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Preface to English edition
Although there have been more than three years after the publication of this work, I’m still really
interested in geothermal energy. I’ve decided to translate my paper into English, to make it more
accessible to students and researchers of other countries. I apologise to all readers for any
possible mistake.
The topic is regulated by the Italian bureaucracy. I’ve considered some of these regulations but
I’ve focused more on the discussion of the thermodynamic model.
I’d like to thank Eng. Anna Maria Giorgio who has dedicated her time to correct this translation
and to teach me a little bit of English.
I hope that I was able to give a small contribution to Research in this field.
Naples, April 2013
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INTRODUCTION
The diffusion of new technologies exploiting the energetic sources and the increased
users’ preference for the environmental- friendly ones have led to an increased number
of plants using the so-called “alternative” energies and to the development of Research.
In recent years, energetic requirement to cool buildings has increased, together with the
electrical requirement to power air conditioners. It has surpassed, at least in our
country, the amount for heating processes.
This work is a contribution to reduce the amount of electricity used for air-conditioning.
It considers many alternative solutions, avoiding the traditional ones.
Energetic saving is one of the most important topic discussed nowadays. This subject is
closely related to the problem of global warming; in fact, while the next exhaustion of
traditional energetic resources isn’t a certainty, greenhouse gases are surely pollutant.
The international scientific community agrees on reducing the emissions produced by
the use of fossil fuels (Tinti, 2008). The comparison proposed, an original and
interesting study, concerns with the performances of two different systems for civil air
conditioning: geothermal heat pump and heat pump with cooled air. The adoption of
an air - conditioning system with a geothermal heat pump is a solution which uses the
soil as a heat reservoir during the year. This process is less influenced by the seasonal
temperature range than by the outside air.
I’ve chosen as case study a commercial building which ideally takes place in three
different Italian climatic regions. I’d like to highlight the peculiarities of the installation
of a geothermal heat pump for air conditioning, during a warmer summer than the one
when this pump has been developed. The pump has been mainly used for winter
heating.
According to this theory, European Union is promoting, especially for the construction
industry, the diffusion of technologies which don’t use gas, oil or coal. They can lead to
a reduction of 80% of energetic consumption and to a great reduction of emissions. In
particular, recently in Brussels some public policies have been focused on the “climate -
energy – innovation” link, according to the limits of the “Protocol of Kyoto” (the limits
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will have lowered gas emissions of the 20% by 2020). This relation must be translated
into new public policies about the development of new technologies. The concept of
sustainability is the corner-stone of energetic policy and the key to competitiveness and
security.
The strategic paper of 10 January 2007 "An Energetic Policy for Europe" calls for a new
industrial revolution, accelerating the transition to a growth with low emissions of
Carbon and increasing, over the years, the amount of produced and used “green
energy”. The challenge must optimise European potential competitiveness, controlling
at the same time the potential costs (Tinti, 2008).
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I LOW-ENTHALPY GEOTHERMAL ENERGY
1.1 Geothermal energy
Geothermal energy is the particular form of energy of the Earth produced by the heat
stored within the crust. This heat flows in huge quantities and is virtually inexhaustible.
It surfaces after its propagation through the rocks or fluids as water and gas. In
particular, Geothermy is the study, Research and exploitation of the terrestrial heat
based on the scientific instruments of Geology, Chemistry, Physics and Engineering.
The terrestrial heat is thus the essence of geothermal energy and, according to most
reliable theories, it has mainly a radioactive origin and in addition a planetary and
chemical one. A portion of terrestrial heat, propagating through the rocks, surfaces and
is called geothermal heat flux. It regularly dissipates through the space (the Earth radiates
endogenous heat through an electrical current of 0.065 Watt/m² (Basta, 2007)). The
intensity of the geothermal flux is about 2500 times smaller than the solar radiation
which arrives on Earth. The average solar radiation is 63 kW/m
2
. Heat disperses by
conduction (without transfer of material) and convection (with transport of material)
through the underground rocks, while the irradiation is practically negligible. The
temperature increases together with an average geothermal gradient
1
of 3 °C/100 m
moving from the depth of 100 m in the crust to the external layer. This gradient isn’t
constant; its value causes positive or negative thermal anomalies, depending on if it is
higher or lower than the average (Sommaruga, 1995).
1
We can define the geothermal gradient as the derivative of the temperature "T" in a certain direction "n",
∂ T / ∂ n. Given the symmetry of the Earth, this partial derivative becomes a total one dT / dn (it’s the
same value for all directions).
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Figure 1.1: Temperatures inside the Earth (source A.R.P.A.V.)
The use of geothermal energy, allowed by particular local geo-lithological conditions,
started immemorial time ago. Geothermal energy can be employed in the infinite range
of human activities, thanks to its extreme versatility. It’s used, for example, for civil,
agricultural and industrial purposes.
There are three different areas, considering the different use of geothermal energy:
- high enthalpy, characterised by the production of electricity and used for industrial
purposes (fluid temperatures above 150 ° C);
- low enthalpy, characterised by direct uses for civil, agricultural and industrial purposes
(temperatures of the fluid are below 150 ° C);
- thermal, characterised by the therapeutic and recreational exploitation.
Nowadays, in our polluted environment, reasons for the direct use of geothermal
energy (in particular for the urban heating) increase more and more, including its high
ecological safety.
The geothermal energy flows in closed circuits without generating smokes, with a
constant process totally independent from climatic factors (Sommaruga, 1995).
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1.1.1 High-enthalpy geothermal energy
The use of geothermal energy has really old origins, and nowadays includes modern
heating systems, and has applications in forestry, fish farming and industry. The first
industrial use was in Larderello in Tuscany, where, since 1904, geothermic energy has
been using to produce electrical energy. The process used the steam flowing from
underground at high temperature and pressure.
The Aquifers in the land of Larderello are so-called vapour-dominated. They use the
pressure generated by the vapour in underground aquifers to move a turbine coupled
to an electric generator; the steam exhausted from this outlet is condensed and purified
from its condensable gases while the condensate can be dispersed on the surface or re-
injected in the subsoil. When the aquifer produces hot water and doesn’t produce
steam, it’s a water-dominated aquifer. The mixture water / steam that exits from
geothermal wells, is separated after a process generating water and steam, which will
be sent to the turbine. High percentage of water (30-80% of total) will disperse or will be
injected again. Most of the geothermal fields in the world belong to this category, as the
Amiata and Travale sites, in Tuscany. Different utilisations can be mixed (electricity and
direct uses). Another heating technology used in civil buildings is the district heating; it
involves taking hot water from the subsoil, which flows through special heat-
exchanger and heats up the water circulating in some terminals ( ex: the terminals are
radiators, fan coils or radiant panels).
The Hard Dry Rock is instead a system exploiting geothermal reservoirs
2
, usually at
considerable artificial depths (3000-5000 m). Cold water is injected in the stock through
a well, then is heated by the hot rocks and is sent back to the surface (eg. cold water can
be used through a heat exchanger to transfer energy to a secondary circuit, feeding a
turbo-generator in order to produce electricity).
2
A “geothermal reservoir” is a rock mass containing a mineral fluid (water, gas, oil). In this case, when
there’s water (or steam), the reservoir may also be indicated with the term "aquifer ".
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There are also:
the magmatic systems : artificial systems exploiting the direct heat of the magma
to heat up a working fluid (they are still in an early stage of testing);
the geo-pressured systems.
The geo-pressured systems are characterised by the water (at high temperatures
like 200 ° C) trapped in tanks at a pressure higher than hydrostatic. They are
able to produce geothermal, mechanical and chemical (methane dissolved)
energy. It’s impossible to produce artificially these systems (Sommaruga, 1995).
The term geothermal system, is referred specifically to all hydro-geological and thermal
systems, where there are hydrodynamic and thermodynamic processes involving the
fluids circulating in the subsoil; the term field indicates the geothermal area where the
cultivation occurs. Geographic localities give usually their names to the fields.
Figure 1.2: Pattern of geothermal system (www.minambiente.it)
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1.1.2 Low-enthalpy geothermal energy
The applications seen in the previous paragraph, are the classic ones and are related to
the "high-enthalpy geothermy”, concerning the exploitation of volcanic or geological
anomalies, that unfortunately isn’t possible for all the type of subsoil. Generally, the
subsoil has a nearly constant temperature at 10 -100 meters of depth, which is called,
omotermia area (from the Ancient Greek omòs = same, termos = heat). The temperature
is calculated as the arithmetic mean of the outside-air temperature in a calendar year
(it’s between 12 and 17 ° C in most Italian regions, regardless of rock disposition,
geological structure and stratigraphy). This constancy depends on the ability of soil to
absorb solar radiation, due to its high capacity of storage. The range of daily-
temperature variation is already reduced from its first centimetres of depth while the
seasonal variation decreases after a few meters. It’s even possible for the normal subsoil
to be exploited as heat reservoir for energetic purposes. It’s used to extract heat during
the winter and to disperse it during the summer (to heat and to cool down buildings,
and to produce hot water for sanitary or industrial purposes). There is still no standard
terminology adopted by international scientists, so we generally refers to this
technology using the terms "low-enthalpy geothermal energy”, just to differentiate it
from the classic one using only underground temperatures above 40 ° C (Basta, 2007).
The primary uses of low-enthalpy geothermal energy are:
- air-conditioning for residential, industrial, public purposes etc..
- in agriculture;
- water crop;
- in industrial processes at controlled temperature.
It’s preferable, in the present technique, to use vertical geothermal probes instead of the
horizontal ones because of the minor seasonal fluctuations in temperature and the
reduced use of land. In free conditions (when the plant isn’t working), there is a zone of
omotermia, at depths of about 10-20 m, where the temperature is constant and it depends
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on several factors. The flows involved are: the flow of geothermal heat (in Europe it’s
about 0.05-0.12 W/m
2
), the solar radiation (about 1000 W/m
2
), rainfall and
underground water flow. Heat pumps are engines that transfer heat from one element
to another, forcing the heating flow to move from lower levels to higher ones. They’re
used to exploit the land as a source of heat.
1.2 Geothermal heat pump
1.2.1 The "heat pump"
The heat pump is a cyclic heat machine whose components operate in order to achieve a
transfer of thermal energy from a source of low temperatures to a one of higher
temperatures. The same transfer happens in a refrigeration system. In many
applications, these machines are called indiscriminately heat pumps. They operate
reversibly, both to heat up in winter and to cool in summer. The name originates from
the analogy of heat pump operation with the hydraulic pump that, similarly, can raise
water from a low level to an higher one. The process happens in the opposite direction
to the natural flow thanks to the mechanical work taken outside. Heat pumps with
vapour-compression, are moved by an electric motor (Electric Heat Pump EHP). They
are rarely moved by combustion engine fuelled by natural gas (Gas Heat Pump GHP).
This study has considered the use of electric heat pumps, in order to verify which of
two compared technologies best fits: air/air heat pump or water/air geothermal heat
pump. The principle of operation of these systems is based on phase-change of
refrigerant fluid along the thermodynamic cycle. Cooling is usually achieved by the
evaporation of a coolant at the lowest temperature and pressure possible within the
loop. It finally becomes superheated steam at the highest temperature and pressure of
the cycle, after a mechanical compression. Heat is obtained by the super-cooling of the
refrigerant and its following condensation. The cycle ends with the expansion of the
fluid through a special organ until the typical starting conditions of evaporation are
reached.
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