100% Solar

All energy used by this house will come from the sun, with the goal being Net Zero. Initially this will include passive energy delivered through efficient windows, “active” solar heat collected via flat plate and possibly evacuated tube collectors, 5 kW of photovoltaics (PV) and about 1/3 of a full cord (face cord: 2’x4’x8’) of cured hardwood per year.

Yes, the best solar collector is a window facing south; however, passive means are not sufficient in our climate without accepting excessive temperature swings and thus would not be considered comfortable by my wife’s definition (the only definition that matters to me) or by using a significant amount of “back-up” heat (fossil fuels).

There are two primary approaches to active solar heating, including heating air or water. Then minimizing temperature swings requires solar storage. There are two main types – dry storage or water storage. For example, hot air is primarily ducted through dry mass (rocks, sand, concrete, etc.) but hot water can be piped through dry mass and/or through a water tank to heat the water. For several reasons I chose to use the sun to heat water and then have the heat stored first in water mass and secondly in compacted sand and concrete mass. I’ll discuss this more in a later section on the heating system.

Finally, for a number of reasons I chose to use the stored solar energy of wood that I find around the yard and neighborhood by burning it in an efficient wood stove to “make ends meet” thermally.

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Net Zero Goal

Net zero energy can be accomplished in many ways, depending on how you define “net zero” (no purchased energy, no fossil fuels). Here are a few of the many ways to “solve” this problem:

You might own a lot of woods and consider any wood off your property as part of the equation. Perhaps you could burn 15 - 20 cords a year, much like our ancestors did. Or you might heat a modestly insulated home with 7 - 10 cords. I don’t think this is sustainable on a large scale or environmentally sound.

You might throw technology and money at the problem: install 10kW PV and 10 kW wind generator and generate about 22,000 kWh a year. With a code built house and a geothermal heat pump, this would work. There is quite a bit of technology to maintain here. I don’t think this is sporting or particularly challenging; nor is it something most people can afford.

You might build a small house (~1000 sf.), with a well insulated shell (Energy Star) and a geothermal heat pump, and install a 10kW PV array. Now we are talking about making some adjustments that are required by the times and a reasonable investment. This might be the best approach if: you can’t orient your house toward the south; a smaller house is acceptable; and you can access the sun for PV and SDHW.

Or you could build a moderate sized home (~2000 sf.), super insulate and air seal the shell, use the sun for heat and hot water, burn 1/3 cord of wood to make thermal ends meet, and then install a modest 5kW PV array for all your electric needs. In such a home the most complicated components would be the inverter and circulator pumps. If such a home could be built for just a “reasonable” premium, then I believe this is be the best answer for my family and perhaps many other families. This is what I decided to do.

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Several Reasons for using Water

There are several reasons for using water as a heat transfer fluid and heat storage medium. Chief among them are:

1. Heating water is more effective in that it has more mass and is easier to move longer distances. Care must be taken so it doesn’t leak, but PEX minimizes fittings and the skill and time required for plumbing.
   
   
2. Water is more flexible. With heated air running through a stone bed or through ducts in a sand or concrete mass the mass is set when the house is built and can’t be easily changed. Water can be routed to different storage units or directly to the house, depending on what is needed, and the amount of water mass can be adjusted, when required. To achieve my goals, I place a priority on heating the water mass that supports domestic and targeted space heating needs, then to reap higher efficiencies from my collectors, I route the sun heated water through the dry mass under the basement floor – the solar battery.
   
   
3. Heat stored in water mass is more easily regulated. I can store thousands of BTUs in an insulated tank and not have it affect the house temperature at this time. Later, the heat can be extracted and used to take a hot shower or directed to space I want to heat.

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Reasons for Wood Stove

1. Wood is 100% solar; it is a form of stored solar energy. Burning wood that would rot otherwise is better than being carbon neutral.
   
   
2. Burning modest amounts of wood, in our region, is sustainable. Still burning less is better; otherwise, you could just build a log cabin and heat with an open fireplace using 10-15 cords a year, as they did 200 years ago!
   
   
3. An air tight wood stove burning 1/3 cord of cured hardwood at 75% efficiency delivers about 5 mmBTUs (million BTUs, 20mmBTU/cord), which is 15-20% of total heat needed. Wood burning is an economically efficient way to “make ends meet,” since the heat can be delivered when I want it, independent of the current weather (the sun is not shining or it is very cold outside).
   
   
4. I live in a lightly populated area where burning solid wood in an EPA Phase II certified stove does not put a burden on the local air quality.
   
   
5. This amount of wood is readily available, usually for free, via property & wood lot management, road & utility access maintenance, or by assisting a neighbor.
   
   
6. Gathering, splitting and moving 1/3 of a cord is not burdensome. Rather it is good exercise.
   
   

If /when #2, #4 or #6 are no longer true, I’ll use a pellet stove. Just 20 bags of pellets per year deliver the same 5 mmBTUs (16 mmBTUs per ton, 80% burn efficiency). Pellet stoves are cleaner burning, pellets are easier to move and store, and pellet stove operation is easier than burning solid wood. Plus pellet stoves have the advantages of more easily regulated heat output and nearly instant on/off. However pellets must be purchased and electricity is needed to run a pellet stove, plus they are have a slightly larger carbon footprint.

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Comfortable

Comfort is critical and must be defined by the family that will live in the home.

In our case, comfort was defined as having an Active Area (the south facing, common area, where we cook, eat, do homework, read, watch TV, play games, access the internet, do hobbies, etc.) that is almost always 68 to 74 degrees – winter and summer, PLUS having bathrooms that are warm for shower time.

This definition is not the fossil fuel burning definition of the past, but it is acceptable to our family and would be happily accepted by many of the people I’ve discussed this with over the years. What is not readily accepted is being cold. As you get older being warm is more important. If you take blood thinning medication being warm is more important. In a civilized world, it seems reasonable to find a sustainable way to be warm enough for the task at hand.

This definition of comfort implies that there will be areas of the home that are allowed to have lower temperatures – in particular the bedrooms. We can comfortably camp at 40 degrees, provided we are properly prepared – why can’t we sleep in comfort at home at 55 or 50 degrees? Also, there may be areas of the house that will be used only when sufficient heat is available, maybe just three seasons. This is not bad; it is the reality of living within our sustainable resources.

Currently we also are accepting the fact that we will not have fully hot showers every day of the year. We want to live within our resources and feel that a lukewarm shower every once in a while is not too terribly bad. We will see… We will see just how many days this might be and just what temperature “lukewarm” is.

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Enduring

There are many aspects of this challenge, but mainly it has to do with the management of water in its various forms – water, ice and water vapor.

1. Footing drainage to daylight is important. This minimizes heat loss by not allowing excess dampness to carry away BTUs faster then dry materials. It also allows for envelope drainage to have a destination where energy is not required to remove the resulting water.
   
   
2. Water management requires that vapor barriers and air barriers be understood and applied correctly. We want to keep materials as dry as possible, but when they get wet, and they surely will, we must allow them to dry adequately.
   
   
3. An energy efficient house must be quite air tight (0.1 Air changes per hour under natural conditions is good). Thus it must be ventilated adequately to assure good indoor air quality and proper levels of humidity.

Other factors in building an enduring home include the building materials. These should be as maintenance free and sustainable as possible.

There are other factors in building an enduring home, chief among them are that the home must be highly functional and economical to maintain. It should also be pleasant to look at too, but alas, beauty is in the eye of the beholder and if it is comfortable and enduring many will look on a simple house favorably.

Lastly, the house can’t be too “expensive” or “experimental” to build. Toward this goal I have utilized conventional building Materials and Techniques, though perhaps used in unconventional ways.

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Materials and Techniques

This is a stick built home with a poured concrete basement. Materials that might be considered fairly “innovative” include: insulated concrete forms (ICFs), a rather new sheathing called the Zip Wall & Roof System, PEX (cross-linked polyethylene) tubing for heating and domestic water, and spray in place foam.

Standard carpentry techniques were used throughout, though I wish I had used “Advance Framing Technique” and will on the next house to save wood and get higher wall insulation levels.

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