- Email: [email protected]
Evaluation of Heat Transfer Performance between Rock and Air in Seasonal Thermal Energy Storage Unit
Available online at www.sciencedirect.com
ScienceDirect
Availableonline onlineatatwww.sciencedirect.com www.sciencedirect.com Available Energy Procedia 00 (2017) 000–000
ScienceDirect ScienceDirect
www.elsevier.com/locate/procedia
Energy (2017) 000–000 576–581 EnergyProcedia Procedia142 00 (2017) www.elsevier.com/locate/procedia
9th International Conference on Applied Energy, ICAE2017, 21-24 August 2017, Cardiff, UK
Evaluation of Heat Transfer Performance between Rock and Air in The 15th International Symposium on District Heating and Cooling Seasonal Thermal Energy Storage Unit b, a a Assessing theAlifeasibility of using the heat demand-outdoor Leyla Amiria, Seyed Ghoreishi-Madiseh *, Agus P. Sasmito , Ferri P. Hassani temperature function for a long-term district heat demand forecast Department of Mining and Material Engineering, McGill University, Montreal, Quebec, Canada aa
bb NBK
NBK Institute of Mining, University of British Columbia, Vancouver, British Columbia, Canada
I. Andrića,b,c*, A. Pinaa, P. Ferrãoa, J. Fournierb., B. Lacarrièrec, O. Le Correc a Abstract IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal b
Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France
Systèmes Énergétiques et Environnement IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, Franceamounts of Ventilation iscDépartement one of the most energy demanding parts of deep -underground mining operations due to the extensive energy needed to cool (or heat) the substantial amount of air flow in underground workings. This considerable amount of energy demand can be partially satisfied by extracting renewable energies or using alternative energy solutions available at mine sites. In Canada, some mining operations have the opportunity to create a Seasonal Thermal Energy Storage (STES) unit by blowing mine Abstract intake air through large volumes of rock mass dumped in a decommissioned pit. This technique allows for creation of the socalled “Natural Heat Exchanger (NHE)” which moderates seasonal air temperature oscillations. This paper signifies a novel heat Districtmodel heating are commonly the literature as onethe of potential the mostfor effective decreasing the transfer fornetworks performance assessment addressed of a NHE in system. It investigates energy solutions savings byfor implementation gas emissions from the sector. Theseinsystems requiremines. high investments which through the heat ofgreenhouse variable number and position of building ventilation trenches underground In this regard, a are 3-Dreturned heat transfer model is sales. Due to the changed climate conditions and building renovation policies, heat demand in the future could decrease, developed to evaluate the thermal storage and the heat transfer between broken rock mass and ventilated air flow and also to prolonging investment return period. specify how the design parameters such as position and number of ventilation trenches will affect the heat transfer performance as The main scope of this paper is to theoffeasibility usingthat theNHE heat demand – outdoor temperature functionof forunderground heat demand well as the total energy savings. Theassess results this studyofshow can be applied in thermal management forecast. district of Alvalade, located in Lisbon was used as a case purposes. study. The district is consisted of 665 mines withThe the aim of decreasing energy consumption for(Portugal), heating, cooling and ventilation vary in both construction ©buildings 2017 Thethat Authors. Published by Elsevierperiod Ltd. and typology. Three weather scenarios (low, medium, high) and three district ©renovation 2017 The Authors. by Elsevier Ltd. intermediate, deep). To estimate the error, obtained heat demand values were scenariosPublished were developed (shallow, Peer-review under responsibility of committee of the 9th International International Conference Conference on Applied Applied Energy. Energy. Peer-review under responsibility of the the scientific scientific committee the 9th compared with results from a dynamic heat demand model,of previously developed and validatedon by the authors. The results showed that when weather considered, margin ofventilation; error could be acceptable for some applications Keywords: Seasonal thermal energy only storage (STES);change NaturalisHeat Exchangerthe (NHE); Mine Porous medium; Energy efficiency (the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation scenarios, the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). The value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the decrease in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and improve the accuracy of heat demand estimations.
© 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and * Corresponding author. Tel.: +1-604-827-2028 Cooling. E-mail address: [email protected]
Keywords: Heat demand; Forecast; Climate change 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 9th International Conference on Applied Energy.
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling.
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 9th International Conference on Applied Energy. 10.1016/j.egypro.2017.12.096
2
Leyla Amiri et al. / Energy Procedia 142 (2017) 576–581 Author name / Energy Procedia 00 (2017) 000–000
Nomenclature Abbreviations: TES Thermal Energy Storage STES Seasonal Thermal Energy Storage LTNE Local Thermal Non-Equilibrium UDF user-defined functions NHE Natural Heat Exchanger
577
Symbols: f fluid phase s solid phase ρ density u fluid velocity p pressure ε porosity μ dynamic viscosity of the fluid cp specific heat k thermal conductivity T temperature
1. Introduction The industrial sector consumes about 37% of the world’s total delivered energy and among them, mining industry contributes about 9% [1]. Within mining activities, ventilation is one of the most energy demanding parts of deep underground mining operations. This energy demand can be partially fulfilled by deploying the renewable energies or using alternative energy solutions available at mine sites. Thermal energy storage (TES) systems are one these solutions which fall will within three main categories including sensible heat storage systems, latent heat storage systems and thermochemical storage systems. The selection of the TES systems mostly depends on the storage period required (i.e. short-term vs. long-term), availability of the medium, operating conditions, purpose of projects, etc. In recent years, TES systems come out to be one of the most attractive thermal applications [2]. Sensible heat storage generally utilizes rock beds or water tanks or aquifers as a storage medium and is available for short-term and long-term storage. In these systems heat/cold energy mostly is stored by increasing/decreasing the storage medium temperature [3, 4]. Some mining operations (e.g. Creighton and Kidd Creek mines in Canada) have the great opportunity to make use of the enormous mass of their fragmented waste rock as an immense seasonal thermal energy storage (STES) unit with the purpose of creating a unique type of heat exchanger; namely “Natural Heat Exchanger”. In this type of heat exchanger, fresh air is passed through the huge body of broken rocks dumped in to an open pit. The heat exchange between the waste rocks and the fresh air will lead to considerable decrease in ventilation costs in these mines [4-6]. Very limited studies [2, 4] have focused on the 3D analysis of heat transfer and fluid flow in the rockpit STES. Numerous computer programs have been developed to investigate the flow of compressible and incompressible fluids through strata [7, 8]. Most of these programs are based upon the porous media concept in which it is assumed that any one stratum contains evenly distributed interconnected pores [8-10]. An extensive study of fluid flow and heat transfer in porous media was undertaken by Whitaker [11], Vafai and Tien [12] and Ghoreishi-Madiseh et al. [2, 4, 13]. Limited number of research work, most important of them [5, 9], have been dedicated to the study of heat transfer in large scale thermal energy storage systems for mine ventilation purposes. The empirical approach suggested by Sylvester [9] can be used to estimate the heat storage capacity of Creighton mine rock-pit. Afterward, Schafrik [14] developed a numerical simulation model for Creighton mine and aimed to validate its results in laboratory scale. To overcome such limitations, researchers have proposed using 3D models capable of simulating heat transfer and fluid flow in the large scale Se-TES units [2, 9, 14]. To conclude, there is a fundamental need for a generally applicable engineering design method based on which the thermal energy storage capacity of rock mass of any mine can be assessed. The main aims of this paper are (i) developed a 3D transient mathematical model of fluid flow and heat transfer of STES; (ii) quantitatively compared the effect of position and number of ventilation trenches on outlet air temperature as well as total energy savings. 2. Model description Here, a 3D model of the rockpit - a large mass of fragmented rock - for STES system of Creighton mine in Canada is proposed. This rockpit model is assumed to be placed over a cone-shaped mine pit which is 697 (m) in diameter and 330 (m) deep. Filling the pit with waste rocks has created a porous rockpit mass. The thickness of the rockpit mass at its very bottom is 133 (m). This porous medium is accessed from the underground through trenches. These trenches are connected to mine ventilation shaft(s) where ventilation fans will continuously move fresh air
Leyla Amiri et al. / Energy Procedia 142 (2017) 576–581 Author name / Energy Procedia 00 (2017) 000–000
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3
through rockpit, trenches, mine shaft(s) and other underground openings. It is expected that due to seasonal temperature difference, thermal energy (hot and cold) is stored in the broken rock as a sensible heat. As Fig 1 shows there are totally 36 trenches distributed in 9 different depth of rockpit (trench#1, #2, …, #8 and #9) in 4 sides (A,B,C and D). The distance between trenches is 50 (m) and 9 trenches in each side. These trenches are turning on and off to find out the effect of number and location of them on STES performance. It should be noted that during all scenario, the fresh air mass flow rate is kept constant 540±10 ( by adjusting the fan suction pressure of active trenches.
Figure 1. View of the rockpit and trenches
The model considers transient fluid flow and heat transfer between ambient air and the broken rock at local thermal non-equilibrium (LTNE) by accounting for the interphase energy transfer between two phases (namely, fluid air and solid rock). The air temperature at the top of the rock is varied according to weather conditions following sinusoidal function with the lowest temperature in the winter -20 ˚C and the highest temperature in the summer 20 ˚C. The outlet air pressure at the active trenches is prescribed according to the suction pressure from the ventilation fans. Thermo-physical properties of the model are given in Table 1. Air is assumed to be ideal gas and its properties are calculated based on standard thermodynamic charts. The boundary conditions is as follows
at top of the rockpit: we prescribe constant ambient pressure, while the temperature is dynamics depending on the weather condition
ptop pamb , Ttop = Tamb = 25 sin 2 t 7884000 / 31536000
(1)
at side and bottom pit walls: no slip condition and zero heat flux is applied. u 0, n T 0 at the trenches outlet: we set the exhaust ventilation fan pressure. pout pfan , n T 0
(2) (3)
Table 1. Thermo-physical properties of the model Property Thermal conductivity of rock (W/m.K) Specific heat capacity of rock (J/kg.K) Density of rock (kg/m3) Permeability of porous rockpit (m2) Average rock diameter (m) Porosity
Value 2 103 3×103 9.26×10-8 1.2 0.4
The governing equations were solved using a finite volume based user-defined numerical simulation code developed in ANSYS Fluent 15 and user-defined functions (UDF) written in C language to take into account the interphase energy transfer and transient boundary condition. The numerical model was solved with Semi-Implicit Pressure-Linked Equation (SIMPLE) algorithm, second-order upwind discretization and Algebraic Multi-grid (AMG) method.
4
Leyla Amiri et al. / Energy Procedia 142 (2017) 576–581 Author name / Energy Procedia 00 (2017) 000–000
579
3. Results and discussions 3.1. Effect of number and location of ventilation trenches on performance of large scale STES unit As can be seen in the Fig. 2, by activating the trenches which are located in somewhat deeper locations, outlet air temperature from ventilation trenches will differ from the ambient air temperature, consequently one can expect the better performance of the thermal energy storage unit. Also, by having more active trenches in the thermal storage unit, the electricity cost for running fans can be decreased. Nevertheless, it should be noted that to facilitate the same mass flow rate for all cases (9, 18, 27 & 36 active trenches), it is required to adjust fan power. Accordingly, quantity of the ventilation trenches should inevitably defined by performing the cost and benefit analysis which evaluates the economics of saving heating/cooling energy in addition to the all expenses related to the setting up ventilation fans. Fig.3 emphasizes the importance of the location of ventilation trenches when all 36 trenches with the same pressure 432 (Pascal) are blowing air to supply 540 ( fresh air for the underground mine. As it can be seen in Fig. 3, the outlet air temperature of the bottom trenches, middle trenches and top trenches are different and the top ones are very close to the ambient air temperature. This little temperature change between ambient and effluent air of the top trenches is expected, since ambient air travels its shortest path through the natural heat exchanger unit (rock mass) to reach this trench. Ambient air travelling towards middle and bottom trenches will have longer time to exchange heat energy with the rock mass and consequently, its temperature will move toward the rock mass. Fig. 3 shows the results of 4-sided, 3-sided, 2-sided and single side trenches in the rockpit, respectively. Similarly, Fig. 3 (d) shows the average outlet air temperature in the model for different cases with different number of active trenches.
Figure 2 : Ambient air temperature at mine site and trenches outlet air temperature, model with 9 active trenches on side A, mass flow rate= 540 (kg/s)
3.2. Energy savings A further point to note is about the energy savings belonging to heating during the winter time and cooling in the summer days. Fig. 4 shows the resulting energy savings and consumptions for four cases; (i) with 9 active trenches on side A, (ii) with 18 active trenches on side A&C, (iii) with 27 active trenches on side A,B&C, and (iv) with 36 active trenches on side A,B,C&D. On average, respectively for 9, 18, 27 and 36 trench cases, about 6.9, 7.9, 7.7 and 7.8 (GWh) per year of gross energy can be saved for heating and cooling. While the energy required for running the fans for 9, 18, 27 and 36 trench cases is about 5.7, 2.8, 1.9 and 1.5, respectively. Accordingly, total amount of 1.2, 5.1, 5.8 and 6.3 (GWh) energy savings per year for constant mass flow rate 540 ( ) with 9, 18, 27 & 36 trenches, respectively. Note that all trenches have the same diameter of 5 (m).
Leyla Amiri et al. / Energy Procedia 142 (2017) 576–581 Author name / Energy Procedia 00 (2017) 000–000
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(a)
(b)
(c)
(d)
5
Figure 3 : Ambient air temperature at mine site and (a) top trenches outlet air temperature, (b) middle trenches outlet air temperature, (c) bottom trenches outlet air temperature (d) average outlet air temperature for model with 9, 18, 27 and 36 active trenches, mass flow rate= 540 (kg/s)
Figure 4 : Gross energy saving and energy consumption of fans with mass flow rate 540 (kg/s) and different number and location of trenches
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Leyla Amiri et al. / Energy Procedia 142 (2017) 576–581 Author name / Energy Procedia 00 (2017) 000–000
581
Assuming the cost of heating (Natural gas) and cooling (electricity) air to be 0.02 and 0.075 USD per kWh, respectively. Results demonstrate that the highest total energy savings is associated with the model with 36 trenches in all side of rockpit and by decreasing the quantity of the active ventilation trenches and increasing the fan suction power the total saving is reduced. According to result, the total estimated savings will be about 925,000, 3,770,000, 4,300,000 and 4,730,000 USD per year, for the cases with 9, 18, 27 and 36 active trenches, respectively. Note that if the potential of carbon savings also is taken into account, then the total savings can be even larger. 4. Conclusion A 3D conjugate fluid flow and heat transfer model was developed for evaluation of the large-scale STES units efficiency in mine ventilation. Effect of quantity of the active ventilation trenches as well as the location of them on the heat exchange rate between the sucked air and the broken rock as a porous media was examined by comparing four cases with different number and location of ventilation trenches in company with the fix mass flowrate and trench diameter as well. It was shown that total energy savings can be getting better by growing the number of ventilation trenches and decreasing fan power for each single trench. The outcomes of this investigation illustrated that increasing the quantity of the active ventilation trenches, from 9 to 36, and keeping constant the air mass flow rate 540 (kg/s), will end up with the higher energy savings. It should be mentioned that optimization on the trench shape and size with regard to the resistance to air flow through them is essential to arrive at optimum rockpit STES design, which is the focus of our future study. References [1] Abdelaziz E, Saidur R, Mekhilef S. A review on energy saving strategies in industrial sector. Renewable and sustainable energy reviews. 2011;15(1):150-68. [2] Ghoreishi-Madiseh SA, Sasmito AP, Hassani FP, Amiri L. Performance evaluation of large scale rock-pit seasonal thermal energy storage for application in underground mine ventilation. Applied Energy. 2017;185, Part 2:1940-7. [3] Jiang P, Zhang L, Xu R. Experimental study of convective heat transfer of carbon dioxide at supercritical pressures in a horizontal rock fracture and its application to enhanced geothermal systems. Applied Thermal Engineering. 2017;117:39-49. [4] Ghoreishi-Madiseh SA, Sasmito AP, Hassani FP, Amiri L. Heat transfer analysis of large scale seasonal thermal energy storage for underground mine ventilation. Energy Procedia. 2015;75:2093-8. [5] Schafrik S. The use of packed sphere modelling for airflow and heat exchange analysis in broken or fragmented rock: Citeseer, 2014. [6] Envers P. Controlled Air Through Efficient System at Inco. Canadian Mining Journal. 1986;107(7):12-4. [7] Sylvestre MJ-G. Heating and ventilation study of Inco's Creighton Mine. 1999. [8] Dunmore R. Towards a method of prediction of firedamp emission for British coal mines. Conference Towards a method of prediction of firedamp emission for British coal mines. p. 351-64. [9] Sylvestre MJ-G. Heating and Ventilation Study of Inco's Creighton Mine1999. [10] Pruess K. A practical method for modeling fluid and heat flow in fractured porous media. Society of Petroleum Engineers Journal. 1985;25(01):14-26. [11] Whitaker S. Simultaneous heat, mass, and momentum transfer in porous media: a theory of drying. Advances in heat transfer. 1977;13:119-203. [12] Vafai K, Tien C. Boundary and inertia effects on flow and heat transfer in porous media. International Journal of Heat and Mass Transfer. 1981;24(2):195-203. [13] Ghoreishi-Madiseh SA, Hassani FP, Mohammadian A, Radziszewski PH. A transient natural convection heat transfer model for geothermal borehole heat exchangers. Journal of Renewable and Sustainable Energy. 2013;5(4):-. [14] Schafrik S. The use of packed sphere modelling for airflow and heat exchange analysis in broken or fragmented rock: Laurentian University of Sudbury, 2015.
ScienceDirect
Availableonline onlineatatwww.sciencedirect.com www.sciencedirect.com Available Energy Procedia 00 (2017) 000–000
ScienceDirect ScienceDirect
www.elsevier.com/locate/procedia
Energy (2017) 000–000 576–581 EnergyProcedia Procedia142 00 (2017) www.elsevier.com/locate/procedia
9th International Conference on Applied Energy, ICAE2017, 21-24 August 2017, Cardiff, UK
Evaluation of Heat Transfer Performance between Rock and Air in The 15th International Symposium on District Heating and Cooling Seasonal Thermal Energy Storage Unit b, a a Assessing theAlifeasibility of using the heat demand-outdoor Leyla Amiria, Seyed Ghoreishi-Madiseh *, Agus P. Sasmito , Ferri P. Hassani temperature function for a long-term district heat demand forecast Department of Mining and Material Engineering, McGill University, Montreal, Quebec, Canada aa
bb NBK
NBK Institute of Mining, University of British Columbia, Vancouver, British Columbia, Canada
I. Andrića,b,c*, A. Pinaa, P. Ferrãoa, J. Fournierb., B. Lacarrièrec, O. Le Correc a Abstract IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal b
Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France
Systèmes Énergétiques et Environnement IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, Franceamounts of Ventilation iscDépartement one of the most energy demanding parts of deep -underground mining operations due to the extensive energy needed to cool (or heat) the substantial amount of air flow in underground workings. This considerable amount of energy demand can be partially satisfied by extracting renewable energies or using alternative energy solutions available at mine sites. In Canada, some mining operations have the opportunity to create a Seasonal Thermal Energy Storage (STES) unit by blowing mine Abstract intake air through large volumes of rock mass dumped in a decommissioned pit. This technique allows for creation of the socalled “Natural Heat Exchanger (NHE)” which moderates seasonal air temperature oscillations. This paper signifies a novel heat Districtmodel heating are commonly the literature as onethe of potential the mostfor effective decreasing the transfer fornetworks performance assessment addressed of a NHE in system. It investigates energy solutions savings byfor implementation gas emissions from the sector. Theseinsystems requiremines. high investments which through the heat ofgreenhouse variable number and position of building ventilation trenches underground In this regard, a are 3-Dreturned heat transfer model is sales. Due to the changed climate conditions and building renovation policies, heat demand in the future could decrease, developed to evaluate the thermal storage and the heat transfer between broken rock mass and ventilated air flow and also to prolonging investment return period. specify how the design parameters such as position and number of ventilation trenches will affect the heat transfer performance as The main scope of this paper is to theoffeasibility usingthat theNHE heat demand – outdoor temperature functionof forunderground heat demand well as the total energy savings. Theassess results this studyofshow can be applied in thermal management forecast. district of Alvalade, located in Lisbon was used as a case purposes. study. The district is consisted of 665 mines withThe the aim of decreasing energy consumption for(Portugal), heating, cooling and ventilation vary in both construction ©buildings 2017 Thethat Authors. Published by Elsevierperiod Ltd. and typology. Three weather scenarios (low, medium, high) and three district ©renovation 2017 The Authors. by Elsevier Ltd. intermediate, deep). To estimate the error, obtained heat demand values were scenariosPublished were developed (shallow, Peer-review under responsibility of committee of the 9th International International Conference Conference on Applied Applied Energy. Energy. Peer-review under responsibility of the the scientific scientific committee the 9th compared with results from a dynamic heat demand model,of previously developed and validatedon by the authors. The results showed that when weather considered, margin ofventilation; error could be acceptable for some applications Keywords: Seasonal thermal energy only storage (STES);change NaturalisHeat Exchangerthe (NHE); Mine Porous medium; Energy efficiency (the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation scenarios, the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). The value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the decrease in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and improve the accuracy of heat demand estimations.
© 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and * Corresponding author. Tel.: +1-604-827-2028 Cooling. E-mail address: [email protected]
Keywords: Heat demand; Forecast; Climate change 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 9th International Conference on Applied Energy.
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling.
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 9th International Conference on Applied Energy. 10.1016/j.egypro.2017.12.096
2
Leyla Amiri et al. / Energy Procedia 142 (2017) 576–581 Author name / Energy Procedia 00 (2017) 000–000
Nomenclature Abbreviations: TES Thermal Energy Storage STES Seasonal Thermal Energy Storage LTNE Local Thermal Non-Equilibrium UDF user-defined functions NHE Natural Heat Exchanger
577
Symbols: f fluid phase s solid phase ρ density u fluid velocity p pressure ε porosity μ dynamic viscosity of the fluid cp specific heat k thermal conductivity T temperature
1. Introduction The industrial sector consumes about 37% of the world’s total delivered energy and among them, mining industry contributes about 9% [1]. Within mining activities, ventilation is one of the most energy demanding parts of deep underground mining operations. This energy demand can be partially fulfilled by deploying the renewable energies or using alternative energy solutions available at mine sites. Thermal energy storage (TES) systems are one these solutions which fall will within three main categories including sensible heat storage systems, latent heat storage systems and thermochemical storage systems. The selection of the TES systems mostly depends on the storage period required (i.e. short-term vs. long-term), availability of the medium, operating conditions, purpose of projects, etc. In recent years, TES systems come out to be one of the most attractive thermal applications [2]. Sensible heat storage generally utilizes rock beds or water tanks or aquifers as a storage medium and is available for short-term and long-term storage. In these systems heat/cold energy mostly is stored by increasing/decreasing the storage medium temperature [3, 4]. Some mining operations (e.g. Creighton and Kidd Creek mines in Canada) have the great opportunity to make use of the enormous mass of their fragmented waste rock as an immense seasonal thermal energy storage (STES) unit with the purpose of creating a unique type of heat exchanger; namely “Natural Heat Exchanger”. In this type of heat exchanger, fresh air is passed through the huge body of broken rocks dumped in to an open pit. The heat exchange between the waste rocks and the fresh air will lead to considerable decrease in ventilation costs in these mines [4-6]. Very limited studies [2, 4] have focused on the 3D analysis of heat transfer and fluid flow in the rockpit STES. Numerous computer programs have been developed to investigate the flow of compressible and incompressible fluids through strata [7, 8]. Most of these programs are based upon the porous media concept in which it is assumed that any one stratum contains evenly distributed interconnected pores [8-10]. An extensive study of fluid flow and heat transfer in porous media was undertaken by Whitaker [11], Vafai and Tien [12] and Ghoreishi-Madiseh et al. [2, 4, 13]. Limited number of research work, most important of them [5, 9], have been dedicated to the study of heat transfer in large scale thermal energy storage systems for mine ventilation purposes. The empirical approach suggested by Sylvester [9] can be used to estimate the heat storage capacity of Creighton mine rock-pit. Afterward, Schafrik [14] developed a numerical simulation model for Creighton mine and aimed to validate its results in laboratory scale. To overcome such limitations, researchers have proposed using 3D models capable of simulating heat transfer and fluid flow in the large scale Se-TES units [2, 9, 14]. To conclude, there is a fundamental need for a generally applicable engineering design method based on which the thermal energy storage capacity of rock mass of any mine can be assessed. The main aims of this paper are (i) developed a 3D transient mathematical model of fluid flow and heat transfer of STES; (ii) quantitatively compared the effect of position and number of ventilation trenches on outlet air temperature as well as total energy savings. 2. Model description Here, a 3D model of the rockpit - a large mass of fragmented rock - for STES system of Creighton mine in Canada is proposed. This rockpit model is assumed to be placed over a cone-shaped mine pit which is 697 (m) in diameter and 330 (m) deep. Filling the pit with waste rocks has created a porous rockpit mass. The thickness of the rockpit mass at its very bottom is 133 (m). This porous medium is accessed from the underground through trenches. These trenches are connected to mine ventilation shaft(s) where ventilation fans will continuously move fresh air
Leyla Amiri et al. / Energy Procedia 142 (2017) 576–581 Author name / Energy Procedia 00 (2017) 000–000
578
3
through rockpit, trenches, mine shaft(s) and other underground openings. It is expected that due to seasonal temperature difference, thermal energy (hot and cold) is stored in the broken rock as a sensible heat. As Fig 1 shows there are totally 36 trenches distributed in 9 different depth of rockpit (trench#1, #2, …, #8 and #9) in 4 sides (A,B,C and D). The distance between trenches is 50 (m) and 9 trenches in each side. These trenches are turning on and off to find out the effect of number and location of them on STES performance. It should be noted that during all scenario, the fresh air mass flow rate is kept constant 540±10 ( by adjusting the fan suction pressure of active trenches.
Figure 1. View of the rockpit and trenches
The model considers transient fluid flow and heat transfer between ambient air and the broken rock at local thermal non-equilibrium (LTNE) by accounting for the interphase energy transfer between two phases (namely, fluid air and solid rock). The air temperature at the top of the rock is varied according to weather conditions following sinusoidal function with the lowest temperature in the winter -20 ˚C and the highest temperature in the summer 20 ˚C. The outlet air pressure at the active trenches is prescribed according to the suction pressure from the ventilation fans. Thermo-physical properties of the model are given in Table 1. Air is assumed to be ideal gas and its properties are calculated based on standard thermodynamic charts. The boundary conditions is as follows
at top of the rockpit: we prescribe constant ambient pressure, while the temperature is dynamics depending on the weather condition
ptop pamb , Ttop = Tamb = 25 sin 2 t 7884000 / 31536000
(1)
at side and bottom pit walls: no slip condition and zero heat flux is applied. u 0, n T 0 at the trenches outlet: we set the exhaust ventilation fan pressure. pout pfan , n T 0
(2) (3)
Table 1. Thermo-physical properties of the model Property Thermal conductivity of rock (W/m.K) Specific heat capacity of rock (J/kg.K) Density of rock (kg/m3) Permeability of porous rockpit (m2) Average rock diameter (m) Porosity
Value 2 103 3×103 9.26×10-8 1.2 0.4
The governing equations were solved using a finite volume based user-defined numerical simulation code developed in ANSYS Fluent 15 and user-defined functions (UDF) written in C language to take into account the interphase energy transfer and transient boundary condition. The numerical model was solved with Semi-Implicit Pressure-Linked Equation (SIMPLE) algorithm, second-order upwind discretization and Algebraic Multi-grid (AMG) method.
4
Leyla Amiri et al. / Energy Procedia 142 (2017) 576–581 Author name / Energy Procedia 00 (2017) 000–000
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3. Results and discussions 3.1. Effect of number and location of ventilation trenches on performance of large scale STES unit As can be seen in the Fig. 2, by activating the trenches which are located in somewhat deeper locations, outlet air temperature from ventilation trenches will differ from the ambient air temperature, consequently one can expect the better performance of the thermal energy storage unit. Also, by having more active trenches in the thermal storage unit, the electricity cost for running fans can be decreased. Nevertheless, it should be noted that to facilitate the same mass flow rate for all cases (9, 18, 27 & 36 active trenches), it is required to adjust fan power. Accordingly, quantity of the ventilation trenches should inevitably defined by performing the cost and benefit analysis which evaluates the economics of saving heating/cooling energy in addition to the all expenses related to the setting up ventilation fans. Fig.3 emphasizes the importance of the location of ventilation trenches when all 36 trenches with the same pressure 432 (Pascal) are blowing air to supply 540 ( fresh air for the underground mine. As it can be seen in Fig. 3, the outlet air temperature of the bottom trenches, middle trenches and top trenches are different and the top ones are very close to the ambient air temperature. This little temperature change between ambient and effluent air of the top trenches is expected, since ambient air travels its shortest path through the natural heat exchanger unit (rock mass) to reach this trench. Ambient air travelling towards middle and bottom trenches will have longer time to exchange heat energy with the rock mass and consequently, its temperature will move toward the rock mass. Fig. 3 shows the results of 4-sided, 3-sided, 2-sided and single side trenches in the rockpit, respectively. Similarly, Fig. 3 (d) shows the average outlet air temperature in the model for different cases with different number of active trenches.
Figure 2 : Ambient air temperature at mine site and trenches outlet air temperature, model with 9 active trenches on side A, mass flow rate= 540 (kg/s)
3.2. Energy savings A further point to note is about the energy savings belonging to heating during the winter time and cooling in the summer days. Fig. 4 shows the resulting energy savings and consumptions for four cases; (i) with 9 active trenches on side A, (ii) with 18 active trenches on side A&C, (iii) with 27 active trenches on side A,B&C, and (iv) with 36 active trenches on side A,B,C&D. On average, respectively for 9, 18, 27 and 36 trench cases, about 6.9, 7.9, 7.7 and 7.8 (GWh) per year of gross energy can be saved for heating and cooling. While the energy required for running the fans for 9, 18, 27 and 36 trench cases is about 5.7, 2.8, 1.9 and 1.5, respectively. Accordingly, total amount of 1.2, 5.1, 5.8 and 6.3 (GWh) energy savings per year for constant mass flow rate 540 ( ) with 9, 18, 27 & 36 trenches, respectively. Note that all trenches have the same diameter of 5 (m).
Leyla Amiri et al. / Energy Procedia 142 (2017) 576–581 Author name / Energy Procedia 00 (2017) 000–000
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Figure 3 : Ambient air temperature at mine site and (a) top trenches outlet air temperature, (b) middle trenches outlet air temperature, (c) bottom trenches outlet air temperature (d) average outlet air temperature for model with 9, 18, 27 and 36 active trenches, mass flow rate= 540 (kg/s)
Figure 4 : Gross energy saving and energy consumption of fans with mass flow rate 540 (kg/s) and different number and location of trenches
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Assuming the cost of heating (Natural gas) and cooling (electricity) air to be 0.02 and 0.075 USD per kWh, respectively. Results demonstrate that the highest total energy savings is associated with the model with 36 trenches in all side of rockpit and by decreasing the quantity of the active ventilation trenches and increasing the fan suction power the total saving is reduced. According to result, the total estimated savings will be about 925,000, 3,770,000, 4,300,000 and 4,730,000 USD per year, for the cases with 9, 18, 27 and 36 active trenches, respectively. Note that if the potential of carbon savings also is taken into account, then the total savings can be even larger. 4. Conclusion A 3D conjugate fluid flow and heat transfer model was developed for evaluation of the large-scale STES units efficiency in mine ventilation. Effect of quantity of the active ventilation trenches as well as the location of them on the heat exchange rate between the sucked air and the broken rock as a porous media was examined by comparing four cases with different number and location of ventilation trenches in company with the fix mass flowrate and trench diameter as well. It was shown that total energy savings can be getting better by growing the number of ventilation trenches and decreasing fan power for each single trench. The outcomes of this investigation illustrated that increasing the quantity of the active ventilation trenches, from 9 to 36, and keeping constant the air mass flow rate 540 (kg/s), will end up with the higher energy savings. It should be mentioned that optimization on the trench shape and size with regard to the resistance to air flow through them is essential to arrive at optimum rockpit STES design, which is the focus of our future study. References [1] Abdelaziz E, Saidur R, Mekhilef S. A review on energy saving strategies in industrial sector. Renewable and sustainable energy reviews. 2011;15(1):150-68. [2] Ghoreishi-Madiseh SA, Sasmito AP, Hassani FP, Amiri L. Performance evaluation of large scale rock-pit seasonal thermal energy storage for application in underground mine ventilation. Applied Energy. 2017;185, Part 2:1940-7. [3] Jiang P, Zhang L, Xu R. Experimental study of convective heat transfer of carbon dioxide at supercritical pressures in a horizontal rock fracture and its application to enhanced geothermal systems. Applied Thermal Engineering. 2017;117:39-49. [4] Ghoreishi-Madiseh SA, Sasmito AP, Hassani FP, Amiri L. Heat transfer analysis of large scale seasonal thermal energy storage for underground mine ventilation. Energy Procedia. 2015;75:2093-8. [5] Schafrik S. The use of packed sphere modelling for airflow and heat exchange analysis in broken or fragmented rock: Citeseer, 2014. [6] Envers P. Controlled Air Through Efficient System at Inco. Canadian Mining Journal. 1986;107(7):12-4. [7] Sylvestre MJ-G. Heating and ventilation study of Inco's Creighton Mine. 1999. [8] Dunmore R. Towards a method of prediction of firedamp emission for British coal mines. Conference Towards a method of prediction of firedamp emission for British coal mines. p. 351-64. [9] Sylvestre MJ-G. Heating and Ventilation Study of Inco's Creighton Mine1999. [10] Pruess K. A practical method for modeling fluid and heat flow in fractured porous media. Society of Petroleum Engineers Journal. 1985;25(01):14-26. [11] Whitaker S. Simultaneous heat, mass, and momentum transfer in porous media: a theory of drying. Advances in heat transfer. 1977;13:119-203. [12] Vafai K, Tien C. Boundary and inertia effects on flow and heat transfer in porous media. International Journal of Heat and Mass Transfer. 1981;24(2):195-203. [13] Ghoreishi-Madiseh SA, Hassani FP, Mohammadian A, Radziszewski PH. A transient natural convection heat transfer model for geothermal borehole heat exchangers. Journal of Renewable and Sustainable Energy. 2013;5(4):-. [14] Schafrik S. The use of packed sphere modelling for airflow and heat exchange analysis in broken or fragmented rock: Laurentian University of Sudbury, 2015.