Operating parameters of a membrane-based parallel-plate liquid desiccant dehumidification system are investigated in this paper. The liquid desiccant and air are in a cross-flow arrangement, and separated by semi-permeable membranes to avoid carry-over problem. A numerical model is developed to simulate the system performance, and validated by experimental and analytical results. Impacts of main operating parameters on the system performance (i.e. sensible, latent and total effectiveness) are evaluated, which include dimensionless parameters (i.e. solution to air mass flow rate ratio * and number of heat transfer units ), solution properties (i.e. concentration and temperature ) and inlet air conditions (i.e. temperature , and relative humidity , ). It is found that * and are two of the most important parameters influencing the system effectiveness. Even though the system performance can be improved by * and , its increasing gradient is limited when * and exceed 1 and 4 respectively. Decreasing solution temperature does not make a great improvement to the system performance, however, increasing solution concentration is a good approach to enhance the latent effectiveness without influencing the sensible effectiveness. The system shows the broad adaptability in various weather conditions, and has the ability to provide relative stable state supply air.
Air dehumidification is of vital importance in building air conditioning and production safety. Semi-permeable membrane module is a novel heat and mass exchanger, which separates the air and liquid desiccant to overcome desiccant droplet carry-over problem in traditional direct-contact systems. Recently, some research works have been carried out in mathematical modelling and experimental testing of membrane-based liquid desiccant dehumidification technology. Compared with the experimental testing, the mathematical modelling has advantages of significant time and cost reductions, practically unlimited level of detail, more profound understanding of physical mechanism and better investigation of critical situation without any risks. This paper presents a comprehensive review of various modelling methods for two types of membrane-based liquid desiccant modules: flat plate and hollow fiber.
Compact linear Fresnel reflector (CLFR) system employing multiple receivers is promising with better optical performance and cost effectiveness compared to linear Fresnel reflector (LFR) system, especially for applications with limited ground availabilities. Nevertheless, only few researches have been conducted to evaluate optical design and performance of the CLFR system. In this study, geometrical models for the CLFR system with flat mirrors and receivers are developed on the basis of polar orientation. A comparative study of concentration characteristics among the LFR, CLFR-complete and CLFR-hybrid systems is conducted based on numerical, ray tracing simulation and experimental results. In addition, optical design analyses of the CLFR-hybrid system are carried out from various design aspects. It is noteworthy that the mirror arrangement and focal length should be optimized for the CLFR-hybrid system with considerations of the associated geometrical characteristic and optical performance. For a small-scale CLFR-hybrid system with a solar field width of 2100mm and a focal length of 1500mm, the geometrical concentration ratio of 15.14 and ground utilization ratio of 0.95 are achieved respectively. The findings demonstrate the feasibility of the CLFR-hybrid system with flat mirrors and polar orientation, which provide progress to the concentrated solar power technology.
A membrane-based liquid desiccant dehumidification cooling system is studied in this paper for energy efficient air conditioning with independent temperature and humidity controls. The system mainly consists of a dehumidifier, a regenerator, an evaporative cooler and an air-to-air heat exchanger. Its feasibility in the hot and humid region is assessed with calcium chloride solution, and the influences of operating variables on the dehumidifier, regenerator, evaporative cooler and overall system performances are investigated through experimental work. The experimental results indicate that the inlet air condition greatly affects the dehumidification and regeneration performances. The system regeneration temperature should be controlled appropriately for a high energy efficiency based on the operative solution concentration ratio. It is worth noting that the solution concentration ratio plays a considerable role in the system performance. The higher the solution concentration ratio, the better the dehumidification performance. However simultaneously more thermal input power is required for the solution regeneration, and a crystallization risk in the normal operating temperature range should be noted as well. The system mass balance between the dehumidifier and regenerator is crucial for the system steady operation. Under the investigated steady operating condition, the supply air temperature of 20.4°C and system COP of 0.70 are achieved at a solution concentration ratio of 36%.
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