Experimental Investigation of a Passive Thermo Siphon Solar Heating System with Phase Change Material in Makurdi

Transcription

1 Nigerian Journal of Solar Energy, Vol. 27, 216. Solar Energy Society of Nigeria (SESN) 216. All rights reserved. Experimental Investigation of a Passive Thermo Siphon Solar Heating System with Phase Change Material in Makurdi 1 *Kuhe, A., 1 Edeoja, A.O. and 2 Itodo, I.N. 1 Department of Mechanical Engineering, University of Agriculture, Makurdi 2 Department of Agricultural and Environmental Engineering, University of Agriculture, Makurdi Abstract - Solar water heating is a process of tapping energy from the sun for the purpose of raising the temperature of water from local water supply to some desirable higher temperature. This paper discusses improving the performance of a flat plate collector by introducing a phase change material (bee wax) to a flat plate collector heating system incorporating the thermosyphon principle. A solar water heater was designed, constructed and tested in Makurdi, North Central Nigeria. Tests were conducted using copper pipes in header and riser with different dimensions. The tests involved the measurement of ambient, water outlet and other collector temperatures as well as the wind speed and relative humidity. The mean collector efficiency of about 7 % and maximum outlet temperature of about 7 C represent appreciable improvement in performance over the conventional flat plate collector water heating systems previously reported for Makurdi for similar periods of the year. Keywords: Thermo-siphon, phase change material, Bee wax, flat plate collector, passive system, renewable energy. 1. INTRODUCTION Solar energy is one of the renewable energy resources that hold a great potential for developed and developing countries in the future. Nowadays, it is being widely used for both heating and electricity generation. For heating applications, solar water heating systems can therefore be a cost-effective way to generate hot water. They can be used in any climate, and the fuel they use the most, sunshine, is absolutely free (Revees, 29). Currently in Nigeria, most households in the urban areas use electricity, kerosene and wood for heating water. These options have their attendant adverse environmental and economic effects. The need to therefore introduce a cost effective and sustainable alternative has in recent times become imperative. The ever increasing cost of electricity in Nigeria coupled with depleting nature of our cur forests which is a major source of fuel wood are pointers to this assertion (Agbo and Unachukwu, 27). Many different designs of solar water heating systems are possible and they are classified as either passive or active and direct or indirect (Kreider and Kreith, 1981; Agbo, 26). Passive systems rely on natural convection created by buoyance effects which is as a result of density difference to circulate the water through the collectors. Thermosyphon systems are passive systems. Active systems use electrically driven pumps and valves to control the circulation of the heat absorbing liquid. This allows greater flexibility than their *Corresponding author Tel: amkuhe@uam.edu.ng 1 passive counterparts since the hot water storage tank does not have to be above or near the collectors. Also, active systems are designed to operate year round without any danger of freezing. In indirect or closed-loop systems, the heat transfer fluid is treated water, a refrigerant, or a non-freezing liquid such as an antifreeze solution, hydrocarbon oil, or silicone. The heat it draws from the absorber plate is transferred to the portable water through a heat exchanger such as a coil either inside or wrapped around the storage tank. (Kreider and Kreith, 1981). A passive solar design works without any pumps or electric components. This makes them less vulnerable to mishaps, easier to maintain, and possibly longer-lasting than active systems. The simplicity also tends to mean that they are less expensive than active systems, but the trade-off is that they are typically less efficient (Marshall, 29). According to (Kishor et al., 21), flatplate collectors are the most economical and popular among domestic solar water heating systems since they are permanently fixed in position, have simple construction, and require less maintenance. The collector, which is the most critical part of the solar heating system, is where solar radiation is absorbed and heat energy is transferred to the fluid in the risers. The flat plate collector absorbs both beam and diffuse radiation, as such it still functions when beam radiation is cut off by cloud covering. This advantage, together with favorable cost are the reasons why flat plate solar collectors are generally used in thermosyphons. The performance and environmental life cycle analysis of thermosyphon solar water heaters using

2 various types of collectors including flat-plate, compound parabolic, evacuated tube, parabolic trough, Fresnel lens, parabolic dish and heliostat field collectors was reported by Kalogirou (24). He reported an optical, thermal and thermodynamic analysis of the collectors and a description of the methods used to evaluate their performance. The thermal performance of the solar collector was determined by obtaining values of instantaneous efficiency for different combinations of incident radiation, ambient temperature, and inlet fluid temperature. Design and experimental analysis of flow inside the collector of a natural circulation solar water heater had been performed and the result shown was that the system performance depends very much on both the flow rate through the collector and the incident solar radiation and the system exhibited optimum flow rate of.1 kg/s-m 2 (Bukola, 26). A side by side experimental investigation was performed to evaluate the influence of the thermal performance of solar domestic hot water systems. The system was a direct solar hot water system utilizing a natural circulation return tube to the storage tank. Result of the system show improvements in the overall system performance as a result of lowering the collector fluid flow rate (Dilip et al., 212; Fanney and Klein, 1998). The results of an experimental study conducted in a water solar flat plate collector with laminar flow conditions to analyze the flow distribution through the collector. LDA (equivalent pipe length, pipe diameter and cross sectional area) measurements were carried out to determine the discharge in each riser, as well as pressure measurements to investigate the relation between junction losses and the local Reynolds number. Analytical calculations based on the measured relations are used in a sensitivity analysis to explain the various possible flow distributions in solar collectors (Volker et al., 22; Chaouchi et al., 27; Edeoja and Ikpambese, 215). An annual simulation to monitor the thermal performance of a direct solar domestic hot water system operated under several controlled strategies has been performed (Sarmal et al., 214; Sivakumarl et al., 212). According to Duffie and Beckman (1991), higher flow rate leads to higher collector efficiency factor. However, it also leads to higher mixing in tank and therefore, a reduction in the overall solar water heating system efficiency (Ashikur et al., 25). Experiment of several collectors with parallel connection and which can be interpreted as a single collector where the number of risers were multiplied by the number of collectors were analyzed. Studies have shown the importance of performance improvement of the collector in solar water heating. In these studies fluid flow systems of a density gradient solar water heater were designed and constructed with the aim to reduce the cost and to bring out better efficiency (Wang and Wu, 199; Kalogirou, 24; Rehim and Lasheen, 27). 2 In an attempt to improve of the efficiency of the thermo syphon system, phase change materials have been incorporated in the collector system (Mohammed and Farshad, 211). Phase change materials (PCM) have been used to improve the productivity of solar stills (Muftah et al, 214) when applied in latent heat thermal energy storage systems (LHTESS). The PCM melts before the temperature of the absorber temperature rises to the melting temperature of the PCM. During phase change, heat will be stored in the melted PCM as a sensible heat. After the peak hour of solar radiation when the solar radiation begins to drop, the molten PCM continue to transfer heat to the concentrator absorber and from the latter to the risers until the molten PCM changes to solid phase. In other words, the PCM will act as a heat source for the water in the risers during low intensity solar radiation periods. This method relies on heat being release from the bottom of absorber of flat plate of the thermosyphon as latent heat. Bee wax was selected for use in this study as the phase change material in attempt to increase the temperature of the absorber plate and water flowing through the risers. This wax was packed underneath the plate of the collector to serve as heat retention material. The properties appear to be good since its latent heat of fusion is J/Kg and it melting point range from C. This implies that at 62 C the wax melt and produce enough heat underneath the plate of the collector, thereby allowing the water to absorb more heat, since the heat will absorb through the absorber plate of the collector, which will lead to increase in temperature of the hot water produce. The application of the wax also keys into the principle of converting waste to wealth while enhance environmental quality. Several researchers have conducted studies on solar thermosyphon systems in Nigeria. However, documented works on performance analysis of an improved thermosyphon system in Nigeria and Benue state in particular are not rare. It, therefore, calls for immediate need to conduct a study on how to improve the efficiency of thermosyphon system in Nigeria can be achieved. Although projects have been carried out to improve efficiency of solar thermosyphon system but there still exist the challenge of getting high efficiency from sun rise to sun set (Edeoja et al., 213; Edeoja et al., 29a). The objective of this work is to improve the efficiency of a thermosyphon solar water heating system under Makurdi conditions by packing a phase change material, beewax, under the collector absorber. 2. MATERIALS AND METHOD Materials selection and primary consideration were focused on ultimate cost of delivered energy in the design and fabrication of the thermo syphon system. The respective materials were duly selected taking into consideration the various inherent factors. Plane window glass of dimensions mm was selected for

3 the glazing. The absorber was made of aluminium sheet painted black. The casing was of plywood of thickness 12 mm. Fiber glass of thickness 2 mm was used as the insulation layer. The storage tank was made of 2 mm mild steel sheet. Copper pipes of 25 mm and 12.5 mm were used for the header and runners of the collector. About 2.4 kg of bee wax was used as the phase change material packed underneath the collector absorber and placed in an insulated casing with a glass cover. A water storage tank placed above the collector. The water gets heated up and flows into a storage tank through thermosyphon principle. Heat retention in the tank which acts a reservoir is dependent on insulation (lagging) of the tank. The performance of the thermosyphon system depends upon the size and capacity of the storage tank, the thermal capacity of the collector, and the connecting pipes including fluid flow and on the pattern of hot water use. All components were designed for and constructed in line with the design values obtained. The system was tested on a normal sunny day and cloudy day between the hours of 9: a.m. to 4: p.m. Solar radiation passes through the glass and strikes the flat black surface of the absorber plate where the solar energy is absorbed as heat (i.e., by increasing the internal energy). This causes the flat-plate collector to become very hot, and so the water contained in the risers and headers bounded to the plate absorb the heat by conduction. The water inside the tubes (risers/headers) expands and so becomes less dense than the cold water from the storage cylinder. By the principle of thermosyphon, hot water is pushed through the collector and rises by natural convection to the storage tank and cold water from the tank simultaneously descends to the bottom header of the collector by gravity. Therefore, there is circulation as a result of an increase in temperature and volume of the warmer water to the top of the storage tank. The circulation continues as hot water goes out, while cold water comes in. The domestic solar water heater is divided into the following components, namely storage tank, absorber plate and fluid passage pipes. The efficiency of the collector can be predicted from equation 1 (Duffie and Beckman, 1991). The collector area can be obtained from equation 2. where useful thermal energy, solar radiation and lost thermal energy and collector area or aperture. The collector area can be obtained from equation 2 (Ogie et al., 213). where actual thermal energy required to increase the temperature of the water in the collector. The volume of water in the collector was computed from equation 3 (Ogie et al, 213). 3 where, Reynold s number, viscosity of the water, density of water and pipe diameter. The actual thermal energy absorbed is given by equation 4. (4) where specific heat capacity of water, are the respective outlet and inlet temperatures and time. The results were then used to select the collector area used which makes allowance for losses. The diameter of the cold water storage tank was computed using equation 5 (Bolaji, 26) by fixing the height as 6 mm. The pressure within the cold and hot water tanks at full capacity are given by equations 6 and 7 (Bolaji, 26). (6) (7) where and are the densities of cold and hot water respectively. By conducting a stress analysis on the tanks, the spacing between the pipes and hence the number of run of pipes on the absorber were obtained. The size of pipe and length was then obtained. In the selection of materials needed for construction stage of this system, two essential factors, namely, the economic consideration (cost) and properties of the materials were considered. A transparent cover is needed in solar collector to help provide the greenhouse effect necessary to heat up the water. A good cover material (transparent cover) should have high transmittance to ultraviolet radiation and low transmittance to infra-red radiation in order to trap the radiated heat from the absorber plate. Low-cost glass was preferred to plastic as the cover material in this work because it has high transmittance to visible light, low transmittance to infrared radiation, and stability. It suppresses the convective and radiative losses from the top of the solar collector plate. Aluminum was selected for absorber plate because it is relatively cheap compared to copper, has good means of welding or attachment to other materials despite its low weldability property, and has good thermal conductivity. The insulating material used for the set-up was fiber glass which has very low thermal conductivity and was obtained from old ovens and other heating systems. Plywood was used in the construction of the collector casing because of its low conductivity and ease of fabrication into the required shape and size. Honey wax used underneath the absorber plate of the collector to serve as heat retention material to enable

4 temperature stability in flat plate collector. Bee wax possess some properties that seem to be good since its latent heat of fusion is J/kg and it melting point range from C. This implies that at 62 C the wax melt and produce enough heat underneath the plate of the collector. The bee wax was used underneath collector because of its ability to melt within a short time when exposed to thermal energy. It also has the ability to store thermal energy during the phase change, known as latent heat thermal storage. These two unique properties informed the decision to consider it as heat retention material. Copper pipes were used due to its high conductivity and resistance to corrosion. This is very important since they function to hold the water to be used domestically. The water tank is made of rolled galvanized mild steel plate from the design calculation of the 2-liter tank, properly lagged to prevent heat loss to the environment. In order to obtain an optimum slope of collector for the thermosyphon system, the collector has to be set to an angle that will yield the optimum result in Makurdi location. This was achieved by using the acceptable relationship which gives collector tilt angle (β) = latitude The ambient temperature, inlet and outlet temperature of the collector, inner and outer glass surface temperature and the temperatures of the plate and air in the collector were measured with thermocouple at intervals of 3 minutes from 9. to 16. hours. The solar radiation was measured with the aid of a solarimeter. Wind speed was measured with the aid of wind gauge. The relative humidity was measured using a humidity meter. The daily collector efficiency was then computed. Figure 1 shows the side and front view of the system. (i) (ii) Fig. 1: Side view (i) and Front view (ii) of the thermo-siphon water heater 3. RESULTS AND DISCUSSION Figure 2 shows the mean hourly variation of the measured solar radiation with time of the day. It shows the usual trend of radiation distribution on a normal day. The trend indicates that solar radiation was relatively high between 11:3 am to 2:3 pm. The peak value of solar radiation was recorded between 1: pm to 2: pm. Hence, the temperature of the hot water produced from the collector is usually high within this period. Figure 3 which show the mean hourly distribution of the outlet water temperature. It confirms that the higher water temperatures correspond to the peak period of solar radiation (Edeoja et al., 29b; Edeoja et al., 29c). The maximum mean outlet water temperature recorded was 6 C as shown in figure 3. This is appreciable considering the fact that the study was conducted in January during the harmattan period with the attendant hazy and dusty weather. 4

5 Collector Efficiency Outlet Temperature ( C) Radiation (W/m 2 ) Time of the Day Fig. 2: Hourly variation of mean solar radiation Time of the Day Fig. 3: Hourly variation of mean outlet temperature Figure 4 shows the distribution of the daily collector efficiency. The maximum efficiency was observed during the course of day 1 of the experiment. The efficiency was 71% and the corresponding maximum temperature of the hot water and plate obtain were 7 C and 96 C respectively. This shows a reasonable improvement compared to values obtained by using other configurations within the location (Edeoja et al., 215; Ibrahim et al., 29). The improvement could be attributed to the incorporation of the phase change material (Mohammed and Farshad, 211; Farshad and Mohammed, 21). It was also observed that the temperature of the plate is fairly stable during the experiment. The reason is because of the improved heat Days Fig. 4: Daily variation of collector efficiency 5 retention caused by phase change material used underneath the plate of the collector. The fluctuations of the efficiency values reflects the performance of the system over the interval as shown in figure 4. Collector efficiency drops over time as system temperature increases (Agbo and Unachukwu, 27). The maximum daily average collector efficiency recorded over the six day interval is 71% on day 1 and the minimum on day 6, 67.5%. According to Agbo and Unachukwu (27), the heat loss factor increases with increasing system temperature, thus resulting in a decrease in the collector efficiency over time. Following from this, the collector efficiency can be improved if the heat loss factor is minimized.by a suitable choice of design materials and specifications.

6 Radiation (W/m 2 ) Collector Efficiency Figure 5 shows the distribution of the measured solar radiation collector efficiency with time of the day. The distribution indicates that high values of solar radiation do not necessarily translate to higher efficiency. Factors such as the geometry of the collector and the material selection also play the vital roles. The use of the phase change material may be seen to have impacted on the performance of the system in two ways from the figure. First, the efficiency values were fairly stable fluctuating between 6 and 8 %. This stability can be traced to the effect of the bee wax in stabilizing the heat transfer process by its energy retention capacity. Secondly, the high efficiency values recorded between 9. and 1. hours are also indicative of this quality of the phase change material insolation efficiency 9: 1: 11: 12: 1: 2: 3: 4: Time of the Day Fig. 5: Variation of mean solar radiation and collector efficiency with time. 4. CONCLUSION The current study has shown that honey wax which is usually regarded as waste product after extraction of honey can positively influence the performance of solar thermal systems in Makurdi location. The results of the study show an improvement in the performance of a thermo-siphon water heating system in terms of collector efficiency and outlet water temperature. Further work will be attempted in other aspects of solar thermal applications such as water distillation with the incorporation of bee wax as a heat retaining medium. REFERENCES Agbo, S.N. (26). Effect of Hot Water Withdrawal Rate on the Mean System Temperature of a Thermosyphon Solar Water Heater; The Pacific Journal of Science and Technology, 7(2): Agbo, S.N. and Unachukwu, G.O. (27). Design and Performance features of a thermosyphon solar water heater for an average size family in Nsukka Urban. Trend in Applied Science Research, 2 (3): Ashikur, M.D., Rahman, K. and Ashraful, I. (25). Performance analysis of a single-phase thermo-syphon system in solar water heating. Proceedings of the 6 th International Conference on Mechanical Engineering (ICME), 28-3 December 25, Dhaka, Bangladesh. Bukola O. B. (26). Flow design and collector 6 performance of a natural circulation solar water heater. Journal of Engineering and Applied Sciences, 1(1): Chaouchi, B., Zrelli, A. and Gabsi, S. (27). Desalination of brackish water by means of a parabolic solar concentrator, Desalination, 217, Dilip, J., Ashok, Y. and Ravi, V. (212). Study of solar water heaters based on exergy analysis. YMCA University of Science & Technology, Faridabad, Haryana. Duffie, J.A. and Beckman, W.A. (1991). Solar Engineering of Thermal processes. 2nd Ed. John Wiley and Sons New York, USA. p Edeoja, A. O., Aliyu, S. J. and Ameh, J. A. (215). Small Scale Performance Evaluation of A Multi- Effect Humidification-Dehumidification System in Makurdi. American Journal of Engineering Research (AJER), 4(7), Edeoja, A. O., Bam, S. A. and Edeoja, J. A. (29a). Comparison of a Flat Plate Collector and a Manually- Tracking Concentrating Collector for Air Heating in Makurdi, Journal of Science and Technology Research, 8(2). Edeoja, A. O., Ibrahim, J. S. and Ekoja, M. (29b). Effect of a Thermal Storage Medium on the Performance of a Basin Solar Still in Makurdi. International Journal of Engineering Science, 1(2).

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