Design of concrete spheres for energy storage at the ocean bed

Abstract

In the energy sector, a transition is made from fossil fuels to sustainable energies. Due to an irregular supply of these sustainable energies, it often occurs that there is a mismatch between supply and demand. Energy storage is a promising technique to overcome this. In this thesis, the aim is to store gravitational energy in concrete spheres which are placed on the seabed.

Therefore, this thesis aims to answer whether the most optimal construction method uses 3D Concrete Printing (3DCP) and if so, what the optimal construction method will be using 3DCP. Based on a specific case, five main construction methods are qualitative investigated, from which two are with 3DCP. From an evaluation, it followed that a combination of 3DCP and conventional casting is the most promising method and is therefore selected for more in-depth research. In this construction method, the formwork is made with 3DCP and concrete is cast in the formwork. A prefab bottom and top part are required, because the maximum inclination (nowadays) for printing is limited to 45°.

Based on the selected construction method, three different 3DCP-variants are investigated: construction in the dry (1), submerged construction (2) and construction on a pontoon (3). The two main topics that are investigated are the use of 3DCP for the formwork and the suitability of unreinforced concrete for the sphere itself. The possible use of 3DCP is verified based on two failure mechanisms: plastic collapse (results in minimal free height of the formwork) and elastic collapse (results in maximum free height of the formwork). The suitability of unreinforced concrete is verified based on strength requirements.

Both in case of construction in the dry and construction on a pontoon, h1 is 0.20 m and h2 is 0.65 m. In case of submerged construction, h1 is 0.25 m and h2 is 0.72 m. The unreinforced wall thickness should not exceed 2.5 m because it is required that the sphere is buoyant. For this wall thickness, a concrete strength class of C60/75 is required for construction in the dry, submerged construction and construction on a pontoon. An evaluation based on feasibility and reliability, combined with the results above, shows that construction in the dry is the most promising method.

An adiabatic calculation is made of the temperature increase of the concrete, based on five different cement compositions. It followed that unreinforced concrete will crack. However, in reality, the temperature increase is semi-adiabatic, resulting in lower tensile stresses. It is therefore highly recommended to further investigate this temperature increase in order to verify the suitability of unreinforced concrete. Measures that could be taken to lower the temperature differences are: addition of liquid nitrogen or addition of ice instead of water for the concrete mix. Besides that, it is recommended to investigate the possible use of fibre-reinforced concrete, which could significantly increase the tensile strength of concrete.