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Based on the lifting design method of the axial flow hydraulic turbine and axial-flow pump impeller, an air turbine design method considering the optimal turbine pressure drop ratio for the solar chimney power plant was presented in this paper. Numerical simulations for the Spanish prototype system incorporating the turbine impeller were conducted to investigate the feasibility of the present turbine design method on the improvement of the system output performance. Furthermore, the influence of the solar radiation on the turbine performance was investigated. The results indicates that the turbine performance can be improved by using the proposed design method. The maximum turbine power output is 53.49 kW, 30.46% higher than that of the Spanish prototype and the turbine is less sensitive to rotational speed. In addition, the turbine high-performance rotational speed range is larger under higher solar radiation.

A detailed model of three-dimensional computational fluid dynamics (CFD) on a finned-tube CO2 gas cooler has been developed and validated. The model is then applied to investigate the effect of uniform and mal-distribution inlet airflow profiles on the coil performance. The airflow mal-distribution velocity profiles include linear-up, linear-down and parabolic while the effected coil performance parameters contain airside pressure drop, average airside heat transfer coefficient, approach temperature and coil heating capacity. The model also enables to predict the CO2 refrigerant temperature profile along the coil pipes from refrigerant inlet to outlet at different operation conditions. The simulation results reveal that different types of inlet airflow velocity profiles have significant effects on the gas cooler performance. The uniform airflow velocity profile case shows the best thermal performance of gas cooler. Compared with the cases of linear-up and parabolic air velocity profiles, the linear-down airflow profile can influence more on the coil heat transfer performance. Due to the thermal conduction between neighbour tubes through coil fins, reversed heat transfer phenomenon exists which can be detected and simulated by the CFD model. It is predicted that the linear-down airflow profile can increase greatly the reversed heat transfer phenomenon and thus lead to the highest approach temperature and the lowest heating capacity amongst these four types of airflow profiles. The research method and outcomes presented in this paper can have great potentials to optimize the performance of a CO2 gas cooler and its associated refrigeration system.

In this study, ultrasonic assisted osmotic pretreatment and pulsed vacuum assisted osmotic pretreatment were applied to investigate their effects on water migration and volatile components of heat ...

Air cooled CO2 finned-tube gas coolers are widely used in CO2 trans-critical refrigeration systems due to simplified structural designs. However, the longitudinal heat conduction along fins can lead to inverse heat transfer between adjacent tubes and thus capacity degradation of the heat exchanger. To cope with this adverse effect, the heat exchanger design with split fins between tube rows can be applied although further verification and analysis are expected. Subsequently, as explained in this paper, detailed CFD models have been purposely developed and simulated for the CO2 gas coolers with both continuous and split fins to quantify the effect of the heat conduction through fins. At various operating conditions of both air and refrigerant sides, totally 36 cases were simulated to study the influence of air inlet velocity, air inlet temperature, CO2 pressure and CO2 mass flow rate as well as the heat conduction along fins on the performance of the finned-tube gas coolers with and without split fins. The simulation results show that the approach temperature can be reduced by 0.6–7.5 K for the heat exchanger with split fins and a specific tube circuitry arrangement. Subsequently, the maximum and average heating capacities can be increased up to 22% and 10% respectively when the finned-tube CO2 gas cooler with split fins is applied. Furthermore, the simulation results show that the application of split fins should be designed and implemented appropriately based on different designs of tube circuitry arrangement in the heat exchanger.

This paper deals with the dynamic experimental investigation on low-grade power generation systems with CO2 transcritical power cycles (T-CO2) and R245fa organic Rankine cycles (ORC). These two systems were heated indirectly by exhaust gases of an 80 kWe micro-turbine CHP unit through a hot thermal oil system. The main components of each test system included a plate-type gas generator/evaporator, a turboexpander with high speed generator, a finned-tube air cooled condenser and a liquid pump. Both test rigs were fully commissioned and instrumented from which comprehensive dynamic experimental investigations were conducted to examine the effects of some important dynamic operational processes on system performance. These included the system start-up, variation of working fluid pump speed, change of thermal oil pump speed and system shutdown. These can lead to fully understand the dynamic and inertia behaviours of the system operations and thus to obtain robust controls. The experimental results reveal that working fluid mass flow rates are affected significantly by the start-up and shutdown processes, followed by the temperatures and pressures at turbine inlets and outlets. The research outcomes can contribute significantly to understand the system dynamic operating processes and thus instruct the system controls and safety operations.

In this study, experimental investigations were conducted on two different test rigs to investigate and compare the performances of CO2 transcritical power cycles (T-CO2) and R245fa organic Rankine cycles (ORC) for low-grade heat power generations. Each test rig consisted of a number of essential components including a turboexpander with a high speed generator, finned-tube air cooled condenser, liquid pump and plate-type gas generator/evaporator. The exhaust flue gases from an 80 kWe micro-turbine CHP unit were utilised as heat sources for both T-CO2 and R245fa ORC power generation systems and hot thermal oil flow was applied commonly as a heat transfer medium. Both test rigs were fully commissioned and instrumented from which comprehensive experimental investigations were carried out to examine the effects of various important operational parameters on system performance. These include working fluid mass flow rate and heat source input etc. at constant heat sink (ambient) parameters. Results showed that with a fixed heat source input, the turbine power generation and overall efficiency of the R245fa ORC or T-CO2 system could be improved significantly at higher working fluid mass flow rates. Quantitatively, when the CO2 and R245fa mass flow rates increased respectively from 0.2 kg/s to 0.26 kg/s and from 0.23 kg/s to 0.27 kg/s, the corresponding turbine power generation increased by 88.2% and 27.3% while the respective turbine overall efficiency enhanced by 35.4% and 7.5%. On the other hand, the turbine power generation and overall efficiency of the R245fa ORC or T-CO2 system increased variably with higher heat source input when the working fluid mass flow rate is fixed. In percentage, when the heat source inputs of the T-CO2 and R245fa ORC systems increased respectively from 52 kW to 60 kW and 61 kW to 68 kW, the corresponding turbine power generation increased 47.7% and 63% while the respective turbine overall efficiency enhanced 8.65% and 1.08%. In addition, the cycle point temperatures and pressures of both systems revealed similar increments at higher working fluid mass flow rates or at higher heat source inputs. Furthermore, heat transfer analyses of both CO2 gas generator and R245fa evaporator can be used to set up efficient controls of working fluid superheating at the heat exchanger outlet. The test results and analyses are essential in evaluating and comparing both systems’ operations at different operating conditions, design structures and components, and can significantly contribute towards optimal component selections and system performance controls.

In this paper, the economic analysis model of solar chimney power plant combining seawater desalination and waste heat (SCPPDW) and wind supercharging solar chimney power plant combining seawater desalination and waste heat (WSCPPDW) was established. Based on net present value (NPV) method, the economic performance of two systems and that of WSCPPDW after optimization were studied. It was found that the addition of wind supercharging device increased the net income (NET) and NPV by 42.8% and 102.5%, respectively. With the increase of time, the annual NET of two systems increased, and the annual NPV firstly increased and then decreased. The annual NET and NPV of WSCPPDW were always larger than those of SCPPDW, which were negative in the first four years, but those of WSCPPDW were only negative in the first year. After sequentially optimizing the turbine rotational speed, nozzle length, chimney outlet radius and mixing section length, the power and freshwater output of WSCPPDW were improved by 455.8% and 11.7%, respectively. And the total net income (TNET) and total net present value (TNPV) were increased by 183.4% and 442.8%, respectively. The declining inflection point of annual NPV was brought forward by 10 years compared to that of initial WSCPPDW.

In this study, a small-scale recuperative Organic Rankine Cycle (ORC) system capable of generating electric power using low-grade thermal energy was setup and experimentally investigated at different operating conditions. The experimental setup consisted of typical recuperative ORC system components, such as a turboexpander with high speed electricity generator, plate recuperator, finned-tube air cooled condenser, ORC fluid pump and plate evaporator. R245fa was used as the working fluid in the recuperative ORC system due to its zero ozone depletion potential (ODP) and appropriate thermophysical properties for ORC system. The working fluid was evaporated in a plate evaporator by hot thermal oil flow which was circulated and heated by exhaust flue gases of an 80kWe microturbine CHP unit. The test rig was fully instrumented and extensive experiments were carried out to compare and examine the influences of various important parameters including heat source temperature and ORC pump speed etc. on the performances of ORC systems with and without recuperator integration. The test results are essential in understanding the system operations and can contribute significantly to optimal system design, component selections and controls.

Integration of renewable energy with energy conversion application for hot water production is a significant technology to be studied for industrial, commercial and residential proposes. It has been further enhanced since the international energy agency (IEA) puts effort to standardize such kind of technology. In this paper, a novel study has been carried out experimentally to control an indirect solar assisted heat pump (IDX-SAHP) system integrated with a latent heat storage (PCM) tank. The PCM heat exchanger tank was designed specially and controlled automatically which allowed storing excess energy during the day and releasing it when needed. In addition, a system control strategy has been purposely designed and implemented in a Building Management System (BMS) to ensure the stable and reliable system operations. The experimental results show that the PCM heat exchanger tank installation has a significant effect on the system operation stability and can improve the COP of the IDX-SAHP system at different weather conditions and a specified hot water load profile.

A significant amount of national energy in the UK has been consumed in the residential sector mainly for space heating and domestic hot water production. Currently, gas boilers are mostly applied in the domestic water heating which contribute significantly to excessive CO2emissions and consumption of primary energy resources. The issues can be solved by popularizing heat pump systems that are attributable to higher performance efficiency than those of traditional gas boilers. Even so, the heat pump system performance can be further enhanced and improved if the dissolved gases in its hot water circuit can be efficiently discharged. To achieve this target, the bubble behaviors in the heat pump water heating system need to be extensively investigated. Based on a purposely built test rig of an air-water heat pump system, the effects of various heat pump hot water side parameters on gas microbubble diameters were measured and analyzed through different experimental conditions. The operational parameters include water flow rate, water flow pressure, system heating load and water flow dissolved gas saturation ratio. It is found from the measurements that the hot water flow rate is the dominate parameter which affects the average bubble diameter and bubble number distribution at different bubble sizes. The research outcomes can significantly contribute to the understanding of gas bubble behaviors at domestic heat pump water heating systems and thus the efficient way for the discharging of the associated dissolved gases.

In this study, a small-scale test rig of CO2 transcritical power cycle (T-CO2) system driven by low-grade heat has been developed and investigated experimentally. The test rig consists of a number of essential components including a CO2 turboexpander with high speed generator, finned-tube air cooled condenser, liquid receiver, CO2 liquid pump and CO2 gas generator. The CO2 is heated in the plate-type gas generator by hot thermal oil flow which is circulated and heated by exhaust flue gases from an 80kWemicroturbine CHP unit. The test rig has been fully commissioned, instrumented, controlled and is ready to operate experiment as required. Subsequently, at constant heat sink (ambient) and heat source (thermal oil) temperatures, a series of experiments have been carried out to examine the effects of various important parameters on the T-CO2 system performance. These include thermal oil flow rate and CO2 mass flow rate etc. Preliminary experimental results show that the CO2 TPC is applicable for the low temperature heat sources to generate power although further efficiency improvements are needed. In addition, the experimental outcomes can instruct future optimal system design and controls.

In CO 2 refrigeration systems, the system can operate in subcritical or transcritical cycle depending on ambient temperature. When the system is operating in transcritical cycle the heat exchanger at the high-pressure side of the system is well known as gas cooler. The gas cooler rejects heat from the superheated refrigerant gas to ambient air without condensation in single phase heat transfer process. The understanding of its performance parameters under different conditions and control strategies is essential to adequately design and optimise for specific applications. In this paper, a detailed mathematical model is developed in the Engineering Equation Solver (EES) platform, and validated with experimental results obtained from a test rig at the National Centre for Sustainable Energy use in Food Chains (CSEF). The model is investigated two different gas cooler designs when separately installed and test into a CO 2 refrigeration system, an attempt to formulate the design guidelines for CO 2 gas coolers. Then the detailed gas cooler models were integrated with the CO 2 refrigeration system model to investigate the effects of the gas cooler design into system COP. Parameters such as intermediate pressure, evaporating temperature and cooling capacity were defined from the experimental results and then inserted to the model. The results of the detailed gas cooler model and CO 2 refrigeration system model were validated against the experimental test results based on the gas cooler performance and overall system performance.

The dehydration of a tilapia fillet is conducive to its preservation. However, dehydration results in high energy consumption and quality variation. Ultrasound-assisted osmotic dehydration as a pre-treatment combines the advantages of strong permeability by preservatives and the cavitation effect and can improve the quality of tilapia fillets dried by heat-pump drying while decreasing the drying time and maintaining the quality of the dried products. The whiteness, texture, rehydration rate, Ca 2+ adenosine triphosphate synthase (Ca 2+ -ATPase) activity and the comprehensive score were used as criteria in single-factor experiment son ultrasound power, ultrasonication and permeate concentration. Based on the results of the single-factor experiment, the Box-Behnken central composite design was adopted to achieve response-surface optimisation. The results indicated Ultrasound-assisted osmotic dehydration as a pre-treatment with suitable ultrasound parameters can significantly improve the quality of heat-pump dried tilapia fillets and provides references for optimising the drying process.

This work conducted an experimental investigation of a small-scale organic Rankine cycle (ORC) system at designed operating and control conditions for low-grade thermal energy recovery application. In the ORC system, R245fa was selected as working fluid while a turboexpander (turbine) with a high speed and permanent magnet synchronous electricity generator was installed to produce electric power and two-plate type heat exchangers were designed as an evaporator and condenser. The effects of condenser cooling water temperatures and R245fa superheat at the turbine inlet on the system performance were measured and analyzed. Practically, to ensure safe operation of the ORC expander, the R245fa superheat at the expander inlet is controlled to remain constant. The experimental results showed that at constant heat source parameters (temperature and flow rate), the turboexpander power output and cycle efficiency increased with lower cooling water temperatures. Under the specified test condition ranges, the maximum turboexpander power generation could achieve 5.405 kW when the cooling water temperature and the pressure ratio were set at 23.0 °C and 7.3 respectively. On the other hand, at certain cooling water temperatures, the superheat at the expander inlet exerted a negative impact on the turboexpander and system performances when the evaporating pressure was kept constant. Ultimately, the superheat was found to be an important control parameter to ensure efficient and safe system operation.

The authors would like to acknowledge the support received from GEA Searle and Research Councils UK (RCUK) for this research project.

Low‐temperature (20 ~ 40°C) heat pump drying technology for food products has advantages over conventional high temperature (45 ~ 90°C) hot air drying but requires proper pretreatment to ensure high‐quality drying products. Subsequently, experimental investigations were carried out for drying of tilapia fillets with heat pump and at different pulse vacuum pretreatment parameters including vacuum cycle rate, cycle number, vacuum pressure, and concentration of trehalose impregnation liquid. The quality of the completed drying product was then evaluated and compared by eight quality indexes containing whiteness, ratio of water loss to solid content increase rate during osmosis dehydration (dehydration efficiency index, DEI), Ca²⁺‐ATPase (adenosine triphosphatase) activity, rehydration rate, hardness, elasticity, glueyness, and chewiness. These indexes are thus generalized into one comprehensive score based on the effective weight of each index on the product quality and analysis of the fuzzy hierarchy process. The experimental measurements were applied to examine the effect of the pretreatment parameters on the product quality indexes and comprehensive score. Accordingly, an artificial neural network model has been developed to train and achieve the optimal pretreatment parameters based on the maximum product quality comprehensive score. The obtained optimal pretreatment parameters have been validated with the measurements. Ultimately, the optimal heat pump drying process is significant and can be applied to an actual application for different food product dryings. PRACTICAL APPLICATIONS: Heat pump technology is one of the prospective processing methods for the drying of tilapia fillets. However, the relative low‐temperature heat pump drying process would increase the drying time and thus deteriorates the risk of food product. To cope with this, a high efficient pretreatment method needs to be designed and applied. Correspondingly, in the project described by this paper, extensive measurements were designed and carried out on the product drying with a heat pump. Consequently, a fuzzy hierarchy process was applied to process the relevant experimental data and generate a comprehensive score for each test. Ultimately, an artificial neural network model has been developed to train and achieve the optimal pretreatment parameters. The optimal heat pump drying process is significant and can be applied to an actual application for different food product dryings.

This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) As a main component in a refrigeration system, finned-tube CO2 gas cooler plays an important role to the system performance and thus needs to be thoroughly investigated. To achieve this, some effective parameters including the CO2 and air fluid velocity fields, temperature profiles and heat transfer characteristics at different operating conditions are predicted and analysed by means of Computational Fluid Dynamics (CFD) modelling and simulation. It is noted that CFD modelling can accurately obtain the local heat transfer coefficients of both air and refrigerant sides, which are difficult to be predicted by conventional empirical correlations. This paper investigates the effect of varied operational parameters on local heat transfer coefficients and temperature profiles of the working fluids in a finned-tube CO2 gas cooler by means of CFD modelling. As one of the simulation results, it is found that the approach temperature decreases with increased air inlet velocity. The model has been compared and validated with experimental measurements and literature correlations. The research methods and outcomes can be used for further investigation and optimization in this area. GEA Searle and Research Councils UK

The optical performance of a solar dish concentrator/cavity receiver system based on the Monte Carlo Ray Tracing Method (MCRTM) was carried out under ideal optics. The study was precipitated by a need to analyze the influence of geometrical parameters and surface properties of a cavity receiver systematically and to understand the mechanism of the optical performance under various parameters. These include diameter ratio (the ratio of aperture diameter to absorber outer diameter), height ratio (the ratio of absorber height to absorber outer diameter) and sidewall absorptivity of cavity receiver. In addition, an optical efficiency of the absorber is defined and presented for the first time. The analysis indicates that the optical efficiency increases and then decreases with higher height ratio, while optical efficiency curves are reduced monotonically with diameter ratio. Simultaneously, the effects of the height and diameter ratios on optical efficiency are both influenced by sidewall absorptivity. Based on the results, a correlation of optical efficiency was further developed to quantify the influences of these factors. The maximum optical efficiency and its corresponding optimal height ratio from both simulation and correlation are presented and obtain reasonable agreement. The optimized selection of height ratio along with diameter ratio are suggested using this result. The height ratio is found to be a pure controlling parameter of the radiation flux distribution.

Due to natural refrigerant applied, CO2 transcritical refrigeration and heat pump systems have been widely applied and attracted more attentions. As a main component, CO2 gas cooler plays an important role in the system performance and thus requires further development for design and control optimizations with advanced technology. Correspondingly, a new coupled 1D and 3D Computational Fluid Dynamics (CFD) model on a finned-tube CO2 gas cooler has been proposed and developed. The CFD model has been validated by comparing with literatures for parameters including airside heat transfer coefficient, refrigerant side temperature profile as well as heating capacity. The model has been applied to predict the heat exchanger performance at different operating conditions of both air and refrigerant sides and maldistributions of air flow inlet. It is found from the simulation results that the refrigerant temperature decreases abruptly in the first coil row and the refrigerant temperature profile along the heat exchanger tubes is affected by thermal conduction between two adjacent tube rows through fins. In addition, the higher air flow inlet velocity can reduce greatly the coil approach temperature and thus improve the system efficiency. Similar effect can also be found from refrigerant pressure. Furthermore, the non-uniform air flow patterns can affect considerably the coil performance in terms of the refrigerant temperature profile, coil approach temperature, coil heating capacity and system energy efficiency. The developed CFD model can be an efficient tool for the performance evaluation and optimisation of the CO2 gas cooler and its associated system.

In this paper, the performance of a refrigerated open-type multi-deck display cabinet with integrated phase change material (PCM) was investigated experimentally. The PCM (water gel) was purposely selected and made by mixing different compositions at specific mass ratios to obtain the appropriate melting and freezing temperatures for this particular application. It was then charged into two single panel radiators, installed immediately after the cabinet evaporator in the main back flow channel, such that the cooled air flow could pass through both external radiator sides. The display cabinet, with or without PCM integration, was then mounted in an air-conditioned chamber and tested at the same operating condition to measure and compare the performances of the system and components. The tests results showed that by installing the PCM radiators, up to 5% of energy savings and lower cabinet temperatures could be achieved. Further benefits could also be obtained, such as a greater stabilization of product temperatures during the defrost period for the modified display cabinet. Department of Environment, Food and Rural Affairs (Defra: AFM280)

A modern supermarket energy control system has a concurrent need for electricity, food refrigeration and space heating or cooling. Approximately 10% of this energy is for conventional gas-powered heating. In recent years, the use of CO 2 as a refrigerant in supermarket systems has received considerable attention due to its negligible contribution to direct greenhouse gas emissions and excellent thermophysical and heat transfer properties. CO 2 refrigeration systems also offer more compact component designs over a conventional HFC system and heat recovery potential from compressor discharge. In this paper, the heat recovery potential of an all-CO 2 cascade refrigeration system in a supermarket has been investigated using the supermarket simulation model “SuperSim” developed by the authors. It has been shown that at UK weather conditions, the heat recovery potential of CO 2 refrigeration systems can be increased by increasing the condenser/gas cooler pressure to the point where all the heat requirements are satisfied. However, the optimum level of heat recovery will vary during the year and the control system should be able to continuously optimize this level based on the relative cost of energy, i.e., gas and electricity.

As a natural working fluid, CO2 has been widely applied in refrigeration and heat pump systems where heat is conventionally rereleased to ambient through external airflow. Owing to extraordinary thermophysical properties, especially its low critical temperature, the CO2 heat release through a high-pressure side heat exchanger will inevitably undergo either supercritical or subcritical processes, depending on ambient air temperatures and head pressure controls. Correspondingly, the heat exchanger will act intermittently as either a gas cooler or condenser within the system during an annual operation. Such evidence should therefore be taken into account for an optimal design of the heat exchanger and head pressure controls in order to significantly enhance the performance of both components and the associated system.To achieve these targets, two CO2 finned-tube gas coolers/condensers with different structural designs and controls have been purposely built, instrumented and connected with an existing test rig of a CO2 booster refrigeration system. Consequently, the performance of the CO2 gas coolers/condensers with different structure designs, controls and system integration at different operating conditions can be thoroughly investigated through experimentation. In the meantime, models of the finned-tube CO2 gas coolers/condensers have been developed using both the distributed (detailed model) and lumped (simple model) methods. The former is employed to give a detailed prediction of the working fluid temperature profiles, localized heat transfer rates and effects of pipe circuitry arrangements, while the latter is suitable for the simulation and optimisation of system integration with less computation time. Both models have been validated with measurements, and moreover the simple model has been integrated with other component models so as to create a system model. The effects of the CO2 gas cooler/condenser sizes and controls on the system performance can thus be compared and analysed.

A modern supermarket energy control system has a concurrent need for electricity, space heating or cooling, and food refrigeration. The power supply to the supermarket is primarily from the national grid, where losses in efficiency are due to the processes of energy conversion and transmission. Combined heat and power (CHP) offers the potential to locally produce electrical power and heating which could save energy and reduce CO2 emissions in the long run. During the summer months, as the space heating requirement in a supermarket is relatively small, the energy efficiency of a CHP installation can be improved by using excess thermal energy to drive a sorption refrigeration system to provide space cooling or refrigeration. This process is also known as tri-generation. In recent years, the use of CO2 as a refrigerant in supermarkets has received considerable attention due to its negligible contribution to direct greenhouse gas emissions and excellent thermophysical and heat transfer properties. Consequently, the application of a tri-generation system in a supermarket with CO2 refrigeration merits further investigation. In this paper, to evaluate the performance of a tri-generation system in the supermarket, a previously tested 80 kWe microturbine device was applied into an operational supermarket to generate power and provide space heating and cooling through exhaust heat. The performance evaluation and comparison for this tri-generation application is based on the prediction from an integrated model of supermarket energy control, cascade CO2 refrigeration and tri-generation systems. The results from this simulation demonstrate the feasibility of a tri-generation system in the supermarket and pave the way for further consideration towards designs in future.

In a domestic central heating system, the phenomenon of microbubble nucleation and detachment on the surface of a boiler heat exchanger finds its origins in the high surface temperature of the wall and consequential localised super saturation conditions. If the surrounding bulk fluid is at under-saturated conditions, then after exiting the boiler, the occurrence is followed by bubbly flow and bubble dissolution. A comprehensive understanding of the fundamentals of bubble dissolution in such a domestic wet central heating system is essential for an enhanced deaeration technique that would consequently improve system performance. In this paper, the bubble dissolution rate along a horizontal pipe was investigated experimentally at different operating conditions in a purpose built test rig of a standard domestic central heating system. A high speed camera was used to measure the bubble size at different depths of focal plane using two square sectioned sight glasses at two stations, spaced 2.2 m apart. A dynamic model for bubble dissolution in horizontal bubbly flow has been developed and compared with experimental data. The effects of several important operating and structural parameters such as saturation ratio, velocity, temperature, pressure of the bulk liquid flow, initial bubble size and pipe inside diameter on the bubble dissolution were thus examined using the model. This model provides a useful tool for understanding bubble behaviours in central heating systems and optimising the system efficiency.

In this paper, 2D CFD models have been developed for a prototype refrigerated open type multi-deck display cabinet with and without integrated phase change material (PCM). The models can predict the effect of adding a PCM container on cabinet efficiencies, air temperature distributions, product temperatures and air flow patterns inside the cabinet at a range of operating conditions including space air temperatures and evaporator air velocities. To validate the cabinet models, the prototype cabinet was mounted in an air conditioned chamber and extensive experiments were conducted at constant space air temperature and relative humidity. The cabinet models have therefore been validated through comparison with experiment results for air temperatures at different locations of the airflow path and of food products. Simulation results show that significant energy can be saved through the installation of a PCM container. Further benefits include greater stabilization of product temperatures during defrost periods for the modified display cabinet. Consequently, the validated models can be used to explore and analyse the cabinet performance and control strategies at various operating and design conditions. Work presented in this paper was carried out with financial support from the Department of Environment, Food and Rural Affairs (Defra: AFM280). The authors would like to acknowledge the support of Defra and the input from the collaborating partners.

In this paper, a theoretical study is conducted to investigate and compare the performance of CO2 transcritical power cycles (T-CO2) and R245fa organic Rankine cycles (ORCs) using low-grade thermal energy to produce useful shaft or electrical power. Each power cycle consists of typical Rankine cycle components, such as a working fluid pump, gas generator or evaporator, turbine with electricity generator, air cooled condenser and recuperator (internal heat exchanger). The thermodynamic models of both cycles have been developed and are applied to calculate and compare the cycle thermal and exergy efficiencies at different operating conditions and control strategies. The simulation results show that the system performances for both cycles vary with different operating conditions. When the heat source (waste heat) temperature increases from 120 °C to 260 °C and heat sink (cooling air) temperature is reduced from 20 °C to 0 °C, both thermal efficiencies of R245fa ORC and T-CO2 with recuperator can significantly increase. On the other hand, R245fa ORC and T-CO2 exergy efficiencies increase with lower heat sink temperatures and generally decrease with higher heat source temperatures. In addition, with the same operating conditions and heat transfer assumptions, the thermal and exergy efficiencies of R245fa ORCs are both slightly higher than those of T-CO2. However, the efficiencies of both cycles can be enhanced by installing a recuperator in each system at specified operating conditions. Ultimately, optimal operating states can be predicted, with particular focus on the working fluid expander inlet pressure for both cycles. The authors would like to acknowledge the support received from GEA Searle, Mentor Graphics Corp. and Research Councils UK (RCUK) for this research project.

The theoretical and experimental aspects of heterogeneous bubble nucleation are reviewed in the context of the micro bubbles present in a domestic gas fired wet central heating system. Such systems are mostly operated through the circulation of heated standard tap water through a closed loop circuit which often results in water supersaturated with dissolved air leading to micro bubble nucleation at the primary heat exchanger wall. This study will report the micro bubble nucleation in a standard domestic central heating system at typical operating conditions. Bubble nucleation rates have been calculated in the range of 0.3–4 bubbles/cm2 s with total system bubble production rates measured in the range of 784–6920 bubbles per second. Bubble nucleation rates have been calculated through the consideration of the heat exchanger surface under super saturation conditions. The lack of experimental research in this area is evident from the non-existence of experimental correlations developed for similar physical scenarios where low levels of super saturation predominate. Hence, the classical nucleation models are considered inadequate for adaptation to the present study. A correlation through the application of the model for non-classical heterogeneous nucleation is proposed, based on the experimental data of the present study.

The CO 2 refrigerant is widely applied in retail refrigeration systems to replace HFC fluids such as R404A, but its system performance requires further improvement. One feasible method would be to pinpoint the matching components of the system, in particular its evaporator coils. This paper focuses on the potential optimization of the CO 2 evaporator design in an existing multi-deck medium temperature display cabinet charged previously with R404A refrigerant. To achieve this target, a display cabinet model with an integrated CFD and detailed evaporator sub models was utilized. This model could readily imitate the actual cabinet performance under varied operating conditions and component designs. Various design options of the CO 2 cabinet evaporator were considered and their impacts on the cabinet performances were predicted by the integrated model. Ultimately, the optimized designs of CO 2 cabinet evaporator coil geometries and refrigerant side operating parameters have been attained.

Wet central heating systems are used throughout the world and the domestic variants are known to account for 16% of the carbon dioxide emissions in the UK. It is an industry wide fact that significant quantities of air leak into the system’s closed loop circuit, mostly due to thermal cycling during routine use. The water in the system results in high dissolved gas saturation levels thus resulting in microbubble nucleation and detachment at the boiler wall. An in depth knowledge into the behaviour of microbubbles in such systems could result in better deaeration systems thus improving the overall system efficiency. In this study, we will report a measurement of the bubble detachment sizes in this system at different operating conditions, these being, the system flow rate, pressure, heating load and saturation ratio. The Winterton models for zero and finite contact angles for the prediction of the bubble detachment diameters in supersaturated solutions were used as a comparison with the test results. The model for the prediction of bubble detachment diameters at finite contact angles in round pipes has yielded good predictions. A new correlation has been proposed so as to include the effects of pressure and heat flux on the bubble characteristics which were not represented through the Winterton model.

Due to less environmental impact, the CO2 booster refrigeration system has been widely applied in the modern supermarket as a substitute for the conventional R404A multiplex system. However, the performance efficiency of the CO2 system still requires further improvement in order to save energy; thus, one of the most efficient techniques would be to investigate and employ the optimal controls for refrigerant high side pressures at various operating states. In this paper, the possible parameters affecting system efficiency of the CO2 system in the transcritical cycle at a higher ambient air temperature are identified through thermodynamic analysis, but cannot be quantified mathematically because of the high non-linearity involved. Instead, sensitive analyses of the system by means of the thermodynamic model is used to examine the effects of parameters including high side refrigerant pressure, ambient air temperature, refrigerant intermediate pressure, and medium and low evaporating temperatures, superheating, effectiveness of suction line heat exchanger, and compressor efficiency on system performance. Consequently, the optimal high side pressure in the transcritical cycle is established and derived as a function of three important parameters consisting of ambient air temperature, the effectiveness of suction line heat exchanger and compressor efficiency. In addition, optimal operating parameters such as the intermediate pressure are also proposed to improve the system performance.

This is the post-print version of the final paper published in International Journal of Refrigeration. The published article is available from the link below. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. Copyright @ 2010 Elsevier B.V. Conventional supermarket refrigeration systems are responsible for considerable CO2 emissions due to high energy consumption and large quantities of refrigerant leakage. In the effort to conserve energy and reduce environmental impacts, an efficient design tool for the analysis, evaluation and comparison of the performance of alternative system designs and controls is required. This paper provides a description of the modelling procedure employed in the supermarket simulation model ‘SuperSim’ for the simulation of the performance of centralised vapour compression refrigeration systems and their interaction with the building envelope and HVAC systems. The model which has been validated against data from a supermarket has been used for the comparison of R404A and CO2 refrigeration systems and the optimisation of the performance of transcritical CO2 systems. These results are presented in Part II of the paper. DEFRA

This is the post-print version of the final paper published in International Journal of Refrigeration. The published article is available from the link below. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. Copyright @ 2010 Elsevier B.V. As described in Part I, the supermarket simulation software “SuperSim” with its integrated refrigeration, building and HVAC system models, can be used to evaluate, compare and optimize alternative supermarket refrigeration systems. In Part II the model was used to evaluate and compare the performance of a CO2 booster refrigeration system with that of a conventional R404A multiplex system in a supermarket application. Floating head pressure control was implemented for both systems when they were in subcritical cycles. For the CO2 system, when the system was in transcritical cycle due to higher ambient air temperature, the head pressure was optimized through extensive thermodynamic cycle analysis as a function of ambient air temperature. The performance of the CO2 booster system in the supermarket was then simulated during a one year period and compared with that of the R404A system. As a result, the system performance will benefit from a lower ambient temperature and a sizeable heat recovery for the CO2 system.

The total annual CO2 emissions associated with the energy consumption of the major retail food outlets in the UK amount to around 4.0 MtCO2. The energy consumption and emissions from supermarkets varies widely and can depend on many factors such as the type and size of the store, business and merchandising practices and refrigeration and environmental control systems used. This paper provides energy consumption data of a sample of 2570 retail food stores from a number of major retail food chains in the UK. The sample covers all major store categories from convenience stores to hypermarkets and includes approximately 30% of the total number of stores in the UK having a net sales area more than 280 m2. The data show a wide variability of energy intensity even within stores of the same retail chain. A power law can be used to describe the variation of the average electrical energy intensity of the stores in the sample with sales area. If the electrical intensity of the stores above the average is reduced to the average by energy conservation measures, annual energy savings of the order of 10% or 840 GWh can be achieved representing 355,000 tonnes annual reduction in CO2 emissions. The paper also discusses the major energy consuming processes in retail food stores and identifies opportunities for energy savings.

This is the post-print version of the final paper published in Applied Energy. The published article is available from the link below. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. Copyright @ 2010 Elsevier B.V. In this paper, the model for the multi-deck medium temperature display cabinets is developed with the integration of CFD and cooling coil sub-models. The distributed method is used to develop the cooling coil model with the airside inputs from the outputs of the CFD model. Inversely, the airside outputs from the cooling coil model are used to update the boundary conditions of the CFD model. To validate this cabinet model, a multi-deck medium temperature display cabinet refrigerated with a secondary refrigerant cooling coil was selected as a prototype and mounted in an air conditioned chamber. Extensive tests were conducted at constant space air temperature and varied relative humilities. The cabinet model has been validated by comparing with the test results for the parameters of air at different locations of the flow path, and temperatures of refrigerant and food product, etc. The validated model is therefore used to explore and analyse the cabinet performance and control strategies at various operating and design conditions. DEFRA

Refrigeration has become an essential part of the food chain. It is used in all stages of the chain, from food processing, to distribution, retail and final consumption in the home. The food industry employs both chilling and freezing processes where the food is cooled from ambient to temperatures above 0 °C in the former and between −18 °C and −35 °C in the latter to slow the physical, microbiological and chemical activities that cause deterioration in foods. In these processes mechanical refrigeration technologies are invariably employed that contribute significantly to the environmental impacts of the food sector both through direct and indirect greenhouse gas emissions. To reduce these emissions, research and development worldwide is aimed at both improving the performance of conventional systems and the development of new refrigeration technologies of potentially much lower environmental impacts. This paper provides a brief review of both current state of the art technologies and emerging refrigeration technologies that have the potential to reduce the environmental impacts of refrigeration in the food industry. The paper also highlights research and development needs to accelerate the development and adoption of these technologies by the food sector.