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Integrated Wood Energy Systems for Sustainable Housing

Mario Kani, P.Eng.
Allen Associates
Toronto, Canada

prepared for:
The Research Division
Canada Mortgage and Housing Corporation
Project Manager: Peter Russell

Abstract

In the context of sustainable housing Allen Associates has developed projects with renewable energy systems including wood heating. The Toronto winning design of the CMHC (Canada Mortgage and Housing Corporation) Healthy House Competition highlighted the need for a wood fired integrated appliance which combines space and domestic water heating, cooking and electricity production. This device will service reduced thermal and electrical end uses and complement photovoltaic electricity generation and solar thermal water heating.

This paper discusses environmentally sustainable housing principles and illustrates their application with three built projects. All projects used a major wood heating appliance with integrated domestic water heating.

A current CMHC project is described which is studying the technical and commercialization aspects of wood-fired cogeneration. Technologies such as heat (or Stirling) engines, steam engines and steam turbines in the range of 500W to 3000W are described as promising technologies. Wood gasification as well as the thermionic effect are also discussed. Two integrated appliances are proposed for further development.

Our present change in direction towards environmental sustainablity will need to promote green technologies and necessarily relegate many established products and practices to the recycling pile. Several industries can benefit directly, including wood stove manufacturers, the mechanical tooling industry and the electrical conversion system manufacturers.

Introduction

There is an increasing recognition that all of our activities and the design of devices which support those activities must move towards environmental sustainability. Sustainable housing means that the design and construction of our houses and energy systems as well as practices in our households must move towards sustaining the ecosystems that sustain us. The concepts of high efficiency, low embodied energy, and renewable energy sources are consistent with sustainability.

It is interesting to note that though properly managed wood supply can be considered a renewable biomass, it is often considered unsustainable due to its combustion emissions, however, the problems are typically rooted in sustainability issues of the application not of the fuel source. Emission problems can occur for the following reasons:

  • unique bioregion (climate)
  • unsustainable urban living (car dominated, density)
  • low efficiency wood burning devices
  • high heat loss houses (requiring excessive amounts of wood )

Sustainable Housing Principles

Housing development that is environmentally sustainable should have no net negative environmental impact in terms of global and local bioregions or ecosystems. Very simply the resource inputs and outflows crossing the site boundaries should be benign whether they be energy or water. The site itself should be life sustaining as an ecosystem. Healthful indoor conditions are part of sustainable design.

CMHC has focused on "healthy housing" as a development goal in the broadest sense for their housing activities (Ref. 1). By comprehensively and consistently designing for occupant health (air, water, sensory), energy efficiency (embodied energy, renewable energy), resource efficiency (materials, waste, water) and environmental responsibility (emissions, waste water, site planning, garbage), sustainable housing is a natural result.

Three Buildings Approaching Environmental Sustainability

Allen Associates has been moving toward environmentally sustainable building designs, resulting in some notable projects. These projects have the following characteristics:

  • high insulation levels
  • low air leakage envelope
  • high performance glazing
  • passive solar design
  • controlled ventilation with heat recovery
  • high efficiency lights, appliances, fans
  • renewable energy resources
  • passive, non-ozone depleting cooling
  • low-tox materials
  • appropriate, low embodied energy construction
  • extensive water conservation
  • benign waste water management
  • site and building greening

The following are three recent examples of existing projects that exemplify these principles. Note that all three projects have major wood heating devices with integrated water heating.

Boyne River Ecology Centre

The Boyne River Ecology Centre, designed with Doug Pollard Architect, is an 500 m2 educational facility at the Toronto Board of Education's natural sciences school located on the Niagara escarpment, 100 km northwest of Toronto. A brief description of environmental features is as follows:

  • Highly efficient thermal envelope, mass construction with earth coupling and sod roof for minimal heat load.
  • Natural ventilation via low/high windows for summer cooling. Passive heat exchange ventilation and displacement type distribution for good indoor air quality in winter
  • Efficient compact fluorescent and halogen lighting with unique controls for reducing demand on the limited electricity supply
  • Living machine bioregenerative wastewater treatment yielding effluent of higher quality than pond supply water
  • Off-grid renewable electrical supply from 650 W photovoltaics, 1.5 kW wind turbine and two small 200 kW hydraulic turbines
  • Space heat is primarily passive solar with a 7 kW central wood fireplace as back-up
  • Domestic hot water heating is solar thermal augmented with wood heat in winter via heat exchangers at the perimeter of the fireplace.

Kitchener-Waterloo YMCA Environmental Learning Centre

For the YMCA Camp KI-WA-Y Allen Associates designed mechanical systems for two buildings, the Earth Residence, a 40 person 300 m2 dormitory and the Day Centre, a 250 m2 administration and special function building (Architect: Charles Simon).

The Earth Residence has many common features with the Boyne Ecology Centre:

  • Earth covered roof for site greening
  • High performance envelope and passive heat recovery ventilation
  • Clivus Multrum composting toilets and Waterloo Biofilter for grey water
  • Off-grid renewable energy supply using 14 kW masonry wood heater, solar thermal, 2 kW photovoltaics and wind generation
  • Recycled wood construction materials

The Day Centre will be the central focus with offices and an auditorium and features:

  • Greenhouse featuring Living Machine waste treatment "garden" supplies passive solar heating via an air-coupled mass floor
  • The rest of the building space and water heated by 50 kW Portage & Main wood boiler and solar thermal
  • Provision for future grid-connected renewable electricity supply and future district heating of other buildings on campus from wood boiler.
  • These projects were primarily designed to minimize environmental impact of resource consumption balanced with measures to mend the ecosystem support structure. However, the cumulative effect of executing these projects is that we now have the technology and design principles to make new and retrofit building developments, whether a single house or a community, become "environmental clean-up modules". Each construction of a housing unit is an opportunity to export renewable power, to clarify water and consume emissions by increased greening, in short, to help restore the actual ecosystem.

Crainford Residence

This 250 m2 house is located in Toronto and includes an at-home workplace.

It has no renewable electricity supply but has a number of important features:

  • Highly efficient envelope allows heating and cooling to be supplied via ventilation air stream (from heat recovery ventilator) in a combined radiant/convective mode
  • Principal heating is a masonry wood heater with gas hot water tank as back-up
  • Water heating is solar thermal augmented with wood heat in winter via heat exchanger in fire box
  • Rainwater collected in cistern for displacing treated water
  • Passive, night-sky radiation cooling (no CFC's)

Sustainable Household Energy Profile

A sustainable building design results in severely reduced energy budgets. By definition, sustainability also does not allow for waste and a wasteful energy lifestyle. As the energy and ecological designers of the Toronto winning entry of the CMHC Healthy House Competition (with Architect Martin Liefhebber) we developed a 100% renewable energy system, in effect the Unplugged House. However, renewable energy is not sustainable if it is used to power inefficient end uses. To make this proposition economically viable and sustainable, the following energy budgets were developed.

Space Heating/Cooking ...............2500 kWh

Domestic Hot Water....................1000 kWh

Electricity.....................................1500 kWh
==============================
..........................................Total.....5000 kWh

Note that a conventional household is about 30,000 kWh of total thermal and electrical load and an efficient household is about 20,000 kWh.

The 5000 kWh is to be supplied by external renewable resources. The energy system consisted of photovoltaic (PV) electricity production, solar thermal domestic water heating and integrated wood-fired pace heat, cooking and electricity. The rationale for wood-based electricity production recognizes the poorer solar potential in winter when space heat is required.

The 5 m2 PV system is responsible for 1000 kWh of electricity and the 3 m2 solar water heater for 700 kWh. The remaining 3300 kWh (2500 kWh for space heat and cooking, 300 kWh for domestic hot water and 500 kWh for electricity) is supplied by wood heat. Overall combustion efficiency is a minimum of 70%. This is equivalent to about 4500 kWh or three quarters of a full cord. This is our best definition of a sustainable rate of wood consumption for a household on 0.2 hectares of woodlot which should allow for sustainable forest practices.

Integration of Water Heating

Our use of water heat exchangers have proceeded without any rigorous existing design principles or test results. The masonry heaters were supplied by Norbert Senf of Masonry Stove Builders, complete with heat exchangers. Our systems have both thermosyphoning and pumped loops. The systems are safeguarded from boiling by ensuring an automatic heat dump to additional heating devices, e.g. fin-tube convectors. These can be located where potential discomfort is not a problem, such as an unconditioned basement.

Issues of CSA approval of heat exchangers and impact on emission remain.

Integrated Wood-fired Cogeneration

As a result of the CMHC Toronto Healthy House project the need for development of a low-output, wood-fired cogenerator at a reasonable cost was identified. Allen Associates has been asked by CMHC to conduct a study into the technical and commercial feasibility of wood-based thermal devices that could also provide electrical power (Ref. 2).

The concept typically focuses on wood combustion to generate mechanical output via steam power or directly from heat via an "external combustion" engine known as a Stirling engine. However, wood gasifier and thermopile technologies are also being assessed.

Market Feasibility

Wood burning appliances exist in a total of 1,400,000 Canadian households Wood or biomass fueled stoves are the sole source of heat in close to 500,000 homes. For these users it is typically less costly to heat a home in this manner than with electricity or oil (if available). The economics would be further improved if electrical production for appliances and lighting were included in the scenario.

A potential market for single dwelling wood-fired cogeneration needs to be defined. There are established market for metal wood stoves, masonry heaters, cook stoves and whole-house fireplaces. In terms of housing types there are essentially three groups:

  • off-grid rural low-density
  • on-grid rural low density with high non-wood fuel costs
  • on-grid high density (urban) with lower non-wood fuel costs

These groups of dwellings will be quantified and market penetration rates defined. In rural communities, the household scale application will compete with community systems; however, many of the houses are separated by significant distances, making it uneconomic for hook-up to district energy systems. For the off-grid case, clearly any reliable self-generation, including PV and wind, is attractive and integration with thermal functions should be a winner. Grid-connection has the attraction of export and not requiring electric storage if the meter can spin forward and backward. Ontario Hydro is just embarking on a pilot project of this type. The above identified market niches will have significantly different expectations of the technology which will need to be addressed.

Alternative and complementary technologies will be reviewed to note opportunities as well as potential competition. An alternative technology is methane-producing digesters fed by compost and/or human waste, and a complementary technology is PVs which supplement electricity year-round but maximize output in summer when thermal output from wood heat has lower demand.

Technology description

The attraction of wood-fired cogeneration is the utilization of high grade heat to produce high-grade energy first (i.e. electricity); then use the thermal by-product for lower temperature demands. In essence, a thermodynamic cascading of energy outputs.

Typically the cogenerator consists of an energy source, a mechanical driver and an electric generator. A different technology using the thermionic principle, can convert heat directly to electricity.

The primary fuel source is assumed to be wood-based: pellets, chips and cord wood. A related renewable fuel source is organic solid wet waste (compostables) which can be conditioned with wood chips, sawdust or straw. This is attractive in agricultural applications. Other agricultural waste, such as rice husks (and perhaps soon hemp stalks), may be feed stock for such appliances. Dual fuel combustion units with propane back-up may also have applications.

The derived energy for input to the mechanical driver can be in the form of heat , steam, wood gas, and methane. Steam is conventionally produced in small boilers operating at pressures as low as 15 psi. Wood gas is produced in gasifiers, a technology originally developed during the second world war when gasoline was in short supply (Ref. 3). The devices are bulky and the fuel supply is "dirty" requiring special cleaning for use in conventional engines. Methane, the major component in natural gas, can be produced by anaerobic digestion of compostables. Some cleaning is also required but digesters exist that produce sufficiently clean biogas for conventional engine cogenerators, as well as a high grade compost for agricultural purposes.

Heat-based mechanical drivers are Stirling engines (Ref. 4). This device is a piston based engine utilizing a low pressure working fluid, typically helium or air. Heat is applied to one end, expanding the working fluid thereby moving the piston. The working fluid is then cooled (or "regenerated") to allow the piston to return. The technology is intrinsically quiet in operation.

Low efficiency units have a long tradition in the third world, particularly in India. Modern designs of Stirling engines have mechanical efficiencies over 20% which makes them the highest mechanical efficiency for small scale direct thermal conversion from a thermal source. However, these units have been typically natural gas fired and availability and costs are a concern. Temperature requirements for small stirling engine is an issue. Operating temperatures drop when one goes from natural gas to pellets to chips to cord wood.

A Stirling engine could be driven off wood gas combustion but overall efficiency would be low. More traditionally, wood gas has been used to drive internal combustion piston engines. While mechanical efficiency may be reasonable, the gasifier is bulky, complex and costly.

For small steam drivers there is a range of options available. Piston steam engines are typically produced for historic markets where looks are as important as operating characteristics. There also exist low pressure, paddle-wheel steam turbines. These technologies are typically no higher than 15% mechanical efficiency. There is development in small high performance steam piston engines and turbines; however, there seems to be no commercialization for lack of a defined market. Safety concerns about operating steam devices in a residential setting is also a perceived barrier.

Availability of small electrical generators does not appear to be a barrier. Conceptually they are simply electric motors run in reverse: a mechanical input results in electrical ouput. As with motors, efficiencies can exceed 80%.

Electricity can be generated by the thermionic effect, that is, the use of dissimilar metals can cause electrons to flow in the presence of heat. This effect is used in thermocouples which measure temperature based upon change in electric flow. A collection of thermocouples is called a thermopile. The technology has the attraction of being simply located on top of a hot surface, e.g. wood stove; however, electrical production efficiency is very low, less than 5%.

Equipment Sizing

The size and profile of end use loads is critical to the equipment requirements for design purposes . This includes loads such as space heating, space cooling, water heating, cooking, refrigeration, ventilation and miscellaneous electrical uses. However, for market considerations a certain degree of efficiency in end uses should be assumed even for the high end of the range. The low end of the range is the sustainable energy profile discussed in Section 4. This will facilitate the equipment sizing by limiting the applications to possibly two electric output values in the 500 W to 3000 W range.

This sizing is sufficient for a comfortable electric lifestyle; however, electrical storage combined with automated or resident energy management is assumed. Significant electric loads of short duration, e.g. power tools, may be supplied by an additional gas generator or by power from the electric grid.

Feasible Systems and Applications

There are a number of reasons why large scale commercialization has not yet proceeded. These range from technical feasibility issues to expectation in the residential setting. Refined products need to be developed. Particular attention must be paid to the user interface issues: location, automation, loading, maintenance, safety issues and noise. However, it is both sobering and comforting to know that around the globe there are many operating systems at different stages of refinement.

The following are two possible configurations for future commercialization.

Integrated Masonry Heater/Stirling

Masonry wood heaters are thermal storage stoves which operate with fast, clean, high temperature burns. This design has an additional high temperature mass (e.g. a soapstone slab) located at the hottest part of the flue to provide several hours of stored heat for the Stirling engine. Lifting insulated lids on the slab will allow cooking to take place. An oven is also part of the design. The stove top is insulated when not used for cooking. In summer, when cooking only is desired, most of the mass can be bypassed by the flue gasses . The electrical output is between 500W and 1000W. This appliance is assumed to operate with complementary PV electricity production and solar thermal water heating. Water is heated at the heat rejection of the Stirling engine to assist the solar water heater.

Separated Steam Generator

Due to potential noise and safety concerns a steam generator would also more likely be separated from living space. The combination is a small, low-pressure steam boiler feeding a piston engine or turbine. Space heating and domestic water heating can be supplied via heat exchangers from the steam directly or possibly after the mechanical device. This device may be fired on electricity demand all year, if no other renewable sources are available. In summer heat will need to be dumped to outside if no thermal loads exist. However, absorption cooling and refrigeration may be considered to improve the utility of the steam.

Prototypes of the best technologies with less integration exist both on this continent and globally. Research and development will be required to produce market ready product. Private sector funding must take the lead as it is in the industry's interest to shape future market opportunity. Prudent, strategic government support during development is amply paid back in job creation and new economic activity.

Conclusion

We must embark seriously and rapidly on the road to environmentally sustainable energy consistent with the development of sustainable housing. New wood biomass integrated energy systems, likely complemented with other renewable energy forms, will play a very significant role in our energy future.

Their success depend upon appropriate application, economics, reliability, user friendliness and currency with environmentally appropriate advancements.

It is always difficult to bring new product to market. Having identified key markets and penetrations rates, the challenge is to position the developed product correctly for sales to accrue. The barriers are typically numerous in any endeavor; by sustained effort, all but the most fundamental can be overcome. The key is to turn barriers as much as possible into opportunities.

For example, the downturn of the housing industry and North American restructuring of manufacturing allows for significant opportunity to set up local manufacturing and importing of advanced components, with a better climate of implementation than would exist in an overheated residential market.

Our present change in direction towards environmental sustainablity will need to promote 'green' technologies and necessarily relegate many established products and practices to the recycling pile. Several industries can benefit directly, including wood stove manufacturers, the mechanical tooling industry and the electrical conversion system manufacturers.


ELEMENTS OF SUSTAINABLE HOUSING

  • high insulation levels
  • low air leakage envelope
  • high performance glazing
  • passive solar design
  • controlled ventilation with heat recovery
  • high efficiency lights, appliances, fans
  • renewable energy sources
  • passive, non-ozone depleting cooling
  • low-tox materials
  • appropriate, low embodied energy construction
  • extensive water conservation
  • benign waste water management
  • site and building greening

References

1. Canada Mortgage and Housing Corporation. CMHC's Healthy Housing Competition: Guide and Technical Requirements. Ottawa, 1993.

2. Allen Associates. Technical and Commercialization feasibility of Household Scale Wood-fired Cogeneration. Study for CMHC, in progress.

3. Solar Energy Research Institute. Handbook of Biomass Downdraft Gasifier Engine Systems. Golden, Colorado, 1988.

4. Walker, G. et al. The Stirling Alternative. University of Calgary, Calgary, Alberta, 1990.


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