR, above which the COP at rated capacity becomes relatively stable. A higher supply-and-return temperature differential is able to lower the distribution pump power consumption, but will increase the heat loss at pipe surfaces [1]. Chilled water can be alternatively produced by absorption refrigeration through trigeneration technology [2], [3]. A thermal storage system with the use of chilled water, eutectic salt or ice can reduce the installed chiller capacity. Recent research work includes the use of ice-water slurries [4], [5] and phase change materials [6].

The three forms of distributing network are: radial, loop and tree-shape. These forms are usually applied in an integrated manner, forming a mixed mode of pipework distribution. The pipework layout is to overcome all physical constraints such as soil condition, geographic layout, pipe diameters, points of supply and distribution, and supply reliability [7], [8]. The use of utility tunnel increases the maintainability and security of the service. Although the applications of seawater cooling are not so common in western countries, seawater is generally used in Asian cities as a means of condenser cooling for water-cooled refrigeration systems at buildings in the vicinity of the seafront. Recent studies indicate that energy savings of around 20–30% are possible in Hong Kong versus the use of air-cooled chiller schemes [9].