Fundamentals of Resilient Design #7: Renewable Energy Systems for Emergency Use

Posted by on Sep 8, 2012

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

House location and design are the starting points in achieving resilience—with such considerations as 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 renewable energy systems for back-up heat, water heating, and electricity. In this blog I’ll review a few of 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 “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).

Masonry heaters

Masonry heaters are cleaner. These also burn cordwood, but usually cut to smaller dimensions. A small, intense fire charges up the thermal mass, which keeps delivering heat for several hours.

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 AE have 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.

A photovoltaic system in southern Vermont; special provisions are required for a PV system to provide back-up power during power outages. Photo: Alex Wilson.

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. With most PV systems, an inverter 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 (net-metered) PV systems is that when the grid goes down you can’t use the electricity. This is a partly 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 necessary to install some battery back-up. The system can either be stand-alone (not connected to the grid at all) or “hybrid,” allowing it to be connected to the grid during normal operation, but then isolated from the grid during a power outage so that it can power key loads in the house.

These systems, sometimes referred to as “islandable” PV systems, are more complex (and costly), because they include a battery bank as well as a specialized inverter that can either tie into the grid or work in this islandable mode. Only a few inverters on the market can function in this capacity, including the Radian Series line from Outback Power.

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.

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 a heat-exchanger in a wood stove to ensure adequate hot water during the winter months when there is less solar energy. If considering heating water using a wood stove, be sure to have someone with experience carry out that modification to the wood stove.

Along with founding the Resilient Design Institute in 2012, 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.

3 Comments

  1. Alex, You report above that a PV system requires some form of battery storage to act as an a credible emergency back-up. But batteries are a pain, they are costly, and they have limited life — and a well designed (for passive survivability) does not need a continuous feed of electric power. It can run on pulse inputs for one sunny period to the next — mainly I think it is freezers that need this. Heating, DHW, battery-power electronic devices, radios, can be functionally enabled thusly, and lighting — well candles and battery power lanterns make a memorably pleasant change in the evening ambiance. So the question: why can’t we simply have a utility-acceptable protocol for feeding PV-generated energy DIRECTLY through the inverter to the household load when the sun is shining? Is there really no institutionally acceptable, safe and sound way to achieve this and safeguard the grid from the (potentially lethal to linemen)back-feed? Because, to my way of thinking, this is the simple, durable, reliable, sufficient, and obvious way of achieving the small amount of remaining back-up power — when you have already installed a PV. And more and ,ore and more people are installing PV.

    • Bruce,
      You make great points, and this is a question I have been asking the industry for years. I would like a reasonably affordable inverter that would work normally in a grid-connected mode but then–with a robust disconnect to prevent backfeeding into the grid–be able to function when the sun is shining to supply power to the house.

      The issue is that you apparently need electric current of the proper waveform to operate the inverter. In an “islandable” system you need a small amount of battery power to establish the proper current or waveform to get the inverter functioning. The system we plan to install at our new house (new old house) will use a Sunny Island from SMA along with a standard inverter and a very small amount of battery back-up–probably a few hundred amp-hours of storage (one to several deep-cycle batteries). That will give us a small amount of back-up power at night (enough for a few lights, a radio, etc.), but then establish the waveform needed to get the standard inverter to operate at full output when the sun is shining. So we would use high-current-draw appliances, pumps, mini-split air-source heat pump, heat-pump water heater, etc. only during the day.

      That said, I understand that SMA is coming out with a “standard” inverter this year that will have an outlet on it so that a small amount of power can be accessed even during a power outage. I think the available power will be limited to something like 20 amps–not nearly the entire potential output of the PV array, but it’s better than nothing.

      I continue to believe that it would be possible to create an inverter that would do what you describe–and what I’ve been asking for for a long time. I’ll certainly report on such a product if it emerges.

  2. Alex,

    Your reply gives me hope. Actually, it seems like quite a reasonable system. My main concern with batteries-as-emergency-backup is (as I understand battery technology) they need to cycle; to receive and give their energy in some sort of regularity. And this (if true) would make batteries an unsuitable solution concept for emergency back-up because Murphy will make sure that the power outage happens when the batteries are drawn down.
    But the arrangement that you describe above side-steps that dysfunctionality. And if the system needs an electric current and a small battery arrangement can provide that, then be drawn down at night and charged up during the day …. then it’s a functional solution concept. And now we are just looking to make it better.

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