Cooling potential of an evaporative cooling tower: Parametric studies

Dependence of the cooling potential of an evaporative cooling tower on the tower parameters (height h, cross-sectional area A_t, evaporative pad area A_p, packing factor of evaporating pads F_p and flow resistance f) has been investigated. The performance of the tower is studied for two different climates, namely hot-dry and composite, typified by Jodhpur and Delhi.     

Thermal performance of a building coupled to an evaporative cooling tower

The thermal performance of a building fitted with an evaporative cooling tower has been evaluated in terms of discomfort index for two climates, namely, composite and hot-dry, typified by New Delhi and Jodhpur, respectively. The effects of various evaporative cooling parameters (height and cross-sectional area of the tower, packing factor, area of the pads, resistance offered to the air flow and local wind conditions) on the performance of the building have been analysed. It was found that, for given parameters of the tower and wind conditions, there is an optimum height of the tower for which the thermal discomfort condition in the building is minimum. The optimum values of the tower height for comfort conditions in the building for various other tower parameters have been obtained for each climate.     

Numerical study of solar-wind tower systems for ventilation of dwellings

Numerical study is carried out of a simplified system of solar-wind tower for ventilation of dwellings. The conservation equations for mass, momentum and energy for mixed convection in two-dimensional coordinates are solved by the control volume method and Simpler algorithm. The governing parameters of the problem are Rayleigh number Ra, Reynolds number, Re (or Richardson number, Ri), the dimensionless conductivity of the solid medium, k_r and the geometrical parameters. Nusselt number Nu and dimensionless volume flow rate are calculated as a function of the governing parameters, and streamlines and isotherms are produced. The results show that the important parameters affecting the ventilation performance are Ra, Re (or Ri), and the geometrical parameters, the aspect ratio A, the exit port size h₁/L and to a lesser degree, the wall thickness ℓ₁/L.     

Performance prediction of a multi-stage wind tower for indoor cooling

A theoretical model is developed to establish an indepth understanding of the performance of a three-stage wind tower with a bypass system for indoor cooling in rural dry and hot climates. Model simulations are presented for a wide range of ambient conditions that include inlet wind speed, inlet temperature and relative humidity. Simulation results provide an insight into the desirable water flow rates and air-to-water loadings for comfort zone tem-peratures and relative humidity levels at the exit of the wind tower. Simulations show wind towers with variable cross-sections provide an increase in the cooling power for the same inlet wind speed, inlet air temperature and relative humidity when compared to wind towers with a constant cross-section. The study shall lead to a better understanding to designing wind towers that are both environmentally friendly and energy efficient.     

Experimental validation of a thermal model of an evaporative cooling system

A periodic thermal model for an evaporative cooling system over the roof has been presented. Open roof pond, water film and flowing water layer are the the special cases of the analysis. The time dependency of solar radiation, ambient air, sol-air and room air temperatures has explicitly been taken into account by expressing as a Fourier series of time for a 24 h cycle. Experimentally observed air temperature of rooms, treated with and without evaporative cooling over the roof, has been found in good agreement with theoretical results.     

Efficiency control in a commercial counter flow wet cooling tower

This paper presents open and closed-loop analyses of a counter flow wet cooling tower. The closed-loop system analysis was based on a comparative evaluation of three control strategies. The first and second comprised a split-range control of the cooling water temperature and an index of thermal performance (efficiency), respectively, and the third strategy comprised a combination of override and split-range control in order to control two performance indexes (efficiency and effectiveness). In this case, a SISO (Single-Input Single-Output) loop for each controlled variable is considered. In each case the water loss through evaporation and the energy consumption in the cooling tower (pump and fans) were estimated in order to analyze its eco-efficiency. All the simulation tests were carried out considering the same regulatory problem and the results show a notable improvement in the tower’s performance when compared to open-loop operation, thus attesting the potential benefits of the use of an efficient control strategy for such equipment.     

Simulation and optimization of a LiBr solar absorption cooling system with evacuated tube collectors

Solar radiation is a clean form of energy, which is required for almost all natural processes on earth. Solar-powered air-conditioning has many advantages when compared to a conventional electrical system. This paper presents a solar cooling system that has been designed for Malaysia and similar tropical regions using evacuated tube solar collectors and LiBr absorption unit. The modeling and simulation of the absorption solar cooling system is carried out with TRNSYS program. The typical meteorological year file containing the weather parameters for Malaysia is used to simulate the system. The results presented show that the system is in phase with the weather, i.e. the cooling demand is large during periods that the solar radiation is high. In order to achieve continuous operation and increase the reliability of the system, a 0.8 m³ hot water storage tank is essential. The optimum system for Malaysia's climate for a 3.5 kW (1 refrigeration ton) system consists of 35 m² evacuated tubes solar collector sloped at 20°.     

Performance of a coupled cooling system with earth-to-air heat exchanger and solar chimney

Buildings represent nearly 40 percent of total energy use in the U.S. and about 50 percent of this energy is used for heating, ventilating, and cooling the space. Conventional heating and cooling systems are having a great impact on security of energy supply and greenhouse gas emissions. Unlike conventional approach, this paper investigates an innovative passive air conditioning system coupling earth-to-air heat exchangers (EAHEs) with solar collector enhanced solar chimneys. By simultaneously utilizing geothermal and solar energy, the system can achieve great energy savings within the building sector and reduce the peak electrical demand in the summer. Experiments were conducted in a test facility in summer to evaluate the performance of such a system. During the test period, the solar chimney drove up to 0.28 m³/s (1000 m³/h) outdoor air into the space. The EAHE provided a maximum 3308 W total cooling capacity during the day time. As a 100 percent outdoor air system, the coupled system maximum cooling capacity was 2582 W that almost covered the building design cooling load. The cooling capacities reached their peak during the day time when the solar radiation intensity was strong. The results show that the coupled system can maintain the indoor thermal environmental comfort conditions at a favorable range that complies with ASHRAE standard for thermal comfort. The findings in this research provide the foundation for design and application of the coupled system.     

Overall heat transfer coefficient in the presence of the axial dispersion coefficient for a forced draught cooling tower

An overall heat transfer coefficient was calculated for a forced draught counterflow cooling tower by using the pulse response technique. The presence of an axial dispersion coefficient for both gas and liquid was considered. Results indicate that, on neglecting the axial mixing and assuming a plug flow, the overall heat transfer coefficient is overestimated and can lead to errors in design applications.     

Experimental investigation of the hydraulic characteristics of a counter flow wet cooling tower

Thermal and nuclear electric power plants as well as several industrial processes invariably discharge considerable energy to their surrounding by heat transfer. Although water drawn from a nearby river or lake can be employed to carry away this energy, cooling towers offer an excellent alternative particularly in locations where sufficient cooling water cannot be easily obtained from natural sources or where concern for the environment imposes some limits on the temperature at which cooling water can be returned to the surrounding. This paper concerns an experimental investigation of the hydraulic characteristics of a counter flow wet cooling tower. The tower contains a “VGA.” (Vertical Grid Apparatus) type packing which is 0.42 m high and consists of four (04) galvanised sheets having a zigzag form, between which are disposed three (03) metallic vertical grids in parallel with a cross sectional test area of 0.15 m × 0.148 m. The present investigation is focused mainly on the effect of the air and water flow rates on the hydraulic characteristics of the cooling tower, for different inlet water temperatures. The two hydrodynamic operating regimes which were observed during the air/water contact operation within the tower, namely the Pellicular Regime (PR) and the Bubble and Dispersion Regime (BDR) have enabled to distinguish two different states of pressure drop characteristics. The first regime is characterized by low pressure drop values, while in the second regime, the pressure drop values are relatively much higher than those observed in the first one. The dependence between the pressure drop characteristics and the combined heat and mass transport (air–water) through the packing inside the cooling tower is also highlighted. The obtained results indicate that this type of tower possesses relatively good hydraulic characteristics. This leads to the saving of energy.     

Numerical simulation of cooling energy consumption in connection with thermostat operation mode and comfort requirements for the Athens buildings

A model and a corresponding numerical procedure, based on the finite-difference method, have been developed for the prediction of buildings thermal behavior under the influence of all possible thermal loads and the “guidance” of cooling control system in conjunction with thermal comfort requirements. Using the developed procedure analyses have been conducted concerning the effects of thermostat operation mode and cooling power in terms of the time, on the total cooling energy consumption for the ideal space cooling, as well as for various usually encountered real cases, thus trying to find ways to reduce cooling energy consumption. The results lead to suggestions for energy savings up to 10%. Extensive comparisons between the ideal and various real cooling modes showed small differences in the 24-h cooling energy consumption. Because of the above finding, our detailed ideal cooling mode predictions gain considerable value and can be considered as a basis for comparison with real cases. They may also provide a good estimate of energy savings obtained if we decide to increase thermostat set point temperature. Therefore, as the extent of cooling energy saving is a priori known, one can decide if (and how much) it is worthy to increase thermostat set point temperature at the expense of thermal comfort. All results of the study, which refer to the Typical Athens Buildings during the typical Athens summer day, under the usual ranges of thermal loads, may be applicable to other regions with similar conditions.     

Fouling characteristics of internal helical-rib roughness tubes using low-velocity cooling tower water

This paper provides long-term cooling tower water fouling data in seven 15.54 mm I.D. copper, helically ribbed tubes taken at low water velocity (1.07 m/s). The ranges of geometric parameters were number of rib starts (18-45), helix angle (25-45°), and height (0.33-0.55 mm). These geometries provide a new class of internal enhancement that is typical of commercially enhanced tubes presently used in water chillers. There are two ranges of fouling characteristics based on internal dimensions: linear range and non-linear range. The ratio of the enhanced-to-plain tube fouling resistance linearly increases with increasing the product of area indexes and efficiency indexes in linear range and the linear relationship fails in non-linear range. A series of semi-theoretical fouling correlations as a function of the product of area indexes and efficiency indexes were developed. They were applicable to different internally ribbed geometries within the above dimensional range. A series of linear multiple regression correlations as a function of geometric variables and Reynold numbers were also developed. The average deviation of the two series of correlations was 4.4% and 4.8%, respectively. The correlations can be directly used to assess the fouling potential of enhanced tubes in actual cooling water situations.     

Simulation of solar-powered absorption cooling system

With developing technology and the rapid increase in world population, the demand for energy is ever increasing. Conventional energy will not be enough to meet the continuously increasing need for energy in the future. In this case, renewable energy sources will become important. Solar energy is a very important energy source because of its advantages. Instead of a compressor system, which uses electricity, an absorption cooling system, using renewable energy and kinds of waste heat energy, may be used for cooling. In this study, a solar-powered, single stage, absorption cooling system, using a water–lithium bromide solution, is simulated. A modular computer program has been developed for the absorption system to simulate various cycle configurations and solar energy parameters for Antalya, Turkey. So, the effects of hot water inlet temperatures on the coefficient of performance (COP) and the surface area of the absorption cooling components are studied. In addition, reference temperatures which are the minimum allowable hot water inlet temperatures are determined and their effect on the fraction of the total load met by non–purchased energy (FNP) and the coefficient of performance are researched. Also, the effects of the collector type and storage tank mass are investigated in detail.     

Liquid axial dispersion coefficient in a packed cooling tower

A dynamic method is used to predict the liquid axial dispersion coefficient in cooling towers. A KCl tracer was employed on the water inlet on the top of the tower. Input and response signals were measured. Cooling tower parameters were determined by predicting the response tracer signal and curve fitting technique in the time domain.     

Accuracy of theoretical models in the prediction of solar chimney performance

A solar chimney is a solar power plant which generates mechanical energy (usually in terms of turbine shaft work) from a rising hot air that is heated by solar energy. The present paper compares the predictions of performances of solar chimney plants by using five simple theoretical models that have been proposed in the literature. The parameters used in the study were various plant geometrical parameters and the insolation. Computational Fluid Dynamics (CFD) simulation was also conducted and its results compared with the theoretical predictions. The power output and the efficiency of the solar chimney plants as functions of the studied parameters were used to compare relative merits of the five theoretical models. Models that performed better than the rest are finally recommended.     

Energy balance of solar absorption and vapour compression cooling systems

Solar absorption cooling systems are viewed as potential alternatives to fossil-fuel-based conventional cooling systems. This view is investigated in this paper from the point of view of the energy balance of solar absorption and conventional systems. The paper investigates the primary energy needs of three cooling systems; dry and wet cooled vapour compression systems and wet cooled solar absorption. The sources of energy demand in the three systems are identified and their primary energy needs determined. The paper, then, investigates the conditions under which the energy inputs to the solar system breaks even with the other two systems. The investigation is conducted with particular reference to the operational and environmental conditions in Kuwait.     

Critical evaluation of solar chimney power plant performance

This paper evaluates the influence of a recently developed convective heat transfer equation, more accurate turbine inlet loss coefficient, quality collector roof glass and various types of soil on the performance of a large scale solar chimney power plant. Results indicate that the new heat transfer equation reduces plant power output considerably. The effect of a more accurate turbine inlet loss coefficient is insignificant, while utilizing better quality glass enhances plant power production. Models employing Limestone and Sandstone soil produce virtually similar results to a Granite-based model. The plant collector height is found to differ from previously obtained optimal values.     

Enhancement of cooling capacity in a hybrid closed circuit cooling tower

Performance characteristics of the Hybrid Closed Circuit Cooling Tower (HCCCT) have been tested experimentally. Bare-type copper tubes and fin tubes were used in staggered arrangement at the heat exchanger of the HCCCT and the cooling capacity and pressure drop have been studied from various perspectives. The relevant temperatures and velocities were selected based on the typical East Asian meteorological constraints for the year round operation of the HCCCT. Performance characteristics were compared and found that for the fin tubes, cooling capacities were about 22% and 260% higher than those of the bare tubes in wet and dry modes, respectively. The pressure drop for the fin tubes was nearly two times higher than that of the bare tubes in both modes. The experimental results show considerable enhancement of cooling capacity.     

Lighting and cooling energy consumption in an open-plan office using solar film coating

In subtropical Hong Kong, solar heat gain via glazing contributes to a significant proportion of the building envelope cooling load. The principal fenestration design includes eliminating direct sunlight and reducing cooling requirements. Daylighting is an effective approach to allow a flexible building façade design strategy, and to enhance an energy-efficient and green building development. This paper studies the lighting and cooling energy performances for a fully air-conditioned open-plan office when solar control films together with daylight-linked lighting controls are being used. Measurements were undertaken at two stages including the electricity expenditures for the office using photoelectric dimming controls only (first stage) and together with the solar control film coatings on the windows (second stage). Electric lighting and cooling energy consumption, transmitted daylight illuminance and solar radiation were systematically recorded and analysed. The measured data were also used for conducting and validating the building energy simulations. The findings showed that the solar film coatings coupled with lighting dimming controls cut down 21.2% electric lighting and 6.9% cooling energy consumption for the open-plan office.