Latest developments in not-in-kind refrigeration technologies

It is not surprising to note that the vapor compression refrigeration has consistently remained a predominant technology for well over a century. Although this technology offers advantages of good efficiency, satisfactory performance, and reasonable cost, it has a major weakness in terms of its environmental unfriendliness causing global warming. There is a growing concern that the increasing demand for heating, cooling, and refrigeration services world-wide may consequently lead to an increase in the related CO₂ emissions. This trend could, however, be alleviated by the performance enhancement of current heat pumping technologies and/or the development of new energy efficient alternative technologies. There has been considerable effort in this area, where researchers (Bansal et al. 2012; Goetzler et al. 2014) have stressed the need for the development of efficient, low-cost, and not-in-kind refrigeration systems.

There is, therefore, a strong need to develop not-in-kind technologies to replace conventional vapor compression refrigeration technology that can improve the energy efficiency and environmental friendliness of residential and commercial building equipment. Such technologies will be critical to provide energy savings or other environmental benefits for space conditioning, water heating, and refrigeration.

This special section of Science and Technology for the Built Environment (STBE) presents selected high-quality articles in the field of next generation not-in-kind technologies for HVAC&R. Altogether, 10 articles made it into this special issue that cover a wide range of topics, including elastocaloric refrigeration (three articles), magneto-caloric refrigeration (five articles), Stirling cycle (one article), and compressor derived metal-hydride heat pumps (one article). Elastocaloric and magnetocaloric are the solid-state refrigeration technologies that avoid the use of “conventional” refrigerants with adverse global warming impact. Lately, materials having giant elastocaloric and magnetocaloric effect have underscored the potential for the fabrication of efficient refrigerators at room temperature. An ideal Stirling cooler is a reversed Stirling engine. Metal hydride systems operate using reversible adsorption and desorption of hydrogen from metallic compounds and can be incorporated in a cycle having a work input (compressor) or thermal energy input (generator).

These technologies are still developing due to the current limitations posed by the state-of-the-art in materials research. A significant amount of research has recently been pursued in these areas where fast developments are occurring both in new materials and systems architecture. It is heartening to note that the future of these technologies is looking promising. Hold on to your seats and read on! Note: There is also a special bonus section of general interest articles.

Elastocaloric

Schmidt et al. present an experimental investigation of a conceptual elastocaloric air conditioning device and the caloric properties of electrocaloric materials in a purpose built test facility. The efficiency of various elastocaloric cooling cycles is evaluated using a graphical approach that was validated with experimental data.

Tušek et al. provide preliminary results of a proof-of-concept regenerator-based elastocaloric cooling device using a commercial Ni-Ti plate. The simulation results indicated that a device (with a plate thickness of 0.1 mm) can produce a specific cooling capacity of up to 7 kW/kg and a coefficient of performance (COP) of up to 5 at the temperature span of 30 K. Results were encouraging for a regenerator-based cooling device using thin NiTi plates with small spacing between the plates.

Qian et al. demonstrate the design of an elastocaloric cooling system using NiTi tubes, driven by hydraulic actuators. A prototype was built and tested under the single-stage reverse Brayton cycle. Preliminary test results indicated that the Ni-Ti tube could provide latent heat of more than 10 kJ.kg⁻¹ under 4.5% strain. The temperature lift of 24.6 K and a system COP 11.0 are predicted from a previously developed dynamic model.

Magnetocaloric

Trevizoli et al. present an overview of the design principles and challenges of an active magnetic regeneration (AMR) that is regarded as one of the most promising alternative technologies for heat pumps and cooling systems at room temperature. The main loss mechanisms responsible for reducing the performance of AMRs are addressed. In order to improve the performance, a design optimization methodology is proposed based on entropy generation minimization.

Benedict et al. present the design details and preliminary results of an active magnetic refrigeration Halbach Array prototype, developed by General Electric (GE) with Gadolinium (Gd) particulate for room temperature cooling. The maximum no-load span of 21 K and the maximum power of 26 W at a span of 1 K were recorded. The unique modularity and control of the new prototype will allow for a detailed understanding of the operation of commercial AMR designs.

Eriksen et al. report experimental results of a novel rotary AMR refrigeration prototype developed at the Technical University of Denmark with cylindrical regenerator being divided into 11 beds filled with a total of 1.7 kg of closely packed spheres of Gd and Gd_(1-x)Y_x. The Second Law efficiency of 18% was obtained at a cooling load of 81.5 W, resulting in a temperature span of 15.5 K and a COP of 3.6. There is a potential to achieve Second Law efficiency of 30% by eliminating parasitic losses and replacing the packed spheres with a theoretical parallel plate regenerator.

Capovilla et al. present the experimental performance of a rotary magnetic refrigerator prototype (made of Gd spheres) developed at the Federal University of Santa Catarina (Brazil). The COP and Second Law efficiency of 2.5 and 3.7%, respectively, were obtained at system temperature spans of 4 and 6 K, while the maximum cooling capacity at zero-span was approximately 150 W. The losses in the flow management sub-system (pumping power and valve losses) were responsible for limiting the system performance.

Torregrosa-Jaime et al. present parametric results of an active magnetic regenerator refrigerator (AMRR) for mobile air-conditioning (MAC). A permanent-magnet parallel-plate AMRR made of Gd-like material is evaluated for a cooling load of 3.03 kW at a temperature span of 29.3 K. The results show that AMRR works optimally for the MAC system (COP = 2.5 and weight 12 to 15 kg) with fluid flow rates at least three times larger than the vapor compression system.

Stirling

Huang et al. present the test results of a coaxial free-piston Stirling engine when reversely operated as an ultra-low temperature freezer. A no-load cooling temperature 142.4 K was obtained at a maximum power input of 1164.5 W. The relative Carnot coefficient of performance of the Stirling unit as a cooler reached 18.6% at 203 K.

Metal hydride heat pump

James et al. investigate the use of a metal-hydride slurry in conjunction with various isothermal compression techniques to overcome the challenges of poor heat transfer and high compressor discharge temperatures faced by compressor driven metal hydride heat pumps. Liquid-flooded, electrochemical, and liquid piston compressors are modeled and integrated into a system model in order to assess their impact on the performance of the slurry-based metal hydride heat pump system.