WORLD ENERGY [R]EVOLUTION A SUSTAINABLE WORLD ENERGY OUTLOOK
box 9.3: sorption cooling units
A thermo-chemical refrigerant cycle (sorption) provides cold by either ab- or adsorption cooling. Absorption occurs when a gaseous or liquid substance is taken up by another substance, e.g. the solution of a gas in a liquid. Adsorption takes place when a liquid or gaseous substance is bound to the surface of a solid material.
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The absorption cooling circle can be described as follows: A liquid refrigerant with a very low boiling point is vaporised at low pressure withdrawing heat from its environment and therefore providing the desired cool. The gaseous refrigerant is then absorbed by a liquid solvent, mostly water. The refrigerant and solvent are separated again by adding (renewable) heat to the system, making use of the different boiling points. The gaseous refrigerant is now condensed, released and returned to the beginning of the process. The heat, which is needed in the process, can be provided e.g. by firing natural gas, combined heat and power plants or solar thermal collectors.
energy technologies |
9.3.9 geothermal, hydrothermal and aerothermal energy
RENEWABLE ENERGY TECHNOLOGIES
The three categories of environmental heat are geothermal, hydrothermal and aerothermal energy. Geothermal energy is the energy stored in the Earth’s crust, i.e. in rock and subsurface fluids. The main source of geothermal energy is the internal heat flow from the Earth’s mantle and core into the crust, which itself is replenished mainly by heat from the decay of radioactive isotopes. At depths of a few meters, the soil is also warmed by the atmosphere. Geothermal energy is available all year round, 24 hours a day and is independent from climatic conditions. Hydrothermal energy is the energy stored in surface waters rivers, lakes, and the sea. Hydrothermal energy is available permanently at temperature level similar to that of shallow geothermal energy. Aerothermal energy is the thermal energy stored in the Earth’s atmosphere, which originally comes from the sun, but has been buffered by the atmosphere. Aerothermal energy is available uninterruptedly, albeit with variations in energy content due to climatic and regional differences. Deep geothermal energy (geothermal reservoirs)
On average, the crust’s temperature increases by 25-30°C per km, reaching around 100°C at 3 km depth in most regions of the world. High temperature fields with that reach over 180°C can be found at this depth in areas with volcanic activity. “Deep geothermal reservoirs” generally refer to geothermal reservoirs more than 400m depth, where reservoir temperatures typically exceed 50°C. Depending on reservoir temperature, deep geothermal energy is used to generate electricity and/or to supply hot water for various thermal applications, e.g. for district heat, balneology etc. Temperatures in geothermal reservoirs less that 400m deep are typically below 30°C which is too low for most direct use applications or electricity production. In these shallow 254
fields, heat pumps are applied to increase the temperature level of the heat extracted from shallow geothermal reservoirs. The use of geothermal energy for heating purposes or for the generation of electricity depends on the availability of steam or hot water as a heat transfer medium. In hydrothermal systems, hot water or water vapour can be tapped directly from the reservoir. Technologies to exploit hydrothermal systems are already well established and are in operation in many parts of the world. However, the there is limited availability of aquifers with sufficient temperature and water production rate at favourable depth. In Europe, high temperature (above 180°C) hydrothermal reservoirs, generally containing steam, are found in Iceland and Italy. Hydrothermal systems with aquifer lowers temperatures (below 180°C) can also be used to produce electricity and heat in other regions. They contain warm water or a water-steam mixture. In contrast to hydrothermal systems, EGS systems do not require a hot aquifer; the heat carrier is the rock itself. They can thus virtually found everywhere. The natural permeability of these reservoirs generally does not allow a sufficient water flow from the injection to the production well, so energy projects require the artificial injection of water into the reservoir, which they do by fracturing rock underground. Water is injected from the surface into the reservoir, where the surrounding rock acts as a heat exchanger. The heated water is pumped back to the surface to supply a power plant or a heating network. While enhanced geothermal systems promise large potentials both for electricity generation and direct use, they are still in the precommercialisation phase. Direct use of geothermal energy
(Deep) geothermal heat from aquifers or deep reservoirs can be used directly in hydrothermal heating plants to supply heat demand nearby or in a district heating network. Networks provide space heat, hot water in households and health facilities or low temperature process heat (industry, agriculture and services). In the surface unit, hot water from the production well is either directly fed into a heat distribution network (“open loop system”). Alternatively, heat is transferred from the geothermal fluid to a secondary heat distribution network via heat exchangers (“closed loop system”). Heating network temperatures are typically in the range 60-100°C. However, higher temperatures are possible if wet or dry steam reservoirs are exploited or if heat pumps are switched into the heat distribution circuit. In these cases, geothermal energy may also supply process heat applications which require temperatures above 100°C. Alternatively, deep borehole heat exchangers can exploit the relatively high temperature at depths between 300 and 3,000m (20 – 110°C) by circulating a working fluid in a borehole in a heat exchanger between the surface and the depth. Heat pumps can be used to increase the temperature of the useful heat, if required. The overall efficiency of geothermal heat use can be raised if several thermal direct-use applications with successive lower temperature levels are connected in series (concept of