Geothermal Energy 2024 PDF

Renewable energy technologies IEN GEOTHERMAL ENERGY D. Del Col Dipartimento di Ingegneria Industriale Università di Padova Source: Mary H. Dickson, Mario Fanelli Geothermal Energy, Utilization and Technology Geothermal Energy Definition: it is...

Renewable energy technologies IEN GEOTHERMAL ENERGY D. Del Col Dipartimento di Ingegneria Industriale Università di Padova Source: Mary H. Dickson, Mario Fanelli Geothermal Energy, Utilization and Technology Geothermal Energy Definition: it is the energy stored in the form of heat beneath the surface of the solid earth (VDI 4640, guideline in Germany) Energy balance (Stacey and Loper,1988) Total heat flow from Earth 42 * 1012 W Just over half of it is radiogenic heat (from the decay of radioactive isotopes) 19 % from the crust (it represents only 2% of the volume, rich of radioactive isotopes) 77 % from the mantle (it represents 82% of the volume) 4 % from the core (16% of the volume, it does not contain radioactive isotopes) Net of radiogenic heat, the Earth is cooling down. The cooling process is very slow (at the mantle, 300 °C decrease in 3 billion years !) POWER GENERATION In the early 1800s a chemical factory was set up in the zone now known as Larderello, to extract boric acid from the hot waters flowing naturally or from specially drilled boreholes. The boric acid was produced by evaporating the hot fluids in iron boilers, using local wood as fuel. In 1827 Francesco Larderel, founder of this industry, invented a system for utilizing the heat of the boric fluids in the evaporation process, rather than the wood from the rapidly depleting forests Larderello – Early days of the boric acid industry The mechanical energy of the natural steam was first exploited at Larderello in the late 19th century. The steam was used to raise liquids in primitive gas lifts, and later in reciprocating and centrifugal pumps and in winches, all used for well drilling or in the boric acid industry. Larderello – First machine for power generation from geothermal energy (1904) NON ELECTRIC APPLICATIONS Geothermal (ground- source) heat pumps have the largest geothermal use worldwide, accounting for 71.6 % of the installed capacity and 59.2 % of the annual energy use GEOTHERMAL GRADIENT Up to depths that can be reached with drilling technologies, over 10000 m, the average geothermal gradient is about 2.5-3°C / 100 m (e.g. + 50-60°C at 2000 m depth) There are many areas with a geothermal gradient very different from the average one Crust – Mantle - Core Litosphere – Asthenosphere Lithosphere: thickness 80 km in oceanic zones, 200 km in continental zones; it behaves as rigid body Asthenosphere: 200-300 km in thickness; more plastic behaviour PLATE TECTONIC PROCESSES  (cresta) (fossa) Subduction is a geological phenomenon that plays a key role in plate tectonics theory. This term refers to the sliding of a lithospheric plate under another plate and its consequent dragging deep into the mantle, connected to the production of new oceanic lithosphere in the mid-ocean ridges, which would tend to increase the overall surface of the planet, if there were no subduction. This phenomenon occurs along the converging edges of the plates, where the oceanic crust is then destroyed by subduction. Example: Andes mountains in South America Example: Himalaya mountain range Example: Island arc between Kamčatka and Alaska  The edges of the plates are densely fractured areas of the crust The most important geothermal areas are located at the edge of the plates GEOTHERMAL SYSTEM - heat source - reservoir - fluid The source must be of natural origin Fluid and reservoir can be "artificial" The fluid can be injected through special injection wells HDR (Hot Dry Rock) projects: high pressure water is injected at a depth of several thousand meters The rock is artificially fractured through hydraulic fracturing USE OF GEOTHERMAL RESOURCES Lindal diagram ELECTRIC POWER GENERATION 3-15 bara 0,1 bara To generate electricity from medium and low temperature sources (85-170°C) Working fluid: n-pentane or others Why the cooling tower ? Dry Bulb, Wet Bulb, and Dew Point Temperatures Dry Bulb Temperature - Tdb The Dry Bulb temperature is usually referred to as air temperature. The Dry Bulb Temperature refers basically to the ambient air temperature. It is called "Dry Bulb" because the air temperature is indicated by a thermometer not affected by the moisture of the air. Wet Bulb Temperature - Twb The Wet Bulb temperature is the temperature of adiabatic saturation. This is the temperature indicated by a moistened thermometer bulb exposed to the air flow. Wet Bulb temperature can be measured by using a thermometer with the bulb wrapped in wet muslin. The adiabatic evaporation of water from the thermometer and the cooling effect is indicated by a "wet bulb temperature" lower than the "dry bulb temperature" in the air. The rate of evaporation from the wet bandage on the bulb, and the temperature difference between the dry bulb and wet bulb, depends on the humidity of the air. The evaporation is reduced when the air contains more water vapor. Dew Point Temperature - Tdp The Dew Point is the temperature at which water vapor starts to condense out of the air, the temperature at which air becomes completely saturated. Why the cooling tower instead of a condenser exchanging heat with external air? Because the limiting temperature in this way is the wet bulb temperature, instead of the dry bulb temperature  ORC – Organic Rankine Cycles Optimal characteristics of a working fluid for Rankine cycle, in order to maximise the thermal efficiency of the cycle : Critical temperature higher than the maximum cycle temperature; Moderate maximum cycle pressure; Pressure at the condenser higher than the atmospheric value; Small specific volume of the vapor at the end of expansion to contain the dimensions of the low pressure stage of the turbine and the exchangers; Vertical dry saturated vapor curve (to avoid the appearance of liquid in the expansion) and expansion in superheated conditions; For applications of limited power, the fluid should have a high molecular mass to reduce the rotation speed and the number of stages of the turbine, and have not too low mass flow rates and areas of passage in the blades; The fluid should be liquid at ambient pressure and temperature, to facilitate its handling; The solidification temperature should be below the lowest foreseeable ambient temperature; The fluid should have good heat exchange characteristics, be inexpensive, thermally stable under conditions of use, non-flammable, non-corrosive, non-toxic, not harmful to the environment. IN GEOTHERMICS, THE ORGANIC FLUID ALLOWS US BETTER EXPLOITATION OF THE GEOTHERMAL RESOURCE Water High molar mass fluid Working fluid Attention: R114 is a CFC (ODP = 1) see also R245fa (HFC, but GWP = 1000) R1233zd (GWP = 1, ODP > 0 but very very low)  ORC CYCLE TURBINE COMPARISON between WATER (M=18 kg/kmol) and R245fa (M=134 kg/kmol) Sat temp in the evaporator Sat temp in the condenser pIN pOUT hIN hOUT,IS hIN-hOUT,IS IN OUT,IS bar bar kJ/kg kJ/kg kJ/kg kg/m3 kg/m3 Water 1,122 0,036 R245fa 119,6 9,51 - Pressure ratio: much higher for water - Enthalpy change: much higher for water - Density: the vapour of the high molar mass fluid is much more dense. For one single expansion stage ORC CYCLE TURBINE Isentropic work in one stage: it is limited by the tip speed for ideal gas The expansion ratio for a single expansion stage increases with molecular mass  ORC CYCLE TURBINE - There is a limit for the maximum value of u2 (tip speed) - When the isoentropic work is high, the tip speed u2 is high too - If mass flow rate is low, since we cannot vary much u2 we must reduce D and increase n; this leads to small impellers working at high rotation speed - When using a high molar mass fluid: the isentropic work is lower than water, we can accept lower u2 ; at head factor low flow rates, n can be lower compared to water and D can be higher; thus using a high molar mass fluid we expect higher efficiency volume when the power is reduced flow factor Comparison between the turbine isentropic efficiency values obtainable with water vapor (a low molecular mass substance) and a high molecular mass working fluid (HMM). Turbine isoentropic efficiency [%] Power range H2O HMM >10 MW 70-80 75-80 1-5 MW 50-70 75-80 200-500 kW 30-50 75-80 10-100 kW 25-50 60-75 When using an organic fluid, as compared to water: the use of an organic fluid allows us to extract more heat from the water of the geothermal source (see the comparison with the same pinch-point at the exchanger) the entire expansion occurs outside the saturation zone the enthalpy jump is lower than water and this allows to design a single-stage turbine with high efficiency: in the case of water, the enthalpy jump in the turbine is high, a low flow rate is needed but also the pressure ratio is high and thus more than one expansion stage and a high rotation speed are needed; in the case of organic fluid the enthalpy jump is smaller, it is possible to operate with a single stage and greater flow rate with an organic fluid, for the same power, a greater flow rate is required than water but the vapour is denser and therefore there is no need to increase the passage area the density of the organic fluid at the discharge is greater: the volumetric flow rate of the water vapor at the discharge is considerably higher and therefore the dimensions of the steam turbine are much greater the pressure of the organic fluid is always greater than the atmospheric pressure Scheme of a machine operating with Rankine ORC cycle ORC cycle Why an organic fluid with high molar mass instead of water ? Technical advantages of ORC Operating advantages of ORC  Applications of ORC by Turboden Geothermal energy Solar energy Biomass Geothermal applications  Geothermal applications The power plant in Soultz started to produce electricity in 2016, it was installed with a gross capacity of 1.7 MWe. The 1.7 MW test plant is purely a demonstration plant. Normal geothermal power are much cheaper than HDR system, with shallower wells, that produce orders of magnitude more energy. It seems possible that breakthroughs will occur that allow us to access the tremendous amounts of stored heat energy in deep rock using HDR technology but for the moment few breakthroughs appear to be on the horizon. Solar applications  THERMAL APPLICATIONS DISTRICT HEATING (REYKJAVIK) GREENHOUSE HEATING AQUACULTURE AND FOOD ANIMALS Map of temperature at 2000 m depth Map of temperature at 5500 m depth SPACE HEATING (GROUND COUPLED HEAT PUMPS) Temperature at given depth HEATING AND COOLING 4 way valve Ciclo Kalina