Canadian Consulting Engineer

Earthly Power

The first ground source heat pumps in North America were mostly in single family residential buildings. They usually had two or three boreholes per system.

May 1, 2009   By Robert Mancini, P. Eng. R. Mancini &Associates

The first ground source heat pumps in North America were mostly in single family residential buildings. They usually had two or three boreholes per system.

Today, however, the size of these geoexchange systems is limited only by the area available for the installation of the ground exchanger. The largest system in the world is installed at Fort Polk, Louisiana. This system provides heating, cooling and domestic hot water for 4,000 homes on the military base. It reduced electrical consumption by 33%, eliminated 27,429 megajoules of natural gas, reduced peak demand by 43% and reduced CO2 emissions by 22,400 tons per annum.

Known as “ground source heat pumps,” “geothermal heat pumps,” “earth energy systems,” or “geoexchange systems,” the technology uses the earth as a heat source, a heat sink and as an energy storage device. Geoexchange is now a mature technology that has been in use for over 50 years. it not only can reduce a building’s carbon footprint but also its operating costs. Some systems use water sources and aquifers, but the commercial and institutional building applications mostly use the ground as the thermal exchanger and have vertical closed loop systems.

Geoexchange systems design and installation is governed by the Canadian Standards Association CAN/CSA-448 Series, “The Design and Installation of Earth Energy Systems.” The standard reflects several years of Canadian experience.

Past problems

In the 1980s and 1990s, the application of geothermal heat pump systems to larger projects such as schools presented challenges for all involved. The designers had no tools to rely on, and with no standards for geothermal equipment and materials, they were at the mercy of equipment and material manufacturers. There were only a few installers available and they were untrained.

Due to the absence of the above infrastructure, problems surfaced during this period and as a result some designers abandoned ground exchangers. Many lessons were learned the hard way, lessons that cost designers and installers millions of dollars and in some cases, their businesses.

Following are some typical problems that I have encountered over 25 years in the industry. I also have some suggestions for avoiding similar pitfalls and ensuring that the systems are reliable.

Learning The Hard Way

In the early 1980s there was no requirement to test the ground conditions, and since design tools did not exist, most designers used rules of thumb developed in the residential industry. By the mid to late 1980s, drilling contractors in the commercial and institutional sectors had learned that it was easy to increase their profits by claiming that the drilling conditions encountered at the site were not what they expected.

In an effort to contain these costs, designers started asking owners to provide test bores to a prescribed depth so that the drilling conditions could be logged and inserted in the specifications. My firm, Mancini, Saldan & Associates, was the first to ask for this form of testing, specifically for the Baltimore School for the Northumberland and Newcastle school board in southern Ontario.

The school board officials hired a local driller to perform two test boreholes to a depth of 260 ft. for the 30,000 sq. ft. elementary school. The drilling logs were inserted in the project specifications, the project was tendered, a low bidder was selected and contracted to perform the work.

The report showed overburden conditions all the way down to 260 ft.. But the first borehole by the drilling contractor found hard Canadian Shield limestone at about 240 ft.. Further investigation revealed that one test borehole had been drilled only to 220 ft., the other to 140 ft.. The problem cost the owner close to $30,000. All subsequent projects from our office required that test boreholes be conducted by a registered geologist or a hydrogeologist.

Borehole testing done right

Steps in the design of a ground exchanger are:

• site survey

• determination of regulatory requirements (environment ministries as well as local authorities)

• Site testing (test boreholes)

• Building energy analysis (to determine energy flows to and from the ground)

• Computer simulation to determine performance and borehole length over long term

• Energy balancing to minimize system size.

The first step is to evaluate the site for a suitable location for a ground exchanger. A flat site is ideal, while sloping sites can be challenging.

Tests must be done to determine the geological and hydrogeological conditions. For vertical closed loop systems this involves drilling boreholes to a prescribed depth, and determining the thermal conductivity of the material adjacent to the borehole.

The information derived from the tests will include the stratigraphy, aquifer locations, water quality of aquifers, soil properties and soil contamination.

Test boreholes must be treated as system boreholes since they will be incorporated into the overall ground exchanger as working units. All ground exchanger boreholes must be exact copies of the test boreholes. Depth, pipe and grout must be the same.

Loop design -Beware of overloads

Designing systems using rules of thumb worked fairly well in single family homes. If the ground exchanger did not perform well in heating the home, the mandatory back-up electrical heat would take over. Cooling requirements in homes are smaller than heating requirements; therefore, there were no issues with cooling.

But in schools located in most climate zones in Canada and the U. S., the cooling requirement is dominant. Using rule of thumb to design the loop for the ground exchanger could result in either over-sizing or under-sizing. Oversizing resulted in higher capital costs but the systems performed better. Under-sizing resulted in over-heating or over-cooling the ground around the boreholes and pipe.

Many designers used to assume that the earth could take the abuses of the energy imbalance without effect, but the result was overheated or overcooled loops, resulting in possible system failure.

Computerized solutions to the complex heat exchange formulas involved did not appear until the early 1990s. The original electronic solution was from Lund University, in Sweden. It was awkward and available only in metric units.

Energy Extracted = Energy Rejected

With larger projects, then, it is necessary to analyze the heating and cooling energy demands of the buildings.

Particular attention must be paid to the “thermal balance” –the balance between energy extracted from the ground and energy returned to the ground on an annual basis. Energy Extracted = Energy Rejected. A thermally balanced ground exchanger is an optimized ground exchanger, and an imbalance will cause long term performance degradation.

The balance can be achieved by finding a use for the excess energy. For example, a typical school in southern Ontario will likely be imbalanced due to excessive heat rejection. The value can be estimated and the energy can be used to heat or reheat fresh air, produce domestic hot water, and even provide energy for snow melting. Most of our projects in southern Ontario and in the U. S. have been designed to take advantage of this resource rather than reject it to the atmosphere. In essence, this is energy that the building owner has paid for.

The building system design therefore cannot be sepa-rated from the ground exchanger design, and a closed loop geothermal heat exchanger must be designed by a competent HVAC engineer trained to design such systems.

The designer must be well versed with building energy simulation, energy efficient design and LEED design requirements.

The ground exchanger size is calculated based on the building’s monthly energy requirements as well as its monthly peak energy needs.

In addition the following information is required to design the ground
exchanger: formation thermal conductivity and diffusivity, grout thermal conductivity, borehole thermal resistance, pipe arrangement within borehole or trenches, heat pump performance information, undisturbed ground temperature, circulation fluid properties (specific heat, density and flow rate, design heat pump inlet temperatures in heating and cooling mode, additional system power (pumps, fluid cooler etc.).

Installation and materials -Know your enemies

Several problems with geothermal systems can be traced to materials and installation.

Dirt in the piping systems. This is the number one culprit and has been the cause of thousands of call-backs and thousands of hours of wasted time. The designer must take every precaution in the specifications and drawings to address this problem. The installer must take every precaution to ensure that dirt does not enter the system during construction. I have witnessed everything from welding slag, to ” gravel, to pop cans in piping systems. Pipe flushing and cleaning in both the ground exchanger and the internal piping system must be done properly. Cleaning solvents must be used to clean metallic piping systems. The ground exchanger piping and internal system piping must be cleaned separately. The designer must witness both and sign off in both instances.

Grout. In the first systems installed in Ontario schools boreholes were “backfilled” with materials such as limestone screenings. Although the thermal conductivity of packed limestone screenings approaches that of the original limestone rock, it soon became clear that the limestone screenings were not covering the entire length of the borehole. This was due to “bridging” at locations where the U-bend piping became twisted (the case in many Ubend down-hole exchangers, especially those with smaller diameter pipe and pipe with thinner wall thicknesses). With the increasing number of geothermal systems installations, and ice lensing problems encountered at the Bayview Hills subdivision near Toronto, government authorities began to scrutinize the installation of ground exchangers and were especially concerned with protecting aquifers. It became clear that a better solution was required.

Air Entrainment. To prevent corrosion destroying metallic components in the system it is necessary that any trapped air that is not removed during flushing and cleaning can be vented either manually or automatically. Air separators and automatic air venting devices are a must. Failure to address this issue has resulted in corrosion at pipe joints with dissimilar metals and has been the cause of poor performance and many call-backs, even when rust inhibitors were introduced in the system.

Geothermal Piping. High Density Polyethylene (HDPE) pipe is the industry pipe of choice for ground exchanger systems. Typical borehole piping will have a nominal diameter of 1″ . Collector or sub-manifold piping connecting boreholes will have pipe diameters ranging from 1 1/4″ to 2″.

This genre of pipe has been used extensively in the distribution of natural gas. The principal reason for adopting the material for geothermal building systems relates to its strength and durability. It is not the perfect conductor of thermal energy, but it is unlikely to fail if manufactured and installed properly.

It is important to realize that HDPE pipe is not all the same. There were at least three instances where 2″ pipe installed in projects in southern Ontario failed in the 1990s. The pipe was found to slit longitudinally and curl inward. All the pipe involved was manufactured in the same factory in the U. S. The materials set forth in the engineer’s specifications were correct.

What went wrong? One can only point to a manufacturing defect or a quality control problem at the factory. However, at the end of the day all involved were sued and had to pay dearly. The moral of this story is to make sure that HDPE pipe is manufactured to Canadian Standards Association CSA-448 and carries the CSA logo.

Antifreeze. Several projects were provided with potassium acetate antifreeze in the 1990s as a result of a ban on ethylene glycol and methanol. This new antifreeze was approved by the Ontario Ministry of the Environment as an “earth friendly” antifreeze. What no-one knew was that potassium acetate in water would channel through threaded joints and leak out. When exposed to air it turned into acetic acid. In one case terrazzo floors in the corridors of a school were destroyed by such leakage. Do not use potassium acetate in a threaded piping system.

Capital and operating costs measure up

Geoexchange is recognized by organizations such as the U. S. Environmental Protection Agency, Natural Resources Canada (NRCan), the EU Commission and in new laws such as the proposed Ontario “Green Energy Act.”

In terms of energy savings, a new building with a well designed and constructed geoexchange system with a vertical closed loop geothermal exchanger should not consume more than 10 kWh/sq. ft./year. As examples from my firm’s current inventory of projects, the Gorham Middle School in Maine consumes 9.2 kWh/sq. ft./year. The College of Education, Health and Rehabilitation at the University of Maine consumes 9.1 kWh/sq. ft./year.

As for capital costs, an example is one of our current Canadian projects, a 135,000-sq. ft. seniors’ building in Sarnia, Ontario:

Geoexchanger: $499,000

Building HVAC system: $1,401,000

Total: $1,900,000, or $14 per sq. ft.

The cost of the ground exchanger should be amortized over its 50-year life.

In general buildings with geoexchange systems operate in the range of $1 per square foot per annum. Conventional systems will typically run above $1.50 per square foot per annum. Geoexchange systems can also provide a hedge against rising fossil fuel costs.

The maintenance costs of several buildings with geoexchange systems have been studied by the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE). The results showed long-term maintenance costing of maximum $13.5/sq. ft./year a year compared to $24 to $35/sq. ft/year for buildings with conventional systems.

Robert Mancini, P. Eng., is principal of R. Mancini & Associates and Mancini, Saldan & Associates, consultants specializing in geothermal heat pump systems design, based in Toronto and Bolton, Ontario.

———

What is GeoexchanGe Good for?

Can provide hot water for

• heating

• domestic water heating

• in-floor heating

• snow melting

• heating outdoor air for ventilation

Can provide chilled water for

• refrigeration

Can store heat energy in the ground

• energy rejected during cooling cycle is used for winter heating

• energy extracted from the ground in winter heating cycle cools the ground, therefore reducing energy consumption in summer cooling

Can provide heat or cool any time of the year

• Zones can be as small as a single office or as large as required


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