Ocean Thermal Energy Conversion (OTEC) Bridge to 2050 Carbon-Neutral Goals by Luis Vega. PhD
Kona OTEC plant photo from SOEST
Imagine a source of energy that is completely natural and could be used to generate as much as half the energy consumed by humanity. It has no smokestack discharge into the atmosphere, does not interfere with the world’s fisheries, or prime agricultural land. And unlike solar and wind has been demonstrated to generate electricity 24/7/365.
This source of energy has been known for nearly 150 years since the publication of “Twenty Thousand Leagues under the Sea” by Jules Verne. Ocean thermal energy conversion (OTEC) technology uses the temperature difference between surface warmer water and deeper colder water encountered in tropical oceans as the source of thermal energy. There are two OTEC cycles, closed and open, whose technology has been proven and for which all required equipment is available. The first concept uses the relatively warm (24°C–30°C) surface water of tropical oceans to vaporize pressurized ammonia through an evaporator. The resulting vapor is then employed to drive a turbine generator. The cold ocean water transported to the surface from 800–1,000 m depths, with temperatures ranging from 8°C–4°C, condenses the ammonia vapor through a condenser. Because the ammonia circulates in a closed loop, this concept has been named closed-cycle OTEC (CC-OTEC). The CC-OTEC concept was demonstrated in 1979, when Hawaii and a consortium of United States companies provided more than 50 kW of power from a small plant mounted on a barge off Hawaii. Subsequently, a 100 kW land-based plant was operated in Nauru by a consortium of Japanese companies. These plants were operated for a few months to demonstrate the concept, and they were too small to be scaled to commercial-sized systems.
Alternatively, the second cycle uses ocean water as the working fluid. The surface water is flash evaporated in a vacuum chamber. The resulting low-pressure steam is used to drive a turbine generator, and the relatively colder deep seawater is used to condense the steam after it has passed through the turbine. This cycle can, therefore, be configured to produce desalinated water as well as electricity. This cycle is referred to as open-cycle OTEC (OC-OTEC), because the working fluid flows once through the system. The concept was demonstrated in Hawaii in the 1990s with a 210 kW plant.
It was also determined that, excluding the cost of environmental pollution due to the generation of electricity with fossil fuel plants, for OTEC there is a marked economy-of-scale such that plants of about 100 MW (say 100 times bigger that the experimental plants) are required to be cost competitive. For example, 10 of these 100 MW plants deployed in the ocean around Oahu would satisfy all residential, land transportation (with electric vehicles) and industrial energy requirements. However, as important as the experimental plants have been in demonstration the technology there are too small to extrapolate costs. A pre-commercial plant sized at about 5 MW and requiring a budget of as much as $500 million must be operated in situ and for at least one continuous year to obtain the records required to evaluate what is considered to be commercial sized plantships that could be deployed world-wide. This argument excluded niche markets in small-island-developing states (SIDS) wherein land-based plants sized at a few MW could be cost-effective.
Ninety-eight nations with access to the required ocean thermal resources within their 200-nautical mile exclusive economic zone (EEZ) have been identified. There is also a market for industrialized economies that could manufacture and supply the equipment required for OTEC plants, even if they do not have the required ocean thermal resources within their EEZs. The worldwide resource is equivalent to more than 7 Terawatts (i.e., equivalent to 70,000 plants with 100 MW capacity). Each 100 MW plant requires a capital investment of about $750 million, so the ultimate market, in a few decades, would be valued in the trillions of dollars.
Unfortunately, by the middle of the 1980’s USA government funding was curtailed before an OTEC plant of significant size was operated. This was due to the abundance of relatively cheap oil. This is relatively because costs due to the environmental effects of generating energy with fossil fuels were not accounted for. It has been estimated that currently accounting for this pollution would be equivalent to at least doubling the cost of a barrel of fuel.
Currently three relatively small experimental plants (20 to 100 kW) are operational in Hawaii, Okinawa and South Korea. These are extremely important as teaching tools and as the common refrain goes: 1 experimental plant is worth 1000 reports.
The major challenge, however, continues to be: How to finance relatively high capital investments that must be balanced by the expected yet to be demonstrated low operational costs?
Perhaps a lesson can be learned from the successful commercialization of Wind Energy due to consistent government funding of pre-commercial projects that led to appropriate and realistic determination of technical requirements and operational costs in Germany, Denmark and Spain. In this context, by commercialization we mean that equipment can be financed under terms that yield cost competitive electricity. This of course depends on specific conditions at each site.
Given that it takes decades for new energy technologies to reach maturity, it seems sensible to once more consider the ocean thermal resource as a renewable energy for the future. OTEC plantships providing electricity and desalinated water to shore stations would be implemented. In addition, OTEC plantships deployed along equatorial international waters could produce energy carriers, like ammonia and hydrogen, to support the post-fossil fuels era.
Perhaps the time has finally arrived for OTEC commercialization
