Cost effective OTEC (ocean thermal energy conversion) electrical power plant by OTEC beats warming
Ocean thermal is the most abundant renewable energy resource on earth. This cost effective OTEC proposal provide electricity at 2 cent/kwh.
OTEC (ocean thermal energy conversion) was first proposed in 1881. This concept has not been properly developed. The oceans on earth already stored more than two years of solar energy transmitted to earth in the thermal form in the top 1000 meter depth. Tropical ocean with surface to 1000 meter depth temperature difference of 20 C can power Rankine cycle heat engines using ammonia as working fluid to provide more than 50 tera-watt of electricity at 2 cents per kilowatt hour.
The OTEC electrical power plant design procedure starts with selecting a potential power plant site and measure the seasonal ocean temperature profile (surface to 1200 meter depth). Rankine cycle analysis using the ammonia temperature/entropy/enthalpy data is conducted. The pumping power requirements for moving deep ocean water to submerged cold heat exchanger (at 40 to 50 meter depth) through a main cold water pipe with water flow speed of less than 2 meter/sec and pipes in the heat exchanger is calculated using the Moody graph. Pumping power required for moving warm surface water through submerged hot heat exchanger (at 25 to 35 meter depth) is also calculated. The masses and displacements for the marine grade aluminum heat exchangers, steel connecting pipes to collect the vaporized ammonia to the inlets of turbine/ inductors, steel pipes to distribute ammonia from turbine outlet to condensers for condensing into liquid, and supporting structure for all these connecting tube components are calculated. The complete structure has slightly positive buoyancy.
The proposed structure has some portion above the ocean surface. The structure can be assembled in the ocean. The cost of a proposed 50 megawatt (MW, net power) OTEC plant is approximately 75 million dollars. The Moody graph can be used to support the design of smaller scale 7 MW or 140 KW models at cost of 20 and 2 million dollars. Gravity anchors using steel cables can keep the OTEC operating power plants at the selected locations.
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About this proposal
Some initial design information will be in this 30000 character portion of the proposal. All interested person are invited to join this proposal. The 30000 character limit is not enough for the file already generated. More details and other revisions will expand this initial published information toward the 30,000 character limit. The proposal in separate file will get revisions to add more information.
I am sure there are many MIT alumni that know physics, mechanics, thermodynamics, fluid mechanics and material science better than me and can make this proposal much better. I am not MIT alumni, but I understand this contest is not limited to MIT alumni. I know my proposal is already posted in this contest. I hope to get some MIT alumni to join this proposal effort.
The initial action is to post this proposal and share the design concept. After reading this posting people can contact me to get more information, I hope there is fund to expand the analysis and conduct tests to validate the analysis. Eventually, we are going to solve the global energy and warming problem. The Paris climate meeting without OTEC is bound to fail.
Energy problem by itself is serious. Using fossil fuels will produce green house gas and make the global warming problem worse. The best solution for both problem is to use renewable energy without green house gas generation. Carbon capture after burning fossil is at best taking one giant step backward before trying to move one step forward. The backward step is easy, but the forward step is not technologically feasible now. I hope information in this proposal and other information can convince people with funding to look at OTEC more carefully and realize OTEC is the solution.
Global energy resource identification and realistic renewable energy solution
Solar energy is delivered to earth at the rate 5000 times the human energy demand. The sun is delivering energy to earth at the rate of 160,000 tera-watt. Solar energy delivered to earth is a year is 1,400,000,000,000,000,000 kilo-watt-hour (10 to 18 power in kwh, or 5 x 10 to 21 power Btu). The stored thermal energy in the top 1000 meter depth is more than two years of delivered solar energy, at 100 % conversion efficiency, the stored energy is sufficient for 10,000 years of human use. The stored thermal energy is completely renewable so that ocean thermal energy use will not reduced the stored energy quantity. Actual ocean thermal energy conversion efficiency at any location is likely to be around 3 %. When OTEC is implemented in 5 % of the earth surface area (which is 7% of the ocean covered area,less than 15 % of the tropical ocean area), the 3% conversion efficiency will be sufficient to provide energy at 0.15.% of the total solar energy level, which is 7.5 times current total human energy need. OTEC is more than adequate for total human energy demand for any realistic long term energy production planning.
Ocean surface temperature range is from -2 to 32 C. The salt content (about 2.5 % by weight) in ocean water caused the freezing temperature to be around -2 C, with slight variation related to salt content level. The -2 C high salt content water with the highest possible density cannot stay near the ocean surface. This cold water has the tendency to fill the bottom of all oceans. At ocean area with more than 200 meter depth, near surface mixing will make surface temperature below 32 C. Only small, shallow bays can reach higher temperature.
OTEC electrical power plant design procedure
Identify ocean temperature condition for the power plant site
The proposal description (calculation) is based on surface water temperature of 27 C and deep water temperature of 5.5 C. At the hot heat exchanger outlet, the ocean water temperature is reduced by 1 C to 26 C. With the temperature difference of 0.5 C between the ocean water and the working fluid, the ammonia working fluid is heated to 25.5 C. 5.5 C cold water from the 1000 meter depth is heated to 9 C at the cold heat exchanger outlet. With the same 0.5 C temperature difference between the ocean water and working fluid, the working fluid is cooled to 9.5 C. The Rankine cycle operation temperature range is from 9.5 C to 25.5 C. At the OTEC plant operation site, ocean water temperature may vary. The corresponding Rankine cycle temperature range will be adjusted according to the actual seasonal ocean water temperature. The OTEC power plant electricity generation capacity will have up to 25 % seasonal change. The 25 % electrical power plant output change is much less than wind turbine power change. During the time the wind speed is below the wind turbine cut off limit, wind power generator cannot generate any electricity at all.The following Rankine cycle analysis is just an example of the necessary analysis procedure. Different water temperature range will lead to different Rankine cycle analysis result. Rankine cycle is used for more than 90 % of the thermal engines for global electricity supply. Most of the Rankine cycle engines use water/steam as working fluid. For OTEC, ammonia or Freon with higher pressure at 20 C is used as working fluid.
Conduct Rankine cycle analysis
At 25.5 C, ammonia entropy at the vapor/liquid mixture to pure vapor phase boundary is 10.0084 Joules/gram-K. The enthalpy is 520.65 J/g. The isentropic (constant entropy) expansion will intercept the 9.5 C line with enthalpy at 455.60 J/g. The enthalpy change is 65.05 J/g. The pumping energy required to push the condensed liquid ammonia into the higher pressure evaporator (hot heat exchanger) is 0.675 J/g. The enthalpy increase from the pure liquid ammonia at 9.5 C to pure vapor at 25.5 C is (718.58 + 520.65)J/g. The ideal conversion efficiency is energy converted to electricity divided by thermal energy input at the evaporator and is 65.05/(718.58 + 520.65 +0.675) = 65.05/1239.91 = 0.0525 = 5.25 %. The OTEC power plant with well designed real turbine/inductor probably can achieve close to 5 % energy conversion efficiency. The global OTEC electrical generating capacity estimate in the previous paragraph use the reduced 3 % as the energy conversion efficiency, lower than the theoretical Rankine cycle limit. The temperature/entropy /enthalpy values used in this paragraph is from a 1978 paper by L.Haar and J. S Gallagher. Entropy and enthalpy values are tabulated using zero points selected by the investigator.
At the rate of 65.05 J/g energy conversion rate, OTEC power plant with 55 MW gross electrical power generation capacity will need ammonia working flow rate requirement of 8,460,000 gram/sec, or 8.46 metric ton per second. Using the reduced 4 % conversion efficiency (instead of the 5.25 %), the required ammonia flow rate will be 11 metric ton per-second.
This proposed OTEC plant use 9.3 meter internal diameter main cold water pipe with average ocean water flow speed of 1.6 meter/sec. The cold ocean water flow rate is 107 cubic meter per-second. The temperature increase of 3.5 C will allow each cubic meter of ocean water to absorb 14.7 mega-Joule (MJ) of thermal energy. The cold heat exchanger is able to discharge 1570 MJ of thermal energy.
For the cold heat exchanger to remove 1570 MJ of thermal energy, the hot heat exchanger must absorb 1650 MJ of thermal energy. With each gram of warm ocean water supplying 4.2 Joules of thermal energy, the required warm ocean water flow rate is 400 cubic meters per second.
With direct sunlight, the maximum rate of energy transfer to earth is 1350 watt-per-square-meter. The rotation of the earth, spreading solar energy onto circular ring will cause the maximum time averaged energy rate to reduce by a factor of pi to 430 watt-per-square meter, or 430 mega-watt-per-square-kilometer. The required 1650 MW thermal input need 4 square kilometer area energy collecting area. The ocean is a much better solar energy concentrator than most devices build to concentrate solar energy for phto-voltaic purpose.
Compared to other Rankine cycle thermal engines (nuclear or fossil fuel power plants) for electricity generation (conversion), OTEC has much smaller working fluid temperature range. This constraint is well known to all OTEC technology investigators. Ammonia (or Freon) working fluid is adequate for the small temperature range. At the moderate temperature and pressure range corrosion is not a serious problem for necessary heat exchanger or turbine components materials.
Off shore OTEC plant site is more cost effective
Few OTEC investigators pay sufficient attention to the 1000 meter minimum distance between the thermal energy source (ocean surface warm water) and sink (deep ocean cold water). At a result, cost effective OTEC power plant design has not been developed. Vertical cold water pipe is the shortest path to bridge this distance. Moody diagram (or Darcy Weisbach method) widely used for fluid mechanic calculation indicated that the pressure required to sustain fluid flow within a circular flow channel is P = [F(density)(V)(V)(L)]/(2d). where F is a coefficient related to the Reynolds number of the fluid flow condition, V is flow velocity (squared in the equation), L is channel length and d is channel diameter. The pressure (and the corresponding power) required to move fluid is proportional to the flow velocity squared. For any specific flow volume, flow velocity is inversely proportional to the channel diameter square. The Moody equation indicates that pumping pressure is inversely proportional to the flow channel diameter to the fifth power. In order to minimize the power to connect the thermal energy source and sink, the cold water pipe need large flow channel diameter.
Vertical cold water pipe is much shorter than ocean bottom terrain following cold water pipe connected to shore. The moody diagram also indicated that pressure requirement is proportional to flow channel length. On shore OTEC plant need cold water pipe ten or more times longer and is therefore not cost effective. On shore OTEC plant design concept can be rejected based on cold water transport distance factor alone. Ocean depth of 1000 meter or more are likely to located beyond 12 nautical miles from shore. Many OTEC power plants will be in the economical exclusion zone. It is highly desirable to provide adequate international law and security protection for operating OTEC power plants outside of the territorial water region.
Calculate component masses and displacements
Floating structure in the ocean must follow the rise and fall of the waves so that the attached rigid cold water pipe must be subjected to strain proportional to the motion amplitude and stress proportional to the displacement (mass). Submerged structure with neutral buoyancy will be under much less strain and stress. The proposed OTEC power plant therefore use submerged structure. The cold water pipe will also be flexible so that strain and stress in any part of the complete power plant will be transmitted to other parts through slightly flexible connection at reduced stress level.
The production OTEC plant will have 7 units with 2352 evaporators and condensers in each unit. There is also a cold water distribution block. All units and the cold water distribution block are built inside protected harbor. The final assembly using just nuts and bolts can be made at the assembly site with more than 1000 meter water depth. Anchors at the assembly site can keep the assembly operation from drifting in the ocean. The steel reinforced concrete cold water distribution block with ballast adjustment capability is also used to support the main cold water pipe vertical assembly. Lower sections of the main cold water pipe is vertically lowered through the center opening of the block. After the next section is attached, the assembled pipe portion is lowered, ready for the attachment of the next section. When the main cold water pipe is completely assembled, the cold water distribution block is lowered into the ocean, allowing the central unit to reach the region above and make cold water channel connection. The six outside units are than connected. The completed assembly depth can be controlled by ballast tanks in the units and cold water distribution block. All 7 units with 2352 evaporator and condenser cells and the connecting tubes can be supported by steel reinforced concrete blocks. The completed OTEC plant can be towed to the selected electricity generating site with previously installed anchors waiting.
The 7 units together have 16464 evaporators and condensers. The cylindrical shape evaporator is 5.2 meter tall with 0.75 m diameter. Within each evaporator, 9 pairs of inverted U shape ocean water flow channels with 12 cm diameter is used to move heating ocean water. Working fluid filling the space outside the ocean water flow channel can absorb thermal energy through heat conducted across the thin marine grade aluminum flow channel wall. The length of each inverted U tube is 10 meters. Heat conduction area for each evaporator is approximately 34.2 meter square. Inside each condenser, 8 meter length inverted U tube is used. The heat transfer area in each condenser is 27.3 meter square.
Making ocean water to flow through the 1000 meter main cold water pipe represents the primary ocean water pumping requirement. To move the ocean water at the rate of 107 cubic meter per second inside the channel with 9.3 meter diameter the required pumping power is 0.8 MW, more than 1 % of the OTEC power plant net power. Moving hot and cold ocean water through the channels inside the evaporators and condensers will bring the total ocean water pumping requirement to no more than 1 MW, which is less than 2% of the power plant electricity generation capacity.
The pressure change due to ocean water flow in the main channel can be calculated using the Moody diagram equation. The Reynolds number for the main cold water channel is about 18,500,000,000, with the corresponding friction factor of 0.005. The pressure change in 1000 meter length is 1.06 psi, similar to height of 0.71 meter in earth's gravitational field. The power required to move cold water at the rate of 107 cubic meters per-second is 0.76 MW. The pressure required to move hot and cold water in the 10 and 8 meter long heat exchanger ocean water channels can be calculated in similar manner. The total ocean water pumping power requirement can be maintained at 1 MW level.
The mass of each (of the 7) unit is around 10,800 metric ton, with enclosed volume of 90,000 cubic meter. The mass of the cold water distribution block is 6,000 metric ton with enclosed volume of 45,000 cubic meter. The mass of all blocks is around 82,000 metric ton with enclosed volume of close to 700,000 cubic meter. The cold water pipe has 1100 metric ton mass and enclosed volume of 80,000 cubic meter. The massive submerged structure is attached to many buoys at the ocean surface using flexible steel cable connection. In case of severe weather, the submerge portion can be lowered into greater ocean depth with less motion and induced stress. I am sure the OTEC plant can survive any tropical storm because at the submerged depth the strain and stress is low.
Transmission of generated power to users
For OTEC power plants within 50 miles of shore, HVDC (high voltage direct current) cable is the power transmission device. Electrical power transmission at higher voltage can reduce current and ohmic loss. The higher voltage will need more insulting dielectric material between conductor at different electrical potential. The trade off for voltage and insulation is likely to resulted in voltage of 400 kilo-volt. AC (alternate current) time average dielectric load is at about 50 % of the material dielectric strength. DC (direct current) is better because the time average use of the material strength is nearly 100 %. HVDC is the new preferred means for electrical power transmission in the world. For OTEC plants located more than 50 miles from shore, the proposed power transmission system is more complicate. Using the generated electricity and other resource to synthesize ammonia is tentatively selected. Ammonia energy supply system will consisted of electrolytic water decomposition, ammonia synthesis, liquid ammonia storage under pressure (no more than 200 psi) and tanker transport. Liquid ammonia tanker is probably better than liquid hydrogen tanker because liquid ammonia energy density by volume is twice as high as liquid hydrogen energy density. The distance the ammonia tankers need to travel to the energy user is much shorter than the distance that crude oil tanker need to travel. The ammonia fuel distribution system maintenance should be easier than the existing crude oil distribution system.
Materials for OTEC plant construction
Marine grade aluminum with thickness (and geometry) sufficient to resist 200 psi pressure can be used as the primary heat exchanger material.The connecting tubing to feed the working fluid vapor to the turbine inlet and collecting the vapor from the turbine exhaust can use lower cost steel material. In the sketches I made to show the evaporator /condenser connection, hexagon cell arrangement is used. The 2352 cell in each unit is 7 to the fourth power (2401) subtracting space sufficient for 49 cells at the center used for the turbines/inductors.
Thermal conduction and pressure calculations
Heat conduction capability for exchanger with thin aluminum wall separating the ocean water and working fluid at the minimum 0.5 C temperature difference can be calculated. The basic thermal conduction equation is Q = KAT/t, where K is the thermal conductivity of the heat conduction material (aluminum at 2w/[cm-C]), A is the cross sectional area of the heat flow path, T is the temperature difference across the heat flow path and t is the heat flow path length (where 0.5 C is the minimum temperature difference). For the 16464 evaporators, the total thermal energy transport area is 560,000 square meter. Across 0.06 cm thick wall, the total heat quantity transfer will be 94,000 MW. The initial calculation using this equation indicates that this heat transfer result would be 50 times higher than the required heat conduction rate of 1650 MW. Calculation for the condenser will lead to similar result.
The main cold water pipe is placed in depth from 60 to 1000 meters with external pressures of up to 1500 psi. There is no internal cavity in the main cold water pipe structure so that the cold water pipe wall will not be subjected to significant static pressure. Ocean water movement will cause the OTEC plant to move. The flexible connection among the rigid portions of the OTEC plant units allowed the rigid parts to transmit motions through reduced force level. The hot and cold heat exchangers are placed in depths from 20 to 60 meters with pressures from 30 to 90 psi. The thermal conduction walls and working fluid transport channels are designed for internal/external pressure difference of 250 psi. During the time of tropical storm, the OTEC plant can be lowered into the calmer, deeper position at higher pressure difference.
The analysis presented here is for the production OTEC power plant with 50 MW power generation capacity. In order to minimize development cost and project risk, smaller ocean OTEC power plant model needs to be tested and all difficulties resolved before the production scale plant construction. The first ocean test model probably should have 48 evaporator and condensing cells. The central space that would have one set of evaporator and condenser cell is the location for turbine /inductor. Using the Moody diagram to scale the ocean water pumping power, the cold water pipe diameter is 0.86 meter. This first ocean model is smaller than the production model by a factor of 343, so that the electricity generating capacity is 140 kilo-watt. The important pre-production OTEC model is a unit with 2352 evaporators and condensers. This OTEC test device at 7 times smaller than the production should be able to generate 7 MW of electricity. The main cold water pipe diameter for the unit size model (2352 evaporators and condensers) is 4.3 meter. If the 50 MW production OTEC plant can be built with 75 million US dollars, the proportional cost of the 140 kilo-watt and 7 MW OTEC plant would be 200,000 and 11 million dollars. It may be possible to test the two smaller models with 2 and 40 million dollars. The cost estimate should be able to include some component redesign and testing. More analysis is needed to make better cost estimates. Smaller model testing temperature data can provide corrections for the Rankine cycle analysis parameter so that more precise prediction of production model generating capacity can be determined. Small model testing for more than a year can provide accurate seasonal electricity generation capacity prediction.
It is highly desirable that during the time of smaller model testing, there is strong tropical storm in the OTEC plant testing area. The stress and strain on the smaller model can then be measured. The stress and strain change related to OTEC plant submersion depth can be recorded. The concept of using greater submersion depth to survive storms can be validated with small and production OTEC power plant models. OTEC power plants can be expected to generate electricity during a storm. The greater depth with less temperature difference between the hot and cold heat exchanger is expected to reduce the electricity generating capacity by no more than 10 %.
Turbine and inductor design analysis has not been included. There are several companies that can design the turbines and inductors. Turbine using ammonia fluid at 10 to 30 C temperature should be easy task for turbine experts. Both steel and aluminum material should be able to meet the temperature and structural strength need easily. If we use 30 tera-watt as the projected future global peak electrical energy requirement, the global need is 600,000 OTEC power plant at 50 MW generating capacity. Using two set of turbines (for redundancy) in each unit, the total global demand for turbine/inductor is on the order of 9 million. The large turbine quantity definitely justify extensive design and testing effort to make sure the turbines are reliable. It is not reasonable make plans for OTEC dominance of global energy supply now. I am sure successful initial demonstration of the OTEC promise after about 10 production OTEC plant operation will make OTEC the recognized global energy and global warming mitigation solution.
More detailed information related to the volume and mass of the aluminum and steel materials will be added later. Aluminum is the material for heat exchanger walls.The total aluminum mass is about 1700 metric ton. At 5000 dollars a ton, the aluminum material cost is 8.5 million dollars. Steel is the material for ammonia working fluid flow and ocean water flow channels .The total mass for steel material is 9600 metric ton. At 1000 dollars a ton, the steel material cost is 9.6 million dollars. The total steel reinforced concrete material mass (for all 7 units, used to support the steel and aluminum components and cold ocean water distribution block) is 82,000 metric ton. At 300 dollars a ton, the cost is 25 million dollars. The raw total cost for these three important materials is 43 million dollars. Allocation of 29 million dollars should be able to cover the cost of other much smaller components such as turbines, inductors pumps and operational control devices. The components will use aluminum, steel and many other materials not mentioned in this proposal.
Three gravity anchors made with steel reinforced concrete arranges as triangle at the ocean bottom should be able to prevent the OTEC power plant from drifting in the ocean. With 2000 metric ton mass and 400 metric ton steel cable, the cost of each anchor is around 1 million dollars. The cost of 3 anchor will bring the total estimate cost for a production OTEC plant to 75 million dollars. Ocean current or tropical storm at the OTEC plant operation site may generate force to move the OTEC plant. The anchors must provide sufficient force to prevent the OTEC plant from drifting. The force to move the OTEC plant is expected to be dependent on ocean water flow speed. The anchors probably need to resist water flow speed up to 3 knots. Water flow speed is expected to diminish at greater depth. The anchor cables lengths need to be adjusted so that at the OTEC plant depth the force level is below the capability of the anchors. Heavier anchors (and cables) would be necessary for OTEC plants operating in locations with stronger ocean current or stronger tropical storms.
Development time minimum requirement
It is possible to complete the small 48 evaporator/condenser OTEC model design within 1 year of funding at 10 million dollar level. The funding should be sufficient for construction and initial test of the 140 kilo-watt OTEC plant in the ocean. After successful conclusion of the 2352 cell (evaporator and condenser) model construction and test with about 40 million dollars and two years' time, the production scale OTEC plant can begin. 5 to 10 year is sufficient to demonstrate OTEC is the global renewable energy solution. The projected 600,000 units of 50 MW OTEC plant would take more than 30 years to build. The reduced electricity price and profit for the OTEC plant builder may be able to speed up the OTEC adoption schedule.
OTEC can also be used for ocean water desalination. The dominant natural desalination is the evaporation/condensation distillation process. The theoretical energy requirement for making 1 cubic meter of fresh water from ocean water is 1 kwh (3,6 MJ). Even though reverse osmosis is able to get 1 cubic meter of fresh water from ocean water at 3 kwh energy. The energy need to be in the form of electrical energy or some mechanical form. A better desalination procedure using the ocean is to use ocean thermal energy directly. The warm ocean surface water can provide the thermal energy to generate water vapor at the evaporator. The cooler deep ocean water can condense the vapor into water. The energy required to move the heating and cooling ocean water through the heat exchangers can be much less than the theoretical energy requirement of 1 kwh for each cubic meter. Ocean thermal energy can easily provide 40 MJ of thermal energy to get 1 cubic meter of fresh water. The cost of desalinate ocean watercan be less than 10 cents for each cubic meter.
Ocean already make the world more comfortable
The ocean of the earth already help the earth to be in more comfortable living condition. Based on the area available to intercept sunlight, the area below 30 degrees latitude would receive about 61 % of solar energy. The area above 30 degree latitude would only receive 39 % of the solar energy. These two regions of 50 % of the earths' surface area radiate energy at roughly 53 % to 47 % ratio. The warmer region (below 30 degree latitude) therefore transfer about 8 % of the solar energy to the colder region already. Ocean current is a major cause for moving some received solar energy from the lower latitude region to the higher latitude region make the lower latitude region cooler and higher latitude region warmer. Having OTEC power plant in the lower latitude region may slightly reduce the ocean current and the related excessive thermal energy (polar) movement from the lower latitude region. If the OTEC energy is transported to the higher latitude region consumers, the net thermal energy transfer difference may be reduced. Weather forcast and reporting agencies usually say that tropical storms moving from lower latitude to higher latitude transfer thermal energy. With more OTEC plants making the tropical surface temperature slightly cooler, the strength of tropical storms and storm related damage may reduce.
Invitation for contest proposal partners
Any interested person wanting to contact me can use my email address firstname.lastname@example.org. I am not yet sure about how to get my Word file information (30 pages, with sketches) into this proposal format. You can be my team member and participate in this proposal editing. I am going to type in close to 30,000 characters to provide the maximum information under the limit. I will also edit my input to improve this proposal.