BEQS MARINE PUMPED-STORAGE CONCEPTS FOR FLOATING … · 2020. 3. 30. · E-proceedings of the 38th IAHR World Congress September 1-6, 2019, Panama City, Panama 6019 Figure 1 Schematic

E-proceedings of the 38th IAHR World CongressSeptember 1-6, 2019, Panama City, Panama

doi:10.3850/38WC092019-1823

6018

BEQS – MARINE PUMPED-STORAGE CONCEPTS FOR FLOATING CITY EXTENSIONS

ROBERT KLAR (1), MATTHIAS TONNEL (2,3), BERND STEIDL(4) & MARKUS AUFLEGER(5)

(1, 2, 4, 5) Unit of Hydraulic Engineering, Department of Infrastructure, University of Innsbruck, Innsbruck, Austria [email protected]

(3) Ecole Nationale Supérieur des Techniques Avancées, France

[email protected]

ABSTRACT

Electricity storage and regulation is the key to reliable and sustainable future energy systems based on wind and sun. Proven pumped-storage hydropower (PSH) accounts currently for over 94 per cent of installed global energy storage capacity and over 99 per cent in terms of energy stored (IHA 2018). A growing decentralization of energy production systems caused by a raising number of renewable sources requires decentralized storage systems as well. Buoyant Energy (BE) a hydraulic storage approach transfers PSH main features to marine environments. In essence, BE consists of large floating water reservoirs and hydraulic pump-turbine/motor-generator systems. In a charge-discharge cycle, electrical power is converted to gravitational energy and vice versa by moving water between sea and reservoir against a pressure difference.

The application of BE systems along coastal areas is of particular interest due to the progressive urbanization and the associated strong growth of coastal cities. The idea of ‘Buoyant Energy Quarters’ (BEQs) emerged to cover energy storage and building ground needs by one solution. BEQs are floating urban neighborhoods. They are uniquely well suited for resource-efficient and environmentally friendly concrete construction. Composed of modules, BEQs can be adjusted, expanded and re-combined easily to adapt to different social needs over time.

Keywords: pumped-storage, hydroelectricity, floating architecture

1 INTRODUCTION Traditionally, Austria is known for its excellence in pumped-storage hydroelectricity (PSH). To transfers the

PSH key features to an offshore environment, experts from the University of Innsbruck developed and patented (EP 2681445 B1, US 9617969 B2) an idea named ‘Buoyant Energy’ (BE). In essence, BE consists of large and floating water reservoirs and hydraulic pump-turbine/motor-generator systems for energy conversion (Klar et al. 2017, 2018). In a charge-discharge cycle, electrical power is converted to gravitational energy and back again. BE is suitable for nearshore and offshore storage needs and uses the well-established PSH technology in a new arrangement.

Currently the ‘K-Regio’ research project BEQs - Buoyant Energy Quarters, funded by the ERDF (European Regional Development Fund), is under investigation to develop concepts of floating city extensions with the add-on value of energy storage capacity using BE technology. Since January 2019, various experts from such different fields as diverse as architecture, civil engineering and the energy industry have formed a strong consortium and are working on this 3-year project.

Figure 1 shows the discharged (left side) and charged state (right side) of a schematic BEQ (Buoyant Energy Quarter). The platform top acts as a building ground for floating architecture. The lower habitable levels are from time to time above or below the water surface depending on the charging state. A pumped-storage seawater reservoir is situated at the lower part. Conventional pumped storage projects store and generate energy by moving water between two reservoirs at different elevations. Here, the second reservoir is the sea and the immersion depth caused by the structure geometry and weight provides the elevation difference between the reservoirs. At times of low electricity demand, excess energy is used to pump water out of the seawater reservoir. The whole structure then rises slowly. During periods of high electricity demand, water flows into the reservoir through turbines generating electricity. As a result, the BEQ is lowered in a controlled manner.


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Figure 1 Schematic sketch of a section through one BEQ (Buoyant Energy Quarter) module

The multi-use space on the platform roofs and inside the structure are suitable for such different purposes like e.g. floating residential and commercial buildings, sports and recreational facilities, parking garages, sewage treatment plants, aquaculture or server farms.

The following chapter 2 focuses on the structural design in order to provide rules for the subsequent architectural design process and to give indications for promising BEQ shapes

to reduce construction costs by developing standardized modules,

to maximize energy storage capacity,

to provide flexibility for the architectural design and

to fulfill the floating stability as well as security and comfort criteria.

The final chapter 3 summarizes the vision and motivation behind BEQs and gives an outlook on future project outcomes.

2 ENERGY STORAGE AND FLOATING STABILITY This section focuses on designing the platform (bottom part of the structure, which will support the top

architecture and ensure the energy storage). The aim is to extract some main design criteria in order to build a model scale-testing platform that will be used for experimenting purposes (static and dynamic behavior checks, inner sloshing analysis) as well as educational, demonstration and promotional purposes.

2.1 Geometry, weight and storage capacity of a BEQ The bottom part of the BEQ (assuring support and floating stability) is assumed wall sided (sidewalls are

vertical). Figures 2 and 3 show the platforms main components and dimensions as schematic illustrations.


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Figure 2 Vertical section of a BEQ structure showing the main geometric parameters. The architecture is

symbolized by a simple gray box on the platform top.

Figure 3 Horizontal cross section of a 5-sided BEQ structure showing the vertical walls and the quartered

seawater reservoirs.

𝐴 and 𝐴𝑤 represent the outer bottom surface area and inner water tank surface area respectively. The horizontal cross section of the platform is a regular polygon with 𝑛 edges and the water reservoirs can be divided by some inner walls (Figure 3). 𝐺 and 𝐺𝑎 are the center of mass of the total structure (platform and added mass from the architecture on top and of total mass 𝑀) and of the architecture on top (building, people, vehicles…) of mass 𝑀𝑎 respectively. For a given bottom shape of surface 𝐴, maximum draft ℎ𝑚𝑎𝑥 and wall thicknesses, it is possible to calculate the energy storage capacity of the BEQ, its draft function of the water filling as well as the maximum water capacity that maximize storage capacity. This has already been studied in an ideal case without wall thicknesses (Klar et al. 2018 and 2017). For a given water level ℎ𝑤, the draft ℎ and pressure head 𝐻 can be calculated:

ℎ = 𝑀 + 𝜌𝑤𝐴𝑤ℎ𝑤

𝜌𝑤𝐴and 𝐻 = ℎ − ℎ𝑤 − 𝑒𝑏 [1]

The energy capacity is then calculated based on the pump-turbine flow rate. The power of the pump-turbine is defined as 𝑃 = 𝜂 ∙ 𝜌𝑤 ∙ 𝑔 ∙ 𝐻 ∙ 𝑄(𝑡), where 𝜂 is the efficiency of the pump-turbine and 𝑄(𝑡) the flow rate. All unknowns (𝑀,𝐻, ℎ, ℎ𝑤) can be expressed as functions of geometrical parameters and 𝑄(𝑡), as an example:

ℎ𝑤(𝑡) = ∫𝑄(𝑡)

𝐴𝑤

𝑡

0

𝑑𝑡 = 𝐹(𝑡)

𝐴𝑤[2]

where,

𝐹(𝑡) = ∫ 𝑄(𝑡)𝑑𝑡𝑡

0

[3]

Where 0 < 𝑡 < 𝑡𝑚𝑎𝑥 and 𝑡𝑚𝑎𝑥 defined by 𝐹(𝑡𝑚𝑎𝑥) = ℎ𝑤max ∙ Aw. Whatever flow rate 𝑄 is chosen, the energy

capacity 𝐸 is given by (in the case of a constant efficiency 𝜂):

𝐸 = 𝜂𝜌𝑤𝑔𝐴𝑤ℎ𝑤𝑚𝑎𝑥 (ℎ𝑚𝑎𝑥 − 𝑒𝑏 −(𝐴 + 𝐴𝑤)

2𝐴ℎ𝑤max) [4]

Maximizing this quantity in regard of ℎ𝑤max gives:

{

ℎ𝑤max =𝐴

𝐴 + 𝐴𝑤(ℎmax − 𝑒𝑏)

𝐸 = 𝜂 𝜌𝑤𝑔𝐴𝐴𝑤

2(𝐴 + 𝐴𝑤)(ℎmax − 𝑒𝑏)

2

𝑀 = 𝜌𝑤 𝐴

(𝐴 + 𝐴𝑤)(𝐴ℎmax −𝐴𝑤𝑒𝑏)

[5]

Knowledge about the construction material and dimensions of the platform enables us to calculate the maximum added mass allowed (𝑀𝑎 = 𝑀 −𝑀𝑠𝑡𝑟𝑢𝑐𝑡) with 𝑀𝑠𝑡𝑟𝑢𝑐𝑡 the mass of the platform.

In order to ensure comfort and security of the goods and person on the platform, a floating stability analysis of the platform needs to be performed. This starts with a static analysis (chapter 2.2) and will be followed by a dynamic analysis of the platform behavior (dynamic stability and water sloshing matters) in future investigations.

2.2 Static floating stability of a BEQ This part focuses on the static stability of the floating platform. The analysis is conducted in a similar way

as in Van Santen (2009). The energy, the righting arm and moments are calculated for different axis directions (𝜓) and heeling angles (𝜙) as well as different filling levels (0 ≤ ℎ𝑤 ≤ ℎ𝑤𝑚𝑎𝑥).

2.2.1 A preliminary study (some basic rules) In a first time, different shapes are studied in order to orientate a shape choice for the platform. The main

parameters for the shape are the number of edges of the cross section (regular polygons: 𝑛 = {3,4,… ,8} sides


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are tested) and the aspect ratio (𝛼 = ℎ𝑚𝑎𝑥2/𝐴 = {0.03 … 0.8}). The influence of quartering the water reservoir

is also studied (a single water reservoir or 𝑛 triangular reservoirs). A first series of calculations is conducted using an idealized platform (no wall thicknesses and a total mass 𝑀 placed at a height of 0.75 ∙ ℎ𝑚𝑎𝑥 . From this analysis, several important designing rules can be extracted:

For a same cross section area (𝐴), the more sides 𝑛, the more stable is the platform. The stabilityresults and curves (energy, righting arms, …) are less sensitive to the axis direction (the stability of adisk shaped cross section platform would be independent of the axis direction, Figure 4).

The smaller the aspect ratio (𝛼) is, the more stable is the platform (Figure 5).

Quartering the reservoir increases the stability of the platform (no change for empty and full reservoirs,Figure 6).

The lower the center of mass is, the more stable is the platform

Those results suggest to avoid triangular based platforms and to focus on aspects ratios smaller than 0.5.

Figure 4 Righting arms of 𝑛 = 3 sided platforms and different axis directions as a function of heel angle

Figure 5 Righting arm of 𝑛 = 3 sided platforms for

different aspect ratios 𝛼 as a function of heel angle

Figure 6 Righting arm of 𝑛 sided platforms of ratio

𝛼 = 0.25 and for quartered and none quartered reservoirs as a function of heel angle

Figure 7 Wind heeling moment, righting arm curve, and parameters used in the intact stability analysis


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2.2.2 Security and comfort criteria In order to ensure the security of the installation, the stability curves (righting arm Figure 7) of the platform

should follow some criteria. Some existing criteria exist for offshore floating platforms as the ones given in HSE (2005). Those criteria are:

The ratio of areas, 𝐴𝑅, under the heeling and righting moment curves has to satisfy the criterion:

𝐴𝑅 = (𝐴 + 𝐵)/(𝐵 + 𝐶) ≫ 1.4 [6]

where the areas 𝐴, 𝐵 and 𝐶 are integrated up to an angle 𝜙𝑅, which is either the second intercept angle, 𝜙2, or minimum downfooding angle, 𝜙𝐷, whichever is smaller.

The second intercept angle,𝜙2 must be not less than 30° in the severe storm condition, and not lessthan 20° (neglecting wind heel) during a location move.

The static heel angle, 𝜙1 , must be not greater than 15°. The metacentric height, 𝐺𝑀, must be not less than 0.5 m.

The right arm parameter, 𝐺𝑍, must satisfy the relationship 𝐺𝑍 ≫ 0.5𝐺𝑀0𝑠𝑖𝑛(𝜙) over the range0 ≪ 𝜙 ≪ 𝑚𝑖𝑛(𝜙𝐷 , 𝜙𝑀 , 15°), where 𝜙𝑀 is the angle of maximum righting lever, and 𝐺𝑀0 is the minimum

permissible value of 𝐺𝑀 specified above.

The dimensions of the platform have been chosen to get the maximum energy capacity for a given 𝐴 and ℎ𝑚𝑎𝑥. But in this case, as soon as the platform heels, the deck is flooded meaning that 𝜙𝐷 is close to zero. In order to satisfy the safety criteria, this angle needs to be increased. The platform sides must be elevated of ℎ𝑎 from their initial value.

2.2.3 Adjusting the size and shape of the platform A finer choice of the platforms dimensions can now be achieved. The choice has been orientated in chapter

2.2.2 to a 4 and 5 sided cross-section area. The wall thickness is taken into account (reinforced concrete walls of density 𝜌𝑐 = 2600 𝑘𝑔.𝑚

−3). From the preliminary study, a new set of platforms geometry is tested for a cross-

section area 𝐴 = 900𝑚2 m, 𝛼 = {0.1, . . . ,0.5} and different positions of the center of gravity 𝐺𝑎 (Figure 2) of theadded mass. The elevation ℎ𝑎 is chosen so that 𝜙𝐷 = 6°. It is possible to estimate the energy storage capacity of each configuration as well as the mass division between platform structure and added mass. Figures 8, 9 and 10 show some exemplary results:

Figure 8 Mass division for a 4-sided shape Figure 9 Mass division for a 5-sided shape


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After processing the results, it is possible to generate easy-readable diagrams that illustrate the range of geometrical parameters for which the platform satisfies the security criteria. These diagrams serve as a basis for the subsequent architectural and mechanical engineering design. Figure 11 shows exemplary stability curves for a 𝑛 = 4 sided BEQ structure with different wall thicknesses. The smaller the aspect ratio 𝛼 is, the higher the center of added mass 𝑀𝑎 of the architecture on top (e.g. building, people, vehicles) can be. The added mass position 𝐺𝑎 is expressed by the distance 𝛾 ∙ ℎ𝑚𝑎𝑥 (Figure 11, horizontal axis) from the platform bottom.

Figure 10 Energy storage capacity for 𝑛 = 4 sides Figure 11 Range of secure stability of the platform

3 CONCLUSIONS AND OUTLOOK Accelerated change is affecting all of us in almost all countries today. It is shaped by the rapid increase of

technological disruptions, economic shifts, environmental threats and social transformations. Cities are a primary arena where change takes place. Today, they are home to 54 percent of the world’s population, and by the middle of this century that figure will rise to 66 percent (Habitat 2016). While cities face major problems, from poverty to pollution, they are also powerhouses of economic growth and catalysts for innovation. Therefore, new visions are necessary to get closer to the United Nations Sustainable Development Goal number 11, to make cities and human settlements inclusive, safe, resilient, and sustainable.

In the course of the ongoing K-Regio research project BEQs a number of diverse experts develop new concepts to address some of the big challenges facing cities at coastal areas, including the rapid urbanization, the rise of the sea levels caused by global warming, the integration of renewable energy, housing affordability and new ways of urban accessibility. Since work on the BEQs project has recently started, this paper outlines the first results of the past six months. The aim is to provide basic rules for the architectural design that enables both

flexibility for the floating architecture (giving simple but efficient and weakly constraining criteria for the architects) and ensures stability and security. The next steps include a study of the dynamical behaviour of BEQs in operating conditions as well as mooring and sloshing for a complete floating stability analysis.

BEQs overall vision is to serve as a model for sustainable neighborhoods designed in a modular way and thus can be easily extended or re-arranged, to adapt for future social-economic realities. BEQs could be a new kind of place that combines the best ideas in floating city architecture and creates an attractive location for creative people, businesses and start-ups, and that serves as a hub for urban innovation to improve the quality of life. Over time, marine life will colonize BEQs underwater concrete faces and thereby transform them into floating reefs, which increases biodiversity and water quality as a side effect.

In terms of energy-storage, up to now most attention was paid either to household or to large storage facilities. Community storage solutions at the meso-level have so far been neglected (Wawer et al. 2018). The BE pumped storage technology is a cost-effective and long-lasting option for energy storage acting as a future provider of ancillary services including network frequency control and reserve generation. With an ability to respond almost instantaneously to changes in the amount of electricity BEQs are able to balance electricity supply and demand at community level. Storing and deploying electricity for people living and working at floating BEQs and connected coastal cities would lead to increased electricity self-consumption from decentralized renewable generation.


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Variable resources of wind and solar as a major part of the baseload generation, comes with a new set of opportunities and challenges. The main opportunities are positive environmental benefits in terms of reduced greenhouse gases and potentially lower energy costs. The main challenge is how to balance the variable resources at a reasonable cost without compromising reliability.

Therefore, BEQs could serve as a model for both the balancing challenge by offering electricity storage capacity and sustainable city extension providing floating building ground.

ACKNOWLEDGEMENTS This work is part of the K-Regio project BEQs, co-financed by the European Regional Development Fund.

REFERENCES HSE (2005), The Health and Safety Executive. Research report 387 : Stability. Habitat, U. N. (2016). World Cities Report: Urbanization and Development: Emerging Futures. Retrieved on

January, 28, 2018. Klar, Robert; Steidl, Bernd; Aufleger, Markus (2018): BEQs and BEL: two new opportunities in pumped-storage

hydropower. In: Aronne Armanini und Elena Nucci (Hg.): New challenges in hydraulic research and engineering. 5th IAHR Europe Congress. Trento, 12.6.-14.06.2018. Trento: IAHR, S. 581–582. Available online at http://rpsonline.com.sg/rps2prod/iahr2018/pdf/205.pdf.

Klar, R., Steidl, B., Aufleger, M. (2018). A floating energy storage system based on fabric. Ocean Engineering. 165. 328-335.

Klar, R., Steidl, B., Sant, T., Aufleger, M., Farrugia, R.N., (2017). Buoyant Energy - balancing wind power and other renewables in Europe's oceans. Journal of Energy Storage, 14 (2), 246–255.

Van Santen J. (2009), The use of energy build up to identify the most critical heeling axis direction for stability calculations for floating offshore structures, 10th International Conference on Stability of Ships and Ocean Vehicles, pp. 65-76.

IEA (2018), ‘Renewables 2018’, International Energy Agency of the Organisation for Economic Cooperation and Development, Paris, France.

IHA (2018), IHA (International Hydropower Association) working paper, December 2018, Title: The world’s water battery: Pumped hydropower storage and the clean energy transition, https://www.hydropower.org/publications/ (last accessed on May 30th, 2019)

Wawer, Tim; Griese, Kai-Michael; Halstrup, Dominik; Ortmann, Manuel (2018): Stromspeicher im Quartier. In: Zeitschrift für Energiewirtschaft 42 (3), S. 225–234. DOI: 10.1007/s12398-018-0230-6.

https://www.hydropower.org/publications/the-world%E2%80%99s-water-battery-pumped-hydropower-storage-and-the-clean-energy-transition

Documents

BEQS MARINE PUMPED-STORAGE CONCEPTS FOR FLOATING … · 2020. 3. 30. · E-proceedings of the 38th IAHR World Congress September 1-6, 2019, Panama City, Panama 6019 Figure 1 Schematic