An experimental evaluation of direct flow evacuated tube solar collector integrated with phase change material
Introduction
Due to the dramatic increase in the energy consumption and greenhouse emissions over the last decades, several countries tend to utilize renewable energy in various applications. Since around 35% of the total produced energy is consumed in the building sector with 75% for space and water cooling/heating applications [2], a considerable attention towards the utilization of solar energy in domestic water heating applications is observed. Even though the use of solar energy is facing the challenge of intermittency and predictability, which causes a gap between the supply and demand for energy, thermal energy storage (TES) offers a solution to problem [3], [4]. Li [5] and Liu et al. [6] reviewed different innovative TES techniques including sensible heat storage (SHES), latent heat storage (LHES) through using PCM, and the thermochemical heat storage. Owing to their high storage density, and isothermal operation during the charging and discharging phase [7], [8], many researchers attempt to integrate PCM in solar water heating systems (SWHS) in the water storage tank or the solar collector itself.
Cabeza et al. [9] investigated experimentally the effect of utilizing sodium acetate-graphite compound modules with melting temperature, , of 58 °C in enhancing the thermal storage capacity of a 146 L hot water TES tank. The results indicated that utilizing the PCM modules in the water storage tank increased the energy density of the TES tank. Furthermore, Khot [10] developed a laboratory model for integrating twelve spherical capsules of HS-58, as a PCM with °C, in a 10 L hot water TES tank. The results indicated that adding the PCM capsules increased the TES capacity by 22%. Fazilati and Alemrajabi [11] presented an experimental investigation on integrating spherical paraffin wax capsules with °C in a hot water TES tank. The authors found that the use of the PCM was very attractive when it achieved the liquid phase. Bedecarrats et al. [12] experimentally investigated the charging and discharging processes of encapsulated mixed water and nucleation agent when introduced as PCM in a 1000 L hot water TES tank. The results illustrated that the heat transfer fluid flow rate, the inlet charging temperature, and the outlet discharging temperature had a significant effect on the charging and discharging durations. Mazman et al. [13] experimentally investigated the effect of integrating various PCM modules in hot water TES tank. Three PCM mixtures (i) paraffin with stearic acid (PS), (ii) paraffin with palmitic acid (PP), and (iii) stearic with myristic acid (SM) were used and integrated in the upper section of the 150 L hot water TES tank. The results indicated that the utilization of PS gave the best thermal performance among the other PCM mixtures during the cooling and reheating phases.
Kurklu et al. [14] and Al-Kayiem and Lin [15] integrated paraffin wax in a flat plate solar collector. The results indicated that integrating PCM in flat plate solar collector had a significant advantage in enhancing the performance of the solar collector since the stored water was kept above 30 °C during the whole night. Furthermore, Khalifa et al. [16], [17] designed and built an integrated solar collector with a back container including paraffin wax, as a PCM with °C, as TES medium to experimentally investigate its performance. The authors indicated that using the paraffin wax in the solar collector was a promising technique for enhancing the thermal performance. Mettawee and Assassa [18] designed and built a compact solar collector integrated with paraffin wax, as a PCM with °C, to investigate its performance experimentally during the charging and discharging phases. They found that the discharge mass flow rate and the natural heat transfer coefficient have a significant effect on the performance of the proposed system.
Besides integrating PCM in flat plate solar collectors, recently several researchers attempted to integrate the PCM in heat pipe evacuated tube solar collectors. Naghavi et al. [19], developed a theoretical model to investigate the performance of integrating PCM to the manifold of an evacuated tube heat pipe solar collector system based on the Stefan problem. Papadimitratos et al. [20], experimentally integrated the PCM in a heat pipe evacuated tube solar collector. The results showed the ability of the proposed system to supply substantial amount of energy regardless of the weather conditions.
Even though the integration of PCM in SWHS tends to be a promising technique in enhancing the overall thermal performance, the low thermal conductivity of the PCM adversely affect the phase time transition and the ability of the proposed systems to be commercially utilized [21], [22]. Therefore, many researchers attempted to enhance the heat transfer characteristics of the PCM through the use of extended surfaces (fins) or the addition of conductive additives [23], [24], [25], [26], [27].
The preceding review shows that significant amount of research has been devoted to incorporating PCM in hot water TES tank, flat plate solar collector, or heat pipe evacuated tube solar collector. The uniqueness of this research lies in the fact that it focuses on integrating the PCM in a typical direct flow evacuated tube solar water heater, with a U-shape heat exchanger, which is hardly addressed in the literature. This research work targets this unique area and explores deeper understanding while offering wider opportunity for the use and operation of this configuration.
The objective of this research work is to evaluate the thermal performance of direct flow evacuated tube solar water heater integrated with PCM with finned and un-finned HX. Two sets of experiments are considered to evaluate the performance of the proposed system; (1) experiments to evaluate the charging and overnight heat loss phases based on the CNS B7277-12558 [1], and (2) experiments to investigate the ability of the proposed systems to discharge energy under various loads using the design of experiments concept.
Section snippets
Experimental setup and methodology
An outdoor experimental test rig is designed and constructed to investigate the thermal performance of a typical Sydney type direct flow evacuated tube solar collector [28]. The experiments are performed at the solar research site of the American University in Cairo, located at a latitude and longitude angles of 30° and 31.5°, respectively. Experiments were performed from January to April 2016. The solar collector with an aperture area of 2.13 m2 has an optical efficiency of 74.2%, and
Thermal performance tests during the charging phase and the overnight period
Fig. 5, Fig. 6 show the time history of the PCM temperature, at different axial locations, and the solar irradiance during the charging phase of systems A and B. For system A, the total solar radiation incident on the solar collector, during the test day, is 26.53 MJ/m2-d. Fig. 5 shows that at the start of the experiment, the temperature of the PCM does not vary along the vertical direction since conduction is the main mechanism for heat transfer. Once melting starts, the PCM at the upper
Conclusions
An experimental investigation is conducted to evaluate the thermal performance of direct flow evacuated tube solar water heater integrated with paraffin wax as LHES. Two geometrical configurations are considered in this study; finned and un-finned U-tube HX. The performance of the SWHS, during the charging and discharging phases, is evaluated by running (1) experiments to evaluate the charging and overnight heat loss phases based on the CNS B7277-12558 [1], and (2) experiments to investigate
Acknowledgement
This research was partially supported by Deutsche Gesellschaft für Internationale Zusammenarbeit (GIZ) GmbH Egyptian-German Private Sector Development Programme (GIZ-PSDP). The authors would also like to thank the Egyptian National Cleaner Production Center-Ministry of Trade and Industry (ENCPC-MTI).
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