I found something interested on last weekends Edmonton Eco Solar tour: A wood fired fireplace / boiler. Made by Wallnofer (from Germany, I believe) and distributed in Canada by Power Strength & Energy Solutions Ltd. (PSES, http://www.pses.ca), the boiler has two combustion chambers. When a control arm of the side of the boiler is opened, an internal plate is lifted and the upper chamber of the boiler behaves like a normal fireplace. This setting is used for 8 or 10 minutes after lighting the wood. Once up to temperature, the side control arm is used to close the upper chimney port which forces the exhaust gases down to the lower combustion chamber where additional air is added causing the gases to burn a second time. The gases then circulate up a chamber at the back of the boiler, past a water jacket that picks up some of the heat, and them up and out the chimney. Interestingly, this design does NOT require the use of an electric fan to push to exhaust gases down. (Obviously the pumps to circulate the water through the water jacket within the boiler require power.) Note that in the pictures I took with my phone, a cover was removed from the side of the boiler to show the piping connections. Under normal use, these would not be visible.
The results are impressive: an efficiency rating of 93%, 14.9kw/51,000BTU with 70% of the heat going into the water to be used for space heating or domestic hot water and 30% radiating into the room. Given how much heat goes into the water, the representative I spoke to said that a storage tank of at least 600L was required and that 1000L of storage was better.
Here's a link to more information on the boiler: http://www.pses.ca/downloads/walltherm.pdf
The Riverdale Net Zero duplex (http://www.riverdalenetzero.ca) has a series of solar thermal collectors that store heat in a 16,000L water tank - practically the size of an in-ground swimming pool - built with concrete walls when the basement was poured. I've understood from discussions with some of the people that worked on that project that there's a certain amount of heat loss just trying to store the heat from September into January and that the cost, complexity and performance means that design probably won't be used again. When considering how we could store solar energy collected in the summer to use months later in the winter, it seems to me that nature provides us with the perfect solution: trees. I find the Walltherm boiler to be a genuinely interesting design that allows us to use wood for more than just space heating.