Geothermal energy is one of the growing new renewable sources, albeit less exploited than others, wind and photovoltaic in the head. Its basic principle is the use of heat in the underground to power building heating systems or the production of electricity. A technology that in its low temperature variant,already has important applications in international level facilities such as the seat of the new European Parliament.
Technical features and operation:
A geothermal heating system is able to exploit the heat contained in the first layers of the subsoil (maximum depth about 100 meters), in Italy the estimates speak of values between 12 and 17 ° and allows for a dual use: heating in winter and cooling in summer. In this case, we talk about low temperature geothermal energy or even so called low enthalpy geothermal energy.
In this case, the plant will include a heat capture system, a heat pump, and a low temperature heat storage and distribution system. The capture system allows the concentration of dispersed heat to allow the pump to move it from the subsoil to the plant (or vice versa, during the summer months).
In case of proximity of hydrothermal sources, the use of geothermal technology can be an important opportunity for the generation of renewable energy. Temperatures in this case will be between 50 and several hundred degrees. It will be the steam that these thermal bags will release to enable the operation of power plants, made up of special turbines connected to a generator for the actual production of electricity.
There are two types of main geothermal systems: vertical and horizontal
Geothermal – Vertical Sensor System:
The most widely used geothermal energy sampling system is geo-probes.
They are in effect a closed-circuit heat exchanger between the heat pump and the ground.
These systems are made up of high-density polyethylene pipes that exchange heat with the soil by means of closed-loop circulation of a fluid bearing heat (glycolated water).
The average depth of the perforations for laying the probes is about 100/150 meters, the diameter of about 15 cm .; Is determined by several factors, mainly of geological or hydrogeological nature of the site in question, as well as the availability of external surface to the building appropriate to the purpose.
In the digged hole, the geothermal probes, consisting of 2 or 4 U-shaped pipes, are descending and ascending in one circuit; barrels fixed at the probe foot are often used to facilitate descent.
When placed in the probe, a pre-mixed bentonite cement product is injected from the bottom of the hole in order to saturate the spaces and create the best conductivity between circulating fluid in the probes and surrounding soil and to prevent communication between possible aquifers crossed by perforation.
If the facility consists of a single probe, it will be directly connected to the heat pump.
If, as is normally the case, the probes are several, they converge through horizontal connections made about 1 meter from the surface, into a geothermal collector, which in turn will then be hydraulically connected to the heat pump with individual delivery and return circuits.
The length of the geothermal heat exchanger, and consequently the length and number of probes, are sized according to the power of the heat pump required for the building to be air-conditioned.
Usually, to obtain 1 kw of geothermal energy from glycolic fluid, 10 to 15 meters of vertical probe are needed, varying in relation to the geological characteristics of the soil.
The heat capture system with vertical probes is the most widespread and even the most constant, since at about 100 m in depth the temperature is stable all year at around 10 ° C, it does not undergo seasonal variations as it is the case in the first meters from the surface, where, for example, the horizontal probes are installed.
This thermal stability with respect to the external temperature variation in the seasons, results in a good chance and exchange capacity and consequently improved heat pumps efficiency in heating and cooling.
Another important advantage of this solution is the minimum space required, and therefore occupied, by the probes field.
The probes, being the actual energy exchangers with the ground, have to keep minimum distances among each other to avoid thermal interference, usually about 8 meters. When the space available is very limited, the probes can also be installed underneath the building under construction.
The thermal conductivity of the soil and the thermal resistance of the probes are verified by running theGround Response Test (GRT).
After finishing the drilling and the laying of the probes, the ground is reset without leaving any trace, inspection well or constraints for its use. Obviously, however, the points where the probes have been placed must be known and reported.
In conclusion, the vertical geothermal probes system is is valid, efficient and long-lasting..
Notwithstanding the high incidence of the construction cost, caused by the use of special machinery during the drilling and laying phase, there is a minimal management cost together with long-term benefits, which justify the initial investment extensively.
Geothermal – Horizontal Probe System:
The use of this solution can be evaluated whenever is available a large green area around the building.
The goal is to keep low the cost for implementing the probes field, but it inevitably allows only superficial geothermal energy to be used.
In this case, the probes field is installed horizontally, by laying Polythene pipes disposed in different configurations (whole field, trench, etc.) and also in this case for practicality several circuits converge into two delivery and return manifolds, from which branch links to the heat pump.
Normally, the depth at which it is laid is about 1.5 meters from ground level to remain largely below the possible freezing level of the ground.
The green surface occupied by a horizontal probe system is, at least, twice that of the surface to be heated in the building. Depending on the type of soil, the ratio to be considered is about 25 square meters to produce 1 kw.
The main advantage over the classic vertical probes solution is the lower incidence of the costs of the geothermal catchment part (the central layout does not change).
On the other hand, the following disadvantages are highlighted:
– Less efficient system because its efficiency is influenced by external temperatures
– Not recommended for summer cooling
– When combined with a high temperature distribution system, it must be suitably oversized
– On the area affected by the probes field it is impossible to build, to make flooring, to plant tall trees and evergreens (there is no problem for using it as garden or orchard).
Then there are also other less common solutions.
Idrothermic – Groundwater system:
In geographic areas where stable aquifers are present at a limited depth (0/50 m depending on the size of the building), water can be used as a thermal vector. By simplifying, areas close to rivers, lakes or paddy fields, and plains in general, usually have these characteristics, which are often found in urban areas, such as two structures known to everyon: in Milan the La Scala Theater and in Turin The Egyptian Museum, are air-conditioned buildings with funnel water systems.
The use of groundwater can be done in two distinct ways:
– Extraction of groundwater and disposal in superficial water bodies
– Extraction of groundwater (pickup shaft) and subsequent re-injection into another well (dispersing)
In both cases we talk about open circuit, with two great benefits in terms of plant efficiency:
1. Water temperature is higher than that found in probes circuits: normally 10 ° / 14 ° C, but 16 ° C is not unusual
2. Unlike the fluid temperature in a “closed loop” where a seasonal drift occurs, in a geothermal ground-water plant, the thermal fluid has a constant temperature throughout the year.
These two factors directly result in a better performance of the Heat Pump, but it is to be considered, however, that in this type of installation the presence of a pump in the collecting well, causes additional electrical consumption. Because of this last aspect, it is not energy-efficient to use deep aquifers.
The greatest advantage of the water system compared to that of the probes is however given by the much smaller initial investment.
This advantage is proportional to the installed power, therefore greater for medium and large buildings, in which case the thermal sampling is carried out with one or two wells, against the need for more drilling to develop all the linear meters needed to achieve the same results.
The groundwater used for the operation of the plants is brought back to the surface or to a surface discharge.
It is not any chemically altered; it thermally drops or acquires an amount of about 4 ° C in relation to the use of the Heat Pump in heating or cooling. It can also be reused for other uses (irrigation, sanitation, etc.).
In absolute terms, the open loop system is the most efficient and achieves very high values of the performance coefficient (COP).
It requires careful probing of the hydrogeological structure of the area concerned, verifying the sustainability of both withdrawals and re-inputs in a way that allows an optimum use of the plants, and the carrying out of more or less articulated authorization procedures.
In some particular climatic or operating conditions it may be interesting to evaluate, as an alternative to a classical geothermal system, a Heat Pump that directly uses air as a thermal source (through water circuits).
In fact, even from the air it is possible to extract heat or give it when needed, and through a heat pump obtain the necessary temperatures for the building’s air conditioning: common air conditioning systems have been using this principle for years.
Conceptually, however, if the strength of the geothermal Heat Pump plants is the temperature of the thermal vecto (water/probes) constant throughout the year, the disadvantage of the ARIA-WATER plants is precisely the temperature of the heat carrier (air) .
The Heat Pump in winter has to generate heat by using cold air, in contrast to summer it must produce cooling by using air when it is warmer: technically speaking, however, it is possible to achieve the purpose but obviously the efficiency and operating costs are not comparable with those of geothermal plants.
This limit is partially offset by very low installation costs (in fact, no drilling or other construction work is required).
With quality aerodynamic Heat Pumps, however, it is possible to completely replace traditional systems in the building, obtaining benefits and savings on annual operating costs, especially for LPG or gas boilers, that are very interesting.
As with geothermic plants, even for aerothermic ones, it is possible to provide one single plant for all building requirements: generate heat, cooling and produce sanitary hot water.
Usually, these machines provide an exterior assembly to be placed in a well-ventilated area where the suction/evaporation/compression assembly is located, and an internal one that includes condensation and all the logic and steering part of the machine. Often in a monobloc that also accrues to the collecting of sanitary water (kettle), usually side by side there is an inertial accumulation.
More recently, the “hydronic” machines also include the condensation unit in the outdoor unit. In this case the connection to the technical room where the accumulations are present is hydraulic and is not part of the refrigerator circuit as in the case of the above mentioned machines.
These systems are particularly suitable for buildings with low consumption, with limited use (second homes) or where, for limits of compliance, no drilling is possible.
Incentives and funding:
The funding system available for geothermal power plants is essentially summarized in the Energy Efficiency Titles, also known as White Certificates in Italy. Only those who are connected to district heating networks fed through geothermal energy can benefit from a tax credit.
Energy Efficiency Titles, similarly to solar thermic, represent the energy saved in TOE (tonnes of oil equivalent) and can be “monetized” by selling them to the market organized by ESO (Energy Services Operators) or through bilateral agreements between obliged subjects (buyers) and volunteers (sellers).
Access to financing for geothermal plants is made available by various lenders.