USE OF GEOTHERMAL ENERGY IN AGRICULTURAL GREENHOUSES
Geothermal heat
Soil and water below ground contain a vast reservoir of thermal energy. Geothermal energy refers to the heat stored beneath the Earth’s surface, which can be harnessed for various purposes, including electricity generation, heating, and cooling.Geothermal heating systems recover this energy and convert it to heat that can be utilized in greenhouses and other buildings. Geothermal heat resources can be classified into three categories: low, medium, and high temperature.
One of the primary applications of geothermal energy in greenhouses is heating. By tapping into the Earth’s natural heat reservoirs, greenhouse operators can maintain stable temperatures even in cold climates, enabling year-round cultivation of crops. This not only extends the growing season but also reduces the dependence on conventional heating methods powered by fossil fuels, thereby lowering carbon emissions and mitigating environmental impact.
Moreover, geothermal energy can be utilized for cooling purposes in regions with high ambient temperatures. By circulating underground water or utilizing geothermal heat pumps, greenhouses can effectively dissipate excess heat, preventing overheating and heat stress in plants. This passive cooling mechanism enhances productivity and ensures optimal growing conditions, especially for heat-sensitive crops.
Temperature of geothermal fluids
- Low temperature (10oC +/-). The soil temperature at the surface varies considerably over the year and closely follows the air temperature. At the 1.5 to 2 m depth, the temperature is more uniform, averaging about 10oC with a variation that depends on soil characteristics and the environmental conditions above ground level. There is also a lag time of about 8 weeks between the maximum surface temperature and the maximum soil temperature at this level which is helpful in winter heating and summer cooling. For the greenhouse production of perennials, herbs, nursery stock and some vegetables. This heated air can be used directly, or, for heating the greenhouse to a higher temperature, a heat pump can be used. Heat pumps are available as air to air, air to water, water to water or water to air systems.
- Medium temperature (60-150°C). Thermal wells and springs in some parts of the world can provide hot water that can be used directly for heat. In USA there are dozens of greenhouse operations in the Rocky Mountain and West Coast states that are heated by medium temperature geothermal energy. The heated water that comes from the ground is distributed through fin radiation or root zone heating.
- High temperature (>150°C). The steam from geysers in California, Nevada and Utah, USA is being tapped for power generation but not for greenhouse heating. Currently there are about 20 sites in operation with several more under construction. The cost of the generated electricity is estimated at 5-8 cents/kWh.
Heating greenhouses with geothermal energy
In most countries the economical choice for geothermal heating is with low temperature heat, because it offers a reasonable payback, typically ten years or less, depending on the system design and the cost of fossil fuel replaced. Before considering the installation of a geothermal system in the greenhouse it is important to carefully calculate the estimated payback and to implement energy conservation measures that usually have a payback period of just a few years. These include: reducing air infiltration, installing energy curtains, insulating sidewalls and the foundation perimeter, making good use of growing space and installing electronic environmental controls. These measures should be done first since they can considerably reduce heat losses and thus reduce the size of the geothermal heating system needed to heat the greenhouse.
Air systems
Earth tubes are piping that is buried 2 to 4 m below the soil surface. The simplest and least expensive systems gather heat during the winter by drawing air through corrugated plastic tubes and direct it into the space to be heated. The air passing through the tubes is warmed by the soil that has a higher temperature than the air. During the summer the system can be used to cool building space by drawing outside air in the greenhouse through the buried tubes. The heat is absorbed by the cooler earth. For example, the average soil temperature 3 m below the surface in central Massachusetts, USA varies between 15oC in early Fall to 8oC in early March. To increase the temperature in the range of 25°and 30°C for air heating an air-to-air heat pump could be employed. The heat pump operates as a reversible refrigerator: it can heat or cool the air used to maintain the optimum growing environment.
Liquid systems typically utilize either the soil heat or heat from a well, pond, or other body of water to warm a liquid such as propylene glycol or methyl alcohol in a closed-loop system from which heat is extracted. Where adequate land is available, horizontal loops may be used to capture heat from the ground. Pipes are placed in trenches in lengths up to 120 m. Multiple loops are used to capture the amount of heat needed to heat the greenhouses. Vertical loops are an alternative solution when the land area is limited. In this case well-drilling equipment is used to bore small diameter holes from 25 to 150 m deep. The hole may be filled with a grout to transfer the soil heat to the pipes.
Closed-loop systems circulate an antifreeze solution through loops of small diameter underground pipes. In cold weather this solution absorbs heat from the ground and carries it to a heat exchanger that extracts the heat. The solution may also go to a heat pump that amplifies the temperature increase. Closed loop systems using pond or lake loops are economical to install when a body of water is nearby. This system eliminates the excavation cost. Antifreeze is circulated through coils of pipe that are placed in the bottom of the pond or lake. A depth of at least 4 m is needed to avoid the influence of the freezing that occurs on the surface during the winter. The pond or lake volume should be large enough to absorb the energy needed for heating, or the energy released by cooling, without significantly impacting the pond/lake water temperature.
Open loop systems utilize ground water directly. Water is usually pumped from one well and returned to a second, adjacent well. The distance between wells has to be far enough so that the return water doesn’t influence the intake water. The water may also be pumped out of a pond or lake at one location and returned a distance away. Open loop systems can be economical if the source of water is located nearby.
The feasibility of geothermal energy varies depending on geographic location and geological conditions. Not all regions possess suitable geothermal resources for cost-effective utilization in greenhouse agriculture, necessitating site-specific assessments and feasibility studies prior to implementation.
Greenhouse Geothermal Heat Classifications
Geothermal heat is classified in 3 categories.
- The first is a low temperature at 10oC. This temperature is reliably found about 3.5 m underground. Harnessing this heat level requires the use of a heat pump in various configurations.
- The next heat level is medium in the range of 60-150oC. This heat comes from thermal wells and springs that occur naturally in certain parts of the world.
- The third category is high temperature, at 150oC. Such extreme heat stems from geysers and is not standardly available for home gardeners.
In most regions, low temperature heat is the most feasible for greenhouse heating. Before considering installing the components necessary for harnessing this natural heat, it is best to consider the payback period and the amount of fossil fuel that can be saved. In many cases, 10 years is the estimated payback time.
Basic Geothermal Greenhouse Design
It’s easiest to install a geothermal heating system prior to erecting the greenhouse. Trenches need to be dug 1.8-3.6 m below the surface of the soil. These trenches will house the hoses or tubing that will carry heat from the ground. Perforated tubes at a diameter of 10 cm will draw warm air from the soil and direct it into the structure. To maximize heat storage and cooling capacity, the greenhouse itself can be partially buried in soil. This acts as an insulator, allowing the system to work more effectively.
Benefits of using Geothermal Energy in Greenhouses
The integration of geothermal energy into greenhouse agriculture offers a multitude of benefits, both environmental and economic.
Firstly, it reduces reliance on non-renewable energy sources, fostering energy independence and resilience against fluctuating fuel prices. This, in turn, enhances the economic viability and long-term sustainability of greenhouse operations.
Secondly, geothermal heating and cooling systems have minimal operational costs once installed, resulting in significant savings over time. Unlike conventional heating systems that require constant fuel supply and maintenance, geothermal systems have lower maintenance requirements and longer lifespans, translating into reduced operating expenses for greenhouse operators.
Furthermore, geothermal energy aligns with the principles of sustainable agriculture by minimizing carbon footprint and environmental degradation. By transitioning to renewable energy sources, greenhouse operations contribute to mitigating climate change and preserving natural resources, thus promoting ecological stewardship and environmental conservation.
Conclusions
Today’s geothermal equipment is more reliable at a lower cost than a few years ago. That, plus recent volatility in fossil fuel prices, have made the use of geothermal heat increasingly popular for residential and commercial applications. Due to the higher temperatures typically needed for greenhouse heating, a heat pump may be required with geothermal systems. Where low temperature heat is needed, such as maintaining an air temperature just above freezing, direct use of the heat is possible.As the cost of fossil fuels increases, the payback for alternative heating systems decreases. For most geothermal systems the payback is typically less than ten years. Occasionally, it can be significantly better than that, in the range of 5 to 7 years, depending on the geothermal source, the cost of the system, and the quantity of fossil fuel displaced.
