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Algae’s potential oil productivity, as compared to terrestrial crops, is significant. However, meeting algae’s full production potential requires year-round growth. This can only be accomplished if the ponds’ optimum growing temperature is maintained throughout the year. Except in tropical regions, this requires some sort of supplemental heat to be added to the bioreactor for at least part of the year. Heat loss for any open bioreactor system occurs as a result of 4 major processes: evaporation, convection, radiation and, to a lesser degree, conduction. Evaporative is generally the largest component of the total heat loss from a pond. Approximately 1,000 BTU are lost for every pound of water that evaporates. As the temperature of the pond increases or the relative humidity of the air is decreased, evaporation increases. Convective loss is the next major mechanism of heat loss. This is caused by cooler air passing over the pond surface. The 2 most important factors in convective heat loss are wind velocity and temperature differences between the air and the pond. Radiant heat loss is dependant primarily on the temperature difference between the pond surface and the surrounding air temperature. A 4th component of heat loss is conductive (through the sides of the pond); it is very small with in-ground ponds and is ignored in the discussions that follow. The table below was generated by our proprietary heat loss model which generally follows well-known heat and mass transfer formulae. It shows the calculated hourly and monthly heat loss for a 1-hectare pond in Dallas kept at 25C year-round. Historical climate data provide the primary basis for the calculations.
Although presented as negative numbers, the red numbers in the Radiant Heat Loss rows represent a radiant heat gain for June through September. The greatest heat loss occurs in March, rather than in the cooler winter months as might be expected. This is due primarily, although not entirely, to historically higher average wind velocities during March in Dallas. It is also important to note that although June through July in Dallas average above 25C, there is not a single month that does not require some supplemental heat (remember that evaporation has a cooling effect). Following is a summary of the types and quantity of heat loss for our hypothetical 1-acre pond over 1 year:
Planners now have a decsion to make: to operate the ponds for less than a full year and accept lower productivities, or find a source of supplemental heat. This is basically a question of economics. Where can we get heat and at what cost? And does the additional productivity justify this cost? In our work with open bioreactors in Nevada, we use geothermal heat, which is abundant and relatively inexpensive on a BTU basis. But what about areas where geothermal resources are not available? Other potential sources include solar and unused heat from existing industrial processes. Power plants are attractive because they produce large quatities of heat as well as CO2 (the benefits of CO2 will be discussed in another section). Let's look at a few of the actual power plants in the Dallas area for our project:
Note that the carbon figures represent tons of CO2 emitted annually and the energy figures are MWh. Using the Lake Hubbard plant as an example, around 6.3x10^5 MWh were generated in 2007. Assuming an efficiency of around 40% (we haven't researched the actual value), as much as 3.2x10^12 BTUs are lost as heat each year. If only 60% of that heat can be captured, nearly 1.6x10^11 BTUs are available each month. That's enough energy to keep around 21 acres of ponds at optimal growing temperatures year-round. (The 21 acres figure was calculated by dividing total available monthly heat by highest single monthly heat demand.) It should be clear that the availability of supplemental heat is another limiting factor on the size of proposed open bioreactor systems. As with water demand, project designers should have a thorough understanding of the economic tradeoffs involving heat, the amount of heat needed and potential sources of unused heat. NEXT MONTH: CO2- Gotta have it, where to get it and an honest assessment of how much can be captured or sequestered. FEEDBACK: Several visitors have wondered if the heating requirements, which they believe are severely limiting, argue in favor of closed bioreactor systems. We would like to note that closed systems are not unaffected by the climate in which they are constructed. Anyone contemplating such a system will need to calculate the energy necessary for heating and/or cooling as well, and where and at what cost this energy can be obtained. -JWB |
