Learning To Conserve by A.J. Both
Greenhouses can be net energy producers, not consumers.
November 11, 2008
Rising energy costs this century have had a particularly severe impact on the greenhouse industry because structures are generally designed for maximum light transmissions and not maximum heat retention. While fluctuations in future energy prices are likely, the general consensus is that prices will remain high.
Energy use and management will continue to have a significant impact on our industry. While it is likely conventional and alternative energy sources will continue to be used by the greenhouse industry, new improved energy collection and storage technologies offer the potential for future commercial greenhouses to be net energy producers rather than energy consumers.
The past three decades have shown that before considering the installation of new energy equipment, it literally paid to operate existing energy systems as efficiently as possible. Over the next decades, it is likely the cost of implementing energy conservation measures will continue to be less than the cost of installing new high-efficiency energy technologies. These conservation measures include, but are not limited to:
- Installing twin-layered covering materials.
- Using energy curtains.
- Installing insulation where appropriate.
- Reducing unwanted air exchange through unintended openings in walls and roofs.
- Periodically verifying that environmental control systems and strategies are operating as intended, and keeping track of energy usage and expenses.
Recent heating technology developments enable growers to further reduce their energy consumption. These technologies include high-efficiency direct-fired unit heaters that mix and discharge heat and carbon dioxide with ambient air and condensing boilers that recover heat by allowing condensation of the exhaust gases. Another improvement is the use of combined heat and power (CHP) systems that generate electricity for use in the greenhouse while excess electricity is exported to the local utility grid, and heat from heat exchangers incorporated in the exhaust system that removes the combustion gases.
Heat pumps are reversible refrigerators that can heat or cool the greenhouse environment with a single system depending on the flow direction of the refrigerant. Designing heat pump systems for the maximum heating and cooling loads of greenhouses results in reduced overall system efficiencies, but reasonable efficiencies can be attained for systems designed to handle the first few heating and cooling stages.
The use of heat pumps, heat exchangers and insulated water storage allows for closed greenhouses that no longer require air exchange with the outside environment for cooling purposes. The benefits of closed greenhouses include increased production from higher carbon dioxide concentrations, reduced insect pressures and reduced water use because transpiration water can be recovered.
Finally, the use of light emitting diodes (LEDs) for supplemental lighting of greenhouse crops promises to significantly reduce the overall energy use of year-round greenhouse production that relies on artificial lighting during the darker winter months.
The energy efficiency of greenhouse systems can be greatly increased by adding capability to store excess energy for later use. "Later" can be later in the day, later in the week or even later in the year. Excess energy is typically stored in insulated water tanks or in underground aquifers. Combining a heat pump with heat exchangers and energy storage makes for a particularly efficient energy system, because the heat pump can operate at its highest efficiency over long periods of time. Excess energy can also be stored in phase-change materials (eutectic salts) that are selected for their characteristic melting points. Eutectic salts absorb significant quantities of energy during their liquid phase and release it during the freezing process. For greenhouse systems, eutectic salts with melting points in the 50-100°F range would be particularly useful.
While new sensor technology and calibration procedures typically improve the control of the greenhouse environment, additional gains can be realized by developing advanced control strategies that better incorporate economic implications of control actions.
For example, while lowering the greenhouse temperature by a few degrees during a hot summer day may be technically feasible, it may not be economical when the costs associated with dropping the temperature are larger than the return expected from improved crop production and/or quality. This approach requires inputs from all greenhouse system components as well as external influences such as weather, energy prices and marketing.
In order to better understand the energy usage of individual greenhouse system components, energy usage meters can be installed so that the impact of different control strategies can be monitored instantaneously as well as over longer periods of time.