Geothermal energy

Geothermal heat pump

Geothermal heat pump

The geothermal heat pump is an air conditioning system for buildings that exploits the heat exchange with the superficial subsoil, by means of a heat pump. Since the heat in the subsoil comes largely from the interior of the Earth, geothermal energy of low enthalpy is classified as a source of renewable energy, although the heat pump itself consumes electricity, generally produced from other sources of energy (for example, fossil fuels).

The heat pump allows the exchange of heat between a "source" at a lower temperature than the "well", or the point where the heat is introduced. In a heating system, the building (more precisely: the circuit of the heating terminals of the building) represents the "hot well"; vice versa, in an air conditioning system, the building is the "cold source" from which the heat is extracted. The economic and energy advantage of the heat pump is given by the relationship between the heat introduced or extracted from the building and the energy consumption (usually electrical or heat in an absorption heat pump), called COP (coefficient of performance). A ratio between 3 and 6 for geothermal heat pumps.

The floor represents for the heat pump a "source" (when it works in heating) or a "well" (in cooling mode) of heat. In comparison with the atmospheric air, which is the source of aerothermal heat pumps, the temperature of the soil at a certain depth is subject to much smaller annual variations: a depth of 5-10 m the temperature of the soil is almost constant during all year round and is roughly equivalent to the average annual air temperature, or around 10-16 ° C. This means that the soil, compared to air, is warmer in winter and colder in summer, to the benefit of the efficiency of the heat pump.

The exchange of heat with the subsoil can take place in three ways:

  • direct exchange, where the evaporator / condenser circuit of the heat pump is in direct contact with the subsoil;
  • closed circuit systems, where the heat pump performs the heat exchange with the soil indirectly, by means of a hydraulic circuit in which a heat transfer fluid flows;
  • open circuit systems, in which groundwater is taken in which the heat exchange takes place.

In cold climates, the building where the heat load is unbalanced in favor of heating, the floor could be cooled due to heat removal is, however, possible to couple the geothermal heat pump for an installation of thermal solar panels and store accumulated heat in the summer in the soil.

The installation costs of the plant are much higher than conventional solutions (natural gas or diesel), but lower maintenance costs allow an investment to be recovered in less than 10 years, with a plant life of at least 25 years. years.

Earth Heat Exchanger

Horizontal closed circuit geothermal system

Geothermal heat pumps provide (or subtract) heat energy to the building, exchanging it with the ground at a shallow depth (1-200 m). The components of the system are therefore three: ground heat exchanger, heat pump and heating / cooling terminals.

As mentioned above, ground heat exchangers are divided into three categories:

  • direct exchange;
  • closed circuit;
  • open circuit.

The exchangers can have different configurations, classified by fluid type and by scheme. In direct exchange systems, the refrigerant circuit of the heat pump is in direct contact with the ground; in closed loop systems a fluid containing water and antifreeze additives is circulated; open-circuit systems operate heat exchange in groundwater.

Direct exchange

In the direct-exchange geothermal heat pump, the heat exchange takes place with the soil. The refrigerant that leaves the heat pump circulates in a pipe inserted in direct contact with the ground, exchanges heat with it and returns to the heat pump. The name "direct exchange" therefore implies the absence of an intermediate (and fluid) circuit between the ground and the heat pump. However, there are no direct interactions between the refrigerant and the earth, if not the heat exchange, and the water does not circulate in the exchange circuit with the ground.

Direct exchange systems are much more efficient than closed loop systems. This is due to the absence of an intermediate circuit (each heat exchanger, however, involves losses) and high thermal conductivity of the copper tubes used for the heat exchanger, which on the other hand are much more expensive compared to the tubes in HDPE used in geothermal probes. In comparison with geothermal probes, the required length is 70-85% smaller and the diameter of the pipe is approximately half. More quality control is required in the pipes, since the refrigerant gas could escape even very small cracks. Copper must be protected against corrosion in acid soils with cathodic protection or with a sacrificial anode.

Closed circuit

Most low enthalpy geothermal systems are composed of three circuits:

  • air conditioning circuit;
  • primary circuit of the heat pump;
  • secondary circuit of heat exchange with the floor.

The secondary circuit is generally made of high density polyethylene, in which mixtures of water and antifreeze (propylene glycol, ethylene glycol, methyl alcohol, methanol or calcium chloride) are used. Ethylene glycol is cheap, but it is toxic even at low concentrations; even the remote possibility of its spilling on the ground has led many control authorities to ban its use. Propylene glycol has replaced ethylene glycol in many cases, although it is more expensive and consumes less energy. Methanol and denatured alcohol are flammable and, therefore, their use is inadvisable.

The circulation pump can be external or included inside the heat pump. In the secondary circuit there are also expansion tanks and safety valves for pressure control

The closed circuit can be installed horizontally at a depth of 1-3 m, or vertically in a specially designed hole (geothermal probes) or on a foundation post (geothermal).

Vertical closed circuit

A vertical closed circuit consists of two or more tubes installed vertically on the ground, forming a closed circuit in which the heat transfer fluid flows. The length of the perforation can be between 20 and 200 m. The drilling can be carried out specifically (vertical geothermal probe) or for a foundation post (geothermal cells or energy cells).

The geothermal probes can have U-shaped configuration (two tubes, round-trip, connected to the bottom), double T or coaxial (two concentric tubes, with the flow in the inner tube and the return in the outer ring , or vice versa). Inside the hole, the space around the pipes is usually filled with a geothermal slurry, that is, a concrete prepared with siliceous inerts and high thermal conductivity additives.

Geothermal probes are widely used when there is not enough space for a horizontal closed circuit plant, or an exploitable groundwater table for an open circuit system. In the fields of the probe, the distance between the perforations is between 5 and 10 m. Indicatively, geothermal probes can provide power between 40 and 70 W per meter of drilling.

At the geothermal poles, however, the hydraulic circuit is inserted into a foundation pile. In this way, it is possible to limit the installation costs, since the drilling is not carried out specifically for the probes. On the other hand, the efficiency of the plant is lower, both due to the lower thermal conductivity of the clay soils in which this type of foundation is used, as well as the presence of long horizontal fluid distribution pipes, which imply significant thermal losses .

Horizontal closed circuit

 

The closed circuit can be placed horizontally in a trench, placed at depths greater than those at which freezing of the ground can occur. The tube can be linear or spiral (earth coils); Another configuration used sometimes is the geothermal basketball, or a spiral pipe 2-3 m high, inserted into the ground. The interchangeable power depends on the length of the pipe and the occupied area: approximately, the power exchanged with the floor is 15-40 W / m². Indicatively, a house with a maximum load of 10 kW requires three pipes DN20 or DN 32 of 120-180 m long.

The pipes are installed at a depth of 1-3 m: the greater the depth of installation, the greater the thermal inertia and the better the efficiency of the heat pump. Compared with vertical geothermal probes, the efficiency of the heat pump is lower, but the lower installation costs make this solution competitive. A variant of the horizontal closed circuit are the systems installed in small ponds, which exploit the thermal inertia of the water.

Open loop

In an open circuit, heat exchange is carried out with groundwater or, more rarely, from surface water bodies (rivers and lakes). The extracted water can be reintroduced into a body of surface water, or in the same aquifer from which it was extracted, by draining ditches or wells. The two wells (withdrawal and reintroduction) must be installed at a sufficient distance, in order to avoid thermal short circuits, which occurs when the thermally altered water re-enters the well (thermal cloud) reaches the collection axis.

The advantage, compared to closed-loop systems, is:

greater efficiency of the heat pump: the extracted water, in fact, is not affected by the exchange of heat (in comparison with the soil around a probe, in which a thermal gradient is formed), until it is Take the thermal shorts; especially for high-power systems, lower installation costs and fewer occupied spaces, compared to geothermal probe systems and even more than horizontal closed-loop systems.

The main disadvantage of these plants is the risk of cracks and incrustations, which shorten the life of the plant. For this reason, the installation of open-circuit geothermal systems in the presence of high contents of dissolved salts is not recommended.

Column of good standing

The standing column well is a particular open circuit system, in which the same well is used for extraction and re-entry. In fact, the water is taken from the bottom of the well and, after the exchange of thermal energy heat with the heat pump, returns to the top of the well. Then, descending to the bottom of the well, the water exchanges heat with the surrounding rock.

Heat pump and air conditioning terminals

The heat pump is the central unit of low enthalpy geothermal systems. With the same machine it is possible to heat and cool the building, produce domestic hot water and feed coils to melt ice and snow (for example, for garage ramps).

Heat transport inside the building can be done by air or liquid. The most suitable air conditioning terminals for the geothermal heat pumps are the radiant panels, since they work at lower temperatures in heating and higher in cooling, thus guaranteeing a greater efficiency of the heat pump. The fan coil can still be used: however, it must take into account the fact that, taking into account the lower fluid temperatures reached by the heat pump, in the case of retrofitting an existing plant, it is necessary to increase the flow of fluid and therefore the section of the distribution tubes.

Underground / Aquifer Thermal Energy Storage

In cold climates, where the energy consumption for heating is much higher than that of air conditioning, the energy balance of the soil may be inadequate, leading to its progressive cooling, with the consequent reduction of the efficiency of the heat pump.

One way to remedy this problem is the storage of heat in the subsoil, using solar thermal panels that receive heat from the sun and, without the help of the heat pump, introduce heat into the subsoil, raising the temperature. In this way, during the winter, the heat pump will work with greater efficiency. This solution is called underground thermal energy storage (UTES) or, in the case of open-circuit systems, aquifer thermal energy storage (ATES).

Energy efficiency

The COP of a geothermal heat pump varies between 3 and 6: this means that for every kWh of electricity consumed, 3-6 thermal kWh are produced. The primary energy efficiency of the electricity generation system in Italy is around 40%: this means that, to produce 1 kWh of electricity, it is necessary to consume 1 / 0.4 = 2.5 kWh thermal. As a result, a geothermal heat pump can produce 3 to 6 kWh thermal by consuming 2.5 kWh of heat (which, in turn, are used to produce 1 kWh of electricity). The primary energy efficiency of a geothermal heat pump is, therefore, variable between 120% and 240%, while the best condensing boilers obtain 90% yields. A geothermal heat pump, compared to a condensing boiler,

The COP of the heat pump depends to a large extent on the temperatures of the two thermostats (fluid exchange circuit in the ground and fluid of the air conditioning system): the smaller the difference, the higher the COP. As a result, the air conditioning terminals that allow the highest performance are the radiant panels, which work at <29 ° C in heating and 16 ° C in cooling, followed by the fan (45 ° C in the heating and 7 ° C in cooling mode).

Environmental aspects

According to the US Environmental Protection Agency (EPA). UU., Geothermal heat pumps are the most efficient, least polluting and economically viable air conditioning system. One of the biggest advantages is undoubtedly the absence of emissions on the site, which makes these plants suitable for urban areas. Greenhouse gas emissions occur, however, in the stage of electricity generation, and therefore depend on the combination of energies adopted by each country. In Sweden, for example, the production of electricity is only carried out at 2% with fossil fuels, so the use of geothermal heat pumps allows reductions in CO 2, which are released around 65-77%; In Poland, where coal is still widely used in thermoelectric plants, geothermal plants cause more emissions that alter the climate than methane or diesel boilers. In Italy, the savings of emissions compared to fossil fuels is around 30%. Another potentially significant impact is the escape of refrigerant from the heat pump: although CFCs have been eliminated due to their ozone-disrupting effect, the fluids used instead (HFC) still have a very high greenhouse effect (GWP), even more than 1000 times that of CO 2. However, given the limited amounts of refrigerant contained in the heat pump, this environmental impact is marginal compared to the production of carbon dioxide. A potential environmental impact is represented by the leakage of the thermovector fluid from the geothermal probes: however, given the modest amounts used and the use of low toxicity fluids, this impact is almost negligible.

Open-loop systems can cause depletion of the aquifer, pollution between different aquifers and, in some cases, even subsidence.

Economic aspects

Geothermal heat pumps are characterized by high installation costs and low maintenance costs. As a result, they represent a medium to long-term investment.

In general, savings in plant maintenance costs vary between 20 and 60% compared to traditional fossil fuel plants.

Regarding amortization times, there is not much information about it, but on average they are less than 10 years old and depend on:

  • Installation dimensions: especially in open-loop systems, there are substantial savings in larger plants (economies of scale);
  • installation costs: in more mature markets, such as in northern Europe, the costs of drilling and installation (especially for closed-loop systems) are lower;
  • cost of electricity and fossil fuels: the energy mix for the production of electricity, competition between operators, taxes and indirect taxes on fuels determine the strong differences between one country and another;
  • incentives, tax relief, subsidized loans.

Also geothermal heat pumps can be installed by the ESCO, companies that charge the installation costs of energy efficiency measures, sharing the gains derived from energy savings.

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Last review: September 27, 2018