Discharging Energy
In the discharging cycle, everything reverses: the gas and the material flows change in direction, the compressor becomes an expander, the expander becomes a compressor, and the electric motor becomes a generator to return energy to the grid.
Schematic of PTES system during charge (storing energy)
Schematic of PTES system during discharge (generating back to network)
Features
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Flexible Arrangement
Unlike some other energy storage technologies, the PTES architecture allows power rating and storage depth to be completely decoupled. This allows the PTES to be optimised for different end users who may be targeting very different market use cases. For example a wind farm operator may wish to install a relatively large storage depth to offset wind curtailment, whereas a PV operator may wish to focus on 10-12 hours to optimise diurnal operation. A peaking plant may chose a shorter storage time of approximately 4 hours. Given the very low marginal cost of the PTES storage, it is expected that the system will be most economically deployed in applications requiring storage depths in excess of 10 hours.
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Unlikely to raise local environmental objections
The PTES can be considered to be ‘site agnostic’ in that no special climatic, geographical or physical features are required for its location. It could be installed in virtually any site that has been zoned for industrial or light-industrial use, with the ideal locations being adjacent either to electrical distribution or power generation assets. There are no hazardous materials on site, and during normal operation there would be no fume-generating process. The land area requirement is modest, thanks to the high power density and stacking of systems in a mostly vertical arrangement. There is no risk of land contamination, posing few challenges to decommissioning or site re-purposing.
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Synchronous operation
The PTES has a synchronous motor-generator with good capability of ‘fault ride through’ – the ability of the generator to support the network and/or reconnect rapidly whilst a transient fault at some other point in the network is automatically cleared. Unlike the electronic-power-converter connected generators of most wind and solar plants, synchronous generators are naturally good at fault ride through and, in fossil and nuclear power stations, traditionally provide network resilience. The ability to provide grid inertia presents an attractive opportunity to the grid which is seeing increasing numbers of spinning inertia being decommissioned.
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Fully reversible within milliseconds
The PTES can be change mode of operations within tens of milliseconds meaning the system can be seen as fully dispatchable generation and can also respond to the ancillary services market. Examples include peaking and frequency response. This allows the PTES to offer ancillary services to the grid.
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Very low energy loss when charged
Energy loss from the storage silos is forecast to be very low. Using appropriate thicknesses of high—performance insulation, loss can be limited to less than 0.5% energy loss per day if the system is dormant for longer periods.
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Long system life
The system components and storage material are projected to have a long life. Apart from some wear & tear typical of industrial equipment (which can be managed through planned maintenance) there is no substantial degradation of the storage material itself. A plant life of 20 years is a reasonable minimum This is in stark contrast to the materials in battery cells for example, which can require complete replacement during the expected life of a BESS.