Multiple integrated applications for low-to medium-temperature geothermal resources in Iceland

There is an increasing global demand for a faster, more expansive development in the energy sector, in order to improve the standard of living of the world's population by the creation of more jobs and better living conditions. The public is, however, well aware of the damage that has been done to the environment, in the form of deforestation, despoiling of lakes and rivers and, in particular, greenhouse effects, and it is unwilling to further sacrifice its natural environment. This decision puts pressure on scientists, engineers and developers to find ways and means of attaining “sustainable energy development”. In other words, the challenge now is to achieve the sustainable development of alternative renewable energy resources. Sustainability may be achieved in a number of ways, but the one most likely to result in a rapid increase in energy output without a deleterious impact on the environment is the revamping and integration of what we already have. This paper attempts to address sustainability as it applies to geothermal energy. We describe the concept of a multiple integrated use of geothermal energy, including the tenable benefits that can be obtained from applying this concept, such as a longer reservoir lifespan, a lower specific environmental impact, and greater marketing flexibility and profitability. The paper also emphasises the importance of achieving a maximum effective temperature drop across the application, commensurate with a minimum flow rate, optimal pumping characteristics and minimal fluid extraction from the geothermal reservoir. In geothermal house heating systems this means using large and effective radiators, dual-pipe heating systems, and thermostatic controls on each radiator. Where modifications to existing house heating systems are not feasible, e.g. by conversion from a single-pipe to a dual-pipe system or installation of larger radiators, an alternative solution is to adopt a cascaded flow of the geothermal fluid through a combination of heating systems operating at different temperature levels. For economic reasons it is always better to use the geothermal water directly if its chemical quality permits us to do so, otherwise heat exchangers made of resistant materials will be needed to isolate the geothermal fluid from the heating fluid in order to avoid corrosion or scaling in the pipes and radiators. The heat exchangers should be designed in such a way as to obtain a maximum temperature drop of the geothermal fluid. The paper also describes some heating system configurations, the characteristics of geothermal heating systems and their automatic control systems, as well as recommended geothermal field management and monitoring systems. The paper also includes a few examples of existing projects to demonstrate what has already been achieved and what could be done in the future; some suggestions are also made for new developments and innovations to make geothermal energy more generally attractive and useful worldwide.     

Thermal analysis of indirect geothermal district heating systems

Low- and moderate-temperature geothermal resources have been discovered in many areas of the world, and are being used increasingly for district heating. Due to the corrosive action of some geothermal waters, heat exchangers are used to avoid circulating the geothermal fluid directly through the district heating systems, in what are called Indirect Geothermal District Heating Systems (IGDHS). In this case, the geothermal water acts as a heat source directly heating the network fluid through a heat exchanger. However, it is different from that of conventional systems in which hot water from a fossil fuel boiler is used directly. In the former (IGDHS), the geothermal water is regarded as a heat source with constant temperature, and in the latter the boiler is considered a heat source with variable heat flux. This paper presents a thermal analysis of a simple IGDHS, and discusses the selection of heat exchangers and optimum operating conditions.     

Theoretical study on the performance of an integrated ground-source heat pump system in a whole year

Being environmental friendly and with the potential of energy-efficiency, ground-source heat pump (GSHP) systems are widely used. However, in southern China, there exists large difference between cooling load in summer and heating load in winter. Thus the increase of soil temperature gradually year-by-year will decrease the COP of the GSHP system. In this paper, the configuration of a vertical dual-function geothermal heat exchanger (GHE) used in an integrated soil cold storage and ground-source heat pump (ISCS&GSHP) system, which charged cold energy to the soil at night and produced chilled water at daytime in summer, and supplied hot water for heating in winter, is presented. This is then followed by reporting the development of the mathematical model for the GHE considering the impact of the coupled heat conduction and groundwater advection on the heat transfer between the GHE and its surrounding soil. The GHE model developed was then integrated with a water-source heat pump and a building energy simulation program together for a whole ISCS&GSHP system. Then the operation performance of the ISCS&GSHP system used for a demonstration building is studied. These simulation results indicated the system transferred 71.505% of the original power consumption at daytime to that at nighttime for the demonstration building. And the net energy exchange in the soil after one-year operation was only 2.28% of the total cold energy charged. Thus we can see the feasibility of the ISCS&GSHP system technically.     

贵州岩溶区一酒店地源热泵设计与制热运行分析

作为一种以浅层地热能为冷热源的供暖制冷技术,地源热泵是国家大力推广的节能环保中央空调系统。简要论述了贵州省遵义市一酒店地源热泵系统设计方案,测试分析并评估了地源热泵系统的冬季制热运行效果,从而提出系统的优化设计和运行建议。     

Parametric sensitivity study of operating and design variables in wellbore heat exchangers

A numerical study was conducted to evaluate the potential for using Wellbore Heat Exchangers (WBHX) to extract heat for use in electricity generation. Variables studied included operational parameters such as wellbore geometries, working fluid properties, circulation rates, and regional properties including basal heat flux and formation rock type. Energy extraction is strongly affected by fluid residence time, heat transfer contact area, and formation thermal properties. Water appears to be the most appropriate working fluid. The effects of tubing properties and casing lengths are of second-order.On the basis of a sensitivity study, a Best Case model was simulated, and results compared against the geothermal fluid requirements of existing power generation plants that use low-temperature geothermal fluids. Even assuming ideal work conversion to electricity, a WBHX cannot supply sufficient energy to generate 200 kWe at the onset of pseudo-steady-state (PSS) conditions. Using realistic conversion efficiencies it is unlikely that the system would be able to generate 50 kWe at the onset of PSS.     

Parametric Study of the Effect of Reference State on Energy and Exergy Efficiencies of Geothermal District Heating Systems (GDHSs): An Application of the Salihli GDHS in Turkey

A parametric study of the effect of reference state on the energy and exergy efficiencies of geothermal district heating systems is presented. In this regard, the work consists of two parts: a modeling study covering energy and exergy analysis and a case study covering the actual system data taken from the Salihli Geothermal District Heating System (SGDHS) in Manisa, Turkey. General energy and exergy analysis of the geothermal district heating systems is introduced along with some thermodynamic performance evaluation parameters. This analysis is then applied to the SGDHS using actual thermodynamic data for its performance evaluation in terms of energy and exergy efficiencies. In addition, a parametric study on the effect of varying dead state properties on the energy and exergy efficiencies of the system that has been conducted to find optimum performance and operating conditions is explained. Two parametric expressions of energy and exergy efficiencies were developed as a function of the reference temperature. Both energy and exergy flow diagrams illustrate and compare results under different conditions. It has been observed that the exergy destructions in the system particularly take place as the exergy of the fluid lost in the heat exchanger, the natural direct discharge of the system (pipeline losses), and the pumps, which account for 31.17%, 8.98%, and 4.27% of the total exergy input to the SGDHS, respectively. For the actual system that is presented, the system energy and exergy efficiencies vary between 0.53 and 0.73 and 0.58 and 0.59, respectively.     

Is it only CO2 that matters? A life cycle perspective on shallow geothermal systems

Shallow geothermal systems such as open and closed geothermal heat pump (GHP) systems are considered to be an efficient and renewable energy technology for cooling and heating of buildings and other facilities. The numbers of installed ground source heat pump (GSHP) systems, for example, is continuously increasing worldwide. The objective of the current study is not only to discuss the net energy consumption and greenhouse gas (GHG) emissions or savings by GHP operation, but also to fully examine environmental burdens and benefits related to applications of such shallow geothermal systems by employing a state-of the-art life cycle assessment (LCA). The latter enables us to assess the entire energy flows and resources use for any product or service that is involved in the life cycle of such a technology. The applied life cycle impact assessment methodology (ReCiPe 2008) shows the relative contributions of resources depletion (34%), human health (43%) and ecosystem quality (23%) of such GSHP systems to the overall environmental damage. Climate change, as one impact category among 18 others, contributes 55.4% to the total environmental impacts. The life cycle impact assessment also demonstrates that the supplied electricity for the operation of the heat pump is the primary contributor to the environmental impact of GSHP systems, followed by the heat pump refrigerant, production of the heat pump, transport, heat carrier liquid, borehole and borehole heat exchanger (BHE). GHG emissions related to the use of such GSHP systems are carefully reviewed; an average of 63 t CO₂ equivalent emissions is calculated for a life cycle of 20 years using the Continental European electricity mix with 0.599 kg CO₂ eq/kWh. However, resulting CO₂ eq savings for Europe, which are between ?31% and 88% in comparison to conventional heating systems such as oil fired boilers and gas furnaces, largely depend on the primary resource of the supplied electricity for the heat pump, the climatic conditions and the inclusion of passive cooling capabilities. Factors such as degradation of coefficient of performance, as well as total leakage of the heat carrier fluid into the soil and aquifer are also carefully assessed, but show only minor environmental impacts.     

公共建筑储热式地热供暖系统多目标协同优化

储热技术是解决办公建筑地热供暖系统供需难以良好匹配及提高地热能利用率的有效手段之一,然而储热装置的引入将增加供暖系统的投资与维护成本,在一定程度上使系统发展受限。以位于河北省沧州地区的某办公建筑为研究对象,构建了储热式地热供暖系统模型,以综合成本、地热能利用率与碳排放量为优化目标,对系统设备选型及运行策略开展协同优化设计。研究表明,与基准系统相比,增设储热水箱可以明显改善地热供暖系统的性能;合理地控制储热水箱储、放热与热泵机组运行是降低系统成本与碳排放量、提高地热能利用率的关键。在此基础上,确定了储热式地热供暖系统的最优运行策略以及对应的设备选型优化参数。最优运行策略下相较于基准系统综合成本降低30.24%,日均地热能利用率提高11.12%,碳排放量减少46.65%。     

Optimum Design and Running of PHEs in Geothermal District Heating

Plate heat exchangers and terminal heat radiators usually take a major initial capital investment in a low-temperature geothermal heating project. In this article, a method of arriving at an optimum design of the plate heat exchanger and the corresponding terminal heat radiators in a geothermal heating system is proposed. A comparison is made between the results of the optimized design and those by the conventional design. The effects of the configuration and some parameters of plate heat exchangers on the design results are discussed. Emphasis is also given to how to obtain the best operation status while a geothermal well is producing at constant flow rate and wellhead temperature.     

Regenerative vapor cycle with isobutane as working fluid

One of the methods of generating geothermal power is to use a suitable working fluid which extracts heat from geothermal fluids and generates power in a closed cycle. This paper presents a discussion of an improvement of the basic closed cycle with isobutane as a working fluid. A regenerative heat exchanger is added for heating the cold condensate of isobutane with the highly superheated exhaust. The addition of this heat exchanger can result in a significant reduction in the size of heat rejection equipment. Furthermore, the waste brine of the improved system is at such a high temperature that the waste heat can be economically utilized for desalting water for industrial uses.     

Geothermal energy in Turkey: the sustainable future

Turkey is an energy importing nation with more than half of our energy requirements met by imported fuels. Air pollution is becoming a significant environmental concern in the country. In this regard, geothermal energy and other renewable energy sources are becoming attractive solution for clean and sustainable energy future for Turkey. Turkey is the seventh richest country in the world in geothermal energy potential. The main uses of geothermal energy are space heating and domestic hot water supply, greenhouse heating, industrial processes, heat pumps and electricity generation. The district heating system applications started with large-scale, city-based geothermal district heating systems in Turkey, whereas the geothermal district heating centre and distribution networks have been designed according to the geothermal district heating system (GDHS) parameters. This constitutes an important advantage of GDHS investments in the country in terms of the technical and economical aspects. In Turkey, approximately 61,000 residences are currently heated by geothermal fluids. A total of 665 MW_t is utilized for space heating of residential, public and private property, and 565,000 m² of greenhouses. The proven geothermal heat capacity, according to data from existing geothermal wells and natural discharges, is 3132 MWt. Present applications have shown that geothermal energy is clean and much cheaper compared to the other fossil and renewable energy sources for Turkey.     

A study on the optimized control strategies of geothermal heat pump system and absorption chiller‐heater

The world is becoming increasingly interested in renewable energy including geothermal energy. The utilization of geothermal systems is currently low because geothermal systems and existing source systems are used independently, but the supply rate of a geothermal system is increasing. Therefore, suggesting efficient operation plans and evaluations of the energy consumption and efficiency of a geothermal system is needed. This paper reports the results of a field study and survey of the present applications and operation conditions of a geothermal system. In addition, this paper proposes an efficient operation strategy for a geothermal system and compares this operation strategy with an existing operation strategy through simulation. The problems of existing operation condition were found out through a field study, and alternatives were proposed. The improvements were evaluated using the transient systems simulation program. And it would be possible for the reduction of the energy consumption through the comparative analysis of equipment efficiency and energy consumption. The result of analyzing the proposed combination header method through simulations compared with existing operation conditions can increase the use of geothermal systems, but the combined cooling and hot water of a geothermal heat pump and existing thermal source system reduced the efficiency of the heat pump. As a result of simulation on individual load‐sharing method, efficiency of geothermal system is increasing compared with the combination header method. This method was especially made to separate geothermal system's water loop and existing thermal source system's water loop. Copyright © 2013 John Wiley & Sons, Ltd.     

Geothermal space heating

The use of geothermal resources for space heating dominates the direct use industry, with approximately 37% of all direct use development. Of this, 75% is provided by district heating systems. In fact, the earliest known commercial use of geothermal energy was in Chaudes-Aigues Cantal, France, where a district heating system was built in the 14th century. Today, geothermal district space heating projects can be found in 12 countries and provide some 44,772 TJ of energy yearly. Although temperatures in excess of 50 °C are generally required, resources as low as 40 °C can be used in certain circumstances, and, if geothermal heat pumps are included, space heating can be a viable alternative to other forms of heating at temperatures well below 10 °C.     

Thermodynamic analysis of a hybrid geothermal heat pump system

A thermodynamic analysis of a hybrid geothermal heat pump system is carried out. Mass, energy, and exergy balances are applied to the system, which has a cooling tower as a heat rejection unit, and system performance is evaluated in terms of coefficient of performance and exergy efficiency. The heating coefficient of performance for the overall system is found to be 5.34, while the corresponding exergy efficiency is 63.4%. The effect of ambient temperature on the exergy destruction and exergy efficiency is investigated for the system components. The results indicate that the performance of hybrid geothermal heat pump systems is superior to air-source heat pumps.     

Thermodynamic and economic assessments and multi-criteria optimization of a novel poly-generation plant using geothermal energy and multi heat recovery technique

Smart use of clean energy sources for achieving higher performance and designing cost-effective systems is recognized as an essential solution for reducing fossil fuel consumption. In this regard, this study supports a comprehensive evaluation and multi-criteria optimization of a novel poly-generation plant embracing geothermal energy from thermodynamic and thermoeconomic perspectives. Hence, the utilization of modified subsystems and smart use of multi heat recovery processes are projected and appraised. In this regard, the plant consists of a double-flash binary geothermal subsystem, an organic Rankine cycle in combination with an ejector refrigeration cycle considering a zeotropic working fluid (a mixture of pentane and R142b), a heating production heat exchanger, and a proton exchange membrane electrolyzer with the combined production of cooling, heating, power, and hydrogen. The crucial thermodynamic and thermoeconomic variables are investigated against key parameters and concluded that the sensitivity of outcomes is more evident with the variation in zeotropic working fluid composition and the vapor quality at the heating production heat exchanger's outlet. The attained results at the optimum mode demonstrated, the energy and exergy efficiencies of the plant as well as total unit costs of products are as being 44.5%, 35.8%, and 18.8 $/GJ, respectively.     

A study on the performance of a geothermal heat exchanger under coupled heat conduction and groundwater advection

Being environmental friendly and with the potential of energy-efficiency, more and more ground-source heat pump (GSHP) systems are being widely used. However, the influence of groundwater advection on the performance of the geothermal heat exchanger (GHE) in a GSHP is not still clearly known. In this paper, the configuration of a vertical dual-function GHE used in an integrated soil cold storage and ground-source heat pump (ISCS&GSHP) system, which charged cold energy to soil at night and produced chilled water in daytime in summer, and hot water for heating in winter, is firstly presented. This is then followed by a report on a mathematical model for the GHE considering the impact of the coupled heat conduction and groundwater advection on the heat transfer between the GHE and its surrounding soil. The GHE model developed was then integrated into a previously developed simulation program for an ISCS&GSHP system, and the operating performances of the GHE in an ISCS&GSHP system having a vertical dual-function GHE have been studied by simulation and reported. These simulation results, firstly seen in open literature, are much helpful to the design of a GHE buried in soil and widely used in GSHP systems or ISCS&GSHP systems.     

A spectral model for transient heat flow in a double U-tube geothermal heat pump system

This paper introduces a semi-analytical model based on the spectral analysis method for the simulation of transient conductive-convective heat flow in an axisymmetric shallow geothermal system consisting of a double U-tube borehole heat exchanger embedded in a soil mass. The proposed model combines the exactness of the analytical methods with an important extent of generality in describing the geometry and boundary conditions of the numerical methods. It calculates the temperature distribution in all involved borehole heat exchanger components and the surrounding soil mass using the fast Fourier transform, for the time domain; and the complex Fourier and Fourier-Bessel series, for the spatial domain. Numerical examples illustrating the model capability to reconstruct thermal response test data together with parametric analysis are given. The CPU time for calculating temperature distributions in all involved components, pipe-in, pipe-out, grout, and soil, using 16,384 FFT samples, for the time domain, and 100 Fourier-Bessel series samples, for the spatial domain, was in the order of 3 s in a normal PC. The model can be utilized for forward calculations of heat flow in a double U-tube geothermal heat pump system, and can be included in inverse calculations for parameter identification of shallow geothermal systems.     

Experimental–numerical study of heat flow in deep low-enthalpy geothermal conditions

This paper presents an intensive experimental–numerical study of heat flow in a saturated porous domain. A temperature and a flow rate range compared to that existing in a typical deep low-enthalpy hydrothermal system is studied. Two main issues are examined: the effect of fluid density and viscosity on heat flow, and the significance and effect of thermal dispersion. Laboratory experiments on a saturated sand layer surrounded by two impermeable clay layers, subjected to different flow rates under cold and hot injection scenarios, and for both vertical and horizontal flow directions, are conducted. A temperature range between 20 °C and 60 °C is studied. The finite element method is utilized to analyze the experimental results. Backcalculations, comparing the numerical results to the experimental results, are conducted to quantify the magnitude of thermal dispersion. A constitutive model describing thermal dispersion in terms of fluid density, viscosity and pore geometry, taking into consideration different injection scenarios, is developed. This study demonstrates the importance of taking the variation of formation water density and viscosity with temperature into consideration in predicting the lifetime of deep low-enthalpy geothermal systems. It shows that if ignored, the lifetime of a system with hot injection will be overestimated, and that with cold injection, will be underestimated.     

地热间接供热系统的优化设计及运行方法研究

从间接式地热供热系统的热力特性出发，导出了间接式地热供热系统主要设备终端散热器和板式换热器的经济优化方法及最佳运行条件，为指导实际地热间接式集中供热系统的优化提供了科学手段。