M. Belusko *

Institute for Sustainable Systems and Technologies, University of South Australia, Australia
Corresponding Author, martin. belusko@unisa. edu. au

Abstract

Chilled beam systems for sensible cooling enable the use of high temperature heat sinks. The use of cooling towers as a heat sink has significant advantages in dry climates due to the high coefficient of performance of these systems. However, this approach tends to only work during mild conditions due to the high wet bulb temperatures experienced in hot weather. Integrating thermal storage into the system, in which water is cooled by the cooling tower at night for use in the chilled beam system during the day, makes use of the significantly lower dry and wet bulb temperatures experienced at night. Variations of this concept have been trialled but no investigation has been made into this concept for multi storey buildings. A simulation in TRNSYS of this concept was conducted and compared to a conventional chiller system cooled by a cooling tower. Significant savings in energy and meaningful savings water were identified for most medium to high capacities of thermal storage. For a building with a floor area of 8000 m2, a storage volume of 2000 m3 would achieve a saving in energy and water of at least 50% and 12% respectively.

Keywords: chilled beam, cooling tower, thermal storage

1. Introduction

Chilled beam technology has proved an effective technology at delivering sensible cooling in commercial buildings [1]. The ability to utilise 15 — 18 oC water as the heat sink instead of the typical 6 — 7 oC has increased the coefficient of performance of chiller systems and enabled the use of low energy heat sinks such as cooling towers [2].

Cooling towers provide a cooling potential which is defined by the wet bulb temperature. There are many climates in which the wet bulb is such that chilled water at 15-18 oC is achievable. Given that cooling towers can have a coefficient of performance (COP) of the order of 40 to 60, sensible cooling loads can be significantly reduced with this arrangement. This concept was investigated for UK conditions where cooling loads are generally mild, and it was shown that a significant portion of the cooling load is possible, but not during hot conditions [2]. This method was investigated further through optimisation of cooling tower operation [3]. The performance of the cooling tower is often defined by the approach, the temperature difference between the chilled water leaving the tower and the inlet air wet bulb temperature. It was shown that a cooling tower can be optimised and achieve low approaches of 1-4 oC. However, it was also presented that even the 2 percentile design wet bulb temperatures for most European cities are 17 oC, meaning a cooling tower cannot be used in hot conditions. In semi arid climates such as southern Australia, summer conditions are dry and hot with sensible cooling the dominant cooling demand. Although an ideal candidate for cooling tower driven chilled beam systems, average wet bulb temperatures are also typically around 16 — 18 oC during cooling periods. Therefore, the scope for sensible cooling using cooling towers is limited to mild cooling load periods.

To make use of this method of cooling for hot periods, an alternative approach is proposed involving thermal storage. Particular in dry climates night time dry bulb and wet bulb temperatures are significantly lower than day time temperatures. Therefore operating a cooling tower at night when most commercial buildings are idle, and storing this chilled water for day time use, may achieve the required chilled water temperatures for chilled beam applications. Variants of this method have been trialled in large commercial buildings in Australia. The Council House Building for the Melbourne city council (CH2 building) applied shower cooling towers in combination with conventional cooling towers operating at night to store cooling in phase change materials which would be used in a chilled ceiling system during the day. This project combined a number of novel technologies, however the cooling system was unable to achieve low enough temperatures to charge the phase change system [4]. Another system, implemented in a 2 storey building in Melbourne (Kangan Batman TAFE building), consists of a 130 m3 water tank for the chilled ceiling system. A night time roof spray cooling method was applied for cooling the water [5]. Adequate water temperatures were achieved and the system was designed to meet the bulk of the cooling load. Due to a roof spraying system having a low cooling capacity this technology cannot be implemented in a multi storey building. Although roof spray systems can produce lower temperatures than cooling towers as they also utilise radiative cooling, the system demonstrates the effectiveness of storage of night time water subject to evaporative cooling.

Cooling towers are not only existing infrastructure and available for use, but also have a high cooling capacity. Secondly, particular in the Australian case, due to water conservation issues and/or peak power requirements, large storage tanks are becoming a standard feature of new commercial buildings. This trend is of particular relevance to buildings attempting to achieve high ecological ratings since cooling towers are generally the biggest consumer of water in a commercial building. Therefore the proposed system may prove a low cost solution. Consequently, a study is warranted to investigate the use of cooling towers operating at night in combination with large storage tanks to drive chilled beam systems during the day. The principle advantage of storage is that rather than providing cooling during mild conditions, cooling can be delivered during extreme conditions which reduces peak power and can improve the overall COP of the chiller system.

An additional advantage of night time operation of the cooling tower is that water temperatures below 20 oC are produced. Legionella, a major concern of cooling tower operation, does not readily grow at these temperatures [6]. Therefore, if a system was developed in which the cooling tower is dedicated to night time operation, the need for chemical treatment could be eliminated. For daytime systems as presented in [3], the chilled water for the chilled beams is usually cooled through a heat exchanger with the cooling tower water to prevent any transfer of Legionella into the chilled beam water. As investigated in [3] this heat exchanger reduces the overall effectiveness of the system. Therefore, by working with lower temperatures this additional heat exchanger may be made redundant and the cooling tower water can directly be used within the chilled beam system.

As highlighted in [3], a principle concern of using a cooling tower at low dry and wet bulb temperatures is the resultant lower cooling driving force. Therefore greater heat and mass transfer is required to achieve the same level of cooling. This results in increased energy usage of the fan as well as greater water usage through evaporation. However, by operating the tower at night sensible cooling of the cooling water is achieved. Furthermore, a cooling tower providing direct cooling to the building, no longer needs to remove the heat of the compressor, as occurs in conventional cooling tower operation. Therefore, potential exists for an overall water saving. This paper presents a simulation of this system for Adelaide, a semi arid city in Australia.