Within the first 12 months of this project, project-management is a standard task. The Kraftblock-Team designed the project-execution-plant, which also includes the risk-management plan, as well as a communication strategy. It furthermore builds and confirmed the core-know how to design a standardized 30MWh storage and executed the essential long-time-performance test.
A 30 MWh KRAFTBLOCK unit was laid out for 15,000 cycles at 1,300°C storage temperature. Heat flow simulations, stress simulations had to be performed. Based on the analysis of those results, the optimization process started. Based on the optimized system, a simulation of the systems results (hth/htp) was executed.
The whole project, especially with respect to the integration on customer’s premises, is highly complex. Permits and safety-audits take a lot of time, less because of high temperatures, more because of the usage of gas. Furthermore, the flare-gas-stack is a security option at steel-industry. Avoiding potential problems and issues that could affect the function of the flare-gas stack must be eliminated completely.
During the first 12 months of this project, Kraftblock designed an energy-system, including charging-device, thermal storage and discharging device.
Charging takes place at Dillinger/Saarstahl, a steel mill located in the southwest of Germany. As an integrated steel-mill they also flare coke-oven gas. The caloric value was confirmed at 9203MJ/m³. The density is 1,339 kg/m³and the oxygen-need to avoid the formation of NOx was determined to 0,367m³ oxygen per m³ coke-oven gas.
Main focus was put on the storage-design.
a) Storage material
With an improved bulk density of app. 1,4tons/m³ it was possible to design an 40 foot standard container with a storage capacity >40MWh (for large-scale units) and >30MWh for a single-stand-alone-unit. The difference in a “large-scale-unit” and “stand-alone-unit” is, that a stand-alone-unit is fully insulated, whereas a “large-scale-unit” only is partially insulated. As the primary goal to achieve a unit with more than 30MWh capacity at 1300°C was achieved in a short time, this point the major goal was stretched to reach 30MWh capacity for a 20 foot container. Using the same material, with improved bulk-density, it is only possible to achieve app. 23MWh (large scale module) and app. 16MWh in a stand-alone module. To achieve the stretch-goal of 30MWh thermal capacity in a 20foot container, bulk density as well as capacity should be improved.
Based on those results the team decided to further-improve the capacity and still pushes that limit to >1,3kJ/kgK. Beneath the improvements of the materials properties, Kraftblock also tested another shape. Instead of pellets the team used a kind of honeycomb-brick, with a bulk-density of >2,1tons/m³ which resulted in a gross-capacity >35MWh for a 20foot-full-scale-module and >24MWh in a 20foot-single storage module.
For the target-application, the maximum temperature-load and temperature cycle stability are essential.
As for the maximum temperature, the material can take, a hemisphere-test and softening-point-tests were performed – both with a positive result up to 1500°C (limit of the lab-furnace). This temperature already is 200°C higher than the planned maximum-application-temperature, so that the test passed and the design-property was approved.
To test the cycle stability, a test-bench was developed. The thermal energy is generated by an electric heater, as temperature-control and mass-flow of the main-heat-transfer-medium (air) can be controlled. The hot air was driven through a packed bed of the pellets. As soon as the latest layer reached maximum temperature, the heater was turned off and cold air cooled the pellets down to ambient conditions again. The controller then repeated the cycle automatically. Each cycle takes app. one hour in time. Up to now 4700cylces were executed. As this process is quite slow, the test-bench was improved, so that 15000cylces now can be executed within 7 months.
During this process, the pellets are observed to mechanical degradation and thermophysical degradation. Up to now, no degradation could be observed.
b) Designing a lightweight container
Steel is a critical and temperature sensitive construction material. Building large-scale thermal energy storage, based on standardized container modules, means a massive construction based on steel. This leads to high CAPEX, potential supply-chain-issues and long lead times. Based on those facts, it was essential to design a lightweight-container structure with outstanding mechanical properties that can be stacked together without an additional static construction that takes the full mechanical load. The more container got stacked on top of the others, the more important the lightweight structure is. Additionally, the choice of alternative construction materials is a chance for further-improvements. After a few brainstorming-sessions, the team decided to follow a top-down approach. This means that Kraftblock first takes a look to a larger structure, consisting of four modules, to determine the mechanical properties and forces. This approach is more promising, as there are only a few iterations necessary to “reduce” the design, compared to a bottom-up approach of the modules. This approach even helped to optimize the usage of the predefined unit volume. The unit volume was further improved by taking a close look at the interface to the charging- and discharging system, especially by redesigning in- and outlets of the storage container. This redesign improved the unit-volume, pressure-drop and compactness of the interfaces.
Two out of three design phases are finished. The structural optimisation already reduced the unit-weight by nearly 30% which means an additional available capacity of up to 2,5MWh.
During this process, FEM and CFD simulations were executed to see the structural effects of the changed interfaces as well as the effects of the lightweight structure itself.
