GroNaS energy converter cell

Top view of a GroNaS energy converter cell

The key elements of the GroNaS technology is our patented energy converter.
An energy converter cell of the GroNaS design is shaped like a flat round disc. This design of the cell makes it possible to design the entire energy converter as a cell stack and to carry out the contacts of both electrodes as a common bipolar plate.

Cut through a stack of GroNaS energy converter cells

The spaces for the positive electrode are linked together. In the energy delivery mode, sulfur is fed to the bottom of the stack and the reaction product (sodium sulfide) exits the topmost cell. In the energy absorption mode, the flow direction is reversed.

The liquid sodium-containing space for the negative electrode of each energy converter cell has a terminal for the supply or discharge of sodium. Each connection line to the common sodium tank initially leads to a simple device, also patent pending, which lets through liquid sodium without creating an electrical connection. It serves to separate the electrical potentials, without which a series connection and thus the generation of a sufficient voltage would not be possible.

In the latest cell design, the solid electrolyte membrane is no longer designed as a flat component, but as an array of many small tubes. As a result, the membrane surface could be increased significantly. A state-of-the-art GroNaS energy conversion cell enables hundred times the power of a conventional sodium-sulfur battery cell.

▷ diameter: 0.8 -1.2 m
▷ Height: 6 – 12 cm
▷ Voltage: 2V
▷ Power: 10 – 20 kW

A GroNaS energy converter cell, including the device for electrical isolation, only consists of approx. 12 immovable, simple metal and ceramic parts as well as a ceramic composite membrane carrier plate and approx. 300 membrane sleeves. The metal parts can be manufactured conventionally. There is also an established manufacturing technology for the membrane sleeves, but existing production lines for similar sleeves made of ordinary ceramic materials must be adapted to the special
sodium β-aluminate material. All membrane sleeves are connected together in one step with the carrier plate.

The performance increase over conventional Na-S battery cells results from the combination of two effects:

▷  Enlargement of the solid electrolyte membrane surface (power increase approx. by factor 20).
▷  Exploiting a higher current density. This is possible because Na2S2 crystal formation does not occur as a result of forced flow through the positive electrode space. (Performance increase by factor 2). Because of the increased power of the energy converter cell and its straightforward design, far fewer components have to be produced and assembled for the energy converter to function in the storage system than in a conventional design consisting of small battery cells.
▷ conventional Na-S storage technology: approx. 35000 components / MWi
▷ GroNaS technology: approx. 1200 components / MWi

GroNaS Energy converter cascade

Energy converter cascade (stack of cells)

A single energy converter cell provides a voltage of 2V. To operate an inverter, a multiple of that is needed. Therefore, a stacked cascade of about 100 cells is assembled. The energy converter cells are connected electrically and fluidically in series. The stacked arrangement of energy converters with bipolar plates has the advantage that the electrical resistance between the energy converters is minimal due to the short distances. Furthermore, this way there is no manufacturing effort for the production of low resistance electrical connections (the cascade flows a current of up to 10,000 A, otherwise relatively complex electrical contacts would be necessary).

▷ Height: 10 – 20 m
▷ Diameter: 2 – 3 m
▷ Voltage: approx. 200 V
▷ Power: approx. 2 MW
▷ Feed pressure sulfur / sodium sulfide: <5 bar
▷ Sodium pressure: <6 bar

In the energy extraction mode, the cascade is fed with sulfur. The liquid subsequently flows through all energy converters and absorbs sodium. The flow through the cascade is controlled so that at the end of the full conversion of the energy carrier material is done and a 1:1 mixture of Na2S4 und Na2S2 leaves the cascade.

Because liquid sodium is itself a very good electrical conductor, the energy converters cannot be supplied with sodium from a common conduit. In the machine house, at the head of the cascade, there is a device for electrical isolation. A line for liquid sodium leads from there to each of the disk-shaped energy converter cells. Only small diameter, thin-walled tubes, are needed, because there is only a pressure of 10 bar at the most, and the delivery rate of sodium is only about one cubic centimeter per second.
In the energy absorption operation, reversing the flow direction, the reconversion of sodium sulfides into sulfur takes place.

The energy converter cascades are placed in concrete tubes, which are lowered into the earth. To increase fire safety (in the case of a leak in one of the sodium supply pipes), the remaining space between the wall of the concrete pipe and the cascade can be filled with sand.

GroNaS Storage facility

GroNaS storage facility

A GroNaS storage system consists essentially of two heat-insulated tanks and a machine house (M). The machine house houses the energy converter cascades (W) and electrical and chemical process engineering functional units. Furthermore, a protective gas system (G), a cooling device (K), a high-voltage field with transformer (T) and switching devices as well as the control and communication devices belong to the storage plant.

The figure shows a particularly safe variant of the storage facility, which is characterized by sodium and sulfur storage tanks lowered into the ground. According to recent findings, an equivalent, low risk potential can also be achieved with above-ground tanks. The construction costs would be significantly lower in this case.

One tank is for liquid sodium. It is full when the accumulator is charged with the maximum energy and empty when the accumulator has exhausted all the energy. In the energy delivery mode, sodium is pumped to the energy converter cascades. During energy absorption, sodium forms in the energy converters and is pumped back into the tank.

The second tank contains sulfur when energy is stored in the storage tank and sodium sulfide when the energy is released. For power generation, sulfur is taken from the top of the tank and pumped through the energy converter cascades. In the process, it converts to sodium sulphide, absorbing sodium. After leaving the cascade, the sodium sulfides are fed down into the same tank from which the sulfur was taken. This tank thus serves at the same time as the reservoir for the high-energy and the low-energy form of the positive electrode. This is possible because sulfur and sodium sulfides are immiscible, like oil and water. Down in the tank is the sodium sulfide mixture with the higher density, and above the sulfur with the lower density.

A cooling device for the energy carrier materials is required, because the energy converters generate more waste heat than is needed to maintain the operating temperature.