取排水の無い発電所冷却水システムに関する基礎研究: 深層熱交換器のファウリングに関する予備的実験

摘要

蒸気タービンを用いる汽力発電(steam power generation)では，ボイラーで発生させた蒸気が復水器で冷やされ水の状態に収縮する時のエネルギーによってタービンが回される.冷却には地下水•河川水•空気などが用いられることもあるものの，我が国の大規模汽力発電所では海水が用いられることがほとんどであり，沿岸海域の生態系への影響等を勘案して，取水した海水よりも7℃程度高い温度の温排水が海に戻されている.Fig.1に従来型の冷却水システムの概念図を示す.図では取水•排水の経路を配管で描いてあるが，発電所構内に海と連通する取水路や排水路を設ける場合も多く，また配管を用いる場合でもその距離は短い.温排水の温度を抑えるためには所要の用水量が必要であり，例えば我が国の50万kW級の汽カ発電設備（火力•原子力）では定格運転時におよそ20〜30 m~3/秒の取排水が行われている.%Most all of the thermal power plants in Japan are located on the shore and use seawater for cooling the turbine steam. The discharge of heated seawater should be restricted not to affect on the coastal ecosystem, and in addition, the seasonal variation of temperature of intake seawater is recognized as the significant factor of the power generating efficiency. One of the authors et al proposed a new cooling water system so as to avoid such problems, that is, the cooling water would be contained in a closed circuit pipe system and circulated between cold regions of the deep water and the condenser for turbine steam cooling. Fouling on surface of the heat exchanger may cause the degradation of heat exchanging performance. It is known that there is a quite small chance in the dark and clean deep sea water to produce the fouling, but we have not so many knowledge in case that the temperature is high at the circumstances in deep water. Therefore, the effect of temperature on the fouling on the heat transfer surface is experimentally investigated in this study. A slender titanium pipe is immersed in the actual deep sea water obtained with the deep water intake facilities in Ohshima Island, off the Izu Peninsula, and held in a container keeping 10 degC in temperature and darkness without light. Heated water at 80 degC is continuously put into the pipe and the decline of temperature along the pipe and the situations of the surface of pipe are observed for 30 days. As a result, bio-fouling, a thin bacteria film is observed on the heat transfer surface when the temperature is higher than about 40 degC.