Numerical modeling of heat transport for ground-coupled heat pump (GCHP) systems and associated life cycle assessments
Başlık çevirisi mevcut değil.
- Tez No: 402762
- Danışmanlar: PROF. JAMES TINJUM, PROF. CHRISTOPHER CHOI
- Tez Türü: Doktora
- Konular: Jeoloji Mühendisliği, Çevre Mühendisliği, İnşaat Mühendisliği, Geological Engineering, Environmental Engineering, Civil Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 2016
- Dil: İngilizce
- Üniversite: University of Wisconsin-Madison
- Enstitü: Yurtdışı Enstitü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 208
Özet
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Özet (Çeviri)
Space heating and cooling accounts for 48% of the total energy consumed worldwide. As part of the growing effort to mitigate the negative effects of greenhouse gases, increasing numbers of commercial enterprises and home owners are turning to ground-coupled heat pump (GCHP) systems as a more efficient and economical alternative. Understanding the heat transfer process between ground loop exchanger and surrounding ground that GCHP systems use and the sustainable utilization of a specific type of ground formation are crucial to any efforts made to optimize the efficiency of heat extraction and/or injection. Historically, studies of GCHPs have focused on the mechanical aspects of the system, but few have systematically accounted for the geological conditions with respect to questions of thermal sustainability and efficiency. Consequently, a significant majority of the current systems assume that both the temperature averages and the thermal conductivity of the surrounding ground remains constant over the entire vertical length of the borehole and throughout the operational life-span of the GCHP system. These assumptions neglect the variations in thermo-physical properties that actually may occur in the subsurface geology, variations that may well play a crucial role in the heat transfer process in the ground. Importantly, the ground temperature can decrease or increase depending on the respective lengths and energy balance of the heating and cooling cycles. Over a period of many years, this fluctuation can cause thermal imbalances, such as overheating due to a high amount of heat injected into the ground during a period when the system is being used to cool the interior of a building or home. Such an imbalance is especially likely to develop when the system relies on a large number of boreholes to cool sizeable commercial buildings.In this study, a series of numerical models have been developed to evaluate the effects of a GCHP system on the thermal conditions of the ground into which the system's borehole(s) have been drilled. A three-dimensional (3D), time-dependent heat conduction process of a 300 m single borehole embedded in ground with varying lithologies was simulated using a finite-element software, ABAQUS. The monthly average surface temperature change and the natural geothermal gradients were considered as boundary conditions. More complex 3D, time-dependent, fluid-coupled models including all borehole components (e.g., fluid, pipe, and grout) were simulated using a finite-volume software, ANSYS computational fluid dynamic (CFD). This model was validated with previous experimental and analytical studies done by Erol and François (2014). Due to the increased computational burden of a 3D model, a 2D model equivalent to the 3D model of one meter depth and with adiabatic upper and lower surfaces was developed. The accuracy of enhanced 2D modeling was compared with 3D modeling results and the results have been validated with data collected from an actual GCHP system in Grand Marsh, WI. The long-term ground thermal response is important for the design of district-scale GHXs and the sustainability of the GCHP's performance over time. To analyze the sustainability of a current borefield under operation in Wisconsin, 2D, non-linear, district-scale borefield was simulated using pre-designed heating and cooling loads (cooling-dominate). This model considers hydrogeological conditions (e.g., groundwater flow, porosity) to assess the advective heat transfer process, as well as the heat conduction occurring in their transmissive aquifer. This model was coupled with enhanced 2D model to simulate potential future mitigation strategies to resolve overheating problems, which often occur in cooling-dominant commercial operations. Finally, to predict the CO2 emissions from different GCHP systems (vertical, horizontal, and unconventionally deep single GHXs), a comprehensive“cradle-to-grave”life cycle analysis (LCA), which is implemented using SimaPro was conducted. Simulated results were compared with a conventional natural gas air conditioning unit for a case study in Wisconsin. The results show that a single unconventionally deep (300 m) GHX system emits 272 metric ton CO2 equivalent emission over its 25-year operation phase lifetime, which is 29% less CO2 emissions than a conventional natural gas air conditioning system. For heat-pump systems, the highest emission contribution comes from the electricity used to operate it (93.3%). Therefore, CO2 emissions could be decreased by reducing the use of fossil fuels, resulting in a cleaner grid. Our sensitivity analyses revealed that use of renewable energy sources at 50% could reduce CO2 emissions of the GHX in our case study by 68%. In summary, step by step a numerical model were developed for the best representative and computational affective model in terms of capability of simulating a dynamic thermal conditions that results in interaction between ground and ground heat exchangers inside the borehole as well as the surrounding ground. Each modeling step was validated according to their boundary conditions and complexity. Ultimate model, improved 2D model single geo-exchange model, aimed to account for geological and hydrogeological conditions as well as the building thermal conditions due to heating and cooling loads being applied by circulating fluid. The study's results show that, absent of appropriate mitigation strategies, overheating problems in the ground may occur even in the first few years of cooling-dominate GCHP operation due to building heating and cooling load imbalance. To balance the energy inputs/outputs to the ground, an operating scheme utilizing cold-water circulation during the off-season was evaluated. Under the simulated conditions, an energy imbalance was fully recovered, and thus the proposed mitigation strategy would seem a viable way to sustain the operating scheme of district-scale borefields in general. The positive impact, up to 50% less temperature rise in the ground was found by accounting groundwater flow at velocity of 1 m/d.
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