درباره استفاده از اصل لوله حرارتی برای بهره برداری از منابع زمین گرمایی با حرارت متوسط و ​​کم

In the direct use of geothermal energy without fluid extraction, heat transfer takes place with no alteration of the natural hydrogeological balance of the basin. An interesting solution both for thermal energy and power production could be the application of heat pipe principle, in particular the Closed Loop Two Phase Thermosyphons (CLTPT). In shallow or not much deep geothermal reservoirs with temperature below 100 °C, the two phase closed loop thermosyphon can transfer heat very efficiently. In this case the most important task is the enhancement of the heat transfer mechanism between the heat exchanger and the aquifer. In the first part of the paper a review of particular applications connected to the geothermal heat pipe applications is proposed, then an analysis of the main technical elements regarding the geothermal aquifer exploitation through two phase thermosyphon systems are given. Some guidelines for power systems sizing are discussed and a proposal for the design of a single borehole heat extraction system with binary cycle utilization is provided.

The utilization plants for low and medium temperature (70–100 °C) geothermal reservoirs based on geofluid extraction, like ORC plants have some inconveniences. The mass withdrawal from the aquifer can alter the natural balance of the basin, while over-exploitation causes temperature and pressure reduction during the lifetime of the plant [1]. The use of extraction pumps for the circulation of corrosive or chemically aggressive geothermal fluids leads to high installation and operation costs and short machinery useful life. Moreover a second well is usually necessary for reinjection, due to evident technical and environmental reasons. In the exploitation of the great geothermal reservoirs, in particular for power purposes, other important issues have to be considered, like for example micro-seismicity, waste water treatment and in a more significant way scaling, chemical deposition phenomena and corrosion [2]. These problems can be avoided using devices that only allow heat transfer with the aquifer, basing on the concept of the Single Borehole Extraction System (SBES), proposed in 1986 by Lockett [3]. For this application a secondary fluid (e.g. water or a low boiling point organic fluid) is needed and a downhole heat exchanger (DHE) is necessary. A DHE consists of a coil or a U-tube located in a well, in which the working (or secondary) fluid circulates (by natural convection). The use of DHE has been discussed extensively in literature both numerically and experimentally in a period of over 30 years. Some papers well summarize the various activities and problems related as for example Lund [4] and Carotenuto et al. [5]. Several types of downhole heat exchangers have been proposed in order to extract heat directly from shallow geothermal aquifers or ground: thermosyphon type heat pipe, concentric tube thermosyphon, downhole coaxial heat exchanger (DCHE), U-tube downhole heat exchanger, as well as others. The main disadvantages of DHE systems respect to fluid extraction systems deal with the absence of heat flow induced into the aquifer by fluid extraction. The heat flow that can be extracted from the well is then tied only to the natural convection that occurs in the aquifer-well system. In case of low aquifer permeability, the convective transport can be particularly weak and the heat extraction can only reach limited values. However, even in favorable conditions, that is for permeability greater than about 10−4 m/s or 10−3 m/s, DHEs are suitable systems for moderate temperature geothermal applications (typically above 70 °C). An alternative and more advantageous pathway with respect to DHE is the application of the heat pipe concept, both in the basic version and in the Closed Loop Two Phase Thermosyphon (CLTPT) configuration [6] and [7]. The principle of Geothermal heat pipe has been proposed in applications with some renewable energy sources and utilization of deep underground heat sources. A particular case of Closed Loop Thermosyphon, named geothermal convector (GTC), for the heat extraction from geothermal aquifers was proposed and tested by Carotenuto et al. [8]. Rieberer [9] has carried out theoretical and experimental studies on ground-coupled heat pumps using carbon dioxide based thermosyphons. The utilization of the thermosyphon concept for geothermal heat extraction avoids the need of a downhole pump. Downhole pumps have two different drawbacks: firstly, they have relatively short lifetimes as they operate in harsh conditions, and secondly it is less thermodynamically efficient to pump and pressurize the fluid and then exchange heat on the surface. Compared with the conventional systems based on the extraction of water, CLTPT permits to exchange heat at a constant temperature and it has also other advantages: - it can be used even in dry geothermal areas (or characterized by very low fluid circulation); - a loop type heat pipe can control the heat transfer rate by controlling the flow rate of the returning working liquid. The use of heat pipe technology has also been proposed also for geothermal power production since the early 1990s [10], [11] and [12]. An interest to the utilization of two phase thermosyphon is demonstrated in connection with development of Enhanced Geothermal Systems (EGS) for power purposes by Ziapour et al. [13] and Wang et al. [14]. Recently the use of Thermosyphon Loops has been proposed for heat extraction from the ground and for domestic space heating [15] and different applications. The advantage of heat pipe, using carbon dioxide as working fluid in conjunction with a ground source heat pump, is shown also by Ochsner in Ref. [16] and its use is prospected for moderate temperature aquifer with the working fluid in transcritical conditions (the critical temperature of CO2 is only 31.1 °C). Notwithstanding the scientific interest of the concept, commercial systems are not available today even if a meaningful research work has been carried out. Moreover the organic fluids that were used in most of the application are refrigerant (like R11), that are no longer acceptable, since they are banned. In the present paper an analysis on the possible use of the CLTPT principle for the geothermal heat extraction from resources at temperatures below 100 °C is analyzed both in case of direct use and in case of power production from a binary cycle plant system based on CLTPT operating principle, with the aim of a complete knowledge for the development of such kind of system. The paper is structured as follows: the next section describes the general aspects of the use of geothermal energy without liquid extraction, while Section 3 presents an analysis of geothermal energy systems that were experimentally investigated in the past both theoretically and experimentally. In Section 4 the approach used for the definition of the potential of an aquifer is presented. In Section 5 the use of a CLTPT system for power production (through a single borehole ORC plant) is illustrated. Section 6 reports a brief discussion, while conclusions are drawn in the last section.

The use of heat pipe principle (in particular in the CLTPT version) can be an interesting pathway for geothermal energy extraction in case of shallow resources (depths below 150 m and temperatures lower than 100 °C). A real advantage consists in the direct solution to resource depletion and reinjection problems. The concept can be used both for direct heat uses, like in geothermal convector (GTC) Systems and for power production. The idea has been critically reviewed starting from the analysis of various applications. The analysis reveals a reduced number of installations and utilization schemes available in literature. Technological issues mainly deal with the sizing of the system due to heat exchange problems in the evaporator section and reduced efficiency due to the combination of heat and mass transfer. This requires a very careful sizing of the loop thermosyphon and it is characterized by a difficult standardization. In particular the analysis of data from some real cases shows meaningful problems for the utilization of Heat Pipe Turbine systems, characterized by very low efficiency values (below 0.01). The development of CLTPT appears to be very interesting not only for direct heat uses, but particularly in connection with the development of small (single well) power systems based on the use of the two pressure levels concept. In this case, introducing a pump at the ground level, it is possible to produce sensible efficiency increases reaching a theoretical value of 7% for small systems of 1–2 kW peak power extracting heat from shallow aquifers at 70–80 °C. Technological issues mainly concerning the optimum design of CLTPT systems should be further investigated for practical system implementation: in the present paper only a preliminary thermodynamic evaluation of the total power output is estimated. In the next step of the work the authors will refine the preliminary analysis about these devices with particular reference to the single borehole binary plant, investigating the impacts of several parameters on the overall system performance. In particular it will be important to experimentally investigate the effect of the subcooling on the operation of a CLTPT.