Solar powered air conditioning system using rotary honeycomb desiccant wheel

A solar powered air conditioning system using liquid desiccant is proposed. A solar air heater containing a porous material is used for regeneration purpose in the proposed system. The honeycomb desiccant rotary wheel is constructed from iron wire and clothes layer impregnated with calcium chloride solution, in honeycomb form, is utilized for the regeneration and absorption processes. The effect of airflow rate and solar radiation intensity on the system regeneration and absorption processes are studied. The obtained results show that the system is highly effective in the regeneration process. An empirical equation to calculate the removed moisture as a function of air flow rate at solar noon is obtained. Also empirical equation for wheel effectiveness as a function of air flow rate for regeneration and absorption process was obtained.     

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.     

An optimum performance of flat-type solar air heater with porous absorber

This investigation is concerned with the design and performance of a flat-type solar air heater in which air flows perpendicularly from the transparent cover to a porous absorber plate. The design phase involved a stability analysis to determine the critical distance (maximum allowable distance) between the absober and transparent cover, for suppressing convection currents, at various environmental and operating conditions. These results are useful to designers of solar collectors of the proposed type. In addition, the thermal performance of this solar heater at its optimum design conditions was computed for a wide range of system parameters illustrating the contribution of conduction and radiative modes of heat transfer. The results indicate that the best operating efficiency can be obtained when running the collector with a mass flow rate of m > 40 kg/m².h. Furthermore, the collector thermal performance is superior than channel type solar air heaters operating under similar conditions and much simpler than honeycomb porous bed solar air heaters.     

Performance analysis of dehumidification rotating wheel using liquid desiccant

In the present work, theoretical and experimental evaluation of the effect of bed configuration and operating conditions on the performance of desiccant dehumidification system has been carried out. A new rotating absorption disk has been designed and constructed to be tested in the experimental work. The desiccant wheel has a cylindrical shape of 50-cm diameter and 10 cm thickness. The flow area of this bed is consisted of 350 narrow slots, which are uniformly distributed over the cross section of the cylindrical bed. Each slot has a cylindrical shape and constructed from a steel spring of 100 mm length and 20 mm inside diameter. To form the absorbing surface in the bed, each spring is coated with a thick cloth layer impregnated with lithium chloride solution, which is used as the working desiccant in these experiments.In the theoretical part of this study, a mathematical model has been developed where its output results are compared with the experimental data. The effect of different design parameters and operating conditions on the absorption and regeneration processes is discussed. The effect of regeneration air temperature, the process air and regeneration air inlet humidity, the rotational speed, the process and regeneration air velocity (or flow rates), the bed length, etc. on the amount of water absorbed/desorbed in a cycle is investigated.For the specific bed design parameters, actual recorded data show that an amount of 95 g of water can be absorbed in the absorption cycle per hour. This value changes with varying the operating conditions. From the theoretical investigation, it is found that at regeneration temperature of 85 °C, the amount of water absorbed is nearly equal to the amount of water desorbed (i.e. equilibrium condition) for a complete cycle. It is seen also that for moderate operating conditions (50% RH, 30 °C) and lower regeneration temperature which is suitable for solar energy application, the reduction in the humidity ratio of the process air reaches about 13% of its initial value. Finally, comparisons between theoretical and experimental results show good agreement.     

转轮复合式空调系统的数值计算及能耗分析

主要研究了以太阳能作为再生热源的转轮除湿和蒸气压缩制冷相结合及转轮除湿、蒸气压缩和蒸发冷却相结合的2种复合式空调系统,同时对电能作为再生热源的上述空调系统进行研究,建立了系统的物理模型,并对系统性能参数进行数学描述。通过与相同条件下常规蒸气压缩空调系统的比较分析,得出复合式空调系统制冷剂质量流量分别减少50.20％和66.67％；压缩系统性能系数COP分别提高了26.49％和32.16％；压缩机能耗分别节省了62.64％和76.92％。电能作为再生热源时,总负荷能耗分别节省了32.68％和42.00％；当采用太阳能作为再生热源时,总负荷能耗节省更多的能量,分别为61.86％和71.16％(认为1kW电能等价于3kW热能)。研究还发现,室内相对湿度相同,随室内设计温度的提高,复合式系统压缩机能耗明显减少,节能率呈上升趋势；相反总负荷能耗的节能率呈下降趋势。干热气候条件下,系统节能较为明显：71.75％和85.96％(电能再生)。热湿气候条件下,系统节能不明显,甚至消耗更多能量,而采用太阳能时,复合式系统均具有明显节能效果。     

An experimental investigation into a solar assisted desiccant-evaporative air-conditioning system

In this experimental investigation a solar assisted open adsorption cooling system has been designed and tested under the local weather conditions of Basrah, Iraq. Data were obtained from June to September, inclusive, 1984. Tests were carried out hourly with a directly supply of hot air from a corrugated absorber solar air heater for regeneration. Also, tests were conducted at a constant regeneration temperature of 70°C using auxiliary heat. Adsorption was carried out by a rotary disk of silica gel. Three mass flow rates of process air were employed without recirculation. The performance of the solar air heater was obtained for both seasons, and the instantaneous efficiency was evaluated experimentally and analytically with results compared. Daily and seasonal coefficients of performance were obtained for the cooling system for the mass flow rates employed. A maximum seasonal average value of 2.8 was obtained for a mass flow rate of 0.075 kg/s. The system performance improved with higher regeneration temperature, higher process air mass flow rate and dry weather. It was possible to generate a cool supply of air at satisfactory conditions using solar energy only for all clear days under the local weather conditions.     

EVALUATION OF PERFORATED PLATE SOLAR AIR HEATER

Perforated plates had been successfully used in recent years to achieve high heat transfer coefficient from the absorber plate to the flowing air stream in solar air heaters. Since pumping pressure to maintain a particular flow in the solar air heater utilizing this type of absorber has significant influence on collected energy, so the design of perforated plate configuration must be based on the net energy gained from that collector which is the difference between energy collected and energy paid to overcome pumping pressure. A mathematical model had been constructed and validated experimentally for perforated plate solar air heater to study the effect of plate configurations and airflow rate on both energy gained and pressure loss. The results show that, the flow rate of air and plate configurations have a great effect on net energy gained from the air heater. The results also show that a plate of certain configurations operates most efficiently at certain flow rate and more than one configurations can give optimum value of net energy gained for a particular flow rate.     

Theoretical and experimental investigation on the application of solar water heater coupled with air humidifier for regeneration of liquid desiccant

Theoretical and experimental investigation on the application of flat plate solar water heater coupled with air humidifier for regeneration of liquid desiccant has been presented in this work. The heated water from the storage tank of the solar heating system is circulated in a finned tube air heater. Hot air from the air heater is blown through a packing of a honeycomb type for the purpose of regeneration of calcium chloride (CaCl₂) solution. An experimental system has been designed and installed for this purpose. The system comprises a solar water heater with a storage tank connected to an air/water heat exchanger. Hot air from the heat exchanger is blown to the air humidifier, which functions in this study as a regenerator. Calcium chloride solution is applied as the working desiccant in this study. Solution concentration is determined at the end of regeneration process and the mass of evaporated water is evaluated. It is observed that the heating temperature varies, at day time, in a range of about 5 °C. This limited variation in hot water temperature demonstrates the importance of the storage tank to attain a nearly steady state operation of the system. Experimental results show that solution with 30% concentration can be regenerated up to 50% using solar energy. In the theoretical part of this study, a multiple-layer artificial neural network (ANN) model has been applied to study the performance of a solar liquid-desiccant dehumidification/regeneration system when calcium chloride solution is applied as the working desiccant. The experimental results of the present study are used to construct and test the ANN model. Then the model has been utilized to describe and analyze the effect of the inlet conditions of air on the regeneration process. Good agreement between the outputs from the ANN model and the corresponding results from the experimental data has been found. The proposed model can work well as a predictive tool to complement the experiments.     

Quick performance prediction of liquid desiccant regeneration in a packed bed

The major energy requirement associated with any liquid desiccant-based systems is the low-grade energy for desiccant regeneration. This paper presents the results from a simplified model of a packed bed regeneration process in which the desiccant solution is heated in any of the two ways. With method A, the desiccant solution is heated in a heat exchanger with a fluid (water) heated by any low-grade thermal energy such as solar energy or waste heat sources. While in method B, the desiccant solution is heated by a conventional energy source such as a line heater. A closed form solution is obtained for both methods of heating through two dimensionless performance parameters to estimate the water evaporation rate from the weak desiccant solution to the scavenging air stream in terms of known operating parameters. Good agreement is shown to exist between the predictions from the simplified model and the experimental findings available in the literature. The influences of the heating fluid (water) inlet temperature and the effectiveness of the heating fluid-to-desiccant heat exchanger on the performance of the regenerator are studied for method A whereas the effects of energy input on the evaporation rate of water with the scavenging air flow rate are investigated for method B and the results are reported in this paper.     

Numerical and experimental investigation of operating parameters of solar–powered desiccant wheel in India

A mathematical model has been used to predict the operating parameters of a desiccant wheel for performance analysis of the desiccant wheel. The model considered both gas and solid side resistance. The model shows a good agreement with experimental data. An experimental setup was fabricated using an evacuated tube solar air collector with a desiccant wheel. The hot air needed for regeneration is produced by the evacuated tube solar air collector, which has a collector surface area of 4.44 m² . The regeneration can be started from 40 °C. The temperature of outlet air obtained is in the range of 40–65 °C in this evacuated tube solar air collector. The experimental setup was installed at NIT Kurukshetra, India, 29° 58′ (latitude) North and 76° 53′ (longitude) East. Numerical results showed that both the moisture removal and the temperature increment of the process air increases with an increasing regeneration air temperature, regeneration air inlet velocity, and process inlet moisture. But both the moisture removal and the temperature increment of the processed air decreases by increasing the process air inlet velocity and regeneration air moisture at the inlet. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library (wileyonlinelibrary.com/journal/htj). DOI 10.1002/htj.21031     

Thermal performance investigation of double pass-finned plate solar air heater

In this paper, the double pass-finned plate solar air heater was investigated theoretically and experimentally. An analytical model for the air heater was presented. Numerical calculations had been performed under Tanta (latitude, 30° 47′N and longitude, 31°E) prevailing weather conditions. The theoretical predictions indicated that the agreement with the measured performance is fairly good. Comparisons between the measured outlet temperatures of flowing air, temperature of the absorber plate and output power of the double pass-finned and v-corrugated plate solar air heaters were also presented. The effect of mass flow rates of air on pressure drop, thermal and thermohydraulic efficiencies of the double pass-finned and v-corrugated plate solar air heaters were also investigated. The results showed that the double pass v-corrugated plate solar air heater is 9.3–11.9% more efficient compared to the double pass-finned plate solar air heater. It was also indicated that the peak values of the thermohydraulic efficiencies of the double pass-finned and v-corrugated plate solar air heaters were obtained when the mass flow rates of the flowing air equal 0.0125 and 0.0225 kg/s, respectively.     

INFLUENCE OF ELEVATED PRESSURE ON SORPTION IN DESICCANT WHEELS

Rotary desiccant wheels have been employed to dry pressurized air streams. In these systems, depending on the moisture content in the air stream and the operating pressure, condensation can occur in the regeneration portion of the wheel. In this article, a numerical method using an implicit finite-difference scheme is developed and applied that enables condensation to be detected and simulated in the regeneration portion of a desiccant wheel operating at high pressures. Using this model, performance analysis of desiccant wheel under these conditions is investigated. It is found that, depending on the value of the separation factor and regeneration temperature, condensation could occupy as much as 40% of regeneration section of the wheel. In this region, regeneration of the desiccant is not possible and usually dehumidification of regeneration air occurs. Also, as the operating pressure increases, the adsorption and desorption characteristics are dramatically affected and the optimum separation factor of desiccant material increases with operating pressure.     

Evaluation of a jet plate solar air heater

To achieve higher heat transfer from the absorber plate to the flowing air stream with an intention to increase the amount of the collected energy, and hence, to improve the efficiency of an air-based solar collector, a unique jet impingement concept has been advanced for evaluation in the present study. To investigate the effects of various geometrical parameters such as the ‘hole’ or ‘nozzle’ diameter on the jet plate, their interspacings, the nozzle height, the distance between the absorber and the jet plate and the operational parameter such as the velocity of air impinging out of the holes/nozzles on to the back side of the absorber surface on the performance parameters of the jet impingement concept air heater, a detailed theoretical parametric analysis has been made on the design for different mass flow rates of air and different lengths of air channel. A parallel study has also been carried out on a conventional parallel plate air heater in order to compare its air temperature increment and performance efficiency with those of the jet plate air heater. The gain in air temperature increment and performance efficiency of the jet-concept air heater over that of the parallel plate air heater with duct depth 10 cm and length 2 m is 15.5°C to 2.5°C and 26.5% to 19%, respectively, for air flow rates in the range 50 to 250 kg/hm².     

Evaporation rate of a novel tilted solar liquid desiccant regeneration system

The regeneration system represents a vital part of any desiccant air conditioning system. The need of a solar assisted desiccant regeneration system is more important today. In this paper, an experimental study of a novel regeneration system modified from solar tilted still is carried out. A corrugated blackened surface is used to heat the desiccant and an air flow is used to regenerate calcium chloride solution. The effect of the liquid to air flow rate ratio; the desiccant temperature; the desiccant concentration and the inlet air humidity ratio on the evaporation rate has been studied experimentally. A wide range of liquid to air flow rate ratios are employed. The optimum value of the liquid to air flow rate ratio for higher evaporation rate is reported.     

Augmentation of the performance of solar regenerator of open absorption cooling system

In open cycle liquid desiccant air conditioning, the solar collector regenerator is one of the effective ways of regenerating liquid solution. In this work, the regeneration of liquid solution using cross flow of air stream with flowing film of desiccant on the surface of a solar collector/regenerator has been investigated. To evaluate the effect of cross flow of air stream on the performance of the unit, two identical units are constructed and tested in the same conditions of operation. One of the two units was augmented with air blower. The absorber plate is a black cloth layer. The forced air stream, which flows across the absorber removes the moisture from the liquid solution. The regeneration in the other collector/regenerator unit is free. The results show enhancement of regeneration efficiency for the forced cross flow compared with the free regeneration. The effect of concentration and flow rate on the performance is discussed. Two relations for regeneration efficiency as a function of concentration for the two units are introduced.     

Development of a solar-assisted dryer and evaluation of energy requirement for the drying of onion

A solar-assisted forced convection dryer was developed to study the effect of airflow rate (2.43, 5.25, 8.09 kg/min), air temperature (55, 65, 75 °C), and fraction of air recycled (up to 90%) on the total energy requirement of drying of onion slices. The dryer was provided with a flat plate solar air heater having both the corrugations and triangular fins to the absorber plate. For drying of onion slices from initial moisture content of about 86% (wet basis) to final moisture content of about 7% (wet basis), the energy required per unit mass of water removed during without using recirculation of air was found between 23.548 and 62.117 MJ/kg water. The percent energy contribution by the solar air heater, electrical heater, and blower was found between 24.5% and 44.5%, 40.2% and 66.9%, and 8.6% and 16.3%, respectively. The savings in total energy due to fraction of air recycled were determined at 65 and 75 °C air temperature for the above three airflow rates. The maximum saving in total energy up to 70.7% was achieved by recycling of the exhaust air. The energy required per unit mass of water removed was found between 12.040 and 38.777 MJ/kg water. The percent energy contribution by the solar air heater, auxiliary heater, and blower was found between 22.4% and 40.9%, 33.6% and 62.6%, and 11.2% and 37.2%, respectively.     

Energy consumption analysis on a dedicated outdoor air system with rotary desiccant wheel

A dedicated outdoor air system (DOAS) with rotary desiccant wheel is the combination of a desiccant dehumidification system and a vapor compression refrigeration system. An energy consumption model of this hybrid DOAS is established for its analysis. Coefficient of performance, COP, is appropriately defined for evaluation on performance of the hybrid DOAS. The results indicate that, compared with a conventional DOAS, energy savings are possible for the suggested DOAS, when solar energy or natural gas is used for regeneration. Ventilation air flow rate, temperature or humidity of outdoor air, as well as regeneration-to-process air ratio, influence the energy consumption and the COP of the hybrid DOAS, greatly.     

Experimental investigation of forced convection and desiccant integrated solar dryer

An indirect forced convection and desiccant integrated solar dryer is designed and fabricated to investigate its performance under the hot and humid climatic conditions of Chennai, India. The system consists of a flat plate solar air collector, drying chamber and a desiccant unit. The desiccant unit is designed to hold 75 kg of CaCl₂-based solid desiccant consisting of 60% bentonite, 10% calcium chloride, 20% vermiculite and 10% cement. Drying experiments have been performed for green peas at different air flow rate. The equilibrium moisture content M_e is reached in 14 h at an air flow rate of 0.03 kg/m² s. The system pickup efficiency, specific moisture extraction rate, dimensionless mass loss, mass shrinkage ratio and drying rate are discussed in this paper.     

Cooling potential of ventilated PV façade and solar air heaters combined with a desiccant cooling machine

Thermal energy collected from a PV-solar air heating system is being used to provide cooling for the Mataro Library, near Barcelona. The system is designed to utilise surplus heat available from the ventilated PV facade and PV shed elements during the summer season to provide building cooling. A desiccant cooling machine was installed on the library roof with an additional solar air collector and connected to the existing ventilated PV façade and PV sheds. The desiccant cooling cycle is a novel open heat driven system that can be used to condition the air supplied to the building interior. Cooling power is supplied to the room space within the building by evaporative cooling of the fresh air supply, and the solar heat from the PV-solar air heating system provides the necessary regeneration air temperature for the desiccant machine. This paper describes the system and gives the main technical details. The cooling performance of the solar powered desiccant cooling system is evaluated by the detailed modelling of the complete cooling process. It is shown that air temperature level of the PV-solar air heating system of 70 °C or more can be efficiently used to regenerate the sorption wheel in the desiccant cooling machine. A solar fraction of 75% can be achieved by such an innovative system and the average COP of the cooling machine over the summer season is approximate 0.518.     

On the performance and entropy generation of the double-pass solar air heater with longitudinal fins

The performance and entropy generation of the double-pass flat plate solar air heater with longitudinal fins are studied numerically. The mathematical models described the heat transfer characteristics of the double-pass flat plate solar air heater derived from the conservation equations of energy. The predictions are done at air mass flow rate ranging between 0.02 and 0.1 kg/s. The effects of the inlet condition of working fluid and dimension of the solar air heater on the heat transfer characteristics, performance, and entropy generation are considered.