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•Presentation of a thermoeconomic approach to model energy storage.•Modelization of a real polygenerative district.•Thermoeconomic analysis of the role of three different kind of storage (electrical, hot and cold thermal).•Analysis of costs and benefits.

The production of β-galactosidase by Kluyveromyces fragilis was studied in different culture systems, with dissolved oxygen concentration control and using defined media. An operating strategy of fed-batch culture with automatic control of substrate addition regulated by dissolved oxygen concentration, consisting of the replacement of variable volumes of broth by fresh medium (once the fed-batch culture has finished), was designed. The volumetric enzyme productivity (Q_p, 13 600 UI dm^-3 h^-1) obtained was 38% higher than that reached in continuous culture of K fragilis with dissolved oxygen concentration control and far higher than that obtained by batch culture of K fragilis under the same aeration conditions.© 2002 Society of Chemical Industry