Applied Science, Faculty of
Mining Engineering, Keevil Institute of
DSpace
Applied Sciences 10 (17): 6022 (2020)
Pokhrel, Sajjan
Kuyuk, Ali Fahrettin
Kalantari, Hosein
Ghoreishi-Madiseh, Seyed Ali
This paper performs a techno-economic assessment in deploying solar photovoltaics to provide energy to a refrigeration machine for a remote underground mine. As shallow deposits are rapidly depleting, underground mines are growing deeper to reach resources situated at greater depths. This creates an immense challenge in air-conditioning as the heat emissions to mine ambient increases substantially as mines reach to deeper levels. A system-level design analysis is performed to couple PV with a refrigeration plant capable of generating 200 tonne of ice per day to help to mitigate this issue. Generated ice can directly be used in cooling deep underground mines via different types of direct heat exchangers. State-of-the-art technology is used in developing the model which aims to decrease the size and cost of a conventional refrigeration system run on a diesel generator. Costs associated with deploying a solar system are computed as per the recent market value. Energy savings, carbon emissions reduction, and net annual savings in employing the system are quantified and compared to a diesel-only scenario. In addition, two different energy storage strategies: an ice storage system and a battery storage system, are compared. A detailed economic analysis is performed over the life of the project to obtain the net cash flow diagram, payback period, and cumulative savings for both systems. Moreover, a sensitivity analysis is proposed to highlight the effect of solar intensity on solar system size and the area required for installment. The study suggests that the use of solar PV in mine refrigeration applications is technically feasible and economically viable depending on the sun-peak hours of the mine location. Additionally, the economics of deploying an ice storage system compared to the battery storage system has a better payback period and more cumulative savings.
applied sciencesArticleTechno-Economic Trade-Off between Battery Storageand Ice Thermal Energy Storage for Application inRenewable Mine Cooling SystemSajjan Pokhrel , Ali Fahrettin Kuyuk, Hosein Kalantari and Seyed Ali Ghoreishi-Madiseh *NBK Institute of Mining Engineering, The University of British Columbia, Vancouver, BC V6T 1Z4, Canada;sajjan12@mail.ubc.ca (S.P.); alikuyuk@mail.ubc.ca (A.F.K.); hoseink@mail.ubc.ca (H.K.)* Correspondence: ali.madiseh@ubc.caReceived: 18 July 2020; Accepted: 24 August 2020; Published: 31 August 2020Abstract: This paper performs a techno-economic assessment in deploying solar photovoltaics toprovide energy to a refrigeration machine for a remote underground mine. As shallow deposits arerapidly depleting, underground mines are growing deeper to reach resources situated at greaterdepths. This creates an immense challenge in air-conditioning as the heat emissions to mine ambientincreases substantially as mines reach to deeper levels. A system-level design analysis is performed tocouple PV with a refrigeration plant capable of generating 200 tonne of ice per day to help to mitigatethis issue. Generated ice can directly be used in cooling deep underground mines via different typesof direct heat exchangers. State-of-the-art technology is used in developing the model which aims todecrease the size and cost of a conventional refrigeration system run on a diesel generator. Costsassociated with deploying a solar system are computed as per the recent market value. Energy savings,carbon emissions reduction, and net annual savings in employing the system are quantified andcompared to a diesel-only scenario. In addition, two different energy storage strategies: an ice storagesystem and a battery storage system, are compared. A detailed economic analysis is performed overthe life of the project to obtain the net cash flow diagram, payback period, and cumulative savings forboth systems. Moreover, a sensitivity analysis is proposed to highlight the effect of solar intensity onsolar system size and the area required for installment. The study suggests that the use of solar PVin mine refrigeration applications is technically feasible and economically viable depending on thesun-peak hours of the mine location. Additionally, the economics of deploying an ice storage systemcompared to the battery storage system has a better payback period and more cumulative savings.Keywords: deep mine cooling; solar electric refrigeration; ice thermal energy storage; techno-economicassessment; solar PV design; renewable energy in mining; solar-powered ice maker1. IntroductionIndustrial energy usage accounts for 40% of the overall global energy demand in 2018, greater thanany other end-use sectors [1]. Mining is the second most energy-intensive industry after construction,draining 6.2% of the global energy share [2]. Mine ventilation and refrigeration could account for about30–40% of a deep underground mine’s total energy expense [3]. Mines are going deeper and deeper tomeet global consumer needs. In ultra-deep mines (2500 m or deeper), gravitational compression of theintake air sinking in the downcast shaft and geothermal heat acting on the mine ambient can cause asubstantial increase in the temperature in association with the depth (auto-compression). Additionally,virgin rock temperature in such mines could be as high as 65 ◦C [4] due to the geothermal gradient.However, regardless of auto-compression impact and virgin rock temperature, mine ambient airwet-bulb temperature should not exceed 28 ◦C [5]. Consequently, more cooling would be required forAppl. Sci. 2020, 10, 6022; doi:10.3390/app10176022 www.mdpi.com/journal/applsciAppl. Sci. 2020, 10, 6022 2 of 16such mines to maintain safe climatic conditions. The design of a mine cooling system is usually basedon the peak summer condition to ensure year-round, stable coolth generation. However, a refrigerationscheme designed over peak surface load requires a relatively large size of chillers, cooling towers,and condensers due to overestimation of annual demand. This overestimation is inevitable as thepeak load acting on these systems are usually seasonal or happen for a very limited period of the year.This results in redundant project cost and could affect the project feasibility negatively. Fortunately,this problem could be solved by employing a ‘by-pass’ system that could trim the peak cooling demandand help downsize the capital cost.The literature suggests that energy savings in a mine ventilation and refrigeration system could beexpanded to 22% of the current trend, reducing it from 17.5 MJ/tonne of ore to 13.6 MJ/tonne of ore [6].Zietsman et al. [7] reports annual savings of ZAR 1.65 million (South African Rand) by implementingpeak clipping via thermal storage arrangement in deep platinum mine. Plessis et al. [8] simulated anenergy efficiency strategy to forecast the potential savings of implementing a peak shaving strategy ina real-life mine cooling system and concluded that a decrease of 33% is possible in overall electricalenergy consumption. A cooling load management initiative taken in two different South Africanmines has shown that about 25% of electricity savings is an attainable figure in mine refrigeration [9].Another energy-saving strategy for a mine cooling system with a power usage of 22 MW resulted in132 GWh savings, equivalent to 30 million ZAR or 38% of annual refrigeration cost [10]. Undoubtedly,gradual deepening of mines calls for more research in energy generation, storage, and management toovercome future mine refrigeration challenges. This paper proposes a strategy to manage the peakcooling demand of a remote underground mine using ice cubes generated from a solar photovoltaics(PV)-driven refrigeration system.The concept of cooling underground mines via ice goes back to the 1980s [11]. However, back then,the complex and delicate architecture of ice plants made it viable only for mines greater than 3000 min depth. With improved technology, the reduced cost of mine cooling creates more opportunity todeploy such systems at shallower depths more feasibly. Studies show that the feasible depth hasmoved from 3000 m in 1980 to 2500 m in 2010 to 1760 m in 2014 [12,13]. It is important to note that suchmines otherwise are cooled with chilled air or chilled water from the surface. It is also worthwhile tomention that the specific heat capacity of water and the latent heat of fusion of ice is about 4.2 and334 times the specific heat capacity of air, respectively. Chilled air is the most cost-effective coolingsolution, but lower specific heat capacity makes it unsuitable to remove extra heat from greater depths.Hence, chilled water is used to remove heat at higher depths. However, as the depth further increases,the pumping requirements for returning water from underground leads to considerable operation cost.Due to the higher specific heat capacity of ice over water, the volume of ice required to dissipate anequal amount of thermal energy is between 20% to 30% of the chilled water [14,15]. In such ultra-deepmines, ice is used as a coolant to enhance the system performance. Belle et al. [14] mentions thatwith the development in technology, the coal mines in the Bowen basin in Australia might have tomove towards ice-cooling in the near future. Although notable mines including Phakisa gold mineand Impala gold mine in South Africa are currently using this technology, the potential use of solarelectricity for ice generation is a novel concept in mine sites.1.1. Motivation of the ResearchToday, amidst the gradual depletion of mining resources, more and more mine enterprises aremoving to remote locations with no electric grid access. In remote sites, fossil fuels including dieseland natural gas are the main sources of energy. In 2017, 18,364 k tonne of diesel is consumed in mining:about 18% of the total industrial share [2]. Diesel emissions are the major contributors to the CO2accumulation in the earth’s atmosphere by adding 123 ppm between the pre-industrial era and 2016 [16].In mining, 71% of the emission is from fossil fuels while 29% is due to electricity usage [17]. In additionto its environmental impacts, diesel dependence also created an escalation in the cost of electricitygeneration from diesel generators. Utility cost of electricity generation from a diesel generator canAppl. Sci. 2020, 10, 6022 3 of 16be as high as 0.76 USD/kWh for some remote locations excluding carbon tax [18]. Thermal duties ofrefrigeration units in deep mines are typically in the range of 0.5 and 20 MW [14,19]. The use of dieselgenerators to generate this huge energy is a financial and an environmental concern. The motivationof the research is the need to lower the high cost of energy that remote mines are currently usingin addition to their higher environmental footprints. The penetration of renewable energy in minechilling application not only assists to break the complicated relationship of mining and renewableenergy, but also improves the public perception of environmental consciousness towards mining.1.2. Relevant WorksRelevant works on this domain are the works that discuss about employing solar technologies inmine sites and the energy storage technology that they opt for. Deployment of solar energy in miningis relatively a new topic, contrary to residential and commercial sectors. The first MW-scale solarPV system in a mine site was installed in 2012 at Cronimet’s South African chromium ore operationgenerating up to 1.8 GWh of energy per year [20]. A historical breakthrough of solar penetration inmining took place in 2013 after the installation of a 10-MW solar thermal plant in a Copper mine inthe Atacama Desert. Owned by Minera El Tesoro, the plant replaced more than 55% of the diesel fuelconsumed in the heating process for electro-extraction of copper [21]. Musselwhite mine in Canada,Chevron molybdenum mine in the USA, and few other mines in Chile are utilizing solar energy in minesites. As a matter of fact, many of the world’s remote mine activities are located between 35◦ latitudenorth and south where the sun is intense and reliable [22]. An ice storage technology run on solarPV can aid in reducing the cooling plant maximum capacity in a Canadian mine [23]. Baig et al. [24]recommends four mine sites in the Northern Territory of Australia for Concentrating Solar Panelinstallation at the regional cluster level. Kim Trapani demonstrated the potential of floating solarpanels in a remote McFaulds lake in Ontario to generate electricity of 400 MW that could provideelectricity to 16 mines in this area [25]. A comprehensive literature review suggests that the solartechnology is penetrating slowly but surely in the mining industry. However, most of the currentand proposed applications include heating and electricity generation while only two publications arediscovered for cooling [23,26].Solar cooling has various technological options. Most widely investigated are solar electric andsolar thermal. Before 2000, only solar thermal option was considered for chilling applications due tothe high price of PV as compared to thermal collectors. However, the price in solar PV dropped byhalf from 2000 to 2010 and another halving took place from 2010 to 2015 [27]. The literature suggeststhat the Vapor Compression Refrigeration System (VCRS) based on solar electric has surpassed solarthermal schemes in 2013 [28–30]. Today, PV-driven solar cooling costs about half of the best thermallydriven systems [27]. Different works of the literature are reported in solar electric cooling for diversedomestic applications [31–33]. However, applications in mining remain largely undiscovered.Solar electric cooling couples VCRS with solar PV. Solar PV installed for industrial applicationslike mining, requires large scale energy storage systems to resolve the intermittency problem. A batterystorage system, which stores energy in a chemical medium, is the most widely used solar energystorage technology [34]. However, in recent years, a new realm in solar refrigeration is added after thebattery bank is completely replaced or partially substituted with Ice Thermal Energy Storage (ITES)system. In this scheme, energy is stored in ice which later is adapted for chilling purposes. The ITESfrom solar PV has a great advantage as it can be used in reducing peak refrigeration demand resultingin peak shaving and capital cost savings. Xu et al. [35] performed an experimental research on solarPVs operated as energy generators for ice thermal storage chillers for air-conditioning applicationsand concluded that a battery bank could completely be replaced by the ITES system. Axaopoulosand Theodoridis [33] successfully presented an autonomous battery-less solar-powered ice maker.Salilih et al. [36] performed an analysis and simulation of solar PV-driven VCRS in a real weathercondition without battery storage. National Renewable Energy Laboratory (NREL) projects the energystorage cost in using Li-ion battery at a utility-scale solar PV is about 80% of the total cost of a PVAppl. Sci. 2020, 10, 6022 4 of 16installation [37]. The average total annualized cost of a thermal storage system is about 12% of a Li-ionbattery, while the efficiency is about half [38]. It would be interesting to compare these two energystorage technologies in an industry-scale refrigeration application.This paper examines the techno-economic aspects of installing solar PV for operating anice-generating machine to cool an underground mine. A thorough literature review suggests onlytwo publications to study the use of solar energy in mine cooling applications [23,26]. Both of theanalyses are performed for a specific mine site and are unsuitable to apply in decision making forthe other mine locations. They also compare thermal storage strategies with a battery system but donot consider the battery replacement cost after standard charging and discharging cycles. This paperpresents a detailed numerical code developed in Excel to design a solar PV system and to comparetwo different energy storage technologies for the proposed application: battery storage and ITES.A system-level analysis is performed for a PV-run refrigeration system capable of generating 200 tonneof ice per day. Carbon emission reduction is quantified in substituting a diesel-operated conventionalrefrigeration system with a solar-operated arrangement. The financial impact of substituting batterywith ITES is performed in terms of cash flow analysis, capital cost, cumulative savings, and paybackperiod considering the time value of money. To make an analysis independent of refrigerationrequirement and mine locations, results are presented in terms of sun peak hours enabling easy decisionmaking for investors. Finally, a sensitivity analysis is proposed to investigate the variation of solarintensity on energy and economic indexes.2. MethodologySolar PV is an integral component of the proposed ice refrigeration system, as shown in Figure 1.Electricity from solar PV operates the refrigeration plant to generate 200 tonne of ice per day. This icecan be fed into the vertical shafts of underground mines which later can be used for refrigerationpurposes. A conveyer belt is used to carry ice from the refrigeration plant to the vertical shaft.Appl. Sci. 2018, 8, x FOR PEER REVIEW 4 of 17 of a Li-ion battery, while the efficiency is about half [38]. It would be interesting to compare these two energy storage technologies in an industry-scale refrigeration application. This paper examines the techno-economic aspects of installing solar PV for operating an ice-generating machine to cool an underground mine. A thorough literature review suggests only two publications to study the use of solar energy in mine cooling applications [23,26]. Both of the analyses are performed for a specific mine site and are unsuitable to apply in decision making for the other mine locations. They also compare thermal storage strategies with a battery system but do not consider the battery replacement cost after standard charging and discharging cycles. This paper presents a detailed numerical code developed in Excel to design a solar PV system and to compare two different energy storage technologies for the proposed application: battery storage and ITES. A system-level analysis is performed for a PV-run refrigeration system capable of generating 200 tonne of ice per day. Carbon emission reduction is quantified in substituting a diesel-operated conventional refrigeration system with a solar-operated arrangement. The financial impact of substituting battery with ITES is performed in terms of cash flow analysis, capital cost, cumulative savings, and payback period considering the time value of money. To make an analysis independent of refrigeration requirement and mine locations, results are presented in terms of sun peak hours enabling easy decision making for investors. Finally, a sensitivity analysis is proposed to investigate the variation of solar intensity on energy and economic indexes. 2. Methodology Solar PV is an integral component of the proposed ice refrigeration system, as shown in Figure 1. Electricity from solar PV operates the refrigeration plant to generate 200 tonne of ice per day. This ice can be fed into the vertical shafts of underground mines which later can be used for refrigeration purposes. A conveyer belt is used to carry ice from the refrigeration plant to the vertical shaft. Figure 1. (a). Flow schematics of Battery Storage System (BS) and (b). Flow schematic of Ice Thermal Energy Storage System (ITES). The refrigeration plant is assembled in two unique configurations. These two different arrangements with corresponding component denomination are presented as options 1 and 2 in Figure 1. In both configurations, solar PV is used to generate electricity that ultimately powers the refrigeration plant. The configurations only differ in terms of their internal design aspects; however, they should still share many commonalities (i.e., number of conveyor belts, size of PV required, and so on.) In option 1, DC from solar PV is stored in a lithium-ion battery bank that is connected to the inverter. This inverter provides AC power needed to operate the refrigeration plant. Then, the refrigeration plant generates ice to be fed to the vertical intake shaft with the help of conveyors and belt system. Option 2 is different than option 1 in terms of the battery bank and the ice storage tank. The battery bank in option 2 is smaller as compared to option 1 and only acts as a buffer stock between solar panel and compressor of the refrigerator. Here, the use of the battery is just to supply the Figure 1. (a). Flow sche atics of Battery Storage Syste (BS) and (b). Flo sche atic of Ice Ther alEnergy Storage System (ITES).The refrigeration plant is asse ble i t i fi ti s. ese t o differentarrangements with cor espondi c i r r se ted as options 1 and 2 inFigure 1. In t fi ti s, solar PV is used to g nerate electricity ha ultimately powersth efrigeration plant. The configurations only differ in terms of their inter l design aspects;however, they should still share a y commonalities (i.e., numbe of conv yor belts, s z of PVrequired, and so on.) In ption 1, DC from solar PV is stored in a lithium-ion battery bank atis conn cted to the inverter. This inverter provides AC power needed to operate the refrigera ionplant. Then, the refrigeration plant generates ic to be fed to the ver ical intake shaft with the help ofconveyors and belt system. Option 2 is different than option 1 in terms of the battery bank and the iceAppl. Sci. 2020, 10, 6022 5 of 16storage tank. The battery bank in option 2 is smaller as compared to option 1 and only acts as a bufferstock between solar panel and compressor of the refrigerator. Here, the use of the battery is just tosupply the constant voltage with minimum noise, and the initial surge current requirement rather thanenergy storage. It is significant to note that option 2 can only operate during sun hours and the systemis designed to generate a fixed amount of cooling per day (200 tonne of ice). Hence, the capacity factorof the refrigeration plant for this system is lower than the option 1 it requires a greater number ofrefrigerators. In option 2, ice generated from solar configuration is stored in the storage tank whichlater is used for refrigeration purposes during evening or night hours when there is no sun in the sky.For convenience, option 1 is referred to as Battery Storage System (BS) and option 2 is referred to as IceThermal Energy Storage System (ITES) hereafter.2.1. State-of-the-Art ComparisonState-of-art technology is used in designing the solar-refrigeration system. Components withimproved durability and lowered costs, and which are available in the market are employed. Today,independently confirmed efficiencies of solar cells and modules are easily higher than 20% [39];however, the industrially manufactured solar modules have an efficiency in the range of about 20%.The PV module used in this analysis is a poly-crystalline module with an efficiency of 19.2% [40].Several companies in Europe and America manufacture ice-generation refrigerators for differentapplications ranging from concrete cooling to mine cooling to fish processing to beverage cooling.These machines can generate ice from the water with a temperature of up to 35 ◦C and an ambienttemperature as high as 40 ◦C [41]. Similarly, several companies manufacture industrial-scale ice storagesystems that could store any type of ice for several days. The typical size of these devices ranges from20 to 250 tonne per container. They have a fully automatic ice discharge system with a conveyer beltand are suitable for standard or tropical climate. The refrigeration plant and the ice storage systemconsidered for this analysis are manufactured by KTI-Plersch Kältetechnik, Germany. These refrigeratorplants rely on ammonia as a refrigerant, and have a designed evaporator temperature of −13 ◦C andcan generate pure ice from the intake water [42]. Lead-acid batteries were the best options for solarPV systems a few years ago. However, with recent developments in technology, lithium-ion batteriesoutperformed lead-acid batteries [43,44]. In addition to a higher life, they have low losses and extendedcycle life with lower storage depletion rates as compared to lead-acid batteries [45]. Hence, in thisanalysis, the lithium-ion batteries with a battery management system is used.2.2. Mathematical Modeling2.2.1. Solar PV DesignMathematical modeling of the system starts with the rated power of the refrigerator. This rated ACpower of the machine is obtained from the manufacturer datasheet and is denoted as Prated. The powerneeded is provided by the solar PV and battery system which is connected to the refrigerator throughinverter of efficiency ηinv. With this efficiency, the maximum power needed in DC from the solarsystem is calculated using Equation (1). Inverter efficiencies of up to 99% can be found thanks to theresearch and development in power electronics. Inverter used in this analysis has a DC-to-AC ratio of1.0 and an efficiency of 97% [46].Maximum power required in DC (Pmax, DC) =Pratedηinv ×DC−AC ratio (1)This power is consumed by the refrigeration machine throughout a day to generate 200 tonneof ice. Consequently, the total energy required in kWh by the system per day is calculated byusing Equation (2).Total energy demand per day in DC (Etotal) = Pmax,DC × 24 (2)Appl. Sci. 2020, 10, 6022 6 of 16This is the total amount of energy to be generated using solar PV each day. The solar system isdesigned to be a stand-alone solar power system. Solar PV module chosen for this application is a400 W poly-crystalline module manufactured by Canadian solar with a rated efficiency of 19.2% [40].Electrical data including optimum operating voltage and optimum operating current are obtained fromthe datasheet available from the manufacturer. To calculate the design operating voltage, Equation (3),a voltage efficiency of 90% is used. This analysis is performed to ensure acceptable module outputcurrent which is less than optimum operating current.PV operating voltage(Voperating)= V opt. power voltage × ηvoltage (3)The guaranteed power output from a single PV module at STC is obtained from relation (4)with a safety factor of 1.10. This number is also verified from the I–V curve available from themanufacturer’s datasheet.Gurrented power(Pgurrented)=Voperating × IoperatingS.F.(4)Output power from a solar PV also depends on ambient operating temperature. So, thermalefficiency is accounted for obtaining an output power form PV by using Equation (5). An equation todetermine the thermal efficiency of the module is given by Equation (6). Proposed by Evans [47], βre fin the equation represents the temperature coefficient of the module, while Tc and Tre f are the solarcell temperature and reference temperature respectively. Since, the system is not designed for specificmine location, no ambient temperature data is available to calculate exact thermal efficiency of thesystem. Thermal efficiency considered for this design analysis is 90%.Poperating = Pgurrented × ηthermal (5)ηthermal =[1 − βre f(Tc − Tre f)](6)Total energy output from each module per day is calculated using peak sun hours for the desiredlocation using Equation (7) with a safety factor of 1.10. Peak sun hours (SHPeak ) is the number ofhours during which the intensity of sunlight is 1000 W/m2 for that specific place per day. The codeis developed in such a way that; with the input of sun peak hours for any mine location, a systemfeasibility result can be concluded.Eper module per day =Poperating × SHpeakS.F.(7)Using this number, the total number of modules required to meet the energy demand is calculatedusing Equation (8)Ntotal =(EtotalEper module per day)(8)Equation (9) gives the number of modules required per string. These are the numbers of solarpanels that should be connected in series to provide an adequate voltage to the battery supply. This isalso equal to the inverter input DC voltage supply.Nseries =Vinverter, DCVoperating(9)The total number of strings in parallel is calculated by using the following relation (10).In computing the number of series and parallel panels, every panel number is adjusted to thenext higher integer.Nparallel =NtotalNseries(10)Appl. Sci. 2020, 10, 6022 7 of 16Finally, nominal rated array output is calculated using Equation (11). This is the rated moduleoutput in watts as per the manufacturer.Nominal rated array output = Ntotal × PVrated power (11)The total area of the solar field required is calculated by multiplying the individual area of the solarpanel with total numbers of solar panels as following. A solar panel used is of size 2108 × 1048 × 40 mmin dimensions.Total area of solar field = Ntotal × area of a module (12)2.2.2. Battery System DesignThe solar system can generate the energy required to operate the machine by converting energyfrom the sunlight into a flow of electrons using the photovoltaic effect. However, this energy isgenerated only when there is solar irradiation. This section describes the design of the battery bank,which is the essential component for BS. The total energy required for the refrigeration machine fromEquation (2) is used as a design criterion for the battery bank. This amount of energy is to be suppliedby the battery system in a single day to operate the system efficiently. This battery storage systemis designed independently from the photovoltaic array. To size the battery bank, the total electricalload is converted from watt-hours to amp-hours. Battery capacity in amp-hours is calculated byusing Equation (13).Total amp-hour demand per day =Etotal required × 1000Batterybus voltage(13)This battery bus voltage provides the nominal operating DC for the system. The battery busvoltage for this application is picked as 600 V, which also corresponds to the DC input voltage for theinverter. The maximum fraction of capacity that can be withdrawn from the battery, also known asthe allowable depth of discharge, is considered in designing the battery system. For this application,0.8 depth-of-discharge is considered. This gives the actual required battery capacity, as shownin Equation (14).Actual battery capclity =Total amp-hour demand per daydepth-o f -discharge(14)Once the total required capacity of the battery bank is estimated, the battery to be used in thesystem is selected. The Tesla Powerball battery specification data sheet is used for the selection of thebattery. Accordingly, the battery selected in this case has a usable energy storage capacity of 13.5 kWh.The total number of batteries in series and parallel is given by the Equations (15) and (16).Nparallel =Actual battery capacityamp-hour capacity of selected battery(15)Nseries =Battery bus voltagevoltage of selected battery(16)Batteries in series are arranged in such a way that the DC voltage provided by the battery systemcorresponds to the inverter DC input voltage. Additionally, the number of batteries in parallel is fixedto provide the battery required capacity. The overall idea is to make sure that the required amp-houris supplied with the required voltage supply. Equations (17) and (18) provide the total number ofbatteries required and total battery capacity, respectively.Ntotal = Nseries ×Nparallel (17)Total battery capacity = Ntotal ×Usable energy of sin gle battery (18)Appl. Sci. 2020, 10, 6022 8 of 162.2.3. Calculations of SavingsIn this section, power savings, energy savings, diesel savings, reductions in carbon emissions,and carbon tax savings are quantified. Here, rated AC power of the refrigerator is used as a referenceto perform an analysis. Energy savings per year in kWh is given by Equation (19),Esavings = Powersavings × 24 × 365 (19)The equivalent amount of diesel savings is calculated by converting energy savings to joulesand dividing by the calorific value of diesel. The calorific value of diesel used in this analysis is35.9 MJ/liter [48]. The efficiency of a diesel generator is accounted for in calculating the total amountof energy used. In this study, the efficiency of the diesel generator is used as 33% and the amount ofdiesel savings is given by Equation (20).Dieselsavings =Esavingscalori f ic value × ηgenerator (20)The quantification of diesel emission is made based on the amount of diesel saved over a year.This quantity is given by relation (21). For this calculation, an emission rate of 2.63 kg of CO2 per literof diesel is used while a carbon tax rate of USD 35 per tonne of CO2 is used in the analysis [16].CO2 savings = quantity of diesel savings × emission per liter (21)Finally, total cost savings is the summation of energy cost savings from diesel generators andcarbon savings. This is calculated by using formula (22).Cost savings =(Energysavings × Diesel cos t)+ (CO2 emissions × carbon tax) (22)2.2.4. Economic and Financial CalculationsNet Present Value (NPV) of the project is calculated based on Equation (23), where i is the discountrate and Fn is the net cash flow in year n. Net cash flow is the difference between net savings and netexpenditure over the year. Net savings include the savings from energy and carbon emissions savings,while the net expenditure includes the capital cost, principal and interest payment, and Operation andMaintenance cost.NPV =N∑n = 0Fn(1 + i)n(23)Annual loan payment for the project is calculated using the following Formula (24), where i and nare interest rate and the loan payback period in years, respectively.Annual loan payment amount = Total loan amount ×[i× (1 + i)n(1 + i)n − 1](24)Levelized cost of energy (LCOE) is that cost that, if assigned to every unit energy produced by thesystem over the analysis period, will equal the total life-cycle cost of the project discounted back to thebase year. LCOE is calculated using an Equation (25). The numerator in Equation (25) represents allcosts of the project over its life while the denominator is the total energy generated from the projectover its life, both converted to a present value. Here, Cn is the total cost and Qn is the energy producedin year n.LCOE =∑Nn = 0Cn(1 + i)n∑Nn = 1Qn(1 + i)n(25)Appl. Sci. 2020, 10, 6022 9 of 16Table 1 summarizes the different economic parameters used in the analysis. The solar installationand system commissioning time is assumed to take a year. For both systems, half of the total capitalcost comes from the company’s equity whereas another half from a loan. The borrowed loan is paidback in 10 equal installments in 10 years from the time after the system is commissioned. This loanrepayment schedule is also known as ‘even total payment’ method. The installation cost of PV andbattery systems are obtained from the National Renewable Energy Laboratory (NREL) report as of 2018pricing structure [37]. Additionally, the cost of the panel, inverter, and battery is verified with othermanufacturing companies [46,49–51]. The cost of a refrigeration plant and its accessories are obtainedfrom a plant manufacturing company. Fixed cost, variable cost, direct cost, and indirect cost associatedwith the project execution are also accounted for in the analysis. The proposed system assumes that thesolar system and battery system are co-located. The cost of the solar system is expressed in USD/kWwhile the cost of the storage system in USD/kWh and the cost of refrigeration plant in USD/tonne ofice generation.Table 1. Economic evaluation parameters of proposed systems.Time Parameters Solar Cost Share (USD per W) Battery Cost Share (USD per kWh)Analysis year 2020 Solar Panel 0.47 Battery 209Year construction starts 2020 Inverter 0.08 Structural 13Construction period (year) 1 Structural and electrical 0.25 Electrical 36Project life (years) 20 Installations and labor 0.13 Installations and labor 23Inflation/discount rate (%) 2 Shipping and handling 0.16 Shipping and handling 12Financing Parameters PII 0.09 PII 8Equity (%) 50 Sales tax 0.05 Sales tax 22Loan (%) 50 Developer overhead 0.36 Developer overhead 10Financing rate (%) 8 Contingency 0.04 Contingency 10Loan term years 10 Developer net profit 0.11 Developer net profit 18Salvage factor (%) 5 Total 1.74 Total 361Note: PII = Construction permitting, interconnection, testing, and commissioning.3. Results and Discussions3.1. Solar PVThe total power drawn by the compressor and its accessories is 443 kW, which provides a coolingcapacity of 1.2 MW with a COP of 2.8. To run this device merely with solar system, nominal arrayoutput of 2.5 MW is needed for a place with 6-h of sun peak. Since most of the remote mines arelocated in high solar intensity zones, this solar irradiation intensity is considered as a standard numberand is used in analysis hereafter. Additionally, 3.4 acres of land is needed to install the proposedsolar photovoltaic system. Figure 2 illustrates the design change in the solar PV system, and thecorresponding change in the Levelized Cost of Energy (LCOE) and the area of the solar field requiredfor different sun peak hours. As expected, there is a strong correlation between solar intensity, LCOE,and the area of the solar field required associated with sun peak hours. An increase in sun peak hoursleads to an obvious decrease in installation capacity, LCOE, and area of installation field required.Appl. Sci. 2020, 10, 6022 10 of 16Appl. Sci. 2018, 8, x FOR PEER REVIEW 10 of 17 Figure 2. PV size, Levelized Cost of Energy (LCOE), and area variation with solar intensity. 3.2. Battery To operate a refrigeration plant for a day, 11.5 MWh of energy is required. This energy is to be stored in the battery bank. The battery bank is designed with an allowable depth of discharge of 0.8 and provides 600 V of battery bus voltage to the inverter. With these design criteria, the total battery capacity of 23.9 KA-h and 14.3 MWh is required to operate the system. However, in designing the battery system, since the refrigeration plant is in operation throughout the day, the only energy required to operate the system during no-sun hours is to be stored in the battery bank. That means, the refrigerator is in operation during the sunshine hours for which the energy is not stored. With this approach, the total battery capacity required for the system is 10.6 MWh and 17.9 KA-h. 3.3. Savings Table 2 summarizes the key technical findings, along with the annual savings after the project comes to execution. On replacing the conventional refrigeration system with a solar-powered refrigeration system, total energy savings of 10.6 MWh per day is achieved. It is worth mentioning that this savings is just for a single refrigeration unit. If multiple units are to be used depending on the refrigeration demand, this number will get multiplied accordingly. Similarly, energy savings per year per unit is 3.9 GWh. In addition, considering capital cost and operation and maintenance cost, the Levelized Cost of Energy (LCOE) from the solar system is calculated as USD 0.06/kWh. This energy is achieved at the cost of 1180 Kilo liters of diesel with a diesel generator running at an efficiency of 33%. The calorific value of diesel considered is 35.9 MJ/liter [48]. With global average diesel price [52], and including transportation and storage costs, total savings from fuel is USD 0.95 million per annum. Unit cost of diesel per liter is USD 0.81 and the cost of energy from diesel generator is calculated as USD 246 per MWh. Moreover, electricity generated from diesel plants is ineffective and comes at the cost of significant environmental risks. These risks may be associated with diesel particulate matter, vapor, and greenhouse gases. Although the effect of particulates and vapor is not accounted for, the GHG emission effect is evaluated. A total of 2.63 kg of CO2 is released per liter of diesel combustion [16] and a total emission of 3103 tonne is saved in penetrating solar system to run a refrigerator. With a carbon tax rate of USD 35 per tonne of CO2, total carbon savings is computed to USD 108k per annum. 0246810121401000200030004000500060003 4 5 6 7 8 9Area of field (acres) & LCOE (United States Cents)PV size (kW)Number of sun peak hoursPV sizeArea of fieldLCOEFigure 2. PV size, Levelized Cost of Energy (LCOE), and area variation with solar intensity.3.2. BatteryTo operate a refrigeration plant for a day, 11.5 MWh of energy is required. This energy is to bestored in the battery bank. The ba tery bank is designed with an allowabl depth of discharge of0.8 and provides 600 V of battery bus voltage to the inverter. With these design criteri , the totalbattery capacity of 23.9 KA-h and 14.3 MWh is required to operate the system. However, in designingthe ba tery system, since the refrigeration plant is in operation throughout th day, the only energyrequired to operate the system during no-sun hours is to be stored in the battery bank. That m ans,the refrigerator is in operation during the sunshine hours for which the energy is not stored. With thisapp oach, the total battery capacity required for the system is 10.6 MW and 17.9 KA-h.3.3. SavingsTable 2 summarizes the key technical findings, along with the annual savings after the project comesto execution. On replacing the conventional refrigeratio system with a solar-powered refrigerationsystem, total energy savings of 10.6 MWh per day is achieved. It is worth mentioning that thissavings is just for a single refrigeration unit. If multi le units are to be used depending on therefrigeration demand, this number will get multiplied accordingly. Similarly, energy savings peryear per unit is 3.9 GWh. In addition, considering capital cost and operation and maintenance cost,the Levelized Cost of Energy (LCOE) from the solar system is calculated as USD 0.06/kWh. This energyis achieved at the cost of 1180 Kilo liters f diesel with a diesel generator running at an efficiency of33%. The calorific value of diesel considered is 35.9 MJ/liter [48]. With global average diesel price [52],and including transportation and storage costs, total savings from fuel is USD 0.95 million per annum.Unit cost of diesel per liter is USD 0.81 and the cost of energy from diesel generator is calculatedas USD 246 per MWh. Moreover, electricity generated from diesel plants is ineffective and comesat the c st of significant environmental risks. These risks may be associated with diesel particulatematter, apor, and greenhouse gases. Although the effect of particulates and vapor is not acc untefor, the GHG emission effect is evaluated. A total of 2.63 kg of CO2 is released per liter of dieselcombusti n [16] and a total emission of 3103 tonne is saved in penetrating solar system to run arefrigerator. With a carbon tax rate of USD 35 per tonne f CO2, t tal carbon savi gs is computed toUSD 108k per annum.Appl. Sci. 2020, 10, 6022 11 of 16Table 2. Key technical findings.S.N. Variable Value Unit1 Solar PV capacity 2.5 MW2 Energy storage capacity 10.6 MWh/day3 Levelized Cost of Energy from PV 0.06 USD/kWh4 Energy savings from Diesel 3.9 GWh/year5 Diesel savings 1180 Kilo liter/year6 Total cost of diesel per liter 0.81 USD7 Diesel energy cost per MWh 246 USD8 Carbon emission savings 3103 Tonne/year9 Operating expenses savings 1.1 million (USD)/year3.4. Capital CostWith the cost breakdown from Table 1, the total capital cost of the project is calculated.Figure 3 depicts the waterfall chart showing the capital cost requirement for two different scenarios.The total initial investment for solar panel and its accessories is found to be USD 4.3 million includingAC/DC inverter. With the battery management cost included, the capital cost of the battery systemis calculated as USD 3.8 million. Considering the decrease in electrochemical performance duringcharging and discharging cycles of the lithium-ion battery, battery replacement is required every10 years or after 3600 cycles [53]. Therefore, for the battery-based system, an additional cost isneeded at the end of each 10-year operational period. This is shown as ‘battery replacement’ costin Figure 3b. The price of lithium-ion batteries after 10 years is projected based on the availablepublished resources [54–56], and accounting the time value of money. The capital cost of a singlerefrigeration plant is USD 910k including installation and commission charge. The storage cost of ice isUSD 32 per kWh and the total cost of the required storage system is USD 350k [38]. For ITES, a smallamount of battery is required to supply sufficient voltage and current for the refrigerator. This numberat least should be equal to the number of batteries in series given by Equation (15). However, to be onthe safe side, the battery system is designed to supply continuous energy demand for half an hour.Corresponding total cost of such a designed battery system is USD 100k.ppl. ci. 2018, 8, x FOR PEER REVIEW 11 of 17 Table 2. Key technical findings. S.N. Variable Value Unit 1 Solar PV capacity 2.5 MW 2 Energy storage apacity 10.6 MWh/day 3 Levelized Cost of Energy from PV 0.06 USD/k h 4 Energy savings from Diesel 3.9 GWh/year 5 Diesel savings 1180 Kilo liter/year 6 Total cost of diesel per liter 0.81 USD 7 l energy cost per MWh 246 USD 8 Carbon emission savings 3103 Tonne/year 9 Operating expenses savings 1.1 million (USD)/year 3.4. apital ost ith the cost breakdown from Table 1, the total capital cost of the project is calculated. Figure 3 depicts the waterfall chart showing the capital cost requirement for two different scenarios. The total initial investment for solar panel and its accessories is found to be USD 4.3 million including AC/DC inverter. With the battery management cost included, the capital cost of the battery system is calculated as USD 3.8 million. Considering the decrease in electroche ical erfor ance ring charging an ischarging cycles of the lithium-ion battery, battery replacement is required every 10 years or after 3600 cycles [53]. Therefore, for the battery-based system, an additional cost is needed at the end of each 10-year operational period. This is shown as ‘battery replacement’ cost in Figure 3b. The price of lithium-ion batteries after 10 years is projected based on the available published resources [54–56], and accounting the time value of money. The capital cost of a single refrigeration plant is USD 910k including installation a d commission charge. The storage cost of ice is USD 32 per kWh and the total cost of the required storage system is USD 350k [38]. For ITES, a small amount of battery is required to s pply sufficient voltage and curre t for the refrigerator. This number at least should be equal to the number of batteries in series given by Equation (15). However, to be on the safe side, the battery s stem is designed to sup ly continuous energy demand for half an r. rres i total cost of such a designed battery syste is S 100k. (a) (b) Figure 3. (a) Capital cost breakdown for ITES and (b) Capital cost breakdown for BS. Clearly, the investment cost of BS is higher than that of ITES. Although both systems shown here share the same solar PV cost, the difference can be attributed to the battery cost. Another important point that should be noted is the cost of a refrigerator in an ITES. The refrigeration cost for this option is four times the BS. As mentioned earlier, both systems are designed to generate an equal amount of ice each day, and ITES does not have a battery bank to operate during non-sun hours. Hence, to ensure the required amount of ice generation, multiple refrigerators should be used. This analysis assumes the number of sun-peak sours to be six, hence four such refrigerators are needed which increases the capital cost of refrigerators in ITES. i r . ( ) it l c st re f r I ( ) it l c st re f r S.l rl , the invest ent cost of BS is i er t t t f .t s s l r c st, t e ifference can be at ributed to the bat ery cost. t r i rt ti t t at s l e te is t t f i t r i . i ti t f i ii f i . , t st s re si e t r t l t fi each day, and ITES does not have a battery bank to operate during no -sun hours. Hence, to ensurethe requir d amount of ice generation, multiple refrigerators should be used. This analysis assumeAppl. Sci. 2020, 10, 6022 12 of 16the number of sun-peak sours to be six, hence four such refrigerators are needed which increases thecapital cost of refrigerators in ITES.3.5. Cash Flow AnalysisThe financial model of the analysis considers that half of the total investment cost in both cases isfrom the company’s equity while the remaining half is loaned from a financial institution. As mentionedin Table 1, inflation and interest rates used in the analysis are 2% and 8%, respectively [57]. With thishypothesis, as can be inferred from Table 3, the total financed amount is equal to USD 4.1 million inITES and USD 5.6 million in BS. In both cases, loan financing is done at the beginning of the projectand is paid back in 10 equal installments over the first 10 years after the project comes in operation.Although the total capital cost of the project is higher in BS, this amount also includes the batteryreplacement cost that will take place after 10 years of operation. It is worth mentioning that the usefullife of the mine considered is 20 years and the economics will change for a shorter or longer minelifecycle period. Since half of the total project investment comes from a loan at the beginning of theproject, battery replacement cost is the part of company’s equity. Due to this reason, as shown inTable 3, initial equity in ITES is considered as USD 4.1 million, which accounts for USD 500k more thanof BS’ equity share. Although the equity amount of ITES at the beginning is more than that of BS, due tothe larger loan amount, annual loan payment of the project is higher in BS than ITES by USD 218k.Table 3. Key economic findings (Currency: USD).Option Total Capital CostLoan EquityAnnual Loan Payment Total Savings Payback PeriodYear ‘0′ Year ‘0′ Year ‘10′ITES 8.3 million 4.1 million 4.1 million - 612k 7.8 million 10 yearsBS 11.2 million 5.6 million 3.6 million 2.0 million 830k 4.8 million 13 yearsFigures 4 and 5 represent the net cash flow diagram for two different setups. The red color inthe figures indicates the net cash outflow, while the green color represents total cumulative savingsat the end of each fiscal year. Cash outflow includes capital cost, loan payment (principal andinterest), and operation and maintenance cost while cumulative savings is the operating income fromannual savings. Net Present Values are used in calculating cumulative savings for a sound economiccomparison. As bank loan is paid in the first 10 years, cash outflow after the 10th year represents onlyoperation and maintenance costs. Year ‘zero’ is the construction year and no savings are made untilthe project comes in operation, which is at the beginning of the year ‘one’. This means, cash flow inyear ‘zero’ refers only to the initial investment made from the company’s equity. It should be recalledthat both proposed systems generate equal cooling effect irrespective of the energy storage technologychosen. This results in an equal amount of operating income or savings over a year. Equal annualsavings with unequal loan payment amount results in different cumulative savings over the period.As depicted in Figures 4 and 5, investing in ITES returns more savings. With project implementation,at the end of the project life, total savings for BS and ITES are estimated at USD 4.8 million and7.8 million, respectively.Hence, from the company’s viewpoint, with an initial equity investment of USD 4.1 million,ITES pays USD 7.8 million in total at the end of the project. On the other hand, BS returns onlyUSD 4.8 million for an initial investment of 5.6 million. This indicates that both options are in profit atthe end of the project. But, while comparing these two systems, cumulative savings for the ITES isgreater than the BS by USD 3.0 million.Cash flow diagram also depicts the payback period for both projects. As initial equity of theITES is higher than the BS by USD 500k, the cumulative saving of ITES is more negative than BSat the beginning of the project. But once the project comes in operation, things go in the oppositedirection due to more amount of loan payment in BS. One more factor to deaccelerate the paybackperiod of the BS is the additional capital requirement in replacing the Li-ion battery at the end of theAppl. Sci. 2020, 10, 6022 13 of 1610th year. This decreases the cumulative savings by USD 2 million and hence increases the paybackperiod. For ITES, a period of 10 years is enough to pay back the investment. However, this timeincreases to 13 years for the BS due to larger capital size and higher annual loan of the associatedsystem. Hence, the ITES storage system is a better option out of two considered alternatives in termsof cumulative savings and payback period.Appl. Sci. 2018, 8, x FOR PEER REVIEW 13 of 17 Figure 4. Cash flow diagram for ITES. Figure 5. Cash flow diagram of BS. Cash flow diagram also depicts the payback period for both projects. As initial equity of the ITES is higher than the BS by USD 500k, the cumulative saving of ITES is more negative than BS at the beginning of the project. But once the project comes in operation, things go in the opposite direction due to more amount of loan payment in BS. One more factor to deaccelerate the payback period of the BS is the additional capital requirement in replacing the Li-ion battery at the end of the 10th year. This decreases the cumulative savings by USD 2 million and hence increases the payback period. For ITES, a period of 10 years is enough to pay back the investment. However, this time increases to 13 years for the BS due to larger capital size and higher annual loan of the associated system. Hence, the ITES storage system is a better option out of two considered alternatives in terms of cumulative savings and payback period. Figure 4. Cash flow diagra for ITES.Appl. Sci. 2018, 8, x FOR PEER REVIEW 13 of 17 Figure 4. Cash flow diagram for ITES. Figure 5. Cash flow diagram of BS. Cash flow diagram also depicts the payback period for both projects. As initial equity of the ITES is higher than the BS by USD 500k, the cumulative saving of ITES is more negative than BS at the beginning of the project. But once the project comes in operation, things go in the opposite direction due to more amount of loan payment in BS. One more factor to deaccelerate the payback period of the BS is the additional capital requirement in replacing the Li-ion battery at the end of the 10th year. This decreases the cumulative savings by USD 2 million and hence increases the payback period. For ITES, a period of 10 years is enough to pay back the investment. However, this time increases to 13 years for the BS due to larger capital size and higher annual loan of the associated system. Hence, the ITES storage system is a better option out of two considered alternatives in terms of cumulative savings and payback period. Figure 5. Cash flow diagram of BS.4. ConclusionsThis techno-economic analysis explored the potential of using solar photovolt ics in cooling remoteunderground mines. The proposed solar syste provid s electric ener y to operate a r frig rationni capable of ge erating 200 tonne of ice per day. The s udy finds that to generate a ooling effectof 8.3 MWh per day, 2.5 MW of solar photovoltai s should be installed in a place with an aver gesun-peak hour of 6. The Levelized Cost of Energy from solar PV for a project with life of 20 yea sis found as 0.06 USD/kWh. The penetration of t solar system c uld save the carbon footprint by3100 t nne per year in ddition to 1180k liters of diesel. Two scenarios are developed in storing theAppl. Sci. 2020, 10, 6022 14 of 16electric energy from the solar system: Ice Thermal Energy Storage System (ITES) and Battery Storagesystem (BS). Economic analysis suggests that capital cost of USD 11.2 million is required to establishthe BS which is about USD 2.9 million greater than ITES. While both systems return enough to payback the investment during their lifetime, a higher initial investment cost leads to a relatively slowerpayback period for BS. While ITES pays back in 10 years, BS takes 13 years to pay back the amountinvested. In addition, ITES and BS generate cumulative savings of USD 7.8 million and USD 4.8 millionrespectively at the end of the project lifetime.In conclusion, this paper recommends the use of solar PV for mine cooling applications inplaces with average sun peak hours of 6 or greater. Besides, Ice Thermal Energy Storage System isrecommended over Battery System due to lower capital investment cost, higher savings, and a smallerpayback period.Author Contributions: S.P.: Conceptualization, investigation, methodology, software, validation,writing—original draft, writing—review & editing. A.F.K.: conceptualization, validation, writing—review& editing. H.K.: conceptualization, validation, writing—review & editing. S.A.G.-M.: conceptualization,validation, resources, supervision, writing—review & editing. All authors have read and agreed to the publishedversion of the manuscript.Funding: This research received no external funding.Conflicts of Interest: The authors declare no conflict of interest.References1. Energy Information Administration. International Energy Outlook 2019 with Projections to 2050; EnergyInformation Administration: Washington, DC, USA, 2019.2. International Energy Agency. 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eng
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Vancouver : University of British Columbia Library
Multidisciplinary Digital Publishing Institute
CC BY 4.0
Faculty
deep mine cooling
solar electric refrigeration
ice thermal energy storage
techno-economic assessment
solar PV design
renewable energy in mining
solar-powered ice maker
Techno-Economic Trade-Off between Battery Storage and Ice Thermal Energy Storage for Application in Renewable Mine Cooling System
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