Fuel-Free Back Up Power

Usually PV systems are backed up with generators, but they are loud, burn fuel, and are inconvenient and expensive to buy and to run.

We needed to upgrade our electrical system to 240V to run electric appliances like a stove, and we needed more energy storage capacity to get through a few cloudy days. We added another battery bank (more about that in another post) and an 8 kW 240V Outback inverter.

Our solar contractor Zeke Yewdall of Dark Forest Solar offered to find buyers for the old 120V equipment, but I had a crazy idea that just might work. Why not use the old solar PV system to back up the new one, I asked?

We hired our neighbors Robert and Max of Max Electric to rewire our electrical panel for 240V, and relocate the breakers for the circuits we wanted to back up (well pump, essential lights, electronics) to one leg of the panel, putting less important circuits on the second leg.

When the new solar system is running, we have 240V and can power the stove, as well as all the 120V circuits on both legs of the panel. When the energy in the batteries falls below a setpoint (which is pretty low since the new batteries are not lead acid and can tolerate being deep-discharged without reducing their lifespan), the old 120V system takes over and we have another 10 kWh of capacity on backup power. All without running a generator or using fuel!

Radiant heat

Though the previous owner designed the house to be thermally efficient, we found that the existing dark ceramic tile/concrete slab floor was always colder than the air, and was unpleasantly cold in the winter. It did not function as a thermal mass to heat the house because there is not sufficient thermal gain through the windows to bring such a large thermal mass above the room temperature.

So we added radiant thermal tubing to the floor, and in the living areas we added insulative wood on top. The house is already much warmer, without even running hot water through the hydronic system. Once we finish the flooring, the next step will be to add the equipment to heat and circulate the water.

Carbon-friendly refrigeration

One of the top climate actions recommended by Project Drawdown is to switch to non-HFC zero global warming potential hydrocarbon refrigerants (i.e. R600a) for refrigeration. Hydrocarbon refrigerants developed by GreenPeace in the 1990s are coming online that have zero global warming potential AND don't damage the ozone. The new refrigerators using "R-600a" are also more efficient (=save you money long term and take less space). Refrigerators and air conditioners using hydrocarbon refrigerants are in wide use abroad and have become available for commercial and residential applications in the US in the past couple of years. Refrigerator salesmen have no idea which refrigerant their refrigerators use, and many spec sheets don't even say which refrigerant a model uses (this is changing as the industry makes the switch to R600a). It can be hard to find this information online, but you can always find it out if you read the fine print on the label inside the door or on the back of the refrigerator.

Our off-grid house came with an efficient 24-VDC chest freezer in the garage, which runs off our backup PV system. That's where we keep things that really shouldn't thaw, like the 1/4 grass-fed cow from our neighbor's pasture.

During our first winter, when we had inadequate PV generation and energy storage, we turned off our refrigerator and stored food in a metal cabinet in our garage, which was at about the same temperature as a refrigerator (34-40 degrees F). We found that food actually kept better in the cabinet in the garage, perhaps because of the air circulation, or lower humidity. But that's maybe a little hard-core, even for us. It is so convenient to store your food in the kitchen! So of course, we needed a regular 120 VAC refrigerator. But we wanted one that was efficient and climate-friendly.

Our search was made more difficult by the fact that new refrigerators were back-ordered for months (this was during the COVID-19 pandemic), and not all models were available.

So we were thrilled to find an efficient R600a refrigerator in stock at a local store! If you're interested, the graphic above is the specs for the model we found: it's super simple and uses about 2/3 the energy of the larger fancy fridges (which was also important for us, since we are off-grid). And it was cheap, too. It's made in Mexico by Winia (Daewoo's appliance brand). We've had zero issues with it, it only uses 1 kWh/day, it's relatively quiet and it works quite well.

When your fridge or AC breaks, it is also important to be sure the HFC refrigerant (e.g. R134a) is captured before trashing the unit.

https://medium.com/…/climate-friendly-fridges-are-truly-coo…

https://www.environmentalleader.com/…/epa-approves-three-a…/

https://www.katom.com/…/true-refrigeration-unveils-new-hc-r…

https://products.geappliances.com/…/gea-support-search-cont…

https://news.samsung.com/…/samsung-earns-energy-star-emerg…/