Today, July 10, my family and I took a trip to western New York state to visit the Brocton factory of Dunkirk Metal Products.  This is where their line of wood gasification boilers is produced. Alternative Fuel Boilers, LLC has been mentioned in earlier posts in this thread.  I noted that we were contemplating the model EBW-100, a 100,000BTU unit.  As with European style gasification boilers, peak efficiency is reached by maximizing the time that the boiler spends in the “secondary combustion” stage for a given load of wood.  This necessitates the use of a large hot water storage tank to soak up the excess energy from the boiler beyond what the house currently needs so that heat can be later drawn from the tank as needed.  This also greatly simplifies the addition of a domestic hot water loop and even a supplemental solar loop.

The control system of the EBW-100 however allows this particular boiler to be used in a tankless system as well, making it essentially a drop-in replacement for our current oil fired boiler.  Once heat demand for the house has been satisfied, the combustion air blower shuts down and the fire dies back.  The ceramic refractory retains its heat so when heat is needed again the boiler relights almost instantly (provided there is sufficient fuel remaining) when the fan is restarted.  While somewhat less efficient than the system with storage tank, it’s still much better (and cleaner) than the typical outdoor wood furnace.  Plus, all heat that does not go up the flu ends up dissipating into the house.

We still have not solved the storage tank issue, and it’s starting to look like this might be a custom project all unto itself.  But it’s nice knowing that it can be added later and we can build the system in stages rather than trying to do it all at once.

Background:

For 10 years FOM has been running its heating system utilizing a conventional oil fired boiler. With the cost of oil around $3 per gallon these days oil is simply no longer economical. In fact it’s beyond the break even point with electric resistance heating for what we pay so something has to be done. At this latitude full time solar heating is out of the question, although with the recent decline in the price of vacuum tube collectors it’s becoming viable as a supplemental heat source in winter and could possibly provide all domestic hot water needs in the summer months. The use of a large insulated water tank for heat storage is also anticipated in order to increase the efficiency of a wood fired boiler. This would make adding supplemental solar heating very simple once the economics of its installation make more sense.

Searching for a boiler:

First of all we’ve already ruled out the use of an outside wood furnace. Why? Well, we don’t like to go outside on those nasty cold days for one. Second, we don’t have unlimited access to wood. Third, although we are in a rural setting, we are located in a development and one of these things would probably not be welcome in the neighborhood. As long as we are firing the boiler, the heat that escapes from it may as well be put to use heating the basement.

There are various designs out there of wood fired boilers that vary widely in their efficiencies. Again, as in other things energy-related, Europe seems to be ahead of North America in the efficiency department. Two European manufacturers I looked at were Tarm based in Denmark, and another from Poland. Both of these manufacturers use boilers designed to burn pyrolytic wood gases at high temperatures, and as such reach their peak efficiencies when the entire load of wood can be completely burned off during the cycle. There are at least two USA based manufacturers of such boilers. One I’ve looked at in detail is the EBW-100 from Dunkirk Metal products. They claim an efficiency of up to 87%, but again, this kind of efficiency is achievable only by burning off a complete load of wood at the highest possible temperature. Therefore some kind of heat storage is required with this kind of boiler.

Heat storage options

Since this is a hot water based system conceptually the simplest heat storage device is a large well insulated tank of water. Getting a large enough tank into our basement to suit the system without having to construct one in place is a problem, but now with the advent of collapsible insulated tanks based on foil faced urethane walls and a rubber lining this becomes much more feasible. Step one will be to determine just how big of a storage tank we could possibly fit in the area currently occupied by the oil storage tank and shop dust collection system.

For Further Consideration

With a water storage (heat storage) tank in place, adding heat exchangers for domestic hot water and eventually a solar array becomes a trivial exercise. What isn’t so trivial is the control system necessary to tie all the systems together and keep everything operating at optimum efficiency. Since we are primarily electronics oriented and not plumbing oriented the control system will be the main focus of future articles. The concept at present for the control system that will ultimately be installed is a modular based design that focuses on pluggable software modules and generic input/output routines, so that the hardware can be general in nature to suit whatever the system designer wishes to use. There are I/O modules on the market that interface via USB, RS485, Dallas 1-wire, CAN, and even ordinary RS232 (see Weeder Technologies). Sensors run the gamut from simple type K thermocouples for temp sensing, analog temp sensors such as the LM34 type parts, and digital 1-wire sensors from Dallas Semiconductor. At the core of the control system is a web server to allow remote monitoring and control of the system.  This is similar to the system which currently runs the oil boiler system, and is simply a standard Unix application running on a StrongARM based embedded Linux system.