Introduction to earth energy systems

* Source: Natural Resources Canada (NRC)

• What is Earth Energy?
• How Earth Energy Systems Work?
• Variations of Earth Energy Systems

WHAT IS EARTH ENERGY?

The sun has always provided heat for the earth. Its energy warms the earth directly, but also indirectly. Its heat evaporates water from the lakes and streams, which eventually falls back to earth and filters into the ground. A few metres of surface soil insulate the earth and ground water below. The warm earth and ground water below the surface provide a free, renewable source of energy for as long as the sun continues to shine. The earth under an average suburban residential lot can easily provide enough free energy to heat and cool the home built on it.

The free energy has only to be moved from the ground into your home. This is done by drawing ground water directly from a well and using a heat pump to extract heat from it. As well, a circuit of underground piping called a loop can be buried in the soil outside the home through which fluid – water or antifreeze – is pumped. The fluid, called the heat transfer fluid, absorbs the heat in the ground water or soil and transfers it to the heat pump. The heat absorbed by the fluid from the solar-heated ground is extracted from it by the heat pump, and the now-chilled fluid is circulated through a heat exchanger over and over again to extract more heat from the earth.

If your home is located near a suitable pond or lake, you can use an Earth Energy System (EES) to draw on this excellent source of free energy.

Burying a loop in the ground around your home is like owning your own oil well, but instead of pumping oil from an underground pool and burning it to create heat (and greenhouse gases), you tap into clean energy that will be there for as long as there is a sun.

A well-designed ground loop will not hurt the earth or plants growing above it. There is no visible part to show that it is buried in your yard. If your system uses ground water, it has no effect on the water other than changing its temperature by a few degrees. Finally, a well-designed ground water system will not waste the water, but put it back into the ground by means of a return well.

How Earth Energy Systems Work?

The heat energy taken from the ground by your EES is considered low-grade heat. In other words, it is not warm enough to heat your home without being concentrated or upgraded somehow. However, there is plenty of it – the average temperature of the ground just a few metres below the surface is similar to (or even higher than) the average annual outdoor air temperature. For example, in Toronto, the average annual air temperature is about 8.9°C, but the average ground temperature is 10.1°C. It is important to note that this ground temperature is 10.1°C on the hottest day of summer as well as on the coldest day of winter. That is why some of the first humans lived in caves – the caves would protect them from the temperature extremes of winter and summer. That is also why an EES works so efficiently – it uses a constant, relatively warm source (ground or water) from which to draw energy.

Basic Components of an EES:

The figure below illustrates a typical EES. It is made up of three main parts: a loop, the heat pump and the distribution system. The following section describes some of the various loop designs, heat pumps and distribution systems commonly used in a Canadian EES.

Components of a typical Earth Energy System

The loop is built from plastic pipe which is buried in the ground outside your home either in a horizontal trench (horizontal loop) or through holes drilled in the earth (vertical loop). The loop may also be laid on the bottom of a nearby lake or pond (lake loop or pond loop). Your EES circulates liquid (the heat transfer fluid) through the loop and to the heat pump located inside the home. The heat pump chills the liquid and distributes the heat collected from it throughout the home. The chilled liquid is pumped back into the loop and, because it is colder than the ground, is able to draw more heat from the surrounding soil. These loops are often referred to collectively as a closed loop, as the same liquid circulates through the closed system over and over again.

A coiled loop can be installed in the ground or in a pond or lake.

Another way is to pump ground water or well water directly through the heat pump. An EES that uses ground water is often referred to as an open-loop system. The heat pump cools the well water, which is usually returned to the ground in a return well. To run an open-loop EES, you need two reliable wells with water that contains few dissolved minerals that can cause scale build-up or rust over the long term, as it is pumped through the heat pump’s heat exchanger.

In both cases, a pump circulates liquid through the loop and the heat pump. The heat pump chills (or collects the heat stored in) the liquid when it is being used as a source of heat, and circulates it back through the loop to pick up more heat. A system for a large home will require a larger heat pump and ground loop, with a circulation pump to match.

After the EES has taken the heat energy from the ground loop and upgraded it to a temperature usable in your home, it delivers the heat evenly to all parts of the building through a distribution system. It can use either air or water to move the heat from the heat pump into the home. Forced air is the most common distribution system in most parts of Canada, although a hot-water or hydronic system can also be used. 

Forced-Air Systems

A heat pump in a forced-air EES uses a heat exchanger to take the heat energy from the refrigerant to heat the air that is blown over it. The air is directed through ducts to the different rooms in the home, as with any forced-air fossil fuel or electric furnace. The advantages of a forced-air EES are as follows:

• it can distribute fresh, outside air throughout the home;
• it can air-condition the home (by taking the heat from the air in your home and transferring it to the ground loop) as well as heat it; and
• it can filter the air in your home as it circulates through the system.

An EES is designed to raise the heat of the air flowing through the heat pump by between 10 and 15°C; fossil fuel or electric furnaces are designed to raise it by 20 to 30°C. That difference means an EES must move more air through the home to distribute the same amount of heat as a conventional furnace. So to design an efficient, quiet forced-air EES, the contractor designing the ductwork must take into account the larger amount of air to be moved. The ductwork should also have acoustic insulation installed inside the plenum and the first few metres of duct, as well as a flexible connection between the heatpump and the main duct to ensure quiet operation.

Hydronic (Hot-Water) Heating Systems

As we said earlier, a heat pump can heat either air or water. The latter type distributes the heat by means of a hydronic (or hot-water) heating system. If you choose it for your home, keep in mind that currently available heat pumps can heat water to no more than about 50°C.

This limits your choices for equipment to distribute the heat to your home. Hot-water baseboard radiators are designed to operate with water heated to at least 65 to 70°C; they are less effective when the water is not as warm. As a result, you will need larger radiators – or more of them – to distribute the same amount of heat. Or you can reduce the heat loss from your home by installing more insulation, so you need less heat.

In-floor hydronic systems are primarily used for heating

You can also install radiant floor, or in-floor, heating systems. These are becoming more common because they can increase comfort and improve system efficiency. Again, you must make sure that your radiant floor heating system is designed to operate within the temperature capabilities of your EES.

The temperature difference between the ground loop and the hot water distribution system depends on the efficiency of the EES; the greater the difference, the less efficient the system. Typically, an EES will extract heat from the earth at about 0°C. If a radiant floor heating system requires a temperature of 50°C to heat your home, the heat pump will produce about 2.5 units of heat for every unit of electricity used to operate the system. If the system can be designed to operate with water at 40°C, it will produce 3.1 units of heat for every unit of electricity used to operate it. In other words, it will be about 25 percent more efficient.

Think about it this way – if you have hot spring water to heat your home, you do not need a heat pump. The hot spring is a totally free, 100 percent-efficient source of energy. But if the temperature of the water from the well needs to be raised 5°C to be high enough to heat your home, you need some additional energy. If it has to be raised 20°C, you need even more energy. The greater the temperature difference, the greater the additional energy need.

If you are thinking of installing a radiant floor heating system in your home, you should tell the person designing it that you are planning to use an EES. Make sure you take the following factors into account:

• placing your floor pipe 20 cm (rather than 30 cm) apart reduces the water temperature required to heat your home by 4 to 5°C and increases the efficiency of your EES by about 10 percent;
• laying your floor heating pipe in concrete or Gypcrete rather than using aluminum reflective plates with the pipe reduces the required temperature by 12 to 15°C, increasing the efficiency of your EES by 25 to 30 percent;
• suspending pipe in the joist space under a floor means that you will need temperatures higher than what your EES can produce, unless the heat loss in the space is very low;
• placing insulation under a slab-on-grade floor or under a basement floor reduces heat loss to the ground below; and
• installing a control system that lowers the water temperature pumped through the floor as the outdoor temperature rises increases the efficiency of the EES. This type of control is commonly called an outdoor reset control.

Earth Energy System Variations

Overview

EESs, 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 is buried 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 was designed to be buried in the ground; it is also used for small natural gas pipelines or water lines. 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.

Canadian Standards Association International (CSA) and the industry have developed standards for EES installation. In addition, most heat pump manufacturers have developed guidelines or proprietary software for their products to ensure that EESs using them are designed and installed correctly. Most provide training for contractors that install their equipment as well as technical support for their dealers. As a homeowner considering the installation of an EES, ask your contractor for proof of training, experience and competence of its staff in loop design and installation.

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.

Horizontal ground loops can both heat and cool your home. They are buried underground.

Coiled or "slinky" loop

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. Typically, these boreholes are 15–100 m deep, and 10–12 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 is filled with clay grout. Some contractors add sand, finely crushed stone or cement to the grout. This is to ensure good contact with the soil and prevent surface water from contaminating the ground water. CSA standards specify that 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.

Vertical ground loops are similar to horizontal loops except that they are

placed vertically and use less ground area.

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 an EES 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 EES contractors who specialize in lake loop installation handle all the permission paperwork for their clients.

Lake loop systems (pond) can be used in either heating or cooling mode.

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 EESs, 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 EES 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 an EES vary with each province.

Ground water systems (open loop) can both heat and cool your home, depending on your needs.

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 an EES. 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 EES 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 15–30m, 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 EES with well water. Ask EES contractors in your area about their experience with open-loop systems when deciding on the best option for your home.