Design analysis of a hybrid jet-pump CO2 compression system

Transport refrigeration contributes to anthropogenic global warming directly because of leakage of refrigerant, usually using high global warming potential (GWP) refrigerants, and indirectly because of the greenhouse gases emitted in driving the vehicle and the refrigeration system. A hybrid jet-pump CO2 compression system is being designed for transport refrigeration so that the GWP of the system is reduced and its performance improved. The jet-pump utilises waste heat from the exhaust gases of the engine to subcool the refrigerant and so enhance performance, reduce energy required from the engine and minimise GWP of the system. The hybrid jet-pump CO2 compression system has been simulated and its performance determined for different operating conditions and optimised using entropy generation minimisation. At an evaporator temperature of -18°C, an ambient temperature of 35°C and a generator temperature of 120°C, the COP increases from about 1.0 to 2.27 as the degree of subcooling increases from 0K to 20K. Similarly, compressor work is reduced by 24% at 20K subcooling. The optimum degree of subcooling was approximately 10K for the operating conditions described above. An improved COP is achieved whilst the size of heat exchangers required to operate the jet-pump are minimised with respect to the overall weight of the system and thus its impact on indirect emissions

Intelligent thermoregulation and homeostasis: lessons from nature

Mankind has developed sophisticated means of maintaining comfortable living conditions around the world. We use many technologies that allow us to achieve thermoregulation and homeostasis and so live, work and survive in otherwise hostile environments. Many species in nature also practice thermoregulation and homeostasis, with some of the most interesting being the social insects. Biomimetics is defined as design inspired by nature, and so a biomimetic approach to thermoregulation might enable engineers to develop novel strategies that have less of an impact on natural resources and are more responsive to the environment. The mound building termites of South America, Africa and Australasia construct sophisticated structures that enable the environment within the nest to be regulated. Wind energy is captured to exchange gases between the nest, the mound and the outside by elaborate internal galleries, tunnels, and ducts. Gas exchange and ventilation could be achieved through responsive envelopes and fabric analogous to termite homeostasis. The oriental hornet can maintain a nest temperature of 28-32°C in tropical, sub-tropical and temperate areas. It has been found that when the adult population are removed from the nest, its temperature is maintained for a number of days afterwards. The oriental hornet nest houses both the adult population and combs that contain the brood. Eggs are placed in the walls of the comb and when they hatch the pupa spin a silk weave and form a silk cap at the open entrance, sealing it from the outside. In studying the thermoregulatory properties of hornet nests, it has been shown that the silk cap and walls of the comb have thermoelectric properties that could help regulate the temperature in the comb. It was shown that as ambient temperature increases, the current intensity increases. When the ambient temperature falls the energy stored is discharged with a flow of electric current from high to low potential. The engineering materials with properties closest to the Hornet silk are the conductive polymers, although the thermoelectric properties may be more than 10,000 times lower than semiconductors used in modern devices they may be useful as heat storage and discharge systems in buildings

An investigation of a jet-pump thermal (ice) storage system powered by low-grade heat

This thesis investigates a novel combination of a jet-pump refrigeration cycle and a thermal (ice) storage (TIS) system that could substantially reduce the electrical energy requirements attributable to comfort cooling.Two methods of TIS were identified; spray ice TIS would use evaporative freezing to store ice on a vertical surface,and encapsulated ice TIS would freeze a bed of encapsulated elements by sublimation freezing.Thestudy also investigates jet-pump refrigeration at partload and a convergent-divergent design manufactured from a thermoset plastic to make recommendations for performance enhancement for a system that has a low COP. An experimental rig was built to investigate the novel concepts in the laboratory. Encapsulated ice TIS was superior to spray ice TIS because, for the same nominal secondary flow, sublimation freezing causes an increase in coolth storage rate of about 10 % compared to evaporative freezing. Encapsulated ice stores experience difficulties in fully discharging their coolth (approximately 6% in this case), but spray ice TIS can be used to produce an ice/brine slurry enabling all of the ice to be used, and so may be more suitable if the unmelted ice represents a large proportion of the cooling capacity. Approximately 85 % to 90 % of the ice formed on the vertical surface during spray ice TIS testing was formed by evaporative freezing from a falling film. At high saturation conditions, heat is transferred mainly by conduction across the falling film. Both the growth of an ice layer on a vertical surface and freezing of encapsulated elements were found to be successful, but a large data spread was observed during spray ice TIS testing. It was thought that a variation in the steady-state saturation conditions in the evaporator/ice store was caused by variability of droplet size distribution from the spray nozzle flow, which may make a full-scale system unreliable. The COP of the spray ice TIS system was approximately 0.15 compared to a COP of approximately 0.25 found during encapsulated ice TIS testing. The difference was because of the use of an over-expanded primary nozzle, which restricted secondary flow and increased momentum losses. A primary nozzle that expands close to the design evaporator saturation conditions should be used to maximise entrainment ratio. The COP of a jet-pump TIS is low, but a system designed to operate at off-peak periods could increase the COP to about 0.8 by taking advantage of the lower ambient conditions. The measurement of entrainment ratio was used successfully to determine ice storage rate and COP. This was valid because of the assumption that the saturation conditions in the evaporator/ice store approached steady-state. However, over longer periods that would be found in large-scale systems, the ice storage rate and entrainment ratio may fall substantially. The steady-state assumption could still be used to observe the change in evaporator conditions by sampling over short time intervals (30 minutes). At part-load, increases in evaporator saturation temperature could increase entrainment ratio substantially (50 % increase) for only a small reduction in critical pressure lift ratio Ns *(15 % reduction). A variation in chilled water temperature could be used to boost entrainment ratio at the peak demand. The variation in Ns* is too small to use this strategy to control the jet-pump with respect to condenser operating conditions. The entrainment ratio is approximately proportional to the diff-user to primary nozzle area ratio. A doubling of entrainment ratio was attained for only a 15% reduction in Ns*. The change in geometry from a constant area throat to a convergent-divergent design caused the flow through the jet-pump to vary with outlet conditions indicating that secondary flow was not choked. Higher entrainment ratios and pressure lift ratios were observed, but the entrainment ratio varied with outlet conditions in the form of peaks and troughs, making its operation unpredictable. This was thought to be caused by the restriction in secondary flow area due to the interaction of the primary jet and the curved wall. The convergent-divergent design manufactured from a thermoset plastic was successfully tested, showing that a plastic material can be used as a material of construction. In principle, a large number of jet-pump units could be manufactured from a single mould, reducing the first cost. The investigation proved the concept of jet-pump TIS. Waste-heat could be utilised over 24 hours and year round, increasing the efficiency of the process. The use of a convergent-divergent throat design, multiple geometry jet-pumps and operation at off-peak periods can maximise the performance over a cooling season, and be competitive with other TIS and chiller systems. The mass production of jet-pumps using injection moulding techniques could reduce substantially the capital cost of a system. All of these factors should encourage the development of such systems, so that the harmful emissions caused by the use of air conditioning systems can be minimised

An experimental investigation of a micro-tubular SOFC membrane-separated liquid desiccant dehumidification and cooling tri-generation system

This paper reports the results of experimental work carried out on a micro-tubular solid oxide fuel cell tri-generation systemthat uses the waste heat from the fuel cell for dehumidification and cooling though the integration of an open cycle liquid desiccant dehumidification and cooling system. The experimental results demonstrate regeneration of the potassium formate solution using the thermal output from the SOFC in the first of its kind tri-generation system. Optimisation has shown that a 2.2L.min-1 regenerator desiccant volumetric flow facilitates best performance.When integrated with the micro-SOFC, the open cycle desiccant system demonstrates a COP of approaching 0.7, an encouraging value for a waste heat driven cooling system of this capacity. A tri-generation performance analysis is presented which serves to demonstrate the novel system operating in a building. The system achieved an electrical efficiency of 11% and regeneration efficiency of approximately 37%. The electrical efficiency is lower than that predicted by the company supplying the micro-tubular SOFC, because the unit suffered sulphur poisoning during preliminary tests. The electrical power output decreased from 250W to 150W, which reduced the electrical efficiency from around 18% to 11% and the overall efficiency from approximately 45% to just over 37%. Low temperature (33-36°C) regeneration was demonstrated

Experimental comparison of a DC PV cooker and a parabolic dish solar cooker under variable solar radiation conditions

Solar cookers are not all-weather cooking devices and operate poorly during cloudy and low sunshine conditions. Their performance is evaluated usually during high solar radiation conditions. The objective of this study is to compare two solar cookers under variable non-ideal weather conditions. The comparison is carried out under variable solar radiation conditions to compare the all-weather performance of these two cookers. This is a major novelty compared to previous work reported where solar cookers are tested under high and ideal solar radiation conditions. Experiments to compare a PV DC battery-powered solar cooker and a parabolic dish solar cooker are presented in this paper. A total of six water heating tests are carried out to comprehensively compare these two types of solar cookers under different solar radiation conditions. Also, four food cooking tests are carried out with different types of food. The PV solar cooker shows almost constant input electrical power in the range of 160–180 W during the experimental tests whereas the input thermal power for the parabolic dish is highly variable depending on the solar radiation conditions (200–1200 W). The output water heating powers obtained using the PV cooker (66–100 W) are comparable to those obtained with parabolic dish solar cookers (78–142 W), regardless of the significantly lower input heating power. Water is boiled in all the heating tests with the PV cooker, whereas water is boiled for tests with low solar radiation variability for the parabolic dish solar cooker. Higher water heating efficiencies within a small range (0.38–0.57) are obtained for the PV cooker compared to the parabolic dish solar cooker (0.11–0.42). The water heating efficiency of the parabolic dish solar cooker is highly affected by ambient solar radiation and windspeed conditions. Food was well cooked with the PV cooker in all four food cooking tests, whereas food in only two tests with low solar radiation variability was well cooked for the parabolic dish solar cooker. The PV cooker proves to be an all-weather cooker from the experimental results obtained. Future work will extend the use of the PV system for other domestic applications such as lighting and refrigeration together with solar cooking for a multipurpose DC decentralized system for communities without grid connectivity

Theoretical and experimental investigations of different area ratios of a supersonic ejector driven by compressed air

Ejectors have some advantages such as being simple, reliable and no moving parts. They can be used in air-conditioning and refrigeration applications. This paper presents a comparison of ejector performance, primary pressure (Pp), back pressure (Pb) and area ratios of ejectors (A2/At) predictions by an analytical model and a computational fluid dynamics model for different operating conditions. Six different area ratios of ejector using air as working fluid in this study were proposed and tested experimentally. The variable area ratios of ejectors (A2/At) were used with a range from 10.68 to 30.62. Two sets of ejectors (A and C) are studied and examined depend on the kind of nozzle. The aim of this study was to investigate these ejectors under variation of primary pressure (Pp) (1.5–6.0 bar) and adjustable spindle position (0 to −25 mm). Two groups of ejectors (A and C) were categorized based on the type of nozzle. The experimental results validate the solutions of the main parameters of ejectors using air as working fluid. The results show that group A is more appropriate for higher values of back pressure, while group C is more suitable for high performance of the ejector. Finally, the main parameters were carried out on six different ejectors to find the best combination based on various nozzles and constant area sections

The microencapsulation, thermal enhancement, and applications of medium and high-melting temperature phase change materials: A review

Microencapsulated phase change materials (MEPCMs) have made tremendous advancements in recent years, owing to their increased demand for a variety of energy storage applications. In this paper, current microencapsulation techniques, enhancement, and use of medium- and high-melting phase change materials (PCMs) are reviewed, as well as their potential benefits and limitations. The most frequently employed PCMs for medium- and high-temperature applications were recognized as salt-based, metallic, inorganic compound, and eutectic. Meanwhile, polymethyl methacrylate (PMMA), polystyrene-butylacrylate (PSBA), polyethyl-2-cyanoacrylate (PECA), and polyurethane were widely used as polymer shell materials for encapsulating medium- and high-melting point PCMs via chemical method, whereas inorganic silica shell was synthesized via various techniques. Hydrolysis followed by heat-oxidation treatment has been extensively studied since 2015 to encapsulate either metal or alloy within Al2O3 shells. Different techniques were developed to generate void between core and shell material to accommodate volume expansion during phase transition. Numerous approaches, including the incorporation of metal particles, carbon, and ceramic, have been found as ways to enhance the thermal performance of PCMs. Multiple storage arrangements were also established to be an effective way of enhancing the overall efficiency of medium-high melting PCM storage systems. Finally, the paper highlights the potential of medium- and high-melting temperature PCMs for solar power generation, solar cooking, and industrial waste heat recovering applications

Gender Diferences in Heart Failure Self-Care: A Multinational Cross-Sectional Study

Background Despite a common view that women are better at self-care, there is very little evidence to support or challenge this perspective in the heart failure (HF) population. Objective The purpose of this study was to determine if there are cross-cultural gender differences in self-reported HF self-care and to describe gender differences in the determinants of HF self-care. Design, setting, and participants A secondary analysis was completed of cross-sectional study data collected on 2082 adults with chronic HF from the United States, Australia and Thailand. Methods Comparisons were made between men and women regarding self-care maintenance, management and confidence as assessed by the Self-Care of Heart Failure Index, as well as the proportion of subjects engaged in adequate self-care. Multivariate comparisons were made to determine if gender explained sufficient variance in HF self-care and the likelihood of reporting adequate self-care, controlling for nine model covariates. Results The sample was comprised of 1306 men and 776 women. Most (73.5%) had systolic or mixed systolic and diastolic HF and 45% had New York Heart Association class III or IV HF. Although small and clinically insignificant gender differences were found in self-care maintenance, gender was not a determinant of any aspect of HF self-care in multivariate models. Married women were 37% less likely to report adequate self-care maintenance than unmarried women. Comorbidities only influenced the HF self-care of men. Being newly diagnosed with HF also primarily affected men. Patients with diastolic HF (predominantly women) had poorer self-care maintenance and less confidence in self-care. Conclusion Differences in HF self-care are attributable to factors other than gender; however, there are several gender-specific determinants of HF self-care that help identify patients at risk for practicing poor self-care

Solid–Gas Thermochemical Energy Storage Materials and Reactors for Low to High-Temperature Applications: A Concise Review

Thermochemical energy storage materials and reactors have been reviewed for a range of temperature applications. For low-temperature applications, magnesium chloride is found to be a suitable candidate at temperatures up to 100 °C, whereas calcium hydroxide is identified to be appropriate for medium-temperature storage applications, ranging from 400 °C up to 650 °C. For the high-temperature range (750–1050 °C), oxides of cobalt, manganese, and copper are found to have the redox behaviour required for thermochemical heat storage. However, some of these materials suffer from low thermal conductivities, agglomeration, and low cyclability and, therefore, require further improvements. The concept of enhancing thermal conductivities through additives such as nanomaterials has been encouraging. From an operational point of view, fluidized-bed reactors perform better than fixed- and moving-bed reactors due to better particle interactions. There is, however, a need for the reaction bed to be further developed toward achieving optimum heat and mass transfers. Agitated fluidized-bed reactors have shown encouraging results and are suggested for further exploration. A combination of appropriate computational tools can facilitate an in-depth understanding of bed dynamics

A novel hybrid dew point cooling system for mobile applications

A novel dew point cooling system for truck cabin cooling has been designed and tested, which shows excellent performance in environments of low to medium relative humidity. Experimental results showed that cooling capacity of over 1kW could be achieved with up to 12K of sensible cooling of the supply air and a COP of up to 7. The DPC unit can provide comfort cooling to a vehicle using a rooftop unit whose DPC core is no larger than 0.5m x 0.5m x 0.25m