ICR2019: overview of current refrigeration research topics (part III)

Energy storage, desiccant air conditioning, high-temperature heat pumps and ejector technologies were some of the salient topics of the latest IIR Congress. Continuation of the synthesis of key presentations of ICR2019.
We continue to review the major themes discussed during the IIR International Congress of Refrigeration (ICR2019) held in August 2019, through the last six keynotes presented during this event. The other eight keynotes were featured in the September issue of the Newsletter (see https://bit.ly/2qkF0Lr).

Phase change materials and more generally the issue of energy storage, were a major theme of the congress since no fewer than 56 papers (out of a total of 640) dealt with this topic.

In her keynote [1], Laurence Fournaison compares four different types of phase change material (PCM) slurries – ice slurries, PCM emulsions, encapsulated PCMs and hydrate slurries – as two-phase secondary fluids in indirect refrigeration systems. This comparison focuses on generation process, industrial deployment and issues hampering their development. In the context of the ongoing HFC phase-down, indirect refrigeration is a particularly interesting technology since it can divide up to a factor 10 the amount of refrigerant contained in a system.

However, two-phase slurries are not currently widely used as secondary refrigerants. Apart from ice slurries for applications below 0 °C, only hydrate slurries have been used in industrial applications but their efficiency or technical performances can be improved to expect further spreading of the technology. PCM emulsions, encapsulated PCMs and gas hydrates slurries require further research to consider industrial development. The drawback of the emulsions and encapsulated slurries is the aging of the fluid under extended freezing and melting cycles and for gas hydrates slurry, it is the high pressure of the system.

Ejectors, which are used to recover expansion work and thus improve the COP of refrigeration systems, were addressed in 28 papers, which illustrates their importance in terms of research and development.

Stefan Elbel’s paper [2] summarizes some of the recent developments in the area of two-phase ejectors. While most attractive for fluids and cycles with high throttling losses, such as transcritical CO2, the use of ejectors still makes sense for various applications using lower pressure refrigerants. Numerous integration options for ejectors exist and the recovered expansion work can be utilized in different ways, for example to raise the compressor suction temperature, to provide cooling at different temperature levels, or to improve evaporator performance through liquid overfeed. Ejector design plays an important role in achieving high work recovery efficiencies, but the design of other system components, such as the evaporator or the vapor/liquid separator that many of the cycle architectures require are equally influential and offer large improvement potentials.

It needs to be ensured that potential inefficiencies of these other system components do not outweigh the positive performance impact provided by the ejector. Numerous approaches exist to implement ejector and cycle control, including adjustable motive nozzle throat area mechanisms, parallel ejectors, or vortex control. The author concludes that “ejectors remain an interesting and intriguing topic and the onset of successful commercialization warrants continued research and development”.

Air conditioning (addressed in 62 papers) and more particularly desiccant cooling systems (16 papers) was an important topic of ICR2019.

- In its keynote “Renewable energy technologies in air conditioning: State of the art and perspectives” [3], Renato Lazzarin estimates that renewable energy technologies can satisfy a relevant fraction of the increasing cooling demand, particularly where solar radiation is abundant. He distinguishes passive (or almost passive) technologies and active technologies.

Among passive technologies, radiative cooling toward the sky, night ventilation cooling, coupling of the room to be cooled with the ground or an aquifer and evaporative cooling are presented. However, the author considers that passive cooling technologies usually offer a marginal contribution to a building air-conditioning, so that hybrid systems coupling passive and active technologies are often used.

Among active technologies, solar thermal cooling (using the sorption process), desiccant cooling (also thermally driven and using the sorption process) and PV driven solar cooling are put forward.

The steady increase in the efficiency of PV modules and the sharp decrease in their costs over the last ten years now make solar photovoltaic cooling the most cost-effective solution in terms of overall cost and an option that can compete with the traditional air-conditioning systems.

- Improving energy efficiency with adequate humidity control capacity and sufficient fresh air ventilation is a long-standing focus of HVAC systems. A traditional vapor-compression air conditioner, which adopts the cooling-based dehumidification method to handle sensible and latent loads altogether, operates at a lower evaporation temperature (5–7°C). This results in a lower COP (2.8-3.6, in general). Temperature-and humidity-independent control (THIC) systems have been proposed to overcome the challenges; however, their COP is hindered by the sorption heat effect. In their keynote [4], Ruzhu Wang, Lingji Hua, Tianshu Ge, Yaodong Tu, and Xiangyu Sun report on a solid desiccant heat pump based on desiccant coated evaporators/condensers (DCEs/DCCs). The desiccants adsorb moisture almost isothermally and can be regenerated by the condensation heat. With the optimized refrigerant and desiccant, their proposed DCCs/DCEs feature a weakly-coupled heat and mass transfer. The system exhibits a COP of 6-7, doubling the efficiency of heat pumps currently used in household, which opens a window for the future development of HVAC systems and also a broad range of THIC applications.

See part IV

These keynotes are available in the Fridoc database (see links below).

All the other communications of the Montreal congress can be downloaded here.

IIR members benefit from a quota of free downloads.

(1) Laurence Fournaison, Practical aspects on slurries for refrigeration and air conditioning: https://bit.ly/2qisBaI

(2) Stefan Elbel, Recent developments in HVAC&R systems utilizing two-phase ejector for expansion work recovery: https://bit.ly/33AQ2KM

(3) Renato Lazzarin, Renewable energy technologies in air conditioning: State of the art and perspectives: https://bit.ly/2Mh6HwZ

(4) Ruzhu Wang, Lingji Hua, Tianshu Ge, Yaodong Tu, Xiangyu Sun, Highly efficient heat pump with desiccant coated evaporator and condenser: principle and application: https://bit.ly/2IUkJT0