# Characterizations of Interfacial Solar Water Evaporation
## 3 Performance Evaluation
### Conversion Efficiency  
[PERSON] et al. (2018) derived the following equation to precisely calculate the photothermal conversion efficiency \(\eta\), which they used to evaluate the effectiveness of their ECB-based solar steam generation:  
\[\eta=m\;h_{\text{LV}}/C_{\text{opt}}\;q_{\text{in}}, \tag{3}\]  
where \(C_{\text{opt}}\) is the optical concentration, \(q_{\text{in}}\) is the solar irradiance on the surface, \(m\) is the net rate of steam production, and \(h_{\text{LV}}\) is the liquid-vapor phase transition enthalpy. This may be calculated as  
\[h_{\text{LV}}=\lambda_{\text{LV}}+C_{\text{P}}\;\Delta T, \tag{4}\]  
where \(C_{\text{P}}\) is the specific heat capacity of bulk water (4.2 kJ kg\({}^{-1}\) K\({}^{-1}\)), \(\lambda_{\text{LV}}\) is the latent heat of water evaporation at standard atmospheric pressure (2.257 MJ kg\({}^{-1}\)), and \(\Delta T\) is the temperature change throughout the evaporation process.  
[PERSON] et al. (2018) found that black Ag nanostructures transform solar energy into heat quite well. Ag assemblages had a steam production efficiency of up to 95.2% compared to just 7.3% for the substrate (i.e., filter paper). [PERSON] et al. (2018) combined tetrakis (4-carboxyphenyl) porphyrin (TCPP) and Co\({}^{2+}\) to form porphyrin paddlewheel frameworks-3 (PPF-3) with an improved photothermal effect. The PPF-3 membrane had a photothermal conversion efficiency of 70.3%, which was a 420% increase over that of the TCPP membrane. [PERSON] et al. (2018) found that carbonized daikon had a highly developed honeycomb cellular structure with many interconnecting channels. It absorbed more than 95.5% of solar energy in the visible spectrum and more than 93.5% in the near-infrared spectrum thanks to the close-packed pores acting as photon traps. [PERSON] et al. (2018) presented an easy, scalable, and inexpensive method of generating a high-efficiency solar steam generator based on one-step calcination of commercial melamine sponges (MS) in air. The in-air calculated MS (AMS) with a thermal insulator achieved a high photothermal efficiency of 92% when irradiated by 1 kW m\({}^{-2}\).  
In general, the heat loss caused by an evaporator has three components: radiation, convection, and conduction. The calculations of these three components are presented below.  
Radiation: The radiation loss is calculated by using the Stefan-Boltzmann law:  
\[\Phi=\varepsilon\;\text{A}\;\sigma\;(T_{1}{}^{4}-T_{2}{}^{4}), \tag{5}\]  
where \(\Phi\) is the heat flux, \(\varepsilon\) is the emissivity, \(\text{A}\) is the evaporation surface area, \(\sigma\) is the Stefan-Boltzmann constant, \(T_{1}\) is the average temperature of the absorber after irradiation, and \(T_{2}\) is the initial temperature.  
Convection: The convection loss is calculated by using [PERSON]'s law of cooling:  
\[\text{Q}=\text{h}\;\text{A}\;\Delta T, \tag{6}\]  
where \(\text{Q}\) is the heat, \(\text{h}\) is the convection heat transfer coefficient, \(\text{A}\) is the evaporation surface area, and \(\Delta T\) is the temperature difference between the absorber after irradiation and the initial temperature. The convection heat transfer coefficient is about 5 W m\({}^{2}\) K.  
Conduction: The conduction loss is calculated as follows:  
\[Q=C\ m\ \Delta T, \tag{7}\]  
where Q is the heat, C is the specific heat capacity of water (4.2 J g\({}^{-1}\)\({}^{\circ}\)C\({}^{-1}\)), t is the irradiation time, m is the water mass, and \(\Delta T\) is the temperature variation of water within 60 min.  
[PERSON] et al. [9] reported that carbonized mushrooms are efficient solar steam generation devices with radiation, convection, and conduction losses of 6%, 5%, and 1%, respectively (Figure 11a). [PERSON] et al. [36] presented an exceedingly straightforward and independent solar energy converter made entirely of a 3D crosslinked honeycomb graphene foam material that is already commercially available. The radiation, convection, and conduction losses were 4.4%, 2.9%, and 2.7%, respectively (Figure 11b).