# Modeling and Analysis of Contactless Solar Evaporation for Scalable Application
## 1 Introduction  
Two-thirds of the global population is affected by water shortage which has become a major global risk for human beings [1]. According to the World Economic Forum, water crises are mainly caused by the depletion of natural fresh water resources by agriculture, industry and urban use, as well as an ever-growing amount of wastewater [2]. Except for different wastewater produced from municipal and industrial processes, the freshwater supply technology itself could produce a huge amount of hypersaline wastewater. For instance, desalination of seawater [3; 4; 5; 6] or brackish water [7] produces concentrated brine as an undesirable by-product. Conventional strategies for concentrated brine disposal include surface water discharge, deep well injection, evaporation ponds and land application [8], which are improper for wastewater treatment and detrimental to soil, land vegetation and the aquatic ecosystem [9; 10; 11; 12]. Zero-liquid discharge (ZLD), a wastewater management strategy that can completely separate salt from water with minimized impact to the aquatic environment, has attracted worldwide interests [7; 12; 13]. Currently, the ZLD wastewater treatment is mainly divided into two stages [7]. First, the wastewater is concentrated by either membrane or thermal methods. Then, the concentrated brine is turned into solid waste by using an evaporation pond or crystallizer [14]. With a sophisticated mechanical structure, the crystallizer has a smaller footprint, but it is highly energy intensive (>50 kWh\(\cdot\)m\({}^{-3}\)) and produces large amounts of carbon emissions [14; 15]. On the contrary, the evaporation pond harvests solar energy to evaporate the water in concentrated brine, which has lower carbon emissions and operating costs. However, it has a low evaporation efficiency and large footprint(\(>\)1000 m\({}^{-2}\)), due to the low energy density of sunlight [16; 17]. In order to improve the competitiveness of this low-carbon technology, it is critical to enhance the energy efficiency of solar evaporation ponds [18; 19; 20]. In recent years, solar-driven interfacial evaporation has been intensively investigated, due to its great potential to achieve high-efficiency desalination and wastewater treatment [6; 21; 22; 23; 24; 25]. The porous evaporator with low thermal conductivity localizes the solar thermal energy at the air-liquid interface to achieve high solar evaporation efficiency. However, the porous evaporator also constrains the salt transport, resulting in fast salt accumulation and crystallization. The salt crystallization will reduce the light absorption and water supply, resulting in a low evaporation rate or even complete evaporation failure [26; 27; 28; 29]. To overcome the salt accumulation issues, different strategies have been proposed, including salt diffusion enhancement [30; 31; 32], salt crystals rinsing [33], multi-layer design [34], and other strategies. Nevertheless, the salt crystallization problem cannot be completely solved as the solar absorber and air-water interface are still in contact. To totally eliminate the salt crystallization on the solar absorber, solar-driven contactless evaporation (SCE) was proposed by [PERSON] et al. [35], as shown in Figure 1. In this design, the solar energy is converted into thermal energy by the solar absorber, and then emitted in the form of infrared thermal radiation. The infrared radiation can be efficiently absorbed by water within a thin layer (\(\sim\)10 \(\upmu\)m) due to the high absorptivity of water [36]. The contactless design archives interfacial heating via radiative heat transfer, which can thoroughly prevent salt accumulation and fouling issues. Due to the application of high-temperature steam generation, the evaporation rate was lower than 0.39 kg\(\cdot\)m\({}^{-2}\cdot\)h\({}^{-1}\). Later, [PERSON] et al. investigated solar-driven contactless heating to enhance the evaporation performance of solar pond [37].