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[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] 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.
[en] Research highlights: → We studied a point focus Fresnel solar collector using different cavity receivers. → The collector heat removal factors are derived to find the optimal cavity shape. → Numerical and experimental analysis shows that the conical cavity is optimum. -- Abstract: A high concentration imaging Fresnel solar collector provided with different cavity receivers was developed and its behavior was investigated. Round copper pipes winded into different spring shapes were used as receiver by placing in the cylindrical cavity to absorb concentrated solar energy and transfer it to a heat transfer fluid (HTF). The collector efficiency factor and collector heat removal factor were derived for the cavity receivers to find out heat transfer mechanism and to propose an effective way for evaluating the performance of Fresnel solar collector and determining the optimal cavity structure. The problem of Fresnel solar collector with synthetic heat transfer oil flow was simulated and analyzed to investigate heat loss from different cavity receivers. Solar irradiation as well as convection and heat transfer in the circulating fluid and between the internal surfaces of the cavity and the environment are considered in the model. The temperature distribution over its area as well as the collector thermal efficiency at nominal flow rate was used in order to validate the simulation results. It was found that the simulated temperature distribution during operation and the average collector efficiency are in good agreement with the experimental data. Finally, the optimal shape of solar cavity receiver, as well as its thermal performance, are deeply analyzed and discussed.
[en] Highlights: → Use two kinds of circulation media in the solar collector. → Air heating and pebble bed heat storage are applied with different operating modes. → Design parameters of the system are optimized by simulation program. → It is found that the system can meet 32.8% of the thermal energy demand in heating season. → Annual solar fraction aims to be 53.04%. -- Abstract: The application of solar air collectors for space heating has attracted extensive attention due to its unique advantages. In this study, a solar air heating system was modeled through TRNSYS for a 3319 m2 building area. This air heating system, which has the potential to be applied for space heating in the heating season (from November to March) and hot water supply all year around in North China, uses pebble bed and water storage tank as heat storage. Five different working modes were designed based on different working conditions: (1) heat storage mode, (2) heating by solar collector, (3) heating by storage bed, (4) heating at night and (5) heating by an auxiliary source. These modes can be operated through the on/off control of fan and auxiliary heater, and through the operation of air dampers manually. The design, optimization and modification of this system are described in this paper. The solar fraction of the system was used as the optimization parameter. Design parameters of the system were optimized by using the TRNSYS program, which include the solar collector area, installation angle of solar collector, mass flow rate through the system, volume of pebble bed, heat transfer coefficient of the insulation layer of the pebble bed and water storage tank, height and volume of the water storage tank. The TRNSYS model has been verified by data from the literature. Results showed that the designed solar system can meet 32.8% of the thermal energy demand in the heating season and 84.6% of the energy consumption in non-heating season, with a yearly average solar fraction of 53.04%.
[en] Ongoing research and development works suggest that good system configurations have significant potential for improving the performance and reducing the cost and size of rotary desiccant dehumidification and air conditioning system. In this paper, a novel desiccant cooling system using regenerative evaporative cooling and a one-rotor two-stage desiccant cooling system are analyzed and compared under Air-conditioning and Refrigeration Institute (ARI) summer, ARI humid and Shanghai summer conditions. The objective of this paper is to compare the thermodynamic performance of the two systems and obtain useful data for practical application. It is found that compared with the conventional desiccant cooling system, the novel desiccant cooling system with regenerative evaporative cooling can handle air to a much lower temperature while maintaining good thermal performance. Under ARI summer, ARI humid and Shanghai summer conditions, the minimum attainable supply air temperatures are reduced from 13.5 °C to 7.9 °C, from 14.2 °C to 9.2 °C and from 18.0 °C to 13.0 °C respectively. It is suggested that the novel desiccant cooling system with regenerative evaporative cooling is beneficial to breaking the obstacle of limited temperature reduction encountered by conventional desiccant cooling system, especially in the case of extreme high humid conditions. - Highlights: ► Desiccant cooling system with regenerative evaporative cooling (REDC) has been studied. ► Comparison between REDC and conventional desiccant cooling system (DCS) has been performed. ► REDC is superior to conventional DCS in thermal utilization, air conditioning and energy saving. ► REDC has significant potential for breaking the obstacle of limited temperature reduction.
[en] Highlights: ► A solar desiccant cooling/heating system is simulation studied. ► The mean deviation is about 10.5% for temperature and 9.6% for humidity ratio. ► The 51.7% of humidity load and 76% of the total cooling can be handled. ► About 49.0% of heating load can be handled by solar energy. ► An optimization of solar air collector has been investigated. - Abstract: To increase the fraction of solar energy might be used in supplying energy for the operation of a building, a solar desiccant cooling and heating system was modeled in Simulink. First, base case performance models were programmed according to the configuration of the installed solar desiccant system and verified by the experimental data. Then, the year-round performance about the system was simulated. Last, design parameters of solar air collectors were optimized that include collector area, air leakage and thermal insulation. Comparison between numerical and experimental results shows good agreement. During the simulation, the humidity load for 63 days (51.7%) can be totally handled by the two-stage desiccant cooling unit. For seasonal total heating load, about 49.0% can be handled by solar energy. Based on optimized results, the thermal energy subsystem functioned to its expected performance in solar energy collection and thermal storage
[en] Multi-source systems for the fulfilment of electric, thermal and cooling demand of a building can be based on different technologies (e.g. solar photovoltaic, solar heating, cogeneration, heat pump, absorption chiller) which use renewable, partially renewable and fossil energy sources. Therefore, one of the main issues of these kinds of multi-source systems is to find the appropriate size of each technology. Moreover, building energy demands depend on the climate in which the building is located and on the characteristics of the building envelope, which also influence the optimal sizing. This paper presents an analysis of the effect of different climatic scenarios on the multi-source energy plant sizing. For this purpose a model has been developed and has been implemented in the Matlab® environment. The model takes into consideration the load profiles for electricity, heating and cooling for a whole year. The performance of the energy systems are modelled through a systemic approach. The optimal sizing of the different technologies composing the multi-source energy plant is investigated by using a genetic algorithm, with the goal of minimizing the primary energy consumption only, since the cost of technologies and, in particular, the actual tariff and incentive scenarios depend on the specific country. Moreover economic considerations may lead to inadequate solutions in terms of primary energy consumption. As a case study, the Sino-Italian Green Energy Laboratory of the Shanghai Jiao Tong University has been hypothetically located in five cities in different climatic zones. The load profiles are calculated by means of a TRNSYS® model. Results show that the optimal load allocation and component sizing are strictly related to climatic data (e.g. external air temperature and solar radiation)
[en] A linear Fresnel reflector (LFR) solar collector with modified V-shaped cavity receiver was investigated both experimentally and theoretically in this paper. Simplified ray tracing technique was employed to optimize the optical design of the LFR system. The Monte Carlo ray tracing method was used to predict the optical performance of the proposed LFR system. A 2D mathematical model was developed to investigate the effect of receiver surface temperature on the overall heat transfer coefficient which reflects the thermal performance of the modified linear cavity receiver. CFD simulation was carried out for the modified cavity receiver treated at various surface temperatures within a range of 90–150 °C, by taking into account the conductive, convective and radiative heat losses. Experimental results show that the overall heat loss coefficient varied from 6.25 to 7.52 W/m2 K for the tested surface temperature range, with an average deviation of about 12% when compared with simulation results. Also, at higher surface temperatures, heat loss through radiative mode was predominant and the system stagnation was found to be about 260 °C with optimal operating temperature of about 121 °C. The thermal efficiency decreased from 45% to 37% as the average surface temperature increased from 90 °C to 150 °C. -- Highlights: ► An LFR solar collector was investigated both experimentally and theoretically. ► Simplified ray tracing technique was used to optimize the optical design. ► The MCRT method was used to predict the optical performance of the LFR system. ► Thermal performance of the LFR system was evaluated by CFD method and experiments. ► The LFR system is a promising technology in generating mid-temperature heat