Alok Srivastava

The relative performance of different approaches to the passive cooling of roofs

Abstract This communication presents a discussion of the relative performance of five different approaches to the reduction of heat flux through the roof. These are roof shading by plants, use of removable canvas, evaporative cooling, a roof garden and use of earthen pots over the roof. Explicit expressions have been obtained for the time-dependent heat flux entering into the living space through the roof; the living space is assumed to be at constant temperature (corresponding to air conditioning). Results of numerical calculations corresponding to a typical hot day (16 May 1981) at New Delhi have been presented. For optimum thermal load levelling and least average heat flux into the room, a shaded roof (due to a vegetable pergola) with a water film is the best choice.

Experimental validation of thermal model of open roof pond

Abstract This note presents a simple periodic thermal model for the performance of an open roof pond. The results of this model are seen to be in good agreement with experiment.

Thermal performance of “still on roof” system

This paper presents an analysis of periodic heat flux through a roof, on which a solar still has been placed. It is seen that on a typical hot day in Delhi, for high reflectivity of the bottom of the basin (on account of deposits or otherwise) the daily heat flux in the room gets reduced by 40% and the production of distilled water is 0.6 kg/m2 day; the produced swings in the heat flux also get reduced in magnitude. For a typical cold day in Delhi, if a black dye is mixed with basin water the daily heat flux in the room increases by a factor of two and the yield of the distillate becomes 5.0 kg/m2 day.

Second law (exergy) analysis of various types of coal

Abstract This communication presents a second law, or exergy, analysis of various types of coal from major mines of the world. The customary first law analysis gives only the quantity of energy, while the second law defines the quality of energy also. The projected increase in coal utilization in power plants makes it desirable to evaluate the energy content of coal both quantitatively and qualitatively, which will result in proper design, matchong and installation of equipment.

Design and thermal performance of a passive cooled building for the semiarid climate of India

Abstract This communication presents design and thermal performance of a hostel building using passive cooling approaches at Jodhpur in India. Jodhpur is a representative of the semiarid climate in India where, in summer, on average, ambient air temperature is 34.4 °C, relative humidity is 28% and substantial wind in the southeast direction is present. A thermal model, based on Fourier series (in time) expansions of solar insolation and ambient air temperature, is developed and applied to study the effectiveness of various cooling approaches such as a wind tower, a desert cooling fan and evaporative cooling on the roof. Numerical calculations show that the best cooling occurs in the rooms which are partially underground and when the roof is treated by evaporative cooling. The living room becomes thermally very compartable when roof evaporative cooling is used along with a desert cooling fan.

Thermal design of a roof as an inexpensive solar collector/storage system

This paper presents the thermal performance of a roof as a solar collector/storage system which is important for the thermal design of buildings. The system consists of a mass of concrete or concrete insulation, one face of which is blackened/glazed and exposed to solar radiation and ambient air, while the other is in contact with room air at constant temperature. The heat can be extracted by the passage of water through the network of tubes in this block. It is seen that, by increasing the depth of the tubes, the rise in water temperature decreases but the time difference between the maxima of the solair temperature and that of the outlet water temperature increases. At a tube depth of 0·10 m, the maximum temperature rise of the water is 33·5°C. The corresponding efficiency of the system is 28·0% while the flow rate of water is 5·0 litre/h m2; the heat flux entering the room is also reduced considerably.

An inexpensive solar collector/storage system in the ground

Presents an analysis of an inexpensive solar collector/storage system consisting of a tank or network of pipes buried in the ground; the ground above the pipes/tank may be replaced by concrete, sand or bricks for different applications. The heat can be extracted by flow of a fluid in the manner which keeps the collection temperature constant. The effectiveness of the collection/storage system can be increased by blackening and glazing of the surface of the ground. The authors have derived an expression for the transient rate at which heat can be retrieved to keep the collection temperature constant. Numerical calculations for typical days in Kuwait are reported.

Performance of a constant flow sand solar collector

An analysis is presented for the performance of an inexpensive sand collector for solar energy which has been validated experimentally. The collector consists of a plane network of polythene pipes buried in a sand mass which is kept in a wooden case, the top surface of the sand being fixed by mixing with sodium silicate, blackened with blackboard paint spray and suitably glazed. The heat can be extracted by fluids passing through the pipes. For a 2 cm depth of the plane of heat retrieval and 0·46 litre/min flow rate of water, the collector efficiency is about 55 per cent. The efficiency increases with the flow rate and decreases with the depth of the plane of heat retrieval.

Thermal performance of ground as an inexpensive solar collector and storage system

Abstract This paper presents an analysis of the performance of an inexpensive integrated solar collector and storage system. The flow of fluid and heat capacity of the storage material in the container is considered simultaneously and an evaluation is made for the resulting outlet fluid temperature for various flow rates and materials by solving the Fourier heat conduction equation with appropriate boundary conditions in different regions. This evaluation, combined with energy balance considerations, yields an expression for the transient rate at which heat can be retrieved—by keeping flow rate constant. Numerical calculations, corresponding to a typical winter day in Delhi have been carried out. The heat retrieval efficiencies are of the order of 50-25%. A phase difference of about 12–18 h is obtained between the maxima of solair temperature and outlet water temperature. The efficiency of the system increases with flow rate and decreases with depth of plane of heat retrieval. Concrete is found to be the best inexpensive sensible heat storage material out of those materials considered.

Heating and cooling buildings by flow of water over the roof

This paper presents an analysis for the reduction of the heat flux that occurs by constant flow water in a network of pipes buried in a roof. An air gap is introduced into the roof to achieve maximum thermal load levelling. A periodic solution of the one-dimensional partial differential equation (describing the temperature distribution in the roof) which also satisfies the appropriate boundary conditions, has been obtained in an explicit form. It is seen that, for a hot summers day in New Delhi (the 26th of May. 1978), and for a water flow rate equal to 6 litres/h m2, the heat flux entering the room is reduced, and that the collection efficiencies for a water heating system with glazed/blackened and bare top, surfaces are 49·0% and 8·27%, respectively. For a cold winters day in New Delhi (the 9th of March, 1979), glazing and blackening the top surface of a room causes heating of water along with an enhancement of heat flux coming into the room. The time difference between the maxima of the heat flux entering the room and tha maxima of solair temperature is 8-10 h.