Design, manufacturing and testing of a compact thermoacoustic refrigerator

Highlights

• A novel compact thermoacoustic refrigerator is designed, manufactured and tested.

• A method for thermal design of heat exchangers for thermoacoustic devices is proposed.

• Comparison between the measurements and DeltaEC design model results are performed.

Abstract

Thermoacoustic refrigeration technology is a green alternative for the conventional refrigeration system in vehicles. The former uses environmentally friendly inert gases whereas the latter uses chemical refrigerants that can have severe impacts on the environment. In order to apply thermoacoustic technology in the automobile industry, the size and the weight of plausible thermoacoustic refrigerators should be decreased.

The design of a new compact thermoacoustic refrigerator is described in this study. This thermoacoustic refrigerator uses two electroacoustic components and one thermoacoustic core. The technical details of design, fabrication, and testing processes are presented. A methodology for thermal design of the heat exchangers is proposed and validated by measurements. The effects of different parameters, such as the driver piston displacement amplitude, the phase shift between the voltage signals applied to the electroacoustic components, and the cold side temperature on the performance indices of the refrigerator are investigated.

The performance of the prototype is compared with the performance calculated by the DeltaEC design model. The overall agreement between the calculated and the measured performance parameters is fair, but further insight into the temperature distributions reveals that non-linear effects yield discrepancies between the model and the experimental results. A preliminary discussion of the origin of these discrepancies is proposed.

Introduction

Chemical refrigerants (such as CFCs and HFCs) used for vapor-compression refrigeration in air conditioning systems are recognized as a source of pollution from a vehicle. Also, cooling systems cause significant increase in power consumption and reduce the overall efficiency of the vehicle. The automotive industry therefore devotes efforts to downsizing thermal needs (isolation, optimized diffusion …) but also to improve the performance of the refrigeration loop, especially the efficiency of high performance heat pumps. For this purpose, alternative “green” technologies are being considered (e.g. [1], [2], [3], [4], [5]). Thermoacoustic cooling devices are systems that offer potentially attractive alternatives [6], especially with the rapid development of hybrid and electric vehicles. Thermoacoustic refrigerators use acoustic energy to pump heat from a cold source to a hot sink. They include an acoustic resonator coupled to an acoustic source and are typically filled with an inert gas as working fluid. Inside the gas column is inserted the thermoacoustic core (TAC), that consists of an open cell porous medium referred to as stack or regenerator, sandwiched between two heat exchangers which are responsible for the heat exchange processes between the working gas and the thermal reservoirs.

Thermoacoustic technology is characterized by relatively simple construction and potentially low maintenance cost. However, to date, very few thermoacoustic refrigeration systems [6], [7], [8], [9] have been investigated for adaptation to the automobile. One of the main reasons, among others, is that the automotive applications are associated with strict restrictions on weight, size, and cost. An innovative device needs to be no larger or heavier than the current Heating, Ventilation Air Conditioning (HVAC) systems, yet ensuring comparable performance.

In usual thermoacoustic devices, the size is driven by resonance conditions yielding metric devices with working frequencies of the order of 10² Hz. Such devices do not fit size and weight criteria for automotive applications. The objective of the present study is to propose a new thermoacoustic refrigerator that is competitive compared to the existing technology in terms of space (the target set for the volume is about 24 L) and performance (the target set for the performance coefficient COP is about 1.1); the temperature of the air supplied to the passenger compartment of the vehicle has to vary from 5 to 15 degrees.

Since the pioneering works of John Wheatley and Gregory Swift at Los Alamos National Laboratories at the beginning of the 1980′s, thermoacoustics has spread over the research world and the research in thermoacoustics is still a topical issue today [10]. The field was very active in the 1990s, with a great deal of fundamental research undertaken. From the 2000s, more applied research devoted to specific applications has appeared such as cryogenics [11], natural gas liquefaction [12], electricity generation [13], domestic refrigeration [14], and thermal management of electronic devices [15]. Thanks to both fundamental and applied research, the performance of thermoacoustic devices has progressed. Twenty-five percent of Carnot coefficient-of-performance was obtained by a thermoacoustic refrigerator driven by an electroacoustic source developed at the Energy Research Center of the Netherlands [16]. This device consists of a 2.9 m half wavelength resonator with a coaxial thermoacoustic core and is driven by an electroacoustic source. In 2015, a multi-stage thermoacoustic cryocooler with a 11 m loop length was developed [17] with a cooling capacity of about 1 kW at liquefied natural gas temperature range. As discussed in the review by Tartibu [18], several other highly efficient travelling wave thermoacoustic devices have been developed in the last decades. The associated refrigerators geometrical configurations consist of single or multiple regenerators connected in series inside a closed-loop tube together with an acoustic source that is either in series with the regenerator(s) inside the loop (e.g. [19], [20]) or coupled to the loop via a side branch (e.g. [14], [21]). Such devices are associated with typical cooling capacities of few hundreds of Watts. Their design is driven by resonance conditions yielding metric devices so that their architecture is not adapted to automotive application. Because many applications are associated with size constraints [22], [23], some compact thermoacoustic devices have already been developed. Among those, the Ben & Jerry’s “bellows-bounce” freezer has a 25 cm diameter and a 48 cm length and pumps 120 W at a −20  °C load temperature with a 19% of Carnot coefficient-of-performance [24].

In the current work, we used the concept of two-source co-axial compact thermoacoustic refrigerator proposed by Poignand, et al. [25], [26] to reach compactness. This concept is based on replacing the resonator historically used to reach high acoustic power and appropriate phase relationship between particle displacement and instantaneous temperature by an effective acoustic impedance provided by a second acoustic source. It follows that the thermoacoustic cavity is fitted with two electroacoustic components working at the same frequency whose amplitudes and phases can be independently tuned for optimizing the performances of the device. Acoustic pressure and particle velocity are therefore no longer linked by quasi-standing wave or quasi-travelling wave conditions as was the case in standing wave devices or travelling wave devices. The working frequency can thus be set so that the wavelength is much greater than the dimensions of the resonator and the compactness of the device is thus significantly improved. This concept of a compact refrigerator has been validated with an academic setup that showed both stack-based and regenerator-based compact devices can reach the same performance as non-compact geometries in terms of temperature difference, heat flux or COP [27]. The purpose of the present work is to develop a compact thermoacoustic device that reach high specific cooling capacity (i.e. the ratio between the cooling capacity and the total volume of the device). This requirement imposes many challenges, especially in thermal and mechanical design, and fabrication processes.

In Section 2, a description of the device is presented, followed by a detailed description of the thermal and mechanical design of the heat exchangers. Then, the instrumentation and control systems of the device are explained. In Section 3, the results of the experiments are discussed and compared with the numerical predictions. In Section 4, the conclusions are drawn.