Free cooling系统在制冷系统经济运行中的研究与应用

卷烟制造工厂在过渡季节需开启制冷机组,为组合式空调机组提供低温冷冻水以确保工房温湿度指标合格,导致制冷机开启周期增长,使整个制冷系统包括制冷机、冷却塔、冷却水泵、冷冻水泵等设备的电耗增加。某卷烟厂在动力中心制冷系统中研究并引进Free cooling(自然冷却)系统。通过系统设计与改造,在过渡季节通过利用冷却塔供冷的方式降温,不仅达到控制工房温湿度指标的目的,同时减少了大功率制冷机组的开启时间,节省了运行费用,提高了设备使用寿命,达到节能降耗的经济运行目的。开展Free cooling系统在制冷系统经济运行中的研究与应用十分必要。     

Experimental investigation of a natural zeolite–water adsorption cooling unit

In this study, a thermally driven adsorption cooling unit using natural zeolite–water as the adsorbent–refrigerant pair has been built and its performance investigated experimentally at various evaporator temperatures. The primary components of the cooling unit are a shell and tube adsorbent bed, an evaporator, a condenser, heating and cooling baths, measurement instruments and supplementary system components. The adsorbent bed is considered to enhance the bed’s heat and mass transfer characteristics; the bed consists of an inner vacuum tube filled with zeolite (zeolite tube) inserted into a larger tubular shell. Under the experimental conditions of 45 °C adsorption, 150 °C desorption, 30 °C condenser and 22.5 °C, 15 °C and 10 °C evaporator temperatures, the COP of the adsorption cooling unit is approximately 0.25 and the maximum average volumetric cooling power density (SCP_v) and mass specific cooling power density per kg adsorbent (SCP) of the cooling unit are 5.2 kW/m³ and 7 W/kg, respectively.     

A solar-powered adsorption cooling system using a silica gel–water mixture

Solar-powered adsorption cooling is an attractive solar energy application. Metallic solar collectors with fins have been used to increase the thermal conductivity in solar collectors. This approach has a negative effect due to solar energy loss by reflection and heat loss resulting from the sensible heat of the metal. For these reasons, a direct-radiation absorption collector is proposed here. The effects of the wavelength of the absorbed light, types of silica gel used and additives to improve the absorptivity have been investigated. We have verified that blue silica gel has a better absorptivity in the near-infrared region than white silica gel. The addition of activated carbon to the silica gel improves the desorption rate and regeneration temperature of the packed bed.     

Energy audit case studies II—air conditioning (cooling) systems

This paper discusses a novel methodology for determination of the efficiencies of each segment in a centralized air conditioning (A/C) plant. The methodology is useful analytical tool in energy audit where it can be used for decision support for evolution of energy conservation measures and techno-economic evaluation of the various options. The procedure is applied to three A/C plants of capacity range from 50 to 500 TR. Losses in chilled air ducts and cycling losses in the refrigerant circuit are identified to be the major causes for decrease in the efficiency and capacity under rating of the systems. Effective controls and insulation of air ducts: have been shown to be the most cost effective solutions to improve the energy efficiency of the A/C plants. The generalized experimental data on overall performance of sixteen A/C plants is also provided.     

Simulation–optimization of solar-assisted desiccant cooling system for subtropical Hong Kong

Solar cooling is a novel approach, which primarily makes use of solar energy, instead of electricity, to drive the air-conditioning systems. In this study, solar-assisted desiccant cooling system (SADCS) was designed to handle the cooling load of typical office in the subtropical Hong Kong, in which half of the building energy is consumed by the air-conditioning systems. The SADCS mainly consisted of desiccant wheel, thermal wheel, evaporative coolers, solar air collectors and gas-fired auxiliary heater, it could directly tackle both the space load and ventilation load. Since the supply air flow is same as the outdoor air flow, the SADCS has a feature of sufficient ventilation that enhances the indoor air quality. Although it is inevitable to involve the auxiliary heater for regeneration of desiccant wheel, it is possible to minimize its usage by the optimal design and control scheme of the SADCS. Through simulation–optimization approach, the SADCS can provide a satisfactory performance in the subtropical Hong Kong.     

A new generation cooling device employing CaCl2-in-silica gel–water system

This article presents the dynamic modelling of a single effect two-bed adsorption chiller utilizing the composite adsorbent “CaCl₂ confined to KSK silica gel” as adsorbent and water as adsorbate, which is based on the experimentally confirmed adsorption isotherms and kinetics data. Compared with the experimental data of conventional adsorption chiller based on RD silica gel + water pair, we found that the new working pair provides better cooling capacity and performances. From numerical simulation, it is also found that the cooling capacity can be increased up to 20% of the parent silica gel + water adsorption chiller and the coefficient of performance (COP) can be improved up to 25% at optimum conditions. We also demonstrate here that the best peak chilled water temperature suppression, and the maximum cooling capacity can be achieved by the optimum analysis for both cycles.     

Influence of the cooling rate on the ageing of lead–calcium alloys

Cast lead–calcium alloys were known to be sensitive to experimental parameters, which cause large variations on the ageing and overageing behaviour. From the study of these parameters, the quenching rate was the only significant parameter. A critical cooling rate was defined based on hardness, electrical resistivity and metallographical observations. The inconsistencies in the literature noticed on the evolutions of lead–calcium alloys can now be explained by whether or not this critical cooling rate was respected.     

Spatial optimization of an underhood cooling module – Towards an innovative control approach

The present paper reports a numerical investigation of spatial optimization of heat-exchanger by acting on its positioning in the vehicle’s cooling module. This analysis also elucidates how to act on the different parameters influencing heat-exchanger performance in order to optimize their functioning. A two-dimensional computation code permits optimizing the performance of the cooling module by positioning different heat exchangers, in both the driving and stop phases of the vehicle. The ultimate aim is to apply new control approaches to real vehicles so as to reduce pump and compressor energy consumption and thus fuel consumption. Compared to a reference “in-series” configuration of the cooling module HXs (in which the different HXs are superposed in the airflow direction), an “in-parallel” configuration (in which the different HX surfaces are in a row with respect to the air flow direction) increases the thermal power of the HXs by 4.4% and decreases the pressure losses by 0.9%.     

Numerical study of the thermal–hydraulic performance of evaporative natural draft cooling towers

In this paper, the mathematical and physical models governing the flow, mass and heat energy of moist have been set up for an evaporative natural draft cooling tower. The models consider the effect of non-spherical shape of water drops on the flow, heat and mass transfer. Experimental data has been adopted to validate the numerical scheme. Average difference between the measured and the predicted outlet water temperature is 0.26°C. Distributions of the velocity components of the moist air, density, pressure, enthalpy and moisture content, the water temperature and its mass flux have been predicted. The simulation shows that some recirculation exits under the lower edge of the shell, where the air enthalpy, temperature, humidity and moisture content are higher, but the density is lower. The simulation also proves that the main transfer processes take place in the fill region where the percentage of latent heat transfer is predicted as 83%. However, about 90% of the heat energy is transferred via evaporation in the rain region although the total heat transfer rate there is very small compared to the fill region. Hourly performance of a natural draft cooling tower under the meteorological condition of Singapore has also been predicted.     

Determination of free cooling potential: A case study for İstanbul,Turkey

A significant portion of energy consumed in buildings is attributed to energy usage by heating, ventilating and air conditioning (HVAC) systems. Free cooling is a good opportunity for energy savings in air conditioning systems. With free cooling, commonly is known economizer cycle, the benefits of lower ambient temperatures are utilized for a significant proportion of the year in many climates. The detailed analysis of local weather data is required to assess the benefits of economizer. In this study, free cooling potential of ?stanbul, Turkey was determined by using hourly dry-bulb temperatures measurements during a period of 16 years. It is found that the free cooling potential varies with supply air temperature and months. It is determined that although there are substantial energy savings during a significant portion of the year especially in transition months (April, May, September and October), the high outdoor air temperatures from June to August, made the system not beneficial for free cooling except at high supply air temperature.     

A review on air flow and coolant flow circuit in vehicles’ cooling system

Engine cooling system plays an important role to maintain the operating temperature of engine. The coolant circuit initiates by picking up heat at water jackets. With the pressure gradient exists in coolant circuit, hot coolant flows out from engine to radiator or to bypass circuit (during cold start). The under hood air flow carries heat away at radiator after the air flows through numerous hood components. The coolant flow circuit and air flow circuit meet each other and exchange heat at radiator. Extensive researches are carried out to study vehicles’ cooling system extensively either numerically or experimentally. The research covers many individual topics which include numerical modelling of engine cooling system, under hood air flow, heat transfer at water jacket, heat transfer at radiator and coolants’ after-boiling phenomenon.     

Adsorption properties of a natural zeolite–water pair for use in adsorption cooling cycles

The equilibrium adsorption capacity of water on a natural zeolite has been experimentally determined at different zeolite temperatures and water vapor pressures for use in an adsorption cooling system. The Dubinin–Astakhov adsorption equilibrium model is fitted to experimental data with an acceptable error limit. Separate correlations are obtained for adsorption and desorption processes as well as a single correlation to model both processes. The isosteric heat of adsorption of water on zeolite has been calculated using the Clausius–Clapeyron equation as a function of adsorption capacity. The cyclic adsorption capacity swing for different condenser, evaporator and adsorbent temperatures is compared with that for the following adsorbent–refrigerant pairs: activated carbon–methanol; silica gel–water; and, zeolite 13X–water. Experimental results show that the maximum adsorption capacity of natural zeolite is nearly 0.12 kg_w/kg_ad for zeolite temperatures and water vapor pressures in the range 40–150 °C and 0.87–7.38 kPa.     

Study on a silica gel–water adsorption chiller integrated with a closed wet cooling tower

A silica gel–water adsorption chiller integrated with a closed wet cooling tower is proposed. This adsorption chiller consists of two vacuum chambers, each with one adsorber, one condenser and one evaporator. Vacuum valves were not adopted in this chiller in order to enhance the reliability. A novel heat recovery process was carried out after a mass recovery-like process to improve the coefficient of performance (COP). Integration of the closed wet cooling tower into the chiller could ensure the cleanliness of cooling water circulating in the chiller and also promote the convenient setup of the chiller. A transient one-dimensional mathematical model was adopted to study this adsorption chiller. The simulated results showed that the cooling power and COP were 10.76 kW and 0.51 respectively when the hot water inlet temperature, the chilled water inlet temperature, the air inlet wet bulb temperature and dry bulb temperature were 85, 15, 28 and 30 °C respectively.     

Cooling performance investigation of electronics cooling system using Al2O3–H2O nanofluid

In this study, the cooling performance of Al₂O₃–H₂O nanofluid was experimentally investigated as a much better developed alternative for the conventional coolant. For this purpose the nanofluid was passed through the custom-made copper minichannel heat sink which is normally attached with the electronic heat source. The thermal performance of the Al₂O₃–H₂O nanofluid was evaluated at different volume fraction of the nanoparticle as well as at different volume flow rate of the nanofluid. The volume fraction of the nanoparticle varied from 0.05 vol.% to 0.2 vol.% whereas the volume flow rate was increased from 0.50 L/min to 1.25 L/min. The experimental results showed that the nanofluid successfully has minimized the heat sink temperature compared to the conventional coolant. It was noticed also that the thermal entropy generation rate was reduced via using nanofluid instead of the normal water. Among the other functions of the nanofluid are to increase the frictional entropy generation rate and to drop the pressure which are insignificant compared to the normal coolant. Given the improved performance of the nanofluid, especially for high heat transportation capacity and low thermal entropy generation rate, it could be used as a better alternative coolant for the electronic cooling system instead of conventional pure water.     

Heat transfer analysis for the design and application of the passive cooling rate controlled device—Box-in-box

BackgroundIn cryopreservation, cooling rate is a dominant factor that influences the survival of cells. Box-in-box (BIB) was recently developed as a reliable, cooling rate controlled and cost-effective cooling device. However, the intrinsic heat transfer characteristic still needs to be further specified for the best of design and application of the device.MethodThe freezing process of samples inside BIB is simulated by developing a one dimensional heat transfer model in which fixed-grid technique is used to solve the solidification problem of the ternary cryopreservation media (water, NaCl and cryoprotectant). Based on the model, several critical factors, including supercooling temperature, structural parameters and application conditions, are evaluated respectively. Several cell free experiments were also conducted to validate the model.ResultsIt was demonstrated that BIB method can achieve uniform and consistent cooling of samples, and the theoretical and experimental results fit quite well. Further analysis reveals that several structural parameters (such as the dimension of insulation layer) and application conditions (such as the cryoprotectant concentration and the sample volume) have significant effect on the freezing process of sample. Thus the design and application of BIB should be carefully conducted to achieve the desired cooling rate.ConclusionThe theoretical model is reasonable for the BIB system. It provides an effective tool to determine the detailed structural parameters when designing BIBs, and it can also be used as a good support for the application of BIB systems.     

Residential solar air conditioning: Energy and exergy analyses of an ammonia–water absorption cooling system

Large scale heat-driven absorption cooling systems are available in the marketplace for industrial applications but the concept of a solar driven absorption chiller for air-conditioning applications is relatively new. Absorption chillers have a lower efficiency than compression refrigeration systems, when used for small scale applications and this restrains the absorption cooling system from air conditioning applications in residential buildings. The potential of a solar driven ammonia–water absorption chiller for residential air conditioning application is discussed and analyzed in this paper. A thermodynamic model has been developed based on a 10 kW air cooled ammonia–water absorption chiller driven by solar thermal energy. Both energy and exergy analyses have been conducted to evaluate the performance of this residential scale cooling system. The analyses uncovered that the absorber is where the most exergy loss occurs (63%) followed by the generator (13%) and the condenser (11%). Furthermore, the exergy loss of the condenser and absorber greatly increase with temperature, the generator less so, and the exergy loss in the evaporator is the least sensitive to increasing temperature.     

Simulation and economic optimization of a solar assisted combined ejector–vapor compression cycle for cooling applications

This paper describes the hourly simulation and optimization of a thermally driven cooling cycle assisted by solar energy. The double stage solar ejector cooling cycle is modelled using the TRNSYS-EES simulation tool and the typical meteorological year file containing the weather data of Florianópolis, Brazil. The first stage is performed by a mechanical compression system with R134a as the working fluid, while the second stage is performed by a thermally driven ejector cycle with R141b. Flat plate collectors and an auxiliary energy burner provide heat to the ejector cycle. The thermo-economical optimization is carried out with respect to the intercooler temperature and the flat plate solar collector area, for given specific costs of the auxiliary energy and electric energy, the capital cost of the collectors, ejector cooler, and the capital cost of equivalent mechanical compression cooler.     

Theoretical analysis of a CO2–NH3 cascade refrigeration system for cooling applications at low temperatures

As a result of environmental problems related to global warming and depletion of the ozone layer caused by the use of synthetic refrigerants (CFC’s, HCFC’s and HFC’s) experienced over the last decades, the return to the use of natural substances for refrigeration purposes, appears to be the best long-term alternative. In this paper, a cascade refrigeration system with CO₂ and NH₃ as working fluids in the low and high temperature stages, respectively, has been analysed. Results of COP and exergetic efficiency versus operating and design parameters have been obtained. In addition, an optimization study based on the optimum CO₂ condensing temperature has been done. Results show that following both method’s exergy analysis and energy optimization, an optimum value of condensing CO₂ temperature is obtained. The compressor isentropic efficiency influence on the optimum system COP has been demonstrated. A methodology to obtain relevant diagrams and correlations to serve as a guideline for design and optimization of this type of systems has been developed and it is presented in the paper.