Since there has not been a posting for nearly 2 years I suppose you think someone died, but no, we are simply very busy. A prime example is the solar photovoltaic (PV) addition we did last month. While working steadily to harvest and store our massive garden’s bounty we spent many evenings discussing how to make life a bit easier. We decided to try reducing the amount of firewood we fell, cut, lop, split, stack, carry, and burn using stored sunshine from a different source.
Sure, firewood is a renewable resource but even our highly efficient masonry stove puts out particulates and unburned gases when it first starts up. The obvious way to attempt this would have been to put up a couple of kilowatts of wind turbine to supplement our off-Grid solar system. Wind often blows at night, when sunshine is not beaming through our large windows, getting passively stored directly into our 30-ton masonry floor slab. But sometimes that same wind would blow when the sun is also shining, overheating our house unless we could think of a way to store the heat, day or night. And at our age we think increasingly about reliability, low maintenance, and less heavy lifting.
So, since we needed to deal somehow with the added heat, we first focused on how to store more heat and where to store it. A heat pump is certainly very efficient since it simply moves heat rather than creating it, but the added cost, complexity, noise, and convective air circulation (moving air makes you feel cooler) or need for a heat storage fluid made little sense to us. It can be far cheaper and simpler to produce heat rather than move it against a temperature gradient, as heat pumps do. And to store heat it’s easiest to work with conducted or radiated energy. There are various sorts of electric heaters on the market, some radiant, some convective, and a few that rely on heat conduction. We settled on some stainless steel conductive heating strips sold by a division of Grainger. Mounting these on the masonry stove’s surface would move extra heat directly into an existing 2-ton mass for gradual release.
And we decided to use more solar PV panels. Raising and maintaining a large wind turbine far from our house, where the wind is strongest, has both technical and physical limitations, while adding 2 kilowatts of solar almost doubles our current PV system, from 2.4 kW to 4.4kW. The familiarity and easy maintenance of solar won out. We wanted to either run 120-volt DC directly into the surface-mount heating elements or channel the energy into a couple of MPPT (maximum power point tracking) controllers, boosting the input into our off-Grid battery bank. The 120-volt figure works nicely with off-the-shelf heaters, and achieving that voltage simply requires finding solar panels whose “Vmp” (voltage at maximum power point) totals 120 when added in series. And 2 kW of energy is no magic figure, but it does represent almost 6900 BTUs per hour. Over a 5 hour sunny winter’s day this totals over 34,000 BTUs, which is a little over a third of our average nightly fire’s total output.
Once the heaters arrived we ordered eight, 255-watt, Sainty mono-crystalline solar panels on E-Bay at $0.85/watt, freight included, on Oct. 10th. These have a Vmp of 30.0 volts, and four in series make up the 120 volts moving to the house. After travelling 250 feet from panels to heating elements they lose roughly 2.7 volts, with still well enough voltage to power the heaters. After toying with various design options we we bought all of the parts to make racks for them on Oct. 15th and the panels arrived from California the next day. We started putting up 2-inch, schedule 40 steel pipe on the 17th and finished the concrete work on the 18th. After cutting and mounting Uni-strut channel steel we began to actually rack the panels on the 19th and finished on the 20th. You can see their new location along the driveway just above our orchard.
Wall penetrations into the house were done on the 21st and we laid out cable, enclosed it in 1-inch polyethylene pipe, and used the electric tractor’s 1-bottom plow to bury 230 feet of cable on the 22nd. The last of the clean-up was done on the 23rd.
Later that week we received two Midnite Solar Classic 200-SL MPPT charge controllers. These were a major investment, at $509 each, that could have been avoided had we only wanted back-up heating, not battery charging. Homes with 24-volt or higher battery bank voltages could get by with a single controller. But since the controllers are limited by the amps they can handle, our lower battery voltage means more amps must be controlled at the same power rating (Watts of power = Volts X Amps). So each controller handles one set of four PV panels. Input breakers were installed for each rack of panels and switches were installed to move input power to either the controller input terminals or to the strip heaters. And output breakers were installed between each controller and the battery bank. When all was wired we started to reap the benefits, even on cloudy days, in the form of quicker charge rates and the ability to run additional daytime loads. And we can independently run each set of panels into either a heater or the batteries.
The strip heater mounting on the masonry stove began next, right after we finished helping a neighbor install a 400-watt wind turbine to supplement her 360 watts of off-Grid PV power. Wind energy is perfect for her ridge-top site, using the simple 33-foot tower kit made for that turbine. The 260-foot distance from the tower site to her house would normally require thick wiring to efficiently charge her 12-volt battery bank. But this turbine puts out roughly 120 volts as 3-phase AC in 14-2 gauge UF Romex, and it uses an MPPT controller to match the house voltage, saving greatly on wire costs.
Above you can see the two strip heaters attached to the stove’s surface. I used some left-over sanded grout to smooth the brick surface and used Tapcon screws to attach the strips over the wet grout. The high temperature white woven wire insulation on the 8-gauge wiring at the bottom and copper screw-on terminals were used to avoid problems with melted solder and stinking plastic wire insulation. The first layer of the bricks that surround the heaters are grouted level into place.
After cutting slots for the strip heaters into the back of 22 more brick using a diamond blade and rotary saw, I simply stacked the brick, creating a “surround” to protect those walking by. Near the top I screwed a thin piece of galvanized steel into the stove using two more Tapcon screws and drilled holes to match the brick channels. Then two pieces of 1-inch galvanized EMT were threaded down the outer brick channels. This allows us to easily unstack the bricks should anything go wrong with the heaters.
The final stack includes some trimmed floor tiles at the top. As I write the heater has really boosted the brick temperatures. Running for nearly 4 hours the temperature of the stove bricks 4 inches from the heater surround is around 165F. A full day’s sun will add to that and make the heat penetrate even deeper into the stove’s mass.
Soon our neighborhood will be snowed in and frozen over but we certainly won’t freeze or starve in the dark!
As an update in May, 2018, I can now report that the heaters did indeed lower our firewood use by a little over 33%. This means less wood to cut for heating but also plenty of solar to use in cooking/baking electrically, even on cloudy days. We no longer have to rely on LP gas (propane) for cooking when the clouds move in, and the extra power will be used this summer to operate a 14,000 BTU portable heat pump. It can be used as a fan, a dehumidifier, an air conditioner, or as a highly efficient heater (about 2.5 times as efficient as the electric strip heaters if you don’t mind the fan noise, the moving air, or the lack of any way to store the heat or cold). We may use it in the house occasionally, but will no doubt also use it to cool our sauna when we need to store freshly picked fruit in the fall.