Liquid Desiccant Air Conditioning With Membrane Dehumidification Control
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Solution Overview
Problem
Existing liquid desiccant systems for air conditioning face challenges in efficiently controlling desiccant concentrations, fluid levels, and reacting to space and outdoor conditions while preventing crystallization and membrane damage, especially in humid climates where conventional vapor compression systems are energy-intensive and prone to overcooling.
Innovation Solution
A desiccant air conditioning system using a microporous membrane with a falling film of liquid desiccant, a heat transfer fluid, and a regenerator that allows independent control of air temperature and humidity through multiple refrigerant or heat transfer fluid loops, with features like siphoning action to maintain membrane flatness and sensors for monitoring membrane porosity and desiccant levels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If concentrated salt solutions are used to dehumidify air, then dehumidification efficiency is improved, but desiccant carry-over to the air stream increases causing corrosion
Solution Approach 1:
A hydrophobic membrane is introduced as an intermediary between the desiccant solution and the air stream. The membrane allows water vapor to pass through while blocking liquid desiccant carry-over, thus maintaining dehumidification efficiency while preventing corrosion in the air stream
Solution Approach 2:
A thin hydrophobic membrane is used to contain the concentrated desiccant solution while allowing vapor transfer. The membrane's selective permeability enables efficient dehumidification without liquid carry-over, resolving the contradiction between productivity and harmful factors
2Object-affected harmful factors
If hydrophobic membranes are used to contain desiccant, then desiccant carry-over is prevented, but membrane adhesion becomes difficult and damage risk increases
Solution Approach 1:
Air pressure is applied to the membrane to hold it in place against the support structure. This pneumatic fastening method eliminates adhesion difficulties and provides secure mounting without compromising the membrane's hydrophobic properties
Solution Approach 2:
Air pressure acts as a counterforce to the gravitational and operational stresses on the membrane, holding it firmly in place. This counterbalancing approach prevents membrane displacement and damage while maintaining its containment function
3Productivity
If desiccant concentration is increased to improve dehumidification, then moisture removal efficiency is improved, but risk of desiccant crystallization increases
Solution Approach 1:
Temperature and concentration sensors provide feedback to the control system, which adjusts heating and desiccant flow rates in real-time. This feedback control maintains optimal desiccant concentration while preventing crystallization, resolving the contradiction between productivity and reliability
Solution Approach 2:
The control system dynamically adjusts temperature and concentration parameters based on operating conditions. By changing these parameters in response to sensor feedback, the system maintains high dehumidification efficiency while staying below crystallization thresholds
4Temperature
If conventional vapor compression systems are used to dehumidify, then cooling capacity is provided, but overcooling occurs and energy intensive reheat is required
Solution Approach 1:
The mechanical vapor compression system is replaced with a liquid desiccant system that uses hygroscopic properties for dehumidification. This substitution eliminates the overcooling problem inherent in compression systems while avoiding the need for energy-intensive reheat
Solution Approach 2:
The system changes the fundamental parameter of moisture removal from temperature-based (compression) to concentration-based (desiccant). This allows independent control of temperature and humidity, providing cooling capacity without overcooling and eliminating reheat requirements
5Measurement precision
If multiple refrigerant loops are used to independently control temperature and humidity, then control precision is improved, but system complexity increases
Solution Approach 1:
The system segments temperature and humidity control into separate functional zones: the refrigerant loop handles temperature control while the liquid desiccant concentration handles humidity control. This segmentation allows independent precision control without requiring multiple complex refrigerant loops
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This system efficiently dehumidifies and conditions air, reduces energy costs by utilizing heat transfer fluids and external sources, and effectively prevents desiccant crystallization, ensuring membrane integrity and extending its service life.
Implementation Method 1
employing micro-porous membranes to separate the liquid desiccant from an air stream
Implementation Method 2
such super-hydrophobic membranes are typically hard to adhere to
Implementation Method 3
liquid desiccants to dehumidify and cool, or heat and humidify an air stream
Implementation Method 4
A desiccant air conditioning system using a microporous membrane with a falling film of liquid desiccant, a heat transfer fluid
Implementation Method 5
The control systems for such 2 and 3-way heat exchangers are unique in that they have to ensure that the proper amount liquid desiccant is applied to the membrane structures without over pressurizing the fluid and without over- or under-concentrating the desiccant. Furthermore the control system needs to respond to demands for fresh air ventilation from the building and needs to adjust to outdoor air conditions
Implementation Method 6
The regeneration side of the system provides a reconditioning function of the liquid desiccant so that it can be re-used on the conditioning side
Data Source
Figure 1
Figure 2A
Figure 2B
AI summary
A desiccant air conditioning system for treating an air stream entering a building space, including a conditioner configured to expose the air stream to a liquid desiccant such that the liquid desiccant dehumidifies the air stream in the warm weather operation mode and humidifies the air stream in the cold weather operation mode. The conditioner includes multiple plate structures arranged in a vertical orientation and spaced apart to permit the air stream to flow between the plate structures. Each plate structure includes a passage through which a heat transfer fluid can flow. Each plate structure also has at least one surface across which the liquid desiccant can flow. The system includes a regenerator connected to the conditioner for causing the liquid desiccant to desorb water in the warm weather operation mode and to absorb water in the cold weather operation mode from a return air stream.