PHYS 1110

Lecture 22

Professor Stephen Thornton

November 20, 2012

Reading QuizWhere was the first tidal barrage power plant opened?

A) United StatesB) FranceC) NorwayD) CanadaE) Switzerland

Reading QuizWhere was the first tidal barrage power plant opened?

A) United StatesB) FranceC) NorwayD) CanadaE) Switzerland

Tidal Energy• Renewable• Two high tides a day• Need 4-5 m tide heights• Water rises steeply in estuaries, lagoons, rivers increasing height of water.• Estimate 3000 GW global, but only 60 GW potential

Tidal barrage

The first commercial tidal wave facility was constructed in Brittany, France on the estuary of the Rance River. It is a tidal barrage system. It began operation in 1966. It has 24 turbines that produce 240 MW of power. The second commercial plant was a tidal barrage system that was built at Annapolis Royale, Nova Scotia, Canada. It became operational in 1984 and produces 20 MW. It is the only commercial tidal barrage in North America.

Rance River estuary tidal barrage. 240 MW

It is doubtful if many more additional tidal barrages will be constructed, because estuaries are among the world’s most productive and sensitive ecosystems. Barrages present a barrier to navigation to both fish and boats, but lock gates like those used in canals can allow shipping. The reduced tidal range (between high and low tides) can destroy the inter-tidal habitat used by wading birds. Sediment collected behind the barrage can also reduce the volume of the estuary over time. Even though the largest tidal power station that began operation in 2011 in South Korea is a barrage facility, the seawall dam had already existed since 1994.

Bay of Fundy, Nova Scotia, Canada. Highest tides in the world, average 11 m.

See camera at http://www.novascotiawebcams.com/bay-of-fundy/halls-harbour-history.html

There are various ways to utilize the power of tides: tidal barrages, tidal turbines, tidal lagoons, and tidal fences.

Tidal turbines – think of wind turbines

SeaGen turbine. Note strong water current.

Tidal turbines have also been suggested for placement in rivers where sufficient current exists. For example, tidal turbines have been tested in the tidal currents of the East River next to Manhattan in New York City. Verdant Power tested six full-size turbines in the East River from 2006-08 and supplied power to New York City businesses. Each three-bladed turbine produced 35 kW power, but they experienced some damage to the tips of the turbine blades due to the swift, powerful tidal currents in the East River.. A permit has been requested to install 30 of the turbines in the near future that will produce 1 MW. A 15 kW proof of concept turbine operated on Loch Linnhe, Scotland in 1994. Another turbine generator was installed in Northern Ireland in 2008 and generates 1.2 MW for about 18 hours a day. These SeaGen rotor turbines can be raised above the surface for maintenance.

Tidal Lagoons

Tidal lagoons can produce power twice during each tide change.

A tidal fence is simply a series of turbines mounted in a row across a bay, an ocean channel or strait.

A large-scale commercial tidal fence is proposed in the Philippines between the islands of Dalpiri and Samar. 2200 MW peak power.

Problem: intermittent supply of energy because of tidal flow.Solution: 1) Generate electricity during rise and fall of tide. 65%2)Two basins.

Tidal energy determination. Similar to wind energy.

31

2

efficiency, Betz limit = 0.59 = density = area swept out by blades = water current speed

P

p

P C Av

C

Av

r

r

=

=

Go over Example 9-2. Typical turbine produces hundreds of kW.

The future of tidal energy is not altogether clear. In 2012 there were only seven tidal power stations operating globally, and three of them operated at less than 2 MW. Only two produced more than 20 MW. Environmental and cost concerns are the primary issues. Tidal power plants have high capital costs to construct, but low costs to operate. That means investors will not receive a return for many years. Tidal energy certainly has potential, but private industry is not convinced that its future is here. More demonstration and pilot plants need to be supported by governments. The concepts of tidal turbines, fences, and lagoons seem to support further investments, but the technology needs to improve. Because of environmental concerns, tidal barrages probably have little future.

Let’s discuss advantages and disadvantages of tidal energy. Divide up.

What kind of facility is being planned for the Philippines?

A) tidal barrageB) tidal turbineC) tidal lagoonD) tidal fenceE) tidal basins

What kind of facility is being planned for the Philippines?

A) tidal barrageB) tidal turbineC) tidal lagoonD) tidal fenceE) tidal basins

Which of the following statements is most untrue?

A) tidal facilities have large capital costs.B) tidal facilities have low operating expensesC) tidal facilities have no environmental issuesD) private investors are not convinced tidal energy is a good investmentE) more demonstration plants need to be built

Which of the following statements is most untrue?

A) tidal facilities have large capital costs.B) tidal facilities have low operating expensesC) tidal facilities have no environmental issuesD) private investors are not convinced tidal energy is a good investmentE) more demonstration plants need to be built

Hydrokinetic energy

1)Describe why and where it is useful.2)Open-center turbine.3)Other devices, start page 9-68.

Divide into groups, discuss, and make reports.

Ocean currents

Ocean currents

Open center turbine

Producing electrical power.

Open-Center Turbine (10 m diameter, 1 MW) before being placed in the Bay of Fundy.

Drawing of Open-Center Turbine device placed on seabed.

Free Flow Power’s hydrokinetic testing turbine.

Hydro Green Energy installed its 35 kW turbine downstream from an existing hydroelectric dam on the Mississippi River in 2008 near Hastings, MN.

Clean Current Power Systems, based in Vancouver, Canada, has developed an underwater turbine.

Ocean waves are due to solar energy, because the sun’s energy causes wind, which in turn generate ocean waves as the wind passes over ocean water.

Periodic up-and-down movement of waves is converted to electricity by placing devices on water that captures mechanical energy, then converts to electrical energy.

Wave energy is distinct from the energies we have discussed previously including tidal currents due to tides and ocean currents like the Gulf Stream. The first experimental wave farm was opened in Portugal in 2008. The wave height depends on the wind speed, the time duration the wind has been blowing and by the depth and topography of the sea floor. Wave power depends not only on the wave height, but also on the wave speed, wavelength, and the water density. Waves propagate long distances over the open ocean with little loss in energy. But as the waves approach the shoreline, the water depth becomes shallower, which causes the wave speed to slow down and increase in size.

Most of the energy of the wave is near the surface. There is a wide range of clever designs that capture the energy of the wave. Some of them are fixed on the shore or the water bottom while others float or are submerged. The ocean is a hostile environment, which creates a huge challenge for developing a device to produce energy. The object is to convert each wave movement into electricity using a generator. We will discuss four basic methods to capture the energy. Divide into groups.

1) point absorbers/buoys2) terminators3) attenuators and overtopping

A form of Point Absorber buoy to generate electricity.

The PowerBuoy system of Ocean Power Technologies. Note the Sea Substation on the ocean floor that takes power from several buoys and sends the power to shore.

Operation of the Salter Duck.

The concept of the oscillating water column is for an ocean wave to force air out through the top of the column to turn a wind turbine and produce electricity.

The rise and ebb of the ocean waves pushes air both ways through the turbine to generate electricity.

Oceanlinx is an Australian wave energy developer that has installed over 750 kW of prototype wave energy generators.

The LIMPET is an OWC device currently operating at 75 kW off the coast of Islay, Scotland. It has a generating capacity of 500 kW and was developed by the Wavegen company. Its mode of operation is shown above. The LIMPET has been in operation since 2000.

Voith Hydro, the parent company of Wavegen, installed the first actual commercial wave energy plant in Spain for the power utility EVE. It has an installed capacity of 300 kW.

Attentuator – The Pelamis P1 wave energy device.

Which of the following would not be a good place to put a water turbine?

A) Florida straitsB) Niagara River above the fallsC) Lake Michigan near ChicagoD) New York City’s East RiverE) Mississippi River near Memphis

Which of the following would not be a good place to put a water turbine?

A) Florida straitsB) Niagara River above the fallsC) Lake Michigan near ChicagoD) New York City’s East RiverE) Mississippi River near Memphis

The attenuator is a device for obtaining which kind of energy?

A) wavesB) hydrokineticC) tidal fencesD) tidal lagoonsE) rivers

The attenuator is a device for obtaining which kind of energy?

A) wavesB) hydrokineticC) tidal fencesD) tidal lagoonsE) rivers

The Salter Duck is a what?

A) extremely inefficient deviceB) an idea that was purportedly killed by nuclear energy proponentsC) tidal fence deviceD) point absorber deviceE) invented by an American for duck hunting in salt marshes

The Salter Duck is a what?

A) extremely inefficient deviceB) an idea that was purportedly killed by nuclear energy proponentsC) tidal fence deviceD) point absorber deviceE) invented by an American for duck hunting in salt marshes

Ocean Thermal EnergyTemperature difference between ocean surface water and 1000 m depth.

The idea of ocean thermal energy conversion (OTEC) is simple; utilize the temperature difference between cooler deep and warmer shallow or surface ocean water to operate a heat engine to produce electricity. Heat energy Qh flows from the hot reservoir (warm surface water) to the heat engine which does work W and deposits heat Qc to the cold reservoir (deep ocean water). The work W drives a turbine to produce electricity. It is generally considered that temperature differences of at least 200C are needed, and the ocean thermal gradients for this temperature difference generally occurs between latitudes 200 N and 200 S. This tropical zone includes parts of two industrial countries, the United States and Australia, as well as 29 territories and 66 developing nations. OTEC has the potential to offer significant amounts of energy, significantly more than other options like ocean wave energy. OTEC power plants can operate continuously generating power around the clock.

The challenge for OTEC is to generate power efficiently from small temperature differences. The most commonly used heat cycle is the Rankine cycle (described in Section 5-6) with a low-pressure turbine. Early OTEC machines only reached 1-3% efficiencies, well below the theoretical maximum of 6-7% for the temperature differences utilized. Because of this, OTEC is considered an emerging technology. Surprisingly, the first operational system was built in Cuba in 1930 and produced 22 kW. A system produced in the United States in 1999 generated 250 kW, and modern designs approach the theoretical maximum Carnot engine efficiencies.

Closed-cycle systems use low-boiling working fluid refrigerants such as ammonia or R-134A. The warm ocean water is used to heat up and boil the ammonia. The ammonia vapor drives the turbine to produce electricity.

The ammonia vapor then passes through the heat exchanger where it is cooled by the cold ocean water and returns to the liquid phase. It is ready for the next cycle.

Open-cycle systems require a low-pressure (partial vacuum) environment to boil ocean water to create steam; water serves as the working fluid. The steam drives a turbine to produce electricity before passing through a heat exchanger cooled by the cold ocean water and changing back to the liquid water phase. The water is desalinated, and the byproducts like salts and other impurities are often saved for other uses. Note that this water is now potable and can be used as fresh water. This is an important natural byproduct. New seawater is used for the next cycle. It has been estimated that a 2 MW OTEC plant could produce 4300 m3 (1.1 million gallons, 4.3 million liters) of desalinated water each day. This is enough water for about 15,000 people according to United Nation estimates.

Attempts to harness OTEC began in the 1880s, but Japan is given much of the credit for boosting the technology in the 1970s when they constructed a 100 kW closed-cycle OTEC plant on the island of the Republic of Nauru. It became operational in 1981 and power was delivered to the electrical grid. Hawaii is the center of OTEC development in the United States because of its warm ocean surface water, access to deep cold water, and high electricity costs. After becoming involved in OTEC technology in 1974, the United States built a couple of pilot plants, but eventually mostly terminated its involvement in OTEC by 2000. More recently the US government has awarded several contracts for design studies of OTEC plants.

The Future of OTEC OTEC is a promising alternative energy resource, especially if it could be produced more cheaply. It is particularly promising for tropical island communities that rely heavily on imported fuel. OTEC plants could provide both much-needed electrical power, but also desalinated water, which often is even more valuable. Careful site selection will keep the environmental impacts of OTEC to a minimum. Appropriate spacing of plants throughout the tropical regions will nearly eliminate negative impacts on ocean temperatures and on marine life. However, there are only a few hundred land-based sites in the tropics where deep-ocean water is close enough to shore to make OTEC plants feasible.

Osmotic Energy

When rivers empty into oceans, the fresh water reduces the salinity of the seawater as the rivers flow into the ocean. If a membrane is placed between the fresh and sea water, the process of osmosis takes effect. The membrane only allows small molecules like water through the membrane leaving the larger salt molecules behind. The water strives for equality for the salt concentration so the fresh water flows through the membrane to the ocean water side to lower the salt concentration of the ocean water. The osmotic process, however, creates a higher pressure in the salt solution side as shown in Figure 9-83. The pressure is then used to drive a turbine and produce electricity. The whole thing sounds incredibly simple. The energy is renewable and it is always available. The biggest disadvantage is the cost of the membranes, but significant breakthroughs in membrane research have occurred. It has been estimated that 2.6 TW of electrical power may be derived from the osmotic process.

The process just shown will eventually stop when the pressure builds up. We need a cyclic process that will work continuously. See below. Fresh water, say from a river, comes in at the top left, and salt water, from the ocean, comes in at the bottom right. Fresh water passes through the semi-permeable membrane through the osmotic process. In the upper right the higher pressure salt water drives the turbine, and the brackish water is expelled.

If we continuously flow through both fresh and salt water that has a salinity difference of 3%, the theoretical potential energy corresponds to a waterfall of 250 m height.

The Norwegian University of Science and Technology and the Norwegian utility company Statkraft have taken the lead. After a decade of collaborative research and development, including a high-performance membrane, Statkraft opened the first prototype osmotic power plant in the world on the Oslo fjord in 2009. Although small, it only produces 2-3 kW, but they have shown that it works. Their system has 10 liters of water flowing through a membrane each second. Stadkraft believes that Norway has a salinity gradient between its rivers and seawater that may allow electrical generation of up to 12 TWh per year.

The commercialization of osmotic energy is still somewhat uncertain, and its role in producing competitive electricity is clearly far into the future. We read projections like 2012, 2015, and 2020, but these dates come and go. The Norwegian pilot plant shows the concept is feasible. The problem is still high capital costs. The next step is a demonstration plant which would scale up the current technology, verify the expected cost of electricity produced, and optimize the operation and maintenance. The improvement in the membranes has been significant, and there are now international conferences on just osmotic membrane development. Early membranes were cellulosic, and more recent ones are thin film. More research and development is needed to improve the cost, maintenance, cleaning, and lifetime of membranes. Nevertheless, we may be only a breakthrough away in membrane development from osmotic energy being truly useful and price competitive.