Authored by: Edited by Leslie Gordon Key points • An important design consideration of a wave-energy device is that it can move out of the way of severe storms. |
In recent years, there has been an increasing emphasis on sustainable-energy sources as part of ongoing efforts to combat climate change. The greatest success in the history of sustainable energy is hydroelectric power, which has been performing reliably for over 100 years. And more-recent technology effectively harnesses wind power, both on land and offshore. In fact, the use of wind power is now so widespread, it is easy to forget how recently the technology became acceptable for generating electric power. The question arises as to why a similar exploitation of the energy of ocean waves has not happened, a principal focus of this article. Also emphasized are the similarities between hydroelectric power and wave power, the principal connection being that water can be stored. A new wave-power device illustrates these similarities.
Wave energy has some significant advantages over wind energy, such as the high density of water compared to air. Wave energy is also more consistent than wind energy. The wind may die down abruptly, bringing wind turbines to a standstill, but waves will often continue for days after the wind that caused them has subsided. In addition, wave energy is more predictable than wind. Finally, and perhaps most significantly, wave energy offers the opportunity for energy storage that wind lacks.
However, there are significant problems associated with wave energy that go a long way towards explaining its relative lack of progress compared to wind energy. The surface of the world’s oceans is a hostile environment, highly unsuitable for mechanical and electrical equipment. Indeed, under severe weather conditions, the ocean surface becomes dangerous for ships. However, today severe storms rarely sink ships because of improvements in weather prediction and in communications. In contrast, wave-energy equipment sitting in a stationary position on the ocean surface typically has no way of avoiding storms. The most-severe storms, with their occasional “killer” waves, can destroy almost any obstacle they confront.
Materials of construction |
Also, the ocean floor is a difficult and expensive location for building equipment foundations, particularly if they must resist overturning moments due to high horizontal loads applied at sea level. The pollution of ocean waters by the release of foreign matter will no longer be tolerated. When such pollution happens, it is difficult and expensive to clean up. In addition, seawater is highly corrosive to many engineering materials. Finally, waves move past an energy-harvesting device intermittently and thus do not yield their energy continuously, but in bursts of high intensity.
In spite of these difficulties, there has been significant progress in wave-energy technology in Europe, particularly in the U.K. For example, the Pelamis system, developed in Scotland, is currently in operation off the coast of Portugal. This is a significant development, as the system is the first example of a wave-power device being connected to a national grid. There are plans for its expansion, which provides encouragement to those committed to harnessing the almost unlimited energy of ocean waves.
Apart from Pelamis, there are many other efforts underway in different parts of the world, using a wide variety of different devices. These generally fall into one of three categories as follows:
Point absorbers are relatively small compared to the wave length. The devices can capture energy from a wave front greater than the physical dimension of the device itself. Point absorbers harvest energy from waves arriving from any direction.
Wave-pump power calculations Float: Diameter = 10.0 ft, length = 138.0 ft, weight = 36,000 lb Case One: Operating pressure = 15 psig Case Two: Operating pressure = 45 psig |
Attenuators have their principal axis parallel to the direction of the incoming wave. Typically, a single mooring attachment permits the device to adjust its orientation to the wave direction. The Pelamis device is of this type.
Finally, terminators have their principal axis placed parallel to the direction of the incoming wave and therefore have the potential to absorb all or most of its energy. The terminator is the most promising of the three types.
Design considerations
A successful wave-power device must satisfy a number of design conditions. They are:
• No complex mechanical or electrical equipment should be located at the ocean surface.
• Power generation should use existing, conventional technology.
• The device must deliver energy continuously, not in an intermittent manner.
• It must be possible to remove surface-mounted equipment from the path of severe storms.
• Energy storage should preferably be a feature of the design.
• Deployment and recovery should be reversible.
• Anchors located on the ocean floor should be gravity type only, not requiring fixed attachment.
• Under no circumstances can foreign matter, particularly liquids, be released into the sea.
• Inexpensive materials should be selected to combat the corrosive effects of seawater.
• Control of the wave device should be land-based.
• The device should be of the terminator type to optimize energy harvesting.
These design conditions make a formidable list, but were nevertheless defined and accepted from the outset in attempting the design of a new wave-power device. The outcome of the design effort, the Glenbower Wave Pump (patent pending), is still in the concept phase. However, a description of the proposed configuration will show that all of the conditions have been satisfied.
The Glenbower Wave Pump uses the heaving action of ocean waves to pump seawater to shore, where it is passed through conventional turbines to generate electric power before being returned to the ocean. The system may include onshore water storage to provide continuous operation, particularly where topographical conditions are favorable. The equipment consists of a series of wave-pump assemblies and pipework to deliver the water to shore under a pressure suitable for power generation on dry land.
Plant-operating strategies Another approach is to use the wave farm to replenish the reservoir on a continuous basis and operate the generating equipment in the same way as a conventional hydroelectric plant. It’s also possible to use the wave farm to fill the reservoir on a 24-hr basis and generate power only during the hours of maximum demand. This is analogous to a pumped storage plant, except that electric power is not used to fill the reservoir. When high waves are present yet another method is suggested. Under these conditions, high-operating pressures produce the most efficient operation, but the pressure of the pumped seawater may be much higher than is required either to overcome the reservoir head, or drive the turbines. In such circumstances, the incoming water can first be passed through the turbines, entering at high pressure and exiting at a considerably lower pressure, but one which is still high enough for it to then enter the reservoir. This method makes the most effective use of high-pressure water being delivered by the wave farm in that it is twice passed through the turbines. A plant consisting of a wave farm and a reservoir can operate quite flexibly using one of the several methods. This should provide low cost, uninterrupted power for many years in the same reliable manner as existing hydroelectric power plants. In the present economic and political climate, in which interdependence is combined with instability, this prospect should be considered attractive. |
A single wave-pump assembly consists of the following components: one float; four horizontal anchors connected to the float by mooring lines; two vertical anchors located directly under the float, two reciprocating pumps connected between the float and the vertical anchors; two surge tanks, one sitting on each of the two vertical anchors; a discharge pipe to carry the water to shore, and a small air pipe to supply pressurizing air from shore to the pump assembly (See “Wave-pump assembly” illustration).
The float is fabricated from mild steel, or from a light, corrosion-resistant material such as fiberglass. Since the float is also used to increase the surge-tank volume, it is designed as a pressure vessel and hence is depicted as a cylindrical vessel with spherical ends. The four mooring lines permit vertical movement of the float, as well as limited horizontal movement — the upwind lines being typically taut and the downwind lines being slack.
The pumps are a reciprocating type, not unlike a traditional hand-cranked pump. The connection at each end of the pump permits rotation about both horizontal axes. This lets the float respond to the random action of the waves, and also ensures that the pumps remain in pure tension or compression under the loading imposed by the rising and falling of the wave. The configuration of the pumps is generally similar to that of conventional air or hydraulic cylinders, except the rod is hollow and is used as the discharge pipe to convey the pumped water into the surge tanks.
All of the anchors, the four horizontal as well as the two vertical, are of similar construction, made from reinforced concrete and shaped in the form of an inverted cup. The cavity formed by the cup is divided into three compartments. When the anchor is being towed to site, the three cavities are partially filled with air so the anchors will float. The provision of three separate cavities allows the air volume in each to be adjusted to ensure anchor stability. Once the pipe assembly is at the site, the air is allowed to partially escape. The anchor is then lowered to the seabed by three winches, the remaining air is released, and the anchor settles, providing the full resistance imparted by its concrete mass. If it is necessary to retrieve the pump assembly for maintenance or inspection, the reverse procedure is employed. The anchors’ design lets the equipment be easily removed at the end of its useful life, while leaving the sea floor as it was found.
The surge tanks are charged with seawater in pulses because of the intermittent action of the waves. To mitigate this pulse effect in the pipe leading to shore, a large volume of air is introduced into the system. This air is stored in the surge tanks mounted on the vertical anchors, and in the float. The surge tanks are partially filled with water and partially with air, while the float contains air only. The volume of water discharged from the pump at each wave stroke is a small fraction of the total volume of stored air and thus the transitory changes in pressure are insignificant and the flow of water is continuous.
Note that while towing a pump assembly to and from the site as a single unit, the pumps, normally vertical or near vertical when operating, will be horizontal when in transit. Because the surge tanks mounted on the vertical anchors will always be vertical, it was necessary to introduce a swivel into the short hose connecting the lower end of each pump to the surge tank (See “Correcting for vertical” illustration).
A separate pipe leads from each wave-pump assembly to an onshore header connected to the generating plant. A shut-off valve and a check valve are provided just before the junction of the pipe and the header. The shut-off valve permits a single wave-pump assembly to be disconnected and removed. The check valve prevents the outward flow of water from the header in the event of a malfunction of a wave-pump assembly. The junction of the pipe and the header is a terminus of the offshore equipment. The header and generating plant are regarded as conventional equipment and therefore not considered a part of the wave-pump equipment.
A complete wave-pump array consists of two separate lines of floats, each parallel to the shoreline, but offset from each other. Floats 1, 3, 5, 7, etc., would be in one line and Floats 2, 4, 6, 8, etc., would be in a second line. The offset between the lines prevents the floats from colliding. The arrangement also ensures that all of the approaching wave would impinge on the floats and that energy would be extracted along the wave’s entire length.
Wave-pump operation and control
The heaving action of the ocean waves causes the float to oscillate vertically and this provides the motion to operate the pump. When the float is descending, water enters the pump above the stationary piston and passes through a check valve in the piston into the lower portion of the cylinder. When the direction reverses and the float is ascending, this water is forced under pressure past a second check valve and into the hollow pump rod and from there to the surge tank.
The pressure at which this second check valve opens is the “operating pressure” which is controlled from the onshore generating station. If the operating pressure is set at atmospheric pressure, the float rises and falls like a ship and no useful work can be done. Thus, atmospheric pressure is the lowest value in the pressure range.
The highest pressure in the range, “critical pressure,” is the pressure that generates a downward force in the pumps equal to the maximum buoyancy force generated by fully submerging the float. Once this pressure is imposed, the float can’t rise, and again no useful work can be done.
So, normal operation of the wave pump falls between atmospheric and critical pressure. Depending on the value of the operating pressure, the rising wave partially submerges the float in the first part of its rise, and then lifts the float during the remainder of its rise. Here, the work done per wave is the product of the rise of the float, the operating pressure, and the net area of the two pumps. Selecting a low value of operating pressure makes the working stroke, and thus the gpm, large. This is a logical operating condition were the water level in a reservoir low. If the reservoir level is high, then a higher pressure is needed to pump water into it, but the gpm would be less.
The wave pump is not intended to operate in extreme storm conditions. These may be expected for only a few days per year, and, as in the case of calm weather, no power is generated during such episodes. Because of the almost limitless capacity of severe storms to destroy all in their path, the capability to remove the float from the ocean surface is an important design condition.
The present system incorporates this significant feature, which will operate as follows: When a severe storm is imminent, the water supply from the pumps is turned off. This quickly raises the operating pressure to the “critical pressure.” The floats cease to rise and come to rest at an elevation corresponding to the trough of two of the high waves being encountered.
This is a relatively safe location but for added protection the floats can be lowered beneath the ocean surface to a depth where there is negligible underwater agitation. The lowering action is effected by further increasing the pressure beyond the critical pressure to a level that opens an additional valve, called the retraction valve. This action causes the pump to descend fully, thereby retracting the float to its lowest position under the ocean surface. The floats will be retained in the retracted position until the storm has abated sufficiently to restore normal operation.