Abstract: The effect of tube spacing as a design factor in the performance of a natural-circulation solar water heater is investigated for copper, zinc and galvanized steel absorber plates. The collector performance in terms of the collector efficiency and the collector fin efficiency are both obtained theoretically and by a computer-aided simulation based on the Hottel-Whiller model of the system. The result indicates that the tube spacing varies inversely with both the collector efficiency and the fin efficiency for the three absorber plates. Copper shows the best performance while galvanized steel shows the least. Performance is optimized with a tube spacing not exceeding 10 cm irrespective of the absorber plate used.
INTRODUCTION
In the light of the present deplorable state of the Nigeria’s power sector, the ever-increasing cost of fossil fuel and the unquantifiable environmental degradation associated with every stage of its production and utilization, solar energy as a renewable energy is a better option as a supplement or even a substitute to our conventional energy sources (Agbo et al., 2005).
Solar water heaters are classified based on a number of designs, which are adopted to suit specific purposes and climatic conditions. Natural-circulation solar water heater is the simplest form of solar heaters given its simplicity of construction, design, utilization and maintenance. The design choice is based on a number of factors: Economic, climatic, availability of materials, among others. Design factors such as the area of the collector, nature of the absorber plate material, storage tank capacity, etc have been shown to affect the performance of a natural-circulation solar water heater in various forms (Garg, 1987; Agbo and Unachukwu, 2006). Eisenmann et al. (2004) has looked at the relationship between the collector efficiency factor and the material content of the collector.
The present study considers the variation of the collector fin efficiency and collector efficiency with tube spacing for copper, zinc and galvanized steel absorber plates.
MATERIALS AND METHODS
The collector fin efficiency and the overall collector efficiency are considered
as performance parameters of a natural-circulation solar water heater (Agbo
and Unachukwu, 2006). These were respectively evaluated theoretically and also
by using a computer-aided simulation done in MATLAB™ software based on
the Hottel-Whiller model of the system. Figure 1 shows the
schematic diagram of the fin attached to each fluid tube. Thus, fin length per
tube is given as W-D/2 as shown:
| Fig. 1: | Absorber plate and tube dimensions |
The fin efficiency describes the fact that there is a decrease in the magnitude of the heat transferred by the fins when compared to the magnitude of the heat transferred if the fins were all at a constant temperature, TK (Duffie and Beckman, 1974). Thus, fin efficiency F is given as:
In terms of the centre-to-centre tube spacing, the fin efficiency is given
by:
| (1) |
where
As presented by Close (1962), the collector fin efficiency and the tube spacing relate to the overall collector efficiency.
Based on the above models, the variation of the fin efficiency with tube spacing for the three absorber plate materials was investigated. Also, the overall collector efficiency and the mean system temperature as a function of the tube spacing were studied by means of a computer-simulation.
RESULTS AND DISCUSSION
Figure 2 and 3 represent the effects of
collector tube spacing on the collector efficiency and the mean system temperature,
respectively. The results indicate that both of these parameters decrease with
increasing tube spacing. For tube spacing not exceeding 10 cm, these performance
parameters are optimized. The result also shows that a collector efficiency
of up to 80% is possible in a natural-circulation solar water heater. This agrees
with the report of an earlier work done by another author. (Danshehu et al.,
1996).
| Fig. 2: | Effect of collector tube spacing on collector efficiency |
| Fig. 3: | Tube spacing and the mean system temperature |
| Fig. 4: | Fin efficiency against tube spacing for various absorber plate materials |
Figure 4 presents the tube spacing effect on the fin efficiency of the collector for the three absorber plates. Generally for all the absorber plates, collector fin efficiency decreases with increasing collector tube spacing. The result shows that copper plate performs best among the three absorber materials. Zinc performs better than galvanized steel. However, at a certain tube spacing (<10 cm), the fin efficiencies for the three plates are optimized and almost equal. Thus, with this tube spacing the performance of the collector is independent of the nature of absorber plate material in use.
CONCLUSIONS
The collector fin efficiency and the overall collector efficiency, as performance parameters of a natural-circulation solar water heater are dependent on the tube spacing and the nature of the absorber plate material. For the three absorber plate materials considered, the fin efficiency decreases with increasing collector tube spacing. With a tube spacing not exceeding 10 cm, the performance of the system is optimized independent of the nature of the absorber plate. However, for higher values of tube spacing, the type of absorber plate becomes significant. This way, the cost implication of using very highly conductive and expensive absorber plate like copper can be avoided or reduced by using other cheaper and readily available plate instead.