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Novel Liquid Desiccant Dehumidification System

Final Report Summary - DEHUMID (Novel Liquid Desiccant Dehumidification System)

Compilation of literature about existing dehumidification technologies was carried out and analysed. Properties of dehumidification systems that are technologically and economically justified were identified.

Functional specifications of all the system components were set and a layout for the system, giving specific attention to compactness, was conducted. The control of the system, being an important energy-reduction issue, was identified and the basic control parameters were defined so as to elaborate on the design and supply a finished product for controlling the system.

Assessment of lithium bromide (LiBr) and of lithium chloride (LiCl) was conducted using facts and findings from the existing literature. The results show that LiCl is a better desiccant material compared to LiBr. However, LiCl should only be used in application where there is no LiCl droplets carryover (i.e. zero carryover). This design restriction together with the aggressive corrosive nature of LiCl has moved design ideas towards the use of non-metal material in the absorber. Hence, the absorber for the proposed system was designed from cellulose fibre, arranged in a packed bed. Further, the cooling of the desiccant solution was integrated into the absorber surface. This choice of design has the following advantages:
- The corrosion problem is eliminated, as cellulose fibre is unaffected by LiCl.
- The LiCl droplets carryover is reduced, as liquid droplets hold better on cellulose fibre surfaces compared to metal surfaces. (Further reduction of the droplets carryover is achieved by U-trapping in the ducting work).

The frequency and the time spent on maintenance are both reduced, as the cellulose fibre absorber is disposable. In the case of metal absorbers, fouling of the metal surface contribute to a reduced performance, and requires considerable cleaning time and effort.

Searching reviewing and analysing the publications about dehumidification systems using liquid desiccants were performed. Over 30 scientific literature sources was studied and shared with other partners of consortium. The information obtained was used in formulations of the requirements of future system, in mathematical modelling of dehumidification system, prediction of performance, economical vitality and other features of future system. Climatic data for each participant' country was analysed and treated. The DEHUMID project participants are from seven countries (Lithuania, United Kingdom, the Netherlands, Poland, Italy, Portugal, Spain). The most extreme places for dehumidification in each country were identified. The climatic data for each country required for designing of air conditioning and dehumidification systems were collected and presented to all participants.

The humidity load model of an office room was elaborated. According to prEN 15251 the humidity load was calculated using three indoor environment categories (three comfort levels). Using available reference year data (climatic parameters of each hour of typical year) the time of possible use and average dehumidification load of future dehumidification system in three countries (Lithuania, United Kingdom, the Netherlands) was calculated. The reference year data for the Netherlands was obtained after treatment of 15 year's (1991-2005) hourly climatic data.

The chart illustrating dehumidification design parameters of all participating countries and internal loads according three indoor climate parameters were prepared as well as chart of humidity ratio duration curves for three countries (Lithuania, United Kingdom, the Netherlands). These curves express required annual amount of dehumidification (tons of moisture to be removed).

Desiccant solution (LiCl-H2O) physical proprieties (solubility boundary, relative vapour pressure, density, thermal capacity, differential enthalpy of dilution), available in literature sources, were programmed as custom MS Excel (MS Visual basic for applications) functions. These functions were tested and used in dehumidification system spreadsheet model. Simultaneously the proprieties of moist air were programmed as psychrometric functions to be used in same model.

Based on the requirements for the control modules, the design has been performed using computer-aided design (CAD) software EAGLE. The inputs have been calculated using assembled sensor data including passive compensation of linearity for the KTY sensors. The temperature inputs have been properly dimensioned using high accuracy resistors. In case after testing it would be required to change the ranges, the resistors (2 for each channel) must be changed.

A state-of-the-art micro-controller (microchip) has been selected. This can be reprogrammed in the switch controls. Before the actual testing has started off, it is not exactly known which functions should be programmed.

The communication was realised using an industrial standard, RS485, 9600 Baud, half duplex. This type is highly resilient against failures. The modules were connected with the personal computer (PC) by a BUS (half-duplex), using master-slave communication protocols. Each module was appointed to an address which can be adjusted by dual in-line package (DIP) switches.

Data protocols were realised as simple as possible. Data processing and correcting the sensors was performed by the PC. Data security was guaranteed by using checksums. All parameters are registered using data logging, allowing for a detailed analysis at a later stage.

Also, an emergency programme is foreseen in case the communication should fail. Themodules will fall back into an emergency mode preventing further damage or even bigger failures. Each module has eight light-emitting diodes (LEDs), which indicate the current status of the outputs, including failures of elements, like sensors. Critical elements are continuously watched. An opto-isolated Interface was integrated in the control system.

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