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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. 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. |