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Our pellet stove has DC fans and a kit that allows us to hook it up to a battery to power those fans in the event of a power outage. Photo: Alex Wilson. Click on image to enlarge.

House location and design are the starting points in achieving resilience--where the house located, how well it can weather storms and flooding, and how effectively it retains heat and utilizes passive solar for heating and daylighting. Beyond that, we should look to more active renewable energy systems for back-up heat, water heating, and electricity. This week we'll review these options.

Wood stoves

In rural areas, clean-burning wood stoves provide an easy option for back-up heat. With a compact, highly energy-efficient (resilient) home, a single, small wood stove can effectively heat the entire house when there is a power outage or interruption in heating fuel. Even in our current home, which is far from what I would call a "resilient" (relative to energy performance), we use a wood stove as our primary heat source--albeit accepting significantly cooler temperatures in parts of the house that are distant from the wood stove. Wood stoves are dirty, though--even EPA-compliant models (as all new wood stoves sold new today must be). In a rural area, such as where I live, reliance on wood heat may be acceptable, but in more densely populated areas extensive use of wood heat would cause significant pollution problems. Even in our area, when there is a power outage and more residents fire up their wood stoves, the air quality deteriorates. Thus, wood heating makes the most sense when the house to be heated is highly energy efficient so that little wood needs to be burned to maintain comfortable, safe conditions. And then, the wood stove should be operated for maximum combustion efficiency (minimum smoke production).

Pellet stoves

Like wood stoves, pellet stoves can do a good job of heating an energy-efficient house. Because of the fan-supplied combustion air, pellet stoves tend to be much cleaner-burning than wood stoves. The need for electricity to operate, though, makes pellet stoves inherently less resilient.

Our pellet stove--the sole heat for the apartment above our garage--works like most pellet stoves when AC electricity is available: electric coils ignite the pellets during start-up, a fan brings combustion air to the burn-pot in the stove, and another fan blows the heated air into the room. In the event of a power outage, however, our pellet stove--unlike most--can still be operated. The fans in our Quadra-Fire Mt. Vernon AEhave DC motors, and we have jumper cables that allow us to operate the stove during a power outage by clipping them to an automotive or other deep-cycle 12-volt battery. This back-up power isn't enough to start the pellet stove (we have to do that manually with pellet starter gel or some kindling), but the battery can power the two fans.

This photovoltaic system in southern Vermont provides back-up power during power outages. Photo: Alex Wilson. Click on image to enlarge.



Solar electricity

The ultimate in resilience can be achieved with a solar-electric (photovoltaic) power system that can be used when the grid is down. Photovoltaic (PV) systems directly convert sunlight into electricity. PV modules can be installed on a roof or on ground-mounted racks. Most use silicon wafers that are specially made so that photons of light excite electrons and generate direct current (DC) electricity. An inverter in most PV systems then converts that DC electricity into alternating current (AC) that can be used by standard household appliances and also fed into the utility grid through a net-metering system.

The problem with most grid-connected PV systems is that when the grid goes down (during a power outage), you can't use the electricity. This is a safety feature with grid-connected PV systems to prevent them from feeding electricity into the power grid when linemen may be repairing down wires. To serve as a power source during a power outage (key to resilience), it is generally necessary to install some battery back-up and a "hybrid" PV system. These systems are more complex (and costly), because they include not only a battery bank, but also controls that send power either to the battery bank or power grid, depending on the charge state of the batteries and status of the grid. There are apparently some specialized inverters that allow electricity to be used in the home (during the daytime when the PV system is producing electricity) even when the system is disconnected from the grid during an outage, but these inverters are uncommon.

This solar water heater on a Guilford, Vermont home is augmented by a heat exchanger in the wood stove, which offers back-up water heating in the event of a power outage--relying on passive thermosiphoning. Photo: Alex Wilson. Click on image to enlarge.



Solar water heating

To heat water when the electric grid is down the best option is a solar water heating system that can operate without AC electricity. Some "active" solar water heaters have DC pumps with integral PV modules that operate the pump when the sun is shining--thus the PV module serves both as the controller and the pumping power. There are also two types of passive solar water heaters that require no electricity. Thermosiphoning systems have the solar collector mounted below the storage tank, and solar-heated water rises through natural convection into the storage tank when the sun is shining. With batch or integral-collector-storage (ICS) solar water heaters, the water is stored right where it is heated (with water pressure delivering that water to a collector on the collector on the roof).

A solar water heating system can be augmented with water heating coils in a wood stove to ensure adequate hot water during the winter months when there is less solar energy.

About this series

Throughout this resilient design series, I'm covering how our homes and communities can continue to function in the event of extended power outages, interruptions in heating fuel, or shortages of water. Resilient design is a life-safety issue that is critical for the security and wellbeing of families in a future of climate uncertainty.

Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. To keep up with his latest articles and musings, you can sign up for his Twitter feed.

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Comments

1 In rural areas, I agree there posted by Robert Riversong on 01/24/2012 at 03:08 pm

In rural areas, I agree there is nothing nicer than a non-electric woodstove for both full-time and power-down heat. However it's not generally possible without a very high mass stove (like a masonry heater or heavy soapstone stove) to burn cleanly (at wide open draft) in a super-efficient home without overheating. More often, the woodstove will be running on a dirtier slow burn. Using clean, very dry wood, makes a great deal of difference. But I will disagree somewhat on the value of a PV and solar thermal system as power-down backups, since there is rarely any sun when winter storms knock out the powerlines. And a far more low-tech approach would be more resilient (as well as more affordable). I used to keep a 50-gallon food-grade plastic water barrel on the second floor plumbed into my water system to offer gravity-feed water in an outage when my well pump couldn't run. And I keep candles and propane or oil lamps for emergency lighting (as well as flashlights and headlamps). Another option where woodstoves are not practical, is a direct-vent wall-mounted propane space heater with piezo-electric ignition. Propane wall lamps also offer excellent lighting. The more we can simplify our technologies and our lives, the more resilient we can be in any unforseen circumstances. I'm fortunate enough to now have no indoor plumbing, so I have no worry about pipes freezing or a failed pump. And my composting outhouse works regardless of the state of the power grid, as does my wood-fired hot tub "bath".

2 I enjoy reading this series a posted by Bill Swanson on 01/27/2012 at 10:26 am

I enjoy reading this series and like seeing most of these are in my "dream home" I designed last year. But that may just be from reading this site often. Now to some day build it.

On thing I'd like to know more about is solar hot water heating. How well does it work in northern climates? How much maintenance? How much more efficient is it than PV? How does it compare in cost and efficiency to a waste heat recovery system on the drain pipe.


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