Ocean Thermal Energy Conversion (OTEC): A source thoroughly researched, but barely implemented

Ocean Thermal Energy Conversion (OTEC) was proposed in 1881 by D’Arsonval. However, OTEC has since failed to be implemented globally, because it has the issue of low efficiency and high costs for OTEC plant production. Currently, the only operating OTEC plants in the world are in Japan and Hawaii.

OTEC uses the thermal energy of the ocean as its source, which is renewable, as this source of energy comes from the Sun's solar radiation which heats the seawater. This creates a temperature difference between the different depths of the ocean, being hotter at the surface and cooler at greater depths.

There are 2 main types of OTEC, in terms of the process of energy generation: closed-cycle or open-cycle, but let’s start with closed-cycle. The key difference between the two lies in the fact that in closed-cycle, the working fluid is permanently contained within the system. The temperature difference of the ocean is harnessed in closed-cycle OTEC by a Rankine cycle, which is a cycle that has the following components: evaporator, condenser and a turbine and generator to ultimately generate the energy. A working fluid (typically ammonia or an ammonia-water mixture) is used in this cycle, and is transferred between the components, by pumps. The working fluid typically has a boiling point that is within the temperature range of OTEC, approximately 5 ºC to 25 ºC. This is so that the working fluid can vaporize into a vapour during the cycle, which is crucial as a vapour produces more power when it spins a turbine than a liquid.

Figure 1. Closed-cycle OTEC Schematic Diagram: https://www.mdpi.com/2077-1312/11/1/140

At point 1, a pump brings warm, surface seawater to the evaporator, in which the working fluid vaporises into a gas. This is then brought to the turbine, spinning it, which runs the generator and so, energy is produced. Since it is renewable, at point 2, another pump brings cold, deep seawater to the condenser, where the vaporised working fluid condenses back into a liquid form, for the cycle to repeat.

Previously, we said closed-cycle is where the working fluid is permanently contained within the system, while in open-cycle it is not. Open-cycle OTEC utilises a similar process as it also uses a Rankine cycle, however this time, water is the working fluid. Since water has a boiling point of 100 ºC, the warm, surface seawater is introduced into the evaporator, which is at a low pressure, so that water can be vaporised at a lower temperature than its normal boiling point, at 1 atm. This is similar to those experiments you may have seen where water boils at high alititudes (this is because at high altitudes the pressure is lower). The vapour then drives a generator, by spinning the turbine, which generates electrical energy. Then at the condenser, the water vapour is condensed by the cold, deep seawater pumped up from below. This condensed water is now desalinated and so, can be used for various human purposes. While the cold, deep seawater pumped up can be used in air conditioning systems. This system is considered “open” because the water is the working fluid and is the same as the ocean water.

Figure 2. Open-cycle OTEC schematic: https://www.engineeringa2z.com/ocean-thermal-energy-conversion/#google_vignette

Comparing these two primary cycles, closed-cycle OTEC offers a more efficient use of the thermal resource as it can generate more electricity, with lower costs, as opposed to open-cycle, which requires large heat energy costs. So, closed-cycle OTEC will be preferred when energy generation is the primary objective, which is the goal of the majority of countries that would implement OTEC. Whereas, open-cycle OTEC has the ability to produce fresh water that has several alternative uses. Therefore, open-cycle will be particularly relevant for countries with water scarcity.

Additionally, OTEC cycles typically have very low efficiencies, which is primarily due to the fact that only low temperature differences are able to be extracted from the ocean. So, this results in the process firstly having a low Carnot efficiency. Carnot efficiency is the theoretical maximum efficiency that a cycle can have as it assumes no energy loss from the cycle. However, since all real cycles have energy loss, the real cycle efficiency of OTEC will be even lower than this already low Carnot efficiency. The typical difference in temperature that OTEC plants can harness is about 25˚C, resulting in a Carnot efficiency of only 8%.

Therefore, this inefficiency of OTEC, coupled with the fact that OTEC plants are very expensive, leaves potential constructors questioning why OTEC plants should be built. Also, OTEC plants are restricted to being situated at coastal locations, like Japan and Hawaii. However, other countries like Singapore and those in the Middle East can be potential locations for OTEC plants, as they are situated at the Equator and so, their seas will have a greater temperature. Currently, research is being conducted to improve OTEC's low efficiency by focusing on the type of working fluid used or by increasing the temperature difference that OTEC utilises, typically by solar heating.