Experimental investigation and analysis of composite silica-gel coated fin-tube heat exchangers

Desiccant coated heat exchanger provides a promising option for desiccant cooling system, since it can handle sensible load and latent load simultaneously within one component. It is fabricated by coating desiccant material on the surface of conventional fin-tube heat exchanger. In order to enhance the performance of conventional silica gel coated heat exchanger (SGCHE), a novel composite silica gel coated heat exchanger (CCHE) is proposed and fabricated. An experimental setup is built to test and compare the dynamic performance of SGCHE and CCHE. Influences of main operation parameters including water temperatures and inlet air conditions on system performance are analyzed in terms of average dehumidification capacity (D_avg) and thermal coefficient of performance (COP_th). Optimization of cycle switch modes is also discussed. Experimental results show that CCHE has better dehumidification performance compared with SGCHE. In addition, pre-cooling before dehumidification process is found to be advantageous to both D_avg and COP_th.     

Experimental study on a two-stage rotary desiccant cooling system

The objectives of this study were to evaluate the performance of a novel two-stage rotary desiccant cooling (TSRDC) system and to obtain useful data and experiences for practical application. Newly developed compound desiccant (silica gel–haloids) was adopted in the system. An experimental set-up was built and used to test the system performance under three typical environmental conditions. System performances were evaluated in terms of moisture removal D and thermal coefficient of performance COP_th. It has found that the required regeneration temperature of TSRDC system is low and COP_th of the system is high. Regeneration temperatures from 65 °C to 80 °C, 65 °C to 75 °C and 80 °C to 90 °C were recommended for each environmental condition. In addition, the effects of some important operating parameters, such as inlet temperature and humidity ratio of process and regeneration air, on system performance were also investigated in this study.     

Cellulose-pad water cooling system with cold storage

The present study develops a cooling system using water as the working medium which is cooled at night by cellulose-pad cooling tower (CWCT) and stored for cooling application at daytime. That is, it utilizes the natural energy drawn from diurnal ambient air temperature difference. A cooling system was built and tested. It is found that the coefficient of performance of CWCT for heat dissipation of water at night, COP_nt, is between 3.8 and 11 and varies linearly with the evaporation temperature glide D_G (difference between cold water temperature in the storage tank and wet-bulb temperature of ambient air). The COP for room cooling at daytime run with air cooler in a room, COP_day, is between 8.8 and 12.6. For day cycle operation, the measured overall cooling COP_o is 5.1. COP_o is expected to reach 9.4 at room temperature 45 °C.     

Investigation on capacity matching in a heat pump and hollow fiber membrane-based two-stage liquid desiccant hybrid air dehumidification system

The heat pump and hollow fiber membrane-based two-stage liquid desiccant hybrid air dehumidification system is promising recently because solution droplets can be prevented from crossing over into the process air. The quasi-isothermal processes are realized by two-stage dehumidification processes and the system performance is improved. In this study, a novel capacity matching index (CMI) is introduced to evaluate the energy capacity matching of the system through modeling study. It is found that CMI is usually lower than 1 under the typical hot and humid weather condition like South China and the demand and supply of energy in the system is mismatching. As inlet air temperature rises, the dehumidification rates, CMI, EER and COP all decrease. But CMI is almost constant with different inlet air humidity. The influence of air inlet temperature to dehumidifiers and regenerators on the system performance is also investigated. The higher the inlet temperature of dehumidifiers is, the larger the CMI, EER and COP are. The dehumidification rates and CMI both grow with an increase in the inlet temperature of regenerators. It is beneficial for energy balance of the system and high moisture loads, but the side effect is that the EER and COP both decrease.     

Experimental and theoretical study on a novel double evaporating temperature chiller applied in THICS using R32/R236fa

A novel chiller with double evaporating temperatures is proposed in this paper, which can be applied in temperature and humidity independent control system (THICS). A zeotropic mixture R32/R236fa is selected as the refrigerant, and chilled water with two different temperatures is produced. The experimental coefficient of performance (COP_exp), theoretical coefficient of performance (COP_th), and second law efficiency (η) of the chiller are studied. The performance of the chiller is studied by varying the mass fraction of R32 in the R32/R236fa (W(R32)), chilled water temperature, and the flow rates of the heat transfer media (chilled water and cooling water). The results show that the high temperature chilled water (T_H,out) can be at 15–18 °C, and the low temperature chilled water (T_L,out) can be at 6–8 °C. When T_H,out is 17 °C and T_L,out is 7 °C, the maximum COP_th and COP_exp are 4.73 and 3.97, respectively. Second law efficiency, η, increases to 31% as W(R32) increases from 0.3 to 0.6.     

Augmented desalination with cooling integration

Humidification–dehumidification (HDH) desalination works based on a water cycle principle and involves air humidification and condensation (dehumidification). The cooling of humid air with chilled water increases the desalination and results in cold air suitable for air conditioning process. The merits of the proposed HDH desalination and cooling are not analyzed and compared in the literature. Therefore, the performance results of HDH with normal water (current technology) and HDH with chilled water (proposed idea) are compared to highlight the merit of this cycle. The combined cycle for desalination and cooling has been solved thermodynamically with psychrometric properties. The results are validated with a laboratory experimental setup. The examined operational process conditions are hot water inlet temperature, efficiency of humidifier, and vapor absorption refrigerator's (VAR's) evaporator exit temperature. The focused results are desalination, cooling and energy utilization factor (EUF). The comparative study recommends the use of chilled water in the final stage. The two stage desalination with dehumidification by normal water in the first stage and chilled water in combined two stages improves the cycle EUF from 0.18 to 0.33. Out of 300 W of cooling generation, 100 W is used for dehumidification and the remaining 200 W is available for air conditioning process at 15 N⋅m³ h⁻¹ of air.     

多级矩阵结构冷却除湿器的除湿性能研究

为了降低空分系统压缩机功耗,提高压缩机运行可靠性,本文提出一种用于压缩机进气除湿的多级矩阵结构的冷却除湿器,并搭建了多级冷却除湿实验台,测试了除湿器的除湿性能。实验结果表明,在进口空气含湿量和温度固定为11.7 g/(kg干空气)和24.4 ℃时,当空气质量流量由0.48 kg/s增至0.78 kg/s,空气出口含湿量由7.1 g/(kg干空气)增至7.7 g/(kg干空气);在进口空气质量流量和温度固定为0.53 kg/s和25.2 ℃时,当冷却水温度由6.9 ℃升至11.9 ℃,空气出口含湿量由7.1 g/(kg干空气)增至9.4 g/(kg干空气)。同时,建立了除湿器内部传热传质过程的稳态数值模型,将模拟结果与实验结果进行对比。结果表明,该模型对于除湿器出口空气含湿量和温度的平均误差分别为8.6%和2.1%,显示出较好的可靠性。进一步模拟研究了多级矩阵结构与单级叉流结构冷却除湿器的除湿性能,发现采用多级结构可以有效提高除湿效率,在进口空气流量和冷却水质量流量分别为0.53 kg/s和0.3 kg/s时,多级结构的除湿量可以提高4.3%,除湿效率可以提高2.5%;通过增加填料模块的长度,可以提高除湿效率。当长方体填料模块体积固定为0.054 m3,模块长度由0.14 m增至0.28 m时,传质系数可由4.3 g/(m2?s)增至6.5 g/(m2?s),除湿效率由66.4%升至79.2%。     

冷冻除湿机最优风量的试验分析

在名义工况下,对调温型除湿机的样机在水冷降温模式和一般升温模式下进行测试,分析风量对制冷量（或制热量）、除湿量、修正输入功率及单位输入功率除湿量的影响,认为存在一个最优风量（4 000m3/h）使得冷冻除湿机单位输入功率除湿量最大.     

除湿换热器串联换热器强化除湿降温性能实验研究

除湿换热器可以同时处理显热与潜热负荷,但由于吸附热的影响,存在热湿负荷处理不同步及显热负荷处理能力不足的问题。本文提出了在除湿换热器后面串联一个显热换热器对空气进行二次处理,搭建了实验台对除湿换热器串联换热器情况下除湿降温过程的动态性能进行测试,并且在实验中分析了水温、进风温度、湿度、速度等主要参数对除湿量、降温量、制冷功率、COP的影响。结果表明:增加显热换热器可以大幅度增加处理空气的平均降温温差,在除湿初期阶段效果尤为明显,同时系统的制冷量也明显提高。此外,分析各参数对实验结果的影响可知,冷水温度与热水温度升高都可以有效提高系统制冷量与COP,空气的温湿度升高会提升系统性能,空气流速变慢对系统平均除湿量与有效除湿时间有明显的提升。     

Development of an ejector cooling system with thermal pumping effect

This paper presents a feasibility study of an ejector cooling system (ECS) that utilizes a multi-function generator (MFG) to eliminate the mechanical pump. The MFG serves as both a pump and a vapor generator. The MFG is designed based on the pressure equilibration between high and low pressures through heating and cooling process. In this design, an ECS that contains no moving components and is entirely powered by heat can be practicable. A prototype using refrigerant R141b as working fluid was constructed and tested in the present study. The experimental results showed that the system coefficient of performance (COP_o) was 0.218 and the cooling capacity was 0.786 kW at generating temperature (T_G) 90 °C, condensing temperature (T_C) 32.4 °C and evaporating temperature (T_E) 8.2 °C. While taking into account the extra heat needed for the MFG operation, the total coefficient of performance (COP_t) is 0.185. It is shown that a continuous operation for the generation of cooling effect in an ECS with MFG can be achieved. This cooling machine can be very reliable since there is no moving part.     

Theoretical analyses of a new two-stage absorption-transcritical hybrid refrigeration system

In this context, a two-stage absorption-transcritical hybrid refrigeration system is proposed. R744 is chosen as a refrigerant for the transcritical heat pump subsystem and LiBr-H₂O working pair for the two-stage absorption refrigeration subsystem. Based on the mathematical and physical models, theoretical investigation is carried out on its performance. The main effects are discussed on COP_net (the ratio of cooling capacity powered by low-grade heat to the low-grade heat consumption for the hybrid system) and COP_mt (the ratio of cooling capacity powered by mechanical work to the mechanical work consumption for the hybrid system). Comparing with the normal two-stage absorption refrigeration system, theoretical results show that COP_net could be improved up to about 55% when the refrigeration temperature is 7 °C. In addition, COP_mt are more than 50% higher than that of the conventional transcritical refrigeration system. It is also found that both 45–55 °C low-grade heat and condensing heat could be used as actuating heat of the two-stage absorption refrigeration subsystem.     

Experimental study on a new internally cooled/heated dehumidifier/regenerator of liquid desiccant systems

For providing good performance of dehumidifier and regenerator with certain dimensions, a new type of internally cooled/heated dehumidifier/regenerator based on the plate–fin heat exchanger (PFHE) was designed. To investigate the behavior of the new equipment, an experimental setup was established in an environment chamber with regulable temperature and humidity air. By the internally cooled dehumidification testing, effects of the cooling water temperature, the air flow rate and the desiccant temperature on the dehumidification performance and the cooling efficiency were presented. The behavior of internally cooled dehumidification process was compared with that of the adiabatic dehumidification process. The results suggested that the cooling efficiency decreased with the increasing of the cooling water temperature and desiccant with low temperature could bring more mass transfer coefficients. There is an optimal air flow rate to achieve the maximum absolute humidity decrease of the air. By the internally heated regeneration testing, effects of the air flow rate and the desiccant inlet temperature on the regeneration performance and air outlet parameters were discussed and also compared with those of the adiabatic regeneration process. It was concluded that the regeneration efficiency of internally heated regeneration was more than that of the adiabatic regeneration, and the internally heated regenerator could offer better thermal performance.     

Theoretical study on a frost-free household refrigerator-freezer

This study deals with a frost-free household refrigerator-freezer, in which frosting can be retarded by dehumidifying the air before it enters the evaporator of the refrigerator-freezer via a desiccant-coated heat exchanger. Because the desiccant can be regenerated via the condensation heat of the refrigerant (which is exhausted into ambient air in conventional household refrigerator-freezers), the proposed system can achieve high energy efficiency. Calculations show that the coefficient of performance of this system (COP) is within the range 1.5–2.5 at an ambient temperature of 15–35 °C. Moreover, it is found that the relative humidity of the refrigerator air (RH_RA) and the temperature of freezer air (T_FA) have a significant effect on COP: COP decreases by about 13% when RH_RA varies from 0.4 to 0.8 and by 10% when T_FA runs from −18 to −23 °C.     

热泵型溶液除湿新风系统性能分析与优化

本文建立了热泵型溶液除湿(HPLD)新风系统数学模型,研究了新风温度、湿度对系统运行性能的影响。结果表明:新风温度升高1℃,系统COP平均下降率为0.9%;新风含湿量增加1 g/(kg干空气),系统COP平均下降率为3.6%,新风湿度增加导致HPLD系统COP大幅下降,系统新风湿度变化的适应性差。为扩大HPLD系统适应范围,提出了冷却除湿与HPLD组合式除湿系统,以组合式除湿系统COP为评价指标,得到组合系统级间新风参数最优状态:温度为21℃,含湿量为14.1 g/(kg干空气)。夏季典型工况下,组合式除湿系统COP为5.40,比单一HPLD系统COP提高87.5%。最后,根据HPLD系统设计参数制作了实验样机,并对模拟结果进行了验证。     

Experimental investigation on heating performance of gas-injected scroll compressor using R32, R1234yf and their 20wt%/80wt% mixture under low ambient temperature

In this paper, an integrated gas-injected scroll compressor heat pump system using R1234yf, R32 and its binary mixtures as working fluid was developed and their heating performances under low ambient temperature were quantitatively evaluated. A composite test system consisting of second-refrigerant calorimeter and water-cooled condenser was used to test the system working performance. The condensing temperature, evaporating temperature, compressor power input and other variables were analyzed to evaluate the system heating capability and energy efficiency. Test results showed that the R1234yf system can run at an evaporating temperature of −25 °C. R1234yf/R32 mixture can run at an evaporating temperature of −20 °C and it has the highest heating COP value among other refrigerants; R1234yf/R32 gas injection system provided very significant performance improvements for heating performance, compared with no gas injection, the heating capacity and heating COP can improve 16%~20% and 13%~16%, respectively.     

Theoretical analysis of an open-cycle absorption heating and cooling system

In warm countries, such as Italy and Spain, the increasing demand for space conditioning cannot be met by the current capacity of installed electric power. This has led to a growing interest in cooling with natural gas. An interesting thermal cooling system uses open-cycle absorption. It treats the air directly without heat exchangers and with very low temperature drops. With absorption dehumidification it is possible not only to obtain a cooling system similar in performance to conventional thermal cooling systems, but also, with few modifications, a high-performance heating system fed by a natural gas burner, which combines simplicity with PER typical of a gas driven heat pump. The proposed new system described in the paper is studied both in winter and summer mode, evaluating the influence of the different parameters.     

Theoretical study of R32 in an oil flooded compression cycle with a scroll machine

R32 is regarded as a potential alternative for R410A, but it has a low slope of isentropic line, high superheat inside a compressor and thus a high discharge temperature. These disadvantages limit its wider adoption. In order to improve the performance of R32 air conditioner, oil flooded compression with regenerator has been suggested. A single stage oil flooded compressor model is developed to obtain a more accurate system-level improvement. In the compressor model, the heat transfer losses between shell and ambient, suction gas and motor, and high-pressure and low-pressure cylinders are considered. By means of parametric studies, it was found that the novel cycle resulted to be beneficial to increase the compressor internal superheating, to decrease the compressor heat losses and to improve its overall isentropic efficiency while cooling capacity or heating capacity is degraded. COP_h improvement can reach up to 16.4% for an evaporating and condensing temperatures of −25 °C and 45 °C, respectively. The discharge temperature resulted to be lower than 110 °C. In addition, a thorough comparison between R32 and R410A with both novel and baseline systems has been carried out. The results indicate that the novel cycle has potential benefits for applications in R32 air conditioners.     

Experimental performance analysis and modelling of liquid desiccant cooling systems for air conditioning in residential buildings

The paper presents a new desiccant cooling cycle to be integrated in residential mechanical ventilation systems. The process shifts the air treatment completely to the return air side, so that the supply air can be cooled by a heat exchanger. Purely sensible cooling is an essential requirement for residential buildings with no maintenance guarantee for supply air humidifiers. As the cooling power is generated on the exhaust air side, the dehumidification process needs to be highly efficient to provide low supply air temperatures. Solid rotating desiccant wheels have been experimentally compared with liquid sorption systems using contact matrix absorbers and cross flow heat exchangers. The best dehumidification performance at no temperature increase was obtained in an evaporatively cooled heat exchanger with sprayed lithium chloride solution. Up to 7 g kg⁻¹ dehumidification could be reached in an isothermal process, although the surface wetting of the first prototype was low. The process then provides inlet air conditions below 20 °C for the summer design conditions of 32 °C, 40% relative humidity. With air volume flow rates of 200 m³ h⁻¹ the system can provide 886 W of cooling power.A theoretical model for both the contact absorber and the cross flow system has been developed and validated against experimental data for a wide range of operating conditions. A simulation study identified the optimisation potential of the system, if for example the surface wetting of the liquid desiccant can be improved.     

Prozessberechnungen für eine CO<Subscript>2</Subscript>-Anlage zur Kälte- und Wärmeerzeugung für Supermärkte. Teil 1: Weitestgehende CO<Subscript>2</Subscript>-Abkühlung durch Umgebungsluft

CO₂ processes are used in supermarkets for medium- and low-temperature refrigeration and by now even for room heating and hot tap-water preparation via heat recovery. Through systematic thermodynamic process calculations, the limits of the heat recovery from a CO₂ plant and its influence on the refrigeration process as well as the complete system are investigated for a supermarket with 100 kW medium-temperature refrigeration capacity. This investigation focuses on the energy efficiency of the heat supply by the extended CO₂ plant. By using the medium-temperature refrigeration capacity as a reference, the results for ambient temperatures between ??15?°C and?+?10?°C are applicable for other supermarket sizes as well.In part 1, a plant is investigated with a ratio between low- and medium-temperature refrigeration capacity of 0.2 and CO₂ cooling via the ambient air as far as possible. The contributions of refrigeration and heat supply are specified separately for a range of high-pressure values for sub-critical and for trans-critical operation of the CO₂ plant. Relevant data for design and operation of the CO₂ plant are made available: the supplied heating capacity (relative to the medium-temperature refrigeration capacity), the coefficients of performance of refrigeration and of combined refrigeration and heating, and the exergy efficiencies of refrigeration and of combined refrigeration and heating. With this knowledge, preferred operation parameters for the CO₂ process can be chosen depending on the ambient temperature. Moreover, the decision is prepared, whether an additional heating system is required or the heat recovery from the refrigeration plant is sufficient. In the following second part, results for limited CO₂ cooling via ambient air will be considered.     

Development of a residential gas absorption heat pump

A residential absorption heat pump system has been developed based on a new absorption pair. It uses R 123a (CCIF₂CHCIF) as the refrigerant and ETFE (ethyltetrahydrofurfylether) as the absorbent. The thermodynamic analysis of the new pair shows the potential of very good performance for heat pump duties.Actual tests show that heating coefficients of performance (COP) in the range of 1.50 are possible, so the system is very competitive with other heating and cooling technologies especially in more northerly climates.