Elsevier

Energy and Buildings

Volume 42, Issue 11, November 2010, Pages 2014-2020
Energy and Buildings

Analysis on the ideal energy efficiency of dehumidification process from buildings

https://doi.org/10.1016/j.enbuild.2010.06.008Get rights and content

Abstract

As dehumidification is one of the most important tasks of environment control of the building, it is necessary to know the energy efficiency of dehumidification processes. The energy efficiency can give the energy cost of drawing moisture from indoor air to the outside environment. This paper presents analysis of the ideal cost of dehumidification process by a liquid desiccant cycle. Formulas to calculate ideal efficiency of dehumidification process are obtained, which is determined by indoor temperature, outdoor temperature, and the temperature of the intersection point of the iso-relative humidity line of indoor air and the iso-humidity ratio line of outdoor air. The ideal efficiency of the condensing dehumidification method is lower than the ideal dehumidification process, due to the fact that condensing dehumidification method must dehumidify the air at the temperature of dew point. Results from this paper can be used as theoretical foundation for the further analysis of various dehumidification methods and the development of new dehumidification processes.

Introduction

Latent load covers 20–40% of the total air conditioning load. Up till now, there are various kinds of dehumidification methods, such as condensing method, compressing method [1], and adsorption or absorption method [2], [3], [4]. Condensing method dehumidifies the air by cooling the air below its dew point, in which moisture being condensed out from the conditioned air. Compressing method dehumidifies the air by compressing it to increase the pressure of the vapor to the saturate pressure and to condense the moisture out. Adsorption or absorption method dehumidifies the air by sorption and regeneration cycle of hygroscopic material including solid desiccant and liquid desiccant [2]. Lazzarin and Castellotti [5] and Li et al. [6] studied the performance of liquid desiccant dehumidification process, in which heat pump was included in the liquid desiccant system. The cooling capacity from evaporator was used to cool liquid desiccant in order to enhance the desiccant's absorption ability, and the heat from condenser of heat pump was used to regenerate liquid desiccant. Similar liquid desiccant system will be adopted in this paper to analyze the dehumidification performance from buildings.

All the dehumidification methods can be regarded as a machine (dehumidification equipment) operating between two humidity sources. Take the dehumidification process from buildings to the outdoor environment as an example, one humidity source is the indoor environment and another source is the outdoor climate. The dehumidification equipment works as a pump to carry humidity between the two sources, which is similar to a heat pump working between two heat sources.

As the Carnot's theorem being used to analyze refrigeration cycle (or heat pump cycle), a similar theorem is required to analyze various dehumidification processes. The condensing method is the most popular dehumidification method, so the cooling capacity of the dehumidification process is usually considered as the thermodynamic input of the process. However, those considerations do not have sufficient theoretical support. As moisture control is one of the most important tasks of environment control of the building, it is necessary to know what the ideal process is and what the ideal efficiency a dehumidification process can achieve. Hence a thermodynamic analysis which tells the ideal efficiency of all the dehumidification methods is needed. The results of the analysis can indicate whether the method can be realized and what efficiency can it achieve.

Based on the basic dehumidification process of removing moisture from buildings to the outdoor environment, this paper focuses on setting up a theorem to analyze the dehumidification process. A simple ideal dehumidification process is then proposed, and the minimum energy input of this process is obtained. Based on the process analysis, formula for the thermodynamic cost of dehumidification process is introduced. The ideal dehumidification process is based on liquid desiccant dehumidification and the analysis extended to the other dehumidification methods. The analysis results are used as guidance for the comparison of various dehumidification processes.

Section snippets

Ideal dehumidification process and its efficiency

The process to dehumidify moisture from buildings to outdoor environment is described in Fig. 1, where Δw is the water vapor released by the humidity source of the building. The moisture comes from indoor occupants, plants, wet surfaces, infiltration, cooking, laundry, etc. The states of indoor air and outdoor air are (TA,wA) and (To,wo), respectively. The aim of dehumidification process is to remove Δw from indoor air to outdoor environment.

The ideal process requires the minimum energy cost.

Discussion of the ideal dehumidification process

Two aspects affect the ideal energy efficiency of the dehumidification process. First, the heat released from liquid desiccant absorption process does not equal to the latent heat of water vapor. This will affect the iso-concentration line of liquid desiccant and drive it away from coinciding with the iso-relative humidity line of air. However, this does not affect the result much [7], because the liquid desiccant is only an operating media, and the above result is not based on the physical

Comparison of different dehumidification methods

At the beginning of this paper, some dehumidification methods have been mentioned. The ideal energy efficiency of each dehumidification method can be calculated. Due to their inherit limit, it is difficult to reach the ideal efficiency for those methods. Therefore some typical dehumidification methods are analyzed here based on the result gotten from above paragraphs.

Conclusion

Dehumidification is very important in building environment control. There are many dehumidification methods. It is necessary to know the ideal energy efficiency of dehumidification process. In this paper the model of dehumidification process is described. A cycle based on liquid desiccant as operating media is then setup to realize the ideal reversible dehumidification process. The ideal dehumidification efficiency is then obtained, which can be used to analyze the thermodynamic cost for all

Acknowledgement

The research described in this paper was supported by National Natural Science Foundation of China (No. 50778094).

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