Go with the Flow

Passive Solar Biodiesel Storage

Inside this Article

Tami Schwerin fills the tank of her diesel Mercedes with biodiesel
Tami Schwerin fills the tank of her diesel Mercedes with biodiesel from Piedmont Biofuel’s solar straw-bale shed.
Piedmont Biofuel’s passive solar cob shed
The thick walls of the passive solar cob shed provide thermal mass, helping moderate temperature swings inside the building.
Tami Schwerin fills the tank of her diesel Mercedes with biodiesel
Piedmont Biofuel’s passive solar cob shed

Cold weather is the Achilles’ heel of biodiesel-fueled vehicles, congealing the fuel to a sludgelike consistency, and clogging fuel lines, filters, and injectors. While onboard heaters in your vehicle can warm the fuel and keep your rig rolling, that only solves half the problem. Cold temperatures can slow the flow at the pump as well, creating a jam for biodiesel home brewers and dispensers who need to keep their fuel flowing through the winter. 

Fortunately, there are options: These easy-to-build, passive solar sheds keep stored fuel warm and flowing—and the best part is, they do it without any additional energy costs. The buildings come with an added bonus—if your plans change and you end up not using the structure for fuel storage, you can still use the space for a garden greenhouse or whatever solar scheme you dream up next.

Chill Factors

Although winter temperatures in the Piedmont region of central North Carolina aren’t typically frigid, they do sink into the teens, causing problems for our biodiesel fuel co-op, Piedmont Biofuels, which distributes 100% biodiesel (B100) to its members.

As ambient temperatures plunge, microcrystal waxes start to form in the fuel. Large crystals precipitate and eventually settle to the bottom of the tank or storage barrel. The lower the temperature drops, the cloudier and thicker the fuel gets. A fuel’s gel point is the temperature at which the fuel freezes and can no longer be pumped or poured. B100 made from new vegetable oil typically has a cloud point (the temperature at which waxes form) of 32°F and starts to gel at 20°F. Biodiesel made from different feedstocks will cloud and gel at different temperatures. (In comparison, petrodiesel typically has a cloud point of 20°F and a gel point of 0°F.)

Winterization methods for pure biodiesel come in many forms. During colder months, some biodiesel users add winter petrodiesel to make a B50 blend with a lower gel point. Others install in-line fuel heaters in their vehicles or plug in their electric engine block heaters overnight. Another, albeit fuel-intensive, method to keep biofuel from gelling is to keep the rig in a heated garage.

Passive Solar Solutions

As a B100 provider, we wrestled with cold-flow fuel pumping issues for several winters. Because our co-op tends to be supported by sustainability addicts who strive to fuel their lives with as little petroleum as possible, winterizing our B100 with petrodiesel was not acceptable. We also were interested in keeping within our mission of leading the sustainability movement in North Carolina, which meant keeping our energy inputs low.

The answer to our dilemma was to design around the problem—literally—so we planned a passive solar storage structure that would heat itself with the sun’s energy. Our previous cob construction experience (see “A Hand-Built Home” in HP112) and access to an abundance of red clay made the decision to build a cob greenhouse a natural choice. We purchased sand and straw from the local lumberyard, and rescued lumber and tin roofing from an old shed that had originally sheltered our 500-gallon biodiesel fuel tank. A glass door and window were hiding nearby in the co-op’s boneyard, waiting for a new home. All told, the materials cost just a few hundred dollars.

We figured that incorporating enough thermal mass—including the tank—into a south-facing structure with adequate glazing could keep the biodiesel above its gel point throughout the winter. We wanted the shed to be big enough to allow access around the tank, but small enough (less than 144 square feet) to count as an agricultural building and not need a building permit. We finally settled on a design that would give us a 100-square foot building, with 1-foot-thick cob walls.

Creating with Cob

Using recycled and local materials helped ensure that the construction of the shed would have a small environmental footprint. A group of co-op members provided labor in exchange for the opportunity to learn how to build with cob. Together, a half dozen of us accomplished the construction in a series of Sunday afternoon workshops after the weekly lunch potluck.

First, we laid landscaping cloth and then tamped gravel to establish a base. Hand-stacked stone, filled in with slip-straw (straw dipped in a water-clay slip), serves as the perimeter foundation. For the cob walls, sand and clay, mixed in a 2:1 ratio, were thrown into a churning cement mixer, watered to the appropriate consistency, and allowed to spin for a minute or so before a few handfuls of straw were added. The finished mix was applied to a slowly growing wall. In appropriate places, lumber was keyed into place for attaching windows and doors.

Cob’s trade-off is that it takes lots of human energy and time. Obtaining and mixing the materials is a slow process, and building up the walls will exercise your muscles—and patience. After about three months of Sunday cobbing, we were ready to put on the roof and frame in the door and windows. During the weeklong intervals between the cob sessions, we covered the walls with several layers of tarps to prevent the walls from drying out too much, which could have compromised the bond between the individual layers of cob.

Despite the up-front work, the finished material has benefits—cob is slow to take up water, slow to degrade, and, most importantly for our purposes, slow to change temperature. With no supplemental heating, the building achieves a constant 20-degree delta T—a difference of 20°F between the ambient (outside) temperature and temperature of the biodiesel in the storage tank—which has suited our fuel-storage strategy very well. Even on 18°F mornings, our fuel is at a cozy 38°F, and flowing nicely.

Initially, we’d considered insulating the north side of the cob greenhouse, either with straw bales or even with used telephone books. This would have improved the building’s thermal performance, but ultimately, the biodiesel in the tank and the cob walls provide enough thermal mass to store the sun’s energy overnight. For our purposes and climate, additional insulation wasn’t necessary, but it may be essential in colder locations.

Biocompatible Bales

As the co-op and demand for biodiesel grew, so did our need for more fuel storage. Although the cob structure performed well and was inexpensive to construct, we needed to be able to build a place for our next solar-heated biodiesel tank in a few weekends instead of a dozen.

We still wanted to hold on to our original criteria of using locally available and salvaged materials to build an inexpensive passive solar shed, and a combined straw bale/stick-frame construction approach fit the bill. For the wall structure, we used leftover pressure-treated lumber donated by a local deck builder and insulated it with straw bales from a local farm. What straw bales lack in thermal mass, they make up for in insulation value, about R-2 per inch. For greenhouse applications, straw bales are a great fit. The material is easy to use and effective. Covered with an earthen plaster, it’s also beautiful.

But beauty is in the eye of the beholder. Built within the city limits, the straw-bale structure proved to be a thorn in the side of local building and fire regulators, who were specifically concerned about the potential hazards of storing 500 gallons of biodiesel inside a straw bale structure. Although inspectors could not find any code references to bring down our straw-bale shed, we allayed their biggest concern by finishing the inside of the shed with drywall. This satisfied the inspectors, who finally signed off on the building, giving us the first legal straw-bale storage shed for biodiesel in North Carolina.

Even with the additional work of sheathing the interior, this project only took two people four days to complete. With hired labor, the associated expense—about $1,000—was significantly more than the cob shed’s entire construction cost.

Passive Performance

To satisfy his inner nerd and prove to skeptics that inexpensive, simple solar sheds made of wood, straw, and clay can offer good performance, co-op member Don Mueller monitored temperatures inside both structures with a datalogger. On average, the straw-bale shed manages to keep stored biodiesel about 25°F above ambient temperatures; the cob structure generally maintains biodiesel about 20°F above outside temperatures. The sun also provides another function for each off-grid fueling shed: A 20-watt PV module charges a 140 amp-hour battery bank, which provides power to a 12-volt fuel pump. 

Though we’re quite proud of our little operation, we have no interest in becoming the next Rockefellers of biodiesel. Our goal is to meet our co-op members’ fuel needs with feedstock that we have on hand. Presently, we meet the transportation fuel needs of about 300 families, with the average member using about 45 gallons of biodiesel per month. Tapping into the sun not only makes this possible—it makes it practical, and keeps our fuel flowing and customers happy. B100 users pull up, grab a fuel nozzle, fill up, and are back on the road in no time flat.


Stephen Hren is a teacher, writer, and carpenter living in Durham, North Carolina. He is currently working on a book with his wife Rebekah called The Carbon-Free Home: 36 Ways to Kick the Fossil Fuel Habit, to be published by Chelsea Green in the spring.

Lyle Estill is a founder of Piedmont Biofuels, and is currently an active board member. He is the author of Biodiesel Power: The Passion, the People, and the Politics of the Next Renewable Fuel (New Society, 2005). He is the publisher of the widely read Energy Blog, which can be found at www.biofuels.coop.

Piedmont Biofuels • www.biofuels.coop

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