Connection of electrodes on polymer based heating elements

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Department of Agriculture
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Phase I
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 Jeffrey Wheeler
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 Jeffrey Wheeler
Title: R&D Engineering Manager
Phone: (269) 964-0271
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Current methods for heating seedling trays in the greenhouse envelope during colder climates include natural gas, geothermal as well as electric. The most common method of heating the greenhouse is with natural gas. Natural gas systems use a forced air commercial furnace to blow heated air onto the environment. There are two ways that the warm air can be directed from the furnace to the seedling. The first way is from above the tray, where the purpose is to provide space heating, or second from under the tray thereby heating the table surface the tray is sitting on, indirectly heating the bottom of the tray. The more modern (and costly to implement) is using a geothermal system. In this case the system captures heat in a closed loop system that absorbs heat from wells drilled into the ground. The fluid can then be circulated in the floor of the greenhouse thereby heating the environment by radiation, the table surface that the trays are sitting on, or through a heat exchanger mounted in the air space that takes the warm fluid and transfers the heat to the air space, acting as a radiator. While geothermal systems are considered to be environmentally friendly, these systems typically use heat transfer fluids which are made of ethylene glycol or refrigerant gases such as refichlorodifluoromethane, which is an ozone depleting substance. Although the heat transfer gases are harmless while contained, leaks and improper end-of-life disposal can contribute to enlarging the ozone hole. Geothermal systems are also very expensive to install. Typical applications of electric heating are space heating, using an electric heat radiator to heat the air in the greenhouse. Another method is to use an infrared emitter to heat the soil, which is more effective but hard to control, can damage plants, and expensive to implement. The low cost method is using a blanket heater, on which the tray is placed, and heat is conducted through the bottom surface of the tray, into the soil. These are typically controlled by a thermostat attached to the heater. This method of temperature control assumes the soil will be at the same temperature. Actually the temperature of the soil will be dependent on how effective the heat is being transferred to the tray, as well as the trays environmental. A failure in the thermostat in this design can result in the tray being sterilized and thus lead to complete loss of the product. In all the examples listed above the product mix that can be germinated at one time is limited to the greenhouse space. If this effort is successful it can lead the way to a new design of horticultural seeding tray that offers a complete no maintenance option for the grower to implement in the greenhouse environment. Further this will allow the user to customize the germination cycles they implement within the common greenhouse space, maximizing production. The expected end result will be a potential reduction in the amount of energy required to heat the target - in this case the soil and seedling - thereby reducing the operation cost for greenhouse operators, up to 92.5% reduction in energy costs.

* information listed above is at the time of submission.

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