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How to Optimize the Design, Size and Operation of Heat-Pump Liquid Desiccant AC Systems

How to Optimize the Design, Size and Operation of Heat-Pump Liquid Desiccant AC Systems

From ASHRAE Journal Newsletter, April 28, 2020

While several studies have addressed heat-pump liquid desiccant air-conditioning (LDAC) systems, there is no information available in scientific literature about how to design, size and operate these systems to optimize energy efficiency, according to a recent Science and Technology for the Built Environment article.

The article, “Optimal design, sizing and operation of heat-pump liquid desiccant air conditioning systems,” presents a novel thermodynamic analysis for heat-pump LDAC systems that is aimed to help engineers and researchers identify the best operating points for heat-pump LDAC systems.

Researchers Ahmed Abdel-Salam, Ph.D., P.Eng., BEMP, HBDP, Member ASHRAE, and Carey Simonson, Ph.D., P.Eng., Fellow ASHRAE, discuss the paper.

1. What is the significance of this research?

The significance of this research is that it provides new guidance to engineers and researchers on how to apply optimal control strategies for the design, sizing and operation of heat-pump liquid desiccant air-conditioning systems. Results from the thermodynamic analysis have shown that the system:

  1. can be used to simultaneously cover latent and sensible loads, and
  2. coefficient of performance (COP) can be improved by up to 50% if the suggested optimization control strategies are implemented.

2. Why is it important to explore this topic now?

Conventional air-conditioning systems are energy-inefficient in the dehumidification of air streams. This is because the air stream has to be overcooled below its dew point temperature to remove moisture by condensation, then the overcooled air stream has to be reheated to a comfortable supply temperature, which results in increasing energy consumption. Liquid desiccant air-conditioning systems are able to directly absorb the required amount of moisture from a humid air stream without the need to overcool or reheat the supply air stream. As a result, liquid desiccant AC systems have the potential to be used for applications with high dehumidification loads.

3. What lessons, facts and/or guidance can an engineer working in the field take away from this research?

This research has demonstrated that the simultaneous regulation of evaporator and condenser temperatures following a given sequence results in optimizing the performance of heat-pump liquid desiccant AC systems. Optimal control strategies have the potential to operate the system under one of the following three operating modes:

  1. to cover latent loads and optimize the COP of the system,
  2. to cover latent loads and balance evaporator and condenser energy, and
  3. to cover latent and sensible loads.

This knowledge about heat-pump liquid desiccant AC systems enables field engineers to apply proposed control strategies to achieve one of the aforementioned operating modes. Moreover, design engineers can benefit from applying the proposed strategies during the sizing and design phase in order to identify the proper size of equipment that enables the system to cover the required loads, while maintaining optimum COP.

4. How can this research further the industry's knowledge on this topic?

Heat-pump liquid desiccant air-conditioning systems have mainly been used by research and industry to cover the latent loads of the system, and an auxiliary system has always been used to cover sensible loads. This research has demonstrated that a heat-pump liquid desiccant air-conditioning system can be used to simultaneously cover sensible and latent loads, while optimizing the COP by applying the proposed control strategies.

5. Were there any surprises or unforeseen challenges for you when preparing this research?

Yes. The main surprise was how dependent the COP of the heat-pump liquid desiccant AC system was on the type of refrigerant used. It was found that the system COP changes by up to 25% depending on the type of refrigerant used.