APPLICATION OF LIGHT-EMITTING-DIODES IN BIOREACTORS - FLASHING LIGHT EFFECTS AND ENERGY ECONOMY IN ALGAL CULTURE (CHLORELLA-PYRENOIDOSA)

Light-emitting diodes (LEDs) were used as the sole light source in con tinuous culture of the green alga Chlorella pyrenoidosa. The LEDs appl ied show a peak emission at 659 nm with a half-power bandwidth of 30 n m. Selection of this wavelength range, which is optimal for excitation of chlorophylls (a) under bar and (b) under bar in their ''red'' abso rption bands makes all photons emitted potentially suitable for photos ynthesis. No need for additional supply of blue light was found. A sta ndardized panel with 2 LEDs cm(-2) fully covered one side of the cultu re vessel. At standard voltage in continuous operation the light outpu t of the diode panel appeared more than sufficient to reach maximal gr owth. Flash operation (5-mu s pulse duration) enables potential use of higher operating voltages which may render up to three times more lig ht output. Flat airlift fermenter-type continuous culture devices were used to estimate steady state growth rates of Chlorella pyrenoidosa a s a function of the light flux (mu mol photons . m(-2) . s(-1)) and th e flashing frequency of the light-emitting diodes (which determines th e duration of the dark ''off'' time between the 5-mu s ''on'' pulses). At the fixed voltage and turbidostat setting applied a 20-kHz frequen cy, which equals dark periods of 45 mu s, still permitted the maximum growth rate to become nearly reached. Lower frequencies fell short of sustaining the maximal growth rate. However, the light flux decrease r esulting from lowering of the flash frequency appeared to reduce the o bserved growth rates less than in the case of a similar flux decrease with light originating from LEDs in continuous operation. Flash applic ation also showed reduction of the quantum requirement for oxygen evol ution at defined frequencies. The frequency domain of interest was bet ween 2 and 14 kHz. LEDs may open interesting new perspectives for stud ies on optimization of mixing in mass algal culture via the possibilit y of separation of interests in the role of modulation on light energy conversion and saturation of nutrient supply. Use of flashing LEDs in indoor algal culture yielded a major gain in energy economy in compar ison to luminescent light sources. (C) 1996 John Wiley & Sons, Inc.