Various fluids have been proposed over the past decades to be used in closed OTEC cycle. A popular choice is ammonia, which has superior transport properties, easy availability, and low cost. Ammonia, however, is toxic and flammable. Fluorinated carbons such as CFCs and HCFCs would be a better choice, if they did not contribute to ozone layer depletion. Hydrocarbons too are good candidates, but they are highly flammable; in addition, this would put OTEC in competition with use of them directly as fuels. The power plant size is dependent upon the vapor pressure of the working fluid. For fluids with high vapor pressure, the size of the turbine and heat exchangers decreases while the wall thickness of the pipe and heat exchangers should increase to endure high pressure especially on the evaporator side
Technical difficulties
Degradation of heat exchanger performance by dissolved gases
A very important technical issue pertaining to the Claude cycle is the performance of direct contact heat exchangers operating at typical OTEC boundary conditions. Many early Claude cycle designs used a surface condenser since their performance is well understood. However, direct contact condensers offer significant disadvantages. As the warm sea water rises in the intake pipes, the pressure decreases to the point where gas begins to evolve. If a significant amount of gas comes out of the solution, designing a gas trap before the direct contact heat exchangers may be justified. Experiments simulating conditions in the warm water intake pipe indicated about 30% of the dissolved gas evolve in the top 8.5 m of the tube. The tradeoff between pre-deaeration of the sea water and expulsion of all the non-condensable gases from the condenser is dependent on the gas evolution dynamics, deaerator efficiency, head loss, vent compressor efficiency and parasitic power. Experimental results have indicated vertical spout condensers perform some 30% better than falling jet types.
Degradation of heat exchanger performance by microbial fouling
Because raw seawater must be passed through the heat exchanger care must be taken to maintain good thermal conductivity. Biofouling layers as thin as 25 to 50 μm can degrade heat exchanger performance by as much as 50%. A 1977 study in which mock heat exchangers were exposed to seawater for ten weeks concluded that although the level of microbial fouling was low, the thermal conductivity of the system was significantly impaired. The apparent discrepancy between the level of fouling and the heat transfer impairment is the result of a thin layer of water trapped by the microbial growth on the surface of the heat exchanger.
Another study, conducted in 1985 at Keahole Point, Hawaii, also concluded that microbial fouling degrades performance over time, as well as studying possible countermeasures to the degradation. The study determined that although regular brushing was able to remove most of the microbial layer, over longer periods of time a tough layer formed on the surface of the exchanger which could not be removed through simple brushing. Additionally the study conducted trials of passing sponge rubber balls through the system. It concluded that although the ball treatment decreases the rate at which fouling occurs it was not enough to completely halt growth and brushing was occasionally necessary to restore full heat transfer capacity. Furthermore, the microbes began to regrow more quickly later in the experiment (i.e. brushing became necessary more often); this confirms the results of a previous study done under similar conditions.[16] The reason for the increased growth rate after subsequent cleanings appears to be the result of selection pressure acting on the microbial colony.
In addition to physical cleaning methods the use of chlorination was examined. Both continuous use of 1 hour per day and intermittent periods of free fouling and then chlorination periods (again 1 hour per day) were studied. Like the foam rubber ball treatment chlorination did not completely stop microbial growth, it merely slowed it; however chlorination levels of .1 mg per liter treated for 1 hour per day slowed microbial growth appreciably and may prove effective in the long term operation of a plant. Finally the study concluded that although microbial fouling was an issue for the warm surface water heat exchanger, the cold water heat exchanger suffered little or no biofouling and only minimal inorganic fouling.
Besides water temperature, microbial fouling also shows a dependence on several other factors. The most obvious factor in microbial growth is nutrient levels, with growth occurring faster in more nutrient rich water. The fouling rate also depends on the material used to construct the heat exchanger. Aluminum tubing slows the growth of microbial life, however the oxide layer which forms on the inside of the pipes makes cleaning more difficult leading to higher accumulated efficiency losses. In contrast, titanium tubing allows biofouling to occur faster but cleaning is more effective than with aluminum.
Improper sealing
The evaporator, turbine, and condenser operate in partial vacuum ranging from 3% to 1% atmospheric pressure. This poses a number of practical concerns. First, the system must be carefully sealed to prevent in-leakage of atmospheric air that can severely degrade or shut down operation. Second, the specific volume of low-pressure steam is very large compared to that of the pressurized working fluid used in the case of a closed cycle OTEC. This means components must have large flow areas to ensure steam velocities do not attain excessively high values.
Parasitic power consumption by exhaust compressor
An approach for reducing the exhaust compressor parasitic power loss is as follows. After most of the steam has been condensed by spout condensers, the non-condensible gas steam mixture is passed through a counter current region which increases the gas-steam reaction by a factor of five. The result is an 80% reduction in the exhaust pumping power requirements.
Degradation of heat exchanger performance by dissolved gases
A very important technical issue pertaining to the Claude cycle is the performance of direct contact heat exchangers operating at typical OTEC boundary conditions. Many early Claude cycle designs used a surface condenser since their performance is well understood. However, direct contact condensers offer significant disadvantages. As the warm sea water rises in the intake pipes, the pressure decreases to the point where gas begins to evolve. If a significant amount of gas comes out of the solution, designing a gas trap before the direct contact heat exchangers may be justified. Experiments simulating conditions in the warm water intake pipe indicated about 30% of the dissolved gas evolve in the top 8.5 m of the tube. The tradeoff between pre-deaeration of the sea water and expulsion of all the non-condensable gases from the condenser is dependent on the gas evolution dynamics, deaerator efficiency, head loss, vent compressor efficiency and parasitic power. Experimental results have indicated vertical spout condensers perform some 30% better than falling jet types.
Degradation of heat exchanger performance by microbial fouling
Because raw seawater must be passed through the heat exchanger care must be taken to maintain good thermal conductivity. Biofouling layers as thin as 25 to 50 μm can degrade heat exchanger performance by as much as 50%. A 1977 study in which mock heat exchangers were exposed to seawater for ten weeks concluded that although the level of microbial fouling was low, the thermal conductivity of the system was significantly impaired. The apparent discrepancy between the level of fouling and the heat transfer impairment is the result of a thin layer of water trapped by the microbial growth on the surface of the heat exchanger.
Another study, conducted in 1985 at Keahole Point, Hawaii, also concluded that microbial fouling degrades performance over time, as well as studying possible countermeasures to the degradation. The study determined that although regular brushing was able to remove most of the microbial layer, over longer periods of time a tough layer formed on the surface of the exchanger which could not be removed through simple brushing. Additionally the study conducted trials of passing sponge rubber balls through the system. It concluded that although the ball treatment decreases the rate at which fouling occurs it was not enough to completely halt growth and brushing was occasionally necessary to restore full heat transfer capacity. Furthermore, the microbes began to regrow more quickly later in the experiment (i.e. brushing became necessary more often); this confirms the results of a previous study done under similar conditions.[16] The reason for the increased growth rate after subsequent cleanings appears to be the result of selection pressure acting on the microbial colony.
In addition to physical cleaning methods the use of chlorination was examined. Both continuous use of 1 hour per day and intermittent periods of free fouling and then chlorination periods (again 1 hour per day) were studied. Like the foam rubber ball treatment chlorination did not completely stop microbial growth, it merely slowed it; however chlorination levels of .1 mg per liter treated for 1 hour per day slowed microbial growth appreciably and may prove effective in the long term operation of a plant. Finally the study concluded that although microbial fouling was an issue for the warm surface water heat exchanger, the cold water heat exchanger suffered little or no biofouling and only minimal inorganic fouling.
Besides water temperature, microbial fouling also shows a dependence on several other factors. The most obvious factor in microbial growth is nutrient levels, with growth occurring faster in more nutrient rich water. The fouling rate also depends on the material used to construct the heat exchanger. Aluminum tubing slows the growth of microbial life, however the oxide layer which forms on the inside of the pipes makes cleaning more difficult leading to higher accumulated efficiency losses. In contrast, titanium tubing allows biofouling to occur faster but cleaning is more effective than with aluminum.
Improper sealing
The evaporator, turbine, and condenser operate in partial vacuum ranging from 3% to 1% atmospheric pressure. This poses a number of practical concerns. First, the system must be carefully sealed to prevent in-leakage of atmospheric air that can severely degrade or shut down operation. Second, the specific volume of low-pressure steam is very large compared to that of the pressurized working fluid used in the case of a closed cycle OTEC. This means components must have large flow areas to ensure steam velocities do not attain excessively high values.
Parasitic power consumption by exhaust compressor
An approach for reducing the exhaust compressor parasitic power loss is as follows. After most of the steam has been condensed by spout condensers, the non-condensible gas steam mixture is passed through a counter current region which increases the gas-steam reaction by a factor of five. The result is an 80% reduction in the exhaust pumping power requirements.
Energy from temperature difference between cold air and warm water
In winter in coastal Arctic locations, the seawater temperature can be 40 degrees Celsius (70 °F) warmer than the local air temperature. Technologies based on closed-cycle OTEC systems could exploit this temperature difference. The lack of the need for long pipes to extract deep seawater might make a system based on this concept less expensive than OTEC.
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