Actually, there are only two main reasons why SVO (Straight Vegetable Oil) doesn't work directly in a Diesel engine. SVO has a lot thicker viscosity than Diesel and, especially if using waste SVO from a restaurant, there are particulate contaminants. There are ways to correct these problems which aren't really that expensive.
First, either use pure virgin SVO or use waste SVO that has been filtered. There are filters which are reusable and washable.
Second, heat the SVO to lower the viscosity. At 150F, SVO has about the same viscosity as Diesel. By using a heated tank and heating to ~180F, the SVO will still be above 150F when it reaches the Diesel engine. The heaters don't take that much energy from rechargable batteries in most areas of the country. Small solar panels to help recharge any extra batteries desired (but not necessary) can also be used.
Using a dual-tank setup as found on many vans and larger pickup trucks, one tank can be set as the feed tank and the other can be set as the fill tank. By putting additional filtering between the fill tank and the feed tank, the engine can run off of the virgin SVO, Diesel, or filter SVO in the feed tank, but the fill tank can be used to put straight waste SVO into. The filtering between the fill tank going to the feed tank will take care of any particulates and keep the waste SVO going into the feed tank properly filtered. Those filters just need to be cleaned periodically. Also, this method allows for virgin SVO or Diesel to be input into the feed tank. There isn't any problem mixing SVO and Diesel in this manner. If you're out and about and need fuel, you can put petro-Diesel right into the feed tank. Also, if you have a known good supplier of waste SVO, you can put it right into the fill tank. (NOTE: not all waste SVO is the same. There are things to watch out for when getting waste SVO from places. Bad waste SVO is worse than no waste SVO!)
An added plus to using SVO in this way is the high lubricating properties it has. Some studies suggest that running a Diesel engine on heated SVO actually increases the life of the engine because of this extra lubrication. Also, most of the information that I've read suggests that lose of power from running SVO rather than petro-Diesel is either nil or minimal.
This method doesn't require any modifications to the Diesel engine (but some engines don't do well running SVO... most are fine, but there are some exceptions such as the Cummins Diesels that were/are used in some Dodge trucks. Evidently they had a high failure rate for a certain pump that those engines use. There isn't any definitive proof that it was caused by the SVO, but never-the-less, those engines aren't currently recommended for trying this with.) Kits for these types of conversions are available.
Finally, if one wishes to farm the vegetables (soybeans, corn, rape seed, etc) to make fuel from, going the SVO route means NOT having the extra refining step of converting the vegetable oil to bio-Diesel. And if you want to go on a trip, just put some jugs of veggie oil in the back of your truck and go!
Fred, I heat my house soley with wood. While I have water-based heat-pumps for "conventional" heating and cooling, I also have wood stoves that have ductwork to other parts of the house and a wood/pellet furnace connected directly into the main ductwork for the house (in addition to the huge fireplace in the living room)... I heat nearly 2700 sq ft for less than $2k a year and, because we have a tendancy to "stoke the fire" a little too much sometimes, the house is usually in the 72F range in the winter. (I like it cooler, but when that wood gets burning, it gets hot fast!)