No single factor is more critical to the design of an open bioreactor, algae-to-biofuels project than its water requirements.  The amount of available fresh water will place some maximum limit on the size of the bioreactor.  In turn, the maximum size of the proposed system will determine the amount of algae that can be grown and the amount of fuel that can be produced.  This is true regardless of whether fresh or saltwater algae are to be grown, because in either case the fresh water lost to evaporation will need to be replenished.

The loss of fresh water from an open bioreactor system is due primarily to evaporation.  For purposes of calculation, the rate of evaporation is controlled by a relatively limited set of variables:  the surface area of the ponds, the water temperature in the ponds, air movement, air temperature and the saturation vapor pressure of the pond water, to name a few.  All of these variables can be either pre-set (pond surface area and temperature, as in this example), found in historical climate records or derived from well-known heat and mass transfer formulae.  Bebout and Associates has used these data to construct a model which can be used to predict water loss anywhere in the world where historical climate data are available.  The model is constantly checked against data from our Reno bioreactor system and adjusted as appropriate.

For our hypothetical Dallas project, we will assume a pond temperature maintained at a minimum of 25 degrees C year-round and a surface area of 1 acre. (Why 25C? This temperature is representative of the optimal growing temperature of many algae species commonly grown for biofuel.)  Once our water loss is calculated for 1 acre, we can easily scale our results to any size bioreactor.

Combining all of the known and inferred climate data for the Dallas area, our proprietary water loss model estimates that around 4.3 million gallons of fresh water will be lost yearly for this size and temperature pond.  This is partially offset by annual rainfall which averages around 915,000 gallons/acre, resulting in a net loss of 3.4 million gallons/acre/year. [To put this amount in perspective, the average person in the U.S. uses around 100 gallons of fresh water per day.  Thus, 3.4 million gallons is equivalent to the amount consumed by less than 100 people over the course of a year.]  As might be expected, the average monthly water losses vary, due primarily to changing average wind velocities and the difference in temperature between the pond surface and the surrounding air. This is illustrated by the first chart below.  Note that the predictive error is quite large because of the inherent vagaries of day-to-day weather. 

It is important to understand that the constant temperature chosen for the ponds can have a significant effect on water loss.  Lowering the temperature from 25C to 20C, for example, reduces the net yearly water loss for our 1-acre pond in Dallas by more than 60% to an estimated net loss of 1.3 million gallons/acre/year. (Note that this effect varies in magnitude from one geographic area to another.)  Be aware, however, that the pond temperature is not infinately adjustable- it is going to be restrained by the optimal growing temperature for the algae species that you plan on growing.

Not surprisingly, there are significant differences in water loss for a given pond size and temperature between different geographic areas.  The graph below illustrates the estimated water loss for 3 different geographic areas:  Dallas, the Middle East and the high desert of the U. S. west.  The Middle East clearly has the highest estimated water loss due to evaporation, but the curve is very similar to the high desert, which is not surprising. The high desert area, although more arid than Dallas, had the lowest predicted water loss from evaporation. This result is somewhat counter-intuitive. The reasons for this become more obvious if you compare the historical climate data for the 3 regions (gotta make you work a little..:-)

In summary, we have estimated the yearly fresh water loss for a 1-acre open bioreactor system located in or around Dallas, Texas.  The amount is not insubstantial, although it can be somewhat mitigated by lower pond temperatures, if that is feasible given the species of algae to be grown.  Using this figure, we can now calculate the probable water loss for any size system.  If the availability of fresh water is unlimited, so is the size of the open bioreactor.  If not, the yearly amount of available fresh water divided by the estimated fresh water loss per acre will give a good approximation of the total acreage of ponds that can be supported. This is an important piece of information that should be understood at the outset of any proposed algae project.