Editing IPCC:AR6/WGIII/Chapter-6 (section)

==== 6.4.2.9 Marine Energy ====

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The ocean is a vast source of energy ( [[#Hoegh-Guldberg--2019|Hoegh-Guldberg et al. 2019]] ). Ocean energy can be extracted from tides, waves, ocean thermal energy conversion (OTEC), currents, and salinity gradients ( [[#Bindoff--2019|Bindoff et al. 2019]] ). Their technical potentials, without considering possible exclusion zones, are explored below. Tidal energy, which uses elevation differences between high and low tides, appears in two forms: potential energy (rise and fall of the tide); and current energy (from tidal currents). The global technically harvestable tidal power from areas close to the coast is estimated as about 1.2 PWh yr –1 (4.3 EJ yr –1 ) ( [[#IRENA--2020b|IRENA 2020b]] ). The potential for tidal current energy is estimated to be larger than that for tidal range or barrage ( [[#Melikoglu--2018|Melikoglu 2018]] ). Ocean wave energy is abundant and predictable and can be extracted directly from surface waves or pressure fluctuations below the surface ( [[#Melikoglu--2018|Melikoglu 2018]] ). Its global theoretical potential is 29.5 PWh yr –1 (106 EJ yr –1 ),which means that wave energy alone could meet all global energy demand ( [[#Mørk--2010|Mørk et al. 2010]] ; [[#IRENA--2020b|IRENA 2020b]] ). The temperature gradients in the ocean can be exploited to produce energy, and its total estimated available resource could be up to 44.0 PWh yr –1 (158 EJ yr –1 ) ( [[#Rajagopalan--2013|Rajagopalan and Nihous 2013]] ). Salinity gradient energy, also known as osmotic power, has a global theoretical potential of over 1.6 PWh yr –1 (6.0 EJ yr –1 ) ( [[#IRENA--2020b|IRENA 2020b]] ). The greatest advantage of most marine energy, excluding wave energy, is that their sources are highly regular and predictable, and energy can be furthermore generated both day and night. An additional use of sea water is to develop lower-cost district cooling systems near the sea ( [[#Hunt--2019|Hunt et al. 2019]] ). The greatest barrier to most marine technology advances is the relatively high upfront costs, uncertainty on environmental regulation and impact, need for investments and insufficient infrastructure ( [[#Kempener--2014a|Kempener and Neumann 2014a]] , b). There are also concerns about technology maturity and performance; thus, not all have the potential to become economically viable ( [[#IRENA--2020b|IRENA 2020b]] ).

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