What is geothermal energy?
Source of geothermal energy
Geothermal energy is generated in the earth’s core, about 5,000 kilometres below the surface. Temperatures hotter than the sun’s surface are continuously produced inside the earth by the slow decay of radioactive particles, a process that happens in all rocks.
The earth has a number of different layers:
- The core itself has two layers: a solid iron core and an outer core made of very hot melted rock, called magma.
- The mantle which surrounds the core is about 3,000 kilometres thick. It is made up of magma and rock.
- The crust is the outermost layer of the earth, the land that forms the continents and ocean floors. It can be 5 to 8 kilometres thick under the oceans and 15 to 50 kilometres thick on the continents.
The earth’s crust is broken into pieces called plates. Magma comes close to the earth’s surface near the edges of these plates, this is where volcanoes occur. The lava that erupts from volcanoes is partly magma. Deep underground, the rocks and water absorb the heat from this magma. The temperature of the rocks and water get hotter as you go deeper underground.
The temperature increases by 30 Celsius each kilometre from the surface to the magma but in certain areas the increase is even higher. In Hungary the temperature gets higher by 50-60 Celsius each kilometre and it often exceeds 100 Celsius 2 kilometres deep. This is the geothermal energy provided by the heat of the rocks. In Hungary the geothermal energy is easily available and there is an abundance of underground waters.
Where is geothermal energy found?
Most geothermal reservoirs are deep underground with no visible clues showing above ground. Geothermal energy can sometimes find its way to the surface in the form of volcanoes, hot springs and geysers. The most active geothermal resources are usually found along major plate boundaries where earthquakes and volcanoes are concentrated.
When magma comes close to the surface it heats ground water found trapped in porous rock or water running along fractured rock surfaces and faults. Such hydrothermal resources have two common ingredients water and heat. Naturally occurring large areas of hydrothermal resources are called geothermal reservoirs. Geologists use different methods to look for geothermal reservoirs. Drilling a well and testing the temperature deep underground is the only way to be sure a geothermal reservoir really exists.
Geothermal energy and the environment
The environmental impact of the geothermal energy depends on the way it is being used. Direct use and heating applications have almost no negative impact on the environment. Geothermal power plants do not burn fuel to generate electricity, so their emission levels are very low. They release less than one percent of the carbon dioxide emissions of a fossil fuel plant. Geothermal plants use scrubber systems to clean the air of hydrogen sulfide that is naturally found in the steam and hot water. After the steam and water from a geothermal reservoir have been used, they are injected back into the earth.
Uses of geothermal energy
Some applications of geothermal energy use the earth’ temperature near the surface while in others require drilling miles into the earth. Notwithstanding such differences the geothermal energy mainly can be used in three ways:
- Hot water can be used for direct heating, hot water supply, in thermal baths, for industry purpose and in the agriculture.
- Electricity can be generated in a power plant using water or steam at very high temperature.
- Stable ground or water temperatures near the earth’s surface can be used to control building temperatures through goethermal heat pump.
Direct use of geothermal energy
The direct use of hot water as an energy source has happened since ancient times. The Romans, Chinese, and Native Americans used hot mineral springs for bathing, cooking and heating. Today, many hot springs are still used for bathing, and many people believe the hot, mineral-rich waters have natural healing powers. In addition to the bathing, the most common use of geothermal energy is for heating buildings through district heating systems. Hot water can be piped directly into buildings and industries for heat. A district heating system provides heat for 95 percent of the buildings in Reykjavik, Iceland. Other direct uses in the industry and agriculture include growing crops, and drying fruits and vegetables.
How geothermal power plants work?
Geothermal power plants require hot hydrothermal resources provided by dry steam wells or hot water wells. In order to use these resources wells should be drilled into the earth and the steam or hot water can be piped to the surface. Geothermal wells are usually 1.5-3 kilometres deep. There are three types of geothermal plants, the dry steam power plants, flash steam plants and the binary power plants.
Dry steam power plants
In case of dry steam power plants the steam goes directly to a turbine, which drives a generator that produces electricity. The steam eliminates the need to burn fossil fuels to run the turbine. This is the oldest type of geothermal power plant. It was first used in Toscana in Italy in 1904, and is still very effective. These plants emit only excess steam and very minor amounts of gases.
Flash Steam Power Plants
Hydrothermal fluids above 180°C can be used in flash plants to make electricity. Fluid is sprayed into a tank held at a much lower pressure than the fluid, causing some of the fluid to rapidly vaporize, or “flash.” The vapour then drives a turbine, which drives a generator. When the steam cools, it condenses to water and is injected back into the ground to be used over and over again. Most geothermal power plants are flash plants.
Binary-Cycle Power Plants
Most geothermal areas contain moderate-temperature water (below 200 Celsius). Energy can be extracted from these fluids in binary-cycle power plants. Hot geothermal fluid and a secondary fluid with a much lower boiling point than water pass through a heat exchanger. Heat from the geothermal fluid causes the secondary fluid to flash to vapor, which then drives the turbines. Because this is a closed-loop system, virtually nothing is emitted to the atmosphere. As moderate-temperature water is by far the more common geothermal resource, most geothermal power plants in the future expected to be binary-cycle plants.
The Kalina technology
The Kalina technology makes it possible to generate electricity from low-heat geothermal areas.
The method is built on generating electricity by using heat-energy from a low-heat source in order to vaporise a mixture of ammonia and water, running in a closed circuit. It is precisely this mixture that makes this technology exceptional. The technique is named after Dr. Alexander Kalina, who is a Russian engineer living in the USA. The “Kalina” technology has been developed over a two decades, however the commercial marketing of the technique started only a few years ago.
It is well known that ammonia and water as other single-component media vaporises and condenses at stable temperature. The uniqueness of “Kalina” technology comes from that the mixture vaporises and condenses at varying temperatures. This gives an opportunity to utilise the waste heat more effectively than with single-component media. Comparing of low-heat circuits demonstrates that by using the “Kalina” technology the effectiveness of the electricity plants can be increased by almost 20-50%.
The „Kalina” technology was first used in Iceland
The “Kalina” technology was first used in connection to a renovation of a geothermal power plant in a small fishing town at Husavik, Iceland. The 2 MWe “Kalina” electrical power plant was built, which uses 124° C hot water for generating electricity before the water is used for heating the town. The power plant in Husavik was put to work in July 2000 and is the first “Kalina” electrical power plant in the world that uses geothermal heat from a lowheat area for generating electricity. The power plant in Husavik provides approximately 75% of the town’s electricity demands.
The ammonia-water mixture is vaporised with geothermal water, which is cooled from 124°C to 80°C. The mixture’s steam, powers a steam turbine, which rotates an electrical generator, which generate the electricity. The geothermal water from the plant is then used for heating the town. After the steam turbine, the ammonia-water mixture is condensed with fresh water, which is heated from 5°C to 25°C. The plant’s cooling water is then used for a fish farming.
How a geothermal heating plant works?
The ultimate source of geothermal energy is magma, and therefore it is independent from weather factors, and can be regarded as a steady, calculable form of renewable energy. The geological endowments of the Carpathian Basin are especially favorable, because the average thickness of the Earth’s crust is approx. 25 km, which is about half of the average thickness round the world, whereas in some places the value of the geothermal gradient can reach the 50 °C/km value, which is about the double of the global average, and consequently the region is an ideal place for the establishment of geothermal power plants.
It was 40–50 years ago when Hungary started to utilize thermal waters in large scales; at that time hydrocarbon fields were explored one after the other, and a large number of wells yielded hot water instead of hydrocarbons.
As depending on the form of water withdrawal, thermal wells have two distinct types: positive and negative wells.
- In positive wells, thermal water comes to the surface by way of free outflow, allowed by reservoir pressure and/or the gas content of water.
- In negative wells, the water level lies lower than the ground level, and therefore it is to be withdrawn with the use of pumps.
In the case of long-term production and large-volume water withdrawal, the water taken from the among the rock formations may decrease reservoir pressure, which reduces the yield, while gradually lowers the water level, and therefore the positive well can turn into a negative one. Towards the maintenance of the hydrodynamic equilibrium of the reservoir pressure and deep reservoirs and their long-term functioning, the reinjection of the extracted water has major significance.
To determine whether or not reinjection is a necessary process, the research areas have to be analyzed separately in their own, broader environs so that the geothermal options be fully explored, which calls for the review of the available geological data. The question is primarily decided in view of geological aspects. In this context, the most important criteria are the refilling of the mainly two types of reservoirs in Hungary, as well as the environmental impacts caused by extraction. The decision may as well be influenced by economic aspects, because an important question is how the costs of reinjection relate to the amount of the water reserve contribution payable on the volume of water conducted into the surface recipient. The operation of the geothermal system impacts the functioning of both its own reservoir and the surrounding wells. For this reason, these studies need to be conducted regionally.
Reinjection is also a proper solution for the safe disposal of large quantities of water often rich in mineral salts without loading the surface recipients. In case the withdrawn thermal water is not reinjected, but let into surface waters, it will change the temperature conditions of the water course concerned, which influences the oxygen contents of the water. Warmer water holds less oxygen than colder water, and oxygen deficiency may lead to fish mortality. Contamination with thermal water can also have negative effects due to the salt content of the thermal water. Salt contamination induces a so-called salting-out effect, which kills fish and the vegetation alike. For this reason, it is harmful to let thermal water enter surface waters.
The form of the utilization of thermal water basically depends on its temperature. The wells operated in Hungary generally yield waters under 85°C enthalpy, while at certain locations their heat content is at the medium 85–100°C, and therefore these sources are suitable for the establishment of geothermal heating plants that can be used – among others – for the heat supply of district heating systems, agricultural and industrial purposes.
The operating principle of the geothermal heating plant
Geothermal heating plants are in fact pieces of equipment used for the extraction, treatment, heat transfer and then reinjection of thermal water, as well as the complexity of pipelines connecting these units.
As depending on its quality, the water withdrawn from the production well undergoes treatment, and then via the insulated pipeline of water transmission it is taken to the place of utilization. The treatment may take the form of physical and/or chemical intervention, yet waters of favorable properties – when they contain just small quantities of gases, small or medium quantities of minerals, or when they are soft or just moderately hard – do not need any manipulation. When the reservoir pressure is not high enough to bring thermal water to the surface without intervention (negative well), pumps (for instance, deep well pumps) need to be operated for extraction and from the place of extraction to the location of heat utilization the forwarding of water is similarly ensured by pumps (booster and accelerating pumps).
The form of heat utilization is also influenced by the chemical composition of water, because non-aggressive waters containing just little dissolved material may serve direct heat utilization – in this case thermal water itself is the heat carrier that is circulated in the system of the heat-utilizing object. This solution is relatively rare, because hot thermal waters coming from large depths are usually too aggressive to be used for direct heating safely. In general, the primary application is indirect utilization during which the heat energy of thermal water is transferred via one or more heat exchangers to the treated, purified heating water, and thereafter the cooled thermal water of unchanged chemical composition is reinjected with the use of pumps. A considerable advantage of this solution is that only the thermal water, so-called primary loop needs to be constructed from resistant materials, whereas the secondary loop for heating can consist of materials generally used for heating systems, thereby making the implementation of the system more economical.
The key constituents of geothermal heating plants are the heat exchangers that at the current state of technological development are usually plate-type, counter-current heat exchangers of small space demand. The most wide-spread solution is counter-current heat exchange where in the equipment the cooled heat-absorbing medium flows in the opposite direction to the heat-transfer medium, and consequently heat transfer and heat balance become steady and optimal in the heat exchanger. Due to the above-detailed corrosive effects of thermal water, the materials of heat exchangers in the geothermal systems are subject to increased stresses, and therefore they are made from resistant materials (for instance, titanium plate heat exchangers). In the heating operations, heating water is also circulated by pumps, and in this system provisions need to be made for the water supply and treatment demands, necessity to maintain pressure because of the volume changes arising from the heat expansion and contraction of water, for which pumps, water treatment units and tanks are used. In heating systems, output changes resulting from the alteration of the ambient temperature can be handled by increasing or decreasing the temperature or mass flow of heating water. In the light of the foregoing, it can be understood that during operations the largest costs are incurred with the electric power consumption needed for the operation of pumps.
When run appropriately, geothermal heating plants have very slight impacts on the environment. They can be used to replace heating with natural gas or solid fuels (e.g. coal), which cuts the quantities of pollutants entering the air – carbon dioxide, nitrogen oxides, sulfur dioxide –, while Hungary’s dependence from imported energy is moderated.