Elsevier

Renewable Energy

Volume 32, Issue 10, August 2007, Pages 1645-1660
Renewable Energy

Performance analysis of new-design solar air collectors for drying applications

https://doi.org/10.1016/j.renene.2006.08.005Get rights and content

Abstract

This paper presents a performance analysis of four types of air heating flat plate solar collectors: a finned collector with an angle of 75°, a finned collector with an angle of 70°, a collector with tubes, and a base collector. In this study, the first and second laws of efficiencies were determined for the collectors and comparisons were made among them. The results showed that the efficiency depends on the solar radiation and the construction of the solar air collectors. The temperature rise varied almost linearly with the incident radiation. The first law of efficiency changed between 26% and 80% for collector-I, between 26% and 42% for collector-II, between 70% and 60% for collector-III, and between 26% and 64% for collector-IV. The values of second law efficiency varied from 0.27 to 0.64 for all collectors? The highest collector efficiency and air temperature rise were achieved by the finned collector with angle of 75°, whereas the lowest values were obtained for the base collector. The effectiveness order of the collectors was determined as the finned collector with angle of 75°, the finned collector with angle of 70°, the collector with tubes, and the base collector.

Introduction

Owing to rapidly rising oil prices (about $60–65 per barrel for today) and greenhouse gases, clean and renewable energy sources, such as solar energy, are receiving greater attention for heating, cooling, drying and power generation applications in Turkey as in other developing and developed countries. Turkey is an energy importing country; more than half of the energy requirement (65% of the total energy consumption) is supplied by imported fossil fuels. However, this situation is not sustainable and environmentally friendly energy policy for Turkey. Turkey has an important potential for renewable energy sources, especially solar, wind, hydropower, biomass and geothermal energy. For many years, open solar drying for granular materials such nut, walnut, grape, etc. in rural regions of Turkey has an important applications due to a great solar energy potential [1], [2].

There has been an increasing interest in using solar air collectors because (1) preheating of fresh air with solar air collectors is a simple and cheap technology, (2) their maintenance and operation are very easy, (3) they do not require a specialized manpower, (4) they can be produced locally, moreover from locally available materials, (5) they are environmentally friendly, and (6) they do not require fuel. On the other hand, solar air heaters are limited in their thermal performance due to the low density, the small volumetric heat capacity and the small heat conductivity of air. Therefore, several types of solar air collectors have been proposed over the recent years in order to improve their performance [3], [4].

Thermal performance of the solar air collectors depends on the material, shape, dimension and layout of the collector. Performance improvement can be achieved using diverse materials, various shapes and different dimensions and layouts. The modifications to improve the heat transfer coefficient between the absorber plate and air include the use of an absorber with fins attached, corrugated absorber, matrix type absorber, with packed bed, with baffles and different configurations are given in the literature [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. For high solar gains, an efficient thermal coupling between absorber and fluid is required, while the electrical power for the fan operation ought to be as small as possible. Increasing the absorber area or fluid flow heat transfer area will increase the heat transfer to the flowing air, on the other hand, will increase the pressure drop in the collector, thereby increasing the required power consumption to pump the air flow crossing the collector.

Metwally et al. [16] conducted an experimental investigation on an advanced corrugated duct solar collector. The collector was constructed of corrugated surfaces similar to those used for compact heat exchangers, with the air flowing normal to the corrugations. The optimum angle of the triangular collector and the effect of the change of the absorber shape factor on the collector performance were deduced by Kabeel and Mejarik [13]. Pottler et al. [20] determined optimized geometries for a solar ventilation air preheater. A single glazed solar matrix air collector was designed in order to overcome the physical problems of conventional air collectors as well as the technical problems of matrix air collectors by Kolb et al. [15]. Three types of solar air collectors, flat plate, finned, and v-corrugated were analyzed to achieve an efficient design of air collector suitable for a solar dryer [14]. The v-corrugated collector was found to be the most efficient collector and the flat plate the least efficient.

Çomaklı and Yüksel [8] presented the experimental results of four types of solar air collectors. Only the exergetic efficiencies of the collectors were calculated and comparisons were made among them on the basis of the exergetic efficiencies. Moummi et al. [17] conducted experiments in solar air collectors with rectangular plate fins inserted perpendicular to the flow. Thermal performance of a solar air heater having its flow channel packed with Raschig rings was presented by Öztürk and Demirel [18]. The characteristic diameter of the Raschig rings, made of black polyvinyl chloride (PVC) tube, was 50 mm and the depth of the packed-bed in flow channel was 60 mm. In another study, Yeh et al. [22] determined that either increasing the collector aspect ratio for increasing fluid velocity or providing fins attached with baffles on the collector for increasing heat transfer area, as well as producing air turbulence, will improve the collector efficiency. Kurtbas and Durmus [12] investigated five types of solar air collectors which had different front absorption surfaces. The more important parameters in order to decrease the exergy loss were found to be the collector efficiency, temperature difference of the air and the pressure loss.

This paper presents the performance analysis of four types of air heating flat plate solar collectors. The analysis includes both the first law and second law of thermodynamics. The collector efficiencies were determined for four types of solar collectors and comparisons were made among them.

Section snippets

Experimental setup and measuring

The experimental system is composed of basically the air heating flat plate solar collectors and its schematic view is shown in Fig. 1. This figure also shows the locations of the sensors connected to the solar air collectors. The system is comprised of solar air collectors, an air circulation fan, air valves, a multimeter, a pyranometer, and some auxiliary and measurement devices. Four flat plate air collectors are used. Looking from right to left are collectors I–IV. The main features of the

Energy and exergy analysis

The first law efficiency of the solar collectors is defined as the ratio of the energy gain to the solar radiation incident on the collector plane: ηI=Q˙cQ˙s,where Q˙c is the rate of heat transfer to a working fluid in the solar collector, and Q˙s the solar energy absorbed by the solar collector surface and is given by Q˙s=IT(τα)Ac,where IT is the rate of incidence of radiation per unit area of the tilted collector surface, Ac the collector area, and τα the effective product

Results and discussion

Four solar air collectors were investigated in this study. The experiments were carried out under climatic conditions of Erzurum city in Turkey. The experiments were performed from March to July in 2004. The useful heat rate and collector efficiency were calculated directly from the data obtained from each collector. The experimental results are presented in the form of graphs that describe temperature increase across the solar air collectors, insolation, useful heat rate, and collector

Conclusions

In the present study, four solar air collectors were tested and a comparison is made among them on the basis of first and second law efficiencies. The following conclusions can be derived:

  • The efficiency of the solar air collectors depends significantly on the solar radiation and surface geometry of the collectors.

  • The efficiency decreases as the reduced temperature parameter increases, meaning, at higher reduced temperature parameter, the overall loss is lower.

  • The efficiency of all solar air

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