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[en] A mini-type solar-powered absorption cooling system with a cooling capacity of 8 kW was designed. Lithium bromide-water was used as the working pairs of the chiller. Solar collectors with an area of 96 m2 were installed. A water storage tank with a volume of 3 m3 was used to store the hot water from the solar collectors. The experimental results showed that the average values of PMV (Predicted Mean Vote) and PPD (Predicted Percentage of Dissatisfied) of the test room were 0.22 and 5.89, respectively. Taking the average value of PMV and PPD into consideration, the solar cooling system could meet the indoor thermal comfort demand with the comfort level of A. The power consumption was reduced by 43.5% after introducing the stepped utilization of energy into the air handling unit. Meanwhile, a theoretical model was established based on Matlab to predict the variations of the system performance with ambient parameters. It is shown that the solar radiation intensity has a greater impact on the performance of the solar powered absorption cooling system compared with the ambient temperature. It is also shown that the indoor air temperature goes down with the increase of the solar radiation intensity as well as the decrease of the ambient temperature. -- Highlights: • The energy step utilization improved the performance of a solar cooling system. • Thermal environment of Class A was achieved by using radiant cooling. • Solar radiation intensity has prominent impact on the solar cooling system
[en] Highlights: • Performance of desiccant coated heat exchanger AC system is predicted. • Effects of main operation parameters and climatic conditions are discussed. • Regeneration temperature of 30 °C is recommended under simulation condition. • Higher ambient humidity ratio results in increased humidity ratio of supply air. • Temperature of ambient air has neglectable effect on supply air. - Abstract: Conventional air source heat pump system faces several challenges when adopted in winter season. Solid desiccant air conditioning system can provide humidification and heating power simultaneously and can be driven by low grade thermal energy; it provides a good alternative for air source heat pump systems. However, conventional solid desiccant air conditioning system adopts desiccant wheel with high cost as core component, which hinders the development of such system. Recently, desiccant coated heat exchanger (DCHE) with low initial cost and high efficiency was developed and this paper aims to investigate performance of DCHE air conditioning system adopted in Shanghai winter season. Performance of the system is predicted by a developed mathematical model where supply air states, mass of humidification and coefficient of performance (COP) are adopted as performance indices to evaluate the feasibility and energy utilization ratio of the system. Effects of regeneration water temperature on system performance are analyzed. It is found that under the simulation condition, relatively low regeneration temperature (such as 20 °C) cannot meet the designed standard and relatively high regeneration temperature (such as 40 °C) provides too much extra heating power, thus moderate regeneration temperature around 30 °C is recommended. Meanwhile, switch time is a crucial operation parameter for the system to obtain satisfied supply air, switch time from 40 s to 80 s and from 70 s to 240 s are recommended for transient and average supply air states, respectively. Both mass of humidification and COP increase with increasing regeneration temperature under simulation condition. Also, influences of ambient air temperature and humidity ratio on system performance are discussed to study the feasibility of such system regarding different climatic conditions. Results show that higher humidity ratio of ambient air results in increased humidity ratio of supply air, temperature of ambient air has neglectable effect on supply air. In conclusion, DCHE air conditioning system can be adopted for winter operation with moderate selection of regeneration temperature as well as switch time.
[en] Adsorption cold storage has lately attracted attention for its large storage capacity and zero cold energy loss during the storing process. Thermodynamic and experimental studies on the cold storage capacity and the cold discharging process, in which the adsorber is either air cooled or adiabatic, have been presented. An adsorption cold storage system with zeolite-water working pair has been developed, and some operating results are summarized. This system is used for providing air conditioning for the driver's cab of an internal combustion locomotive. Unlike a normal adsorption air conditioner, the system starts running with the adsorption process, during which the cold energy stored is discharged, and ends running with the generation process. The adsorbent temperature decreases during the cold storing period between two runs. The refrigeration power output for the whole running cycle is about 4.1 kW. It appears that such a system is quite energetically efficient and is comparatively suitable for providing discontinuous refrigeration capacity when powered by low grade thermal energy, such as industrial exhausted heat or solar energy
[en] Highlights: • A Bi_2Te_3 TEC with silica aerogel encapsulation is proposed. • A three dimensional model for the TEC is developed. • This model first considers the effect of air gap and aerogel. • Different thicknesses of aerogel encapsulation for TEC are discussed. - Abstract: A Bi_2Te_3 TEC with silica aerogel encapsulation is developed. Silica aerogel with different thicknesses is filled in the void spaces around the TE legs and started from cold-side ceramic plate. A three dimensional mathematical model for the TEC is developed. This model considers the effect air gap and silica aerogel. (Bi_0_._2Sb_0_._8)_2Te_3 and Bi_2(Te_0_._9_7Sb_0_._0_3)_3, which have temperature-dependent TE properties, are selected to be p-type and n-type TE materials. Also, an experimental test bench is built to validate the three dimensional model. The performances of non-silica aerogel encapsulated TEC with and without consideration of air gap are investigated. Meanwhile, the effects of different thicknesses of silica aerogel encapsulation under different T_as and V_as are analysed. The results show that the cold side ceramic and interconnector, and cold part of TE legs can be insulated effectively while the hot part of TE legs can be effectively dissipated using part silica encapsulation when T_h ⩾ T_a ⩾ T_c. The maximum Q_c at L_a_e_r = 0.8 mm is nearly increased by 7% as compared with that at L_a_e_r = 0 mm when T_a = (T_c + T_h)/2. Moreover, apart from the cold side interconnector, L_a_e_r should be about 2%, 15% and 25% of the L_l_e_g corresponding to the maximum Q_c condition when T_a = T_c, T_a = (T_h + T_c)/2 and T_a = T_h, respectively. The value of L_a_e_r can be (T_a−T_c)/(T_h−T_c)L_l_e_g corresponding to the optimum COP condition
[en] Highlights: • Solid sorption heat pipe (SSHP) with composite NaBr-NH_3 is proposed for continuous heat transfer. • Both vertical and horizontal SSHPs are investigated. • SSHP features non-isothermal heat transfer performance at sorbent and condenser sections. • The highest radial heat flux in vertical and horizontal SSHPs is 22.1 and 12.4 kW/m"2, respectively. • Both SSHPs have axial heat flux higher than 400 kW/m"2. - Abstract: A novel type solid sorption heat pipe (SSHP) is developed for continuous heat transfer. In contrast to conventional heat pipe (HP), SSHP utilizes the composite sorbent-sorbate as working media to replace the wick structure inside HP. Such a technology is expected to alleviate the heat transfer limits of conventional HP. NaBr is chosen as the sorbent, and the expanded natural graphite treated with sulfuric acid serves as the matrix. A certain molar amount of the sorbate (NH_3) is complexed with the composite sorbent. The desorption, condensation and chemisorption processes of NaBr-NH_3 working pairs are investigated for both vertical and horizontal placed SSHP. The results show that the desorption process of NaBr-NH_3 solid-gas reaction can be carried out while the heating temperature reaches up to 60 °C or above. The highest radial heat flux in both vertical and horizontal placed SSHP is around 22.1 and 12.4 kW/m"2, respectively, while the axial heat flux for both SSHPs is not less than 400 kW/m"2. It can be concluded that the SSHP is characterized by the non-isothermal heat transfer performance and verified to be available for continuous heat transfer. The vertical SSHP has a better overall heat transfer performance than horizontal SSHP under the same condition and NaBr-NH_3 working pairs applied in SSHP is suitable for low-grade thermal energy transfer above 60 °C.
[en] An Organic Rankine Cycle (ORC) with a scroll expander with varying displacement is studied. First, to obtain the isentropic efficiency of the scroll expander modified from an automobile air-condition compressor with a displacement of 66 ml/r, the performance is investigated by experiments on a test rig driven by the compressed air. Second, based on the experimentally obtained isentropic efficiency, thermodynamic and heat transfer models of ORC are established on the basis of sub-models of the main apparatuses. Consequently, energy and exergy efficiencies are analyzed. Based on the simulation, an ORC system is constructed and investigated. Experiments show that for a given heat source temperature of 105 °C, the energy efficiency of the system ranges from 1.7% to 3.2% and the exergy efficiency of the system is 8.6%–16.9%. Additionally, another scroll expander with a displacement of 86 ml/r is utilized to investigate how displacement of the scroll expander influences performance of the ORC system. Finally, experiments also show that the IHX deteriorates the performance of ORC system, which is significantly different from simulation results. - Highlights: • The isentropic efficiency of the scroll expander is investigated by compressed air. • An ORC system with R245fa fluid is designed and simulated. • An ORC system with scroll expanders of different displacements is established. • Reasonable exergy and energy efficiencies of ORC are achieved by experiments. • Experiments show an IHX requires a special design for getting optimal performance
[en] A small pumpless ORC (organic rankine cycle) system with different scroll expanders modified from compressors of the automobile air-conditioner is established, and the refrigerant R245fa is chosen as the working fluid. Different hot water temperatures of 80, 85, 90 and 95 °C are employed to drive the pumpless ORC system. Experimental results show that a maximum shaft power of 361.0 W is obtained under the hot water temperature of 95 °C, whereas the average shaft power is 155.8 W. The maximum energy efficiency of 2.3% and the maximum exergy efficiency of 12.8% are obtained at the hot water temperature of 90 °C. Meanwhile a test rig for investigating the mechanical loss of the scroll expander is established. The torque caused by the internal mechanical friction of the expander is about 0.4 N m. Additionally, another scroll expander with a displacement of 86 ml/r is also employed to investigate how scroll expander displacement influences the performance of the pumpless ORC system. Finally, the performance of the pumpless ORC system is compared with that of the conventional ORC system, and experimental results show that the small pumpless ORC system has more advantages for the low-grade heat recovery. - Highlights: • A small pumpless ORC (organic rankine cycle) system is established, and different scroll expanders are tested. • The maximum energy and exergy efficiency are 2.3% and 12.8% respectively. • A maximum shaft power of 361.0 W is obtained under the heat source temperature of 95 °C. • The small pumpless ORC system has characteristics of the high efficiency.
[en] Highlights: • Activated carbon–ammonia multi-stage adsorption refrigerator was analyzed. • COP, exergetic efficiency and entropy production of cycles were calculated. • Single-stage cycle usually has the advantages of simple structure and high COP. • Multi-stage cycles adapt to critical conditions better than single-stage cycle. • Boundary conditions for choosing optimal cycle were summarized as tables. - Abstract: Activated carbon–ammonia multi-stage adsorption refrigeration cycle was analyzed in this article, which realized deep-freezing for evaporating temperature under −18 °C with heating source temperature much lower than 100 °C. Cycle mathematical models for single, two and three-stage cycles were established on the basis of thorough thermodynamic analysis. According to simulation results of thermodynamic evaluation indicators such as COP (coefficient of performance), exergetic efficiency and cycle entropy production, multi-stage cycle adapts to high condensing temperature, low evaporating temperature and low heating source temperature well. Proposed cycle with selected working pair can theoretically work under very severe conditions, such as −25 °C evaporating temperature, 40 °C condensing temperature, and 70 °C heating source temperature, but under these working conditions it has the drawback of low cycle adsorption quantity. It was found that both COP and exergetic efficiency are of great reference value in the choice of cycle, whereas entropy production is not so useful for cycle stage selection. Finally, the application boundary conditions of single-stage, two-stage, and three-stage cycles were summarized as tables according to the simulation results, which provides reference for choosing optimal cycle under different conditions.
[en] Research highlights: → A dynamic mathematical model is built to predict the performance of DCHE system. → Operation time in dehumidification is a crucial parameter to system performance. → Under ARI summer condition, the largest cooling power can reach to 2.6 kW. → Under ARI humid condition, the largest cooling power can reach to 3.4 kW. → System performs better with smaller fin distance and tube diameter. -- Abstract: Desiccant coated heat exchanger (DCHE) system can handle latent and sensible load simultaneously by removing the released adsorption heat in dehumidification process. The system can also be driven by low grade thermal energy such as solar energy. In this paper, a dynamic one-dimensional mathematical model validated by experimental data is established to predict the performance of DCHE system, using conventional silica gel as desiccant material. Cooling performance of DCHE system is calculated under ARI (American Air-conditioning and Refrigeration Institute) summer and humid conditions. Simulated results show that the operation time in dehumidification process is a crucial factor for cooling capacity of DCHE system, which can be enhanced by eliminating the initial period with higher outlet air temperature, the largest cooling power of DCHE system increase from 2.6 kW to 3.5 kW by eliminating first 50 s of operation time under ARI summer condition. The results also prove that the system can provide cooling power to indoor condition with selective operation time when regeneration temperature varies from 50 oC to 80 oC. Besides, the model is adopted to analyze the effects of some structural parameters on system performance under simulated condition. The system performs well in smaller cobber tube external diameter condition, while both transient heat and mass transfer capacity can be enhanced under the condition of smaller distance between the fins.
[en] Research highlights: → Composite adsorbent 'employing lithium chloride in silica gel' and water as working pair. → A new type adsorbent bed is used to accommodate the composite adsorbent. → A dynamic model of the adsorption chiller is built. → The coefficient of performance (COP) and the cooling capacity will be improved. -- Abstract: This paper presents a novel adsorption chiller using composite adsorbent 'employing lithium chloride in silica gel' as adsorbent and water as adsorbate. A new type adsorbent bed is used to accommodate the composite adsorbent. The mass recovery between two adsorbent beds usually results in the adsorbate unbalance. So a novel auto water makeup unite is used to solve the problem. A dynamic model of the adsorption chiller is built based on the adsorption isotherms to predict the performance. The simulation result shows that the coefficient of performance (COP) and the cooling capacity will increase by using this new composite adsorbent. When the temperatures of hot water inlet, cooling water inlet, and chilled water inlet are 363, 303 and 293 K, COP will be 0.43, and the cooling capacity will be 5.295 kW. Also operation strategy is optimized. Different temperatures of hot water inlet, cooling water inlet and chilling water inlet will result in different COP and cooling capacity.