|Fig. 1: Byproducts from Open Cycle OTEC. (Source: Wikimedia Commons.)|
According to the EIA, in 2010 the world consumed 524 quadrillion Btu of energy and the world's energy consumption is expected to continue to increase.  Ocean thermal energy conversion (OTEC) is a way to renewably produce energy from the difference in water temperature from the surface to the depths of the ocean. OTEC was first proposed by J. A. D'Arsonval in 1881 and in 1930 G. Claude tested a concept of OTEC off the shores of northern Cuba.  In 1979, Lockheed Missiles and Space Company and Dillingham Corporation in partnership with the Hawaiian Government began building the first floating OTEC plant off the western coast of Hawaii.  Around 1996, another OTEC plant in Hawaii, operated by Pacific International Center for High Technology Research (PICHTR) produced 255 kW of gross electrical power and 103kW of net electrical power.  This particular OTEC plant run by PICHTR is an open cycle OTEC system.  There are multiple different types of OTEC systems and I will be focusing on open cycle OTEC and closed cycle OTEC. 
Closed cycle OTEC is D'Arsonval's original concept of OTEC.  In a closed cycle system, the working fluid is evaporated by the temperature of the warm seawater and then the vapor expands through the turbogenerator producing electricity.  In order to complete the cycle, the expanded vapor goes through the condenser, condensed by the cold seawater, and then pressurized by the boiler feed pump.  The seawater supply system accounts for most of the parasitic power consumption.  The best working fluids for a closed cycle OTEC system have a very low boiling points; for example, ammonia, chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and hydrofluorocarbons(HFCs).  Due to the Montreal Protocol, CFCs and HCFCs are (or already are) being phased out of production. This reduces the list of working fluids to ammonia and HFCs. The disadvantages for using closed cycle OTEC are the properties of the working fluid and the possible biofouling of the heat exchangers.  Ammonia is toxic even at low concentrations and HFCs are a greenhouse gas.  To alleviate these concerns, Claude proposed using steam generated by using warm seawater as the OTEC working fluid and thus the open cycle OTEC system was born. 
Open cycle OTEC starts by flash evaporating the warm seawater in a partial vacuum, of pressures around 1% to 3% of the atmosphere.  Then the expansion of the steam through the turbine generates the electricity after which the vapor condenses as it contacts the cold seawater.  Finally, any condensate and residual noncondensable gas is compressed and discharged.  The reason this process is called open cycle OTEC is due to the discharge of the steam after only one pass through the cycle.  The majority of the parasitic power consumption comes from the initial evacuation of the system and other operations performed by the vacuum compressor along with the seawater and discharge pumps.  There are two different types of condensers that can be implemented into the open cycle OTEC design,direct contact condenser (DCC) and surface condenser. The DCC is responsible for dispensing cold seawater over the water vapor and is inexpensive and efficient due to the direct contact between the different temperature fluids.  The surface condenser is more expensive and harder to maintain since it operates using a physical separator between the warm and cold water, but the byproduct it produces is fresh water.  Some of the disadvantages of open cycle OTEC systems is that operating at partial vacuum conditions is vulnerable to "air-in-leakages" and promotes the production of noncondensable gases.  As a result, some power is consumed to pressurize and remove these gases.  Furthermore, the low steam density requires a larger volumetric flow rate to produce a unit of electricity.  Let's now calculate the seawater flow rate (Q) for the 255kW open cycle OTEC plant in Hawaii that is operated by PICHTR. The seawater flow rate (Q) can be expressed by the equation below.
η ρ Cp ΔT
where P is the power (255 kW), η is the system's net efficiency (0.061 for an 85% efficient turbine), ρ is the density of the seawater (106 gm m-3), Cp is the specific heat of the seawater 4.2 joules gm-1 °C-1), and ΔT is the difference in temperature between the warm and cold seawater (21.5 °C).  The seawater flow rate for a 255 kW open cycle OTEC plant is 0.046 m3 sec-1. This is equivalent to filling 46 liter bottles full of seawater every second. This may not sound like a lot but as the power demand for the OTEC plant is increased to hundreds of MW of power, the seawater flow rate will also increase dramatically. For example if the power of this plant is increased to 100 MW, then Q will equal 18 m3 sec-1, which is an increase of about 400 times. However, a portion of the generated power is needed to move the fluid through the pipes. In order to calculate this part of the parasitic power consumption for the 255kW OTEC plant in Hawaii, the change in pressure between the warm seawater to the cold seawater is multiplied by the flow rate that was previously calculated. The maximum theoretical pressure difference, for the open cycle plant in Hawaii, is approximately 2700Pa.  Using the calculated flow rate of 0.046 m3 sec-1, the parasitic power consumption is about 124 watts. This means that of the gross total power of 255kW, 124 watts of power are being used to keep the cycle running. This is why it is important to pay attention to the net power produced by the OTEC plant.
It is predicted that in the year 2040, the world will consume 820 quadrillion Btu of energy.  More renewable energy methods will have to be integrated into our society in order to meet energy needs as our supply of fossil fuels runs out. Ocean thermal energy conversion is one method for producing renewable energy. A common concern with developing OTEC is the impact on the environment. For smaller scale OTEC plants, reports by the NOAA have concluded that the environmental impact is small but that there is not enough evidence to determine the magnitude of the environmental impact from commercial OTEC plants.  More information on the environmental impact of OTEC will be discovered once more large scale OTEC plants are operational. As seen in Fig. 1, OTEC is not just a great way to produce electrical energy but can also be used to desalinate seawater, provide cold water for air conditioning and irrigation, as well as provide nutrient rich water for mariculture. 
© Andrea Eller. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 "International Energy Outlook 2013," U.S. Energy Information Administration, DOE/EIA-0484(2013), July 2013.
 S. M. Masutani and P. K. Takahashi, "Ocean Thermal Energy Conversion," in Wiley Encyclopedia of Electrical and Electronic Engineering, Vol. 15, ed. by J. G. Webster (Wiley, 1999), p. 93.
 J. T. Harrison, "The 40 MWe OTEC Plant at Kahe Point, Oahu, Hawaii: A Case Study of Potential Biological Impacts," U.S. National Oceeanic and Atmospheric Administration, Technical Memorandum NOAA-TM-NMFS-SWFC-68, February 1987.