Wood Gasification / Solar Wiki: Introduction

The purpose of this document is to chronicle and share experience in developing a domestic wood boiler / solar heating system, as well as to provide an interactive forum for others to share relevant insights, suggestions, and inputs. The hope is that this might evolve into a helpful resources for others who are undertaking similar projects. Welcome, and please feel free to contribute.

Content areas will include the following:

Background

Most woodstoves and wood-fired boilers emit noxious fumes and create fire hazards from creosote deposits in the chimney. A notable exception is wood gasification boilers, which employ a two-stage process to first create flammable gasses (primarily carbon monoxide and hydrogen) and then burn those gasses at very high temperatures to achieve very high efficiency and relatively low emissions.

In the US, this type of boiler is common in commercial / industrial applications but almost unknown in residential use. Initial research uncovered only two brands with US distribution, both quite expensive and with long lead times. Eventually a boiler of Eastern European manufacture was obtained and installed.

Gasification boilers are more complex than traditional wood boilers, and require a more sophisticated control system. The system that this project is based on came with an onboard controller that monitors water jacket temperature and controls both the circulator pump and a variable-speed fan that effectively regulates the combustion rate.

Initial Installation

The wood boiler was initially installed in parallel with an existing oil boiler, with a manual process to switch between them. The house has three zones of hot water baseboards, a zone for domestic hot water, and a zone which heats a hot tub. There is a circulator pump for each boiler feeding a common manifold with motor operated zone valves for each zone.

The house is in Vermont at about 44° North Latitude. It has three full floors and about 3500ft² (about 325m²) of floorspace. In the first heating season, fuel consumption was about 3.2 cords (11.6m³) of wood (and virtually no oil) between November 21 and March 15.

Problems

While the first season’s experience with this boiler was satisfactory, there were several opportunities for improvement:

There was no heat storage other than the thermal mass of the house. This meant that hot water would quickly run out if the boiler was not operating, and it meant that a fire ahd to be built every day.
There is not very much baseboard - only 52’ for the whole house. This is nowhere near enough to absorb the full heat output of the boiler. This meant that the boiler would have to throttle back, which is less efficient. It also means that it takes a long time to raise the temperature of the house.
Baseboards are designed to operate at 80°C, and the controller shuts off the circulator at 60° C. However, domestic hot water and the hot tub zones could use water down to 50° C or even lower. This means that there is residual heat energy that is lost or not available for use every time the boiler shuts down.
Switchover to oil heat was a manual process - a switch had to be thrown to activate a relay to make the necessary changes to control signals.
There was no convenient way to know that the boiler needed more wood, or that the gasification / combustion process was working optimally.
Since it’s not desirable to fire up the wood boiler in warm weather, hot water has to be heated with oil.

Heat Storage

Part of the solution is to add heat storage capacity. A heavily insulated 750 gallon stainless steel tank will be installed for this purpose. This tank will have three heat exchanger coils:

Boiler / Zone coil for heating tank or extracting heat to feed zones
Domestic hot water preheat
Solar panel

The tank will be designed to provide maximum thermal stratification, with the goal of having the tank top near 80°C during winter months, and 50-55°C in the summer.

In the winter, the boiler will heat the tank whenever the boiler is making more heat than the house zones can absorb. In turn, the tank will provide heat to the house zones when the boiler is not active.

Part of the challenge is in determining the best insulation. The tank will be located outside, where temperatures can reach -30°C (-25°F).

Solar Panel

Experiments are underway with a solar swimming pool heater. This unit is 4’ x 20’ and made of extruded black polyolefin with multiple small water passages. It will be placed on a south-facing slope below the heat storage tank. Circulation will be via thermosiphoning. Part of the experiment is to see what temperature this panel can attain in open air. We also will likely try enclosing the panel in a glazed enclosure.

Controller

In reviewing the system, there are several areas where additional controls and control logic would be helpful:

To automate the switchover to the backup oil-fired boiler (a completely independent system)
To make better decisions about the control of the circulator pump. Baseboard zones require water at 80° C, while the domestic hot water, hot tub, and radiant heat zones will work quite well at much lower temperatures.
To control the thermal storage tank.
To monitor overall system health and performance.
To log data for subsequent analysis.
To provide a mechanism for remote monitoring and control of system settings.

More information on the controller is at: