Last edited 17 Oct 2016

Geothermal pile foundations

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Geothermal Energy

What is geothermal energy?

Geothermal energy is the second most abundant source of heat on earth, after solar energy. It is the natural heat energy stored in the earth. This energy is contained in about 260 billion cubic metres of rocks and metallic alloys, just below the outer surface of the earth, that are at or near their melting points (Lanterman & Lee, 2007). Calculations show that the Earth, originating from a completely molten state, would have cooled and become completely solid if the only energy input was that of the Sun, so it is believed that the ultimate source of geothermal energy is the decay of naturally radioactive isotopes (Dincer et al., 2007).

The thermal energy from the earth continuously flows outwards. This heat transfer from the core to the surrounding mantle is principally via conduction. When the temperature and pressure of the system becomes high enough, some of the rocks that make up the mantle melt and form magma. As the liquid magma is less dense than surrounding rocks, it slowly rises, convecting thermal heat towards the earth’s crust (Lanterman & Lee, 2007).

Geothermal temperature increases with depth in the earth’s crust. Using the technology available at present, it has been found that the average geothermal gradient is about 3°C per 100m (Dincer et al., 2007). According to Lund (2009) the approximate total thermal energy above surface temperature to a depth of 10 kilometres is 1.3x10² Joules, equivalent to using 3x10¹ barrels of oil. As global energy consumption is equivalent to about 100 million barrels of oil per day, the thermal energy to a depth of 10km would supply all of mankind’s energy needs for six million years. However, based on current technology, only a fraction of this energy is available as a recoverable source. The rest of the energy is too widely spread over the surface of the earth or is too deep for it to be practical to reach.

History

Geothermal water from natural pools and hot springs has been used by humans for tens of thousands of years for cooking, bathing and heating. The Romans used geothermal energy for space heating, and direct heating has been used universally for agricultural purposes for many years, for example for greenhouse heating (Lanterman & Lee, 2007). The world’s first geothermal district heating system was developed in the 1300s at Chaudes-Aigues in France and it is still operational. The oldest and still functional geothermal district heating system in the United States of America is in Boise, Idaho. It became operational in 1892, is powered directly by a deep geothermal well and provides space heating for up to 450 homes. In Iceland, municipal heating was provided using hot geothermal sources in the 1930s and they are still a major source of heating today. Early industrial applications of geothermal energy included chemical extraction in the Larderello region of Italy, with geyser steam being used to extract boric acid for commercial use in the 1800s (Lund, 2009).

Geothermal energy was first used for electrical power generation with experimental work by Prince Gionori Conti in the Larderello field in Tuscany, Italy in 1904. This was followed by the first commercial power plant being commissioned in Larderello in 1913. In Japan, an experimental site for geothermal work in Beppu in 1919 led to a pilot plant in 1924 (Lanterman & Lee, 2007). These developments were followed by a plant in Wairakei, New Zealand in 1958 and an experimental plant at Pathe, Mexico in 1959. The first plant in the USA was then set up at The Geysers in 1960 and is currently the largest geothermal power producer in the USA (Collie, 1978). Due to the corrosive properties of most groundwater and steam, there were complications with the utilisation of geothermal energy for power generation until 1950, as metallurgy was not advanced enough to enable the manufacture of corrosion-resistant steam turbine blades (Dincer et al., 2007).

Current usage

Geothermal energy has been used on a commercial scale for over 100 years and more than 70 countries now exploit geothermal resources (Batchelor, 2005). At present, the USA remains the biggest producer of electricity from geothermal energy (Lanterman & Lee, 2007). Due to Iceland’s geological location, it has abundant geothermal resources and leads in the use of geothermal energy for space heating, hot water supply and agricultural uses, with over half its population living in houses heated by geothermal energy (Collie, 1978).

The main benefits of geothermal energy are:

  • The resources are continuous, reliable, sustainable and clean.
  • The cost of geothermal energy is not prone to fluctuation.
  • It provides a large resource, readily available in one form or another in every country, leading to a reduction of energy imports, therefore lowering dependency on external economical or political situations.
  • It helps reduce dependence on fossil or nuclear fuels.
  • It can be cost-competitive in providing base-load electricity, heating, cooling and hot water.
  • There is diversity of use: electricity generation and direct use of heat.
  • It can be used simultaneously for both power generation and direct-use applications.
  • It has low operating and maintenance costs.
  • There is a low land area requirement for geothermal power plants.
  • Geothermal systems can be installed at remote locations without requiring other infrastructure; the region can prosper without pollution.
  • Geothermal energy can easily be combined with other energy systems.

(Lanterman & Lee, 2007)

The existing uses of geothermal energy can be separated into three broad categories, based on delivery temperature from the ground:

High-temperature applications

High temperatures are primarily used for power stations and require temperatures of greater than 150°C. Typically the geothermal fluids used are at 200-280°C and are from wells 1500-2500 metres deep (Batchelor, 2005). High-temperature reservoirs are only found in regions with active volcanism and tectonic events on major plate or fault boundaries (Batchelor, 2005). The characteristics of the hydrothermal resources (resources containing water and/or steam) determine the power cycle of the geothermal power plant. In rare and geographically limited locations, dry steam is produced and this can be used directly to turn the turbines. However, in most cases the hot water resources need to be flashed, by reducing their pressure, in order to produce the steam required (Lund, 2009). Although geothermal power generation only accounts for a fraction of the world total, it is very important locally in many countries.

Medium-temperature applications

Temperatures between 40°C and 150°C are used for large-scale heating and process applications, and some limited power generation. Some medium-temperature thermal energy reservoirs are found in the same regions as the high-temperature reservoirs (plate and fault boundaries), where the heat-source is more diffused in reaching the surface or less completely trapped (Collie, 1978). Another type of medium-temperature reservoir exists where poorly conducting rock strata in the crust accumulates regional heat flows (Collie, 1978). These resources are extensively used in countries such as Hungary and Iceland, for space and district heating and agriculture. The difference between space and district heating is that space heating systems only supply heat to one structure, whereas district heating systems serve many structures from a common set of wells (Lanterman & Lee, 2007). The countries that have the highest usage of medium-temperature geothermal resources for direct use are China, the USA, Iceland and Turkey, accounting for 68% of the geothermal energy used directly as heat (Batchelor, 2007).

Low-temperature applications

Low temperatures are used with heat pumps on ground source systems to provide heating, cooling and hot water, using temperatures of less than 40°C. As the ground temperatures required for this application reduce, the area where geothermal utilisation is possible expands rapidly, making them suitable for small-scale and even domestic use in almost any location (Batchelor, 2007). The systems can be either open-loop, using ground water directly through an evaporator heat exchanger, or closed- loop, using a water-based antifreeze mixture circulating through sealed pipes (Batchelor, 2007). Although open-loop systems provide the highest energy yield, they require the highest financial input and pose the highest technical risks (Boennec, 2008).

NB The sun is the major contributor to heat stored in the earth at these lower temperatures. See Ground source heat pumps for more information.

Environmental and economic considerations

Geothermal energy can be considered a clean and environmentally friendly energy source as it generates no (or minimal) greenhouse gases (such as carbon-dioxide and nitrous oxide) because the conversion and utilisation processes do not involve any chemical reactions, in particular, combustion (Lanterman & Lee, 2007). It is also classified as both renewable and sustainable (Lund, 2009). Furthermore, geothermal energy is available continuously, regardless of weather conditions, a stark contrast to solar and wind power.

Noise pollution when drilling wells is not an issue, as drilling only takes place during the day.

The current economic benefits of geothermal energy are that it is flexible and can be used centrally (power plants) or locally (district heating), aiding economic development of small, isolated communities (Dincer et al., 2007). The deeper the boreholes required, the more capital-intensive the systems are, so as technology develops, allowing us to use lower temperatures for electricity generation and heating, the more economically attractive geothermal energy becomes. This will further aid the development of remote communities that do not have ties to main electricity distribution systems.

Most of the emissions of geothermal power plants are associated with the cooling towers that produce water vapour and possibly carbon dioxide, sulphur dioxide, nitrous oxides and hydrogen sulphide, but in amounts that are a fraction of those produced by fossil-fuel-fired power plants (Lund, 2009). Geothermal power plants are also generally not visually intrusive as can be made to blend in with the landscape and use minimum land (Lund, 2009).

Heating and hot water provision do not pose many environmental issues. The extraction of heat leads to a temporary drop in ground temperatures but in times of low heat demand the ground partly recovers due to underground water flows and geothermal flow (Boennec, 2008). The extraction of water is also managed as in most cases once the water is used it is returned to the aquifer, albeit at a different temperature.

They are also very economical, aside from initial installation costs, as there are no heating and hot water bills and maintenance costs are low.

Pile Foundations

Pile foundations are long, slender, columnar elements in a foundation that are installed into the ground. They are typically made from steel or reinforced concrete and possibly timber. A foundation is described as piled when its depth is more than three times its breadth (Atkinson, 2007).

Pile foundations are principally used to transfer the loads from a superstructure, through weak, compressible strata or water onto stronger, more compact, less compressible and stiffer soil or rock at depth, increasing the effective size of a foundation and resisting horizontal loads (Tomlinson & Woodward, 2008). They are used in very large buildings, and in situations when the soil under a building is not suitable to prevent excessive settlement.

Piles can be classified by their function:

  • End bearing piles are those where most of the friction is developed at the toe.
  • Friction piles are those where most of the pile bearing capacity is developed by shear stresses along the sides of the pile (Atkinson, 2007).

There are two types of pile foundation installations: driven piles and bored piles:

  • Driven piles are normally made from pre-cast concrete which is then hammered into the ground once on site.
  • Bored piles are cast in situ; the soil is bored out of the ground, underreaming is performed and then the concrete is poured into the hole. Alternatively, boring of the soil and pouring of the concrete can take place simultaneously, in which case the piles are called continuous fight augured (CFA) piles (O’Sullivan, 2010).

The choice of pile used depends on the location and type of structure, the ground conditions, durability of the materials in the environment and cost. Most piles use some end bearing and some friction, in order to resist the action of loads. Driven piles are useful in offshore applications, are stable in soft squeezing soils and can densify loose soil. However, bored piles are more popular in urban areas as there is minimal vibration, they can be used where headroom is limited, there is no risk of heave and it is easy to vary their length (O’Sullivan, 2010).

Geothermal Piles

Geothermal piles consist of pile foundations combined with closed-loop ground source heat pump systems. Their purpose is to provide support to the building, as well as acting as a heat source and a heat sink. In effect, the thermal mass of the ground enables the building to store unwanted heat from cooling systems and allows heat pumps to warm the building in winter (Boennec, 2008). Generally, ground source heat pumps used in domestic situations extract heat from the ground over a certain number of hours per year, by way of underground pipes which are laid either horizontally or vertically in a hole in the ground (Boennec, 2008). In geothermal piles, the pipe loops are laid vertically, in order for it to be possible for them to be incorporated into the pile foundations.

Construction of Geothermal Piles

Structural piles are turned into heat exchangers by adding one or more loops of plastic pipes down their length. In the construction of geothermal piles, the pile diameter and length should be designed to resist the applied structural loads, and not increased to suit the geothermal requirements. When constructing the piles, initially the soil is bored out of the ground and a rigid, welded reinforcement cage is inserted. Several close-ended loops of high density polyethylene plastic absorber pipes (generally 25mm diameter and 2-3mm wall thickness) are then fixed evenly around the inside of the reinforcement cage for the full depth.

Loops are fabricated off-site and filled with heat transfer fluid (water with antifreeze or saline solution) and fitted with a locking valve and manometer at the top of the pile cage. Before concreting, the absorber pipes are pressurised for an integrity test, and to prevent collapse due to the fluid concrete. This pressure is maintained until the concrete hardens and reapplied before the absorber pipes are finally enclosed. When concreting, the tops of the pipes are held back to avoid damage and a tremie pipe is placed to the base of the pile. Concrete is poured through the tremie and it is raised up as the concrete fills the pile. Once the pile is finished, the absorber pipes are connected to a heat exchanger which is then connected to a secondary circuit of pipes in the floors and walls of the building (Tomlinson & Woodward, 2008).

How closed-loop ground source heat pumps work

‘Geothermal heat exchanger technology is the most efficient method of heating, cooling or refrigerating any enclosure that can be conditioned’ (Tinkler, 2007:p.753). The principle of a ground source heat pump system is to transfer heat to and from the earth. In cool weather, the earth’s natural heat is collected through the loops and carried by heat transfer fluid to a unit in the building. This unit uses electrically driven compressors and heat exchangers to concentrate the earth’s heat and release it inside the building at a higher temperature.

In warm weather, the process is reversed in order to cool the building. The excess heat is drawn from the building and transferred to the heat transfer fluid, using the heat exchanger in the indoor unit. The heat then travels along the loop and is absorbed by the earth.

Although ground source heat pumps have the same basic mechanism as air source heat pumps, they offer the distinct advantage that the ground is warmer than the air in winter (and therefore able to provide more heat) and cooler than the air in summer (and therefore able to absorb more heat) (Lanterman & Lee, 2007).

Benefits of Geothermal Piles

In a move to reduce the effects of climate change, planners, regulators and local authorities have encouraged technologies for saving carbon to be integrated into new buildings. Ground source heat pump systems are becoming more widely used because they are both renewable and energy efficient (Tinkler, 2007). In the United Kingdom, many councils have introduced the ‘Merton Law’, which requires all new medium and large buildings to have 10% on-site renewable energy supplies (Boennec, 2008). To help achieve this, geothermal piles have become particularly attractive to developers in city centres as most large developments already require pile foundations, so these offer the lowest total cost whilst offering the highest renewable contribution and having the lowest spatial requirements (Boennec, 2008).

Geothermal piles are also economically beneficial in the long term. Although they commonly require similar or higher initial investment costs, they have lower running costs and hence lower life-cycle costs than comparable systems. They also have a very long life span (Brandl, 2009).

Other benefits include that due to low temperatures and pressures and the fact that the absorber pipes are embedded in concrete, there is virtually no risk of pipe damage or groundwater pollution. The comfort of the people can also be better, due to the lower temperature, high surface area of heated floors and walls. Additionally, they are space saving and visually unobtrusive.

Possible challenges and how to overcome them

There are some potential challenges that may have to be faced when constructing and using geothermal piles. Firstly there are issues related to the newness of this technology, namely that there is a severe skills shortage at all levels of the procurement chain. For example, there is difficulty finding good drilling operatives with the right kind of experience, leading to flooded construction sites, failed drilling, damaged pipes and poorly working systems (Boennec, 2008). Design consultants also lack training which, along with a lack of UK design standards, leads to ‘open’ specifications and poor integration of ground source heat pumps into buildings. This leaves contractors with the opportunity to deliver lower quality equipment, materials and workmanship than may be expected. Some contractors offer solutions that are intended to minimise carbon dioxide emissions while others optimise their offer to minimise installed costs. However, the Ground Source Heat Pump Association recognises these issues and is working with industry to address the skills shortage; telling consultants to better train engineers, produce tighter specifications and monitor the projects’ delivery very closely (Boennec, 2008).

There has also been significant concern about the effect of cyclical heating and cooling on pile performance. There have been two major studies into the impacts of this repeated heating and cooling: at the Swiss Federal Institute of Technology in Lausanne in 2006 and at Lambeth College in London in 2009. In Lausanne, thermal testing was carried out on a single geothermal test pile at intervals during the construction of the building: heating and recovery cycles were applied as increasing loads were added to the piles (Bourne-Webb et al., 2009). This study indicated that the thermal loads on geothermal piles induce additional stresses on surrounding structural piles, causing a decrease of lateral friction. It confirmed that geothermal piles can be designed to absorb these thermal effects without causing undue subsidence of the foundations (Boennec, 2008).

In the Lambeth College project there were a total of 146 piles at a depth of 25 metres. The study of the pile response to heat cycles was performed by Faber Maunsell, Skanska Cementation and Geothermal International (Boennec, 2008). Pile-loading tests that incorporated temperature cycles while under an extended period of maintained loading, were undertaken for seven weeks. It was found that concrete stresses in addition to those due to static loading were generated when the pile was heated. However, the shear stresses mobilised at the pile/soil interface during thermal cycling were not excessively large and it was concluded that the geotechnical capacity of the piles was unlikely to be affected and that minimal settlement occurred (Bourne-Webb, 2009).

Another issue is the risk of long-term ‘below ground global warming’ or ‘below ground global cooling’, which is caused by an imbalance in the heating and cooling demands of the buildings above, especially as geothermal piles become more popular in densely populated areas. The solutions to this problem are to diversify the profile of buildings served by geothermal piles in the local area and to design buildings in such a way that the heating and cooling demand is balanced ( for example, if there is a high cooling demand, incorporate water heating into the system to balance this). However, if in the long term these strategies fail, the ground can be artificially helped back to its undisturbed temperature using dry coolers to cool the ground or waste heat recharge of the ground when the heating demand across the year is imbalanced (for example, tarmac solar collector systems) (Boennec, 2008).

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