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There are a lot of buzz about algal biofuel. I'd like to use algae as a clean fuel for space heating. I wonder if there are technologies out there that convert algae directly to heat without having to extract oil first. I need something like an enzyme-catalyzed fermentation process that slowly releases the heat without polluting the air.

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Combustion. Algae will need to be dried first, of course, and you will need to adjust an oven to cleanly burn algae. Or you extract the oil first and combust that, but then you have a complicated process for a relatively simple task.

As you asked for fermentation, I doubt that's a way to go: While all or most fermentative processes produce some waste heat, most of the energy in the feedstock is used for cell growth. Alternatively you can digest the algae anaerobically, produce biogas, use the digestate as fertilizer for your algae, and heat your house with the gas. Again a complicated process for simple problem.

Algae are grown for oil because oil is a high grade fuel, easier transportable than gas and suitable for vehicles. I doubt that algae are a good option for space heating, as a far wider range of fuels and energy sources can be used here and a lot can be achieved with simple insulation.

  • Combustion would contribute to smog. – George Chen Feb 14 '14 at 11:10
  • no necessarily. Look up smog. – mart Feb 14 '14 at 11:16
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If you think of algae as heat engines -- turning sunlight into algae -- they will produce waste heat. The problem with the waste heat is the temperature. You will end up with a lot of low temperature algae soup. Indeed, the gain in temperature just from the sunlight is much larger than the waste heat of the algae.

Algae are a large fraction water. 80-90 percent at a guess. Let's look at the dry weight: Oils and fats are all about 9 kcal/gram. Carbohydrates and proteins both run about 4 kcal/gram. The fraction of ash in algae is very small. Under 1%. So if 10% of the dry weight is oil, then that accounts for close 10 20% of the energy value.

If you want to calculate: Consider a 100 gram sample. If 20% is oil, that 20 grams at 9 kcal/gram = 180 kcal, and 80 grams of carbohydrate and protein at 4 kcal/gram = 320 kcal. Total is 500 kcal. So the 20% oil accounts for 36% of the energy.

For conventional use, algae are selected for high oil production. Unfortunately high oil production means that the algae are making oil, instead of more algae. A mutation back to the original form will outstrip the population of the oil variant very quickly. Serious energy production from algae will require cleaning out a loop, and restocking it on a regular basis. This will become a technical management issue with journals and conferences.

To get energy out of algae there are several alternatives:

  1. Blend it, separate out the oil, use the rest as a feed stock for the algae loop. This oil can then be used as a feed stock for whatever fuel fantasies you can conjure up.

  2. Dry it. As a powder it is the equivalent of greasy sawdust. Burning this could be done using the same technology as coal fired generating stations that use powdered coal. This last one seems quite attractive to me. I wonder if the waste heat from the generating station is sufficient to dry the algae.

    One of the issues here is that with the loss of protein with the burned algae, you lose the nitrogen that the algae need to make more algae. In an ideal system you are only

  3. Compost it. The decompostion reaction produces heat. Done right this can supply water at temperataures up to 140 F. With suitable breeding of composting bacteria it may be possible to raise this some. This also has the nitrogen loss issue, as bacteria convert the nitrogen either to ammonia or to nitrous oxides, or to plain nitrogen.

removing CH chains from the system.

If yu are willing to settle for common algae, then you may be able to grow the algae in the cooling pond of the power plant.

Overall, however using algae as fuel is not a great idea. Far better to use it to make chicken feed, or rabbit feed. Or maybe as the coloring agent in Soylent Green.

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An interesting option would be to convert the algae to oil in the current manner, then to burn it for electricity, then use the electricity to power a heat pump. This is a form of co-generation, where the heat that would normally be wasted is treated as a useful output, but the electricity is directly used to generate (more) heat.

The reason for the added complexity is the gain in the heat pump. Roughly, the co-gen plant will put out 1/3 electricity and 2/3 heat. The heat pump will have a gain of 3-6, so you'll get between 3/3 heat and 6/3 heat from the electricity, for a total output of 5/3 to 8/3. In other words, for each kWh of combustable oil you get from your algae, you get between 1.6kWh and 2.6kWh of heat.

The question then is how much heat energy you lose by converting the algae to oil. This article suggests nearly 30% usable hydrocarbon by weight, but there's considerable heating required to extract them. HowStuffWorks discusses other methods, but it seems likely that 30% oil by weight will be an upper limit for a while yet. But the 70% remaining can probably be burnt or composted. The hydrocarbon content is likely to be the most energy-dense, so even though it's only 30% of the mass it'll probably have more than half the energy content (for example, even dry algae has some water content, and water doesn't burn).

Taking a guess at the overall cycle efficiency is hard, especially if we want to consider the energy cost of all the equipment. It's even more complicated because you wouldn't do this for one house, you'd do it on a municipal or national level. One honking great refinery, co-gen plants around the city and heat pumps in houses with piped steam to distribute the co-gen heat to nearby consumers.

But if we assume the energy cost of the equipment is low compared to the total energy involved, we could say 30% of the mass/50% of the energy becomes oil which is used at an overall efficiency of about 200%, and the remaining 50% of the energy is used at 100% (ie, burnt) that still gives an overall efficiency of 150%. Which is enough that it's likely to be better than just burning the algae at home (where you will have significant heat loss up the chimney)

  • I totally like the heat pump idea. But I still don't think oil is necessary. If we generate heat by combustion, we can use sterling-engine to power heat pump and bypass the oil and electricity stage. – George Chen Feb 16 '14 at 1:33
  • yes, but the practicalities of growing algae, drying it and moving it around mean that it's unlikely to happen on a large scale. And on a small scale, most co-gen units have quite specific fuel requirements. It could be done, definitely, but I fear the cost would be high. You might go through a number of co-gen burners before you find one that will work with algae, for example. – Móż Feb 16 '14 at 1:38

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