by Tamara Scully
“Heating is typically a very significant component of the overall energy use in a greenhouse operation,’ A.J. Both, Associate Extension Specialist, Bioresource Engineering, Department of Environmental Sciences Rutgers, said in a presentation for The Farm Energy IQ (FEIQ) program.
Greenhouse heating systems typically consist of forced air unit systems, or hot water boiler systems. No matter the system for heat delivery, the heating system needs to be sized to meet the heating needs of the greenhouse. Sizing a system properly requires calculating the expected amount of heat loss.
The greenhouse as well as site conditions play major roles in heat loss. Windy conditions or a set-point temperature high above the external temperature both increase energy needs, and require increased capacity in the greenhouse heating system.
The energy generated by the boiler, versus the energy lost from the system, is the conversion efficiency of the system. Each fuel source has its own conversion efficiency, too.
“If you don’t include conversion efficiency, you undersize the heating capacity needed for your particular application,” Both said.
Keeping heat inside
Structural heat loss depends upon the materials used in the structure, the surface area of the structure, and the difference between the internal greenhouse temperature set point and the outside temperature conditions. A larger surface area corresponds with an increase in heat loss potential.
Infiltration loss is that which occurs around openings. Sealing leaks and repairing glazing cracks will increase efficiency. Adding vent louvers to fans will create a tighter seal and prevent unnecessary air loss.
Perimeter loss is due to escaped heat from the greenhouse to the soil, which is primarily a problem in areas where groundwater is moving close to the surface. Adding insulation to the sides of the greenhouse, continuing below the soil line, will decrease perimeter loss.
“It pays to consider using a horizontal layer of insulation underneath your entire floor,” if you are in a wet area, Both said. “If there is no barrier, we can see a fair amount of heat escaping. Dry soil is not a good conductor of heat.”
Adding together the amount of heat loss due to structural, infiltration and perimeter losses provides a total number that can be plugged into a mathematical formula to determine the cost of providing a unit of heat to the greenhouse. This cost can be compared across fuel sources by calculating the Btu’s produced per unit of heat, knowing the unit cost of the fuel, including the conversion efficiency, and finding the cost per Mbtu (millions Btu).
This Mbtu calculation allows “apple to apple comparison” to determine “how much it costs heating a space with various sources” of fuel, Both said.
Heating system design
Forced air units are less expensive, but hot water systems deliver more uniform heating, and offer more flexibility in delivery options.
Hot air systems can be modified with polytubes, either overhead or on the floor, which allow the warm air generated by the system to mix in a more uniform manner throughout the greenhouse. A polytube has small openings which allow the warm air to escape. The velocity of this escape creates turbulence, which determines how well the air mixes.
“Make sure that warm air is mixed properly with the rest of the greenhouse air,” Both said.
Horizontal airflow fans can create a circulatory pattern which allows all plants to be grown in a uniform environment, rather than having pockets of different temperatures. This keeps the growth rate steady, an important factor if the plants are to be harvested at one time.
Hot water systems allow the pipes to be placed in the ground, under benches, or around perimeters to better distribute heat uniformly to the plants. One advantage of hot water systems embedded in the floor is that the materials absorb heat, and this residual heat will be released should the heating system shut down.
Embedded water pipes also allow the potential for “ebb and flood” irrigation. Holes are drilled into the plumbing system, allowing irrigation to occur by flooding the floor as needed. Hot water systems can be used to deliver root zone heating.
Root zone heating can potentially allow the overall air temperature of the greenhouse to be reduced, as heat is being delivered directly to the plant roots. However, the challenge is that the system cannot be turned up high. Abnormally cold outside temperatures can require a supplemental heating system.
Other energy savers
Adding curtains is a recommended measure to increase energy efficiency. Curtains reduce radiant heat loss, keeping solar energy inside the greenhouse. While many growers try to save costs by using a combined curtain for shade and for heat retention, separate curtains for each function allow better management and energy efficiency.
Condensing boilers combust fuel, releasing water vapor, which is then condensed back into water, recollecting the heat. This energy is then typically used to preheat the return water going to the boiler, increasing the overall fuel efficiency.
Having properly placed sensors also keeps energy use down. Sensor should be inside an aspirated box, to allow airflow while protecting them from moisture. The sensor should be located near the plants. This method provides the most accurate readings of actual plant conditions.
Solar panels can be added to greenhouse walls or roofs. Particularly on roofs, shadows can become a problem, creating conditions of non-uniform light reaching plants.
Wind turbines are not commonly used in many regions, but can reduce energy consumption by a significant amount under the right conditions.
Heat pumps, using geothermal energy, have excellent potential for reducing greenhouse energy consumption. There are both horizontal and vertical system installations. When combined with a heat storage unit, very high efficiencies can be achieved.
Burning waste residue, such as shredded wood, in a biomass boiler holds great potential if the resource is readily available. Designing a biomass boiler to operate at maximum efficiency, and then storing any excess energy for use when needed, is the most efficient approach.
Other options include wood burning stoves of various sizes, from a small unit to larger pelletized units that accept pallets of fuel at a time. Smaller woodstoves will need fuel added frequently.
At the Rutgers EcoComplex, a combined heat and power system is used in the greenhouse. Landfill gases are captured and utilized as an energy source to generate electricity. The electricity is then used onsite in the greenhouse, as well as sold back to the grid. Next, the waste heat generated from the heating system is recollected via a heat exchanger, and used to heat the water for the greenhouse heating system. A conventional heating system typically operates at under 50 percent efficiency. A combined system operates at over 85 percent efficiency.
While growers may be interested in implementing new systems for energy reduction, including alternative energy sources, Both recommends that they implement “conservation measures before you think about implementing any strategies for reduction in consumption.”
Energy: a greenhouse primer
by Tamara Scully