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.     

Low enthalpy geothermal energy utilisation schemes for greenhouse and district heating at Traianoupolis Evros, Greece

A socio-economic study has been made of the possible use of low enthalpy geothermal resources for district and greenhouse heating in the Traianoupolis Evros region. The thermal energy potential of the Aristino-Traianoupolis geothermal field has been estimated at 10.8 MW_th (discharge temperature of 25 °C). Geothermal wellhead water temperatures range from 53 to 92 °C, from 300 m deep wells yielding over 250 m³/h. Our conclusions show, amongst the different scenarios examined and on the basis of a market study, that utilisation of this geothermal energy capacity for district heating of nearby villages, and/or greenhouse heating directed at serving local vegetable markets, would be an attractive investment.     

Titanium in the geothermal industry

Titanium resists seawater and brine at temperatures as high as 260 °C, and is also resistant to corrosion by sulphur dioxide; hydrogen sulphide; and aqueous solutions of those gases. Titanium is fully resistant to corrosion and stress corrosion cracking in the standard NACE test solution containing 3000 ppm dissolved H₂S, 5% NaCl, and 0.5% acetic acid (pH 3.5). To avoid pitting at temperatures above 80 °C, titanium alloys containing nickel, molybdenum, palladium or ruthenium are used. Examples of equipment fabricated in titanium in order to withstand the corrosive fluids present in some geothermal installations are plate heat exchangers and well casing. By careful selection of the grade of titanium, material thickness (with no corrosion allowance) and fabrication method, an economic fabrication with low maintenance costs and high availability can be achieved. A prime example of the application of titanium in the geothermal industry is the use of Grade 29 well casing in the Salton Sea, USA, which enables the exploitation of a geothermal resource containing highly corrosive brine. Advances in production technology are being applied to reduce the cost of the casing pipe. This technology may enable the use of sea water injection to augment weak or depleted aquifers, or to generate steam from Hot Dry Rocks.     

Silica removal from Mokai, New Zealand, geothermal brine by treatment with lime and a cationic precipitant

Preliminary experiments using two chemicals (CaO, a quicklime, and a cationic nitrogen-bearing precipitant, EC-004) to remove silica from geothermal brine were undertaken at the Mokai geothermal plant, New Zealand. The brine was mixed with the reagent (CaO or EC-004). The reaction was studied from the start of the experiment (NRT, 0 min, no retaining time) and after 15 min (15RT) at 90 °C. The concentration of silica in the brine was initially 954 mg/l, and decreased linearly with increasing reagent concentration. When CaO is added, the silica concentration at 15RT was 200 mg/l lower than at NRT and became almost zero on addition of 1.5 g/l. In contrast, when EC-004 is added, the total silica concentration nearly reaches the solubility of amorphous silica at 90 °C. In order to prevent silica scaling in Mokai brines cooled to 90 °C, the CaO and EC-004 added should be individually adjusted to 0.5 g/l and 80 mg/l, respectively.     

Hydrogeological and isotopic survey of geothermal fields in the Buyuk Menderes graben, Turkey

The main high and low enthalpy geothermal fields in the Buyuk Menderes graben (Western Anatolia) and their reservoir temperatures are as follows: Kizildere (242 °C), Germencik (232 °C), Aydin-Ilicabasi (101 °C), Yılmazkoy (142 °C), Salavatli (171 °C), Soke (26 °C), Denizli -Pamukkale (36 °C), Karahayit (59 °C), Golemezli (101 °C) and Yenice (70 °C). The geothermal systems are controlled by active graben faults. The reservoir rocks in the geothermal fields are the limestone and conglomerate units within Neogene sediments and the marble-quartzite units within Paleozoic metamorphic formations. There are clear δ¹⁸O shifts from the Mediterranean Meteoric Water Line (MMWL) in the Kizildere, Germencik and Aydin fields, where a good relation between high temperatures and δ¹⁸O shift has also been observed, indicating deep circulation and water rock interactions. In the Pamukkale, Karahayit, Golemezli and Yenice fields and in Soke region, low temperatures, small isotope shifts, shallow circulations and mixing with shallow cold water have been noted.     

Geothermal development in Hungary—country update report 2000–2002

This paper describes the status of geothermal energy utilization—direct use—in Hungary, with emphasis on developments between 2000 and 2002. The level of utilization of geothermal energy in the world increased in this period and geothermal energy was the leading producer, with 70% of the total electricity production, of all the renewable energy sources (wind, solar, geothermal and tidal), followed by wind energy at 28%. The current cost of direct heat use from biomass is 1–5 US¢/kWh, geothermal 0.5–5 US¢/kWh and solar heating 3–20 US¢/kWh. The data relative to direct use in Hungary decreased in this period and the contribution of geothermal energy to the energy balance of Hungary, despite significant proven reserves (with reinjection) of 380 million m³/year, with a heat content of 63.5 PJ/a at ΔT=40 °C, remained very low (0.25%). Despite the fact that geothermal fluids with temperatures at the surface higher than 100 °C are available, no electricity has been generated. As of 31 December 2002, the geothermal capacity utilised in direct applications in Hungary is estimated to be 324.5 MW_t and to produce 2804 TJ/year. Geothermal heat pumps represent about 4.0 MW_t of this installed capacity. The quantity of thermal water produced for direct uses in 2002 was approximately 22 million m³, with an average utilization temperature of 31 °C. The main consumer of geothermal energy is agriculture (68% of the total geothermal heat dedicated to direct uses). The geothermal water is used only in five spas for space heating and sanitary hot water (SHW), although there are 260 spas in the country, and the thermal water produced has an average surface temperature of 68 °C. The total heat capacity installed in the spas is approximately 1250 MW_t; this is not provided by geothermal but could be, i.e., geothermal could provide more than three times the geothermal capacity utilized in direct uses by 31 December 2002 (324.5 MW_t).     

Geothermal rice drying unit in Kotchany, Macedonia

A geothermal field in Kotchany (Macedonia) has very advantageous characteristics for direct application purposes. Low content of minerals, moderate temperature (78°C) and substantial available geothermal water flow (up to 300 l/s) enabled the establishment of a district heating scheme comprising mainly agricultural and industrial uses.A rice drying unit of 10 t/h capacity was installed 8 years ago, using the geothermal water as the primary heat source. A temperature drop of 75/50°C enables the adaptation of conventional drying technology, already proven in practice in the surrounding rice growing region. Water to air heat exchanger and all necessary equipment and materials are of local production, made of copper and carbon steel.The use of such drying units is strongly recommended for the concrete district heating scheme because it offers a very simple geothermal application and enables improvement in the annual heating load factor without high investments in geothermal water distribution lines.     

Geothermal resource development in agriculture in Kebili region, Southern Tunisia

Geothermal energy in the Kebili region, south of Tunisia, is used in a number of applications, but mainly in agriculture. Approximately 95% of the thermal water is used for irrigation of oases and heating greenhouses. Generally, when the water temperature is less than 40–45 °C it is used directly for irrigation, but when it exceeds 45 °C it is cooled by means of atmospheric towers before being used to irrigate 16,000 hectares of oases (half of the total area of the oases in Tunisia). Geothermal energy is also used for heating and irrigating greenhouses, which are considered promising and economically feasible applications. The total area of heated greenhouses in the country has increased considerably and is today at 103 ha, 44% of which are located in the Kebili area. Utilization of the geothermal resources will, without a doubt, increase in the near future once we have implemented the last phase of the greenhouse project. By the end of 2003, 13 ha will be added in the region, representing an increase of 29%.     

Chemical and isotopic measurements of geothermal discharges in the Acqui terme district, Piedmont, Italy

Chemical and isotopic studies have been carried out on samples from the Acqui geothermal district (Piedmont, Italy). The results indicate that the waters represent mixtures of meteoric waters and a fossil brine; the contribution of meteoric waters ranges between 93 and 98%. The recharge zone of meteoric waters is most likely in the Voltri-Savona massif at an isotopic recharge altitude of 590 m. On the basis of chemical and isotopic data the reservoir temperature has been estimated at about 200°C. This high value renders the Acqui district possibly the most promising geothermal system in northern Italy.     

Shallow gravel aquifers and the urban ‘heat island’ effect: a source of low enthalpy geothermal energy

Northern European countries with no high temperature geothermal resources can utilise the urban ‘heat island’ effect to generate low enthalpy geothermal energy for space heating/cooling systems in buildings, provided a suitable aquifer underlies the urban area. Buried valleys, formed at the height of the Pleistocene glaciation 15,000 years ago, when sea level was 130 m lower than present, and infilled with gravels as sea level rose again at the end of the Pleistocene, underlie many European cities. These high yielding aquifers exist at only a few metres depth, and can provide a supply of groundwater at temperatures elevated 3–4 K above the average rural groundwater temperatures. This can produce a marked improvement both in the output and in the efficiency of a geothermal system making use of this source. When passed through a heat pump operating at a Coefficient of Performance (COP) of 4.5:1, a well yielding 20 l/s of groundwater at 13 °C can generate 865 kW heat, sufficient to supply space heating for buildings with a footprint in excess of 12,000 m² with a peak heating intensity of 70 W/m². The economics of this low enthalpy geothermal energy source are outlined. Although development costs are minimal, at current low natural gas fuel prices in Ireland, heating-only applications will be less attractive, and a real cost saving will only accrue if dual heating/cooling functions can be developed.     

Feasibility evaluation of gas-assisted heating for mold surface temperature control during injection molding process

Dynamic mold surface temperature control has the advantage of improving molded part qualities without significant increases in cycle time. In this study, a gas-assisted heating system combined with water cooling and different mold designs to achieve dynamic mold surface temperature control was established. The feasibility of using gas-assisted heating for mold surface temperature control during the injection molding process was then evaluated from experimental results. The effect of mold design as well as heating conditions including hot gas temperature, gas flow capacity, and heating time on the heating efficiency and the distribution uniformity of mold surface temperature were also studied. Results showed that as hot gas temperature and gas flow capacity increased, as well as increasing heating times from 2 s to 4 s, mold surface temperature increased significantly. Fan shaped gas channel design exhibits better mold surface temperature distribution uniformity than tube shaped gas channel design. During gas-assisted heating/cooling, it takes 2 s to increase mold surface temperature from 60 °C to 120 °C and 34 s for mold surface to return to 60 °C. In addition, under specified heating conditions and using the best composite mold designs, the heating rate can reach up to 30 °C/s, a rate well-suited to industrial applications.     

Routine instrumental procedures to characterise the mineralogy of modern and ancient silica sinters

Tightly constrained determinative methods can be used to characterise the silica minerals (opal-A, opal-CT, opal-C, quartz, moganite) and physical properties of silica sinters. Optimal X-ray powder diffraction operating parameters indicate silica lattice order/disorder using untreated, dry, <106 μm powders scanned at 0.6° 2θ/min with a step size of 0.01° from 10–40° 2θ and an internal Si standard. Simultaneous differential thermal and thermogravimetric analysis of 15.0±0.1 mg sinter samples of <106 μm grain size, at a heating rate of 20°C/min in dry air, identify thermal events associated with dehydration, organic combustion, and changes of state. Where abundant organic matter is present, nitrogen is the preferred atmosphere for thermal analysis. Thermogravimetric-determined water contents of sinters differ from Penfield determinations reflecting the differing nature of the two techniques. Laser Raman microprobe techniques can be used to explore the mineralogy of particular sinter morphologies and habits down to 10 μm diameter. The nature of the silica species present can assist in characterising individual sinter deposits and, combined with textural, density and/or porosity determinations, can lead to a better understanding of the hydrology and paleohydrology of a geothermal prospect.     

Geothermal district heating for reno, Nevada, U.S.A.

The City of Reno is one of the most obvious candidates for geothermal district heating in the United States. Lying within a helt of major thermal anomalies, it has within its boundaries the Moana Hot Springs geothermal reservoir and probably other reservoirs, and only 14 km to the south is the major reservoir at Steamboat Hot Springs.This paper discusses the alternative heat sources that can be used, and selects Steamboat as the most conservative choice for a “worst-case” analysis of the details and economics of a model district heating system. A closed 16 km transmission loop between Steamboat and downtown Reno is envisaged, carrying 121°C water in the supply line and 65°C in the return line. This loop is isolated by heat exchangers from both the 176°C geothermal fluids at the Steamboat end of the line, and from the Reno user systems at the other. Detailed analysis of thermal demand densities in different parts of the City led to a model distribution network 48 km long, that serves the optimum grouping of zones of concentrated heat users. A large proportion of existing heating systems are hydronic and retrofitting of the selected buildings to the district heating system is relatively straightforward. Total peak load for the proposed system is 138 MJ s⁻¹ (139 MW_t) and annual consumption is 1.1 × 10¹⁵ J. Capital costs total about $55.4 million, in 1981 dollars. The economic analysis shows this system could provide considerable savings relative to the cost of natural gas, if revenue bond financing at 13% is employed for the construction and startup of the system. Even more favorable economic results could be achieved if a geothermal resource could be developed closer to downtown Reno, eliminating the high cost of the 16 km transmission pipeline. Reducing the service area to the most concentrated area of heat use in downtown Reno produces an even more viable system, about half the size of the full system.     

Evidence for thermal mining in low temperature geothermal areas in Iceland

This study deals with thermal mining in several geothermal systems in Iceland. A number of 2500- to 3000-m deep drillholes have been drilled into low temperature geothermal areas in the country. The conductive gradient outside active geothermal areas has also been mapped, and shows a systematic variation from lower than 50°C/km in the outer parts of the Tertiary basalts to over 100°C/km on the borders of the volcanic zones (rift zones). The difference between formation temperatures inside geothermal systems and the surrounding conductive gradient can be computed as a function of depth. This difference is termed ΔT in this paper. The ΔT-curves show that the upper parts of the geothermal systems are heated and the lower parts are cooled compared to the undisturbed conductive gradient. In many cases the cooling of the lower part is greater than the heating in the upper part, so that a net thermal mining has occurred. This thermal mining is calculated for several geothermal systems, and the systems are compared. The net thermal mining in the top 3000 m appears to be much greater in formations of Pleistocene and Pliocene age. It gradually decreases to zero for formations older than 6 million years. However, the net thermal mining is critically dependent on the maximum depth of water convection in these systems, which is unknown.     

Low- to medium-temperature geothermal reserves in Mexico: a first assessment

In this work we make a first, partial, assessment of the low- to medium-temperature geothermal reserves of Mexico. The assessment covers about 30% of the identified geothermal surface manifestations. For reserve assessment we use the volume method, supplemented by Montecarlo simulations and statistics, in order to quantify the inherent uncertainties. We estimate these reserves as lying between 7.7 × 10¹⁶ and 8.6 × 10¹⁶ kJ, with 90% confidence. The distribution of most likely reservoir temperatures is in the 60–180 °C range, with a mean of 111 °C. These massive amounts of recoverable energy and the associated temperatures are potentially important for the economic development of the associated geothermal localities.     

Analysis of tracer test data, and injection-induced cooling, in the Laugaland geothermal field, N-Iceland

The Laugaland geothermal system in N-Iceland is hosted by low-permeability fractured basalt and its productivity is limited by insufficient recharge, even though substantial thermal energy is in-place in the 90–100 °C hot rocks of the system. The purpose of a 2-year reinjection experiment, completed in late 1999, was to demonstrate that some of this energy could be extracted economically through long-term reinjection. A comprehensive monitoring program was implemented as part of the project, including three detailed tracer tests. More than 1400 tracer samples were collected during the tests. Tracer return data indicate that the injected water travels through the area bedrock by two modes: first, along direct, small volume flow-paths, such as fractures or interbeds; second, by dispersion and mixing throughout a large volume of the reservoir. Based on the tracer test results, and assuming 15 l/s average future reinjection, the temperature of water produced is predicted to decline by 1–3 °C in 10 years. It can be asserted, in spite of measurement uncertainties, that the 2-year reinjection experiment did not cause a temperature decline greater than about 0.5 °C, conforming to predictions. It is estimated that future reinjection at 15 l/s will enable an increase in energy production amounting to about 24 GWh_th/year, which equals roughly of the average yearly energy production at Laugaland during the last decade. Reinjection has continued after the experiment and is already an important part of the management of the Laugaland geothermal system.     

The geothermal industry in Iceland

Direct (non-electrical) uses of geothermal energy in Iceland in 1984 amounted to 5517 GWh and the installed power was 889 MW_t, assuming 35°C discharge temperature. The bulk of this thermal power was for district heating, called hitaveita in Icelandic. In recent years this utilization has increased moderately. The installed geothermal electric power is currently 41 MW_e and is unlikely to change in the near future. Icelandic personnel have participated in many geothermal projects of the United Nations during the last 35 years. Contract work has been carried out by Icelandic consulting firms in several developing countries.     

Research and development of geothermal energy production in Hungary

The basement of the Pannonian (Carpathian) basin is represented by Paleozoic metamorphic and Mesozoic dolomite and limestone formations. The Tertiary basin gradually subsided during the Alpine orogeny down to 6000 m and was filled by elastic sediments with several water horizons.A heat flow of 2.0 to 3.4 μcal/cm²s gives temperature gradients between 45 and 70 °C/km in the basin. At 2000 m depth the virgin rock temperature is between 110 and 150°C. 80 geothermal wells about 2000 m deep have shown the great geothermal potential of the basin.The main hot water reservoir is the Upper Pliocene (Pannonian) sandstone formation. Hot water is produced by wells from the blanket or sheet sand and sandstone, intercalated frequently by siltstone. Between a 100–300 m interval, 3 to 8 permeable layers are exploited resulting in 1–3 m³/min hot water at 80–99°C temperature.Wells at present are overflowing with shut-in pressures of 3–5 atm.The Pannonian basin is a conduction-dominated reservoir. Convection systems are negligible, hot igneous systems do not exist. The assessment of geothermal resources revealed that the content of the water-bearing rocks down to 3000 m amounts to 12,600 × 10¹⁸cal. In the Tertiary sediments 10,560 × 10¹⁸cal and in the Upper Pannonian, 1938 × 10¹⁸cal are stored. In the Upper Pannonian geothermal reservoir, below 1000 m, where the virgin rock temperature is between 70 and 140°C, the stored heat is 768 × 10⁸cal. A 10¹⁸ cal is equivalent to the combustion heat of 100 million tons of oil. The amount of recoverable geothermal energy from 768 × 10⁸cal is 7.42 × 10¹⁸cal, i.e. about 10,000 MW century, not considering reinjection.At present the Pannonian geothermal reservoir stores the greatest amount of identified heat which can be mobilized and used. Hungary has 496 geothermal wells with a nominal capacity of 428 m³/min, producing 1342 MW heat. 147 wells have an outflow temperature of more than 60°C producing 190 m³/min, that is, 845 MW. In 1974 290 MWyear of geothermal energy was utilized in agriculture, district heating and industry.     

Stockholm CHP potential—An opportunity for CO2 reductions?

The potential for combined heat and power (CHP) generation in Stockholm is large and a total heat demand of about 10 TWh/year can be met in a renewed large district heating system. This model of the Stockholm district heating system shows that CHP generation can increase from 8% in 2004 to 15.5% of the total electricity generation in Sweden. Increased electricity costs in recent years have awakened an interest to invest in new electricity generation. Since renewable alternatives are favoured by green certificates, bio-fuelled CHP is most profitable at low electricity prices. Since heat demand in the district heating network sets the limit for possible electricity generation, a CHP alternative with a high electricity to heat ratio will be more profitable at when electricity prices are high. The efficient energy use in CHP has the potential to contribute to reductions in carbon dioxide emissions in Europe, when they are required and the European electricity market is working perfectly. The potential in Stockholm exceeds Sweden's undertakings under the Kyoto protocol and national reduction goals.     

Geothermal development in Romania

Exploration for geothermal resources began in Romania in the early 1960s, based on a detailed geological exploration program for hydrocarbon resources that had a capacious budget and enabled the identification of eight geothermal areas. Over 200 wells drilled to depths between 800 and 3500 m have indicated the presence of low-enthalpy geothermal resources (40–120 °C). Completion and experimental production from over 100 wells during the past 25 years has led to the evaluation of the exploitable heat resources of the geothermal reservoirs. The proven reserves, with the wells that have already been drilled, amount to about 200,000 TJ for 20 years. The main geothermal systems discovered on Romanian territory are in porous permeable formations such as sandstones and siltstones (Western Plain and the Olt Valley) or in fractured carbonate formations (Oradea, Bors, and north of Bucharest). The total thermal capacity of the existing wells is about 480 MW_t (for a reference temperature of 25 °C). Only 152 MW_t of this potential is currently being exploited, from 96 wells (35 of which are used for health and recreational bathing), producing hot water in the temperature range 45–115 °C. In 2002 the annual energy utilisation from these wells was about 2900 TJ, with a capacity factor of 0.6. More than 80% of the wells are artesian producers, 18 wells require anti-scaling chemical treatment and six are reinjection wells. During the period 1995–2002, 15 exploration-production geothermal wells were drilled and completed, two of which were dry holes. Drilling was financed by the geological exploration fund of the State Budget, to depths varying between 1500 and 3500 m. Progress in the direct utilisation sector of geothermal resources has been extremely slow because of the difficulties encountered during the transition period from a centrally planned to a free-market economy; geothermal production is at present far below the level that could be expected from its assessed potential, with geothermal operations lagging behind in technology. The main obstacle to geothermal development in Romania is the lack of domestic investment capital. In order to stimulate the interest of potential investors from developed countries and to comply with the requirements of the large international banks, an adequate legal and institutional framework has been created, adapted to a market-oriented economy.