GeoExchange Systems Variations

Overview

 

GXS, by definition, use the earth as their energy source. As noted earlier, there are basically two ways to move energy from the ground and into your home – an open loop or well-water system, or a closed loop.

In a closed loop system, a loop isburied in the earth around the home, or laid in a nearby lake or pond. Virtually all loops built today use high-density polyethylene (HDPE) pipe. This type of pipe is specifically designed to be buried in the ground and is marked “geothermal” or “geo”. It cannot be marked “potable”. Joints are made by fusing or melting the pipe and fittings together, which makes a nearly leak-proof connection. Mechanical joints are not used in the ground. A loop made out of HDPE can last 50 years or more. 

A mixture of antifreeze and water is circulated continuously through the loop and heat pump, transferring heat from or to the soil respectively, as heating or air conditioning is needed. In a closed-loop system, the fluid never comes in contact with the soil. It is sealed inside the loop and heat pump.

In an open-loop system, ground water is drawn up from a well and through the heat pump, then typically pumped back into a return well. New water is always being pumped through the system when it is in operation. It is called an open-loop system because the ground water is open to the environment.

Closed Loops

 

Closed loops can have many configurations. There are three basic types: vertical, horizontal and lake (or pond). The loop type and configuration most suitable for your home depend on the size of your property, your future plans for it, its soil, and even your contractor’s excavation equipment. Most often, the loop configuration is selected on the basis of cost. If the loop is designed and installed properly, by taking into account the heating and cooling requirements of the home, one type of loop will operate with the same efficiency as another, and provide years of free, renewable energy.

Over the years the industry has developed standards for GXS installation. In addition, most heat pump manufacturers have developed guidelines or proprietary software for their products to ensure that GXS using them are designed and installed according to their specifications. The CGC also developed best practices and a user friendly designs software, GeoAnalyser.

As a homeowner considering the installation of a GXS, ask your contractor for proof of training, experience and competence of its staff in loop design and installation. Since 2007, the Canadian GeoExchange Coalition trains and accredits industry professionals. The best way to ensure your installation contractor’s competence is to verify whether he is CGC accredited. A list of industry specialists is available at www.geoexchange.ca.

Horizontal Loops

 

As the name implies, these loops are buried horizontally, usually at a depth of about 2 to 2.5 m, although it can vary from 1.5 to 3 m or more. Usually trenches are excavated with a backhoe; a chain trencher can be used in some soil types. Fill can sometimes be used to cover a loop in a low-lying area of the property. The trench can be from 1 to 3 m wide. Four or even six pipes can be laid at the bottom of a wide trench, while some loop designs allow two layers of pipe to be stacked in a trench at different levels. Loop configurations may even use a “slinky” or coiled configuration that concentrates additional pipe in a trench. Many different configurations have been tested and approved. Make sure you ask your contractor for references. Contractors can often show you photographs of loops they have installed.

The area you need to install a horizontal loop depends on the heating and cooling loads of your home, the depth at which the loop is to be buried, the soil and how much moisture it contains, the climate, the efficiency of the heat pump and the configuration of the loop. The average 150-m2 home needs an area of between 300 and 700 m2. Your contractor will use computer software or loop design guidelines provided by the heat pump manufacturer to determine the size and configuration of your earth loop.

Vertical Loops

 

Vertical loops are made out of HDPE pipe, which is inserted into holes drilled in the soil. Taking in to account different canadian geological conditions and drilling equipment used, these boreholes are 15–150 m deep, and 10–15 cm around. Two lengths of pipe are fused into a “U-bend” (two 90° elbows) and inserted into the borehole. The size of pipe used for the loop varies, depending on the cost of drilling and the depth of the borehole; 32 mm pipe is common in some areas, 19 or 25 mm pipe in others. After the pipe has been placed in the borehole, it must be grouted to prevent potentially polluted surface water infiltration into lithostratisgraphic units and aquifers. A bentonite grout is normally used. This is to ensure good contact with the soil and prevent surface water from contaminating the ground water. The borehole around the pipe is to be filled by means of a tremie line, or a pipe inserted to the bottom of the borehole and retracted as it is filled with grout. This process is designed to eliminate air pockets around the pipe and ensure good contact with the soil.

The main advantage of a vertical loop is that it can be installed in a much smaller area than a horizontal loop. Four boreholes drilled in an area of 9 m2 – which fits easily into an average city backyard – can provide all the renewable energy you need to heat an average 150-m2 home.

The cost of installing a vertical loop can vary greatly, with soil conditions the single most important factor. Drilling into granite requires much heavier, more costly equipment, and is much more time-consuming than drilling into soft clay. It is even more time-consuming when the soil
contains a mix of materials, such as layers of boulders, gravel and sand.

The installation of a vertical loop in this type of soil is three to four times more costly than that of a horizontal one.In areas like southern Manitoba and Saskatchewan, however, where glacial Lake Agassiz has left 15–50 m of soft clay deposits, a vertical loop can be installed for about the same cost as a horizontal one.

The depth of borehole needed for a vertical loop depends on the same factors that determine the land area required for a horizontal one. The land area needed for the vertical loop, however, depends on the depth to which the boreholes can be drilled cost-effectively. For example, if a GXS requires 180 m of borehole in total, and is to be installed where bedrock is found at 20 m, it would usually be cheaper to drill nine boreholes to a depth of 20 m than three to a depth of 60 m. Nine boreholes would require an area of about 150 m2, and three, an area of about 60 m2.

Lake or Pond Loops

 

These types of loops can be installed very cost-effectively for a home located near a lake or pond. Many homes in northern Ontario, for example, are within metres of a lake that soaks up the sun’s energy all summer. The water temperature at the bottom of an ice-covered lake is about 4 to 5°C even during the coldest blizzard. And in the summer, the lake water can easily absorb the heat you are trying to expel to cool your home. All you need is a year-round minimum depth of 2–2.5 m of water in which the loop can be protected from wave action and ice pile-ups.

Unless you own the lake, however, you need permission from the provincial government, and in some cases from the Government of Canada, to install a lake loop. Some jurisdictions do not allow them. Destruction of fish spawning grounds, shoreline erosion, obstruction of traffic on navigable waters and potential damage to the environment concern several government departments. In some jurisdictions, enough lake loops have been installed that permission is simply a matter of filling out forms. Some GXS contractors who specialize in lake loop installation handle all the permission paperwork for their clients.

In the Prairies, farm ponds are often excavated to provide water for irrigation or livestock. A 750– 1000-m2 pond with a constant depth of 2.5 m can do double duty as a clean source of energy. The oceans can also supply vast amounts of energy, but care must be taken to protect an ocean loop from tide and wave damage. Many homes on the West Coast already benefit from free,renewable ocean energy.

Open Loops

 

Open loops, or ground water GXS, take heat from well water that is pumped directly through the heat exchanger in a heat pump. The required flow of well water is determined by the capacity of your heat pump. In the coldest part of the winter, heating a typical 150-m2 new home takes 20 000–30 000 L of water per day, or a flow rate of 0.4–0.5 L per second (a typical backyard pool contains about 60 000– 70 000 L). A larger home will need proportionally more water. You need a reliable well to supply this volume of water. Typically, you will also need a second or return well to dispose of the water by pumping it back into the ground. Most provinces regulate the use of wells, and can advise you on the use of well water for GXS applications. For example, you must take care to avoid affecting your neighbors’ wells when pumping continuously. Regulations on the use of well water as a heat source for a GXS vary with each province. You should contact the department with jurisdiction over ground water resources for the regulations in your province.

To ensure that the well is capable of supplying the water on a sustainable basis, and that the return well has the capacity to accept the water after it has circulated through the heat pump, you need to carry out a pump test on your wells. In some locations, the capacity of the aquifer is well known, and you can find out the capacity of your new well within a few hours. In other areas, it will be necessary to perform a test by measuring the drop in water levels at specified intervals while the well is pumped at a known rate for as long as 24 hours.

As well water circulates through the heat pump, corrosive water can damage the heat exchanger over time; additionally, water with a high mineral content can cause scale buildup. Most manufacturers can supply heat pumps made out of resistant materials like cupronickel or stainless steel that are more suitable for use in open-loop systems. Manufacturers will specify the quality of water that is acceptable for their equipment.Again, you may need to have your water tested to ensure it falls within the guidelines. The department that regulates the water resources in your province may be able to advise you on where the water can be tested.

Mechanical equipment lasts longer if it does not have to start and stop repeatedly. Well pumps are no exception. The contractor installing the well pump and pressure system must be told that it will be used to supply water for a GXS. For efficient operation, the pump design and horsepower must be chosen to supply the correct amount of water. Bigger is not better. The water requirements for the system, the height the water is lifted from the well and the piping from the well to the house and to the return well must be taken into account. To prevent the well pump from short-cycling, you may need to install a larger pressure tank. These details can affect the overall efficiency of your GXS by as much as 25–30 percent. 

The temperature of ground water is very constant, ranging between 5 and 12°C across Canada. The temperature of the fluid pumped through a closed loop used in a home normally drops to slightly below freezing during the winter. When well water is used as the energy source during the winter, the heat pump produces more heat and will be more efficient. However, since the water must actually be lifted from the ground, sometimes as much as 15–30 m, you will need a more powerful pump than the one required for a closed-loop system. In addition, the same pump often supplies water for both the heat pump and general household use. The cost of operating the larger well pump often offsets the efficiency of running the GXS with well well water. Ask GXS contractors in your area about their experience with open-loop systems when deciding on the best option for your home.

When you are planning any excavation, you must make sure the site is surveyed and that the location of any other services, such as electrical lines, gas lines, water lines, sewer lines, septic fields or underground storage tanks, is determined. Also, when you are deciding where to install a loop on your property, keep in mind that heavy equipment cannot operate under overhead electrical lines. Wherever you install the ground loop or water wells and lines for a GXS, they must be added to your site plan. This will avoid costly future repairs. The homeowner must be provided with a copy of a drawing showing the location of a closed-loop system, and that a tracing wire or tracing tape must be laid in the ground above any closed-loop pipes to make finding the system easier in the future. In addition, the contractor must keep a copy of your closed-loop layout for seven years.

 

 

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