Study on Renewable Energy Resources, Oman - authority for ...

Renewable Energy Resources in Oman
5.5 Geothermal power plants
Energy conversion principle
Geothermal heat originates from weak radioactive activity of volcanic activity in
the underground. The temperature in the ground normally raise with 30 o C per
kilometres depth meaning that a borehole normally need to be more than 3 km in
order to be able produce steam or superheated water, that can be used to drive a
low pressure steam turbine or other equipment.
If volcanic activity is present usable heat at higher temperature and less depth
will be available.
Technology
The water from geothermal wells is normally extreme saline and contains many
salts solutions, particles and compounds. Clogging and corrosion problems are
therefore often seen and need to be considered. Used water from the geothermal
well can be drained off or re-infiltrated in the ground, which also can create
problems with clogging of re-infiltration wells.
Applications
Geothermal is used in different applications depending on the temperature of the
soil water.
In a temperature range between 60 o C and 100 o C the energy can be used for district
heating. Via a heat exchanger the energy content in the hot water from the
well is transferred to the circulated water in the district heating system.
In a temperature range between 100 o C and 170 o C the energy can be utilised for
electricity generation. The heat from the hot water is transferred through a heat
exchanger to another liquid with a low boiling point and is used for electricity
generation in a power plant.
At temperatures above 170 o C the hot water can be utilised directly in steam
plants after the water is cleaned. When the hot water is pumped up from the underground
the pressure decreases and the water is transferred into steam.
The hot water could also be used to desalinate seawater in thermally driven desalination
systems based on vapour compression cycles. This implies that the
well is located close to the coast line.
Electricity generation
If steam is generated from the well, it can be used to drive a low pressure condensing
steam cycle to generate electricity at low efficiency depending
An alternative is to use an ORC plant (Organic Rankine Cycle), which would be
able to produce electricity based on superheated water of around 120 o C with an
electricity production of typically 8-12% electricity of the incoming energy before
subtraction of pumping energy for the wells. Pumping energy has to be considered
from case to case. It is not unusual that electricity consumption for pumping
energy amounts to 5-10% of the usable heat in the well water.
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