On what basis can we make the claim 'Efficiency up to 95%'? Here’s our Lead Scientist’s take on the efficiency of our energy storage system.
Resistive heating of sand is essentially 100% efficient, but the efficiency is inevitably lowered by heat loss through the boundaries of the system. However, there are several ways our solution tackles this problem.
1. Since sand is solid material, the heat is transferred inside the storage only by conduction. As the heat conductivity of the sand is rather low, the outer parts of the storage act effectively as insulators for the core and thus there always is considerably steep radial temperature profile inside the storage.
Simply put, unlike for water-based storages that have constant temperature everywhere, the outer layers of a sand-based heat storage have temperatures much below the average temperature of the system and the heat does not flow effectively from core to the outer layers and finally to the ambient space.
2. Obviously, even if the sand has the self-insulating property described below, we use good amount of conventional insulation at the boundaries of the system.
3. The heat transfer pipe system inside the sand allows us to prioritise the boundaries when discharging the storage and prioritise the core when charging the storage.
This means that even if some of the heat is about to be conducted to the outer layers, we can make use a good proportion of it instead of it getting lost. For the heat charged to the core of the system it takes very long time to reach the boundaries.
4. The scale of the storage affects a lot to the efficiency. This is so, because a smaller storage has more surface area compared to its volume than a bigger one, and the heat loss is essentially proportional to the surface area.
Simply put, the core of the storage can hold the heat for very long time without it getting lost, and the core is bigger for bigger storages.
5. The usage of the heat storage obviously affects on its efficiency. The shorter the storage period, the less heat is lost between charging and discharging. The desired storage period completely depends on the application and the needs of the customer, and it can vary from one day to several months.
Taking all these factors into account in our modelling, we claim that the large 1 GWh storages we offer will have the efficiency of around 95% when used for normal wind-power oriented storing cycles of 1 to 2 weeks.
Instead, our first heat storage, the world-famous 'sand battery' in Kankaanpää cannot reach these efficiency numbers, mainly due to its limited capacity of 8 MWh.
Text: Ville Kivioja, Lead Scientist
This article was conducted under the project NewSETS – New energy storages promoting sustainable energy transition in societies.
This project has received funding in the framework of the joint programming initiative ERA-Net Smart Energy Systems’ focus initiatives Smart Grids Plus and Integrated, Regional Energy Systems, with support from the European Union’s Horizon 2020 research and innovation programme under grant agreements No 646039 and 775970.
The content and views expressed in this material are those of the authors and do not necessarily reflect the views or opinion of the ERA-Net SES initiative. Any reference given does not necessarily imply the endorsement by ERA-Net SES.