Armstrong House Project: Energy System Concept
Updated: April 2010
Space heating
The space heating needs will be met by a combination of wood burning stoves (one in the living room and one in the kitchen) and low temperature radiant tubing installed in the floors (downstairs) and walls (upstairs). Firewood will be sourced from abundant supplies on the property. Carbon emissions from burning wood are equal to carbon released when wood is allowed to decay on the ground. The primary system used to create 95 +/- degree water for space heating will be a 98% percent efficient propane fired boiler with outdoor temperature reset controls. Design of the low temperature radiant heat system (95° vs.165°) makes it possible to reduce fossil fuel used to supply warm water to the radiant system and to increase the potential of solar energy as viable supplement. In the future solar, thermal panels could be added to preheat the water used in the radiant the space heating system.
Domestic hot water
Hot water for sinks and showers will be provided by the same propane fired boiler used for space heating in combination with preheated water provided by roof mounted solar hot water panels. The solar preheated water helps reduce the use of propane. Further propane savings will be accomplished by designing the plumbing system to recover waste heat, for example, from hot water that goes down the drain during a shower.
Off grid electricity for lighting and appliances
The primary electrical source will be a propane fired generator with an estimated capacity of 8.5 KW. The current plan anticipates running the generator twice per day, once in the morning (30-45 mins.) and once in the evening (30-45 mins.). The generator charges a battery bank capable of sustaining a maximum load for 4 hours; with a maximum load equivalent to 25 amps at 120 vac. Also under consideration is a plan to recover waste heat from the generator exhaust system in order to help provide hot water for space heating and/or domestic use.
A ground mounted, 1320 watt solar photovoltaic array, already in place, generates supplemental electricity to help charge the batteries, thereby reducing use of the propane generator. An inverter converts DC current from the batteries to normal AC house current to power super efficient lighting and appliances. The generator will be programmed to come on automatically when the batteries dip below a specified charge level. The existing solar electric array is capable of running certain critical appliances, (i.e., computer, emergency lighting and smoke alarms.) It is now being used to provide electricity for tools and temporary lighting during the construction phase. Expanding the solar electric array and battery storage system in order to further reduce the use size of the generator is also under consideration.
Insulation
The exterior walls (R-19), crawl space, (R-24 to R-28) ceilings (R-24 to R-28) and garage are all insulated with a bio-based spray foam type insulation that provides a tight seal against air infiltration. In addition the exterior walls are covered with a perforated, foil faced, closed cell polyethylene foam house wrap which provide a wind and breathable vapor barrier. All cracks and penetrations (outlets and window openings) in the exterior walls have been carefully sealed with caulk and foam prior to installation of the vapor barrier.
General Information
The power supply system design requires occupants to run major electrical loads (i.e. washer, dryer, vacuum cleaner etc.) during specified times, mostly, if not always, when generator is scheduled to run. The propane generator has a useful life of 15 years and the solar panels have a useful life of approximately 25 years. The off grid system has smaller carbon footprint and is not subject to electrical rate increases brownouts, blackouts or local outages from storm damage.