Elsevier

Energy and Buildings

Volume 42, Issue 12, December 2010, Pages 2457-2464
Energy and Buildings

Performance analysis of liquid desiccant based air-conditioning system under variable fresh air ratios

https://doi.org/10.1016/j.enbuild.2010.08.027Get rights and content

Abstract

In conventional air-conditioning system, fresh air volume is always restricted to save energy, which sacrifices indoor air quality (IAQ) to some extent. However, removing the latent load of air by liquid desiccant rather than by cooling is an alternative way of reducing energy consumption. Therefore, IAQ can be improved by increasing the volume of fresh air introduced into an air-conditioning system. In this paper, a liquid desiccant based air-conditioning system is studied, with the system performance under various fresh air ratios analyzed using simulation tests. In addition, the proposed system and a conventional system are compared. In the proposed system, with the increase in fresh air ratio, the heating load for solution regeneration rises, the dehumidification efficiency increases and the regeneration efficiency drops. The coefficient of performance (COP) of the liquid desiccant based system decreases sharply when the fresh air ratio exceeds 60%. The results also show that the proposed system can save power notably. The maximum power saving ratio is 58.9% when the fresh air ratio is 20%; however, the ratio drops when the fresh air ratio increases. These findings will be beneficial in the selection of fresh air ventilation strategies for liquid desiccant based air-conditioning systems.

Introduction

In conventional air-conditioning systems, the dominant way of treating air is to simultaneously dehumidify and cool it. The supply air is cooled by coolants with temperatures below the dew point of the air to remove the moisture. Latent load removal needs a much lower temperature than sensible load removal. The cold and dry air after cooling must always be reheated, which leads to considerable energy waste. In addition, the fresh air volume is always restricted to save energy, and mold is apt to breed in the damp conditions unavoidably caused by condensation, both of which deteriorate indoor air quality (IAQ). Removing the latent and sensible loads separately is believed to be a good way of reducing power consumption and improving IAQ in conventional air-conditioning systems [1]. There are several means including solid desiccant, liquid desiccant and membrane dehumidification can be used to remove the latent load of air independently [2], [3], [4].

As an energy-efficient, environmentally friendly and healthy means of air dehumidification, the liquid desiccant system can be used to achieve the decoupling of the air latent load and sensible load removal. The driving force of moisture removal by the liquid desiccant is the water vapor pressure difference between the desiccant solution and the air. The moisture in air is absorbed by the strong desiccant solution rather than condensed by low temperature coolants. The energy saving makes more fresh air available in the supply air. In addition, the sensible load can be removed by relatively high temperature coolants, which means that damp conditions can be avoided and the performance of chilling equipment can be improved. Moreover, the desiccant solution itself has certain sterilization effect, which also improves IAQ.

The liquid desiccant system using low-grade heat resource was proposed by Lof [5], and its application in air-conditioning systems has been widely investigated [6], [7]. Dai et al. conducted an experimental and theoretical study of a hybrid system consisting of liquid desiccant, evaporative cooling and vapor compression air-conditioning. They showed that the performance improved as a result of changes to the inlet air states entering into the vapor compression air-conditioning system [8]. The thermodynamic performance of a hybrid absorption refrigeration and liquid desiccant system was studied by Ahmed et al. [9], the coefficient of performance was about 50% higher than that of a conventional vapor absorption machine. Oliveira et al. [10] studied an air-conditioning system with novel needle impeller rotors, liquid dehumidifier and heat pipes, and their results showed that the coefficient of performance without indoor air recirculation was always higher than 0.7.

Most of the aforementioned research analyzed the performance of liquid desiccant in latent load removal. However, the performance of the liquid desiccant based air-conditioning system when more fresh air is introduced has seldom been studied. There is no doubt that more fresh air can be introduced if liquid desiccant is used for dehumidification (latent load removal) in air-conditioning, which is beneficial to IAQ. However, the performance of the liquid desiccant based air-conditioning system may vary with the increase in fresh air volume. In this paper, a liquid desiccant based air-conditioning system operating under typical climate conditions in Hong Kong is studied through a mathematical approach. Variations in the solution heating load, sensible load, efficiency of the dehumidifier and regenerator, and the overall performance of the system under different fresh air ratios are simulated. The energy saving potential compared with a typical conventional air-conditioning system with primary return air is obtained.

Section snippets

System description

The liquid desiccant based air-conditioning system proposed in this study is shown in Fig. 1. The dehumidifier and regenerator used in the system are adiabatic. They have been well developed and are widely used. The air flows upward and the solution flows downward in the packing of the dehumidifier and the regenerator: that is, the air-solution is under counter flow. The heights of the dehumidifier and the regenerator are 1 m and 0.5 m respectively, and the transverse section areas are both 1 m2.

Results and discussion

First, the states of the mixed air (C) are determined, as listed in Table 2. As the states of the air to be handled are different, the corresponding latent loads and sensible loads in air handling are also varied. Fig. 4 indicates that, both the sensible load and the latent load increase with the increase in the fresh air ratio. However, the trends of the increments of both loads are not the same. The increment of the latent load is much sharper than that of the sensible load. The latent load

Conclusions

To study the performance and energy-saving potential of the liquid desiccant based air-conditioning system, simulations were conducted based on the climatic conditions of Hong Kong with fresh air ratios varying from 20% to 100% at intervals of 10%. The power consumption of the liquid desiccant based system and a conventional air-conditioning system with primary return air was compared, with the findings summarized as follows.

The desiccant solution flow rate needed in the liquid desiccant system

Acknowledgements

The research reported herein is supported by an Internal Competitive Research Grant (ICRG) of the Hong Kong Polytechnic University (G-YG49) and the Environment and Conservation Fund (ECF) of Hong Kong. The support is gratefully acknowledged.

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