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Pictures of the Future


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Pictures of the Future
The Magazine for Research and Innovation

Smart Grids and Energy Storage

A Lot More Than Just Hot Air

Experts from Siemens are optimizing a simple and cost-effective method for storing electricity from renewable sources. They use wind power to heat up air that is then blown into a bin of stones until they glow. When there is a demand for electricity, the balls release the energy again to a steam boiler and from there to a generator.

Electricity generated with excess wind power can be stored in stones as hot air. Whenever needed, the stones can release their trapped heat to drive a generator. Experts at Siemens are now optimizing this affordable and scalable technology. Their goal is to create an efficient and cost-effective energy storage system.

Some storms have the potential to do great things.  On December 1, 2016, for instance, a low-pressure zone called Theresa caused more than 31,000 megawatts of wind power to be temporarily fed into the grid throughout Germany – a tremendous surge of power compared to an average day.  Unfortunately, however, much of that power was probably never used because of the absence of energy storage systems. .

But this frustrating picture is set to change. A new energy storage system called Future Energy Solution (FES) is being developed by Siemens in cooperation with Hamburg University of Technology and municipal utility company Hamburg Energie. The system, which is located in Hamburg and promises to store the excess power generated by wind parks for several hours or even a whole day, is being created as part of the Future Energy Systems project, which is funded by Germany’s Ministry for Economic Affairs. This approach is designed to help solve the problem of how the energy generated by wind turbines can be affordably stored when it’s not needed in the grid or cannot be transmitted because of potential overloads.

In cooperation with Hamburg Energie and Hamburg University of Technology, experts from Siemens’ Wind Power and Renewables Division have developed a heat storage system based on the use of electricity generated with surplus wind power. CT is investigating and improving the heat flow within the facility.
The system is the essence of simplicity. It converts surplus energy into heat, which is blown into an insulated bin of stones.
Marco Prenzel prepares the FES storage system for a test run. To do so, he connects the system’s heating unit with a supply line from a container loaded with heated stones.

Simple Storage Technique

The Hamburg system – a small prototype facility – is the essence of simplicity. It converts surplus energy into heat, which is blown into an insulated bin of stones. The rocks heat up until the temperature rises to more than 600 degrees Celsius. If there is demand for this stored energy, the rocks heat an airflow that drives a steam cycle and generates new electricity in the process. This straightforward design uses natural stones, is cost-effective, and could potentially be used to supplement existing storage systems.

Siemens Corporate Technology (CT) is playing an important role in the effort to optimize FES’s overall concept. Since 2016 CT has been running a test facility at a Siemens campus in Erlangen, where experts are examining how heat is transported inside containers filled with stones. “The thermal storage system is the centerpiece of the Hamburg facility,” says CT project manager Vladimir Danov. “It’s very important that we understand the heat transport phenomena within the storage system so that we can increase its overall efficiency and build a full-scale power station.” Danov and his team are interested in the distribution of heat and currents as well as in fluctuations and and energy storage losses. The better they can understand these phenomena, the better the heat accumulator can be adjusted to the requirements of the facility as a whole.

Manfred Wohlfarth carefully places some 13,000 ceramic balls into the facility. The balls are ideal for examining and optimizing heat flows. In the next step, the balls will be replaced by real stones.

Simplified Measurements

The Hamburg facility is located in an approximately five-meter-long container. Instead of stones,  it contains around 13,000 ceramic balls. “The advantage of using ceramic balls is that they all have the same size and shape, which makes it easier to calculate heat transport and the processes within the bin,” says Danov. “However, we will use ceramic balls only in the current test phase. In the next step we will fill the storage system with natural stones so that we can study how irregular shapes and a variety of material data influence heat transport.”

Ceramic balls cannot be used in a large-scale facility, because they would be too expensive. The researchers are currently searching for the best types of stones to use and have identified several favorites. Clearly, the higher the stones’ thermal stability, the more durable and efficient the storage system will be.

Sensors in Action

Regardless of whether balls or stones are used, the biggest challenge is how to measure heat transport processes inside the container. In order to obtain findings that are as detailed as possible, researchers have installed around 50 thermocouples in the storage system. These sensors measure temperatures at a variety of points in the container, as well as airflow volumes and pressure gradients.  “Our findings will make it easier to scale up the facility in the future,” says Jochen Schäfer, Head of the Distributed Energy Systems and Heat Conversion Research Group at CT.

Complete Facility Planned for Early 2018

However, before the FES concept can be scaled up, researchers want to build a complete facility — including the technology for turning heat back into electricity. The prototypes in Hamburg and Erlangen lack this technology because the focus to date has been on examining the flow of heat through stones. In early 2018, a complete facility encompassing storage system and technology for converting heat back into electricity is scheduled to be built in Hamburg. That initial  facility is project to have an efficiency of 25 percent. However, the efficiency of a large-scale facility with an output of more than one hundred megawatts could be as high as 50 percent. The storage system would then become a key part of Germany’s energy transition and fully live up to its name of “Future Energy Solution.”

Ulrich Kreutzer