Review of liquid desiccant air dehumidification systems coupled with heat pump: System configurations, component design, and performance
Introduction
Nowadays, Vapor Compression System (VCS) is the most common method used for air conditioning. VCS is a commercially available and reliable technology, although they present limited latent cooling capacity. VCS provides latent cooling through condensation by cooling the air lower than the dew point (e.g., typically lower than 13 °C). This low temperature, combined with the velocity of supplied air, generates local discomfort conditions for the building occupants [1]. Thus, VCS typically reheats the dehumidified air to comfortable temperatures before delivering it indoors. This method is applied mainly in large commercial systems. The excessive cooling for condensation-based dehumidification and the later reheating are energy wasteful.
As an alternative to VCS, Liquid Desiccants Dehumidification (LDD) has been studied. LDD systems remove water content from the air using the Liquid Desiccant (LD) to absorb water vapor in the air. After the LD absorbs water vapor in the air, it must be regenerated in a desorption process to recover its initial concentration and dehumidification potential for repeatable uses. However, absorption and desorption are not energy-free processes. They require sources of cooling and heating energy, respectively.
The earliest efforts on LDD technology from Löf used triethylene glycol for air dehumidification 70 years ago [2]. Early designs for LDD systems used high-temperature heating sources to power the regeneration process to reach a high LD concentration. These temperatures ranged between 60 °C and 140 °C [3], [4], [5], [6], [7], [8], [9]. Combustion or electric resistances were commonly used to reach high temperatures [3], [4], [5], [6], [7], [8], [9]. After regeneration, the LD at high concentration and relatively high temperature was used in the dehumidifier to remove water from the air. This meant that after dehumidification, the supply air needed significant sensible cooling [3], [4]. The use of solar energy and waste heat energy sources [3], [4], [5], [6] was studied to reduce the energy used in the regeneration process. For the dehumidification side, direct evaporative cooling and cooling towers were evaluated as a low energy consumption sensible cooling source. However, an auxiliary sensible cooling source was still needed [3], [4]. The simulation and experimental test proved the feasibility of the designs despite of complexity and large volume [3], [4].
Recently, as an improvement over the LDD systems, the use of heat pumps (HP) as cooling and heating sources has been studied [10], [11], [12], [13]. HP in LDD systems is more energy efficient than its direct use for air cooling only. The HP in LDD systems does not need to reach air dewpoint, so it operates at a smaller temperature lift with a higher Coefficient of Performance (COP). Additionally, LD in the new system operates at a lower temperature and concentration. It leads to a much lower regeneration temperature, which can be provided by the HP condenser (32 °C and 55 °C) [14], [15], [16], [17], [18], [19]. Thus, HP-coupled LDD systems (HP-LDD) can address the drawbacks of early LDD systems and make them energy efficient, compact, and simpler in construction. Nevertheless, HP-LDD systems present their own challenges in the control and load balancing of the HP. In the past 16 years, the number of studies (63 papers) have been increasing on the HP-LDD systems. Fig. 1 shows the number of papers on the HP-LDD systems found on the database Scopus. The figure shows that most of the studies on HP-LDD are modeling based.
The keywords for finding relevant papers for this review were “Heat Pump” and “Liquid desiccant”. These keywords were applied to Scopus and ScienceDirect academic search engines. The keywords selected were wide enough to try to capture all relevant papers but also provide results from papers not directly related to the goal of this review. For this reason, several search results had to be discarded manually. From the total results, the following categories were discarded to filter for only the relevant papers: papers not related to air-conditioning dehumidification, papers that only included liquid desiccant dehumidification systems without heat pumps coupling, papers related to heat pumps not including liquid desiccant dehumidification, papers that included VCS only to provide sensible cooling to the supply air not connected to the dehumidifier and regenerator. In addition to the general search described, certain sections of this review are required to look for specific terms. These sections reviewed earlier LDD systems designs and components. In these cases, the filters used are publication year and keywords such as “liquid desiccant dehumidifier”, “liquid desiccant regenerator”, “liquid desiccant storage”.
Currently, several authors have studied HP-LDD systems through models and experimentally. Additionally, some authors have included HP-LDD systems into broader review encompassing all types of LDD systems [10]. However, among the results shown, no review paper focused on HP-LDD systems could be found. Therefore, there is a need to review, summarize and systematize the findings of these authors. The review presents the state-of-the-art HP-LDD systems, existing knowledge gaps, and an outlook on the areas where further research is needed to advance HP-LDD technology. This paper is organized as follows: a review of the working principle behind HP-LDD, analyzing their different components, the possible configurations and operations modes, energy and dehumidification performance, discussing overall system integration, and finally presenting the main findings from the review, a discussion about the identified gaps, an outlook for future research based and conclusions.
Section snippets
Working principle
LDD has two mass exchange processes between LD and air: dehumidification and regeneration. The dehumidification process is the removal of water content from the air, by exposing it to LD for the absorption process to take place. As more water is added to the LD solution, it diminishes its concentration and water absorption capacity. Dehumidification, as an exothermic process, heats air and LD. This increase in temperature reduces the water vapor pressure differential between air and LD, which
Components review
The following section reviews the characteristics of the main components of HP-LDD systems. Some of these components are similar to those used in earlier LDD systems. Therefore, sources form both types of systems are included in the review.
System integration
The integration of HP with LDD systems presents three challenges: the load balancing of the HP itself, dehumidification and regeneration capacity balancing, and operation at variable outdoor and internal loads conditions. The balancing of dehumidification, regeneration capacity and operation under variable conditions are common to all LDD systems, but the combination with the HP load balancing is particularly complex.
HP-LDD systems have the same primary components, including a dehumidifier,
Findings
The paper critically reviewed all the existing studies on HP-LDD systems. Additionally, studies on LDD system and components were also reviewed. Next, the main findings of each section are summarized.
Section 3 describes the main components of an HP-LDD system.
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HP-LDD systems can present two types of dehumidifiers by construction: Packed bed and membrane-based. Of these two, packed bed dehumidifiers are the most commonly used. Additionally, the availability of a cooling source allows generating
CRediT authorship contribution statement
Tomas Venegas: Conceptualization, Data Curation, Formal Analysis, Investigation, Methodology, Visualization, Writing - original draft, Writing - review & editing. Ming Qu: Conceptualization, Formal Analysis, Investigation, Methodology, Project Administration, Supervision, Visualization, Writing - original draft, Writing - review & editing. Lingshi Wang: Conceptualization, Formal Analysis, Investigation, Methodology, Writing - review & editing. Xiaobing Liu: Conceptualization, Formal Analysis,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was funded by the Emerging Technologies Program of the Buildings Technology Office at the US Department of Energy. This work has been authored by the staff of UT Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy (DOE). The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this
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