Transforming Citrus Waste into Carbon Aerogels for Enhanced Solar-Thermal Energy Storage and Conversion

From Waste to Functional Material—Carbon Aerogels from Citrus Biomass Infiltrated with Phase Change Materials for Possible Application in Solar-Thermal Energy Conversion and Storage

Abstract: The conversion and storage of green energy materials have recently gained significant attention, particularly in energy-intensive buildings. Phase Change Materials (PCMs) have become increasingly popular, not only for energy storage but also in composites aimed at solar energy conversion. This research explores a sustainable approach to transforming orange biomass waste (OBW) into advanced porous carbon aerogel (PCA) composites, specifically designed for solar-thermal energy harvesting and storage in building applications. Utilizing potato starch as a binder, the study outlines a scalable process for producing a uniform and lightweight carbon matrix. The optimal results were achieved with PCA containing 2.5% starch, which exhibited the lowest mass loss (8.2, 0.5, 11.2% pt) during leakage testing. This study underscores the potential of OBW-derived aerogels as effective matrices for PCM impregnation and shape stabilization, paving the way for their future use in solar-thermal energy conversion and storage, and contributing to reduced energy consumption in buildings. By repurposing agricultural waste, this work promotes sustainable material development and enhances the application of renewable energy technologies.

Keywords: biomass; phase change materials; solar energy harvesting; biomass waste; citrus waste; carbon aerogels

1. Introduction

Energy consumption has surged in recent decades, leading to the extensive use of fossil fuels and a corresponding rise in greenhouse gas emissions due to the ongoing reliance on nonrenewable energy sources. To address these challenges, researchers are exploring new technologies for harvesting green energy and developing more sustainable methods for energy generation and storage. Attention has increasingly focused on renewable energy sources such as solar, wind, hydropower, biogas, and geothermal.

According to the Global Status Report for Buildings and Construction (Buildings-GSR) in 2022, the building sector accounted for 30% of the final energy demand, primarily for heating and cooling, with an additional 4% used for producing construction materials. The energy demand in this sector has risen by 1% in just one year. Research indicates that buildings typically lose energy through two main mechanisms: ventilation heat loss (19%) and fabric heat loss (81%). The latter is associated with doors, windows, floors, exterior walls, and roofs, with significant percentages attributed to various components.

Energy storage is crucial for optimizing energy utilization. PCMs, which store energy during heating and release it during cooling, can mitigate energy losses. These materials undergo phase changes, which can be solid-liquid, solid-solid, or liquid-gas transitions. PCMs are categorized into three main types: organic (paraffin and non-paraffin compounds), inorganic (hydrated salts and metals), and eutectic (combinations of organic and inorganic substances).

Numerous studies have demonstrated the application of PCMs in building materials, such as walls and ceilings. Research has also suggested enhancements to PCM performance, particularly in energy-related materials, and the implementation of these materials across various building elements. For instance, the use of paraffin-based PCMs as modifiers for triple-pane and double-pane windows has shown promising results in reducing energy consumption and temperature fluctuations.

The global economy is shifting towards sustainable solutions and circular economy models that focus on optimizing resource utilization and minimizing production waste. This framework emphasizes the restorative use of resources to prevent them from becoming discarded waste. Utilizing materials from natural sources and their modifications has gained traction in recent years, and there is a growing emphasis on recycling biomass waste and incorporating it into material design processes.

2. Materials and Methods

2.1. Materials

Orange biomass waste was collected from local cafes. Ethanol (99.9% wt) and starch (ST, 99.9%, POCH) were sourced from Avantor Performance Materials, Poland. The phase change materials (PCMs) used included PEG 6000 (99.9%, Sigma Aldrich), palmitic acid (99.0%, Chempur), and octacosane (99+%, Thermo Scientific Chemicals).

2.2. Preparation of Porous Carbon Aerogels (PCA)

The PCA preparation followed established literature methods. The OBW was washed, blended, and dried before preparing the ST binder in deionized water. The blended OBW was then mixed with the ST gel, subjected to ultrasonic treatment, and frozen. After freeze-drying, the samples underwent a four-step carbonization process in a nitrogen atmosphere.

2.3. Preparation of PCM-PCA Composites

The PCM-PCA composites were prepared using vacuum-assisted impregnation. The PCMs were melted in a vacuum oven and then introduced to the PCA under vacuum conditions to ensure thorough impregnation.

2.4. Characterization

The characterization involved CHN elemental analysis, thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and differential scanning calorimetry (DSC) to evaluate the thermal properties of the composites.

3. Results and Discussion

3.1. SEM Analysis

The SEM images revealed that PCA samples exhibited highly porous 3D structures, with variations in porosity based on ST content. The optimal ST content led to a more uniform pore structure, enhancing the potential for PCM loading.

3.2. TGA Results

TGA results indicated that the carbon content in PCA samples significantly increased post-carbonization, correlating with higher ST content. The thermal stability of PCM-PCA composites showed promising results, particularly for composites with lower ST content.

3.3. DSC Results

The DSC analysis demonstrated shifts in melting and crystallization temperatures for PCM-PCA composites compared to pure PCMs, indicating the influence of the carbon matrix on phase transition characteristics.

3.4. Leakage Tests

The leakage tests highlighted that PEG-PCA samples exhibited minimal leakage, attributed to the viscosity of the PCM, while PA and OC composites showed more significant mass loss.

4. Conclusions

This study successfully developed PCA-PCM composites for solar-thermal energy conversion using orange biomass waste and starch as a binder. Key findings include:
1. The carbonization process effectively increased carbon content, making the material suitable for solar energy applications.
2. The addition of starch significantly influenced the morphology and porosity of PCA samples, impacting PCM loading capacity.
3. Optimal binder ratios were critical in achieving materials with effective PCM impregnation and minimal leakage during phase changes.
4. The performance of the composites indicated potential for long-term durability and consistent functionality.

The research highlights the promise of OBW-derived carbon aerogels in various applications, including energy storage and thermal management systems. Further investigation into long-term performance and efficiency of solar-thermal conversion is warranted.