Energy Conservation Research and Recommended Practices Revisited!by Ted H. Short Rising energy prices over the last six months have greatly renewed grower interest in our extensive energy research done more that 20 years ago after the oil embargo of 1973. Fortunately, much of that research is still relevant and much is summarized in a 1979 OSU Extension Bulletin 651 on Conserving Energy in Ohio Greenhouses. Since that time, the greenhouse industry has implemented many positive practices that remain in-use today. It's now time, however, to review what we are doing and continue to pick the best practices for the future. It is also time to review the possible energy savings of new technologies such as polystyrene pellets pumped into and out of acrylic and polycarbonate panels. This technique, developed by engineers at Ohio State to reduce night time energy use up to 80% is called Select-A-Shade for it's additional summer use as a controlled shading system. Select-A-Shade is currently being demonstrated on a full scale commercial type greenhouse compartment at Toledo Botanical Garden, Toledo, Ohio. Most energy conservation practices can be classified as either modifications or maintenance techniques applied to the structures and heating systems. Structural modifications usually reduce infiltration and add insulation to the greenhouse, sometimes just during cold nights. Heating system modifications seek to optimize heat recovery from the burners and deliver the heat directly to the plants. Any heat going up the chimney or any heating of the air above the plants will essentially be significant losses to the system. A summary of conservation methods compared to single pane glasshouses can be found in Table 1. The wide range of savings is mostly related to the wide range of air leakage rates for glasshouses. For instance, if your old lapped glasshouse is poorly sealed, there could be a 40% savings in fuel by simply sealing all glass laps as well as the mounting frames. If the glasshouse is relatively new and already sealed, more work on sealing may only yield a 5% annual savings. Likewise, if the glasshouse is well sealed, adding a nighttime curtain may have no more than a 20% savings while replacing the glass with double acrylic will give at best a 40% savings. The actual annual savings realized will also vary with the severity of weather conditions. Many of the data in Table 1 were developed for northern Ohio conditions where temperatures, wind, and skies are highly variable throughout the year. The potential savings of any combined system must be considered with great caution. Adding the potential savings listed for two or more systems in Table 1 will almost never indicate the total expected savings. For instance, the addition of double plastic and an internal curtain system to a glasshouse cannot possibly give a 100% fuel savings. Essentially, part of the heat flow that the curtain would have stopped has already been stopped with the double glazing. This example also demonstrates another pitfall in calculating energy savings economics. In the preceding example of double poly and night curtain savings, if both systems had been installed in a greenhouse, the amount of heat saved by one system would jeopardize the economics needed to justify the investment in the other system. Anything you can do to the worst case will almost always yield the best economic savings. Maintenance procedures essentially seek to preserve the maximum designed operating conditions of the heating system or a structure. While maintenance procedures usually provide fewer savings than modification methods, they usually are easier to do and require a smaller investment. Proper maintenance will also prolong the life of the structure and equipment and insure against failure during critical crop periods. |
