Technical-economic aspects of the utilization of geothermal waters

0375-6505/87 $3.00 + 0.00 Pergamon Journals Ltd. ~/1987 CNR.

Geothermk's, Vol. 15, No. 5/6, pp. 857-879, 1986 Printed in Great Britain.

T E C H N I C A L - E C O N O M I C ASPECTS OF THE U T I L I Z A T I O N OF G E O T H E R M A L WATERS ENRICO BARBIER

International Institute for Geothermal Research, Piazza Solferino 2, 56100 Pisa, Italy A b s t r a c t - - A brief description is given of the physico-chemical parameters characterizing a hot water

geothermal reservoir and of its exploitation by means of single or coupled (doublet) wells. The technical aspects of geothermal heat to the users is then discussed, beginning with corrosion of materials caused by seven main agents: oxygen, hydrogen sulphide, carbon dioxide, ammonia, hydrogen, sulphates and chlorides. A brief mention is made of scaling due to calcium carbonate, silica and calcium sulphates. The basic components of a geothermal plant for non-electric uses are then discussed: production pumps, surface pipelines, heat exchangers, heat pumps and reinjection pumps. The advantages and disadvantages of the different equipment and materials used in the geothermal sector are also presented. A list is also given of the criteria used in the energy and economic balance of a geothermal operation. FOREWORD This r e p o r t deals with n a t u r a l hot waters whose t e m p e r a t u r e is generally no higher t h a n boiling p o i n t at the surface. This sector o f g e o t h e r m a l energy is r e f e r r e d to as the low enthalpy sector. T h e r e are f o u r m a i n aspects o f a g e o t h e r m a l i n s t a l l a t i o n o f this type: - - f i n d i n g the g e o t h e r m a l heat, - - t h e g e o t h e r m a l heat to the users at the surface, --equipment and materials, - - t h e energy a n d e c o n o m i c b a l a n c e o f the g e o t h e r m a l o p e r a t i o n . FINDING THE GEOTHERMAL

HEAT

In o r d e r to exploit g e o t h e r m a l energy we m u s t first f i n d a reservoir o f hot fluids, t h a t is, a p e r m e a b l e geological f o r m a t i o n whose stored fluids can be b r o u g h t to the surface t h r o u g h wells. A geothermal reservoir has physical a n d h y d r o d y n a m i c characteristics t h a t g o v e r n well p r o d u c t i o n . T h e m o s t i m p o r t a n t physical c h a r a c t e r i s t i c s are the depth o f the reservoir, its extension, thickness a n d its pattern. T h e m a i n h y d r o d y n a m i c characteristics are permeability, which is a m e a s u r e o f the ease with which the g e o t h e r m a l fluid flows within the reservoir, transmissivity, which is the p r o d u c t o f p e r m e a b i l i t y a n d reservoir thickness a n d c o n s e q u e n t l y the p a r a m e t e r with the greatest influence on well p r o d u c t i o n , a n d , finally, static pressure o f the fluid c o n t a i n e d in the reservoir, which m a y o r m a y not p e r m i t the wells to p r o d u c e s p o n t a n e o u s l y w i t h o u t using p u m p s . T h e geothermal fluid in the reservoir also has its o w n p a r a m e t e r s , the m a i n ones being temperature, salinity a n d dissolved gases. Once we have d e f i n e d all these characteristics we can e v a l u a t e the f l o w - r a t e s a t t a i n a b l e in a set p e r i o d o f t i m e (the c o m m e r c i a l life o f the reservoir) a n d the p o w e r o f the p u m p s used to extract the fluid. E v e n in the case o f a r t e s i a n wells, the fluid has quite o f t e n to be p u m p e d to reach suitable flow-rates, d e p e n d i n g o n the utilization. E x p l o i t a t i o n o f a g e o t h e r m a l reservoir t a k e s p l a c e t h r o u g h wells. T h e r e are t w o possible types of production: - - a single, p r o d u c t i o n well - - a couple o f wells (doublet), one o f which is the p r o d u c t i o n well and the other a reinjection well. 857

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t:nrico Barbier

The single well solution can only be adopted when the geothermal fluid can be discharged on the surface after its utilization; in other words, if there is no risk of polluting the environmenl with the salts and gas contained in the fluid. The economic convenience of using a single ~ell is obvious, as drilling can represent as much as 70% of the total cost of a geothermal projecl. This solution, therefore, entails a smaller, more accessible capital investment, especially when the project is on such a small scale tha~ there is no possibility of amortizing the cos! of a reinjectior~ well. The drawbacks to the single well solution are initially of a technical nature but eventually have an adverse effect on the economics of the project. A pressure drop will, in fact, probably occur in the reservoir as a result of exploitation, but this could in part be avoided with a reinjection well. Experience has also shown that exploitation of a reservoir by means of single wells only (i.e. with no reinjection) does not lead to intensive production because of the negative interaction between neighbouring wells, which can also lead to a considerable decrease in flow-rate. Reducing the density of the production wells in order to avoid these negative effects unfortunately also entails lower total flow-rates. The coupled wells solution (doublet) offers the obvious advantage of not polluting on the surface, of reducing the pressure drops in the reservoir by recharging the system through the reinjection well and, consequently, of permitting a more intensive exploitation of the resource. O f course, the location of the reinjection well must be chosen with due care in order to avoid cooling the zone around the production well. For example, the wellbottoms are usually from 1000 to 1500 m apart, whereas on the surface the two wellheads are right next to one another; that is, the two wells can be drilled without moving the rig, allowing considerable savings in dismounting and re-assembling the drilling rig elsewhere. There are various possible solutions (vertical production well, directional reinjection well, both wells directional, etc., Fig. 1) (AFME, 1983). The effects of the interaction between the production and reinjection wells on reservoir behaviour can be predicted by simulation models. These models describe the advance of cold fronts in time from the reinjection wells to the production wells (i.e. the temperature decrease in the fluid produced); they can thus be used to predict the lifespan of the reservoir and thus optimize development of the associated geothermal utilization project (Fig. 2). The drawbacks to the doublet are that drilling costs twice as much as a single well, the added cost of running the reinjection pump, of installing pipelines and instrumentation for monitoring reinjection. However, a reinjection well is usually capable of taking in more fluid than can be produced by a production well, which means that less reinjection wells are needed than production wells.

T H E G E O T H E R M A L H E A T TO T H E USERS

General problems." corrosion and scaling Geothermal waters usually contain variable quantities of elements and chemical compounds, sometimes more than 300 g/l, that are responsible for corrosion or incrustation in geothermal plants. The temperature of the geothermal fluid and its chemical composition are closely related, and a temperature increase normally corresponds to an increase in the salts dissolved in the fluid. The chemical composition of the fluid is clearly tied to the type of rock forming the geothermal reservoir and thus would be expected to change accordingly. However, some elements and chemical compounds are more or less present everywhere. There are seven main corrosive agents: oxygen (gas in the fluid), hydrogen sulphide (gas), carbon dioxide (gas), a m m o n i a (gas), hydrogen (as an ion), sulphates and chlorides (as solids).

Technical- Economic Aspects of the Geothermal Waters

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Oxygen is the most aggressive chemical in the geothermal fluid and it usually is of atmospheric origin, the result of air penetrating the plant. Dissolved oxygen is not a natural component of geothermal fluids between 50 and 100°C if these fluids also contain traces of hydrogen sulphide. Where an oxygen concentration of more than 10 ppb (parts per billion) is detected in the presence of hydrogen sulphide (another strong aggressive agent), then we can assume that there is either a leakage of air in the geothermal plant or a mixing of very shallow waters with the hot fluid (GRC, 1979). Oxygen and hydrogen sulphide are not usually found in the fluid simultaneously as one tends to eliminate the other. It appears from the many case histories that the concentration of oxygen is closely tied to the rate of corrosion of steel that is in contact with geothermal waters. For example, a carbon steel pipeline carrying the same type of fluid (pH 4.5 - 6.0) corroded at a rate of less than 0.1 mm/yr,

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but this increased to 25 m m / y r (250 times as fast) in the presence of oxygen and hydrogen sulphide (EEC, 1983). It is rather difficult to inhibit contact with oxygen once the geothermal fluid has reached the surface, especially if submersible pumps are not used. Even when the fluid is kept circulating at a pressure above atmospheric it is difficult to prevent air inleakage at the pump seals, especially if plant maintenance is not up to standard. However, in the design stage it is always prudent to assume that the geothermal fluid may contain traces of dissolved oxygen (Smith and Ellis, 1983). Hydrogen sulphide is nearly always present (at least a few ppb but usually a few ppm) in geothermal fluids above 50°C (Ellis and Conover, 1981). This is a very corrosive chemical, especially on copper or cupronickel alloys. Concentrations of a few ppb only are capable of considerable damage and this is further exacerbated if oxygen is also present. Carbon dioxide is normally present in m a n y geothermal fluids: it affects the pH and corrodes carbon steel. Experience has, however, shown that the higher the p H of a geothermal water, the less corrosive is this gas. This factor is taken into due consideration in the chemical treatment of geothermal fluids to reduce their corrosion effects (Karlsson, 1982). Ammonia, even in traces, can give rise to stress-corrosion cracking in some copper alloys and can also increase the corrosion rate of mild steel. Hydrogen, as an ion, has the effect of decreasing the pH of the fluid, and consequently making it more aggressive; this results in local breakages of the passivation film. The latter is a thin layer of oxide that forms on the surface of a metal (for example, stainless steel) and increases its resistance to corrosion. Sulphates play a minor role in material corrosion, although they may become the most aggressive anion in fluids with low chloride concentrations. Chlorides in geothermal fluids accelerate corrosion in that they increase the electric conductivity of the water. They often cause local breakages of the passivation film, which are far more dangerous than an increase in uniform corrosion rate, the usual consequence of an increase in chloride concentration. Nevertheless, it would appear that corrosion in low-alloy steel increases with the concentration of chlorides, but remains within acceptable limits if there is no carbon dioxide, hydrogen sulphide or oxygen present (EEC, 1983).

Technical- Economic Aspects o f the Geothermal Waters

861

Other chemical elements in the geothermal fluids, especially the heavy metal ions (copper, lead, mercury), can have a detrimental effect on the materials of the plant, although exactly what effect they do have, particularly in low concentrations, has not yet been clearly defined (Reeber, 1980). Various micro-organisms are also conducive to material corrosion. The most important of these are the sulphate-reducing bacteria and iron-oxidizing bacteria. Microbiological research is now being directed at defining these relatively unknown phenomena and at developing suitable biocides for combatting them (Citernesi et al., 1985). Scaling, which sometimes develops in plants with a circulation of geothermal fluids, may originate in two ways: from the deposition of minerals contained in the geothermal fluid as a result of supersaturation and from the accumulation of corrosion products. Scaling reduces the efficiency of the geothermal plant by increasing resistance to fluid movement and, consequently, decreasing heat transfer. The most common form of scaling is calcium carbonate (calcite) and silica. In some cases there are incrustations of calcium sulphate and even of heavy metal sulphides such as copper, lead and silver, if these are present in large concentrations in the fluid. Scaling due to calcium carbonate can be avoided by keeping the CO2 partial pressure of the fluid at bottomhole values. Calcium carbonate solubility tends to increase as temperature decreases, and as solubility increases the carbonate obviously tends not to precipitate. However, in the 4 0 - 90°C interval, which is the most common with geothermal waters, temperature has little influence on the solubility of calcium carbonate (Coury & Assoc., 1980). The solubility of silica, on the other hand, increases with temperature and its deposits are also far more difficult to remove from the geothermal plants. Dissolved silica frequently deposits at spots where corrosion has already begun to develop, so that it is considered a corrosion inhibitor. However, if the silica content of the geothermal water is not large enough to create a protective layer of scaling, then there is an increased risk of pitting corrosion (Karlsson, 1982). The rate of scale deposition of the chemicals is critically dependent upon the chemical being deposited. Silica deposition is very slow and often takes days to reach equilibrium, as opposed to calcium carbonate scaling (calcite), which can reach equilibrium within less than a second. The rate of scaling of calcium sulphate is intermediate between that of silica and calcite (GRC, 1979). E Q U I P M E N T AND M A T E R I A L S A geothermal plant that uses hot waters either for space-heating, in industrial processes or for generating electricity with binary cycles, will always require all or part of the following basic components: --production pumps, --surface pipelines, - - h e a t exchangers, - - h e a t pumps, --reinjection pumps.

Production pumps In some cases the geothermal wells are artesian, so that there is no need for pumps, or at least not in the early stages of exploitation of the reservoir. In the majority of cases, however, this artesian flow is inadequate and geothermal water must be pumped to achieve a high enough flow-rate; in some cases as much as 250 m3/h (70 l/s) can be obtained from one well. One important benefit of pumping a self-flowing well is the constant pressure of the liquid, which prevents flashing in the borehole, and eventual scaling. Furthermore, by not allowing

862

Emico Barbier

the fluid to flash, the pump discharge temperature can be much higher than the surl~tcc temperature of a self-flowing well. This is an important consideration when high-temperature geothermal applications are desired (GRC, 1979). There are lwo types of production pump currently available, one with a motor at well-hcad and vertical shaft (line shaft pumps), the other is the submergible pump. The line shaft pumps, or vertical turbine putnps (Fig. 3), usually have an electric motor at wellhead, so that maintenance is easier and cheaper. There is also the added benefit that the flow-rate can be varied, either by means of a mechanical regulator (in which case power consumption varies very little) or by an electric regulator (which cuts o f f part of the stator coils from the motor circuit). Thus the pump can be operated at two rates o f flow, corresponding to two different powers. The borehole must obviously be fairly vertical to permit the pump shaft to operate efficiently (AFME, 1983).

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Technical-Economic Aspects of the Geothermal Waters

863

The line shaft pumps have been successfully used in geothermal wells. They increase fluid pressure by the centrifugal force imparted on the liquid by a shaft-driven impeller. In order to achieve the high pressure required in some geothermal service, pumping against very high heads, these pumps frequently contain more than one stage arranged in series. Although the flow through each stage is the same, each stage of the pump successively increases the liquid pressure. The pump suction is at the bottom of the bowl assembly. The fluid progresses upward through each stage and exits in the annulus between the column pipe and the shaft-enclosing tube. Tubing bearings support the shaft at regular intervals ( 1 . 5 - 2 m). The rubber bearings have a temperature limit around 110 - 120°C. The best results have actually been obtained (in Iceland) with Teflon bearings, every 1.5 m, and steel shafts with diameters of 1 in. 3/16 (30 mm) down to 250 m depth; from this depth to 300 m the diameter increases to 1 in. 11/16 (43 mm), with flow-rates between 100 and 180 m3/h (30 - 50 l/s). Lubrication of the Teflon bearings is by filtered geothermal water pumped down the shaft-enclosing pipe. The life of the Teflon bearings in Iceland is 3 - 4 years, as opposed to the 3 - 4 months of other types of bearings (rubber, bronze, babbitt) (Karlsson, 1982). Maintenance of a vertical turbine pump of the above type depends greatly on the fluid being pumped. In low-temperature, low-solids content applications, these pumps can provide years of service without maintenance. In corrosive service, or where solids readily deposit on the metal surfaces, the pumps may operate less than a year before cleaning and repairs are required. Pumping hot fluids obviously leads to expansion of the metals forming the pumping system. The line-shaft is usually made of a different material from the column and tubing, so that their expansion rates will differ. Axial expansion of the shaft increases with the length of the latter, in other words, with the depth at which the pump impeller is placed within the bore; it should therefore be assembled under the assumption that the shaft will have to be raised or lowered after the entire system has equilibrated at the pumping temperature. The power of the pump motor at wellhead is obtained from empirical equations, using tables supplied by the manufacturers. One formula used in hydrodynamics (Streeter, 1961) is: P(kW) =

pump capacity (l/s) × head (m) × specific gravity 76 × 0.735.

As an order of magnitude, flow-rates of 100 m3/h (30 l/s) with a wellhead pressure of 2 kg/cm: and water-level at - 2 2 0 m from groundlevel, require a pumping power of about 120 kW (Orkustofnun, 1984). This figure is about twice the value obtained from the above equation, but it is better to overestimate the size of the pump and have it operate at reduced power, an expedient which will prolong its working life. There are also a few drawbacks to the line shaft pumps, deriving mainly from their long drive shaft; it is important to achieve a good verticality of the well, and this obviously requires greater care during drilling (and costs), especially if the pump is to be installed at considerable depth (maximum depth is around 300 m). The noise of the motor of the wellhead pump is another drawback, particularly if the well is sited near a built-up area. The second type of pumps, and probably the most commonly used in geothermal service, clearly came from the oil field: these are downhole or submergible pumps. The electric motor-pump section is, in this case, under water, in the well, suspended from a production casing and connected to the surface by electric cables (Fig. 4). These pumps, whose diameter ranges from 4 to 14 in., must be installed in wells of wider diameter than usual, generally 13 3/8 in. (34 cm), so as to lower the motor-pump section (around 9 in.) with the production casing (7 in., 18 cm), (AFME, 1983). The induction, three-phase engine is oil-filled for cooling and lubrication and completely sealed. Motor cooling is achieved through heat transfer to the well fluid moving by, and these motors

864

Enrico Barbier

Fig. 4. Submergible pump (TRW REDA pump). have been operating successfully in well fluid temperatures in excess of 150°C. These motors are available in a wide range of diameters, powers and voltages. The pump section (Fig. 5) consists of a multistage centrifugal arrangement quite similar to the vertical turbine p u m p (line shaft pumps). Capacity ranges up to 530 m-'/h (147 l/s) and lifts up to 4500 m (Fig. 6). Due to hydraulic limitations, combined with diameter limitations, tile lift per stage is relatively low; however, as m a n y as 500 stages have been used to meet high head requirements. Through the use of corrosion-resistant materials, pump wear and corrosion are minimized and long-term predictable performance in all normally encountered fluids can be assured (GRC, 1979). A special downhole pump, the Turbopress, has been developed by the French company Pompes Guinard, with the aim of overcoming the severe environmental conditions encountered by conventional electric submergible pumps in geothermal wells (Fig. 7). It has first been developed for use in 13 3/8 in. casings, which is a widely used casing diameter in geothermal wells, but other diameters are available. The downhole set consists of a hydraulic turbine and a centrifugal pump, both driven by the same shaft. The fluid which operates the turbine is taken from the well fluid lifted by the pump. It is supplied to the turbine by a surface " c h a r g e " p u m p and

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through a standard tubing from which hangs the downhole pump set. The advantages are that there is no need to send electricity down into the well, high voltage power cables are vulnerable in the corrosive environment of the wells; the pump is well adapted to high temperature (up to 270°C); there is no use of any plastic materials; the equipment is resistent to corrosion from NaCI and H2S and to abrasion by solid particles; finally, the design of the pump set is such that there are no mechanical seals between the turbine and the pump. This is generally a major source of failure in conventional submergible pumps. Submergible pumps are advantageous because high speed pumps at deep pump settings can be used to increase well production rates above those possible with line shaft pumps. Past

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experience, especially in Iceland, has shown that the submergible pumps are economically more convenient for wells whose water-levels are more than 250 m below ground-level. The main pump manufacturers are the U.S.A. companies Byron Jackson, Reda, £1oway, Peirless and Centrilifl, KSB of the Federal Republic of Germany and Guinard of France.

Pipin~ m#teri#ls Surface production and transmission piping systems are extremely important in a geothermal plant as the viability of a project depends to a great extent on being able to transport the hot water as economically as possible. The pipes are usually well insulated as excessive heat losses lead to a drop in delivered fluid temperature or even non-viability in the user system application. The mechanism of heat transfer in buried or surface pipelines is rather complicated and depends on piping material, insulation, soil and rock characteristics, as well as the entry temperature of the fluid, its velocity and the diameter of the pipes. The pipes used to transfer the geothermal fluid from wellhead to the heat exchangers or directly to the users (if the chemical composition is such as to permit this) can be divided into two categories: metal and nommet#l. The most frequently used metal pipes are made of carbon steel and are chosen for their wide availability, strength and capacity to resist strong pressures. Steel pipes are also quite easy to install and usually there is no problem finding workmen capable of installing them. The joints of these pipes may be threaded, welded or by gland- type couplers with elastomeric gaskets, such as tapered joints with O-tings. These gaskets must be chosen carefully so as to ensure a sufficient pipe expansion and resistance to the operative temperatures and pressures ( o R C , ]979). If buried underground, steel pipes must be well protected from corrosion, especially if laid in very moist or acid soil. The major drawbacks to steel pipes are their high cost and high heat loss if no insulation is used. An uninsulated bare steel line will lose up to ]0 times as much heat as a comparably sized

Technical-Economic Aspects of the Geothermal Waters

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pre-insulated asbestos-cement pipe length. Insulation via wrapping with spun fiberglass or rock wool is one possible alternative. It is not, however, recommended for buried lines since moisture encroaches over time, thereby reducing the efficacy of the insulation. Field wrapped carbon steel is comparatively expensive. Pre-insulated carbon steel is available but costs are high (Goering et al., 1980). Another type of metal piping available on the market are copper pipes and one of the main benefits in this case is that pre-insulated copper piping can be found in the small diameters (< 3 in.) that are not available in asbestos-concrete, cement or synthetic plastic. Handling and installation characteristics of copper pipes are good and, with insulation, the heat loss is low. The main drawback to pre-insulated copper piping is its high cost (Goering et al., 1980), but it has the added disadvantage of interacting electrochemically with the rest of the plant, usually of another metal such as steel. The damage is minimal the lower the oxygen content, but in this case expensive copper or copper alloy pipes would not be required anyway (EEC, 1983). The non-metal piping includes asbestos-cement pipes, fiberglass reinforced polyester pipes (FRP) and some patented plastic pipes, such as polyvinyl chloride (PVC), chlorinatedpolyvinyl chloride (CPVC), high density polyethylene (PE) and polypropilene (Moplen) pipes.

868

Enrico 13arbier

Asbestos cement or concrete piping is commonly used for geothermal transmission lines a~ld these pipes are attractive because of their low initial cost compared to steel pipes. On supply pipes or pressure pipes there is the possibility of oxygen entering the water due to the porosity of the pipes (GRC, 1979). However, there are also other drawbacks, such as the much greater heat losses than in the insulated steel pipes, since the asbestos cement pipe is generally used with no insulation other than the soil and grass cover. Moreover, this type of pipe is much weaker structurally than the steel type, and thus requires special bedding and extra care when backfilling the trench. This entails much higher maintenance costs. These pipes are, therefore, mainly used when the geothermal water has to be transported over long distances (even a few tens of kilometres) and when fluid production costs are pretty low. In this case the heat losses caused by the reduced insulating properties of the material itself are not of great importance (Karlsson, 1982). Last but not least, the asbestos cement pipes are prone to corrosion caused by the geothermal fluid, as revealed by the Iceland case histories. The calcium from the pipe walls is dissolved and mixes with the water. This is, however, a slow process, and it appears that the speed of corrosion is greatest at the beginning, and gradually decreases with time. In any case, it seems safe to assume that the asbestos cement pipe in geothermal service will last at least 20 years (Karlsson, 1982). The fiberglass reinforced polyester pipes (FRP) are capable of withstanding temperatures of 150°C and pressures of 8 kg/cm: (some types only). Maintenance costs over a 12-year period have been nil and the pipe shows no sign of deterioration (GRC, 1979). Fiberglass in general will not carry steam as it breaks the plastic down. The main attraction of fiberglass piping is its resistance to corrosion and scaling. These are also light pipes that cost less to install. The major disadvantage with this material is that extra care must be taken when backfilling the pipeline trench to avoid damage to the outer epoxy coating of the pipe. In fact, once the outer protective coating is broken, soil moisture or rainwater tends to infiltrate the damaged filaments of fiberglass, thus compromising the resistance of the piping. We should also mention that experience suggests that different types of FRP should not be used on the same circuit. Any subsequent replacements and repairs should also be made with material of the same type and manufacturer (Coury & Associates, 1980). The patented plastic pipes are made of plastic material such as PVC, CPVC, polyethilene, polypropilene, etc., which are usually fairly resistant to chemical attack and easy to join, but also have temperature limits (Goering et al., 1980). To date there has been little experience of these materials in the geothermal sector (GRC, 1979). Polyvinyl chloride (PVC) is very attractive because of its low cost. On the other hand, heat loss is high and it begins to weaken at temperatures above 27°C. At 60°C its strength is 22% less than its initial value. For example, the ASTM D2241 160 psi (11 k g / c m 2) rated pipe, one of the highest rated PVC pipelines, would be derated to only 28 psi (2 k g / c m 2) at 60°C. Thus, when buried and subjected to temperatures of more than 27°C, the pipe tends to bend or buckle. Great care must also be taken when backfilling the trench in which it is to be buried (Goering et al., 1980). Chlorinated polyvinyl chloride (CPVC) is better than the previous plastic, but is rather expensive. Heat loss is again high and similar to that of PVC piping. It does not appear to be available in diameters of more than 10 in. The CPVC begins to weaken above 38°C, although it is not as bad as PVC. It loses from 20 to 40% of its original strength at 65°C only. Once again backfilling of the pipeline trench must be conducted with extreme care. The high-density polyethylene (PE) pipes have a high corrosion and scaling strength. However, the major drawback is their pressure derating at elevated temperatures and high costs. For example, a pipe with an initial rating of 100 psi (7 k g / c m 2) at 24°C would be rated at only 50 psi (3.5 kg/cm-') if operating with fluids at temperatures of 65°C. This piping material could be used where pressure limitations are not a major concern.

Technical- Economic Aspects of the Geothermal Waters

869

The polypropilene (Moplen) pipes have also quite a high resistance to attack from both acid and basic chemicals, but are somewhat instable in the presence of oxygen, especially at temperatures above atmospheric. We have already mentioned that the pipes carrying the hot water to the users should be insulated if possible. Heat losses in the system have, in fact, an adverse effect on plant efficiency and, consequently, on its economics. The type and thickness of the insulation is, of course, determined on the basis of its cost, but also on the position of the pipelines (buried or above ground), on the environmental temperature and temperature of the fluid to be transported (Fig. 8). In an underground system, the resistance to moisture absorption and to superimposed loads must be considered. Above ground insulations must be evaluated for fire and smoke protection, from damage by people and from damage to thermal efficiencies. Most of the criteria f6r proper installation of insulation can be obtained from the insulation manufacturers. In order to assess the different types of insulation available in relation to certain situations, it is worth consulting the Handbook of the American Society of Heating, Refrigerating & Air Conditioning Engineers ( 1 9 7 5 - 78). For example, the Handbook reports that some interesting results have been obtained with pre-insulated asbestos concrete (Goering et al., 1980), which has proved to be an attractive piping material except where carbon steel is required to overcome pressure limitations. This pre-insulated pipe consists of two concentric concrete asbestos pipes with a polyurethane foam insulation in between. Heat loss is small and the pipe is relatively simple to install; the special bedding requirements for a singular concrete (or cement) asbestos pipe are, in fact, unnecessary with this product due to the strength imparted by bonding two concrete asbestos pipes with foam insulation, which

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870

E n r i c o Burbler

is an excellent thermal insulator, l h c potential for interior corrosion problems is mitigated b\ coating the interior of the pipe with epoxy. As an alternative to the pre-insulated pipes, an insulating urethane foam can be sprayed onto the pipe after it has been laid in the trench. The major advantage of this technique is that i~ is cheaper than the pre-insulated pipes, but the unprotected foam unfortunately decays \~ith time, primarily as a consequence of soil moisture. It has been estimated that the buried urelhanc insulation with no extra coating (such as PVC) becomes ineffective after 5 years (Goerin~ et al., 1980). On the other hand, PVC coating brings the cost of the pipe up to the same price level as the pre-insulated pipes, thus eliminating the commercial benefit of the alternative. The effects of insulating the pipe system, in terms of fluid heat and temperature losses are shown in Table 1 (Coury & Assoc., 1980, modified and Goering et al., 1980), as the Huid is transported through various types of pipeline. ] a b l e 1. Estimated temperature loss (%7) per 305 m pipeline (1000 ft) buricd in soil Situation Fluid temperature Ground temperature Burial depth Fluid velocity

93°C 1.6°C 0.9 m 1.8 m / s

Nominal pipe diameter (in.) Bare carbon steel Insulated carbon steel Bare fiberglass Insulated fiberglass Bare concrete asbestos Insul. concrete asbestos

4 1.9°(7 0.3 1.7 (t.2 0.8 0.1

8 0.6°C

12 0.3~C

0. I

0.1

O.4 0.0 0.2 0.0

0.3 0.0 0. I 0.0

16 0.3iX; 0.0 0.3 0.0 0.0 0.1)

It is surprising to note that the material used for the pipelines, be it non-insulated carbon steel or fiberglass, has little influence on heat loss. Furthermore, they both show the same behaviour when insulated thermically. Consequently, we can deduce that it is insulation that is the main factor and that the supply pipes for both the geothermal water and the recirculating water must be insulated. Table 2 can be used for a rapid comparison of the cost of the various pipings discussed above. Installation costs are not included. The figures refer to 1980 and are thus no longer valid except for comparison (in U S $ / m , Goering et al., 1980, modified). When field installation costs are added, the costs given in the table escalate between 30 and 50%. Fable 2. Cost of various pipings Nominal type diameter Material

4 in.

8 in.

12 in.

16 in.

Bare carbon steel, 40 Bare carbon steel, 80 Reinforced fiberglass PVC (polyvinyl chloride) CPVC (chlorinated polyvinyl chloride) High density polyethylene ¢~ High density polyethylene ~-~' Concrete asbestos Pre-insulated asbestos

13.9 21.5 11.7 7.9 23.0 5.3 19.4 6.4 23.8

34.7 54.0 36.2 17.3 116.6 18.2 35.4 13.3 39.6

60.2 80.8 53.7 31.2 * 39.8 86.9 24.8 62.7

78.2 104.0 81.8 69.5 62.7

*Pipe not readily available. kg/cm: rating at 24°C, or 3.5 kg/cm: at 6 6 ' ( '. cq4 kg/cm: rating at 24°C, or 7 kg/cm: at 66°C. '~'7

*

37.4 12g.0

Technical-Economic Aspects of the Geothermal Waters

871

Heat exchangers With the exception of a few particularly favourable situations in which the geothermal fluid has few aggressive properties, a heat exchanger must nearly always be installed between the production well and the users. It is in the exchanger that the heat from the geothermal water is transferred to a clean secondary water, which is then carried to the plants utilizing this heat. These two fluids never come into contact in the exchanger, and the corrosive geothermal fluid, after heat exchange, is reinjected into special wells or discharged on the surface. The heat exchanger in a geothermal plant has, therefore, the crucial role of maintaining circulation of the frequently corrosive and incrusting geothermal fluid in controlled equipment. In fact, it is easier and cheaper to select suitable materials and clean and replace parts in the exchanger. Consequently, complicated piping and plants are protected from the harsher environment of the geothermal fluid. One final advantage offered by the exchanger is that the pressure values of the geothermal fluid are independent of the pressures of the clean secondary water sent to the utilizers. The presence of the heat exchanger, in an intermediate position between the well and the users, obviously entails a drop in temperature between the primary (geothermal) fluid and the secondary (fresh water) fluid. The lower the drop in temperature, the better is the performance of the exchanger. Usually the temperature drop can be kept around 2 - 3 ° C . There are various types of heat exchangers available, but they all belong to one of two categories: downhole heat exchangers or surface heat exchangers. The downhole heat exchanger (Fig. 9) consists of a pipe, or a system of pipes suspended within the geothermal well. Clean secondary water circulates in this pipe, either naturally or driven by pumps. The advantage of this type of exchanger is that it eliminates all the problems (and costs) pertaining to disposal of geothermal fluids, as the latter remain in the production well. This system is also cheaper than using surface heat exchangers where only one well is required. The downhole heat exchangers are generally installed no deeper than 150 m. In order to optimize efficiency, that is, attain a good transfer of heat from the geothermal fluid to the secondary fluid circulating in the piping in the well, there should be an annular space between the casing and the walls of the borehole. In this case, natural convection circulates the water down inside the casing, through the lower perforations, up through the annulus and back inside the casing through the upper perforations (Fig. 9). If the design parameters of bore diameter, casing diameter, heat exchanger length, tube diameter, number of loops, flow-rate and inlet temperature are carefully selected, the velocity and mass flow of the natural convection cell in the well may approach those of a conventional surface shell-and-tube heat exchanger (GRC, 1979). Corrosion in the well piping is a factor to be taken into consideration and experience has shown that it is prone to occur at the a i r - w a t e r interface at static water-level. Past experience has also shown that the downhole heat exchanger is economic, greatly reduces corrosion problems, eliminates environmental pollution (as there is no waste with geothermal fluids) and certainly helps preserve the geothermal aquifer (Barbier and Fanelli, 1977). The surface heat exchangers include a few types of exchanger, the most commonly used being the shell-and-tube and plate heat exchanger. The shell-and-tube heat exchangers consist of a set number of parallel tubes enclosed within a cylinder. A fluid (generally the geothermal) circulates in the tubes, while the clean secondary water circulates outside the tubes, but inside the cylinder. There are two possible configurations for the tubes within the cylinder, either U-shaped or in straight parallel lines with removable heads at both ends to facilitate cleaning of the tubes. The straight tube configuration appears to be the easiest to clean (GRC, 1979). Baffles are usually inserted in the shell side flow to provide

Enrico Barbier

872

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turbulence and a tortuous path for the shell side fluid (Denver Res. Ind., 1980). The materials used in the exchanger must obviously be chosen on the basis of the chemical composition of the geothermal fluid. If mild steel shells and copper or silicon bronze tubes are used, then the shell-and-tube heat exchanger will prove cheaper than the plate exchanger (GRC, 1979). The plate heat exchangers (Fig. 10) consist of a Series of plates held in a frame by clamping rods. The primary and secondary fluids are generally passed through alternating passages between the plates, which are sandwiched together and manifolded such that the fluids being heated and cooled flow in alternate channels, hot fluid on one side and cold fluid on the other side of each plate. However, stamping of the plates provides a variety of flow-patterns and sizes. This design lends itself quite well to annual disassembly and cleaning to remove any scale deposits that may have formed. It also allows the area of heat exchange surface to be increased simply by the addition of more plates. An increased heat exchange area can be used to accommodate increased flow or to cause a greater temperature difference (inlet to outlet) to occur in the hot and cold side fluids. Plate exchangers provide higher heat transfer coefficients (by a factor of about 3) than the shell-and-tube exchangers, due to high fluid velocity attained in the relatively narrow flow channels. This high velocity generates high shear forces that enhance heat transfer and has the added advantage of reducing the rate of plate surface fouling, which is of particular concern when the geothermal fluid has a high chemical content (Denver Res. Inst., 1980). The improved heat transfer coefficients permit the plate heat exchangers to be constructed with much smaller heat transfer surface area. The units are therefore lighter than shell-and-tube

Technical-Economic Aspects of the Geothermal Waters

873

Plate heat exchanger

t

Fig. 10. Plate heat exchanger. Top, general assembly (Alfa-Laval, Sweden). Bottom, a plate and the flow circuit (Barriquand, France). and cost less. The penalty paid for this is a greater pressure drop through the plate exchanger, although with proper design it need not be excessive. The data supplied by manufacturers indicate that there is a linear response of heat transfer to decreasing flow-rate from the rated maximum. This is of crucial importance when matching the geothermal heat supply to system demand. Experience recommends that titanium plates be used, rather than the standard stainless steel. Titanium is far more resistant to corrosion and greatly increases the reliability of the exchanger. The only drawback is its cost, about three times that of stainless steel. The operative limits for plate heat exchangers are 150°C and 20 k g / c m 2 of pressure. They take up less space than the shell-and-tube and measure 1.50 × 1 × 1 m for a geothermal flowrate of 150 m3/h.

874

Enrico Barbier

The main manufacturers of exchangers on the international market are the Swedish compan3 Alfa-Laval, the French Vicarb and Barriquand, the German GEA and British APV. Heat p u m p s

In the direct use of geothermal waters, it may prove economically advantageous to increase a low temperature ( 1 5 - 50°C) by means of heat p u m p s . The latter exploit the properties of a volatile working fluid (freon, isobutane, etc.), which circulates in the pump in an evaporator and in a condenser, to increase the temperature of the secondary water to the users. In order to better explain the role of this thermal device, the heat p u m p , one could compare it with the pump traditionally used to raise water. The water pump is a device that, driven by electricity, transports water from one place to another, e.g. lifting it from a certain depth and raising it to a higher level. In a similar fashion, the heat pump extracts the heat from a source, at a temperature that is lower than is desired, and hands it back at a higher temperature. Thermodynamically, of course, this passage is not a spontaneous or natural one, as heat always passes from hotter bodies to colder ones and not vice versa. So in order to extract heat from a low temperature source and use it at a higher temperature we must again consume a certain quantity of energy, such as electricity, to get the heat pump to complete this operation. The best known heat pump of all is the domestic refrigerator, in which the heat is extracted from the food (corresponding to our low temperature geothermal water) and is handed back at a higher temperature to the environment around the fridge itself. It must be emphasized that the role of the heat pump is not that of transforming electric energy into thermal energy as in a normal electric resistance, but of extracting the thermal energy from a source (geothermal water, in this case) and transferring it at a higher temperature, for example in the ambient that is to be heated. It is thus possible to obtain a far greater quantity of heat, per unit of electricity consumed, than would be produced by direct conversion in an electric resistance. The main parts of a heat pump are the compressor, condenser, evaporator and expansion device. By consuming energy (e.g. electricity) the compressor raises the temperature of the working fluid (e.g. freon) while compressing and liquefying it. The freon yields its heat to the clean secondary water in the condenser, and increases the water temperature as desired. The freon then leaves the condenser, passes through the expansion device, where it expands, undergoes flashing and cools. It then enters the evaporator, where it extracts heat from the circulating geothermal fluid, passes on to the compressor and completes the cycle again (Fig. 11). The thermal energy obtained at the exit of a heat pump is closely related to the high and low temperature limits of the working fluid (e.g. freon). In order to evaluate the correlation between the thermal energy produced by a heat pump and the electric energy consumed in its operation, we will take a practical example. Let us assume that our geothermal water is at 40°C, which is too low for space heating and must therefore be raised to 80°C. The freon must be compressed and liquefied in the compressor at temperature of roughly 85°C; the 5°C difference is so that it can yield its heat to the clean secondary water which is to have a temperature of 80°C at the outlet. The freon must then be flashed in the evaporator to a temperature of about 35~C so as to acquire heat from the geothermal water (which is at 40~C). The temperature difference for the working fluid will, therefore, be 8 5 - 3 5 = 50°C. The theoretical maximum thermodynamic efficiency of the pump will be COP =

Q.o,

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~ ....

W

Q . o t - Q~ot~t

T, ot - T~o~,~

85°C + 273 358 K = 7.16 (85 + ~'/~) Z (35 + 273) = 50

Technical- Economic Aspects of the Geothermal Waters

875

, oce Condenser

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valve J

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which is termed the Coefficient of Performance of the pump and corresponds to the ratio between the total heating output of the pump (Qhot) and the total electrical input ( W = Qhot - O c o l d ) . Qho, is the amount of heat available at the outlet temperature of 80°C and is equal to the sum of the heat recovered from the geothermal water (Qco~) and the work (W) performed on the fluid by the compressor consuming the electric energy supplied to the pump. However, taking all the performances of the different parts of the pump (motor, compressor, etc.) and assuming that they total 60°70, the actual COP will then be: C O P a c t u a l = 7.16 × 0.60 = 4.3. In other words, the pump will be capable of supplying 4.3 times as much heat as an electric heater consuming the same amount of electricity, thanks to the heat extracted from the geothermal water, which, in our case, we have assumed at 40°C (Marion and Hornyak, 1982). The values of the COP obviously vary according to the type and working temperature range of the pump (Fig. 12) (Olivet, 1981). The higher the COP the higher the energy and economic

876

Enrico Barbier

~0

~0 -20

0 20 40 Water temperature at outlet of evaporator (°C)

Fig. 12. ~oefficient of Performance (COP) of heat pumps (Olivet, 1981).

savings obtained with the pump. The COP increases with the decrease in difference between temperature in the evaporator (temperature of the geothermal water) and in the condenser (outlet temperature of the secondary water). Water-source heat pumps (i.e. pumps that use the geothermal water) appear to have applicability for space heating with source fluids in the temperature range of slightly above 0°C and around 50°C. Most common units of this type at present on the market are recommended for operation at source fluid temperatures that lie in the range of about 15 to 35°C. At the lower end of this temperature range (15 °C) the pumps commonly available on the market appear to have reasonable COPs but are usually designed for a relatively small temperature drop of the source fluid (geothermal water, for example). The small temperature drop for the fluid results in a large mass flow-rate and, therefore, large quantities of fluid. This large usage of fluid becomes a limiting factor when the fluid is being taken from an exhaustible supply such as a geothermal well. At the upper end of the applicable temperature range (35°C) for the majority o f the pumps commonly available on the market, these units have poorer COPs. Substantial improvement of the COPs and large temperature drops for the source fluid can, therefore, be obtained by redesign of the heat pump at source temperatures above 15°C. As the source temperature increases above this value, the benefit of redesign increases, entailing an improved COP and reduced fluid consumption per unit of output (Reistad and Means, 1980). In addition to the basic components of the heat pump already mentioned, geothermal heat pumps can be either non-isolated or isolated systems (Smith and Ellis, 1983). In a non-isolated system the geothermal water is circulated directly through the water side of the evaporator. In an isolated system, the geothermal water is, instead, circulated through an isolation heat

Technical-Economic Aspects of the Geothermal Waters

877

exchanger, where heat is transferred to treated water flowing in a closed loop between the isolation heat exchanger and the evaporator. The isolated system suffers a thermal penalty because of the temperature drop across the isolation heat exchanger, and it does require an additional heat exchanger, piping, pump and controls. Isolation systems have the advantage of protecting the heat pump or pumps from the corrosive and scaling problems produced frequently by the geothermal waters. Plate heat exchangers in titanium are commonly used to minimize the costs of the isolation heat exchanger, as well as to simplify cleaning. Due to the need for minimal initial costs to allow geothermal heat pump systems to have an acceptable payback period, it is considered likely that most systems will be compelled to use heat pumps that are commonly available on the market. Most manufacturers offer evaporators in steel, copper, cupronikel or stainless steel. If the designer of the geothermal plant concludes that these standard materials are not suitable for service for a specific geothermal fluid, then an isolation system will probably be his best option. It should be emphasised that numerous factors other than corrosion resistance must be considered if a successful system is to be achieved. Therefore, a heat pump should be selected only after system design by specialists and consultation with the heat pump manufacturer (Smith and Ellis, 1983). The best-known manufacturers of heat pumps are the U.S.A. companies York, Carrier, Westinghouse, the Swiss Sulzer, the German Man and Neunert and the Italian Zanussi.

Reinjection pumps Reinjection pumps are, as their name says, used to reinject the extracted and utilized geothermal water back into the geothermal reservoir or into other permeable horizons considered suitable for this purpose. The reinjection pumps are generally horizontal centrifugal pumps located at the wellhead (Fig. 13). The geothermal water can be reinjected after circulating in the utilization system or, chemical content not permitting, it can be reinjected immediately after crossing the heat exchanger which, in these circumstances, is located near the production well. The reinjection pump usually increases the pressure of the geothermal water from a few atmospheres, which is the pressure at the exchanger exit, to a few tens of atmospheres, which can be the reinjection pressure. The choice of materials for the reinjection pumps is governed by the type of geothermal fluid with which it will come into contact. If this fluid is aggressive then stainless steel casing and impellers are recommended. For manufacturers of reinjection pumps on the world market, the reader is referred to those listed previously for the production pumps.

c //////////

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bor

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. 2: ~ ~ - 2 - 2 - _

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Fig. 13. Reinjection pump in a geothermal circuit (AFME, 1983).

878

Enrico Barbier

ENERGY A N D E C O N O M I C BALANCE OF T H E G E O T H E R M A L ( ) P E R N I I O N No matter to what use the geothermal resource will eventually be destined when ils I~cal i, exploited directly, whether for space heating or industrial processes, the first step must alway, be an assessment of the economic feasibility of the project. In other words, once the size of the resource and the compatibility or' its temperature a~ad potential flow-rate with direct use have been ascertained, an analysis must be made of ~he economic viability of the type of application envisaged. That is, the geothermal plant mus~ guarantee sufficient benefits, savings or revenues to justify the amount of capital investment necessary to bring it on-line. Various factors contribute to determining the feasibility of direct-use geothermal energy (GRC, 1979), viz.: - - t h e geologic parameters of the resource - - t h e engineering criteria or the technical viability of the project - - t h e economics of the venture, i.e. will the annual savings provide sufficient return to the investors to justify the capital expenditure? The most important factors influencing heat production costs are well costs, well flow-rate, resource temperature, distance separating demand and supply, population density (in domestic heating), size of demand and the system load factor. Life cycle cost analysis is the method generally used to determine economic feasibility. It combines all the techniques of projecting and evaluating total system costs over the expected life of the system (Higbee, 1984). These costs include capital investment, annual costs of operating and maintaining the system, financing costs, taxes and insurances. Once the life cycle costs have been calculated, it is possible to determine the rate of retnrn on invested capital and the payback period. It must be remembered that the direct use of geothermal heat is capital intensive, that is, the principal costs are initial investment costs. Over 75% of the production costs are fixed costs related to the initial capital investment (Bloomster et al., 1977). The required capital investment is found by estimating all equipment and installation costs associated with the system. These costs refer to: - - p r o d u c t i o n wells --injection wells - - p u m p s (production, circulation, reinjection) - - h e a t exchangers - - h e a t pumps (if required) --pipes - - m e t e r s , valves, storage tanks --buildings, etc. The operating and maintenance costs should be broken down into electrical operating costs for: - - p r o d u c t i o n , circulation and reinjection pumps --fans - - h e a t pumps (if required) - - e q u i p m e n t maintenance and labour. Inflation must be considered carefully for maintenance and labour and for the electric power for operating the system. At this point, the viability of the geothermal option must be assessed on the basis of a comparison with a system operating on conventional energy. We must thus evaluate the capital investment necessary for the latter, including maintenance and labour. These costs would obviously include the cost of fuel which, in the conventional energy design weighs very heavily indeed on the final cost of the thermal energy produced.

Technical-Economic Aspects of the Geothermal Waters

879

Upon completion of all these forecast cash flows for the life of each system, the geothermal costs will be subtracted from the conventional system costs to arrive at the annual savings occurring in each year of operation (Higbee, 1984). The analysis of the economic feasibility of a geothermal project is the work of specialists, and to have any real significance, it must be broken down into considerable detail. Some computer programs have already been devised for a rapid analysis in certain circumstances. The readers are referred to the bibliography (EEC, 1982; Blair et al., 1982). REFERENCES AFME (1983) Guide du maitre d'ouvrage en g6othermie. BRGM, Manuels et Methodes, No. 8, Orl6ans, p. 188. AFME (1983a) La g~othermie. Paris, p. 32. ASHRAE-Amer. Soc. Heating, Refrig. and Air-Condit. Engng, Inc., A S f t R A E HandbooK's." Equipmenl (19751. Fundamentals (1977), Systems (1977), Applications (1978). New York. Barbier, E. and Fanelli, M. (1977) Non-electrical uses of geothermal energy. Prog. Energy Combust. Sci. 3, 7 3 - 103. Blair, P. D., Cassel, T.A.V. and Edelstein, R. H. (1982) Geothermal Energy: Investment Decisions and Commercial Development, p. 184. John Wiley, New York. Bloomquist, R. G., Basescu, N., Higbee, C., Justus, D. and Simpson, S. (1980) Washington: a guide to geothermal energy development. U.S.D.O.E., OIT Geo-heat Center and Washington State Energy Office, contract EY-77-C-06-1066. Bloomster, C. H., Fassbender, L. L. and McDonald, C. L. (1977) Geothermal energy potential for district and process heating applications in the United States and economic analysis. Battelle Pacific North-west Lab Report, BNWL-2311/UC-66i, p. 44. Citernesi, U., Benvenuti, G. and Ferrara, G. C. (1985) Microbiological aspects of concrete and iron deterioration in geothermal powerplants. Seminar on Utiliz. of Geoth. En. for Electric Power Produc. and Space Heating, Florence 14-17 May 1984. Geothermics 14, 315-326. Coury & Assoc. (1980) A feasibility analysis of geothermal district heating for Lakeview, Oregon. U.S.D.O.E. Rep., DOE/ET27229-TI, p. 85. Denver Research Institute (1980) Municipal geothermal heat utilization plan for Glenwood Springs, Colorado, U.S.D.O.E. Rep., DOE/ID/12049-3, p. 266. E.E.C. (1982) Technical and economic feasibility of low-enthalpy geothermal projects in the E.E.C. Three reports. EUR8241EN, Commission of the European Communities, Brussels. E.E.C. (1983) Low-enthalpy geothermal process, material selection, corrosion prevention and scaling problems. Final Report, EUR7835EN, Commission of the European Communities, Brussels. Ellis, P. F. and Conover, M. F. (1981) Material selection guidelines for geothermal energy systems. U.S.D.O.E. Rep, DOE/RA/27026-1. GRC-Geothermal Resources Council (1979) Direct utilization of geothermal energy: a technical handbook. Spec. Rep. No. 7, Eds, Anderson D. N. and Lurid J. W. Geothermal Resources Council, Davis, CA 95617. Goering, S. W., Garing, K. L., Coury, G. E. and Fritzler, E. A. (1980) Residential and commercial space heating and cooling with possible greenhouse operation. U.S.D.O.E. Rep. DOE/ET/28455-3. Higbee, C. V. (1984) Life cycle cost analysis for direct-use geothermal systems: an introduction. Geother. Ener.~y 12, 3,7--8. Karlsson, T. (1982) Geothermal district heating: the Icelandic experience. U.N.U. Geothermal Training Programme, Reykjavik, Rep. 1982-4, p. 116. Marion, J. B. and Hornyak, W. F. (1982) Physics for Science and Engineering. Part 1, p. 743. Saunders College Publishing Holt, Reinhart & Winston, Philadelphia. Olivet, J. (1981) Solaire et g~othermie contre petrole, p. 240. Editions du Moniteur, Paris. Orkustofnun (Icelandic National Energy Authority) (1984) Persona[ communication. Reeber, R. R. (1980) Coatings in geothermal energy production. Thin Solid Films 72, 3 3 - 47. Reistad, G. M. and Means, P. (1980) Heat pumps for geothermal applications: availability and performance. U.S.D.O.E. Rep., DOE/ID/12020-TI, p. 67. Smith, C. S. and Ellis IIP. F. (1983) Addendum to material selection guidelines for geothermal energy utilization systems. U.S.D.O.E. Rep., DOE/ET/27026-2, p. 213. Streeter, V. L. (Ed.) (1961) Handbook ~2fFluicl Dynamics. McGraw-Hill, New York, 1st Edn. -

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