heat exchangers

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TESTS WITH PCM IN GROUTING OF UTES BOREHOLE HEAT EXCHANGERS
Conference Paper · June 2021
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Javier F. Urchueguia
Universitat Politècnica de València
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2. ENERSTOCK 2021 Oral lectures
39
9 – 11 June 2021, Ljubljana, Slovenia
TESTS WITH PCM IN GROUTING OF UTES BOREHOLE HEAT EXCHANGERS
Burkhard Sanner1
, Javier F. Urchueguía S.2
, José M. Cuevas C.2
, Borja Badenes B.2
, Patrick Fontana3
, Ali Nejad
Ghafar3
, Ojas Arun Chaudhari3
, Michael Shuster4
1
Ubeg Dr. Mands & Sauer GbR, Wetzlar, Germany,
2
Universitat Politècnica de València, Valencia, Spain,
3
RISE Research Institutes of Sweden AB, Stockholm, Sweden,
4
Carmel Olefins Ltd., Haifa, Israel
INTRODUCTION: Within the EU-funded GEOCOND project (Urchueguía and Sanner, 2019), one of the
research objectives considered the improvement of short-term heat and cold storage capabilities for UTES
(Underground Thermal Energy Storage) applications. While UTES usually is intended for seasonal storage of
heat or cold, additional short-term storage requirements nevertheless occur often in practical applications.
Examples include heat storage from quickly varying sources like solar thermal or discharge to changing loads
in district heating networks.
In ATES (Aquifer Thermal Energy Storage), control of the pump rate can help to adjust to short-term load
variations. This option does not exist in BTES (Borehole Thermal Energy Storage), where heat exchange is
limited to conductive heat transfer. Here Phase Change Materials (PCM) are seen as a solution, as they can
increase storage capacity of a given volume of material, in this case the filling of the borehole annulus. In a
combination of quick reactions of the PCM inside the borehole and the slower heat transfer to and from the
surrounding ground, the overall storage behaviour of a BTES can be optimized. This energetic advantage of
this approach had already been investigated theoretically (e.g. Rabin and Korin, 1996), but not yet with PCM
dispersed in the grout matrix. Hence, in the frame of developing new grout formulas, mixtures with
inclusion of PCM were devised and tested.
MATERIALS AND METHODS: The development concerned two separate fields of application, with the
respective PCM selection. One application is for individual ground source heat pumps (GSHPs) for heating
and cooling and is intended to improve cooling efficiency, e.g. after colder nights. The required working
temperature here is in the range of 25-35 °C. The other application concerns UTES in district heating, where
short-term fluctuations can be smoothened at an elevated temperature. The PCM for this application
should have a working temperature within the range of 50-80 °C.
The additional requirement for the material in both cases is the compatibility with the other components of
the grout and the retention of the required grout properties (handling and sealing). The PCM thus must be
shape-stable in fine grain size, well mixable for good dispersion and low separation, and stable in the
alkaline environment of a typical grout. The rheological and mechanical properties as well as the low
permeability of the grout must not be deteriorated. Material selections were tested in the laboratory, in a
lab-size reproduction of a typical borehole section (“sandbox” experiment), in a field test with reduced
borehole depth at the campus of the Universitat Politècnica de València (UPV) in Spain and in a full-size
UTES borehole at the RISE facilities in Borås, Sweden.
RESULTS: Early tests in the laboratory allowed to exclude materials that did not prove stable in the presence
of other components in the grout (mainly cement and other additives). Also separation by gravity such as
segregation while mixing had to be addressed, e.g. by adjusting the viscosity of the grout mixture, while
taking into account the needs for handling and pumpability. The resulting grout containing PCM is
characterised by quite uniform distribution of the organic PCM particles throughout the cementitious
matrix, which results in good interaction between the matrix and the PCM particles, implying good thermal
contact as required for substantial energy storage.
PROGRAMME

3. ENERSTOCK 2021 Oral lectures
40
9 – 11 June 2021, Ljubljana, Slovenia
Prototype tests of grout with PCM were done in a “sandbox” of about 1 m3 at the RISE facilities in
Stockholm. The ground was simulated by water-saturated sand, and a PE-pipe embedded in grout was
installed inside the sand to represent the borehole heat exchanger. A row of sensors recorded the
temperature from the wall of the sandbox through the sand and grout to the wall of the PE-pipe. Different
types of grout with and without PCM were tested for comparison. When heated water circulated in the
pipe, PCM embedded grout shows less temperature increase in comparison with non-PCM grout, moreover,
PCM grouts showed a high temperature gradient. This initial behaviour changed after PCM reached the
melting temperature (29 °C). When PCM absorbed the heat during phase change (solid to liquid), it could no
longer hinder the heat transfer, and the temperature profile approached to that of non-PCM grout, which
implies a thermal storage effect.
In two boreholes of about 12 m depth at the UPV campus borehole heat exchangers grouted with different
types of PCM were installed, both for the lower temperature range in GSHP-applications (<30 °C).
Performance tests were done in a kind of TRT set-up, and first evaluations hint to a thermal effect within
the PCM activity range.
In the RISE facilities in Borås, a borehole of 50 m depth was used to test the grout with PCM for the
application >50 °C. Several heating and cooling tests were carried out with temperatures up to 70 °C. To
achieve this temperature within about one day, a heat injection rate of 210-220 W/m was applied; for
comparison, another run was performed with just 130 W/m of heat injection, keeping the temperature at
40 °C, well below the PCM activity range. Simulations are intended to verify the impact of the PCM.
Deeper evaluation of all tests is ongoing in order to quantify the effects encountered, by using numerical
simulation of thermal behaviour with or without PCM. We will include some results from that ongoing work
in the final presentation.
While the current cost of the PCM would prevent wider application, a thermal effect of short-term heat
storage using PCM dispersed in the grout can be seen, and justifies further work on refinement of the grout
formulas and cost-reduction of the materials.
REFERENCES:
Y Rabin, E, Korin, Incorporation of Phase Change Materials into a Ground Thermal Energy Storage System:
Theoretical Study, J. Energy Resour. Technol. 118(3), 237-241 (1996)
J.F. Urchueguía, B. Sanner, Project GEOCOND: Advanced Materials and Processes to Improve Performance
and Cost Efficiency of Shallow Geothermal Systems and Underground Thermal Energy Storage, Proc. EGC
2019 #228 (2019)
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