Perpetual Harvest Greenhouse System

By Chris Marron and Mark Hoffman

The Perpetual Harvest Greenhouse System provides an indoor ecosystem capable of growing equal yields of organic produce 52 weeks per year. This system creates 365 ideal growing days per year by optimizing light, carbon dioxide enrichment, and soluble nutrients in conjunction with continuous planting and harvesting. Because the geo-hydroponics (organic) based Perpetual Harvest system can economically simulate warm season growing conditions, crops that would otherwise be shipped from warmer climates can be grown profitably in colder climates during winter months. Such off-season production significantly increases return on investment of the Perpetual Harvest system in comparison to conventional greenhouse systems because heating and cooling costs could be up to 75% less than for the standard three-season greenhouse operation. This system also allows a greenhouse operator to create growing conditions unique to specific crops such that almost any crop can be harvested at any time of year, even in colder climates.

The Perpetual Harvest Greenhouse system accomplishes profitable year round production by optimizing two primary features of greenhouse operation – Growing techniques and Energy management. This system integrates the latest innovations in greenhouse design and operation with emerging understanding of growing techniques to create production levels not possible in an outdoor system, or in a three-season greenhouse. Because this system can operate for four seasons, its yearly energy usage exceeds that of the three-season greenhouse, however its overall profitability is 6-8 times that of the conventional three-season greenhouse or outdoor plantings because the system can provide organic produce when other systems can not. The uniqueness of the Perpetual Harvest system lies not in any one feature, but instead in the integration of many innovative aspects of greenhouse design and operation. All the features utilized in the Perpetual Harvest system have been successfully applied in existing growing systems; however, research indicates that no single publicized greenhouse system currently in operation utilizes the combination of features integrated into the Perpetual Harvest system. Furthermore, the Perpetual Harvest system can be easily integrated with renewable energy systems such as a bio-diesel plant, ethanol still, methane bio-digester, and/or co-generation unit, thus improving energy efficiency, driving down operating costs, and producing marketable fuel by-products.

Optimizing Growing Conditions

The Perpetual Harvest system utilizes unique growing techniques to maximize plant growth. Enhanced growing techniques include: providing artificial light, carbon dioxide (CO2) enrichment, and maximizing soluble nutrients absorbed through roots and leaves. The system enhances growth by proportionally increasing the five most important growing conditions at certain times of the day, thus producing a 'supercharged' growing environment causing plants to reach erectly for the light while rapidly absorbing nutrients. The result is a significant and rapid growth surge. Plants can process approximately twice as many nutrients if light, CO2, and soluble nutrients are increased in balance at the same time. Standard greenhouse growing temperature is ~85°F, while experience indicates temperature can be successfully increased to 95°F with increased light, CO2, and soluble nutrient levels, along with additional water. Growing at increased temperature has the added advantage of allowing the greenhouse to remain sealed longer from the outdoor atmophere each day, leaving the higher CO2 concentration available for a longer period. With normal light, CO2, and soluble nutrient levels, plants become stressed at temperatures above 85°F - not so, with the Perpetual Harvest system. Operating at higher greenhouse temperatures effectively utilizes periods where it is difficult to maintain greenhouse temperatures less than 85°F.

Light:

In the Perpetual Harvest system, plants receive the same amount of light from the fall equinox until spring equinox by adjusting day length with artificial sunlight. Experience indicates that ~11 ½ hours is optimal daylight length for most common food plants in temperate zones. Additionally, applying supplemental light for three hours each morning, every day of the year, at the same time that the CO2 concentration is enriched, has been seen to maximize plant growth. Increased light supports CO2 absorption by stimulating plants to open their stomata. Supplementing the red, blue, and yellow light frequencies during this enhanced growth period optimizes utilization of the added light. Red and blue frequencies (from halide lamps) enhance vegetative growth while yellow frequencies (from high-pressure sodium bulbs) enhance fruit set and development. 

Carbon Dioxide Enrichment:

Normal atmospheric CO2 concentration is ~370 ppm, however, experience indicates that some plants prefer up to 2000 ppm CO2 (approximately five times normal). In the Perpetual Harvest system this increased level is maintained for only 3 hours in the mid morning. During this 3 hour period, the plants store CO2 that will be used to boost plant growth later in the day after CO2 level has returned to normal. CO2 is primarily produced by a flame (propane or natural gas) CO2 generator. The flame can serve as a 'peaking CO2 generator' and baseline CO2 levels could be provided by decomposing compost or other continuous low producing sources. A digital CO2 monitor determines when CO2 generators will cycle, and also serves as an alarm for humans to take precaution when in the greenhouse during the high CO2 period.

Soluble Nutrients:

The Perpetual Harvest system utilizes the ebb and flow style of geo-hydroponics, passing organic nutrients through a soil-less growing medium placed in plastic lined beds. Pearlite, pumice, vermiculite, and decomposing organic matter (potting soil) comprise the soil-less growing medium. Using a soil-less growing medium greatly reduces the likelihood of soil borne diseases and pests that can proliferate in the enclosed greenhouse space. Soluble nutrients are provided by addition of organic compost tea created using the traditional Indore compost method developed by Sir Albert Howard. This method, based on years of compost experimentation, produces compost from decomposing cellulose products such as peat moss, straw, and last season's crop residue mixed with already composted animal manure along with a small amount of real soil and recently finished compost as an inoculant.

In the Perpetual Harvest system, Indore compost is made using only organic ingredients mixed in a 25:1 ratio of carbon to nitrogen. Earthworms are added to the pile after the initial heating period (~8 days) to convert the existing nutrients into worm castings, a nutrient form more easily accessible to plants. After 14 days, compost is old enough to use as a nutrient base for making compost tea and/or growing medium. Foliar feeding of this compost tea, applied to the underside of leaves, is also performed in conjunction with the three-hour mid-morning light/CO2 enrichment period. After worm digestion, the compost can be mixed with last season's used growing medium at a mixture rate determined by muscle testing. During this enhanced mode of operation, daily muscle testing (kinesiology) is utilized to provide the data needed to fine-tune light, nutrient, and temperature levels.

Energy Management System

Energy costs are the most expensive aspect of greenhouse operation. The Perpetual Harvest system capitalizes on recent innovations in greenhouse design to significantly reduce energy inputs. This reduction is primarily achieved through two aspects – Insulation design and Energy storage and transfer. Other aspects, such as greenhouse layout and temperature control also enhance efficiency, but to a lesser extent.

Insulation Design:

The south facing wall of the Perpetual Harvest greenhouse is composed of double layers of polyethylene, between which are injected biodegradable soap bubbles. The soap bubbles are fed into a distribution plenum at the top of the greenhouse where they emerge at intervals along the length of the greenhouse, and flow down to fill the space between the polyethylene sheets. Recent developments in bubble making equipment designed for commercial fire suppression systems have resulted in equipment that can fill the polyethylene gap within minutes. The Perpetual Harvest system employs a bubble indication system that senses bubble collapse and auto starts the bubble making machine when the bubble wall drops below a specified height. The soap bubbles resist convective heat transfer, and with an 'R' value of approximately R-1 per inch of bubbles, significantly increases R-value over that of single sheet polyethylene walls, or even double sheet polyethylene walls with an air gap in between. Soap bubbles also block infrared light but not visible or ultraviolet light. This attribute creates an ideal greenhouse situation since the light frequencies required for photosynthesis (visible light) pass through the bubbles but the frequencies that would result in radiant heat loss (infrared) are moderated. This means that light needed for plant growth is available even though unwanted heat transfer is minimized. Bubbles can impede unwanted heat transfer in either direction using this system. For example, draining the bubbles during the day can increase internal heat gain, while injecting bubbles during the day can reduce internal heat gain. Bubbles can be produced at night to prevent heat loss and maintain inside temperature. This process was developed in the Stelle greenhouse over twenty years ago by residents who received their funding through a State of Illinois grant and has been successfully used in Canada.

Energy Storage and Transfer Systems:

The Perpetual Harvest greenhouse design employs redundant energy storage and transfer systems. These systems listed by priority of use are:

• Subterranean heating/cooling system (SHCS)
• Hydronic radiant heat system with the following heat sources:
• Solar/thermal heater
• Co-gen unit waste heat
• Babington burner

• Natural gas/propane forced air heat as the final back-up heat source

The subterranean heating system is comprised of several hundred feet of thin walled 4" perforated, polyethylene drainage tubing buried under gravel inside the greenhouse base. A fan connected to the tubing via a common plenum provides forced flow of greenhouse air through the tubing. Because daytime greenhouse air is warm and humid and the greenhouse base is cool, moisture will condense as the air passes through the buried drainage tubing, thus removing heat from the air. Upon returning into the greenhouse air space, the air is cooler and less humid. In this condition, the returned air can absorb moisture, thus cooling the greenhouse air. The uniqueness of this cooling system lies in the phase change that has occurred in the buried tubing. Besides cooling the greenhouse air, this process also heats the greenhouse base. At night, the fan can be run to heat air as it again passes through the buried tubing, thus convectively transfering heat stored in the greenhouse base to the greenhouse atmosphere as the air reenters the greenhouse. In this manner, the subterranean heat storage system can provide both heating and cooling. The SHCS is equiped with dual speed fans to allow for finer temperature control. Experience in Colorado indicates that this system can meet the greenhouse heating and cooling needs for all but approximately 50 days per year.

The Perpetual Harvest heating and cooling system design integrates a multi-fuel fired hydronic radiant heating system with the SHCS (primarily for climates without the solar resources of Colorado). The hydronic radiant heating system consists of tubes placed inside the SHCS tubes. This system includes a large water storage tank and is needed only during colder months, storing heat during daytime that can be withdrawn at night or during cloudy days by airflow of the SHCS.along the tubes of the radiant heating system. To some extent, the radiant floor heating system also transfers heat into the greenhouse base/floor. Heat is desirable at floor level to keep the root zone warm. As long as roots are warm, plants can withstand air temperatures up to 15°F less than the root zone temperature.

The hydronic radiant heating system is heated by three sources: a solar/thermal system, a co-generating unit, and a Babbington burner. The solar/thermal heating system is essentially a solar and/or wood boiler powered pool heater circulating hot water into the storage tank. The co-gen waste heat systems and Babbington burner are also connected to the radiant heating system as backup heat sources. The Babbington burner burns oil (waste vegetable or motor oil) or biodiesel and can quickly provide a significant amount of heat (the U.S. military heats all the meals served in the field using this system). The co-gen unit provides both heat and electricity and can be powered from a variety of renewable fuels such as ethanol, biodiesel, or methane.

Greenhouse Layout:

Although the Perpetual Harvest greenhouse system can be retrofitted to just about any existing greenhouse design, due to low angle of sun in northern winters, the optimal Perpetual Harvest greenhouse would have a tall northern wall and the planting beds would be vertically stacked in terraces stepping upward toward the northern wall. Looking externally at the greenhouse from one end, it would appear similar to an A frame with the northern wall earth bermed. Ideally, the greenhouse would be built into a south facing hill and include a short southern wall at ground level. Besides terraced beds, it would be possible to apply the verti-grow method that utilizes pots hanging one above the other. It would also be possible to build the terraces out of enclosed concrete fish tanks, thus allowing fish to be raised (aquaponics), providing another income stream. 

Temperature/Humidity Control:

The Perpetual Harvest control systems are designed to regulate temperature using thermostats, timers, and/or programmable controllers, all with the option for manual override. The energy management systems are operated with the intent of maintaining the desired greenhouse temperature and humidity with the minimum energy input. The greenhouse should be maintained below 60% humidity at all times, if possible.

General temperature control in a northern climate is as follows. The SHCS is operated at all times, unless its outlet air temperature drops below 55°F. Should the SHCS air outlet temperature drop below ~60°F, the radiant heating system automatically initiates flow, thus transfering its heat to the air in the SHCS tubing, maintaining or increasing the SHCS outlet air temperature. During the mid-morning enhanced growth period of operation, heat addition from solar gain, the CO2 generators, and artificial lights could cause significant heat buildup, especially on sunny days. If such heat buildup causes interior air temperature to reach 96°F, CO2 generation and artificial lighting are automatically terminated and the greenhouse atmosphere is exhausted to the outdoors. After the cool incoming outside air causes interior temperature to drop to 80°F, exhaust fans are stopped and CO2 generation and artificial lighting are reinitiated, provided the three hour enhanced growing period has not reached completion. Subterranean heating operates to provide heat at night and in the morning until needed. Cooler temperatures may be needed to improve fruit set and possibly enhance fruit sweetness. Most berries need cooler night time temperatures to produce fruit, so the Perpetual Harvest system utilizes a solar air conditioning system to draw evening temps down to around 50°F for a short period during hot weather.

A TYPICAL DAY IN THE GREENHOUSE

Temperature/humidity regulation and plant maintenance activities during a normal Spring or Fall day in a northern climate typically occur as follows:

Sunrise - 7AM:   Interior temp. - 60°F,   Exterior temp. - 35°F

Remove bubbles to allow solar heat gain and turn on fans to recharge SHCS (if not already running). Turn on all interior air circulating fans to promote plant strength.

9AM:   Interior temp. – 80°F,  Exterior temp. - 50°F

Refill bubble cavity to minimize heat input

Water plants with soluble nutrient solution

  9:30AM   no change in temp.

Foliar feed plants

10AM:  Interior temp. – 85°F,   Exterior temp. – 60°F-80°F

Turn on CO2 generator and gro-lights

Leave greenhouse for three hours to avoid high CO2 concentration

11AM:   Interior  temp. – 95°F

No human activity in greenhouse

12PM    no change in interior temp.

1PM   Interior temp. - 95°F

Shut down CO2 generators and lights

Give greenhouse a long exhaust fan cycle to lower interior temp. to 85°F

2PM   Interior temp. – 85°F 

Remove any dead foliage

Prepare plants for taking cuttings

 3PM   Interior temp. maintained at 85°F

 4PM   Interior temp. maintained at 85°F

Begin daily harvest

Plant seeds

If afternoon is cool or cloudy, remove bubbles to allow for solar gain

5PM   Interior temp. – 75°F   Exterior temp. – 60°F

Turn off half of interior fans

Start gro-lights

6PM    Interior temp. – 75°F   Exterior temp. – 55°F

Refill bubble cavity to hold in heat

Transplant seedlings and cuttings

6:30 PM   no change in temp.

Turn off gro-lights

Give greenhouse a long exhaust cycle to remove humidity and lower temp. to below 60°F to sweeten fruit

8PM   Interior temp. - 60°F

Cooler night time temperatures may be needed for fruits and berries at certain times of their growing cycle to improve fruit set and possibly enhance fruit sweetness. Through use of the SHCS, the Perpetual Harvest system can produce these lower temperatures for a short period even during hot weather.

Integration of Renewable Energy Systems

Although the Perpetual Harvest Greenhouse system can operate profitably with the systems already described, overall energy efficiency can be improved by addition of a variety of renewable energy systems. Higher energy efficiency can lead to more profitable long term operation despite the higher capital expense of additional systems.

Perhaps the most viable and efficient energy component to integrate into the Perpetual Harvest system is the co-generation unit. This is because the co-gen unit produces multiple useful outputs. The co-gen unit produces electricity, which is needed for lighting, fans, and electronics. As described earlier, it also produces heat which can be stored in the hydronic radiant heating system. If the co-gen unit is powered by ethanol, methane, or bio-diesel it might even be possible to feed its exhaust into the greenhouse as a CO2 source (depending on completeness of combustion) and/or heat source. Furthermore, the exhaust line and cooling system lines could be buried into the greenhouse base where their heat can be transfered into the greenhouse substructure, much like the heat in the radiant heating system.

A system to produce the bio-fuel consumed by the co-gen unit could also be added. For example, if the co-gen unit is powered by a diesel engine, a bio-diesel plant could be built alongside to feed the engine. The same would be true for an ethanol still if the generator were powered by an engine designed to burn ethanol and/or gasoline. An ethanol plant has the added benefit of producing CO2 as a distillation by-product. As described earlier, it is desirable to enhance CO2 enrichment in the greenhouse, therefore CO2 produced by an ethanol still would displace the need for some of the CO2 generated through igniting propane or natural gas torches during the mid-morning enhanced growth period. The still would also produce waste heat that, if it could be captured, could heat water in the radiant heating system.

Addition of a methane digester to the mix of energy systems could produce at least two useful byproducts. The first would be the methane gas itself, which could be used at least three ways: 1) to power a gas engine for the co-gen unit, 2) burned during the enhanced growing period as a CO2 generator, 3) used to heat an ethanol still. A less obvious byproduct of a methane digester is the nutrient rich sludge left over from anerobic digestion. The liquid from this sludge can function as an important nutrient source for the hydroponic solution being fed to the plants, and any undigestible sludge can be applied as garden fertilizer.

Regardless of which renewable energy systems (if any) are integrated with the Perpetual Harvest system, a building separate from the greenhouse will be needed to ensure the mechanical components are isolated from the humid greenhouse environment. This building would likely also house the composting and vermiculture operations.

Choice of renewable energy systems integrated into the Perpetual Harvest system will likely depend on availability of local biomass resources. It should be noted that for cases where a bio-fuel waste product (for example, methane digester sludge) is to be used in growing greenhouse produce, the biomass inputs may need to be of certified organic origin in order to retain the ability to certify the greenhouse produce as organic. This could be problematic unless the operation has access to organic biomass inputs.

Competitive Features and Profit Centers

The Perpetual Harvest Greenhouse system has numerous unique features that enhance its competitiveness in comparison to a standard three-season greenhouse. These features are:

• Simple, yet highly efficient heating and cooling design
• Continuous year-round growing and harvesting of organic fruits and vegetables, providing 'just in time' availability for buyers
• Ability to grow 'designer' fruits and vegetables by artificially creating 'seasons', thus capitalizing on increased prices for out of season crops
• Reduced need for pest control due to compost based nutrient application bringing balance to plants and keeping soil borne insects and diseases out of the greenhouse biome
• Higher plant brix (sugar) levels, resulting in longer produce shelf life
• Maximized sunlight harvesting through use of tiered beds
• Integrating renewable energy systems to:
• reduce energy costs,
• provide additional profit centers – such as sales of bio-fuels,
• establish local energy self-sufficiency
• Significantly reduce shipping costs by raising food crops locally

Besides the advantages just listed, it should be noted that the Perpetual Harvest food production system can become a uniquely closed resource loop if it is integrated with nearby restaurant(s). A resource sharing relationship with a local restaurant would allow waste cooking oil to be utilized as a bio-diesel source. It would also allow food scraps to be recycled, either directly into a bio-digester, or indirectly via feeding animals such as hogs and chickens. In turn, these animals could provide another income stream in the form of meat and eggs. It can be seen that as the Perpetual Harvest system integrates greater numbers of resource utilizing components, additional income streams arise due to the efficient utilization of energy and biomass. Ultimately, reduced waste increases profit, while greatly minimizing the challenge of waste elimination and removal (pollution) so prevalent in modern, large scale, industrial agriculture systems. See Figure 1, Perpetual Harvest Energy and Resource Flowpaths for a diagramatic representation of possible resource flows within the Perpetual Harvest system.

 

Summary

 

The Perpetual Harvest Greenhouse system derives its effectiveness and economic competetiveness from the integration of its many innovative features. Those features include high R-value bubble wall insulation, integrated methods of heat storage and temperature management, and an enhanced mid-day growing period stimulated by increased carbon dioxide concentration, enhanced lighting, and increased soluble nutrient levels. Although the construction costs of the Perpetual Harvest system exceed that of the standard three season greenhouse, the extended harvest season and significantly reduced long term energy costs should result in a higher return on investment for this system than for other greenhouse systems currently in operation. (See the article titled, "Packin' snacks for trip to Mars" to learn of a successful greenhouse in New Jersey that implements many, but not all of the features of the Perpetual Harvest system.) Inclusion of renewable energy systems into the overall design produces multiple income streams not typical of a greenhouse system. Ideally, the Perpetual Harvest system would be completely energy self sustaining – deriving all its energy needs directly from the sun or from locally harvested sunlight via biomass. Some general benefits of this system are:

• · High quality, fresh-picked, organic produce with superior flavor.
• · Local Grown. Minimal trucking costs.
• · Can produces seasonal crops all year long if desired.
• · Holds potential to integrate agri-business into metropolitan areas.
• · Diversifies income streams, providing a vehicle for reviving rural farm communities.
• · Sustainable, renewable, environmentally sound.
• · Profitable. Weekly crops/weekly income. Income can be steady instead of seasonal.
• · Promotes self-sufficiency and independence.
• · Could be used to reduce food and energy costs for prisons, schools, hospitals etc.

 

Lastly, it deserves to be stated that not only does the Perpetual Harvest system provide local employment and a possible means of regenerating local farm economies, it also can serve as the physical life blood of a sustainable community or co-housing unit. Considering that human societies are typically organized around and through sharing of both food and energy, the fully developed Perpetual Harvest system provides for these two most basic human needs.

 

At this time, a prototype of this fully integrated energy/food system is needed so that performance of the Perpetual Harvest system can be optimized. Once proven effective and profitable, this system can serve as an example of how a community can function in a self sustaining manner by efficiently using the resources at its immediate disposal.