Abstract
NTT is developing a phosphoric-acid fuel-cell energy system for telecommunication co-generation systems to reduce energy costs and help preserve the environment. We have studied a thermoelectric generation system using the exhaust vapor heat from a 200kW fuel-cell, which can convert heat energy directly into electrical power. The system consists of 170°C hot-plates heated by the exhaust heat, small cooling plates connected to a heat pipe, an outdoor heat-radiating fin, and Bi-Te thermoelectric modules, which are set up between the hot-and cold-plates. This system generates power efficiently without loss of the subsidiary cooling pump power. The output power density increases with decreasing outdoor temperature and with decreasing thickness of Bi-Te. The system generates a maximum power density of 2.4kW/m2 at 5°C, the lowest tested outdoor temperature, with Bi-Te thickness of 1mm. The thermoelectric conversion efficiency is 1.4 to 2.1% at 5°C in the Bi-Te thickness range from 1 to 5mm. We estimate that the thermal contact resistance is 4 to 5×10-4m2/kW from a simulation using the thermal resistance model. Total power is estimated to be of 1.9kW for Bi-Te thickness of 5mm and 1.3kW for 1mm when using all of the exhaust vapor heat from a 200kW fuel cell. The thermoelectric module using 1-mm-thick Bi-Te generated electricity more economically than the 5-mm-thick one because it generated electricity efficiently with less Bi-Te.
