Geothermal Systems
Updated: 2021-10-19

Geothermal energy – from the Greek, “geo”, meaning “the Earth”, and “therme”, meaning “heat” – is generally defined, nowadays, as the available thermal, or heat, energy stored in the Earth’s crust, down to a depth of 3 miles, or so. You do not need, however, to go to great lengths – or, indeed, depths – to take advantage of geothermal systems, for heating, hot water, and cooling in your home, and the principle on which they operate is actually very straightforward. Wherever you live on the surface of the Earth, you experience, at least, some temperature variation throughout the course of a year, and annual fluctuations of 20°C, or 30°C, are not uncommon in many locations. The surface of the Earth, however, absorbs almost 50% of the radiation that it receives from the Sun – the energy from that radiation is stored in the soil, rocks and groundwater that constitute the upper part of the Earth’s crust – and the temperature beneath the surface remains fairly constant, all year round. This, in turn, effectively creates of reservoir of free, renewable energy, which can be tapped into by an appropriate geothermal system. Geothermal systems can greatly reduce the demand for non-renewable resources, such as coal, oil and natural gas, in the average home, and may therefore be a practical, and affordable, method of lessening greenhouse gas emissions, and minimising your overall carbon footprint. They also incur lower operating, and maintenance, costs than other heating, or cooling systems, and are generally more reliable. Geothermal systems have been widely adopted in the United States, and Scandinavia – which have relatively large fluctuations in surface temperature – where their heating and cooling potential can be fully exploited, but are also becoming popular in many other countries.

How Geothermal Systems Work

When a geothermal system is required to provide heating, heat from the soil, and groundwater, is absorbed, usually by a water and antifreeze solution, or by water from a surface, or underground, source – in a system of horizontal, or vertical, underground pipes – and travels to a unit where it is transferred to the air. This warm air is circulated through a conventional duct system – similar to that generally used in forced air heating systems – to provide domestic heating, including under floor heating, and hot water. Under floor heating is, in fact, one of the most energy efficient ways of heating your home, and is ideally suited to a ground source heat pump.

When cooling, or air conditioning, is required, however, the system works in reverse. Heat is extracted from the air in a home, and can be used, once again, to provide hot water. Alternatively, some, or all, of the heat extracted can be returned to the underground system, where it is returned to the soil, and groundwater. In either case, the air, minus its heat, is circulated around the home, to provide a cooling effect.

Types of Geothermal System

A geothermal system requires three main components: a method of transferring heat to, and from, the Earth – an “Earth”, or “ground”, loop, composed of piping, or a natural water source – a furnace unit, and a distribution system, within a building.

A geothermal heat pump may transfer heat between the Earth and your home via a network of underground pipes – usually constructed from high-density polyethylene – sunk into the ground horizontally, in trenches, or vertically, in boreholes. This is the most common form of geothermal system, and is known as a “closed loop” system. If a sufficient area of land is available, a horizontal ground loop is often the more economical to install, and to operate. A number of different configurations are possible, depending on the area of land available, but a depth of 4 feet, or more, is usually required for satisfactory operation. The other alternative, of course, is a vertical ground loop, and this may be more applicable if the area of land available is at a premium, or if underground conditions – an abundance of stones, or rocks, for example – make digging horizontal trenches impractical, or impossible. Vertical boreholes are usually drilled to a depth of between 150 and 300 feet, but, once again, the precise depth can be adjusted to suit a particular installation, and the total length of buried piping required.

If you have access to a source of water – either on the surface, or underground – another, less common, form of geothermal system, known as an “open loop” system, may be an option. Water, typically from a well, or a lake, or a pond, is pumped into a heat exchanger, where the heat is extracted, before being discharged back to its original source. Do bear in mind, however, that poor water quality, high mineral, or organic matter, content, and/or high acidity, can have severely adverse effects on an open loop geothermal system. Consider, too, the volume of water that is required to pass through a heat exchanger per unit time; a typical domestic dwelling may require heat to be exchanged from 10, or perhaps even 15, gallons of water, per minute in order to provide adequate heating, or cooling.

Installation Issues

Geothermal systems are, perhaps, best installed in conjunction with the building of a new home. Not only is the installation, itself, likely to be less expensive, but a new home also provides an opportunity for the owner to select doors, windows and insulation that will make the “building envelope” – in other words, the exterior of the building – as “tight”, or energy efficient, as possible. This reduces the interior capacity that needs to be heated, or cooled, which, in turn, means that a geothermal system can be smaller, and more economical. That is not to say, however, that geothermal systems are in any way unsuitable for existing buildings. Indeed, they offer a degree of flexibility, and are especially suited to homes where the combustion of fossil fuels is not possible. Geothermal systems involve no combustion, and therefore no danger of fire, explosion, or carbon monoxide poisoning. If a geothermal system is to replace an existing, forced air, heating system, it is usually possible to reuse the existing ducting. If not, then ductwork must be installed as part of the installation. If you are interested in under floor heating – which is another popular application of geothermal systems – it may be possible to find flooring that contains pre-fitted piping, with a view to installing a geothermal system at a later date.
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