Thermal performance of a packed bed double pass solar air heater with a latent heat storage system: an experimental investigation

Abstract

Purpose Solar energy applications are limited because of its intermittent and discontinuous availability with respect to time. Hence, solar energy thermal conversion systems need integration with thermal storage units (TSUs) to use solar energy in off sunshine hours. This paper aims to perform thermal analysis of a solar air heater (SAH) integrated with a phase change material (PCM)-based TSU to supply hot air during night period. Design/methodology/approach An experimental setup with TSU as main component was prepared with SAH at its upward side, food chamber at its downward side as subcomponents. In TSU, paraffin wax was used as thermal energy storage material. Mass flow rate of air considered as an input parameter in the experiment. Two different absorber plates, namely, plane and ribbed absorber plates were used for the experimentation. Each day for a fixed mass flow of air, observations were made during charging and discharging of PCM. Findings Nusselt number and convection heat transfer coefficients were analytically calculated by considering flow through TSU as external flow over bank of tubes in a rectangular duct. A temperature drop of around 7-8°C during charging of PCM and temperature rise of around 4-5°C during discharging of PCM was observed from the experimental results. The average practical efficiency of TSU with ribbed absorber plate SAH during charging and discharging of PCM was 22 and 6 per cent, respectively, higher than that of TSU with plane absorber plate SAH. Research limitations/implications There are no limitations for research on SAH integrated with TSU. Different PCM including paraffin wax, Glauber’s salt, salt hydrates and water are used for thermal storage. Only limitation is lower efficiency of SAH integrated with TSU because of lower heat transfer coefficients with air as working medium. If it can improve heat transfer coefficients of air then heat transfer rates with these units will be higher. Practical implications There are no practical limitations for research on SAH integrated with TSU. Sophisticated instrumentation is needed to measure flow rates, temperatures and pressure variations of air. Social implications In poultry farms during night, chicks cannot survive at cold climatic conditions. Hence, hot air should be supplied to poultry farms whenever the atmospheric temperature drops. It is proposed that, in combination with TSUs, heat produced by SAH is stored in day time in the form of either sensible or latent heat and is retrieved to provide hot air in the night times. This will reduce total operating costs in poultry farms. Originality/value Conventionally, people are producing hot air by combusting coal in poultry forms. This cost around Rs. 75,000 per month for a batch of 225 to 250 chicks in a poultry form. Hot air could be produced economically during off sunshine hours from SAH integrated with TSU compared to the conventional method of coal burning. Present experimental investigations conducted to fill the literature gap in this area of research and to design a SAH integrated with TSU to produce hot air for poultry forms.

A comprehensive review of experimental investigation procedures and thermal performance enhancement techniques of solar air heaters

“…113 Latent heat storage systems using phasechange materials provide much storage density as well as extended operating hours. 113,114,120,121 The phasechange materials have the fundamental characteristics, such as melting and solidifying, by which they can store and discharge substantial measure of energy. PCMs also have the ability to store energy at constant temperature.…”

“…Therefore, many researchers are working to find the effective ways of increasing paraffin wax thermal conductivity by employing encapsulation technology of wax, 122 nanoparticles in the unit 113,114 and the usage of fins 119 or porous materials (graphite foams 114,118 , and granular carbon 123,124 ) in the paraffin wax. Dosapati et al 120 performed experimental investigation on double-pass packed-bed SAH integrated with PCM-based storage unit obtained average values of efficiency as 68 and 65.8 during charging and discharging of PCM, respectively. Saxena et al 121 performed an experimental study on three different models, incorporating carbon granular powder and paraffin wax under natural convection and forced convection, and found that the combination of carbon granular powder and paraffin wax produced highest thermal efficiency.…”

“…Table 14 represents the summary of various SAH configurations with PCM as heat storage material. From the literature, it is revealed that SAHs assisted with PCM provide high thermal energy storage density compared with sensible heat storage materials [114][115][116][117][118][119][120][121][122][123][124][125] and increased collector working hours. 106,114,116,118,122 The solar air heater assisted with combination of granular carbon and paraffin wax, as shown in Figure 11 exhibited better thermal performance when compared to flat plate with PCM as heat storage…”

“…The PCMs are generally used due to high thermal stability and chemical balance at moderate operating temperatures and are usually introduced with capsules of different shapes under the absorber plate 113 . Latent heat storage systems using phase‐change materials provide much storage density as well as extended operating hours 113,114,120,121 . The phase‐change materials have the fundamental characteristics, such as melting and solidifying, by which they can store and discharge substantial measure of energy.…”