Exploring Microalgae as Solutions to Global Fuel Issues

[Ivan Seeking]

Since the idea of using the generator as a nitrogen source was mentioned, I thought I had better do some seat-of-the-pants calculations to see if we are even in the proper order of magnitude of nitrogen mass to be useful. I couldn’t remember how far I took this.

This is not a precise calculation as there are far too many variables that cannot be precisely determined at this time, but enough information is available to see if we might have a chance of getting close.

The easiest way to address this was to consider first the typical NOx emissions from a diesel engine. I find a range of 2.5 to 6 grams NOx per mile, depending on the size of the engine and the mileage. For large diesels, 6 grams per mile or ~ 3.5 moles of N per gallon of fuel seemed to be the best number to use for engines meeting the current or recent emissions standards. The majority of the NOxs produced also seem to be NO and NO₂, both of which go to nitric acid when combined with atmospheric moisture via the path

2 NO₂ + H₂O → HNO₂ + HNO₃
3 HNO₂ → HNO₃ + 2 NO + H₂O
4 NO + 3 O₂ + 2 H₂O → 4 HNO₃

Which is how we get acid rain. The dissociated NO₃^- is then taken up by the algae.

There are a number of assumptions made here. The first are the ratios of NO to NO₂, which I took to be 50% each of the total moles of NOX produced. Also assumed is that these are the only oxides of nitrogen that are significant as a percentage of the total.

The required mass of nitrogen per gallon of water was based on the recommended standards using a commercial liquid algae fertilizer. I show this to be approximately 1.8 grams of N per gallon of water, per batch. Assuming batch cycles of once a month, seven doubling periods with adjust water volumes, using 8 inches of water as a maximum level, and assuming that we are using the standard V ditch, approximately 32,000 gallons of water are required per acre. [note that we have about three times the water, but this assumes that we starve the algae for nitrogen at the end of the life cycle in order to increase the % oil yield by weight]

If we assume 6000 gallons of fuel produced per acre year, and 40% of that is required for power generation, we expect to generate something around 700 moles of nitrogen, or enough N for just over 2700 gallons of water per batch, or just under 10% of the nitrogen required.

So we would appear to be at least in the proper order of magnitude. Also, since a great deal of effort has been made to reduce diesel NOx emissions, it would seem that yields might be increased significantly if we seek to increase emissions.

Edit/correction: Note also that it was not entirely clear if we are considering the grams of N required, or the grams of nitrate required for the algae. If we are talking about grams of nitrate, then the results are far more favorable. I now seem to recall that the industry standard is to specify the mass ratio of nitrate, as for a 15-2-0 liquid fertilzer in this case - 10mL per gallon of water, and 1.2 grams per mL. This would push our result to approximately 40% of the required nitrogen without making any modifications to the engine.

Key point: By increasing the compression ratio of the engine, our NOx emissions increase, which in turn should allow us to increase the return on free nitrates for the algae. We also increase the thermodynamic efficiency of the generators and reduce the operating energy costs. NOx production can also be increased by making ajustments to the injection timing, which may or may not be beneficial to energy costs, and also by adjusting the size of the fuel particles in the combustion chamber, which would likely reduce the efficiency of the generator.

 

[mheslep]

Since the idea of using the generator as a nitrogen source was mentioned, I thought I had better do some seat-of-the-pants calculations to see if we are even in the proper order of magnitude of nitrogen mass to be useful. ...

Interesting overview. I take it then that most of the nitrogen would end up end the waste stream at every harvest? (I understand this doesn't mean an actual landfill). If so, this really illustrates the advantage of the Exxon/Venter synthetic strain that supposedly excretes the oil, the cell stays otherwise intact, thus no nitrogen replenishment required.

 

[Ivan Seeking]

Interesting overview. I take it then that most of the nitrogen would end up end the waste stream at every harvest? (I understand this doesn't mean an actual landfill). If so, this really illustrates the advantage of the Exxon/Venter synthetic strain that supposedly excretes the oil, the cell stays otherwise intact, thus no nitrogen replenishment required.

Perhaps. If a viable strain can be made to excrete the oil, then the advantages in harvesting alone are enormous. Note however that having oil-excreting algae does not automatically mean that we have a viable strain. Nor would one expect the algae cells to be perpetual. It seems a bit much to hope that we could charge the system and close the lid forever. I also wonder about viability of any continuous-yield [perpetual] system. The best information that I have is that no such system has ever proven to be reliable. Bypassing the centrifuges and presses [or the supercritical extraction system, or whatever] is one thing. Making a system perpetual is another.

But I don't know that the nitrogen is a bad thing. I would imagine that this is really more a question of the economics of biomass than an environmental question of nitrogen. If we can use the algae biomass as animal feed, in turn helping to feed the hungry of the world for example, not to mention keeping my BBQ steaks coming, it could be a good thing.

...
concentrations than those of any other nutrient except carbon, hydrogen and oxygen,
nutrients not of soil fertility management concern in most situations. Nitrogen is an
important component of many important structural, genetic and metabolic compounds in
plant cells. It is a major component of chlorophyll, the compound by which plants use
sunlight energy to produce sugars from water and carbon dioxide (i.e. photosynthesis).
It is also a major component of amino acids, the building blocks of proteins. Some
proteins act as structural units in plant cells while others act as enzymes, making
possible many of the biochemical reactions on which life is based. Nitrogen is a
component of energy-transfer compounds, such as ATP (adenosine triphosphate) which
allow cells to conserve and use the energy released in metabolism. Finally, nitrogen is a
significant component of nucleic acids such as DNA, the genetic material that allows
cells (and eventually whole plants) to grow and reproduce. Nitrogen plays the same
roles (with the exception of photosynthesis) in animals, too. Without nitrogen, there
would be no life as we know it...

http://www.rainbowplantfood.com/agronomics/efu/nitrogen.pdf

In nature, nitrogen fixers are considered to be a good thing. Just think of the generator engine as a giant legume.

 

[mheslep]

Perhaps. If a viable strain can be made to excrete the oil, then the advantages in harvesting alone are enormous. Note however that having oil-excreting algae does not automatically mean that we have a viable strain. Nor would one expect the algae cells to be perptual. It seems a bit much to hope that we could charge the system and close the lid forever.

No individual cell lives forever - the idea would be that in a stable tank the replication rate is in equilibrium with the death rate.

But I don't know that the nitrogen is a bad thing. ...

Oh, I don't mean that normal nitrogen levels are at all bad for the environment. But solely from an economic break-even take on oil from algae, keeping the required nitrogen in a continually harvested system is a cost, whether it comes from direct fertilizer injection or some other (NOx) clever method. I'm just looking to minimize the costs.

 

[joelupchurch]

I came across a article that claims that a MWh of power from Soy based Biodiesel consumes over 180,000 liters of water. I thought this might make an interesting contrast to Algae based Biodiesel that doesn't require fresh water.

http://spectrum.ieee.org/energy/environment/how-much-water-does-it-take-to-make-electricity"

 

[mheslep]

I came across a article that claims that a MWh of power from Soy based Biodiesel consumes over 180,000 liters of water. I thought this might make an interesting contrast to Algae based Biodiesel that doesn't require fresh water.

http://spectrum.ieee.org/energy/environment/how-much-water-does-it-take-to-make-electricity"

Those hydrogen atoms in hydrocarbons have to come from somewhere. One mole of, e.g, CH4 requires two moles of H2O, minimum.

 

[turbo]

It might be possible to extract nitrogen from diesel exhaust by running the exhaust through a scrubber. Envision a vertical SS cylinder filled with (ceramic?) substrate. Feed the exhaust into the bottom and let it propagate upward as it travels through and around the ceramic, which is constantly wetted by water sprays at the top of the scrubber. The water should pick up the nitrates, which can then be pumped to the reaction trenches.

We used similar scrubbers in the pulp mill where I was a process chemist. The substrate was ceramic molded into the form of half-cylinders with pinched centers and flared edges with ridges. Lots of surface area, little resistance to gas-flow.

 

[Ivan Seeking]

What is the potential financial value of diesel-generated nitrates? NaNO₃ is apparently the preferred form of nitrate fertilizer for algae.

Based on a 22 ton [metric] minimum order, I received a bid of $545/mt for NaNO₃ [85% pure]. As is always true, hopefully someone will check my math, but based on 1.8 grams of NO₃ per gallon of water, and 32,000 gallons of treated water per acre-month, I get a price of about $650 per acre-year.

Recall that based on the assumptions made, our gross revenues are approximately $5400 per acre-year. Just the cost of nitrates accounts for 12% of our gross revenues. This brings our budget down from 12 cents per sq-ft per year, to 10.5 cents per sq-ft per year.

 

[turbo]

You shouldn't have to buy commercial fertilizer to get the nitrates, though. As long as the algae waste is not intended for human consumption, you might be able to use waste-treatment plant sludge. Farmers around here are loathe to use it, even to fertilize silage crops for dairy farms, and it's got to get disposed somewhere. You might even get it for free, because it saves the municipalities owning the plants the tipping fees associated with landfilling the waste. Sludge is often pressed to reduce water content (thus weight), since tipping fees are generally based on load weight. If you can accept unpressed sludge, you save the municipalities the cost of dewatering the sludge AND the tipping fees associated with landfilling it.

 

[Ivan Seeking]

You shouldn't have to buy commercial fertilizer to get the nitrates, though. As long as the algae waste is not intended for human consumption, you might be able to use waste-treatment plant sludge. Farmers around here are loathe to use it, even to fertilize silage crops for dairy farms, and it's got to get disposed somewhere. You might even get it for free, because it saves the municipalities owning the plants the tipping fees associated with landfilling the waste. Sludge is often pressed to reduce water content (thus weight), since tipping fees are generally based on load weight. If you can accept unpressed sludge, you save the municipalities the cost of dewatering the sludge AND the tipping fees associated with landfilling it.

That begins to complicate matters wrt issues of purity, process efficiency, etc, but one long-term goal is to use algae farms as nitrate sinks. It potentially makes pollution cleanup profitable.

The other things to keep in mind are the scale of a large algae farm, and the supply and demand cost curve. What is the nitrate requirement for several million acres of algae, which is what we need before we begin to make a dent in the fuel market? I was caught several times by seemingly good source solutions that were dwarfed by the demand for a real operation.

 

[turbo]

The other things to keep in mind are the scale of a large algae farm, and the supply and demand cost curve. What is the nitrate requirement for several million acres of algae, which is what we need before we begin to make a dent in the fuel market? I was caught several times by seemingly good source solutions that were dwarfed by the demand for a real operation.

Obviously, for this to work, the algae production facilities need to be diffuse (spread all over the place) in order to have access to cheap nitrate-rich waste. You'd also need to site the facilities near major cities, to get the best bang for the buck, which would increase the cost of the land - suburban property tends to be expensive. Of course, if the government got involved and subsidized the transport and use of the sludge, that could change, but you'd have to have some pretty impressive pilot programs running to convince Congress to cough up the bucks.

Over 20 years back, I did a plant evaluation for an ethanol plant in Iowa prior to bidding on the creation of a system analysis and training materials. I expressed my skepticism about the economic viability of the operation, and was told by a manager that without Federal subsidies, the plant would never even break even. There is was - a small chemical plant in the middle of corn country that was designed, built, and operated entirely on subsidies based on the premise that diverting valuable food from human consumption and animal feed could fuel our vehicles.

 

[Ivan Seeking]

I do think it is important to remember that should it be practical, generating pure nitrate onsite is likely the ideal solution to the nitrogen problem. One way or another we produce tons of the stuff. We can also increase in our generator efficiency. Otherwise, the only practical option that I could see TODAY was to buy sodium nitrate for $550 a ton.

The use of fish, sludge, waste water, fowl and cattle manure, agricultural runoff, or other waste products, introduces a whole new set of problems. I would expect that those sorts of applications could differ significantly from a high-yield farm aimed at fuel production. Again, the first thing a biologist wants to do is to put everything in an autoclave for 24 hours. That was not an exaggeration.

 

[turbo]

I do think it is important to remember that should it be practical, generating pure nitrate onsite is likely the ideal solution to the nitrogen problem. One way or another we produce tons of the stuff. We also increase in our generator efficiency.

The use of fish, sludge, waste water, fowl and cattle manure, agricultural runoff, or other waste products, introduces a whole new set of problems. I would expect that those sorts of applications could differ significantly from a high-yield farm aimed at fuel production.

That being the case, it should be a solid case for NOx scrubbers. They are already in wide usage, and your major costs would be in buying the scrubbers, running the water-showers, and pumping out the sumps to the trenches. Not real high-tech, but I just don't know if your generating facilities (diesels) could generate enough nitrates to feed the algae.

 

[mheslep]

The other things to keep in mind are the scale of a large algae farm, and the supply and demand cost curve. What is the nitrate requirement for several million acres of algae, which is what we need before we begin to make a dent in the fuel market? I was caught several times by seemingly good source solutions that were dwarfed by the demand for a real operation.

Well for this to work at large scale, algae biodiesel has to start displacing corn ethanol. So demand for nitrates should simply shift from corn to algae.

 

[mheslep]

I do think it is important to remember that should it be practical, generating pure nitrate onsite is likely the ideal solution to the nitrogen problem. One way or another we produce tons of the stuff. We can also increase in our generator efficiency. Otherwise, the only practical option that I could see TODAY was to buy sodium nitrate for $550 a ton. ..

At full scale, full replacement of petroleum scale, I assert you have no choice but to recycle most of that nitrogen. It has to be mostly a closed cycle over 20 million acres.

 

[Ivan Seeking]

Well for this to work at large scale, algae biodiesel has to start displacing corn ethanol. So demand for nitrates should simply shift from corn to algae.

I have no idea how agricultural demands compare to algae farming on a tons/acre-year basis, but I see algae as an entirely new demand for nitrates. It would be interesting to go back to global demands for fuel and the required acreage of algae, and see if the nitrate supply for this even exists.

I seem to recall that we once estimated that it would take 40,000 sq miles of algae to entirely replace the US petro supply. That would be something over 25 million acres, so we might expect a need for something like 30 million metric tons of nitrate per year.

The chemistry of nutrient recovery does seem to be a critical consideration. This would especially be true for a farm designed to produce electrical power using a completely closed system.

 

[Ivan Seeking]

That being the case, it should be a solid case for NOx scrubbers. They are already in wide usage, and your major costs would be in buying the scrubbers, running the water-showers, and pumping out the sumps to the trenches. Not real high-tech, but I just don't know if your generating facilities (diesels) could generate enough nitrates to feed the algae.

Considering the baseline estimates, I am encouraged. This goes back many years for me but I do believe NOX production is easy to increase significantly - maybe even double or triple. It would require that engines be modified or special builds, but it is fairly easy to do. There may even be a brand of marine engines or generators that are exempt from emissions standards and still use the higher compression ratios. For example, I could easily see the military using something like this.

 

[chemisttree]

Solar energy could be used to convert N2 into nitrate. I have a bit of information if anyone is interested.

 

[mheslep]

I have no idea how agricultural demands compare to algae farming on a tons/acre-year basis, but I see algae as an entirely new demand for nitrates. ...

I can't see how. For algae biofuel to work, it must eventually become more cost effective than corn biofuel. At at ~40X higher yield per acre for algae this is surely possible. Edit: main point is that at full scale one has to keep track of where your nitrogen molecules go. You can't just throw them away. Thus while the nitrate requirements probably are right for start up, later they should decrease as the recycling takes place, somehow.

 

[mheslep]

[...]
I seem to recall that we once estimated that it would take 40,000 sq miles of algae to entirely replace the US petro supply. That would be something over 25 million acres, so we might expect a need for something like 30 million metric tons of nitrate per year.

I reran the numbers for an internal energy group the other day - 29m acres / 45k sq miles at 10,000 g/a-y, for 2009 US petro total all sinks. Edit: 5x that using Exxon 2000 g/a-y numbers. Compares to 93 million US acres planted just for corn.

 

[Ivan Seeking]

I reran the numbers for an internal energy group the other day - 29m acres / 45k sq miles at 10,000 g/a-y, for 2009 US petro total all sinks. Edit: 5x that using Exxon 2000 g/a-y numbers.

Hopefully they were being unduly conservative. I am hoping for a multiplier no worse than 3. But given your check of the information, I would bet that we need a minimum of 100 million acres, with a little over one mt of nitrate required per acre-year.

Compares to 93 million US acres planted just for corn.

This acreage is currently dedicated to corn?

 

[Ivan Seeking]

Did you include efficiency gains for gasoline use? Note that we get something like a 0.75 multiplier on the energy demand by converting gasoline use to biodiesel use - i.e., the efficiency of diesel compared to gasoline engines.

 

[Ivan Seeking]

Solar energy could be used to convert N2 into nitrate. I have a bit of information if anyone is interested.

Those nitrates are a little hard to scoop-up, aren't they?

Can you offer any thoughts as to the reaction time required and the efficiency [completeness] of the reaction, going from NO or NO2, to HNO3, in a scrubber?

We also high high concentrations of CO and CO2 to in the exhaust stream. Carbonic acid is good. In fact, one typically adjusts the pH level of the water by increasing the available CO2 through aeration. But I have no idea what potential interactions there might be with CO, CO2, NO, and NO2 reacting in unison with water.

As an aside, note that we also recapture 40% of our water lost to hydrocarbon production.

 

[chemisttree]

Those nitrates are a little hard to scoop-up, aren't they?

Can you offer any thoughts as to the reaction time required and the efficiency [completeness] of the reaction, going from NO or NO2, to HNO3, in a scrubber?

The "nitrates" are soluble. In the process air is pumped through a silica substrate doped with some transition metals. In the sun it produces ammonia, not nitrates. Sorry for that.

I don't have any information about reaction rates for NO to nitrate. I imagine that would be pretty straightforward to find though.

Nitric acid is produced by bubbling NO2 into water. NO is produced in this process and is rapidly oxidized to NO2 which is then captured by water. It seems that the scrubbing/aeration processes already in use for HNO3 could be adapted for scrubbing NOx but I'm not sure how the process would be affected for ppm levels of NOx compounds.

 

[mheslep]

..This acreage is currently dedicated to corn?

Yes just to corn, 2007 peak. It's down a bit now.
http://www.nass.usda.gov/Charts_and_Maps/Field_Crops/cornac.asp
From the chart spike one can guess 10-15 million acres is dedicated to ethanol.

 

[mheslep]

Did you include efficiency gains for gasoline use? Note that we get something like a 0.75 multiplier on the energy demand by converting gasoline use to biodiesel use - i.e., the efficiency of diesel compared to gasoline engines.

No I did not include that. I used current US consumption 291 billion gallons per year, 2009, that's unrefined petroleum. Imports are ~60% of that.

 

[Ivan Seeking]

The "nitrates" are soluble. In the process air is pumped through a silica substrate doped with some transition metals. In the sun it produces ammonia, not nitrates. Sorry for that.

Ah, actually, I thought you were joking. My mistake.

 

[Ivan Seeking]

Yes just to corn, 2007 peak. It's down a bit now.
http://www.nass.usda.gov/Charts_and_Maps/Field_Crops/cornac.asp

That's a great point of reference. We need about as much algae acreage as we already have dedicated to corn.

 

[Ivan Seeking]

Here is an interesting doc that I found by chance. It provides a very practical baseline for comparing the economics of algae farming [based on the discussion here] to food crop farming.
http://www.cias.wisc.edu/curriculum/modV/secd/Costs and Returns worksheet.doc

I noticed a few specific numbers in the examples, that were interesting.

 

[chemisttree]

Ah, actually, I thought you were joking. My mistake.

No joke, really!

http://www.pnas.org/content/80/12/3873.full.pdf

 

[Ivan Seeking]

The design tends to migrate towards the notion that the heart of the bioreactor is really the generator. Note again that if we only want to generate electrical power, the system might be closed with all water and nutrients preserved, though one always expects that there are some practical limitations and losses.

One business strategy considered was that agreements could be made with power companies to sell power to the grid, esp during peak demand periods, in order to help offset fluctuations in fuel prices. In the limit this could be accomplished by over-sizing the generating capacity of the farm by 250%. Assuming 6000 gallons per acre-year with a net yield of 60%, we get 3600 gpay. At 125KBTUs per gallon, 40% efficiency for the diesel, 90% efficiency for the generator, we get 162 MBTUs, or 47,500 KW-Hrs per acre-year. Given this and our assumed maximum operating cost of $5400 per acre-yr, we can break even about 11 or 12 cents per KW-Hr. With this in mind, it might make sense to locate a farm near a area with a growing demand for relatively expensive electrical power. By accepting the up-front equipment costs, we can help to at least recover operating costs should the price of fuel drop significantly below 3 or $3.5 a gallon. It was a risk management strategy. It is also a way to recover costs if you have millions of gallons of oil that for some reason don't meet the fuel standards. But it is also interesting to know when we might begin to sell electrical power.

Given Exxon’s numbers we would seem to be in the range of 20 cents per KW-Hr.

Another observed advantage in using a simple or modified V ditch having a 45 degree or similar slope, is that algae growth might be maximized during periods of low light levels, as follows: As I understand all of this, by increasing the volume of water according to an ideal optical set point, we maximize the number of algae cells optically active at any time. The only active cells are those at the surface of the water, so the density of the cells at the surface determines the rate of photosynthesis for the system. If the optical density is too great, the cells in effect begin to shade each other, which is what happens as the system approaches equilibrium. If the density is too low, we aren’t maximizing the exposed area of the reactor. By using a V shape and adjusting the water volume, we reduce the surface area of the water, rather than the time spent at the surface by any particular algae cell - we reduce our energy input to the system either way. This is reasonably compatible with our cause because have less algae [mass per unit area] to “power” in the early stages of growth. However, if we use a white or even a clear plastic liner, and if the reactors are oriented properly wrt the sun, we get reflected sunlight from the walls of the reactor into the water. Much of this is due to semi-specular reflection. This can be true for much of the day and year, again, depending on the orientation of the reactor, the latitude, and the slope of the walls. If for example we only have one inch of water in an eight inch reactor using a 45 degree slope, we have one-eighth of the exposed area as compared to the total possible. But we might hope to get an average adder of [8 x 0.1 or 0.2], or an 80-160% gain, for example, for the total incident light acting on the algae.

Likewise, it would seem that during the summer months, simple shading tricks like that mentioned earlier could be used to provide a surprising degree of control over the amount of light incident on the algae. This would need to be targeted according to the strain or strains of algae selected and the associated light preferences and limits.

 

[turbo]

Regarding shading - if you are introducing nitrogen-rich water from scrubber-sumps via perforated pipes lying on the bottom of the trenches, you may get enough up-welling to mix the algae and optimize light exposure. Of course, if you've got thick mats of algae at the surface of the water in the trenches, that might not hold true.

 

[mheslep]

The design tends to migrate towards the notion that the heart of the bioreactor is really the generator. Note again that if we only want to generate electrical power, the system might be closed with all water and nutrients preserved, though one always expects that there are some practical limitations and losses.

One business strategy considered was that agreements could be made with power companies to sell power to the grid, esp during peak demand periods, in order to help offset fluctuations in fuel prices. In the limit this could be accomplished by over-sizing the generating capacity of the farm by 250%. Assuming 6000 gallons per acre-year with a net yield of 60%, we get 3600 gpay. At 125KBTUs per gallon, 40% efficiency for the diesel, 90% efficiency for the generator, we get 162 MBTUs, or 47,500 KW-Hrs per acre-year. Given this and our assumed maximum operating cost of $5400 per acre-yr, we can break even about 11 or 12 cents per KW-Hr. With this in mind, it might make sense to locate a farm near a area with a growing demand for relatively expensive electrical power. By accepting the up-front equipment costs, we can help to at least recover operating costs should the price of fuel drop significantly below 3 or $3.5 a gallon. It was a risk management strategy. But it is interesting to know when we might begin to sell electrical power. ...

11-12 c / kWh sounds overly optimistic, but I don't see anything wrong with your figures. Note that's the bus-bar, or wholesale price of electricity. Even large industrial customers would pay an extra 20%, double for residential daytime.

We discussed this before - there was that Science paper showing more useful energy delivered if biofuels went directly to electricity generation, and they (Science) didn't even address the increased efficiencies you identify in such a closed system. One problem, however, is that direct solar thermal generation plants will necessarily always beat algae in efficiency per acre. Algae is a great organism for photosynthesis (the best?) but it can not match man made solar collection per acre.

At the same time algae has the advantage of providing a direct method to store the solar energy indefinitely as diesel, where as solar arrays catch a week of snow or clouds and shut down. That is, algae can 'do baseload', solar concentrators not, not more than ~24 hours. Given these pros and cons, its not clear to me whether algae biofuel works better economically as a transportation fuel or for electric generation.

 

[Ivan Seeking]

11-12 c / kWh sounds overly optimistic, but I don't see anything wrong with your figures. Note that's the bus-bar, or wholesale price of electricity. Even large industrial customers would pay an extra 20%, double for residential daytime.

The key assumptions are $5400 per a-y [12 cents per sq-ft per year] total operating costs, including all amortized costs, and 3600 gallons per a-y net yields.

We discussed this before - there was that Science paper showing more useful energy delivered if biofuels went directly to electricity generation, and they (Science) didn't even address the increased efficiencies you identify in such a closed system. One problem, however, is that direct solar thermal generation plants will necessarily always beat algae in efficiency per acre. Algae is a great organism for photosynthesis (the best?) but it can not match man made solar collection per acre.

In large part it comes down to the cost per KW-Hr. Algae likely can't compete with the $10k per sq-meter solar cells on the Mars rovers either.

At the same time algae has the advantage of providing a direct method to store the solar energy indefinitely as diesel, where as solar arrays catch a week of snow or clouds and shut down. That is, algae can 'do baseload', solar concentrators not, not more than ~24 hours. Given these pros and cons, its not clear to me whether algae biofuel works better economically as a transportation fuel or for electric generation.

The two goals do not appear to be mutually exclusive. In fact it would seem that any algae farm is a hybrid of both. Looking at the economics of things, especially until this is a proven technology, one expects to produce substandard fuels from time to time. Having the ability to recover those losses by selling electrical power is pretty easy to justify. So the ideal generating capacity of any farm is an interesting issue to explore.

 

[Ivan Seeking]

Regarding shading - if you are introducing nitrogen-rich water from scrubber-sumps via perforated pipes lying on the bottom of the trenches, you may get enough up-welling to mix the algae and optimize light exposure. Of course, if you've got thick mats of algae at the surface of the water in the trenches, that might not hold true.

That's where it gets a little tricky. It is easy to kill the energy budget with aeration. I was landing on a compromise between aeration duty cycles and volumetric flow rates, and mechanical circulation - a simple rotating cylinder [pipe], perhaps one inch in diameter with simple paddles extrusions, that rests in the bottom of the V just above the aeration pipe, and turns very slowly, or at least with a very small average demand for energy.

It seems most efficient to use aeration only to the extent that required CO2 is delivered to the system, and to satisfy the circulation requirements mechanically. But this issue was never resolved to my satisfaction. It is a very difficult practical problem. One has to count the watt-minutes very carefully.