Other system designs for algae production are possible. The Japanese, French, and German governments have invested significant R&D dollars on novel closed biore­actor designs for algae production. The main advantage of such closed systems is that they are not as subject to contamination with whatever organism happens to be carried in the wind.

When designing a photobioreactor, design parameters such as reactor dimen­sion, flowrate, light requirements, culture condition, algae species, reproducibility, and economic value need to be taken into consideration. Depending on the reac­tor dimensions, site location, and local climate, these parameters can determine the type of cultivation system needed (open versus closed). Reactor design should have good mixing properties, efficiency, and reproducibility and be easy to maintain and sterilize. An efficient photobioreactor not only improves productivity but also is used to cultivate multiple strains of algae. The performance of a photobioreactor is measured by volumetric productivity, areal productivity, and productivity per unit of illuminated surface (Riesing 2006). Volumetric productivity is a function of biomass concentration per unit volume of bioreactor per unit of time. Areal productivity is defined as biomass concentration per unit of occupied land per unit of time. Produc­tivity per unit of illuminated surface is measured as biomass concentration per area per unit of time.

Closed bioreactors support up to fivefold higher productivity with respect to re­actor volume and consequently have a smaller “footprint” on a yield basis. Besides saving water, energy, and chemicals, closed bioreactors have many other advantages that are increasingly making them the reactor of choice for biofuel production, as their costs are lower (Schenk et al. 2008). Closed bioreactors permit essentially single-species culture of microalgae for prolonged periods. Most closed bioreactors are designed as tubular reactors, plate reactors, or bubble column reactors (Weiss — man et al. 1988; Pulz 2001). Other less common designs like semihollow spheres have been reported to run successfully (Sato et al. 2006).

Enclosed photobioreactors have been employed to overcome the contamination and evaporation problems encountered in open ponds (Molina Grima et al. 1999). These systems are made of transparent materials and are generally placed outdoors for illumination by natural light. The cultivation vessels have a large surface-area — to-volume ratio.

The main problems in the large-scale cultivation of microalgae outdoors in open ponds are low productivity and contamination. To overcome these problems,

a closed system consisting of polyethylenes sleeves was developed. In a study con­ducted outdoors. The closed system was found to be superior to open ponds with respect to growth and production in a number of microalgae. In both closed and open systems, growth and production under continuous operation were higher than in batch cultivation (Cohen et al. 1991).

Sananurak et al. (2009) have designed, built, and operated a closed, recirculat­ing, continuous culture system to produce microalgae and rotifers in seawater (25% salinity) for larval fish culture. The system opens up a new perspective in terms of automated production of rotifers without labor cost. Rotifers can be easily harvested daily by a conical harvest net, and there is no routine maintenance work. This new, automated system has three components: a microalgae culture, a rotifer culture and storage with harvest, and a water treatment and reuse component.

The preferred alternative is closed photobioreactors, where the algae fluid re­mains in a closed environment to enable accelerated growth and better control over environmental conditions. These glass or plastic enclosures, often operated under modest pressure, can be mounted in a variety of horizontal or vertical configura­tions and can take many different shapes and sizes. Rigid frameworks or structures are usually used to support the photobioreactor enclosures.

Open systems using a monoculture are also vulnerable to viral infection. The en­ergy that a high-oil strain invests in the production of oil is energy that is not invested in the production of proteins or carbohydrates, usually resulting in the species being less hardy or having a slower growth rate. Algal species with lower oil content, not having to divert their energies away from growth, have an easier time in the harsher conditions of an open system.

Closed systems (not exposed to open air) do not have the problem of contami­nation by other organisms blown in by the air. The problem for a closed system is finding a cheap source of sterile CO2. Several experimenters have found the CO2 from a smokestack works well for growing algae.

In hybrid systems, both open ponds as well as closed bioreactor system are used in combination to get better results. Open ponds are a very proficient and lucrative method of cultivating algae, but they become contaminated with superfluous species very quickly. A combination of both systems is probably the most logical choice for cost-effective cultivation of high yielding strains for biofuels. Open ponds are in­oculated with a desired strain that had invariably been cultivated in a bioreactor, whether it is as simple as a plastic bag or a high-tech fiber-optic bioreactor. Impor­tantly, the size of the inoculums needs to be large enough for the desired species to establish in the open system before an unwanted species. Therefore, to minimize contamination issues, cleaning or flushing the ponds should be part of the aquacul­ture routine, and as such, open ponds can be considered as batch cultures (Schenk etal. 2008).

Abundant light, which is necessary for photosynthesis, is the third requirement. This is often accomplished by situating the facility in a geographic location with abundant, uninterrupted sunshine such as the American Southwest (Brown and Zeiler 1993). This is a favored approach when cultivating in open ponds. When working with bioreactors, sunlight quantity and quality can be further enhanced through the use of solar collectors, solar concentrators, and fiber optics in a system called photobioreactors (Scott and Bryner 2006; Chisti 2007). These technologies allow optimal sunlight to reach algal cells either by allowing them to float in arrays of thin, horizontal tubes or by directing light, through a fiber-optic matrix, through the bioreactor chamber itself.

The pH level generally increases as the microalgae consume CO2. Addition of carbon dioxide along the reactor would sustain microalgal growth by preventing car­bon limitation and an excess rise in pH. However, tubular photobioreactors do not work well in large-scale production because the surface-to-volume ratio is lower, causing poor light absorption. Length of tubes is another concern of tubular photo­bioreactors. As the tube length increases, the time for microalgae exposure to light increases, hence increasing the absorption of available CO2 and increasing photo­synthesis rate. However, the dissolved oxygen level also increases, which can easily lead to oxygen poisoning, and photoinhibition can result from the excess light ex­posure (Ogbonna and Tanaka 1997).