Page - 41 - in Our Car as Power Plant

41 The demand for electricity has a certain pattern during the day. During the night there is a minimum demand. In the morning offices, schools and industries start to work and demand goes up. In the afternoon people go home to eat, watch television or do some other activities during the evening. Electricity consumption decreases and when we go to sleep we shut down many of our electronic appliances. Electricity demand during night is at a minimum again. To be able to follow this electricity demand, we have built a power system with base-load, intermediate-load and peak-load power plants. Base-load plants run almost all of the time. These are our nuclear plants, coal fired power plants and large and modern Combined Cycle Gas Turbine (CCGT) plants. Their characteristics are a long start-up time (many hours), a high efficiency at full load and decreasing efficiency when operated at partial load. Peak- load power plants on the other hand, can start and stop very quickly (minutes). So when there is a sudden fluctuation in demand, these peak-load power plants will produce. But their full-load efficiency is much lower than the base-load power plants. In many cases these peak-load power plants are gas turbines with efficiencies between 25 and 35%. The intermediate-load power plants have indeed inter - mediate characteristics: start-up in one to two hours and reason ably high efficiencies. Normally these are the older combined cycle gas turbine plants. So in the total power system all the power plants are operated based on a certain merit order.