Home / Blog / November 2012 / Power and Energy Primer: Part 3 - Load Curves and Generation

Power and Energy Primer: Part 3 - Load Curves and Generation

Posted by Lester Shen | Date November 1, 2012

In the previous Power and Energy Primer, we introduced the concept of “peak loads” on the electricity system. Peak loads are important because a utility needs to plan for that maximum demand, even if it only occurs during a handful of hours through the year. The daily and seasonal pattern of customer demand will dictate a utility’s sources of electricity.

Many North American utilities can buy or sell electricity across a wide geographic region to help meet that demand through their independent system operator (ISO). This video shows how the daily electric load curve varies over the year for the New England ISO.

The following graph shows the average daily electric load for the New England ISO for the months of January, April, July, and October 2011.

Jan,-April,-July,-oct,-load-curves.png

The demand changes dramatically across the course of a day, but also from season to season. In April and October the maximum demand is about 15 GW, while the maximum load in July is almost 21 GW. The shapes of the curves show that during January, April, and October, maximum demand occurs in the evening, when people are returning home and starting their evening activities. In July, however, commercial demand for air conditioning results in an afternoon peak.

So, what does this mean for the utility? It means that they need to have enough power plants to meet this demand at the time it is called for, or they risk causing a blackout. All utilities have a mix of electricity sources, including coal, natural gas, and nuclear power. Utilities decide which power plants to run based on a number of factors, including cost, environmental regulations, and location. The important thing is that different plants run for different amounts of the year.

Some run all the time (so called “baseload” plants), and some run only to meet the maximum demand (“peaking” plants). Nuclear and coal-fired power plants are typically used as baseload power plants and run continuously. They have low fuel costs but may take hours to days to reach their steady state output from startup. Gas turbine power plants can quickly respond to demand and so are used as peaking power plants. Typically they are cheaper to build but are more expensive to operate.

Let’s consider again the average January day shown in the above graph (darker blue line). From 4pm to 10pm electricity demand peaks, to a high of 19 GW. Additional generators, i.e., peaking power plants, need to be brought online to account for this demand. In the summer (the pink line for July), the peak is about 21 GW, and two additional GW are needed.

As demand increases, so will the need for more power plants. However, utilities are looking more and more at energy efficiency and other demand-side strategies to reduce peak and the need for building new power plants. There is growing interest in having customers reduce their demand on call, or shift demand to a different part of the day, as a way of managing the highest demands of the year.