An assessment of ocean thermal energy conversion resources in the South China Sea

Abstract

The South China Sea (SCS) is a marginal sea of the western Pacific Ocean, encompassing a large area over 3.5 million km2. It is characterized by warm tropical and subtropical water with the surface temperature around 24–30 D°C throughout the year, which makes it a suitable region for the Ocean Thermal Energy Conversion (OTEC). However, the estimates of this new type of renewable energy in the SCS are still a back-of-the-envelope calculation with great discrepancy. Wu and Jiang (1988) estimated the total ocean thermal energy resources using the heat capacity method and gave a very rough result of 33 TW under the assumption of homogeneous horizontal temperature distribution due to the paucity of data. Nihous (2005) presented a new method of estimating the ocean thermal energy using a simple one-dimensional time-domain model of the ocean thermal structure. This method takes into account that the possible disruption of the ocean vertical thermal structure by the massive seawater flow rates required for sustainable OTEC operations, and therefore can make a more accurate and realistic estimation of the OTEC resource. In this study, the OTEC resources over the entire SCS are assessed following Nihous' method based on the most recent World Ocean Atlas data (2013). The estimated maximum steady-state OTEC electrical power is about 0.5 TW with consideration of Carnot efficiency. We also calculated the distribution of the power density in the SCS following the heat capacity method. The results show that except the shallow northern area the power density is all above 300 kW/km2 with small annual and interannual variations, suggesting a long-term steady and rich power resource. Through the analysis of the temperature difference, the offshore distance and the bathymetrical and socio-economical characteristics, the area around the Xisha and Nansha Islands is found to be ideal for the implementation of OTEC operations, where the temperature difference between the warm surface and 1000 m depth always ranges from 20 ° (in winter) to 26 °C (in summer) and the cold water intake depth is about 500–600 m within a short offshore distance of 10 km. Under present conditions and standardized OTEC operations, three candidate sites that are suitable for OTEC power plants are selected around the Xisha Islands and Nansha Islands. The selection rules used here are 1) the sea surface temperature ranges from 24–28 ° with an annual average of 26°; 2) the temperature at 1000m depth is between 4–5 ° with an annual average of 4.5 °; 3) short distance from cold seawater with a steep topographic slope (15–20) and a relatively smooth seafloor (Vega et al., 2010). With a 10MW designed power output of OTEC facility, all of the three candidate sites can yield an annual gross electrical power of 13–22 MW and 10–19 MW net power. And these plants could produce about 80 GWh of electricity per year and about 22,000 cubic meters of desalinized water per day.