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Exploitation of the waste-heat from hydro power plants
Energy xxx (2014) 1e6
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Energy journal homepage: www.elsevier.com/locate/energy
Exploitation of the waste-heat from hydro power plants D. Gori canec a, *, V. Pozeb b, L. Tomsi c b, P. Trop a a b
University of Maribor, Faculty of Chemistry and Chemical Engineering, Smetanova ulica 17, 2000 Maribor, Slovenia Hydro Power Plant Maribor, Obrezna ulica 170, 2000 Maribor, Slovenia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 December 2013 Received in revised form 24 June 2014 Accepted 29 June 2014 Available online xxx
This paper presents the possibility of exploiting low-temperature heat from the generators' cooling system within a hydro power plant, using a HTHP (high temperature heat-pump) that enables heating at up to 85 C. The results based on theoretical calculations are presented for heat-flows, the powers of the compressors, and COP (coefficient of performance) values for the cases when using the refrigerant R717 and a single stage high pressure compressor (up to 50 bar) under varying operational conditions. Real possibilities are presented for heat production based on measurements of a closed cooling system of generators, thus showing that the total efficiencies of generators can be enhanced by up to 1% whilst reducing the electricity consumption during the electric heating of buildings. In addition, the simulations of cost and revenue, and cumulatively discounted cash-flows of the investment in HTHP are presented using the MS Excel computer program. The payback period for the investment in a 500 kW high-temperature heat-pump for exploiting lowtemperature heat of the generators' cooling system would be approximately 2 years for the case of heating the commercial buildings of the hydro power plant, and 7 years for the case when heating the fluid within the nearby district heating systems of urban settlements. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Renewable energy sources Low-temperature heat High temperature heat-pump Heat
1. Introduction World oil and fossil fuel supplies are rapidly dwindling [1] and over the last decade energy demand has doubled [2]. More research and developments are needed for engaging energy problems and to reduce the emissions of greenhouse gases [3]. Consequently environmental and spatial planning for renewable energy sources are becoming increasingly important values; therefore environmental problems need to be considered, even at the international level, regarding the fields of heating buildings, and the development of common approaches and new attitudes towards the natural environment. It is for these reasons that one of the principally agreed measures is the reduction (or at least slowdown of the growth) of energy-demand, especially through its efficient usage and the usage of renewable-energy, whilst ensuring the same or a higher quality of life. One of the possible measures for reducing CO2 emissions is exploiting the waste-heat from electrical generators within a hydro power plant that produces renewable electricity.
* Corresponding author. Tel.: þ386 31 817 073. E-mail address: [email protected] (D. Gori canec).
Recently, more attention has been paid to the integration of high-temperature heat pumps within energy systems with the goals of enhancing their efficiencies, and reducing the CO2 emissions. Integration of a heat pump into existing ammonia refrigeration units is reported in Refs. [4,5]. However, they considered the high performance compressor with the maximum ammonia pressure of 40 bar. Additional scientifically applied research has brought [6] higher performance single stage compressors with maximum ammonia pressure of 50 bar. High temperature heat pumps integrated within a cogeneration unit can also help towards a better coexistence between cogeneration and intermittent renewables [7], because the overall cogeneration efficiency increases from 88.9 to 95.5% [8]. The generators waste-heat have a low-temperature potential and in the case of a hydro power plant it is usually released into river water using a cooling system. However, the low-temperature potential can be exploited for the production of high temperature water for high-temperature heating using a high temperature heat pump. It is easy to see that the overall efficiencies of generators would increase because of the development of new technologies for waste heat utilisation, whilst meanwhile decreasing the electricity consumption of electric boilers for the heating of office buildings. This
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work presents the summary of a scientifically applied study that is presently in the implementation phase. The main focus is the possibility of using low-temperature-heat that is released by the cooling of generators within a hydro power plant by integrating an HTHP (high temperature heat-pump) within the plant. The design of the system only became possible after 2009, when the efficient HTHPs emerged on the global market, and they are now manufactured on the basis of the results of the EUREKA research project [6]. The expedience of the presented system for exploiting the low-temperature heat from the generators' cooling system using HTHP, is supported by the following facts: the proposed innovative reconstruction of generators' cooling systems has not been presented until now, the efficiency of the renewable energy source is enhanced, the waste-heat obtained by cooling the generators is practically free, the investment in a high-temperature heat-pump is small, with a short-term return, it is possible to integrate HTHP within the heating systems, and meets the requirements regarding the rational usage of energy and the environmental protection.
2. Cooling system of generators Two cooling systems for regulated cooling of three synchronous generators (SIEMENS, type 1HD 7339-3WE24-Z) were installed in the hydro power plant Mariborski otok. A closed system for the regulated cooling of the generators was implemented for lowtemperature heating. Now the system would be upgraded with the HTHP to meet the needs of high-temperature heating of the plant's office buildings. Additionally, any excess heat produced would be transferred into the nearby district heating system (2 km away) in order to extend the exploitation time of the HTHP into the summer months. An open cooling system would be used in the event of a HTTP failure or in the case of any excess of lowtemperature heat. A cooling water volume flow of 7.5 l/s (27 m3/h) with a temperature regime of 30e40 C is available per single generator. The released heat can be partly used in the high-temperature heatpump at the highest temperature level and the remainder stored in two heat storage tanks of 50 m3 each e Fig. 1 The air which cools the synchronous generator windings is cooled by water-cooled coolers on the outer side of the stator housing.
Fig. 1. Schematic representation of exploiting the potential of low-temperature generators' cooling system using HTHP.
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The regulator of the open system compares the actual temperature of the stator winding with the desired temperature every 10 min, and if necessary opens or closes the regulation valve at the outlet pipework. Opening of the valve depends on the deviation of the actual temperature from that desired, meanwhile the closing always occurs within the same time interval. The closed system is much faster because the measurements of the actual and desirable temperatures are taken every 30 s, whilst the opening and closing is carried out using the same principle as in the case of the open system. The closed system has been functioning for quite some time. There is no direct link between the closed and open cooling systems within the regulation algorithm. The regulation of both cooling systems depends on the required average temperature of the stator coils of the generators. Both systems differ only in the outlet temperatures of the cooling waters. When the open cooling system is in operation, the temperature of the stator winding is maintained at between 50 and 60 C. The valve closes at a temperature below 50 C and the valve opens at a temperature above 60 C. The closed system operates in the same way except that the cooling water temperatures are set at 20 C higher.
2.1. Heat losses of the generators Losses in the form of heat occur during the operations of the generators (generation of electricity). For generator No3 the calculated heat losses are given in Table 1. The measurements were done at a potential difference of 10 kV and power factors of cos4 ¼ 0.8 and cos4 ¼ 1.0. Heat losses were estimated by a calorimetric method according to the standard IEC 34-2/34-2A. Under this method the data given in Table 1 are determined as follows: The losses due to air ventilation and friction: the bulk of these losses are transferred to the generator coolers. The total losses due to ventilation and friction amount to 42.9 kW. The losses in the iron: at the rated voltage of the generator (10.5 kV), the bulk of these losses (85.7 kW) are transferred to generator coolers. The losses in the copper stator winding amount to 174.8 kW. The bulk of this heat flow is conveyed through cooling air to generator coolers. The losses due to the excitation current of the rotor at 75 C amount to 122.9 kW. The bulk of this heat is conveyed through cooling air to generator coolers. The sum of the losses by radiation. Here the heat is transferred by radiation through the surfaces of the individual parts of the generator such as the lower and upper lid, the oil arrester, and the concrete wall around the generator (19.9 kW). The heat flow due to
3
radiation is not discharged through the generators' cooling system, but it is transferred directly into the surroundings. The total heat flow that occurs at 100% capacity of the generator (210 MWe) amounted to 406 kW (1.943% losses). 3. HTHP (High-temperature heat-pump) The development of heat pumps began during the time of the infamous oil crisis, when many manufacturers were looking for solutions to replace fossil fuels with alternative energy sources. One of the solutions was the usage of waste heat or ambient heat. Subsequently, because these heat pumps did not provide the expected results, they were forgotten during the post crisis period. However, nowadays heat pumps are attracting more and more interest. This is mainly because of higher ecological awareness and increases in the prices of fossil fuels. Furthermore, its usage for heating is more energy efficient and environmentally acceptable. It can be stated that the usages of heat pumps has increased significantly with the development of new technologies, the increases in technological efficiencies, and due to smaller process units. New types of low-temperature heat pumps can operate at outside air temperatures from (20) C, with COP (coefficient of performance) up to 3. Following the development of the HTHP that has been manufactured based on the results of the research project EUREKA [2], it can be predicted that, in the regions with low-temperature sources available, the HTHPs will represent one of the more important pieces of equipment for high temperature heating. The development of the HTHP and its application within process systems was supported by intensive research work [2e7] The technical innovations of the EUREKA project were in the choice of refrigerant, and the design of a single-stage high-pressure compressor (50 bar) as the basic component of HTHP that allows COP even higher than 9, and hot water production with temperatures up to 85 C. 3.1. Mathematical model 3.1.1. Parameters of the heat pump Heat flow FHTH (kW) of the high-temperature heat-pump is determined by the following equation [4]:
FHTP ¼ qm $ðh2 h3 Þ
(1)
Where h2 is the specific enthalpy of the vapour refrigerant at the compressor outlet (kJ/kg), h3 is the specific enthalpy of the refrigerant after condensation and before reduction of pressure (kJ/kg) and qm the mass flow-rate of the refrigerant (kg/s). COP (coefficient of performance) of a high-temperature heatpump is defined by the following equation:
Table 1 Heat losses at power factor cos4 ¼ 1.0 for generator No3 (manufacturer data). P (%) P (kW) (generator power) Losses due to air ventilation and friction Losses in the iron Losses in the copper (75 C) Losses due to excitation current e rotor (75 C); (I2R) Total losses (kW)
40 8322 42.9 85.7 27.9 53.5 210
60 12,483 42.9 85.7 62.9 71.4 263
80 16,644 42.9 85.7 111.9 94.3 335
90 18,724 42.9 85.7 141.6 120.5 391
100 20,805 42.9 85.7 174.8 122.9 426
The sum of the losses by radiation (through upper and lower lid, concrete, and the oil vent) The heat that is transferred through air coolers F (kW) F/P (%) The calculated value of the losses
19.9 190 2.285 1.943%
19.9 243 1.947
19.9 315 1.892
19.9 371 1.980
19.9 406 1.954
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Table 2 Power necessary for the compressor, the produced heat-flow and COP, depending on the temperature of the low-temperature energy source. Temperature of hot water
Inlet source temperature ( C)
15
20
25
30
35
40
45
50
55
Outlet source temperature ( C)
5
10
15
20
25
30
35
40
45
108.6 35.3 3.08
136 38.5 3.54 128.7 40.1 3.21
166.7 41.4 4.03 158.9 43.6 3.65
200.7 43.9 4.58 192.5 46.7 4.13
238 45.7 5.21 229.5 49.3 4.66
278.9 46.8 5.96 269.7 51.1 5.28
323.2 47 6.88 313.6 52.1 6.02
371.5 46.3 8.03 361 52.1 6.93
423.3 44.4 9.54 412.1 51.1 8.07
229.4 70.1 3.28
279.2 74.7 3.74
334.4 78.5 4.26 321.5 83.5 3.86
395 81.3 4.86 381.5 87.5 4.36
461.8 83.1 5.56 447.2 90.4 4.95
534.4 83.5 6.4 518.7 92 5.64
613.1 82.3 7.45 596.2 92.1 6.48
698.1 79.5 8.79 679.5 90.4 7.52
Speed of the compressor e 970 rpm Heat flow (kW) 65 C Power necessary COP 70 C Heat flow (kW) Power necessary COP Speed of the compressor e 1600 rpm 65 C Heat flow (kW) Power necessary COP Heat flow (kW) 70 C Power necessary COP
COP ¼
(kW)
(kW)
(kW)
(kW)
FHTP P
(2)
where Pad is the adiabatic power of the compressor (kW), V1 is the operation volume of the compressor, and hk is the compressor's efficiency.
where P is the compressor power (kW). 3.1.2. Parameters of the compressor The temperature at the compressor outlet is determined by the equation
TT ¼ TS $rK
c1 c
(3)
where TT is the temperature at the compressor outlet (K), TS is the temperature at the compressor inlet (K), rK is the pressure ratio, and c is the adiabatic index of the refrigerant. The pressure ratio of the compressor is determined by the following equation:
rK ¼
p2 p1
(4)
where p1 is the pressure at the compressor inlet (Pa), and p2 is the pressure at the compressor outlet (Pa). The required power of the compressor for adiabatic refrigerant compression for the operation of a heat pump is defined as:
2 6 c 6 c1$p1 $V1 $4 P¼
Pad ¼ hk
c1 p2 p1
c
3 7 17 5 (5)
hk
Table 3 Measurements of the heat flows, and the inlet and outlet temperatures of the cooling water in the open generators' cooling system. Date
Time
Heat flow (kW)
Inlet temperature ( C)
Outlet temperature ( C)
3/30/2012 3/30/2012 3/30/2012 3/30/2012 3/30/2012 3/30/2012 3/30/2012 3/30/2012
12:18:43 PM 12:28:43 PM 12:38:43 PM 12:48:43 PM 12:58:43 PM 1:08:43 PM 1:18:43 PM 1:28:43 PM
117.16 116.47 114.98 114.68 117.15 115.69 115.48 114.59
9.9 9.9 9.9 9.9 10.0 10.0 10.0 10.0
13.1 13.1 13.1 13.1 13.2 13.2 13.2 13.2
3.2. Purpose HTHPs [9e13] are devices that offer high-added value and provide an outstanding contribution to reducing energy dependence because they can be used throughout every branch of industry using the emerging waste heat-flows of different fluids. HTHP enables: efficient usage of the heat potentials of various low-temperature heat sources, replacement of energy pollutants with an alternative energy source, increase in the use of renewable energy sources, significant reduction in the cost of heat produced for the hightemperature heating of urban centres, high COP, reliable heat supply, reducing of greenhouse gas emissions, reduction of environmental pollution and contribution to the protection of the environment, acceptable investment with a short payback period. HTHP extracts heat from low-temperature sources such as geothermal water, flue gases, the cooling waters of industrial heat sources, the heat of industrial refrigerator systems for the food industry, etc. The low temperature heat is used for evaporating the working fluid in the evaporator. The vapour working fluid is then compressed within a high-performance compressor, condensed in the condenser, and then expanded through an expansion valve. The released heat in the condenser is used for heating the water of a heating system up to 85 C. The efficiency of HTHP is specified with the coefficient of efficiency COP, which can be even higher than 9 (Table 2). The HTHPs that are now on the market and represent world class innovation also play an important role in the mitigation of CO2 emissions.
3.3. Operation parameters The results of heat-flow calculations, necessary powers, and COPs of HTHP are represented in Table 2. The results are given for various operating conditions using R717 (NH3) working fluid, and
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Table 4 The data and calculations of heat production using HTHP. Month
Average power of generators (MW)
Average low-temperature heat of the generators' cooling system (MWh) per day
Production of heat per month (MWh) COP ¼ 6
Operation of HTHP at 1600 rpm (h/day)
Time of operation at 1600 rpm (h/month)
1 2 3 4 5 6 7 8 9 10 11 12
17.199 17.199 22.230 33.579 47.502 53.352 45.747 36.270 31.824 27.729 26.325 21.411
2.98 2.98 3.85 5.82 8.23 9.24 7.93 6.28 5.51 4.80 4.56 3.71
107.8 97.4 139.3 203.6 297.7 323.6 286.7 227.3 193.0 173.8 159.7 134.2
7.0 7.0 9.0 13.6 19.2 21.6 18.5 14.7 12.9 11.2 10.6 8.7
216 195 279 407 595 647 573 455 386 348 319 268
Maximum heat flow at 1600 rpm (COP ¼ 6) 500 kW. Maximum number of operating hours per year 4688 h/a. Planned heat production per year using HTHP (COP ¼ 6) 2344 MWh. The difference between the actual and planned flow rate of the river þ8%.
commercially available 50 bar compressor, for the hot water temperatures of 65 and 70 C. Other refrigerants proved to be less appropriate due to the lower enthalpy difference between the vapour and liquid phases, smaller heat-flow, and lower COP. The capacity of the compressor within HTHP can be controlled stepwise (970; 1450; and 1600 rpm) or by continuously variable regulation of the electric motor that drives the compressor. According to the data in Table 2, if the desired hot water temperature is 65 C, the inlet temperature of the source flowing into HTHP is 55 C and its outlet temperature 45 C at 1600 rpm of the compressor, the obtained heat flow is equal to 689.1 kW. The needed electric power is 79.5 kW; therefore COP is equal to 8.79. However, at lower temperatures of the low-temperature source, COP is correspondingly lower.
and its surroundings. The difference would result in an increase in the heat-flows of the cooling and heating systems. Table 4 includes the generation of power, production of lowtemperature heat from the generators' cooling system, calculated production of heat using the HTHP, and daily and monthly operation hours in the discussed hydro power plant. Based on the measurements, it was determined that the maximum heat flow of each generator is 170.3 kW. The lowtemperature potential of this heat flow can be usefully exploited using a high-temperature heat-pump, with the rated power of 500 kW, which corresponds to the heat potential (510 kW) of the cooling water from three generators within the hydro power plant.
5. Economics of heat extraction from the generators' cooling system
4. Production of heat using HTHP As part of the research, measurements of heat flow were carried out as well as the inlet and outlet temperatures of the cooling water within the open cooling system. Some data on the measurements that have been carried out every 10 min are stated in Table 3. Flow measurements were performed with a WP-Helix Serie 400 instrument, No. 402,136, and measurements of the temperature with a Pt 100 conductivity-thermometer. In regard to the measurements outside the heating season, we can conclude that approximately 40% of the generated heat is transferred to the storage heating system, whilst the remaining heat is lost into the surroundings through the casing of the generator and other thermal bridges. More thermal energy could be obtained by reducing the maximum temperature of the generators' cooling system from 40 to 30 C because the heat-losses are lower due to the smaller temperature difference between the generator
Simulation of the costs and revenues of the investment in HTHP was carried out using MS Excel. The basic data is stated in Tables 5 and 6, and the cumulatively discounted cash-flow is presented in Figs. 2 and 3. The calculation takes into account the installation of insulated feed and return lines of 2 km in length, which connects the hydro power plant and the pipes of the district heating system. Based on the computer program created in MS Excel for investment calculation, it was found that the payback period for investment (730,000 V) in HTHP and other peripherals (2000 m preinsulated pipes for connection to the district heating pipeline, pumping station, etc.) is approximately seven years at 4688 operating hours per year. Economic capital calculation for HTHP for the heating of the hydro power plant office buildings (without the investment in the pre-insulated pipes) shows that the return on investment (163,000 V) is approximately 2 years at 3200 operating hours per year.
Table 5 Data for the heat production. Operation factor Nominal accounting power HTHP Average heat flow HTHP Number of operating hours per year Heat production HTHP Heat losses of insulated pipe network The total length of the pipe network Heat losses of the pipe network Sold thermal energy
100 500 500 4688 2,344,000 0.010 2000 93,760 2,250,240
% kWth kWth h/a kWh/a kWh/m/h m kWh/a kWh/a
Table 6 Operating costs of HTHP. COP HTHP Consumption of electricity for compressor HTHP Price of the produced heat HTHP Clearing price for power Price of heat production from natural gas Price of electricity (production cost)
6.00 0.17 0.0092 18.88 0.0658 0.0550
kWhe/kWhth EUR/kWh EUR/kWth/a EUR/kWh EUR/kWh
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Fig. 2. Cumulatively discounted cash-flow for the case of HTHP and connection to the district heating system (IRR ¼ 14.8%).
of electric power amount to 0.55 kg/kWh. Compared to fossil fuels, a high-temperature heat-pump is very economically and environmentally efficient because it indirectly (from the use of electricity powering the compressors) emits 54, and 65% less CO2 than natural gas and heating oil, respectively. The economic calculations of the cumulatively discounted cashflows presented in Fig. 2, show that the payback period of the investment (HTHP and pre-insulated pipes for connection to the district heating pipeline, pumping station etc.) for the purpose of district heating would be seven years (IRR ¼ 14.8%) at approximately 4700 operating hours per year. However, in the case of the investment in HTHP only for the purpose of heating the commercial buildings of the hydro power plant, the cumulatively discounted cash-flow calculations (Fig. 3) showed that the payback period would be a little over two years (IRR ¼ 48.5%) at 3200 operating hours per year. Implementation of the heat exploitation from the generators' cooling system within the hydro power-plant, presents a significant environmental contribution because the generated heat is extracted from a low-temperature renewable energy-source. The overall efficiencies of the generators can be increased in this way by at least 1%, meanwhile reducing the consumption of electricity for heating the commercial buildings and hydro power-plants. References
Fig. 3. Cumulatively discounted cash-flow for the case of HTHP only (IRR ¼ 48.5%).
However, in the case of exploiting a low-temperature source with a higher temperature, the return on investment would be even shorter due to higher COP. 6. Conclusion A high-temperature heat-pump is an economically and ecologically very efficient piece of equipment for the exploitation of liquid or gaseous low-temperature heat sources. According to current legislation, the heat produced by a high-temperature heatpump is considered as a renewable energy-source. A hightemperature heat-pump does not emit greenhouse gases into the atmosphere during its operation (same as electric cars). However, because it needs electricity for the compressor drive of the HTHP, it has an indirect emission of 0.092 kg/kWh CO2 (COP ¼ 6). This is because in Slovenia the average CO2 emissions from the generation
[1] Olabi AG. The 3rd international conference on sustainable energy and environmental protection SEEP 2009-Guest Editor's Introduction. Energy 2010;35(12):4508e9. [2] Olabi AG. State of the art on renewable and sustainable energy. Energy 2013;61:2e5. [3] Olabi AG. Developments in sustainable energy and environmental protection. Energy 2012;39(1):2e5. [4] Kapustenko PO, Ulyev LM, Boldyryev SA, Garev AO. Integration of a heat pump into the heat supply system of a cheese production plant. Energy 2008;33(6): 882e9. [5] Korfitsen E, Kristensen APK. Ammonia high pressure heat pumps in food refrigeration applications. Int J Refrig 1998;21(3):212e8. [6] Eureka. EUREKA SUCCESS STORY > E! 4117 HTH PUMP. http://www. eurekanetwork.org/showsuccessstory?p_r_p_564233524_ articleId¼362473&p_r_p_564233524_groupId¼10137, [accessed 03.03.14]. [7] Blarke MB. Towards an intermittency-friendly energy system: comparing electric boilers and heat pumps in distributed cogeneration. Appl Energy 2012;91(1):349e65. [8] Blarke MB, Dotzauer E. Intermittency-friendly and high-efficiency cogeneration: operational optimisation of cogeneration with compression heat pump, flue gas heat recovery, and intermediate cold storage. Energy 2011;36(12): 6867e78. [9] Kulcar B, Goricanec D, Krope J. Economy of exploiting heat from lowtemperature geothermal sources using a heat pump. Energy Build 2008;40(3):323e9. [10] Kulcar B, Goricanec D, Krope J. Economy of replacing a refrigerant in a cooling system for preparing chilled water. Int J Refrig 2010;33(5):989e94. [11] Jotanovi c M, Mis cevi c M, Kul car B, Gori canec D. Optimal proportion of refrigerants in single- stage high-temperature heat pump. Int J Syst Appl Eng Dev 2012;6(6):365e75. [12] Jotanovi c M, Tadi c G, Krope J, Gori canec D. Analysis of two-stage high- temperature heat pump efficiency. Int J Syst Appl Eng Dev 2012;6(5):350e63. [13] Torha c E, Gori canec D, Andrejevi c S, Saljnikov S, Krope J. New high temperature heat pumps for exploiting low-temperature sources. Res J Chem Environ 2011;15(2):1e6.
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Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Exploitation of the waste-heat from hydro power plants D. Gori canec a, *, V. Pozeb b, L. Tomsi c b, P. Trop a a b
University of Maribor, Faculty of Chemistry and Chemical Engineering, Smetanova ulica 17, 2000 Maribor, Slovenia Hydro Power Plant Maribor, Obrezna ulica 170, 2000 Maribor, Slovenia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 December 2013 Received in revised form 24 June 2014 Accepted 29 June 2014 Available online xxx
This paper presents the possibility of exploiting low-temperature heat from the generators' cooling system within a hydro power plant, using a HTHP (high temperature heat-pump) that enables heating at up to 85 C. The results based on theoretical calculations are presented for heat-flows, the powers of the compressors, and COP (coefficient of performance) values for the cases when using the refrigerant R717 and a single stage high pressure compressor (up to 50 bar) under varying operational conditions. Real possibilities are presented for heat production based on measurements of a closed cooling system of generators, thus showing that the total efficiencies of generators can be enhanced by up to 1% whilst reducing the electricity consumption during the electric heating of buildings. In addition, the simulations of cost and revenue, and cumulatively discounted cash-flows of the investment in HTHP are presented using the MS Excel computer program. The payback period for the investment in a 500 kW high-temperature heat-pump for exploiting lowtemperature heat of the generators' cooling system would be approximately 2 years for the case of heating the commercial buildings of the hydro power plant, and 7 years for the case when heating the fluid within the nearby district heating systems of urban settlements. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Renewable energy sources Low-temperature heat High temperature heat-pump Heat
1. Introduction World oil and fossil fuel supplies are rapidly dwindling [1] and over the last decade energy demand has doubled [2]. More research and developments are needed for engaging energy problems and to reduce the emissions of greenhouse gases [3]. Consequently environmental and spatial planning for renewable energy sources are becoming increasingly important values; therefore environmental problems need to be considered, even at the international level, regarding the fields of heating buildings, and the development of common approaches and new attitudes towards the natural environment. It is for these reasons that one of the principally agreed measures is the reduction (or at least slowdown of the growth) of energy-demand, especially through its efficient usage and the usage of renewable-energy, whilst ensuring the same or a higher quality of life. One of the possible measures for reducing CO2 emissions is exploiting the waste-heat from electrical generators within a hydro power plant that produces renewable electricity.
* Corresponding author. Tel.: þ386 31 817 073. E-mail address: [email protected] (D. Gori canec).
Recently, more attention has been paid to the integration of high-temperature heat pumps within energy systems with the goals of enhancing their efficiencies, and reducing the CO2 emissions. Integration of a heat pump into existing ammonia refrigeration units is reported in Refs. [4,5]. However, they considered the high performance compressor with the maximum ammonia pressure of 40 bar. Additional scientifically applied research has brought [6] higher performance single stage compressors with maximum ammonia pressure of 50 bar. High temperature heat pumps integrated within a cogeneration unit can also help towards a better coexistence between cogeneration and intermittent renewables [7], because the overall cogeneration efficiency increases from 88.9 to 95.5% [8]. The generators waste-heat have a low-temperature potential and in the case of a hydro power plant it is usually released into river water using a cooling system. However, the low-temperature potential can be exploited for the production of high temperature water for high-temperature heating using a high temperature heat pump. It is easy to see that the overall efficiencies of generators would increase because of the development of new technologies for waste heat utilisation, whilst meanwhile decreasing the electricity consumption of electric boilers for the heating of office buildings. This
http://dx.doi.org/10.1016/j.energy.2014.06.106 0360-5442/© 2014 Elsevier Ltd. All rights reserved.
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D. Goricanec et al. / Energy xxx (2014) 1e6
work presents the summary of a scientifically applied study that is presently in the implementation phase. The main focus is the possibility of using low-temperature-heat that is released by the cooling of generators within a hydro power plant by integrating an HTHP (high temperature heat-pump) within the plant. The design of the system only became possible after 2009, when the efficient HTHPs emerged on the global market, and they are now manufactured on the basis of the results of the EUREKA research project [6]. The expedience of the presented system for exploiting the low-temperature heat from the generators' cooling system using HTHP, is supported by the following facts: the proposed innovative reconstruction of generators' cooling systems has not been presented until now, the efficiency of the renewable energy source is enhanced, the waste-heat obtained by cooling the generators is practically free, the investment in a high-temperature heat-pump is small, with a short-term return, it is possible to integrate HTHP within the heating systems, and meets the requirements regarding the rational usage of energy and the environmental protection.
2. Cooling system of generators Two cooling systems for regulated cooling of three synchronous generators (SIEMENS, type 1HD 7339-3WE24-Z) were installed in the hydro power plant Mariborski otok. A closed system for the regulated cooling of the generators was implemented for lowtemperature heating. Now the system would be upgraded with the HTHP to meet the needs of high-temperature heating of the plant's office buildings. Additionally, any excess heat produced would be transferred into the nearby district heating system (2 km away) in order to extend the exploitation time of the HTHP into the summer months. An open cooling system would be used in the event of a HTTP failure or in the case of any excess of lowtemperature heat. A cooling water volume flow of 7.5 l/s (27 m3/h) with a temperature regime of 30e40 C is available per single generator. The released heat can be partly used in the high-temperature heatpump at the highest temperature level and the remainder stored in two heat storage tanks of 50 m3 each e Fig. 1 The air which cools the synchronous generator windings is cooled by water-cooled coolers on the outer side of the stator housing.
Fig. 1. Schematic representation of exploiting the potential of low-temperature generators' cooling system using HTHP.
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The regulator of the open system compares the actual temperature of the stator winding with the desired temperature every 10 min, and if necessary opens or closes the regulation valve at the outlet pipework. Opening of the valve depends on the deviation of the actual temperature from that desired, meanwhile the closing always occurs within the same time interval. The closed system is much faster because the measurements of the actual and desirable temperatures are taken every 30 s, whilst the opening and closing is carried out using the same principle as in the case of the open system. The closed system has been functioning for quite some time. There is no direct link between the closed and open cooling systems within the regulation algorithm. The regulation of both cooling systems depends on the required average temperature of the stator coils of the generators. Both systems differ only in the outlet temperatures of the cooling waters. When the open cooling system is in operation, the temperature of the stator winding is maintained at between 50 and 60 C. The valve closes at a temperature below 50 C and the valve opens at a temperature above 60 C. The closed system operates in the same way except that the cooling water temperatures are set at 20 C higher.
2.1. Heat losses of the generators Losses in the form of heat occur during the operations of the generators (generation of electricity). For generator No3 the calculated heat losses are given in Table 1. The measurements were done at a potential difference of 10 kV and power factors of cos4 ¼ 0.8 and cos4 ¼ 1.0. Heat losses were estimated by a calorimetric method according to the standard IEC 34-2/34-2A. Under this method the data given in Table 1 are determined as follows: The losses due to air ventilation and friction: the bulk of these losses are transferred to the generator coolers. The total losses due to ventilation and friction amount to 42.9 kW. The losses in the iron: at the rated voltage of the generator (10.5 kV), the bulk of these losses (85.7 kW) are transferred to generator coolers. The losses in the copper stator winding amount to 174.8 kW. The bulk of this heat flow is conveyed through cooling air to generator coolers. The losses due to the excitation current of the rotor at 75 C amount to 122.9 kW. The bulk of this heat is conveyed through cooling air to generator coolers. The sum of the losses by radiation. Here the heat is transferred by radiation through the surfaces of the individual parts of the generator such as the lower and upper lid, the oil arrester, and the concrete wall around the generator (19.9 kW). The heat flow due to
3
radiation is not discharged through the generators' cooling system, but it is transferred directly into the surroundings. The total heat flow that occurs at 100% capacity of the generator (210 MWe) amounted to 406 kW (1.943% losses). 3. HTHP (High-temperature heat-pump) The development of heat pumps began during the time of the infamous oil crisis, when many manufacturers were looking for solutions to replace fossil fuels with alternative energy sources. One of the solutions was the usage of waste heat or ambient heat. Subsequently, because these heat pumps did not provide the expected results, they were forgotten during the post crisis period. However, nowadays heat pumps are attracting more and more interest. This is mainly because of higher ecological awareness and increases in the prices of fossil fuels. Furthermore, its usage for heating is more energy efficient and environmentally acceptable. It can be stated that the usages of heat pumps has increased significantly with the development of new technologies, the increases in technological efficiencies, and due to smaller process units. New types of low-temperature heat pumps can operate at outside air temperatures from (20) C, with COP (coefficient of performance) up to 3. Following the development of the HTHP that has been manufactured based on the results of the research project EUREKA [2], it can be predicted that, in the regions with low-temperature sources available, the HTHPs will represent one of the more important pieces of equipment for high temperature heating. The development of the HTHP and its application within process systems was supported by intensive research work [2e7] The technical innovations of the EUREKA project were in the choice of refrigerant, and the design of a single-stage high-pressure compressor (50 bar) as the basic component of HTHP that allows COP even higher than 9, and hot water production with temperatures up to 85 C. 3.1. Mathematical model 3.1.1. Parameters of the heat pump Heat flow FHTH (kW) of the high-temperature heat-pump is determined by the following equation [4]:
FHTP ¼ qm $ðh2 h3 Þ
(1)
Where h2 is the specific enthalpy of the vapour refrigerant at the compressor outlet (kJ/kg), h3 is the specific enthalpy of the refrigerant after condensation and before reduction of pressure (kJ/kg) and qm the mass flow-rate of the refrigerant (kg/s). COP (coefficient of performance) of a high-temperature heatpump is defined by the following equation:
Table 1 Heat losses at power factor cos4 ¼ 1.0 for generator No3 (manufacturer data). P (%) P (kW) (generator power) Losses due to air ventilation and friction Losses in the iron Losses in the copper (75 C) Losses due to excitation current e rotor (75 C); (I2R) Total losses (kW)
40 8322 42.9 85.7 27.9 53.5 210
60 12,483 42.9 85.7 62.9 71.4 263
80 16,644 42.9 85.7 111.9 94.3 335
90 18,724 42.9 85.7 141.6 120.5 391
100 20,805 42.9 85.7 174.8 122.9 426
The sum of the losses by radiation (through upper and lower lid, concrete, and the oil vent) The heat that is transferred through air coolers F (kW) F/P (%) The calculated value of the losses
19.9 190 2.285 1.943%
19.9 243 1.947
19.9 315 1.892
19.9 371 1.980
19.9 406 1.954
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Table 2 Power necessary for the compressor, the produced heat-flow and COP, depending on the temperature of the low-temperature energy source. Temperature of hot water
Inlet source temperature ( C)
15
20
25
30
35
40
45
50
55
Outlet source temperature ( C)
5
10
15
20
25
30
35
40
45
108.6 35.3 3.08
136 38.5 3.54 128.7 40.1 3.21
166.7 41.4 4.03 158.9 43.6 3.65
200.7 43.9 4.58 192.5 46.7 4.13
238 45.7 5.21 229.5 49.3 4.66
278.9 46.8 5.96 269.7 51.1 5.28
323.2 47 6.88 313.6 52.1 6.02
371.5 46.3 8.03 361 52.1 6.93
423.3 44.4 9.54 412.1 51.1 8.07
229.4 70.1 3.28
279.2 74.7 3.74
334.4 78.5 4.26 321.5 83.5 3.86
395 81.3 4.86 381.5 87.5 4.36
461.8 83.1 5.56 447.2 90.4 4.95
534.4 83.5 6.4 518.7 92 5.64
613.1 82.3 7.45 596.2 92.1 6.48
698.1 79.5 8.79 679.5 90.4 7.52
Speed of the compressor e 970 rpm Heat flow (kW) 65 C Power necessary COP 70 C Heat flow (kW) Power necessary COP Speed of the compressor e 1600 rpm 65 C Heat flow (kW) Power necessary COP Heat flow (kW) 70 C Power necessary COP
COP ¼
(kW)
(kW)
(kW)
(kW)
FHTP P
(2)
where Pad is the adiabatic power of the compressor (kW), V1 is the operation volume of the compressor, and hk is the compressor's efficiency.
where P is the compressor power (kW). 3.1.2. Parameters of the compressor The temperature at the compressor outlet is determined by the equation
TT ¼ TS $rK
c1 c
(3)
where TT is the temperature at the compressor outlet (K), TS is the temperature at the compressor inlet (K), rK is the pressure ratio, and c is the adiabatic index of the refrigerant. The pressure ratio of the compressor is determined by the following equation:
rK ¼
p2 p1
(4)
where p1 is the pressure at the compressor inlet (Pa), and p2 is the pressure at the compressor outlet (Pa). The required power of the compressor for adiabatic refrigerant compression for the operation of a heat pump is defined as:
2 6 c 6 c1$p1 $V1 $4 P¼
Pad ¼ hk
c1 p2 p1
c
3 7 17 5 (5)
hk
Table 3 Measurements of the heat flows, and the inlet and outlet temperatures of the cooling water in the open generators' cooling system. Date
Time
Heat flow (kW)
Inlet temperature ( C)
Outlet temperature ( C)
3/30/2012 3/30/2012 3/30/2012 3/30/2012 3/30/2012 3/30/2012 3/30/2012 3/30/2012
12:18:43 PM 12:28:43 PM 12:38:43 PM 12:48:43 PM 12:58:43 PM 1:08:43 PM 1:18:43 PM 1:28:43 PM
117.16 116.47 114.98 114.68 117.15 115.69 115.48 114.59
9.9 9.9 9.9 9.9 10.0 10.0 10.0 10.0
13.1 13.1 13.1 13.1 13.2 13.2 13.2 13.2
3.2. Purpose HTHPs [9e13] are devices that offer high-added value and provide an outstanding contribution to reducing energy dependence because they can be used throughout every branch of industry using the emerging waste heat-flows of different fluids. HTHP enables: efficient usage of the heat potentials of various low-temperature heat sources, replacement of energy pollutants with an alternative energy source, increase in the use of renewable energy sources, significant reduction in the cost of heat produced for the hightemperature heating of urban centres, high COP, reliable heat supply, reducing of greenhouse gas emissions, reduction of environmental pollution and contribution to the protection of the environment, acceptable investment with a short payback period. HTHP extracts heat from low-temperature sources such as geothermal water, flue gases, the cooling waters of industrial heat sources, the heat of industrial refrigerator systems for the food industry, etc. The low temperature heat is used for evaporating the working fluid in the evaporator. The vapour working fluid is then compressed within a high-performance compressor, condensed in the condenser, and then expanded through an expansion valve. The released heat in the condenser is used for heating the water of a heating system up to 85 C. The efficiency of HTHP is specified with the coefficient of efficiency COP, which can be even higher than 9 (Table 2). The HTHPs that are now on the market and represent world class innovation also play an important role in the mitigation of CO2 emissions.
3.3. Operation parameters The results of heat-flow calculations, necessary powers, and COPs of HTHP are represented in Table 2. The results are given for various operating conditions using R717 (NH3) working fluid, and
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Table 4 The data and calculations of heat production using HTHP. Month
Average power of generators (MW)
Average low-temperature heat of the generators' cooling system (MWh) per day
Production of heat per month (MWh) COP ¼ 6
Operation of HTHP at 1600 rpm (h/day)
Time of operation at 1600 rpm (h/month)
1 2 3 4 5 6 7 8 9 10 11 12
17.199 17.199 22.230 33.579 47.502 53.352 45.747 36.270 31.824 27.729 26.325 21.411
2.98 2.98 3.85 5.82 8.23 9.24 7.93 6.28 5.51 4.80 4.56 3.71
107.8 97.4 139.3 203.6 297.7 323.6 286.7 227.3 193.0 173.8 159.7 134.2
7.0 7.0 9.0 13.6 19.2 21.6 18.5 14.7 12.9 11.2 10.6 8.7
216 195 279 407 595 647 573 455 386 348 319 268
Maximum heat flow at 1600 rpm (COP ¼ 6) 500 kW. Maximum number of operating hours per year 4688 h/a. Planned heat production per year using HTHP (COP ¼ 6) 2344 MWh. The difference between the actual and planned flow rate of the river þ8%.
commercially available 50 bar compressor, for the hot water temperatures of 65 and 70 C. Other refrigerants proved to be less appropriate due to the lower enthalpy difference between the vapour and liquid phases, smaller heat-flow, and lower COP. The capacity of the compressor within HTHP can be controlled stepwise (970; 1450; and 1600 rpm) or by continuously variable regulation of the electric motor that drives the compressor. According to the data in Table 2, if the desired hot water temperature is 65 C, the inlet temperature of the source flowing into HTHP is 55 C and its outlet temperature 45 C at 1600 rpm of the compressor, the obtained heat flow is equal to 689.1 kW. The needed electric power is 79.5 kW; therefore COP is equal to 8.79. However, at lower temperatures of the low-temperature source, COP is correspondingly lower.
and its surroundings. The difference would result in an increase in the heat-flows of the cooling and heating systems. Table 4 includes the generation of power, production of lowtemperature heat from the generators' cooling system, calculated production of heat using the HTHP, and daily and monthly operation hours in the discussed hydro power plant. Based on the measurements, it was determined that the maximum heat flow of each generator is 170.3 kW. The lowtemperature potential of this heat flow can be usefully exploited using a high-temperature heat-pump, with the rated power of 500 kW, which corresponds to the heat potential (510 kW) of the cooling water from three generators within the hydro power plant.
5. Economics of heat extraction from the generators' cooling system
4. Production of heat using HTHP As part of the research, measurements of heat flow were carried out as well as the inlet and outlet temperatures of the cooling water within the open cooling system. Some data on the measurements that have been carried out every 10 min are stated in Table 3. Flow measurements were performed with a WP-Helix Serie 400 instrument, No. 402,136, and measurements of the temperature with a Pt 100 conductivity-thermometer. In regard to the measurements outside the heating season, we can conclude that approximately 40% of the generated heat is transferred to the storage heating system, whilst the remaining heat is lost into the surroundings through the casing of the generator and other thermal bridges. More thermal energy could be obtained by reducing the maximum temperature of the generators' cooling system from 40 to 30 C because the heat-losses are lower due to the smaller temperature difference between the generator
Simulation of the costs and revenues of the investment in HTHP was carried out using MS Excel. The basic data is stated in Tables 5 and 6, and the cumulatively discounted cash-flow is presented in Figs. 2 and 3. The calculation takes into account the installation of insulated feed and return lines of 2 km in length, which connects the hydro power plant and the pipes of the district heating system. Based on the computer program created in MS Excel for investment calculation, it was found that the payback period for investment (730,000 V) in HTHP and other peripherals (2000 m preinsulated pipes for connection to the district heating pipeline, pumping station, etc.) is approximately seven years at 4688 operating hours per year. Economic capital calculation for HTHP for the heating of the hydro power plant office buildings (without the investment in the pre-insulated pipes) shows that the return on investment (163,000 V) is approximately 2 years at 3200 operating hours per year.
Table 5 Data for the heat production. Operation factor Nominal accounting power HTHP Average heat flow HTHP Number of operating hours per year Heat production HTHP Heat losses of insulated pipe network The total length of the pipe network Heat losses of the pipe network Sold thermal energy
100 500 500 4688 2,344,000 0.010 2000 93,760 2,250,240
% kWth kWth h/a kWh/a kWh/m/h m kWh/a kWh/a
Table 6 Operating costs of HTHP. COP HTHP Consumption of electricity for compressor HTHP Price of the produced heat HTHP Clearing price for power Price of heat production from natural gas Price of electricity (production cost)
6.00 0.17 0.0092 18.88 0.0658 0.0550
kWhe/kWhth EUR/kWh EUR/kWth/a EUR/kWh EUR/kWh
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Fig. 2. Cumulatively discounted cash-flow for the case of HTHP and connection to the district heating system (IRR ¼ 14.8%).
of electric power amount to 0.55 kg/kWh. Compared to fossil fuels, a high-temperature heat-pump is very economically and environmentally efficient because it indirectly (from the use of electricity powering the compressors) emits 54, and 65% less CO2 than natural gas and heating oil, respectively. The economic calculations of the cumulatively discounted cashflows presented in Fig. 2, show that the payback period of the investment (HTHP and pre-insulated pipes for connection to the district heating pipeline, pumping station etc.) for the purpose of district heating would be seven years (IRR ¼ 14.8%) at approximately 4700 operating hours per year. However, in the case of the investment in HTHP only for the purpose of heating the commercial buildings of the hydro power plant, the cumulatively discounted cash-flow calculations (Fig. 3) showed that the payback period would be a little over two years (IRR ¼ 48.5%) at 3200 operating hours per year. Implementation of the heat exploitation from the generators' cooling system within the hydro power-plant, presents a significant environmental contribution because the generated heat is extracted from a low-temperature renewable energy-source. The overall efficiencies of the generators can be increased in this way by at least 1%, meanwhile reducing the consumption of electricity for heating the commercial buildings and hydro power-plants. References
Fig. 3. Cumulatively discounted cash-flow for the case of HTHP only (IRR ¼ 48.5%).
However, in the case of exploiting a low-temperature source with a higher temperature, the return on investment would be even shorter due to higher COP. 6. Conclusion A high-temperature heat-pump is an economically and ecologically very efficient piece of equipment for the exploitation of liquid or gaseous low-temperature heat sources. According to current legislation, the heat produced by a high-temperature heatpump is considered as a renewable energy-source. A hightemperature heat-pump does not emit greenhouse gases into the atmosphere during its operation (same as electric cars). However, because it needs electricity for the compressor drive of the HTHP, it has an indirect emission of 0.092 kg/kWh CO2 (COP ¼ 6). This is because in Slovenia the average CO2 emissions from the generation
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