ASK THE EXPERTS: Solar Biodiesel

Solar collectors used for biodiesel processing
These nine collectors provide ample heating for biodiesel processing at this Colorado-based plant.
Solar collectors used for biodiesel processing

I just finished the article on “Solar-Powered Biodiesel” by Laura Mezoff Christy (HP148). I was quite impressed by the installation. How can one get 200 gallons of 185°F water from flat-plate collectors? I have not installed solar hot water collectors because for years I have been told that flat-plate solar collectors will not produce temperatures greater than 125°F.

I heat my house and domestic hot water in the winter with 150°F water from an outdoor wood burner. The water piping goes underground to a heat exchanger, and then to cast-iron radiators. If I could get a couple hundred gallons of 185°F or even 150°F water per day, in conjunction with the wood burner, I’d be golden.

Ed Lowe • Assonet, Massachusetts

Flat-plate collectors are easily capable of heating water above 125°F. Systems using flat-plate collectors can overheat (exceeding 180°F) if the daily use of hot water decreases or the collectors are oversized. However, the collectors are less efficient at elevated temperatures due to more heat being lost to the surrounding air. As the operating temperature of the collectors exceeds the ambient temperature, the collectors’ output is diminished. Extra collectors are needed to overcome the heat loss of higher operating temperatures.

In most of the United States, a simple sizing rule used in solar-heating water is to use 1 square foot of collector per every 1.5 gallons of water to be heated. On sunny days, this configuration can heat a tank of water to between 120°F and 150°F, depending on the local groundwater temperature and, to some extent, the season. Using this simple rule, about 100 square feet of collectors are needed to heat 150 gallons of water to 125°F or so. In southwestern Colorado (and the Southwest in general), the sizing rule is 1 square foot of collector per every 2 gallons of water to be heated (so about 100 square feet of collector area will heat 200 gallons of water to 125°F). But the oil for biodiesel production needs another 60°F to meet the required 185°F. The extra six collectors are more than enough to make up for the heat lost to the ambient temperature at the elevated operating temperature.

Another factor is that oil is being heated, not water. Water has a specific heat of 1—it takes 1 Btu to heat 1 pound of water 1°F. Canola and other vegetable oils have a specific heat of about 0.45, which means it takes 0.45 Btu to heat 1 pound of oil 1°F. Additionally, the oil is only about nine-tenths as dense as water. This fact, combined with other system specifics, means that it will take about half as many Btu or, in this case, about half as many collectors, to heat the same volume of oil as it would take to heat water. Volume, temperature rise, specific heat, and density are all factors in calculating the amount of heat needed to raise a given liquid to a certain temperature. It is not surprising that the nine collectors are presently supplying enough heat for 100% of the almost 200 gallons of oil per day production. The 270 square feet of collector surface area may be adequate to keep up with their future requirements.

Chuck MarkenHome Power solar heating editor

The standard, cast-iron radiators that you are using with your wood boiler system are designed for high temperatures, like steam-heat. You might be able to use lower-temperature solar hot water by switching to radiators designed for lower temperature operation. See ”Renewable Hydronic Heating” in HP152 for more details.

The Home Power Crew

Comments (1)

halasell's picture

That was a lot of good information. Sometimes it's helpful to think about the extreme case to understand the limits of the system. Heat loss out of the collector will match solar thermal into the collector when you've reached the highest liquid temperature the collector will support. At this point, more collectors won't increase the temperature further since each collector has reached the same balance.

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