Utilizing solar and wind energy can help balance the costs of production in your greenhouse facilities.
Because solar panels do not transmit light, they can be attached to south-facing roofs or other support structures, or on a tracking device that follows the position of the sun across the sky.
Photo courtesy of A.J. Both, Rutgers University
As you know, since the start of the millennium oil prices have risen significantly, peaked at over $140 per barrel in the summer of 2008 and have since fallen to around $100 per barrel (approximately a five-fold increase compared to average oil prices between 1980 and 2000). Natural gas prices have not risen as dramatically because of increased production from domestic shale gas reserves.
As our economy proceeds to recover since the financial crisis of 2008, consumers appear to be spending a bit more on non-essentials, including ornamental plants. But the operative term there is “a bit.” There’s still plenty of competition for that rare extra dollar, and if growers are to succeed in an era of doing more with less, they need to save as much as possible on production costs.
It is no surprise, then, that many growers with greenhouse facilities are wondering how they will do so. One strategy is to lower energy consumption by using alternative energy sources. Though a lot of attention has been given to biomass as an alternative fuel source (for example, wood, corn or switchgrass), solar and wind energy also hold tremendous promise as sustainable and economical means of powering your production programs.
Power from the sun
Plants need sunlight for photosynthesis, so typical greenhouse glazing materials are designed for maximum light transmission. However, because sunlight can become too bright for optimum plant production, shade curtains are used to control the light intensity at crop level. The glazing material and the shade curtain have a significant impact on the heat loss (and gain) of a greenhouse, so designers typically compromise between light transmission and energy management.
While plants convert sunlight into biomass production (with efficiencies of only a few percent), photovoltaic (PV) panels convert it into electricity. The conversion efficiencies of PV panels have increased over the years to as high as 20 percent or more at maximum light intensity. Some experimental PV cells have achieved efficiencies of 40 percent.
PV panels should be mounted for maximum light interception. In the Northern hemisphere, panels can be attached to south-facing roofs or other support structures (see photo, opposite), or on a tracking device that follows the position of the sun across the sky. Typical PV panels do not transmit sunlight, so installation on greenhouse roofs is not recommended unless the greenhouse is used to grow crops that require reduced light intensities (for example, orchids).
PV panels generate DC power that can be utilized to operate DC greenhouse equipment, or converted to AC power to operate most conventional greenhouse equipment. PV systems can be interconnected with the local electrical grid, ensuring that electrical power is available even when there is little or no sun, unless there are local power outages. In a grid-connected system, excess power from the solar installation can be sent to the grid. (Interconnection requirements vary from state to state and utility to utility.) Off-grid PV systems require some form of electrical storage to provide power during periods of little or no sun. Typically, banks of batteries are installed for this purpose. Off-grid systems are best suited to applications where there is no nearby electrical grid and electrical loads are predictable and fairly consistent.
A significant portion of sunlight reaches the surface of the Earth as heat radiation. We can use this energy to heat water. Typically, not much water is needed for washing and cleaning purposes, but growers can use warm water to heat the greenhouse. The largest amount of solar energy can be collected during the middle of the day, so storing the warm water for use during the night is a good strategy to reduce the use of heating fuel. Fluctuating heating fuel prices make long-term storage of warm water attractive. This strategy would allow for energy collection during the sunnier months of the year, to be used during the darker (and colder) months. Long-term storage requires large, insulated holding tanks or other means (such as underground aquifers) to contain the energy. In some cases, both warm and cold-water storage systems are used, allowing for heating as well as cooling of the greenhouse environment. In that case, the use of a heat pump (a reversible refrigerator that can be operated in heating or cooling mode) can prove economical.
There are many technologies for converting incoming solar radiation into heat. The most common systems are flat plate collectors, in which water or other fluids flow through a panel that is oriented toward the sun. Very simple flat plate collectors are often used for heating swimming pools. Slightly more complex systems that may incorporate water storage, insulation behind the panels and transparent covers in front of the panels are used around the world for heating hot water for domestic use. Flat plate collectors work most efficiently at lower water temperatures. Other systems for converting sunlight into heat include evacuated tube collectors and parabolic reflectors. These products are capable of generating higher temperatures, but are significantly more expensive and often depend on using tracking systems to maintain an optimal orientation.
The American Solar Energy Society provides additional information on their web site (www.ases.org).
Small wind generators (up to 100 kW in generating capacity) can be used to operate much of your greenhouse equipment, except for possibly a supplemental lighting array, simply because of its rather significant power consumption. Some farming cooperatives may help to finance large installations.
Photo courtesy of A.J. Both, Rutgers University
Harnessing the wind
Using wind to pump water and generate power is not a new idea. Before the start of rural electrification in 1936, wind energy was widely used across the U.S. During the last dozen years or so, technology improvements and rising energy prices have significantly increased the number of wind energy installations. In many cases, large installations occurred on farmland, but often the farmers are not the main users of the generated energy, nor do they own the equipment. Many farmers only receive a land lease payment for the land area used by wind turbines. Today most new wind installations are used to generate electric power.
Wind energy is fast becoming an alternative of choice in the U.S.
Obviously, the success of wind energy installations depends on site-specific wind conditions. The height of the generator above the ground also plays an important role. Coastal and mountainous areas and the Central Plains typically experience higher average wind speeds and are thus more attractive as generating sites. Wind maps have been compiled for all regions of the U.S., and these maps are useful for a first approximation of the average wind speed at a given location. For a good example, visit www.nrel.gov/gis/wind.html. However, local topology, vegetation and surrounding building structures significantly affect the average wind speed. Where possible, use long-term (preferably 1 year or longer) local wind speed measurements to determine whether a site is appropriate for wind generation. Currently, an average wind speed of 9 mph for small wind generators and 13 mph for large generators (measured at the height of the generator) is considered necessary for the economical use of wind power.
So what are the options if you want to power your greenhouse with a wind generator? Small wind generators (up to 100 kW in generating capacity) can be used to operate much of the greenhouse equipment, except for possibly a supplemental lighting array, simply because of its rather significant power consumption. (Such a system can be operated off-grid, but a connection to the grid is preferable.) Grid connected systems have the advantage of having power available when the wind system is not functioning at full capacity, and do not require batteries for electrical storage. Large wind installations require a significant investment, are subject to environmental impact reviews and have not always been embraced by local communities. In some cases, farmers have formed cooperatives to finance such projects, gain community approval and reduce investment risks. The American Wind Energy Association provides additional information on its web site at www.awea.org.
Depending on local regulations and laws, so-called net-metering systems can be installed that allow your electricity meter to run “backwards” when your power generator is producing more than you need. Often, net-metering arrangements allow a producer to deliver to the grid up to their typical annual use determined over a period of a few years. Beyond that, any excess power delivered to the grid will not result in additional reimbursements. Therefore, larger generating systems that plan to export significant quantities of electricity to the grid require special arrangements with the local electric utility. In many states, electrical utilities are mandated to purchase a specific percentage of their energy from “green” or renewable sources and could very well be interested in making special arrangements with larger energy producers.
Grant, loan and rebate programs
Spending extra dollars retrofitting your facility to save money on utility costs may seem illogical. In the long run, however, you can realize real savings. Plus, local utilities, state and federal organizations offer a variety of grant, loan and rebate programs for alternative energy installations. Each of these programs comes with its own set of requirements and often require cost-sharing. Nevertheless, these programs can reduce the investment costs and/or reduce the payback period.
Many of these programs are announced on web sites requiring some effort to learn about them. In some states, energy-regulating commissions such as a Board of Public Utilities or state energy agencies have programs for renewable energy systems. Your local Extension service, state departments of agriculture, the USDA and the NRCS (National Resource Conservation Service) are good places to start investigating the various opportunities. And don’t forget to talk to your local utility.
Renewable energy certificates
Some states administer renewable energy certificate (REC) programs that allow certified producers of eligible renewable energy to sell these certificates that represent proof that 1,000 kWh of electricity was produced. Thus, in addition to reducing your electric power consumption from the utility grid (i.e., by lowering your monthly electricity bill or receiving payment for excess electricity you exported to the grid), the RECs generated by your system can provide additional income when sold, for example, to a power company that was mandated to deliver a certain percentage of its total output as renewable energy. While prices for RECs fluctuate, REC programs provide additional financial incentives for renewable energy production. Find more information at www.eere.energy.gov/greenpower/markets.
Before you consider a solar or wind system for your greenhouse facilities, however, the first step in any renewable energy project is ensuring that the existing system is functioning efficiently. The reason is quite simple: The cost of implementing energy efficiency measures is less than the cost of installing renewable energy technologies to compensate for inefficient use of conventional energy sources.
Some of the many ways to achieve better energy performance in greenhouses include using thermal curtains where possible and checking that they seal properly (i.e., form a continuous barrier); verifying that environmental control systems are doing what they are supposed to (including sensor calibration); and sealing glazing leaks through unintended openings in walls and roofs.
Additional energy conservation ideas and techniques can be found at www.flor.hrt.msu.edu/energy. A useful reference source is the book titled “Energy Conservation for Commercial Greenhouses,” published by PALS Publishing (http://palspublishing.cals.cornell.edu). And you can read about one company’s experience with energy audits in the September 2012 issue of American Nurseryman; see www.amerinursery.com/article-8626.aspx.
Dr. A.J. Both is Associate Extension Specialist in Controlled Environment Engineering in the Department of Environmental Sciences at Rutgers University, New Brunswick, NJ. He can be reached at