Page 243 - Handbook of Battery Materials

212 7 Bromine-Storage Materials

7.5
Safety, Physiological Aspects, and Recycling
7.5.1
Safety

Safety risks and environmental impact are of major importance for the practical
success of bromine storage system. The nonaqueous polybromide complexes
in general show excellent physical properties, such as good ionic conductivity
−1
(0.1–0.05 cm ), oxidation stability (depending on the nature of the ammonium
ion), and a low bromine vapor pressure. The concentration of active bromine
in the aqueous solution is reduced by formation of the complex phase up to
−1
0.01–0.05 mol L , hence ensuring a decisive decrease of self-discharge.
Figure 7.2 demonstrates that the bromine vapor pressure over a complex phase
remains remarkably low with increasing temperature and is not a critical factor
restricting battery operation. Even at ∼60 C, vapor pressures of Br 2 and elemen-
◦
tal bromine reaching only a small percentage of the atmospheric pressure are
obtained.
Moreover, calculations on the evaporation rate of bromine from the complex
phase were carried out assuming a worst-case scenario, namely a complete spillage
of the total bromine inventory (as polybromide complex) of a fully charged (100%
2
SOC) 15 kWh module, which means ∼32.5 kg of available Br 2 , forming a 10 m pool
on the ground as a consequence of battery damage. Rates of bromine evaporation
from the complex phase in air were measured in laboratory tests [89] under various
conditions (flow, temperature). The results are presented in Figure 7.8.
Figure 7.9 shows the distribution of bromine emissions (concentrations) as a
function of distance from the source of emission, assuming various atmospheric
◦
conditions (air flow) at 20 C. The maximum admissible concentration (MAK
value) of 0.7 mg m −3 (0.01 ppm) is reached within about 50 m under worst-case
atmospheric condition, whereas higher and dangerous values are observed close
to the place of complex spillage. However, the assumptions serving as the basis
for these estimates and calculations appear rather unrealistic, considering the low
probability of a complete release of the complex phase at 100% SOC from the
reservoir and of the distribution on the ground, and moreover neglecting the role
of the aqueous electrolyte solution which is also present and tends to spread on the
complex surface due to its lower density.
The most important safety-relevant measures involving automatic control during
battery operation are:
• thermal management, at a maximum of 42 C;
◦
• automatic pumps and flow control, leakage sensor and pumps that stop automat-
ically in the case of bromine escaping from the stack;
• overcharge control.
As shown by several investigations [89], the bromine-rich polybromide phase
by itself is hardly flammable and fire-extinguishing properties have been reported