Cooling performance of roof ponds with gunny bags floating on water surface as compared with a movable insulation

A roof pond with gunny bags floating on water surface (RPWGB) has been proven to be an efficient evaporative based cooling technique in the previous studies of the authors due to the creation of thermal stratification inside the pond. In this paper, a mathematical model is developed for the investigation of its cooling performance as compared to a roof pond with a movable insulation (RPWMI) under hot dry climatic conditions. This one-dimensional model is based on the new empirical correlations of water evaporation rate from a wetted surface and a free water surface to the ambient air proposed by the authors, and takes into account the response of buildings as a whole to evaporative cooling. Results by simulations indicated that RPWGB performed slightly better than RPWMI, which is widely considered as the most efficient evaporative based roof cooling techniques, except when they are applied to a building with poorly insulated light walls and roof.     

International aspects of air conditioning with movable insulation

Natural air conditioning, involving heating with solar energy and summer cooling to the night sky, is accomplished with ceiling ponds and movable insulation. Pertinent international aspects of climate, economies, mores and needs are related to industrialized and to developing countries. Tested throughout 15 months, in Phoenix, Arizona, when temperatures varied from below freezing to 115°F days, extensive data were obtained for very hot-dry and hot-humid climates. The Sky Therm system met these weather types with six means for modulating ambient conditions: solar heating, heat capacity, nocturnal radiation, water evaporation; radiation, evaporation and fan-coil operation; and evaporation, fan-coil and pond-blower use. Typical results are presented with a chart indicating that average daily room temperatures between 70° and 80°F can be maintained throughout a normal year, in Phoenix, without supplementary heating or cooling. During low dewpoint periods, water evaporation is not required until maximum temperatures exceed 100°; until 105° is exceeded, no electricity is needed. With dewpoints above 50° and dry-bulb maxima over 100°, both evaporation and electricity were used. In most developing countries nearer the equator, ample heating with horizontal pond collectors can be expected at normal altitudes. Acclimatization of people to somewhat higher summer temperatures may make the use of electricity and water evaporation unnecessary. The required plastics are being increasingly produced in industrializing countries, but cost may restrict use of the new method to buildings of higher priority than those to which the system would be applied in industrialized countries. Dual effects obtainable simultaneously with the same movable insulation include keeping one building or room warm and another one cold. The efficiency of a solar water-heater (or a solar still) can be improved by covering it at night with insulation moved from over a container in which water is cooled (or frozen) by nocturnal radiation; this water is maintained cold throughout the day when the insulation is positioned over it.     

A parametric study of the small-scale solar pond (SSSP)

A parametric study of salt-gradient solar ponds of size less than 100 m² is presented. The study is based on a dynamic model of the pond which takes into account the variation of solar radiation, ambient temperature and the amount of heat extracted with time. Furthermore3 it considers a small-scale pond whose top is covered by a transparent cover, thus considerably reducing the thickness of the top convective zone. The parameters investigated include: pond dimensions, depths of the different layers, starting dates for pond operation and load application, pond insulation and the value of the thermal load.     

Saturated solar ponds: 2. Parametric studies

The effects of following parameters on the performance of saturated solar ponds are studied: thickness of upper convective zone, nonconvective zone, and lower convective zone; starting time of the pond; water table depth below the pond; ground thermal conductivity; transmissivity of salt solution; incident radiation; ambient air temperature, humidity, and velocity; thermophysical properties of salt solution; pond bottom reflectivity; convection, evaporation, radiation, and ground heat losses; temperature and rate of heat removal; type of salt. Magnesium chloride and potassium nitrate salt ponds located at Madras (India) are considered for the parametric study. A comparison is also made with an unsaturated solar pond.     

The ice pond—production and seasonal storage of ice for cooling

The ice pond is a scheme of making ice in winter and storing it for cooling in summer. During winter a large excavated reservoir is filled with a snow/ice mixture produced by a snow machine of the type used in ski resorts. At the end of the cold season the ice is covered with insulation. When cooling is needed, chilled water is pumped from the pond, and, after being warmed by the load, the water is returned to the pond. Since the ice in the ice pond is porous, it forms a very effective heat exchanger. Water can be extracted at temperatures between 0 and 1°C, thus providing excellent potential for dehumidification. Experimental ice ponds have been built and operated from 1980 to 1982, and a full scale ice pond (with 10,000 m³ capacity) has been built to supply cooling for a 12,000 m² office building. This paper describes the operating experience gained so far. The basic processes of ice making, ice preservation and ice utilization are described, and the economic prospects for ice ponds are analysed.     

Mathematical modeling of passive dry cooling for power plants in arid land

Radiative cooling from surfaces gives a promising alternative to conventional cooling techniques. A new means for cooling power plants through passive dry cooling is explored utilizing convection and infrared radiation from a covered cooling pond, such that evaporation is suppressed. A mathematical model is established to predict the temperature distribution along the proposed prototype pond. A quantitative study of the heat rejection process is presented for 24-hour operation of the cooling system. The numerical study shows that a total heat rejection of 150 W/m², on the average, could be obtained under normal circumstances.     

On the selection and design of the proper roof pond variant for passive cooling purposes

The present paper aims to fully summarize the current scientific and technological experience focusing on the comparative characteristics of roof pond variants. The design guidelines and pond characteristics provide the opportunity to make the proper decision of a roof pond variant for cooling purposes. The following systems are under detailed investigation: covered/uncovered pond with/without sprays, skytherm, energy roof, coolroof, walkable pond, wet gunny bags, cool-pool shaded and ventilated pond. A brief background of the motivation behind the creation of the above variants is provided. The advantages and disadvantages of ponds as well as the design considerations and state of the art are discussed. Additionally, in the present study a detailed comparison is performed in terms of effectiveness and cooling demand reduction. A complete set of criteria affecting the choice of the proper roof pond are also analyzed. Finally, a decision support flowchart, is provided based on the various criteria and parameters.     

Theoretical and experimental analysis of a novel low emissivity water pond in summer

The present study focuses on the research of a new passive roof cooling technique, based on the combination of low emissivity materials and water. A novel roof pond is chosen as the most advantageous in terms of both energy efficiency and less maintenance or functional demands. The pond – referred as “Roof Pond with Gunny Bag” (RPWGB) – is covered by a cloth floating on water level, encouraging evaporative heat losses. A mathematical model describing the energy flux through the RPWGB is developed. The following sensitivity analysis marks the parameters that reduce bottom pond temperature thus improving the efficiency of the system. The experimental study analyses alternative ways to reduce bottom pond temperature. For this purpose, the low emissivity material is placed in different positions, above, below and floating on water level. Heat dissipation occurs by means of radiation losses and water evaporation.     

Effectiveness of nocturnal ventilative passive cooling strategy in the maritime desert climate of the Arabian Gulf region

This paper presents results of the first set of experiments carried out to investigate the potential of nocturnal ventilative cooling techniques in detached residential buildings in the Arabian Gulf region. This experiment is part of a wider research programme in the field of passive solar cooling strategies at the King Faisal University, sponsored by the “Joint United States-Saudi Arabian Program for Cooperation in the field of Solar Energy” (SOLERAS). The experiments were structured to identify the comfort enhancement potential of direct and induced nocturnal convective cooling techniques through the use of controlled ventilation during predetermined periods of favourable ambient conditions. Horizontally placed, vertically hung openings were designed in accordance with the results of the analysis of weather data and located in the North and South walls of the optimum orientation of the full-scale prototype test house. Conventional concrete block, load bearing construction with external insulation and heavy internal thermal mass were used. Fanger Predicted Mean Vote, as a function of dry bulb temperature, wet bulb temperature air velocity, and mean radiant temperature, were calculated and recorded continuously every 3 min of the duration of the experiment. These values have been averaged to evaluate hourly comfort conditions in various zones of the test house. These were compared with the ambient weather conditions during the period of the experiment to evaluate the potential of this passive cooling technique in conserving a very substantial part of the energy wasted in mechanical cooling.     

Freshwater floating-collector-type solar pond

A new type of solar pond is introduced in an effort to provide an inexpensive, renewable heat source, mainly for space-heating purposes. The suggested freshwater solar pond is covered with floating collectors made of insulation boards and thin plastic sheets, and requires very little, if any, constructional work. Forced flow in the collector layer of the pond provides the reduction of the mean collector and pond temperatures and ensures high efficiency. The collectors also provide the necessary thermal and evaporation insulation so that freshwater may be used and the cumbersome and delicate task of salt-gradient maintenance is eliminated.Both simulation and experimental results are reported. A one-dimensional quasi-steady-state simple model is the basis for the simulation. The presented experimental data validate the simulations. Longterm performance characteristics as well as possible modes of seasonal storage utilization of such solar ponds are discussed.     

Passive solar heating of a non-airconditioned building with movable insulation over the roof pond

This paper presents a simple mathematical model for solar space heating in a non-airconditioned building with movable insulation over the roof pond. The building room considered is of rectangular shape (6 m × 5 m × 4 m) based on the ground. The effects of heat conduction to the ground, heat transfers to furnishings and heat losses due to air ventilation/infiltration have been taken into account in the general heat transfer analysis. The day-to-night change of insulation over the roof pond has been represented by a rectangular step function variation of the heat transfer coefficient at the pond's surface. An increase of 3 to 4°C in the room air temperature is achieved by means of movable insulation over the roof pond on a mild winter's day (17th February, 1982) in New Delhi.     

Cooling load reduction of buildings using passive roof options

A model is presented to predict the thermal performance of a building with shading and roof ponds. A computer program is written to calculate hourly cooling load requirements by the numerical solution of the energy balance equation for the building. This simulation is validated for a bare roof by comparison with field data taken from an actual house in Shiraz, Iran. The effectiveness of the different roof options for passive cooling have been examined. Results indicate that for the house under study (of popular size and building material for Shiraz) cooling load demand reductions of 79.0%, 58.1% and 43.6% may be obtained by using shaded-pond, pond, and shaded roofs respectively.     

A water requirement model for salt gradient solar ponds

A model for predicting the salt gradient solar pond (SGSP) area that could be maintained with a given water supply is presented together with several specific applications. For example, based on 30-year average water flows, the model predicts that 1.93 × 10⁹ m² (477,000 acres) of solar ponds, 1.02 × 10⁹ m² (253,000 acres) of evaporation ponds to recycle salt, and 0.51 × 10⁹ m² (125,000 acres) of freshwater storage reservoirs could be maintained at the Great Salt Lake of Utah. Water use requirements per unit of electrical energy from solar ponds are calculated as 600,000 m³/MW·yr. This is roughly 30 times the water evaporated per unit of electrical energy from coal-fired generating plants using wet cooling towers, but substantially less than water evaporation losses per unit of electrical energy produced from typical hydropower dams and reservoirs. It is concluded that water use requirements for solar ponds, although not necessarily prohibitive, are substantial; and in many locations may be the physical factor that limits solar pond development.     

MODELLING AND SIMULATION OF A SOLAR POND-GREENHOUSE SYSTEM

To maintain prescribed temperature levels inside a greenhouse for plant growth, diesel fuel was used in the past, however due to increased fuel prices, greenhouse heating expenses have reached such levels that conservation and alternative methods are becoming attractive. Such alternatives include solar passive heating, and solar ponds.

Although there are several methods of heating a greenhouse, the present paper focused on the potential use of solar ponds as a primary heating system. The possibility of supplying all heating requirements of a greenhouse through a solar pond has been theoretically investigated.

Models to simulate both the solar pond and the greenhouse thermal behavior were developed based on conventional energy balance equations. Numerical techniques were used to estimate daily greenhouse heating and cooling requirements and the performance of the solar pond as a heating system.     

Thermal management of solar photovoltaics modules for enhanced power generation

Industry and government interest in solar energy has increased in recent years in the Middle East. However, despite high levels of solar irradiance in the Arabian Gulf, harsh climatic conditions adversely affect the electrical performance of solar photovoltaics (PV). The objective of this study is to compare the annual performance characteristics of solar PV modules that utilize either sun-tracking or water cooling to increase electrical power generation relative to that of stationary, passively cooled modules in the Middle East climatic conditions. This is achieved using an electro-thermal model developed and validated against experimental data acquired in this study. The model is used to predict the annual electrical power output of a 140 W PV module in Abu Dhabi (24.43°N, 54.45°E) under four operating conditions: (i) stationary geographical south facing orientation with passive air cooling, (ii) sun-tracked orientation with passive air cooling, (iii) stationary geographical south facing orientation with water cooling at ambient air temperature, and (iv) stationary geographical south facing orientation with water refrigerated at either 10 °C or 20 °C below ambient air temperature. For water cooled modules, annual electrical power output increases by 22% for water at ambient air temperature, and by 28% and 31% for water refrigerated at 10 °C and 20 °C below ambient air temperature, respectively. 80% of the annual output enhancement obtained using water cooling occurs between the months of May and October. Finally, whereas the annual yield enhancement obtained with water cooling at ambient air temperature from May to October is of 18% relative to stationary passive cooling conditions, sun-tracking over the complete year produces an enhancement of only 15% relative to stationary passive cooling conditions.     

Ditigal simulation of indoor temperatures of buildings with roof ponds

A theoretical model is presented to predict the thermal performance of a building with roof ponds. Equations have been derived for the estimation of steady periodic heat fluxes through the roof slab and the outer walls. Energy storage and release by the partition walls and the floor has been considered. The other cooling loads have been estimated using the methods recommended in the ASHRAE Guide and Data Book. Hourly indoor temperatures are obtained by the numerical solution of the energy balance equation for the building. The algorithm that has been developed for digital simulation of the indoor temperatures is presented. The effectiveness of different kinds of roof-pond systems, i.e. shaded ponds, “Sky-therm”, etc. for passive coolings have been examined. The studies indicate that the indoor temperatures of a building located in Delhi can be maintained below 30°C in summer while the maximum dry-bulb temperatures are above 40°C.     

被动蒸发冷却技术在双层皮玻璃幕墙的应用

双层皮玻璃幕墙较之传统幕墙更好地为建筑提供夏季隔热和冬季保温。本文提出将被动蒸发冷却技术应用于双层皮玻璃幕墙结构,通过在双层皮玻璃幕墙内设置水被动冷却蒸发系统,当夹层温度过高时水自动喷出或流出,达到降低夹层温度、提高隔热性能的目的,以提高双层皮玻璃幕墙对夏季炎热气候的适应性。     

Prospects and scopes of solar pond: A detailed review

Solar pond is an artificially constructed pond in which significant temperature rises are caused to occur in the lower regions by preventing convection. To prevent convection, salt water is used in the pond. Those ponds are called “salt gradient solar pond”. In the last 15 years, many salt gradient solar ponds varying in size from a few hundred to a few thousand square meters of surface area have been built in a number of countries. Nowadays, mini solar ponds are also being constructed for various thermal applications. In this work, various design of solar pond, prospects to improve performance, factors affecting performance, mode of heat extraction, theoretical simulation, measurement of parameters, economic analysis and its applications are reviewed.     

Heating performance investigation of a bidirectional partition fluid thermal diode

A novel thermal diode, bidirectional partition fluid thermal diode (BPFTD) that is fabricated by integrating a thermal insulation partition and a movable control blade into a water tank, is proposed. The bidirectional configuration allows the BPFTD to serve both passive solar heating in winter and passive cooling in summer. BPFTD heating performances are tested with two side-by-side hot boxes and compared experimentally with a water-wall having optimum thickness. Two stages of experiments are conducted. The first stage is to investigate an appropriate position of BPFTD partition, and the second compares the BPFTD with the water-wall. The test results show that the BPFTD has much better heating performances than the water-wall. Analysis indicates that the BPFTD may increase heat supply by around 140% when a single glazing cover without night insulation is used and by around 70% in case of using a double glazing cover without night insulation.     

Cooling of water flowing through a night sky radiator

Experimental and theoretical studies of nocturnal cooling of water flowing through a night sky radiator unit were carried out. The system used for this study is two parallel plates night sky radiator with the top one (radiator) being a painted black aluminum plate. The radiator plate was covered by a polyethylene windscreen cover. The experimental study was conducted for a gravity flow open loop system. The parameters studied include the effect of average water mass flow rate, uncovered from the ambient versus covered supply warm water tank, and windscreen cover thickness on the performance. A new definition of the performance was expressed by an overall efficiency. A comparison between nocturnal cooling of stagnant and flowing water was also done. In the theoretical study a thorough radiation model was developed which calculates the radiation heat losses from both the cover and the radiator plate considering multiple reflections and absorptions between cover and plate. Energy balance was then applied on the cover, the radiator plate and the water. The cover and insulation were considered one lump each, while each of the radiator plate and the water were divided into discrete number of lumps. Energy storage in the radiator frame and insulation, and water elements was considered. Experimental results were used for verification of the theoretical model.