Parametric effects on the performance of flat plate collectors of a solar water heating system

Parametric effects on the performance of flat plate collectors of a solar water heating system

C.B Joshi, T.R Bajracharya, B.R Maharjan, H.M Nakarmi, N.L Shrestha

Department of Mechanical Engineering Pulchowk Campus, IOE, TU

GPO Box 1175, Pulchowk, Lalitpur ,Nepal

“Corresponding author, FAX: 977 1 5525830, email address: triratna@ioe.edu.np

ABSTRACT

Among all water heating devices that are based on renewable energy sources, solar water heating systems are the most popular ones. As a result, uses of such systems have been increasing significantly. However, no in depth attempt has been made to analyze and determine the performance of these systems in Nepal. Such kind of endeavor is, however, necessary to explore the possibilities of such systems. This will bring benefits to all concerned. In this study, a benign attempt has been made to determine the combine effects of different interdependent parameters on the efficiency of Flat Plate Collector, the heart of the system, by using analytical method and computer program. Based on it, some parametric changes in a Flat Plate Collector have been made and their effect on its performance has been studied. The findings of the study that the performance of the prevailing Flat Plate Collector of thermo-syphon type Solar Water Heating System has been found satisfactory with instantaneous efficiency of 61.43% and daily average efficiency of 37.86%. Its efficiency has found to be increased by 67.47% by changing some of the major independent parameters.

1.0 Background

Nepal relies heavily on its traditional energy resources such as fuel wood agricultural energy requirements; water-heating is the main one. Hot water is being used in almost every sectors of the life. The useful energy consumption for water heating on rural residential, urban residential, commercial and industrial sectors in 1911/92 were 6, 71,456 GJ, 1, 17,970 GJ, and 80,000 GJ respectively [1]. Presently the energy requirement for this purpose is being met through electric geysers, liquefied petroleum gas (LPG), kerosene etc. In the urban areas and agricultural wastes, animal wastes, fuel, woods and charcoal in the rural areas. Hot water in industrial sectors is used for process heat applications and the one in commercial sector is used for general purposes. It is produce by using electrical geysers and fuel oil-fired boilers. This has led to large scale demand for electricity and petroleum products. Besides, excessive use of firewood and fossil fuel has resulted into excessive environmental pollution. Furthermore, since fossils fuels are not available in the country, expenditure on the import of petroleum products has drained the country’s limited foreign exchange reserves. On the other hand, in spite of the large hydro power resource, the cost of the electricity in Nepal is still one of the highest in the world. So using electricity for heating water is highly uneconomic in the context is an everlasting, non-pollution and freely locally available source of energy, can be used easily and comfortably for providing hot water.
 So far as water heating, use of Solar Water Heaters (SWH) in household sector, commercial sector and public sectors saves in an average 20%, 42%, and 40% of the total electricity consumption respectively[2]. The entire scenario indicates that the use of SWH can be economical and hence benefices to their users on long run and helps preserve country’s ecology, environment and economy. In the view of all above alternatives; we presently have, for heating water.

In spite of all these positive aspects of SWHs very little effort has been made in their development in Nepal as compared to the one made in other countries. Thus the status of their technology, deigns etc. have remained almost unchanged. Likewise, the type of their users has also not changed much. A as result, the performance of SWHs presently available in the local markets is visibly decreasing, The main reason behind this scenario is the lack of theoretical and technical knowledge among the ever growing new manufactures whose products are nothing but the copied version of others. If solar energy has to be harnessed in a greater extend, efforts should be made to provide the concerned entrepreneurs of this sectors with the updated information and guidance on the basis of knowledge and recent development in SHW [3].

2.0 Parameters

A large number of parameters the performance of any flat plate collector used in solar water heating system. The major parameters are glazing, tube spacing, absorber plate thickness, plate spacing, absorber plate coating, collector tilt, dust on glazing, insulation, bonding between absorber plate and tubes and fluid inlet temperature. In the present study of tube spacing between absorber plate to glazing, thickness of bottom insulation, thickness of side insulation and in a design of a collector have been studied.

The study is carried out on specific date and time with fixed radiation data for a collector with the constant area and is based on assumption that the installation is ideal. Parametric changes are carried out only by dimensional variations.

3.0 Theoretical Analysis

The analysis based on the standard mathematical equation and empirical relations have been carried out, using computer program. The graphical representations of the analysis are presented below.

3.1 Tube Spacing

Fig: 1 Effect of tube-to-tube spacing on efficiency for different glazing systems.

Fig.1. shows that as tube spacing increases efficiency increases efficiency for all glazing system decreases. The main reason behind it is as tube-to-tube spacing increases, the mean plate temperature also increases resulting in to loss due to radiation and convection. The rate of decrease of efficiency of single glazing system. This is due to the long wave transmittance of single glazing system.

The figure also shows that the rate of decrease in efficiency foe a single glazing system glazing system is higher than that of double and triple glazing with the increasing tube spacing. For tube spacing greater than 385 mm, double glazing system gives higher efficiency than the single one.

The instantaneous efficiency for the prevailing single glazing FPC having an average tube spacing 100 mm, has been found to be 61.43%. The efficiency increases significantly as the tube spacing decreases. The rate of increase in efficiency decreases however as the tube spacing decreases below 45 mm.

3.2 Absorber Plate Thickness

Fig: 2 Effect of absorber plate thickness on efficiency for different glazing systems.

Fig. 2 shows that with the increase in plate thickness of the collector the efficiency increases. This trend is highly pertinent for the plate thickness up to 0.5 mm for a single glazing system and less for double and triple ones. This range of plate thickness, the efficiency does not increase significantly for all the three systems. For the same plate thickness, the efficiency of a single glazing system. In the prevailing FPC with single glazing systems the average plate thickness is 0.977 mm. The corresponding efficiency of such system has been found to be 61.43%. If the plate thickness increases beyond this thickness, the efficiency increases only insignificantly. For higher efficiency; it is recommended that the plate thickness be in the range of 0.3 mm to 1mm depending upon the number of glazing provided it is safe for practical purposes.

3.3 Plate to Glazing Spacing

Fig: 3 Effect of Plate to glazing space on efficiency for different glazing systems

Fig.3 Shows that for single glazing system, at lower plate to glazing spacing, efficiency, is low due to glazing spacing, efficiency is low due to high conduction heat transfer from air inside the collector. The efficiency increases rapidly up to 11mm spacing down from that point to 21 mm spacing. At 11 mm spacing efficiency reached maximum. Because at this spacing the convection heat loss is suppressed and the heat transfer mechanism through the gap is by radiation and convection only. After that, efficiency is approximately constant with increasing spacing. At high spacing heat transfer coefficient between plate and glazing becomes low but shading factor goes high so efficiency goes down. For double and triple glazing system, top loss coefficient is low but shading area of collector becomes high. Therefore, for achieving the maximum efficiency the spacing should be in the range of approximately 18mm for all numbers of glazing.

3.4 Glazing Thickness

Fig: 4 Effect of grazing thickness on efficiency for different glazing systems

Fig.4 shows that as glazing thickness increases efficiency decreases for all types of glazing system due to decrease in transmissivity of the 3-4mm glazing thickness is used, for that thickness the efficiency varies from 63.51-61.44 %. While selecting the thickness of glazing strength, impact and vibration capacity are to be considered. Hence in the present study of thickness of 4 mm has been used.

3.5 Bottom Insulation Thickness

Fig: 5 Effect of bottom insulation thickness on efficiency for different glazing systems

Fig.5 shows that from a certain value of bottom insulation thickness i.e. 60 mm increase in efficiency is small with respect to increase in thickness .The bottom heat loss coefficient play a significant role in total heat loss coefficient for small insulation thickness. However, from 70mm thickness, increase in insulation thickness has less significant role in total heat loss coefficient. Therefore, for high efficiency, it is recommended that the bottom insulation of 40-60 mm be used.

3.6 Side Insulation Thickness

Fig.6 Effect of side insulation Thickness efficiency for different glazing system

Fig 6 indicates that for all types of glazing system efficiency increases rapidly, when insulation thickness increases to 30 mm. However, beyond the insulation thickness of 50 mm increase in the thickness of side insulation does not increase efficiency significantly. Then after efficiency decreases with higher insulation thickness due it decreases the aperture area of the collector.

4.0 Proposed Changes in Parameters and Their Results

Based on above analysis change in performance of a FPC with new figures in proposed as shown in table 1.

Table 1

Parameters	Proposed	Prevailing
Tube to tube spacing	45 mm	100 mm
Absorber plate thickness	1 mm	0.98 mm
Plate to glazing spacing	18 mm	25 mm
Glazing thickness	3 mm	4 mm
Bottom insulation thickness	50 mm	65 mm
Side insulation thickness	50 mm	65 mm

Same mathematical model and program, as used for the analysis of prevailing FPC was used for the calculation. Following results has been obtained.

In prevailing FPC, mean plate temperature was 47.30°C, overall heat was 780.98 W, mass flow rate was 0.04189 kg/s and efficiency was 61.43 %. In FPC with the proposed changes in parameters, mean plate temperature had been changed to 41.72°C, overall heat loss coefficient was 6.62 W/m²K, useful heat gain was 904.53 W, mass flow rate was 0.03701 kg/s and efficiency was 67.47%. These results were obtained by changing all parameters. It was found that tube spacing ad glazing thickness are major parameters that increased the efficiency of proposed one. By changing tube-to-tube spacing to 45mm glazing thickness to 3mm, efficiency was found 65.52% and 63.45% respectively.

5.0 Conclusions

From the analysis, the efficiency of prevailing FPC is found not so poor; still it is less compare to the once achieve elsewhere. This is mainly due to absorber plate materials, glazing materials, tube material, bonding technology between plates and tube materials, insulation material, and absorber profile. Efficiency of collector in the range of 70-80% was reported. It is higher than prevailing FPC used in Nepal due to use of convection suppression device between glazing and absorber plate, selective coating on absorber plate, and highly conductive material for tube and plate.

The efficiency of FPC with the proposed changes in parameters was still lower than that of achieved elsewhere. Further improvement on efficiency can be obtained by using reflective foil inside the wall selection of optimum tilt angle, using insulation with low thermo conductivity, decreasing bond resistance between tube and plate, using glazing with high short wave transmittance and low for long wave heat radiation.

References

[1] Central Bureau of Statistics, National Planning Commission Secretariat, HMG-Nepal, Statistical Pocket Book Nepal 2002, 2002

[2] Joshi, C.B and Maharjan, R., ‘Solar Water Heating Technology in Nepal’, Science Universal, Vol.4, No.4, December, 1994

[3] Water and Energy Commission Secretariat, Final Report For Inventory On solar Water Heating System and its Technological Assessment for Household Adoption in Nepal-HMG/N, 1998