Most of the commercial available products from algae are produced in open ponds (Figure 1). The main reason for using these systems are that they are very cheap to build, operation is simple and scaling up is very easy. Disadvantages are low volumetric productivities, vulnerability to contamination and high evaporative losses. The most commonly used open system is a raceway pond, single or several of these ponds connected in series. These ponds are usually no more than thirty centimeters deep and are mixed via a paddle wheel that circulates water with nutrients and microalgae. Ponds can produce about 20 ton biomass/ha/year.

However, many species cannot be grown in these open ponds because they are easily overgrown by other microorganisms and because some products (used in cosmetics and for human consumption) require a constant quality and safety is a major parameter. For these products, closed systems that can be fully controlled have to be used. As closed photobioreactors, only the tubular photobioreactor (Figure 2) is used commercially because it can be easily constructed and it can have a ten times higher volumetric productivitiy than open ponds. This type of reactor consists of many tubes of about 3-10 cm in optical path (path that light has to travel, i.e. tube diameter) and 25-100 meter long and is operated at biomass concentrations of 1 to 5 grams per liter dry weight. The system has as main drawbacks the high investment and operation costs and a poor mass transfer which can lead to oxygen build up in the reactor to inhibiting levels and consequently growth inhibition. Biomass production is about 20-60 ton/ha/yr depending on optical density and the capacity of the degassers.

Flat panel photobioreactors have been considered too expensive to be viable. However, innovative ways to construct flat panels cheaper are being developed and for example plastic bags in metal racks can favor this type of reactor over tubular reactors. In the future, innovative cheap designs of this type of reactor will probably be used instead of tubular systems.

Figure 1: Raceway pond at Cyanotech (Hawaii)	Figure 2: Tubular reactor of Salata GmbH(Ritschenhausen, Germany)