Geothermal Flashcards
Where does geothermal heat come from?
Geothermal heat comes from the inside of the earth. 30 to 50 % accretion and 70-50% radioactive decay
Is the lifetime of a geothermal borehole limited?
Yes
Where is the geothermal potential especially high?
Close to plate boundaries
Discuss possible applications of geothermal heat with (a) T ≈ 30°C, (b) T ≈ 60°C, (c) T > 100 ºC
a) Swimming, Mining,De-icing
b) Greenhouses by combined space and hotbed heating
c) Drying of organic products, fresh water by distillation, canning of food, crying of fish meal, drying of diatomaceous earth, refrigeration by ammonia absortion
What type of systems do you use to exploit near-surface energy
(1-1,5 m). What are the elements/sources the heat gets extracted
from?
Geothermal ground collectors.
Soil and ambient air
Give two different types of ground collectors.
Plain ground collectors and trench ground collectors.
What is necessary so that you can install a two well system?
Only if ground water is available.
What can you use if you want to utilize the energy in deeper ground
(up to 150 m)?
A deep borehole heat exchanger.
How does a vertical heat exchanger work?
They work with gravity and the density difference between liquid and vapor phase. These pipes are sealed and they contain a liquid work fluid. This fluid becomes evaporated by absorving heat at the bottom, the vapor rises and due to the heat sink it recondenses and the liquid flows down again. There is an adiabatic zone at the middle.
Give 2 types of drilling technologies. What are differences?
Rotary drilling process has the motor above ground, and deflections of a few degrees possible.
Directional drilling has hydraulic underground motor, deflection from vertical borehole possible and MWD - measurement while drilling - allows a permanent control of drilling direction.
What is a casing of a borehole and what do we need it for?
It’s a large diameter pipe that is assembled and inserted into a recently drilled section of a borehole. We need it for preventing fluid loss, sealing off and avoiding blowout, isolating different zones with different pressures, and providing a smooth internal bore for installing production equipment.
What is the Thermal Response Test (TRT) for? What can we
measure with it?
It’s an experimental determination of the thermal conductivity, in order to design borehole and number of bore holes.
With it we can measure the thermal conductivity.
What is hydraulic conductivity?
It’s the ability of water to go through soil or rock
Give three possibilities for direct utilization of geothermal heat.
Space heating with heat pumps
Electric power production with binary cycles
Conventional power production
Is a direct use of near surface geothermal energy possible? If not,
how could you make it work though (still using surface near surface
geothermal energy?
No, heat pumps could make it work.
Define the COP in the context of heat pumps.
Coefficient of Performance is the useful heat/electrical energy
How does a geothermal heatpump works?
Works with the compression of a working fluid in gaseous state (which was heated by geothermal heat), which then rejects heat at the level of a condenser
Τhe higher the COP the higher the … temperature and the lower
the … temperature
The higher the COP the higher the evaporation temperature and the lower the condensation temperature.
Give 2 possible heat sources for a geothermal heat pump.
Underground, water, air, industrial waste heat
What happens, when the actual ground temperature is lower than
the design temperature? Compare to the thermodynamic cycle in q21.
What happens to the COP?
More mechanical work is needed in order to provide same amount of heat. COP decreases.
What happens if your heat exchanger is built too small?
You can’t obtain the desired temperature.
What is a thermal driven heat pump? Which part of the heat pump
is now thermally driven?
It is a pump that instead of a compressor has been replaced with a thermally driven compression, it has a absorber and a absorber.
How do you express the efficiency of a gas heat pump (which is a
thermal driven heat pump)?
GUE=Q/Gas
There more local the district heating the … the effiency.
higher
By which parameter is the heat exchanger efficiency limited? How
can it be decreased?
Is limited by the return remperature of the district heating.
Return temperature can be reduced by floor, ceiling and wall heating concepts.
What has the function of the steam generator in the clausius rankine cycle when using geothermal energy?
It’s the underground heat.
What is a single flash plant?
It’s a cycle where saturated liquid is extracted from the underground, flash evaporation happens in the throttle, which makes the steam contains no impurities and particles, and then the elecrticity is generated.
When do you prefer an ORC plant?
When the reservoir temperature is low.
Why is there no need for a recuperator in geothermal applications?
The recuperator makes the system more complex and will only compensate at the long term, which is a thing that most investers don’t want.
What is the difference between the Kalina Cycle and the ORC? Give
an advantage of the Kalina cycle over the ORC.
Kalina cycle has higher live steam temperatures and less exergy loss when compared to ORC.
What is the working fluid in the Kalina cycle?
A mixture of water and ammonia.
What are typical design criteria for a geothermal plant?
Design criteria are high full load hours for the geothermal plant. Peak load is covered by peak load boilers. Residual heat can be used for power production.
The higher the CHP coefficient the more … a CHP-configuration
flexible
The more full load hours of geothermal the higher the
profitability
Larger complexity enables a higher … in heat and power
production
flexibility
What are general environmental risks of deep geothermal system/
near-surface geothermal systems (2 each)
Deep geothermal system: Risk of hydraulic short circuit and release of minerals.
Near-surface geothermal systems: Harmful working fluids and horizontal heat exchangers may affect vegetation.
Which part of a geothermal project is the most expensive?
Drilling
How high are power generation costs and heat generation costs
compared to other energy sources
Power generation is sometimes more expensive than some renewable energies.
Heat generation is usually cheaper than any other energy sources.
Is geothermal renewable
Half-life of the isotopes is much larger than the age of the earth
what is the order of geothermal heat?
10^20-10^21
descending order of geothermal sources with respect to depth
hot dry rock, two well, collectors
Latest two modifications in ORC than conventional
2 pressure process, 2 pressure split process
head ranges of turbines
Pelton (150 to 1800m), Francis (30 to 800m), Kaplan (15 to 75m), bulb (2 to 20m)
Flow rates of turbines
Pelton (60 m^3/s), Francis (800 m^3/s), Kaplan (wide range of flow rates)
Risk of hydro power plants
Environmental impact,
Hydrological uncertainty(e.g.precipitation),
Force majeur(e.g.earthquake),
Geological risks
Explain cavitation
Under some operational conditions, the static pressure can locally
drop to very low levels which leads to evaporation of water drops.
When the local pressure rises again, the tiny bubbles implode causing
a jet stream with high velocities. When this jet and the resulting
pressure wave hits solid material (e.g. turbine blades) substantial
damages are caused over time.
Most vulnerable to cavitation are the edges of the blades but also the
entire surface depending on the operations mode of the turbine.
