Design and constructions of test units

As comparison of both storage materials did not provide a clear preference for one material, both materials have been considered for the test storage units. Therefore two storage units have been designed and constructed. Dimensioning of the storage has been performed with the simulation environment “StorageTechThermo”, developed at DLR, where the storage is described by physical models, allowing also for dynamic simulation of the system. This model is described in detail in [3].

To allow for good comparison, both units are of same dimensions. The storage capacity for the ceramic storage unit is around 350 kWh storage capacity and for the high temperature concrete unit it is approx. 20% lower. Each of theses storage units is build up of two parallel storage modules.

The heat exchanger is composed of 36 tubes of high-temperature steel with nominal diameter of 21×2 mm. They are distributed in a square arrangement of 6 by 6 tubes with a distance of 80 mm. Collectors and distributors have been designed for best equal flow distribution in all tubes.

One storage module has the total dimensions of 0.48×0.48×23 m3. The thermal expansion of one module of castable ceramic with steel heat exchanger, when heated up from ambient to maximum operation temperature of 390°C, is 125 mm, for the concrete modules it’s 120 mm, so the modules will move approx. 60mm to each side. During cycling at operating temperature the temperature difference is only 40 K so the movement is only less than 10 mm to each side. To allow for this movement, the storage modules are bedded on two metal sheets as sliding planes.

All four blocks have been built in parallel. For practical reasons, due to the different production methods, the two outer modules have been filled with the castable ceramic and the two middle modules with high temperature concrete as storage material. Fig. 1 shows the construction of the storage units. All four heat exchangers have been welded together on site. On this picture the first concrete module has just been casted and the worker in the background is compacting the material with a vibrator. The shuttering boards for the second concrete module are already fixed.

Fig. 1 Heat exchangers with collectors and distributors of the four storage modules, first high temperature concrete module just casted

The storage units are equipped with 208 thermocouples casted into the storage modules, 2 flow meters, 2 differential pressure sensors and 8 Pt-100 for measuring oil temperature. Fig. 2 shows all four storage modules after casting and curing, with all piping connected. The sensors are being connected to the three switch cases on the left side.

Fig. 2 Storage module without insulation