Ideal efficiency analysis and comparison of condensing and liquid desiccant dehumidification
Highlights
► Ideal dehumidification process is gained by both condensing and desiccant methods. ► Unified expression of ideal COP of the two dehumidification methods is derived. ► The ideal COP of condensing method will be lower than or equal to desiccant method. ► Main performance factor of desiccant method is humidity ratio of regeneration air. ► Main performance influencing factor of condensing mode is temperature of heat sink.
Introduction
Temperature control and humidity control are two important objectives of HVAC system, which include removing cooling load and moisture load from indoor environment to outdoor environment. Commonly used dehumidification methods are condensing dehumidification and desiccant dehumidification.
In condensing dehumidification, air is cooled below its dew point, then moisture is condensed out from the air. In desiccant dehumidification, the driving force of moisture between air and desiccant is the difference in vapor pressure [1]. Desiccant can be solid desiccant or liquid desiccant, and in this paper liquid desiccant dehumidification will be analyzed specifically. During dehumidification process, vapor pressure of liquid desiccant is smaller than that of air, so moisture will transfer from air to desiccant, and vice versa for regeneration process.
There are considerable researches focusing on liquid desiccant dehumidification in several aspects, including heat and mass transfer performance of dehumidifier and regenerator by experiment [2], [3], numerical simulation and analytical calculation [4], [5], new flow patterns [6], the performance of liquid desiccant system combined with solar regenerator or evaporative cooling [7], etc. Most of researches above are based on the first law of thermodynamics and characteristics of heat and mass transfer. And from perspective of the second law of thermodynamics, particularly exergy analysis, which has appeared to be an essential tool for system design, analysis and optimization of thermal systems [8], researches are still limited. At present, concepts and principles of exergy analysis in HVAC system are proposed [9], [10]. For desiccant dehumidification, including solid desiccant [11] and liquid desiccant [6], [12], exergy is used as a tool to explore irreversible loss and indicate the potential of improvement. In particular, analysis of ideal liquid desiccant process [13], [14] helps to improve understanding about advantages and limitations of liquid desiccant dehumidification.
Condensing dehumidification and liquid desiccant dehumidification are widely used in HVAC systems. The objective of this paper is to establish a unified expression of the two dehumidification methods based on the ideal dehumidification process. Dehumidifying is one of the main tasks of indoor environment control. This paper intends to compare the two dehumidification methods in indoor environment control processes and answer the following questions: Which dehumidification method is higher in efficiency in ideal process? Which parameter is the most important influencing factor for the two dehumidification methods? What are the connections and essences of the two dehumidification methods?
Section snippets
Definition of ideal dehumidification process
The ideal dehumidification process means removing moisture per unit mass from indoor air state (TI, ωI) to outdoor air state (TO, ωO) with minimum input work. The minimum input work of the process is W, which can be divided into two parts Wc and Wd, as shown in Eq. (1). Wc is minimum input work to change the temperature of moisture per unit mass from indoor air temperature (TI) to outdoor air temperature (TO). Wd is minimum input work to remove moisture per unit mass from indoor humidity ratio (
Achieve ideal dehumidification process by condensing cycle
The difference between liquid desiccant dehumidification and condensing dehumidification is that liquid desiccant needs to be regenerated in order to complete a dehumidification circle. And for both dehumidification methods, the latent heat of removed water vapor and the input work of chiller have to be exhausted into the ambient environment. In condensing dehumidification cycle, there is no need for regeneration solution, but the heat of condenser has to be exhausted.
For condensing
Equivalent presentations of ideal dehumidification
From a thermodynamic point of view, exergy is defined as the maximum amount of work which can be produced by a stream of matter, heat or work as it comes to equilibrium with a reference environment [8]. The exergy of moist air in standard atmosphere pressure is the sum of thermo mechanical exergy and chemical exergy [16]:
The input work of ideal dehumidification to remove moisture per unit mass from indoor to outdoor
Explanation of ideal dehumidification process by exergy
The input work to remove moisture per unit mass from indoor to outdoor can be expressed as the partial derivative of exergy between outdoor air state and indoor air state, as shown in Eq. (13), which indicates the essence of dehumidification is transportation of moisture between two different humidity ratio ωI and ωO.
For liquid desiccant dehumidification, solution absorbs moisture at ωI and releases at ωO. For condensing dehumidification, transportation of moisture is achieved by chiller, which
Conclusion
This paper analyzes the minimum input work and COP of ideal liquid desiccant and condensing dehumidification. From theoretical derivation and calculation above, the main conclusions of this paper are listed as:
- (1)
Ideal dehumidification process is achieved by both condensing dehumidification and liquid desiccant dehumidification methods. The minimum input work and ideal COP of dehumidification process are derived and presented by a unified expression.
- (2)
For liquid desiccant dehumidification, the
Acknowledgements
The research described in this paper was supported by National Natural Science Foundation of China (no. 51138005) and the foundation for the author of National Excellent Doctoral Dissertation of China (no. 201049).
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