THEORETICAL BACKGROUND

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Under the auspice of:

Division of Earth Sciences

Chapter 2.2

THEORETICAL BACKGROUND

Kiril Popovski, Sanja Popovska Vasilevska

Faculty of Technical Sciences – Bitola, Macedonia E-mail: [email protected]

1. Introduction

In principle, the use of geothermal heat pumps is interesting in three cases:

- When temperature of geothermal water is below the needed ones for particular direct uses;

- When in integrated projects with geothermal water use in cascades appear one or more installations in the end part of the cascades, but with higher temperature demands than the others; and - When appear economically justified

possibility to use the effluent water of other direct application uses.

However, now-a-days also an other possibility becomes more and more interesting, i.e. the use of very low temperature natural waters of geothermal or surface origin and natural heat of the earth in shallow depths.

In all the cases, the technological background is the possibility offered by the heat pump plant to increase the temperature of the heating fluid in much more economical way that any other classical device for heat production. With addition of only a part of the needed energy in the system, it offers applicable temperature levels for different heating purposes.

During the early eighties this particular technical device has been very popular and seemed to be the solution not only for the low-temperature energy sources, but for all the heating purposes in general. However, changes of the prices at the world energy market and a list of limitations which ap- peared in exploitation of commercial projects, made the heat pumps only one of possible technical/technological solutions for low-temperature direct application projects. Sometimes it is economically feasible, but sometimes also not. As already said, it is now-a-days only one of possible solutions on disposal which should be taken in account . In opposite, the development of heat pumps constructions based on the use of low temperature heat of shallow waters and soil is becoming very popular in some developed countries in the world, and proved itself as a competitive solution to any other classical heat production.

In any case, during all these years of development of this particular heat production technology it was possible to realize a kind of resistance to its introduction even by the energy people and people who otherwise think in a clever way for energy development problems. Simply, most of them do not understand properly the basic characteristics

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of the technology, i.e. how is it possible to produce energy without proper burning or using natural energy sources (solar heat, water flow energy, etc.) and, what’s the most strange, with a negative energy balan- ce. – less energy enter in the plant than is going outside!! It doesn’t look as a serious and secure solution for practical purposes, i.e. it looks more as a “perpetum mobile”

play of scientists which shall be finally paid by the poor user who believed in such

“impossible” solutions. Can any “normal”

person believe in heat production based on the cooling production technology?

Before going further to the elaboration of the different parts of the technology of heat pump, characteristics of different constructions, possibilities for proper and economical exploitation, etc. it’s probably necessary, step by step, to explain the theoretical background of it, if possible in a simple way, understandable for different professions involved in it’s development and exploitation.

That’s the task of this chapter, i.e. to make a summary of the theory in an acceptable way, from the essential roots to the final composition of an commercial heat pump device. Additional information, connected directly to the “ground source” heat pumps construction can be found in the following chapters.

2. Cooling and Working Processes

In order to gain useful work from different energy sources, there are developed a list of so called cycle processes. One of the most applied is certainly the Carnot cycle composed of two isotherms and two adiabats (Fig.1).

The surface encircled with the points 1, 2, 3 and 4 represents the work, gained with the cycle process. The gas didn’t change its state after execution of the cycle. Therefore it was not the one which produced some work. It is obvious that the work is results of the difference between the supplied and subtracted heats to the process. They are also the basic condition for execution of the Car- not cycle. The question is in which direction

should be realized the process, and what shall be the final results of execution i different ones. If the execution is in the right hand direction (Fig.1 and 2.a), the heat is supplied at T1 = T2 and subtracted at T3 = T4 < T1 = T2. As a result, a work is gained, equal to the surface closed in the area 1-2-3- 4. In opposite (Fig.2.b), when the execution goes in the left hand direction, the heat is supplied at T1 = T2 < T3 = T4. Taking into account, according to the first law of the thermodynamics, that it is not possible to realise a process in which the heat is transferred from a body with a lower temperature to a body with a higher one, the left hand cycle conditions supply of additional work to enable its execution. That work is equal to the surface closed in the area 1-2-3-4.

1 2

3 4

T

s Qh

Qcond

5 6

Fig.1. The Carnot cycle in T-s diagram The conclusion is that the right hand direction of the execution of the cycle results with production of work, and the left hand direction with consumption of work.

Heat is transferred from environment with higher temperatures to the one with lower ones for the right hand directionned process

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(Fig. 3), and from lower to the higher ones for the left hand directionned process (Fig.

4). It is obvious that the second one conditions that temperatures of environments should be Te.h. < T3 = T4 =

T, and Te.l. > T1 = T2 = To. Thermical efficiency of such a process shall be:

hc = W/Q = (T-To)/T = 1 - To/T [1]

1 2 3

4

1 2

3 4

Q °

T T

s s

T

To cond h Qh

Qcond

5 6

Fig. 2. Right hand and left hand directionned cycle of Carnot

1 4

2 3

PUMP

TURBINE STEAM PRODUCER

CONDENSER

Qk

Q E

Fig.3. Basic composition of a plant for execution of the right hand cycle Taking into account that both T and To

are much higher than the absolute zero one, the hc shall be always much smaller than 1 (one). If going further, the hc is the highest possible efficiency because not conditioning temperature differences for heat transfers realization. However, that is not realistic because no one heat transfer can be realised without the existence of a temperature difference between the bodies which take parti- cipation in it. Therefore, the Carnot cycle is of theoretical nature. Real processes consist temperature differences which result with

efficiencies which are always lower than the Carnot one for the same process temperatures.

It is also evident that the right hand processes condition the incorporation of a ex- panding device enabling production of work (Fig. 3 and 5.a), and for the left hand pro- cessses a compressing one enabling addition of work in order to enable the process realization (Fig. 4 and 5.b & c).

Useful result of the first ones is that enabling production of useful work or other types of energy (electricity), and of the second

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ones that enabling cooling of bodies or rooms (cooling plants) and heating bodies

and rooms with the use of low-temperature heat sources (heat pumps).

2 1

3 2

Qe Q

CONDENSER

EVAPORATOR REGULATION

VALVE M

COMPRESSOR k

E

Fig.4. Basic composition of a plant for execution of the left hand cycle

o C 300

200

100

0

-100

-200

-273 -300

ENTROPY "s"

1.55 1.8 1.9 2.15

Temperature "t"

1

2 3

4

1 2 3

4

1 2 3

4

ELECTRICITY

HEAT LOSS TO THE COOLING FLUID HEAT GAIN OF THE COOLED FLUID HEAT EXTRACTION OF THE HEAT SOURCE

Qo

Qk

Qo

Qo ELECTRICITY

PRODUCTION REFRIGERATION HEAT PUMP

Fig.5. Different character of Carnot cycles depending on the direction of realisation Therefore, a cooling plant is composed

(Fig.6) of four characteristical processes:

- Compression from “1” to “2” in order to lift the energetic level of the working fluid above the one of the environment;

- Heat supply to the environment by the process of working fluid condensation from “2” to “3”;

- Expansion of the working fluid from “3”

to “4”, resulting with its temperature

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drop below the one of the environment;

and,

- Heat collection from the cooled body or room from “4” to “1” by the process of working fluid evaporation.

log p

i 1

3 2

4

ENERGY GOING TO THE CONDENSER

ENERGY TAKEN OF THE COMPRESOR ENERGY TAKEN FROM

THE COOLED ROOM

Qo Q

c Qcond

STEAM

LIQUID

SATURATION CURVE

Fig.6. Representation of a cooling process in log p- i diagram After the realization of each cycle the

state of the working fluid is at the starting level, but the process influenced the environment in the way that took the cooling energy Qo from the cooled body or room, used the energy for compressing the fluid Qc and supply to the environment the heat of condensation Qk which is equal to:

Qk = Qo + Qc (W) [2]

Therefore, the useful heat (coldness) of the process is:

Qo = Qk - Qc (W) [3]

It can be seen at the diagram that as the temperature difference (T – To) is bigger, more mechanical work is necessary to be supplied in the process and v.v. For estimation of the quality of a cooling process (i.e. efficiency of the consumption of supplied work in it) so called process cooling grade is in use, or cooling multiplicator:

ε = Q/Q^c [4]

For the process realised under conditions of Carnot, it is:

εc = Q/Qc.c = To/(T - To) [5]

where Qc.c is the heat equal to energy supplied to the compressor for lifting the state of working fluid from “1” to “2”.

Therefore, the cooling multiplicator is reciprocal value of the working efficiency of the plant and it is as bigger as temperature differences are smaller. Each enlargement of the temperature difference influences the value of cooling multiplicator negatively.

Therefore, it is necessary to avoid in practice any cooling which is with lower To than necessary.

3. Absorption cooling

The therm “absorption” means charac- teristic of some materials which under determined working conditions absorb the gaseous substances of other materials with the whole own volume and, other working conditions, to free them back. Also, under

“absorption” the absorption on the surface layer is understood, and under “sorption” the absorption in general.

The absorption cooling plants work on the principle of evaporation of gaseous materials, for which necessary heat is supplied to the process (except the energy for compression in compression plants) to enable execution of the left hand cycle. With other words, the compressor is changed with

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some kind of a “heat” compressor in the absorption plants. All the other parts are the same as in the compressor cooling plants.

Principle scheme of an absorption plant is given at the Fig.7. The “compression” is realized in the plant generator (2-7) in the way that the steam of the cooling fluid is absorbed under the pressure po by the non- saturated solution of the absorption fluid (poor solution) in the absorber (4-5), which becomes afterwards saturated (rich). In the generator the “rich” solution is put under higher temperatures and pressure p, which results with its decomposition. The cooling fluid evaporates and goes to the condenser (8), and the (again) “poor” solution back to the absorber.

The cooling fluid steam goes to the condenser in the same way like in the compressor plants. There, it condenses by supplying the heat to environment (water, air), but keeping the pressure constant. From the condenser the liquid goes to the expansion device where expand to po (8-9) and its temperature drops below the To. Coming into contact with warmer fluid (water, air, body) in the evaporator it takes heat of it (9- 10) and evaporates again before going to the absorber with the state po and To. In that way the cycle is closed, resulting with the useful production of “coldness”, i.e. extraction of heat of cooled fluid or body.

The heating fluid (for example the mixture NH3/H2O passed during the cycles execution the following state changes (Fig.8):

1-2 Evaporation of NH3 from the saturated solution under the condition of constant pressure p and continual heat supply (by means of boiler or geothermal steam or water);

2-3 Streaming or flow of the solution from the generator to the absorber through the expansion valve;

3-4 Absorption of cooling fluid steam in the poor solution under the condition of continual pressure po and heat extraction; and

4-5 Flow of saturated solution from the absorber to the generator.

The theory of absorption cooling plants is based on the thermodynamics of two- component solutions. In difference with the usual materials, the state of two-component solutions is determined with three values:

the pressure “p”, temperature “T” and con- centration “j” in kg/kg cooling fluid in the solution. That is the reason that each solution has own specific characteristics and dia- grams of changes of components depending on the changes of above listed values.

The economy of work of a compressor cooling plant is characterised by the heat multiplicator which depends on the temperatures of evaporation and condensation of the cooling fluid. However, for the absorption plant also the temperature of the supplied heat should be taken into account and the heat relation zc has the following shape:

zc = [ To/(Tk-To)] * [(Tg - Tk)/Tg] [6]

where the meaning of signs is as given in Fig.9. The real heat relation of an absorption working process is given as a relation between the gained cooling effect Qo and heat supplied to the process Qg:

z = Qo/Qg [7]

Fig 10 enables orientation for the exergy flows in compression and absorption units, showing that much more heat should be transferred to environment for the same cooling effect if using the later one. That makes them more expensive and is one of the main reason why have been pushed out of the market by the compressor cooling units. However, their advantage that can use cheap or free of charge heat energy except the expensive electrical one makes them again attractive, particularly in combination of practical absence of the need for main- tenance during the exploitation.

Geothermal steam or hot water can be convenient heat supplier for this type of cooling plants, as it is proven experimentally in laboratories but also in a list of com- mercially used plants in U.S.A., New Zea- land and some small number in Europe.

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A P S O R B E R

E V A P O R A T O R G E N E R A T O R

CONDENSER

HEAT EXCHANGER

S O L U T I O N COLD WATER

1 2

3

4

5 6

7

8

9

10 p

t

t 1

2

po

GEOTHERMAL WATER

Fig. 7. Scheme of work of an absorption cooling plant

-30 -20 -10 0 10 20 30 40 50 60 70 80 90100120 1

2 3 4 5 6 7 8 9 10

Temperature t °C

Condension preassure pbar

P = 1.94

Pr (resorption) 6

4 3

4' 3'

5 1 2

1' 2'

NH3concentr.

100% 60% 50% 30% 20% 10%

Fig.8. Process flow in an absorption and resorption cooling plant with a mixture of NH3/H20 in log p-t diagram

4. Cooling Fluids for Absorption Plants Not the same cooling fluids are convenient for absorption plants as for the compressor ones. Some of the most known and applied two-component solutions shall be mentioned here. These are:

a) NH3/H2O: That is up to now the most used solution and enables satisfactory exploitation of absorption plants. Onliest negative side are the relatively high condense pressures, but still the advantages of this solution are bigger than the disadvantages. It is in rather wide use.

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log p

To Tk Tg 1/T

ß ß

kc kc

(1/T ) - (1/T )

o k (1/T)-(1/T )p

1 2

4 3

= 1 = 0

Fig.9. Cooling fluid process flow in an absorption plant shown in the lop p - 1/T diagram

COMPRESSOR COOLING PLANT ABSORPTION COOLING PLANT

Q Q

k k

ENVIRONMENT ENVIRONMENT

COOLED ROOM COOLED ROOM

Qo Qo

Qc (Elec- tricity)

Qc (Sup- plied heat) Qc

Qc 1

2

To To

Qc3

Fig.10. Exergy flows presentation in a compression and in absorption cooling unit b) LiBr/H2O: It is in use mainly for the

compact construction plants for space air conditioning. This solution is not very good for large temperature differences, like it is the previous one.

Evaporation temperatures below 0°C are not allowed for it.

c) NH3 as cooling fluid with LiNO3 as ab- sorbent: This solution has the advan- tage in front of the previous one because the rectification of gaseous cooling fluid is not necessary. However, there is a possibility for crystallisation in some working conditions.

d) CH3NH2 as cooling fluid with CH3NH2

as absorbent: This couple conditions low working pressures, has good thermodynamic characteristics and relatively large enthalpy of evaporation. Its

disadvantage is that conditions rather expensive rectification devices, it is poisonous and inflammable.

e) CH3OH as cooling fluid with LiBr as absorbent: The advantages of this coup- le is in the high enthalpy of evaporation, good thermodynamic characteristics and the possibility to work with evaporation temperatures below 0°C. However, also here the danger of crystallisation exists.

f) R-22 as cooling fluid and E-181 as ab- sorbent: It is in clean steamy state and is not dangerous. However, R-22 has relatively small steaming enthalpy and low critical temperature of only 96°C.

Depending on the field of use, two basic configurations are in most wide use today.

For the temperatures above 0°C, i.e. air conditioning, that is lithium bromide as the

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absorbent and water as refrigerant. For the ones below 0°C that is ammonia as refrigerant and water as absorbent.

In addition to the listed ones there are some organic cooling fluids and different combinations of absorbents under experi- mentation in U.S.A. It is possible to expect significant improvements in near future in this particular scientific field of work.

5. Heat pump theory

It is already discussed in the part 1, which are the main differences between the right hand or working processes and left hand ones. The first ones (Fig.2) produce useful work (Surface 1-2-3-4) and the second ones enable heat transfer from the lower to higher temperature levels and, in that way, cooling of bodies or rooms (Surface 1- 4-5-6).

In both cases the efficiency strongly depends on the temperature difference between the level 1-4 and 3-2. Fact is that in both type of processes a large quantity of heat is thrown out in the environment, however with low exergy content which make s them unusable.

The question is what shall happen if the evaporation temperature range (4-1) is lifted to the level which is higher than the environment ones? It is evident that the first cones- quence (Fig.2.b) is that there shall not be anymore cooling as a result of the process.

Process is not able to produce temperatures which are lower than the environment ones.

Second consequence id that the condensation temperatures are now much higher than the environmental ones, i.e. for some cases high enough to supply heat for some purposes. The left hand process changed its character from the “cooling” to the “heating”

one.

The composition of the plant is the same as for the cooling purposes (Fig.4).

However, the evaporator takes now heat at the temperature level which is higher than the environment one, and the condenser supply heat to some process or heat user in general. Depending on the character of the

“environment” and heat users, construction

solutions of these heat exchangers is changing.

Fig.11. Heat pump with heat extraction of a natural water flow

For instance (Fig.11), the evaporator can be a heat exchanger put in a water flow (river, channel) with temperatures near to the environmental air ones. The plant lifts working fluid temperatures high enough to serve for heating the fluid of a central heating installation. Then, condenser is a heat exchanger where the heat is supplied to the flow of the heating fluid. Temperatures disposition of such an installation are presented at the Fig.12.

The useful heat gained with such a plant is:

Q = Qo + W [8]

Thermal multiplicator of such a process is:

ε

c = Q/W = (Qo + W)/W = T/(T-To) [9]

For instance, for the plant at Fig.11, it is:

ε

c = Q/W = 323/(323-283) = 8.075 That means that from the mechanical work of the compressor of 1 kWh, it shall be produced Q = 8.075 * 3600 = 29070 kJ of useful heat.

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With the work of the compressor K, some kind of “pumping” the heat from a lower to a higher level is enabled. That is the reason that the name “heat pumps” is accep- ted for such plants.

However, the previous values are valid only for the ideal process of Carnot. The real thermal multiplicator is smaller for the thermal and mechanical losses in the process:

ε

^r^{= e}^c^{* h}ⁱ [10]

ε

^r^{= h}ⁱ^{* T/(T-T}^o^{) [11]}

The value of the internal efficiency depends on the power compressor power, quality of production of the heat pump parts, temperature levels and character of the heat transfer process in question. Some kind of

“average” values are given in the Table 1.

Table 12.1. Internal Efficiency (thermal and mechanical) of Heat Pumps

Heat Type of compressor Internal Mark

Power efficiency

(kW)

300 - 3000 Open, centrifugal 0.55 - 0.75 e1 50 - 500 Open, piston 0.50 - 0.65 e2 20 - 50 Half closed 0.45 - 0.55 e3 2 - 25 Closed, with R-22 0.35 - 0.50 e4 0.5 - 3 Closed, with R-12 0.20 - 0.35 e5 < 0.5 Closed < 0.25 e6

………

The losses appear in more phases:

- Temperature of the cooler fluid, of which the heat is extracted, is several degrees higher than the evaporation temperature;

- Temperature of the heating fluid is several degrees lower than thee condensation temperature;

- Heat loses from the connection pipes appear in the plant, even normally heat isolated;

- Friction losses appears in the compressor;

- Characteristics of the cooling fluid dif- fers of the characteristics of ideal one;

- Mechanical and electrical losses in the electro motor appear; and

- Different other losses.

Fig.12. Temperatures disposition in the heat pump at Fig.11 Really gained heat is therefore:

Q = Qk = Qo/[1 - (1/

ε

r)] [9]

Fig.13. Dependence of

ε

^{c on the}

temperatures of condensation and evaporation

The value of thermal multiplicator for the process of Carnot depends on the values of temperatures of condensation ad evaporation (Fig.13). It is evident that not their height is in question but their relation,

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i.e. difference. That is very important factor for the quality of concrete process estimation.

In the Fig.14. lines of relations of condensation temperature tk, differences between the condensation and evaporation temperatures and thermal multiplicator for the process of Carnot are given. The most used in practice interval between 40° and 80°C is taken.

The Fig.15 illustrates very good the influ-ence of temperatures for different types of compressors, as given in the Table 1.

Fig.14. Thermal multiplicator of a heat pump for constant evaporation temperature

of 0°C and different temperatures of the heating fluid

In the Fig.14 the relation between the thermal multiplicator of the process of Car- not and the real one can be followed. The dotted surface is the field of real thermal multiplicators for internal efficiencies between 0.45-0.65, which are the most often in practice.

However, it is necessary to take into account that the value of thermal multiplicator is not constant for a plant in exploitation.

If, for example, a residential heating installation is in question, when lower external temperatures appear higher temperatures of the heating fluid are necessary, i.e. also higher condensation ones. That gives bigger temperature difference between the condensation and evaporation temperature, which

results with a lower value of the thermal multiplicator.

Fig.15. Thermal multiplicator of the process of Carnot for different temperatures of

condensation and evaporation

Fig.16. Real thermal multiplicators of heat pumps for the evaporation temperature of 0

°C and efficiencies as given in the Table 1 For the transition period some lower temperatures of the heating fluid are necessary, which means smaller temperature difference between two temperatures and in- creasing of the value of thermal multiplicator.

Above described changes are illustrated at the Fig.17., where also the consumption of mechanical work of the compressor is given. It is evident that mechanical work dec-

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rease with the external temperatures decree- asing, and v.v.

Fig.17. The flow of the real thermal multiplicator and requested compressor power of one and more heat pumps depending on the difference of external and internal temperature of heated room

The values of thermal multiplicator show that the use of left hand process for heating purposes is energetically a very convenient method. Efficiency of such a process is much higher than direct burning of fuels or using of thermal stoves, air heaters, etc.

The problem is, when geothermal energy is in question, what is more efficient - to use it only as energy source for heat extraction, or also for lifting the energy stage of the heating fluid, i.e. are compressor or absorption units more efficient and economical for commercial use. Before discussing the question it is initially necessary to underline that geothermal fluid is convenient for use in absorption units only when having enough high temperature performances. Low-temperature geothermal fluids are not convenient for such type of direct application.

Fig. 18. Exergy flows in a compressor and in an absorption heat-pump unit

At the Fig.18. exergy flows of both types of use are given. It is evident that, for the same influence (i.e. heat extraction) to the environment, heat supply of the absorption unit is significantly bigger than the one of the compressor unit. However, also the heat supply to the environment is much bigger, which result with bigger dimension (higher investment costs) of the plant. This negative difference is covered in exploitation by the use of cheaper heat energy for driving the unit and not the electricity, as it is for the compressor one. Therefore, where possible, it is recommended to use absorption units, however only if the heat loading factor justifies higher investment costs in comparison with the compressor ones.

6. Ratings and Performances

Equipment ratings are generally based upon a temperature drop through the evaporator of 5.6°C and a temperature rise through the condenser also of 5.6 °C. Stan- dard equipment may be operated at Dt’s up to 11.2 °C. For bigger temperature rise in the condenser, alternate circuiting of four or eight passes is available.

Off the shelf type heat pumps are, because of the use of single-stage compressors, limited in terms of temperature boost (difference between the condenser and evaporator leaving water temperatures). Most of equipment (Raferty, 1989) is limited to 55- 80 °C boost, with a maximum of 120 °C leaving condenser water temperature (100

°C for centrifugal machines). For applications that exceed these temperature limits, systems of conventional fuel peaking or multi stage heat pump must be used (serial connection of separate machines with condenser of one connected to the evaporator of the other). However, such solutions are not very much recommended because the thermodynamic losses in heat exchangers make them impractical from an economic stand- point.

When considering the performances of heat pumps, data can be found in the litera- ture of manufacturers.

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7. Schemes for Connection to Geother- mal Systems

The way of connection of heat pumps to geothermal systems depends on the characteristics of the geothermal water (chemical composition, temperature, etc,), purpose, technical characteristics of the heat using installation, etc. Each case condition careful technical, technological and economy study of influencing factors in order to come to locally optimal design of heat pump project.

Taking into account that this course is devoted to the ground heat source heat pumps and that for them a list of schemes and case studies shall be given and discussed, only the heat pumps connections to hydrogeothermal sources shall be discussed in this chapter.

The most simple case is for non-cor- rosive geothermal waters and heat use by consumers of the same or very similar heat using characteristics. Than direct connection heat pump scheme (Fig.19) can be applied which is the most simple technical solution and usually condition the lowest investment costs.

When different types of heat users are in question, the design should be changed in the part of heat supply regulation to separate installations (Fig.20). Connection of separate installations to the heat pump can be in parallel or serial mode, depending on requested heating temperatures. If they are near, the parallel connection should be applied. If differing enough, the serial (cas- cade) use can be applied which enable better use of temperature difference on disposal and better heat loading characteristics of the system.

Sanitary warm water preparation is a very economical use of geothermal water because offering rather uniform heat consumption all over the year and, in that way, very high heat loading factor. Technical design of the connection to geothermal schemes is usually very simple (Fig.21), but also depends on the geothermal well and water characteristics. When very low temperature of geothermal water is on disposal or when looking for improvement of the heat loading

factor of more complicated geothermal schemes by use of effluent water of other users, the same scheme can be improved by incorporation of a heat pump (Fig.21). In that way much better use of heat on disposal can be reached and lower temperatures of the effluent geothermal water, which is important from the environmental point of view.

Direct connection of heat pumps to geothermal schemes is not always possible.

When aggressive or waters with inclination to scale deposition are in question, then indirect connection should be applied. Heat pump in such cases is not anymore in a open loop system but in a close loop one.

Therefore, consequences which influence the technical design of such systems should be taken into account. Heat pump plant should be then treated as a heat consumer of the geothermal system, like other heating installations, unique or one of more consumers connected in one closed loop.

The best examples of geothermal heat pump schemes can be found in U.S.A. and France, where practically initially introdu- ced and afterwards developed. In France, that was the low temperature and aggres- siveness of geothermal waters which con- ditioned heat pumps application, and in U.S.A. the improvement of efficiency of direct application geothermal systems.

Fig.19. Direct connection of heat pump to geothermal well

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There are also some good examples in Slovenia, Italy and other European and other countries. However, development in most of them is stopped as consequence of the low energy prices at the world market. Neces- sary high investment costs can be economically justified only when applications with

rather high annual heat loading factors are in question (sanitary warm water preparation, continual industrial uses, etc.). Space heating installations can justify the heat pump application only in the regions with colder climates, where heating season lasts 8-9 moths (Iceland).

Fig.20. Two types of heating installations connected to the same heat pump

Fig.21. Sanitary warm water preparation with geothermal water. (1) Geothermal water supply line, (2) Geothermal water accumulator,

(3) Heat exchanger, (4) Sanitary water accumulator, (5) Sanitary water pump, (6) Warm

water, (7) Cold water from the town supply system, (8) Effluent water

Fig.22. Sanitary warm water preparation with the use of heat pump. (1) Geothermal water supply line, (2) Geothermal water accumulator, (3) Evaporator of the heat pump, (4) Sanitary water accumulator, (5) Sanitary water pump, (6) Sanitary warm water, (7) Cold water from the town supply system, (8) Effluent water, (9) Channel for effluent water, (10) Compressor, (11) Expansion valve, (12) Condenser