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Energy Storage customers in the process industry and energy sectors . “ Finland was a great place for us to start , as it relies a lot on district heating . But we want to move into bigger industrial applications ; heat generation for industrial processes is responsible for up to 20 % of the global CO2 emissions – so there is a lot of work to do in this sector . And we want to help .” In terms of limitations , the sand battery has very few . A modular system capable of scaling up without a foreseeable limit offers an ideal solution to one of renewable energy ’ s biggest challenges . However , Markku admits that the system does require short distances between the storage unit and the customer . “ Transferring heat over long distances , either through hot air or water , is not efficient . So we ’ d need to practically limit the storage site to 10 GW hours of capacity because going larger than that , the benefits of scaling up start to diminish ,” he says .
Sand is a cheap but incredibly effective medium for storing heat over long periods of time . Source : Polar Night Energy storage is definitely something we are looking into . It would especially lend itself to these large-scale projects that we ’ d like to focus more on .”
How does the sand battery work ? The heat storage system is made up of an insulated steel silo , filled with sand and heat transfer pipes . The only other equipment required consists of automation components , valves , a fan , and a heat exchanger or steam generator . “ We have the large heat exchanger buried in the sand , which is our own technology . And then outside of the storage is a quite conventional heat exchanger ; in the Kankaanpää case , it is an air-to-water heat exchanger ,” says Markku . “ We have this closed air loop , which is our patented technology , so we heat air using electric resistors when discharging the storage and that hot air passes to the air-to-water heat exchanger .” Depending on the needs of the application , the sand can reach temperatures between 600 and 1000 degrees Celsius . Markku explains that “ in practice , the maximum temperature of a sand-based heat storage is not limited by the properties of the storage medium , but by the heat resistance of the materials used in the construction and control of the storage .” Although the company remains close-lipped on the exact details of its heat transfer system , Markku confirms that stainless steel is used .
Scaling up But the Kankaanpää unit is still only the beginning , as it sits at the lower range of PNE ’ s system capabilities . “ The Kankaanpää system includes boosting up waste heat from a data centre , so this means that a small storage can work if there is an additional business model included ,” says Markku . “ But we really want to focus on scaling up much larger than this . The larger the storage , the better the ratio ; there is more capacity and the relative losses are lower .” And the system is certainly scalable . PNE ’ s proprietary heat exchanger buried amongst the sand is as easy to arrange for large-scale as it is for small-scale , and the control system remains simple even in a unit 100 times larger . The project has gained attention and interest from around the world , and PNE is now in negotiations with
Data and simulations To aid in maximising the efficiency of the design , PNE used COMSOL Multiphysics ® software to create dynamic models of the sand battery system . Through the Comsol system , they could also access a wide database of realworld heat transfer data . Markku explains that they opted to create time dependent models because of the importance of understanding the dynamic nature of the heat transfer . The team use the platform as a design tool for the heat transfer side , but not the mechanical structure . It gave them the ability to model the entire system on selected timeframes . “ For example , we could look at hourly based data for a full year , then we ’ d get a fairly good estimate for what we can deliver each hour of the year with our system in practice ,” he says . “ The demo plant in Tampere , which we set up before the first commercial system in Kankaanpää , was used to validate our models to compare real life data and data from the programme ." The PNE team used the programme to confirm whether or not certain designs were capable of storing heat for very long periods of time , eventually coming to the conclusion of using evenly distributed hot air ducts throughout the sand storage unit . This software enables engineers to build predictive models rather than full-size prototypes , meaning they can test multiple ideas before committing to the most efficient . Having real-world data on hand gives PNE the assurance that its systems are successful and meaningful contributions to a global dilemma .
Key figures
Temperatures : 600 - 1000 ° C ( depending on the design ) Nominal power : Up to 100 MW Capacity : Up to 20 GWh Efficiency : Up to 90 % Storing Cycle : Hours to months Lifespan : Tens of years Investment cost : < 10 euro / kWh of storage capacity Safety : 0 poisonous or hazardous materials , minimal emissions Running costs : Minimal , no consumables , fully automised
36 Heat Exchanger World September 2022 www . heat-exchanger-world . com