I wrote and illustrated this booklet for Living Energy Farm in 2022.
Sun to Table: Food Processing with a Solar Direct-Drive DC Microgrid
Introduction
At Living Energy Farm, a small, intentional community in Louisa, VA, we take an unusual approach to sustainability. Unlike most “Eco-friendly” homes and communities in the US, we don’t try to generate enough clean power to support the typical US lifestyle. We could never generate (not to mention store) enough energy for the planet’s 7.9 billion people to live like people in the US; the average US resident uses a staggering 74,000kWh of power every year (from electricity, fossil fuels, and other sources), nearly five times what people use in Mexico and 37 times what people use in Mozambique.1 If we want a future that’s livable and equitable, we have to develop sustainable solutions that everyone in the world can access. And the most privileged people on the planet have to learn to live with less. That’s what LEF is about.
At LEF, we grow our own organic food, share housing and resources, live in well-insulated buildings, heat buildings and water with solar-thermal heating systems, and use solar electricity directly and efficiently. We live comfortably with about 2kW of solar generation capacity, using around 1kWh of electricity per person per day,2 one-twelfth what the average US resident uses.3
We rely heavily on electricity for food production, including pumping water, winnowing, drying, grinding, cooking, and refrigeration. How do we do all this using only 2kWs of power? The answer involves a simple but very non-standard electrical system we call a “solar direct drive DC microgrid.”
“Microgrid:” our electricity comes from a series of PV panels, not the local power company.
1“Energy Use per Person.” ourworldindata.org. https://ourworldindata.org/grapher/per-capita-energy-use?country=IND~TWN~SWE Accessed 17 March, 2022.
2 Calculated based on NREL insolation data reported in Murphy, T. (2021). Energy and human ambitions on a finite planet: assessing and adapting to planetary limits. eScholarship, University of California. https://escholarship.org/uc/item/9js5291m. p. 208.
3 Based on US EIA data from 2018. Ibid. p. 105.
“Solar direct drive:” instead of storing electricity in batteries, the way most off-grid systems do, we wire most of our household appliances (pump, washing machine) directly to solar panels. We do have a few batteries to charge gadgets and power lights, but 90 percent of our electricity never passes through a battery. This means we get most of our power while the sun is shining. We adjust our energy use accordingly; for example, we only bake bread on sunny days.
“DC:” our appliances run on DC electricity, not AC. We save a lot of money and power by using DC electricity straight from our PV panels. For one thing, it saves us the trouble of using inverters, which are expensive to maintain and sometimes diminish system efficiency in low-power systems like ours. For another thing, it allows us to do more mechanical work with less power, since DC motors tend to be very tolerant of variable power inputs, making it possible to run a lot of them at once off the same panel.
Most US. appliances don’t use DC electricity; they use AC. Your washer, your fans, and your oven probably have motors specialized for AC. They are also optimized for the US 120V power grid. So a big part of what we do at LEF is develop machines that work with our power. For machines that turn—washers, mills, fans, saws—we often take off their original motors and belt them to DC-compatible motors. For machines that produce heat (ovens, hot plates), we rewire them and give them heavy-duty switches, since DC electricity is apt to arc and will burn out regular switches fast. In some cases, we build our own machines from the ground up.1
This zine tells the stories of nine appliances we use with our DC microgrid to process food. We hope to inspire you and your community to find more sustainable and equitable ways to live.
1 Please only try this at home if you have the help of—or are—an electrician.
Well Pump
Direct-drive DC well pumps have been around for 30 years, so we didn’t have to do any engineering or modifying to get our pump set up. We simply bought one (our pump is a Grundfos SQFlex, but there are many different kinds available), wired it to a 1,400W PV array, and we were good to go. What’s unique about our pump is what happened after we installed it.
Early in the farm’s life, before we had residential buildings, we only used the pump occasionally, to irrigate the fields when it didn’t rain enough. Those 1,400W of power mostly went unused. So we tried an experiment: when we weren’t pumping water, we hooked the PV array to a blower with a simple DC motor to dry seeds. We weren’t sure what would happen when a cloud passed over the sun. When power fluctuates like that, AC motors burn up. But to our surprise, the DC blower motor tolerated ups and downs in the power supply just fine, simply slowing down when a cloud passed. That was the beginning of our DC microgrid.
Still, we were careful never to run the pump and the blower at the same time. We were sure that would strain the motors to the breaking point, like what happens when you overload an AC circuit. Then, one day, it happened: somebody forgot to turn the pump off when they turned on the blower. But the pump kept running, and so did the blower. They slowed down, but they didn’t break. Over time, we learned that you don’t have to worry about overloading a DC circuit the way you do with an AC circuit. Even simple DC motors can tolerate huge power fluctuations without damage. That means it’s OK to power multiple appliances at once on a DC circuit.
Today our pump still works fine. We often run it at the same time as several other lower-energy appliances, such as the food dryer and the winnowing fan. Our DC microgrid is alive and well.
Winnowing Fan
This fan is one of our simplest food processing appliances: no belts, just a 22-inch triple fan blade coupled directly to a DC motor and stuck in a screen cage. Though it‘s not fancy, the winnowing fan is essential for processing our most important crops, the grains and legumes. On sunny days in fall and winter, after the wheat is dry, the Lima beans are threshed, and the corn and peanuts are shelled, people will say, “This is a good day to winnow. Get out the tarps.” As we pour the grain/beans/nuts in front of the fan, the chaff/pods/shells blow away, and the (mostly) clean grain falls into a bin, ready for grinding.
Grain Mill
This artisan Grainmaker mill is what we use to grind wheat, corn, and peanuts. As with many of our machines, it consists of a main apparatus belted to a separate DC motor. But when we set the mill up, we faced a challenge: the motor spins at up to 1800 RPM, but mills like this need slow speed and high torque. If they turn too fast, the grinding plates will overheat and actually cook the flour, which clogs the machine. Our solution was to belt the motor to the mill using a two-pulley system and a hanging “Jack shaft” with double bearings. This reduces the RPM from 1800 to 50, a perfect speed.
Active Solar Food Dryer/Heater
This is the system we use for drying and dehydrating fruits, vegetables, and grains. It starts on the roof, where air gets heated (up to 260 degrees in summer) by about 240 square feet of solar thermal panels. From there, a blower belted to a DC motor pulls the hot air down a 12-inch duct and into a closet off the kitchen where the drying racks are. With good sun, sliced peppers dry in a day or two. Sliced persimmons take 3 to 4 days. Dense grains like wheat and corn take two weeks. In winter, when there’s nothing to dry, we can redirect the warm air under the kitchen floor where it is stored in crushed rock and radiates heat up into the kitchen all night.
One advantage of our active solar food dryer is that it takes very little energy to run. It’s powered by our 1,400W PV panel, but the blower motor doesn’t need nearly that much energy, so it still works even if it’s cloudy. If it’s sunny, we can run lots of other appliances off the panel at the same time. In fact, we can tell by the sound of the blower if another appliance, such as the hotplate, has turned on properly: the hum of the blower will drop in pitch as the energy-hungry cooker starts up.
Blender
This is our source of salsa, hummus, smoothies, and pumpkin butter. Like most portable household appliances, it has a universal motor, compatible with DC or AC electricity. Pretty much any universal-motor blender will work with a DC system (provided the voltage is similar to the AC voltage the motor is optimized for), with one important modification: wiring.
Our blender used to have lots of buttons on the front that controlled the speed, but those electronic controls would have blown out the minute we plugged it into our DC system. Before we could use our blender, we had to open up the blender’s base and snip the wires to the control panel. Then we installed a heavy-duty, DC-friendly external switch to replace the one we disabled. Now there’s a piece of tape over the old control panel, and the blender just has two settings: on and off. It’s not fancy, but it works fine.
Roxy ISEC
Insulated solar electric cookers (ISECs) are simple, low-powered devices that cook food with direct-drive DC electricity inside thickly insulated containers. Over the last decade, people around the world have created many different models. Here at LEF, with our limited electricity supply, ISECs are an ideal way to cook, so we design and build our own.
The Roxy, named for its thick Rockwool insulation, is one of the first ISECs we designed. Think of it as a cross between an oven and a crock pot. It consists of a burner (homemade, made of nichrome wire set in cement) inside an insulated sheet metal box. Using only 330W, the Roxy is great for simmering things like soup, potatoes, rice, and corn. The Roxy is also good for baking squash and sweet potatoes.
One of the nicest things about the Roxy is that it has its own dedicated PV panel, so it doesn’t compete for power with other appliances. We mounted the panel on a swiveling “tracker” so it can get as much sun as possible all day long.
Big ISEC
The Big ISEC is our biggest, most powerful homemade electric cooker and the first to have rudimentary temperature controls. The Big ISEC can be set to low, medium, or high thanks to a series of switches. The switches adjust the resistance on the circuit that runs through the burners, which (unlike the Roxy burner) are just normal, store-bought, stove top nichrome heating elements. The more burners the current runs through, the higher the resistance, and the lower the heat output. The Big ISEC can run at 1400W (one burner), 700W (two burners), or 300W (three burners). The temperature control feature makes the ISEC ideal for baking. This is what we use to bake bread, roast peanuts, or just reheat leftovers.
But beware! Cooking with the Big ISEC takes practice! For one thing, the temperature can vary considerably depending on the weather. With full sun and full power, the Big ISEC can get up near 400 degrees F, but with clouds, it doesn’t get as hot. Also, because of the way the burners are wired, high heat is concentrated on the right side of the oven. It’s common to see cooks in our kitchen arranging potatoes on a baking sheet by size, with the big potatoes (which take more energy to cook) on the right and the small potatoes on the left. It’s also common to smell burning peanuts if the cook forgets to stir them while they’re roasting!
Another thing that makes it tricky to cook in the Big ISEC is the fact that it shares a power source—our big 1,400W panel—with lots of other appliances. Especially on a cloudy day, it’s common to hear cooks say, “Why isn’t my oven heating up? Oh, the pump must be on.” Or an exasperated cook may dash out to the shop and yell at the mechanics, “Turn those drills off, I’m trying to finish lunch!”
Hotplate
The hotplate is what we use if we want to get things really hot, really fast. It’s great for sauteing vegetables, making pancakes, or heating up water for a quick pot of tea.
The hot plate takes a lot of energy from the 180V PV panel. In order to get the most out of the hotplate, most other appliances need to be turned off, especially at the beginning when it is heating up.
Much like the blender, the hotplate is a store-bought device that we modified only by replacing the switch.
Refrigerator
How do you keep food cold in a refrigerator that only runs when the sun’s out? First, you super insulate it. Our Sundanzer fridge has four-inch thick walls. Second, you use a chest design. Cold air sinks, right? What happens when you open the door of an upright refrigerator? All that cold air slides right out the bottom, and the motor has to cool it back down again. With our fridge, we can open it again and again without losing much cold air. It stays cold for days at a time even when there’s no sun.
For folks used to a conventional upright refrigerator, it takes time to get used to a direct drive fridge. Because the compressor only runs whens the sun's out, the cold food itself is the "battery" that maintains the temperature. Therefore it works best if the fridge is kept mostly full with something dense- water bottles work fine if you don't have a lot of leftovers. Also, we teach people to avoid putting anything warm in the fridge at the end of the day. We let the leftovers cool down thoroughly before putting them away for the night.
More information
About LEF
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livingenergyfarm.org
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lowtechlab.org/en/news-blog/nomade-des-mers-living-energy-farm-une-communaute-sans-petrole
DC microgrids
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livingenergylights.com/learn-about-dc-microgrids/
Insulated solar electric cookers
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sharedcurriculum.peteschwartz.net/solar-electric-cooking/
