Name : Nuttachanok Thedmuang M.57 No.6
Reference : https://attra.ncat.org/publication/geothermal-greenhouses-exploring-the-potential/
INTRODUCTION: GEOTHERMAL ENERGY FOR AGRICULTURAL GROWING STRUCTURES :
Agricultural growing structures, such as greenhouses and high tunnels, are used for plant propagation, season extension, and to enhance and control crop production. Greenhouses and high tunnels can have the same basic structure: metal frames and plastic polyethylene, polycarbonate, or glass walls and roof. Both provide varying degrees of temperature and climate control to extend the growing season. High tunnels, sometimes called hoop houses, differ from greenhouses in that they include non-automated climate controls (like vents or roll-up sides) and are typically used for season extension rather than year-round growing. Traditional greenhouses are primarily used for plant propagation, require heat usually provided through automated climate controls, and generally built with more durable materials. This also allows for more consistent year-round growing. A third variation, passive solar greenhouses, consists of structures typically smaller than high tunnels that do not use power-generated heat sources like greenhouses do.
Active Systems :
An active system utilizes a water-to-air heat pump, a liquid (either water or a glycol solution) as the medium for heat exchange, and evaporator and condenser coils. It uses the basic refrigeration cycle of evaporation, compression, condensation, and expansion to heat or cool the air before it is delivered to the space. The piping can be organized in the ground horizontally (trenches) or vertically (bore holes). Horizontal trenches are most applicable for small-scale operations because a vertical system requires drilling several deep holes (about 200 to 500 feet) to install the piping. This can become expensive because of the deep drilling that is required, although it does have greater ability to offset energy needed for heating or cooling in more sophisticated buildings and large operations (EPA, 2016). In this system, the liquid runs through piping in the ground, either absorbing or diffusing heat, depending on the temperature needs of the building above.
Traditional greenhouses protect crops in temperature extremes by using temperature, humidity, and ventilation controls to maintain an ideal climate (Rimol Greenhouse Systems, 2019). Generating such a climate can result in significant energy costs. Heating and electric combined is typically the third-largest cost for operating a traditional greenhouse (Penn State Cooperative Extension Service, 2020). By contrast, high tunnels and solar greenhouses are passive in design and do not require extra energy for heating and cooling, making them a more cost-effective option for certain operations. However, growers can further benefit from these structures when they are supplemented with an equally cost-effective climate-control system. Perhaps surprisingly, an efficient source of supplemental energy can be found right below our feet.
Geothermal heat comes in 3 categories.
How Geothermal Systems Work :
The flow of heat energy within a geothermal system can be utilized to heat or cool a space, depending on seasonal needs. Air is moved from inside a building and pushed through a network of underground piping. Convection through this piping heats or cools the air, which is then exhausted back into the structure at a more desirable temperature. When the underground temperature is warmer than the ambient temperature above ground, the heat pump or fan pushes the warmer air up to the growing structure that needs to be heated. When the temperature in the growing structure is too warm, the pump or fan removes the warm air from the building and pushes it into the pipe below ground, returning cooler air to the buildin
An open loop geothermal heating system that harnesses energy from deep in the soil requires several components.
- First is a geothermal heat pump unit which will circulate the cool air to warm soil in winter and vice versa in summer. This helps retain a consistent temperature in the structure.
- To use this pump effectively, tubing needs to be installed under the structure to circulate the heat.
- Accompanying the pump are a solenoid valve, transformer, and water pump.
A closed loop system requires many more parts. The open loop will also require a thermostat, tubing or hose kit, manifold, a non-pressurized flow center, and adaptor kits. A closed loop system also relies upon propylene glycol which acts as an antifreeze for circulating water.