It’s currently around zero degrees F (-18 C) outside on a clear, sunny day. We heat our home both with sunshine and by burning dead or thinned-out trees. Every so often, usually during the annual process of firewood cutting late in early Winter, we reconsider how we heat our already highly efficient home. We’ve pretty well maximized every conservation method using R-50 (U-factor of 0.02) walls, an R-60 (U-0.01666) roof, and R-15 insulating shutters over the large south-facing windows that heat us on sunny days. And our masonry wood-stove very efficiently supplies both plenty of heat and heat storage for hours after the fire is out. But cutting more firewood than necessary can be hard on the body, possibly hazardous at times, and it can sometimes feel like an ordeal.
We considered various methods to make firewood cutting, splitting, and moving easier, but we have pretty well maximized that too. The solar-charged electric chainsaws (both Makita corded, and Greenworks cordless) are light, quiet, low in vibration, and quick to get the job done, and smaller limbs and branches are quickly cut into kindling using various ratcheting loppers.
We considered adding some sort of additional active or passive solar system that could store the heat of a sunny day for some additional time. This could be a vertical air or water heating panel on the south wall of the house, a full-length greenhouse addition on the south side, or a free-standing structure optimized for the process. But at a design temperature of zero degrees F, 240 square feet of double glazing shining on 17 cubic yards of concrete, all enclosed in a heavily insulated structure would only supply the heat needed at night after one sunny day, plus one day and night extra. That’s a huge expense, plenty of embedded energy and CO2 footprint in the building materials, and locally rare flat, sunny space taken up by a structure that has limited year-round utility.
We considered using wind power or additional PV panels to supply radiant electric heat. We need about 175,000 BTUs to heat our home at zero degrees F for a full day. We already receive about 90,000 BTUs per day through direct sunshine in our south-facing windows. That leaves a deficit of about 85,000 BTUs on a sunny day, and the full 175,000 BTUs on a cloudy day.
A cloudy day would require a 3000-watt wind-turbine running “flat-out” for 17 hours. If we could add enough to our already large off-Grid battery bank to store the needed power, and utilize a “mini-split” heat pump to remove latent heat from the outdoors instead of simply turning amps directly into heat, we’d still need at least a 1000-watt turbine, again running at full design speed for 17 hours per day. Just supplying what sunshine doesn’t on a sunny day would cut these figures in half.
Converting solar PV power directly into 85,000 BTUs of heat would require an additional 6 kilowatts of panels, assuming a 4.2-hour sunny day, mid-Winter, at about 45 degrees of latitude, just to supply what we’d need at night. The heat pump would still need an additional 2 kilowatts of PV panels, nearly doubling our current solar system, and in either case we’d need plenty of added battery storage. And this is just for the deficit on a sunny day, not a fully cloudy day!
Additional PV and wind resources are sometimes plentiful, often sporadic, but never cheap. whether purchased individually or combined into a smaller hybrid system. A house running on the Grid might consider a heat pump as the lesser of two evils, but it still burns non-renewable resources whether you consider the Grid as your sole source, or as the “battery” (which it isn’t, it’s a temporary, non-renewable, energy buffer) in a Grid-tied renewable system.
We checked the total BTU usage of our house using real-world experience, just to be sure. Burning a cord of local boxelder (soft maple) trees, with their fairly low BTU content of 17.9 million BTUs per cord, or 6194 BTUs per pound (figuring in a 20% moisture content) over a typical 4 month, or 120 day heating season, means that we average about 24 pounds of firewood per heating day. The small fires use about 20 pounds and the really cold nights burn around 30, so we’re right in the ballpark. As tedious as the firewood cutting may seem, the single cord (4 feet by 4 feet by 8 feet, 128 cubic feet) that we need each year is already growing on our land, needs cutting and thinning anyway for the best growth of the trees, and it would turn into CO2 whether we burned it or not, simply from rotting on the ground. It feels hard to neglect or waste such an abundant resource.
There’s an old expression from Henry Ford, “Chop your own wood and it will warm you twice“. While sawing, lopping, splitting, stacking, and hauling all indeed generate some personal heating, the benefits outweigh the merely caloric. Firewood not only heats our home but gives us domestic hot water for about a third of the year (when excess sunshine doesn’t). It also cooks our meals and supplies wood ashes for liming the garden, orchard, and pasture when necessary. Firewood is a renewable fuel, often found locally, and the tools needed to process it are only as extensive and expensive as you decide to make them.
As we age further, even if we decide at some point that dealing with trees is just too difficult, we can either hire some help for the process, buy already dried, split, delivered, and stacked wood from someone locally, or simply swap wood-stoves to use bagged wood pellets, purchased locally by the ton, in a more automated stove. Trading convenience for added expense may seem wimpy at some point in your life, but if it keeps renewables in the loop it could be just another option in a wide continuum of possibilities.