Periodic theory of an open roof pond

This paper describes an analysis of the reduction of the heat flux coming into a room through a roof with an open water pond. A periodic analysis of the system, based on the linearisation of Dunkle's⁶ expressions for convective, radiative and evaporative losses, is presented. Numerical computation for the heat flux has been carried out for a typical hot day, the 19th of June, 1979, in New Delhi. These results were compared with those for a water-sprayed system. It was found that, for typical parameters, there are reductions in the maximum heat flux entering the room of 48% and 41% for the roof pond and water-sprayed systems, respectively (for a relative humidity of 0·8). The corresponding reductions in daily heat input into the room are 20% and 35%, respectively. Load levelling is, of course, much better in the case of the open roof pond.     

Non-convective roof pond for passive space heating

This paper reports on an investigation of a passive system comprising a non-convective pond placed on the roof of a building in order to heat the building. Periodic analysis of the solar heat transfer process in the system, which is exposed to solar radiation and atmospheric temperature on one side and is in contact with room air at constant temperature (corresponding to an air-conditioned room) on the other, indicates that this system provides better thermal storage than a thick concrete roof or the convective roof pond system. In sunny winter climates, such a system can provide 100% of all heating needs. A viscosity stabilised non-convective pond of shallow depth (10–15 cm) is envisaged to be the most suitable for the present application.     

Improving the thermal performance of a roof pond system

A roof pond is an effective passive technique for the year-round heating and cooling of buildings. Data obtained from an experimental roof pond building show that the summer performance of such a building can be further improved by allowing vapours to escape from the pond into the atmosphere. Escape of vapours during winter operation can, however, cause deterioration in the performance. It is, therefore, suggeested that a control mechanism be used to allow the vapours to escape only when it is desirable     

Periodic analysis of a ventilated trombe wall

A periodic analysis of a ventilated Trombe wall is presented. The analysis is able to take into account the storage effects and predict the daily or long term performance of the system. The thermal flux entering an air-conditioned space (maintained at 20°C) through the wall has been evaluated. Numerical calculations have been performed corresponding to the meteorological data on 19 January 1981 at New Delhi, a typical cold winter day. Subsequent parametric studies using the simulation indicated that the critical parameters were the mass flow rate of air, depth of plane of heat retrieval and the length of the network of pipe.     

智能型太阳能屋顶系统

太阳能屋顶与建筑结合是太阳能应用领域的新趋势，上海交通大学的太阳能屋顶系统是把太阳能发电与建筑、实用与科研结合在一起的一个实例。同时，为了增强其示范效果，在整体设计时考虑了安装数据采集器及远程监控装置，使其成为一种智能型的太阳能发电系统。本文提出该系统设计思路同大家探讨，以共同促进太阳能建筑事业的发展。１设计目的a解决光伏组件安装在屋面上而不是地面上所带来的一系列问题，以满足建筑一体化的要求；b在有电地区做到最大限度地降低造价；c如何配置更精确的数据处理系统。２设计方案为了达到上述目的，采取了如下…     

Winter temperature control in a non-airconditioned building with non-convective roof pond

This paper presents an investigation of the internal thermal environment of a building having a non-convective roof pond. A self consistent periodic heat transfer analysis has been developed to evaluate the indoor heat flux and the resultant inside air temperature. The effects of air ventilation/infiltration, furnishings (assumed to be isothermal masses) and the basement ground heat storage have been incorporated in the analysis. Numerical computations, corresponding to a typical winter day at New Delhi, have been made to illustrate the analytical results. It is found that the placement of a non-convective roof pond of about 10 cm depth considerably enhances the heat flux entering the living space and increases the indoor temperature to comfortable conditions in mild winter climates.     

Design and control tradeoffs for rockbins in passively solar heated houses with trombe walls, direct gain and high solar fractions

Performances of active and passive rockbins are compared in Albuquerque, NM, Santa Maria, CA, and Madison, WI. The basic house is assumed to contain both Trombe wall and direct gain. The Trombe wall and direct gain are assumed to be optimally sized and controlled for each weather zone. It is demonstrated that, provided charging is done from the trombe wall, rockbins can be used to advantage in reducing the early morning auxiliary energy consumption peak common to passive houses with night setback thermostats. The paper also analyzes performance sensitivity to rockbin configuration (active, radiant slab top down charge, radiant slab bottom up charge), and to control strategies for charging and discharging. Effects of fan energy are included.     

Performance of antisolar insulated roof system

Rooms with concrete slab roofs directly exposed to the sun become unbearably hot during summer and very cold during winter. Huge amounts of energy are required to keep them comfortable. Application of thermal insulation on roofs significantly reduces energy required for heating and cooling. The effectiveness of roof insulations may be further enhanced if a layer of antisolar coating is applied on top of the insulation. The antisolar coating reflects most of the incident sunlight and prevents the roof from heating up. This reduces the daily cycles of thermal expansion and contraction which cause cracks in the roof slabs for the rainwater to leak through. The antisolar coating prolongs the useful life of the building structure as well as the life of the insulation that evaporates with heat. The method of application of the antisolar coating has been specially developed to eliminate thermal bridges formed between the edges of the tiles. This report presents the results of an experiment conducted at the Attock Refinery Limited (ARL) Rawalpindi to assess the performance of the antisolar insulated roof system. Record of the room temperature before and after the installation of the system shows a significant reduction in the indoor temperature. The room occupants, who used to experience a very high thermal stress after 10:30 am in spite of the 1.5-ton air conditioner operating in the room, felt much relieved after the installation. They had to turn back the thermostat of the air conditioner and even had to switch it off occasionally. A detailed thermal analysis of the room shows that cost of an antisolar system is paid back in less than a year in the form of savings of energy required for air-conditioning in summer and for gas heating in winter. In addition, the system prevents the addition of 150 kg per year of green house gases to the atmosphere for each square meter of the area covered by the system. It also provides a quieter environment by reducing the operational duration of the air-conditioning and gas heating appliances.     

Variation of discomfort with south window area and insulation on walls/roof

Dependence of the discomfort index on the south window area and the cost of optimally distributed insulation over the walls and roof (corresponding to different available investments) of a non-air-conditioned one room building is investigated. It is assumed that, in the winter, the wooden shutters provided in the window are opened during the day and are closed during the night, and that, in the summer, the shutters are closed during the day and are opened during the night. It is seen that the discomfort decreases with increasing window area and an increasing amount of insulation.     

Effect of sloping walls on salt concentration profile in a solar pond

The effect of sloping walls on the salt concentration profile in solar ponds is studied. The variation of the area of the pond at different depths is expressed in terms of the top surface area and a single non-dimensional parameter defined in terms of the geometrical characteristics of the pond. This variation is then introduced into the differential equation governing the upward salt diffusion. The dependence of the molecular diffusivity of salt on temperature and the resulting vertical variation of the molecular diffusivity in solar ponds with sloping walls is also considered. The differential equation is then solved and the general solution for the salt concentration as a function of depth is obtained. Results for different pond configurations and also different top and bottom salt concentrations are presented and discussed. It is shown that as a result of sloping walls the density gradient in the top region assumes a smaller value than at the bottom of the solar ponds. If this effect is not considered in the design of solar ponds the density gradient in the top region may decrease well below the stability limit which can then result in an undesired growth of the top mixed layer.     

Performance analysis of a bittern-based solar pond

An attempt has been made to analyze the performance of the bittern-based solar pond of a 1600-m² area located in Bhavnagar, India (latitude 21°, 45'N and longitude 72°, 12'E). Solar radiation transmission in the pond has been measured with a silicon solar cell module and the results are used in the calculation. Thermal efficiency of the pond is worked out by the correlations proposed by Kooi and Hull. Its value is very very low and reasons for this are discussed in the text. Theoretical temperature profiles in the NCZ and optimum thickness of the NCZ are calculated based on the correlations developed by Kooi. Calculated temperature profiles and observed profiles in NCZ match quite satisfactorily.     

Experimental studies on a novel roof pond configuration for the cooling of buildings

A new evaporation based passive cooling technology was tested. The technology is based on the exposure of “floating” wetted cloth to the ambient air. It was compared to various other passive cooling techniques, with very favorable results. Two identical shallow ponds were constructed. One of them was covered with white cotton towels stretched on a densely perforated PVC panel supported by pieces of waterproof polystyrene keeping it just floating on the water surface. Five comparable experiments of different cooling techniques have been carried out. The results indicate that the new cooling technique performed slightly better than the pond with movable insulation, which is widely considered as one of the best roof cooling techniques based on evaporation. It seems that the higher efficiency of the tested technique is due to the thermal stratification created in the water inside the pond, which more effectively resists the transfer of heat gains from the sun and ambient air into the deep water of the pond. In turn, the water temperature near the floor of the pond is lower, thus heat flow from the building to the pond is increased. During the experiment, all the ponds which were compared were ranked according to performance (from best to worse): shaded pond with towels floated on it, pond with towels floated on it and pond with movable insulation, shaded open pond, open pond, covered pond.     

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.     

Performance estimation and experimental measurements of a photovoltaic roof

The market for photovoltaic systems is rapidly expanding. Currently, there are a few large utility photovoltaic power plants, thousands of residential systems, and tens of thousands of remote power systems in use. Even if photovoltaics is a technology that has already demonstrated its effectiveness and holds great promise in electrical generation, the costs are still too high to guarantee a commercial competitivity.The study presented in this paper is part of a European XVII Thermie project entitled “Pilot project for photovoltaic, energetic and biohousing retrieval in a school”, the aim of which was to install a photovoltaic plant and solar air collectors coupled with a sun breaker structure at a scientific high school in Umbertide, in central Italy. A 15 kWp photovoltaic power plant was mounted on the roofs of two blocks of the school building in spring 2001. The system consists of 220 modules for a total of 22 arrays, which are connected to inverters to allow conventional appliances to be powered by photovoltaic electricity.The photovoltaic plant is remotely controlled and data on sun radiation, ambient temperature, modules temperature and power production are continuously acquired by a PC. The measured power plant performances during the year are presented in this paper.Furthermore, the climate in the area has been simulated through the available experimental data and the system behavior under these conditions is predicted. The experimental data have been used to validate a predicting numerical model for photovoltaic plants performance.     

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.     

Performance assessment of a magnesium chloride saturated solar pond

This paper deals with the experimental investigation of a magnesium chloride saturated solar pond and its performance evaluation through energy and exergy efficiencies. The solar pond system is filled with magnesium chloride containing water to form layers with varying densities. A solar pond generally consists of three zones, and the densities of these zones increase from the top convective zone to the bottom storage zone. The incoming solar radiation is absorbed by salty water (with magnesium chloride) which eventually increases the temperature of the storage zone. The high-temperature salty water at the bottom of the solar pond remains much denser than the salty water in the upper layers. Thus, the convective heat losses are prevented by gradient layers. The experimental temperature changes of the solar pond are measured by using thermocouples from August to November. The densities of the layers are also measured and analysed by taking samples from at the same point of the temperature sensors. The energy and exergy content distributions are determined for the heat storage zone and the non-convective zone. The maximum exergy destructions and losses appear to be 79.05 MJ for the heat storage zone and 175.01 MJ for the non-convective zone in August. The energy and exergy efficiencies of the solar pond are defined as a function of solar radiation and temperatures. As a result, the maximum energy and exergy efficiencies are found to be 27.41% and 26.04% for the heat storage zone, 19.71% and 17.45% for the non-convective zone in August, respectively.     

Thermal load levelling of heat flux in PCCM collection/storage walls/roof system

In this communication, a steady state analysis of a phase change component material (PCCM) collection-cum-storage walls/roof system has been presented by incorporating the effect of water flow through a parallel plate placed at the liquid and solid interface. The effect of various parameters, i.e. fraction of melted region, walls/roof thickness, flow rate and latent heat of fusion of PCCM on heat flux entering the air conditioned room has been studied in detail. Numerical calculations have been made for two typical days at Delhi (18 June 1983 and 18 January 1984). The following conclusions have been made: (i) thermal load levelling decreases with an increase of the fraction of melted region and vice versa for total thickness of walls/roof; and (ii) phase shift of 10 h is observed for 40–50% fraction of melted region for walls/roof thickness of 0.10 m.     

Performance of an inexpensive underground shallow solar pond water heater

In this communication, an inexpensive underground shallow solar pond (SSP) water heater has been suggested, and its performance has also been studied in detail. An explicit analytical expression for temperature has been obtained. The effect of various parameters, viz duration of covering the system by insulation, duration of flow rate, flow rate, insulation thickness, water mass, etc. have also been discussed. Numerical calculations have been made for a typical cold day.