Elsevier

Renewable Energy

Volume 96, Part A, October 2016, Pages 756-764
Renewable Energy

Evaluation of temperature and efficiency in relation to mass flow on a solar flat plate collector in Mexico

https://doi.org/10.1016/j.renene.2016.05.027Get rights and content

Highlights

  • The effect on temperature and efficiency under different flow conditions was studied.

  • Using a small submersible pump, the output temperature was reduced from 90 °C to 45 °C.

  • Working under forced flow, the solar heater increased its efficiency from 0.37 to 0.49.

Abstract

This study presents the thermal analysis of a solar flat plate water heater working under real operating conditions in a hot sub-humid region, in the city of Merida, Yucatan, Mexico. This water heater was evaluated under thermosyphon conditions and, with the use of a submersible pump, under forced flow conditions. In both cases the thermal performance of the collector was studied in order to determine the impact of flow on the working temperature and efficiency of the collector.

A comparison of the temperature values registered for both thermosyphon and forced flow regimes showed that there were significant reductions both in the temperature registered at the outlet of the collector, and in the temperature differential between the inlet and the outlet of the collector. Moreover, greater efficiency values were found when the water heater was working under forced flow.

These results suggest that the use of a submersible pump in a solar flat plate water heater could be an efficient way to control the maximum working temperature in a solar collector in the region, while also providing a positive effect on the daily efficiency.

Introduction

Solar heaters are widely used to heat water given their performance in the use of solar energy. Islam et al. [1] report that from 2007 to 2010 there has been an increase in existing solar hot water capacity from 125 to 185 GW, with an average annual growth rate of 16% in solar hot water/heating between 2005 and 2010. Solar flat plate water heaters are used in both domestic and industrial sectors [2], and represent a viable option in low temperature industrial applications [3]. There are other studies in the literature, which not only analyse the potential for growth of this technology and its applications, but also address the problems of its implementation and perspectives for the future. A number of publications focus on the implementation of solar water heaters in a specific geographical region (see Refs. [4], [5], [6], [7], [8]) given that each region offers a diversity of operating conditions, with its own corresponding problems for the use of water heaters. In the case of this article, the Yucatan region is located in the southeast of Mexico, with a hot sub-humid climate and daily solar incident radiation up to 7 kWh/m2 in summer [9], which is favourable for the use of solar thermal energy technology; however, solar heaters are still not widely used in the region. This may be due to a variety of reasons, such as lack of awareness, high acquisition costs or uncertainty in their operation, and since the water in this region has a high concentration of calcium carbonates, which results in lime deposits in the pipes, there are further doubts regarding the performance of solar heaters in Yucatan. Additionally, it has been found that locally-available high performance solar heaters (such as evacuated tube solar heaters), might not be suitable for the regional conditions, since a water temperature of up to 95 °C [10] has been recorded in the thermo-tank, which is an extremely dangerous temperature for residential use.

Therefore, further research studies are necessary to determine the daily performance of a flat plate heater operating in this region under different conditions. One particular topic of interest is the effect of the flow on temperature and efficiency. It is well known that using forced circulation leads to better performance. Various types of analysis, such as experimental [11], energetic and exergetic [12], demonstrate that increasing the flow rate leads to an increase in the thermal efficiency of the collector. This effect has also been found when testing solar heaters with alternative configurations, such as heaters equipped with rectangle conduits [13], collectors made with a copolymer [14] or with fins attached under various arrays [15], but its effect is uncertain in this region. Moreover, it is important to maintain a low temperature in the collector, particularly in collectors made from plastic materials, such as those proposed by Refs. [14] and [16], where it has been observed that a high temperature can have an adverse effect on a collector made from these materials, giving rise to structural failures [17]. In solar plants mass flow manipulation is used to match the temperature requirements of load processes [18], [19], which has been found to work at a smaller scale also. In laboratory studies, while studying heat transfer enhancement devices in flat-plate solar collectors, Hobbi and Siddiqui [20], found a clear correlation between the temperature in the collector and the magnitude of water flow circulating, with a greater flow resulting in a lower temperature difference between the inlet and outlet of the collector. With this in mind, a reduction in the operating temperature would be useful in order to protect and extend the useful life of this type of collectors, an aspect which has received attention. Wallner et al. [21] conducted a theoretical study on the prevention of overheating in a polymeric flat plate collector implementing thermotropic layers; however, overheating could be avoided by installing a miniature submersible pump in the outlet of the thermo-tank given that a forced flow should result in a reduction of the difference in temperature between the inlet and outlet of the collector.

To the best of the authors' knowledge, no studies have been carried out on the effect of mass flow on the working temperature of solar collectors in the region; thus, the focus of this work was to study, analytically and experimentally, the performance of a solar flat plate water heater operating under real working conditions of thermosyphon and forced flow in the hot sub-humid region of Yucatan over whole days of service.

Section snippets

Experimental set-up

The solar heater (Fig. 1) consists of a storage tank with isolation and a flat plate solar collector located in the city of Merida, Yucatan, at 21°01′47.8″ latitude North and 89°38′17.0″ longitude West, in the Yucatan peninsula.

A 200-L graduated thermo tank was constructed by covering a polypropylene tank, 0.7 m in diameter, with sheets of galvanized metal laminate in the shape of a tetrahedral arrangement with a square base of 0.76 m each side and a height of 0.91 m, which included 25.4 mm of

Analytical study

In this study two types of water circulation tests were carried out: thermosyphon and forced flow (pump). In the second case, the pump was kept functioning the whole day in order to evaluate the analytical equations without the presence of radiation, thus forced flow was required permanently.

Fig. 2 shows the temperatures measured on the collector and the ambient data for one day with thermosyphon (Fig. 2a) and one day with the pump (Fig. 2b).

Fig. 3 presents a comparison of the useful heat rate

Conclusions

In this study a solar flat plate water heater was built and installed in Yucatan, Mexico, where the system was evaluated under working conditions, functioning in two operating regimes: forced flow (under different mass flow values), and thermosyphon. Two submersible pumps, 6W and 14W, were evaluated at the collector inlet to induce a mass flow which was varied on different days, maintaining a constant value, from 5 to 20 gs−1, throughout each day.

Given the temperatures registered, it was

Acknowledgments

The authors would like to thank Ricardo Gamboa-Castellanos and Diego Medina-Carril for their support on the construction and instrumentation of the thermosolar systems, and to CONACYT for the postdoctoral grant provided in the call 2014-4.

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