Energy storage is a hot topic in today’s electricity industry – thanks to the increasing amount of clean, renewable energy connecting to the grid to support our environment. Because of the variable and intermittent nature of renewable energy (wind solar), energy storage has become a sort of “holy grail” – grid operators are looking for ways to store the energy produced by these sources during the off peak. New and innovative storage technologies are coming to market in large numbers, but there is a type of cost-effective storage that certain utilities are already using, taking advantage of naturally occurring gravity and water.
Traditional Pumped Storage: What is it and how does it work?
Many electrical utilities have hydroelectric facilities that can store energy, and it is most common to think of these as pumped storage systems. Pumped storage has been a popular means of storing energy for many years. Its usage grew with the advent of nuclear power generation, because the nuclear plants had capability to generate large amounts of power, but restricted ability to vary the capacity required to follow the fluctuations in demand that occur each day. Pumped storage made an ideal companion to nuclear plants, as they could store energy when power was not needed at night, releasing it during peak hours the following day.
A pumped storage plant is an intelligent system that pumps water from below a powerhouse to a lake, often far above the plant. The water is stored in the lake, and used to generate power some time later when it is needed, draining through the power plant. Pumped storage plants generally have a specialized turbine that can be used to either generate power or to pump water. In the latter case, the generator operates as a large motor, driving the turbine.
The Inefficiencies of Traditional Pumped Storage
The good news about pumped storage is that it can store large amounts of energy, but the bad news is that its return efficiency is usually around 70%. Today, with the low cost of natural gas and efficient combustion turbines that are available, it can be somewhat difficult to justify operation of pumped storage. When the losses are included, the combustion turbine may be the less costly way to meet peak requirements.
Hydroelectric Generating Stations: Efficiently utilizing water by-product
Some hydro utilities have a slightly different method of utilizing water for energy storage that is often poorly understood, but is incredibly efficient. A hydroelectric generating station with a reservoir that can store water for even a few hours of operation can be an extremely useful energy storage tool. The system works in a straightforward manner.
When a utility company that owns a hydro plant with storage finds that they can purchase power at a very low price, they do so, and shut down their own hydro generation. This causes the reservoir to fill slowly. A few hours later, when prices are higher, the same utility can sell the surplus energy saved previously not by pumping, but by simply not generating. The water inflow to the reservoir fills it directly, and the reduction in capacity while storing that the net amount of water in the reservoir grows slowly.
Organizations across the globe have been able to profit using hydro storage, capturing revenues from the large differences between day and night energy prices. In Canada, BC Hydro, Manitoba Hydro and Hydro Quebec are three large utilities that have also utilized hydroelectric plants to their full, valuable potential. Some private generating companies have been able to use similar methods to maximize their revenues, generating only when prices are high. Brookfield Renewable Energy Partners L.P., a subsidiary of Brookfield Asset Management (a global alternative asset manager), owns a variety of hydro facilities across the United States that execute this type of storage.
Utility owners, like Brookfield, have found that putting extra generation capacity into a large project is a low-cost investment, since the actual dam and associated infrastructure make up the major costs. Older hydro plants were often built based on “firm” or minimum capacity, but the truth is that adding extra capacity is not only cheap but very effective for storage. Because of this, new hydro plants are now generally equipped to generate far more capacity than the river flow can support – it is this extra capacity that enables this form of cost-effective and very efficient energy storage.
Hydro storage without pump capacity is an excellent means to store energy. It is flexible, efficient, and relatively cost effective. As energy storage becomes increasingly important in today’s complex and evolving power system, solutions like “pumpless” hydro storage will become more crucial and more valuable to utilities and a reliable power system.