## 1 Introduction  
The rapid consumption of fossil fuels and the increasing contradiction between energy supply and demand are compelling researchers to utilize energy more effectively and develop renewable energy [1]. A solar energy source, which is inexhaustible, economical and environmentally friendly, has been widely recognized as an ideal form of renewable energy [2]. In recent years, the conversion of photo-thermal technology is the most successful and popular technique among the different forms of solar energy utilization, such as light-thermal conversion, light-electricity conversion, light-biology conversion [3]. Photo-thermal applications not only possess the merits of relatively high photo-thermal energy conversion efficiency but also can be achieved without complicated and expensive instruments [4; 5]. However, there are still some nonnegligible drawbacks in the use of solar energy, such as low conversion efficiency, diurnal fluctuation of optical radiation, which limit the efficient utilization of solar thermal energy storage [4]. Latent heat thermal energy storage (LHTES) system has proved to be a promising technique for overcoming the drawbacks of solar energy conservation because of its outstanding advantages, such as high heat storage density, constant operating temperature, isothermal characteristics. In recent years, LHTES has been widely applied to various thermal storage management applications [6; 7; 8; 9]. Phase change materials (PCMs), as representative of advanced LHTES materials, are the most prevalent and effective technique for thermal energy storage because of their high enthalpy change, non-toxic, and reusability characteristic, which can be utilized in a range of applications, such as thermal management of solar energy, insulation clothing, thermal insulation buildings, spacecraft thermal control and so on [10; 11; 12; 13; 14]. Among various organic PCMs (OPCMs), polyethylene glycol (PEG) is universally recognized as a kind of outstanding solid-liquid transformation PCM because of its excellent properties, including economically available, high phase change enthalpy, biodegradation, little subcooling, low vapor pressure, non-toxicity [14; 15; 16]. In addition, it is convenient to obtain the suitable phase change temperature and enthalpy via simply changing molecular weight of PEG [17]. However, the pristine PEG as OPCMs in the utilization of solar energy has intrinsic shortcomings, including low thermal conductivity and poor absorptive performance in the optical light, which accounts for 50% of the solar radiation energy. The former refers to the speed of absorbing and releasing thermal energy, which can reduce the energy efficiency [18]. The latter leads to low solar energy efficiency, which limits its applications of solar thermal energy to a great extent. Consequently, the thermal conductivity and photothermal performance of OPCM are imperative to be enhanced. Hence, many prominent works have been done to obtain the PCMs with much higher thermal conductivity. A prevalent approach against this shortcoming is to incorporate the highly thermal conductive fillers into the PCMs, such as silver nanowire [19], Cu [20], TiO\({}_{2}\) nanoparticles [21], [PERSON] et al. [19] achieved PEG-Ag/expanded vermiculite PCMs by the physical blending and impregnation method. The thermal conductivity increased to 0.68 W/(mK) for 19.3 wt % silver nanowire in the composite PCMs, which was 11.3 times higher than that of pristine PEG. [PERSON] et al. [20] prepared composite PCMs by adding Cu powder to PEG/SiO\({}_{2}\) through the sol-gel method, and discovered that the thermal conductivity of the composite PCMs reached up to 0.431 W/(mK) by an addition of 3.45 wt % Cu powder, which was enhanced by 49.13% in comparison with pure PEG. [PERSON] et al. [21] used a two-step method to prepare stearic acid-TiO\({}_{2}\) nanofluids PCMs. For composite with only 0.3 wt % TiO\({}_{2}\), the enhancement of thermal conductivity was measured to be 70.52%.