Phase change heat transfer in a rectangular enclosure as a function of inclination and fin placement

Abstract

In this paper, melting of a phase change material (PCM) inside a rectangular enclosure, possibly finned and inclined, is studied numerically. The application of this work is related to the temperature control of a finned PV panel filled with PCM and installed at different tilt angles. The studied system is modeled as a 2D rectangular enclosure filled with PCM (RT25) and packed between two aluminum plates, where the front side is exposed to a constant heat flux of 1000 W/m² for 2 h. Four geometries were considered including a non-finned PCM enclosure, a PCM enclosure with one centered full-width fin, one half-width fin attached to the front plate, and one half-width fin attached to the back plate. Results have shown that the most efficient thermal management of the PV-PCM panel is obtained when the PCM enclosure is equipped with a full-width fin simultaneously attached to the front and back plates. With such a PV panel design, the PCM melting is dominated by natural convection heat transfer from both sides of the PCM enclosure at an early stage, with added heat losses from the back plate to the external environment. Accordingly, low values of the front and back plates temperatures can be maintained during a stabilization time of 80 min as long as the tilt angle is varied from 0° to 75° from the vertical. The efficient temperature control resulting from the full-width fin geometry is mainly related to the high overall heat transfer coefficient obtained during the whole melting process.

Introduction

The electrical efficiency of a solar photovoltaic (PV) device is closely related to its operating temperature. Depending upon the type of solar cell and the climatic conditions, 6–25% of the incident solar radiation is converted to electricity whereas the rest is transformed to heat, which can result in an excessive increase of the PV module temperature and a significant decrease of its efficiency in the absence of an adequate cooling method [1]. In case of solar PV panels, a drop in the conversion rate of approximately 0.3–0.65%/K over a nominal cell operating temperature of 298.15 K has been reported in the literature [2,3]. Among the various developed solutions to improve the thermal management of solar PV panels, the integration of a phase change material (PCM) on the back of the device has been proven to be an effective method of passive temperature control [2].

During the sunshine hours, the temperature of the PV panel can be kept slightly above the melting temperature of the PCM as the excess heat is stored in the PCM undergoing melting in a nearly isothermal process. When the sun goes down and the PV panel temperature decreases, PCM solidification takes place and the stored energy is released back into the environment. The reduction in the PV panel temperature depends on the amount of PCM used, its thermophysical properties as well as the design of the PCM filled cavity. In many studies [[3], [4], [5]], RT25, a commercial paraffin-based PCM was selected due to its low melting temperature barely exceeding the PV characterization temperature and its high latent heat of fusion. This PCM was then used to fill a rectangular cavity in contact with the rear surface of the PV panel.

Huang et al. [4] were the first to develop a finite volume numerical model based on a full-sized PV-PCM panel geometry, and validated it successfully with realistic experimental conditions. A parametric study was conducted to assess the thermal performance of the PV panel for different geometric properties of the rectangular PCM enclosure in cases without and with fins and under different insolation ranging from 750 to 1000 W/m². Results showed that although the use of PCM prevented the increase of the PV panel temperature for a certain interval of time, the thermal performance of the device was still limited by the low thermal conductivity of the PCM. A significant improvement of the thermal management was achieved by equipping the PCM enclosure with 2–5 fins. In a subsequent study [5], the same authors experimentally investigated the thermal performance of a finned PCM-based PV panel with a maximum of 32 fins, and for two PCM materials, RT25 and GR40. Results showed that although GR40 still delayed the temperature increase of the PV panel, it was less effective compared to RT25 due to its smaller latent heat of fusion and reduced bulk thermal conductivity owing to its granular structure. The thickness of the PCM media as well as the width and spacing between the fins leading to the lowest front temperature of the PV panel were identified under specific insolation conditions. Using the same test rig, the authors [6] experimentally analyzed the effect of full size fins (fins in contact with both the front and back side of the PCM enclosure) spacing and number on natural convection inside the PV panel filled with RT27 once it starts to melt. The measurement of PCM temperature at different enclosure positions proved that when the fin spacing was reduced by increasing their number, natural convection in the molten PCM was inhibited and conduction became the prevailing heat transfer mode. For a maximum number of 32 fins, the time interval under which the PV panel temperature could be kept lower than 303 K was shortened to less than 150 min, and a faster increase of the front PV panel temperature was observed once the PCM was fully melted.

Biwole et al. [3] developed a detailed finite element numerical model to study heat transfer in a vertical PV-PCM panel. First, the authors validated their numerical results using their own experimental setup. Then, a parametric study was carried out based on the same geometry used in Ref. [4], and recommendations regarding the architecture of the PV-PCM panel and the use of fins to control the front PV panel temperature were issued. Based on the same mathematical formulation, the same author and his co-workers [7] conducted a more detailed numerical study to optimize the number, dimension, and positioning of fins in the PCM enclosure. The efficiency of the PV-PCM panel was assessed based on the time interval required to keep the front PV panel temperature as close as possible to the ambient for a longer time, the total energy stored in the PCM and the heat transfer rates between the front plate and the PCM.

The PCM melting process in a finned rectangular enclosure has also been investigated for possible integration in passive solar buildings through the use of rectangular and triangular shaped fins [8], rectangular fins with different lengths [9], and inclination angles [10]. All studies reiterate the dependence of the PCM melting process on the geometric properties of the used fins and their distribution throughout the rectangular PCM enclosure.

In the aforementioned studies, the thermal behavior the PCM in the rectangular enclosure was assessed for a vertically oriented system where the main focus was to look at the impact of improving the PCM thermal conductivity by adding fins. However, it is well recognized that in order to maximize the incident solar radiation on a PV panel, the inclination angle should be carefully chosen. Its value depends on the geographical position (latitude) and the period of the PV panel use [11,12]. Such a change in orientation will affect the thermal performance of the PCM-PV panel. Indeed, through the studies exploring the influence of the tilt angle on the thermal behavior of a PCM-filled enclosure, without fins [11,[13], [14], [15], [16]], with rectangular fins [[17], [18], [19]], and with combination of triangular fins and nanoparticles [20], it was found that the melting process is dominantly controlled by natural convection until the system, heated from above, is inclined nearly horizontally, at which point, logically, conduction is the only mode of heat transfer present.

Groulx and Biwole [15] showed numerically that in the case of a non-finned PV-PCM panel, the thermal behavior of the device varies only slightly over inclination angles ranging from 0° to 60° from the vertical, and a front plate temperature of 23 K lower than the one obtained for a horizontally oriented PV-PCM panel could be achieved and maintained for a period up to 60 min. Kant et al. [21] studied a realistic non-finned PV panel attached with RT35 PCM at its back, with actual wind velocity, ambient temperature and solar radiation data. By simulating both melting and solidification in the PCM, they found the maximum panel operating temperature to be 54.90 °C for the considered location. They also shown that higher wind velocities and lower tilt angles from vertical lead to lower operating temperature of PV panels. Khanna et al. [22], starting from the study of Groulx and Biwole, added the actual electricity efficiency conversion as a function of temperature to the study to determine that the PV-PCM panel would operate, on average, at a 2% greater efficiency (19% up from 17%) for nearly 120 min due to the temperature control of the PCM. However, no study of temperature control of a PV-PCM panel, as well as overall heat transfer within the PCM cavity as a function of inclination, has been performed when fins are added to the cavity.

Therefore, this paper presents an investigation into the effect of the inclination angle in a finned PCM-filled PV panel. The objectives are to numerically quantify the rate of heat transfer found in the PCM filled enclosure for systems with different internal fin configurations and inclinations and to determine how this affects the thermal performance of the PV panel in terms of temperature control and thermal energy storage. For this study, the PCM used is RT25 due to its suitability for PV panels temperature control, its well-defined thermophysical properties and for comparison purposes since it has been extensively used in previous studies found in the scientific literature. The following sections present a detailed description of the studied geometry of the PCM filled enclosure, the mathematical model used to model the melting process, and the results showing the characteristic melting behavior of the PCM-filled PV panel for each of the considered configurations.