Evaluation of photobioreactor heat balance for predicting changes in culture medium temperature due to light irradiation

Microalgal photosynthesis requires appropriate culture medium temperatures to achieve high photosynthetic performance and to maintain production of a high-quality biomass product. Enclosed systems, such as our conical, helica l tubular photobioreactor (HTP), can accomplish high photosynthetic efficie ncy and the small amount of culture medium used by these systems means that the culture medium temperature may be effectively controlled. On the other hand, because a high ratio of surface area to culture medium volume leads to rapid heating under the illumination condition and substantial heat loss at night, maintaining a suitable culture medium temperature is necessary t o achieve efficient, commercially practical biomass production. In order to predict changes in the culture medium temperature caused by changes in sol ar irradiance and ambient temperature, it is necessary to understand the he at balance within the photo bioreactor. We therefore investigated the heat balance in three major parts (photostage, degasser, and helical heat exchan ger) of our conical HTP, analyzed the time-dependent changes in medium temp erature at various room temperatures and radiant energy inputs, and predict ed changes in the culture medium temperature based on the characteristics o f heat transfer among the three parts. Using this model, the predicted chan ges in culture medium temperature were very similar to the changes observed experimentally in the laboratory and under field conditions. This means th at by calculating the time-dependent changes in the culture medium temperat ure, based on measurements of solar energy input and ambient temperature, w e should be able to estimate the energy required to maintain the culture me dium temperature within a range where photosynthetic performance of microal gae is high. (C) 2001 John Wiley & Sons, Inc. Biotechnol Bioeng 74: 466-475 , 2001.