使用Turgo渦輪與同軸閉迴路地熱取熱系統之全流式地熱發電廠創新設計

Abstract

本研究提出了一種全流式地熱發電廠的創新設計，並對其進行相關理論、實驗及數值分析，最後於宜蘭清水進行現地試驗。本設計的創新在於地底取熱及發電機組兩個子系統。其中於地底取熱子系統的設計，本研究使用了同軸雙套管於地底形成流體的閉迴路進行取熱；而於發電機組子系統中，本研究設計使用了Turgo渦輪與超音速二相流噴嘴以進行全流式發電。 地底取熱系統的設計利用了水通過同軸的雙套管於地底進行閉迴路取熱。當液態水由同軸雙套管間的環形空間流往地底，液態水會被熱岩層加熱及加壓。在流至同軸雙套管尾端時，高壓的過冷液態水以閉迴路方式流向內管，再沿內管流回地表供發電使用。本研究使用了自行開發的半經驗數值方法進行同軸閉迴路地底取熱系統的數值分析。由於流體熱對流的經驗式以隱式方式與岩層固體熱傳導方程式耦合，得以避免進行同軸閉迴路套管中複雜流體運動的模擬。因此，此半經驗數值方法能有效率地計算同軸閉迴路套管中流體與岩石圈中乾熱岩岩層中岩石固體的流固耦合熱傳問題。本研究模擬了在不同井深和不同流體質量流率下，於宜蘭清水地區以此系統取熱的熱傳。首先，此數值方法以宜蘭大學進行的現地試驗進行驗證。模擬所得在不同流體質量流率下2000公尺深井的套管出口流體溫度與現地試驗結果十分吻合。再來，進行在不同質量流率下，經過連續20年的取熱模擬。模擬顯示不管質量流率為何，5000公尺深井取熱影響區域的最大半徑均約為100公尺。流體質量流率越大，取熱速度越大且井壁溫度越低。此方法可於極短的計算時間內計算出井壁溫度，與國家高速計算中心使用Ansys CFX的三維模擬結果亦十分吻合。 1978年美國勞倫斯實驗室(Lawrence Livermore Laboratory)以理論預測全流式地熱發電的發電效能可高過主流的閃發式地熱發電。本研究為了更多地利用高壓井水的熱焓並且避免於渦輪葉片表面發生孔蝕現象，發電機組的設計使用了Turgo渦輪與漸縮漸擴噴嘴進行全流式發電。當高壓的過冷液體通過漸縮漸擴噴嘴後會形成高速的閃發噴射流，並以斜角度衝擊Turgo渦輪的渦輪葉片來驅動Turgo渦輪。此新穎設計可在不使用任何汽水分離器或熱交換器下，直接轉換來自高壓井水的地熱能，也因此設計出的系統簡單且易於維護。由簡單的控制體積和速度三角分析可得知，噴嘴出口射流速度及射流衝擊葉片的角度決定了系統的熱效率和發電量，且系統的最高功率發生在渦輪葉片的尖端速度約為噴嘴出口射流速度的一半時。為了驗證理論分析的結果，本研究建造了全流式地熱發電機的原型並於2015至2020間在宜蘭清水地熱九號井進行現地試驗。長達四年的現地試驗顯示此設計成功避免孔蝕問題的發生，並且全流式發電機原型的發電效能與勞倫斯實驗室的理論曲線相當吻合。與傳統的有機朗肯循環發電機相比，此新穎設計的發電效能在熱源為中低焓值熱庫(熱庫溫度≥150℃)時較有競爭力。 結合同軸閉迴路地底取熱系統與全流式斜衝擊型發電機而成的全流式地熱發電廠，是有可長時間發電、簡單易於維護、適用熱源溫度範圍廣等優點的穩健系統。因此，此新式全流式地熱發電廠，不管對地熱能源的學界或業界來說，都是有發展性、極具潛力的應用。

In this study, we propose a novel design of total flow geothermal power plant. This design is analyzed theoretically, experimentally and numerically, and field-tested in Qingshui, Yi-Lan. The innovation of this design is in two subsystems: the underground heat extraction system and the geothermal power generator. In the underground heat extraction system, fluid flows in the co-axial tubes to form a closed loop underground for heat extraction; in the geothermal power generator, our design uses Turgo turbine and supersonic two-phase nozzles for total-flow power generation. The design of the heat extraction system utilizes co-axial tubes for underground closed-loop heat extraction. As the liquid water flows in the annulus of the co-axial tubes to the underground, it is heated and pressurized by the hot rock layers. At the end of the co-axial tubes, the high-pressure compressed liquid water flows toward the inner tube in a closed-loop manner and then flows toward the ground surface along the inner tube for power generation. We numerically investigate the co-axial closed-loop heat extraction system using an in-house semi-empirical numerical method. Empirical equations for convective heat transfer of the fluid are coupled implicitly with heat conduction equation of the solid rock layers to avoid detail simulation of the complex fluid motion in the co-axial closed-loop tubes. Therefore, this semi-empirical method is able to efficiently solve the heat transfer problem involving flow-structure interaction between the fluid flow in the co-axial closed-loop tubes and the solid hot-dry-rock layers in the lithosphere. We simulate the heat transfers of the system in Qingshui, Yi-Lan with various well depths and various fluid mass flow rates. First, the method is validated by the field-tests performed by Yi-Lan University. The output fluid temperatures for a 2100m-depth well at various fluid mass flow rates obtained from simulations show well agreement with the field tests. Second, continuously 20-year heat extractions at various fluid mass flow rates are simulated. Simulations show that regardless of the fluid mass flow rates, the temperature-drop area affected by a 5000m-depth well has a maximum radius of about 100 m. The larger fluid mass flow rate, the greater heat extraction rate and the lower well surface temperature. The well surface temperature simulated by the in-house code in a competitively short computational time is also well agreed with 3D simulation performed by National Center for High-Performance Computing (NCHC) using Ansys CFX. In 1978, Lawrence Livermore Laboratory in the United States theoretically predicted that total-flow generators have a better performance than main-stream flash generators. In order to maximize the enthalpy utility of high-pressure compressed liquid from the well and to prevent the onset of cavitation on the surfaces of turbine blades, the design of power generator uses Turgo turbine and converging-diverging (CD) nozzles for total-flow power generation. The high-speed flashing jets are formed by high-pressure compressed liquid from the well through the CD nozzles and impinge turbine blades obliquely to drive the Turgo turbine. This novel design is able to convert geothermal power directly from the geothermal fluid without implementing any phase separator or heat exchanger, which results in a simple and easy-to-maintain system. From a simple control-volume analysis, the thermal efficiency and the power generation of the system are determined by the jet speed at the nozzle exit and the impact angle between the flashing jet and the blade surface, and the maximum power generation occurs when the tip speed of Turgo turbine blade is about half of the jet speed at the nozzle exit. In order to validate the results from the theoretical analysis, a prototype of the total-flow geothermal power generator is built and field-tested at the No.9 Qingshui Geothermal Well in Yi-Lan from 2015 to 2020. The four-year-long field testing shows that the design successfully prevents cavitation on blade surfaces and the performance of the prototype agrees well with the theoretical curve predicted by the Lawrence Laboratory. Compared with traditional Organic Rankine Cycle (ORC) generators, the new design has a competitive geothermal efficiency when it operates at moderate reservoir enthalpy (reservoir temperature≥150℃). The total-flow geothermal power plant that combines the co-axial closed-loop heat extraction system and the total-flow power generator is a robust system that is capable of long-time geothermal power generation, simple, low-cost, easy-to-maintain, and adaptable for a wide range of reservoir temperatures. Therefore, for both academy and industry in geothermal energy, this novel total-flow geothermal power plant is an intriguing and promising application with potentials.