Optimization of the working fluid in a Joule–Thomson cold stage

Vibration-free miniature Joule–Thomson (JT) coolers are of interest for cooling a wide variety of devices, including low-noise amplifiers, semiconducting and superconducting electronics, and small optical detectors used in space applications. For cooling such devices, coolers are needed which have operating temperatures within a wide temperature range of 2–250 K. In this paper, the optimization of the working fluid in JT cold stages is described that operate at different temperatures within that range. For each temperature, the most suitable working fluid is selected on the basis of the coefficient of performance of the cold stage, which is defined as the ratio of the gross cooling power to the change in Gibbs free energy of the fluid during compression. In addition, a figure of merit of the heat exchange in the counter-flow heat exchanger is evaluated that depends only on the properties of the working fluid.     

Optimization of the working fluid for a sorption-based Joule–Thomson cooler

Sorption-based Joule–Thomson coolers operate vibration-free, have a potentially long life time, and cause no electromagnetic interference. Therefore, they are appealing to a wide variety of applications, such as cooling of low-noise amplifiers, superconducting electronics, and optical detectors. The required cooling temperature depends on the device to be cooled and extends into the cryogenic range well below 80 K. This paper presents a generalized methodology for optimization in a sorption-based JT cooler. The analysis is based on the inherent properties of the fluids and the adsorbent. By using this method, the working fluid of a JT cooler driven by a single-stage sorption compressor is optimized for two ranges of cold-tip operating temperatures: 65–160 K and 16–38 K. The optimization method is also extended to two-stage compression and specifically nitrogen and carbon monoxide are considered.     

电磁式发动机水冷系统流固耦合传热研究

针对电磁式发动机电磁线圈由工作温度过高导致其内阻过大、发动机工作效率降低的问题,应用流固耦合传热理论对发动机水冷系统的传热性能进行研究.以电磁式发动机水冷系统流场和固场为研究对象,建立流固耦合传热的数学模型;利用UG(Unigraphics)软件建立水冷系统流固耦合模型,对水冷系统的流场和温度场分布进行仿真分析.结果表...     

太阳同步轨道辐射制冷技术的发展

辐射制冷装置由于具有寿命长、无机械振动、无电磁干扰、本身无功耗等非常适合于空间应用的一系列特点、优点，因而得到了广泛的应用。随着各国空间项目的实施，辐射制冷技术得到了长足的发展，并且逐渐成熟。迄今为止，这种类型的制冷器在空间应用中还是最广泛和最实用的。     

In-storage softening of kiwi fruit: effects of delayed cooling

Rapid softening of kiwi fruit while in storage at 0°C limits the marketing period for this crop and contributes to economic loss. The time required to bring fruit to optimum storage temperature is affected by delays between harvest and placing fruit in the cooling facility and by the completeness of cooling. Incomplete cooling of fruit in commercial forced-air coolers results in overall above-optimal average fruit temperatures for undesirable lengths of time and also results in a wide range of temperature among individual fruits. The half cooling time for palletized kiwi fruit packed in wooden trays with liners was 7 h in the coolers tested; thus seven-eighths cooling required 21 h. Delays of 24 h or more before the start of cooling accelerated the softening of fruit, enhanced soluble solids content, and increased the incidence of rotting and shrivelling during storage.     

Evaluation of standing-wave thermoacoustic cycles for cooling applications

The most promising applications for standing-wave thermoacoustic cooling were investigated from the perspective of the ratio of coefficient of performance (COP) to the reversible COP or COPR. A design optimization program based on the thermoacoustic simulation program known as DELTAE was developed. The program was applied to two standing-wave thermoacoustic cooler configurations in order to determine the best possible COPRs for various temperature spans between hot-side and cold-side stack-end temperatures. It was found that the COPR of standing-wave thermoacoustic coolers increases with temperature span and reaches a maximum for temperature lifts around 80 °C. Analysis of the results and the losses clearly shows that the efficiency of these systems may be good for refrigeration, but not for air-conditioning and cryogenic cooling. The COPRs determined from measurements for various thermoacoustic coolers developed so far show similar trends, and generally support the optimization results.     

Passive coolers for pre-cooling of JT loops for deep space infrared imaging applications

Infrared instruments (IR) for deep space imaging missions, such as the James Webb Space Telescope (JWST) and Planck, require cryogenic cooling for proper operation of their focal plane arrays (FPA) in far infrared and sub-millimeter wavelength ranges. The FPA is sometimes located meters away from the spacecraft. To meet such remote cooling requirement, a Joule-Thomson (J-T) loop becomes a convenient choice for either direct cooling for the FPA or for serving as a heat sink for a cascade cooling system. The refrigerant lines of the JT loop inevitably suffer parasitic heat leak primarily due to IR backload as they traverse from the spacecraft to the FPA. An actively cooled JT loop using a mechanical pre-cooler located at the spacecraft will experience the highest parasitic heat leak since the lines are cold through the entire length whereas a passively cooled JT loop can utilize a number of radiators to cool the lines down gradually in stages and hence reduce the heat leak. In addition to savings in power and mass, a passive cooler offers consistent and predictable performance with practically no performance degradation in a thermally stable orbit, such as one around the Sun-Earth L2 point. Passive coolers are less popular in low temperature applications when their cooling capacity diminishes rapidly in proportion to T4 until the temperature reaches a point where either the parasitic heat leak becomes so significant or its size becomes so excessive that the passive cooling scheme becomes impractical. Despite the limited capacity, passive cooling may still prove to be a viable alternative to active cooling depending on the operating temperature and heat dissipation rate of the FPA. The current effort aims at evaluating the merit of using passive coolers as an alternative to using a mechanical cooler for pre-cooling of a JT loop for remote IR instrument cooling. A parametric study is conducted to explore the merits of passive cooling of a JT loop in a temperature range below 30 K. Correlations between cooling capacity, heat leak from supporting structure, and the operating temperature are investigated to provide design guidelines. Radiator staging options will also be presented and discussed.     

SPICA sub-Kelvin cryogenic chains

SPICA, a Japanese led mission, is part of the JAXA future science program and is planned for launch in 2018. SPICA will perform imaging and spectroscopic observations in the mid- and far-IR waveband, and is developing instrumentation spanning the 5–400 μm range. The SPICA payload features several candidate instruments, some of them requiring temperature down to 50 mK. This is currently the case for SAFARI, a core instrument developed by a European-based consortium, and BLISS proposed by CALTECH/JPL in the US.SPICA’s distinctive feature is to actively cool its telescope to below 6 K. In addition, SPICA is a liquid cryogen free satellite and all the cooling will be provided by radiative cooling (L2 orbit) down to 30 K and by mechanical coolers for lower temperatures. The satellite will launch warm and slowly equilibrate to its operating temperatures once in orbit. This warm launch approach makes it possible to eliminate a large liquid cryogen tank and to use the mass saved to launch a large diameter telescope (3.2 m). This 4 K cooled telescope significantly reduces its own thermal radiation, offering superior sensitivity in the infrared region.The cryogenic system that enables this warm launch/cooled telescope concept is a key issue of the mission. This cryogenic chain features a number of cooling stages comprising passive radiators, Stirling coolers and several Joule Thomson loops, offering cooling powers at typically 20, 4.5, 2.5 and 1.7 K. The SAFARI and BLISS detectors require cooling to temperatures as low as 50 mK. The instrument coolers will be operated from these heat sinks. They are composed of a small demagnetization refrigerator (ADR) pre cooled by either a single or a double sorption cooler, respectively for SAFARI and BLISS. The BLISS cooler maintains continuous cooling at 300 mK and thus suppresses the thermal equilibrium time constant of the large focal plane.These hybrid architectures allow designing low weight coolers able to reach 50 mK. Because the sorption cooler has extremely low mass for a sub-Kelvin cooler, it allows the stringent mass budget to be met. These concepts are discussed in this paper.     

Test and simulation of a solar powered solid sorption cooling machine

At the Institut für Thermodynamik und Wärmetechnik (ITW) a solar powered cooling machine with no moving parts has been built for demonstration purposes. The main part of the device is an absorber/desorber unit which is mounted inside a concentrating solar collector. The heat of absorption is transported out of the solar collector by means of two horizontally working heatpipes. The working pair consists of NH₃ used as the refrigerant and SrCl₂ as the absorbing medium. The performance of the solar refrigeration unit was measured in a field test.The working principle of a discontinuously working, solid sorption cooling machine will be explained using the demonstration machine as an example. Results obtained from a field test performed in 1995 are presented and discussed. Furthermore, a simulation program for the numerical simulation of a solar powered solid sorption cooling machine has been developed and tested.Finally, to confirm a problem-free continuous operation of a solid sorption system, a long term study over 14 months was carried out using NH₃/SrCl₂ as the working pair.     

Thermosyphon-assisted cooling system for refrigeration applications

Residential refrigeration is one of the largest contributors to home appliance energy consumption in the U.S. This energy use is nearly doubled if commercial refrigeration is included. By using cold outside temperatures for cooling, however, energy savings can be realized. In this work, a thermosyphon-assisted cooling system for a household refrigerator is presented. A single finned copper-tube thermosyphon with methanol as the working fluid is used. An environmental chamber that can reproduce outside ambient temperatures to −5 °C is located above the refrigerator. Results are presented for different outside temperatures, fan speeds, and filling ratios in the thermosyphon. The electricity use for both the original equipment refrigeration and the thermosyphon system is reported. The interior temperature is found to vary depending on the number of fans that are used to blow air over the thermosyphon evaporator in the refrigerated space. This can be used to produce localized micro-climates within the refrigerated space for further energy savings.     

Performance analysis of a two-stage mechanical compression–ejector cooling cycle intended for micro-trigeneration system

This paper provides the results of a performance analysis of a two-stage mechanical compression–ejector cooling cycle. In the proposed cooling system the compression process is realized in two stages: by a mechanical compressor as the first stage and by an ejector as the second stage. Ammonia (R717) is investigated as the working fluid for the cooling system in the present study. The influence of the middle pressure, and evaporating and condensing temperatures on the characteristics of the cooling system is analyzed. Based on the obtained results a pilot small-scale two-stage refrigeration unit with cooling capacity of 10 kW intended for application in micro-trigeneration systems is designed.     

Baseline design of a sorption-based Joule-Thomson cooler chain for the METIS instrument in the E-ELT

METIS, the Mid-Infrared E-ELT Imager and Spectrograph, is one of the proposed instruments in E-ELT (European Extremely Large Telescope). Its infrared detectors require multiple operating temperatures below 77 K. Therefore, active coolers have to be deployed to provide sub-liquid-nitrogen (sub-LN2) temperature cooling. However, the sensitive imaging optical detecting system also demands very low levels of vibration. Thus, the University of Twente proposed a vibration-free cooling technique based on physical sorption. In this paper, we describe the baseline design of such a sorption-based Joule-Thomson cooler chain for the METIS instrument, that is able to deliver cooling powers of 0.4 W at 8 K, 1.1 W at 25 K and 1.4 W at 40 K from a 70-K heat sinking. This design is based on working fluid selection, cascading cooler stages and operating parameter optimization. Also, the performance of the resulting cooler design is analyzed.     

Role of working fluids on the cooling of discrete heated modules: a numerical approach

The study has focused on the role of working fluids (air, water and FC-72) on the cooling of discrete heated modules under free, forced and mixed convection medium. Three non-identical protruding discrete heat sources are arranged at different positions on a substrate board following golden mean ratio (GMR). Numerical simulations for these heat sources are carried out using a commercial software (ANSYS-Icepak R-15) to simulate their flow and temperature fields under three different modes of heat transfer. Results suggest that the temperature of the heat sources is a strong function of their size, position on the substrate board, the velocity of the fluid and type of working fluid used. A correlation has been proposed for the temperature of these heat sources keeping in mind their strong dependence on the afore-mentioned parameters. It has been found that water can be used for better heat removal from the heat sources due to its high boiling point. The whole idea gives a clear insight to the electronic cooling engineers regarding the selection of working fluids and modes of heat transfer for the cooling of electronic components.     

Conceptual design of a high-efficiency absorption cooling cycle

The search for high-efficiency, gas-fired cooling cycles has led to the development of dual-loop absorption machines with cooling coefficients of performance (COPs) in the 1.2 to 1.7 range. This increased performance may call for high generator temperatures, new working fluids or new materials of construction. In most cases, two different sets of working fluids are required. The conceptual design presented here is aimed at obtaining high efficiencies with relatively low temperatures, employing only one set of fluids. The concept consists of two loops coupled in a configuration aimed at minimizing the loss of thermodynamic availability incurred when transferring refrigerant between the loops. The working fluid pair is a solution of lithium bromide-water. The calculated COPs are of the order of 1.8. The cycle relies on an elaborate evaporator-absorber combination. The paper presents the conceptual design, the critical assumptions, and the performance calculations for the concept.     

50 mK cooling solution with an ADR precooled by a sorption cooler

CEA/SBT is currently developing a 2.5 K-50 mK cooling solution composed of a small demagnetization refrigerator (ADR) precooled by a sorption cooler, equivalent to the high temperature stage of a two-stage ADR system. Thanks to the use of this dual technology, a low weight cooler able to reach 50 mK with a heat sink up to 2.5 K can be designed. Because the sorption cooler is probably the lightest solution to produce sub-Kelvin temperatures, these developments allow us to propose a solution to face the drastic reduction in the mass budget of space missions like SPICA or IXO. The European Space Agency (ESA) is funding the development of an engineering model able to produce 1 μW net heat lift at 50 mK. It is sized so that the sorption cooler provides an additional 10 μW at 300 mK. The ESA main requirements are an autonomy of more than 24 h and a recycling time smaller than 8 h. We present the design of the system able to meet these requirements as well as the expected performances and preliminary measurements.     

Investigation on gradient thermal cycle for power and refrigeration cogeneration

In order to improve energy utilization efficiency of low grade heat, a novel gradient thermal cycle for power and refrigeration cogeneration is proposed. The cycle is cascaded with two stages based on different thermal driven temperature. The first stage is pumpless Organic Rankine Cycle (PRC) while the second stage is two-stage sorption refrigerator. R245fa is selected as the working fluid of PRC, whereas CaCl₂-BaCl₂-NH₃ working pair is chosen for two-stage sorption refrigerator. Different heat source temperatures from 80°C to 95°C are adopted for analysis and comparison. Results indicate that the highest average power output and cooling effect are able to reach 204 W and 0.91 kW under the condition of 95°C heat source temperature and 10°C refrigeration temperature. For different heat source temperatures, total energy and exergy efficiency of the gradient thermal cycle for power and refrigeration cogeneration range from 9.49% to 9.9% and 10.9% to 11.8%, respectively. For gradient thermal cycle exergy efficiency of heat utilization ranges from 24% to 18.8% which is 126.5% and 70.9% higher than the PRC and two-stage sorption refrigerator, respectively, when the heat source temperature is 80°C.     

State of the art in sorption heat pumping and cooling technologies

Heat transformation with sorption systems has received increased attention in recent years. In this review it is intended to discuss current trends as well as forthcoming applications with the respective appropriate technology as it is deduced from activities in the field. Especially, we report about the papers and discussions of the International Sorption Heat Pump Conference (ISHPC'99) which was held in March 1999 in Munich, Germany. The review is grouped into a fundamentals part, a part about thermodynamic cycles, and an applications part. In the fundamentals part the discussion about working pairs and heat and mass transfer is reflected. Thermodynamic cycles which are being discussed are special solid sorption cycles, cycles fit for low-temperature driving heat, compression-sorption hybrids, and open cycles. In the applications part the classical cooling business is the main issue. The review comprises chillers and refrigerators which may be direct fired or waste heat driven. Interest is given to the improvement of efficiency on the one hand as well as to adaptation to low temperature waste heat use on the other hand — two very different developments. The use of solar energy as a heat source belongs to that area also. An additional important role — for decades — is played by automotive application. The area of heat pumping for heating purposes is less prominent but not negligible. Systems with a large capacity are being installed every once in a while, but the small scale domestic market still is not really covered with appropriate technology. Finally, industrial heat pumping involves the reverse cycle (heat transformer) also. Activity in this field is rather small. In summary, no unexpected developments can be reported on, but progress is steady and the market increases continuously, especially in the far east.     

A new generation of porous non-evaporable getters

The application of non-evaporable getters is increasing. In many of these applications the getters have to work at relatively low temperatures to avoid damaging internal components and their excessive degassing. At these low temperatures the bulk diffusion of the sorbed gases is normally very small (except for H₂) and therefore the active surface must be maximized. This means that large specific surfaces and porous structures have to be used. Already existing porous getters normally require activation at high temperatures (700–900°C) but in several cases this is not easy or even possible to achieve. A new generation of porous getters has therefore been prepared and studied to fulfill these requirements. The getters are based on a recently developed ZrVFe getter alloy, which is characterized by a high activity for the residual gases. The new porous getters are obtained via a sintering process between zirconium and ZrVFe alloy powders, which, however, maintains a relatively large surface area and porosity. Therefore, they combine the large specific surface and porous structure necessary for surface sorption of gases at low temperatures with the characteristics of high diffusivity, which makes possible efficient sorption also after activation at relatively low temperature (400–500°C). Besides these already remarkable properties, the getters show particularly attractive mechanical characteristics to meet the special working conditions of some tubes (vibrations, shocks, etc.). The getter performances for the main gases (H₂, CO, H₂O) are presented and discussed; the tests are performed using standard dynamic sorption techniques and the microbalance method (for H₂O). The results obtained for sorption at room temperature confirm the good performances due to the high surface area. Higher sorption temperatures (200°C, 400°C) tested show the large effects of the high diffusivity of the material used, which is responsible for the further noticeable improvement of the sorption characteristics obtained in the lower temperature ranges. These getters appear, therefore, to be a real contribution to particular problems of many vacuum devices such as special electron tubes.     

Carnot-COP for sorption heat pumps working between four temperature levels

The theoretical efficiency limits of heat driven heat pumps operating between three and four temperatures are derived from the fundamental thermodynamical laws, i.e. the energy balance and the entropy balance. While in the three temperatures case the system is fully determined by specification of the three temperatures and the cooling capacity, a four temperature heat pump needs, in addition to the four temperatures and the cooling capacity, specification of an additional operating parameter. This can be, for example, the ratio of the two heat flows which are released at the two different intermediate temperatures. Various assumptions regarding this proportion are discussed with respect to their relevance for both the combination power cycle/vapor compression cycle as well as for single-effect sorption cycles. The present analysis shows that a single-effect sorption heat pump is principally not able to operate reversibly in an environment of four externally specified temperatures unless the four temperatures follow, incidentally, a correlation that is given by the equilibrium properties of the employed working fluids. Therefore, in endo-reversible models for four-temperature sorption cycles only three rather than four operating temperatures may be specified independently.     

In-storage softening of kiwi fruit: effects of delayed coolingRamollissement des kiwis au cours de l'entreposage: influence du retard de la réfrigération

Rapid softening of kiwi fruit while in storage at 0°C limits the marketing period for this crop and contributes to economic loss. The time required to bring fruit to optimum storage temperature is affected by delays between harvest and placing fruit in the cooling facility and by the completeness of cooling. Incomplete cooling of fruit in commercial forced-air coolers results in overall above-optimal average fruit temperatures for undesirable lengths of time and also results in a wide range of temperature among individual fruits. The half cooling time for palletized kiwi fruit packed in wooden trays with liners was 7 h in the coolers tested; thus seven-eighths cooling required 21 h. Delays of 24 h or more before the start of cooling accelerated the softening of fruit, enhanced soluble solids content, and increased the incidence of rotting and shrivelling during storage.