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Few factories can produce standard and non-standard products with equal speed and quality.  Thanks to its confident, skilled workforce and superb engineering skills, this plant does it every day. Success comes from tight control of works orders executed by highly flexible, confident operators. The first measurements by thermometer were probably performed in 1740 by De Gensanne, in a mine near Belfort, in France (Buffon, 1778). In more recent estimates, based on a greater number of data, the total flow of heat from the Earth is about 6 percent higher than the figure utilized by Stacey and Loper in 1988.
In many areas of life, practical applications precede scientific research and technological developments, and the geothermal sector is a good example of this. By 1904 the first attempt was being made at generating electricity from geothermal steam; again, it was to take place at Larderello (Figure 3).
The success of this experiment was a clear indication of the industrial value of geothermal energy and marked the beginning of a form of exploitation that was to develop significantly from then on. The utilization of geothermal energy in developing countries has exhibited an interesting trend over the years. The difference in temperature between deep hotter zones and shallow colder zones generates a conductive flow of heat from the former towards the latter, with a tendency to create uniform conditions, although, as often happens with natural phenomena, this situation is never actually attained. The temperature increase with depth, as well as volcanoes, geysers, hot springs, etc., are in a sense the visible or tangible expression of the heat in the interior of the Earth, but this heat also engenders other phenomena that are less discernible by man, but of such magnitude that the Earth has been compared to an immense 'thermal engine'. Because of the difference in temperature between the different parts of the asthenosphere, convective movements and, possibly, convective cells were formed some tens of millions of years ago.
In zones where the lithosphere is thinner, and especially in oceanic areas, the lithosphere is pushed upwards and broken by the very hot, partly molten material ascending from the asthenosphere, in correspondence to the ascending branch of convective cells. These phenomena lead to a simple observation: since there is apparently no increase in the Earth's surface with time, the formation of new lithosphere along the ridges and the spreading of the ocean beds must be accompanied by a comparable shrinkage of the lithosphere in other parts of the globe. Spreading ridges, transform faults and subduction zones form a vast network that divides our planet into six immense and several other smaller lithospheric areas or plates (Figure 5).
The mechanism underlying geothermal systems is by and large governed by fluid convection. Convection, by its nature, tends to increase temperatures in the upper part of a system as temperatures in the lower part decrease (White, 1973).
The phenomenon we have just described may seem quite a simple one but the reconstruction of a good model of a real geothermal system is by no means easy to achieve.
Of all the elements of a geothermal system, the heat source is the only one that need be natural. Artificial recharge through injection wells can also help to replenish and maintain 'old' or 'exhausted' geothermal fields. The Los Alamos project was the forerunner for other similar projects in Australia, France, Germany, Japan and the UK. Frequently a distinction is made between water- or liquid-dominated geothermal systems and vapour-dominated (or dry steam) geothermal systems (White, 1973). Geothermal field is a geographical definition, usually indicating an area of geothermal activity at the earth's surface. The most critical factor for the classification of geothermal energy as a renewable energy source is the rate of energy recharge. A large number of methods and technologies are available in order to reach these objectives.
Geochemical surveys (including isotope geochemistry) are a useful means of determining whether the geothermal system is water- or vapour-dominated, of estimating the minimum temperature expected at depth, of estimating the homogeneity of the water supply, of inferring the chemical characteristics of the deep fluid, and of determining the source of recharge water. Geophysical surveys are directed at obtaining indirectly, from the surface or from depth intervals close to the surface, the physical parameters of deep geological formations. Thermal techniques (temperature measurements, determination of geothermal gradient and terrestrial heat flow) can often provide a good approximation of the temperature at the top of the reservoir.
Conventional steam turbines require fluids at temperatures of at least 150 A°C and are available with either atmospheric (back-pressure) or condensing exhausts. With this type of unit, steam consumption (from the same inlet pressure) per kilowatt-hour produced is almost double that of a condensing unit.
The condensing units, having more auxiliary equipment, are more complex than the atmospheric exhaust units and the bigger sizes can take twice as long to construct and install.
Generating electricity from low-to-medium temperature geothermal fluids and from the waste hot waters coming from the separators in water - dominated geothermal fields has made considerable progress since improvements were made in binary fluid technology. By selecting suitable secondary fluids, binary systems can be designed to utilise geothermal fluids in the temperature range 85-170 A°C. Binary plant technology is a very cost-effective and reliable means of converting into electricity the energy available from water-dominated geothermal fields (below 170 A°C). A new binary system, the Kalina cycle, which utilizes a water-ammonia mixture as working fluid, was developed in the 1990s. Small mobile plants, conventional or not, can not only reduce the risk inherent to drilling new wells but, what is more important, they can help in meeting the energy requirements of isolated areas. The convenience of the small mobile plants is most evident for areas without ready access to conventional fuels, and for communities that would be too expensive to connect to the national electric grid, despite the presence of high voltage transmission lines in the vicinity. Space cooling is a feasible option where absorption machines can be adapted to geothermal use.
As engineers already know, heat pumps are machines that move heat in a direction opposite to that in which it would tend to go naturally, i.e. Ground-coupled and ground-water heat pump systems have now been installed in great numbers in at least 30 countries, for a total thermal capacity of more than 9500 MWt (in 2003). We doesn't provide plants to plant products or service, please contact them directly and verify their companies info carefully. By 1870, modern scientific methods were being used to study the thermal regime of the Earth (Bullard, 1965), but it was not until the twentieth century, and the discovery of the role played by radiogenic heat, that we could fully comprehend such phenomena as heat balance and the Earth's thermal history. In the early part of the nineteenth century the geothermal fluids were already being exploited for their energy content.
The geothermal steam was used to raise liquids in primitive gas lifts and later in reciprocating and centrifugal pumps and winches, all of which were used in drilling or the local boric acid industry. In the five years between 1975 and 1979 the geothermal electric capacity installed in these countries increased from 75 to 462 MWe; by the end of the next five-year period (1984) this figure had reached 1495 MWe, showing a rate of increase during these two periods of 500% and 223%, respectively (Dickson and Fanelli, 1988). Installed geothermal generating capacities world-wide from 1995 to 2000 (from Huttrer, 2001), and at the end of 2003. The physical and chemical characteristics of the crust, mantle and core vary from the surface of the Earth to its centre. Their extremely slow movement (a few centimetres per year) is maintained by the heat produced continually by the decay of the radioactive elements and the heat coming from the deepest parts of the Earth. This is indeed what happens in subduction zones, the largest of which are indicated by huge ocean trenches, such as those extending along the western margin of the Pacific Ocean and the western coast of South America. Because of the huge tensions generated by the Earth's thermal engine and the asymmetry of the zones producing and consuming lithospheric material, these plates drift slowly up against one another, shifting position continually.
It can be described schematically as 'convecting water in the upper crust of the Earth, which, in a confined space, transfers heat from a heat source to a heat sink, usually the free surface' (Hochstein, 1990).
Figure 7 describes schematically the mechanism in the case of an intermediate-temperature hydrothermal system. It requires skill in many disciplines and a vast experience, especially when dealing with high -temperature systems.
For example, in The Geysers field in California, USA, one of the biggest geothermal fields in the world, production began to decline dramatically at the end of the 1980s because of a lack of fluids.
High-pressure water is pumped through a specially drilled well into a deep body of hot, compact rock, causing its hydraulic fracturing. After a period of relative neglect, these projects have been given renewed impulse from the discovery, firstly, that deep rocks have a certain degree of natural fracturation, and further, that the methodologies and technologies adopted will depend on local geologic conditions.
The resources are divided into low, medium and high enthalpy (or temperature) resources, according to criteria that are generally based on the energy content of the fluids and their potential forms of utilization.
To make it as simple as possible, let us take the example of a pot filled with liquid water in which pressure can be kept constant at 1 atm (101.3 kPa). In cases without surface activity this term may be used to indicate the area at the surface corresponding to the geothermal reservoir below (Axelsson and Gunnlaugsson, 2000). Renewable describes a property of the energy source, whereas sustainable describes how the resource is utilised. In the exploitation of natural geothermal systems, energy recharge takes place by advection of thermal water on the same time scale as production from the resource.
Consumption can obviously be sustained over any period of time in which a resource is being created faster than it is being depleted. These include anticipated utilization, technology available, economics, as well as situation, location and time, all of which affect the exploration programme.
Many of these methods are in current use and have already been widely experimented in other sectors of research. Valuable information can also be obtained on the type of problems that are likely to arise during the re-injection phase and plant utilization (e.g. Nor can they be used indiscriminately in any situation or condition, as a method that produces excellent results in a determinate geological environment may give very unsatisfactory results in another. The data provided by exploratory wells should be capable of verifying all the hypotheses and models elaborated from the results of surface exploration and of confirming that the reservoir is productive and that it contains enough fluids of adequate characteristics for the utilization for which it is intended.
During each of these phases we gradually eliminate the less interesting areas and concentrate on the most promising ones.
However, the atmospheric exhaust turbines are extremely useful as pilot plants, stand-by plants, in the case of small supplies from isolated wells, and for generating electricity from test wells during field development. The specific steam consumption of the condensing units is, however, about half that of the atmospheric exhaust units. The upper limit depends on the thermal stability of the organic binary fluid, and the lower limit on technical-economic factors: below this temperature the size of the heat exchangers required would render the project uneconomical.


The working fluid is expanded, in super-heated conditions, through the high-pressure turbine and then re-heated before entering the low-pressure turbine. The standard of living of many communities could be considerably improved were they able to draw on local sources of energy.
The expense involved in serving these small by-passed communities is prohibitive, since the step-down transformers needed to tap electricity from high voltage lines cost more than US$ 675,000 each, installed, and the simplest form of local distribution of electricity, at 11 kV using wooden poles, costs a minimum of US$ 20,000 per kilometre (US$ 1994). The main costs are initial investment costs, for production and injection wells, down-hole and transmission pumps, pipelines and distribution networks, monitoring and control equipment, peaking stations and storage tanks. The technology of these machines is well known, and they are readily available on the market. All modern thermal models of the Earth, in fact, must take into account the heat continually generated by the decay of the long-lived radioactive isotopes of uranium (U238, U235), thorium (Th232) and potassium (K40), which are present in the Earth (Lubimova, 1968). The temperature of the mantle has decreased no more than 300 to 350 A°C in three billion years, remaining at about 4000 A°C at its base.
A chemical industry was set up in that period in Italy (in the zone now known as Larderello), to extract boric acid from the boric hot waters emerging naturally or from specially drilled shallow boreholes. Between 1850 and 1875 the factory at Larderello held the monopoly in Europe for boric acid production. The geothermal power installed in the developing countries in 1995 and 2000 represents 38 and 47% of the world total, respectively. The number of countries with direct uses has very likely increased since then, as well as the total installed capacity and energy use. These values are based on 24,774 measurements at 20,201 sites covering about 62% of the Earth's surface. The outermost shell of the Earth, known as the lithosphere, is made up of the crust and the upper layer of the mantle.
Immense volumes of deep hotter rocks, less dense and lighter than the surrounding material, rise with these movements towards the surface, while the colder, denser and heavier rocks near the surface tend to sink, re-heat and rise to the surface once again, very similar to what happens to water boiling in a pot or kettle.
A relatively tiny fraction of the molten rocks upwelling from the asthenosphere emerges from the crests of these ridges and, in contact with the seawater, solidifies to form a new oceanic crust. The margins of the plates correspond to weak, densely fractured zones of the crust, characterised by an intense seismicity, by a large number of volcanoes and, because of the ascent of very hot materials towards the surface, by a high terrestrial heat flow. Convection occurs because of the heating and consequent thermal expansion of fluids in a gravity field; heat, which is supplied at the base of the circulation system, is the energy that drives the system. Geothermal systems also occur in nature in a variety of combinations of geological, physical and chemical characteristics, thus giving rise to several different types of system.
For example, the geothermal fluids extracted from the reservoir to drive the turbine in a geothermal power-plant could, after their utilization, be injected back into the reservoir through specific injection wells. The first project of this type, the Southeast Geysers Effluent Recycling Project, was launched in 1997, to transport treated wastewater for 48 km to the geothermal field.
The water permeates these artificial fractures, extracting heat from the surrounding rock, which acts as a natural reservoir. The most advanced research in the HDR sector was that being conducted in Japan and under the European project at Alsace (France). If we then heat the water, it will begin boiling once it reaches a temperature of 100 A°C (boiling temperature at a pressure of 1 atm) and will pass from the liquid to the gas (vapour) phase. The term sustainable development is used by the World Commission on Environment and Development to indicate development that 'a€¦.meets the needs of the present generation without compromising the needs of future generations'. The techniques and methodologies that have proved successful in mineral and oil or gas exploration will however not necessarily be the best solution in geothermal exploration.
These methodologies are, therefore, more suited to defining details during the final stages of exploration, before the exploratory wells are sited.
In order to reduce costs, it is therefore very important that the geophysical method(s) be selected very carefully beforehand by geophysicists working in close collaboration with geologists (Meidav, 1998).
Siting of the exploratory wells is therefore a very delicate operation.Exploration programmeBefore drawing up a geothermal exploration programme all existing geological, geophysical and geochemical data must be collected and integrated with any data available from previous studies on water, minerals and oil resources in the study area and adjacent areas.
The methods used also become progressively more sophisticated and more detailed as the programme develops.
The steam, direct from dry steam wells or, after separation, from wet wells, is passed through a turbine and exhausted to the atmosphere (Figure 11). They are also used when the steam has a high non-condensable gas content (>12% in weight). Apart from low-to-medium temperature geothermal fluids and waste fluids, binary systems can also be utilised where flashing of the geothermal fluids should preferably be avoided (for example, to prevent well sealing). Their cost depends on a number of factors, but particularly on the temperature of the geothermal fluid produced, which influences the size of the turbine, heat exchangers and cooling system. After the second expansion the saturated vapour moves through a recuperative boiler before being condensed in a water-cooled condenser. Electricity could facilitate many apparently banal, but extremely important operations, such as pumping water for irrigation, freezing fruit and vegetables for longer conservation. Operating expenses, however, are comparatively lower than in conventional systems, and consist of pumping power, system maintenance, control and management. The absorption cycle is a process that utilises heat instead of electricity as the energy source.
Aquifers and soils with temperatures in the 5 to 30 A°C range are being used in these systems.
The greatest drawback in irrigating with warm waters is that, to obtain any worthwhile variation in soil temperature, such large quantities of water are required at temperatures low enough to prevent damage to the plants that the fields would be flooded.
Added to radiogenic heat, in uncertain proportions, are other potential sources of heat such as the primordial energy of planetary accretion.
The boric acid was obtained by evaporating the boric waters in iron boilers, using the wood from nearby forests as fuel.
Between 1910 and 1940 the low-pressure steam in this part of Tuscany was brought into use to heat the industrial and residential buildings and greenhouses. Empirical estimators, referenced to geological map units, enabled heat flow to be estimated in areas without measurements. Ranging in thickness from less than 80 km in oceanic zones to over 200 km in continental areas, the lithosphere behaves as a rigid body.
Most of the material rising from the asthenosphere, however, divides into two branches that flow in opposite directions beneath the lithosphere.
Part of the lithospheric material returns to a molten state and may rise to the surface again through fractures in the crust. The heat source can be either a very high temperature (> 600 A°C) magmatic intrusion that has reached relatively shallow depths (5-10 km) or, as in certain low-temperature systems, the Earth's normal temperature, which, as we explained earlier, increases with depth.
Heated fluid of lower density tends to rise and to be replaced by colder fluid of high density, coming from the margins of the system. This project has led to the reactivation of a number of power plants that had been abandoned because of a lack of fluids.
This 'reservoir' is later penetrated by a second well, which is used to extract the heated water. Several projects launched in Japan in the 1980s (at Hijiori, Ogachi and Yunomori), and heavily financed by the Japanese government and industry, have produced interesting results both from the scientific and industrial standpoints.
Figure 9 illustrates in graphic form these and other terms that may be used by geothermal specialists.
A standard method of classification, as with terminology, would avoid confusion and ambiguity but, until such a method exists, we must indicate the temperature values or ranges involved case by case, since terms such as low, intermediate and high are meaningless at best, and frequently misleading. In the case of hot, dry rocks, and some of the hot water aquifers in sedimentary basins, energy recharge is only by thermal conduction; due to the slow rate of the latter process, however, hot dry rocks and some sedimentary reservoirs should be considered as finite energy resources (Stefansson, 2000).
In this context, sustainable development does not imply that any given energy resource needs to be used in a totally sustainable fashion, but merely that a replacement for the resource can be found that will allow future generations to provide for themselves despite the fact that the particular resource has been depleted. Information on the existence of geothermal fluids in the geological structures can be obtained with the electrical and electromagnetic prospectings, which are more sensitive than the other surveys to the presence of these fluids and to variations in temperature; these two techniques have been applied widely with satisfactory results.
This information frequently plays an important role in defining the objectives of the geothermal exploration programme and could lead to a significant reduction in costs. The size and budget of the entire programme should be proportional to its objectives, to the importance of the resources we expect to find, and to the planned forms of utilization. The atmospheric exhaust units can be constructed and installed very quickly and put into operation in little more than 13-14 months from their order date.
In this case, down-hole pumps can be used to keep the fluids in a pressurised liquid state, and the energy can be extracted from the circulating fluid by means of binary units.
The total size of the plant has little effect on the specific cost, as a series of standard modular units is joined together to obtain larger capacities. The Kalina cycle is more efficient than existing geothermal ORC binary power plants, but is of more complex design. A crucial factor in estimating the initial cost of the system is the thermal load density, or the heat demand divided by the ground area of the district. The refrigeration effect is obtained by utilising two fluids: a refrigerant, which circulates, evaporates and condenses, and a secondary fluid or absorbent. Any refrigeration device (window air conditioner, refrigerator, freezer, etc.) moves heat from a space (to keep it cool) and discharges that heat at higher temperatures. One possible solution to this problem is to adopt a subsurface irrigation system coupled to a buried-pipeline soil-heating device. Although complex bespoke assemblies can take up to 40 days, average lead time for its major products is four days. However, it was not until a period between the sixteenth and seventeenth century, when the first mines were excavated to a few hundred metres below ground level, that man deduced, from simple physical sensations, that the Earth's temperature increased with depth. Realistic theories on these models were not available until the 1980s, when it was demonstrated that there was no equilibrium between the radiogenic heat generated in the Earth's interior and the heat dissipated into space from the Earth, and that our planet is slowly cooling down. The thermal energy of the Earth is therefore immense, but only a fraction could be utilized by mankind.


In 1827 Francesco Larderel, founder of this industry, developed a system for utilising the heat of the boric fluids in the evaporation process, rather than burning wood from the rapidly depleting forests (Figure 2).
In 1919 the first geothermal wells in Japan were drilled at Beppu, followed in 1921 by wells drilled at The Geysers, California, USA.
There are, however, vast areas in which the geothermal gradient is far from the average value. Below the lithosphere is the zone known as the asthenosphere, 200-300 km in thickness, and of a 'less rigid' or 'more plastic' behaviour. The continual generation of new crust and the pull of these two branches in opposite directions has caused the ocean beds on either side of the ridges to drift apart at a rate of a few centimetres per year.
The reservoir is a volume of hot permeable rocks from which the circulating fluids extract heat. For many years now re-injection has also been adopted in various parts of the world as a means of drastically reducing the impact on the environment of geothermal plant operations. In the second system, the Santa Rosa Geysers Recharge Project, 41.5 million litres per day of tertiary treated waste-water will be pumped from the Santa Rosa regional sewage treatment plant and other cities through a 66-km pipeline to The Geysers field, where it will recharge the reservoir through specially drilled boreholes. The system therefore consists of (i) the borehole used for hydraulic fracturing, through which cold water is injected into (ii) the artificial reservoir, and (iii) the borehole used to extract the hot water. The European HDR project, on the other hand, has been implemented over a number of phases, including the drilling of two wells, one of which has reached bottom-hole at 5060 m. Depending on temperature and pressure conditions, they can produce hot water, water and steam mixtures, wet steam and, in some cases, dry steam. The vapour coexisting with the liquid, and in thermodynamic equilibrium with it, is wet steam. In the static systems(also known as stagnant or storage systems) there is only minor or no recharge to the reservoir and heat is transferred only by conduction. Thus, it may not be necessary that a specific geothermal field be exploited in sustainable fashion. Geological and hydrogeological studies have an important role in all subsequent phases of geothermal research, right up to the siting of exploratory and producing boreholes. As the geochemical survey provides useful data for planning exploration and its cost is relatively low compared to other more sophisticated methods, such as the geophysical surveys, the geochemical techniques should be utilised as much as possible before proceeding with other more expensive methodologies. The magnetotelluric method, which exploits the electromagnetic waves generated by solar storms, has been greatly improved over the last few years, and now offers a vast spectrum of possible applications, despite the fact that it requires sophisticated instrumentation and is sensitive to background noise in urbanised areas. The programme schedule should be flexible and reassessed as the results come in from the various surveys of each phase; similarly the geological-geothermal model should be progressively updated and improved. A high heat density determines the economic feasibility of a district heating project, since the distribution network is expensive.
For applications above 0 A°C (primarily in space and process conditioning), the cycle uses lithium bromide as the absorbent and water as the refrigerant. The only difference between a heat pump and a refrigeration unit is the desired effect, cooling for the refrigeration unit and heating for the heat pump. Heating the soil in buried pipelines without the irrigation system could decrease the heat conductivity of the soil, because of the drop in humidity around the pipes, and consequent thermal insulation. In its fastest moving area, 96% of today’s orders will go out tomorrow – when competitors often quote in weeks. So far our utilization of this energy has been limited to areas in which geological conditions permit a carrier (water in the liquid phase or steam) to 'transfer' the heat from deep hot zones to or near the surface, thus giving rise to geothermal resources; innovative techniques in the near future, however, may offer new perspectives in this sector. In 1892 the first geothermal district heating system began operations in Boise, Idaho (USA). In 1958 a small geothermal power plant began operating in New Zealand, in 1959 another began in Mexico, in 1960 in the USA, followed by many other countries in the years to come.Present status of geothermal utilizationAfter the Second World War many countries were attracted by geothermal energy, considering it to be economically competitive with other forms of energy. In other words, on a geological scale in which time is measured in millions of years, this part of the Earth behaves in much the same way as a fluid in certain processes. Where the trenches are located in the ocean, as in the Western Pacific, these magmatic arcs consist of chains of volcanic islands; where the trenches run along the margins of continents the arcs consist of chains of mountains with numerous volcanoes, such as the Andes.
The reservoir is generally overlain by a cover of impermeable rocks and connected to a surficial recharge area through which the meteoric waters can replace or partly replace the fluids that escape from the reservoir through springs or are extracted by boreholes.
The entire system, complete with surface utilization plant, could form a closed loop (Garnish, 1987) (see Figure 8).
If we continue to heat the pot and maintain the pressure at 1 atm, the liquid will evaporate entirely and the pot will contain steam only. Perhaps we should direct our geothermal sustainability studies towards reaching and then sustaining a certain overall level of geothermal production at a national or regional level, both for electric power generation and direct heat applications, for a certain period, say 300 years, by bringing new geothermal systems on line as others are depleted (Wright, 1998).Table 3. They also provide the background information for interpreting the data obtained with the other exploration methods and, finally, for constructing a realistic model of the geothermal system and assessing the potential of the resource. The main advantage of the magnetotelluric method is that it can be used to define deeper structures than are attainable with the electric and the other electro-magnetic techniques. These periodic re-assessments of the programme should ideally eliminate any operations that are no longer necessary and insert others, according to the results attained at each stage. Some economic benefit can be achieved by combining heating and cooling in areas where the climate permits. A second distinguishing factor of many heat pumps is that they are reversible and can provide either heating or cooling in the space.
In 1928 Iceland, another pioneer in the utilization of geothermal energy, also began exploiting its geothermal fluids (mainly hot waters) for domestic heating purposes. It did not have to be imported, and, in some cases, it was the only energy source available locally. On the other hand, in some 'geothermal areas' the gradient is more than ten times the average value.
The University of North Dakota is currently providing access via internet to an updated heat flow database comprising data on oceanic and continental areas. The ridges are cut perpendicularly by enormous fractures, in some cases a few thousand kilometres in length, called transform faults. The geothermal fluid is water, in the majority of cases meteoric water, in the liquid or vapour phase, depending on its temperature and pressure.
Geothermal systems of this type, the best-known of which are Larderello in Italy and The Geysers in California, are somewhat rare, and are high-temperature systems.
The information obtained from the geological and hydrogeological studies may also be used in the production phase, providing valuable information for the reservoir and production engineers.
The Controlled Source Audiomagnetotelluric method (CSAMT), developed recently, uses artificially induced waves instead of natural electro-magnetic waves.
Clearly any reduction in the number and size of the prospectings will lead to a decrease in costs, and also a corresponding increase in the risk of error or failure. The load factor in a system with combined heating and cooling would be higher than the factor for heating alone, and the unit energy price would consequently improve (Gudmundsson, 1988).
Geothermal fluids provide the thermal energy to drive these machines, although their efficiency decreases with temperatures lower than 105 A°C. The heat pumps, of course, need energy to operate, but in suitable climatic conditions and with a good design, the energy balance will be a positive one (Figure 16). The chemical composition of the geothermal waters used in irrigation must be monitored carefully to avoid adverse effects on the plants. The duration and cost of exploration can be appreciably reduced by a good exploration programme and an efficient coordination of the research.
The penetration depth is shallower with this technique, but it is quicker, cheaper, and provides far more detail than the classic MT method. In the first case the systems will be characterised by low temperatures, usually no higher than 100 A°C at economic depths; in the second case the temperatures could cover a wide range from low to very high, and even above 400 A°C.
The following are some of the most common definitions and classifications in this discipline.
Finally, increasing the temperature to, say, 120 A°C, and keeping the pressure at 1 atm, we will obtain superheated steam with a superheating of 20 A°C, i.e.
The geopressured reservoirs consist of permeable sedimentary rocks, included within impermeable low-conductivity strata, containing pressurized hot water that remained trapped at the moment of deposition of the sediments.
The hot water pressure approaches lithostatic pressure, greatly exceeding the hydrostatic pressure.
The medium-to-low temperature resources (< 150 A°C) are suited to many different types of application. At other temperatures and pressures, of course, these phenomena also take place in the underground, in what one author many years ago called a€?nature's tea-kettlea€?. The classical Lindal diagram (Lindal, 1973), which shows the possible uses of geothermal fluids at different temperatures, still holds valid (Figure 10, derived from the original Lindal diagram, with the addition of electricity generation from binary cycles. The geopressured systems could produce thermal and hydraulic energy (pressurized hot water) and methane gas. Fluids at temperatures below 20 A°C are rarely used and in very particular conditions, or in heat pump applications. These resources have been investigated extensively, but so far there has been no industrial exploitation.
The Lindal diagram emphasises two important aspects of the utilization of geothermal resources (Gudmundsson, 1988): (a) with cascading and combined uses it is possible to enhance the feasibility of geothermal projects and (b) the resource temperature may limit the possible uses.
Bathing, space and district heating, agricultural applications, aquaculture and some industrial uses are the best known forms of utilization, but heat pumps are the most widespread (12.5% of the total energy use in 2000). There are many other types of utilization, on a much smaller scale, some of which are unusual. The flow of geothermal fluid is in red, the secondary fluid in green, and the cooling water in blue.Electricity generation mainly takes place in conventional steam turbines and binary plants, depending on the characteristics of the geothermal resource.



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