By Caroline de Monasterio
Large-scale production of algae biodiesel is not a viable solution in the displacement of petroleum-based fuels. The technology to efficiently produce biodiesel from microalgae is not competitive with more advanced and emerging renewable technologies. At present production efficiencies, algae biodiesel has an approximate cost of thirty-three dollars per gallon (6). According to an NREL report (5), wind-generated electricity costs between six and nine cents per kilowatt-hour, and photo-voltaics have an estimated range of eighteen to twenty-three cents. The high costs of algae biodiesel production are due in part to the energy required to circulate gases, fluids and other materials in the growth environment.
Currently, large-scale production of algae biodiesel is not a viable solution in the displacement of petroleum-based fuels. The technology to efficiently produce biodiesel from microalgae is not competitive with more advanced and emerging renewable technologies. Algae convert solar energy to chemical energy via photosynthesis. The efficiency of this conversion process ranges between three to six percent (1). An ideal system consumes the minimum available energy while producing maximum yield. Efficiency is a measure of the success associated with energy conversion, and it is constrained by the laws of thermodynamics. The less efficient an energy conversion, the more energy is transformed to a lower quality form. Therefore, in order to maintain a desired output, as efficiency decreases, initial resources must increase proportionally. While sunlight is an inexhaustible source, the energy and additional resources used in processing, refining and transporting the biodiesel are not. Microalgae cultivation has several novel applications separate from oil production, specifically wastewater treatment and carbon sequestration. However in terms of current technology, the operational considerations associated with production highlight the impracticality of extensive research and development invested in turning algae biodiesel into a core fuel supply.
At present production efficiencies, algae biodiesel has an approximate cost of thirty-three dollars per gallon (6). According to a National Renewable Energy Laboratory report (5), wind-generated electricity costs between six and nine cents per kilowatt-hour, and photo-voltaics have an estimated range of eighteen to twenty-three cents. Plug-in hybrid electric vehicles will be introduced to the mass consumer market in 2010. President Barack Obama has stated the goal of one million plug-in hybrid electric vehicles to be in operation by 2015. While market penetration on this scale is highly improbable, it is very likely there will be considerable incentives instituted to meet this goal. As a result, there will be limited incentives to invest limited time and money on necessary research and development required needed to improve algae biodiesel production. The high costs of algae biodiesel production are due in part to the energy required to circulate gases, fluids and other materials in the growth environment. Additional resources are required in the oil extraction and refinement to obtain the final product (2). There is also the issue of energy density. When compared to other biofuel sources, the most attractive feature of algae biodiesel, is the amount of land required for production. Other biofuels require significantly more land in order to produce what is a relatively small amount of usable energy. Although less land area is involved in algae production than needed for other bio-crops, there remains significant surface area required to match current US consumption of petroleum-based fuels. The following calculations validate this hypothesis*:
The Oak Ridge National Laboratory reports the energy density (lower heating value) for conventional and diesel fuel (7) as follows:
Gasoline = 115,000 Btu/gallon
Petro-diesel = 130,500 Btu/gallon
According to another NREL study (4), the United States consumption of motor fuel is 390 million gallons a day or approximately 142 billion gallons a year.
As biofuel energy density is less than that of petroleum-based fuel, the fuel economy is decreased. The following estimate represents an upper limit estimation substituting the energy density of gasoline for that of petroleum:
- 142 x 109 gallons * [115000 Btu/gal] * [1 kWh / 3413 Btu] = 478.5 x 109 kWh (per year).
- Assuming eight hours per day of sufficient solar energy this translates to 59.8 billion kW a year.
- The same NREL paper reported an approximate number of incident solar energy to be 600 W/m2 for the United States. Thus, 59.8 billion kW * (1 m2 / 0.6 kW) = 99.7 x 109 square meters.
- If we make the assumption that future technological advances will increase the overall production efficiency to 15% this equates to 184.4 x 109 square meters.
- The surface area of the United States (excluding water bodies) is 9,161,922 square kilometers (wikipedia.org/List_of_U.S._states_by_area). Thus, approximately twenty percent of land in the United States would need to be devoted to algae production in order to match petroleum fuel consumption. Based on these calculations, it would appear that the presumptive savings in land use are rather overstated. Based on these assumptions, the required land area precludes the efficacy of this operation.
Some may make the argument that the additional benefits of algae cultivation, such as wastewater treatment and carbon dioxide sequestration, have not been taken into consideration in this valuation. As a response to this argument, there are many variables associated with this process, but these variables have not been tested or proven beyond the theoretical stage. Successful algae cultivation depends on several factors, and different algal strains have different requirements (8). Two of these factors are adequate water supply and a stable range of temperature. Uncertainties related to these components are climate, location and system characteristics. Open systems, such as raceways and ponds, are currently the most economical and practical systems. However in this basic system, there remain multiple risks and variables present. Some autotrophs tolerate variations in their environment, but the conditions required for efficient algae cultivation have a more limited range. In general, water temperature must be maintained n the range of twenty to thirty degrees Celsius (8). The energy that can be associated with maintaining these temperatures is dependent on climatic conditions found in the environment. An additional concern is the risk of evaporation from these open systems. Covering these structures in order to prevent evaporation would be costly, while risking elevating water temperature above a desirable level. Therefore, it is proposed to focus on saltwater tolerant species for future research and development. One problem with saltwater cultivation is the higher the salt content, the lower the oil production and doubling rate. Additionally, there is the need to remove salt molecules from algae grown in salt water prior to oil extraction.
In conclusion, with examination one can see that the processes involved in certain aspects of algal-oil production may potentially require a greater amount of energy than originally thought. At this moment, the majority of this energy would come from fossil fuels, which violates a key principle of sustainability. Although people argue of the benefits of algae biofuel production, when considering algae biofuel as a plausible fuel supply, there is no existing data that supports the theory that algae biofuel could be a viable solution.
* Currently the most feasible/predominant/economical method for algae cultivation is open systems (ponds/raceways), and thus assumptions and calculations are based on this presumption.
(3) A Look Back at the U.S. Department of Energy’s Aquatic Species Program—Biodiesel from Algae July 1998; NREL study