Now that the epic saga of the Project From Hell XIX, Return of the Spawn of the Scope Creep is over, it seems an appropriate time to gather my thoughts about other sources of next generation biofuels. One of the ones widely acknowledged to be slightly farther off, but promising, is the use of algae as a feedstock for so-called third-generation fuels. However, I'm very, very skeptical of any of the claims being made about algae (I spent about a year trying to work with the finicky little bastards in the lab and know their peculiarities) and even more so about extremely widespread algal biofuel cultivation. Here's why.
Algae is one of the holy grails of biofuel advocates. The algal approach to the resource scarcity problem is to simply look at overall biomass productivity, and observe that the biomass doubling time of an algae bloom is on the order of 1 week, regardless of culture size, whereas for even miscanthus giganteus, the sterile tallgrass hyrbid that grows faster than bamboo, it's significantly lower. Ditto for sunlight utilization, and, depending on the construction of your growth vessel, land use. All of this is fair enough; however, algal cultures might produce biomass, but they produce a different kind of biomass.
What can algal biomass offer?
Unlike lignocellulosic biomass, algae are not manufacturing cellulose or sugars. What they are producing is raw monocellular biomass, which is not easily fermentable. I've said that about woody biomass and lignocellulosic biomass in general, so let me clarify: there is no way to break this stuff up into sugars. What you will get as a result of destroying the cells is a mix of proteins, nucleic acids, phospholipids and cellular membrane - not good for feeding to yeast, the best ethanol-producing organisms out there, or anything else that depends on sugars for that matter. Detrivoric bacteria eat this, but what they spit out is not fuel. In order to engineer detrivores to spit out fuels, you'd have to be able to engineer pathways for every single conceivable feedstock. In addition, (more on this below), algal plants will almost always face severe septic control problems, so what's to prevent your hacked detrivoric bug, which will - guaranteed - be metabolically inferior to any naturally occurring detrivore, from being completely outcompeted at the first sign of contamination? And more importantly, if you're already (as is likely) engineering your algae platform, why hack two bugs instead of one?
This is why whatever you're going to get out of algae, you're not going to do it by digesting. It generally leads to another distinguishing feature of algal plays: the algae has to be both the feedstock and the product. So what's the product?
Most likely, oils. There are a lot of algal species out there that produce, relative to their weight, copious quantities of natural oils. Relatively high-producing species of algae will yield a good deal of oil. The dream of biofuels boosters is to produce biodiesel from this oil. There are a lot problems with that dream.
As I said before, algae are finicky little bastards. Their media is simple, but their growth requirements are difficult. They need light. They need to not be crowded. In order to grow, they need agitation or else they will form cell aggregates, followed by a death spiral from most of the culture. If they grow too much, optical density goes too high and they die. If pH changes, they die in droves. If too many dead cells are present, they also die from poisonous decomposition products. If you are growing algae in a carefully controlled laboratory environment, control is a breeze; however, if you are culturing on a scale necessary to produce fuels, it is much harder.
In general, it is also good idea to keep your culture alive and without competition. This is true in any bioprocess. Yeasts, obligate anaerobes and thermophiles usually have either great antibiotic resistance or the ability to live in media that are completely inhospitable (through pH or temperature constraints) to other organisms. For most bioprocesses, these are used to prevent the growth of competing cultures that might, say, eat the sugar meant for turning into ethanol and turn it into lactic acid instead.
For algae, this is different. Algae also have a lot of predators and cannot, to my knowledge, tolerate microzoocides. They also have no defense mechanism; their weapon in nature is sheer abundance and dispersion. In an enclosed environment with ridiculous algal density, if a single herbivorous zooplankton or mollusk larva manages to make its way into the photobioreactor or open pool, you lose your culture. Game over.
Processing is Expensive
While for many other bioproducts, processing requires new technology but thereafter can be run relatively cheaply, with microalgae you run into problems that are technically feasible but difficult to do cheaply. In order to harvest algal oil, you must
- Harvest the algal culture (draining the tank or photobioreactor)
- Separate the algae from the media (settling, followed by centrifuging the media)
- Destroy the algae cell walls to release the oil (sonication, heat treatment or pH adjustment)
- Extract the oil from the raw lysate (generally, solvent extraction with hexane followed by evaporation of the hexane).
Feedstock is Expensive
This one sounds really counterintuitive: after all, all you need is sunlight, water and air, right?
Well, turns out, sunlight is not costless. While the marginal cost of sunlight is zero, the up-front cost you need to pay is for the land on which to collect that sunlight is very significant. Think of it this way: if you want to make algal fuels, you are converting sunlight to energy at a rate of efficiency far, far below that of a 13% efficient polysilicon solar panel. So for an equivalent amount of liquid fuel energy from an algal plant compared to a solar plant powering an electric car, you will need at least several times the land, probably in the 10X-20X range. Additionally, since there are no merchant algae suppliers, the producer of the algae fuels will need to own or lease all of the land. In contrast, cellulosic biofuels manufacturers can operate off of a logging rights model or offer to harvest corn stover on others' land.
Water is also sometimes a problem. A vast amount of water is required to culture algae. Open pond areas lose a lot of water from evaporation. Areas with a lot of sunlight are also often water-poor. Water loss can be mitigated by constructing photobioreactors that condense and return evaporated water, but this adds to capital cost (see below).
Air is also expensive to get. Because algae grow so fast, they can run out of feedstock - that is, carbon dioxide - very quickly. Algae must be constantly aerated to maintain growth. That costs electricity and it adds up. It depends on your growth vessel - a photobioreactor will need sparging (essentially, bubbling) fed by compressors, while an open pond will need something akin to paddle wheels. It is also noteworthy to point out that the amount of carbon dioxide in air is low, so many algae hopefuls are planning on utilizing flue gas from nearby power plants instead. While this comes with more carbon dioxide, it's also not costless, as no power plant is giving away something that another manufacturer needs for free.
Growth Vessels are Expensive
Unlike other dedicated biomass producers like miscanthus or even wood, algae require specialized growing vessels. Because of this, capital costs for any algal enterprise are very high. The cheapest kind of growth vessel, the open pond, involves lining a shallow pit with plastics. Imagine the material costs of covering dozens of square miles with injection-molding grade polyethylene (think plastic forks). Now add on the fabrication costs, since specialized equipment is needed. Finally, recall that we are talking about huge land area, and that this is merely the simplest solution - in fact, most algal ventures are choosing full-on photobioreactors, since they make septic control easier, so at the very least double the figure compared to open ponds.
That's a lot of money. Not as much as covering the equivalent area with solar panels, but ouch.
So, will it ever work?
Since I've just spent a good amount of time heaping crud on the algae model, to put it mildly, you might be relieved to know that I think that there are some circumstances where algae growing as a commercial enterprise will work.
As I've said before, do not be fooled by high oil prices: fuels are low-value products. With algae, what you want to be making are specialty chemicals or food rather than fuels.
For all of their faults, algae still are one of the most productive biomass sources around. Compared to, say, corn, they produce as much biomass in a week as corn does in 9 months. For some types of algae all of that biomass is edible, whereas with corn we take only a small fraction of the plant. It should be no surprise that algae for food has been commercially viable for decades. Just look in your local store and find Spirulina dietary supplements - those same oils that people dream of turning into biodiesel are rich in omega-3 and omega-6 fatty acids, and carry with them a nice protein rider too.
There are also some companies using algae as their organism for producing chemicals. Solazyme is the best example of this type of company. Rather than focusing on low value-added products, such as fuels, they are producing tailored oils. These could be used as lubricants, or extremely high-value nutrition supplements (imagine 100% omega-3 fatty acids), or what have you. The point is that they are targeting a higher value portion of the spectrum. They are also sidestepping many algal production issues by using only the high growth rate and oil production capabilities of algae, feeding them sugar instead of growing them by sunlight. It is certainly more expensive to do things that way, but you know what? Since they are producing ridiculously specialized products, they can afford it.
While there might still be some situations in which algae biofuels are viable - especially if there is government support, like free land, subsidies, loan guarantees, or support like carbon mitigation credits - I see very dim prospects for the long-term viability of algae. The antics of companies like Algenol, which has been flogging $1 a gallon ethanol from algae (apparently genetically modified) since I was a freshman in undergrad, are to me increasingly resembling a comedy routine.