Construction, Fabrication and Performance Analysis of an Indigenously Built Serpentine Type

Thermosyphon Solar Water Heater Md. Sayeed-Ur-Rahim Mahadi#1, Muhammad Faisal Hasan#2, Akil Ahammed#3, Mohammad Tawheed Kibria#4,

Saiful Huque#5 # Institute of Renewable Energy, University of Dhaka,

Dhaka-1000, Bangladesh. 1sayeed.mahadi@gmail.com

2faisal.buet@gmail.com 3q.gar.bd@gmail.com

4kibria.tawheed@gmail.com 5saiful.ire@du.ac.bd

Abstract— A serpentine type thermosyphon flat-plate solar water heater has been constructed and fabricated with locally available material. The hourly system performance of the solar water heater has been experimentally investigated for several days in Dhaka city (lat 23.7° N, long 90.38° E) of Bangladesh. The single glazed collector was made from copper sheet and tubes. The collector was tilted at an angle of 27 degree facing south on a supporting structure. The working fluid of the closed thermosyphon heated at the serpentine type flat-plate collector enters the water tank due to density gradient caused by temperature differences. Such collectors are adequate for water heating and this at present is an economical way to use solar energy for water heating. In performance evaluation of the serpentine type solar water heater, inlet and outlet water temperature of collector and tank water temperature was recorded for consecutive hours. Maximum collector outlet water temperature obtained was 67°C and maximum tank water temperature obtained was 61°C during June-July period. From the collected data, various temperature graphs are plotted and analyzed for different solar time, various amount solar insolation and different weather condition. A pH test was also carried out to study the pH value of water within the aluminium built water tank. Some recommendations are made after evaluating the performance of thermosyphon, thermal insulation of water tank, collector radiation absorption, effects of glazing and losses of the collector. This is a field level experiment to improve the device performance and cost reduction for the construction. Conclusion can be drawn after evaluating the performance over a certain period of time for different designs and modifications. Keywords— Serpentine type thermosyphon flat-plate solar water heater, Thermosyphon, Flat-plate collector, Insulated tank and pipe, Solar Water heater, solar collector.

I. INTRODUCTION It has become obvious that fossil fuels resources are fast

depleting and are gradually coming to an end. Conventional energy has also created many problems. The most serious of these are they are finite, they cannot be regenerated and their harmful effect on the environment. Now man embarks on the search for alternative sources of energy. The primary sources

of alternative energy that hold potential for future the solar energy is one of them. At earth’s surface the solar radiation varies over the day and the year. In Bangladesh, monthly averaged daily solar radiation is in the range 4.0-4.5 kWh/m2-day [1].

The per capita energy consumption in our country is one of the lowest amongst the sub continent [2]. In peak demand conditions energy usage should be appropriate and meticulous. Our technology for energy harvest, conversion and efficient application is still lacking behind with that used in developed countries. We should give emphasis for the development of indigenous technology. In Bangladesh there is ample thermal energy available from the sunshine [3]. So, solar water heating for domestic as well as industrial usage could be an effective way of saving conventional energy in Bangladesh [4]. Utilization of solar thermal energy is relatively in a nascent state in our country. Therefore, before taking any wider initiative in this field smaller units should be experimented first in terms of performance evaluation [5].

A typical solar water heater consists of a hot water storage tank and heat collectors. A black absorbing surface (a selective chemical coating surface), inside the collector, absorbs energy from solar radiation and transfers the energy to water flowing through the pipes. Hot water is less dense or lighter than cold water. This hot and lighter water naturally moves to the upper part of the storage tank which is insulated to prevent heat loss. Therefore in a tank containing water at different temperatures, the hot water will tend to rise to the top and cold water will tend to sink to the bottom. The simplest type of collector in a solar water heating system is the flat plate collector. The two basic designs of flat-plate collectors are the header and riser type and the serpentine type [6]. A serpentine type flat-plate solar collector is very significant due to its wide range of potential application. It is basically a black surface which is called collector that is placed at a convenient angle to the daily motion of the sun for collecting maximum solar radiation, and provided with a transparent cover for trapping the heat; appropriate insulation for reducing

heat losses around the sides, top and rear can quite effectively act as an energy converter. Water is used as heat transfer fluid which moves through the tubes due to density gradient caused by temperature differences. Such collectors are adequate for heating a working fluid at temperature around 60°C to 70°C and at present it is the economical way to use solar energy for water heating. In this experiment, a serpentine type thermosyphon flat-plate solar water heater has been constructed and fabricated with locally available material which has an insulated storage tank of capacity 120 litre (maximum) and has a serpentine collector coated with ordinary black coating. This system is a passive natural circulation heating system or non-pump system which works using heat transfer technique, designed on a thermosyphon mechanism. It is basically useful for household work when the normal water needs to be heated.

This project work has been conducted employing single-phase heat transfer process in a solar flat-plate collector using insulated water tank and insulated connecting pipe. There is a still scope to reduce a huge amount of heat loss from the tank and also from the connecting pipe to the environment. The ultimate result is the enhancement of water temperature and the efficiency. Our objectives are to construct, fabricate and install a low cost thermosyphon serpentine type solar water heater to utilize solar energy for water heating and to analyse the performance of the system with insulated tank and pipe.

II. EXPERIMENTAL SETUP The general assembly incorporates two major components-

the collector, and the storage tank [5]. Collector: The collector receives the sun’s radiation and

heats the water. A serpentine type flat-plate solar collector is constructed indigenously for this experiment. Its main components are:-

a) Absorber Plate: The main purpose of the absorber is to receive and absorb the solar energy. To develop serpentine type collector, absorber plate is made corrugated to which copper tubes attached. The absorber plate material is 32 gauge copper sheet. A black paint coating of high-absorptance and low emittance was used for further improvement.

b) Tubes or channels: Copper tube of 10mm inner diameter and 1.5 mm thickness is used to circulate the water. The tubes are attached to the plate by gas welding.

c) Transparent cover or glazing: The radiation transmitting cover allows solar energy to pass through (convection) and traps the heat into the system but minimizes heat loss from the absorber. We have used a 5 mm thick, 105cm × 81cm tempered glass sheet as our cover.

d) Heat-transport fluid: The fluid, flowing through tubes, receives heat from the absorber plate. Water is being used as heat transfer fluid.

e) Heat insulating backing: Its main purpose is to provide thermal insulation to the system which minimizes the heat loss from the back and sides of the collector. Glass wool and cork sheet were used for insulation purpose.

Fig. 1 Fabricated copper tubes of Fig. 2 Top view of experimented Solar Collector Collector

Collector Housing or container: The exterior box, which integrates the other components that make up the collector. The box is made of a layered assembly of rubber and metallic sheet (aluminium) compacted by sealing gasket. The whole collector setup is shown in Fig. 3.

Water Storage Tank: A rectangular tank of 61cm length, 45 cm width and 45 cm height has been used for the purpose of supply and storing hot water in the closed loop thermosiphon system in this experiment. It is a multilayered tank with the outer covering made of aluminium sheet with powder coating. The innermost layer is made of aluminium sheet, while the space in between is filled with layers of glass wool, cotton and cork sheet, as seen in Fig. 4. The intermediate assembly is provided for insulation purpose.

Fig. 3 Top view of indigenously built serpentine type thermosyphon solar

collector

Fig. 4 Cross section of water storage tank

A close loop water circulation system has been introduced into the system. The close loop system allows the transport fluid (water) to flow from the storage tank to the collector. The working fluid then is heated up through solar insolation and circulates back to the storage tank while cold water from the storage replaces it [5]. Schematic diagram of the single-phase thermosyphon type flat–plate collector with insulated water tank is shown in the Fig. 5.

Fig. 5 Schematic diagram of the experimental setup of the solar water heating

system

Fig. 6 Insulated (Glass wool) inlet and outlet pipe

Fig. 7 Experimental setup of the solar Fig. 8 Insulated water storage tank water heating system. (Complete system)

III. WORKING PROCEDURE

The serpentine thermosyphon system is installed at Institute of Renewable Energy (IRE), University of Dhaka, Bangladesh, where the average solar radiation is available in the range of 4.5-5.5 kWh/m2-day. The serpentine collector has been settled at a tilted angle of 27 degree (on 06 June, 2013) facing south on a supporting structure after construction. The angle of tilt is determined through optimization. The setup does not allow flexibility in axis tracking. So a fixed angle of tilt is determined which allows maximum absorption of solar radiation throughout the year. IRE lies in 23.7° latitude east. The general angle of tilt is to be in the range of (latitude ± 15) degrees [7]. Considering this, the collector has been set at an angle of 27° with respect to the ground level. During the experiment, instantaneous solar flux (global radiation) was measured by a Pyranometer (KIPP&ZONEN DELFT/Holland, Model CM11, and Sensitivity 5.02 x 10-6 V/Wm-2). At about 60 minute’s interval, instantaneous solar insolation and water temperature in the water tank is recorded carefully. The study was done for several days throughout the month of June and July to get a better comparison. Data has been collected for different weather conditions as well as with different amount of storage water.

Water samples have been collected for every experimentation day with different acquired water temperatures. These sample water has been subjected to various tests to realize any kind of contamination from the storage unit, or the absorber unit due to solar radiation and system material.

IV. EXPERIMENTAL RESULTS AND DISCUSSION Under various conditions a series of experiments were

conducted and a range of conclusions were drawn from them. Firstly, to realize the maximum achievable water temperature, the setup was exposed to solar radiation over a period of one

month. Data were collected at regular intervals on the days of experimentation, at different weather conditions for various volumes like 20, 40, 60, 80, 100 litters in water storage tank. The first experiment was conducted on 10th June, 2013. The collector was then exposed to solar insolation from 10 a.m. to 6 p.m. Maximum water temperature was found to be 61°C. It was observed that collector outlet temperature heats up the storage water and loses its temperature. In the end of the day, collector outlet temperature is lower than the storage water temperature due to insufficient solar insolation. The initial water that was drained from the storage tank gave a considerable lower temperature (referred to as ‘collected water temperature). The data obtained for a typical sunny day are shown in Table 1 and Table 2. The maximum water temperature acquired dwindled noticeably with the change in the weather condition as seen from Fig. 15. Temperatures obtained were comparatively much higher on sunny days than those found in rainy or stormy condition.

Table 1: Performance of the solar collector over 8 hours for a typical sunny day (Continued)

Date: 16/06/2013 Storage Water Volume = 60L Time Solar

Insolation W/m2

Ambient Temperature (°C)

Water temperature in Collector(°C) Inlet Outlet T

10:00 637 30 36 50 14 11:00 756 33 37 55 18 12:00 836 34 38 62 24 13:00 916 36 43 67 24 14:00 876 34 45 63 18 15:00 796 33 45 55 10 16:00 512 31 43 49 6 17:00 199 30 43 44 1 18:00 99 30 41 41 0

Table 2: Performance of the water storage tank over 8 hours on the same date (16/06/2013)

Date: 16/06/2013 Storage Water Volume = 60L Time Solar

Insolation W/m2

Ambient Temperature (°C)

Tank water temperature(°C) Upper surface

Lower surface

10:00 637 30 43 40 Maximum Temp = 55°C

11:00 756 33 47 41 12:00 836 34 50 42 13:00 916 36 55 46 14:00 876 34 54 49 15:00 796 33 50 47 16:00 512 31 50 46 17:00 199 30 49 46 18:00 99 30 49 46

As can be seen from the table 1, the outlet water

temperature in the collector was more than 50 oC when the solar insolation was high. The highest water temperature

(more than 60oC) in the collector was obtained around the solar noon. The temperature difference between the inlet and outlet of the collector was also higher during this time. A diminishing tendency in temperature difference was seen as the sun moves toward horizon. Due to heat loss and lower value of global radiation the collector performance was very low at the time of sunset.

From table 2, we see that there was a considerable variation of temperature between the upper surface water and the lower surface water inside the water tank. Obviously this temperature gradient is the sole cause of the thermosyphon effect. The temperature gradient was found maximum around solar noon. A maximum upper surface water temperature of 55 oC was obtained at 13:00 hours. This temperature difference diminished with the reduction of solar insolation value.

01020304050607080

Tem

per

atu

re(°

C)

Time (Hours)

Temperature Vs Time Graph for Solar Collector

Ambient Temperature

Water Temperature in Collector Inlet

Water Temperature in Collector Outlet

Fig. 9 Hourly variation of inlet and outlet water temperature of solar collector for a typical sunny day

0

10

20

30

40

50

60

Tem

per

atu

re(°

C)

Time (Hours)

Temperature Vs Time Graph for Insulated Water Tank

Ambient Temperature

Insulated Tank Upper Surface Temperature

Insulated Tank Lower Surface Temperature

Fig. 10 Hourly variation of upper and lower surface water temperature of water storage tank containing 60 litre water for a typical sunny day

Fig. 11 Variation of average inlet and outlet temperature of solar collector with solar insolation

Various performance indicating graphs, obtained through series of experiments, are shown here. Fig. 9 shows the average temperature in collector inlet and outlet as well as ambient temperature variation throughout the day. Fig. 10 shows the similar temperature vs. time curve for insulated water tank. Average inlet and outlet water temperature variation in the solar collector with respect to solar insolation is shown in Fig. 11. It is observed that the water temperature from the collector outlet increases with the increased solar insolation. It is also seen that storage water temperature decreases with time due to heat loss from the water storage tank. From each of the three figures it can be said that the Solar Water Heater system performance was high, when the insolation was high, mostly around the solar noon.

Fig. 12 Hourly variation of collected water temperature from water storage tank containing 60 litre water on different dates

Fig. 13 Storage water volume vs. Average maximum collected water temperature curve

Fig. 14 Hourly variation of water temperature of collected water from water storage tank for different water volumes

Fig. 12 depicts collected water temperature variation with time in different dates. The output temperature was found similar in different dates.

The collected water temperature is dependent on the storage water volume, which can be realized from Fig. 13. As the storage water volume increases more heat from the sun is required to raise the water temperature, which resulted in lower output water temperature with the increase in storage volume.

And Fig. 14 depicts the performance of the water heater in the form of output water temperature vs. storage volume. The output water temperature is higher for lower storage volume. An optimization of desired output water temperature and required water volume can be done by this graph. A selection of desired temperature and required quantity of water could be based on this graph.

Fig. 15 Variation of average maximum temperature of collected water from water storage tank containing 60 litre water in different weather conditions

Fig. 15 shows the variation of average maximum temperature of collected water from water storage tank at different weather conditions. The maximum temperature is obtained at very sunny day and logically it is reduced for sunny, mildly sunny, cloudy, windy, rainy and stormy condition respectively. A final experimentation was conducted where 60 litre water was drawn out and refilled instantaneously over a 6 hour

period at a 0.15litre/min rate, and at a regular interval of 0.5 hour. The maximum water temperature found under this test was 45°C. Around $250 US dollar equivalent amount has been spent for the construction and fabrication of this serpentine type Solar water Heater System.

A. Water treatment Water samples have been collected at different stages of the

experimentation period with varying degrees of water temperature. The pH values of the samples were found to increase steadily with time. The initial water with which the experiment was started had a temperature of 30.5 °C, pH value of 6.9. The final sample that was obtained on the eighth day of the experiment had a temperature of 55 °C, pH value of 7.1. The maximum temperature acquired throughout the experiment was 67 °C. This sample had pH value of 7.6.

To compare with a practical case, water from the same input source (temperature 31°C, pH 6.9) was manually heated up to 67 °C with the help of an electric water heater. This heated sample gave a pH value of 7.2.

V. CONCLUSION AND FUTURE WORKS In this experiment, this serpentine type solar water heater

provides a maximum temperature of 61°C and a maximum rise of about 30° C within a span of four hours in a typical sunny day (solar insolation is of the range of 700 - 1000 W/m2-hr). In normal condition (average solar insolation is of the range of 600 W/m2-hr), it has been able to steadily provide about 22°C rise in water temperature. Maximum collected water temperature can be obtained in high solar insolation. It is observed that collected water temperature increases gradually as storage water amount was reduced from maximum 100 litres to 20 litres. With ambient temperature in the range of 30-32°C, the maximum increase in water temperature was about 12°C at a 9litre/hour circulation rate. Heat loss during sunless hours (6:00 pm - 10:00 am) was measured on several days with varying degrees of weather condition. Better insulation is to be introduced in order to minimize the heat loss parameter. In this work, the pH readings obtained were all within safety range of usable water. We can obtain a highly efficient solar water heater by using different materials (which is not easily available and a little bit costly), but the cost of the system will be high and go beyond the reach of user. So, we used some low cost material (which is locally available and a little bit cheap) to reduce the cost as well as to obtain the performance at an acceptable level for user. It is worth mentioning that the collected water temperature and instantaneous efficiency of the system with insulated tank and pipe are more than that of the system with non-insulated tank and pipe. It can be also added that the performance of the collector with glazing is far better than that of the collector without glazing.

This type of solar heating system can be used as an alternative to electric water heater if desired temperature is less than 60°- 65° C. This is an ongoing experiment. The results obtained till now are promising. However data have only been collected for a period of two months. To realize

actual performance efficiency, data have to be collected throughout the year, especially for the three distinct seasons - winter (November - February), summer (March - July), and autumn (August - October). Performance variation with and without glazing/insulation is another aspect that should be measured to realize economic efficiency. Structural adjustment in the form of providing variable axis of tilt can be introduced to acquire solar energy more proficiently.

ACKNOWLEDGMENT The authors are grateful to the authority and staff members

of Institute of Renewable Energy, University of Dhaka for allowing to perform the experiment in their premises.

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