TheMatIK: Metallic latent heat storage for process heat from 250 to 550 °C

In the joint project TheMatIK, Fraunhofer UMSICHT, together with project partners, investigated and developed metallic phase change materials and encapsulations for high-temperature heat storage for use in industry and in combined heat and power plants. A pilot-scale test rig for simulating process steam backup was set up and operated with a zinc-aluminium alloy, ZnAl6, as the phase change material.

Starting point: process steam supply for industrial processes

Many industrial processes require an uninterrupted supply of process steam, which is usually ensured by combined heat and power plants. If the CHP plant fails or is shut down, fossil-fired backup boilers (auxiliary boilers, standby boilers) generally take over steam generation. Since steam supply typically drops rapidly within a few minutes when a power plant shuts down, backup boilers are usually kept permanently on standby in minimum-load operation. This entails high fuel consumption and correspondingly high costs. At the start of the project, the share of renewable energies in industrial process heat was stagnating at around 6 percent. In order for more renewable energies to be used, conventional power plants must be operated more flexibly. This requires alternatives to conventional process steam backup.

Integration options for the high-temperature storage system

At the start of the project, possible integration scenarios for a metallic latent heat storage system were systematically evaluated. Thermodynamic target criteria (pressure, temperature, storage capacity) were compared with process engineering boundary conditions. The highest-rated integration scenario – process steam backup – was then investigated in detail over the further course of the project, both theoretically (flow and process simulation) and practically (test rig).

(1) The storage unit temporarily takes over steam generation to start up the steam turbine when the power plant is restarted, until the regular steam boiler takes over the supply.

(2) Application specifically for hard coal-fired power plants: charging via flue gas, discharging to preheat the ambient air for coal conditioning during power plant start-up.

Development steps of the high-temperature storage system

The joint project encompassed the development levels of material, component and system, with the following work packages:

• Definition of integration concepts and requirement profiles
• Selection and characterization of metallic phase change materials
• Flow simulation of the coupled phase change and evaporation process
• Development and testing of encapsulation materials
• Construction and operation of a pilot-scale test rig
• Determination of cycle and long-term stability
• Dynamic process simulation and economic/potential analysis

Material selection: metallic phase change materials

Aluminium, copper, magnesium, nickel, lead, antimony, silicon and zinc, among others, were investigated as alloying elements. In total, the project team identified around 25 alloys with eutectic temperatures between 252 °C and 548 °C. Eight particularly promising candidates were characterized in detail. Simultaneous thermal analysis (STA), differential scanning calorimetry (DSC), laser flash analysis (LFA) and dilatometry were used to determine the thermodynamic properties – phase change temperature and enthalpy, heat capacity, thermal conductivity and thermal expansion.

The zinc-aluminium alloy ZnAl6 was selected for the construction of the test storage unit, as its phase change temperature of 381 °C lies favourably between the temperature of the heat source (410 °C) and that of the heat sink (296 °C):

Compared with classic storage salts such as sodium or potassium nitrate or solar salt, metallic phase change materials such as ZnAl6 have a significantly higher thermal conductivity and volumetric storage density. At the same time, they have a lower gravimetric phase change enthalpy, so that more material is required to store the same amount of energy.

Encapsulation and cycle stability

Since liquid metallic phase change materials can react with the encapsulation material, the project team investigated the corrosion resistance of eight alloys in combination with three types of steel. The samples were both exposed continuously in the liquid state for 2,500 hours and subjected to 1,000 melting and solidification cycles. Each cycle comprised 45 minutes in the furnace zone and 15 minutes of air cooling. The samples were subsequently examined by scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM/EDX).

With the exception of the combination of ZnAl6 and steel C, for which only very low reactivity was found, all the aluminium-rich alloys investigated exhibited reactive interactions with the respective encapsulation material. Brittle FeAl phases formed in the process, which did not constitute an effective diffusion barrier. By contrast, the zinc-magnesium alloy ZnMg46.3 consistently showed very low reactivity in combination with all three types of steel tested.

In order to investigate the test storage unit under realistic operating conditions, Fraunhofer UMSICHT set up a fully automated test rig to simulate process steam backup:

Initial discharge tests confirmed the results of the previously performed flow simulations: the steam temperature rose to over 340 °C within a very short time and remained above the sink temperature of 296 °C for more than 20 minutes. This demonstrated the fundamental suitability of metallic phase change materials for highly dynamic process steam supply.

Process simulation and economic viability

To transfer the test rig results to real industrial parks, the project team created a dynamic process model in Aspen Plus Dynamics. This model represents the power plant shutdown, the storage discharge, the start-up of the backup boiler and the behaviour of the process steam network. Researched characteristic values from German industrial parks and industrial CHP plants, grouped into representative clusters, served as the data basis.

The simulations showed that power drops occurring at the steam consumers can be almost completely compensated for with the aid of a latent heat storage system – with particularly high benefits for smaller steam networks and a high proportion of failing steam generators. The supplementary economic and potential analysis (taking into account fuel, CO₂ certificate and maintenance costs) revealed short payback periods: the main savings arise from the difference between the steam generation costs of the power plant and those of the backup boiler, since the latter can be shifted from permanent minimum-load operation to more efficient base-load operation.

Conclusion

The results achieved in the TheMatIK project show that latent heat storage systems with metallic phase change materials can provide superheated steam within a very short time. They bridge the period between power plant shutdown and backup boiler start-up, make the boiler's permanent minimum-load operation unnecessary, save fossil fuel, contribute to reducing CO₂ emissions and increase the share of combined heat and power in process steam.

Continuation

The methodology developed in TheMatIK – from material characterization through cycle stability testing to process simulation of storage integration – forms the technical basis for the subsequent demonstration project ISSDEMO, in which a metallic high-temperature storage system will be tested for the first time under real production conditions at an industrial partner.