Biofuels are crops grown for their energy. They have one important difference compared to other forms of renewable energy: they include their own storage. However, they do require some elements of processing to make them suitable for use.
Because of this, and because biofuels require acreage that would otherwise be used for something else, in general biofuels will be used as a backup for the other energy sources.
In particular, the liquid fuels - vegetable oil and alcohol - are used to generate mechanical and electrical energy, and so the size of the crop grown will depend only on the amount used in the previous year. If none was used, none will be grown.
Wood is only really suitable for heating. However, wood can also be used to make charcoal, which has a few other useful properties.
It is anticipated that some wood will be generated by normal farming activities: cutting hedges and pruning fruit trees will both produce a significant amount of wood. If we end up with sea frontage there will also be driftwood, and if there is involvement in the building trade that will generate a certain amount of wood scrap which can be burned.
Coppicing is the process of letting trees grow for maybe five years, and then cutting them right back to let them re-grow. As such, coppicing requires some land dedicated to growing trees.
The obvious candidate trees for the part of the world we are interested in are alder, which will spread over bare rock; and willow, which will grow in places too damp to grow other things. Alder typically yields 6 to 7 tons per acre, and willow typically yields 3 to 5 tons per acre.
All of this wood needs to be cut into small enough chunks for the burner used, and then stored long enough to dry: maybe up to a year.
The intended use of wood is in two ways: burning it in a wood-burning kitchen range, to provide cooking, hot water and space heating; and use in an open fire, to provide some heat and also to provide ambience for the living room. A back boiler in the open fire will extract some heat into the heat store, for hot water and for heating the rest of the house.
A wood-burning kitchen range like a Rayburn Heatranger 355M delivers about 16.1kW; or a Stanley Super Star delivers about 17.6kW. An hour of this provides about 20% of the daily average heat requirement for a winters' day; or enough energy to heat 460 litres of hot water from 10oC to 40oC.
It is only anticipated that the open fire will be used when the weather is cold. The heat store and underfloor heating should provide most of the winter heat, but it might be nice to have a fire going too. The open fire is not efficient, so any use of this should be considered to be luxury rather than necessity.
It is anticipated that the kitchen range will be used primarily for cooking. In this use it would be lit maybe half an hour before cooking begins, and allowed to burn out shortly after cooking ends. During this time it would be putting heat into the heat store.
If a prolonged period of calm overcast results in the heat store becoming cold, then the kitchen range (and, to a lesser extent, the open fire) can be used to put heat into the heat store. Five hours of the range should provide all the heat for an average winters' day, and if the heat store stores five days' heat, it takes a little over a day of continuous operation to get it fully heated.
In practice, since a range needs to be loaded with wood by hand, the likely use is going to be to burn for a few hours, enough to provide another day or three, and then go back to normal use for the next few days.
Oil may be grown using a number of crops. Olives, if we can persuade them to grow, will give us 1019kg per hectare; if not, sunflowers will give 800kg per hectare. Both of these oils are good eating, which is an important detail about oil: a litre of oil provides about 6200 dieticians Calories, enough to feed someone for three or four days. That's an interesting backup energy plan: it wouldn't be healthy eating, but you wouldn't starve.
Alcohol is made from carbohydrate: the best yielding crop for carbohydrate is potatoes, which yield about 14 tonnes per acre, or about 3500kg of carbohydrate for the best varieties. Potatoes grow well in the territory we're thinking of moving to.
Alcohol and oil are used, along with sodium hydroxide, to make biodiesel.
In all the obvious crops, oil is obtained by pressing it out of the stock. For use in a diesel engine, it should be filtered.
The oil may be dried and sterilised prior to storage by heating it to 200oC.
The conversion from carbohydrate to alcohol is done using the following chemical process:-
C6(H2O)6 -> 2C2H5OH + 2CO2
So a mole of carbohydrate (RMM = 180) produces 2 moles of ethanol (RMM=46). So that's a bit less than 50%: 3500kg of carbohydrate will only result in 1700kg of alcohol.
The alcohol must be dried using a distillation process. If it is to be used in alcohol burning stoves and engines, this 95% pure alcohol is probably sufficient: if it is to be used in biodiesel manufacture, it must be further dried with zeolite or quicklime.
Drying the alcohol means distilling it from the 12% to 18% solution left by the yeast; and then removing the last 5% or so using a drying agent. The drying agent that would be available to the self-sufficient farmer is quicklime, obtained by burning seashells, chalk, marble or limestone.
Converting oil to biodiesel is a complex process, and one that is moderately wasteful. Commercial biodiesel is an ester of a fatty acid and methanol, but the farm biodiesel will be an ester of a fatty acid and ethanol. Methanol is harder to make on the farm, and is rather poisonous, making it dangerous to handle.
The main reaction is:-
oil + ethanol -> biodiesel + glycerine
C3H5-(C18H37O2)3 + 3C2H5OH -> 3C2H5-C18H37O2 + C3H5(OH)3
So a mole of oil (RMM = 896) plus three moles of alcohol (RMM = 46) produces three moles of biodiesel (RMM=314) plus one mole of glycerine (RMM=92). The raw materials are 87% oil and 13% alcohol; the products are 91% biodiesel and 9% glycerine.
The reaction occurs in an alkaline environment; dry caustic soda is dissolved in absolutely dry alcohol, and mixed with the dry oil. The glycerine separates out from the ester, which is the biodiesel. The presence of any water at all allows the glycerine and ester to mix, and they don't separate out.
To create an alkaline environment, sodium hydroxide is used. The best source of this is the electrolysis of seawater in a divided vessel, ensuring the current densities in the bridges prevent the wrong ions from making any progress. The byproduct of this reaction is chlorine bleach, which is useful in brewing for sterilisation.
Alternatively, for now at least, quicklime (or zeolite), caustic soda and bleach are all easily obtained from civilisation.
The normal uses for these liquid biofuels are to run engines, and to run lamps.
The engine for using straight vegetable oil is the Lister CS stationary engine. This is a water-cooled single-cylinder engine designed in the 1920s, and known for reliability. It is governed at 600RPM, so a 10-pole permanent magnet motor on it might generate 3kW to 4kW of mains electricity for charging batteries or running appliances.
It turns about 20% of the energy of the fuel into useful work, and produces about 4kW at the shaft: so with a heat exchanger on the exhaust it might be expected to recover 80% of the remaining 16kW of heat. So it might be expected to put 12.8kW into the heat store when running at full power.
It runs typically for 1000 hours between major services - about 40 days continuous operation. But if it were the only supply, it would be run only when major appliances were running, and the battery bank would provide the rest of the power. Even in winter it would only need to run for five hours to put 64kWh into the heat store and 20kWh into the batteries. In this duty cycle, it might be expected to last a year or more between services.
As well as running on straight vegetable oil it will also run on well-filtered waste oil, as well as mineral diesel, parrafin or heating oil.
Lister-designed engines are available new from a number of importers, as they are manufactured in India.
However, the Lister engine is unlikely to be used in normal operation other than as a range-extending trailer for an electric car. In this mode, it doesn't contribute anything to the heat store, as we don't have 200 miles of hose.
Vegetable oil will burn as-is, although with a rather sooty flame. Biodiesel will burn anywhere parrafin or heating oil would burn. In lamps, it's probably better than parrafin, because it will smell nicer.
For use just for heat generation, the Lister engine can be used to run immersion heaters in the heat store. But this should only be done when the battery bank is full: if the sun shines or the wind blows after the battery bank is full, the energy will only end up dumped in the heat store anyway.
These fall into two main categories: generators and transport devices.
Straight vegetable oil can be burned in the Lister CS engine.
Biodiesel can be used in a wide variety of engines, including an aero engine designed for parrafin. For transportation, a reserve of biodiesel might make a difference to our transport options.
The main application for fuel oil for backups is to provide electricity. The heating is better provided by wood, which produces more than double the energy for a given acreage.
Because gas systems are dangerous, where we live there is a strong regulatory framework around the use of gas appliances. This regulatory framework is there for a good reason - so there are no plans to implement any kind of methane system.
This page is some notes on Domestic Power from Renewable Sources, and is written and maintained by Simon. At this stage these pages are constantly under revision. Thoughts and comments are welcome.