| Abstract | Razvoj svijesti o utjecaju čovjeka na okoliš doveo je do niza mjera s ciljem povećanja
energetske učinkovitosti, smanjenja emisija CO2 i povećanja udjela energije iz obnovljivih
izvora. Učinkovito rješenje za zadovoljavanje potreba za grijanjem i hlađenjem jesu dizalice
topline povezane s tlom koje iskorištavaju plitku geotermalnu energiju. Iskorištavanje tog
niskotemperaturnog izvora moguće je zbog postojanja toplinskog toka iz središta Zemlje uslijed
zaostale topline formiranja jezgre i radioaktivnog raspada minerala. Potonji proces je
kontinuiran te se stoga geotermalna energija ubraja u obnovljive izvore. Iako postoje različiti
oblici povezivanja dizalice topline s tlom, u ovom radu obrađuje se indirektni sustav s
vertikalnim bušotinskim izmjenjivačem u tlu.
Složene termogeološke uvjete u tlu u literaturi, kao i u stručnoj praksi, uvriježeno je opisivati s
temperaturom toplinski neporemećenog tla, efektivnom toplinskom provodnosti i efektivnim
toplinskim otporom bušotine. Sva tri parametra određuju se ispitivanjem toplinskog odziva tla
na izvedenoj bušotini pri čemu se tlo zagrijava ili hladi poznatim toplinskim tokom kako bi se
dobile temperaturne krivulje medija na ulazu i izlazu iz bušotine. Za obradu toplinskog odziva
tla najčešće se koristi analitički model beskonačnog linijskog izvora, dok se za složenije procese
koriste modeli s konačnim izvorom te numerički ili hibridni numeričko-analitički modeli.
U odnosu na konvencionalno ispitivanje odziva tla u kojem se mjere samo temperature polaza
i povrata iz bušotine, više podataka moguće je prikupiti distribuiranim mjerenjem temperature
po dubini bušotine. Upotrebom većeg broja temperaturnih osjetnika ili jednog kabela s optičkim
vlaknom, moguće je odrediti temperaturni profil neporemećenog tla te praćenjem odziva
pojedinih slojeva odrediti vertikalnu razdiobu svojstava.
Za istu aritmetičku sredinu toplinskih provodnosti nehomogenog tla, redoslijed slojeva utječe
na izlaznu temperaturu medija iz bušotine te u konačnici na efektivna svojstva dobivena iz
ispitivanja toplinskog odziva. Različite konfiguracije heterogenog tla utječu i na razdiobu
toplinskog toka po dubini bušotine za razliku od idealiziranog homogenog tla koje karakterizira
linearna razdioba kao posljedica smanjenja razlike srednje temperature medija i tla s porastom
dubine bušotine. Na temelju pregleda dostupne literature uočen je nedostatak istraživanja o
utjecaju heterogenosti tla na dugoročni rad sustava dizalice topline povezane s tlom te je u tu
svrhu na lokaciji Fakulteta strojarstva i brodogradnje izvedena istražna bušotina opremljena
kabelom s optičkim vlaknom. Na samoj bušotini provedeno je prošireno distribuirano
ispitivanje toplinskog odziva tla s dugotrajnim praćenjem oporavka tla nakon faze grijanja.
Određena je vertikalna razdioba svojstava tla iz faze grijanja i faze oporavka te je izmjeren
temperaturni profil toplinski neporemećenog tla s primjetnim negativnim gradijentom kao
posljedicom utjecaja zgrade fakulteta na okolno tlo. Numeričkim modelom simuliran je
dugogodišnji utjecaj zgrade Fakulteta i uspješno je repliciran negativni temperaturnim
gradijent. Dodatno je razvijen i model za proširivanje spoznaja o toplinskom otporu
izmjenjivača u izvedbi s dvostrukom U cijevi zbog nedostatka analitičkih izraza u literaturi pri
čemu je analiza provedena za slučajeve kada cijevi unutar bušotinskog izmjenjivača topline
nisu simetrično postavljene.
Za potrebe simuliranja utjecaja heterogenog tla na rad dizalice topline odabran je otporničkokapacitivni
pristup modeliranju izmjenjivača topline u tlu. U navedenom modelu domena tla
podijeljena je na n koncentričnih cilindara i m vertikalnih slojeva tla. Svaka ćelija tla u
toplinskom je dodiru sa susjednim ćelijama iznad, ispod te ispred i iza nje. Izuzev ćelija u
zadnjem radijalnom segmentu tla koje su u dodiru s toplinski neporemećenim tlom te u prvom
segmentu gdje je definirana veza tla i bušotinskog izmjenjivača preko temperature zida
bušotine. Unutar bušotine ispuna je podijeljena u dva dijela, središnji i vanjski, a svaka cijev i
fluid u pojedinom presjeku modelirani su zasebno. Ulazni parametar u model bušotine i tla su
protok i temperatura medija kroz izmjenjivač topline, a izlazni temperaturno polje u
promatranom trenutku, odnosno izlazna temperatura iz bušotine. Validacija modela tla
provedena je s beskonačnim linijskim izvorom za dugoročni odziv tla, a model bušotine i tla s
dostupnim rezultatima numeričke simulacije za kratkoročni odziv tla. Model uspješno replicira
izlaznu temperaturu medija iz bušotine izmjerenu tijekom ispitivanja toplinskog odziva tla, a u
sezonskoj simulaciji ta je temperatura ulazni podatak za model dizalice topline. Model dizalice
topline rekonstruiran je na temelju provedenih višednevnih mjerenja sustava u radu te je
uspješno primijenjen za analizu nehomogene razdiobe svojstava tla na sezonsku učinkovitost
sustava u sezoni grijanja i hlađenja za različite temperaturne profile toplinski neporemećenog
tla. |
| Abstract (english) | Increased awareness about the anthropogenic impact on the environment is the basis of efforts
made to increase energy efficiency and environmental conservation in different aspects of
human activity. This applies in particular to the building sector in the European Union, which
is an important contributor in the final indicators such as CO2 emissions and final energy
consumption. The European Energy and Climate Strategy 2030, besides emissions reduction,
places emphasis on the production of energy from renewable sources. One of the most efficient
solutions for meeting the needs for heating and cooling are ground coupled heat pumps that are
classified into renewable energy sources.
Heat pumps do not need high temperature sources as they exploit shallow geothermal energy
resulting from the existence of a heat flux from the center of the Earth due to the residual heat
of core formation and radioactive decomposition of minerals. The latter process is continuous,
therefore geothermal energy is considered to be renewable source. Although different types of
connection of the heat pump to the ground exist, in this thesis vertical borehole heat exchanger
with double U pipe is investigated. The main obstacle to the wider application of this technology
is high investment costs. In order to exploit the shallow geothermal energy for heating and
cooling in efficient and financially competitive way, it is necessary to know the thermal
properties of the ground and to properly size the system of the heat pump coupled to the ground.
Complex thermogeology conditions in the underground and undisturbed ground temperature
usually cannot be modeled directly using analogy for differing geographical locations. It is
common in literature to represent borehole conditions by means of undisturbed ground
temperature, effective thermal conductivity and effective borehole thermal resistance.
The latter being influenced by flow regime, geometrical and physical properties of used
materials. All three parameters are obtained by application of thermal response test (TRT) on
borehole heat exchanger, during which the ground is heated or cooled by prescribed heat flux
while fluid inlet and outlet temperatures from the borehole are monitored. Application of
suitable model results in thermal properties needed for sizing procedure. Commonly, infinite
line source model is used to evaluate the thermal response test data, as long as the test is carried
out in accordance with simplifications introduced by the model. More complex phenomenon’s
are modeled using finite line modifications or by application of numerical models.
Compared to the conventional TRT, which measures only borehole inlet and outlet
temperatures, more data can be collected by distributed temperature measurement along the
borehole depth. Using a larger number of temperature sensors or one fiber optic cable, it is
possible to determine the temperature profile of the undisturbed ground and to determine the
vertical distribution of underground properties by monitoring the response of the individual
layers.
The temperature profile in the upper layers up to a depth of 10-20 m is influenced by
atmospheric conditions and artificial heat flows from the soil surface. At a depth at which the
atmospheric effect disappears, a static temperature constant in time exists and with increasing
depth temperature changes depending on the present geothermal gradient.
It has been shown that the order of the layers of heterogeneous ground profile, having same
arithmetic mean thermal conductivity, influences the leaving temperature of the fluid from the
borehole and consequently effective properties obtained from the thermal response test. The
results of numerical simulations for different ground configurations indicate that the
distribution of properties affects the distribution of the heat flow along the depth of the borehole,
as opposed to the idealized homogeneous ground characterized by linear distribution resulting
from a decrease in the temperature difference between the fluid and the surrounding ground.
Based on a review of the available literature, a lack of research into the impact of ground
heterogeneity on the long-term operation of the ground-coupled heat pump system is found.
For this purpose on the premises of Faculty of Mechanical Engineering and Naval Architecture,
an exploratory double U pipe borehole heat exchanger equipped with an optical fiber cable is
coupled to a heat pump used for the heating and cooling of two classrooms. Borehole heat
exchanger was subjected to 480 h long distributed thermal response test (DTRT) with heating
and recovery phase observed. Vertical distribution of ground thermal properties is obtained for
both phases and conducted measurements were used for the comparison of different fluid
temperature averaging procedures.
Undisturbed ground temperature profile show evidence of negative gradient characteristic to
urban environment with artificial heat flows from the surface. Numerical 2D model was used
to simulate long term effect of faculty building on surrounding ground and temperature profile
was successfully replicated enabling the evaluation of the size of the temperature zone affected
by heat transferred from building to the ground.
The numerical model has also been developed to expand the knowledge about the thermal
resistance of the exchanger in a double U pipe configuration due to the lack of analytical
expressions in the literature that are applicable to a wide range of configurations. Analysis is
extended to cases where the tubes within the borehole are not symmetrically positioned.
For the purposes of influence evaluation of the heterogeneous ground on the operation of the
heat pump system, a resistance-capacity model approach is used for the description of heat
exchanger and ground. In this model, the ground domain is divided into n concentric cylinders
and m vertical layers. The temperature distribution around the periphery of the concentric
cylinders is neglected, so that each cylinder is described with one vertex and a corresponding
thermal capacity. Each cell of the ground is in thermal contact with adjacent cells above, below
and before and behind it. Except for the cells in the last cylinder that are in contact with the
undisturbed ground and in the first cylinder where the connection between the ground and the
borehole is defined with temperature of the borehole wall. Inside the borehole, grouting is
divided in two parts, one inner between pipes and other outer between pipes and borehole wall.
Each tube and fluid inside it is modeled separately, where the heat capacity of the pipe is added
to the grouting and the vertical heat transfer is modelled for the grouting part, but not for the
pipes. Temperature distribution of the fluid in pipe cross section is disregarded as the fluid flow
is modelled as 1D phenomenon and convective resistance is employed as coupling between
fluid and pipe surface. The input parameter for borehole model is mass flow and inlet
temperature whereas model calculates resulting temperature field for observed time step and
outlet temperature. The ground model validation was performed quantitatively with the infinite
line source for ground long-term response and borehole model is validated with available
numerical simulation results for the short-term response. Comparison of the DTRT results
model showed that the model successfully replicates the output temperature from the borehole,
which is the used as input data for the heat pump model. The latter is based on long term
measurements of system in operation and is successfully used to analyze how the system
seasonal efficiency is influenced by different parameters with emphasizes on heterogeneous
ground.
Objective and hypothesis of the research
The application of distributive temperature measurements during the thermal response test of
the ground enables the determination of the vertical distribution of thermal properties and the
actual temperature profile of undisturbed ground. The heterogeneity of the underground has a
visible impact on the distribution of the heat flow along the depth of the well and influences the
exit temperature of the fluid from the borehole heat exchanger. However, the long-term impact
of the heterogeneous ground in relation to the idealized homogeneous ground on the operation
of heat pump system has not been investigated in the available literature. Therefore, objective
of the research is the experimental investigation of the geothermal heat pump system in
operation, as well as the detailed simulation of the interaction of the heat exchanger and the
surrounding ground.
Hypothesis of the research are:
- the development of the numerical model of the borehole heat exchanger in ground allows
estimating the influence of individual factors on the efficiency of the exchanger, the analysis of
non-stationary changes, and validation with the measurements of the temperature distribution
in the borehole
- system simulation and comparison with measured results allows quantifying the effect of
ground heterogeneity on the seasonal efficiency of the geothermal heat pump and evaluation of
the heat exchange between the conditioned area and the underground as a heat sink or source
- developed model enables better understanding of ground changes during system operation
Scientific contribution
The scientific contribution of the research includes the modeling of interaction between the heat
pump and the heat exchanger in heterogeneous ground, and the analysis of the influence of the
vertical distribution of thermal properties on the performance of the heat exchanger and the
resulting temperature fields during the operation of the system.
Expanding the knowledge of the heat resistance of the borehole heat exchanger in a double U
pipe configuration for which there are no analytical expressions applicable to a wide range of
layouts as for the configuration of a single U pipe. By modeling the segment of the borehole, a
possible range of thermal resistance in the case of asymmetric position of the tube inside the
borehole is shown.
Analysis of the temperature profile of the thermal undisturbed ground with the aim of
simplifying the determination of underground temperature based on limited available data and
analyzing the temporal and spatial zone of the impact of the building on the temperature field
in the underground.
Conclusion
The vertical distribution of thermal conductivity and thermal resistance was determined on the
basis of the heating and the recovery phase, where it was observed that the prolongation of the
recovery phase results in higher effective values of thermal conductivity and thermal resistance.
Comparing the results from different stages of the DTRT, the effective properties determined
at the earlier recovery stage best replicate the temperature curve for the overall measurement,
and in relation to the heating phase, the specified conductivity is greater by 5 % and the
resistance by 13 %. Comparison of different methods of averaging of fluid temperature showed
that the arithmetic mean of the input and output results in higher effective properties than the
actual profile, but without the distributed temperature measurement, the actual temperature
profile cannot be accurately determined.
The temperature profile of the thermal undisturbed ground showed a deviation from the
geothermal gradient in the higher layers of ground, which is the result of years of interaction
between the faculty building and the surrounding ground. The local minimum temperature was
recorded about 42 meters below which the effects from the soil surface disappear and the
temperature profile with depth changes according to the measured geothermal gradient of 0,037⁰C/m. The extrapolation of the geothermal gradient to the surface results in a temperature
similar to the mean annual air temperature, while the development of the temperature profile
with the analytical model replicates the temperature profile only if the ground temperatures are
known. Along with the assumed heat flow of 2.35 W m-2, the temperature field in the ground
around the faculty building was simulated, and the underground temperature profile was
replicated for a simulation duration of 40 to 50 years. Although the thermal disturbance does
not spread in space at significant distances, in case of interference of the two parts of the
building a local temperature increase of about 3 ⁰C is found.
The developed model was used to analyze the effect of underground heterogeneity on the
efficiency of the system and showed that in addition to the sequence of ground layers, the effect
on efficiency also has existing ground temperature profile which, depending on the depth of the
borehole and the geothermal gradient. Different ground profiles have an impact on the
efficiency of the system, but not to the extent that optimizing the field of the boreholes based
on ground layers will achieve noticeable difference in seasonal efficiency. |
