A simple graphical method for sizing flat-plate solar air heaters based on transversal and longitudinal aspect ratios

Abstract

Solar air heaters (SAHs) are widely used for drying vegetables and fruits or for domestic heating. Certain sizing parameters are necessary to obtain the right dimensions for the required air temperature, flow rate, and thus useful thermal energy. The well-known Hottel–Whillier–Bliss equation was used and made dimensionless by applying the collector longitudinal and transversal aspect ratios (r_l and r_t) of a double-glazed flat plate solar air heater (DG-FPSAH). The steady-state equations are solved to determine the average temperatures. Thereafter, one could calculate the overall loss heat coefficient and efficiency factor, obtained analytically. A Matlab code was developed to estimate primarily unknown temperatures, useful energy, and the Nusselt number. An iterative numerical method is used until convergence occurs. The inlet cross-sectional area and air flow velocity are defined as input data. The proposed sizing method depends on the output temperature required by the customer. This temperature can be determined from the plotted curves of the dimensionless ratios. Hence, the SAH-needed dimensions are determined graphically depending on the functional requirements for construction planning, such as technology choice, work breakdown, and budgeting. In the present case study, based on the input parameters, an airflow rate of 1.2 kg/s entering a DG-FPSAH with an output temperature of 41.5°C yields the dimensions of L_in = 3.824 m, W_in = 2.735 m, and H_in = 0.1825, specifying the collector length, width, and air duct height. The gathered energy and thermohydraulic efficiency are: Q_u = 7.91 kW, η_col = 0.752, respectively.