A fuel's energy density is a measure of how much energy can be released from a given volume of the fuel. For example, the energy densities of coal and gasoline are both much higher than that of wood, because wood contains about 50% water, plus a substantial amount of other materials which cannot be burned. Thus, the energy released from burning a cubic foot of wood is much less than that derived from burning a cubic foot of coal or gasoline.
Low energy density for biomass leads to problems in gathering and transporting sufficient amounts of it to locations where the raw fuel can either be burned or processed. Biomass tends to be bulky and inconvenient to handle, contributing to the expense and difficulty of using it as a source of fuel.