Heat Pumps

Using heat from low temperature geothermal sources with a high temperature heat pump E. Torhac1, D. Goricanec2, J. Krope2, A. Saljnikov3 A szerzõk a távfûtésnél alkalmazható kishõmérsékletû geotermális energiaforrások használatát gazdasági számításokon keresztül igazolják, arra az esetre, amikor a termálvíz energiáját részben egy nagyhõmérsékletû hõszivattyú segítségével mûködtetett elsõdleges hõátadó közeg veszi fel. Mivel a legtöbb vizsgált épületben nagyhõmérsékletû fûtési rendszer található, a fûtési rendszer nagyhõmérsékletû közeget igényel. Ezt a hõmérsékletet már egy egyfokozatú hõszivattyú is elõ tudja állítani, nem elhanyagolhatóan olcsóbban. A vizsgálat tárgyát emiatt ezek a hõszivattyúk és az elõforduló hûtõközegek, azaz a H2O, NH3, R407c és R600a adták. A mûködés szempontjából a legjobb eredményeket az NH3 és az R600 adta. A mûködési vizsgálatok mellett a szerzõk gazdaságossági és megtérülési vizsgálatokat is végeztek, amelyek eredményei táblázatos formában láthatók a cikkben.

Abstract The article treats the usage of heat from low temperature geothermal sources for a high temperature heating of buildings with a high temperature heat pump. Due to lower investment costs, a single-stage high temperature heat pump was chosen. The calculations of the heat pump were carried out with H2O, NH3, R407c and R600a as refrigerants. Main operating characteristics of the single-stage high temperature heat pump are given in the form of diagrams. Economic viability of the investment to a high temperature heat pump with the net present value method was carried out together with the return coefficient and time period for repayment of the investment, which were established for the use of geothermal heat at a temperature of 42 °C. Key words: Geothermal energy, heat pump, coefficient of performance (COP), refrigerant, economy

Introduction Due to the increased use of fossil fuels, the amount of greenhouse gas emissions has increased rapidly, causing climatic changes. The mankind is therefore expected to reduce the use of fossil fuels which cover the needs for energy. This need inspired the search and use of renewable energy sources, which are more environment friendly and at the same time economically viable. 1 Nafta-Geoterm

d.o.o., Mlinska ulica 5, 9220 Lendava, Slo-

Renewable energy sources include geothermal energy and its use is gaining in competitiveness with the increase of fossil fuel prices. Geothermal energy is the heat from the Earth’s inside and comprises of three components. – energy current through the Earth’s crust in the form of mass transfer, – flow of heat due to thermal conductivity of rock and – energy stored in rocks and fluids in Earth’s crust. Geothermal energy is used mainly for heating buildings, balneology, greenhouses, farms and industry. Town Lendava decided for the use of geothermal energy and for this purpose a geothermal well Le-2g was drilled. At wellhead pressure of 0.1 MPa and a temperature of 66 °C the production of a geothermal well is 25 kg/s. Geothermal water is to be used for district heating of buildings in the immediate center of the town Lendava. The current plan includes only the use of geothermal water for district heating of buildings so that the geothermal heat from the primary geothermal circuit is transmitted through the heat exchanger to the secondary system of district heating with the temperature regime 40 °C/60 °C. The temperature of the used geothermal water is 42 °C, and is still high enough to be exploited for high-temperature heating of buildings by using high temperature heat pumps. The purpose of the research is to establish the economic viability of the use of a high-temperature heat pump in the district heating system in buildings where the heat source used would be lowtemperature geothermal water. In this way energy from the geothermal water could be used to a maximum before returning back to earth through a reinjection well. Through reinjection, the used geothermal water is re-warmed in the lower geological layers, which is the reason geothermal water count as a renewable energy source. Most buildings in the center of Lendava are older, with poor insulation, and are heated by a high-temperature radiator heating system, only a few newer buildings have the low-temperature heating system. Due to the needed high temperature of the water in the heating system for high-temperature heating of buildings, a high temperature heat pump is required and the heat source would be the used geothermal water with the temperature around 40 °C.

Heat pump

venia University of Maribor, Faculty of Chemistry and Chemical Engineeing, Smetanova ulica 17, 2000 Maribor, Slovenia 3 University of Belgrade, Faculty of Mechanical Engineering, Kraljice Marije 16, 11120 Belgrade, Serbia

Heat pump for heating is a device that operates by taking the heat from the environment at a lower temperature level and emits it into the heating system at a higher level. The important parameters for designing a hightemperature heat pump are the coefficient of performance

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(COP), heat flow of the condenser and evaporator, the compressor power consumption and the pressure ratio of the compressor. The coefficient of performance of the heat pump gives the ratio between the heat flow generated in the condenser and the energy used to power the compressor. Heat flow of the evaporator indicates the amount of heat generated from geothermal water. Heat flow of the condenser indicates the amount of heat the high temperature heat pump produced for the purpose of heating. Establishing the required compressor power indicates the amount of electric energy used for compression of the refrigerant. According to the operation method of the heat pump there are four different regimes in the heating system: – monovalent operation of the heat pump – bivalent alternative operation of the heat pump – bivalent parallel operation of the heat pump – bivalent partially parallel operation of the heat pump For the purpose of district heating of buildings, three basic types of heat pumps would be suitable: – Single-stage heat pump – Two-stage heat pump with a flash expander – Two-stage heat pump with a heat exchanger. [1, 2] By using a single-stage heat pump for heating buildings, depending on the selected compressor and the refrigerant, the water temperature for district heating could rise up to 80 °C or more, depending on the initial temperature of geothermal

water we use. A two-stage heat pump with flash expander and an appropriate refrigerant could be used for high temperature heating systems. The problem is in the higher price of such pump and the appropriate choice of refrigerant. The use of a single-stage heat pump is recommended for the existing district heating system, where the heat source for the heat pumps would be the used geothermal water (Tgm = 42 °C) [3, 4].

Mathematical model Discharged heat flow in the condenser without cooling the refrigerant: Φ C = qm ⋅ (h2 − h3 ) (1) Coefficient of performance (COP): Φ εC = C P

The main physical and chemical characteristics of the refrigerants are given in the following Table 1. Table 1: Physical and chemical characteristics of refrigerants Refrigerant

Chemical sign

Molar mass (g/mol)

Evaporization enthalpy (kJ/kg)

R 718

H2O

18

2500

R 717

NH3

17

1371

R 125

C2HF5

120

159.7

R 32

CH2F2

52.02

382.5

R 134a

C2H2F4

102.0

216.1

R 143a

C2H2F4

84.0

226.8

R 600a

i-C4H10

58.1

355.4

The calculation of heat pumps has been carried out at different temperatures of evaporation and condensation and at different temperatures of geothermal water.

Results of the calculation for a single-stage heat pump For calculations of the single-stage heat pump the following four refrigerants were used: H2O, NH3, R407c and R600a. Based on the results for each refrigerant the characteristic values of the heat pump were calculated. Fig. 1 shows the results of the calculations for single-stage heat pump using water as a refrigerant. Fig. 2 and 3 show the coefficient of performance and heat pump compressor pressure ratio, depending on the outlet temperature of geothermal water and the temperature of condensation. For a better understanding of diagrams and their legends, the signs are as follows: P – power of the compressor (W), Φc – heat flow of the condenser (W) Φr – heat flow of the evaporator (W) tk – condensation temperature (°C) tg1 – temperature of geothermal water (42 °C) 200

(2)

(3)

Thermodynamic data of the refrigerant Each separate variation of the single-stage high temperature heat pump was calculated using the chosen refrigerants. The chosen refrigerants were: H2O, NH3, R407c, R404a, R134a and R600a [1, 5, 6]. Refrigerants were chosen based on the use of refrigerants of well-known heat pump producers and ecological criteria. 16

Power ( k W )

Needed power for the compressor:

q ⋅ (h − h ) P= m 2 1 η

P tk = 57 °C

180 160

P tk = 67 °C

140

Pi tk = 77 °C

120 100 80 60 40 20 0

5

10 15 20 25 30 Geothermal water outlet temperature (°C)

35

Fig.1: Single-stage heat pump with refrigerant (H 2O) Magyar Épületgépészet, LVIII. évfolyam, 2009/7-8. szám

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P tk = 77 °C Qc tk = 77 °C

140 120 100 80 60 40 20 0

5

tk = 77 °C

5

10 15 20 25 30 Geothermal water outlet temperature (°C)

35

Fig. 4: Single-stage heat pump with refrigerant (NH 3)

4

7

3

tk = 57 °C tk = 67 °C

6 5

10 15 20 25 30 Geothermal water outlet temperature (°C)

35

Fig. 2: Coefficient of performance for a single-stage heat pump (H2O)

tk = 77 °C

5 COP

2

P tk = 67 °C

160

tk = 67 °C

6

COP

180

tk = 57 °C

7

P tk = 57 °C

200

Power (kW)

With single-stage heat pump and H2O as refrigerant the condensation temperature rises with the increase in compressor power and heat flow in the condenser. Heat flow of the evaporator also increases in accordance with the outlet temperature of the cooling geothermal water. The coefficient of performance for such single-stage heat pump increases with the increase of the outlet temperature of geothermal water and decreases with the increase of the condensation temperature.

4 3 2

P re s s u re ra tio

45 40

tk = 57 °C

35

tk = 67 °C

30

tk = 77 °C

25

1

20

8

4 3

1

15 25 Geothermal water outlet temperature (°C)

35

Fig. 3: Pressure ratio for the compressor of a single-stage heat pump (H2O) Fig. 4 shows the results of calculations for single-stage heat pump with ammonia (NH3) as refrigerant. Fig. 5 and 6 show the interdependence of the coefficient of performance, pressure ratio of the compressor and outlet temperature of geothermal water at different condensation temperatures. For single-stage heat pump with ammonia (NH3) as refrigerant it is evident that by increasing the condenser temperature the needed compressor power and the heat flow of condenser also increase. The coefficient of performance for such a pump is between 2.4 in 3.4. Magyar Épületgépészet, LVIII. évfolyam, 2009/7-8. szám

tk = 77 °C

5

10

5

35

tk = 67 °C

6

2

0

10 15 20 25 30 Geothermal water outlet temperature (°C)

tk = 57 °C

7

15 5

5

Fig. 5: Coefficient of performance for a single-stage heat pump (NH3)

Pressure ratio

Coefficient of performance is between 3.0 and 4.2 at 10°C outlet temperature of geothermal water. An important parameter for production of heat pumps is also the pressure ratio of the compressor. This parameter needs to remain at or under the value 3. As it is shown in diagram on Fig. 3, for a single-stage heat pump with water as refrigerant the pressure ratio is considerably above the recommended value. Therefore the choice of such heat pump is not recommended.

5

10 15 20 25 30 Geothermal water outlet temperature (°C)

35

Fig. 6: Pressure ratio of the compressor for a single-stage heat pump (NH3) Fig. 7 shows the results of calculations for a single-stage heat pump with R407c mixture as refrigerant. Fig. 8 and 9 show the change in coefficient of performance and compressor pressure ratio, depending on the outlet temperature of geothermal water and the temperature of condensation. With single-stage heat pump and R407c as refrigerant, the used compressor power increases. Heat flow of the condenser also increases in comparison to the first two cases. The coefficient of performance is between 1.24 and 1.77 and the pressure ratio is also above the recommended value, meaning that the production of such a heat pump is not economically justified. 17

450 400

P tk = 67 °C

350

90

P tk = 77 °C

70 Power (kW )

200 150 100 50

60 50 40 30 20

5

15 25 Geothermal water outlet temperature (°C)

35

10 0

Fig. 7: Single-stage heat pump with refrigerant (R407c) tk = 57 °C

3

5,5 COP

1 0,5 5

15 25 Geothermal water outlet temperature (°C)

35

5 4,5

3

Tk = 77 °C

22

4,5 4

3

3,5

Pressure ratio

P re s s u re ra tio

tk = 77 °C

2

5

15 25 Geothermal water outlet temperature (°C)

27 32 Geothermal water outlet temperature (°C)

37

Fig. 11: Coefficient of performance for a single-stage heat pump (R600a)

Tk = 67 °C

4

1

tk = 67 °C

3,5

7 Tk = 57 °C

tk = 57 °C

4

Fig. 8: Coefficient of performance for a single-stage heat pump (R407c)

5

37

6

1,5

6

27 32 Geothermal water outlet temperature (°C)

6,5

tk = 77 °C

2

22

Fig. 10: Single-stage heat pump with refrigerant (R600a)

tk = 67 °C

2,5 COP

P tk = 67 °C

80

250

0

P tk = 57 °C

100

P tk = 77 °C

300

P ow er (kW )

three. The disadvantage of refrigerant R600a is the fact that it only starts evaporating around the temperature of 27 °C.

P tk = 57 °C

35

Fig. 9: Pressure ratio for the compressor of a single stage heat pump with refrigerant (R407c)

tk = 57 °C tk = 67 °C tk = 77 °C

3 2,5 2 1,5

22

27 32 Geothermal water outlet temperature (°C)

37

Fig. 10 shows the results of calculations for a single-stage heat pump with R600a (isobutane) as refrigerant. Fig. 11 and 12 show the change in coefficient of performance and compressor pressure ratio, depending on the outlet temperature of geothermal water. The use of refrigerant R600a (isobutane) with a singlestage heat pump shows that the use of compressor power decreases slightly and the heat flow of the condenser decreases as well. The coefficient of performance is between 4.3 and 5.5. The compressor pressure ratio is also around

Any economic analysis is based on two presumptions: – the investment, which is the amount needed to achieve the project and

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Magyar Épületgépészet, LVIII. évfolyam, 2009/7-8. szám

Fig. 12: Pressure ratio of the compressor for a single-stage heat pump (R600a)

Economy of exploiting energy of geothermal water using a heat pump

– the excess of income over expenditure (the difference between the revenue brought by the project and operational costs) Economy of the project means establishing the value of these two assumptions with methods varying in complexity and accuracy [7, 8]. The choice of method depends on the degree of development of the project at a given moment. Net present value (NPV) is very often used in the preparation of investment projects in the later stages of project development, when sufficient data is available. Net present value is the sum of the current value of all cash flows. The rule for the decision to invest on the basis of the NPV is that the investment is accepted if the NPV is greater than zero, and rejected if it is negative. The cost of investment can be covered from own resources, bank loans or a combination of both. Present value of investment costs CINV is determined by discounting the annuities as follows:

CINV

(4)

The annuity factor - an is determined as follows:

r ⋅ (1 + ra ) n1 an = a (1 + ra ) n1 − 1

(5)

CS = ∑ j =0

0.02 ⋅ CTÈ ⋅ ( 1 + rj ) j (1 + rj + r ) j

(6)

NPV of electricity costs to power the compressor CPS is determined as follows: N

C E ⋅ PE ⋅ t1 ⋅ t 2 ⋅ (1 + rj ) j

j =0

(1 + rj + r ) j

C PS = ∑

(7)

NPV of revenues from the produced heat with account of inflation and discounting the annuities CP is determined as follows: N

Qk ⋅ CT ⋅ t1 ⋅ t 2 ⋅ (1 + rj ) j

j =0

(1 + rj + r ) j

CP = ∑

(8)

NPV profits from heat considering the costs of investment, maintenance and the cost of electricity to power the compressor are determined as follows:

C = CP − (CINV + CS + CPS )

(9)

The success of the investment is estimated by the coefficient of profitability:

K=

CP CINV + CS + CPS

Value

Units

Own funds

1/3·CTC

EUR

Price of a heat pump

24 000.0

EUR

10

years

Discount rate

0.07

–

Discount rate of annual installments

0.07

–

Inflation rate

0.012

–

Power used by the compressor

51.79

kW

Heat flow of a condenser

187.3

kW

Price of electricity

0.07

EUR/kWh

0.0325

EUR/kWh

20

years

0.02·CTÈ

EUR

Operational time of a heat pump per day

18

h/day

Operational time of a heat pump per year

120

days/year

Number of years for repayment of a heat pump

Price of heat

The maintenance cost for the heat pump CS is estimated to 2% of the purchase price. NPV of costs considering inflation is determined by the equation: N

Table 2: Data for economic analysis of a single-stage heat pump

(10)

Operational life of a heat pump Maintenance costs

The results of calculation for net present value (NPV) of a heat pump with which geothermal water is cooled to 10 °C are shown in Table 3. The coefficient of profitability is 1.1, repayment time for the investment is 3.5 years. Fig. 13 shows the interdependence of NPV and the outlet temperature of used geothermal water. 50000 Net present value (NPV)

a ⋅C = C0 + ∑ n TÈj j =0 (1 + r ) N

The second criterion for the success of the investment is the repayment period, this is the time of operation that is necessary for all the costs to be repaid. Here is an example of determining the economic viability for two-stage heat pumps for district heating. The data for economic calculation of single stage high-temperature heat pump are given in Table 2:

45000 40000 35000 30000 tk = 67 °C

25000 20000 15000

5

10 15 20 25 30 Geothermal water outlet temperature (°C)

35

Fig. 13: Interdependence of NPV and outlet temperature of geothermal water

The coefficient of profitability K represents the ratio between the NPV of income from produced heat, and the sum of all the NPV costs. Magyar Épületgépészet, LVIII. évfolyam, 2009/7-8. szám

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Table 3: Net present values N (years)

Cinv EUR/ year

Cs EUR/ year

Cps EUR/ year

Cp EUR/ year

NPV EUR/ year

0

8 000.0

0.0

0.0

0.0

-8 000.0

1

2 661.0

523.8

7 324.0

12 300.0

1 791.2

2

2 487.0

489.9

6 850.0

11 500.0

1 673.1

3

2 324.0

458.2

6 407.0

10 760.0

1 570.8

4

2 172.0

428.6

5 993.0

10 060.0

1 466.5

5

2 030.0

400.8

5 605.0

9 411.0

1 375.2

6

1 897.0

374.9

5 242.0

8 802.0

1 288.1

7

1 773.0

350.6

4 903.0

8 233.0

1 206.4

8

1 657.0

328.0

4 586.0

7 700.0

1 129.0

9

1 549.0

306.7

4 289.0

7 202.0

1 057.3

10

1 448.0

286.9

4 012.0

6 736.0

989.1

11

0.0

268.3

3 752.0

6 300.0

2 279.7

12

0.0

251.0

3 509.0

5 893.0

2 133.0

13

0.0

234.7

3 282.0

5 511.0

1 994.3

14

0.0

219.6

3 070.0

5 155.0

1 865.5

15

0.0

205.3

2 871.0

4 821.0

1 744.7

16

0.0

192.1

2 686.0

4 509.0

1 630.9

17

0.0

179.6

2 512.0

4 218.0

1 526.4

18

0.0

168.0

2 349.0

3 945.0

1 428.0

19

0.0

157.1

2 197.0

3 690.0

1 335.9

20

0.0

147.0

2 055.0

3 451.0

1 249.0

NPV

27 998.0

5 971.1

83 494

140 197

22 733.9

Conclusion The use of low temperature geothermal energy for district heating of buildings, where the heat of geothermal water is already partially used in the primary transmitter by using a high-temperature heat pump, is justified as economic calculation shows. Due to high-temperature radiator heating system in most buildings, the heating system requires a high-temperature heat carrier. Already a single-stage heat pump can achieve the necessary high temperature. At calculations, the best results were obtained by using NH3 and R600a as refrigerants. Economic calculations were also carried out. The method of net present value (NPV) has been used to determine the economic viability of the investment. Economic calculations demonstrate that the investment in a heat pump is justified, because the investment is repaid after a few years only.

Nomenclature: an C C0 20

– annual installment factor – profit (EUR/year) – investors own funds (EUR)

CE CT CTÈ CS cp h h2 h3 K N p0 pT pS P R ra n1 rj t1 t2 tk tg1 tg2 T0 T βK εc η qm Φk Φc Φr ΦR ΦC

– price of electricity (EUR/kW h) – price of heat (EUR/kW h) – price of a heat pump (EUR) – expenses (EUR/year) – specific heat capacity (kJ/kg·K) – specific enthalpy (kJ/kg) – specific enthalpy on the discharge side of a compressor (kJ/kg) – specific enthalpy of a refrigerant in a condenser (kJ/kg) – coefficient of profitability – operational life of a heat pump (years) – atmospheric pressure (Pa) – pressure at the discharge side of the compressor (Pa) – pressure at the suction side of the compressor (Pa) – power of the compressor (W) – discount rate – discount rate of annual installments – time period of paying off annual installments (years) – inflation rate – operational time of a heat pump (h/day) – operational time of a heat pump in days per year (days/year) – condensation temperature (°C) – inlet temperature of geothermal water (°C) – outlet temperature of geothermal water (°C) – temperature at standard conditions (298 K) – temperature (K) – efficiency of the compressor – coefficient of performance (COP) – efficiency of the compressor – mass flow rate of a refrigerant (kg/s) – heat flow of the condenser (W) – heat flow of the condenser (W) – heat flow of the evaporator (W) – inlet heat flow of the evaporator (W) – outlet heat flow of the evaporator (W)

Literature [1] Ibrahim Dincer, Refrigeration Systems and applications, John Wiley & Sons, 2003 [2] www.viessmann.de [3] Vasic V., Krope J., Goricanec D., An analysis of energy flows in an absorption chiller. Stroj. vestn., 2000, Vol. 46, Iss. 8, pp. 517-524 [4] Torhac E., Crepinsek L. L., Krope J., Goricanec D., Saljnikov A., Stipic R., Kozic Dj.: Profitability evaluation of the heating system using borehole heat exchanger and heat pump. IASME Trans., 2005, Vol. 2, Iss. 8, pp 1381-1388 [5] Stoecker, W. F., Industrial refrigeration handbook, Updated ed. of: Industrial refrigeration. McGrawe-Hill Companies, 1998 [6] Hirrschberg H. G., Handbuch Verfahrenstechnik und Anlagenbau, Springer Verlag, 1999 [7] Kozic D.., Krope J., Goricanec D. Optimization of Large Heat Pumps in Long Distance Transit Heat Transportation. Int. J. of Power and Energy Systems, Vol. 14. No.1,1994. [8] Kurtz, Ruth, Handbook of engineerin economics, Guide for engineers, technicians scientists, and managers, McGrawe-Hill, 1984

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