Achieving better energy-efficient air conditioning - A review of technologies and strategies

A review of chemical heat pumps, thermodynamic cycles and thermal energy storage technologies for low grade heat utilisation

Hydrogen could play an important role as an energy vector in the coming decades in the framework of Sustainable Development. It is the universe's most abundant element and thus a never-ending source of energy. Hydrogen can be directly converted into electric energy by using fuel cells without producing toxic gases. It can also be produced by renewable sources such as biomass, solar and wind energies with no impact for the environment. However, although hydrogen represents a promising eco-friendly solution for energy transition, several issues related to its storage and delivery remain to be solved if it is to be widely used in both stationary and automotive applications. Hydrogen has the lowest volumetric energy density among the commonly used fuels, i.e., 0.01079 MJ/L at atmospheric pressure. Compression is the direct solution to overcome this obstacle. High pressure levels can give satisfying energy densities. The present review summarises the state of the art of the most classical hydrogen compression technologies. We shall present the technical and design features of mechanical compressors, i.e., reciprocating, diaphragm, linear and ionic liquid compressors, as well as of innovative non-mechanical technologies specifically conceived for hydrogen applications, such as cryogenic, metal hydride, electrochemical and adsorption compressors. The basic operating principles and the potential performance levels for each compression technology are analysed. Specifically, their current uses in hydrogen applications and their technological limits are described along with proposals of possible ways of improving their performance levels.

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Review of the current technologies and performances of hydrogen compression for stationary and automotive applications

Vapor compression air source heat pump (VCASHP) system has been widely applied due to its simple structure and low initial cost. However, there are a series of problems when the system is operated for space or water heating in cold regions, and a number of research groups have done much work to improve VCASHP systems for solving these problems over the last two decades. This review presents an update on the recent advances in VCASHP systems applied in cold regions. In this paper, the advanced systems are classified into three major types: single-stage, dual-stage and multi-stage compression systems. In addition, the dual-stage compression system is divided according to the number of compressors and separate loops into quasi-two-stage, two-stage and cascade compression systems. The heating performances and characteristics of each type of compression systems with different working fluids and intermediate configurations have been analyzed and summarized. Meanwhile, several issues of these advanced systems that impede their widespread applications in cold regions are proposed. It is hoped that both the reviewed contents and proposed issues may be useful to the research fellows working in this field.

Advances in vapor compression air source heat pump system in cold regions: A review

https://pubs.rsc.org/en/content/articlepdf/2015/cc/c4cc07849g

Fouling in microstructured devices: a review

A comprehensive model of a miniature-scale linear compressor for electronics cooling is presented. Linear compressors are appealing for refrigeration applications in electronics cooling. A small number of moving components translate to less theoretical frictional losses and the possibility that this technology could scale to smaller physical sizes better than conventional compressors. The model developed here incorporates all of the major components of the linear compressor including dynamics associated with the piston motion. The results of the compressor model were validated using experimental data from a prototype linear compressor. The prototype compressor has an overall displacement of approximately 3 cm 3 , an average stroke of 0.6 cm. The prototype compressor was custom built for this work and utilizes custom parts with the exception of the mechanical springs and the linear motor. The model results showed good agreement when validated against the experimental results. The piston stroke is predicted within 1.3% MAE. The volumetric and overall isentropic efficiencies are predicted within 24% and 31%, MAE respectively.

/pdf/a-comprehensive-model-of-a-miniature-scale-linear-compressor-4cwbs2qzys.pdf

A Comprehensive Model of a Miniature-Scale Linear Compressor for Electronics Cooling

https://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=1002&context=herrick

Air-side particulate fouling of microchannel heat exchangers: Experimental comparison of air-side pressure drop and heat transfer with plate-fin heat exchanger

While economizing can improve the performance of vapor compression cycles, the cost of multi-stage compressors has limited its implementation to large-scale applications. However, the development of compressors with injection ports presents new opportunities for economizing in smaller-scale applications. In addition to eliminating the need for a costly multi-stage compressor, multiple injection ports can be added at relatively low cost to further improve cycle performance. In the current study, a model was developed to study the effect of the number of injection ports on performance and determine the limit to performance with injection. The model predicts that continuous injection, in which economized refrigerant is injected at an infinite number of ports to maintain saturated vapor in the compressor, provides very significant performance improvements for air-conditioning and refrigeration applications. At standard operating conditions, the COP increases between 18% and 51% depending on the application, with higher temperature lift cycles benefiting most significantly.

/pdf/performance-limit-for-economized-cycles-with-continuous-47kkby39qq.pdf

Performance limit for economized cycles with continuous refrigerant injection.

Measured effect of airflow and refrigerant charge on the seasonal performance of an air-source heat pump using R-410A

Vapor compression refrigeration technology has seen great improvement over the last several decades in terms of cycle efficiency through a concerted effort of manufacturers, regulators, and research engineers. As the standard vapor compression systems approach practical limits, cycle modifications should be investigated to increase system efficiency and capacity. One possible means of increasing cycle efficiency is to flood the compressor with a large quantity of oil to achieve a quasi-isothermal compression process, in addition to using a regenerator to increase refrigerant subcooling. In theory, compressor flooding and regeneration can provide a significant increase in system efficiency over the standard vapor compression system. The effectiveness of compressor flooding and regeneration increases as the temperature lift of the system increases. Therefore, this technology is particularly well suited towards lower evaporating temperatures and high ambient temperatures as seen in supermarket refrigeration applications. While predicted increases in cycle efficiency are over 40% for supermarket refrigeration applications, this technology is still very beneficial for typical air-conditioning applications, for which improvements in cycle efficiency greater than 5% are predicted. It has to be noted though that the beneficial effects of compressor flooding can only be realized if a regenerator is used to exchange heat between the refrigerant vapor exiting the evaporator and the liquid exiting the condenser.

https://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=1000&context=herrick

Eckhard A. Groll

Papers

A Comprehensive Model of a Miniature-Scale Linear Compressor for Electronics Cooling

Air-side particulate fouling of microchannel heat exchangers: Experimental comparison of air-side pressure drop and heat transfer with plate-fin heat exchanger

Performance limit for economized cycles with continuous refrigerant injection.

Measured effect of airflow and refrigerant charge on the seasonal performance of an air-source heat pump using R-410A

Performance of Vapor Compression Systems with Compressor Oil Flooding and Regeneration