Transient simulation of household refrigerators: A semi-empirical quasi-steady approach

This paper outlines a simulation model for the cycling behavior of household refrigerators and, therefore, for predicting their energy consumption. The modeling methodology follows a quasi-steady approach, where the refrigeration system and also the refrigerated compartments are modeled following steady-state and transient approaches, respectively. The mass, energy and momentum conservation principles were used to put forward the equation set, whereas experimental data were collected and used to reduce the model closing parameters such as the thermal conductances and capacitances. The experiments were carried out using a controlled temperature and humidity environmental chamber. The model predictions were compared to experimental data, when it was found that the energy consumption is well predicted by the model with a maximum deviation of ±2%. A sensitivity analysis was also carried out to identify opportunities for energy savings.     

供冷系统能耗评价模型及应用

随着人民生活水平提高,供冷系统日益普及。受全球气候变暖和能源资源短缺问题影响,供冷系统能耗控制日益受到广泛关注。为科学表征供冷系统能源消耗状况,将供冷系统划分为六个子系统,从总能系统层次上给出了适合于各类供冷系统的广义化的能耗评价一般模型。在此基础之上,结合单耗分析方法,提出了供冷系统能耗的定量计算方法。采用了热力学第一定律、热力学第二定律分析,对供冷过程的热力学本质、冷量?、供冷能力等方面展开研究,系统阐述了供冷系统的理论节能极限及理论最低能耗,并立足总能系统对供冷过程的理论极限－可逆供冷作出了说明。在北京市目前设计标准下,供冷系统理论最低燃料单耗为0.82kg/GJ,当前供冷系统能耗水平（25.27 kg/GJ）与之相比,节能潜力甚大。为验证所构建的供冷系统能耗评价一般化模型,结合北京市通州区供冷系统的实施方案,对分散电制冷、集中电制冷、分散热制冷、集中热制冷四个基本类型的供冷模式进行了实例计算分析,并与四种模式的?分析计算结果进行了比较,结果显示：所提能耗模型与?分析结果一致,电制冷燃料单耗低于热制冷,分散制冷燃料单耗低于集中制冷,集中热制冷能耗较高,应慎重发展。所提出的能耗分析评估模型为各种类型供冷系统的一般化对比评估提供了理论分析方法,同时可为城市供冷系统的规划设计提供参考。     

基于风电消纳的多类型储能系统联合经济调度

为使电力系统尽可能提高风电接纳能力,建立了基于风电消纳的实时失调成本模型,考虑到安全性、环保性,分别构建了基于风电-火电调度总成本模型和基于风电-火电-联合储能系统调度总成本模型,并采用HS-PSO混合算法求解调度总成本模型。算例仿真结果表明,抽水蓄能电站、蓄电池和超级电容器混合储能系统在增加经济效益、提高风电接纳能力方面优势明显;同时验证了储能系统容量与弃风率的关系,为含风电并网运行的系统经济调度提供了依据。     

Simulation-based design and optimization of refrigeration cassettes

A model-driven design and optimization methodology for sizing the components of refrigeration cassettes for light commercial applications (i.e., cooling capacities ranging from 0.5 to 1.5 kW) is presented. Mathematical models were devised for each of the system components and their numerical results were compared with experimental data taken with different cassettes. It was found that the model predictions for the working pressures, power consumption, cooling capacity and coefficient of performance (COP) showed maximum deviations of ±10%. A genetic optimization algorithm was used to design the condenser and evaporator and also to select the compressor model based on an objective function which considers both the COP and cost. The optimization led to two improved cassette designs, which were assembled and tested. One of the optimized cassettes showed a COP/cost ratio approximately 50% higher than that of the baseline system.     

Simulation-optimization of solar-thermal refrigeration systems for office use in subtropical Hong Kong

The solar-thermal refrigeration systems, covering the solar absorption refrigeration system (SAbRS) and the solar adsorption refrigeration system (SAdRS), were designed for typical office in the subtropical Hong Kong. The approach of simulation-optimization was adopted in order to determine the optimal design parameters for SAbRS and SAdRS against the conventional design practice. For simulation, dynamic model of each system was refined on the TRNSYS platform. For optimization, the objective was to minimize the annual primary energy consumption of SAbRS or SAdRS in response to the changing loading and climatic conditions throughout a year. This is a constrained optimization problem since the upper limit of comfort temperature was stipulated, such that the minimization of system energy would not sacrifice the indoor thermal comfort. Due to the complex, multidimensional and constrained nature of the dynamic simulation models, the differential evolution (DE), which has been proven effective in evolutionary computation (EC), was used for optimization purpose. Through the simulation-optimization run, the optimized designs of SAbRS and SAdRS were determined, and their corresponding primary energy consumptions could be 12.2% and 7.1% less than those based on the general design practice. The results provide useful guidelines for the equipment design of SAbRS and SAdRS.     

Assessment of the controlling envelope of a model-based multivariable controller for vapor compression refrigeration systems

This paper explores the controlling characteristics of a first-principles model-based controller specially developed for vapor compression refrigeration systems. Mathematical sub-models were put forward for each of the system components: heat exchangers (condenser and evaporator), variable-speed compressor and variable-orifice electric expansion device. The dynamic simulation model was then used to design a multivariable controller based on the linear-quadratic-Gaussian technique using a Kalman filter for the estimator design. A purpose-built testing apparatus comprised of a variable-speed compressor and a pulse-width modulated expansion valve was used to collect data for the system identification, and model and controller validation exercises. It was found that the model reproduces the experimental trends of the working pressures and power consumption in conditions far from the nominal point of operation (±30%) with a maximum deviation of ±5%. Additional experiments were also performed to verify the ability of the controller of tracking reference changes and rejecting thermal load disturbances. It was found that the controller is able to keep the refrigeration system running properly when the thermal load was changed from 340 to 580 W (460 W nominal), and the evaporator superheating degree was varied from 9.5 °C to 22 °C (16.6 °C nominal).     

高铁客运站分布式多能源系统供能协同优化策略研究

为解决传统高铁客运站供能系统中能源利用率较低的问题,以日运行购气费用和购电费用最优为优化目标,以系统运行过程中实时能量平衡为约束条件,以可再生能源出力和吸收式制冷占比为优化变量,建立多能源协同供能的分布式能源系统,并将该模型应用于北方某高铁客运站,分析可再生能源的利用率、制冷系统中可再生能源电出力的电制冷占比以及电网出力的节电率。仿真计算结果表明,分布式能源系统的使用提高了可再生能源的利用率,其中风电机组出力占其出力极限的96.5%,光伏机组出力94.7%;相比于参比系统,分布式能源系统的成本节约率为12.5%;电制冷占比为13%;电网的节电率为53.9%。     

Theoretical Investigation of Waste Heat Recovery from an IC Engine Using Vapor Absorption Refrigeration System and Thermoelectric Converter

This study proposes the preliminary simulation of a single cylinder spark ignition engine with waste heat recovery system. To harvest waste heat energy from the engine exhaust a thermoelectric generator coupled to a vapor absorption refrigeration (VAR) system was proposed in this simulation work. Parametric simulation of engine, thermoelectric generator and VAR using thermodynamic relations was carried out in MATLAB – Simulink software. An attempt has been made mathematically to integrate engine, thermoelectric generator and VAR system to study the effect of engine load, speed, equivalence ratio on thermoelectric output and coefficient of performance (COP) of a VAR system. In this study, the VAR system runs by taking heat energy from the exhaust gas and the electric power produced by a thermoelectric generator was utilized to run the pump of the refrigeration system. It was found that COP of the absorption refrigeration system depends on engine load, speed and air fuel equivalence ratio. The study also reveals that about 10% to 15% of the total exhaust energy can be harvested using this system.     

Experimental Analysis of the Freezing Process in a Horizontal Plate Freezer With CO2 as Refrigerant in a Cascade Refrigeration System

An experimental evaluation of a cascade refrigeration system prototype using NH₃ and CO₂ as refrigerants has been performed. The prototype is used to supply a 9-kW refrigeration capacity to a horizontal plate freezer at an evaporation temperature of –50°C as design conditions. The prototype includes a specific control system and a data acquisition system. The article describes the prototype and the experimental program carried out to study the freezing process in the horizontal plate freezer. The experimental results include the freezing time, the temperature evolution in the product to be frozen, the refrigeration capacity of the facility, the electric power of the compressors, the coefficient of performance (COP) of the low- and high-temperature systems, and the overall system COP. The results obtained would confirm that the use of CO₂ in plate freezers represents a viable alternative to refrigerants currently in use.     

Modeling and simulation of solar absorption system performance in Beirut

An analytical study is performed on solar energy utilization in space cooling of a small residential application using a solar lithium bromide absorption system. A simulation program for modeling and performance evaluation of the solar-operated absorption cycle is done for all possible climatic conditions of Beirut. The results have shown that for each ton of refrigeration it is required to have a minimum collector area of 23.3 m² with an optimal water storage tank capacity ranging from 1000 to 1500 liters for the system to operate solely on solar energy for about seven hours a day. The monthly solar fraction of total energy use in cooling is determined as a function of solar collector area and storage tank capacity.An economic assessment is performed based on current cost of conventional cooling systems. It is found that the solar cooling system is marginally competitive only when combined with domestic water heating.     

Performance of hybrid vaccine freezer that combined thermoelectric coolers with vapor compression refrigeration: Experimental assessment

In the medical field, refrigeration systems are used to store and transport vaccines, blood, and other medical supplies that require specific temperature ranges to remain effective. As technology continues to advance, the demand for more efficient and sustainable refrigeration systems is also increasing. The freezer compartment is typically designed to maintain a temperature of −18°C to −23°C for storing frozen items. Consequently, this work aims to develop a hybrid refrigeration system that combines a thermoelectric cooler system (TEC) and a vapor compression refrigeration cycle (VCC) system to achieve lower temperatures than conventional refrigerators. Also, the performance of the proposed hybrid refrigeration system is experimentally assessed with various operating conditions, including varying the voltage delivered to the system. The experimental results exhibited that the temperature inside the freezer room reached −33°C, while the cold side temperature is −47°C. Also, the maximum coefficient of performance of the VCC system, TEC, and hybrid system is 2.07, 1.06, and 0.37, respectively, at a DC voltage applied of 6 V. Moreover, the results revealed that the hybrid system combining a TEC and a VCC system can be a valuable technology for specific applications with low temperatures and limited capacity requirements.     

Energy efficiency standards for refrigerators in Brazil: A methodology for impact evaluation

In Brazil energy efficiency standards for cold appliances was established in 2007. A specified single set of MEPS (minimum energy performance standards) for refrigerators, freezers and freezer refrigerators was implemented without evaluating its impacts and estimation of potential electricity savings. This paper presents a methodology for assessing the impacts of the Brazilian MEPS for cold appliances. It uses a bottom-up approach to estimate residential end-use consumption and to evaluate the energy saving potential for refrigerators. The household electricity consumption is projected by modeling appliance ownership using an econometric approach based on the recent household survey data. A cost–benefit analysis for more stringent standards is presented from the perspective of the society and electricity customers. The results showed that even considering the current market conditions (high discount rate for financing new efficient equipment) some MEPS options are advantageous for customers. The analysis also demonstrates significant cost-effective saving potential from the society perspective that could reach 21 TWh throughout the period of 2010–2030—about 25% of current residential consumption.     

Assessment of methods to reduce the energy consumption of food cold stores

Energy is a major cost in the operation of food cold stores. Work has shown that considerable energy savings can be achieved in cold stores. Results from 38 cold store audits carried out across Europe are presented.Substantial savings could be achieved if operation of cold storage facilities were optimised in terms of heat loads on the rooms and the operation of the refrigeration system. Many improvements identified were low in cost (improved door protection, defrost optimisation, control settings and repairs). In large stores (>100 m³) most improvements identified were cost effective and had short payback times, whereas in small stores there were fewer energy saving options that had realistic payback times. The potential for large energy savings of at minimum 8% and at maximum 72% were identified by optimising usage of stores, repairing current equipment and by retrofitting of energy efficient equipment. Often these improvements had short payback times of less than 1 year.In each facility the options to reduce energy consumption varied. This indicated that to fully identify the maximum energy savings, recommendations need to be specific to a particular plant. General recommendations cannot fully exploit the energy savings available and therefore to maximise energy savings it is essential to monitor and analyse data from each facility.     

Design, performance, and cost characteristics of high temperaturesuperconducting magnetic energy storage

A conceptual design for superconducting magnetic energy storage (SMES) using oxide superconductors with higher critical temperature than metallic superconductors has been analyzed for design features, refrigeration requirements, and estimated costs of major components. The study covered the energy storage range from 2 to 200 MWh at power levels from 4 to 400 MW. A SMES that uses high temperature superconductors (HTS) and operates at high magnetic field (e.g. 10 T), can be more compact than a comparable, conventional low-temperature device at lower field. The refrigeration power required for a higher temperature unit (20 to 77 K) will be less by 60% to 90%. The improvement in energy efficiency is significant for small units, but less important for large ones. The material cost for HTS units is dominated by the cost of superconductor, so that the total cost of an HTS system will be comparable to a low temperature system only if the superconductor price in $/ampere-meter is made comparable by increasing current density or decreasing wire cost     

The energy consumption performance of roof lawn gardens in Thailand

Statistics have confirmed that a lawn placed on a roof top can reduce the cooling load. In this study, the VISUAL DOE 4.0 energy simulation program was used to determine the effects of roof lawn gardens (RLG) on the annual energy required for cooling, and annual electrical usage in an experimental model. The overall heat transfer coefficient (U-Value) of the RLG was estimated by using data from site measurements and the effect of the RLG on the building’s energy consumption was calculated by the simulation program. The effect of different thicknesses of soil was also investigated. From the field measurements, it was confirmed that the RLG could reduce building heat gain. Thus, an experiment was set up to find the energy benefits of the RLG. With evaporative cooling by the RLG, it was found that the consumption of energy in a building with a RLG with 0.20 m of soil was 37.11% less than in a building with a bare roof surface (without evaporative cooling). Moreover, a building with 0.10 m of soil in the RLG consumed 31.07% less energy than a building with a bare roof surface. The results show that an increase in the thickness of the soil layer reduces the building energy consumption.     

节能技术在冰箱经济运行中的应用分析

本文运用大量的实验数据和对比分析,传统冰箱在运行过程中降低能耗的几种成熟技术的现状及发展趋势。并浅析VIP技术将会对能量消耗产生的影响,以及该技术在规模生产中降低成本和提高效益的可行性。     

湿热地区农村居住建筑应用沼气空调的前景分析

随着我国新农村建设的不断推进,农村建筑能耗呈现持续快速上升的趋势。沼气空调以沼气为燃料,具有运行经济性突出、节能、清洁环保等优点,能解决农村生活水平提高带来的能源瓶颈问题。文章根据湿热地区农村居住建筑对室内舒适性的要求,利用能耗模拟软件,分析了沼气空调系统在农村应用的可行性,计算可知沼气空调年户均投入仅为电制冷空调年户均投入的74.7%,可认为其在农村建筑中使用沼气空调具有较好的经济和环境效益。     

Performance characteristics of HCFC-123 ejector refrigeration cycles

Ejector refrigeration systems can use low grade thermal energy, at temperatures as low as 60°C, to provide space cooling. Since this waste energy is widely available and the cost of its supply is negligible in many cases, cooling costs can be lower than conventional systems, which makes the method very attractive. The present study describes a computer simulation model for ejector refrigeration systems that was developed using an existing ejector theory. This model allows for internal irreversibilities within the ejector to be included and caters for the addition of a regenerator and a precooler for improving the system coefficient of performance. The study shows that HCFC-123 is a suitable replacement for CFC-11 in this application. Results also indicate that the use of a variable geometry ejector can maintain the optimum performance of refrigeration systems when operating conditions change.     

Neural network based optimization for cascade filling process of on-board hydrogen tank

Compressed hydrogen storage is widely used in hydrogen fuel cell vehicles (HFCVs). Cascade filling systems can provide different pressure levels associated with various source tanks allowing for a variable mass flow rate. To meet refueling performance objectives, safe and fast filling processes must be available to HFCVs. The main objective of this paper is to establish an optimization methodology to determine the initial thermodynamic conditions of the filling system that leads to the lowest final temperature of hydrogen in the on-board storage tank with minimal energy consumption. First, a zero-dimensional lumped parameter model is established. This simplified model, implemented in Matlab/Simulink, is then used to simulate the flow of hydrogen from cascade pressure tanks to an on-board hydrogen storage tank. A neural network is then trained with model calculation results and experimental data for multi-objective optimization. It is found to have good prediction, allowing the determination of optimal filling parameters. The study shows that a cascade filling system can well refuel the on-board storage tank with constant average pressure ramp rate (APRR). Furthermore, a strong pre-cooling system can effectively lower the final temperature at a cost of larger energy consumption. By using the proposed neural network, for charging times less than 183s, the optimization procedure predicts that the inlet temperature is 259.99–266.58 K, which can effectively reduce energy consumption by about 2.5%.     

Electricity end-uses in the residential sector of Brazil

The residential energy consumption has been studied in many countries as it usually accounts for a large percentage of the total energy consumption. Energy end-uses have also been a matter of concern as they can assist energy system planning. The objective of this paper is to assess the actual scenario of electricity consumption and estimate electricity end-uses in the residential sector of Brazil for different bioclimatic zones. The analysis is based on a survey performed by 17 energy utilities enclosing a total of 17,643 houses or flats over 12 states in Brazil. The survey was performed to obtain electricity consumption data for all household appliances found in houses and flats. The electricity end-uses were estimated by performing weighted averages according to the location of the dwellings in each bioclimatic zone. Results indicate that the largest end-uses are for refrigerator and freezer together, which account for about 38–49% of the electricity consumption in dwellings in Brazil. Air-conditioning and electric shower are the end-uses that are more dependent on the climatic conditions. The main conclusion that can be made from the analysis is that air-conditioning should be a major concern in the residential sector of Brazil in the near future as its ownership is still low, but its electricity consumption is already significant mainly over summer.