Self-thermal storage integrated evacuated tube solar collectors using thermally enhanced phase change materials
Creators: name: Aramesh, Mohamad
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401703
subject: Energy generation, conversion and storage (excl. chemical and electrical)

description: On average, 15- 27% of total energy demand of a typical residential building in Australia, is dedicated to water heating. Additionally, water heating contributes to approximately 25% of the greenhouse gas emissions associated with such buildings. Residential buildings form one of the highest energy consuming sectors in many countries. In Australia, residential buildings consume more energy than the commercial, agriculture, construction, and water and waste sectors altogether. Such statistics clearly imply the necessity of developing a green and sustainable solution for residential hot water generation.
Solar thermal collectors (STCs) are commercially developed technologies thar offer promising renewable solutions for residential water heating. Evacuated tube solar collectors (ETSCs) specifically have taken 60% of the global STC market. Their wide operational range of 50 °C to 200 °C and as well as their high year-round efficiency of 50-90% are some of the key reasons for this popularity. 

Due to the intermittent nature of solar energy, ETSCs cannot supply a residential hot water demand uninterruptedly without the support of a properly designed thermal storage component. Current commercial ETSCs come with large hot water storage tanks. This external bulky component itself has been a great barrier facing widespread implementation of STCs in general including ETSC technology in particular. These hot water tanks can weigh about 4-6 times more than the collector itself and normally have a storage volume of 100-300 litres. Such tanks require occupy large spaces within inside premises. 
On the other hand, phase change materials (PCMs) offer strong alternatives to conventional hot water storage tanks due to their high thermal storage capacity. However, many commonly used PCMs suffer from the known shortfall of low thermal conductivity that affects their heat transfer performance. Several thermal enhancement methods are used to address this challenge, among which embedding metal foam (MF) structure into PCM body, has proven to be highly effective.

A through literature review reveals that internal integration of PCM-MF composite inside the tubes of ETSCs (ETSC-PCM-MF), being proposed by this research, has not been studied before and no experimental data has been reported on that. This is while this combination can potentially offer a high performing compact solution for uninterrupted hot water supply in residential applications. Guided by a thorough review study, paraffin-copper foam composite is identified to be a proper choice for the purposes of this work. Both materials are readily available at competitive prices and the composite is physically stable, and is non-toxic and non-corrosive, making this a desirable choice for use in a residential hot water application. 

The primary goal of this study is to explore the possibilities and challenges associated with the proposed design. To achieve this, the study begins by analysing the interactions within a PCM-metal foam composite to identify the ideal composition for the intended application. Following this, an experimental setup is created to assess the performance of the proposed design in supplying hot water to a specific residential scenario. The study then delves into theoretical investigations to understand how various design parameters affect the overall thermal performance. Ultimately, based on the findings, the study outlines future steps required to further develop and successfully implement the proposed design in real-world applications.

To achieve the goals of this study, the experiments mainly focus on performance evaluation of the proposed design and consists of two main stages. At the first stage, the self-storing ETSC-PCM-MF design is compared with conventional and other previously introduced storage-enhanced designs. Then at the second stage, this design performance in supplying residential hot water demand is measured. On the other hand, the theoretical part concerns the effective design parameters and delivers a parametric study. Ansys Fluent software is chosen to support the theoretical investigations and the reliability of the simulation is validated using the experimental data.

Relatively close designs previously presented in the literature are based on filling the collector tubes with pure PCM (ETSC-PCM) or a PCM-fin composite (ETSC-PCM-fin). The first stage of experiments focuses on comparing those designs with the new ETSC-PCM-MF design proposed here. The experimental data shows the novel self-storing design outperforms the earlier above-mentioned designs. The results also suggest that the ETSC-PCM-MF design compared to the ETSC-PCM and ETSC-PCM-fin designs, contains 8% and 6% less amount of PCM, respectively. However, through a 35% and 8% higher heat transfer rate compared to those cases with the same order, the proposed design obtains 10% and 5% more thermal energy storage, respectively. Furthermore, overall thermal efficiencies of the ETSC-PCM, ETSC-PCM-fin, and ETSC-PCM-MF designs are obtained to be 52%, 54%, and 57%, respectively. 
In the next step of experiments, the self-storing ETSC-PCM-MF design is compared with a conventional design in a residential hot water scenario. The results show that the proposed design reaches a maximum hot water output temperature of about 25% higher than that achieved by the conventional design. Moreover, overall thermal efficiency of the conventional and ETSC-PCM-MF designs are measured to be about 49% and 72%, with thermal storage efficiency values of about 57% and 75%, respectively. 

Following these promising results, the self-storing design is further investigated to determine how it helps with eliminating or reducing the size an external hot water storage unit that is usually used with ESTCs. According to the experimental results the self-storing design requires 55% smaller external hot water tank. It must be noted that the collectors used for this experimental study are scaled down units that offer less space for accommodating the proposed PCM-MF composite storage medium. Larger scale commercially available ETSCs, which are usually used for domestic hot water supply can accommodate 89% more PCM-MF composite for every 1 litre of hot water delivered compared to the volume offered by the scaled down ETSC used in this experimental study.  In other words, implementing the proposed design on a commercial scale product can help reduce the size of the hot water tank more than achieved by using a scaled down ETSC unit in this study (i.e., 55%).

To further develop the proposed design, a theoretical parametric study is conducted using Ansys Fluent software. Choices for PCMs are very limited due to the specific requirements of the intended application, i.e., residential hot water generation. However, many variations are possible in choosing MF specifications. When comparing common MF materials, using copper foam leads to 29% and 61% faster heat transfer compared to nickel and aluminium foams, respectively. MF porosity is shown to significantly affect the collector performance in different ways. An 80% increase in the MF porosity increases heat storage capacity by 88% and decreases the heat transfer rate by 60%. Considering the importance of both performance parameters, the MF porosity of 90% is selected as the optimal value for the collectors employed in the present study. MF pore density on the other hand has a mild impact on the collector performance: that is 80% increase in MF pore density leads to 6% decrease in heat transfer rate and subsequently 4% decrease in the amount of heat storage capacity.

This study informs a pathway for optimising the system for its practical implementation by considering every aspect of the collector including the dimensions and geometry as well as its operational procedure and conditions. This optimum design then can be further investigated over a year-round operation to fine tune its design and performance with emphasis on its ability for continuous supply of hot water in the context of a real application.