Reducing Heating Costs in Greenhouses

BACKGROUND AND GOAL OF PROJECT

Transport cost for produce from the southern and west coast USA, Mexico and Central America is increasing due to rising costs of truck fuels. Transport distances often more than 2,000 miles to the northern central USA, northeast USA and Canada consume large amounts of fuel. Production cost in southern and western regions is increasing due to rising costs of irrigation water and electricity for pump operation to lift ground water from deeper reservoirs and pressurize irrigation spray systems. Delivery distances are much shorter for locally grown produce, soils are more naturally fertile, and rainfall is more adequate as a water source in northern growing regions.

Consumer demand for fresh produce year-round has been established throughout the USA and Canada. Demand for specialty and high-quality produce has increase in all regions. Greenhouses that are efficient for extended time operation in northern climate conditions allow northern growers to be reliable suppliers of many varieties of fresh vegetables, salad greens, herbs and small fruits for much of the year to consumers in their region. Organic fresh produce can be grown in the protected stable environment of a well designed greenhouse.

The goal of the project is to develop greenhouse technology which provided adequate levels of light, temperature and fresh air for plant growth over the longest possible period of the year in northern latitude regions with low operating expense. Moderate cost technology is of course needed so that investment is paid back with lower operating cost and improved yields of high-quality produce. Extending the feasible operating time period of greenhouses in northern regions enables year-round utilization of land, water systems, property taxes, equipment, vehicles, packing and storage facilities and other overhead items.

High-Efficiency Greenhouse is the term used in this paper regarding several aspects of greenhouse design and operation that will be developed in the project: (1) enhancement of natural daylight to reduce need for electric lighting, (2) insulation methods to conserve heat, (3) efficient ventilation systems including recovery of sensible and latent heat from exhaust air to preheat incoming fresh air, (4) heat storage systems to absorb excess passive solar heat in the day and release useful heat at night, (5) efficient internal air circulation and distribution, (6) renewable sources of heat for greenhouses including better use of the existing solar heating effect in glazed structures and renewable local fuels (wood waste, agricultural residue, bio-fuels, etc.

Northern Climate and Plant Growth
Extreme changes of light and temperature occur during the annual weather cycle in northern regions, defined as north of 40 degrees latitude or approximately Zone 4 and farther north. Natural light availability and outdoor temperature range from extreme low to extreme high values. Daily integral of light energy (accumulated daily value, called Daily Light Integral or DLI) is the combined effect of the day-length and light intensity on the ground surface. DLI varies from very low values in the winter to very high values in the summer. Day-length is the time period from sunrise to sunset and ranges from four hours in the winter to twenty hours in the summer in the farther north latitudes. Shortest day-length occurs at the Winter Solstice (December 21) with only 4-8 hours of daylight per day) depending on latitude), Longest day-length occurs at the Summer Solstice on June 21.

Progress to Date with Experimentation
Five experimental free-standing small low-tunnel greenhouses have been built for initial evaluation of new concepts. All the test greenhouses are oriented south so the side with the largest glazing area faces due south, This orientation appears to provide the maximum possible natural light during the low-light period of the year. All the greenhouses are covered with clear 6 mil polyethylene plastic film stretched over rounded arch hoops. Various end closure methods are used including rigid foam plastic, extruded clear polycarbonate sheet cut to fit the end shape, or gathering the film at the ends. Rolling the cover up on the sides allows access into the greenhouses on either side in most cases. Three test greenhouses are situated over raised soil beds. Two test greenhouses are placed over table areas for seedling trays. Several designs of heat storage and soil bed warming methods are being studied.

Nearby buildings shelter the greenhouses form wind to some extent. Some blockage of sunlight occurs in early morning and late afternoon due to proximity of buildings. The greenhouse structures stand about 8-12 feet away from the buildings. Underground air ducts connect the greenhouses to the nearby buildings to supply warm air for supplemental heating when needed. Moderate temperature air (70-90F) is supplies by a wood-fires heating system in the basement of the nearby home. Supply and return ducts provide a continuous loop for air circulation through the length of the tunnel. Air ducting and airflow are used to remove the excess daytime solar heat when this occurs inside the greenhouses. Greenhouses #1, #2, and #5 have individual small blower-fans that boost airflow for more rapid transfer of excess heat to the heat storage systems in these greenhouses. Recycled multi-speed blower-fans salvaged from forced-air gas furnaces (800-2,000 CFM) are used for the warm air supply and return airflow system. When supplemental heating or removal of excess solar heat is not needed, air circulation is provided at a minimal level to supply fresh air for plant respiration.

Active solar heat collection panels on the roof of a workshop and a wood-fuel furnace in the nearby home provide supplemental heat for the small experimental greenhouses. The active solar panels produce warm air which is well-suited to greenhouse heating or the active solar panel output can be stores in a separate heat storage system. The active solar panels have high-absorbing, low-emitting selective coated copper absorber plates covered by double-layer insulating glass units. The active solar system and a large rock bed heat storage unit have been operating since 1978 in a research project funded by the Minnesota State Energy Office.

Rainwater and snow-melt water are collected and stored for irrigation use. The unit cost of city water is quite high, such that investment in rain and snow water collection systems is worthwhile. Chlorine-free, mineral-free water may also be more favorable to plants. Runoff water from greenhouses is collected in the aisles between greenhouses and drains toward grates at several points. Piping from the grate drains conveys the water to storage tanks in the workshop and the basement of a nearby home. Simple low-cost rain and snow water collection systems for runoff from roofs of larger produce growing greenhouses and may be economically feasible to reduce need to pump water up from a well in rural areas.

Details of Experimental Greenhouses:

Greenhouse#1 is a 5’x 35’ plan with 5’ height and is used to grow transplant seedling after the seedlings have been started with electric lights indoors. The north side is 5’ high vertical wall made of treated wood 2x8s, treated plywood and 8” thick polystyrene foam insulation. The wall is painted white on the interior and sheathed with western red cedar on the outside. Hoops made ¾’ PVC pipe are attached to the vertical north wall and allow natural light to enter through the south side and top of the enclosure space. Seedlings are moved into #1 in late March when the sun us at a higher angle and blockage of diffuse light from the north direction by the insulated north wall is not as much a concern as would be if this greenhouse was used during the winter.

Trays of seedlings are places on a full-width table in greenhouse #1. The table is made of four layers of hollow-core concrete blocks laid on their sides for the entire area inside the greenhouse. The massive amount of concrete in the table structure in greenhouse #1 stores all available excess daytime heat. Warm air collected at the top of the tunnel space is circulated through the core holes of the side-laid blocks. The blower-fan that circulates the sir through heat storage system also provides HAF (horizontal Air Flow) in the length direction in the growing area in greenhouse #1. Electric lights may be added at some point so that seedlings can be germinated in greenhouse #1 in January and February, to compensate for the blockage of north side diffuse daylight in this design due to the insulated north sidewall.

Greenhouse #2 is a 5’ x15’ plan with 5’ height. Hoops made of ¾” PVC pipe arch over a 2’ high raised soil bed. The soil is held in a sturdy rectangular box made of treated plywood with 3” plastic foam insulation on the inside of the box. A heat storage system made of side-laid concrete blocks with core holes aligned in the length direction was installed 16” under the soil bed. The flow of warm air through the block holes also warms the soil bed for additional heat absorbing capacity for excess daytime heat and more favorable growing environment.

Greenhouse #3 is 5’ x 15’ plan with 5” height. Hoops made of ¾” PVC pipe arch over a 2’ high raised soil bed. The soil is held in a treated plywood box with 3” plastic foam insulation on the inside of the box. Four runs of 4” diameter corrugated PVC plastic tubing were placed 16” under the soil bed to warm the soil from underneath. The south sidewall of the raised bed box is glazed with thermo pane glass (recycled insulating glass windows with aluminum frames) to collect additional solar heat input.

Greenhouse #4 is 6’ x 15’ plan with 5’ height. A tunnel-inside-tunnel cover system of hoops and stretched plastic is attached over a box of 8x8 landscape timbers that hold a soil bed. The outer tunnel is made of ¾” PVC pipe hoops covered with stretched plastic film. The inner tunnel is made of ½” OVC pipe hoops covered with light-duty stretched plastic film during cold weather. The “tunnel-inside-tunnel” design creates an insulating air space in the cover without need for a pressurizing blower. Concrete blocks stacked along the south sidewall of the raised bed box are covered by the outer clear plastic film layer to absorb and store additional solar heat input. The wood timber box has a lining of plastic foam insulation to hold warmth in the soil and prevent leaching from the treated wood timbers.

Greenhouse #5 is 6’x12’ plan with 5’ height and is connected to greenhouse #4 (end-to-end connection). Hoops arch over a large table made of side-laid concrete blocks. Sidewalls are 2’ high with treated 2x6 wood framing and insulation. The table is made of three layers of side-laid concrete blocks with the core holes aligned to circulate airflow from a centerline plenum under the table area toward the sidewalls. Warm air is circulated from the top of the tunnel space into the center plenum by a blower fan. A small space (4”wide) between the concrete blocks and sidewall provides a plenum by a blower-fan. A small space (4” wide) between the concrete blocks and sidewall provides a plenum along the inside of the sidewalls. Airflow circulating into each sidewall plenum turns upward and is then diverted to horizontal flow by the stretched plastic cover and flows toward the center of the table growing area. This pattern of flow provides horizontal air flow (HAF) in the growing area as well as transferring excess solar heat in the concrete blocks in the daytime. At night the airflow moves heat from the blocks to the growing area.

Limitations of the Current Experimentation
Small greenhouses as built initially in this project serve well for studying concepts. Small structures allow easier experimentation with ideas for hoops, raised beds, integral table structures, heat storage, movable insulation, end closures, cover fastening methods, airflow, ect. Modifications can be made at relatively low cost. Structures under six feet high are allowed by city codes without building permits. Small greenhouses, however, have a thermal disadvantage due to their greater exposed surface area in proportion to the growing area inside, as compared to larger greenhouses. Heat loss in small greenhouses is proportionately high in relation to the square footage of growing area when compared to larger greenhouses. Heat loss from exposed envelope surface is the largest single factor in supplemental heating expenses. However, heating needed due to air exchange (heating of cold incoming fresh air) is also a major factor, however, and is theoretically the same per square foot of growing are regardless of size of greenhouse, since the air requirement is based on the area of plants growing in a greenhouse.

Large greenhouses will need to be built to accurately test achievable thermal efficiency of new designs. Efficiency will be rated in terms of fuel heat required per square foot of growing area for various design. In future research sited, each test greenhouse must have its own heating system to accurately monitor fuel use independently. A fuel type that can be accurately measured must be used (propane, natural gas, fuel, oil, or metered electricity). Supplemental heating equipment of known certified efficiency is needed when field-testing thermal efficiency of a greenhouse structural envelope, solar effects, and movable night insulation. Accurate instrumentation to record interior light intensity levels, indoor temperature, and indoor humidity is needed to study the performance in terms of the environment provided by different types of greenhouses. Accurate instrumentation is needed to record outdoor temperature and daylight at test sites to correlate greenhouse performance to the variation of actual local weather during the testing period (versus “normal” long-term statistical average weather).

The project is reviewing historical weather records to help conceptualize new greenhouse designs that can work well with existing weather patterns in various northern regions. Cold periods appear to have a tendency of greater frequency of clear sky conditions. A greenhouse design that captures more sunlight on clear days but does not compromise light on cloudy days can utilize solar heat input for 20-50% of the heating requirement (if solar heat input can be stored when excess occurs). Solar energy collection surfaces can be added nearby but separate from a greenhouse. An example could be vertical-mounted warm air active solar collector panels extending as “wing walls” from the east and west ends can provide a higher percentage of cold weather heating needs of a greenhouse. Early morning and late afternoon sun reflection from glazed surfaces on vertical warm air solar collector panels) may increase early morning and late afternoon natural light brightness is a greenhouse to help meet the light needs of plants that require a longer photoperiod, if properly positioned and oriented in relation to the greenhouse. Added light reflected into the east end of the greenhouse in the morning and the west end in the afternoon may provide a longer effective photoperiod.

USDA Funded Research Work on Heat Storage

Research work in this project is currently focused on heat storage in greenhouses to better utilize the heat energy created by sunlight during the daytime. Heat storage systems function as a heat absorber during periods of excess heat and can release heat later at night when heating is needed. Anchoring-Heat Storage (AHS) units weighing 300 to 3,000 pounds have been designed which serve multiple functions, including anchoring, heat storage, support of tabletops, and air circulation. Design detailing, cost estimates, and thermal analysis are in progress. Anchoring weight needed per unit will depend on the size and structural design of the greenhouse, spacing of anchoring units, and wind conditions at the site. AHS units constructed of low-cost dense materials which serve multiple functions may be cost-effective.

The most practical heat transfer fluid medium in greenhouses for cold regions appears to be air. Systems using airflow offer simplicity, freeze-tolerance, and possibility to combine with other function such as air circulation for plant health. Some heat transfer will occur wherever heat storage materials receive sun directly, from the radiant heating effect of sunlight. For rapid and efficient heat transfer, however, air passages are needed within any heat storage system so that fan-forced air circulation can be used for rapid heat transfer by forced convection on multiple surface areas. At night, the slow natural radiant release of heat from storage units may be adequate with minor natural convection in the air passages.

Positioning of AHS units will correspond to spacing of the major structural members of the greenhouse, e.g. 4ft or 6 ft spacing, or heavier AHS units can be positioned at every other upright structural member and heavy-duty horizontal purlins installed for load transfer. Weighted systems work well in high gusting wind whereas stake anchoring systems may gradually pull loose after repeated gusts of wind.

Two types of AHS units are being studied. The first concept is to make units by clamping together a number of standard concrete blocks (10 to 60 standard construction blocks depending on weight needed). This system can be built without special materials using standard tools and manual labor. Standard hollow-core concrete construction blocks (two holes) can be stacked with the cores aligned and mechanically clamped together with steel brackets. Aligned core-holes provide air passages and multiple heat transfer surfaces within an AHS unit. Clamping brackets are made of standard angle-iron brackets on the ends of an AHS unit and provide secure fastening points for the greenhouse structure, e.g. upright hoops or posts. Draw members or “tension struts” that pull the end brackets tight on the blocks are made of steel tubing. Bolts pull the end brackets tight to the draw members.

The second concept is to use large heavy units of precast concrete for AHS unit. Large precast units must have air passages cast in during manufacturing. Special casting form are needed which may be costly for concrete plant to purchase. Large precast units would likely be economical only if the units can also be used for other purposes such as bunkers and retaining walls, and foundation for other types of structures such as livestock shelters, equipment sheds and crop storage buildings. Heavy equipment such as a large forklift, front-end loader or truck crane are needed to move and place large precast units.

Future Work with Cooperators

The Northern Greenhouse Research Project is seeking rural or urban cooperators who wish to begin or expand in greenhouse growing with innovative structures and have adequate land area to build larger greenhouses. Fresh produce and horticulture growers may be eligible for agricultural loans and producer research grants for this type of project. Schools, park boards, churches, or non-profit agencies with open land in urban areas may be eligible for grants to develop innovative sources of food for urban residents and/or educational facilities. Research and teaching staff at Colleges and Universities are invited to collaborate. Horticultural-floral growers may participate or utilize the results of this project as well as food growers. Improved thermal and daylight efficiency will provide better growing conditions and reduced operating expense in the usual greenhouse operation period (Mar-Apr-May) as well as for extended operation.

As research funds become available, the scope of this project may included studying shape factor of greenhouses to collect maximum natural light, internal reflectors to maximize light levels in the growing area or increase hours per day that interior light level is above threshold level for photosynthesis, external reflectors to direct more daylight into a greenhouse, moveable insulation methods which do not compromise natural light such as moveable layers places in strategic position at night for insulation, and low-cost fixed perimeter insulation at ground level. Reflector concepts in design stage are multi-mode and will not block diffuse light entry during cloudy or hazy sky conditions. Further topics may include automatically opened greenhouse covers where a plastic film cover is retracted or moved to allow full sunlight to reach the plants on mild days, scale factor in relation to thermal efficiency, and urban greenhouses that utilize building ventilation exhaust heat from nearby buildings.

Greenhouse Growing Operations in Northern Region in Current Practice

Greenhouses are currently used primarily for ornamental crops (flowers, house plants, hanging baskets, patio container plants, bedding plants, etc.) and vegetable transplant seedling (tomatoes, peppers, herbs, cabbage, broccoli, etc). Bedding plants, hanging baskets, patio plants, and vegetable transplant seedling are saleable only in the spring and summer in northern markets. Greenhouse growing in northern regions is therefore primarily a springtime operation at present. The products listed above generally need only 9-12 weeks growing time to be ready to sell. Planting seeds or cuttings typically begins in early to middle March to allow enough growing time to be ready in proper timing for the beginning of outdoor gardening in early June.

Some greenhouse growers in northern area grow or finish the growth of specialized crops at other times of the year, such as autumn and the early year (e.g. Poinsettias for Christmas and flowers for Valentines, Easter and Mothers Day). Production costs are high for these crops in northern regions since electric lighting is require and conventional greenhouses are expensive to heat in cold weather. Precise timing of growth and bloom development is needed. Production of these specialized crops has been shifting to more southerly areas but not northern greenhouses are still used to store and display the products for retailing. High efficiency greenhouses for northern latitudes may help to shift some of this production back to northern regions if transport costs increase for shipping plants from southern regions.

Summer use of greenhouses in northern regions can assist crops that prefer very warm conditions, e.g. tomatoes, melons, peppers, and Mediterranean herbs; however, caution must be used to prevent excessive heat. Automatic ventilation systems must be kept in good working order or manually opened vents used properly to prevent extreme heat buildup. Fine mesh screen on ventilation openings can provide protection against many types of insects as well as bird and animal damage to crops.

Closing Notes and Ideas

“Solar Greenhouses” as developed in the 1973-85 period were not adopted for agricultural or horticultural production. The structures cost more to build than standard greenhouse structures since steep angle glazing, insulation of the north side, added thermal storage materials and free-standing structures cost more to build. Research data was not available on specific performance in northern latitude cold regions to justify higher investment cost. Horticultural production of bedding plants, a large part of the market, is concentrated in the 10-12 week period prior to last frost dates, i.e. beginning in greenhouses 10-12 weeks ahead of the time when extensive outdoor gardening begins, and this period and sector of production is fairly well served by standard greenhouse design such as gutter-connected multi-bay truss roof designs which collect light efficiently with the higher angle sun in April and May.

Natural daylight transmission into the growing area inside a greenhouse is important during all daytime hours for plant growth and crop yields. Although supplemental electric lighting can compensate for lack of natural light, lighting systems are costly to install and expensive to operate in most regions. On cloudy or hazy days when natural light is incoming from all directions in the sky, blockage of north side daylight has the effect to reduce interior light brightness in the growing area. Research studies in the late 1980s and 1990s analyzed daylight transmission through the sides and roof areas of various shape greenhouses during various weather conditions and confirmed a need for north side light entry in cloudy or hazy conditions. Inventions of mechanisms to move reflect surface materials or reflective-insulating materials, are available to develop. Research funds are sought to build full-size prototypes and test for a period of years in a northern latitude location to determine actual average performance in terms of improved light levels and fuel savings. Variation of PPS (Percent Possible Sunshine) from year to year and outdoor average temperatures variations require testing for a period of 4-5 years to determine representative performance data.

Because a substantial number of days in northern latitudes have diffuse daylight, particularly in the low light period of the year, light entry from a northerly direction is important for plant growth if extended operation of a greenhouse is desired. Multi-mode light enhancement systems (e.g. movable reflective surfaces that automatically accommodate natural light conditions) must be evaluated in terms of cost, reliability, and benefits for reduced operating expenses and improved crop quality and yields.

A specific goal is to design greenhouses that work for growing warm-preference high-light requirement vegetables and herbs during the moderate low light periods of the year in northern regions, which is not currently a common practice. The rationale includes several aspects. Extending the months of feasible greenhouse operation allow spreading overhead cost over more production volume and spreading farm workload more evenly through the year. Premium prices are paid for certain crops when fresh delivery can be achieved at non-standard times of year. Reliability of production is important, e.g. to assure customers that delivery will be made at agreed times. Several crop rotations per year or successive plantings could help achieve better profitability for northern growers.

The early experimentation period in greenhouse design during 1973-1985 achieved designs with low heating expense, however, average natural light brightness inside many experimental greenhouses was compromised compared to standard greenhouses because methods of insulating the north side caused blockage of diffuse light from the northerly direction on hazy and cloudy days. Reflective interior surfaces on the north side which improved brightness on clear sunny days caused blockage of north side diffuse light on cloudy days and appear to result in no net gain of average light brightness. Movable insulation or reflectors were not developed in the early experimentation phase. The current project builds upon previous work in some respects but recognizes the key importance of natural light for plant growth particularly when greenhouses are used fro food crop production.

New features for greenhouses have become possible with recent technology advances in materials, microprocessor computer controls, and recent inventions of mechanisms to move reflective and insulating layers into multiple positions inside a greenhouse. A broad term for a greenhouse with south-orientation, substantial solar gain, moveable insulation to reduce night heat loss and north side daytime heat loss on clear days, a moveable internal reflector (likely combined with moveable insulation layer), heat storage, ground level perimeter insulation, and proper automatic controls could be “High-Efficiency Greenhouse.” A universal greenhouse design that suits all northern regions, however, will not likely be developed. Greenhouses must be design for regional climate factors, however, will not likely be developed. Greenhouses must be design for regional climate factors and may need special design for particular crop production techniques.

The author reviewed about 200 published articles and books as background for this project, through a literature search in the AGRICOLA, AGRIS, CAB, NTIS and CANADIAN RESEARCH INDEX databases. The author has a background in heat transfer, thermodynamics, airflow, heating, solar energy and heat storage, and graduated Cum Laude in Mechanical Engineering at the University of Minnesota.

Several invention disclosures have been filed with the US Patent office for this project, regarding mechanism for light enhancement, movable insulation, greenhouse structures and greenhouse heat storage systems. A recently filed disclosure describes a system of using low-cost dense material (standard concrete blocks) to perform four function in a greenhouse: (1) store daytime excess heat for later use to reduce the cost of supplemental heating fuel, (2) anchor and brace a greenhouse structure against wind forces, (3) support table areas inside a greenhouse, (4) air circulation in growing areas in a greenhouse. The system or parts thereof can also be used in other types of agricultural buildings where moderation of temperature by heat storage is desirable, such as livestock shelters, egg production in poultry shelters or storage of crops (i.e. potatoes, sugar beets, carrots, etc). With suitable storage of daytime warmth from couth-oriented glazing area or active solar energy collection of some form, temperature can be maintained above freezing inside a protective enclosure during a major portion of the below-freezing weather period of the year without use of supplemental heating fuel.