Renewable Energy (Solar/Wind)

Solar

All greenhouses collect solar energy. Solar greenhouses are designed not only to collect solar energy during sunny days but also to store heat for use at night or during cloudy periods. They can either stand alone or be attached to houses or barns. A solar greenhouse may be an underground pit, a shed-type structure, or a hoophouse. Large-scale producers use free-standing solar greenhouses, while attached structures are primarily used by home-scale growers.

Passive solar greenhouses are often good choices for small growers because they are a cost-efficient way for farmers to extend the growing season. In colder climates or in areas with long periods of cloudy weather, solar heating may need to be supplemented with a gas or electric heating system to protect plants against extreme cold. Active solar greenhouses use supplemental energy to move solar heated air or water from storage or collection areas to other regions of the greenhouse.

Use of solar electric (photovoltaic) heating systems for greenhouses is most cost-effective in the production of high-value crops.

Solar Greenhouse Designs

Solar greenhouses differ from conventional greenhouses in four ways. Solar greenhouses:

  • Have glazing oriented to receive maximum solar heat during the winter.
  • Use heat-storing materials to retain solar heat.
  • Have large amounts of insulation where there is little or no direct sunlight.
  • Use glazing material and glazing installation methods that minimize heat loss.
  • Rely primarily on natural ventilation for summer cooling.

Implementing these basic principles of solar greenhouse design is imperative in the successful design, construction and maintenance of an energy-efficient structure.

Attached solar greenhouses, which typically incorporate a passive solar design, are lean-to structures that form a room jutting out from a house or barn. These structures provide space for transplants, herbs or limited quantities of food plants.

Commercial freestanding solar greenhouses are large enough for the production of vegetables, ornamental plants or herbs. There are two primary designs for freestanding solar greenhouses:

  • shed type.
  • hoophouse.

A shed-type solar greenhouse is oriented to have its long axis running from east to west. The south-facing wall is glazed to collect the optimum amount of solar energy, while the north-facing wall is well-insulated to prevent heat loss. This orientation is in contrast to that of a conventional greenhouse, which has its roof running north-south to allow for uniform light distribution on all sides of the plants. To reduce the effects of poor light distribution in an east-west oriented greenhouse, the north wall is covered or painted with reflective material.

Freestanding hoophouses are rounded, symmetrical structures. Unlike the shed-type solar greenhouses, these do not have an insulated north side. Solarization of these structures involves practices that enhance the absorption and distribution of the solar heat entering them. This typically involves the collection of solar heat in the soil beneath the floor, in a process called earth thermal storage (ETS), as well as in other storage materials such as water or rocks. Insulation of the greenhouse wall is important for minimizing heat loss. In addition, polymer paneling works as a selective transmission medium, perfectly diffusing sunlight into different spectral frequencies. The effect is to trap energy within the greenhouse, which heats both the plants and the ground inside.

Solar/Wind

Solar/wind hybrid renewable energy systems can power high density vertical hydroponic equipment used in greenhouses. Wind energy displaces costly fuel and polluting carbon emissions, provides a hedge against rising fuel prices (natural gas, coal), can be cost-competitive with other forms of energy generation, and may reduce electricity costs. For a commercial greenhouse operation, a wind turbine will generate electricity and any excess electricity can be delivered to the power grid. During calm periods, electricity can be drawn from the power grid.

According to many renewable energy experts, a small “hybrid” electric system that combines wind and solar (photovoltaic) technologies offers several advantages over either technology used as a single system.

In much of the U.S., wind speeds are low in the summer when the sun shines brightest and longest. The wind is strong in the winter when less sunlight is available. Because the peak operating times for wind and solar systems occur at different times of the day and year, hybrid systems are more likely to produce power when needed.

Many hybrid systems operate “off-grid,” which means that they’re not connected to an electricity distribution system. For the times when neither the wind nor the solar system are producing, most hybrid systems provide power through batteries and/or an engine generator powered by conventional fuels, such as diesel. If the batteries run low, the engine generator can provide power and recharge the batteries.

Adding an engine generator makes the system more complex, but modern electronic controllers can operate these systems automatically. An engine generator can also reduce the size of the other components needed for the system. Storage capacity must be large enough to supply electrical needs during non-charging periods.

Battery banks are typically sized to supply the electric load for one to three days.

Photovoltaics

Photovoltaics (or PV) is the field of technology and research related to the application of solar cells for energy by converting solar energy (sunlight, including ultra violet radiation) directly into electricity. Due to the growing demand for clean sources of energy, the manufacture of solar cells and photovoltaic arrays has expanded dramatically in recent years.

Energy efficient greenhouses powered by PV panels require a control design capable of making efficient use of energy. The goal of this control design involves improving PV cell efficiency, developing PV modules adapted for greenhouse use, optimizing PV panels and their integration into the greenhouse, and managing the energy generated (electrical converters and accumulators, etc.).

The first step requires defining a computational climate model of the greenhouse. The model predicts the temperature, velocity and humidity of the air inside the greenhouse as a function of the greenhouse architecture and farming. This requires a knowledge of and experience with FVM (finite volume method) simulation tools, thermodynamics, fluid mechanics, radiation, materials and farming biology. Once the model is created, a parametric analysis needs to be carried out in order to define the most efficient greenhouse configuration (components: type and location, ventilation, constituent materials, etc.) according to the local land topography.

The model is also used to define the HVAC (Heating, Ventilation and Air Conditioning) of the greenhouse. Then comes defining and developing a control strategy that best meets greenhouse behavior along with the necessary hardware for this control.

Thin-film PV and copper-indium-gallium-selenide (CIGS) offer reduced materials usage, lower weight and smaller size compared to wafered silicon, as well as the potential for more efficient manufacturing processes and the potential for new applications in Building Integrated Photovoltaic (BIPV) and flexible substrates.

BIPV glass skin modules function, not as an add-on, but rather as an integral part of a buildingā€™s skin. They can be used just like standard building materials, but with the added value of generating electricity directly from sunlight. BIPV manufacturers include:

  • SCHOTT North America, Inc.
  • The Carvist Corporation
  • Suntech Power
  • Visual Communications GlassOnWeb
  • Rainbow Solar

Other Surface Technologies

Translucent “NansulateĀ® Greenhouse” from Industrial Nanotech, Inc., Naples, Fla., is a thin-film thermal insulation coating designed to be applied to the clear panels of greenhouses to reduce energy consumption and costs without reducing the passage of light critical to healthy plant growth. In early 2009, the company completed spectrophotometer testing on its patented Nansulate Greenhouse product to measure light transmission and absorbance through the thermally insulating and mold resistant coating. The data demonstrates that Nansulate does not interfere with the important visible light needed for healthy plant growth, while offering effective thermal insulation benefits to reduce the energy needed to maintain optimal temperatures in greenhouses, therefore significantly reducing operating expenses.

Liquid crystal panels developed through a partnership between Kent State University and the Cleveland Botanical Garden are showing promise. The Botanical Garden conducted an experiment pitting a conventional glass greenhouse against a greenhouse made from liquid crystal panels. In the study’s early stages, the plants inside the liquid crystal greenhouse performed somewhat better than those in the conventional greenhouse.

Environmental Control

Even during cold weather a greenhouse can get too warm on bright, sunny days. So ventilation equipment must be built into the greenhouse structure to control temperatures in all seasons as well as minimize the chance of infestation from airborne pathogens. Hand-operated roof vents require frequent temperature checks. As outdoor weather changes, sashes must be opened and closed manually to keep plants from getting too hot or cold.

Automatic ventilation eliminates the manual work and is the best way to cool a greenhouse. If a structure has roof vents, a special electric motor and thermostat will open and close the vents. Fresh outside air is brought in through the roof vents. Warm air flows out through escape vents. Besides reducing heat, the change of air improves growing conditions. Responding to this air transfer, the thermostat will turn off and on to keep temperatures right for plants. Fans provide good ventilation and are needed in both large and small greenhouses. Exhaust fans must be large enough to change the air within the structure every minute. To accomplish this, the capacity of the fan in cubic feet per minute at 1/8-inch static pressure should equal the volume of the structure. The volume can be calculated by multiplying the floor area by 7.

If the structure is high enough, the exhaust fan and the motorized intake louvers can be placed above the doors at opposite end-walls. This will exhaust the hottest, most humid air, and prevent a direct draft on the plants near the intake.

Fan and duct HVAC can also be used for automatic HVAC. Plastic ducts are suspended by wires or straps from the roof of the structure. The fan-heater-louver unit gives positive air flow and the polyethylene duct distributes the incoming air evenly throughout the greenhouse.

Environment control typically encompasses air temperature, supplemental light, air movement (circulation and mixing), and CO2 concentration. Some degree of relative humidity control may also be included. Integrated control by computer, found in most modern greenhouses, provides the flexibility of zoned control of each environmental parameter without conflicting control signals (for example, ventilating while supplementing CO2.)

Structural insulation opportunities in greenhouses are minimal and heat requirements are high in comparison to most other types of buildings. Efforts to conserve heat, such as insulating the north wall and part of the north roof, have often shown negative benefits by reducing natural light and degrading plant growth and quality. Movable, horizontal, indoor curtain systems (which also double as movable shade systems) can save approximately one-quarter of the yearly heat in a cold climate.

Greenhouses can also be heated by oil or natural gas. Heat delivery is either hydronic (hot water) or by steam.

Solar loads in greenhouses are so great that mechanical cooling, as by air conditioning, is prohibitively expensive. Options for greenhouse cooling are thus limited. The most typical cooling mechanism is ventilation, either natural or mechanical.

The next step of cooling is to use evaporative means. Cooling can be accomplished by spraying a fine mist into the ventilation air or by pulling outside air through matrices or structures that are wetted to cool the air flowing past them.

Greenhouse lighting may be used for photoperiodic reasons, or for enhancing growth. Photoperiodic lighting is a very low intensity light during the night to break the darkness period and induce plant responses representative of summer (short nights and long days). Greenhouse supplemental lighting is usually provided by high-pressure sodium (HPS) lights because of their relatively high energy efficiency.

Lighting

Innovative light-emitting diode (LED) technologies, an increasingly popular alternative to HPS lighting, are in some ways revolutionizing the way indoor crops are grown. LED electronic light sources are based on the semiconductor diode. When the diode is forward biased (switched on), electrons are able to recombine with holes and energy is released in the form of light. This effect is called electroluminescence and the color of the light is determined by the energy gap of the semiconductor. The LED is usually small in area (less than 1 mm2) with integrated optical components to shape its radiation pattern and assist in reflection.

Modern LEDs are available across the visible, ultraviolet and infrared wavelengths, with very high brightness. LEDs present many advantages over traditional light sources, including lower energy consumption, longer lifetime, improved robustness, smaller size and faster switching. However, they are relatively expensive and require more precise current and heat management than traditional light sources.

LED grow lights consume less power by delivering only the light that plants need. A single LED grow light directly can replace a 1000-watt HPS light while consuming 80 percent less energy. Cool-running LED lights also eliminate the need for ballasts, reflects, noisy fans or expensive cooling systems.

Economics

Modern greenhouse technologies have mirrored developments in most of agriculture in that increased labor efficiency, larger sizes of greenhouse operations, and mass production of a few crops, or even a single crop, have become the rule to be profitable. The current dynamic in the greenhouse industry in the United States is characterized by the entry of many growers in small, specialized operations, and consolidations and mergers of large operations.