- Email: [email protected]
Symbiotic offshore energy harvesting and storage systems
Sustainable Energy Technologies and Assessments xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Sustainable Energy Technologies and Assessments journal homepage: www.elsevier.com/locate/seta
Original Research Article
Symbiotic offshore energy harvesting and storage systems Alexander H. Slocum ⇑ Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139, USA
a r t i c l e
i n f o
Article history: Received 7 August 2014 Revised 8 October 2014 Accepted 9 October 2014 Available online xxxx Keywords: Offshore energy harvesting Water turbines Energy storage Symbiotic system
a b s t r a c t Taken separately, various offshore energy harvesting and storage machines can have a difficult time competing with land-based systems with regard to both initial capital cost and variable costs. However, collocating systems in a symbiotic way can lead to competitive advantages for offshore systems. This paper explores such combinations based on using the support structure for an offshore wind turbine as a basis for an energy storage system, a wave energy harvesting system, and a uranium-from-seawater mining system. Further considerations could also include aquaculture facilities as well as enhanced sport fisherman opportunities. Finally, far offshore systems are not only less likely to negatively impact birds, such as birds-of-prey, while experience with offshore oil platforms have been shown to act as undersea wildlife havens. Ó 2014 Elsevier Ltd. All rights reserved.
Introduction Consider the history of the cellular phone or the automobile: both started as simple devices but have evolved into platforms for multiple functions. Smart phones allow for voice communication but also are used to take pictures, play games, surf the web, give directions. . . Automobiles not only provide a means for transportation and self-expression, they serve as entertainment and communication centers. It is hypothesized here that renewable energy systems can also serve as multipurpose devices, whose added value can push renewables’ economic viability past the tipping point. ‘‘Symbiotic’’ strictly refers biologically to ‘‘living together’’, but the concept can be a powerful catalyst for reducing energy systems’ environmental impact and ROI. Identifying multi functional opportunities can be accomplished by considering all the characteristics of a deployment zone that are not directly concerned with the primary energy-gathering device, especially the negative issues or risks associated with the primary energy-gathering device. For example, consider solar panels, which are often deployed in sunny regions. A negative aspect of such devices is they get dirty and need to be cleaned, with scarce water. A characteristic of sunny regions is they receive little rainfall. However, a large flat panel can be a great water-gathering device from rainfall or even fog. In the case of the latter its not just gathering water from fog condensing on the panels but fog condensing on other structures ⇑ Address: MIT Room 3-445, 77 Massachusetts Avenue, Cambridge, MA 02139, USA. Tel.: +1 617 253 0012. E-mail address: [email protected]
placed among the solar field [1]. Collected water can be stored and used sparingly over the course of many months for cleaning the cells. The water is not taken from the environment but rather very slowly released back into it because cleaning is done at night when evaporation is low and the dirty runoff can be delivered to the ground. This catalyzes another thought, water with dust falling on the ground potentially carries nutrients as well as moisture and to what good purpose can this be used for? What high value crops can be grown in partial shade that require only modest water resources? This sort of branching thought pattern can continue on with thoughts such as a robot PV panel cleaner/inspector/position adjuster could also tend the crops. . . Considering the entire product life cycle of different systems together can thus lead to reduced overall systems capital and variable costs. Reciprocity The central theme of this paper is thus the creative application of the principle of reciprocity [2]:
1 ¼ opportunity problem
ð1Þ
With this thought process in mind, consider wind turbines. In general, the closer the gathering of a resource to its market, the better. In much of the world, population density is highest, but as beautiful and renewably compelling as a single wind turbine may be, dozens, hundreds, or thousands of them along a scenic ridge or offshore horizon is seen by many as a form of pollution worse than believ-
http://dx.doi.org/10.1016/j.seta.2014.10.004 2213-1388/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Slocum AH. Symbiotic offshore energy harvesting and storage systems. Sustainable Energy Technologies and Assessments (2014), http://dx.doi.org/10.1016/j.seta.2014.10.004
2
A.H. Slocum / Sustainable Energy Technologies and Assessments xxx (2014) xxx–xxx
ing/hoping that global warming is not really the result of human activity. Fortunately, whether by intelligent design, luck, or physics, the earth is round, which means if one goes about the distance of a marathon offshore, large wind turbines are no longer present a serious scenic problem because the horizon drops off by about 130 m. However, the water does often tend to get deep in some places and undersea high power cables are expensive.
Table 1 Potential parameters of an offshore site for wind turbines. Parameter
Physics
Problem
Opportunity
Wind Currents
P = f(V^3) P = f(V^3)
Energy harvesting Energy harvesting
Waves
P = f(h, s)
Intermittent Structure loading Structure loading Dissolved elements Deep-water mooring Transmission line cost
Sea water
The answer could be blowing in the wind, (moving with the waves, flowing in the currents. . .) If it is accepted that offshore wind turbines should be far offshore so they are visually unobtrusive, then what are the parameters associated with being far offshore, and what are the physics associated with those parameters? Moreover, consider not only energy, but also the other human need for food and protein in particular. A partial list is shown in Table 1. Right from the beginning it is apparent that any offshore wind farm will have to assess loadings from currents and waves, and those same assessments can be used to consider/optimize machines that could be attached to the wind tower structure in order to gather additional energy from the site. Since the strength of a tower is proportional to the diameter squared and loading (and cost) is proportional to area (diameter), added energy harvesting methods, even though additional energy harvesting mechanisms may increase tower loading, they should be able to reduce the overall cost of energy harvested from the site. Also apparent are other potential revenue generating means including energy storage [3], aquaculture [4], and harvesting uranium from seawater [5,6]. Current energy harvesting Even though water is 800 denser than air, water turbine blade diameters are typically modest in comparison, so to be competitive with wind energy, flow velocities on the order of 2–4 m/s are needed to economically harvest energy from ocean currents. Note the average speed of the Gulf Stream is about 1.8 m/s with a top speed near the surface of up to 2.5 m/s.1 Most offshore wind farms will probably not be located in such strong currents, but if they are, it could make sense to mount underwater turbines onto the structure that supports the wind turbine as shown in Fig. 1. To help with the trade-off of high-speed currents causing vortex-shedding vibrations on mooring cables or scouring base mountings, ocean turbines may need to be developed with larger blades to operate in slower speed currents. These turbines could be compatible with wave energy harvesting systems, but might interfere with uranium harvesting or aquaculture systems depending on how they are configured.
Deep water
E = mgh
Underwater distance
$=>depth, distance
Energy harvesting Uranium harvesting Pumped hydro energy storage Cleaner water for aquaculture
bine itself. Note that when there is wind there are usually waves, but sometimes when the wind is too slow, significant ocean swells still pass as they can travel hundreds of miles. Uranium harvesting Nuclear fission has the potential to greatly reduce carbon dioxide emissions from power generation. However, a 2011 study by the Organization for Economic Cooperation and Development estimated that at the current consumption rate the global conventional reserves of uranium (7.1 million tonnes) could be depleted in roughly a century [7]. Moreover, as reserves decrease, uranium mining shifts to lower quality sites leading to a higher extraction cost. Fortunately, uranium is present uniformly in ocean water in the form of uranyl ions at a low concentration of 3–3.3 lg/L3. The amount of uranium in the ocean is estimated at 4.5 billion tonnes, nearly a thousand times the conventional reserves, and it is uniformly distributed due to the action of currents. Uranium adsorption by chelating polymers has been found to be most promising in terms of cost, adsorption capacity and environmental footprint [8]. Fig. 3 shows a 3D net-based system, which is inspired from the concept proposed by the Japan Atomic Energy Agency [9], minimizes the amount of chains and ships needed. The concept, shown in Fig. 3, is a net of adsorbent attached to two chains and ballast masses lying at the bottom of the ocean. A single ship continuously
Wave energy harvesting Where there is wind, there are often waves, hence it can make sense to include wave energy harvesters with wind turbine structures. There are many different types of wave energy harvesting devices, but a cylindrical structure that utilizes Wells turbines, concept shown in Fig. 2, in particular may be most appropriate considering the other potential uses for the offshore structure of uranium harvesting and aquaculture. The simple circular system shown in Fig. 1 mounted around the base of the wind tower could be made from sections bolted together. In severe storms it could be submerged to avoid damage. An estimate of the total power that could be generated from this wave power generation ring is also shown in Fig. 1. This is nearly as much power as from the wind tur1
http://oceanservice.noaa.gov/facts/gulfstreamspeed.html.
Fig. 1. Water turbine(s) mounted on an offshore wind turbine tower.
Please cite this article in press as: Slocum AH. Symbiotic offshore energy harvesting and storage systems. Sustainable Energy Technologies and Assessments (2014), http://dx.doi.org/10.1016/j.seta.2014.10.004
A.H. Slocum / Sustainable Energy Technologies and Assessments xxx (2014) xxx–xxx
3
Fig. 2. Wells turbines in a cylindrical structure at the waterline of an offshore wind turbine tower.
recovers the net, runs it through the elution process and replaces it at the bottom. The number of braids in parallel in the net is chosen so that the ship can harvest all the net during the selected harvest period at the recovery velocity taken 4 m/min, and produce 1200 tonnes of uranium per year, enough to supply a 5GW nuclear power plant. Fig. 4 shows a wind turbine tower system approximately to scale [5]. A system of 20 loops of 100 m each, for a total belt length is 4000 meters, loops through once every 38 days and can provide enough uranium to supply 5 MW of electric power during the same period. The central reservoir represents the space required for the elution process. Two storage tanks store enough chemicals to feed the elution process for a month before a crew comes to collect the harvested uranium and refill the tanks. This means, for example, that a 2 GW offshore wind farm could harvest enough uranium to also keep operating a 2 GW nuclear power plant. The net-based system can be located in 100 m deep water and uses a single ship that continuously operates and it is predicted to produce uranium at $326/kg. The continuous system is attached to an offshore wind turbine system to eliminate the need for additional mooring and increase the overall energy gathering ability of the wind farm system. This system could maximize the adsorbent yield but being more dispersed would achieve a production cost of $403/kg of uranium based on current models. Both systems’ costs are expected to drop significantly with materials advances and scale up.
Elution
Sea surface
Ship m
m
m
m
m m m
m
m
m Sea bottom
m m
Fig. 3. Bottom based net mooring and recovery system.
Fig. 4. Wind turbine structure anchored continuous belt system.
Energy storage The offshore environment represents a unique unobtrusive and safe environment for utility scale energy storage that takes advantage of the hydrostatic pressure at ocean depths to create a robust pumped storage device and thus a more consistent and predictable utility scale renewable energy power plant. Fig. 5 illustrates the concept for an undersea pumped hydro system and Fig. 6 shows how it could be deployed in a hexagonal pattern as mooring points for floating wind turbines [3], where concrete spheres act as undersea reservoirs. Fig. 7 shows how concrete pipe could be used so a long series of interconnected pipe feed into one or two large pump/turbine systems. More concrete is needed for a pipe-based system, but fewer pump systems would be required and logistics should be simpler. For either configuration, when the water is pumped out, the concrete is subject to compressive stresses by the deep sea, which wants to flow back in (once a valve to a turbine is opened). Compression is concrete’s most favourable operating mode, and hence great structural efficiency can be obtained. For the pumped hydro system an interesting symbiotic design point exists with regard to the concrete: when the wall thickness is great enough to provide enough weight to keep the structure on the bottom when the water is pumped out (self ballasting), the structure is strong enough to be placed about 700 m deep which also happens to be an ideal head depth for commercially available pump/turbine systems. Fig. 8 shows the output from a spreadsheet for designing a cylinder based ORES, where the major realms of the design are evaluated. This enables the rapid assessment of project feasibility. System properties: The water depth is the single biggest factor with regard to energy density and hence viability. The Internal Pressure can either be vacuum or one atmosphere if a snorkel (tube to surface which could be a mooring line) is used. The concrete properties, especially strength and cost, will also have a major factor on project viability.
Please cite this article in press as: Slocum AH. Symbiotic offshore energy harvesting and storage systems. Sustainable Energy Technologies and Assessments (2014), http://dx.doi.org/10.1016/j.seta.2014.10.004
4
A.H. Slocum / Sustainable Energy Technologies and Assessments xxx (2014) xxx–xxx
Fig. 5. Ocean renewable energy storage system (ORES) concept during charging and discharging operations.
Fig. 6. ORES configuration with floating wind turbines and spherical mooring buoys/pumped hydro systems.
Fig. 7. ORES configuration with floating wind turbine(s) moored to pipe-based mooring/pumped hydro system.
Please cite this article in press as: Slocum AH. Symbiotic offshore energy harvesting and storage systems. Sustainable Energy Technologies and Assessments (2014), http://dx.doi.org/10.1016/j.seta.2014.10.004
5
A.H. Slocum / Sustainable Energy Technologies and Assessments xxx (2014) xxx–xxx System Properties Water depth (m): planned, max allowed for system External Pressure, Po (N/m^2) Internal Pressure, Pi (N/mm^2) Concrete density (kg/m^3) Seawater density (kg/m^3) Modulus of elasticity, E (N/m^2) Poissons ratio, v Concrete strength (Mpa, ksi) Safety factor Emplaced concrete tube cost per tonne (with rebar) Desired ballast for Floating Wind Turbine (Tonnes) Energy Storage Requirements MWhe required at desired depth MWhe at maximum allowable depth Pump/turbnine efficiency Geometry Inside Diameter ID, Di (mm) Wall thickness/ID Length (depth into page), L (m) Outside Diameter OD, Do (mm) Wall thickness, t (mm) Length to diameter ratio Storage Properties Charge capacity (MWh) energy/concrete mass (kWh/tonne) Number of units required Total length of pipe (km) Bouyancy Mass of displaced water (tonnes) Mass displaced water + FWT ballast needed (tonnes) Empty ORES dry mass (tonnes) Total weight of all cylinders (tonnes) Bouyancy: will it stay down? (Y/N must be >1)
Design Stress and Displacement 300 Axial Force on hemispherical ends, Fa (N) 3,000,000 Radial displacement, ur (mm) 100,000 2,600 Radial stress, sigmar (MPa) 1,050 Hoop stress, sigmatheta (MPa) 2.07E+10 Axial stress, sigmaz (MPa) 0.29 Von Mises stress (MPa) 40 Max design stress 1.5 VON MISES/Max design stress (be < 1) $500 Economics 300 Cost of undersea pump/turbine ($/W) Size (MW) 32 Pump/turbine cost 69 Total cost of concrete 80% Cost saved on concrete for FWT ballast Capital recovery period (years) 6100 Maintanance (annual, as a % of initial cost) 15% Number of charge/discharge cycles per day 110 Number of charge cycles during ROI period 7930 At desired (stated) depth 915 Cost of electricity from storage ($/kWh) 14 Initial capital cost $/kWh kWh/tonne concrete 2.29 At optimal (maximum) depth 0.356 Cost of electricity from storage ($/kWh) 14 Initial capital cost $/kWh 1.54 kWh/tonne concrete Comparison to Li ion batteries 6,096 Battery cost ($/kWh) 6,396 Number of full use cycles 6,429 Cost of electricity from storage ($/kWh) 90,004 sinks
148,169,055 -0.755 inner wall 0 -14 -7 12.3 26.7 0.46 $2.00 2 $4,000,000 $45,001,859 $150,000 16 5% 2 11,680 $0.24 $1,527 0.356 $0.11 $822 0.771 $400 2,000 $0.20
Fig. 8. Example spreadsheet design output for a cylindrical based ORES design (inputs in black, outputs in red). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 9. A Grand Challenge worthy of China’s great past, present, and future (inputs in black, outputs in red). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Energy storage requirements: The desired value is entered, and the actual maximum potential is displayed. As shown the latter is larger because in this case the depth of the water specified is not as deep as the structure could tolerate. This is because the
design assumes the pipe wall thickness should also be great enough to also provide negative buoyancy. In this case, it suggests a lower strength, and hence less expensive, concrete could be used.
Please cite this article in press as: Slocum AH. Symbiotic offshore energy harvesting and storage systems. Sustainable Energy Technologies and Assessments (2014), http://dx.doi.org/10.1016/j.seta.2014.10.004
6
A.H. Slocum / Sustainable Energy Technologies and Assessments xxx (2014) xxx–xxx
Geometry: The manufacturability of the pipe sections will drive these values, and hence depend on the resources that can be brought to bear by the project developer. Storage properties: These results indicate how many pipe sections are needed and the total length of pipe needed to achieve the desired storage capacity. Buoyancy: The mass of the system and its elements, and is the pipe heavy enough to stay on the bottom when the water is pumped out are addressed here. The latter does not assume any help from suction that occurs when a rigid body is placed on a muddy bottom. That effect can be considered insurance. Design stress and displacement: In addition to checking the stresses in the pipe, the radial displacement needs to be determined to make sure that other elements that may be attached to the pipe can accommodate the motion (beware of over constraint). Economics: Many different factors are considered here, and form the core of determining if the project is viable at the desired depth, and the optimal (maximum) depth the pipe could withstand. This can help the project manager consider should the pipe be moved to deeper water. Comparison to Li ion batteries: This section reminds the project developer to be objective with regards to energy storage selection. The same basic principles considered here for what is essentially an underwater pumped hydro system would apply to systems that propose to use a flexible bladder to store compressed air, where such systems would also need a similar amount of ballast, but the ballast can be dead weight. A large inflated balloon for storing energy deep undersea would also have significant buoyancy and the design challenge is to enable the system to go through extensive cycles without fatiguing at the attachment points. Designers of such systems may wish to integrate structural fibres for withstanding the stress into the balloon structure itself to minimize the chance of fretting wear between external cables and a bladder. In addition, a roll-sock type structure may prove advantageous for controlled deformation and stress management. Symbiotically, any substantial offshore structure has the potential to act as an artificial reef for the benefit of fish stocks. Seafloor
based energy storage units in deep water can also act as mooring points for other floating energy harvesting devices. With this multi-use purpose in mind, designers and business analysts can maximize the ROI. Aquaculture Last but not least, consider aquaculture, especially given that the world’s fisheries are becoming depleted. Far-offshore systems have the distinct advantage over near-shore systems in that fish growth by-products are quickly carried away from the fish pen and hence less antibiotics should be required to keep the fish healthy. As mentioned above, the cold nutrient rich bottom water pumped out of the storage spheres can be sent to the surface for the benefit of the farmed fish. Although a far-offshore fish farm by itself might not be economical, if it is collocated with one or more of the above systems, service boats and crews can be shared and the system as a whole can thus become more viable. A Planetary Grand Challenge Grand Challenges, such as human powered flight across the English Channel or non-stop round the world flight, seem to be very effective at catalyzing innovation and spirit; therefore it is appropriate that given the above described potential symbiotic systems might be game changers for the planet if they can be engineered to be economical, a Planetary Grand Challenge is thus proposed here to all nations of the world with seacoasts: provide energy and food for a significant portion of your population using sustainable offshore resources. An ideal leader for the first Planetary Grand Challenge would be China. Why? China has over a billion people in need of energy and protein in increasing amounts as it modernizes. The Chinese diet typically includes fish, but at current wild harvest rates soon there will be no more fish anywhere. Meanwhile it is exemplary how China has been able to muster immense resources and willpower to create a high speed rail network from nothing in less than a decade, and is now rapidly developing its robotic research and devel-
Fig. 10. A glorious pacific sunset, as seen from a dive boat by the author, which was a powerful catalyst for much of the work presented in this paper.
Please cite this article in press as: Slocum AH. Symbiotic offshore energy harvesting and storage systems. Sustainable Energy Technologies and Assessments (2014), http://dx.doi.org/10.1016/j.seta.2014.10.004
A.H. Slocum / Sustainable Energy Technologies and Assessments xxx (2014) xxx–xxx
opment capacity. Looking back to the future, also consider ‘‘as long ago as the 12th Century B.C., there were fish rearing records in the Chinese Classics of early Chou Dynasty (1112–221 B.C.).’’ ‘The Chinese Fish Culture Classic’ was originally written by Fan Lee, a politician turned fish culturist, in ancient China during the 5th Century B.C.’’.2 Additional motivation comes from the Romans who challenged their legions to compete with each other to most rapidly build the highest quality sections of roads and other structures, such as Hadrian’s Wall. China could lead the way to the future by challenging its provinces and cities within to create offshore wind energy and aquaculture farms to be the first to realize a Planetary Grand Challenge with goals such as outlined in Fig. 9:
7
be considered and factored into the decision as to whether or not a system is good for the long term health of the planet, and thus for humans as well. Application of the above could then be optimized via the mechanism of a Planetary Grand Challenge where countries compete to see which can achieve a bigger PROI in proportion to their size and influence. It is the author’s belief that engineering is a blend of science and statistics with which managers and politicians paint our future. We are all responsible for the canvas of life, and we CAN work together to create a beautiful future where with every glorious sunset, Fig. 10, we can rest assured that another wonderful day will greet our precious planet. Acknowledgements
Overall system requirements: How many people are to be served, and how much fish and power do they need will be the primary system configuration drivers. Wind farm parameters: How many turbines are needed, how big the wind farm needs to be, and the shape of the region are determined here. Also important is given the number turbines a day to be installed it is determined how many years will be required to complete the system. Aquaculture portion: Given fish growth and size parameters, the size of the confinement zones around each turbine is determined, and a system designer can determine if such a structure is feasible. Comparison with nuclear power: This provides the project manager with a perspective on what the system can do in comparison with how many nuclear plants would need to be otherwise be built, just to serve the power needs of the people. Conclusions and further work Various offshore energy harvesting and storage machines might be independently viable under some circumstances, but when combined with each other or additional operations, such as aquaculture, to share load-bearing structure and maintenance equipment/personnel, not only should their capital and operating costs be reduced, but their profitability should also increase. It is suggested that research systems combining at least two of these systems be developed and deployed to further develop the ability to create symbiotic sustainable systems. Accordingly it is proposed that more than a simple first order short term return on investment (ROI) be considered, rather a new factor ‘‘Planetary ROI’’ (PROI) be developed, similar to life cycle analysis. With a PROI factor, 1st-Nth order effects (+ and )
2
The author would like to thank his coauthors on other papers cited in the references [3] (Gregory Fennell, James Meredith, Guillaume Bettoli, Ruaridh Macdonald, Melissa Showers, Alejandro Gupta, Monique Sager, Alison Greenlee) and [6] (Mathieu Picard, Camille Baelden, You Wu, Le Chang) for their dedication to helping save the planet. The author would also like to thank the reviewers for their time and effort to help with evolving this manuscript. In addition, the author is extremely grateful to his many hosts in China for their hospitality and catalyzing conversations on his recent trip (July 2014). References [1] Park K et al. Optimal design of permeable fiber network structures for fog harvesting. Langmuir 2013;29:13269–77. http://dx.doi.org/10.1021/la402409f. [2] Slocum AH. FUNdaMENTALs of Design, self-published free online http:// web.mit.edu/2.75 , Topic 3, p. 3–14. [3] Slocum AH et-al., Ocean Renewable Energy Storage (ORES) System: Analysis of an Undersea Energy Storage Concept, Proceedings of IEEE, V 101, No. 4, April 2013, p. 906–924. [4] Love MS, Caselle JE, Snook L. Fish assemblages around seven oil platforms in the Santa Barbara Channel area. Fish Bull 1998;1. [5] Kim J, Tsouris C, Mayes RT, Oyola Y, Saito T, janke CJ, et al. Recovery of uranium from seawater: a review of current status and future research needs. Sep Sci Technol 2013;48(3):367. [6] Picard M. Extraction of uranium from seawater: design and testing of a symbiotic system. Nucl Technol 2014;188(2):200–17. [7] Uranium 2011: resources, production and demand, a joint report by the OECD Nuclear Energy Agency and the International Atomic Energy Agency, Organisation for Economic Co-operation and Development (2012). [8] Zhang A, Asakura T, Uchiyama G. The adsorption mechanism of uranium(VI) from seawater on a macroporous fibrous polymeric adsorbent containing amidoxime chelating functional group. React Funct Polym 2003;57(1):67. [9] Tamada M, Seko N, Kasai N, Shimizu T. Cost estimation of uranium recovery from seawater with system of braid type adsorbent. Trans At Energy Soc Jpn 2006;5(4):358.
http://www.fao.org/docrep/field/009/ag158e/AG158E02.htm.
Please cite this article in press as: Slocum AH. Symbiotic offshore energy harvesting and storage systems. Sustainable Energy Technologies and Assessments (2014), http://dx.doi.org/10.1016/j.seta.2014.10.004
Contents lists available at ScienceDirect
Sustainable Energy Technologies and Assessments journal homepage: www.elsevier.com/locate/seta
Original Research Article
Symbiotic offshore energy harvesting and storage systems Alexander H. Slocum ⇑ Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139, USA
a r t i c l e
i n f o
Article history: Received 7 August 2014 Revised 8 October 2014 Accepted 9 October 2014 Available online xxxx Keywords: Offshore energy harvesting Water turbines Energy storage Symbiotic system
a b s t r a c t Taken separately, various offshore energy harvesting and storage machines can have a difficult time competing with land-based systems with regard to both initial capital cost and variable costs. However, collocating systems in a symbiotic way can lead to competitive advantages for offshore systems. This paper explores such combinations based on using the support structure for an offshore wind turbine as a basis for an energy storage system, a wave energy harvesting system, and a uranium-from-seawater mining system. Further considerations could also include aquaculture facilities as well as enhanced sport fisherman opportunities. Finally, far offshore systems are not only less likely to negatively impact birds, such as birds-of-prey, while experience with offshore oil platforms have been shown to act as undersea wildlife havens. Ó 2014 Elsevier Ltd. All rights reserved.
Introduction Consider the history of the cellular phone or the automobile: both started as simple devices but have evolved into platforms for multiple functions. Smart phones allow for voice communication but also are used to take pictures, play games, surf the web, give directions. . . Automobiles not only provide a means for transportation and self-expression, they serve as entertainment and communication centers. It is hypothesized here that renewable energy systems can also serve as multipurpose devices, whose added value can push renewables’ economic viability past the tipping point. ‘‘Symbiotic’’ strictly refers biologically to ‘‘living together’’, but the concept can be a powerful catalyst for reducing energy systems’ environmental impact and ROI. Identifying multi functional opportunities can be accomplished by considering all the characteristics of a deployment zone that are not directly concerned with the primary energy-gathering device, especially the negative issues or risks associated with the primary energy-gathering device. For example, consider solar panels, which are often deployed in sunny regions. A negative aspect of such devices is they get dirty and need to be cleaned, with scarce water. A characteristic of sunny regions is they receive little rainfall. However, a large flat panel can be a great water-gathering device from rainfall or even fog. In the case of the latter its not just gathering water from fog condensing on the panels but fog condensing on other structures ⇑ Address: MIT Room 3-445, 77 Massachusetts Avenue, Cambridge, MA 02139, USA. Tel.: +1 617 253 0012. E-mail address: [email protected]
placed among the solar field [1]. Collected water can be stored and used sparingly over the course of many months for cleaning the cells. The water is not taken from the environment but rather very slowly released back into it because cleaning is done at night when evaporation is low and the dirty runoff can be delivered to the ground. This catalyzes another thought, water with dust falling on the ground potentially carries nutrients as well as moisture and to what good purpose can this be used for? What high value crops can be grown in partial shade that require only modest water resources? This sort of branching thought pattern can continue on with thoughts such as a robot PV panel cleaner/inspector/position adjuster could also tend the crops. . . Considering the entire product life cycle of different systems together can thus lead to reduced overall systems capital and variable costs. Reciprocity The central theme of this paper is thus the creative application of the principle of reciprocity [2]:
1 ¼ opportunity problem
ð1Þ
With this thought process in mind, consider wind turbines. In general, the closer the gathering of a resource to its market, the better. In much of the world, population density is highest, but as beautiful and renewably compelling as a single wind turbine may be, dozens, hundreds, or thousands of them along a scenic ridge or offshore horizon is seen by many as a form of pollution worse than believ-
http://dx.doi.org/10.1016/j.seta.2014.10.004 2213-1388/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Slocum AH. Symbiotic offshore energy harvesting and storage systems. Sustainable Energy Technologies and Assessments (2014), http://dx.doi.org/10.1016/j.seta.2014.10.004
2
A.H. Slocum / Sustainable Energy Technologies and Assessments xxx (2014) xxx–xxx
ing/hoping that global warming is not really the result of human activity. Fortunately, whether by intelligent design, luck, or physics, the earth is round, which means if one goes about the distance of a marathon offshore, large wind turbines are no longer present a serious scenic problem because the horizon drops off by about 130 m. However, the water does often tend to get deep in some places and undersea high power cables are expensive.
Table 1 Potential parameters of an offshore site for wind turbines. Parameter
Physics
Problem
Opportunity
Wind Currents
P = f(V^3) P = f(V^3)
Energy harvesting Energy harvesting
Waves
P = f(h, s)
Intermittent Structure loading Structure loading Dissolved elements Deep-water mooring Transmission line cost
Sea water
The answer could be blowing in the wind, (moving with the waves, flowing in the currents. . .) If it is accepted that offshore wind turbines should be far offshore so they are visually unobtrusive, then what are the parameters associated with being far offshore, and what are the physics associated with those parameters? Moreover, consider not only energy, but also the other human need for food and protein in particular. A partial list is shown in Table 1. Right from the beginning it is apparent that any offshore wind farm will have to assess loadings from currents and waves, and those same assessments can be used to consider/optimize machines that could be attached to the wind tower structure in order to gather additional energy from the site. Since the strength of a tower is proportional to the diameter squared and loading (and cost) is proportional to area (diameter), added energy harvesting methods, even though additional energy harvesting mechanisms may increase tower loading, they should be able to reduce the overall cost of energy harvested from the site. Also apparent are other potential revenue generating means including energy storage [3], aquaculture [4], and harvesting uranium from seawater [5,6]. Current energy harvesting Even though water is 800 denser than air, water turbine blade diameters are typically modest in comparison, so to be competitive with wind energy, flow velocities on the order of 2–4 m/s are needed to economically harvest energy from ocean currents. Note the average speed of the Gulf Stream is about 1.8 m/s with a top speed near the surface of up to 2.5 m/s.1 Most offshore wind farms will probably not be located in such strong currents, but if they are, it could make sense to mount underwater turbines onto the structure that supports the wind turbine as shown in Fig. 1. To help with the trade-off of high-speed currents causing vortex-shedding vibrations on mooring cables or scouring base mountings, ocean turbines may need to be developed with larger blades to operate in slower speed currents. These turbines could be compatible with wave energy harvesting systems, but might interfere with uranium harvesting or aquaculture systems depending on how they are configured.
Deep water
E = mgh
Underwater distance
$=>depth, distance
Energy harvesting Uranium harvesting Pumped hydro energy storage Cleaner water for aquaculture
bine itself. Note that when there is wind there are usually waves, but sometimes when the wind is too slow, significant ocean swells still pass as they can travel hundreds of miles. Uranium harvesting Nuclear fission has the potential to greatly reduce carbon dioxide emissions from power generation. However, a 2011 study by the Organization for Economic Cooperation and Development estimated that at the current consumption rate the global conventional reserves of uranium (7.1 million tonnes) could be depleted in roughly a century [7]. Moreover, as reserves decrease, uranium mining shifts to lower quality sites leading to a higher extraction cost. Fortunately, uranium is present uniformly in ocean water in the form of uranyl ions at a low concentration of 3–3.3 lg/L3. The amount of uranium in the ocean is estimated at 4.5 billion tonnes, nearly a thousand times the conventional reserves, and it is uniformly distributed due to the action of currents. Uranium adsorption by chelating polymers has been found to be most promising in terms of cost, adsorption capacity and environmental footprint [8]. Fig. 3 shows a 3D net-based system, which is inspired from the concept proposed by the Japan Atomic Energy Agency [9], minimizes the amount of chains and ships needed. The concept, shown in Fig. 3, is a net of adsorbent attached to two chains and ballast masses lying at the bottom of the ocean. A single ship continuously
Wave energy harvesting Where there is wind, there are often waves, hence it can make sense to include wave energy harvesters with wind turbine structures. There are many different types of wave energy harvesting devices, but a cylindrical structure that utilizes Wells turbines, concept shown in Fig. 2, in particular may be most appropriate considering the other potential uses for the offshore structure of uranium harvesting and aquaculture. The simple circular system shown in Fig. 1 mounted around the base of the wind tower could be made from sections bolted together. In severe storms it could be submerged to avoid damage. An estimate of the total power that could be generated from this wave power generation ring is also shown in Fig. 1. This is nearly as much power as from the wind tur1
http://oceanservice.noaa.gov/facts/gulfstreamspeed.html.
Fig. 1. Water turbine(s) mounted on an offshore wind turbine tower.
Please cite this article in press as: Slocum AH. Symbiotic offshore energy harvesting and storage systems. Sustainable Energy Technologies and Assessments (2014), http://dx.doi.org/10.1016/j.seta.2014.10.004
A.H. Slocum / Sustainable Energy Technologies and Assessments xxx (2014) xxx–xxx
3
Fig. 2. Wells turbines in a cylindrical structure at the waterline of an offshore wind turbine tower.
recovers the net, runs it through the elution process and replaces it at the bottom. The number of braids in parallel in the net is chosen so that the ship can harvest all the net during the selected harvest period at the recovery velocity taken 4 m/min, and produce 1200 tonnes of uranium per year, enough to supply a 5GW nuclear power plant. Fig. 4 shows a wind turbine tower system approximately to scale [5]. A system of 20 loops of 100 m each, for a total belt length is 4000 meters, loops through once every 38 days and can provide enough uranium to supply 5 MW of electric power during the same period. The central reservoir represents the space required for the elution process. Two storage tanks store enough chemicals to feed the elution process for a month before a crew comes to collect the harvested uranium and refill the tanks. This means, for example, that a 2 GW offshore wind farm could harvest enough uranium to also keep operating a 2 GW nuclear power plant. The net-based system can be located in 100 m deep water and uses a single ship that continuously operates and it is predicted to produce uranium at $326/kg. The continuous system is attached to an offshore wind turbine system to eliminate the need for additional mooring and increase the overall energy gathering ability of the wind farm system. This system could maximize the adsorbent yield but being more dispersed would achieve a production cost of $403/kg of uranium based on current models. Both systems’ costs are expected to drop significantly with materials advances and scale up.
Elution
Sea surface
Ship m
m
m
m
m m m
m
m
m Sea bottom
m m
Fig. 3. Bottom based net mooring and recovery system.
Fig. 4. Wind turbine structure anchored continuous belt system.
Energy storage The offshore environment represents a unique unobtrusive and safe environment for utility scale energy storage that takes advantage of the hydrostatic pressure at ocean depths to create a robust pumped storage device and thus a more consistent and predictable utility scale renewable energy power plant. Fig. 5 illustrates the concept for an undersea pumped hydro system and Fig. 6 shows how it could be deployed in a hexagonal pattern as mooring points for floating wind turbines [3], where concrete spheres act as undersea reservoirs. Fig. 7 shows how concrete pipe could be used so a long series of interconnected pipe feed into one or two large pump/turbine systems. More concrete is needed for a pipe-based system, but fewer pump systems would be required and logistics should be simpler. For either configuration, when the water is pumped out, the concrete is subject to compressive stresses by the deep sea, which wants to flow back in (once a valve to a turbine is opened). Compression is concrete’s most favourable operating mode, and hence great structural efficiency can be obtained. For the pumped hydro system an interesting symbiotic design point exists with regard to the concrete: when the wall thickness is great enough to provide enough weight to keep the structure on the bottom when the water is pumped out (self ballasting), the structure is strong enough to be placed about 700 m deep which also happens to be an ideal head depth for commercially available pump/turbine systems. Fig. 8 shows the output from a spreadsheet for designing a cylinder based ORES, where the major realms of the design are evaluated. This enables the rapid assessment of project feasibility. System properties: The water depth is the single biggest factor with regard to energy density and hence viability. The Internal Pressure can either be vacuum or one atmosphere if a snorkel (tube to surface which could be a mooring line) is used. The concrete properties, especially strength and cost, will also have a major factor on project viability.
Please cite this article in press as: Slocum AH. Symbiotic offshore energy harvesting and storage systems. Sustainable Energy Technologies and Assessments (2014), http://dx.doi.org/10.1016/j.seta.2014.10.004
4
A.H. Slocum / Sustainable Energy Technologies and Assessments xxx (2014) xxx–xxx
Fig. 5. Ocean renewable energy storage system (ORES) concept during charging and discharging operations.
Fig. 6. ORES configuration with floating wind turbines and spherical mooring buoys/pumped hydro systems.
Fig. 7. ORES configuration with floating wind turbine(s) moored to pipe-based mooring/pumped hydro system.
Please cite this article in press as: Slocum AH. Symbiotic offshore energy harvesting and storage systems. Sustainable Energy Technologies and Assessments (2014), http://dx.doi.org/10.1016/j.seta.2014.10.004
5
A.H. Slocum / Sustainable Energy Technologies and Assessments xxx (2014) xxx–xxx System Properties Water depth (m): planned, max allowed for system External Pressure, Po (N/m^2) Internal Pressure, Pi (N/mm^2) Concrete density (kg/m^3) Seawater density (kg/m^3) Modulus of elasticity, E (N/m^2) Poissons ratio, v Concrete strength (Mpa, ksi) Safety factor Emplaced concrete tube cost per tonne (with rebar) Desired ballast for Floating Wind Turbine (Tonnes) Energy Storage Requirements MWhe required at desired depth MWhe at maximum allowable depth Pump/turbnine efficiency Geometry Inside Diameter ID, Di (mm) Wall thickness/ID Length (depth into page), L (m) Outside Diameter OD, Do (mm) Wall thickness, t (mm) Length to diameter ratio Storage Properties Charge capacity (MWh) energy/concrete mass (kWh/tonne) Number of units required Total length of pipe (km) Bouyancy Mass of displaced water (tonnes) Mass displaced water + FWT ballast needed (tonnes) Empty ORES dry mass (tonnes) Total weight of all cylinders (tonnes) Bouyancy: will it stay down? (Y/N must be >1)
Design Stress and Displacement 300 Axial Force on hemispherical ends, Fa (N) 3,000,000 Radial displacement, ur (mm) 100,000 2,600 Radial stress, sigmar (MPa) 1,050 Hoop stress, sigmatheta (MPa) 2.07E+10 Axial stress, sigmaz (MPa) 0.29 Von Mises stress (MPa) 40 Max design stress 1.5 VON MISES/Max design stress (be < 1) $500 Economics 300 Cost of undersea pump/turbine ($/W) Size (MW) 32 Pump/turbine cost 69 Total cost of concrete 80% Cost saved on concrete for FWT ballast Capital recovery period (years) 6100 Maintanance (annual, as a % of initial cost) 15% Number of charge/discharge cycles per day 110 Number of charge cycles during ROI period 7930 At desired (stated) depth 915 Cost of electricity from storage ($/kWh) 14 Initial capital cost $/kWh kWh/tonne concrete 2.29 At optimal (maximum) depth 0.356 Cost of electricity from storage ($/kWh) 14 Initial capital cost $/kWh 1.54 kWh/tonne concrete Comparison to Li ion batteries 6,096 Battery cost ($/kWh) 6,396 Number of full use cycles 6,429 Cost of electricity from storage ($/kWh) 90,004 sinks
148,169,055 -0.755 inner wall 0 -14 -7 12.3 26.7 0.46 $2.00 2 $4,000,000 $45,001,859 $150,000 16 5% 2 11,680 $0.24 $1,527 0.356 $0.11 $822 0.771 $400 2,000 $0.20
Fig. 8. Example spreadsheet design output for a cylindrical based ORES design (inputs in black, outputs in red). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 9. A Grand Challenge worthy of China’s great past, present, and future (inputs in black, outputs in red). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Energy storage requirements: The desired value is entered, and the actual maximum potential is displayed. As shown the latter is larger because in this case the depth of the water specified is not as deep as the structure could tolerate. This is because the
design assumes the pipe wall thickness should also be great enough to also provide negative buoyancy. In this case, it suggests a lower strength, and hence less expensive, concrete could be used.
Please cite this article in press as: Slocum AH. Symbiotic offshore energy harvesting and storage systems. Sustainable Energy Technologies and Assessments (2014), http://dx.doi.org/10.1016/j.seta.2014.10.004
6
A.H. Slocum / Sustainable Energy Technologies and Assessments xxx (2014) xxx–xxx
Geometry: The manufacturability of the pipe sections will drive these values, and hence depend on the resources that can be brought to bear by the project developer. Storage properties: These results indicate how many pipe sections are needed and the total length of pipe needed to achieve the desired storage capacity. Buoyancy: The mass of the system and its elements, and is the pipe heavy enough to stay on the bottom when the water is pumped out are addressed here. The latter does not assume any help from suction that occurs when a rigid body is placed on a muddy bottom. That effect can be considered insurance. Design stress and displacement: In addition to checking the stresses in the pipe, the radial displacement needs to be determined to make sure that other elements that may be attached to the pipe can accommodate the motion (beware of over constraint). Economics: Many different factors are considered here, and form the core of determining if the project is viable at the desired depth, and the optimal (maximum) depth the pipe could withstand. This can help the project manager consider should the pipe be moved to deeper water. Comparison to Li ion batteries: This section reminds the project developer to be objective with regards to energy storage selection. The same basic principles considered here for what is essentially an underwater pumped hydro system would apply to systems that propose to use a flexible bladder to store compressed air, where such systems would also need a similar amount of ballast, but the ballast can be dead weight. A large inflated balloon for storing energy deep undersea would also have significant buoyancy and the design challenge is to enable the system to go through extensive cycles without fatiguing at the attachment points. Designers of such systems may wish to integrate structural fibres for withstanding the stress into the balloon structure itself to minimize the chance of fretting wear between external cables and a bladder. In addition, a roll-sock type structure may prove advantageous for controlled deformation and stress management. Symbiotically, any substantial offshore structure has the potential to act as an artificial reef for the benefit of fish stocks. Seafloor
based energy storage units in deep water can also act as mooring points for other floating energy harvesting devices. With this multi-use purpose in mind, designers and business analysts can maximize the ROI. Aquaculture Last but not least, consider aquaculture, especially given that the world’s fisheries are becoming depleted. Far-offshore systems have the distinct advantage over near-shore systems in that fish growth by-products are quickly carried away from the fish pen and hence less antibiotics should be required to keep the fish healthy. As mentioned above, the cold nutrient rich bottom water pumped out of the storage spheres can be sent to the surface for the benefit of the farmed fish. Although a far-offshore fish farm by itself might not be economical, if it is collocated with one or more of the above systems, service boats and crews can be shared and the system as a whole can thus become more viable. A Planetary Grand Challenge Grand Challenges, such as human powered flight across the English Channel or non-stop round the world flight, seem to be very effective at catalyzing innovation and spirit; therefore it is appropriate that given the above described potential symbiotic systems might be game changers for the planet if they can be engineered to be economical, a Planetary Grand Challenge is thus proposed here to all nations of the world with seacoasts: provide energy and food for a significant portion of your population using sustainable offshore resources. An ideal leader for the first Planetary Grand Challenge would be China. Why? China has over a billion people in need of energy and protein in increasing amounts as it modernizes. The Chinese diet typically includes fish, but at current wild harvest rates soon there will be no more fish anywhere. Meanwhile it is exemplary how China has been able to muster immense resources and willpower to create a high speed rail network from nothing in less than a decade, and is now rapidly developing its robotic research and devel-
Fig. 10. A glorious pacific sunset, as seen from a dive boat by the author, which was a powerful catalyst for much of the work presented in this paper.
Please cite this article in press as: Slocum AH. Symbiotic offshore energy harvesting and storage systems. Sustainable Energy Technologies and Assessments (2014), http://dx.doi.org/10.1016/j.seta.2014.10.004
A.H. Slocum / Sustainable Energy Technologies and Assessments xxx (2014) xxx–xxx
opment capacity. Looking back to the future, also consider ‘‘as long ago as the 12th Century B.C., there were fish rearing records in the Chinese Classics of early Chou Dynasty (1112–221 B.C.).’’ ‘The Chinese Fish Culture Classic’ was originally written by Fan Lee, a politician turned fish culturist, in ancient China during the 5th Century B.C.’’.2 Additional motivation comes from the Romans who challenged their legions to compete with each other to most rapidly build the highest quality sections of roads and other structures, such as Hadrian’s Wall. China could lead the way to the future by challenging its provinces and cities within to create offshore wind energy and aquaculture farms to be the first to realize a Planetary Grand Challenge with goals such as outlined in Fig. 9:
7
be considered and factored into the decision as to whether or not a system is good for the long term health of the planet, and thus for humans as well. Application of the above could then be optimized via the mechanism of a Planetary Grand Challenge where countries compete to see which can achieve a bigger PROI in proportion to their size and influence. It is the author’s belief that engineering is a blend of science and statistics with which managers and politicians paint our future. We are all responsible for the canvas of life, and we CAN work together to create a beautiful future where with every glorious sunset, Fig. 10, we can rest assured that another wonderful day will greet our precious planet. Acknowledgements
Overall system requirements: How many people are to be served, and how much fish and power do they need will be the primary system configuration drivers. Wind farm parameters: How many turbines are needed, how big the wind farm needs to be, and the shape of the region are determined here. Also important is given the number turbines a day to be installed it is determined how many years will be required to complete the system. Aquaculture portion: Given fish growth and size parameters, the size of the confinement zones around each turbine is determined, and a system designer can determine if such a structure is feasible. Comparison with nuclear power: This provides the project manager with a perspective on what the system can do in comparison with how many nuclear plants would need to be otherwise be built, just to serve the power needs of the people. Conclusions and further work Various offshore energy harvesting and storage machines might be independently viable under some circumstances, but when combined with each other or additional operations, such as aquaculture, to share load-bearing structure and maintenance equipment/personnel, not only should their capital and operating costs be reduced, but their profitability should also increase. It is suggested that research systems combining at least two of these systems be developed and deployed to further develop the ability to create symbiotic sustainable systems. Accordingly it is proposed that more than a simple first order short term return on investment (ROI) be considered, rather a new factor ‘‘Planetary ROI’’ (PROI) be developed, similar to life cycle analysis. With a PROI factor, 1st-Nth order effects (+ and )
2
The author would like to thank his coauthors on other papers cited in the references [3] (Gregory Fennell, James Meredith, Guillaume Bettoli, Ruaridh Macdonald, Melissa Showers, Alejandro Gupta, Monique Sager, Alison Greenlee) and [6] (Mathieu Picard, Camille Baelden, You Wu, Le Chang) for their dedication to helping save the planet. The author would also like to thank the reviewers for their time and effort to help with evolving this manuscript. In addition, the author is extremely grateful to his many hosts in China for their hospitality and catalyzing conversations on his recent trip (July 2014). References [1] Park K et al. Optimal design of permeable fiber network structures for fog harvesting. Langmuir 2013;29:13269–77. http://dx.doi.org/10.1021/la402409f. [2] Slocum AH. FUNdaMENTALs of Design, self-published free online http:// web.mit.edu/2.75 , Topic 3, p. 3–14. [3] Slocum AH et-al., Ocean Renewable Energy Storage (ORES) System: Analysis of an Undersea Energy Storage Concept, Proceedings of IEEE, V 101, No. 4, April 2013, p. 906–924. [4] Love MS, Caselle JE, Snook L. Fish assemblages around seven oil platforms in the Santa Barbara Channel area. Fish Bull 1998;1. [5] Kim J, Tsouris C, Mayes RT, Oyola Y, Saito T, janke CJ, et al. Recovery of uranium from seawater: a review of current status and future research needs. Sep Sci Technol 2013;48(3):367. [6] Picard M. Extraction of uranium from seawater: design and testing of a symbiotic system. Nucl Technol 2014;188(2):200–17. [7] Uranium 2011: resources, production and demand, a joint report by the OECD Nuclear Energy Agency and the International Atomic Energy Agency, Organisation for Economic Co-operation and Development (2012). [8] Zhang A, Asakura T, Uchiyama G. The adsorption mechanism of uranium(VI) from seawater on a macroporous fibrous polymeric adsorbent containing amidoxime chelating functional group. React Funct Polym 2003;57(1):67. [9] Tamada M, Seko N, Kasai N, Shimizu T. Cost estimation of uranium recovery from seawater with system of braid type adsorbent. Trans At Energy Soc Jpn 2006;5(4):358.
http://www.fao.org/docrep/field/009/ag158e/AG158E02.htm.
Please cite this article in press as: Slocum AH. Symbiotic offshore energy harvesting and storage systems. Sustainable Energy Technologies and Assessments (2014), http://dx.doi.org/10.1016/j.seta.2014.10.004