FAQ's & A's - Technology
Why are you confident that your technology will work?
Our confidence that our technology will work is corroborated by at least six factors:
Why use DC current instead of AC?
We would typically choose to send the electrical power that we generate back to shore in the form of “direct current” or “DC” instead of “alternating current” or “AC”. This is because AC power can react with the seawater and cause arcing (i.e. sparks) and a loss of efficiency. However, DC power doesn’t interact with the seawater and tends to be the more efficient method of transmitting electrical power along the seafloor.
“…for transmission of large amounts of electric power through long submarine cables, direct current (DC) is preferred over AC, because DC cables require no reactive power. As well, for three phase AC-cables three conductors are necessary, while for DC only 1 or 2 conductors are required.”
Also, high-voltage DC (“HVDC”) tends to be very efficient. Subsea HVDC cables tend to lose only about 3% of the power transmitted through them for every 1,000 km of cable.
Could we build smaller devices to deploy them closer to shore and therefore have a shorter cable? What is the expected power generated by such devices?
We can build lower-power devices, and, while those devices will have smaller diameters, their length will remain unchanged. Our device leverages the motion of the waves at the surface of the sea against the stillness of the water about 20 meters below the waves. Regardless of its diameter, our devices must still extend from the surface to the still water beneath the waves.
We could build shorter devices, and these would still generate power. However, the amount of power they generated would be limited to the degree to which the relative motion of the surface waves and the water around the submerged venturi tube was preserved.
Our confidence that our technology will work is corroborated by at least six factors:
- “Black & Veatch” analyzed our concept and concluded… “B&V… believes that… Spindrift… could potentially be a competitive device and more economic than others…”
- A CFD analysis reported in the literature determined that an analogous device (i.e. similar in design to a Rotech Tidal Turbine) driven by less energetic fluid flow would generate 113 kW.
- A damped oscillator model of our full-scale wave energy device shows an optimal, ¼-phase latency.
- The similar “Rotech Tidal Turbine” has been proven to work and is being deployed.
- The sea trial of our prototype venturi tube was successful. And,
- Our design is consistent with the design of other “impulse turbines” and our device should work for the same reasons that they do.
Why use DC current instead of AC?
We would typically choose to send the electrical power that we generate back to shore in the form of “direct current” or “DC” instead of “alternating current” or “AC”. This is because AC power can react with the seawater and cause arcing (i.e. sparks) and a loss of efficiency. However, DC power doesn’t interact with the seawater and tends to be the more efficient method of transmitting electrical power along the seafloor.
“…for transmission of large amounts of electric power through long submarine cables, direct current (DC) is preferred over AC, because DC cables require no reactive power. As well, for three phase AC-cables three conductors are necessary, while for DC only 1 or 2 conductors are required.”
Also, high-voltage DC (“HVDC”) tends to be very efficient. Subsea HVDC cables tend to lose only about 3% of the power transmitted through them for every 1,000 km of cable.
Could we build smaller devices to deploy them closer to shore and therefore have a shorter cable? What is the expected power generated by such devices?
We can build lower-power devices, and, while those devices will have smaller diameters, their length will remain unchanged. Our device leverages the motion of the waves at the surface of the sea against the stillness of the water about 20 meters below the waves. Regardless of its diameter, our devices must still extend from the surface to the still water beneath the waves.
We could build shorter devices, and these would still generate power. However, the amount of power they generated would be limited to the degree to which the relative motion of the surface waves and the water around the submerged venturi tube was preserved.
Isn’t it true that no power is available inside a tube?
Mass flow in must equal mass flow out. For an incompressible fluid (i.e. water) this means that velocity in equals velocity out - no change in kinetic energy and no energy extraction.
It is true that water is incompressible, and that velocity in must equal velocity out. However, the water entering the venturi tube will be highly pressurized while the water leaving the tube will have a much lower pressure. Pressure is a form of potential energy. The energy that will be extracted by our devices will ultimately be derived from the drop in the water’s pressure potential energy - and that pressure gradient will ultimately be created at the expense of energy in the waves that drive the devices.
The viability of our technology is supported by its similarity to the following:
1. The “Rotech Tidal Turbine” (“RTT”) is essentially a hydrokinetic turbine inside a venturi tube – just like the Spindrift device. It has already been validated, and proven to generate power. 1/20th scale wave tank testing completed in Glasgow, Scotland, in May of 2004. And, large-scale deployments of 1 MW devices are underway, e.g. off the coast of South Korea.
2. Reaction turbines, including “Francis” and “Kaplan” turbines, remove pressure from the water flowing through them. Pressure potential energy is converted, by the turbines, into additional kinetic energy, and that “added” kinetic energy is then extracted by the turbine’s “propeller” or “runner”. So, even though water is incompressible; and, even though the volume of water entering these turbines is identical to the volume of water which they discharge; power is nonetheless extracted.
There are many reaction turbines with this design which are successfully generating electrical power throughout the world. And, these turbines typically operate at efficiencies of over 90%.
3. Impulse turbines, like “Pelton” and “Turgo” turbines, typically first convert the hydraulic potential energy (Ep = mgh) of water (i.e. its “head” pressure) into additional speed and kinetic energy (Ek = mv2/2) by passing the water through a nozzle (i.e. a special type of venturi tube). The resulting “jet” of water is then directed against the blades of the turbine’s runner, where most of its “amplified” speed and kinetic energy are removed. As in the case of reaction turbines, the resulting discharge from an impulse turbine can be carried away through a pipe with a diameter equal to the diameter of the pipe delivering the water to the nozzle. And, as with reaction turbines, impulse turbines typically operate at efficiencies over 90%.
4. “Reverse pump” turbines are made from devices initially intended for use as a pumps. When a pump is turned on, electrical power energizes the motor, which spins the rotor, which results in a decrease in the water pressure at the inlet, and an increase in the water pressure at the outlet. When a pump is operating, water travels into, and out from, the rotor through pipes of equal diameter, and at a constant speed – only the water’s pressure is affected.
When the flow of water is reversed, and high-pressure water is directed into the pump’s rotor in the “wrong” direction, then the attached motor can be forced to run in reverse and generate electrical power. The diameters of the pipe into, and out of, a pump are usually identical (see Figure 4). When run in reverse, water flows into, and out from, the pump at an identical speed, and with identical kinetic energy, and is nonetheless able to drive the pump’s rotor (which is now acting as a hydrokinetic turbine) and cause the motor to generate, instead of consume, electrical power. It does this by extracting energy from the pressure of the water driving the rotor.
If it could be made to work, it will be inefficient. This device would work on following waves through a long stroke, making it incapable of taking the height (power) out of the wave.
Mass flow in must equal mass flow out. For an incompressible fluid (i.e. water) this means that velocity in equals velocity out - no change in kinetic energy and no energy extraction.
It is true that water is incompressible, and that velocity in must equal velocity out. However, the water entering the venturi tube will be highly pressurized while the water leaving the tube will have a much lower pressure. Pressure is a form of potential energy. The energy that will be extracted by our devices will ultimately be derived from the drop in the water’s pressure potential energy - and that pressure gradient will ultimately be created at the expense of energy in the waves that drive the devices.
The viability of our technology is supported by its similarity to the following:
1. The “Rotech Tidal Turbine” (“RTT”) is essentially a hydrokinetic turbine inside a venturi tube – just like the Spindrift device. It has already been validated, and proven to generate power. 1/20th scale wave tank testing completed in Glasgow, Scotland, in May of 2004. And, large-scale deployments of 1 MW devices are underway, e.g. off the coast of South Korea.
2. Reaction turbines, including “Francis” and “Kaplan” turbines, remove pressure from the water flowing through them. Pressure potential energy is converted, by the turbines, into additional kinetic energy, and that “added” kinetic energy is then extracted by the turbine’s “propeller” or “runner”. So, even though water is incompressible; and, even though the volume of water entering these turbines is identical to the volume of water which they discharge; power is nonetheless extracted.
There are many reaction turbines with this design which are successfully generating electrical power throughout the world. And, these turbines typically operate at efficiencies of over 90%.
3. Impulse turbines, like “Pelton” and “Turgo” turbines, typically first convert the hydraulic potential energy (Ep = mgh) of water (i.e. its “head” pressure) into additional speed and kinetic energy (Ek = mv2/2) by passing the water through a nozzle (i.e. a special type of venturi tube). The resulting “jet” of water is then directed against the blades of the turbine’s runner, where most of its “amplified” speed and kinetic energy are removed. As in the case of reaction turbines, the resulting discharge from an impulse turbine can be carried away through a pipe with a diameter equal to the diameter of the pipe delivering the water to the nozzle. And, as with reaction turbines, impulse turbines typically operate at efficiencies over 90%.
4. “Reverse pump” turbines are made from devices initially intended for use as a pumps. When a pump is turned on, electrical power energizes the motor, which spins the rotor, which results in a decrease in the water pressure at the inlet, and an increase in the water pressure at the outlet. When a pump is operating, water travels into, and out from, the rotor through pipes of equal diameter, and at a constant speed – only the water’s pressure is affected.
When the flow of water is reversed, and high-pressure water is directed into the pump’s rotor in the “wrong” direction, then the attached motor can be forced to run in reverse and generate electrical power. The diameters of the pipe into, and out of, a pump are usually identical (see Figure 4). When run in reverse, water flows into, and out from, the pump at an identical speed, and with identical kinetic energy, and is nonetheless able to drive the pump’s rotor (which is now acting as a hydrokinetic turbine) and cause the motor to generate, instead of consume, electrical power. It does this by extracting energy from the pressure of the water driving the rotor.
If it could be made to work, it will be inefficient. This device would work on following waves through a long stroke, making it incapable of taking the height (power) out of the wave.
This conclusion is refuted by the following observations:
- “Damped oscillator simulations” (see Figure 5) indicate that the vertical oscillations of a Spindrift device will have a substantial phase latency relative the vertical oscillations of the waves that drive it. This is a characteristic of point-absorbing wave energy devices that extract power from, and thereby “dampen”, ocean waves.
And, as discussed above, the inherent design and operation of a Spindrift device is typical of the class of hydrokinetic turbines referred to as “impulse” turbines. And those turbines are capable of operating at or near a 90% efficiency level.
Have you considered the effect of cavitation on the turbine blades?
Cavitation (i.e. the formation of tiny bubbles) may be a problem. These devices will be accelerating the seawater to a speed close to, and sometimes reaching, the "choke speed" of the water. Cavitation will occur just before the choke speed is reached. As a result, it may be necessary to swap out turbines every ten years or so - we'll have to see. However, this kind of "grand overhaul" every few years is already factored in to our cost estimates.
However, as water moves through a venturi tube, the pressure of the water decreases as the forward speed of the water increases. And, the damage caused by “cavitation” is caused when the bubbles “pop”, not when they form. Because the pressure of the water will be very low at the time that cavitation occurs, we expect that those bubbles will not “pop”, but will instead “deflate” as slowly as they appeared. If this proves to be correct, then these cavitation-induced bubbles would not be expected to cause any damage to the turbine.
The turbine will have some angular momentum that will have to be reversed with each cycle. Is this a significant factor?
We have two alternate device designs. One uses a rigid turbine which will cause the shaft to reverse its direction of spin each time the water reverses its direction of flow. The other uses a "bi-directional" turbine in which the blades change their angle of attack (i.e. rotate) in response to changes in the flow of water. No matter what direction the seawater comes from, the turbine will turn in the same direction. These types of bi-directional turbines are already used in oscillating water columns. However, we have a custom (i.e. a better) design for our wave energy devices.
The rigid turbine that reverses its direction of rotation should be more reliable since it doesn’t have any moving blades (i.e. its just a solid piece of metal). However, the control system for this design will be more complicated. The bi-directional design may increase maintenance requirements (we believe only slightly). However, it will allow for a much simpler control system. We’ll have to wait for sea trial data to see which proves to be the most advantageous.
Is you device a point absorber?
Yes our device is a point absorber, which means that it doesn't care which direction waves approach from.
What are the risks should a cable become frayed or compromised while the generated energy is being transmitted?
If a cable is damaged or broken then the cable would short out. It would cause a shutdown of the buoys (we would "turn them off" so that they stopped generating power). It would require us to bring the damaged portion of the cable to the surface and repair it.
Have you built some yet? How small can they go before efficiency drops off?
We've built and tested several small prototype venturi tubes. We are beginning to build prototypes that generate power. The small devices will be efficient - the venturi effect works in both small and large venturi tubes.
How will these tube handle higher wave action, such as hurricanes?
Spindrift devices will handle very high wave action without trouble. They will ride up-and-down on the waves just like navigational buoys. In fact, they will keep generating power. The only potential problem would be if the wave climate grew so violent that it exceeded the limits built in to the mooring and power cables interconnecting the devices, e.g. during a "once-in-a-century" type of storm.
However, this network of cables, interconnecting a farm of devices, will be made with a great enough range of dynamic tensioning so that the cabling network should survive almost any storm. But, if the limits of the network are in danger of being exceeded then the devices will be designed to automatically disconnect from each other and allow secondary cables to hold the network together. These secondary cables will be a fail-safes which will allow the devices an even greater range of motion while they ride out the storm. We would then reconnect the buoys to their primary cables after the storm is over. However, this would be a very unlikely event.
If the power-generating buoys are not anchored, and are floating around, how are they connected to the subsea power cable?
All of the buoys are interconnected by mooring cables located at the surface. These cables connect each buoy to its neighbors. These cables are dynamically tensioned (using floats and weights) to allow the buoys freedom of motion, but to still cause them to stay in their proper position within the farm. At two to four points along the perimeter of each farm, special mooring buoys (buoys connected by cable to an anchor on the seafloor) keep the entire farm near its desired location.
Power cables (attached to the associated surface mooring cables) connect each buoy to at least one other buoy, allowing the power to be collected and brought together to be fed into a single power cable. Since the far end of the subsea power cable (i.e. the end near the farm of Spindrift devices) will be lifted from the sea floor to the surface, and since the motion of the buoy supporting that cable will be inhibited to some degree by the weight and inertia of that cable, there will be at least one buoy in every farm dedicated to providing the farm with access to the primary power cable. The lightweight power cables connecting the power-generating buoys will collect at this dedicated power cable buoy so that the collected power can be sent to shore.
Are there any problematic issues regarding the water(humidity) protection of the exposed cables/circuits?
We will have to take care to ensure that humidity and sea water won't be able to damage the cables connecting the Spindrift devices to each other, and connecting the two or three "anchor" buoys per farm to the sea floor. However, this need to protect from corrosion the cables, and other exposed components, used in or near the sea, is an old one. And, right now, we're planning on using "impressed currents" to prevent corrosion. And we'll refine our strategy as we gain experience with our prototypes.
How do you ensure the integrity of the cables especially with regard to mechanical protection? Will the cables be running in flexible HDPE conduits/pipes?
The subsea power cables will likely be high-voltage DC cables. They will be buried about 6 to 10 feet under the sea floor. This will afford them a great deal of protection from mechanical damage, corrosion, etc. The cables will be directly buried, without any casing like HDPE conduits. This is the standard deployment method.
“On a general basis… high voltage cables [have] a proven lifetime of more than 70 years. A large number of the cables in successful service are more than 50 years old… submarine HVDC cables will be [at least] 50-60 years.” There's a reference pdf on HVDC transmission and lifetime expectancy here.
- “Damped oscillator simulations” (see Figure 5) indicate that the vertical oscillations of a Spindrift device will have a substantial phase latency relative the vertical oscillations of the waves that drive it. This is a characteristic of point-absorbing wave energy devices that extract power from, and thereby “dampen”, ocean waves.
And, as discussed above, the inherent design and operation of a Spindrift device is typical of the class of hydrokinetic turbines referred to as “impulse” turbines. And those turbines are capable of operating at or near a 90% efficiency level.
Have you considered the effect of cavitation on the turbine blades?
Cavitation (i.e. the formation of tiny bubbles) may be a problem. These devices will be accelerating the seawater to a speed close to, and sometimes reaching, the "choke speed" of the water. Cavitation will occur just before the choke speed is reached. As a result, it may be necessary to swap out turbines every ten years or so - we'll have to see. However, this kind of "grand overhaul" every few years is already factored in to our cost estimates.
However, as water moves through a venturi tube, the pressure of the water decreases as the forward speed of the water increases. And, the damage caused by “cavitation” is caused when the bubbles “pop”, not when they form. Because the pressure of the water will be very low at the time that cavitation occurs, we expect that those bubbles will not “pop”, but will instead “deflate” as slowly as they appeared. If this proves to be correct, then these cavitation-induced bubbles would not be expected to cause any damage to the turbine.
The turbine will have some angular momentum that will have to be reversed with each cycle. Is this a significant factor?
We have two alternate device designs. One uses a rigid turbine which will cause the shaft to reverse its direction of spin each time the water reverses its direction of flow. The other uses a "bi-directional" turbine in which the blades change their angle of attack (i.e. rotate) in response to changes in the flow of water. No matter what direction the seawater comes from, the turbine will turn in the same direction. These types of bi-directional turbines are already used in oscillating water columns. However, we have a custom (i.e. a better) design for our wave energy devices.
The rigid turbine that reverses its direction of rotation should be more reliable since it doesn’t have any moving blades (i.e. its just a solid piece of metal). However, the control system for this design will be more complicated. The bi-directional design may increase maintenance requirements (we believe only slightly). However, it will allow for a much simpler control system. We’ll have to wait for sea trial data to see which proves to be the most advantageous.
Is you device a point absorber?
Yes our device is a point absorber, which means that it doesn't care which direction waves approach from.
What are the risks should a cable become frayed or compromised while the generated energy is being transmitted?
If a cable is damaged or broken then the cable would short out. It would cause a shutdown of the buoys (we would "turn them off" so that they stopped generating power). It would require us to bring the damaged portion of the cable to the surface and repair it.
Have you built some yet? How small can they go before efficiency drops off?
We've built and tested several small prototype venturi tubes. We are beginning to build prototypes that generate power. The small devices will be efficient - the venturi effect works in both small and large venturi tubes.
How will these tube handle higher wave action, such as hurricanes?
Spindrift devices will handle very high wave action without trouble. They will ride up-and-down on the waves just like navigational buoys. In fact, they will keep generating power. The only potential problem would be if the wave climate grew so violent that it exceeded the limits built in to the mooring and power cables interconnecting the devices, e.g. during a "once-in-a-century" type of storm.
However, this network of cables, interconnecting a farm of devices, will be made with a great enough range of dynamic tensioning so that the cabling network should survive almost any storm. But, if the limits of the network are in danger of being exceeded then the devices will be designed to automatically disconnect from each other and allow secondary cables to hold the network together. These secondary cables will be a fail-safes which will allow the devices an even greater range of motion while they ride out the storm. We would then reconnect the buoys to their primary cables after the storm is over. However, this would be a very unlikely event.
If the power-generating buoys are not anchored, and are floating around, how are they connected to the subsea power cable?
All of the buoys are interconnected by mooring cables located at the surface. These cables connect each buoy to its neighbors. These cables are dynamically tensioned (using floats and weights) to allow the buoys freedom of motion, but to still cause them to stay in their proper position within the farm. At two to four points along the perimeter of each farm, special mooring buoys (buoys connected by cable to an anchor on the seafloor) keep the entire farm near its desired location.
Power cables (attached to the associated surface mooring cables) connect each buoy to at least one other buoy, allowing the power to be collected and brought together to be fed into a single power cable. Since the far end of the subsea power cable (i.e. the end near the farm of Spindrift devices) will be lifted from the sea floor to the surface, and since the motion of the buoy supporting that cable will be inhibited to some degree by the weight and inertia of that cable, there will be at least one buoy in every farm dedicated to providing the farm with access to the primary power cable. The lightweight power cables connecting the power-generating buoys will collect at this dedicated power cable buoy so that the collected power can be sent to shore.
Are there any problematic issues regarding the water(humidity) protection of the exposed cables/circuits?
We will have to take care to ensure that humidity and sea water won't be able to damage the cables connecting the Spindrift devices to each other, and connecting the two or three "anchor" buoys per farm to the sea floor. However, this need to protect from corrosion the cables, and other exposed components, used in or near the sea, is an old one. And, right now, we're planning on using "impressed currents" to prevent corrosion. And we'll refine our strategy as we gain experience with our prototypes.
How do you ensure the integrity of the cables especially with regard to mechanical protection? Will the cables be running in flexible HDPE conduits/pipes?
The subsea power cables will likely be high-voltage DC cables. They will be buried about 6 to 10 feet under the sea floor. This will afford them a great deal of protection from mechanical damage, corrosion, etc. The cables will be directly buried, without any casing like HDPE conduits. This is the standard deployment method.
“On a general basis… high voltage cables [have] a proven lifetime of more than 70 years. A large number of the cables in successful service are more than 50 years old… submarine HVDC cables will be [at least] 50-60 years.” There's a reference pdf on HVDC transmission and lifetime expectancy here.
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