Decoupling dehumidification and cooling for energy saving and desirable space air conditions in hot and humid Hong Kong

Abstract

The combined use of dedicated outdoor air ventilation (DV) and dry cooling (DC) air-conditioning system to decouple sensible and latent cooling for desirable space air conditions, better indoor air quality, and energy efficiency is proposed for hot and humid climates like Hong Kong. In this study, the performance and energy saving potential of DCDV system in comparison to conventional systems (constant air volume (CAV) system with and without reheat) for air conditioning of a typical office building in Hong Kong are evaluated. Through hour-by-hour simulations, using actual equipment performance data and realistic building and system characteristics, the cooling load profile, resultant indoor air conditions, condensation at the DC coil, and energy consumptions are calculated and analyzed. The results indicate that with the use of DCDV system, the desirable indoor conditions could be achieved and the annual energy use could be reduced by 54% over CAV system with reheat. The condensate-free characteristic at the DC coil to reduce risk of catching disease could also be realized.

Introduction

Hong Kong is of subtropical climate. The humid outdoor air combined with the ventilation requirement increases the latent load significantly and hence the energy use for air-conditioning [1], [2]. Conventional air-conditioning systems for commercial and residential applications rely on temperature control to take care of moisture removal, and thus space relative humidity is not controlled. In the process of simultaneously meeting space cooling and dehumidification requirements, cold-coil of the air handling equipment (air handler) needs to operate at very low effective coil temperatures to enable the removal of moisture by condensation on a cold surface. As such, the conditioned supply air is typically at low temperature (10–12 °C dry-bulb) and is close to saturation (90–95% relative humidity) [3]. The space relative humidity is achieved by matching the sensible heat ratio (SHR) of the air handler with the space SHR, but a perfect match is rarely achieved both for full load and part load conditions [4], [5], [6]. This divergence between equipment and space SHR leads to high space relative humidity levels to increase health risk such as respiratory symptoms and unspecific symptoms (e.g. tiredness and headaches) [7]. It has been recommended that the supply air humidity must be maintained below the 70% level to reduce risk of catching disease [8]. Furthermore, there are concerns about the hygienic problem associated with dirt collected at the condensed water drain pan to impair indoor air quality [9]. These problems are getting serious as the trend nowadays is to raise outdoor air flow rates in order to improve indoor air quality (IAQ) [10].

In solving the moisture problems, recent research studies advocate decoupling dehumidification and cooling to make humidity control independent from temperature control in air conditioning systems for better air quality and energy performance [11], [12], [13]. This is often achieved by the combined use of chilled ceiling (or chilled beams) and desiccant dehumidification.

Chilled ceiling (CC) and desiccant dehumidification system have been widely accepted and investigated in European, America and Asian countries not only to resolve the moisture problems but also for better energy performance resulting from raised supply air and chilled water temperatures to improve the efficiency of the central cooling plant [14], [15], [16], [17]. Hao and Zhang [18] investigated the combined use of desiccant dehumidification with CC and displacement ventilation system for achieving the energy saving and thermal comfort objectives. The results revealed that in comparison to the use of conventional all-air system, the proposed system could reduce electricity consumption by 62%. Fong et al. [19] proposed a hybrid system using chilled beams, absorption refrigeration and desiccant dehumidification powered up by solar energy. It was found that the annual primary energy consumption of the solar hybrid cooling system was 36.5% lower than that of the conventional system.

However, unlike cold or moderate climate regions, because of the long, hot and humid summer with an average dew-point exceeding 22 °C, applications of CC system in Hong Kong are much more challenging for the possibility of condensation on the CC surfaces [20]. Insufficient space air movement is another problem associated with the use of CC system because heat removal and humidity control in the space relies on radiant heat transfer. Moreover, air naturally drops from CC may suppress the stratification boundary of displacement ventilation to the occupied zone to cause thermal discomfort, especially in room sections without heat sources. The results of Fitzner’s study [21] indicate that this causes unexpected reverse effects on the air quality and restricts the application of CC system to spaces with total cooling load greater than 100 W/m². Furthermore, chilled water which circulates through CC system is often generated using conventional vapor compression refrigeration and is subsequently raised to the required secondary temperature by means of a mixing arrangement which wastes energy [22].

Regarding the use of active-desiccant technology to decouple dehumidification, research results have identified the alternative use of dedicated ventilation (DV) system to treat outside air alone, while re-circulating air is treated by a separate cooling coil [23], [24], [25].

With the same concept but replacing CC by dry cooling (DC) air handler, together with DV system is an alternative method for decoupling dehumidification from cooling (DCDV system). In DCDV system, the DV coil treats all the latent load and part of sensible load simultaneously while the DC coil treats the remaining sensible load to meet the required space condition. Control of the two devices is independent.

Much research on evaluating the indoor air quality, thermal performance and energy consumption of CC system in hot and humid climates has been conducted [26], [27], [28], [29], [30], while nothing is available in extant literature on DCDV system. This study aims to investigate the feasible use of DCDV system in hot and humid climate city like Hong Kong for enhanced system performance and energy saving potential. The simulation software – EnergyPlus [31] is employed to model the system and its application in a typical office building in Hong Kong.