- Agronomic: corn
- Crop Production: greenhouses
- Energy: bioenergy and biofuels, energy conservation/efficiency, energy use
This project is to identify a flexible fuel biomass furnace that would have the heating capability needed for a hoop style, double polyethylene covered greenhouse and its capability to maintain a stable environment for crop production. Crop profitability would also be evaluated based upon fuel expense, handling costs for the fuel and by-products, and return on equipment investment.
Greenhouse crop production during the winter results in large energy demands to keep the greenhouse environment suitable for plant production. For the smaller grower, the greenhouse typically is heated by forced air furnaces, or unit heaters, utilizing either propane or natural gas as the energy source. On average, the price of propane has increased 16% each of the previous 5 years. Natural gas has had an average increase of 17%, however, the prices have been extremely volatile on a monthly basis whereas propane prices are more predictable.
In an effort to conserve on the increasing fossil fuel costs, many growers have lowered the production temperatures to the lowest they can and still grow plant material. These lower temperatures may result in more plant mortality and diminished plant quality due to higher fungus incidence in cool and highly humid environments. The grower needs to eliminate the concern over heating costs and redirect their attention back to growing quality and profitable plant material. To do this, a more stable and predictable priced energy source needs to be identified.
The most common and least expensive type of heating is by the use of forced air unit heaters using gas (natural or propane) as the energy source. Gas is a fuel which is convenient and requires no labor for use in the unit heater. If a grower was to convert fuels, say from gas to biomass, the simplest conversion would be to select a biomass forced air unit heater. Biomass heating has become more popular in recent years to the homeowner as a form of supplemental heating. Corn and wood pellets prices have remained inexpensive and fairly constant over recent years. Given the fuel price stability, I felt biomass would make an excellent alternative fuel for the large heat demands of a greenhouse. One consideration with biomass is, it requires handling in order to load the furnace and the heating unit requires occasional cleaning.
The purpose of this project was to evaluate the feasibility of using a biomass heating unit for greenhouses. Of particular interest is, whether the biomass unit heater will be able to recover quickly enough for the unique demands within the greenhouse. The amount of labor required to operate this heating method will be monitored and a return on investment calculation will determine if a cost savings can be realized. Finally, the growing environment will be evaluated for temperature stability and humidity or condensate control, as well as, identification of production issues that materialize during the study.
In order to implement the use of a biomass heating unit, I needed to explore the various units currently on the market, learn their construction and how each of them work. With this information I could better select a unit suitable for greenhouse applications.
In doing an internet search, I was able to find several biomass furnace vendors at a Penn State website (pennstate.cas.psu.edu) called Energy Strategies, Burning Shelled Corn as Fuel. I received more detailed information by phone and mail from several of these vendors and narrowed it down to four distinctly different models. I had the opportunity to make a personal visit with two manufacturers, Eagle Manufacturing and Year-A-Round, and saw the other two units at greenhouses I visited. The following is a brief summary of what I learned.
The first unit was a 170,000 btu Golden Grain Stove Model 3101 at a retail price of $3995. This unit was portable, had a 3 bushel hopper, double auger, two speed blower and didn’t require a chimney. The second unit was a 165,000 btu A-Maize-Ing Heat furnace by LDJ Manufacturing at a retail cost of $4,000. It had a 14 bushel hopper, single auger, three speed blower and required a chimney. Third, was a 200,000 btu Eagle Manufacturing Model 200 biomass furnace at a retail price of $4,200. This unit is put in a fixed location and uses a double walled chimney. It has a 7 bushel hopper, double feed augers and a single speed blower. Eagle Manufacturing also offered a model 300 which is a 300,000 btu furnace for $5,100 with a 10 bushel hopper and a larger heat exchanger. At the top of my prices for a suitable unit heater for my greenhouse, was from Year-A-Round Corporation. They had a model 500-FA, or 50,000 – 500,000 btu furnace, for $9,395. This unit had a 17 bushel hopper, manual Glow Plug ignition, burn pot with agitator, separate ash pan and solid state control panel.
Generally, all of the units have a storage bin of various capacities for the biomass; a single or double screw type auger that feeds the biomass from the bin to the burn chamber; some type of “burn pot” for the biomass; a heat exchanger; and a combustion blower. The combustion blower is a critical component of the unit as it is what keeps the corn, or biomass, burning. A fine balance of the fuel feeding system and the combustion fan has been established by each of the manufacturers. Too much, or too little, of either can put out the fire. The more expensive the system, the more detailed the unit becomes. In the more expensive systems, the burn pot (where the actual fire occurs) may have an agitator and a separate ash pan so the fire doesn’t have to be put out in order to clean out the unit. What I did notice with all of the units I investigated was, the initial firing cannot be automated. Whether you use lighter fluid on fire starter blocks or you purchase an expensive system that has electronic ignition rods, some sort of manual labor must occur to fire up the system.
Biomass Equipment Selection
Expense, durability and fuel flexibility were the primary considerations when selecting which biomass heating unit to test in my greenhouse project. The greenhouse environment is harsh due to the frequent incidence of water, constant high humidity and extreme summer temperatures. Of major consideration in this study was to have flexibility in the biomass and not be limited to burning corn. Corn is a commodity which is often in high demand due to its multiple uses as feed for livestock, cooking oils, syrups and the currently booming ethanol fuel market which could greatly affect its per bushel cost. By having fuel flexibility, our greenhouse operation will potentially stabilize energy costs which will allow for more crop profitability. Affordability of the unit is another major concern on selection. Careful consideration must be given for the biomass heaters initial cost and the added labor for operation, as these additional costs could counteract any savings in fuel expense.
Given its fuel flexibility and the apparent durability for harsh environments, I visited with Eagle Manufacturing Incorporated about using their shop model 200 for the study. In discussions with Joe Engle, owner, on my concerns of temperature stability within the greenhouse, we decided to error on the side of caution and use their model 300. Joe supplied us with a unit and also helped with the installation and make any performance adjustments necessary to evaluate the use within greenhouses.
With gas heating units, there is no labor during their normal operation. With biomass heating, one would expect an increase in labor due to fuel handling and unit cleaning. In an effort to reduce handling labor, I chose to purchase a used 235 bushel bulk bin from a local farming operation that no longer was feeding hogs. I attempted to find a used utility auger, however, quickly learned most used augers are in poor shape. I was able to order an inexpensive 4” x 12’ auger through a regional farm store. After installation, I found that loading the furnace is quick and easy.
The easiest way to compare greenhouse energy use between fuels would be to do a side by side comparison. Since we have only one greenhouse at the time, I received information from Penn State’s Agricultural and Engineering website on Equivalent Heating Values. From their equivalent chart, I could estimate the amount of an alternative fuel that would have been used to heat the greenhouse space. As part of the explanation for fuel btu (British Thermal Unit), I found that each of the fuels have a thermal efficiency. Most of the salesman from each of the furnace companies incorrectly compared the btu’s of the fuels and did not adjust them for the thermal efficiency (or transfer of energy from fuel to air) of the individual fuel.
For example, shelled corn has a heat content of 6,800 BTU/lb or 380,800 BTU/bushel. The thermal efficiency of corn is 75% so you have a net of 5,100 BTU/lb or 185,600 BTU/bushel. Propane has a heat content of 91,600 BTU/gallon with an 85% thermal efficiency resulting in a net value of 77,860 BTU/gallon. The net BTU from each of the fuel type make a more realistic comparison.
77,860 BTU(gal Propane)/5,100 BTU(lb corn)
15 lbs corn = 1 gallon propane or
1 bushel of corn (56lbs)=3.7 gallons propane or
1 bushel of corn = 3.4 therms natural gas
Propane and corn would usually be purchased prior to the heating season. However, refueling during the peak would be necessary but a pre-season “lock-in” price could be determined. Natural gas prices fluctuate monthly depending upon demand. If available, the natural gas supplier usually offers a limited number of customers lock in prices prior to the heating season. The history above is assuming an individual was able to lock in prices during September for all three fuels.