Enhanced thermal properties of polyethylene glycol/modified rice husk ash eco-friendly form-stable phase change material via optimizing support pore structure

Highlights

• Polyethylene glycol/modified rice husk ash eco-friendly form-stable PCM was prepared.

• Pore structure of modified rice husk ash (mRHA) was optimized via ultrasonic acid treatment.

• Enlarged surface area and stacked pore volume of mRHA enhanced loading of polyethylene glycol (PEG).

• Higher surface complexity of mRHA leaded to greater nucleation promotion of PEG.

Abstract

In this work, a novel eco-friendly form-stable phase change material (PCM) was synthetized based on latent heat storage material of polyethylene glycol (PEG) and supporting material of rice husk ash (RHA). To enhance the thermal properties of the form-stable PCM, rough RHA (rRHA) was modified by an ultrasonic hydrochloric acid treatment to unblock the pores stacked by non-crystal silica. The modified RHA (mRHA) behaved an optimized pore structures, in which the BJH pore volume and the BET surface area showed 24.8% and 12.8% increases compared with rRHA. The latent heat of 119.3 J/g for PEG/mRHA form-stable PCM was 20.9% higher than that of PEG/rRHA form-stable PCM due to deep filling of PEG into the non-crystal silica stacked pores of mRHA. Moreover, Frenkel-Halsey-Hill (FHH) theory was introduced to quantitatively characterize the surface complexities of rRHA and mRHA. The result showed the increased surface complexity of mRHA, leading to greater nucleation promotion for PEG. Thus, PEG/mRHA achieved the improved crystallization behaviors. In addition, PEG/mRHA showed favorable thermal cycling reliability after 300 thermal cycles. Therefore, PEG/mRHA provides a new insight into the improved resource utilization and functionality of RHA in terms of eco-friendly form-stable PCM for thermal energy storage.

Introduction

Energy crisis is intensifying with tremendous consumption amounts of fossil fuels [1]. It is known that more than 50% of the end demand for energy form is thermal energy, in which the most of them are used in building such as water heater, heating and air conditioning systems and so on [2,3]. Hence, thermal energy storage (TES) technologies applied in buildings are the keys to efficient energy utilization [4]. Phase change materials (PCMs) can absorb a large amounts of energy stored as latent heat and release it over a narrow temperature span during phase transition [5,6]. Due to this thermal energy storage feature of PCMs, they are highly praised in term of solving the energy shortage. According to the difference of chemical composition, PCMs are divided into three classifications: inorganic, organic and inorganic-organic [7]. Therein, organic PCMs have aroused great research interest owing to their excellent performance of high latent heat, thermally stable property and low supercooling, etc [8,9]. Polyethylene glycol (PEG) with hydrophilic long chain is classified to the organic polymer PCM. It has been extensively applied in building energy saving [10,11] due to the excellent features of suitable phase change temperature (46∼65°C), high thermal enthalpy (145∼175 J/g), non-toxic, low-cost, stable chemical and physical properties [12,13]. However, there are still two defects for PEG to be solved: leakage during the phase transition [14] and relatively high supercooling degree due to the incomplete crystallinity [15].

In order to avoid the leakage of PEG, the encapsulation techniques of PEG have been developed to prepare form-stable to solve this issue. The encapsulation techniques include impregnation into the porous supports [16] and core-shell encapsulation methods [17]. Therein, the impregnation of PEG into the porous materials is widely adopted because of the simple preparation process. Many porous supporting carriers have been demonstrated to effectively encapsulate PEG including the types of mesoporous silica [15], porous carbon [18] and porous oxide [19], etc. However, the demand of form-stable PEG used in buildings is very large. Although mesoporous silica and porous carbon show great advantage in term of high PCM encapsulation ratio, their high costs limit the application in buildings. Thus, some porous oxide supports with easy to obtain and cheapness are developed such as diatomite [20], expanded vermiculite [21], expanded perlite [22] and so on. Nevertheless, the porosities of these cheap porous supporting materials are relatively low, leading to inferior thermal energy storage capacity of the formed phase change composite. Therefore, how to develop a low-cost and high thermal energy storage PEG-based form-stable PCM for building energy saving is a challenging issue.

Rice is one of the largest food crops in the world, billions of human beings depend on it for survival. With the gradual increase of the world population, the output of rice is increasing by millions of tons every year [23]. Rice husk coated on rice is commonly removed by milling due to poor nutritional value. It can be further used as biomass fuel to replace coal and natural gas for heat supply. Rice husk ash (RHA) is usually generated in thermal power plants by burning rice husk. The residual RHA is considered as solid waste [24]. The main chemical composition of RHA is non-crystal silica and a small amount of metal oxides covering the non-crystal surface. Although the application and commercial values of RHA in construction have been confirmed in view of current numerous researches [25,26], the universal treatment is still landfill. This will cause a series of environment hazards including land and groundwater pollution, thus endangering human health. Hence, the resource utilization of RHA is an important approach to address the tricky problem.

Fortunately, RHA presents fine powders with the particle size distribution between 1 μm and 80 μm. The rich surface area and porous structure of RHA make it highly suitable for serving as the porous support of PEG-based form-stable PCM. This not only provides a feasible approach for the resource utilization of waste RHA, but also provides a supporting material with nearly zero cost and low carbon for encapsulating PEG. It is noteworthy that the interior of RHA contains a large amount of pores stacked by non-crystal silica nanoparticles [23]. However, these pores are partially covered by metal oxides [27,28]. The stacked pores can be fully exposed when the metal oxides are removed, releasing a huge amount of deep pore channels. This could cause enlarged pore volume and surface area of RHA, thus holding more PEG. To be specific, the larger pore volume and surface area of porous supports can deliver more capillary force and surface tension to PEG [29,30], thus improving the encapsulation ratio of PEG within RHA.

Furthermore, RHA with fine powder feature can be served as heterogeneous nucleation site for PEG, promoting PEG nucleation. For the sites, the roughness of substrate surface has a prominent influence on its heterogeneous nucleation efficiency. To be specific, the increase of the surface roughness is favorable for the nucleation [31,32]. The adequate exposure of the pores stacked by non-crystal silica nanoparticles could cause sharp interface geometrical fluctuations. This will result in the variation in fractal characteristic of RHA surface [33,34], further leading to an increased surface complexity. It can be expected that the elevated complexity of RHA surface has a further influence on the promotion of PEG nucleation, making it easier to induce the nucleation. Moreover, the improved nucleation may further enhance the crystallization behavior of PEG, mitigating the supercooling and improving the heat release characteristic.

In this work, ultrasonic hydrochloric acid treatment was used to remove the metal oxides to unblock the porous channel within RHA to prepare the modified RHA (mRHA). The influences of porous structure change of RHA on the thermal energy storage capacity and crystallization behaviors of PEG were addressed based on the synthesized PEG/rough RHA (PEG/rRHA) and PEG/mRHA form-stable PCMs. The element compositions and surface chemical components were evaluated by X-ray fluorescence (XRF) and X-ray photoelectron spectroscopy (XPS). The phase transition behaviors of the prepared form-stable PCMs was recorded using differential scanning calorimetry (DSC). The chemical compatibility and thermal stability of the composite PCMs were demonstrated by Fourier transform infrared (FT-IR) and thermogravimetric analysis (TGA), respectively. Moreover, the micromorphology, pore structure and fractal characteristics of rRHA and mRHA were investigated by scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) and Frenkel-Halsey-Hill (FHH) theory to figure out the influencing mechanism of optimized pore structure on the enhanced thermal properties of RHA based form-stable PEG. In addition, the thermal cycling reliability was discussed as well.