Energy and Capital Costs of High Tunnel Construction

A version of this material was first presented as a poster: 

The material presented on this page has been modified to reflect a re-analysis of the data. Differences from the original published abstract and poster presentation are shown in green. These changes do not alter the original conclusions.

Abstract

Commercial vegetable growers in Kentucky have used high tunnels for year-round production for the past decade. They suggest it is a more energy-efficient and economical means of supplying off-season vegetables to the region than trucking field-grown produce from warmer regions. In 2005 we erected a 9 x 12 m high tunnel, designed to comply with National Organic Program standards, at the Kentucky State University Research Farm. We recorded the retail cost of each component, and estimated its embodied energy using published figures for common building materials. The materials used for construction were valued at $2,800, and contained 97 GJ of embodied energy. The structure and plastic cladding accounted for 24 and 76% of the total capital cost, and 84 and 16% of the embodied energy, respectively. Assuming that the frame, plastic cladding, and other components last 20, 4, and 10 years, respectively, the average cost of the tunnel is $328 yr^-1, and the average energy input is 12 GJ yr^-1, half of which is energy used to generate electricity for continuous operation of the  blower fan. The plastic cladding accounts for 50% of the annual amortized cost, and 33% of the embodied energy. If the structure is used to grow 2,400 heads of lettuce each winter for sale through direct-market channels  it could generate sufficient income to recover the total cost of construction materials in its first year. Trucking this amount of produce from California to Kentucky would require approximately 7.5 GJ. We conclude that there is an economic incentive for growers to adopt this technology, but no energy efficiency advantage to society. Longer tunnels, such as the 9 x 29 m models more commonly used by commercial vegetable growers in Kentucky, will be more energy- and capital-efficient.

Introduction

Commercial vegetable growers in Kentucky have been using unheated high tunnels for year-round production for more than a decade.¹ Proponents of high tunnel production claim it is a more energy-efficient and economical means of supplying off-season vegetables than trucking field-grown produce from warmer climates.² Increasing energy-efficiency can contribute to both environmental and economic sustainability.
In 2005 we constructed a high tunnel at the Kentucky State University Research Farm as part of an organic vegetable production demonstration. We set out to measure the economic and energy efficiency of using this structure for season extension.

Materials and Methods

A 9 x 12 m high tunnel was constructed on organic land at the  Kentucky State University research farm. Two layers of 6 mil poly were supported by 11 hoops, spaced 1.2 m apart. Wood-framed walls at either end contained a screen door and two windows for passive ventilation. Hoops and end walls were supported by steel pipes, driven 0.6 m into the soil. Poly was attached to the frame with aluminum 'wiggle wire' fastener tracks. A 60 W blower fan was used to maintain an insulating pocket of air between the plastic layers.

The cost and weight of construction materials was recorded, and the energy used to manufacture and transport these materials to the site was estimated. (Table 1).

Cool-season crops were transplanted and direct-seeded in the house beginning in late January. These were gradually replaced with warm-season crops, beginning in April (Fig. 1).

Temperatures were recorded at half-hour intervals 3 cm above the soil surface, 2 m inside and outside the north wall of the tunnel.

Windows were opened on most winter and spring mornings to allow passive ventilation. They were closed in the evening to retain heat through the night. At the end of May windows were opened permanently, and sides were rolled up to 1 m above the soil surface.

Fig. 1. Stages in construction of the KSU high tunnel. A summer cover crop of cowpea was incorporated  at the tunnel site (top left). End walls were prepared off-site (top middle). Steel pipes were pounded into the soil to anchor the structure (top right). Visitors to the KSU field day attached the bows to the anchor pipes (center left). Two layers of plastic were stretched over the frame (center middle) and attached with wiggle wire (center right).  Cool-season crops planted in late January (bottom left) were ready for harvest by March (bottom middle). Warm season crops planted in April were harvested in June and July (bottom right). A more detailed timeline of construction and planting is available here.

Results and Discussion

The materials used to construct our high tunnel required an energy investment of approximately 6 GJ per year over the life of the tunnel (Table 1). Operating the blower fan continuously demands another 6 GJ per year. 12 GJ could be used to truck approximately 4,000 heads of lettuce to Kentucky from California⁴ -- five times as much as the tunnel produces in a single harvest. The tunnel is expected to produce three crops of head lettuce per winter. 

Table 1. Embodied energy and cost of high tunnel construction materials. A cost analysis, including paving stones and paint, is posted here.

		Weight	Life	Energy intensity3	Embodied E		Cost	Amortized
Component	Material	(kg)	(y)	(MJ/kg)	(GJ)	(MJ/y)	($)	($/y)
11 hoops	steel	543	20	31	16.8	842	1,020	51
Poly fasteners	aluminum	45	20	241	10.8	546	310	31
Cladding (2 layers)	6 mil poly	184	4	86	15.8	3,950	660	165
Framing	pine	250	10	12	30.0	300	160	16
Toe boards	plastic lumber	150	10	12	18.0	180	300	30
Screen doors (2)	steel	18	10	31	5.6	56	250	25
Blower fan	steel	1	10	151	0.1	1	100	10
Total		1,191			97.1	5,876	2,800	328

High tunnels can be part of an economically sustainable operation. Our materials cost <$3,000 (Table 1). The value of the harvest will cover the cost of construction materials in the first year, and will far exceed the amortized annual cost of $328.
The tunnel buffered temperature fluctuations during the transition from winter to summer by boosting winter highs and moderating winter lows (Fig. 2). Differences between inside and outside temperatures became less pronounced when the sides were rolled up, and windows were left open for summer ventilation.

Fig. 2. Average daily temperature outside (top) and inside the KSU high tunnel. Vertical bars show temperature range for each day. A longer-term temperature analysis is posted here.

We found linear relationships between inside and outside temperatures for the KSU tunnel, with differences most pronounced at cool temperatures (Fig. 3). Such relationships could be used to measure high tunnel effectiveness.

Fig. 3. Relationships between inside and outside temperatures. A longer-term analysis of these relationships is available here.

Our energy analysis raises questions about the environmental sustainability of even unheated houses using the double layer inflated poly system. This is just one of the systems used by the growing number of producers using high tunnels for season extension in Kentucky: Single layer houses, and double layers separated by stretched lines of T-tape are also used. These should be more energy efficient, as would larger houses using the inflated poly system. We plan to evaluate alternative systems in future studies.

Conclusion

1. High tunnels can allow year-round vegetable production in Kentucky.
2. The double layer inflated poly system is affordable, but energy intensive at the scale tested.
3. Daily minimum and mean temperatures inside a high tunnel are linearly related to outside temperatures.
   

Literature Cited

1. Wiediger, Paul and Alison. 2003. Walking to Spring. Au Naturel Farm, Smiths Grove, KY.
2. Coleman, Eliot. 1999. Four Season Harvest. Chelsea Greens Publishing Co., White River, VT.
3. Wada, Yoshihiko. 1993. The appropriated carrying capacity of tomato production: Comparing the ecological footprints of hydroponic greenhouse and mechanized field operations. M.A. thesis. School of Community and Regional Planning, University of British Columbia.
4. Pimentel, D. and M. Pimentel. 1996. Food, Energy, and Society. University Press of Colorado.

 

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