Interfacial photothermal solar desalination: A call to repentance

Interfacial photothermal solar evaporation (IPSE) is a process that uses direct solar heating to generate water vapour. Research in this process has grown explosively in the past decade and claims about its relevance to commercial application as a desalination method appear with extreme frequency. However, published systems using 2D materials are approximately two orders of magnitude less efficient than current best practice solar energy utilization using photovoltaic technology coupled with reverse osmosis (PV-RO) and there is no realistic prospect of this gap being closed. Large scale pilot trials of IPSE systems have obtained approximately 0.7 kg.m⁻².h⁻¹ of purified water at 1 kW.m⁻² solar illumination, while a commercial PV-RO seawater desalination plant running with commercially available photovoltaic panels would achieve approximately 65 kg.m⁻².h⁻¹ of water at 1 kW.m⁻² solar illumination. The IPSE literature rarely considers the additional energetic and capital requirements for condensation that must accompany any evaporation process, which are substantial. The IPSE literature is also unaware of the state of the art and challenges in thermal desalination, where the issue is not reducing the enthalpy of vaporization, but efficiently capturing and re-using the enthalpy of condensation. While inefficient compared to PV-RO, direct use of concentrated solar power to drive existing thermal desalination processes is also significantly more efficient than IPSE processes. Most IPSE literature also relies on methods for the calculation of ‘effective enthalpy of vaporization’ which have serious conceptual and methodological errors. This review quantitatively addresses these issues and suggests alternative directions for IPSE researchers.