How do we adapt the electrical output of a fossil powerplant according to the demand on the grid ?


How do we adapt the electrical output of a fossil powerplant according to the demand on the grid ?

In: Engineering

In any power generation facility; you basically just ramp the power production up and down in order to meet demand. In the case of fossil fuel plants, that functionally means increasing the rate of fuel consumption.

You can just turn up and down consumption of big coal power stations. However, it’s very slow and can be expensive because it can be inefficient. A large part of the electricity in the system comes from smaller, faster and more responsive electricity generators so usually they’re the ones that will increase or decrease generation to meet demand.

Things like batteries and hydro-electric plants can either draw electricity from the system or generate electricity to put into the system, so they tend to be used to regulate supply and demand on short time scales.

The plant has operators. These people look at the phase of the electricity being generated in comparison to the grid itself. If you are too fast, then you reduce the amount of steam running into the turbines that drive the generators that make electricity. If your plant is lagging, you increase the steam rate. To maintain steam production, you might have to add more fuel to the plant, there are other controls for that. If the steam pressure starts to rise, you add less fuel, or even vent the excess steam (which is sad because it’s a waste of fuel).

Lots of people are doing this, both at each plant and at the central grid operations center.

The power plant have a lot of rotating mass in its turbines and generators which are synchronized to the frequency of the grid through the electromagnets in the generator. When you use more power you will change the magnetic fields in the generator slowing it down. But because there is so much rotating mass this takes time. Meanwhile grid operators will see that the frequency of the power grid is going down and is able to change valves to allow more steam into the turbines to increase their speed. This can be done anywhere in the power grid as long as the transfer cables is able to keep up. When this is done the steam pressure will drop and the steam temperature drops so the power plant operators will need to add more fuel to generate more heat to keep the temperature in the steam up.

One of the issues with replacing fossil fuel power plants is that they have a lot of energy stored up in rotating mass and in the temperature of the water and steam which can be released very fast or stored up very fast as needed. Sometimes even without human or computer control. That is the basis for a very stable power grid where you only need regular shipments of fuel as everything can be throttled as you want. With solar and wind power there is no way to store up the energy and release it to the grid as needed. The only other current options are hydro, nuclear and geothermal but they can not be built everywhere as there are geographical and regulatory issues. So we need to either build more transmission lines so the power can be sent elsewhere when not needed and sent back when it is needed or we need to build storage facilities like batteries and pumped hydro.

The power grid is monitored by several groups. They are monitoring high level grid stability and availability. Like MISO or ERCOT. Then they have intermediaries that are more local and will dispatch orders and do the actual MegaWatt (MW) control to the various power plants. The generator is a huge electromagnet and the field is changed to make the spinning magnet (rotor) stronger or weaker and the steam or fuel to the turbine is adjusted to keep the speed at 3600rpm plus or minus a couple rpmM

Edit. Also, most of the generators are filled with nearly pure hydrogen. Just an interesting bit