An in depth evaluation of matrix, external upstream and downstream recycles on a double pass flat plate solar air heater efficacy

Ali Ahmadkhani,Gholamabbas Sadeghi,Habibollah Safarzadeh

doi:10.1016/j.tsep.2020.100789

Ali Ahmadkhani, Gholamabbas Sadeghi + Show 1 more

Open Access

https://doi.org/10.1016/j.tsep.2020.100789

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Abstract

In the present study, the thermal and thermohydraulic characteristics of different types of double pass solar air heaters (DPSAHs) containing three flow channels were analytically investigated. The analyses were conducted at air mass flow rates of 0.01, 0.015, 0.025 kg/s and different reflux ratios of 0.1 to 1. The effects of upstream and downstream recycling patterns were analyzed. Moreover, the impacts of matrix placed between the absorber plate and the second glass cover with various porosities, and variation of solar radiation intensity on the DPSAH performance were examined. Furthermore, the pressure drop due to the existence of matrix was considered to obtain more realistic outcomes. The results demonstrated that as to the downward recycling pattern, using matrix leads to an increase in the DPSAH thermal efficiency presenting the thermal efficiency of 79%; however, it brings about a reduction in its thermohydraulic efficiency at high mass flow rates and reflux ratios and high fan power cost is incurred, subsequently. The most compelling result is: if we consider an upstream recycling pattern, it is true that we have overlooked a certain amount of the DPSAH efficiency compared to a matrix-based downward recycling DPSAH (about 7% less efficiency); however, at high mass flow rates and reflux ratios the thermal efficiency of DPSAH is similar to that of a matrix-based upward recycling DPSAH, and in this way, not only the hot air demand is supplied, also the extra fan power cost and the cost of providing a suitable matrix are eliminated.

Highlights

Today, solar air heaters (SAHs) are extensively applied to both in dustrial and domestic applications
The impacts of some performance parameters; such as changing the air mass flow rate, altering the reflux ratio, changing the solar radiation intensity on the collector surface, variation of the matrix porosity, and the pressure drop due to the matrix in the second channel on the double pass solar air heaters (DPSAHs) thermal and thermohydraulic performances were inves tigated
Escalation of the air mass flow rate and the reflux ratio leads to a rise in the thermohydraulic efficiency of a DPSAH

Summary

Introduction

Solar air heaters (SAHs) are extensively applied to both in dustrial and domestic applications They mostly serve the purposes of providing heat for cold climatic conditions, and drying food materials leading to reaching net zero greenhouse gases emission. Zheng et al [1] mathematically investigated the effect of corrugated packing on the thermal and hydraulic efficiencies of an SAH They considered the pressure drop within the flow channel and compared the novel proposed SAH performance to that of unglazed and glazed tran spired solar collectors. Zheng et al [3] numerically and experimentally surveyed the effect of using perforated corrugated absorber plate on the thermal performance of a glazed transpired collector for space heating applications. The results indicated that the maximum energy efficiency of the solar collector is about 78% regarding 141 m3/hr flow rate. Charvat et al [6] used phase change materials for Nomenclature

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