Increased flexibility in thermoelectric power plants

Increased flexibility in thermoelectric power plants

Increase Flexibility In Thermoelectric Power Plants

Thanks to the stringent decarbonization policies, a strong increase in the penetration of renewables within the electricity system is expected 

This large increase in non-programmable renewable sources, especially photovoltaics, can lead to overgeneration in specific moments of the day, generating an imbalance in the planning of electricity generation. Most conventional generating units, like coal power plants or Combined Cycle Gas Turbine (CCGT) power plants, needs to adapt to this new operating condition, moving from base load to highly variable load operation. This change can significantly reduce plant performances, damage components and reduce service life.

One of the possible applications of the MGTES is its integration into the steam section of existing thermoelectric power plants. In such plants – Rankine cycle or CCGT – the MGTES can be used to reduce the minimum load and to offer a back up thermal power to be used when needed. This results in a greater flexibility.

The minimum load is the minimum electrical power at which the plant generation section, consisting of a steam generator, a steam turbine and a power transformer, can operate indefinitely with full stability and in compliance with the assigned technical limits and polluting emissions. A power plant operates at its minimum load when producing electricity is not profitable, but it is still more convenient than shutting down and reigniting the plant within a relatively short time.

Every shut down and reignition reduces the plant efficiency and service life, and increases the O&M costs; moreover, the process is slow and, in the transitory phases, prevents the plant from regulating the power with the speed needed to follow the load variations. The lower the technical load, the less unprofitable is to operate the plant when the price of electricity is low. The diagram shows an example of how, the integration of the MGTES in a power plant, modifies its nominal power profile.

When the plant operates at its minimum load, the MGTES stores a part of the thermal energy produced by the steam generator (in green) – thus reducing the minimum load itself from the point of view of the electricity grid the plant is connected to – and returns the stored energy when the demand increases and it is profitable to operate the plant normally. The stored heat is a reserve of available power (in blue) to produce more steam to feed the power-block, increasing the power at which the plant works.

grafico

Benefits on the load profile of a power plant integrated with the STEM-RES (reduction of the minimum load and increase of available power).
 

In general, the minimum load of a steam generator is higher than that of the steam turbine it feeds; therefore, from the point of view of the grid, the minimum load of Rankine cycle plants and of the steam sections of CCGTs is usually that of the steam generator. Introducing a thermal energy storage system like the STEM-RES between the high-pressure and the low-pressure sections of the steam turbine, it is possible to lower the minimum load of the steam generator – and therefore of the entire plant – to that of the steam turbine.

In other words, the STEM-RES can be used to decouple the technical limitations of the steam generator from those of the turbine, lowering the minimum load of the entire plant down to that of the turbine and, in other words, making a power adjustment. To do so, the steam flow that feeds one or more sections of the turbine is reduced. Alternatively, it is possible to extract the steam from different sections of the steam generator and send it to the STEM-RES heat exchanger rather than to the turbine, so that both the steam generator and the turbine can operate at their respective minimum powers.

A further benefit of integrating the MGTES in a power plant is that the steam generator can work with fewer load variations, limiting the thermal stress and improving the efficiency, which results in an extended service life and a lower fuel consumption during transients.

To recap, the integration of the MGTES into a thermoelectric power plant produces several benefits for both the Utility and the Transmission System Operator (TSO). Summing up, for the utility:

  • Increases flexibility of the plant according to the fluctuation of electricity prices on the market.
  • Preserves the integrity of the plant components, reducing the overall numbers starts and stop.
  • Increases the available power for the supply of grid regulation services that can be spent on the Dispatching Services Market (MSD)

Likewise, for the power grid and the system operator, the benefits are mainly:

  • The increase in the flexibility of conventional thermal plant increases the overall flexibility of the national electricity system.
  • The TSO has an additional flexibility instrument that can be managed independently, according to the needs of the electricity grid.

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