As a kind of biomass energy, biodiesel has similar physical properties to diesel and has good stability, excellent environmental protection properties, and combustion performance, and glycerol is an accompanying substance in its production process (i.e., over 10 wt%) (Liu D. et al., 2019). Glycerol has a low vapor temperature and a high boiling point due to its hydrogen bond network. Also, glycerol has good thermal stability, a wide working temperature range, and excellent environmental protection properties, so it is suitable for use in energy transportation. Due to the promotion of biomass energy, the production of glycerol has also increased, resulting in a situation of oversupply. In order to solve the problem of excess glycerol production capacity, many researchers have devoted themselves to the practical application of glycerol. At present, the heat exchanger can exchange heat directly and can also be made into mixed solvents or DES. For example, Moghaddam et al. (2016) checked the stability of dispersion of pure graphene nanosheets in glycerol by calculating the solubility parameter of pure graphene and pure glycerol (Figure 1D). It is shown that pure graphene with five layers can form a stable dispersion in glycerol. Fazel et al. (2013) studied the pool boiling heat-transfer characteristics of water/glycerol binary mixed solvents with different concentrations (0–35% mass glycerol) on horizontal rod heaters and found that when the concentration of glycerol was 0.02% mole fraction, the pool boiling heat-transfer coefficient can be increased by 15–20% (Figures 1E,F). However, the glycerol molecule has three hydroxyl groups, which lead to the formation of an intermolecular hydrogen bond network structure and make it have a high viscosity, which is the main problem limiting the application of glycerol-based nanofluids.

The DES prepared by glycerol as a hydrogen bond donor can effectively inhibit the hydrogen bond interaction between glycerol molecules and plays an important role in reducing the overall viscosity of the solvent. Faraone et al. (2018) of the National Institute of Standards and Technology of the United States systematically studied the relationship between hydrogen bonds and properties of glycerol/ChCl DES and concluded that choline ions were in the hydrogen bond network gaps constructed by glycerol molecules, and this “isolation effect” led to the reduction of glycerol molecular viscosity (Figure 1B). Dziubinska-Kühn et al. (2022) investigated the effect of hydrogen bond donor supersaturation on the properties of DES by titration analysis of glycerol/ChCl DES. Zhao et al. (2013) used glycerol/ChCl as a solvent and soybean oil as a raw material to enzymatically hydrolyze biodiesel, which could convert 88% of triglycerides by enzymatic hydrolysis within 1 day. In addition, Noor et al. (2021) studied the change of thermal conductivity of various glycerol-based DESs in the temperature range of 25–60°C and explored the application of fluids in heat transfer. It was found that the thermal conductivity decreased with the increase of temperature, and the solvent system with a high ChCl ratio had better thermal stability (Figures 1G,H). In addition, Noor et al. (2021) studied the thermal conductivity of glycerol-based DES with different collocations. It is shown from all results that the thermal conductivity decreases with the increase of temperature. This is because the temperature has a negative effect on electrons’ randomness and a positive effect on electrons’ free motion. Additionally, mixtures of a high ChCl ratio showed more stable values of thermal conductivity.

Nanofluids involving metal oxide. As an efficient heat-transfer medium, DESs have been widely used in catalysis, biomass degradation, materials chemistry, and other fields. Changhui Liu et al. (2019b) prepared glycerol/ChCl DES and filled it with TiO₂ and Al₂O₃ nanoparticles to study their thermal properties such as thermal conductivity and specific heat. The results show that the nanofluid can maintain the liquid working state at −35 to 275°C, and the thermal conductivity is increased by 3–11.4%; after filling with nanoparticles, the specific heat capacity is partially decreased compared with the base liquid (Figure 1C).

Nanofluids involving non-metal oxide. In order to further study the heat transfer of glycerol-based DES nanofluids, Changhui Liu et al. (2019a) prepared a new type of Cu-SiO₂ hybrid nanoparticle to enhance the stability and thermal conductivity of DES nanofluids. In this work, 2-butoxy-3,4-dihydropyran (DP) was employed as a metal nanoparticle anchor. It was found that DP was able to enhance the dispersing ability and thermal conductivity of SiO₂ nanoparticle-filled DES-based nanofluids. The thermal conductivity of the Cu-SiO₂-filled DES nanofluid was enhanced by a maximum of 13.6%, which indicated the positive effect of DP on the thermal conductivity improvement of the nanofluid.