The system schematic shows two electrical machines in a WindTP system: a motor to drive the secondary compressor when that is required and a generator to extract electrical power from the expander when that is delivering output power. WindTP can be implemented in this way but since the secondary compressor would usually be located very close to the expander anyway, it is sensible in most contexts to couple these two directly via a mechanical clutch so that we avoid converting mechanical power from the expander into electricity just to consume (some of) that electricity again in the secondary compressor.
The single electrical machine in the system is a synchronous generator connected directly to the grid. Nearly every power station in the world uses the same kind of machine. Grid-connected synchronous generators are very efficient and very easy to control. They have another major advantage due to their inertia. If the total load on the grid increases suddenly, the very first thing that happens is that all of the spinning synchronous generators start to slow down and the kinetic energy being released from their rotors temporarily makes up the difference between the mechanical power being fed in and the (greater) electrical power being drawn out. The rotor inertia gives the system controller enough time to adjust how much mechanical power is being fed in so that the speed is restored again.
Thus, WindTP systems have real inertia that is valued highly by grid operators. Other wind turbine transmission arrangements can be controlled to exhibit inertia also but in most cases it is not a natural property. Note that photo-voltaic generation has no inertia whatsoever: if a cloud passes over a large PV farm, the output drops very suddenly and the inertia of other generators on the system are called-upon to hold up grid frequency. As more PV is introduced into the generation mix, there is a dire need for more synchronous generation also to be introduced.
WindTP has the potential to provide a range of flexibility services to the grid. These include inertia and reactive power compensation as well as the longer-timescale energy storage services.