Soil and water below ground contains a vast reservoir of thermal energy. Geothermal heating systems recover this energy and convert it to heat that can be utilized in greenhouses and other buildings. Geothermal heat can be classified into three categories.
Low temperature (50°F)
The soil temperature at the surface varies considerably over the year and closely follows the air temperature. At the 10-12' depth it is more uniform averaging about 50°F with a variation of about 6°F above and below this level. There is also a lag time of about 8 weeks between the maximum surface temperature and the maximum soil temperature at the 12' level which is helpful in winter heating and summer cooling. For the greenhouse production of perennials, herbs, nursery stock and some vegetables that require a temperature from 32-45°F this low grade soil heated air or water can be used directly. For heating the greenhouse to a higher temperature, a heat pump is necessary. These are available as air to air, air to water, water to water or water to air systems.
Medium temperature (140-300°F)
Thermal wells and springs in some parts of the world including the west coast of the U.S. provide hot water that can be used directly for heat. There are currently over 40 greenhouse operations in Oregon, California and Washington that are heated by geothermal energy. The heated water that comes from the ground is distributed through fin radiation or root zone heating.
High temperature (>300°)
The steam from geysers in California, Nevada and Utah is being tapped for power generation. Currently there are about 20 sites in operation with several more under construction. These produce power for 5-8 cents/kW hr.
Greenhouse heating systems
In New England, the only choice that we have for geothermal heating is with low temperature heat. There are several systems that appear to be feasible that have a reasonable payback. Before considering the installation of one of these systems, it is important to address energy conservation. Reducing infiltration, installing energy curtains, insulating sidewalls and the foundation perimeter, making good use of growing space and installing electronic controls should be done first. This will save considerable heat and reduce the size of the heating system needed.
Earth tubes are piping that is buried 6' to 12' 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 the heated air in the greenhouse through the buried tubes and then returning it to the building. The heat is absorbed by the cooler earth.
In the above system the air can be warmed or cooled to near the soil temperature. For example, the average soil temperature 8' below the surface in central Massachusetts varies between 60°F in early Fall to 46°F in early March. To increase the temperature to 80°F - 90°F for air heating for ornamentals or bedding plants, an air to air heat pump could be employed. This process is similar to what happens in a refrigeration system.
Liquid systems utilize either the soil heat to warm a liquid, such as water or antifreeze or directly use water from ponds or well and extract the heat. There are several systems that have been used successfully.
Closed-loop systems circulate water or 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 it. In may also go to a heat pump that amplifies it so that the temperature is warmer.
Horizontal loops may be used where adequate land is available. Pipes are placed in trenches in lengths to 400'. Multiple loops are used to capture the amount of heat needed to heat the greenhouse. Vertical loops are an alternative were land area is limited. Well drilling equipment is used to bore small diameter holes from 75' to 500'; deep. The hole may be filled with a grout to transfer the soil heat to the pipes.
Pond or lake loops are economical to install when a body of water is nearby. This system eliminates the excavation cost. Water or antifreeze is circulated through coils of pipe that are placed in the bottom of the pond or lake. A depth of at least 12' is needed to avoid the influence of the freezing that occurs on the surface during the winter.
An open loop system utilizes 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 use of ground heat is becoming more popular for residential and commercial applications. Due to the high temperature needed for conventional greenhouse heating, a heat pump is needed. Today's equipment is more reliable at a lower cost than a few years ago. 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 shortens. For most geothermal systems the payback is in less than ten years with energy prices at $25/MBtu. (#2 fuel oil = $2.50/gal) Additional information is available at: Mass.GovJohn W. Bartok, Jr.
Natural Resources Mgt. & Engr. Dept.
University of Connecticut , Storrs CT