Editing Ocean thermal energy conversion (section)

== Technical difficulties ==

=== Dissolved gases ===

The performance of direct contact heat exchangers operating at typical OTEC boundary conditions is important to the Claude cycle. Many early Claude cycle designs used a surface condenser since their performance was well understood. However, direct contact condensers offer significant disadvantages. As cold water rises in the intake pipe, the pressure decreases to the point where gas begins to evolve. If a significant amount of gas comes out of solution, placing 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 evolves in the top {{convert|8.5|m|ft|sp=us}} of the tube. The trade-off between [[degassing|pre-deaeration]]<ref>{{cite web|url=https://www.merriam-webster.com/dictionary/deaerate|title=Definition of DEAERATE|website=www.merriam-webster.com}}</ref> of the seawater and expulsion of 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 indicate vertical spout condensers perform some 30% better than falling jet types.

=== Microbial fouling ===

Because raw seawater must pass through the heat exchanger, care must be taken to maintain good [[thermal conductivity]]. [[Biofouling]] layers as thin as {{convert|25|to|50|μm}} can degrade heat exchanger performance by as much as 50%.<ref name=microbial-countermeasures/> 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.<ref name=microbial-fouling>{{cite journal |vauthors=Aftring RP, Taylor BF |title=Assessment of Microbial Fouling in an Ocean Thermal Energy Conversion Experiment |journal=Appl. Environ. Microbiol. |volume=38 |issue=4 |pages=734–739 |date=October 1979 |pmid=16345450 |pmc=243568 |doi= 10.1128/AEM.38.4.734-739.1979|bibcode=1979ApEnM..38..734A }}</ref> 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.<ref name=microbial-fouling/>

Another study concluded that fouling degrades performance over time, and determined that although regular brushing was able to remove most of the microbial layer, over time a tougher layer formed that could not be removed through simple brushing.<ref name=microbial-countermeasures>{{cite journal |vauthors=Berger LR, Berger JA |title=Countermeasures to Microbiofouling in Simulated Ocean Thermal Energy Conversion Heat Exchangers with Surface and Deep Ocean Waters in Hawaii |journal=Appl. Environ. Microbiol. |volume=51 |issue=6 |pages=1186–1198 |date=June 1986 |pmid=16347076 |pmc=239043 |doi= 10.1128/AEM.51.6.1186-1198.1986|bibcode=1986ApEnM..51.1186B }}</ref> The study passed sponge rubber balls through the system. It concluded that although the ball treatment decreased the fouling rate it was not enough to completely halt growth and brushing was occasionally necessary to restore capacity. The microbes regrew more quickly later in the experiment (i.e. brushing became necessary more often) replicating the results of a previous study.<ref name=microbial-brush-cleaning>{{cite journal |vauthors=Nickels JS, Bobbie RJ, Lott DF, Martz RF, Benson PH, White DC |title=Effect of Manual Brush Cleaning on Biomass and Community Structure of Microfouling Film Formed on Aluminum and Titanium Surfaces Exposed to Rapidly Flowing Seawater |journal=Appl. Environ. Microbiol. |volume=41 |issue=6 |pages=1442–1453 |date=June 1981 |pmid=16345798 |pmc=243937 |doi= 10.1128/AEM.41.6.1442-1453.1981|bibcode=1981ApEnM..41.1442N }}</ref> The increased growth rate after subsequent cleanings appears to result from selection pressure on the microbial colony.<ref name=microbial-brush-cleaning/>

Continuous use of 1 hour per day and intermittent periods of free fouling and then [[Water chlorination|chlorination]] periods (again 1 hour per day) were studied. Chlorination slowed but did not stop microbial growth; however chlorination levels of 0.1&nbsp;mg per liter for 1 hour per day may prove effective for long term operation of a plant.<ref name=microbial-countermeasures/>  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.<ref name=microbial-countermeasures/>

Besides water temperature, microbial fouling also depends on nutrient levels, with growth occurring faster in nutrient rich water.<ref name=biofilm-dynamics>{{cite journal |last1=Trulear |first1=Michael G. |last2=Characklis |first2=William G. |title=Dynamics of Biofilm Processes |journal=Journal of the Water Pollution Control Federation |date=1982 |volume=54 |issue=9 |pages=1288–1301 |jstor=25041684 }}</ref> The fouling rate also depends on the material used to construct the heat exchanger. [[Aluminium]] tubing slows the growth of microbial life, although the [[aluminium oxide|oxide]] layer which forms on the inside of the pipes complicates cleaning and leads to larger efficiency losses.<ref name=microbial-brush-cleaning/>  In contrast, [[titanium]] tubing allows biofouling to occur faster but cleaning is more effective than with aluminium.<ref name=microbial-brush-cleaning/>

=== Sealing ===

The evaporator, turbine, and condenser operate in partial vacuum ranging from 3% to 1% of atmospheric pressure. The system must be carefully sealed to prevent in-leakage of atmospheric air that can degrade or shut down operation. In closed-cycle OTEC, the specific volume of low-pressure steam is very large compared to that of the pressurized working fluid. 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 [[Losses in electrical systems|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.