One of the biggest national challenges posed by climate change is to convert our system of heating and lighting our homes to a zero greenhouse gas emissions. “Electrify everything” is the path . . . but this poses a huge problem in the northeast where renewable solar output is not seasonally matched with heating loads and wind power is not sufficiently reliable to match heating loads. Long term energy storage or long distance transmission will be needed to match output and demand, and both are problematic. Hydrogen from excess solar and wind has been suggested as a possibility, but the inefficiency of electrolyzing hydrogen using existing technologies seems to make it a poor choice.
This winter had been an experiment in my own personal quest for 100% energy self sufficiency. This winter, like last, we have been holed up in our off-the-grid mountain cabin – a privilege made possible by COVID remote work and my phased-retirement teaching schedule. So I am proud to say that from December 20 until this past Wednesday we have been 99.9% energy self-sufficient for heat, transport, lighting, and connectivity using onsite renewable energy.
We have used a combination of the old technologies (wood heat, lead acid batteries, a bicycle) and the newest (solar panels, LED lighting, USB powered electronics, efficient small inverters) to overcome the seasonal, overnight, and cloudy day storage problems.
I know all about the environmental and health challenges of wood heat. Indoor and outdoor air quality suffers even with the best woodstoves. But indoor air quality is not an externality, at least – that’s my own health I put at risk, not others. Outdoor air quality is less of an issue in a sparsely populated rural area like this.
Yes, wood heat has higher carbon emissions per BTU than coal. Even! But these emissions are roughly offset when you can burn exclusively dead and down wood – wood that was going to rot or burn into carbon dioxide and methane anyway, depending on the forest climate and ecology. And some soil carbon.
Wood heat is just stored solar energy from photosynthesis, of course. Wood may still be the most efficient form of seasonal solar storage we have at a not-preposterous cost. Unfortunately.
We burn about two cords of hardwood equivalent in a cold north country winter to keep our tiny 350 square foot cabin comfortable – that’s about 40,000,000 BTUs according to Dirk Thomas’s The Woodburners Companion. Assuming 75% efficiency, that’s about 30,000,000 BTUs of actual heat. An article in Mother Earth News claims that you can count on about a cord of dead and down wood per acre in a mature forest wood lot, which seems slightly optimistic based on my own experience downstate. Up here we have thirty acres of woods, so scrounging two or three cords a year from snags and blowdown is never a problem. Though it’s mostly low-heat softwoods.
The problem, as always, is scaleability – there just aren’t enough acres of mature forest producing a cord per acre of dead wood to heat all the homes in the cold part of the country. Wood plantations are an environmental disaster, with dubious climate benefits.
What would it take to produce and store the equivalent amount of energy using solar panels?
Let’s consider hydrogen electrolysis, first. Modern gas burning heat can achieve close to 95% efficiency, so lets just assume that we would need the same 30,000,000 BTU worth of hydrogen energy. I’ll use 75% as a slightly optimist figure for electrolysis to hydrogen heat value efficiency. That gets us back to needing 40,000,000 BTU of energy. At 3,400 BTU per kilowatt hour – that’s about 12 megawatt hours to cover the seasonal heating needs for this tiny cabin. Using the annualized average daily solar exposure for Rochester, NY of 3.3, one hundred-watt rated solar panel produces about 330 watt hours per day. So 12 mWh annually would require something like one hundred one hundred watt solar panels. At about six square feet and $100 per panel, that’s about six hundred square feet of space and $10,000 to replace our three acres worth of cord wood production. Plus installation. Plus the cost of an electrolyzing plant and hydrogen storage. This piece suggests that a ten kW share of an electrolyzing plant might run £7,000. But an electrolyzing plant is a grid-based utility-level solution, not a form of off-grid energy self sufficiency. Solar panels are much more space efficient than photosynthesis, at least!
What if we tried to get to the same heating self-sufficient place using an electric air source heat pump and lithium ion batteries instead? The good news is that ASHPs have greater than 100% “efficiency” since they suck heat out of the cold air – up to three times more heat than electrical energy consumed. So our 12 megawatt hours of heat might only take 4 mWh of electricity. And lithium ion batteries are much more efficient at storing electrical energy than hydrogen electrolysis.For round numbers, I am going to use 100% battery storage efficiency, though of course no battery actually achieves that. Our 4 mWh of super-efficient LiOn ASHP heat would require only 33 100 watt panels or so, for a much more reasonable $3,300 in panel cost. But 4 mWh of Tesla powerwalls might set you back a bit. At 14 kWh each, that’s about 285 Powerwalls. At about $10,000 each, installed, that works out to a $3 million substitute for a three acre woodlot. Not to mention the space needed for all those Powerwalls (which might be about the same as two cords of wood). I suppose at that price, it might be cheaper to invest in ten times as many solar panels and settle on a few days worth of storage. Then you might have to clear some forest to make room for the solar panels.
Of course, all these back of the envelope musings are just to cover our tiny cabin – you can probably triple all of these numbers for a more typically sized house. Which helps illustrate the challenge of converting to 100% renewable energy for heating in cold regions using existing technology. Heat pumps have a huge efficiency challenge over hydrogen, but hydrogen may solve the storage problem. Either way, it will take utility level transmission, conversion, and storage infrastructure that may be inconsistent with green ideals of distributed, local renewable energy.
Our non-heating energy needs have been much less fraught. Four 100w solar panels on the roof charge three old fashioned deep discharge marine batteries under the floor. The cabin runs on a 12v DC system – there are USB outlets for charging our phones and tablets, auto style 12v outlets for other 12v appliances and micro inverters, and LED lights. Electricity is scarce in January, when on a good day there is only about 45 minutes of direct sunlight on the solar panels and this year it seems the sun did not come out of the clouds for more than three days in the entire month. But there was always enough juice to charge our phones and keep the lights on. We made it to all of our Zoom meetings. It helps that we don’t have to run a water pump (we melt snow camp-style in the winter, and use a composting toilet). I biked into town every few weeks to stock up on milk, eggs, and beer – stuff we can’t economically get delivered to our cabin.
It seems that most people would find this lifestyle too primitive. But we are content – we have thousands of acres of woods connected to the trails on our property for skiing and exploring. There is a great community of like-minded outdoorsy types. And the wonders of modern technology let us be as socially connected as anyone else in these COVID times, and even stream movies until the data hotspot runs out.
But this week I got in the car for the first time since December, since we are now eligible for a COVID vaccine and had an appointment at a clinic 50 miles away, so our near total energy self sufficiency came to a fossil fueled hybrid end.
Why only 99% ? We do most of our cooking on the wood stove – but we have a small butane stovetop we also use. At 10,000 BTUs per canister, that worked out to about 1% of our energy use. So we were only 99% self sufficient.