Elsevier

Energy and Buildings

Volume 43, Issue 8, August 2011, Pages 1895-1903
Energy and Buildings

Performance of temperature and humidity independent control air-conditioning system in an office building

https://doi.org/10.1016/j.enbuild.2011.03.041Get rights and content

Abstract

The temperature and humidity independent control (THIC) system, which controls indoor temperature and moisture separately, may be an attractive alternative to existing conventional HVAC systems for its prominent improvement on the overall system performance and utilization of low grade energy resources. In order to verify the effectiveness of THIC system, a pilot project has been implemented in an office building in Shenzhen, China. In the system, liquid desiccant fresh air handling units driven by heat pumps are utilized to remove the entire latent load of outdoor air supplied for the whole building, and chilled water at the temperature of 17.5 °C from chiller is pumped and distributed into dry fan coil units and radiant panels to control indoor temperature. This paper presents the results of field test of the system, which shows that the system can provide a comfortable indoor environment even in very hot and humid weather. The COP of the entire THIC system can reach 4.0. According to the energy usage data recorded from the year 2009, the energy consumption of the THIC system in the tested office building was 32.2 kWh/(m2 yr), which demonstrates magnificent energy-saving potential compared with the conventional air-conditioning system (around 49 kWh/(m2 yr)).

Highlights

► Temperature and humidity independent control (THIC) system is an attractive HVAC mode. ► We test the THIC system performance in an office building in Shenzhen. ► Such system provides a comfortable indoor environment even in hot and humid climate. ► The COP of the entire THIC system reaches 4.0, much higher than conventional system. ► The THIC system demonstrates magnificent energy-saving potential.

Introduction

In the conventional HVAC system that removes moisture by condensation, air is cooled and dehumidified simultaneously. In most cases, sensible load of building covers the majority part of the whole cooling load while the latent load (moisture load) takes only a small part. However, as the required cooling source temperature of dehumidification is much lower than that of cooling, the chilled water temperature has to be reduced to meet the demand for condensation dehumidification. Moreover, the ratio of sensible load to latent load varies largely due to the changes of outdoor climate, number variance of indoor occupants, indoor equipments and lighting utilization mode and so on. Therefore, the indoor temperature and humidity, the two key parameters, can hardly be satisfied with condensation by the cooling coil only. In practice, the common reaction to the increased humidity is to reduce the set-point temperature and then re-condition the air after passing the cooling coil to the proper temperature, which results in a plenty of energy wastefulness [1].

To avoid the aforementioned problems, temperature and humidity independent control (THIC) air-conditioning system stands out as an appropriate pattern that temperature and humidity can be regulated independently with temperature control subsystem and humidity control subsystem respectively. Besides, the coil temperature for cooling in the temperature control subsystem can be considerably increased, e.g. from current 7 °C to 17 °C, so that improvement on the performance of chillers or even free cooling from ambient could be obtained.

Many investigations have been carried out on the hybrid desiccant dehumidification and air-conditioning system [1], [2], [3], [4], which integrates liquid/solid desiccant units with a conventional cooling system to avoid excess cooling. Liquid desiccant units developed quickly in recent years, for its advantages of dehumidifying at a temperature higher than the air's dew-point to avoid reheat procedure in the system, and regenerating desiccant at a low temperature which can be driven by low-grade heat sources [5], [6]. Many studies focusing on improving its performance with process optimization have been conducted in depth, such as Yadav [7], DryKor Ltd. [8], and Liu et al. [9]. Chen et al. [10] designed an independent dehumidification air-conditioning system with a hot water-driven liquid desiccant and a chiller that provides 18–21 °C chilled water for an office building in Beijing, which saved about 30% cooling cost compared with conventional system. The performance of a hybrid system tested by Ma et al. [11] was 44.5% higher than conventional vapor compression system at a latent load of 30% and this improving could be achieved by 73.8% at a 42% latent load. Besides, the specific research on the feasibility and performance of the hybrid system in hot and humid regions is promoted [12], [13].

This paper will investigate the real operating performance of a THIC air-conditioning system operated in an office building located in Shenzhen, a modern metropolis in southern China of hot and humid climate. In this THIC system, the liquid desiccant fresh air handling units driven by heat pumps are employed to handle the outdoor air to remove the entire latent load and supply enough fresh air to the occupied spaces, and the high-temperature chiller that produces chilled water of 17.5 °C for the indoor terminal devices (radiant panels and dry fan coil units) is applied to control indoor temperature. The operating principle and performance test results of the THIC system will be shown in this paper, and suggestion for performance improvement will also be included.

Section snippets

Description of the THIC system in an office building

The THIC system has been put into practice as a pilot project in an office building in Shenzhen, China. This system has been brought into operation in July 2008 and the basic information about the building and air-conditioning system goes as follows.

Indoor thermal environment

Fig. 7 shows the tested results of indoor temperatures, humidity ratios and CO2 concentrations with the outdoor temperature and relative humidity of 34.9 °C and 61% respectively. As indicated by the figure, the THIC system could provide a comfortable indoor environment with suitable thermal condition and good indoor air quality.

Fig. 8 depicts the tested temperatures and humidity ratios along the vertical direction in the vestibule. In the occupied zone (the height within 2 m), the temperature and

Discussion

According to our knowledge, cooling air can be realized more easily than dehumidification by condensation, since the latter one requires lower temperature of cooling source than the former. However, the COP of the tested temperature control subsystem is lower than or equal to that of the humidity control subsystem in present THIC system. Thus, this section will focus on how to improve the performance of the temperature control subsystem.

According to the performance of each component in the

Conclusion

The operating performance of the THIC system in an office building in Shenzhen is presented in this paper. Liquid desiccant fresh air units are used to supply dry fresh air to control indoor humidity, and chilled water with the temperature of 17.5 °C is pumped and distributed into radiant panels and dry fan coil units to control indoor temperature. The followings are the conclusions based on the tested results:

  • (1)

    The THIC system can provide a comfortable indoor environment that indoor temperatures,

Acknowledgements

The research described in this paper was supported by National Natural Science Foundation of China (No. 51006058) and the foundation for the author of National Excellent Doctoral Dissertation of China. The authors appreciate the valuable help from Mr. Qiang Bin and Mr. Yang Haibo in Shenzhen.

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