AQUATIC ENERGY FOR ELECTRICITY GENERATION. Supplementing lecture notes

Transcription

1 AQUATIC ENERGY FOR ELECTRICITY GENERATION Supplementing lecture notes Tidal power Wave energy Ocean currents Ocean temperature gradients Salt gradients Principles Technology status Sustainable potential Environmental issues Economy Björn Kjellström professor em Luleå University of Technology

2 Present importance of aquatic energy As illustrated by figure 1. aquatic energy does not contribute significantly among the energy sources used by mankind today. Tidal power gives the largest contribution that is equal to a marginal 0,004% of the total supply. Figure 1: Contribution of primary sources to the energy supply to the world human population 2003[IEA, 2006] Tidal power Sustainability of fossil fuels and uranium Energy Source Reserves (TWh) Remaining years Coal Oil Fossil gas Uranium Principles The principles of tidal power is covered adequately in the book edited by Boyle /1/. Technology status The technology status of tidal power is covered adequately in the book edited by Boyle /1/. The possibility to utilise tidal currents is not much discussed however. A Canadian company, Blue Energy, will install a large power plant in the Dalupiri passage located on the South side of the San Bernardino Strait in the Philippines. The Dalupiri tidal fence will have a length of four kilometer. The power production from this site will range from 0 MW at slack tide, to 2200 MW of peak power with a base daily average of 1100 MW. Some more information in the Ocean current power section of these lecture notes. Sustainable potential Along the Swedish coasts the variations in water level because of tidal effects are less than 30 cm. It appears as completely unrealistic to use this for electricity generation.

3 Environmental issues The environmental impacts of tidal power are covered adequately in the book edited by Boyle /1/. Economy The Blue Energy Power System, see /3/, claim that they can overcome the problems of traditional ocean technologies by being both cost effective (US$1200 per kw capacity) and ecologically benign. The "tidal fence" structures are larger arrays of slow moving vertical-axis turbines which allow water and marine life to flow through freely and safely. With an interest rate of 8% and 8760 operating hours per year and an exchange rate of 8 SEK/USD, 1200 USD/kW will give a generation cost of about 0,1 SEK/kWh Wave power Most aspects of wave power are covered adequately in the book edited by Boyle /1/. The potential in Sweden is very small. Ocean currents Recovery of energy from ocean currents is discussed in /2/. The reference is old and more recent information may be available. The conclusions regarding utilisation of this energy source in Sweden are nevertheless still valid. Examples of recent information collected from the Internet are given below. Principles Ocean currents are caused by the rotation of the earth, by tidal effects, wind, variations in atmospheric pressure and outflows of fresh water from rivers. The energy flow in a current can be calculated from: P = ½ ρ v W/m 2 where ρ is the density of water in kg/m 3 and v is the flow velocity in m/s. The energy can be extracted by immersion of a turbine in the flow, see figure 2. The maximum recovery of the power for frictionless flow is: P = 0,3 ρ v A W where A is the swept surface of the turbine wheel. One of the primary advantages of this technology is the energy density. While solar and wind systems are well-suited for remote off grid locations, ocean current energy is ideal for large-scale developments in the multiple gigawatt range. Sea water is 832 times as dense as air, providing a 5 knot ocean current with more kinetic energy than a 3 3

4 350 km/h wind. The system is not dependent upon tidal amplitude, making it viable in many more locations than traditional ocean barrage systems. Technology status Propeller turbines of Kaplan-type, similar to those used in hydropower plants can be used, see figure 2. The rotor speed would be 5 10 rpm. Arrangements for positioning the turbine in the ocean will be very site dependant. Hanging the turbine(s) from anchored pontoons could be a good solution at many sites. (a) (b) Figure 2: (a) Kaplan turbine [VERBUND-Austrian Hydro Power] (b) Vertical Davis turbine [Blue Energy Canada Inc] Another possibility is to use turbines with vertical rotors like those used by Blue Energy Canada, see /3/. The company is commercializing the Davis Hydro turbine, a device that generates electricity from ocean currents. Six prototypes (up to 100 kw)

5 have been built and tested, multiple independent assessments have verified technical feasibility, and private investment groups have approved financing based on economic viability. The applications appear to mainly utilize tidal currents. Blue Energy has recently received a firm funding commitment for US$2.8 billion to construct a largescale ocean power plant in the Philippines. The 2200 MW "Dalupiri" project is scheduled to begin within the year. Figure 3 shows an invention where a wire loop is moved by the current by means of parachutes. The movement of the wire is transmitted to a shaft that drives an electric generator. A model with parachutes of 10 m diameter has been built and tested. The full scale plant would have parachutes with a diameter of 100 m and a loop of 18 km. Figure 3: Water low velocity converter [Steelman, 1974] According to /4/, several projects have been undertaken to investigate the feasibility of extracting energy from ocean currents. These include projects in the Kurushima straits in Japan, Corran Narrows in Scotland and one north of Darwin, Australia. As reported in /4/ neither of these have demonstrated commercially viable technology. An expected problem is growth of mussels and similar marine creatures on the turbine blades. The surfaces may need painting with anti-fouling at regular intervals. Sustainable potential Current velocities are normally low, between 0,1 5 m/s. Higher velocities can be found in narrow straits. According to /3/ the global potential is 450,000 MW. Examples of theoretical energy densities in some currents are given below: California current 0,0017 kw/m 2 (US west coast) Golf stream 1,7 4 kw/m 2 (off Floridas coast) Kuroshio current 0,2 0,6 kw/m 2

6 (West Pacific) Öresund about 0,5 kw/m 2 (between Sweden and Denmark) Environmental issues The only expected environmental issue is the effects of anti-fouling paint. Economy An estimate presented in /2/ for a 3,3 MW ocean current plant located in Öresund, consisting of 20 turbines of 15 m diameter gives an electricity generation cost of 2,6 SEK/kWh 1. See also section on tidal power. Ocean thermal gradients Recovery of energy from ocean thermal gradients is discussed in /2/. The reference is old and more recent information may be available. An Example of recent information collected from the Internet is given below. The conclusions in /2/ regarding utilisation of this energy source in Sweden are nevertheless still valid. Principles Ocean thermal gradient power plants (OTEC) make use of temperature gradients in a thermal (Rankine) cycle process. The working medium is a refrigerant, for instance ammonia. Technology status Figure 4 shows a possible flow scheme for an OTEC-plant. The low temperature differences in the cycle leads to low efficiencies. For example, at the Caribbean Sea, tropic waters meet and flow over artic waters with a temperature difference of C over a m vertical separation. Based on 20 C temperature difference, the Carnot cycle (best possible) efficiency is low, i.e., approximately 6%. The actual efficiency is 2-3% since the water must be pumped and there are thermal losses. 1 Recalculated from cost 0,95 SEK/kWh using Swedish consumer price index KPI.

7 Figure 4: Flow scheme of an OTEC plant [Saga University] In 1929, a French engineer, George Claude, finally constructed a 22 kw machine on the coast of Cuba. He took the warm surface water and put it into an evaporator. The pressure was lowered which caused the water to vaporize. It was forced through a turbine and it produced 22 kw of electricity. Cold water was piped up from lower ocean depths to cool the vaporized water so the cycle could begin again. A problem with this was that the pipes that collects cold water at lower depths kept breaking off during storms. It was eventually abandoned because it was too much work to keep it working. The new designs for OTEC are still mostly experimental. Only small-scale versions have been made. The largest so far is near Japan, and it can create 100 kw of electricity. Another small-scale OTEC is off the coast of Hawaii (Kailua-Kona), producing 50 kw of electricity. If a successful OTEC is built, it is planned to produce 2 MW of electricity. However, a full scale OTEC would cost many millions of dollars, and it would be very difficult to build. To compensate for its low thermal efficiency, OTEC has to move a lot of water. That means OTEC-generated electricity has a glut of work to do at the plant before any of it can be made available to the community power grid. Some 20 to 40 percent of the power, in fact, goes to pump the water through intake pipes in and around an OTEC system. While it took roughly 150 kilowatts of electricity to run the Kailua-Kona test plant, larger commercial plants would use a lower percentage of the total energy produced. That's why, a century after the idea was first conceived, OTEC researchers are still striving to develop plants that consistently produce more energy than is needed to run the pumps, and that operate well enough in the corrosive marine climate to justify the development and construction.

8 Sustainable potential OTEC plants must be located where a difference of above 20 C occurs year round. The map shown in figure 5 indicates areas of interest. Ocean depths must also be available fairly close to shore-based facilities for economic operation. Floating plant ships could provide more flexibility. In the US, ocean thermal energy conversion is limited to tropical regions, such as Hawaii, and to a portion of the Atlantic coast. Figure 5: Areas of interest for OTEC plants In Swedish waters, the temperature gradient during summer months seldom exceeds 10 o C. Results presented in /2/ indicate that under these conditions OTEC power generation is economically un-realistic, with estimated generation costs around 15 SEK/kWh. Environmental issues There is no doubt that mowing around large quantities of water that are exposed to rapid changes in temperature may lead to some concerns for the effects on aquatic life. Economy An OTEC power plant requires the use of large components because of the low thermal efficiency, and hence, a large capital investment is needed for such plants. As mentioned above, OTEC power generation is not realistic in Sweden. Estimates presented in /2/ indicate generation costs of 0,75 1,5 SEK/kWh (in 2003 cost level) at favourable sites.

9 Salt gradients Recovery of energy from salt gradients is discussed in /2/. The reference is old and more recent information may be available. The conclusions regarding utilisation of this energy source in Sweden are nevertheless still valid. Principles The utilisation of salt gradient energy is based on the utilisation of the difference in Gibbs free energy between water with high salt content and water with low salt content. The change in free energy from salt to fresh water can be calculated from: X Gs G f = n R T ln X s f where G is Gibbs free energy, n the number of moles, X the mole fraction of water (1,0 for fresh water). Mixing 1 kg of fresh water (55,5 mole) from the river Göta älv with a large amount of water from the Kattegatt with X = 0,98 releases energy equal to 2600 J. That is equal to the energy released when the same quantity of water falls 265 m. Technology status Three approaches has been studied in Sweden for utilisation of salt gradient energy: Chemo-electric conversion. (using ion-selective membranes) Chemo-mechanic conversion (using fibers that expand and contract when exposed to alternatively salt and fresh water) Back pressure osmosis (using membranes that allow water molecules but not salt molecules to pass through) The principles are illustrated in figure 6 and 7. Laboratory scale experiments have demonstrated that the first two processes work but development work has now been terminated. All the three conversion processes require special types of membranes or fibers and technology break-through can not be expected until such materials are available at reasonable cost. Pollution of the membranes by impurities and organisms in the water is an obvious problem that has not been resolved. Sustainable potential No estimates of the global potential have been found. Estimates for rivers in Sweden indicate a potential for electricity generation of around 8 TWh/year.

10 Environmental issues The environmental impacts have not been very much studied but it is clear that passing a large fraction of a river flow through a membrane raises many environmental concerns. Economy Economic estimates are presented in /2/ but the credibility is very low. References 1. Boyle G B editor Renewable energy Oxford University Press, Akvatisk energi Nämnden för energiproduktionsforskning, resultatrapport NE 1981:

11 OCEAN CURRENTS Principles Ocean currents are caused by the rotation of the earth, by tidal effects, wind, variations in atmospheric pressure and outflows of fresh water from rivers. The energy can be extracted by immersion of a turbine in the flow. The maximum recovery of the power for frictionless flow is: P= 0.3 The primary advantage of this technology is the energy density. Technology status Propeller turbines of Kaplan-type, similar to those used in hydropower plants can be used. The rotor speed would be 5-10 rpm. Another option is a vertical turbine.

12 OCEAN THERMAL GRADIENT ENERGY Principles Ocean thermal gradients power plants (OTEC) make use of temperature gradients in a thermal (Rankine) cycle process. The working medium is a refrigerant, for instance ammonia. The low temperature differences in the cycle lead to low efficiencies. For 20 C temperature difference, the Carnot cycle efficiency is approximately 6%. The actual efficiency is 2-3 % since the water must be pumped and there are thermal losses. Only small-scale experimental versions (power output kw) have been built. The largest so far is near Japan, and it can create 100 kw of electricity.

13 Substainable potential OTEC plants must be located where a difference of above 20 C occurs year round. Ocean depths must also be available fairly close to shore-based facilities for economic operation. Floating plants ships could provide more flexibility. In Swedish waters, the temperature gradients during the summer months seldom exceeds 10C. Results presented in /2/indicate that under these conditions OTEC power generation is economically un-realistic, with estimated generation costs around 15 SEK/kWh. Environmental issues Mowing around large quantities of water that are exposed to rapid changes in temperature may lead to some concerns for the effects on aquatic life. Economy An OTEC power plant requires the use of large components because of the low thermal efficiency, and hence, a large capital investment is needed for such plants. Estimated presented in /2/ indicate generation costs of 0,75-1,5 SEK/kWh (in 2003 cost level) at favourable sites.

14 Technology status Three approaches has been studied in Sweden for utilisation of salt gradient energy: Chemo-electric conversion. (using ion-selective membranes) Back- pressure osmosis (using membranes that allow water molecules but not salt molecules to pass through).

15 Chemo- mechanic conversion (using fibers that expand and contracts when exposed to alternatively salt and fresh water) A. Fibers contracted, cylinders flushed with fresh water B. Fibers expand when drenched in fresh water C. Fresh water replaced by salt water D. Fibers contract and given mechanical work to the fly-wheel The inventor Orvar Elmquist built such a machine in his kitchen and demonstrated it working in All the three conversion processes require special types of membranes or fibers and technology break-through can not be expected until such materials are available at reasonable cost. Pollution of the membranes by impurities and organisms in the water is an obvious problem that has not been resolved.