Comparative life cycle assessment of thermal energy storage systems for solar power plants

Comparative life cycle assessment of thermal energy storage systems for solar power plants

Renewable Energy 44 (2012) 166e173 Contents lists available at SciVerse ScienceDirect Renewable Energy journal homepage: www.elsevier.com/locate/ren...

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Renewable Energy 44 (2012) 166e173

Contents lists available at SciVerse ScienceDirect

Renewable Energy journal homepage: www.elsevier.com/locate/renene

Comparative life cycle assessment of thermal energy storage systems for solar power plants Eduard Oró a, *, Antoni Gil a, Alvaro de Gracia a, Dieter Boer b, Luisa F. Cabeza a a b

GREA Innovació concurrent, Edifici CREA, Universitat de Lleida, Pere de Cabrera s/n, 25001 Lleida, Spain Departament d’Enginyeria Mecànica, Universitat Rovira i Virgili, Av. Països Catalans, 26, 43007 Tarragona, Spain

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 June 2011 Accepted 10 January 2012 Available online 14 February 2012

The present work compares the environmental impact of three different thermal energy storage (TES) systems for solar power plants. A Life Cycle Assessment (LCA) for these systems is developed: sensible heat storage both in solid (high temperature concrete) and liquid (molten salts) thermal storage media, and latent heat storage which uses phase change material (PCM). The aim of this paper is to analyze if the energy savings related to the stored energy of the different systems are enough to balance the environmental impact produced during the manufacturing and operation phase of each storage system. Some hypothetical scenarios are studied using LCA methodology to point out the differences between each TES system. The system based on solid media, due to his simplicity, shows the lowest environmental impact per kWh stored of all three systems compared. In addition, the liquid media (molten salts) shows the highest impact per kWh stored because it needs more material and complex equipment. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: LCA PCM Thermal energy storage Solar power plant EI99

1. Introduction Lately, concentrating solar power (CSP) plants are becoming the best option to produce clean and renewable energy. In addition, the investment for solar thermal projects in the next decade has increased in the meantime significantly [1]. Mainly, four elements are required in CSP plants: concentrator, receiver, transport/storage media system, and power conversion device [2]. It is well known that CSP plants have the perspective to expand their electricity production time using thermal energy storage systems. It is very interesting to support deeply thermal energy storage (TES) in the CSP plants. TES systems would be charged in the peak of solar energy during the day-time, and the stored heat would be released at night time or during parts of the day when the solar power is not enough to produce electricity. Therefore, TES systems have an important role in CSP plants even though it is one of the systems less developed. High temperature TES is starting to play an important role in the industrial field and in solar applications. Different TES technologies have been developed recently [2,3]: (1) sensible heat storage both in solid media and in liquid media, (2) latent heat storage, which use phase change materials (PCM) as thermal media, and (3)

* Corresponding author. Tel.: þ34 973 003576; fax: þ34 973 003575. E-mail address: [email protected] (E. Oró). 0960-1481/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2012.01.008

chemical storage. This work is only focused in the sensible and latent TES systems at high temperature (100  Ce550  C). Sensible heat storage can use solid or liquid media. Concrete is the regular material chosen in sensible heat storage due to its low cost, easy processing, high specific heat, mechanical properties, etc. Moreover, high temperature concrete and castable ceramics systems were developed by Laing et al. [4] as thermal storage systems for parabolic trough power plants using synthetic oil as the heat transfer medium. On the other hand, different materials such as molten salts, mineral oils, and synthetics oils, can be used as liquid media. These materials maintain natural thermal stratification due to density differences between hot and cold fluid. This property allows working using thermal stratification (thermocline tank) and storing the heat with only one tank, or working with a more complex system using two different tanks, one for the cold and the other for the hot media. Gabbrielli et al. [5] studied the optimal design of a molten salt thermal storage tank (a two tanks configuration) for parabolic trough solar power plants. The optimal design turned out to be an internally insulated, carbon steel storage tank characterized by a maximum allowable height of 11 m and a diameter of 22.4 m. The use of latent heat allows storing energy nearly isothermally in some materials, as heat of fusion or vaporization (mainly fusion). The correct PCM in each system has to be chosen depending of the application and its working temperature range. For example, Bayón

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Fig. 1. Solid media storage system developed by Laing et al. [4].

et al. [6] developed a system with a mixture of molten salts (KNO3/ NaNO3) as PCM and expanded graphite fins arranged in a “sandwich configuration” to improve the thermal conductivity to store 100 kWh of latent heat in solar thermal power plant. To compare the different TES systems, the storage capacity must be evaluated as well as the environmental impact of each system. Most of the studies available in the literature concluded, that there are important benefits in terms of reducing the environmental impact in CSP plants compared to other types of plants of electricity generation such as conventional fuel plants and gas power plants [7]. In addition, future development will enable further reduction of environmental impacts that are caused by regenerative energy systems. Moreover, Heath et al. [8] compared the life cycle assessment (LCA) of two TES designs: two tank, indirect molten salt and indirect thermocline, and this analysis focused on estimating the emissions embodied in the production of the materials used in the TES system. They concluded that the embodied emissions of greenhouse gases for the thermocline system are less than half of those for the two tanks used in the 6 h, 50 MWe. Therefore, it is very important to study in more detail the environmental aspects of TES in the CSP plants. Specially, since lately a lot of new information about different methods to implement TES in CSP plants [9], and since these are seen as an environmental alternative to conventional power plants, environmental evaluations of the whole plant and its life should be compared. The method usually utilized to quantify and evaluate the environmental impacts of different systems is known as life cycle assessment (LCA). It analyzes the impact of a product from “cradle” (raw materials extraction), through manufacturing, transportation and use, to “grave” (disposal) [10]. The environmental performance of high temperature molten salts CSP plant compared to conventional and gas power plants was presented in Piemonte et al. [11]. The LCA was performed considering the most important materials used for construction as well as the energy requirements for construction. Lechón et al. [12] evaluated the environmental impacts derived from the electricity production of a 17 MW solar thermal plant with central tower technology and a 50 MW solar thermal plant with parabolic trough technology, both of them hybrid operation power plants. Cavallaro et al. [13] performed a preliminary environmental analysis, assisted by LCA methodology, of a CSP type solar thermal power station using paraboloidal dish concentrators. It was concluded that the overall environmental impact arising from the entire life cycle of a thermal solar power plant that uses linear parabolic concentrators is extremely limited and almost insignificant compared to the impact produced by traditional fossil fuel power stations. From LCA results Pehnt [7] concluded that all renewable energy chains that the inputs of finite energy resources and emissions of greenhouse gases are extremely low compared

with a conventional system. Even though no references were found in LCA studies for the use of PCM in TES systems, the environmental impact of using PCM in other applications were analyzed, such as the use of PCM in building envelopes [14]. This paper addresses the use of TES systems from a global point of view, focussing in the whole life cycle from cradle to grave. The LCA will compare three different systems working under the same operating conditions. The detailed description and justification of these studies can be found elsewhere [4e6]. Moreover, three different scenarios have been studied varying the temperature gradient (DT) of the storage material. 2. Description of the TES systems considered 2.1. Introduction The three TES systems that have been considered in this paper are taken from the literature and have been either tested in pilot plants or designed as theoretical commercial systems. The three systems are very different in size; therefore the environmental impact evaluation will be done per kg of storage material and per kWh stored. The studied systems are:  Solid media system. Sensible heat is stored in this system using high temperature concrete as storage material [4].  Molten salts system. Sensible heat is stored in liquid media; using molten salts based on a mixture of NaNO3 and KNO3 [5].  PCM system. Latent and sensible heat is stored using the molten salts [6] described in the previous system.

2.2. Solid media system This TES system, developed by Laing et al. [4], has a storage capacity of about 350 kWh and can operate with maximum temperatures of 390  C. In this pilot plant, located at the Plataforma Solar de Almeria (Spain), a tubular heat exchanger is integrated into the storage material, which is high temperature concrete. The heat exchanger is based on 36 tubes of high temperature steel with Table 1 Main characteristics of the solid sensible storage system studied in Laing et al. [4]. Characteristics of the solid sensible TES system

Storage capacity design Density Specific heat capacity Total volume of the storage unit Volume of the tubes Volume of the solid media

Units

Value

kWh kg/m3 J/kg$K m3 m3 m3

280 2750 916 10.3 0.57 9.73

168

E. Oró et al. / Renewable Energy 44 (2012) 166e173 Table 2 Main characteristics of the molten salts sensible storage system studied by Gabbrielli et al. [5]. Characteristics of the molten salts TES system

Diameter Height Total volume of the storage unit Weight of steel Thickness of the lateral material insulation Roof insulating thickness FoamglasÒ thickness Number of brick foundation Number of brick vessel (radially) mounted) Number of brick bottom

Units

Value

m m m3 t mm mm mm e e e

22.4 11 4335 279 125 125 40 2 1 5

Fig. 2. Molten salts storage tanks located in Andasol 1 [15].

a nominal diameter of 21 mm. These are distributed in a square arrangement of 6  6 tubes with a separation of 80 mm. The TES system consists of two modules with dimensions of 0.48  0.48  23 m (10.3 m3 of volume). Fig. 1 shows the storage systems. Table 1 presents the main characteristics of the storage material. 2.3. Molten salts system Sensible heat storage in the liquid phase can use two different configurations. The simplest one is based on one storage tank, using thermal stratification (thermocline tank). Not enough data about this configuration are available to carry out an LCA analysis. For this reason, this case will not be considered. The other configuration uses two storage tanks where the hot and the cold thermal fluid are separated (Fig. 2). This work will analyze the environmental impact of the two tanks configuration system with a diameter of 22.4 m and a height of 11 m each [5]. The thermal capacity of the storage system in the research performed by Gabbrielli et al. [5] was 600 MWh, which corresponds to about 5500 t of molten salts with a thermal gradient between the two tanks of 260  C. These tanks have the size of a real installation, and the molten salts were 60 wt.% NaNO3 and 40 wt.% KNO3. The working temperature of the hot and cold storage tanks were 550  C and 290  C, respectively. The density of the mixture of salts at 500  C is 1740 kg/m3 and the specific heat capacity 1437.6 kJ/kg K. As it can be seen in Fig. 3, the molten salts storage tanks are made by different layers. For the lateral walls (inside out): flexible

protective liner (stainless steel), layer of insulating firebricks, tank shell (carbon steel), ceramic fibre insulation (layer), ceramic fibre insulation (exterior), and aluminium sheet. The bottom of the tank is made of: flexible protective liner (stainless steel), layer of insulating firebricks, tank shell (carbon steel), fine sand, insulating firebricks, foamglasÒ, reinforced concrete with water-based cooling system, poor concrete, and foundation piles. And the materials used in the roof of the tank are: flexible protective liner (stainless steel), ceramic fibre insulation, ellipsoid-shaped sheet (carbon steel), and ceramic insulating material. Table 2 shows the main characteristics of these tanks. The TES system needs additional equipment in each molten salts tank such as, a suitable heating system, a water cooling system, and an electrical pump. The water cooling system has to maintain the temperature of the concrete below 100  C in the base of the tanks. The heating system heats the salts in all the circuit to prevent solidification and the molten salts pumps move the salts between the hot and cold tanks. 2.4. PCM system This TES system should operate in a specific range of temperature to work under phase change conditions of the storage material and therefore to store latent and sensible heat. The PCM used was 54 wt.% KNO3 46 wt.% NaNO3 eutectic mixture [6], similar to the molten salts used in the TES system described by Gabbrielli et al. [5]. The total amount of the mixture was 2100 kg and its density was 2040e1950 kg/m3. The TES system consisted of a bundle of 36 parallel tubes comprised by six pipes arranged in six passes with fins; the

Fig. 3. Sketch of the molten salts storage tank foundation (a) And roof (b) construction proposed by Gabbrielli et al. [5].

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169

Fig. 4. Stored energy per kg in the different TES systems in function of the DT.

conductive fins were made of expanded graphite (EG) foil, spaced 10-mm apart and perpendicular to the pipes, and had a section of 490  490 mm2 and a thickness of 1 mm. Fig. 4 shows the PCM storage system module, where the top view and the side view can be seen. The storage prototype was filled with PCM with a melting point of 221  C and latent heat of fusion about 100 kJ/kg Table 3 shows the main characteristics of the PCM TES system.

is widely used in LCA and is provided by an extended database. It divides the environmental burdens into three different damage categories: human health, ecosystem quality, and resources. It is an endpoint or damage-oriented approach. These three damage categories are grouped into an easily understandable single indicator. The evaluation of each impact category is given by:

IMPj ¼

X

dk;j $LCIk

[1]

k

3. Methodology The developed Life Cycle Assessment (LCA) is based on the EcoIndicator 99 (EI99) impact assessment method, extracted from the database EcoInvent 2009 [16]. The EI99 has been chosen because it

Table 3 Main characteristics of the PCM TES system studied [6]. Characteristics of the PCM TES system

TES prototype dimensions Length Height Width Volume Tube bundle dimensions Number of pipes Pipes length Diameter Thickness Volume exterior Net volume Density Mass Graphite properties Dimensions Thickness Number of pipes Volume Density

Units

Value

mm mm mm m3

4310 586 596 1.505

e m mm mm m3 m3 m3/kg kg

6 24 22 3 0.05474 0.02579 7850 202.42

mm mm e m3 m3/kg

490  490 1 320 0.076,832 1000e1100

Where IMPj is the j impact category (human health, ecosystem quality, and resources), dk;j is the coefficient of damage (extracted from database [17]) associated with the component k and impact j, and the LCIk is the Life Cycle Inventory entry. The coefficient of damage of the resources impact category is expressed in MJ of surplus energy needed for future extraction. For the ecosystem quality impact category the coefficient of damage stands for the loss of species over a certain area, during a certain time (% plant, species/m2$year). The damage to human health is expressed as the number of year life lost and the number of years lived disabled (disability adjusted life years, DALYs). The hierarchic perspective is considered and two different phases in the life of the system are taken into account: the manufacturing/dismantling and the operational phase. The Life Cycle Assessment is developed for the previously described TES systems: solid media, molten salts, and PCM system. First a Life Cycle Inventory has to be detailed for the manufacturing and dismantling phase of the TES systems. The correspondence between the experimental components and the EcoInvent database is shown in Table 4. Furthermore, since the molten salts system is the only case with electric demand (molten salts pump, cooling and heating system), the environmental impact during the operational phase must be included. Moreover, the other two TES systems studied in this paper do not present any environmental impact during the operational phase. Electricity used to meet the energy demand considers the production mix corresponding to the Spanish energy production system (#674 in the EcoInvent database).

170

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Table 4 All the components in the TES systems relation with EI99 database [14]. EI99 database relation with all the components studied Component

Name in the database Eco Invent corresponding to the component

Concrete Tubes of steel High temperature concrete Metal sheets Rock wool FoamglasÒ KNO3

Concrete, normal, at plant, CH, [m3] (#504) Drawing of pipes, steel, RER, [kg] (#1163) Concrete, normal, at plant, CH, [m3] (#504)

NaNO3 Stainless steel (metal sheets) Firebricks Carbon steel Ceramic fibre Aluminium sheet Stainless steel (metal sheets) Sand Poor concrete Carbon steel Molten salt pump Water pump Disposal concrete Disposal concrete þ steel bars Disposal Metal sheets Disposal rock wool Disposal foamglasÒ Disposal KNO3

Disposal NaNO3

Disposal stainless steel Disposal firebricks

Disposal Carbon steel Disposal ceramic fibre Disposal aluminium Disposal sand

Disposal poor concrete Disposal of tubes

Sheet rolling, steel, RER, [kg] (#1174) Rock wool, packed, at plant, CH, [kg] (#1001) Foam glass, at plant, RER, [kg] (#7160) Potassium nitrate, as N, at regional storehouse, RER, [kg] (#52) Sodium nitrate, as N, at regional storehouse, RER, [kg] (#52) Selective coating, stainless steel sheet, black chrome, CH, [m2] (#1189) Refractory, fireclay, packed, at plant, DE, [kg] (#498) Sheet rolling, steel, RER, [kg] (#1174) Cellulose fibre, inclusive blowing in, at plant, CH, [kg] (#991) Powder coating, aluminium sheet, RER, [m2] (#1166) Selective coating, stainless steel sheet, black chrome, CH, [m2] (#1189) Silica sand, at plant, DE, [kg] (#479) Poor concrete, at plant, CH, [m3] (#511) Sheet rolling, steel, RER, [kg] (#1174) Pump station, CH, [unit] (#5736) Pump 40 W, at plant, CH, [unit] (#1865) Disposal, building, concrete, not reinforced, to recycling, CH, [kg] (#2148) Disposal, building, reinforced concrete, to recycling, CH, [kg] (#2153) Disposal, steel, 0% water, to inert material landfill, CH, [kg] (#2082) Disposal, insulation spiral-seam duct, rock wool, DN 400, 30 mm, CH, [m] (#10,825) Disposal, insulation spiral-seam duct, rock wool, DN 400, 30 mm, CH, [m] (#10,825) Disposal, salt tailings potash mining, 0% water, to residual material landfill, CH, [kg] (#2202) Disposal, salt tailings potash mining, 0% water, to residual material landfill, CH, [kg] (#2202) Disposal, steel, 0% water, to inert material landfill, CH, [kg] (#2082) Disposal, refractory SPL, Al elec.lysis, 0% water, to residual material landfill, CH, [kg] (#2200) Disposal, steel, 0% water, to inert material landfill, CH, [kg] (#2082) Disposal, building, fibre board, to final disposal, CH, [kg] (#2016) Disposal, aluminium, 0% water, to sanitary landfill, CH, [kg] (#2215) Disposal, slag from MG silicon production, 0% water, to inert material landfill, CH, [kg] (#2081) Disposal, building, concrete, not reinforced, to recycling, CH, [kg] (#2148) Disposal, steel, 0% water, to inert material landfill, CH, [kg] (#2082)

The assumptions used for the LCA in all the studied scenarios are:  Only the TES system is considered in this analysis (the heat transfer fluid circulating inside the TES system is out of the scope of the study).  The expected life time for each TES system is 20 years.

 Because no data is available in the EcoInvent database, manufacturing and disposals of NaNO3 are estimated to be the same as KNO3 and manufacturing and disposals of high temperature concrete are estimated to be the same as concrete.  Not all the elements or dimensions of the different layers in each TES system are correctly defined by the authors describing the systems [4e6], so several hypothesis have been made in each system B Solid media system. The thickness of the concrete base is 50 cm, and of the rock wool is 20 cm. B Molten salts system. The thickness of the flexible protective liner is 1 mm; of the tank covering (sheet of aluminium) is 1 mm; of the sand layer is 20 cm; of the layer of reinforced concrete is 70 cm; and of the layer of poor concrete is 30 cm. B Concerning to the operational phase: - The power of the heating system is 175 kW. - There are two molten salts pumps and each has a power of 300 kW. - The cooling system in each tank has a water pump with 1.2 kW of power. - The working days of the TES system is 200 days per year [18]. Because the information about the foundation piles is not available in Gabrielli et al. work [5] and in the work done by Herrmann et al. [19], they did not mention any foundation piles, the authors of this study did not take into account the foundation piles in the LCA method. B PCM system. From the experience of the authors in the topic, the thickness of the concrete base is 50 cm; the thickness of the rock wool is 20 cm; and the thickness of the foamglasÒ is 20 cm. Due to the big differences between the three studied systems, such as the dimensions, the energy storage capacity, and the initial and final temperatures of the storage material, in the Life Cycle Assessment it is necessary to evaluate hypothetical scenarios. The energy stored in a normal day of operation is calculated for each of the analysed TES systems as the DT of the storage material. To compare the studied TES systems different scenarios are introduced:  Scenario 1. Normal conditions  Scenario 2. DT of the storage material is set to 50  C  Scenario 3. DT of the storage material is set to 250  C In scenario 1, the working conditions (temperature and DT) in each system are the ones detailed in the literature [4e6]; the DT of

Table 5 Considered temperature range in each scenario and each TES system. Temperature range in each scenario and in each TES system

Scenario 1 Normal conditions Scenario 2 DT ¼ 50  C

Scenario 3 DT ¼ 250  C

Solid media Molten salts PCM Solid media Molten salts PCM Solid media Molten salts PCM

High temperature [ C]

Low temperature [ C]

390 550 235 390 550 246 390 550 346

120 290 195 340 500 196 140 300 96

Storeg energy per kg [kWh/kg]

E. Oró et al. / Renewable Energy 44 (2012) 166e173

Table 7 Life Cycle Inventory and impact during manufacturing phase in molten salts system.

0.14 Solid Media

0.12

Molten salts

171

PCM

Material used and life cycle impact for the molten salts TES system

0.10 0.08 0.06 0.04 0.02 0.00 Normal conditions

250

50 Delta T [ºC]

Fig. 5. Stored energy per kg in the different TES systems in function of the DT.

the systems are 40  C for the PCM, 260  C for the molten salts, and 270  C for the solid media system. A common DT is set in scenario 2 and 3 to compare the amount of energy stored in each system under the same operating conditions. Since the DT only reflects the gradient temperature between the initial and final conditions, design specifications must be taken into account to determine the initial and final temperature of each process. According to the characteristics of each system, the molten salts TES can reach 550  C and the solid media allows to work up to 390  C. The authors of the PCM system do not specify any maximum temperature, but looking at the materials of the storage tank (stainless steel) and the storage material (molten salts) it can be concluded that 350  C is the maximum temperature of operation. Note that the molten salts system can reach higher temperatures because there are more layers and components in the tank to work under these conditions. For the molten salts system and for the solid media it is assumed that the highest temperature of each system must be the maximum possible. To use the latent heat of the PCM system, the operating temperature range in this case must be coincident with the phase change range of the PCM (221  C). Therefore the melting point temperature is considered to be in the middle of the temperature range. Table 5 shows the working temperature range for each TES system and scenario. In scenario 2, the gradient temperature is too low for the molten salts and solid media systems but it is interesting to compare these systems with the same conditions as the PCM system (DT ¼ 50  C). On the other hand, scenario 3 is ideal for the molten salts and solid media systems due to the similar temperature gradients (DT ¼ 250  C), but the DT is too high for the

Component

Material used Units Impact points Impact/amount (EI99) material used

KNO3 NaNO3 Concrete Poor concrete Stainless steel (metal sheets) Firebricks Carbon steel Ceramic fibre Aluminium sheet Sand FoamglasÒ Molten salt pump Water pump Disposal stainless steel Disposal firebricks Disposal carbon steel Disposal ceramic fibre Disposal aluminium Disposal KNO3 Disposal NaNO3 Disposal sand Disposal foamglasÒ Disposal concrete Disposal poor concrete Total

3,300,000.00 2,200,000.00 551.71 236.45 3360.95

kg kg m3 m3 m2

1,909,479.00 1,272,986.00 3709.95 887.70 206.59

0.5786 0.5786 6.7244 3.7543 0.0615

1,271,756.92 554,052.81 10,419.79 1548.18 417,726.27 4256.08 2.00 30.00 18,468.34 1,271,756.92 554,052.81 10,419.79 3560.81 3,300,000.00 2,200,000.00 417,726.27 5805.66 551.71 236.45 e

kg kg kg m2 kg kg unit unit kg kg kg kg kg kg kg kg m m3 m3 e

100,307.28 21,345.18 353.05 3.00 575.96 385.83 37,034.00 78.17 19.13 20,557.95 573.78 53.03 0.06 5466.12 3644.08 432.60 526.31 0.85 0.32 3376,261.79

0.0789 0.0385 0.0339 0.0019 0.0014 0.0907 18,517 2.6057 0.0010 0.0162 0.0010 0.0051 0.0000 0.0017 0.0017 0.0010 0.0907 0.0015 0.0014 17,350.11

PCM system even though no problems would be detected when working under these conditions. To compare the storage energy capacity of each system, the total amount of the energy stored is divided by the mass of storage material used. Fig. 5 shows the stored energy per kg of material used (only the storage material) for each system and scenario. It can be observed that the solid media system has the lowest value of stored energy per kg of storage material, followed by the molten salts system. The PCM system presents the highest value of energy per kg due to the latent heat stored during the phase change. Furthermore, the gradient temperature in PCM should be 100  C and 200  C to reach the storage energy per mass of solid media and molten salts system at normal conditions, respectively.

Table 8 Life Cycle Inventory and impact during manufacturing phase in PCM system. Material used and life cycle impact for the PCM TES system Component

Material used

Units

Impact points (EI99)

Impact/amount material used

Concrete Tubes of steel Metal sheets KNO3 NaNO3 Metal sheets Rock wool FoamglasÒ Disposal concrete Disposal of tubes Disposal vessel Disposal KNO3 Disposal NaNO3 Disposal Metal sheets Disposal rock woolpara> Disposal foamglasÒ Total

1.60 202.42 427.33 1134.00 966.00 80.67 49.91 69.36 6600.00 202.42 427.33 1134.00 966.00 80.67 12.93 4.31 e

m3 kg kg kg kg kg kg kg kg kg kg kg kg kg m m e

10.76 0.44 16.46 656.17 558.96 3.11 6.94 6.29 8.98 0.21 0.44 1.88 1.60 0.08 0.07 1.13 1270.23

6.7244 0.0022 0.0385 0.5786 0.5786 0.0385 0.1390 0.0907 0.0014 0.0010 0.0010 0.0017 0.0017 0.0010 0.0056 0.2626 8465

Table 6 Life cycle Inventory and impact during manufacturing phase in solid media system. Material used and life cycle impact for the solid sensible TES system Component

Concrete Tubes of steel High temperature concrete Metal sheets Rock wool Disposal concrete Disposal concrete þ steel bars Disposal Metal sheets Disposal rock wool Total

Material used

Units

Impact points (EI99)

Impact/amount material used

7.2 1516.2 9.7

m3 kg m3

48.42 31.04 65.43

6.7244 0.0205 6.7244

173.3 403.0 19,800.0 28,273.7

kg kg kg kg

6.68 56.01 26.93 43.39

0.0385 0.1390 0.0014 0.0015

173.3 138.0

kg m e

0.18 0.77 278.85

0.0010 0.0056 13.66

e

172

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Table 9 Impact results during manufacturing and each operational phase. Manufacturing þ operation GLOBAL IMPACT/kWh

Scenario 1 Normal conditions

Scenario 2 DT ¼ 50  C

Scenario 3 DT ¼ 250  C

Solid media Molten salts PCM Solid media Molten salts PCM Solid media Molten salts PCM

Eco system quality [Impact/kWh]

Human health [Impact/kWh]

Resources [Impact/kWh]

Total

0.01 0.47 1.12 0.04 2.43 1.02 0.01 0.49 0.38

0.10 2.55 6.31 0.53 13.27 5.76 0.11 2.65 2.14

0.04 2.65 6.36 0.22 13.76 5.80 0.04 2.75 2.16

0.15 5.67 13.79 0.80 29.47 12.59 0.16 5.89 4.68

4. Results and discussion

4.2. Global impact of the LCA analysis

4.1. Life Cycle Inventory

Table 9 shows the results of the global impact during the manufacturing and operational phase of the TES systems studied. To compare the TES systems, the global impact is divided by the energy stored in each scenario. As stated before in Section 3, LCA is only focused on the TES system; therefore, the solid media and PCM systems do not have any impact during the operational phase, while the molten salts system presents impact during the operational phase, due to the use of two salts pumps, a water pump in the refrigerator system on the bottom, and electric heaters. Table 9 and Fig. 6 show that the global impact of the human health and resources damage categories are similar, being 82% higher than the impact of the ecosystem. As it was expected, when the DT increases and therefore the energy stored, the global impact per kWh decreases, being the scenario 3 the best scenario from the point of view of the manufacturing and operational impact per kWh stored. There is a high difference of the global impact/kWh stored between the solid media system and the other two systems, which use salts as storage material. The solid media system does not need any vessel or tank and the system is very simple in comparison to the other TES systems, specifically to the molten salts TES system. Even though the energy storage capacity of the solid media is lower than the salts systems, the global impact per kWh stored is low, being a 0.8%, 1.9%, and 1.5% of the global impact of the molten salts and PCM system in scenario 1, 2, and 3, respectively. Since the molten salts system was designed to reach 550  C, it needs more equipment than the others. To evaluate this system without this equipment the maximum temperature is decreased to levels comparable with the other systems. Analysing the molten salts system without those specific elements (the refrigeration system, the firebricks layers, the sand layer, and 20% decrease in the thickness of concrete on the base), the global impact is only reduced a 4% in comparison to the previous study. In addition, the molten salts systems still have more impact per kWh stored than the PCM system although the maximum temperature working is the same.

A list of all the materials used in the construction of the solid media system is shown in Table 6. The high temperature concrete (used as storage media) presents the highest impact during the manufacturing phase of this system producing a 23.46% of the global impact, followed by the impact of the rock wool (20.09%). Note that the disposal of solid media must include also the dismantling impact of the steel pipes, producing the 15.56% of the global impact during the manufacturing/dismantling phase. The Life Cycle Inventory of the molten salts system is detailed in Table 7. 94.2% of the total impact is due to the storage material which is a mix of two salts (KNO3 and NaNO3). These differences in the impact between the salts and the other materials of the system are due to the high amount of the storage material (5500 t). The impact of all the other materials is very low, being most of them insignificant. Table 8 shows a list of all the materials used in the construction of the PCM system. As it was previously observed, the highest impact belongs to the storage material, which is a mix of salts (KNO3 and NaNO3), being 95.66% of the global impact. The manufacturing impact of the molten salts system is 3,378,626 points (Table 7) while the impact of the solid media is only 279 (Table 6) and the impact of the PCM system is 1270 (Table 8), both being drastically different from the molten salts system. These high differences in the manufacturing impact are related to the difference between the systems such as the dimensions and the final purposes of them. Therefore, to compare the different TES systems, it is necessary to evaluate the impact of each system per kWh of stored energy.

5. Conclusions

Fig. 6. Global impact of each storage system for 3 different scenarios.

In this work the environmental impact of three different thermal energy storage systems (TES) used in the solar power plants (CSP) have been analyzed and compared using Life Cycle Assessment (LCA) methodology based on the Eco-Indicator 99 (EI99). The evaluated TES (solid media, molten salts and PCM system) present different dimensions, storage capacity and final purposes. Therefore, to compare them, the global impact (manufacturing þ operational impact) per kWh stored is analyzed.

E. Oró et al. / Renewable Energy 44 (2012) 166e173

LCA results show for the two systems that use molten salts as storage material, similar impact results during the manufacturing phase, being the impact of each storage material more than 94% of the system. The molten salt system presents higher impact than the PCM system because it requires specific equipment to withstand higher temperatures. Furthermore since the PCM system uses latent heat storage, the impact per kWh stored is reduced, in comparison to a system that only stores sensible heat. Even though the energy storage capacity of the solid media is lower than the salts systems, due to the construction simplicity of it, the global impact per kWh stored is the lowest for all the scenarios, being as far the most environmentally friendly TES system. From this analysis it can be concluded that the commonly used two tanks molten salts storage system is the one with the highest environmental impact and, therefore, should be substituted by any of the other two studied systems. This higher impact is mainly due to the complexity of the system and because it uses more storage material, hence the only way to reduce its impact would be to find more environmentally friendly materials and simpler control strategies. Nevertheless, since these systems have only either been recently commercially implemented or are still in research status or under development, new commercial data would be interesting to corroborate the results obtained here. Acknowledgements The work was partially funded by the Spanish government (projects ENE2008-06687-C02-01/CON, CTQ2009-14420-C02, DPI2008-04099I, A/023551/09, and ENE2011-22722). Antoni Gil would like to thank the Col$legi d’Enginyers Industrials de Catalunya for his research appointment. The authors would like to thank the Catalan Government for the quality accreditation given to their research group (2009 SGR 534). Eduard Oró would like to thank the University of Lleida for his research fellowship. References [1] Becker M, Meinecke W, Geyer M, Trieb F, Blanco M, Romero M, et al. Solar thermal power plants, prepared for the EUREC-agency; May 2003.

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