Rooftop Liquid Desiccant Membrane Unit for Low-Energy Dehumidification
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Solution Overview
Problem
Conventional Roof Top Units (RTUs) are inefficient in dehumidifying air streams, leading to poor energy performance, high humidity, and uncomfortable building conditions, especially in humid climates, and lack the ability to humidify in winter, resulting in dry air and increased energy costs.
Innovation Solution
The use of liquid desiccants running as a falling film over a support plate covered by a microporous membrane, combined with a heat transfer fluid and a refrigerant system, allows for efficient dehumidification and humidification by preventing desiccant carry-over, enabling turbulent air flows and simultaneous cooling and heating while maintaining air quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If conventional vapor compression systems are used to dehumidify air, then cooling capacity is provided, but the systems overcool the air and require energy intensive reheat systems
Solution Approach 1:
The system separates the dehumidification function from the cooling function by using two parallel pathways: a liquid desiccant system for dehumidification and a conventional vapor compression system for cooling, allowing each to operate independently and efficiently without the need for reheat
Solution Approach 2:
The liquid desiccant system performs both dehumidification and heat recovery functions, while the membrane module provides both separation and mass transfer functions, creating a multi-functional system that eliminates the need for separate reheat equipment
2Productivity
If liquid desiccant systems use concentrated salt solutions, then dehumidification efficiency is improved, but desiccant carry-over risk increases causing corrosion
Solution Approach 1:
A hydrophobic microporous membrane is introduced as an intermediary between the liquid desiccant and the air stream, allowing water vapor to pass through while blocking liquid desiccant carry-over, thus enabling the use of concentrated salt solutions without corrosion risk
Solution Approach 2:
The hydrophobic microporous membrane acts as a thin film barrier that selectively permits vapor transmission while preventing liquid penetration, enabling efficient dehumidification with concentrated desiccants without carry-over
3Ease of operation
If packed bed systems are used for liquid desiccant dehumidification, then direct contact between air and desiccant is achieved, but fan power and pressure drops increase
Solution Approach 1:
The packed bed is replaced with a flat sheet membrane structure that provides direct contact between air and desiccant without the high pressure drops associated with packed beds, significantly reducing fan power requirements
Solution Approach 2:
The hydrophobic microporous membrane provides a large surface area for mass transfer while maintaining low flow resistance, enabling efficient dehumidification with minimal fan power compared to packed bed systems
4Productivity
If turbulent air flows are used in membrane systems, then heat and moisture transfer rates are enhanced, but system complexity increases
Solution Approach 1:
The flat sheet membrane configuration naturally accommodates turbulent flow conditions and provides large surface area for enhanced mass and heat transfer, achieving high productivity without complex internal structures
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 achieves efficient dehumidification and humidification, reducing energy consumption, eliminating desiccant contamination risks, and providing cost-effective, thermally efficient air treatment capable of handling high outside air percentages without the need for additional reheat systems.
Implementation Method 1
The water vapor in an air stream that is flowing over the membrane diffuses through the membrane into the underlying desiccant resulting in a drier air stream
Implementation Method 2
Liquid desiccant systems have been used for many years and are generally quite efficient at removing moisture from the air stream
Implementation Method 3
If the desiccant is at the same time cooler than the air stream, a cooling function will occur as well, resulting in a simultaneous cooling and dehumidification effect
Data Source
AI summary
Liquid desiccant air-conditioning systems cool and dehumidify a space in a building when operating in a cooling operation mode, and heat and humidify the space when operating in a heating operation mode.