Andrew J. Pimm

Shape and cost analysis of pressurized fabric structures for subsea compressed air energy storage

In this article, three different methods are presented for finding the deformed shape of pressurized fabric structures underwater. The methods are used here to analyse the shape and cost of ‘energy bags’, inflatable bags that can be anchored to the seabed and used for subsea compressed air energy storage. First, a system of coupled ordinary differential equations is derived which can be solved to find the shape of an inextensible axisymmetric membrane. Then finite-element analysis (FEA) of an axisymmetric natural shape bag is carried out using cable elements, giving the deformed shape of an extensible axisymmetric membrane. Finally, a full three-dimensional FEA is presented which includes cable and membrane elements. A simple optimization is also used to minimize the cost per unit of energy stored in an axisymmetric natural shape energy bag, and it is shown that if only materials costs are taken into account, the cost of surface is approximately equal to the cost of meridional reinforcement in the optimum-sized bag.

Analysis of flexible fabric structures for large-scale subsea compressed air energy storage

The idea of storing compressed air in submerged flexible fabric structures anchored to the seabed is being investigated for its potential to be a clean, economically-attractive means of energy storage which could integrate well with offshore renewable energy conversion. In this paper a simple axisymmetric model of an inextensional pressurised bag is presented, along with its implementation in a constrained multidimensional optimization used to minimise the cost of the bag materials per unit of stored energy. Base pressure difference and circumferential stress are included in the optimization, and the effect of hanging ballast masses from the inside of the bag is also considered. Results are given for a zero pressure natural shape bag, a zero pressure bag with circumferential stress and hanging masses, and a nonzero pressure bag with circumferential stress and hanging masses.

Analysis of a Wind Turbine Power Transmission System with Intrinsic Energy Storage Capability

A wind turbine transmission system is described wherein mechanical power directly from the slow rotation of the shaft of a large wind turbine rotor is carried over to electrical power through a synchronous generator via the circulation of a high pressure gas running in a closed circuit. In the most straightforward mode of operation, power is injected into the gas circuit via special low-speed nearly-adiabatic compressors with very high isentropic efficiency and is extracted using an expander that is also nearly-adiabatic with relatively high isentropic efficiency. In other operating modes, it is possible to exploit the temperature changes arising naturally from adiabatic compression or expansion of gas so as to put energy into storage or recover energy from storage. This paper explores some of the design rationale behind such a power transmission system and uses exergy analysis to explain and evaluate its operation.

The economics of hybrid energy storage plant

Energy storage technologies all have different characteristics that make them suitable to particular purposes. A hybrid energy storage (HES) arrangement can take advantage of these differences. An example of this is a compressed-air energy storage plant with a flywheel mounted on the same shaft as the motor/generator and turbo machinery, where the flywheel would be used for high-frequency transactions with the grid while the compressed air store, with lower efficiency but also lower cost per unit of energy storage, could be used for longer term storage. This paper reports an algorithm which can be used to find the maximum arbitrage value achievable with a given HES plant for a given set of electricity prices. This is used in an optimisation routine to find the configuration of hybrid flywheel/compressed air plant that maximises potential arbitrage value. With realistic costs and efficiencies, it is found that the maximum annual return on investment is 0.29%, corresponding to a plant with 10% of its total energy storage capacity in the form of flywheel and the rest as compressed air.

Compressed Air Energy Storage

Abstract Compressed air energy storage (CAES) is known to have strong potential to deliver high-performance energy storage at large scales for relatively low costs compared with any other solution. Although only two large-scale CAES plants are presently operational, energy is stored in the form of compressed air in a vast number of situations and the basic technologies of air compression and expansion are very familiar. This chapter outlines some of the fundamental elements of energy storage via compressed air and highlights why CAES actually represents a very wide array of possible plant types.

Underwater Compressed Air Energy Storage

At the center of every compressed air energy storage installation is the vessel, or set of vessels, that retains the high-pressure air. Normally, high-pressure air storage also dominates the cost of the installation, and its characteristics play a key role in determining performance. Underwater storage of pressurized air is characterized by three important attributes: (1) it has the potential to achieve very low cost per unit of energy stored, (2) it naturally tends to exhibit an isobaric (constant pressure) characteristic of pressure versus fill-level, and (3) in stark contrast to underground air storage, it is feasible in many locations to establish economically competitive compressed air stores at relatively small scales—measured in tens of megawatt-hours rather than gigawatt-hours.