Synergies in an integrated Ocean Thermal Energy system

Abstract

Small tropical islands are facing major issues. Most of them rely on imported fossil fuels, which makes electricity generation both expensive and unsustainable. Moreover, small tropical islands are more vulnerable to the consequences of climate change than the mainland. Effects of climate change can be: more and more powerful hurricanes, shrinking fresh water resources, crop failures, land loss and coastal ecosystem change. An Ocean Thermal Energy system can be the solution for a tropical island. Cold deep seawater can be pumped up to generated baseload electricity in an onshore Ocean Thermal Energy Conversion power plant. The same deep ocean water can be used to cool houses, hotels, data centers, greenhouses and more. Furthermore, fresh water can be produced in an Ocean Thermal Water Plant and aquaculture and algae farms can use the nutrient-rich, virtually pathogen-free deep ocean water to cultivate fish and algae. In this work, a comparison has been made for the water usage and cold water pipe diameter between a non-integrated and a time-integrated Ocean Thermal Energy system. The cold deep seawater has to be pumped up by a large and long pipe. To reach the depth of about 1000 m, the cold water pipe can be a few kilometers long, depending on the slope of the seabed. To make an Ocean Thermal Energy system economically more attractive, an optimization study can be made for the maximum water demand and cold water pipe diameter. To do this, a modular model of an Ocean Thermal Energy system is developed in Python 2.7. The model exist out of modules for the Ocean Thermal Energy Conversion, Sea Water Air Conditioning, data center cooling, greenhouse cooling and fresh water production.
A case study has been conducted for an Ocean Thermal Energy system in Cura\c{c}ao. Based on the expected electricity and cooling demands, a comparison has been made between a non-integrated and a time-integrated Ocean Thermal Energy system. The results show that the maximum cold deep seawater mass flow and the required cold water pipe diameter in case of an time-integrated Ocean Thermal Energy system are lower, but the difference with a non-integrated system is not significant. Peak demands can be compensated with thermal storages. The optimization with thermal storages are applied to Sea Water Air Conditioning, data center cooling, greenhouse cooling and fresh water production. When the daily fluctuations are compensated, the decrease in maximum water demand and cold water pipe diameter is larger compared to the time-integrated Ocean Thermal Energy system. The optimization to compensate seasonal fluctuations result in an even higher decrease in cold water mass flow and pipe diameter. However, very large storage volumes are required and it has to be researched if the reduction in cold water pipe cost outweigh the cost of the thermal storages. Changing the configuration of the subsystems in the Ocean Thermal energy system also decreases the maximum water demand and cold water pipe diameter. The results show that a significant decrease is possible when only the Ocean Thermal Energy Conversion power plant and the Sea Water Air Conditioning are connected to the cold deep seawater. The maximum water demands of the subsystem are researched in case that the Ocean Thermal Energy system is expanded or when the Ocean Thermal Energy Conversion power plant is shut down in favor of Sea Water Air Conditioning or data center cooling. Furthermore the sensitivity of key parameters have been investigated to determine the impact of a small change on the maximum water mass flow and the cold water pipe diameter