Heat storage in lead-acid accumulators on-board submarines.

Abstract: A Swedish submarine often operates in colder waters, resulting in a cold on-board climate and a low temperature in the batteries. The batteries reaches their maximum capacity when they are at a temperature of 30°C, which they rarely are during the winter months when the temperature often is lower than 7°C. The diesel engines are producing a significant amount of waste heat that is just dispersed into the sea. By using this waste heat and storing it in the batteries would the capacity of the batteries be better and the on-board climate as well. The heat transfer to the battery cell will be through an existing cooling system in the pole bridge together with internal heat generation from different charging levels. This cooling system has a limited reach into the cell making the ability to spread heat inside the cell limited. The main question to investigate is how well this heat will distribute and how much energy that can be stored over time inside a battery cell. The primary method has been to create a realistic CAD model of a battery cell that later on has been used for simulation through a CFD analysis. To create a realistic model previous calculations and schematics have been studied and interviews with the supplier of the cell and Kockums AB employees have been performed. Furthermore, analytical calculations for both simplifications of the geometry and the potential of proposed method have been done. The available waste heat from the diesel engines trough the cooling water is about 2,7MW, which is more than needed since the pole bridge has a maximum delivered heat of 912W/bridge. The heat storage potential is 18,8 MJ per double cell and all the cells together form an energy storage with far greater capacity than a regular accumulator tank in for example a villa. This is estimated to be enough to make the on-board climate more pleasant. This is also confirmed by previous crew members who points out that when the batteries are warm, the climate on-board is also better. The results showed that the heating from the pole bridge was faster than expected. Already after 20 % of the time of heating combined with stage 1 charge the energy storage had reached 30 % of the full potential. After full time this factor had reached 67 % which corresponds to 12,6 MJ stored heat per double cell. The local temperatures inside the cell during the first case varies significantly and reaches its highest value after 40 % of full time when the temperature difference between the top electrolyte and the electrolyte pumped from the bottom is over 20°C. The trend after full time is that the average temperature is flattened out, which suggests that the cell already is near its stead state. Heating and stage 2 charging starts at the values reached after stage 1 charge and since the trend of the temperature rise already was flattened out the temperatures during case 2 are not increasing by so much. The average temperature is only increased by less than 5°C and the energy stored after stage 2 charge is just slightly increased to 72 % of full capacity. The conclusion is that it is possible to store heat in the batteries but further work has to be done. The potential heat storage and available heat has been confirmed to be large enough, the heat transfer from the pole bridge is bigger than expected resulting in a quicker heating of the cell. However, big differences of local temperatures inside the cell where observed, which may result in stratifications that will affect the chemical process when charging. Further work also needs to be done to determine how much of the stored heat is radiated directly out to the sea and for how long the heat source will affect the climate on-board the submarine.