A continuous/discrete simulation of controlled atmosphere (CA) cool storage systems: evaluation of plant performance/design and product quality evolution

In this paper, a controlled atmosphere storage model, developed earlier, is validated using an industrial CA cool storage facility with stepwise product loading strategy. The proposed combined continuous and discrete CA cool storage simulation model was able to represent the step response transients and continuous events, with accuracy less than the observed variability or at best with accuracy level as set by the measurement instrument. Moreover, the handling of discrete/continuous events enabled the implementation of practical operational procedures and to investigate their implication on plant performance/design and directly on the product quality. The model is a valid working model for design and optimization of CA cool storage facilities both with respect to plant performance and final quality of the stored product.     

Passive thermal energy storage in refrigerated warehouses

This paper investigates operational strategies that use stored products as thermal mass to shift refrigeration loads to more favorable operational periods (low energy cost periods, lower outdoor air conditions, etc.) allowing an opportunity to reduce system operating costs. An integrated model of the stored product, warehouse air, and warehouse structure is developed and thermal response characteristics are predicted for a selected warehousing facility. Simulated results are validated with experimental measurements. Food quality impacts associated with the temperature cycling caused by potential operating strategies are discussed. Results from this investigation indicated that a full load-shifting control strategy would save $82,000 (US) ($0.40/ft²/year or $4.28/m²/year) annually over the test facility's current operational strategy, representing 53% of the total cooling cost. Predicted maximum warehouse temperature variation is 5.6°C, which is not expected to cause significant product quality changes in the temperature range (below−18°C) studied.     

Part-mix allocation between automated and conventional manufacturing

The overall part-mix of a production system is allocated optimally between the existing conventional facilities and planned automated system to be acquired. Approximate mean value analysis (MVA) is utilized to evaluate performance of the automated system under trial part-mix allocations. Cost of processing and WIPs are taken as the sources of operational costs. Cost minimizing route allocations when alternatives exist are handled by a search procedure.     

Intelligent control schemes for automated storage and retrieval systems

A new information system approach to the operational controls of automated storage and retrieval systems (AS/RS) is developed and examined. This approach is based on artificial intelligence, state-operator framework for problem solving. Gradually increasing the information level, several operational goal functions are identified for an industrial unit-load food produce AS/RS. These functions use real-time statistical interpolations to select the desired storage and retrieval bins. As a result the AS/RS response adapts itself to stochastic perturbations in the system conditions. Experimental evaluations using multiple variance analysis technique and detailed simulations have shown that the proposed dynamic approach is superior to the common industrial control method currently used in those industrial systems characterized by batch arrivals (and retrievals) of the UL's and non-stationary demand patterns, These evaluations further suggest that improved performance is realized with the increase in the information level. The operational control scheme developed in this paper appears to be an excellent control alternative for unit-load AS/RSs. This is due to its limited computational requirements and the augmented productivity as demonstrated here for a real case study.     

Evaporative condenser control in industrial refrigeration systems

This paper is a result of a research project which focused on optimization of an existing industrial refrigeration system for a large two-temperature level cold storage distribution facility located near Milwaukee, Wisconsin. This system utilized a combination of single-screw and reciprocating compressors (each operating under single-stage compression), an evaporative condenser, and a combination of liquid overfeed and direct expansion evaporators. A mathematical model of the existing system was developed. The model was validated using experimental data recorded from the system. Subsequently, the model served as a tool to evaluate alternative system designs and operating strategies that lead to optimum system performance. The methods, analysis, and results presented in this paper focus on evaporative condenser sizing and head pressure control. Operating system head pressures that minimize the energy costs of the system were found to be a linear function of the outdoor wet-bulb temperature. A methodology for implementing the optimum control strategy is presented. Simulation results for the annual performance of the refrigeration system investigated in this project show a reduction in annual energy consumption by 11% as a result of the recommended design and control changes.     

Ice-water two-phase flow behavior in ice heat storage systems

In this paper, we describe (1) the characteristics of ice-water two-phase flow created in a supercooling-type ice storage system, (2) the numerical analysis models for both processes of ice storing and melting within ice storage tanks, and (3) the design technologies for ice storage systems based on such predictive models, as well as the operational performance of actual equipment built with these design technologies. As the result of this technological development, it has been confirmed that the predictive approaches for the one dimensional ice accumulation in the given tanks and ice melting with low temperature water enough to air conditioning applications are reasonable and widely usable. Furthermore by the design conditions obtained here it is possible to simplify the construction of ice storage tanks and to realize high reliability in the actual equipment.     

Waste heat driven dual-mode, multi-stage, multi-bed regenerative adsorption system

Over the past few decades there have been considerable efforts to use adsorption (solid/vapor) for cooling and heat pump applications, but intensified efforts were initiated only since the imposition of international restrictions on the production and utilization of CFCs and HCFCs. In this paper, a dual-mode silica gel–water adsorption chiller design is outlined along with the performance evaluation of the innovative chiller. This adsorption chiller utilizes effectively low-temperature solar or waste heat sources of temperature between 40 and 95 °C. Two operation modes are possible for the advanced chiller. The first operation mode will be to work as a highly efficient conventional chiller where the driving source temperature is between 60 and 95 °C. The second operation mode will be to work as an advanced three-stage adsorption chiller where the available driving source temperature is very low (between 40 and 60 °C). With this very low driving source temperature in combination with a coolant at 30 °C, no other cycle except an advanced adsorption cycle with staged regeneration will be operational. The drawback of this operational mode is its poor efficiency in terms of cooling capacity and COP. Simulation results show that the optimum COP values are obtained at driving source temperatures between 50 and 55 °C in three-stage mode, and between 80 and 85 °C in single-stage, multi-bed mode.     

Natural working fluids for solar-boosted heat pumps

The option of using natural working fluids as a substitute of R-22 for solar-boosted heat pumps depends not only upon thermal performance and hazardous rating but also on potential impacts on the environment. This paper presents the comparative assessment of natural working fluids with R-22 in terms of their characteristics and thermophysical properties, and thermal performance. Some justification is given for using natural working fluids in a solar boosted heat pump water heater. The results show that R-744 is not suitable for solar-boosted heat pumps because of its low critical temperature and high operational pressures. On the other hand, R-717 seems to be a more appropriate substitute in terms of operational parameters and overall performance. However, major changes in the heat pumps are required. R-290 and R-1270 are identified as candidates for direct drop-in substitutes for R-22.     

Optimal dwell point policies for automated storage/retrieval systems with dedicated storage

This study is to develop an optimal dwell point policy for automated storage/retrieval systems. For square-in-time racks with dedicated storage, we determine the optimal dwell points in closed form. We also confirm the intuitive remit that the input point is a good alternative dwell point for dedicated storage.     

Thermal performance of cold storage in thermal battery for air conditioning

This article studies, experimentally and theoretically, the thermal performance of cold storage in thermal battery for air conditioning. Thermal battery utilizes the superior heat transfer characteristics of heat pipe and eliminates drawbacks found in the conventional thermal storage tank. Experimental investigations are first conducted to study the cold storage thermal performance in two experimental systems: the ratio of distance between heat pipes to outer diameter of heat pipe W/D=6 and 2. Different heat transfer mechanisms including nucleate boiling, geyser boiling and natural convection are identified in different experimental systems with various liquid fills. A theoretical model to determine the thermal characteristics of the thermal battery has also been developed. Comparisons of this theory with experimental data show good agreements in the nucleate boiling stage of cold storage process.     

Possibilities to reduce pavement rehabilitation cost of a collision-free road investment using an LCCA design procedure

In the past decades, many single carriageway roads in Sweden have been converted to collision-free roads as a cost-effective alternative to conventional motorways. Investigations have concluded that the road type has been successful in reducing the number of fatal accidents, despite increased operation and maintenance costs. In recent years, the focus has shifted to converting narrower roads, which are anticipated to further increase pavement rehabilitation cost but also complicate traffic management during road works. This paper first examines the consequences in terms of road agency and road user costs; second, the possibilities to influence future costs by design alternatives. A life cycle cost analysis was employed to study future costs. The analysis indicated significant increase in life cycle cost compared to a single carriageway road. The analysis of design alternatives also indicated that future agency and road user cost can be substantially improved by finding an optimal alternative.     

Screening of pure fluids as alternative refrigerants

Hydrofluorocarbons are now well established as refrigerants because of their zero ozone depletion potential. Since they have a high global warming potential, other alternatives as, e.g. fluorinated ethers or cyclic hydrocarbons are considered as next-generation refrigerants. Screening of alternative refrigerants is difficult because mostly no or only few data are available. To evaluate, e.g. the cycle performance, the thermodynamic properties of the refrigerants must be known and described accurately by an equation of state. Here, the physically based BACKONE equations are used to describe alternative refrigerants, such as natural refrigerants, hydrofluorocarbons, fluorinated cyclic hydrocarbons, and fluorinated ethers. BACKONE needs only a few substance specific parameters to describe thermodynamic properties with high accuracy. Thus, even alternative refrigerants, with very few available experimental data can be described. Calculations with BACKONE of the performance of many refrigerants show that some hydrocarbons and fluorinated ethers are a good alternative.     

A study on the performance of multi-stage heat pumps using mixturesEtude sur la performance de pompes à chaleur multi-étagées utilisant des mélanges

Multi-stage heat pumps composed of a condenser, evaporator, compressor, suction line heat exchanger, and low and/or high stage economizers are studied by computer simulation. Their thermodynamic performance and design options are examined for various working fluids. In the simulation, HCFC22/HCFC142b and HFC134a are studied as an interim and long term alternatives for CFC12 while HFC32/HFC134a and HFC125/HFC134a are studied as long term alternatives for HCFC22. The results indicate that the three-stage super heat pump with appropriate mixtures is up to 27.3% more energy efficient than the conventional single-stage system with pure fluids. While many factors contribute to the performance increase of a super heat pump, the most important factor is found to be the temperature matching between the secondary heat transfer fluid and refrigerant mixture, which is followed by the use of a low stage economizer and suction line heat exchanger. The contribution resulting from the use of a high stage economizer, however, is not significant. With the suction line heat exchanger, the system efficiency increases more with the fluids of larger molar liquid specific heats. From the view point of volumetric capacity and energy efficiency, a 40%HCFC22/60%HCFC142b mixture is proposed as an interim alternative for CFC12 while a 25%HFC32/75%HFC134a mixture is proposed as a long term alternative for HCFC22.     

Use of activity-based costing and economic value analysis for the justification of capital investments in automated material handling systems

This paper develops a two-stage method for the justification of investment in modern material handling systems. The underlying approach places heavy emphasis on the use of cost avoidance data quantified by activity-based costing systems. The first stage collects the life-cycle costs and benefits resulting from the reconfiguration of material handling systems in manufacturing facilities. A recently developed software package for process modelling and management is employed to capture the characteristics of material handling activities and to derive accurate applied cost rates for each activity and material handling system combination. These activity-related cost data as well as various opportunity costs are incorporated in a case-specific, investment decision model that constitutes the second stage of the method. The model performs an Economic Value Analysis for each material handling alternative. In doing so, it offers a sound basis for economic comparison of different systems. A case study illustrates the merit of our two-stage method in decision making by placing an accurate and reliable economic value on the transition from manual to automated material handling in a manufacturing facility. The use of activity-based costs reverses the decision of sustaining the 'status quo' supported by a traditional investment justification approach restricted to solely labour cost reductions     

Sensor economy principles and selection procedures

Integrating multiple sensors into manufacturing systems enables adaptive and flexible automation and increases process adaptivity and quality control. The design of an optimum configuration must be based on a systematic analysis relative to operational and economic considerations. This design should include an evaluation of sensor performance, a decision on how many sensors, of which type they should be, and where to locate them, modes of sensor interaction, and the influence on overall performance based on a cost/benefit analysis. In most real-world applications it is impossible to develop an accurate model to predict sensor performance and therefore, performance must be derived empirically but in a systematic manner. This paper presents research and development of integrated sensor economy principles and selection procedures in addition to a performance assessment model. A systems engineering approach is applied for empirical investigations for assessment of alternative guidance systems. The outcome of this research is a set of general design guidelines for integrating multiple sensors into robotic manufacturing systems.     

Thermoeconomic evaluation and optimization of a double-effect H2O/LiBr vapour-absorption refrigeration system

In this paper, the thermoeconomic concept is applied to the optimization of a double-effect H₂O/LiBr VAR system, aimed at minimizing its overall product cost. A simplified cost minimization methodology based on the thermoeconomic concept is applied to calculate the economic costs of all the internal flows and products of the system by formulating thermoeconomic cost balances. Once these costs are determined, the system is thermoeconomically evaluated to identify the effects of the design variables on cost of the flows and products. This enables to suggest changes of the design variables that would make the overall system cost-effective. Finally, an approximate optimum design configuration is obtained by means of an iterative procedure. The result shows significant improvement in the system performance. The sensitivity analysis shows that the changes in optimal values of the decision variables are negligible with changes in the fuel cost.     

The application of photo-coupler for frost detecting in an air-source heat pump

This experimental study is carried out to investigate reliability and effectiveness of a new method of using photo-coupler for detecting frost formation in an air source heat pump, and further to determine the most efficient initiation point of the defrost cycle. This new method of using photo-coupler as a frost sensing device is evaluated by comparing its performance with conventional time control defrost system in which defrost cycle is set to start at predetermined interval, e.g. about at every 1–1.5 h. Results indicate that overall heating capacity of photo-coupler detection method (case IV) is 5.5% higher than that of time control method. It is also shown that for maximum efficiency the defrost cycle must be initiated before the frost build-up area exceeds 45% of total front surface of the outdoor coil.     

Allocating Items to an Automated Storage and Retrieval System

To reduce material handling costs, distribution centers purchase automated storage and retrieval systems (AS/RS). It is often the case that the capacity of the AS/RS is insufficient to store all items. The distribution center must then decide which items to assign to the AS/RS and in what quantities they will be stored. In this paper, we develop a heuristic procedure to solve this problem. A priori and a posteriori tests of the data for the optimality of this heuristic procedure are provided. The proposed procedure is testedwith data gathered fr oma naval supply center.     

Refrigeration system optimization

It must first be established that refrigeration is necessary and that the required cooling cannot be achieved more cheaply by other means. The operating costs of a refrigeration system depend mainly on power consumption and this can be reduced by a wide variety of methods. The operating and capital costs should be brought together in the net present value for an appropriate life span. Various case studies are presented. Other relevant factors to be considered in system optimization include control methods, plant siting and the form of contract used.     

An extended techno-economic analysis of the utilization of glycerol as an alternative feedstock for methanol production

Glycerol is a by-product of biodiesel production and may become an environmental problem. This paper investigates the utilization of glycerol as alternative feedstock for methanol production. A mathematical model of the methanol plant encompassing the steam reforming and methanol synthesis units is employed to generate data for an economic analysis involving two comparative cases: the conventional operation of the plant using only natural gas and the operation with partial substitution of the natural gas by glycerol. The results indicate that the glycerol injection can reduce the total natural gas consumption by about 11% for a given fixed methanol production. A breakeven analysis procedure is applied to determine the limit price of glycerol that makes this operation economically feasible. Based on a natural gas price of 10.13 US$/MMbtu, this analysis demonstrates that glycerol injection is feasible if its price is lower than 78.5 US$/t. Additionally, a sensitivity analysis indicates that a variation of 10% on the natural gas price causes a 26% variation on the glycerol breakeven point. The complete set of data indicates that it is possible to explore periods of glycerol low prices to reduce the operational costs of methanol plants that suffer from high natural gas prices.