A multi-stage down-draft evaporative cool tower for semi-enclosed spaces: Aerodynamic performance

A multi-stage down-draft evaporative cool tower (DECT) was developed as an improvement to a previously developed single-stage design. The new tower incorporates a secondary air inlet, added to reduce the water consumption required to produce a desired cooling output in a tower of given maximum cross-section and primary inlet geometry. The secondary air, which may be drawn from the interior space being chilled, is cooled by evaporation in the lower section of the tower. This paper reports on the results of experiments conducted to establish the aerodynamic performance of the design prior to installation of a water spraying system. Design of the water spraying system and experiments on cooling performance are discussed in a companion paper.     

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.     

Modeling of water spray evaporation: Application to passive cooling of buildings

The use of water droplet evaporation in shower towers and passive downdraft evaporative cooling needs the estimation of the time needed to completely evaporate the drops. To solve this problem, a cellular approach is proposed in which the spray is considered as a pile of rigid spheres of equal size; each sphere has multiple layers and contains a drop in its center. The evaporation takes place gradually from the superficial layer towards the internal layers. Parametric studies show the influence of each variable on the evaporation time of the droplets. The model may be used for sizing passive evaporative cooling systems and towers for buildings using the passive evaporative down draught effect. A building equipped with a shower tower has been tested in the framework of the European project PDEC/JOULE in Catania (Italy). The spraying system was successfully sized by using the model proposed in this paper.     

A 3D CAD-based simulation tool for prediction and evaluation of the thermal improvement effect of passive cooling walls in the developed urban locations

As a passive cooling strategy aimed at controlling increased surface temperatures and creating cooler urban environments, the authors have developed a passive cooling wall (PCW) constructed of moist void bricks that are capable of absorbing water and which allow wind penetration, thus reducing their surface temperatures by means of water evaporation. Passive cooling effects, such as solar shading, radiation cooling and ventilation cooling can be enhanced by incorporating PCWs into the design of outdoor or semi-enclosed spaces in parks, pedestrian areas and residential courtyards. The purpose of the present paper is to detail the development of a 3D CAD-based simulation tool that can be used to predict and evaluate the thermal improvement effect in urban locations where PCW installation is under consideration. Measurement results for the surface reduction effect of a PCW are introduced in the first part of the paper. In the second part, thermal modeling of a PCW is proposed based on analysis results of experimental data. Following that, a comparison study that integrates the proposed thermal modeling was conducted to validate the simulation method. In order to demonstrate the applicability of the developed simulation tool, a case study was then performed to predict and evaluate the thermal improvement effect at an actual urban location where PCWs were installed. Simulations were performed by modeling the construction location in two scenarios; one where the PCWs were composed of dry bricks, and another where the bricks were wet. The results show that, in terms of surface temperature and mean radiant temperature (MRT), this simulation tool can provide quantitative predictions and evaluations of thermal improvements resulting from the installation of PCWs.     

A new photovoltaic floating cover system for water reservoirs

This paper describes a new photovoltaic floating cover system for water reservoirs developed jointly by the company CELEMIN ENERGY and the Universidad Politécnica de Valencia. The system consists of polyethylene floating modules which, with the use of tension producing elements and elastic fasteners, are able to adapt to varying reservoir water levels.A full-scale plant located near Alicante (Spain) was built in an agriculture reservoir to study the behaviour of the system. The top of the reservoir has a surface area of 4700 m² but only 7% of such area has been covered with the fixed solar system.The system also minimizes evaporation losses from water reservoirs.     

Performance and potential of a novel floating photovoltaic system in Egyptian winter climate on calm water surface

This article investigates the performance of a partially submerged floating photovoltaic system (PSFPV) as a proposal for harvesting solar energy as an electricity production novel system under Egyptian hot climate on calm water surfaces. The proposed system comprised of a floating photovoltaic system with a submerged portion in the surrounding water. The PSFPV system is constructed in addition to the water body and is then extensively examined under Egyptian outdoor conditions. The submerged portion of the PSFPV system keeps the system passively cool by being in direct contact with the surrounding water. A performance comparison between the novel PSFPV system and a similar land-based photovoltaic system (LPV) is also provided. The suggested PSFPV module's thermal and electrical performance was evaluated concerning its submerged length, which ranged from 4 to 24 cm. The results reveal that the PSFPV system achieves a reduction of about 15.10% in operating temperature relative to the LPV system. Also, the PSFPV system produces up to 20.76% more electricity than the LPV system. The PSFPV system is capable of alleviating the emission of CO₂ by about 49.66 kg/summer season. The proposed PSFPV system reveals a reduction in the LCOE from 0.075 to 0.067 ($/kWh) by increasing the submerged length from 4 to 24 cm.     

A new zero energy cool chamber with a solar-driven adsorption refrigerator

A new zero energy cool chamber (ZECC) consisting of two cooling systems, a solar-driven adsorption refrigerator and an evaporative cooling system, was developed and then evaluated as low-cost and eco-friendly cooling storage for storing fruit with moderate respiration rates. The solar-driven adsorption refrigerator, consisting of a solar collector containing activated carbon as an adsorbent, a condenser and an evaporator, cools water based by evaporating methanol and adsorbing it on activated carbon, and then makes ice. The methanol adsorbed on the activated carbon is desorbed by applying solar heat. The ice is then used to cool the storage space, which can be done for a long time without the need for electricity. The evaporative cooling system also cools the storage space by evaporating water from the wet walls containing wet filler. The combined use of two cooling systems reduced the average inside temperature of the new ZECC to 12.07 °C compared with an average outside temperature of 31.5 °C and extended the shelf life of tomatoes from 7 to 23 days. These results suggest that the new ZECC proposed here is low-cost and energy-saving and is useful for storing fruit and vegetables in areas where electricity is unavailable.     

A passive solar water heating system for vineyard frost protection

The threat of frost during spring time (after ‘bud burst’) is an ever present danger to the vineyard owner. To minimise the risk, in addition to good site selection and vineyard management, a number of active frost protection systems are available. Most active methods of frost protection are costly in monetary terms and can also have a detrimental effect on the environment. This work presents the design and performance of a passive solar water heating quilt system under real vineyard operating conditions. Two vineyard sites were selected, and the solar water heating quilt design was evaluated over a three-month period. Detailed measurements of the temperature below and above the soil surface, levels of incident solar radiation and the wind direction and speed were recorded. Field study results indicate that the quilts can improve the solar collection and heat retention of the soil, resulting in increased temperatures during frost events of up to 1 °C in air space immediately adjacent to the solar quilts when compared to conditions off the protected area. In addition, the time period during which the frost remains a danger to the vine is also reduced. When heat collection, storage and extraction rates are investigated, simplified calculations indicate that the solar quilt can improve collection by 38.5% over bare soil, resulting in the release of 32% more heat. Extrapolated to vineyard coverage, this could result in an extra 3500 MJ of heat per hectare per (typical frost event condition) day.     

Performance review of a novel combined thermoelectric power generation and water desalination system

A novel combined thermoelectric power generation and water desalination system is described with a system schematic. The proposed system utilises low grade thermal energy to heat thermoelectric generators for power generation and water desalination. A theoretical analysis presents the governing equations to estimate the systems performance characteristics combined with experimental validation. Experimental set-up consists of an electric heat source, thermoelectric modules, heat pipes, a heat sink and an evaporator vessel. Four heat pipes are embedded in a heat spreader block to passively cool the bottom side of the thermoelectric cells. The condenser of these four heat pipes is immersed in a pool of saline water stored in an evaporation vessel which is maintained at sub-atmospheric pressure. The liquid to vapour phase change cooling method achieve low saturation temperature and offers a high heat transfer coefficient for the cooling of the thermoelectric generators. At the same time this method utilises the low temperature heat extracted from the cold side of the thermoelectric generator for water desalination. It was observed that at low saturation temperatures greater heat flux could be supplied to the thermoelectric generators with less heat losses to the atmosphere.     

Study on the design procedure for a multi-cool/heat tube system

The objective of this study is to contribute to widespread use of earth-to-air heat exchangers by proposing a design procedure. In this paper, it is discussed the design method when an earth-to-air heat exchanger system consists of multiple pipes with a close arrangement.A numerical model for this multi-cool/heat tube system was developed and it was verified by field measurements. With taking into account the thermal interference between tubes, the heat transfer performance was evaluated under various design conditions such as number of tubes, arrangement interval, air velocity and length, and soil properties. Based on these results, an estimation method for the heat transfer rate for the multi-cool/heat tube system is proposed.     

A new approach to analysis and optimization of evaporative cooling system II: Applications

After introducing the concepts of moisture entransy, moisture entransy dissipation and thermal resistance based on moisture entransy dissipation (TRMED) in part I of this study, we further analyze several direct/indirect evaporative cooling processes based on the above concepts in this part. The nature of moisture entransy, moisture entransy dissipation and TRMED during evaporative cooling processes was reexamined. The results demonstrate that it is the moisture entransy, not the enthalpy, that represents the endothermic ability of a moist air, and reducing the entransy dissipation by both enlarging the thermal conductance of heat and mass transfer, and decreasing the temperature potential of the moist air, i.e. the difference between the dry-bulb temperature of moist air over its dew-point temperature, will result in a smaller system TRMED, and consequently a better evaporative cooling performance. Then, a minimum thermal resistance law for optimizing evaporative cooling systems is developed. For given mass flow rates of both moist air and water, with prescribed moist air and water conditions, minimizing the TRMED will actually lead to the most efficient evaporative cooling performance. Finally, the thermal conductance allocation for an indirect evaporative cooling system is optimized to illustrate the application of the proposed minimum thermal resistance law.     

A new approach to analysis and optimization of evaporative cooling system I: Theory

Using the analogy between heat and mass transfer processes, the recently developed entransy theory is extended in this paper to tackle the coupled heat and mass transfer processes so as to analyze and optimize the performance of evaporative cooling systems. We first introduce a few new concepts including the moisture entransy, moisture entransy dissipation, and the thermal resistance in terms of the moisture entransy dissipation. Thereinafter, the moisture entransy is employed to describe the endothermic ability of a moist air. The moisture entransy dissipation on the other hand is used to measure the loss of the endothermic ability, i.e. the irreversibility, in the coupled heat and mass transfer processes – this total loss is shown to consist of three parts: (1) the sensible heat entransy dissipation, (2) the latent heat entransy dissipation, and (3) the entransy dissipation induced by a temperature potential. Finally the new thermal resistance, defined as the moisture entransy dissipation rate divided by the squared refrigerating effect output rate, is recommended as an index to effectively reflect the performance of the evaporative cooling system. In the end, two typical evaporative cooling processes are analyzed to illustrate the applications of the proposed concepts.     

A natural ventilation wind tower with heat pipe heat recovery for cold climates

Commercial wind towers are passive ventilation technology based on traditional wind towers of the Middle East. Typical operation of wind towers in cold – mild climates is generally limited to summer-seasons as the outdoor air is too cold to be introduced into spaces for the majority of the year. In addition, the use of natural ventilation solutions has been seen to increase heat loss in buildings and lead to increased energy cost. Wind towers are normally shut down for the sake of avoiding indoor heating energy losses during winter months. Consequently, the concentration of pollutants has seen to rise above the guideline levels, which can lead to ill health. To improve the year-round capabilities of wind towers, a heat recovery system utilising the combination of heat pipes and heat sink was incorporated into a multi-directional wind tower. This study investigates the potential of this concept through the use of numerical analysis and wind tunnel experiments for validation. The findings showed that the wind tower with heat pipes was capable of meeting the required ventilation rates above an inlet air velocity of 1 m/s. In addition to sufficient ventilation, the integration of heat pipes had a positive effect on thermal performance of the wind tower; it raised the supply air by up to 4.5 K. The technology presented here is subject to a patent application (PCT/GB2014/052263).     

Design parameters and control strategies for a combined passive heating and cooling system in Louisville,KY

ABSTRACT

Conventional passive solar systems can significantly reduce a building's heating load. However, the integration of passive heating and cooling systems in the same building and the benefits of actively controlling passive systems has largely been unexplored. The objective of this study was to determine the relative performance of a passive solar heating and sky cooling system operating with a range of control strategies, with the goal of minimising the overall annual energy use for space conditioning. A combined system (CS) and a separate system (SS) were simulated with thermal networks using MATLAB, with weather data for Louisville, KY. The control strategies simulated included: Seasonal, Ambient, Room and Matrix. The highest fraction of energy supplied by ambient sources for the SS was 0.707 with Matrix control, while for the CS, the highest fraction (0.704) was with Matrix temperature control with switchable attributes for heating and cooling.     

Thermal performance of a closed wet cooling tower for chilled ceilings: measurement and CFD simulation

A closed wet cooling tower, adapted for use with chilled ceilings in buildings, was tested experimentally. The thermal efficiency of the cooling tower was measured for different air flow rates, water flow rates, spray flow rates and wet bulb air temperatures. CFD was also used to predict the thermal performance of the cooling tower. Good agreement was obtained between CFD prediction and experimental measurement. Copyright © 2000 John Wiley & Sons, Ltd.     

A system with a tracking concentrating heliostat for lighting underground spaces with beams of sunlight

The results of the introduction of a solar-power installation for lighting and creating light effects in an underground room using mirror-concentrating systems are described.     

Thermodynamic analysis of a lithium bromide/water absorption system for cooling and heating applications

Absorption systems have the potential of employing thermal energy such as waste heat to produce both chilled water and hot water for building cooling and heating applications. In the present study, a lithium bromide/water (LiBr/H₂O) absorption system for cooling and heating applications was analysed on the basis of the first and second laws of thermodynamics. Simulation was employed to determine the coefficient of performance (COP) and the exergetic efficiency of the absorption system under different operating conditions such as the heat source, cooling water, chilled water, and supply hot water temperatures. Copyright © 2001 John Wiley & Sons, Ltd.     

Design and evaluation methods for a water cooling panel system for decay heat removal from a high-temperature gas-cooled reactor

An experiment was performed to simulate a water cooling panel system for decay heat removal from a high-temperature gas-cooled reactor (HTGR) and to investigate the performance of decay heat removal and the temperature distribution for components of the system. The experimental apparatus is composed of a pressure vessel 1 m in diameter and 3 m in height, containing heaters with a maximum heating rate of 100 kW which simulates the decay heat of the reactor core and cooling panels surrounding the pressure vessel. The analytical code THANPACST2 was applied to analyze the experimental data and to investigate the validity of the analytical method and model proposed. Under conditions using helium gas at a pressure of 0.73 MPa and temperature of 210°C in the pressure vessel, temperatures of the pressure vessel were well estimated to within differences of −29 to +37°C compared to the experimental data. The analyses indicate that the heat removed by the cooling panel is 11.4% less than the experimental value and the heat transferred by thermal radiation is 74.4% of the total heating value. It was also found that the lower head of the pressure vessel is effectively cooled by natural convection through the flow channels at the upper and the lower edges of the skirt-type support of the pressure vessel. © 1998 Scripta Technica, Heat Trans Jpn Res, 26(3): 159–175, 1997     

A steady‐state model for the design and optimization of a centralized cooling system

A simplified steady‐state model has been developed to describe thermodynamically the operation of a centralized cooling system. This model resolves the mass and energy equations simultaneously and uses inputs that are readily available to the design engineer. The model utilizes an empirical relationship for the compressor power as a function of cooling load and key temperatures. The outputs include the chiller coefficient of performance (COP) and the compressor actual power. The model simulation results are validated with a manufacturer performance data and compared with the experimental data collected at Hewlett‐Packard Laboratories site for two chillers: a variable speed and a constant speed chiller. The results of the model are found to closely match the current experimental data with less than 5% average deviation for chiller load over 10% and with a maximum deviation of 18% at 95% chiller load. For the constant speed chiller, the chiller efficiency increases with increasing load and peaks at full load. For the variable speed chiller, the chiller efficiency peaks at part loading between 40 and 80% of the chiller full load depending on the condenser water temperature. This indicates that for variable speed chillers, the chiller design has been optimized for loading less than 100% depending on the ambient conditions, customer specifications and size of the chiller. Copyright © 2010 John Wiley & Sons, Ltd.