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A solar regenerated liquid desiccant evaporative cooling system for office building application in hot and humid climate
Abstract
In this paper, we provide thermal analysis and design methodology of a liquid desiccant assisted dew point indirect evaporative cooling system. The purpose of the system is to serve as an alternative for conventional vapour-compression based building air conditioning systems in providing satisfactory human thermal comfort conditions in the hot and humid climatic regions. The main features of the study are the following: i) novel incorporation of a forced parallel flow direct solar regenerator and a dew point indirect evaporative cooler within the same air conditioning unit; ii) detailed thermal modelling of each of the system components with lesser simplifying assumptions with respect to earlier works; iii) large cooling capacity (∼18.8 TR) under harsh climate; and, iv) a comprehensive year-around case study for system operation in a hot and humid location (Kolkata, India). Our thermal model is validated with a reference model study. The maximum room air temperature predicted by the current system for yearlong analysis is 26.7 °C. The thermal COP of the system for diurnal operation in the most humid month of a calendar year (July) varied between 0.40-0.96. The cooling system can prevent overheating of the conditioned space, as specified by ASHRAE Standard 55-2017, throughout the year.
... The sub-cooling and reheating process in the condensation dehumidification system causes unnecessary energy consumption [6]. Although the liquid desiccant dehumidification system can ensure indoor comfort and energy efficiency, its complex system structure makes it more suitable for application in commercial buildings with a large latent load [7,8]. ...
... The temperature related TCC and EIR modifier curves (MC) are the curve with two independent variables: wet-bulb temperature of the air entering the cooling coil, and dry-bulb temperature of the air entering the air-cooled condenser coil. The form of the curve is shown in Eq. (7). The flow fraction related TCC and EIR modifier curves are the curve with one independent variable: the ratio of the actual air flow rate to the rated air flow rate as shown in Eq. (8). ...
Heat-pump assisted hybrid desiccant cooling (HPDC) systems are generally placed with the evaporators located downstream of the desiccant wheel (HPDC-D) to deal with the adsorption of heat and control indoor temperature; positioning the evaporator upstream of the desiccant wheel (HPDC-U) can improve dehumidification performance. In this study, HPDC systems are established based on an analytic solution-validated desiccant wheel model, and the effect of the system configuration on the indoor temperature and humidity control is quantitatively analyzed in dynamic conditions. The proposed dimensionless parameters clarify the thermal performance of the dehumidification and regeneration processes. The simulation results propose an optimized HPDC-SE system with dual separate evaporators, and its energy consumption is compared. Finally, the feasibility of combining the HPDC system with a heat recovery plate is discussed. Due to the thermal imbalance between the condensing heat required for regeneration and the supply air cooling capacity required for the indoor sensible load, the HPDC-U system could not continuously control the indoor temperature below the control range in dynamic conditions, and the HPDC-D system could not continuously control the indoor humidity. The analysis of the dimensionless parameters shows that the thermal economy of the desiccant wheel decreases with the increase in the process inlet air humidity. The proposed HPDC-SE system can improve the indoor environment control effect and reduce the electricity consumption by 5%, 4.6%, and 11.7% compared to the HPDC-U system, respectively. The heat recovery plate has a low overall utilization and heat recovery rate for HPDC-U and HPDC-SE systems, and affects the indoor humidity control of the HPDC-D system. Therefore, the heat recovery plate is not suitable for HPDC systems.
... can be comfortably combined with a cooling unit to configure a hybrid system [13][14][15][16] . Giampieri et al. 17 presented a review on the thermodynamics and economics of liquid desiccant systems. ...
This study aims to experimentally demonstrate a liquid desiccant systems effectiveness by using thermo-chemical fluid, such as aqueous solution of calcium chloride. This study evaluated the effect of operating temperatures on air properties (temperature, relative humidity, and moisture content) and system effectiveness by varying air flow rates. The system’s functionality was influenced by the operational temperature and air flow rate, and the dehumidification effectiveness was higher at low operating temperatures and low airflow rates. An ANN metamodel-based control strategy is also proposed for implementation in hybrid thermo-chemical networks with the help of system performance data and real-time data. The suggested ANN model’s results were validated using a variety of measuring techniques, including the RMSE, MAPE, correlation (R), and coefficient of determination (R²). The proposed ANN analysis achieved an excellent correlation between predicted and experimentally measured data.
... Buildings account for around 30-40 % of global power use, with space heating and cooling constituting over half of this energy usage. Although vapourcompression air conditioning systems are extensively utilised for indoor comfort, their dependence on highquality energy exacerbates problems such as passive air pollution 5 . Although these systems efficiently provide the appropriate indoor climate, they include environmental disadvantages. ...
Dew-Point Evaporative Cooling (DPEC) is an efficient method for energy conservation, relevant to both energy supply and demand systems by enabling rapid heat and mass transfer. This research examines the intrinsic capacity of evaporative cooling to lower air temperatures without elevating humidity, a concept well-documented in prior investigations. The results demonstrate that the velocity in the wet channel substantially influences the efficiency of the cooling process. Data indicates that an increase in velocity within the wet channel correlates with a decrease in the temperature of the outgoing air, with a reduction of 2.4ºC to 3.2ºC noted when the velocity is elevated from 2.2 meters per second to 8.5 meters per second. Moreover, both wet bulb and dew point efficiencies enhance with increased wet channel velocities, a pattern consistently observed in both electrically driven systems and those utilising solar energy. It was observed that as relative humidity rises while the Dry Bulb Temperature (DBT) remains constant, the efficacy of both wet bulb and dew point cooling diminishes. The results demonstrate the efficacy of evaporative cooling in lowering air temperature without increasing moisture, while also emphasising the significance of wet channel velocity and inlet air conditions in optimising the process. The results obtained in the experimental analysis also demonstrate that the DPEC system shows a promising solution in the wet and dry regions due to its ability to operate without using energy-intensive compressors and harmful refrigerants. Major Findings: During the experimental analysis, it has been found that: 1. The DPEC system shows higher wet bulbs and dew points. 2. Effectiveness due to higher airflow velocity in the wet channel. 3. In the same way, the system is showing higher wet bulb and dew point effectiveness when there is lower relative humidity of incoming air.
... Schematic of a liquid desiccant cooling system for buildings (Modified and reproduced schematic with permission based on reference[169]) ...
In buildings, air conditioning and mechanical ventilation (ACMV) systems are the major shareholders of overall energy consumption. Energy-efficient designs for ACMV systems in building applications are therefore needed. While designing an efficient ACMV system, consideration must be given to the growing concerns of enhanced thermal comfort and improved indoor air quality. The variable refrigerant flow (VRF) air-conditioning system is a widely adopted alternative to the existing building cooling systems due to the higher energy efficiency and individualized temperature control feature. However, it still suffers from shortcomings such as no outdoor air induction for ventilation and higher initial cost. Therefore, this paper reviewed the variable refrigerant flow and mechanical ventilation/air distribution systems, their integrated designs for non-residential buildings, performance evaluation and control optimization of the integrated systems, VRF systems’ faults detection and diagnosis, current application of the VRF systems, and associated challenges. Together with these all, some advanced buildings’ cooling techniques and improvements toward nearly/net-zero energy buildings are briefly discussed. Indoor thermal comfort models and criteria for different climates are also presented for an in-depth understanding of the VRF integrated mechanical ventilation designs. The literature survey shows that the supply air temperature and airflow rate are foremost in parameters that can be optimized in VRF integrated ventilation design as they greatly reduce the energy consumption. Further, policies on elevated indoor temperatures in air-conditioned buildings to mitigate their carbon footprint are strictly being implemented. Therefore, this review provides an insight to the researchers for further improvement in the integrated design and control optimization of the parameters involved. A paradigm shifts from the conventional compression-based electric-powered air conditioning systems to the renewable energy driven advanced air conditioning technologies which is also an emerging research area to be focused on achieving the target of nearly/net-zero energy buildings.
... The findings showcased a maximum monitored CC of up to 4.6 kW, indicating the feasibility of this approach. Ghosh and Bhattacharya [110] introduced a system designed to ensure thermal comfort in large office buildings. This system integrated a solar regenerator, liquid desiccant, and a cooling unit. ...
Recent advancements in single-stage evaporative cooling (EC) have showcased their effectiveness as an energy-efficient and sustainable air-conditioning (AC) solution. However, several challenges hinder the widespread adoption of EC in various applications. These challenges include climate sensitivity, substantial spatial requirements, and limitations in achieving desired output temperatures. To address these concerns, there has been a growing focus on integrating EC with solar energy (SE) systems. With traditional energy resources being depleted, the use of SE has gained prominence as a sustainable solution to meet future energy demands while mitigating environmental pollution. This paper presents a comprehensive review of hybrid EC–SE systems, aiming to elucidate the potential synergies, benefits, and challenges associated with this integration. The review explores the principles and mathematical approaches of various configurations of EC systems to assess their compatibility with SE sources. Furthermore, the review delves into the mathematical model of SE, encompassing both solar power generation and thermal collectors, with the aim of integrating it into the EC model. It delves into key aspects of energy consumption and performance, showcasing advancements in achieving higher efficiency and enhanced cooling capacity through the hybrid systems. Additionally, the review highlights research gaps in the existing literature, emphasizing the need for further exploration in this interdisciplinary field. In conclusion, this paper offers valuable insights into the potential of EC–SE systems to address energy and cooling requirements while promoting sustainable development.
... Most species that grow in greenhouses require temperatures that range between 17 and 27 • C [11]. This poses an issue for countries that reside in hot and arid regions with hot, humid, and arid climates [12,13]. During summer, the ambient temperatures in such countries may reach up to 50 • C and humidity levels over 90 %, making it a necessity to utilize cooling methods in greenhouses to control the temperature and humidity levels. ...
The demand for food items is increasing to match population growth rates. Meanwhile, many countries struggle to guarantee a full year-round production of crops because of the harsh weather conditions in cold and hot seasons. Greenhouses are used to mitigate the harsh weather conditions and allow for a full year-round production of crops by controlling the microclimate and ensuring that the crops acquire the required level of temperature, humidity, water, and light. Several design factors play a crucial role in the successful deployment of the greenhouse. These parameters include shape, orientation, structure, cover material, and climate control technologies. Climate control strategies include cooling and heating technologies. This paper presents a comprehensive review of cooling technologies for greenhouses, especially ones deployed in hot climates. Technologies such as natural ventilation, shading and reflection, evaporative cooling, desiccant cooling, and combined cooling technologies are presented and discussed. Moreover, this paper explores recent advancements in the field of cooling technologies and systems. The paper also explains existing strategies to save energy and improve the efficiency of greenhouses. The results of this review are expected to guide researchers in the process of selecting the most suitable cooling technology for their specific regions.
... While providing excellent dehumidification capacity and energy efficiency, the complex structure and system size of liquid desiccant systems makes it more suitable for commercial buildings with a large latent load and separate equipment room. Ghosh et al. [11] studied the thermal comfort of liquid desiccant dehumidification systems for an office building. Guan et al. [12] conducted a case study on a liquid desiccant dehumidification system in a factory and determined the effect of regeneration temperature on the exergy, which improved from 17.7% to 28.6% due to the reduction in the regeneration temperature. ...
In modern buildings, indoor humidity must be effectively controlled for thermal comfort and to inhibit the breeding of mold and mildew. To exclude the dependence on additional regeneration heat source for desiccant cooling system, this field study used heat pump-assisted hybrid desiccant cooling (HPDC) systems, which place the evaporator upstream of dehumidification side of the desiccant wheel to perform pre-cooling and pre-dehumidification. The main purpose was to investigate the thermal environment control and energy-saving effects of HPDC systems under different load pattern scenarios; Field studies were conducted in residential buildings, public space, and classrooms. When operating in the first scenario, the electricity consumption of the HPDC system was 37.4% and 14% lower than that of conventional electric heat pumps (EHPs) under different wall thermal-storage conditions. In the second scenario, high dehumidification load led to large adsorption heat, causing indoor temperature to rise continually until the indoor humidity reached the design level. Through the field study in the classroom, the dehumidification mode and cooling mode switching approach can effectively improve indoor temperature control problems and reduce energy consumption by 14% compared to EHP. This field study covers possible application scenarios for HPDC systems, demonstrating the feasibility of replacing conventional air conditioning.
Implementing a neoteric practical design for dew-point evaporative cooler (DPEC) rather than the widely used flat plate and corrugated plate types has been challenging since its first invention. Thus far, no study has been dedicated to design and experimentally implement a shell and tube concept for DPEC with an innovative configuration that can contribute to mass production, globalization, and effortless maintenance of such a high-performance cooling machine. Therefore, in this paper, the shell and tube concept has been employed as a core design for DPEC that has been reinforced by practical approaches. The practical approaches include a dedicated novel design for the proposed system and utilizing super cheap materials for the system's construction. Both of the aforementioned approaches provide solutions for overcoming the aforementioned issues. In the meanwhile, the superiority of the proposed DPEC has been approved by comparing it to the flat plate DPEC. During the experimentations, the prototype was able to reduce the ambient temperature by up to 34.1 °C (from 53 °C to 18.9 °C), besides, it could achieve a dew-point effectiveness of 99.6 % and wet-bulb effectiveness of 135 %.
Turbine generator (TG) buildings in thermal power plants require chilled water air conditioning systems (CWAS) to control the temperature and humidity of heat dissipated rooms, which are energy-intensive and non-eco-friendly. As an alternative, a novel refrigerant-free 100% fresh air (FA)-based hybrid liquid desiccant air conditioning system (HLDAS) is proposed and presented the methodology to design it for a case study of TG building of 2X660 MW power plant in the most humid climate. The cooling and heating requirements for HLDAS are met with a cooling tower and potential waste heat from the plant. To assess the saving potential of the proposed system, annual energy consumption for the HLDAS, 10% FA-based CWAS (CWAS-1) and 100% FA-based CWAS (CWAS-2) are quantified and compared. The results revealed that HLDAS consume 55% and 31% of CWAS-1 and CWAS-2 power consumption, respectively. Low-grade potential waste heat from the plant is estimated and found to be sufficient for solution regeneration. The proposed system is extremely energy efficient for 100% FA applications where low-grade waste heat is available. In addition to this, since there are no ODP gases associated with the proposed system, the proposed HLDAS is recommendable in view of environmentally friendly technology and indoor air quality (since it is a 100% FA-based system). This work will support HVAC engineers and researchers in designing the LDAS for a given air conditioning application.
The Center for the Built Environment (CBE) Thermal Comfort Tool is a free online tool for thermal comfort calculations and visualizations that complies with the ASHRAE 55–2017, ISO 7730:2005 and EN 16798–1:2019 Standards. It incorporates the major thermal comfort models, including the Predicted Mean Vote (PMV), Standard Effective Temperature (SET), adaptive models, local discomfort models, SolarCal, and dynamic predictive clothing insulation. Our tool also provides dynamic and interactive visualizations of thermal comfort zones. The CBE Thermal Comfort Tool has several practical applications and each year is used by more than 49,000 users worldwide, including engineers, architects, researchers, educators, facility managers and policymakers.
This study has investigated the feasibility of three different solar-assisted air conditioning systems for typical medium-sized office buildings in all eight Australian capital cities using the whole building energy simulation software EnergyPlus. The studied solar cooling systems include: solar desiccant-evaporative cooling (SDEC) system, hybrid solar desiccant-compression cooling (SDCC) system, and solar absorption cooling (SAC) system. A referenced conventional vapor compression variable-air-volume (VAV) system has also been investigated for comparison purpose. The technical, environmental, and economic performances of each solar cooling system have been evaluated in terms of solar fraction (SF), system coefficient of performance (COP), annual HVAC (heating, ventilation, and air conditioning) electricity consumption, annual CO2 emissions reduction, payback period (PBP), and net present value (NPV). The results demonstrate that the SDEC system consumes the least energy in Brisbane and Darwin, achieving 56.9% and 82.1% annual energy savings, respectively, compared to the conventional VAV system, while for the other six cities, the SAC system is the most energy efficient. However, from both energy and economic aspects, the SDEC system is more feasible in Adelaide, Brisbane, Darwin, Melbourne, Perth, and Sydney because of high annual SF and COP, low yearly energy consumption, short PBP and positive NPV, while for Canberra and Hobart, although the SAC system achieves considerable energy savings, it is not economically beneficial due to high initial cost. Therefore, the SDEC system is the most economically beneficial for most of Australian cities, especially in hot and humid climates. The SAC system is also energy efficient, but is not as economic as the SDEC system. However, for Canberra and Hobart, reducing initial cost is the key point to achieve economic feasibility of solar cooling applications.
In many countries, solar cooling systems are one of the best candidates to tackling global warming and summer peak loads of building air conditioning systems. In this work, based on the collaboration with the Saudi research institute “King Abdullah City for Atomic and Renewable Energy”, two types of solar driven cooling systems are investigated and compared: the open-circuit Desiccant Evaporative Cooling technology and the system based on single-stage LiBr absorption chillers. A computer code has been developed in Trnsys® to simulate a well-insulated single-family residential building, starting from a 3D model of the building architecture, and the solar cooling systems. The simulations have been carried out for two different locations: one in a dry desert area (Riyadh, KSA), the other one on the seaside (Abu Dhabi, UAE). The results show that absorption chillers need a very effective heat rejection system to work properly, whilst DEC systems exhibit a dramatic performance reduction in wet climates.
The current work focuses on the Queen's University Solar Liquid Desiccant Cooling Demonstration project. A solar Liquid Desiccant Air Conditioning system (LDAC) has been installed at a field site in Kingston, Ontario, Canada, and is currently being tested to evaluate the performance of the system when driven by solar energy. The installed system features a low-flow parallel plate liquid desiccant air conditioner, and a 95m2 evacuated tube solar collector array. While summer testing has only recently begun, five test days have shown an overall solar collector efficiency of 56%, solar fraction of 63% and a thermal COP of 0.47. The average total cooling was 12.3 kW and average latent cooling was 13.2 kW. The solar array was also operated between October 2011 and May 2012 and heat was rejected using a dry cooler. Over the heating season 18,800kWh were collected with an average collection efficiency of 61%. TRNSYS simulations over-predicted the amount of energy collected by 13% (21,264kWh) likely due to failed vacuum tubes, snow cover of collectors, and improper sensor location.
This paper is concerned with performance investigation and comparison between two novel configurations (type A and type B) of liquid desiccant air-conditioning system driven by low grade thermal energies. Both of these configurations, compared with other liquid desiccant systems, may have some outstanding advantages: leading to improved indoor air qualities, possessing a compact structure and making the best use of space exhaust air to cool the process air. Computer simulation is performed to analyze the cycle performance of these two system configurations under the effect of five key variables. Simulation results show that type B configuration is superior when the supply air temperature and humidity ratio are required to be relatively low or the ambient air absolute humidity is relatively high. In contrast, system of type A configuration has higher performance under the lower ambient air absolute humidity conditions. For a typical hot and humid summer day of Zhuhai, a city of Guangdong Province in southern China, the supply air temperature of type B configuration is more stable.
A simple linear model has been developed, based on the greenhouse crop heat and mass balances allowing for the calculation of inside air temperature and humidity together with crop temperature. This model is valid for a mature non-stressed crop with low-temperature difference between inside and outside. It can numerically be inverted in order to identify the greenhouse ventilation function together with the soil heat storage parameter, by fitting measured and calculated data of crop temperature and inside air temperature and humidity, and by deducing crop transpiration rate.This model was tested in summer in a ‘classically ventilated tunnel’ with a small opening surface (6%), and in a ‘largely opened tunnel’ with a large opening surface (18%). Measurements were carried out when the tomato crop was mature. On the basis of the experimental measurements, the coefficients of efficiency for ventilation were determined and used to validate the model with respect to inside air measurements. The identified values for the ventilation coefficients are in agreement with the values reported in the literature. Likewise, calculated inside air speed deduced from ventilation in both tunnels was also in good agreement with the measured values. It is shown that this approach allows for a precise estimation of ventilation and transpiration rates using only simple measurement devices such as temperature and air humidity sensors.
A new and simple greenhouse crop transpiration model enabling predictions from outdoor conditions is presented and the parameters involved are discussed with respect to different types of greenhouse-crop systems. This transpiration model was validated against experimental data measured in a soil-less tomato crop cultivation in Avignon in summer conditions, when the greenhouse is open, and early spring climatic conditions, when the greenhouse is kept closed and the inside air strongly confined. The evapo-condensation phenomena on the greenhouse cover, particularly important when the greenhouse is closed, are not considered in the water vapour balance. Model estimation improved from spring to summer and comparisons with previous transpiration models have shown that considering outside climate instead of inside climate as a boundary condition implied a deterioration of transpiration model performances especially when greenhouse air is confined. This deterioration is primarily due to simplifications introduced during the model derivation and model performances were satisfactory when the greenhouse air was closely coupled to outdoor conditions.
In the liquid desiccant dehumidification system, there are mainly two types of dehumidifier/regenerator: the adiabatic type, and the internally-cooled/heated type. Thus, it is valuable to compare the performance difference and application scope of these two types. A comparison between the two types should be conducted at a system level, rather than at a component level. Numerical and analytical models were adopted to investigate the performance difference of the two systems. The solution circulating flow rate was the key influencing factor of the performance difference. When ms/ma was approximately 1.1–1.3, the two systems showed similar performance. The internally-cooled system showed better performance under a small flow rate, because of the higher efficiency of the dehumidifier/regenerator and lower heat-cold offset. In contrast, the adiabatic system performed better under a large solution flow rate, owing to a more stable heat and mass transfer driving force. In addition, both systems showed larger efficiencies and higher coefficient of performance (COP) when the outdoor air was relatively dryer. However, the outdoor air status was found to have little influence on the performance difference between the two systems.
The investigation showed that there were few researches about the real dynamic performance of solar assisted liquid desiccant air conditioning system with the unsteady term in the governing equations. Regarding the unsteady properties of the components, a novel dynamic model was developed for a solar assisted internally-cooled liquid desiccant air conditioning system. In the model, the variable t (time) was involved in the governing equations of each component to take into consideration the influence of the previous condition on the following state of the system. The accuracy of the above mathematical model was verified partly with experimental results of some key components. Then, the model was applied to a real solar assisted internally-cooled liquid desiccant air conditioning system for an office building with the capacity of eight people in Hong Kong. The transient performance of the system on the summer design day of Hong Kong was investigated. The results reflected the necessity of considering the influence of the former state on the following running of the system. To sum up, the novel model could be a reliable tool to predicate the system operation and provide useful information for better control of the system as well.
The large electricity consumption due to the extensive use of vapour compression machines in the air conditioning and industrial sectors dramatically increases the emissions of greenhouse gases. Besides, the synthetic halocarbon refrigerants used in said machines are responsible for ozone depletion. These environmental concerns give an opportunity to solar thermally-driven liquid sorption systems, whose energy consumption primarily lies on the regeneration heat of liquid sorbents. This paper surveys both theoretical and experimental studies on solar thermal regeneration methods of the hygroscopic solutions used in liquid sorption systems. A brief review of some conventional and alternative liquid desiccants utilised in said systems is firstly presented. Furthermore, information about several configurations of regenerators and their performance is covered in detail, putting special emphasis on solar collector/regenerators, which use directly solar energy to reconcentrate the weak desiccant solution. A comparative assessment of relevant thermophysical properties of some hygroscopic liquids is realised in order to determine their suitability for use in sorption systems. The results show that the aqueous solutions of potassium formate and ionic liquids are very promising desiccants due to their low vapour pressures, specific heat capacities and dynamic viscosities, as well as their non-corrosiveness. A performance comparison between typical regeneration units and solar collector/regenerators is also carried out. It could be shown that solar collector/regenerators have a high potential for enhancing the water desorption rate from a diluted desiccant solution and, consequently, the capacity of the air dehumidification process within the absorber.
In this paper, a novel scheme of a partially closed solar regenerated liquid desiccant assisted evaporative cooling system has been proposed. The objective of the system is to provide suitable conditions inside greenhouses for cultivation of high value temperate crops like lettuce throughout the year in hot and humid climates of tropics and subtropics. The partially closed system re-circulated a fraction of return air from the greenhouse and ventilated a certain amount of ambient air. A forced parallel flow solar regenerator has been used to re-concentrate the weak desiccant solution. A thermal model has been developed to predict the greenhouse air temperature, vapour pressure deficit and the coefficient of performance of the system. The predicted greenhouse temperature has been compared with that of a reference model study available in literature. A very good agreement is established on comparing the temperatures predicted by the two models with a root mean square error of 0.82 °C and average percentage difference being 2.4%. The COP of the cooling system varied between 0.64 and 0.74 for daylong operation in the most humid month of the year (July). The maximum greenhouse temperature predicted using our proposed model was about 27 °C for year round system operation for the place under consideration (Kolkata). The predicted vapour pressure deficits were also in the optimum range. Thus, optimum or viable lettuce growing conditions were predicted by our model all year around. The study consequently reinforces the viability of cultivation of target plantation (lettuce) in hot and humid climates prevailing in the tropical or subtropical countries like India with our model.
Challenges on energy and environmental fronts have initiated momentum in increasing the role of renewable energy in air-conditioning industry. Liquid desiccant cooling systems (LDCS), emerging as promising alternative to conventional vapor compression systems, can run on low grade heat which can easily be drawn from solar energy. In the present study a small capacity liquid desiccant evaporative cooling system for small office application is developed. The system is a dedicated outdoor air system (DOAS). Lithium chloride solution is used as a liquid desiccant. The system consists of a dehumidifier, a regenerator, a regenerative evaporative cooler, heat exchangers (solution-solution, air-water, and solution-water) and non-concentrating solar collectors. The dehumidifier in the system is an indirect contact heat and mass exchanger which eliminates the carryover of desiccant. The major energy consumption in the LDCS is for the regeneration process which is tapped from solar energy. The performance of the overall system is presented in terms of its dehumidification effectiveness, moisture removal rate, cooling capacity and thermal COP.
One of the major obstacles to improving solar thermal cooling technologies is the high operating temperature requirements of most solar thermal cooling systems. This paper reviews recent advances that could reduce the required heat source temperatures for solar desiccant cooling to the range of 50°C–60°C. These approaches include (i) isothermal dehumidification (e.g. two-stage dehumidification or internal cooled dehumidification) and (ii) pre-cooling of the entry air with ambient heat sinks (e.g. indirect evaporative cooling or geothermal exchange). These techniques can potentially leads to a more thermodynamically efficient solution for utilising recovered heat from flat plate photovoltaic thermal (PV/T) collectors for desiccant regeneration.
This article is about using solar energy and liquid desiccant to provide evaporative cooling systems of spaces occupied by plants in high ambient humidity climate. The system took full benefit of the regeneration of liquid desiccant by pure solar energy. The effect of airflow on the predicted and measured average daily maximums of greenhouse temperatures obtained using desiccant system and those obtained with conventional evaporative cooling for the month of June was investigated and reported. The desiccant evaporative cooling system lowered the average daily maximum temperatures in the greenhouse by about 6 °C relative to conventional evaporative cooling system. Furthermore, shell and tube heat exchanger configuration was adopted to simulate pipes implanted in the desiccant pad and fed with nanofluids supplied from a cooling tower. The effects of changing volume fractions of nanoparticles on energy effectiveness and heat transfer coefficient of an assumed shell and tube heat exchanger have been analyzed and discussed. Improvements on convective heat transfer coefficient of 7.20–14.40%, 6.20–12.30%, and 5.50–9.01% were obtained for 0.01–0.04 volume concentrations of Al2O3-W, Fe3O4-W, and ZnO-W nanofluids, respectively. In addition, energy effectiveness has been analyzed and enhancements of 27.50–50.10%, 25.01–40.10%, and 24.00–32.02% were obtained for volume fractions from 0.01 to 0.04 of ZnO, Fe3O4, and Al2O3 nanoparticles, respectively, suspended in water with constant mass flow rates of fluids. Dynamic indicators (life cycle and annualized life cycle costs) were applied to evaluate the economic-effectiveness of this energy supply system. The total life cycle cost was found to be /year.
About 1000 solar thermal cooling systems have been installed worldwide, so experience with system sizing and design is limited. To counter the lack of experience and to evaluate the potential of energy and cost efficient solar cooling systems, a systematic system design study has been carried out covering most climatic regions worldwide. For each technology investigated, an energy optimized control strategy was developed which maximizes the primary energy efficiency.
It could be shown that a reduction of nominal chiller power by 30%–40% or more hardly affects the solar cooling fraction for most climates, but significantly increases the machine operating hours and thus improves the economics. Single effect absorption cooling systems easily reach 80% solar cooling fraction for all but very humid climates. Primary energy ratios can be over 3.0 and primary energy savings between 30 and 79%, depending on system design and cooling load data.
The economic analysis shows that solar thermal cooling and heating is more viable in hot climates than in moderate European climates. To achieve payback times of 10 years with today's energy prices, the investment costs have to be reduced by 30–70% depending on the location and dimensioning.
This study aims to improve the performance of existing indirect evaporative coolers. A new dew point indirect evaporative cooler with counter-current heat/mass exchanger was developed in this research by optimal design, material selection, numerical simulation, experimental investigations and economic, environmental, regional acceptance analysis.
A new dew point heat/mass exchanger using a counter-current flow pattern was designed by numerical simulation in terms of material, structure, geometrical sizes and operating conditions. The numerical results indicate that under a typical cooling design condition, i.e., 35oC dry-bulb/24oC wet-bulb temperatures, the heat exchanger could achieve a wet-bulb effectiveness of approximately 1.4. The results of numerical simulation are consistent with some published test data. Based on the numeric results and the material selection determined from a set of related tests, a prototype dew point heat/mass exchanger and the associated air cooler was designed and constructed in laboratory. Testing was carried out to evaluate the performance of the experiment prototype. The results indicate that the wet-bulb effectiveness of the prototype ranged from 55% to 110% for all test conditions. The power consumption of the prototype ranged from 10 to 50 W with energy efficiency (or COP) rated from 3 to 12. It is also found that the water consumption of the prototype was very small which ranged from 0.2-1.3 litre/h. A comparison between the numerical and experimental results was carried out and the reasons for the discrepancy were analysed. This research also investigates the feasibility, economic and environmental potential of using a dew point cooler in buildings in Europe and China.
From the related studies in this thesis, it is concluded that the dew point cooler can achieve a higher performance (in terms of effectiveness and energy efficiency) than the typical indirect evaporative coolers without adding too much cost. It is found that the effectiveness and energy efficiency of the heat/mass exchanger in the cooler are largely dependent upon channel geometries, the intake air velocity, temperature, humidity and the working-to-intake air ratio but less on the feed water temperature. To maximise effectiveness and energy efficiency, it is suggested that 1) the channel height and the length of exchanger should be set below 6 mm and 1-1.2 m respectively; 2) the intake channel air velocity should be controlled to 0.5-1 m/s; and 3) the working-to-intake air ratio should be adjusted to 0.4-0.5. It is also concluded that the dew point system is suitable for most regions with dry, mild and hot climate. It is, however, unsuitable for humid regions where the system is used as a stand-alone unit. Compared to the conventional mechanical compression cooling system, the dew point system has a significantly higher potential in saving energy bills and reducing carbon emission.
A project to construct an 8 kW commercial dew point cooler is currently under development with the assistance of a Chinese company. By the optimisation of material, structure and geometries, the cooler is expected to achieve a cooling output of 8 kW at the inlet air of 38oC dry-bulb/ 21oC wet-bulb temperatures, with a wet-bulb effectiveness of 1.02 at 1530 m3/h of supply air flow and 1200 m3/h of discharge air flow, whereas the power input of the unit is about 450 W and the energy efficiency (or COP) at 18.5.
Experiments and theoretical modelling have been carried out to predict the performance of a solar-powered liquid desiccant cooling system for greenhouses. We have tested two components of the system in the laboratory using MgCl2 desiccant: (i) a regenerator which was tested under a solar simulator and (ii) a desiccator which was installed in a test duct. Theoretical models have been developed for both regenerator and desiccator and gave good agreement with the experiments. The verified computer model is used to predict the performance of the whole system during the hot summer months in Mumbai, Chittagong, Muscat, Messina and Havana. Taking examples of temperate, sub-tropical, tropical and heat-tolerant tropical crops (lettuce, soya bean, tomato and cucumber respectively) we estimate the extensions in growing seasons enabled by the system. Compared to conventional evaporative cooling, the desiccant system lowers average daily maximum temperatures in the hot season by 5.5–7.5 °C, sufficient to maintain viable growing conditions for lettuce throughout the year. In the case of tomato, cucumber and soya bean the system enables optimal cultivation through most summer months. It is concluded that the concept is technically viable and deserves testing by means of a pilot installation at an appropriate location.
Desiccant cooling systems have the ability to provide efficient humidity and temperature control while reducing the electrical energy requirement for air conditioning as compared to a conventional system. Naturally, the desiccant air dehumidification process greatly influences the overall performance of the desiccant system. Therefore, the effects of variables such as air and desiccant flow rates, air temperature and humidity, desiccant temperature and concentration, and the area available for heat and mass transfer are of great interest. Due to the complexity of the dehumidification process, theoretical modeling relies heavily upon experimental studies. However, a limited number of experimental studies are reported in the literature. This paper presents results from a detailed experimental investigation of the heat and mass transfer between a liquid desiccant (triethylene glycol) and air in a packed bed absorption tower using high liquid flow rates. A high performance packing that combines good heat and mass transfer characteristics with low pressure drop is used. The rate of dehumidification, as well as the effectiveness of the dehumidification process are assessed based on the variables listed above. Good agreement is shown to exist between the experimental findings and predictions from finite difference modeling. In addition, a comparison between the findings in the present study and findings previously reported in the literature is made. The results obtained from this study make it possible to characterize the important variables which impact the system design.
In recent years attempts have been made to use packed beds in open-cycle liquid desiccant cooling systems for the dehumidification process and/or for regenerating weak aqueous solutions. A step-wise heat and mass balance across the bed is used to determine the performance of the reconcentration process. However, due to the abundance of variables involved with the packed bed, the analysis becomes increasingly complex. In this paper dimensionless vapor pressure and temperature difference ratios suitable for application in the reconcentration of aqueous solutions are defined. Further, a closed-form analytical solution is obtained to predict the mass of water evaporated from the weak absorbent solution, through a simplified vapor pressure correlation and the dimensionless vapor pressure and temperature difference ratios.
The heat rejected and water evaporated in mechanical and natural draft cooling towers are critically evaluated by employing the Merkel, Poppe, and e-number-of-transfer-units (e-NTU) methods of analysis, respectively, at different operating and ambient conditions. The importance of using a particular method of analysis when evaluating the performance characteristics of a certain fill material and subsequently employing the same analytical approach to predict cooling tower performance is stressed. The effect of ambient humidity and temperature on the performance of cooling towers employing the Merkel, e-NTU, and Poppe methods of analysis are evaluated.
A solar-regenerated liquid desiccant ventilation pre-conditioning system has been installed and experiments were carried out for a period of nine months covering rainy, cold, and hot seasons in a hot and humid climate (Thailand). A heat exchanger was used to cool the dehumidified air instead of typical evaporative cooling to maintain the dryness of the air. The use of solar energy at the regeneration process and cooling water from a cooling tower makes the system more passive. The evaporation rate at the regeneration process was always greater than the moisture removal rate at the dehumidification process indicating that the concentration of the desiccant in the system would not decrease and so the performance would not drop during continuous operation. The system could reduce the temperature of the delivered air by about 1.2°C while the humidity ratio was reduced by 0.0042kgw/kgda equivalent to 11.1% relative humidity reduction. The experimental results were also compared with models in literature.
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.
This study is motivated by the difficulty of cultivating crops in very hot countries and by the tendency for some such countries to become dependent on imported food.Liquid desiccation with solar regeneration is considered as a means of lowering the temperature in evaporatively-cooled greenhouses. Previous studies demonstrated the technical feasibility of the desiccation–evaporation process, but mainly in the context of human dwellings.In the proposed cycle, the air is dried prior to entering the evaporative cooler. This lowers the wet-bulb temperature of the air. The cooling is assisted by using the regenerator to partially shade the greenhouse. The heat of desiccation is transferred and rejected at the outlet of the greenhouse.The cycle is analysed and results given for the climate of the The Gulf, based on weather data from Abu Dhabi. Taking examples of a temperate crop (lettuce), a tropical crop (tomato) and a tropical crop resistant to high temperatures (cucumber) we estimate the extension in growing seasons relative to (i) a greenhouse with simple fan ventilation (ii) a greenhouse with conventional evaporative cooling.Compared to option (ii), the proposed system lowers summers maximum temperatures by 5°C. This will extend the optimum season for lettuce cultivation from 3 to 6 months of the year and, for tomato and cucumber, from 7 months to the whole year.
In this investigation, a desiccant dehumidifier is tested for different ranges of liquid to air flow rate ratios to expand the validity range of the results. Theoretical and experimental studies of the simultaneous heat and mass transfer to evaluate the moisture removal rate are conducted. The model predictions are compared with experimental results with very good agreement. Through the experimental study, the important design variables that affect the moisture removal rate are defined and compared with previous studies. The correlation found in the literature is assessed, and the errors are reported. The parameters that are varied during the experiments included the air and liquid flow rates, the air humidity ratio, the desiccant equilibrium humidity and the packing height. It is found that the liquid flow rate has no significant effect on the moisture removal rate when the liquid to air flow ratio has exceeded the value of 2.
A forced flow solar collector/regenerator is one of the effective ways of regenerating the weak liquid solution in an open cycle liquid desiccant air conditioner using solar energy. In this system, the weak solution flows over the absorber plate of a tilted collector/regenerator as a thin liquid film. The forced air stream, which flows parallel or counter to the solution film, removes the moisture which is evaporated from the liquid solution due to absorption of solar energy. The absorber plate of the collector/regenerator is blackened and glazed to enhance the solar energy absorption and protect it from the environment. To evaluate the thermal performance of the solar collector/regenerator, a computer model has been developed using calcium chloride as the desiccant. A parametric analysis of the system has been performed to calculate the rate of evaporation of water from the solution as a function of the system variables and the climatic conditions.
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.
Liquid desiccant-based dehumidification systems have been widely used to remove water vapour from air in a packed column using different liquid desiccants. The liquid desiccants are usually grouped into two categories: aqueous solutions of inorganic salts and aqueous solutions of organic compounds. In order to design such a desiccant—dehumidification system, correlations of the column performance parameters are necessary. A correlation of column efficiency for different packings and desiccant solutions was developed in this study using lithium chloride (LiCl) as the inorganic salt and triethylene glycol (TEG) as the organic compound. This correlation involves the air and liquid flow rates, air and liquid inlet temperatures, column and packing dimensions, and the equilibrium properties of the desiccant solutions. The correlation was tested for polypropylene Flexi rings, ceramic Berl saddles, glass Raschig rings and polypropylene Pall rings. The average value of the errors between predicted values and experimental data was about 7%.
Preliminary results are presented from an analysis of the forced flow solar regenerator used for regenerating weak absorbent solutions, whose thermal performance depends on the rate of evaporation of water from the weak solution. A simple expression is derived to determine the desorption rate of water, assuming that solution temperature, concentration, and vapor pressure are constant at the arithmetic average of these values at the start and end of the regenerator. It is found that the effect of preheating the solution, or air, increases the rate of desorption due to the mass transfer potential increase, although solution regeneration is more effective while preheating the air than while preheating the solution.
Seasonal performance simulations of liquid desiccant cooling systems provide valuable information for developing cost-competitive and energy efficient alternatives to conventional air conditioning techniques. To ensure reliable simulation results, component models must be based on and verified by experimental data. This paper presents closed-form performance correlations for the effectiveness of heat and mass transfer processes in a packed bed dehumidifier/regenerator. The physical phenomena relevant to the heat and mass transfer in these devices has been used to develop a novel set of correlations based on the relevant nondimensional parameters. A large body of previously reported experimental data, including results recently obtained by the present authors, was used in deriving the correlations. The key improvements offered by these newly developed correlations include the applicability for both dehumidification and regeneration, inclusion of proper nondimensional parameters such as wettability, and extension in range of validity. The number of different studies used makes these correlations valid for broad ranges of conditions encountered in packed bed liquid desiccant dehumidifiers and regenerators.
The current energy crisis, climate change and increased air conditioning demands have generated a need for developing technologies based on renewable energy sources. Foremost amongst the cooling technologies are the sorption technologies working on low grade heat that can be supplied by solar energy. Liquid desiccant technologies seem to be a promising option as these tend to have higher thermal COPs, lower regeneration temperatures, facilitate simultaneous cooling and ease of storage of concentrated desiccant that can be used during the nonsunshine hours. But few concerns like carryover of liquid desiccant in air require further investigations. The liquid desiccant system under study incorporates a double channelled exchanger for air to liquid desiccant heat and mass transfer. It provides a large surface area for air/desiccant contact and reduces the carryover as direct contact between desiccant and air is minimized unlike spray towers, packed bed and falling film designs. Desiccant is heated in a plate heat exchanger using hot water and then regenerated in a regenerator. The set-up comprises of a dehumidifier, along with a regenerator, a cooling tower, plate heat exchangers and a control unit. Experiments were conducted on the system using calcium chloride and lithium chloride as desiccants by varying parameters like inlet air conditions, hot water temperature and desiccant concentration in order to evaluate the performance of the system under different operating conditions. The performance of the system is presented in terms of moisture removal rates, dehumidifier and regenerator effectiveness.Research highlights► Indirect contact between air and desiccant using a porous surface to avoid carryover. ► Humidity effectiveness and moisture removal rate reported under varying conditions. ► Humidity effectiveness with LiCl as desiccant in the range 0.36–0.45. ► Mass transfer characteristic of contactor surface restricted system performance.
The performance of an open absorption-system, energized by low-grade heat such as insolation and/or waste heat, has been investigated. This combined evaporative cooler (CEC) [i.e. an indirect evaporative cooler (IEC) together with a direct evaporative-cooler (DEC)] was used to cool the air. A computer simulation of the cooling cycle was devised, so that the performance characteristics of the system could be predicted for a range of operating conditions: the influences of various design-parameters on the coefficient of performance (COP) were also evaluated. The COP of the CEC system was at least 20% greater than those achieved when employing either the IEC or DEC systems alone.
A solar-regenerated liquid desiccant ventilation pre-conditioning system has been proposed for use in hot and humid climates. The system aims to dehumidify the ventilation air which is the major source of latent load. A heat exchanger is used to cool the dehumidified air instead of typical evaporative cooling to maintain the dryness of the air. The use of solar energy at the regeneration process and cooling water from a cooling tower makes the system more passive. The simulation procedure for the proposed system has been presented. By inputting the climatic data and the physical parameters of all equipments, the operating parameters at each equipment and the performance parameters of the system can be evaluated. The simulation procedure is demonstrated by showing the daily profiles of the operating and performance parameters on a typical day as well as investigating the influence of the selected operating parameters on the system performance. The results suggest that the most influential parameters are solar radiation, ventilation rate, and desiccant solution concentration. The balance between the water removed at the dehumidifier and that evaporated at the regenerator needs to be considered to maintain uniform performance during continuous operation.
This paper describes current trends in solar-powered air conditioning, which has seen renewed interest in recent years due to the growing awareness of global warming and other environmental problems. Closed-cycle heat-powered cooling devices are based mainly on absorption chillers, a proven technology employing LiBr–water as the working fluid pair. Recent developments in gas-fired systems of this type make available double- and triple-effect chillers with considerably higher COP than their single-effect counterparts, which makes it possible to reduce the amount of solar heat required per kW of cooling. These systems require, however, high-temperature solar collectors. The principles of multi-staging absorption systems are described. An economic comparison is provided which shows the total system cost to be dominated by the solar part of the system. At current prices, the high COP, high temperature alternative is still more costly than the low temperature one. Open-cycle desiccant systems employing either solid or liquid sorbents are described. While the main thrust in research on novel closed-cycle absorption systems has been toward increasing the operating temperature in order to improve efficiency through multi-staging, open-cycle absorption and desiccant systems have been developed for use with low temperature heat sources such as flat plate solar collectors. A novel open-cycle (DER) system is described, which makes it possible to use the solar heat at relatively low temperatures, for producing both chilled water and cold, dehumidified air in variable quantities, as required by the load.
The paper presents numerical investigation of a novel counter-flow heat and mass exchanger used in the indirect evaporative dew point cooling systems, a potential alternative to the conventional mechanical compression air conditioning systems. Numeric simulation was carried out to optimise the geometrical sizes and operating conditions of the exchanger in order to enhance the cooling (dew point and wet bulb) effectiveness of the exchanger and maximise the energy efficiency of the dew point cooling system. The results of the simulations indicated that cooling (dew point and wet bulb) effectiveness and energy efficiency are largely dependent on the dimensions of the airflow passages, air velocity and working-to-intake-air ratio, and less dependent on the temperature of the feed water. It is recommended that exchanger intake air velocity should be controlled to a value below 0.3–0.5 m/s; height of air passage (channel) should be set to 6 mm or below and the length of the passage should be 200 time the height; the working-to-intake-air ratio should be around 0.4. Under the UK summer design condition, i.e., 28 °C of dry bulb temperature, 20 °C of wet bulb temperature and 16 °C of dew point temperature, the exchanger can achieve wet-bulb effectiveness of up to 1.3 and dew-point effectiveness of up to 0.9.
One of the main components of a liquid desiccant cooling system is the regenerator. In a liquid desiccant air conditioner, outside air is dehumidified by liquid desiccant and cooled within the absorber. The diluted desiccant solution thus obtained has to be concentrated for reuse, by passing through the regenerator and the cycle is, consequently, repeated. The regenerator used in this application is a forced parallel flow type solar collector/regenerator. The regenerator has been designed and optimized and the prototype of the solar collector/regenerator has been built and tested. Calcium chloride has been used as the absorbent solution. The results of the tests conducted as a parametric analysis indicate that the air and solution mass flow-rates and the climatic conditions affect the regenerator performance. Furthermore, a comparison between the experimental data obtained and a previously developed model for a forced parallel flow solar collector/regenerator reveals that the experiments are in good agreement with the model predictions. Finally, it was concluded that the proposed solar collector/regenerator performs satisfactorily under the summer conditions of Adelaide, Australia.
This paper describes a relatively simple model for the preliminary design of an air dehumidification process occurring in a packed bed using liquid desiccant through dimensionless vapor pressure and temperature difference ratios. An expression is derived using the aforementioned ratios to predict the water condensation rate from the air to the desiccant solution in terms of known operating parameters. The model predictions were compared against a reliable set of experimental data available in the literature, with very good agreement. The effects of the cooling water inlet temperature and the desiccant-to-water heat exchanger effectiveness on the performance of the dehumidifier are also studied and the results are presented in this paper.
Desiccant systems have been proposed as energy saving alternatives to vapor compression air conditioning for handling the latent load. Use of liquid desiccants offers several design and performance advantages over solid desiccants, especially when solar energy is used for regeneration. For liquid–gas contact, packed towers with low pressure drop provide good heat and mass transfer characteristics for compact designs. This paper presents the results from a study of the performance of a packed tower absorber and regenerator for an aqueous lithium chloride desiccant dehumidification system. The rates of dehumidification and regeneration, as well as the effectiveness of the dehumidification and regeneration processes were assessed under the effects of variables such as air and desiccant flow rates, air temperature and humidity, and desiccant temperature and concentration. A variation of the Öberg and Goswami mathematical model was used to predict the experimental findings giving satisfactory results.
A packed column air-liquid contactor has been studied in application to air dehumidification and regeneration in solar air conditioning with liquid desiccants. A theoretical model has been developed to predict the performance of the device under various operating conditions. Computer simulations based on the model are presented which indicate the practical range of air to liquid flux ratios and associated changes in air humidity and desiccant concentration. An experimental apparatus has been constructed and experiments performed with Monoethylene Glycol (MEG) and Lithium Bromide as desiccants. MEG experiments have yielded inaccurate results and have pointed out some practical problems associated with the use of Glycols. LiBr experiments show very good agreement with the theoretical model. Preheating of the air is shown to greatly enhance desiccant regeneration. The packed column yields good results as a dehumidifier/regenerator, provided pressure drop can be reduced with the use of suitable packing.
Growing demand for air conditioning in recent years has caused a significant increase in demand for primary energy resources. Solar-powered cooling is one of the environmentally-friendly techniques which may help alleviate the problem. A promising solar cooling method is through the use of a liquid desiccant system, where humidity is absorbed directly from the process air by direct contact with the desiccant. The desiccant is then regenerated, again in direct contact with an external air stream, by solar heat at relatively low temperatures. The liquid desiccant system has many potential advantages over other solar air conditioning systems and can provide a promising alternative to absorption or to solid desiccant systems.Earlier work by the authors included theoretical simulations and preliminary experiments on the key components of the liquid desiccant system. The objective of the present study has been to construct a prototype system based on the knowledge gained, to monitor its performance, identify problems and carry out preliminary design optimization. A 16 kWt system was installed at the Energy Engineering Center at the Technion, in the Mediterranean city of Haifa. The system comprises a dehumidifier and a regenerator with their associated components operating together to dehumidify the fresh (ambient) air supply to a group of offices on the top floor of the building. LiCl-water is employed as the working fluid. The system is coupled to a solar collector field and employs two methods of storage – hot water and desiccant solution in the regenerated state. The performance of the system was monitored for five summer months under varying operating conditions. The paper describes the operation of the experimental system and presents the measured data and the calculated performance parameters.
A novel dew point evaporative cooling system for sensible cooling of the ventilation air for air conditioning application was constructed and experiments were carried out to investigate the outlet air conditions and the system effectiveness at different inlet air conditions (temperature, humidity and velocity) covering dry, temperate and humid climates. The results showed that wet bulb effectiveness ranged between 92 and 114% and the dew point effectiveness between 58 and 84%. A continuous operation of the system during a typical day of summer season in a hot and humid climate showed that wet bulb and dew point effectiveness were almost constant at about 102 and 76%, respectively. The experiment results were compared with some recent studies in literature.
Desiccant systems find applications in a very large variety of industrial and daily usage products including the new HVAC installations. An overview of liquid desiccant technology has been presented in this paper along with a compilation of experimental performance data of liquid desiccant dehumidifiers, empirical dehumidification effectiveness and mass transfer correlations in a useful and easy to read tabular format. The latest trends in this area suggest that hybrid systems are of current interest to HVAC industry, not only for high latent load applications but also for improving indoor air quality. The paper presents a comprehensive comparative parametric analysis of packed bed dehumidifiers for three commonly used desiccant materials viz. triethylene glycol, lithium chloride and calcium chloride, using empirical correlations for dehumidification effectiveness from the literature. The analysis reveals significant variations and anomalies in trends between the predictions by various correlations for the same operating conditions, and highlights the need for benchmarking the performance of desiccant dehumidifiers.
This paper presents the experimental tests and the theoretical analysis on the chemical dehumidification of air by a liquid desiccant and desiccant regeneration in an absorption/desorption column with random packing.The experimental set-up is fully described together with measurements, procedures, data reduction and accuracy. The experimental tests include dehumidification and desiccant regeneration runs carried out with the traditional hygroscopic salt solutions H2O/LiCl and H2O/LiBr and the new salt solution H2O/KCOOH in the typical operative ranges of air conditioning applications.A theoretical model of the packed column and the relative simulation computer code was developed to predict the performance of the system and to analyse the system sensitivity to the main operating parameters. A fair agreement was found between the experimental tests and the simulation computer code.The experimental tests and the theoretical analysis show that the chemical dehumidification of air by hygroscopic salt solutions ensures consistent reduction in humidity ratio, which is suitable for applications to air conditioning or drying processes. Moreover, desiccant regeneration requires a temperature level around 40–50 °C which can be easily obtained by using solar energy or heat recovered from an industrial process or from a thermal engine.
Principal component analysis of dry-bulb temperature, wet-bulb temperature and global solar radiation was considered, and a new climatic index (principal component Z) determined for two emissions scenarios – low and medium forcing. Multi-year building energy simulations were conducted for generic air-conditioned office buildings in Harbin, Beijing, Shanghai, Kunming and Hong Kong, representing the five major architectural climates in China. Regression models were developed to correlate the simulated monthly heating and cooling loads and building energy use with the corresponding Z. The coefficient of determination (R2) was largely within 0.78–0.99, indicating strong correlation. A decreasing trend of heating load and an increasing trend of cooling load due to climate change in future years were observed. For low forcing, the overall impact on the total building energy use would vary from 4.2% reduction in severe cold Harbin (heating-dominated) in the north to 4.3% increase in subtropical Hong Kong (cooling-dominated) in the south. In Beijing and Shanghai where heating and cooling are both important, the average annual building energy use in 2001–2100 would only be about 0.8% and 0.7% higher than that in 1971–2000, respectively.
The thermal performance prediction of wet-cooling towers is critically analyzed and refined. Natural draft counterflow towers and mechanical draft counterflow and crossflow towers are considered. The Merkel, Poppe and e-NTU heat and mass transfer methods of analysis are derived from first principles, as these methods form the cornerstone of wet-cooling tower performance evaluation. The critical differences between these methods, when applied to fill performance analyses and cooling tower performance evaluations, are highlighted. The reasons for these differences are discussed with the aid of psychrometric charts. A new extended empirical relation for the loss coefficient of fills is proposed where the viscous and form drag effects are accounted for as well as the buoyancy, momentum and fill height effects. The empirical equation for the transfer characteristic of fills is extended to include the effects of fill height and the inlet water temperature. Empirical equations to predict the temperature inversion profile, height of the temperature inversion and the height from which air is drawn into the cooling tower are developed. The influence of temperature and humidity inversions on the performance of wet-cooling towers is subsequently investigated. A comprehensive analytical computer program is developed to predict and optimize the performance of wet-cooling towers. Computer programs are also developed to generate cooling tower performance curves, analyze fill performance test data and plot psychrometric charts. Thesis (PhD (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2003.
Solar radiant energy over India, Indian Meteorological Department
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