For many decades, the electricity demand has followed what can be considered as a predictable daily pattern. This pattern allows utilities to perfectly predict future demand so that they can prepare themselves for buying and selling the electricity as in the energy market.

As more electricity is being generated from renewable resources, with the largest share of solar technologies, this addition to the utility grid introduces changes to the traditional daily profile of the electricity demand. These changes bring challenges with them to utilities to address reliability issues. In this lesson, we will introduce the electricity demand profile and the challenges to utilities after adding solar systems in large capacities. In addition, we will introduce this effect by what is referred to as the "Duck Curve," and later in the lesson we will talk about a proposed solution to that effect.

Ultimately, this lesson helps our solar professionals understand the back-end effect on PV and other renewable energy resources in the utility grid. Whether you work for an electric utility or you are a PV designer at an engineering firm, understanding the bigger picture on deploying PV technology helps in analyzing how the industry is driven and how to adapt to these changes. 

Let’s get started!

Learning Outcomes

At the successful completion of this lesson, students should be able to:

• Describe predicted load profile viewed from the utility side.
• Discuss challenges of excessive PV interconnection on the utility grid.

As we learned previously, the demand for electricity varies throughout the day and year, and so does solar irradiance. For example, the residential electricity demand rises in the morning to peak just before noontime, and then it levels out up until the evening peak, when everyone gets home from work and starts using electricity. And that pattern repeats itself over and over with some variations between summer and winter seasons, as seen in the left curves on Figure 12.1. We can also see that this curve is location dependent. There are key characteristics to this daily demand profile, such as the two daily peaks and then a base load demand.

Figure 12.1: Daily demand profile for different sectors (household-left) (bakery) (offices) (supermarket-right) in Germany (IEA PV Roadmap).

Credit: Based on IEA data from Technology Roadmap: Solar Photovoltaic Energy 2014 Edition, International Energy Agency (IEA) OECD/IEA [5] ©2014. License: IEA [6].

As we said earlier, utilities became experts at predicting these values to better trade their electricity, and also, more importantly, to plan the operating schedule for the power plants. This planning helps optimally and economically operate their power plants to meet the base loads (usually coal or nuclear) and the additional capacity to meet the peak demands (such as Natural gas).

The load demand had been under control up until the distributed generation sources got introduced to the grid, which are variable and unexpected. Although the idea of meeting the peak demand is very appealing and is actually beneficial, excessive addition of these resources such as solar will change the load profile in such a way that utilities have to get out of their comfort zones and address these changes by meeting the new demand profiles.

Since our class focuses on solar systems, let’s take the solar effect as an example: Integrating a small amount of PV capacity doesn’t raise any technical issues to the grid, as long as the PV capacity is not concentrated in areas where the grid is weak and demand is low. However, when adding PV capacity in larger scales, the main concern from the grid and utilities point of view will be the supply and demand balance. One of the main issues that the solar arrays have are the inability to schedule its operating as compared to traditional coal plants, for example. The sun may shine as predicted, or it might not shine at all. In addition, solar only contributed in the best scenario to the daytime demand profile rather than the daily profile, and that contribution lowers the base load on the utility demand, but it disappears in the evening time.

So what is that fundamental change that solar adds to the daily demand profile?

The effect that solar power has on the daily profile is referred to as the "Duck Curve" or "Duck Chart." This change in the load shape of the daily curve starts to look like a duck. If we look at solar from the grid point of view, the additional solar looks like a load reduction and that is at the same time unpredictable and uncontrollable. In other words, the solar disturbs the operation of the bulk power plants such as coal by lowering the base load demand, as seen in Figure 12.2.

As said earlier, there are key characteristics to this daily demand profile, such as the two daily peaks and then a base load demand. Usually loads don’t fall below a certain value, as we see on the 2012 curve provided by the California Independent System Operators (ISO) as shown in Figure 12.2.

Figure 12.2: Daily demand profile and the duck curve(IEA PV Roadmap).

Credit: Based on IEA data from Technology Roadmap: Solar Photovoltaic Energy 2014 Edition, International Energy Agency (IEA) OECD/IEA [5] ©2014, License: IEA [6].

We can see some greater challenges as solar becomes larger in capacity. The load demand can fall down to closer to very small demand value, which means the massive base power plants need to shut down, and that is not an easy task since starting a traditional power plant requires hours, and the process is slow and might not meet the steep ramp demand in the evening after the solar is gone. This can result in a serious stability issue and power outages. For this reason, solar is sometimes viewed as a disruptive technology to the grid and utilities. A prediction of the duck effect on the daily demand curves can also be seen on the 2020 prediction for the daily profile in Figure 12.2.

Proposed Solutions to Duck Effect

Solar arrays may produce more solar energy than the grid needs. When such oversupply exists, there are two main scenarios to propose solutions from the grid point of view - the grid operator side and the load side.

Grid Operator Side

• Utility operators can manually "curtail" solar production, cutting some solar capacity off from the grid. As a result, this wastes the energy produced from the sun, but it saves the grid. This is considered a short term solution.  

• Another solution is interconnection with other electricity networks, such as the Texas and western interconnection at the transmission level to expand the capacity of the grid so it can handle more PV capacity. As it sounds, this is a long term solution and usually it takes a long time to expand the grid. 


Load Side or Demand Side Management (DSM)

• The first DSM strategy refers to Demand Response (DR) including storage systems. This can shift the peak to other times during the day, which is usually done either through incentivizing the customers to use electricity when it is cheaper or by storing the electricity for later use. However, this might require more capital investment at the customer side due to the additional storage systems cost. 

• The other type of DSM is the Energy Efficiency that lowers the total demand in general. This strategy has great potential, as our peak demand keeps growing due to additional appliances and devices connected to the grid.

This week, you will finish up and submit your Final Project.

In this lesson, we talked about the electricity demand and load profile changes due to the addition of solar into the grid. We also discussed the duck effect and the shape of the curve as a result of solar energy availability during the daytime and its absence during the nighttime.

Our class has reached its end, and we hope we covered all information needed to prepare you as a future solar professional and equipped you with the right tools. Our main goal as a solar option is to expose you to various scenarios you might face in the real-world when dealing with solar systems in terms of system main components, sizing and design, permitting, documentation, code compliance, interconnection methods, safety regulations, commissioning, operating and maintenance, monitoring, and most importantly the effect of PV on the grid.