Geothermal Moves From the Ground Up – Part 1

Heat is being taken from the building and being transferred into the ground in summer. In winter, heat from the ground is transferred into the building.

Geothermal

Editor’s note: This article first appeared in the March/April 2015 edition of Business Energy.

Caves aren’t really cold, but they aren’t really warm either. In a nutshell, that explains a bit about how geothermal systems derive their energy. For anyone who has gone caving or simply visited one of the many stunning caverns throughout this country, wearing a light jacket is often advised no matter what time of year the visit happens. Year round, a dependable temperature somewhere between the 50s and 60s can be found.

The number is a sure thing. Geothermal systems use the heat from the earth, that constant underground temperature, to heat your space in the winter and cool it in summer. Geothermal is cost effective, energy-efficient, reliable, and environment friendly. There are no pollutants resulting from the use of geothermal power.

Geothermal can be used in residential and commercial applications, in new construction as well as in existing homes or businesses.

Multifaceted Benefits With the Types of Geothermal Systems
Geothermal can cut heating and cooling costs by as much as 70%. An added benefit is that these systems can also provide hot water for free. Geothermal systems also keep indoor air cleaner by not relying on outside air, keeping buildings free of pollens, outdoor pollutants, mold, spores, and other allergens. Geothermal is more comfortable than many other systems and the systems do not dry the air like many conventional HVAC systems do. They do not eliminate all humidity. However humidity can be added if needed, or, in the summer when dehumidification may be a more pressing problem, there are ways to dehumidify.

The starting point is piping in which liquid such as water or antifreeze is the medium. There are four types of geothermal ground loop systems to choose from: horizontal, vertical, lake, and open. The same loop works for both heating and cooling.

Horizontal, vertical and lake systems are all closed-loop systems. The heat pump and loop form a sealed pressurized system through which the liquid medium is circulated. Open loop systems, however, are not sealed and are open on either end to obtain the liquid medium (water) from an existing well, and to discharge it when it gets too cold or warm.

Vertical and horizontal closed-loop systems are laid vertically in wells or horizontally in trenches. Both are placed in an area adjacent to the buildings they serve. These systems can go in yards, under parking lots, or under playing fields. Pond or lake loops require a nearby body of water with adequate depth in which to put the loops. Open loops require adequate water for the needs of the system.

There are three major components to a geothermal system: a geo-exchange well, ground source heat pump, and a distribution system. Often times consumers have a hard time understanding which of the multiple options is best for them. There are three different types of geo-exchange wells to consider.

A closed loop system is simply a borehole in which two high-density polyethelene pipes with a U bend at the bottom are installed, with a thermally enhanced grout the length of the borehole. This design typically consists of multiple 200- to 400-foot wells. This type of geo-exchange well is strictly reliant on conductive heat transfer, which is the natural flow of heat energy from the earth through the thermally enhanced grout and the high-density polyethelene tubing and into a glycol/water solution. This is the most common type of geo-exchange well across the United States, where geology permits. However, in Maine, New Hampshire, Vermont, and much of New England, closed loop is not as economical as some other options. Two advantages of closed-loop wells are that there is nothing to worry about in terms of water quality, and sediment will not be a problem with this system.

A standing column well is constructed in the same manner as a conventionally drilled water well with a submersible pump system. This type of geo exchange well also has conductive heat transfer characteristics. Heat energy transfers from the bedrock directly into the column of water.

Then the column of water is circulated into the building through the heat pump in which it extracts heat energy, and then returns this colder water back to the earth where it is reheated by the earth’s crust. This type of heat transfer is more efficient than the closed loop as the heat does not have to go through resistant materials such as the grout, HDPE pipe and into the glycol/water solution.

In addition to this conductive heat transfer one can, in most cases, incorporate a bleed into the system. This gives us what is called an advective heat source. This is simply an act of discharging some of the cold water out of the well to allow the well to recover with warmer water.

This bleed can reduce the drilling to 55–85 feet per ton, and add efficiency to the system by keeping the well temperature higher. This will save money on up front cost and add efficiency at the same time. A 150-foot-per-ton borehole is needed with a no bleed scenario. Though this is still less than the 175 feet per ton for the closed loop, and again more efficient because of a better heat transfer. Contractors typically want to use bleed whenever they can responsibly discharge of the water involved.

The advantages of a standing column well include the fact that it can use same well for drinking water as for the geothermal system (a great cost saving), that heat pumps can be sized approximately 20% smaller because of warmer incoming water temperature, and that boreholes can be drilled shallower, thus saving on upfront costs. ESSbug

 

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