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[en] Heat and mass transfer processes in a cross flow liquid desiccant dehumidifier, in which wet durable honeycomb paper constitutes the packing material, is investigated in this paper. The device is expected to be used in hot and humid areas to control the indoor humidity environment. A mathematical model, able to determine the heat and mass transfer between the air and the falling film of liquid desiccant, is developed, and the analysis on Nusselt and Sherwood numbers at the liquid-air interface is performed considering the solution of 40% H2O/CaCl2. Also obtained is the theoretical Nusselt number under assumed conditions and the relevant analysis, as well as the comparison between the two results
[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] 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] In this study, a small scale hybrid solar heating, chilling and power generation system, including parabolic trough solar collector with cavity receiver, a helical screw expander and silica gel-water adsorption chiller, etc., was proposed and extensively investigated. The system has the merits of effecting the power generation cycle at lower temperature level with solar energy more efficiently and can provide both thermal energy and power for remote off-grid regions. A case study was carried out to evaluate an annual energy and exergy efficiency of the system under the climate of northwestern region of China. It is found that both the main energy and exergy loss take place at the parabolic trough collector, amount to 36.2% and 70.4%, respectively. Also found is that the studied system can have a higher solar energy conversion efficiency than the conventional solar thermal power generation system alone. The energy efficiency can be increased to 58.0% from 10.2%, and the exergy efficiency can be increased to 15.2% from 12.5%. Moreover, the economical analysis in terms of cost and payback period (PP) has been carried out. The study reveals that the proposed system the PP of the proposed system is about 18 years under present energy price conditions. The sensitivity analysis shows that if the interest rate decreases to 3% or energy price increase by 50%, PP will be less than 10 years. (author)
[en] Highlights: • Three types of novel DCHEs with variable structure sizes are tested and compared. • Ranks of influence factors in experiments are obtained by using Taguchi method. • Higher surface compactness of DCHE means higher heat and mass transfer capacity. • Heat and mass transfer coefficients are functions of pressure drop. Desiccant-coated heat exchanger (DCHE) is a novel component for handling both sensible and latent heat assisted by desiccant materials. In this paper, three types of DCHEs with the same transfer surface area, DCHE A (fin pitch 2 mm, fin depth 44 mm), DCHE B (fin pitch 3 mm, fin depth 66 mm) and DCHE C (fin pitch 4 mm, fin depth 88 mm), are fabricated to make out the relationships between structure sizes and performance characteristics. The transient heat and moisture transfer performance, as well as the pressure drop passing through DCHEs, are tested and compared in depth. By using Taguchi method, the ranks of influence factors in heat and mass transfer performances are obtained. With the same transfer surface area but different surface compactness, three DCHEs show different heat and mass transfer capacities and different pressure drops. DCHE A with the highest surface compactness shows the highest heat and mass transfer capacity, while the highest pressure drop is shown as deficiency. DCHE C with the smallest surface compactness shows the highest heat recovery efficiency and the lowest pressure drop. Heat transfer coefficient of DCHE A is 14.9% greater than DCHE B, 19.6% greater than DCHE C in dehumidification process. The moisture adsorbed value of DCHE A is 9.6% greater than DCHE B, 18.2% greater than DCHE C. Pressure drop of DCHE A is 50% larger than DCHE B, and 90% larger than DCHE C. The correlations of Nusselt number and Euler number of three DCHEs are summarized by fitting the experimental data.
[en] Highlights: • 3-D dynamic thermal-electrical models are developed and validated for PV/T. • 2-D temperature distributions of PV/T collector are illustrated and compared. • The dynamic response characteristics of different collectors are analyzed. • The comprehensive performances are studied under various operating conditions. • A multi-objective optimization is carried out for the design of PV/T DHW system. PV/T collector turns out to be a promising alternative for the traditional solar thermal collector for domestic hot water (DHW) application. In this paper, four comprehensive thermal and electrical models including unglazed PV/T, glazed PV/T, PV and flat plate thermal collector are established for the purpose of accurate long-term simulation and optimization. The detailed 2-D temperature distributions of the glazed and unglazed PV/T collector are illustrated and compared for the first time. Besides, the dynamic response characteristics of different collectors are discussed and compared. By running the models under a broad combination of the operating conditions, the comprehensive performances are obtained and found to be significantly influenced by flow rate, temperatures, radiation and wind speed. A multi-objective optimization model coupling TRNSYS and NSGA-II tool is established to study and optimize the PV/T DHW system for a complete year. The Pareto frontier of conflicting objectives (life cycle savings and prime energy saving efficiency) is obtained for the optimal system design. The mass flow rate on the Pareto frontier is between 0.0085 kg/s and 0.011 kg/s in this study. The optimal value of tank volume on Pareto frontier shows an equally scattering distribution between 99.5 L and 218.6Lfor a 2 m2 glazed PV/T collector.
[en] Highlights: • Performance of heat pump based on DCHE is predicted. • Effects of refrigerants and climatic conditions are discussed. • R134a is recommended as refrigerant. • Switch time is crucial for avoiding condensation. • DCVC system obtain improved performance under simulation conditions. Hybrid solid desiccant and vapor compression system is developed to realize independent control of temperature and humidity. However the large volume has always hindered further development and heat pump system based on desiccant coated heat exchanger (DCHE) is proposed to solve the problem. It’s DCHE instead of conventional sensible heat exchanger is adopted as evaporator/condenser in this system, which also serves as a bridge to link conventional solid desiccant system and vapor compression system together. Simulation model is established in this paper to analyze system performance operating as separate air conditioner. It is found that under new operation condition with higher evaporation temperature and taking COP, pressure and flammability into account, R134a is recommended as refrigerant. Switch time is identified as crucial parameter to avoid condensation. In full fresh air mode, 20–300 s and 30–180 s are the recommended switch time under ARI summer and humid conditions respectively. Also, switch time around 100 s can obtain the highest cooling capacity. For hot and humid summer time in Shanghai, the system can’t meet the indoor requirement with full fresh air mode. However, decreasing the handled load is proved to be effective method. Also the system can obtain the overall COP as high as 5.8 under simulation conditions.
[en] A two-stage solar powered liquid-desiccant dehumidification system, for which two kinds of desiccant solution (lithium chloride and calcium bromide) are fed to the two dehumidification stages separately, has been studied. In the studied system air moisture (latent) load is separately removed by a pre-dehumidifier using cheap calcium chloride (CaCl2) and a main dehumidifier using stable lithium bromide (LiBr). Side-effect of mixing heat rejected during dehumidification process is considerably alleviated by an indirect evaporative cooling unit added between the two dehumidification stages. The feasibility of high-desiccant concentration difference achieved by reusing desiccant solution to dehumidify air and regenerating desiccant repeatedly is analyzed. By increasing desiccant concentration difference, desiccant storage capacity is effectively explored. It is found that the pre-dehumidification effect of CaCl2 solution is significant in high ambient humidity condition. Also seen is that the desiccant investment can be decreased by 53%, though the cost of equipments is somewhat increased, and the Tcop and COP of the proposed system can reach 0.97 and 2.13, respectively
[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] A novel rotary desiccant cooling cycle is proposed and studied using thermodynamic analysis method. The proposed cycle integrates the technologies of isothermal dehumidification and regenerative evaporative cooling, which are beneficial for irreversibility reduction. Thermodynamic investigation on the basic rotary desiccant cooling cycle shows that the exergy efficiency of the basic cycle is only 8.6%. The processes of desiccant dehumidification and evaporative cooling, which are essentially the basis for rotary desiccant cooling, affect the exergy performance of the cycle greatly and account for about one third of the total exergy destruction. The proposed cycle has potential to improve rotary desiccant cooling technology. It is advantageous in terms of both heat source utilization rate and space cooling capacity. The exergy efficiency of the new cycle is enhanced significantly to 29.1%, which is about three times that of the ventilation cycle, and 60% higher than that of the two-stage rotary desiccant cooling cycle. Furthermore, the regeneration temperature is reduced from 80 °C to about 60 °C. The corresponding specific exergy of the supply air is increased by nearly 30% when compared with the conventional cycles. -- Highlights: ► A novel rotary desiccant cooling cycle is developed using thermodynamic analysis method. ► Isothermal dehumidification and regenerative evaporative cooling have been integrated. ► The cycle is advantageous in terms of both heat source utilization rate and space cooling capacity. ► Cascaded energy utilization is beneficial for cycle performance improvement. ► Upper limits, which will be helpful to practical design and optimization, are obtained.