Simulation of a solar driven aqua-ammonia absorption refrigeration system Part 1: mathematical description and system optimization

A theoretical thermodynamic analysis was performed on an aqua-ammonia refrigeration (AAR) cycle coupled to a solar water heating system using CPC-collectors and augmented with auxialiary energy. Thirteen AAR cycles were considered in the search for the maximum coefficient of performance. Also investigated was the exergetic performance of the AAR cycle. The adopted temperature and mass flow rate control strategy on the storage tank-auxiliary heater-AAR generator loop ensured operation of the refrigeration systems at design conditions. Consequently, the ratio between the pumps' electrical power and the designed evaporator heat transfer rate was kept to a minimum.     

Pressure, flow and temperature transients in refrigeration systems

The pressure, flow and temperature transients which occur in simple refrigeration systems (incorporating both dry expansion and flooded evaporators), when subjected to disturbances such as control and load inputs and when defrosting, are described. The effects of oil in such systems are also considered. It is concluded that such transients have a significant influence on system reliability and that system design still contains a significant element of art as well as technology.     

Electronic energy saving in refrigeration equipment

Electronic technology is now available for use on most types of refrigeration equipment. Loss reduced motors (LRMs), using development of technologgy originally invented for the NASA space programme, tackle a major area of inefficiency. Refrigeration engineers are well aware of the inefficiency of running the compressor drive motor at part load, and in central plant have minimized the problem by using several motors with load switching to match the actual load requirement. However, in smaller single compressor equipment no solution has yet been proposed. LRMs can provide the solution and, moreover, are etremely cost-effective with installation paybacks well within two years.     

Thermo-economic analysis of refrigeration and heat pump systems

This Paper is based on a lecture presented the Austrian Society of Refrigeration Engineers. The optimization of refrigeration and heat pump systems discussed and a thermo-economic analysis is outlined using the Second Law of Thermodynamics.     

Comparison of conventional and automated refrigerated storage systems

High rise warehouses with automated storage and retrieval systems (AS/RS) can frequently be justified as a viable alternative to conventional freezer storage. To determine viability, a design year data base must be developed and both conventional and automated system/facility options tested against this operational standard. To determine which alternative best suits design year requirements, a justification analysis must be completed. This analysis must address investment costs and total operational costs. Operational factors such as expandability, flexibility and maintainability must be considered. This Paper outlines a basic approach to the justification analysis required when considering high rise automated and conventional alternatives for refrigerated warehousing applications.     

Refrigeration throughout the world

The economic impact of refrigeration technology throughout the world is already very impressive, and more significant than is generally believed. While the yearly investment in machinery and equipment may approach US$100 billion, the value of products treated by refrigeration is perhaps ten times this amount. The importance of refrigeration is bound to increase since it will be an essential factor in solving two major problems of the future: the supply of enough food and the development of new energy sources. The use of low temperatures constitutes a major means of conservation of perishable foods during storage and distribution and it is widely applied in the developed countries. In the Third World, however, the use of low temperatures is mainly limited to food for export and the needs of the ‘well-to-do’ sector of the population. Refrigerated or frozen foods, in their present form, are generally priced too high for the undernourished poor. There is a pressing need for simple and inexpensive cooling methods to enable efficient low cost mass production and distribution under more primitive conditions. The future of man, and his food supply in particular, depends on the availability of sufficient energy. At present, fossil fuels carry the main load, but in the course of the next century new energy sources, such as nuclear fusion, will have to be commissioned. Refrigeration technology will play an important part in this development.     

Design and operating characteristics of evaporative cooling systems

Evaporative cooling systems are commonly used in countries where the climate is hot and dry, as found in most zones of India and Australia. The potential energy savings envisaged by replacing conventional refrigerated systems by evaporative systems is ≈75%. Indirect systems can achieve comfort conditions similar to refrigerated systems in climatic zones where the wet bulb temperature is usually <25°C. The comfort afforded by indirect evaporative systems is superior to that achieved by direct evaporative systems. An 8.5 ton indirect-direct evaporative cooling system has been fabricated and tested and its performance compared with a computer prediction. The system's scope for use in India and Australia is analysed.     

Review article from ASHRAE symposium, San Francisco, January 1986: How to save energy in hotel system design

The following papers were presented at this symposium:

1. 1. HVAC systems and energy conservation in hotels by J. R. Wagner, Environmental Engineering Co., Baltimore, Maryland.

2. 2. Cogeneration for resort hotels by T.D. Baker, RTKL Associates, Inc., Baltimore, Maryland.

3. 3. How much sense do room occupancy sensor controls make? by H. P. Becker, Chavenah Keane and Company, New York.

This Paper reviews these three articles.     

Simulation of a solar driven aqua-ammonia absorption refrigeration system Part 2: viability for milk cooling at remote Brazilian dairy farms

Refrigeration at dairy farms in Brazil would contribute substantially to improving the rate and quality of milk production. Farmers would be able to milk twice a day or more, and keep the milk refrigerated until it was transported to the delivery station. The 1980 rural electricity consumption in Brazil's major milk-producing state was only 0.93% of the national total². This low consumption can be attributed to the high costs associated with the electrification of the rural regions. In these areas, cooling cycles based on the absorption principle may be especially suitable. Solar energy, coupled with methane generated from cattle herd wastes, has been investigated as a source of auxiliary energy.     

Refrigeration and the world's food supply — especially in developing countries

The reason for starvation today is poverty. Refrigeration can help the developing countries to export high value products. This helps to improve the economical situation and their buying power. Refrigeration can also help to reduce waste. Primitive distribution systems using refrigeration can be used at an early stage but the infrastructure is the limiting factor for introduction of modern transport equipment. Appropriate refrigeration technology is required, and the IIR should promote this by, for example, introducing an award for good proposals.     

Experimental verification of a mathematical model for simulation of industrial refrigeration plants

A mathematical model, proposed for simulation of food refrigeration processes, was tested against experimental data collected in a New Zealand meat processing plant which had a total refrigeration capacity of 2.5 MW. Predicted air temperatures in freezing and chilling operations followed the same trends as measured data. Differences were shown to be more attributable to uncertainties in data than to deficiencies in the formulation of the mathematical model. The model is considered suitable for use in simulation of a wide range of food refrigeration processes. Such simulations provide useful information about plant performance that cannot be obtained by steady state analysis. Predictions were worst for short times immediately following step changes in plant operation; should more accurate simulation of these periods be required a different form of mathematical model is needed.     

Thermoeconomic optimization of a refrigeration plant

In this paper, thermoeconomic theory is applied to the economic optimization of a conventional refrigeration plant, aimed at minimizing its overall operation and amortization cost.Thermal systems cannot always be optimized by means of mathematical or numerical techniques, because a complete model of the plant is not always available, and, in any case, mathematical difficulties are often great, even for not particularly complex systems, and the help of computerized algorithms is needed.In this paper, a simplified cost minimization methodology is applied, based on the so-called Theory of Exergetic Cost, here utilized to evaluate the economic costs of all the internal flows and products of the installation. As shown in the paper, once these costs have been calculated, a design configuration not far from the real global optimum can be obtained by means of a sequential, local optimization of the system, carried out unit by unit, that is, breaking down the global problem into a sequence of simpler problems.In the paper, the case of a very simple plant is considered to develop a numerical example, and, in spite of the approximations introduced to simplify the optimization procedure, the results obtained show acceptable accuracy when compared with those provided by a conventional and more complex optimization methodology.     

On reducing the size of liquid separators for injector circulation plate freezers

Liquid separators for injector plate freezers are large because the liquid level rises towards the end of the freezing process. To calculate the volume of liquid being collected in the separator during freezing the freezing time and the heat removed must be evaluated. A simple method of freezing time estimation based on the progression of a phase change front is proposed. The size of separators can be reduced considerably by letting part of the liquid feed by-pass the injector during initial freezing. With this arrangement the injector dimensions are based upon a refrigerating capacity lower than the maximum.     

Flow visualization applied to a small refrigeration compressor

This paper describes a flow visualization technique that was used to evaluate qualitatively the gas flow pattern inside a small, hermetically sealed, reciprocating refrigeration compressor. The applicable compressor designs are those in which the suction gas from the evaporator is dumped into the compressor shell, and is then drawn through a muffler into the suction plenum of the compressor. The physical separation of the muffler inlet from the suction gas inlet serves to reduce compressor noise and also provides an easy and convenient means of separating any liquid (compressor oil or liquid refrigerant) from the refrigerant gas. For the flow visualization studies the compressor housing was replaced by a clear plastic shell. Atmospheric air seeded with white smoke was the working fluid. The suction inlet and muffler were parts from a commercial compressor. The flow pulsations were modelled by connecting the muffler outlet to the input plenum of an auxiliary compressor. The flow patterns near the muffler inlet were recorded with a video camera. The mixing of the inlet gas with the gas circulating inside the muffler was studied. The effect of alignment and offset of the muffler inlet relative to the suction inlet, the effect of muffler size, and the effect of a shroud around the muffler were studied. The results were used to guide a companion study of detailed temperature and pressure measurements inside a working compressor.     

Thermal acoustic oscillations in triple point liquid hydrogen systems

Thermal acoustic oscillations are often observed in tubes which penetrate a cryogenic system and are closed at the warm end and open at the cold end. Such tubes are genrally used for filling or vetning the tank, providing relief pressure or inserting instruments taps. Large amounts of heat (of the order of ten to a thousand times more than by normal heat conduction) can be transferred into a cryogenic system when such thermaloscillations occur. A number of studies examining thermal acoustic oscillations in liquid helium systems have been performed by Rott et al. However, only minimal consideration has been given to such oscillationsin liquid and sluch hydrogen systems. This study extends Rott's theory to the stability aspects of thermal acoustic oscillations for a straight tube closed at the warm end and inserted into a Dewar flask filled with triple point liquid hydrogen when the cold open end is located above the liquid surface. These results can also be applied to a slush hydrogen when the pressure in the Dewar flask is reduced to the triple point pressure of hydrogen. Numerical results have been obtained in this study for developing stability curves, establishing oscillation frequency characteristics and identifying critical configurations for initiating such oscillation. The mechanisms associated with the two branches of the stability curves for thermal acoustic oscillations have also been investigated.     

Influence of adsorption cycle limitations on the system performance

The economics of heat driven heat pumps are governed by two thermodynamic quantities: the performance on the one hand and the size of the heat exchangers, which is required to obtain this performance, on the other hand. The aim of this paper is to discuss the influence of the main features of adsorption cycles in comparison to absorption cycles on these quantities. In our case, a Zeolite UCC16×40-type 13X has been taken as the adsorbent. The analysis has already been used for absorption heat pumps. In the case of adsorption and other solid sorption chillers, additional limitations appear, e.g. identical design of all adsorbers, incomplete heat recovery between adsorbers, cycling of inert mass, etc., which all contribute to a lowering of the system performance or to an increase of the exchange area required. To show the basic differences between absorption and adsorption cycle optimization, and also to show the impact of physical or technical limitations on the adsorption chiller performance, a detailed analysis has been performed. It is confirmed that it is mainly the lack of a solution heat exchanger which lowers the adsorption system COP and not physical properties of the working pair, so there is still a lot of room for improvement in the solid-sorption of heat pumps.     

Controlled atmosphere storage of celery with the silicone membrane system

Laboratory experiments were performed to assess the potential of the silicone membrane system for long term storage of celery under controlled atmosphere (CA) conditions. Designed CO₂ levels of 1.5, 3.5 and 5.5% were selected to calculate the silicone membrane areas to be installed on the chambers. Visual observations, mass losses and physiological analyses were the means selected to assess the quality of the product. Pascal celery, imported from California (USA), was stored at 1.5°C for 29 days under regular atmosphere (RA) with 80% relative humidity (RH), and 128 days under averaged carbon dioxide (CO₂)+oxygen(O₂) concentrations of 0.8+17.7, 2.0+12.6 and 4.0%+9.2% with RH close to saturation. The results showed that the first two CO₂+O₂ combinations were the most suitable CA compositions, among those tested, for long term storage of that cultivar, with total mass losses of less than 30 and 23% respectively. Clean cut celery, grown locally in Québec on organic soil, was stored at 1.5°C under RA with 80% RH and under three CA compositions (1.1+ 14.8, 2.3+7.9 and 3.2% CO₂+5.5% O₂) with RH close to saturation for 62 and 71 days, respectively. The total mass loss averaged 25% and no significant difference was observed between the RA and CA stored celery. In the experiment with Clean cut celery, the losses were mainly attributed to bacterial infections. Both celery cultivars stored under CA with high RH showed better texture and crispness than those stored under RA. It was also noted that the total sugar content significantly increased with storage.     

Thermoeconomic optimization of a two stage combined refrigeration system: a finite-time approach

A finite-time thermoeconomic performance analysis based on a new kind of optimization criterion has been carried out for a two-stage endoreversible combined refrigeration cycle model. The optimal performances and design parameters that maximize the objective function (cooling load per total cost) are investigated. In this context, the optimal temperatures of the working fluids, the optimum performance coefficient, the optimum specific cooling load and the optimal distribution of the heat exchanger areas are determined in terms of technical and economical parameters. The effects of the economical parameter that characterizes the investment and energy consumption costs on the general and the optimal performances have been discussed.     

Comparative study of irreversibilities in an aqua-ammonia absorption refrigeration system

Irreversibilities in components of an aqua-ammonia absorption refrigeratio system (ARS) have been determined by second law analysis. The components of the ARS are as follows: condenser, evaporator, absorber, generator, pump, expansion valves, mixture heat exchanger and refrigerant heat exchanger. It is assumed that the ammonia concentration at the generator exit is, independent of the other parameters, equal to 0.999 and at the evaporator exit the gas is saturated vapour. Pressrre losses between the generator and condenser, and the evaporator and absorber are taken into consideration. In the results the dimensionless exergy loss of each component, the exergetic coefficient of performance, the coefficient of performance and the circulation ratio are given graphically for each different generator, evaporator, condenser and absorber temperature.     

Production de froid par transformation thermochimique : expérimentation d'un nouveau système à double effet à contactThermochemical cooling sorption process: experimentation of a new double effect by contact system

The processes proposed in order to improve the energetic performances of thermochemical cooling sorption systems involve an increase of the technological complexity of the installation that can limit their practical interest. The double effect by contact studied allow to consider high energetic performances, simple working mode and also a good compactness of the installation. The analysis of the theoretical working mode of this process, compared to the classic double effect, permits putting forward both advantages and inconveniences of this new process. These are quantified with the help of results supplied from an experimental pilot.