Comparative performance investigation of integrated collector‐storage solar water heaters with various heat loss reduction strategies

A detailed comparative assessment is reported on the thermal performance of integrated collector‐storage (ICS) solar water heaters with various strategies for reducing top heat losses. The objective of this investigation is to assess and compare heat loss reduction strategies. The shape of ICS solar water heater considered in present investigation is rectangular. The thermal performance of the solar water heater is evaluated and analyzed for the following cases: (1) single glass cover without night insulation; (2) single glass cover with night insulation; (3) double glass cover without night insulation; (4) transparent insulation with single glass cover; and (5) insulating baffle plate with single glass cover. Energy balances are developed for each case and solved using a finite difference technique. The numerical assessment of the system performance is performed for a typical July day in Toronto. Each strategy is observed to be beneficial, reducing top heat losses, and improving system performance. The greatest performance enhancements are observed for the water heater with a single glass cover and night insulation and for the system with a double glass cover and without night insulation. Copyright © 2010 John Wiley & Sons, Ltd.     

结构参数对全玻璃真空管太阳热水器夜间热损失的影响研究

文章建立了三维非稳态的全玻璃真空管太阳热水器的数值模型,分析了夜间散热时,该热水器内的流动和传热特征,并且在夜间同一工况下,模拟研究了贮热水箱保温材料的导热系数、保温厚度,以及真空管涂层的发射率对贮热水箱温度、真空管温度和该热水器夜间热损失的影响。分析结果表明:随着散热过程的持续进行,全玻璃真空管太阳热水器内温度分层情况越来越明显,内部流体的流速越来越小,真空管内静滞区域自下往上逐渐扩大;当贮热水箱保温材料的导热系数由0.035 W/(m·℃)减小至0.020 W/(m·℃)时,该热水器的夜间热损失减少了8.5%;当贮热水箱保温厚度由50 mm增加至60 mm时,该热水器的夜间热损失减少了5.0%;当真空管涂层的发射率由0.06减小至0.05时,该热水器的夜间热损失减少了4.0%。     

Thermal optimisation of a built-in storage solar water heater

A simple transient analysis of a built-in storage solar water heater is presented; the results obtained by the present theory are in close agreement with experimental observations as well as with predictions obtained by a more rigorous theory. The water heater consits of an insulated rectangular tank whose top surface is suitably blackened by blackboard paint and then glazed. The unit is exposed to solar radiation during sunshine hours and is covered by adequate insulation at night. The effect of insulation thicknesses (top and bottom insulation) and water mass in the tank is discussed. The importance of using a reflecting sheet instead of insulation is emphasised.     

Short communication simple transient thermal model for solar colector/storage water heaters

This note presents a simple transient model for predicting the thermal performance of some novel solar water heaters which combine both collection and storage of solar energy. These heaters consist of either (i) an insulated rectangular tank whose top surface is blackened and suitably glazed, or (ii) an insulated open shallow tank with black bottom/inner sides and a top glass cover (shallow solar pond). the heaters are adequately covered with an insulation during the night to reduce the heat losses. the proposed model is based on different characteristic equations during sunshine and off-sunshine hours. It is seen that the model predicts the water temperature in close agreement with the experimental observations and earlier theoretical investigations.     

Collector cum storage solar water heaters with and without transparent insulation material

The integrated collector–storage solar water heaters are less expensive and can offer the best alternative for domestic applications particularly to small families to meet hot water requirements. The top heat losses of such solar water heaters are quite high during the night and the temperature of stored hot water is considerably reduced unless covered with extra insulating cover in the evening which is a cumbersome job. The transparent insulation material widely used in Europe for space heating can also minimize top heat losses, if used in such solar water heaters. For this purpose, two units of solar collector cum storage water heaters have been designed to study the relative effect of TI for retaining solar heated hot water for a night duration. Both units were identical in all respects except one of them was covered with TIM. The theoretical exercise was carried out to evaluate design parameters of ISC which revealed total heat loss factor (U_L) 1.03 W/m² K with TIM glazed against 7.06 W/m² K with glass glazed. The TIM glazed has been found to be quite effective as compared to glass glazed SWH and yielded hot water at higher temperature by 8.5 to 9.5°C the next morning. The storage efficiency of such solar water heaters has been found to be 39.8% with TIM glazed as compared to 15.1% without TIM. The TIM glazing means not having to cover the ISC solar water heater with a separate insulator cover in the evening and thus makes its operation much simpler.     

Shallow solar pond water heater: An analytical study

This paper presents a simplified analytical model to predict the performance of a shallow solar pond water heater which combines both collection and storage in a single unit. Essentially, the water heater consists of a metallic (G.I.) tray, whose inner and bottom surfaces are blackened, and is covered with a transparent sheet at the top; the sides and bottom surfaces of the assembly are well insulated. The unit stores a substantial amount of heat for the next day morning's use when the top cover is insulated during the off-sunshine hours. The performance of the water heater, both during the day and night time, can satisfactorily be predicted by this theoretical model. The theoretical calculations are found to be in good agreement with the experimental observations reported earlier.     

Theoretical and experimental studies on collector/storage type solar water heater

Extensive theoretical and experimental studies on a built-in-storage solar water heater which was developed earlier by Garg in India, are carried out. In this water heater the absorber plate performs the dual function of absorbing the solar energy and storing the heated water. In the theoretical study, the transient performance of the system is predicted by solving the mathematical models consisting of energy balance equations which are written on different collector nodes by considering their capacity effects and various heat loss effects. These equations are converted in the finite difference form and then solved by digital computer. Solar radiation and ambient temperature are represented by Fourier series in the theoretical analysis. Its night cooling drawback is somehow checked by covering the collector system by an insulation cover during cooling hours and also by using a insulated baffle plate inside the tank adjacent to the absorber plate. It is observed that by using the insulation cover, the collector performance can be improved by 70 per cent. Use of baffle plate improves the performance during day as well as night time.     

Effect of the baffle plate on transient performance of built-in-storage water heater

This communication presents a study on the effects of variation in the inlet and outlet vent area of the baffle plate, variation in the thickness of upper and lower water columns and use of an insulation cover during the night on the transient variation of water temperature of a built-in-storage solar water heater. The effect of duration of water flow rate in the lower column on its performance has also been studied. Numerical calculations have been made for a typical day in Delhi for December 1979.     

An experimental study on a modified design compact shallow solar pond

In this study a design of compact shallow solar pond modified from that of Kudish (Kudish and Wof, 1978) is studied experimentally. The difference between this design and that of Kudish is that the water depth is allowed to be changed from 2–15 cm and is not fixed as in the mentioned reference design. The thermal insulation of the cover is installed between the reflector and the inner surface of the wooden cover. In the design of the reference (Kudish and Wof, 1978), the insulation is fixed on the outer surface of the cover, where it will probably be damaged by water, wind, dust etc. Also in our design, the level of water in the bag is measured using a static pressure manometer. The tests were carried out both in winter and in summer whereas they have been done only in summer in the mentioned reference. The results show that the compact shallow solar pond can provide a suitable water temperature to be used for low and moderate temperature applications even in a cold winter and with 10 cm water depth.     

Experimental study of an inexpensive solar collector cum storage system for agricultural uses

This paper presents an experimental investigation of an Augmented Integral Rock System. Experiments have been performed on such a solar air heater fabricated at the Institute. Experimental observations of fluid temperature, energy storage and other measures of system performance with variation of air mass flow rate, number of glazings and depth of rock bed are presented. The heat decay characteristics of the rock bed, while using a night insulation cover in addition to glass covers, are also reported in detail.     

Effect of movable insulation on the performance of ground collector

This communication presents an analysis of the thermal performance of an inexpensive underground solar water heater. The system has been studied under two modes of heat collection (a) constant flow rate and (b) constant collection temperature. The effect of variation of different parameters, viz., flow rate of water, depth of plane of heat retrieval, thickness of flowing water column and duration of insulation on top to prevent night heat losses, has also been discussed.     

Transient behaviour of collector/storage solar water heaters for generalised demand patterns

This communication presents a simple transient model for predicting the thermal performance of collector/storage solar water heaters for generalised demand patterns. These heaters consist of either (i) an insulated rectangular metallic tank whose top surface is blackened and suitably glazed (i.e. a built-in storage solar water heater) or (ii) an insulated open shallow tank with black bottom.inner sides and a glass plate at the surface in contact with the water (i.e. a shallow solar pond water heater). The time dependence of the water temperature for the withdrawal of hot water from the system at constant flow rates constantly or intermittently has been explicitly evaluated. Numerical results for the operation of the system in industrial and community service applications are discussed.     

A solar-assisted biogas system: A new approach

In order to increase the production of biogas, particularly in the winter months of northern India, a new design of a collection-cum-storage water heater integrated with the dome of conventional Indian (KVIC) biogas units has been proposed and studied theoretically. During sunshine hours, the biogas system is exposed to solar radiation to increase the slurry temperature, and is covered with movable insulation to avoid night heat losses from the top as well as the sides of the dome. Some interesting conclusions have been drawn.     

Thermal characteristics of a solar tank with aerogel surface insulation

The transient thermal characteristics of a solar tank (a shallow solar-pond water heater) with a silica aerogel surface insulation have been examined both experimentally and theoretically. It is found that the temperature drop of the water within the solar tank after sunset can be appreciably suppressed by introducing the aerogel surface insulation system and that the proposed theoretical model can predict sunny-day hourly variations in the hot-water temperature within the solar tank with acceptable accuracy.     

A low cost collector-cum-storage type solar water heater

A low cost collector-cum-storage type solar water heater has been developed. The cost of the heater is reduced by replacing window glass cover with 0.2 mm thick PVC film to avoid glass breakage in transportation and maintenance. In this paper the performances of solar water heaters having double glazing of PVC and glass have been compared. It was found that their performances are similar. Moreover, by providing an insulating cover in the night, the water remains warm till next morning for taking a bath etc. in the early hours when there is no sunshine.     

带透明蜂窝盖板和辅助反射面的整体式太阳热水器

作者对一种带透明蜂窝盖板和辅助反射面的整体式(ICS)太阳热水器进行了实验研究。该太阳热水器采用截面为三角形的水箱，水箱背面和侧面用30mm聚苯乙烯泡沫隔热，其它两个面为吸热面。底吸热面利用辅助反射面加热，而上吸热面则覆盖5cm的透明蜂窝及2mm的有机玻璃板。这种设计加大了ICS太阳热水器的吸热面积，同时也降低了吸热面向环境的热损。对实验结果的分析表明，该热水器的热效率不高，但保温性能很好。     

A Simple Transient Analysis of an Underground Shallow Solar Pond Water Heater

In this note, a simple transient analysis of an underground shallow solar pond water heater has been analysed. Effect of a movable insulation during off-sunshine hours to reduce the reverse night heat losses from the top of the system as well as intermittent withdrawal of hot water from the system have also been incorporated in the analysis. On the basis of the numerical computation, it is concluded that a quasi-steady state is reached after 3-4 days depending on the flow rate of withdrawal.     

真空管热水器倾角的数值模拟分析

利用fluent软件中的太阳载荷模型对真空管家用太阳热水器进行了三维数值模拟计算,分析了30°、45°、60°倾角下真空管热水器内的流场和温度场随加热时间的变化及传热和流动过程。结果表明,真空管热水器在加热期间,在真空管管口的上壁面出现温度最大值,水箱内真空管管口以下水的温度相对于管口以上的温度分层很明显,水箱内真空管管口以上的热水温度几乎相同,说明水箱内管口以上的水进行了充分混合;对于30°和45°倾角的系统,随着加热时间的增加,水箱内管口以下的水温与管口以上的水温从10 K增加到30 K,以45°倾角为例,当考虑了水箱内管口以下的冷水区时,随着加热时间的增加,真空管与水箱内的温差从0.73 K增大到1.13 K,仅考虑水箱内均匀分布的三点时,随着加热时间的增加,真空管与水箱内的温差从0.15 K减小到了0.03 K;随着系统倾角从30°增加到60°,水箱内管口以下的冷水区域在逐渐减小。因此,为了减小水箱底部的冷水区,插入水箱内的真空管应尽可能短。     

Performance at Colombo, Ceylon, of a pressurised solar water heater of the combined collector and storage type

A solar water heater combining collection and storage has been tested in Ceylon. The heater consists of a square coil of 3 in. diameter pipe, 44·3 ft in length, in a wooden box with heat insulation at the bottom and two glass covers. The glass surface is 20 ft² in area. If water is drawn off whenever the water reaches 120°F, it is possible to obtain 30–50 gal of water a day, the first draw-off being made about noon. The efficiency of collection based on the exposed glass area on the top of the box (which was 1·55 times larger than the horizontally-projected area of the pipe surface) is around 46 per cent.An equation for the performance of the heat collector over the day was devised, and this was applied by means of a computer program to solar radiation data for the whole year. The results showed that no heat could be collected for less than 10 per cent of the year. The total energy collection for the year was about 1250 kWh.Another computer program compared the performance of the heaters made up of 2, , 3, and 4 in. diam pipes respectively. This study indicated that a heater of to in. diam pipe would provide the best performance for most purposes.