NUMERICAL SIMULATION OF HEAT EXTRACTION IN THE GEOTHERMAL PART OF THE EXCHANGER IN ONE-HOLE SYSTEM

Abstract

Geothermal plants and power stations can work as one-hole
systems with an injection and production well, or as two-hole
systems. The one-hole system operates using a vertical geothermal
probe. It is an exchanger that has two parts. One part
is immersed in a geothermal deposit, where the heat extraction
medium takes heat from geothermal water. The other part
of the exchanger, a double-pipe exchanger, is located in the
impermeable rock massif. The extraction medium is conducted
through a rig-shaped channel to the geothermal part of the
exchanger, where its temperature rises. The heat extraction
medium flows up to the ground through the inner pipe. However,
the heat exchange takes place in both parts of the exchanger,
and the part of the exchanger located in the geothermal
deposit has a considerable impact on the amount of
geothermal energy gained. The type of exchanger also influences
the amount of heat extracted. The results of exchanger
calculations on heat extraction for two types of exchanger
(Field's * exchanger and a spiral-tube exchanger) are presented.
The calculations were carried out using two computational
models. In the first model it was assumed that the temperature
field and the overall heat transfer coefficient were
known. In the second, the exchanger was considered to be
a cross-counter-flow heat exchanger with a single non-mixing
fluid. It was also assumed that the temperature field and the
overall heat transfer coefficient were known in this case too.
In both cases, it was assumed that the temperature field in a
geothermal deposit changes linearly in a vertical direction, as
is the case in a rock massif.
To simplify the calculations, in all the cases the same conditions
for the double-pipe heat exchanger part were taken
into account. Also, it was assumed that the inner pipe was
perfectly insulated. Comparisons of the results of the heat extraction
calculations for the Field exchanger and the spiraltube
exchanger, obtained using both computational models,
are presented as graphs. The graphs illustrate the impact of
the overall heat transfer coefficients and heat extraction rates.