- Email: [email protected]
Section 5 Space and process heating: state-of-the-art and prospects
f;eothermics, Vol. 14, N o . 2 / 3 , pp. 165 -- 173, 1985.
0375 - 6 5 0 5 / 8 5 $ 3 . 0 0 + 0 . 0 0 P e r g a m o n Press Ltd. ~ 1985 C N R .
P r i n t e d in G r e a t Britain.
UN Seminar on the Utilization of Geolhermal Energy for Eleclric Power Production and Space Healing. Florence 1984
SECTION SPACE
AND
5
PROCESS HEATING: STATE-OF-THE-ART AND PROSPECTS
General Report prepared by V. N. Kruzhelnitsky, A tomteploelektroprojekt Institute, Ministry of Energy of the U.S.S.R., Riga, appointed General Rapporteur by the Government of the U.S.S.R. R.105
INTRODUCTION Existing reserves of fuel for electric power production and heating are very limited and their depletion is continuing. Additional energy sources such as geothermal energy (including the energy of hot dry rock), which at present are merely highly desirable, may, in the near future, become indispensable to a country's normal, social and economic development. From this point of view, the question of the economic efficiency of obtaining energy from a particular source may be less important than that of its availability. Many specialists dealing with environmental protection problems believe, not without justification, that geothermal energy may prove to be one of the "cleanest" sources of convertible energy at humanity's disposal. This view is based, not on any lack of problems and unsolved questions concerning the environmental aspects of geothermal energy production (discharge of saline waters, noise, emission of associated gases, surface subsidence, seismic effects, etc.) but on the uniqueness of geothermal energy in the sense that all activity, including extraction and conversion, is concentrated in one place in the immediate vicinity of the consumer and that, for this reason, the environmental effects of the development of geothermal reservoirs are completely confined to the area of the geothermal site. Although calculations of the geothermal energy reserves that are exploitable with the stateof-the-art technology have shown that such reserves are not very large (corresponding to approximately 1% of total consumption), the solving of a number of engineering, social and legal problems in the process of commercial utilization of geothermal energy is already making it possible to prepare for the utilization of the vast reserves of energy locked in hot dry rocks. H u m a n utilization of geothermal energy began in sectors other than electricity generation, but was of a haphazard nature and was for the most part confined to sources having a natural outlet to the earth's surface. In recent years, and especially since the beginning of the energy crisis, extensive research has been undertaken in many countries into the detection of geothermal energy reserves and into various aspects of techniques for the utilization, at first, of high-enthalpy sources for electricity generation and, later, of medium- or low-enthalpy sources for industrial, agricultural and domestic use. This research shows that the proven geothermal reserves exploitable with the present technology have, for the most part, medium- or lowenthalpies and that, a most important point, they occur in areas close to existing or prospective energy users. Some geothermal reserves, particularly the highly mineralized ones, can also serve as a source of feedstocks for various enterprises (extraction of rare metal and other mineral substances, production of dry ice, etc.). 165
166
V. N. Kruzhelnitsky
Growing interest is being shown in the utilization of medium-enthalpy geothermal heat sources, despite the fact that they are inadequate for economic electric power generation and, in many cases, too expensive to be used for heating premises or agricultural objectives. For industrial processes, however, such geothermal heat sources are ideal, especially since the industrial use of geothermal heat is in many cases its most rational utilization from the point of view of the constant output and the high annual well utilization factor. It used to be thought that geothermal production sites, which are "industrial" by definition, were incompatible with residential and recreational areas on account of their noise, smell and landscape-disrupting effects. However, recent technical developments (submerged pumps, downhole heat exchangers, closed circuits with reinjection of thermal water), as well as practice in the commercial utilization of geothermal waters, have proved that a satisfactory degree of compatibility can be achieved. All the above explains the continually growing interest in the utilization of geothermal energy in industry, agriculture and the communal and domestic sectors. UTILIZATION IN VARIOUS SECTORS: A G R I C U L T U R A L , INDUSTRIAL AND DOMESTIC Utilization level In 1982, the installed capacity for non-electrical uses of geothermal energy (domestic heating, agriculture, industrial processes), not including balneotherapy, was 4050 MWt. The countries which use geothermal energy most (not for electricity) are the following (in decreasing order of utilization): Iceland, Hungary, U.S.S.R., China, New Zealand, U.S.A., Japan, Italy, France, Romania and Czechoslovakia (Table 1). Besides these countries, geothermal research is being carried out in many other areas. It is particularly advanced in Turkey, Costa Rica, Colombia, Ecuador, Peru, Bolivia, Argentina, U.K., Spain, Federal Republic of Germany, Poland, Bulgaria, Djibouti, Iran and Thailand. More detailed information on some characteristic countries with different levels of achievement in the field of non-electrical uses of geothermal energy is given below. F a b l e 1. A s s e s s m e n t o f direct uses o f g e o t h e r m a l energy: m a x i m u m c a p a c i t y o f plants in o p e r a t i o n in 1982 ( M W t ) Total I n c l u d i n g Not including bathing , bathing
H e a t i n g , airconditioning
Agriculture
Industry
Balneology
Undefined uses
A n n u a l utilization factor
0.4
0.3
0.6
(I.4
--
(I.35
0.4
Austria China France Hungary Iceland Italy Japan New Z e a l a n d Romania Czechoslovakia U.S.A. U.S.S.R. Yugoslavia Others
2 70 165 75 780 107 50 50 27 25 87 140 14 33
-60 15 565 77 56 31 10 9 I0 16 395 . 56
-195 -289 164 +
2 339 180 959 1096 190 90 226 36 35 221 555 14 1(17
1625
1300
4050
Total
3 17
8 4 360
---106 --
17
248
I
5 356 180 1540 1305 566 4484 226 36 43 225 915 14 355
370
6200
755
10,250
14 30 75 27 9 166
581 209 376 4394 --
12 20 .
.
.
Space and Process Heating
167
Italy. In Italy, there are at present three major centres of direct utilization of geothermal sources: (i) At Larderello, where low-enthalpy geothermal steam ( 1 0 5 - 1 3 2 ° C ) is used to heat 348,000 m 3 of premises and greenhouses. (ii) In the Monte Amiata area, where geothermal fluids, after use at a geothermal powerstation, are used to heat greenhouses, a drying plant and premises of various kinds. The entire installation envisaged in the project has been commissioned in 1983, consumption amounts to 1 5 0 , 0 0 0 - 3 5 0 , 0 0 0 Gcal a year. (iii) At Abano, where geothermal energy is used for district heating of hotels and greenhouses. District heating projects are being realised in Milan, Vicenza, Ferrara and Cesano (Rome). Other projects for agricultural or multipurpose use in Tuscany at Pomarance, Monterotondo, Radicondoli and Siena and on Ischia and Sardinia islands are under study. In 1981, geothermal plants for space heating and domestic water supply yielded an oil saving estimated at approximately 33,500 tons, or, with the inclusion of agricultural and industrial uses, 42,500 OET. Under the national energy programme, a saving of the order of 300,000 OET a year is to be achieved by 1990 as a result of non-electrical uses of geothermal energy. Taking account of the consumers' potential, the figure reached by the year 2000 or later may amount to I million p.a. Low-temperature geothermal sources brought into use by June 1983 have an energy equivalence of l million Gcal/year.
Table 2. Principal Italian projects (in operation or planned) for non-electrical uses of geothermal energ,, (1983) Temperature of the fluid or condensate (°C)
Technology Place
Maximum capacity of plant (MWt)
Gcal/year
80 65
80 3.5
150,000 8000
100 60
14 2 23 3 27 6 3 1 9 2 5(1 58 21 6.5 12 12 1.1 3.2 3.2 2 1.2
Direct use Abano Basin of ~vater Galzignano E
7.
.-"z = ..~ e.-,
o '~ :--
:'8 =
g + ~. ~= x: ~ ~ ~ ~ '~ ..= ~= ~ ~ = -~ e-, ~ fi! ~ m
~ ~" '.v =
Ferrara Metanopoli Larderello -) Lago Castelnuovo Castelnuovo Pomarance Bulera Radicondoli Amiata Piancastagnaio
)
Pantani Cesano Siena Vulcano lschia Padova Vicenza Bagni di Romagna Acqui Terme
100
200
95 120 90 140 (80 ~ 90 90 55 150 200 70 20(1 90 70 65 26-42 70
Utilization factor (070)
Stale of project
Areas of use
25 30
O O
H, B G
75,000 5000 40,000 10,000 90,000 9000 7500 7000 23,000 3000 100,000 220,000 50,000 8600 30,000 80,000 8550
70 33 23 44 44 20 33 90 _'~3 20 27 50 31 18 33 85 100
C C
P O P P C O P C G P C P
H H H G IND H G G,H G H,E G,E DR,E G H H DES,E DES
11,000 9000 3000 2000
46 37 20 22
P (7 C P
H H H, B H
O
O: Operational; C: construction; P: planning; H: heating; G: greenhouses; IND: industry; DR: drying; DES: desalination; B: balneology; E: electricity.
168
V. N. Kruzhelnitsky
U.S.S.R. The country's energy potential from underground heat sources is large. According to the Academy of Sciences and the Ministry of Geology, proven thermal water resources are sufficient for the extraction of 2 0 - 22 million m ~a day over a prolonged period. Ninety per cent of these waters have low enthalpy and most are highly mineralized. However, thermal waters are unevenly distributed over the cotlntry's territory. More than 70% of proven thermal water reserves occur in sparsely-populated regions of Siberia and the Soviet Far East, which limits the possibility of their large-scale utilization. Today, low-grade thermal waters are used to heat and supply hot water to residential and industrial premises, to heat greenhouses and hothouses for therapeutic purposes, for intensified fish breeding in ponds and to heat livestock farms. Thus, a thermal well with a flow rate of 1500 m ~/day and a temperature of 60°C at the wellhead supplies heat and hot water to a residential area of the town of Makhacbkala, with a resulting economy of 1200 OET per annum. At present, small areas of Tbilisi and Makhachkala, some small settlements, and a State farm with greenhouses in the outskirts of Grozny are heated and provided with hot water using the heat of underground sources. In the Mostovsk area of Krasnodar territory, geothermal heat is supplied to a 19 ha greenhouse complex, a livestock farm and 200 ha of fish ponds. In the next few years, geothermal heating schemes are to be implemented in A l m a - A t a , Tashkent, P e t r o p a v l o v s k - Kamchatsky, the Tskhaltubo resort and elsewhere. A long-term plan for the integrated use of geothermal energy until 1990 has been drawn tip. It provides for this new energy source to play a substantial part in the country's fnel and energy balance. Prospecting for and extraction of thermal waters will be considerably expanded, leading to the saving of around 1 million OET by 1985. France. France has its own national programme for the development of geothermal energy. In addition to the central area of the Paris Basin, where the Dogger reservoir is being explored, there are more than 30 other developments out of a possible total of 300. Several successes have been achieved in Aquitaine, particularly in the Bordeaux arca, where the wells in operation make possible an annual fuel saving of 2s00 OET. Work on geothermal recovery from oil wells is scheduled to the south of this area. A project for supplying heat to greenhouses with a total area of 3.5 ha is being executed at Lamaz6re and will save 2500 OET a year. The favourable areas are in the Paris region and in Aquitaine, it should be noted, however, that inventory of France's geothermal resources is far from complete. Turkey. Extensive geological, geophysical and geochemical investigations of geothermal sources have been carried out in Turkey. Results indicate that at least 31,000 MW (thermal) will be available for non-electrical uses. The largest geothermal fields known today are the follo~ing: The S a r a y k O y - B u l d a n , where about 1600 t/h of geothermal fluid ~ith a reservoir temperature of 200 212°C are obtained from six productive wells in the Klzllderc geothermal field. After utilization at a geothermal pox~er-station, the fluid (100°C) will be used to heat 500 acres of greenhouses. Satisfactory results have been achieved to date with the heating of greenhouses of an area of 1 ha. Because of its chemical properties, the geothermal fluid will be used in the textile industry for bleaching. Production of 200 t of dry ice a day is planned. The Afyon area, where, as a first stage, the heating of 2 ha of greenhouses has been successfully organized. The area, ,':hich possesses considerable geothermal water reserves, is capable of serving as a reliable source of heal for tourist centres and resorts, induslrial enterprises and agricultural units. A district heating scheme has been prepared for the tOXVlaof Afyon (heat load: 331 Gcal/h).
Space and Process Heating
169
The G e r m e n c i k - Omerbeyli area, where a m a j o r geothermal deposit with a temperature of 231 °C and free fi om scale formation has been discovered. This source is suitable for use in the canning and textile industries as well as for the heating of premises and greenhouses and the development of tourism and of a network of health resorts. Belgium. Geothermal energy is to be used as the energy source in new municipal heating networks at Mons, with a total annual fuel saving of 2600 OET, including 2100 O E T from the heating of buildings and 500 O E T from the heating of greenhouses and the preheating of sludge at a waste water plant. Research operations, including drilling at Douvrain and Ghlin and in the Campine area, are continuing. Greece. Although the main effort in the geothermal energy field is concentrated on highenthalpy wells, a start was also made in 1981 in the use of low-enthalpy sources: one well for heating greenhouses and a second well in the northern part of the country for domestic heating and other initiatives are envisaged in the agricultural sector.
Areas of application The energy of geothermal waters is used in a wide variety of applications. For agricultural uses, systems with a water temperature below 100°C or between 100 and 150°C are of particular interest. Agricultural uses of geothermal energy (principally to heat greenhouses and water and for irrigation and refrigeration) overlap areas of application of solar energy, although, owing to the nature of geothermal fluid, its high temperature and its abundance, some applications (hydroponics, hot beds, winter warm irrigation, anti-parasitic sterilization of greenhouse soil) are of course, typically hydrothermal. In some countries (China, Hungary, Japan), geothermal heat is also used to heat incubators in poultry-farming and livestock sheds. In addition, if its salt concentration permits, geothermal water can be used directly for growing high-protein algae and in fish breeding, as is done in some countries (China, Japan, U.S.A., U.S.S.R.). Preservation of agrozoo-technical products by geothermal preparation drying, freezepasteurization or sterilization is practised in a number of other countries. Algae growing and fish breeding are practised in Greenland, wood drying in New Zealand, lucerne-fodder and vegetable growing in the U.S.A. and soil-heating in hot beds in the U.S.S.R. Geothermal energy can also be used in a wide variety of industrial processes. Table 3 lists a number of industrial sectors and the temperature required for each. The table demonstrates the extensive possibilities for the use of geothermal energy.
M E T H O D S OF U T I L I Z A T I O N OF G E O T H E R M A L E N E R G Y Research has shown that the exploitation of low-enthalpy geothermal heat is based mainly on the following solutions: (a) direct heat exchange, applied when the temperature of the geothermal water is higher than the users require; (b) utilization of heat-pump cycles; (c) utilization of mechanically compressed steam obtained by the flashing of geothermal water. Solutions (b) and (c) are necessary if the temperature required by users is higher than that of the geothermal water. Where the temperature of the geothermal water is not high enough to meet user requirements but is sufficient for preheating purposes, an intermediate technical solution between (a) and (b) or (c) may be proposed. Combinations of all three methods are also possible.
V. N. Kruzhelnitsky
170
Table 3. Temperatures of geothermal fluids required in industry (°C) Energy production Power generation (wet or dry steam) Power generation (Rankine cycle) Heating in the municipal sector, water for domestic use Heat supply with heat pump Refrigeration Air-conditioning
>140 >90 60 - 130 25 - 60 140 180 70 120
Chemical industry Plastics Aluminium oxide (Bayer process) Petrochemistry Explosives Pharmaceutical Synthetic rubber Soaps and detergents
85 240 140 150 120 150 90 - 150 65 125 25 - 95 80 85
Agrozootechnical and food industries Distillery Canning Beer, malt Vegetable oil Fried potatoes Sugar Fodder (dried) Greenhouses Milk Tobacco, cigarettes Dairy industry Meat (slaughtering and processing) Grain (drying) Soft drinks Zootechnics, animal husbandry Cultivation of mushrooms and ferments Warm irrigation, soil heating (hot beds) Hydroponics
100 160 90 150 75 150 70 - 150 60 200 50 - 140 80 -- 135 60 130 70 120 105 40- 95 40 85 40 80 25 80 25 60 20 - 50 20 35 15 35
Extractive industries Building materials Desalination Oil regeneration Other industries Forest industry Paper, pulp, limber Textile industry Balneology
50 I10
150 120 9(1
55 50 90 20-
180 150 155 150
The above-mentioned solutions presuppose the possibility of using a variety of process paths: direct u t i l i z a t i o n o f g e o t h e r m a l w a t e r ; use via a d o w n - h o l e o r u p - h o l e h e a t e x c h a n g e r ; use in c l o s e d circuits, etc. In p r i n c i p l e , m e t h o d s (a) a n d (b) can be r e g a r d e d as t w o v e r s i o n s o f the s a m e m e t h o d if the h e a t p u m p is c o n s i d e r e d to be a v a r i e t y o f h e a t e x c h a n g e r (with an i n t e r m e d i a t e h e a t - t r a n s f e r m e d i u m ) . H o w e v e r , b e a r i n g in m i n d the special f e a t u r e s o f t h e i n t e r m e d i a t e h e a t t r a n s f e r m e d i u m used in h e a t p u m p s a n d t h e r e l a t i v e n o v e l t y o f its use in the g e o t h e r m a l e n e r g y field, t r e a t i n g m e t h o d (b) as d i s t i n c t f r o m direct h e a t e x c h a n g e m u s t be c o n s i d e r e d c o r r e c t . A s a l r e a d y n o t e d , m o s t o f the g e o t h e r m a l fields h a v e l o w - e n t h a l p y , a n d the a p p l i c a t i o n o f m e t h o d s (b) a n d (c) facilitates the e x t e n s i v e use o f g e o t h e r m a l w a t e r s w i t h v e r y l o w - e n t h a l p y o r m o r e c o m p l e t e use o f the e n e r g y o f g e o t h e r m a l w a t e r in s y s t e m s c o m b i n e d w i t h the direct h e a t e x c h a n g e m e t h o d . T h i s e x p l a i n s the c o n s i d e r a b l e i n t e r e s t s h o w n in this aspect by specialists from a number of countries.
Space and Process Heating
171
The COP (Coefficient of Performance) of heat-pumps is of particular interest for temperature differences between condensation and evaporation of up to 40°C, with only very slight dependence on the temperature level, while on the contrary, temperature differences exceeding 80°C are of no interest at all. For intermediate situations between 40°C and 80°C, further investigation of the problem is called for. As to compressed steam, the same conclusions apply, except that they relate to the temperature difference between the thermal level required by users and the temperature on compressor intake. Using compressed steam offers some increase in efficiency compared with that obtainable with heat-pump cycles; however, this solution presents difficult problems, above all low flow rates and low temperatures at the compressor intake. The most efficient use for heat-pump systems is in the introduction of air-conditioning in southern areas, where the duration and energy consumption of the cooling period are commensurate with those of the heating period. For example, a scheme has been devised for the use of heat pumps to provide heating and cooling at the Tskhaltubo health resort (waste water from thermal baths). When implemented, it will yield a saving of 50,000 OET a year. R E I N J E C T I O N AND DISTRIBUTION PROBLEMS The problems with the reinjection and distribution of geothermal fluids derive from the inherent nature of the fluids, i.e. their physical and chemical properties. The most important of these properties are: (i) the high concentration of minerals, including carbonates, in most geothermal fields; (ii) a tendency towards scale formation, especially in the event of changes in temperature conditions; (iii) a change in heat content on transition from the liquid to the gaseous phase depending on pressure and temperature (instantaneous boiling); (iv) the presence of dissolved gases in the geothermal fluids; (v) corrosive effects and (vi) erosive effects. The extent to which these problems can be solved will largely determine the economic profitability, and sometimes even the feasibility, of the commercial exploitation of each particular geothermal field. One way of solving some of the above-mentioned problems is to keep water under pressure inside the well and to extract the water from the well at high temperature. This means that pumps must be installed down the well. This method is dictated by a wish to avoid: instantaneous boiling with the resulting heavy energy loss; the sedimentation of carbonates; and the emission of dangerous dissolved gases. Some problems can also be solved by installing a heat exchanger at the top of the hole in cases where, owing to its heavy mineral concentration, corrosive effects and scale formation, the geothermal fluid cannot be delivered directly to the consumer. The main points to be borne in mind in choosing, designing, building and operating heat exchangers are the following: fouling of the heat-exchange surface and deterioration of the hydraulic and heat-exchange characteristics; difficulty of ensuring a long service life, including reliability; prevention of corrosion and erosion of the heat-exchange surfaces; prevention of hydrodynamic vibration; transient thermal stresses; difficulty of guaranteeing low manufacturing and operating costs; possibility of repairing and cleaning the surfaces; the requirements of compactness and efficiency. Experience with downhole heat exchangers utilizing a single or multiple " U " tube (at Klamath Falls in Oregon, U.S.A., at Rotorua and Taupo, New Zealand and at Afyon, Turkey) have yielded satisfactory results. In addition to solving the same problems as a heat exchanger placed at the wellhead, a downhole heat exchanger solves the same problems as a downhole pump and makes it possible to dispense with the reinjection of waste geothermal water.
172
V. N. Kruzhelnitsky E C O N O M I C FEASIBILITY
A good deal of experience has been acquired in various parts of the world over the past 10 years in the economics of prospecting, drilling and developing the necessary equipment, and solving environmental and operational problems, so that the commercial efficiency of a specific proxen geothermal deposit can now be determined with a fair amount of precision. Some general conclusions can also be drawn, as follows: Ca) of all types of new energy sources, the use of geothermal energy offers the greatest economic advantages; (b) a fairly large number of proven geothermal sources are or could be economically profitable in exploitation and competitive with conventional sources based on fossil fuels; (c) the economics of geothermal energy production is largely determined by drilling costs and by the prices of fossil fuels, especially oil; (d) in view of the existing trend towards more stringent enviromnental protection requirements, the economic efficiency of using geothermal sources will increase even though oil prices are falling; (e) the economic profitability of a system based fully or partly on geothermal energy should be greater than that of a conventional system and the payback period should be shorter; (f) since high initial capital investments have to be regarded as forming part of the fuel costs and must be compensated by low operating and service costs, the overall saving from using geothermal energy is largely determined by the annual utilization factor rather than by the system's efficiency; (g) in systems requiring appropriate volumes of geothermal fluid and thermal installations to regulate the temperature, the scale effect and integration (seasonal or cascade) are indispensable; (h) geothermal systems are generally incorporated in a larger system with a base load such that the annual utilization factor for the system as a whole is not lower than 0.5, that being the minimum value for acceptable economic results; (i) the economic efficiency of highly-mineralized geothermal waters rises considerably if the waters are used in an integrated manner and not only for their energy potential (development of balneotherapeutic complexes in the case of waters with curative properties, extraction of rare metals or other mineral substances, etc.); (j) in many cases, the industrial use of geothermal energy is the most rational method of utilization from the point of view of the volume required and the high annual utilization factor; (k) because of inadequate data and the difficulty of defining geological and physical conditions, it would be risky to estimate reserves in geothermal systems at depths greater than 3000 m; (1) when the temperature of the geothermal fluid is low (around 60°C) and well depth is 1500-2000 m, utilization of the fluid is economically justified only in large-scale projects (e.g. greenhouses with a total area of at least 10 ha); (m) in the case of wells of medium depth (of the order of 1500 2000 m), the minimum annual output which will ensure the viability of the enterprise is at present around 15,000 Gcal; (n) in view of the financial risk involved and the need for preliminary research, the setting up of a " s o u r c e - u s e r " system, promotion activities and publicity, the best preconditions for the development of geothermal energy are to be found in countries with a planned economy and those which have established national programmes of their own or are participating in the programmes of international communities.
Space and Process Heating
173
Further and more intensive development o f geothermal energy p r o d u c t i o n is subject to considerable constraints and difficulties o f an economic nature, viz.: (a) considerable financial risk, owing to the need for heavy capital investment at a time when the geological results are still uncertain; (b) the considerable time-lag between the start o f financing and the attainment o f profitability; (c) where geothermal resources are used in industry, agriculture or the c o m m u n a l sector, the accessibility, suitability and, ultimately, economic efficiency o f the geothermal source itself are by no means always the priority criteria in the choice o f the site for industrial or residential development; (d) the exploitation o f geothermal waters in industry at a utilization factor three to four times higher than in district heating systems usually entails changes in plant design and these are not always compatible with investment payback periods acceptable to industry; (e) the high well-productivity threshold o f an economically justified enterprise (of the order o f 15,000 Gcal a year) entails difficulties both as regards finding the geothermal source and as regards finding a user for the heat; (f) since potential users have only a limited idea o f the possibilities offered by geothermal energy and since there exists no industrial technology for the utilization o f " d i f f i c u l t " fluids, publicity and p r o m o t i o n activities are indispensable. Lastly, local development p r o g r a m m e s do not always correspond to potential geothermal energy reserves. LIST OF REPORTS EP/SEM.9/R.4 EP/SEM.9/R.7
Geothermal fluids in agriculture and animal husbandry. R. Lesmo and C. Sommaruga (Italy) (E). Preliminary investigations o f low enthalpy geothermal energy utilization for industrial process heat consumptions. P. Ceron, L. Faletti, C. G. Palmerini and C. Piemonte (Italy) (E).
EP/SEM.9/R.15 Non-electric uses o f the geothermal resource: economic considerations. G. Calabr6 and G. Trambaioli (Italy) (E). EP/SEM.9/R.21 The demonstration projects o f the European Communities in the field of geothermal energy. G. Gerini (European Economic Communities) (F). EP/SEM.9/R.33 Geothermal model of Denizli, SaraykOy - Buldan area. S. Simsek (Turkey) (E). EP/SEM.9/R.46 Utilization of geothermal energy of Afyon area in domestic heating and agriculture. B. Erisen (Turkey) (E). EP/SEM.9/R.52 Geothermal activity in Italy: present status and future prospects. R. Carella, G. Verdiani, C. G. Palmerini and G. C. Stefani (Italy) (E). EP/SEM.9/R.53 Industrial non-electric uses o f geothermal fluids. M. Brianza, M. Guglielminetti, G. lnvernizzi and C. Sommaruga (Italy) (E). EP/SEM.9/R.54 Sedimentological and diagraphic characteristics o f the Dogger in the geothermal borehole at Aulnay-sous-Bois. J. Rojas and D. Giot (France) (F). EP/SEM.9/R.59 Utilization o f thermal waters in the U.S.S.R. for space heating and industrial processes. V. A. Vasilov, V. K. Muslimov and Y. I. Sultanov (U.S.S.R.) (R). Abstract only. EP/SEM.9/R.75 Utilization o f geothermal energy in heating systems. E. V. Sarnatski (U.S.S.R.) (R). EP/SEM.9/R.78 Geothermal heating systems using heat pumps. P. Jaud (France) (F). (E) Original in English (F) Original in French (R) Original in Russian.
0375 - 6 5 0 5 / 8 5 $ 3 . 0 0 + 0 . 0 0 P e r g a m o n Press Ltd. ~ 1985 C N R .
P r i n t e d in G r e a t Britain.
UN Seminar on the Utilization of Geolhermal Energy for Eleclric Power Production and Space Healing. Florence 1984
SECTION SPACE
AND
5
PROCESS HEATING: STATE-OF-THE-ART AND PROSPECTS
General Report prepared by V. N. Kruzhelnitsky, A tomteploelektroprojekt Institute, Ministry of Energy of the U.S.S.R., Riga, appointed General Rapporteur by the Government of the U.S.S.R. R.105
INTRODUCTION Existing reserves of fuel for electric power production and heating are very limited and their depletion is continuing. Additional energy sources such as geothermal energy (including the energy of hot dry rock), which at present are merely highly desirable, may, in the near future, become indispensable to a country's normal, social and economic development. From this point of view, the question of the economic efficiency of obtaining energy from a particular source may be less important than that of its availability. Many specialists dealing with environmental protection problems believe, not without justification, that geothermal energy may prove to be one of the "cleanest" sources of convertible energy at humanity's disposal. This view is based, not on any lack of problems and unsolved questions concerning the environmental aspects of geothermal energy production (discharge of saline waters, noise, emission of associated gases, surface subsidence, seismic effects, etc.) but on the uniqueness of geothermal energy in the sense that all activity, including extraction and conversion, is concentrated in one place in the immediate vicinity of the consumer and that, for this reason, the environmental effects of the development of geothermal reservoirs are completely confined to the area of the geothermal site. Although calculations of the geothermal energy reserves that are exploitable with the stateof-the-art technology have shown that such reserves are not very large (corresponding to approximately 1% of total consumption), the solving of a number of engineering, social and legal problems in the process of commercial utilization of geothermal energy is already making it possible to prepare for the utilization of the vast reserves of energy locked in hot dry rocks. H u m a n utilization of geothermal energy began in sectors other than electricity generation, but was of a haphazard nature and was for the most part confined to sources having a natural outlet to the earth's surface. In recent years, and especially since the beginning of the energy crisis, extensive research has been undertaken in many countries into the detection of geothermal energy reserves and into various aspects of techniques for the utilization, at first, of high-enthalpy sources for electricity generation and, later, of medium- or low-enthalpy sources for industrial, agricultural and domestic use. This research shows that the proven geothermal reserves exploitable with the present technology have, for the most part, medium- or lowenthalpies and that, a most important point, they occur in areas close to existing or prospective energy users. Some geothermal reserves, particularly the highly mineralized ones, can also serve as a source of feedstocks for various enterprises (extraction of rare metal and other mineral substances, production of dry ice, etc.). 165
166
V. N. Kruzhelnitsky
Growing interest is being shown in the utilization of medium-enthalpy geothermal heat sources, despite the fact that they are inadequate for economic electric power generation and, in many cases, too expensive to be used for heating premises or agricultural objectives. For industrial processes, however, such geothermal heat sources are ideal, especially since the industrial use of geothermal heat is in many cases its most rational utilization from the point of view of the constant output and the high annual well utilization factor. It used to be thought that geothermal production sites, which are "industrial" by definition, were incompatible with residential and recreational areas on account of their noise, smell and landscape-disrupting effects. However, recent technical developments (submerged pumps, downhole heat exchangers, closed circuits with reinjection of thermal water), as well as practice in the commercial utilization of geothermal waters, have proved that a satisfactory degree of compatibility can be achieved. All the above explains the continually growing interest in the utilization of geothermal energy in industry, agriculture and the communal and domestic sectors. UTILIZATION IN VARIOUS SECTORS: A G R I C U L T U R A L , INDUSTRIAL AND DOMESTIC Utilization level In 1982, the installed capacity for non-electrical uses of geothermal energy (domestic heating, agriculture, industrial processes), not including balneotherapy, was 4050 MWt. The countries which use geothermal energy most (not for electricity) are the following (in decreasing order of utilization): Iceland, Hungary, U.S.S.R., China, New Zealand, U.S.A., Japan, Italy, France, Romania and Czechoslovakia (Table 1). Besides these countries, geothermal research is being carried out in many other areas. It is particularly advanced in Turkey, Costa Rica, Colombia, Ecuador, Peru, Bolivia, Argentina, U.K., Spain, Federal Republic of Germany, Poland, Bulgaria, Djibouti, Iran and Thailand. More detailed information on some characteristic countries with different levels of achievement in the field of non-electrical uses of geothermal energy is given below. F a b l e 1. A s s e s s m e n t o f direct uses o f g e o t h e r m a l energy: m a x i m u m c a p a c i t y o f plants in o p e r a t i o n in 1982 ( M W t ) Total I n c l u d i n g Not including bathing , bathing
H e a t i n g , airconditioning
Agriculture
Industry
Balneology
Undefined uses
A n n u a l utilization factor
0.4
0.3
0.6
(I.4
--
(I.35
0.4
Austria China France Hungary Iceland Italy Japan New Z e a l a n d Romania Czechoslovakia U.S.A. U.S.S.R. Yugoslavia Others
2 70 165 75 780 107 50 50 27 25 87 140 14 33
-60 15 565 77 56 31 10 9 I0 16 395 . 56
-195 -289 164 +
2 339 180 959 1096 190 90 226 36 35 221 555 14 1(17
1625
1300
4050
Total
3 17
8 4 360
---106 --
17
248
I
5 356 180 1540 1305 566 4484 226 36 43 225 915 14 355
370
6200
755
10,250
14 30 75 27 9 166
581 209 376 4394 --
12 20 .
.
.
Space and Process Heating
167
Italy. In Italy, there are at present three major centres of direct utilization of geothermal sources: (i) At Larderello, where low-enthalpy geothermal steam ( 1 0 5 - 1 3 2 ° C ) is used to heat 348,000 m 3 of premises and greenhouses. (ii) In the Monte Amiata area, where geothermal fluids, after use at a geothermal powerstation, are used to heat greenhouses, a drying plant and premises of various kinds. The entire installation envisaged in the project has been commissioned in 1983, consumption amounts to 1 5 0 , 0 0 0 - 3 5 0 , 0 0 0 Gcal a year. (iii) At Abano, where geothermal energy is used for district heating of hotels and greenhouses. District heating projects are being realised in Milan, Vicenza, Ferrara and Cesano (Rome). Other projects for agricultural or multipurpose use in Tuscany at Pomarance, Monterotondo, Radicondoli and Siena and on Ischia and Sardinia islands are under study. In 1981, geothermal plants for space heating and domestic water supply yielded an oil saving estimated at approximately 33,500 tons, or, with the inclusion of agricultural and industrial uses, 42,500 OET. Under the national energy programme, a saving of the order of 300,000 OET a year is to be achieved by 1990 as a result of non-electrical uses of geothermal energy. Taking account of the consumers' potential, the figure reached by the year 2000 or later may amount to I million p.a. Low-temperature geothermal sources brought into use by June 1983 have an energy equivalence of l million Gcal/year.
Table 2. Principal Italian projects (in operation or planned) for non-electrical uses of geothermal energ,, (1983) Temperature of the fluid or condensate (°C)
Technology Place
Maximum capacity of plant (MWt)
Gcal/year
80 65
80 3.5
150,000 8000
100 60
14 2 23 3 27 6 3 1 9 2 5(1 58 21 6.5 12 12 1.1 3.2 3.2 2 1.2
Direct use Abano Basin of ~vater Galzignano E
7.
.-"z = ..~ e.-,
o '~ :--
:'8 =
g + ~. ~= x: ~ ~ ~ ~ '~ ..= ~= ~ ~ = -~ e-, ~ fi! ~ m
~ ~" '.v =
Ferrara Metanopoli Larderello -) Lago Castelnuovo Castelnuovo Pomarance Bulera Radicondoli Amiata Piancastagnaio
)
Pantani Cesano Siena Vulcano lschia Padova Vicenza Bagni di Romagna Acqui Terme
100
200
95 120 90 140 (80 ~ 90 90 55 150 200 70 20(1 90 70 65 26-42 70
Utilization factor (070)
Stale of project
Areas of use
25 30
O O
H, B G
75,000 5000 40,000 10,000 90,000 9000 7500 7000 23,000 3000 100,000 220,000 50,000 8600 30,000 80,000 8550
70 33 23 44 44 20 33 90 _'~3 20 27 50 31 18 33 85 100
C C
P O P P C O P C G P C P
H H H G IND H G G,H G H,E G,E DR,E G H H DES,E DES
11,000 9000 3000 2000
46 37 20 22
P (7 C P
H H H, B H
O
O: Operational; C: construction; P: planning; H: heating; G: greenhouses; IND: industry; DR: drying; DES: desalination; B: balneology; E: electricity.
168
V. N. Kruzhelnitsky
U.S.S.R. The country's energy potential from underground heat sources is large. According to the Academy of Sciences and the Ministry of Geology, proven thermal water resources are sufficient for the extraction of 2 0 - 22 million m ~a day over a prolonged period. Ninety per cent of these waters have low enthalpy and most are highly mineralized. However, thermal waters are unevenly distributed over the cotlntry's territory. More than 70% of proven thermal water reserves occur in sparsely-populated regions of Siberia and the Soviet Far East, which limits the possibility of their large-scale utilization. Today, low-grade thermal waters are used to heat and supply hot water to residential and industrial premises, to heat greenhouses and hothouses for therapeutic purposes, for intensified fish breeding in ponds and to heat livestock farms. Thus, a thermal well with a flow rate of 1500 m ~/day and a temperature of 60°C at the wellhead supplies heat and hot water to a residential area of the town of Makhacbkala, with a resulting economy of 1200 OET per annum. At present, small areas of Tbilisi and Makhachkala, some small settlements, and a State farm with greenhouses in the outskirts of Grozny are heated and provided with hot water using the heat of underground sources. In the Mostovsk area of Krasnodar territory, geothermal heat is supplied to a 19 ha greenhouse complex, a livestock farm and 200 ha of fish ponds. In the next few years, geothermal heating schemes are to be implemented in A l m a - A t a , Tashkent, P e t r o p a v l o v s k - Kamchatsky, the Tskhaltubo resort and elsewhere. A long-term plan for the integrated use of geothermal energy until 1990 has been drawn tip. It provides for this new energy source to play a substantial part in the country's fnel and energy balance. Prospecting for and extraction of thermal waters will be considerably expanded, leading to the saving of around 1 million OET by 1985. France. France has its own national programme for the development of geothermal energy. In addition to the central area of the Paris Basin, where the Dogger reservoir is being explored, there are more than 30 other developments out of a possible total of 300. Several successes have been achieved in Aquitaine, particularly in the Bordeaux arca, where the wells in operation make possible an annual fuel saving of 2s00 OET. Work on geothermal recovery from oil wells is scheduled to the south of this area. A project for supplying heat to greenhouses with a total area of 3.5 ha is being executed at Lamaz6re and will save 2500 OET a year. The favourable areas are in the Paris region and in Aquitaine, it should be noted, however, that inventory of France's geothermal resources is far from complete. Turkey. Extensive geological, geophysical and geochemical investigations of geothermal sources have been carried out in Turkey. Results indicate that at least 31,000 MW (thermal) will be available for non-electrical uses. The largest geothermal fields known today are the follo~ing: The S a r a y k O y - B u l d a n , where about 1600 t/h of geothermal fluid ~ith a reservoir temperature of 200 212°C are obtained from six productive wells in the Klzllderc geothermal field. After utilization at a geothermal pox~er-station, the fluid (100°C) will be used to heat 500 acres of greenhouses. Satisfactory results have been achieved to date with the heating of greenhouses of an area of 1 ha. Because of its chemical properties, the geothermal fluid will be used in the textile industry for bleaching. Production of 200 t of dry ice a day is planned. The Afyon area, where, as a first stage, the heating of 2 ha of greenhouses has been successfully organized. The area, ,':hich possesses considerable geothermal water reserves, is capable of serving as a reliable source of heal for tourist centres and resorts, induslrial enterprises and agricultural units. A district heating scheme has been prepared for the tOXVlaof Afyon (heat load: 331 Gcal/h).
Space and Process Heating
169
The G e r m e n c i k - Omerbeyli area, where a m a j o r geothermal deposit with a temperature of 231 °C and free fi om scale formation has been discovered. This source is suitable for use in the canning and textile industries as well as for the heating of premises and greenhouses and the development of tourism and of a network of health resorts. Belgium. Geothermal energy is to be used as the energy source in new municipal heating networks at Mons, with a total annual fuel saving of 2600 OET, including 2100 O E T from the heating of buildings and 500 O E T from the heating of greenhouses and the preheating of sludge at a waste water plant. Research operations, including drilling at Douvrain and Ghlin and in the Campine area, are continuing. Greece. Although the main effort in the geothermal energy field is concentrated on highenthalpy wells, a start was also made in 1981 in the use of low-enthalpy sources: one well for heating greenhouses and a second well in the northern part of the country for domestic heating and other initiatives are envisaged in the agricultural sector.
Areas of application The energy of geothermal waters is used in a wide variety of applications. For agricultural uses, systems with a water temperature below 100°C or between 100 and 150°C are of particular interest. Agricultural uses of geothermal energy (principally to heat greenhouses and water and for irrigation and refrigeration) overlap areas of application of solar energy, although, owing to the nature of geothermal fluid, its high temperature and its abundance, some applications (hydroponics, hot beds, winter warm irrigation, anti-parasitic sterilization of greenhouse soil) are of course, typically hydrothermal. In some countries (China, Hungary, Japan), geothermal heat is also used to heat incubators in poultry-farming and livestock sheds. In addition, if its salt concentration permits, geothermal water can be used directly for growing high-protein algae and in fish breeding, as is done in some countries (China, Japan, U.S.A., U.S.S.R.). Preservation of agrozoo-technical products by geothermal preparation drying, freezepasteurization or sterilization is practised in a number of other countries. Algae growing and fish breeding are practised in Greenland, wood drying in New Zealand, lucerne-fodder and vegetable growing in the U.S.A. and soil-heating in hot beds in the U.S.S.R. Geothermal energy can also be used in a wide variety of industrial processes. Table 3 lists a number of industrial sectors and the temperature required for each. The table demonstrates the extensive possibilities for the use of geothermal energy.
M E T H O D S OF U T I L I Z A T I O N OF G E O T H E R M A L E N E R G Y Research has shown that the exploitation of low-enthalpy geothermal heat is based mainly on the following solutions: (a) direct heat exchange, applied when the temperature of the geothermal water is higher than the users require; (b) utilization of heat-pump cycles; (c) utilization of mechanically compressed steam obtained by the flashing of geothermal water. Solutions (b) and (c) are necessary if the temperature required by users is higher than that of the geothermal water. Where the temperature of the geothermal water is not high enough to meet user requirements but is sufficient for preheating purposes, an intermediate technical solution between (a) and (b) or (c) may be proposed. Combinations of all three methods are also possible.
V. N. Kruzhelnitsky
170
Table 3. Temperatures of geothermal fluids required in industry (°C) Energy production Power generation (wet or dry steam) Power generation (Rankine cycle) Heating in the municipal sector, water for domestic use Heat supply with heat pump Refrigeration Air-conditioning
>140 >90 60 - 130 25 - 60 140 180 70 120
Chemical industry Plastics Aluminium oxide (Bayer process) Petrochemistry Explosives Pharmaceutical Synthetic rubber Soaps and detergents
85 240 140 150 120 150 90 - 150 65 125 25 - 95 80 85
Agrozootechnical and food industries Distillery Canning Beer, malt Vegetable oil Fried potatoes Sugar Fodder (dried) Greenhouses Milk Tobacco, cigarettes Dairy industry Meat (slaughtering and processing) Grain (drying) Soft drinks Zootechnics, animal husbandry Cultivation of mushrooms and ferments Warm irrigation, soil heating (hot beds) Hydroponics
100 160 90 150 75 150 70 - 150 60 200 50 - 140 80 -- 135 60 130 70 120 105 40- 95 40 85 40 80 25 80 25 60 20 - 50 20 35 15 35
Extractive industries Building materials Desalination Oil regeneration Other industries Forest industry Paper, pulp, limber Textile industry Balneology
50 I10
150 120 9(1
55 50 90 20-
180 150 155 150
The above-mentioned solutions presuppose the possibility of using a variety of process paths: direct u t i l i z a t i o n o f g e o t h e r m a l w a t e r ; use via a d o w n - h o l e o r u p - h o l e h e a t e x c h a n g e r ; use in c l o s e d circuits, etc. In p r i n c i p l e , m e t h o d s (a) a n d (b) can be r e g a r d e d as t w o v e r s i o n s o f the s a m e m e t h o d if the h e a t p u m p is c o n s i d e r e d to be a v a r i e t y o f h e a t e x c h a n g e r (with an i n t e r m e d i a t e h e a t - t r a n s f e r m e d i u m ) . H o w e v e r , b e a r i n g in m i n d the special f e a t u r e s o f t h e i n t e r m e d i a t e h e a t t r a n s f e r m e d i u m used in h e a t p u m p s a n d t h e r e l a t i v e n o v e l t y o f its use in the g e o t h e r m a l e n e r g y field, t r e a t i n g m e t h o d (b) as d i s t i n c t f r o m direct h e a t e x c h a n g e m u s t be c o n s i d e r e d c o r r e c t . A s a l r e a d y n o t e d , m o s t o f the g e o t h e r m a l fields h a v e l o w - e n t h a l p y , a n d the a p p l i c a t i o n o f m e t h o d s (b) a n d (c) facilitates the e x t e n s i v e use o f g e o t h e r m a l w a t e r s w i t h v e r y l o w - e n t h a l p y o r m o r e c o m p l e t e use o f the e n e r g y o f g e o t h e r m a l w a t e r in s y s t e m s c o m b i n e d w i t h the direct h e a t e x c h a n g e m e t h o d . T h i s e x p l a i n s the c o n s i d e r a b l e i n t e r e s t s h o w n in this aspect by specialists from a number of countries.
Space and Process Heating
171
The COP (Coefficient of Performance) of heat-pumps is of particular interest for temperature differences between condensation and evaporation of up to 40°C, with only very slight dependence on the temperature level, while on the contrary, temperature differences exceeding 80°C are of no interest at all. For intermediate situations between 40°C and 80°C, further investigation of the problem is called for. As to compressed steam, the same conclusions apply, except that they relate to the temperature difference between the thermal level required by users and the temperature on compressor intake. Using compressed steam offers some increase in efficiency compared with that obtainable with heat-pump cycles; however, this solution presents difficult problems, above all low flow rates and low temperatures at the compressor intake. The most efficient use for heat-pump systems is in the introduction of air-conditioning in southern areas, where the duration and energy consumption of the cooling period are commensurate with those of the heating period. For example, a scheme has been devised for the use of heat pumps to provide heating and cooling at the Tskhaltubo health resort (waste water from thermal baths). When implemented, it will yield a saving of 50,000 OET a year. R E I N J E C T I O N AND DISTRIBUTION PROBLEMS The problems with the reinjection and distribution of geothermal fluids derive from the inherent nature of the fluids, i.e. their physical and chemical properties. The most important of these properties are: (i) the high concentration of minerals, including carbonates, in most geothermal fields; (ii) a tendency towards scale formation, especially in the event of changes in temperature conditions; (iii) a change in heat content on transition from the liquid to the gaseous phase depending on pressure and temperature (instantaneous boiling); (iv) the presence of dissolved gases in the geothermal fluids; (v) corrosive effects and (vi) erosive effects. The extent to which these problems can be solved will largely determine the economic profitability, and sometimes even the feasibility, of the commercial exploitation of each particular geothermal field. One way of solving some of the above-mentioned problems is to keep water under pressure inside the well and to extract the water from the well at high temperature. This means that pumps must be installed down the well. This method is dictated by a wish to avoid: instantaneous boiling with the resulting heavy energy loss; the sedimentation of carbonates; and the emission of dangerous dissolved gases. Some problems can also be solved by installing a heat exchanger at the top of the hole in cases where, owing to its heavy mineral concentration, corrosive effects and scale formation, the geothermal fluid cannot be delivered directly to the consumer. The main points to be borne in mind in choosing, designing, building and operating heat exchangers are the following: fouling of the heat-exchange surface and deterioration of the hydraulic and heat-exchange characteristics; difficulty of ensuring a long service life, including reliability; prevention of corrosion and erosion of the heat-exchange surfaces; prevention of hydrodynamic vibration; transient thermal stresses; difficulty of guaranteeing low manufacturing and operating costs; possibility of repairing and cleaning the surfaces; the requirements of compactness and efficiency. Experience with downhole heat exchangers utilizing a single or multiple " U " tube (at Klamath Falls in Oregon, U.S.A., at Rotorua and Taupo, New Zealand and at Afyon, Turkey) have yielded satisfactory results. In addition to solving the same problems as a heat exchanger placed at the wellhead, a downhole heat exchanger solves the same problems as a downhole pump and makes it possible to dispense with the reinjection of waste geothermal water.
172
V. N. Kruzhelnitsky E C O N O M I C FEASIBILITY
A good deal of experience has been acquired in various parts of the world over the past 10 years in the economics of prospecting, drilling and developing the necessary equipment, and solving environmental and operational problems, so that the commercial efficiency of a specific proxen geothermal deposit can now be determined with a fair amount of precision. Some general conclusions can also be drawn, as follows: Ca) of all types of new energy sources, the use of geothermal energy offers the greatest economic advantages; (b) a fairly large number of proven geothermal sources are or could be economically profitable in exploitation and competitive with conventional sources based on fossil fuels; (c) the economics of geothermal energy production is largely determined by drilling costs and by the prices of fossil fuels, especially oil; (d) in view of the existing trend towards more stringent enviromnental protection requirements, the economic efficiency of using geothermal sources will increase even though oil prices are falling; (e) the economic profitability of a system based fully or partly on geothermal energy should be greater than that of a conventional system and the payback period should be shorter; (f) since high initial capital investments have to be regarded as forming part of the fuel costs and must be compensated by low operating and service costs, the overall saving from using geothermal energy is largely determined by the annual utilization factor rather than by the system's efficiency; (g) in systems requiring appropriate volumes of geothermal fluid and thermal installations to regulate the temperature, the scale effect and integration (seasonal or cascade) are indispensable; (h) geothermal systems are generally incorporated in a larger system with a base load such that the annual utilization factor for the system as a whole is not lower than 0.5, that being the minimum value for acceptable economic results; (i) the economic efficiency of highly-mineralized geothermal waters rises considerably if the waters are used in an integrated manner and not only for their energy potential (development of balneotherapeutic complexes in the case of waters with curative properties, extraction of rare metals or other mineral substances, etc.); (j) in many cases, the industrial use of geothermal energy is the most rational method of utilization from the point of view of the volume required and the high annual utilization factor; (k) because of inadequate data and the difficulty of defining geological and physical conditions, it would be risky to estimate reserves in geothermal systems at depths greater than 3000 m; (1) when the temperature of the geothermal fluid is low (around 60°C) and well depth is 1500-2000 m, utilization of the fluid is economically justified only in large-scale projects (e.g. greenhouses with a total area of at least 10 ha); (m) in the case of wells of medium depth (of the order of 1500 2000 m), the minimum annual output which will ensure the viability of the enterprise is at present around 15,000 Gcal; (n) in view of the financial risk involved and the need for preliminary research, the setting up of a " s o u r c e - u s e r " system, promotion activities and publicity, the best preconditions for the development of geothermal energy are to be found in countries with a planned economy and those which have established national programmes of their own or are participating in the programmes of international communities.
Space and Process Heating
173
Further and more intensive development o f geothermal energy p r o d u c t i o n is subject to considerable constraints and difficulties o f an economic nature, viz.: (a) considerable financial risk, owing to the need for heavy capital investment at a time when the geological results are still uncertain; (b) the considerable time-lag between the start o f financing and the attainment o f profitability; (c) where geothermal resources are used in industry, agriculture or the c o m m u n a l sector, the accessibility, suitability and, ultimately, economic efficiency o f the geothermal source itself are by no means always the priority criteria in the choice o f the site for industrial or residential development; (d) the exploitation o f geothermal waters in industry at a utilization factor three to four times higher than in district heating systems usually entails changes in plant design and these are not always compatible with investment payback periods acceptable to industry; (e) the high well-productivity threshold o f an economically justified enterprise (of the order o f 15,000 Gcal a year) entails difficulties both as regards finding the geothermal source and as regards finding a user for the heat; (f) since potential users have only a limited idea o f the possibilities offered by geothermal energy and since there exists no industrial technology for the utilization o f " d i f f i c u l t " fluids, publicity and p r o m o t i o n activities are indispensable. Lastly, local development p r o g r a m m e s do not always correspond to potential geothermal energy reserves. LIST OF REPORTS EP/SEM.9/R.4 EP/SEM.9/R.7
Geothermal fluids in agriculture and animal husbandry. R. Lesmo and C. Sommaruga (Italy) (E). Preliminary investigations o f low enthalpy geothermal energy utilization for industrial process heat consumptions. P. Ceron, L. Faletti, C. G. Palmerini and C. Piemonte (Italy) (E).
EP/SEM.9/R.15 Non-electric uses o f the geothermal resource: economic considerations. G. Calabr6 and G. Trambaioli (Italy) (E). EP/SEM.9/R.21 The demonstration projects o f the European Communities in the field of geothermal energy. G. Gerini (European Economic Communities) (F). EP/SEM.9/R.33 Geothermal model of Denizli, SaraykOy - Buldan area. S. Simsek (Turkey) (E). EP/SEM.9/R.46 Utilization of geothermal energy of Afyon area in domestic heating and agriculture. B. Erisen (Turkey) (E). EP/SEM.9/R.52 Geothermal activity in Italy: present status and future prospects. R. Carella, G. Verdiani, C. G. Palmerini and G. C. Stefani (Italy) (E). EP/SEM.9/R.53 Industrial non-electric uses o f geothermal fluids. M. Brianza, M. Guglielminetti, G. lnvernizzi and C. Sommaruga (Italy) (E). EP/SEM.9/R.54 Sedimentological and diagraphic characteristics o f the Dogger in the geothermal borehole at Aulnay-sous-Bois. J. Rojas and D. Giot (France) (F). EP/SEM.9/R.59 Utilization o f thermal waters in the U.S.S.R. for space heating and industrial processes. V. A. Vasilov, V. K. Muslimov and Y. I. Sultanov (U.S.S.R.) (R). Abstract only. EP/SEM.9/R.75 Utilization o f geothermal energy in heating systems. E. V. Sarnatski (U.S.S.R.) (R). EP/SEM.9/R.78 Geothermal heating systems using heat pumps. P. Jaud (France) (F). (E) Original in English (F) Original in French (R) Original in Russian.