Performance investigation of a thermal‐driven refrigeration system

Thermal‐driven refrigeration systems have attracted increasing research and development interest in recent years. These systems do not cause ozone depletion and can reduce demand on electricity. The main objective of this work is to perform tests and theoretical analyses of a thermal‐driven refrigeration system using a new sorbent–sorptive pair as the working pair. The active component of the sorbent used in this study is sodium thiocyanate (NaSCN). Ammonia (NH₃) is chosen as sorptive. Based on the thermodynamic properties of the working solution, a mathematical model is introduced to analyze the system characteristics and performance. A series of experimental data is collected to establish the relationships among different system parameters. The results are compared with those of other thermal‐driven refrigeration systems. It is shown that the advantages provided by this system over others include lower generator and evaporator temperatures and a higher coefficient of performance (COP). Copyright © 2008 John Wiley & Sons, Ltd.     

Dynamic simulation and experimental verification on absorption refrigeration driven by vehicle waste heat

Waste heat exhausted from vehicle engine has significant influences on the performances of absorption refrigeration driven by vehicle exhaust heat (VARS), especially for the transient response of VARS to the exhaust temperature and flow rate. Moreover, ambient temperature and flow rate of solution pump also affect its performances. Based on these, a dynamic mathematical model of VARS was proposed and established to observe its dynamic performances in this work. Further, the dynamic model was solved, and the simulation results were compared to the experimental data of a prototype in the same operating conditions. Finally, the transient response of VARS to exhaust temperature and flow rate, ambient temperature, and flow rate of solution pump were analyzed. The simulation results showed that the response time is about 2000 seconds when a disturbance was imposed. The research findings can supply some guidance for a reliable design, optimization, and control strategy for the actual application of the VARS.     

Exergetic performance assessment of a ground‐source heat pump drying system

In evaluating the efficiency of heat pump (HP) systems, the most commonly used measure is the energy (or first law) efficiency, which is modified to a coefficient of performance (COP) for HP systems. However, for indicating the possibilities for thermodynamic improvement, energy analysis is inadequate and exergy analysis is needed. This study presents an exergetic assessment of a ground‐source (or geothermal) HP (GSHP) drying system. This system was designed, constructed and tested in the Solar Energy Institute of Ege University, Izmir, Turkey. The exergy destructions in each of the components of the overall system are determined for average values of experimentally measured parameters. Exergy efficiencies of the system components are determined to assess their performances and to elucidate potentials for improvement. COP values for the GSHP unit and overall GSHP drying system are found to range between 1.63–2.88 and 1.45–2.65, respectively, while corresponding exergy efficiency values on a product/fuel basis are found to be 21.1 and 15.5% at a dead state temperature of 27°C, respectively. Specific moisture extraction rate (SMER) on the system basis is obtained to be 0.122 kg kW^?1 h^?1. For drying systems, the so‐called specific moisture exergetic rate (SMExR), which is defined as the ratio of the moisture removed in kg to the exergy input in kW h, is also proposed by the authors. The SMExR of the whole GSHP drying system is found to be 5.11 kg kW^?1 h^?1. Copyright © 2006 John Wiley & Sons, Ltd.     

Global moment‐independent sensitivity analysis of single‐stage thermoelectric refrigeration system

A mathematical model of single‐stage thermoelectric refrigeration system (SSTRS) was built considering the influence of external heat transfer. Based on the relationship between the input system design variables and the output cooling capacity, the coefficient of performance and the influence of the fluctuation of the design variables on the stability of the thermoelectric refrigeration system output performance parameters were studied using a moment‐independent sensitivity analysis. The Latin hypercube sampling method was used to simulate the fluctuation of the design variables, and their moment‐independent sensitivity indices were calculated by the Monte Carlo method. The design variables were then sorted according to their importance to the output parameters, and the key design variables affecting the coefficient of performance and the cooling capacity were obtained. These results can provide useful guidance for the design and optimisation of SSTRS.     

太阳能两级吸收式制冷机应用分析

在给定冷凝温度和吸收温度的情况下提出了利用太阳能做热源、地下水做冷源的两级吸收式制冷系统,COP可达1.4~1.7,高于其他形式吸收式热泵,节约了冷却水的用量。     

Study on the start‐up characteristics of double‐effect absorption refrigerator driven by waste steam

Absorption refrigerators can be driven by waste heat. In particular, the double‐effect type of absorption refrigerator, which is highly efficient, is drawing a great deal of attention as a waste heat recovery system. This research examines the double‐effect absorption refrigerator driven by steam, assuming waste heat from the fuel cells is applied. In the start‐up of the double‐effect absorption refrigerator, the solution temperature is almost equal to the temperature of the air. Due to the large temperature difference between the solution and the heat sources, the solution is overheated. In the worst case, it will be crystallized. Additionally, some solution is circulated due to the existence of a pressure difference between the heat exchangers. If the solution pump is started before an adequate pressure difference is obtained, the absorption refrigerator cannot be started normally. To investigate these problems, a simulation model is constructed. An experiment is conducted to investigate the performance of this model. As a result, the validity of this model is confirmed and the detailed start‐up characteristics are clarified. © 2000 Scripta Technica, Heat Trans Asian Res, 29(5): 427–445, 2000     

Cooling performance and energy saving of a compression–absorption refrigeration system driven by a gas engine

The prototype of combined vapour compression–absorption refrigeration system was set up, where a gas engine drove directly an open screw compressor in a vapour compression refrigeration chiller and waste heat from the gas engine was used to operate absorption refrigeration cycle. The experimental procedure and results showed that the combined refrigeration system was feasible. The cooling capacity of the prototype reached about 589 kW at the Chinese rated conditions of air conditioning (the inlet and outlet temperatures of chilled water are 12 and 7°C, the inlet and outlet temperatures of cooling water are 30 and 35°C, respectively). Primary energy rate (PER) and comparative primary energy saving were used to evaluate energy utilization efficiency of the combined refrigeration system. The calculated results showed that the PER of the prototype was about 1.81 and the prototype saved more than 25% of primary energy compared to a conventional electrically driven vapour compression refrigeration unit. Error analysis showed that the total error of the combined cooling system measurement was about 4.2% in this work. Copyright © 2006 John Wiley & Sons, Ltd.     

Analysis of a combined power and cooling cycle for low‐grade heat sources

This paper presents a parametric analysis of a combined power/cooling cycle, which combines the Rankine and absorption refrigeration cycles, uses ammonia–water mixture as the working fluid and produces power and refrigeration, while power is the primary goal. This cycle, also known as the Goswami Cycle, can be used as a bottoming cycle using waste heat from a conventional power cycle or as an independent cycle using low‐temperature sources such as geothermal and solar energy. Optimum operating conditions were found for a range of ammonia concentration in the basic solution, isentropic turbine efficiency and boiler pressure. It is shown that the cycle can be optimized for net work, cooling output, effective first law and exergy efficiencies. The effect of rectification cooling source (external and internal) on the cycle output was investigated, and it was found that an internal rectification cooling source always produces higher efficiencies. When ammonia vapor is superheated after the rectification process, cycle efficiencies increase but cooling output decreases. Copyright © 2010 John Wiley & Sons, Ltd.     

Thermal‐economic analysis of a transcritical Rankine power cycle with reheat enhancement for a low‐grade heat source

A thermal‐economic analysis of a transcritical Rankine power cycle with reheat enhancement using a low‐grade industrial waste heat is presented. Under the identical operating conditions, the reheat cycle is compared to the non‐reheat baseline cycle with respect to the specific net power output, the thermal efficiency, the heat exchanger area, and the total capital costs of the systems. Detailed parametric effects are investigated in order to maximize the cycle performance and minimize the system unit cost per net work output. The main results show that the value of the optimum reheat pressure maximizing the specific net work output is approximately equal to the one that causes the same expansion ratio across each stage turbine. Relative performance improvement by reheat process over the baseline is augmented with an increase of the high pressure but a decrease of the turbine inlet temperature. Enhancement for the specific net work output is more significant than that for the thermal efficiency under each condition, because total heat input is increased in the reheat cycle for the reheat process. The economic analysis reveals that the respective optimal high pressures minimizing the unit heat exchanger area and system cost are much lower than that maximizing the energy performance. The comparative analysis identifies the range of operating conditions when the proposed reheat cycle is more cost effective than the baseline. Copyright © 2012 John Wiley & Sons, Ltd.     

Research on performance of mixed absorption refrigeration for solar air-conditioning

A novel lithium bromide/water mixed absorption refrigeration cycle that is suitable for the utilization of solar air-conditioning and can overcome the draw-backs of low system overall efficiency of traditional solar absorption refrigeration air-condition systems is presented. The accessorial high pressure generator was added in the cycle. The lithium bromide solution flowing out from the high pressure generator was mixed with the solution from the low pressure absorber to increase lithium bromide solution concentration and decrease pressure in the high pressure absorber. The performance of a mixed absorption refrigeration cycle was analyzed. The theoretical analysis shows that the highest COP is 0.61, while the highest available temperature difference of heat resource is 33.2°C. The whole coefficient of performance of the solar air-conditioning using mixed absorption cycle is 94.5% higher than that of two-stage absorption. The advantages of solar air-conditioning can be markedly made use of by the cycle. __________ Translated from Journal of Huazhong University of Science and Technology, 2006, 34(8): 62–64 [译自: 华中科技大学学报]     

太阳能吸收式制冷机性能分析

在太阳辐射动态变化的情况下,对制冷量为5kW的水冷式太阳能吸收式制冷机的性能进行了模拟,得出了集热器出口水温随时间变化的规律曲线以及在此规律的影响下吸收式制冷机的性能曲线。模拟结果表明水冷式太阳能吸收式制冷机在理论上是切实可行的,但是集热器出口水温度的变化以及冷却水温度对系统性能有较大的影响。冷却水温度越低、系统的性能系数越高。     

Performance evaluation of a vertical ground‐source heat pump system in Izmir,Turkey

This study summarizes the cooling performance of a ground‐source heat pump system which was installed in a 65‐m² room in the Solar Energy Institute, Ege University, Izmir (568 degree‐days cooling, base: 22°C; 1226 degree‐days heating, base: 18°C) Turkey. The institute, built in 1986, has a liveable floor area of 3000 m², and uses passive solar techniques. The heating and cooling loads of the room were, respectively, 3.8 and 4.2 kW at design conditions. The system was commissioned in May 2000 and the performance tests have been carried out since then. Based upon the measurements, the heat rejection rate to the soil with an average thermal diffusivity of 0.00375 m²h^?1 in the cooling mode was found to be in average 51 W m^?1 of bore depth, while the maximum entering water temperature to the unit was recorded as 35.9°C. The cooling coefficient of performance of the heat pump and the whole system was relatively low when compared to other heat pumps operating under conditions at or near design values. The primary reasons for this were discussed in detail and the potential for performance improvements was also suggested. Copyright © 2002 John Wiley & Sons, Ltd.     

Study on a direct‐expansion solar‐assisted heat pump water heating system

Analytical and experimental studies were performed on a direct‐expansion solar‐assisted heat pump (DX‐SAHP) water heating system, in which a 2 m² bare flat collector acts as a source as well as an evaporator for the refrigerant. A simulation model was developed to predict the long‐term thermal performance of the system approximately. The monthly averaged COP was found to vary between 4 and 6, while the collector efficiency ranged from 40 to 60%. The simulated results were used to obtain an optimum design of the system and to determinate a proper strategy for system operating control. The effect of various parameters, including solar insolation, ambient temperature, collector area, storage volume and speed of compressor, had been investigated on the thermal performance of the DX‐SAHP system, and the results had indicated that the system performance is governed strongly by the change of solar insolation, collector area and speed of compressor. The experimental results obtained under winter climate conditions were shown to agree reasonably with the computer simulation. Copyright © 2003 John Wiley & Sons, Ltd.     

A study on the optimized control strategies of geothermal heat pump system and absorption chiller‐heater

The world is becoming increasingly interested in renewable energy including geothermal energy. The utilization of geothermal systems is currently low because geothermal systems and existing source systems are used independently, but the supply rate of a geothermal system is increasing. Therefore, suggesting efficient operation plans and evaluations of the energy consumption and efficiency of a geothermal system is needed. This paper reports the results of a field study and survey of the present applications and operation conditions of a geothermal system. In addition, this paper proposes an efficient operation strategy for a geothermal system and compares this operation strategy with an existing operation strategy through simulation. The problems of existing operation condition were found out through a field study, and alternatives were proposed. The improvements were evaluated using the transient systems simulation program. And it would be possible for the reduction of the energy consumption through the comparative analysis of equipment efficiency and energy consumption. The result of analyzing the proposed combination header method through simulations compared with existing operation conditions can increase the use of geothermal systems, but the combined cooling and hot water of a geothermal heat pump and existing thermal source system reduced the efficiency of the heat pump. As a result of simulation on individual load‐sharing method, efficiency of geothermal system is increasing compared with the combination header method. This method was especially made to separate geothermal system's water loop and existing thermal source system's water loop. Copyright © 2013 John Wiley & Sons, Ltd.     

渔船发动机废热驱动的两级复叠吸收制冷循环性能分析

针对传统吸收式制冷无法达到较低温度以及自复叠吸收制冷在制得较低温度时系统性能系数过小的缺点,提出发动机废热驱动的两级复叠式吸收制冷循环用于捕获海产品的速冻保鲜。首先采用SRK方程获得了该循环高、低温级工质对R134a/DMF和R23/DMF的热力学性质参数,进而对循环进行了建模分析。通过直接搜索法得到了在不同工况下的最优高温级发生温度。发现在当吸收温度为30℃,冷凝温度为35℃,制冷温度在-40℃以上时,循环最佳蒸发冷凝温度和高、低温级发生温度分别为-3℃、106℃和140℃,此时循环COPint 可达到0.143。但该循环性能受吸收、冷凝温度影响较大,因此不太适合在海水温度过高的海域使用。     

Performance assessment of a liquid‐phase separation refrigeration cycle

Absorption refrigeration cycles are alternatives to conventional vapor‐compression cycles in which the energy required for refrigeration is provided by heat instead of mechanical work. In this paper, a novel refrigeration cycle utilizing the immiscible liquid‐phase separation behavior is simulated and analyzed using Aspen simulator. The two conjugate liquids adopted in this work are triethylamine (solute) and water (solvent). This binary system has a low critical solution temperature of 18 °C. The thermophysical properties of the binary mixture are generated using the universal functional activity coefficient (UNIFAC) and the nonrandom two‐liquid (NRTL) models. The phase splitting phenomenon at the generator temperature is predicted by both models. However, in comparison with the available experimental data for the same binary mixture, NRTL model gives better predictions for the flow rates and compositions of the material streams. Heat duties of the evaporator, absorber, and generator and the power consumption of the solution pump have been calculated using UNIFAC and NRTL models. The cycle COP that plays a major role in determining the cycle economical viability has been predicted for different operating conditions using the two models. Simulation results show that, for a waste heat reservoir at 60 °C and using NRTL model, the COP is about 2.0. Second law analysis conducted for all cycle components of the cycle shows that about 42% of the total exergy destructed occurs in the generator. Finally, the liquid‐phase separation refrigeration cycle is predicted to be a promising cycle in the near future because of hardware and energy savings. Copyright © 2011 John Wiley & Sons, Ltd.     

Thermoeconomic assessment of an absorption refrigeration and hydrogen-fueled diesel power generator cogeneration system

The thermoeconomic assessment of a cogeneration application that uses a reciprocating diesel engine and an ammonia–water absorption refrigeration system for electrical power and cold production from hydrogen as fuel is presented. The purpose of the assessment is to get both exergetic and exergoeconomic costs of the cogeneration plant products at different load conditions and concentrations of hydrogen–diesel oil blends. The exhaust gas of the reciprocating diesel engine is used as an energy source for an ammonia–water absorption refrigeration system. The reciprocating diesel engine was simulated using the Gate Cycle™ software, and the ammonia–water absorption refrigeration system simulation and the thermoeconomic assessment were carried out using the Engineering Equation Solver software (EES). The results show that engine combustion is the process of higher exergy destruction in the cogeneration system. Increased hydrogen concentration in the fuel increases the system exergetic efficiency for all load conditions. Exergy destruction in the components of the ammonia–water absorption refrigeration system is increased with increasing load due to the rise of heat transfer. At intermediate and high loads energy efficiency is increased in the power system, and low values of unit exergetic cost and competitive specific exergoeconomic costs are noticed. The cogeneration system operation at intermediate and high engine loads was proven to be feasible.     

Thermodynamic analysis and performance study for adsorption refrigeration cycle driven by a fuel cell electric vehicle waste heat

Three kinds of adsorption refrigeration cycles are analyzed in this paper, a two‐bed continuous cycle, an adiabatic mass recovery cycle, and an isothermal mass recovery cycle. Operating parameters (including desorption temperature, adsorption temperature, cycle adsorption rate, COP, and period refrigerating capacity) with the change of the evaporating temperature, condensing temperature, heat capacity ratio, and heat resource temperature are discussed. The analysis indicates that performance differences between the mass recovery cycle and the two‐bed continuous cycle are reduced with an increasing of evaporating temperature and heat source temperature. By increasing the heat capacity ratio, COP values for the three kinds of cycle decrease. When the heat source temperature is between 70 and 90°C, the performance of the isothermal mass recovery cycle is best. Through study, this paper puts forward that the isothermal mass recovery cycle is the best cycle for adsorption refrigeration systems driven by fuel cell electrical vehicle waste heat. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res, 39(7): 523–538, 2010; Published online 16 July 2010 in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/htj.20315     

Comparative analysis of a solar‐driven novel salt‐based absorption chiller with the implementation of nanoparticles

The present study exemplifies the comprehensive thermal analysis to compare and contrast ammonia‐lithium nitrate (NH₃‐LiNO₃) and ammonia‐sodiumthiocynate (NH₃‐NaSCN) absorption systems with and without incorporation of nanoparticles. A well‐mixed solution of copper oxide/water (CuO/H₂O) nanofluid is considered inside a flat‐plate collector linked to an absorption chiller to produce 15‐kW refrigeration at ?5°C evaporator temperature. Enhancements in heat transfer coefficient, thermal efficiency, and useful heat gain of the collector are evaluated, and the effect of these achievements on the performance of both absorption chillers have been determined for different source temperatures. A maximum 121.7% enhancement is found in the heat transfer coefficient with the application of the nanofluid at 2% nanoparticle concentration. The maximum coefficient of performance observed for the NH₃‐NaSCN chiller is 0.12% higher than that for the NH₃‐LiNO₃ chiller at 0°C evaporator temperature. Contradictory to this, the average system coefficient of performance of the NH₃‐LiNO₃ absorption system has been found 5.51% higher than that of the NH₃‐NaSCN system at the same evaporator temperature. Moreover, the application of the nanofluid enhanced the performance of the NH₃‐NaSCN and NH₃‐LiNO₃ systems by 2.70% and 1.50%, respectively, for lower generator temperature and becomes almost the same at higher temperatures, which altogether recommends the flat‐plate collector–coupled NH₃‐LiNO₃ absorption system be integrated with a nanofluid.     

Study on a twin‐bed adsorption refrigeration system using R134a‐activated carbon pair

A low capacity twin‐bed adsorption refrigeration system has been built with R134a as a refrigerant and activated carbon as the adsorbent. Simple tube‐in‐tube heat exchangers have been fabricated and have been used as the adsorber beds. Activated carbon (granular type) has been filled in the annular space of the inner tube and outer tube. A plate heat exchanger has been used as the condenser and the temperature of cooling water has been maintained between 25°C and 30°C, also the evaporator has been custom designed as per requirements. A mathematical model has also been developed and the results obtained have been found to be comparable. While operating the system in the single‐bed mode a cooling power of 250.4 W has been obtained with a coefficient of performance (COP) of 0.38 with an average evaporator temperature of 18.4°C against a predicted value of 263.7 W with a COP of 0.41. While operating in the twin‐bed mode a cooling power of 281.3 W with a COP of 0.47 with an average evaporator temperature of 17.6°C has been obtained against a predicted value of 294.5 W with a COP of 0.52.