Pumped storage hydro power plants: Intro
This article is an introduction into pumped storage hydro power plants. The aim is to explain the purpose and functionality of such sources of electric energy. What is the reason that pumped storage power plants experience a renaissance in Europe [1] and other locations worldwide? You should have a basic understanding after reading this article.
Purpose of pumped storage hydro power plants
The electric power demand is not constant. The consumption somewhat varies depending on the season and it also fluctuates during each day [2]. Visualization of variable loading is presented in loading diagrams. These can be drawn on yearly, weekly or daily base. Daily loading diagram is the most “dynamic” one and needs to be considered in power generation and transmission system. Peak demand is typically in the morning hours and then again in late afternoon/evening.
Example of a daily loading diagram is shown in figures 1a and 1b below. Time (daily hour) is on the x-axis with corresponding instant load on y-axis.
Figure 1a was recorded on Monday, September 15, 2003. Daily maximum is 7’922 MW at 6:55 pm. Daily minimum is 5’887 MW at 2:55 am.
Figure 1b was recorded on Sunday, September 14, 2003. Daily maximum is 6’907 MW at 6:55 pm. Daily minimum is 5’229 MW at 5:24 am.
Above example clearly shows variation in power demand during the day or week. On the other side the power generation varies as well. Fossil power plants operate at (almost) constant load, but the renewable power generation, in particular wind and solar, is highly fluctuating and less predictable. In Europe, specifically in the mountain areas, rivers typically have the highest flow in the spring or beginning of summer (melting snow) while there is less water flow during winter time. Therefore, countries with larger amount of hydro power in their energy mix need to compensate such fluctuation as well.
Coverage of loading diagram
This is a strategy how to cover the varying load on power generation side based on economy and ease of power regulation. List below ranks the power generation blocks from cheap and difficult to regulate to expensive and easy to regulate:
– nuclear power stations
– river flow power plants, solar power plants, wind power
– traditional thermal power plants
– pumped storage and accumulation power plants
In order to keep the power grids stable, the power generation and consumption needs to be balanced all the time. However, many power plants are ideally operated with almost constant load. Nuclear power plants are best example of fairly constant power source. Also coal fired power plants are not very well suited for varying load. Therefore, other power generation sources are required to keep the grid balanced. Pumped storage power plants are used for this purpose for several decades. Most of them had been built quite a while ago. However, the increased installed power of wind parks and solar cells increased also the volatility of power generation. Therefore, it is more important than even before to have systems that can consume larger amount of power when the generation exceeds the consumption and release the accumulated energy when the power demand is high.
As already mentioned, nuclear power plants are traditionally designed and operated for base load and kept at constant power (close to maximum). Therefore, the combination of nuclear and pumped storage power plant sounds like a good fit. In Czech Republic, for example, there are two nuclear power plants, multiple coal fired power plants and several river flow hydro power plants. In addition, there are two larger pumped storage power plants to balance the power demand.
Example: Czech Republic has two nuclear power plants: Dukovany and Temelin. The country has several pumped storage hydro power plants. However, two large pumped storage hydro power plants stand out: Dalesice (480 MW installed power) and Dlouhe Strane (650 MW installed power). Their purpose is to balance the variable demand while both nuclear power stations are operated with almost constant load.
Functionality of pumped storage power plants
Traditional pumped storage power plants consist of two reservoirs (lakes). In turbine mode the water flows from upper lake into the lower lake and turns the turbine. In pump mode the water is pumped from the lower lake into the upper lake. The height difference of the two water reservoirs ranges from several meters up to several hundred meters (in Europe mainly in Alpine area). Sometimes, a river flow power plant is combined with a pumped storage power plant. The advantages of pumped storage power plants are relatively high efficiency and possibility to build the power plant for high peak power (several hundred MW).
Pumped storage power plants use a machine set that can operate either as turbine-generator for power generation or pump-motor for power consumption. Important feature, especially for those covering daily peak demand, is a quick response time. Modern pumped hydro storage power plants can ramp up from standstill to full load within 1-2 minutes following the command from the dispatcher of power transmission grid.
Future of pumped storage hydro power plants
Pumped storage power plants are part of the power generation mix since a long time. Several years back the market seemed to be rather saturated and only few new PSHPP were in design phase or construction stage. However, as the share of renewable electricity sources increases, the power grids face challenges with power balance and stability. Not only the power demand fluctuates, but also the power generation. As they are not synchronized, the instant difference between production and consumption becomes even larger. Therefore, successful integration of renewable energy requires sufficient storage capacities. Pumped storage power plants are considered as one of the most suitable solutions with proven track record. In this sense pumped storage power plants experience kind of renaissance. New pumped storage power plants are built and existing pumped storage power plants are retrofitted with eventual power upgrade.
Newly built pumped storage hydro power plants are expected to widely utilize variable speed operation for better economy and further strengthening of grid stability.
What is next?
What can you expect in this series of articles? As our passion is in power electronics, drives and drive systems we will naturally focus on variable speed solutions for hydro power generation. We also plan to introduce selected hydro power plants a bit closer to you. Many of these power plants, at least in Europe (e.g. [3]), actually have a visitor center and you can make a guided or unguided tour and get your own impression. You can’t make it? Don’t worry – we will invite you on a virtual tour with us.
References
[1] Europe to experience pumped storage boom, https://setis.ec.europa.eu/publications/setis-magazine/power-storage/europe-experience-pumped-storage-boom
[2] Electricity demand during the day, U.S: Energy Information Administration, https://www.eia.gov/todayinenergy/detail.php?id=830
[3] Grimselwelt (Switzerland) – Tours in hydro power plants, https://www.grimselwelt.ch/en/tours
[4] Switzerland’s 1,000-MW Linthal pumped-storage plant connected to grid, hydroreview.com
[5] ABB helps to protect Europe’s largest pumped storage power plant in Wales, https://new.abb.com/news/detail/11987/abb-helps-to-protect-europes-largest-pumped-storage-power-plant-in-wales
[6] Switzerland’s first large-scale alpine solar power plant (Axpo), https://www.axpo.com/lt/en/energy-knowledge/pioneer-project-in-the-swiss-alps.html
