Abstract
The exergy conversion efficiency was calculated in the case where a low level exergy of heat discharged from a factory is converted into a high exergy level of electricity via dialytic battery, and was found to be in the range of 0.494% to 0.797%. Efficiency increases with an increase in feed temperature and in the case of counter-current feed flow configuration. The feed to the dialytic battery is composed of sea water and concentrated sea water, which is concentrated by evaporating water by means of a cooling tower. The heat required is extracted from the warm industrial waste water discharged from factories.
The sea water, whose temperature is raised to 40°C, is fed into an ideal cooling tower and placed in contact with the air (temperature 15°C, humidity 70%, the yearly average values in the Tokyo area). At equilibrium, about 5% of the water being fed to the cooling tower is evaporated and the temperature of the concentrate is lowered to 12°C. The ideal cooling tower number is found to be 47 for concentrating the sea water to a saturated state (26.4 wt% salt).
The dialytic battery is composed of 1000 paired anion exchange membranes and cation (area resistivity 2Ω·cm2 for each) and a maximum output of 43.3 kW was calculated at the following operating condition; 40°C, saturated sea water (5415 mol/m3), fresh sea water (598 mol/m3), average feed velocity 0.01 m/s.
Assuming that 10% of one million yen/kW as the initial investment for this system is allocated to the membrane, the cost of the membrane might be 50 yen/m2 in the case of 1 W/(m2·pair), and 150 yen/m2 in the case of 3 W/(m2·pair).
