Environmental profile of latent energy storage materials applied to industrial systems

Science of the Total Environment 473–474 (2014) 565–575

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Environmental profile of latent energy storage materials applied to industrial systems Ana M. López-Sabirón, Alfonso Aranda-Usón, M.D. Mainar-Toledo, Victor J. Ferreira, Germán Ferreira ⁎ CIRCE - Centre of Research for Energy Resources and Consumption, University of Zaragoza, Zaragoza, Spain

H I G H L I G H T S • • • •

PCM shows the highest contribution to global warming category in TES manufacture. HTF has the highest impact in human toxicity during the TES manufacture. The disposal of components in the landfill has negative effect on the final results. Energy saving compensates environmental impacts associated to the use of a TES.

a r t i c l e

i n f o

Article history: Received 8 August 2013 Received in revised form 2 December 2013 Accepted 4 December 2013 Available online 4 January 2014 Keywords: Phase change material (PCM) Life Cycle Assessment (LCA) Thermal energy storage (TES) Net Zero Environmental Impact Times (NZEIT) Environmental impacts

a b s t r a c t Industry sector is an intensive-energy consumer and approximately 20–50% of industrial energy consumption is lost as waste heat. Therefore, there is a great potential for reducing energy consumption and, subsequently, decreasing the fossil fuels used if this lost energy can be recovered. Thermal Energy Storage (TES) based on Latent Heat Storage systems (LHS) using Phase Change Materials (PCMs) has become one of the most feasible solutions in achieving energy savings through waste heat recovery, especially when there is a mismatch between the supply and consumption of energy processes. In this paper, a shell and tube heat exchanger incorporating PCMs has been considered to store the excess energy available in an industrial process. Several attempts have been made to design the most appropriate system considering many cost–benefit and technical criteria to maximise the heat recovery. However, the environmental criterion also is an important factor when determining whether this technology is not only energy and cost-efficient but also environmentally friendly, considering the whole life of the system from its manufacture to its disposal. To this end, this research includes a Life Cycle Assessment (LCA) to determine whether the energy savings of conventional fuels during the operation stage are large enough to balance the environmental impact originated in an industrial TES system including the manufacture, use and disposal phases. Inputs and outputs of each management stage have been defined, and the inventory emissions calculated by SIMAPRO v7.3.2. A midpoint and endpoint approaches have been carried out using two methods, CML 2001 and Eco-indicator 99, respectively. As a preliminary result, a promising reduction in the overall impacts was obtained by the use of this technology. From the environmental impact results, a matrix of possible technical solutions is displayed, to improve the environmental performance. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Thermal energy storage (TES) is an attractive technology for different industrial applications from a technical, economic and environmental point of view (Dincer and Rosen, 2001). In fact, this technology can reduce the size, the operational failures, the environmental impact, and the manufacturing and operating costs of several industrial systems which cannot manage the waste heat generated during their operation.

⁎ Corresponding author. E-mail address: [email protected] (G. Ferreira). 0048-9697/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.scitotenv.2013.12.013

TES can be designed to keep both the hot and cold media in contact or to separate them by using a heat exchanger. Then, the cold medium storage involves two well-known mechanisms for storing waste heat, namely latent heat (LH) or sensible heat (SH). Several authors have studied both mechanisms in order to design systems capable of storing waste heat or excess heat from an industrial system with the purpose of using it in other systems or heat itself when its operation is required (Dinçer and Rosen, 2002). Most of these studies are focused on TES which have been developed and based on experimental (Al-Abidi et al., 2013; Delgado et al., 2012; ElGhnam et al., 2012; Mawire and McPherson, 2009; Regin et al., 2006; Tay et al., 2012a, 2012b; Trp, 2005; Tyagi et al., 2012) and numerical (Banaszek et al., 2000; Guo and Zhang, 2008; Guo et al., 2013; Mawire and

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Nomenclature DALY disability adjusted life years DPO diphenyl oxide ES energy storage ETAP Environmental Technologies Action Plan GHG greenhouse gases HTF heat transfer fluid LCA Life Cycle Assessment LCI life cycle inventory LH latent heat NZEIT Net Zero Environmental Impact Times PCM phase change material PDF*m2yr potentially disappeared fraction times area times year SH sensible heat TES thermal energy storage Hf latent heat Tf melting temperature n number of years m mass Em energy consumption Esaved total thermal storage energy

McPherson, 2009; Oró et al., 2013; Regin et al., 2006; Tay et al., 2012b; Trp, 2005) research. They are mainly focused on the technical design of these storage systems. All these studies have encouraged several authors to publish different original reviews based on them (Al-Abidi et al., 2012; Gil et al., 2010; Kenisarin, 2010; Oró et al., 2012; Pinel et al., 2011; Regin et al., 2008; Rismanchi et al., 2012; Sharma et al., 2009; Soares et al., 2013; Zalba et al., 2003; Zhou et al., 2012). However, there is a significant lack of knowledge on the environmental implications to quantify the environmental benefits associated to the use of this technology. Saving of fossil fuel consumption and, therefore, CO2 eq. emissions generated by using them is one of the most important characteristics of these systems. Then, the continuous increase in the level of greenhouse gas (GHG) emissions and rising fossil fuel prices are the main techno-economic characteristics that promote efforts for using various sources of waste or excess heat recovered (Sharma et al., 2009; Vanneste et al., 2011; Wagner and Rubin, 2014). The first study that addressed the analysis of the environmental implications of TES systems was carried out by Beggs (Beggs, 1994) almost 20 years ago. However, the environmental impact analysis should be performed considering a broader perspective of the product or service's life stages. This perspective should include direct and indirect “cradle-to-grave” environmental impacts. Then, researchers (Castell et al., 2013; Denholm and Kulcinski, 2004) introduced the Life Cycle Assessment (LCA) methodology as a tool to estimate the environmental impact of TES, particularly in solar power plants (Battisti and Corrado, 2005; Oró et al., 2012; Piemonte et al., 2011). These authors suggested that incorporating of PCM substantially reduces the overall environmental impact under the experimental conditions studied. Nevertheless, despite the fact that LCA has been applied to different scenarios, a limited number of studies have been published using this methodology to assess environmental aspects such as the overall TES environmental performance throughout its operational life cycle. In addition, in most of the above-mentioned researches which have analysed the environmental implications of TES systems, LCA was evaluated using Eco-indicator method to model the approach of characterisation of an impact indicator. However, a characterisation at midpoint level has not been found. In this study, based on the authors' knowledge,

for the first time the impact of substances involved in a new TES design on the environment changing natural environmental aspects (level midpoint) is carried out by means LCA methodology using the CML method. Then, besides the midpoint approach and following the trend of recently published works, this study uses LCA methodology and also Ecoindicator 99 in order to determine whether energy savings of conventional fuels during the operation stage are large enough to balance the environmental impact caused in an industrial TES system. The manufacture, use and disposal phases are included along this analysis.

2. Methodology 2.1. Scope of the analysis As mentioned above, the environmental analysis proposed in this work is based on the LCA methodology to determine whether energy savings are large enough to balance the environmental impact caused during the manufacture, use and disposal stage of a TES system. LCA methodology has been reported for analysing direct and indirect “cradle-to-grave” environmental impacts of products, services or processes (Aranda-Usón et al., 2012; Gironi and Piemonte, 2011; Hunt et al., 1996). On the other hand, this has already been fully technically and scientifically proven (Rebitzer et al., 2004; Society of Environmental Toxicology and Chemistry (SETAC), 1993; UNEP/SETAC Life Cycle Initiative, 2011). Additionally, this methodology is strongly encouraged by the European Union policies and regulations, i.e. the Environmental Technologies Action Plan (ETAP) on Sustainable Consumption and Production and Sustainable Industrial Policy (COM-2008 397) or the ETAP action Plan (COM-2004 38 final). The most up-to-date structure of the LCA is proposed by the standard ISO 14040 (Guinee et al., 2001) which mentions that LCA has four main phases. All of them are well described in Aranda et al. (Aranda-Usón et al., 2013) and can be summarised as an iterative process which may be repeated if a need for further information emerges during its implementation (Rebitzer et al., 2004; Tukker, 2000; Udo de Haes and Heijungs, 2007). Thus, in this study the LCA methodology attempts to associate emissions and extractions of life cycle inventory (LCI) on the basis of impact pathways to their potential environmental damages. These impact pathways refer to environmental processes and they show the causal chain of subsequent effects originating from an emission or extraction. In order to study the inventory stage, via impact assessment, two approaches (midpoint and endpoint) frequently used in LCA are performed in this work. Midpoints are considered points in the cause– effect chain (environmental mechanism) of a particular impact category somewhere between stressor and endpoints (Guinee et al., 2001). Whereas, endpoint approach evaluates those elements at the end of an environmental mechanism being themselves of value to society e.g. damage to Human Health or to Ecosystem diversity. Midpoint and endpoint approach assessments were carried out by two methods commonly used in a scientific LCA research, namely CML 2001 and Eco-indicator 99 (Chen et al., 2012; Tiruta-Barna et al., 2007; Wäger et al., 2011). The CML method uses multiple indicators at midpoint level (Guinée, 2002). This involves the impact categories into two groups: Obligatory impact categories, base line impact categories, which are described in detail by several authors (Amores et al., 2013; Renó et al., 2011; Zaman, 2010), and additional impact categories which are operational impact categories that are dependent on the study requirements. In this work, the considered base line impact categories of the CML method are as follows: Abiotic depletion, Acidification, Eutrophication, Global warming, Ozone layer depletion, Human toxicity, Fresh water aquatic ecotoxicity, Marine aquatic ecotoxicity, Terrestrial ecotoxicity and Photochemical oxidation.

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On the other hand, Eco-indicator 99 method includes multiple endpoint indicators that can be combined in a single endpoint indicator (Goedkoop and Spriensma, 2000). Its methodology is based in the calculus of eleven damage categories (Climate change, Ozone layer, Acidification/Eutrophication, Carcinogenic, Respiratory effects, Radiation, Ecotoxicity, Land use, Mineral resources, and Fossil fuel resources), with a time frame of 20 years, which can be aggregated into three global indicators (Human health, Ecosystem quality and Resources). Unlike other most recent methodologies, Eco-indicator 99 also includes the potential impact from future extractions in the impact assessment. Both methods (CML 2001 and Eco-indicator 99) are indicator approaches available in life cycle impact assessment in SIMAPRO v7.3.2 (PRé Consultants, 2007). Thus, inputs and outputs of each management stage were defined. The inventory emissions have been calculated using SIMAPRO v7.3.2 and data provided by the Ecoinvent v2.2 database (Ecoinvent Centre, 2007) has been used to support the calculations. 2.2. System description and boundaries This study is focused mainly on a TES including phase change materials to recover waste heat energy and the effects that this causes on the annual energy consumption of the plant. Therefore, the functional unit used is a TES unit which is fully described in this section. The expected life time for the TES is 20 years. Fig. 1 shows the system boundary including the stages involved into the process. Namely, manufacture of PCM and other materials used in TES manufacturing, and the energy exchanged during the use phase and the disposal step. The transport stages, such as the transport from the material manufacturing plant to the TES manufacture location have been also taken into account. 2.2.1. PCM manufacturing PCMs are particularly attractive to store energy at constant temperature which corresponds to the melting temperature of the material (Sharma et al., 2009). Therefore and as reported by Aranda et al. (Aranda-Usón et al., 2013), the PCM selection must be in accordance with its application, ensuring the performance of the latent heat storage system. In this case, PCM with a melting temperature around 300– 320 °C, compatible with stainless steel, is required. Thus, various storage materials such as, molten salts, nitrate eutectics or pure nitrates, can be included in thermal energy storage systems working at high temperatures to improve their performance. Therefore, taking into account the melting point requirement, sodium nitrate (NaNO3), particularly NaNO3 from Chile, has been selected as suitable PCM in this case. NaNO3 from Chile is a natural material composed of a mixture of nitrates, and is mined from caliche ore that is found in deposits located in the desert of northern Chile. It is a very strong oxidant, with a wide

Fig. 1. Description of the system boundaries.

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Table 1 Main physical and thermal properties of sodium nitrate.

NaNO3

Hf

Tf

[kJ/kg] 178

[°C]

Density (solid) [kg/m3]

Density (liquid) [kg/m3]

Specific heat (solid) [J/kgK]

Specific heat (liquid) [J/kgK]

306

2260

1900

1588

1650

range of uses. Table 1 shows the main thermal properties of the PCM. Temperature of fusion is denoted as Tf and Hf is the latent heat of fusion. The PCM manufacturing process considers the production location. Since about 85% of the commercial sodium nitrate came from as natural product, therefore, this is the perspective included in this environmental study. Natural sodium nitrate is obtained by extracting the caliche ore in the Atacama Desert and then treating it by leaching with brine followed by fractional crystallisation. Fig. 2 represents the process carried out to obtain the final sodium nitrate including the transport stage to Spain. Regarding the energy consumption in PCM manufacturing and according to data provided by the CODEX Alimentarius Commission (CODEX Committee on Food Labelling, 2004), about 4 GJ per tonne of nitrate is originated from renewable energy and 3.04 GJ per tonne of nitrate from non-renewable energy. This latter energy consumption is mainly associated with electrical consumption due to the crushing and milling processes, water pumps, belts and other machinery. The use of water takes place in the lixiviation process. This process is carried out under controlled conditions with a 35–40 °C weak salt/water solution. The only loss of water is through evaporation from large ponds where the solutions are concentrated by solar energy. In addition, the energy supply source used (electricity) is a critical parameter that can significantly influence the environmental burden of a process. Since the energy involved in this process should be based on the Chilean electricity production mix, a specific module has been developed to be incorporated in the LCA evaluation. Fig. 3 shows the main sources contributing to the Chilean case study (International Energy Agency (IAE), 2009). Thus, the results obtained in this research takes into account the impacts due to efficient power conversion processes and primary sources since the model is defined by a specific electrical network into the environmental assessment conducted with SIMAPRO, in this case the electricity mix in Chile. On the other hand, transport stages are taken in account. These are associated with the transportation routes from the mine to the nitrate plant and from the industrial manufacture site to the client plant. In this regard, two transport modules have been incorporated into the environmental model, one model for the road transport and another for maritime transport from Chile to Spain. 2.2.2. TES manufacturing The design of the TES is based on four parts in the tank manufacturing, namely vessel tank–material, tubes-tank–material, PCM-Sodium nitrate and HTF. In detail, the design involves components as shell and tubes heat exchanger made of stainless steel. This consists of a conventional rectangular vessel with a bundle of tubes inside considering a packing factor of 0.57. The PCM is located in the housing of the shell part and the heat transfer fluid (HTF) circulates inside the bundle of tubes. In this case, the commercial synthetic thermal oil based on a biphenyl/diphenyl oxide (DPO) eutectic mixture has been considered and characterised by a diphenylether composition in the evaluation. Table 2 shows the main physical and thermal properties of the HTF and steel used in the manufacture of the TES. The transport stages of the other materials (HTF and stainless steel) have also been developed. In this sense two more transport stages have been considered in different modules with an assumption made for the value of the distances (300 km by road).

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Fig. 2. Flow diagram of sodium nitrate production.

2.2.3. Energy exchanged during the use phase The overall heat storage capability is determined by Eq (1), which allows estimating the total energy exchanged. Esaved ¼ n  m  H f

ð1Þ

Hf is the latent heat content available at Tf and n is the number of phase changes that occur during a specific time of period. The mass of the PCM involved in the TES system is denoted as m. Considering the results obtained by Steinmann et al. (Steinmann et al., 2009), who demonstrated that nitrates have a charging and discharging cycle of about 6 h and after 4000 h of operation the sodium nitrate maintained its effectivity, in this study the system's operational batch performance was analysed with 250 cycles during the year. The total energy exchanged estimated in Eq. (1) can be then used in other processes avoiding the consumption of fossil fuels. Thus, in order to analyse this situation, a theoretical scenario was developed where the energy recovered is used to evaporate water instead of burning natural gas in a furnace. Additionally, considering the operation without degradation, the sodium nitrate is replaced 7 times during the lifetime of the TES. 2.2.4. Disposal The system boundaries also include disposal scenarios for the components of the TES in their end-of-life. Thus, two main case studies are considered: (i) conventional disposal in landfill for all the components and (ii) recycling of 50% of the steel contained in the TES system and disposal in landfill for the rest of the components. 2.3. Life cycle inventories (LCI) As mentioned above, LCI is used in the impact analysis in which the involved energy and material are included. Since the TES is a prototype,

most of the data used in this study comes from the design calculations performed by internal sources of the project development. However, information from literature has been also used to obtain the final LCI input data (CODEX Committee on Food Labelling, 2004; Ecoinvent Centre, 2007; Steinmann et al., 2009). Table 3 shows the most relevant data included in the LCI considering the system boundaries defined in Fig. 1. Finally, in order to ensure that all relevant environmental impacts are represented in the study, the following cut-off criteria are assumed: (i) Materials: flows less than 1% of the cumulative mass of all the inputs and outputs of the LCI model, depending on the type of flow, have been excluded because their environmental relevance is not a concern. However, it was ensured that the sum of the ignored material flows does not exceed 5% of the mass, energy or environmental relevance. (ii) Energy: flows less than 1% of the cumulative energy of all the inputs and outputs of the LCI model, (depending on the type of flow), are excluded from this analysis. Their environmental relevance is not a concern. These criteria were established based on an in-depth analysis of the system performing energy and mass balances for the processes involved. 3. Results 3.1. TES system analysis As mentioned above, midpoint and endpoint approaches are used by applying the LCA methodology. To start with the analysis, only the TES manufacturing stage is considered on the above described parts of the TES system. In the case of midpoint assessment, the results depicted in Fig. 4 show that the PCM manufacture represents the highest percentage of impact from approximately 40% to 80% with respect to all categories analysed, except in Human Toxicity. Considering that this last indicator accounts a total impact of 5.07 ton 1.4-DB equivalent and the PCM manufacturing contributes with 634.86 kg 1.4DB equivalent, this stage only involves a 12.5% of the impacts associated to this environmental category.

Table 2 Main physical and thermal properties of the HTF and steel (Incropera and DeWitt, 2002).

Fig. 3. Distribution of energy sources in the Chilean electrical mix in 2009 (International Energy Agency (IAE), 2009).

HTF Steel

Density [kg/m3]

Thermal conductivity [W/mK]

Specific heat [J/kgK]

654 7900

0.07 14.9

2760 477

A.M. López-Sabirón et al. / Science of the Total Environment 473–474 (2014) 565–575 Table 3 Main parameters of the LCI. Process Input

Output

PCM Natural sodium nitrate Water Electricity Maritime transport Road transport HTF Diphenylether-compounds Road transport Tank Steel production —Tubes Steel production—Vessel Road transport, tubes Road transport, vessel Use phase Esaved Disposal Disposal steel Recycling steel Disposal HTF Disposal NaNO3

Unit/magnitude kg l MJ tkm tkm

1651.5 11560.6 5020.6 19818.2 1024.9

kg kgkm

41.1 12338.9

kg kg kgkm kgkm

146.1 113.7 12338.9 12338.9

GJ

840

% % % %

100/50 0/50 100 100

However, in this latter, HTF production shows the highest value with 64.56% of the total impact in this category. This result is associated with benzene emissions to water, since these represent the 70% of the impact in the human toxicity category, while other compound emissions, such as chromium, selenium, arsenic, polycyclic aromatic hydrocarbons and other metal and semimetals, among others, constitute the remaining impact in the human toxicity. These emissions are mainly related to the use of diphenylether compounds and the production of their major components such as the dichlorobenzene. On the other hand, PCM and HTF reveal similar environmental impacts in photochemical oxidation and ozone layer depletion categories. In the latter, the main contributions are CFC-10 (tetrachloromethane) emissions and Halon 1301(bromotrifluoromethane) which are presented in HTF and PCM productions, respectively. However, in the case of photochemical oxidation category, although both PCM and HTF contribute markedly in the emissions of sulphur

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dioxide and alkenes, PCM production has a higher contribution with respect to HTF in the impact associated to SO2. Nevertheless, HTF entails important contributions of cumene and acetaldehyde emissions. Finally, an important environmental category is global warming since it quantifies the CO2 equivalent emissions and determinates the carbon footprint of these processes. In this category, it can be observed that PCM has the highest contribution with approximately 76% of the total environmental impact while HTF production represents around the 20%. In this regards, the CO2 emissions from fossil sources is the most remarkable contribution, since they represent about 94% of the impact in case of PCM production process. The main sources can be found in the maritime transport and electric energy consumption associated, which account for 47% and 39.3%, respectively, of the total emissions in the material production. In addition, an analysis of the detailed components contained in the tank manufacturing was also carried out in the endpoint assessment. The results are shown in Table 4. Similar to midpoint, the most important contribution is located in the manufacture of the PCM which covers approximately the 62.4% of the damages in case of Human Health, 58.3% in Ecosystem Quality and 70.5% in Resources. These results are consistent with those published by Oró et al. (Oró et al., 2012) which show a similar trend in the analysis of a TES based on PCM which was used in a solar power plant. Their environmental evaluation is based on EcoIndicator 99 and their results concluded that the highest impact (95.66% of the global impact) belongs to the thermal storage material, in that case a mix of KNO3 and NaNO3. In this work, a similar situation is found when the TES manufacturing stage is analysed. In addition, the maritime transport stage accounts for 32–37% of the damages, depending on the indicator considered. On the other hand, considering absolute values, the operation stage of the ship is also the highest value in terms of Human Health and Ecosystem Quality, while crude oil production is the most important contribution to the Resources indicator followed by use of natural gas. The latter is due to the electrical energy consumption that is based on nonrenewable fossil fuel resources, which are depleting in time and produce large amounts of CO2 and consequently have a great impact on the environment (Menoufi et al., 2013). These major contributions are the same when the entire TES system is considered and not only the manufacture of the PCM.

Fig. 4. Midpoint assessment considering the TES manufacturing [%].

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Table 4 Damage assessment considering the TES system manufacturing. Damage category

Human Health ×103

Ecosystem Quality ×10−1

Resources ×10−2

Unit

DALY

PDF*m2yr

MJ surplus

TES tank Vessel tank–material Tubes-tank–material PCM-sodium nitrate HTF

4.310 0.139 0.246 2.690 1.240

35.80 3.28 4.80 20.90 6.85

40.30 1.56 2.38 2.84 7.94

whose emissions such us ammonia, sulphur or nitrogen oxides, ethane or propane, among others, contribute significantly to climate change and respiratory effect impact categories. On the other hand, ferronickel and ferrochromium production contributes to perform significant impacts on the eco-toxicity, acidification, eutrophication and mineral impact categories, since a high quantity of metals (chromium, nickel or lead) and gases (nitrogen or sulphur oxides) are involved. 3.2. Analysis of the TES system manufacture and end-of-life scenarios As mentioned previously, two different end-of-life scenarios have been considered:

Table 5 Midpoint impact categories for both end-of-life scenarios (Case A and Case B) and the main stages. Impact category

Abiotic depletion Acidification Eutrophication Global warming Ozone layer depletion Human toxicity Fresh water aquatic ecotox. Marine aquatic ecotoxicity Terrestrial ecotoxicity Photochemical oxidation

Unit

TES tank

kg Sb eq

Case B End-oflife step

18.44

−1.28

Case A Total 17.16

End-of- Total life step 0.25

18.69

23.16 −0.48 22.68 0.10 kg SO2 eq kg PO4 eq 5.72 −0.27 5.46 0.02 kg CO2 eq 2721.15 −143.25 2577.90 17.68 mg CFC-11 eq 366.89 3.38 370.27 6.30

23.25 5.75 2738.83 373.18

kg 1.4-DB eq kg 1.4-DB eq

5072.98 786.29

−59.58 5013.40 −40.94 745.35

7.27 1.41

5080.25 787.69

t 1.4-DB eq

1385.83

−90.25 1295.58

3.16

1388.99

kg 1.4-DB eq kg C2H4 eq

• Case A: a conventional disposal in landfill for all the components • Case B: recycling of the 50% of the steel content in the TES system and as a disposal in landfill the rest of components.

24.32

−0.31

24.01

0.04

24.35

0.79

−0.10

0.69

0.004

0.79

However, in global terms, the second highest contribution in the Human Health can be attributed to the use of diphenylether compounds, ferronickel in Resources and ferrochromium in Ecosystem Quality. Diphenylether compounds are the main component of the HTF used in the TES system, while ferronickel and ferrochromium are implicated in the steel production process being the basic material in the tubes and vessel manufacturing. It has been reported that the production of diphenylether involves different materials and processes

3.2.1. Midpoint approach assessment As shown in Table 5 and Fig. 5, an environmental benefit in all impact categories for the end-of-life scenario (case B) is observed, except in ozone layer depletion category. The main reason to this situation can be found when the impacts avoided in the steel recovery are compared to those associated to the landfill of the rest of TES materials and other component contributions, especially in terms of Halon 1301 emissions, the value of the impact avoided being lower. On the other hand, it can be observed that global warming, abiotic depletion and photochemical oxidation are the main categories affected by environmental benefits associated to the recycling process. In fact, the most important reduction is observed in photochemical oxidation where approximately 13% of the impacts are avoided with respect to case study A. This is due to a decrease in CO2 and SO2 emissions generated during the steel production. Finally, the greater contribution, from a positive point of view, is based also on the reduction of CO2 emission from the combustion of fossil sources focusing on the global warming category and, in the case of the abiotic depletion, the environmental benefits came from the decrease of coal consumption associated also to the steel production. 3.2.2. Endpoint approach assessment Fig. 6 shows impact categories involved in endpoint approach for both end-of-life scenarios (Cases A and B). Case A has the higher damage in all categories when it is compared to Case B. A minor difference is observed in individual damage categories corresponding to the

Case A Case B Photochemical oxidation [kg C2H4 eq] Terrestrial ecotoxicity [kg 1.4-DB eq] Marine aquatic ecotoxicity [kg 1.4-DB eq] Fresh water aquatic ecotox. [kg 1.4-DB eq] Human toxicity [kg 1.4-DB eq] Ozone layer depletion [kg CFC-11 eq] Global warming [kg CO2 eq] Eutrophication [kg PO4--- eq] Acidification [kg SO2 eq] Abiotic depletion [kg Sb eq] 75

80

85

90

95

Enviromental impact (%) Fig. 5. Impact categories for both end-of-life scenarios: Case A and Case B considering the midpoint approach.

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Case A Case B Fossil fuels [MJ surplus] Minerals [MJ surplus] Land use [PDF*m2yr] Acidific./ Eutrophic. [PDF*m2yr] Ecotoxicity [PDF*m2yr] Ozone layer [DALY] Radiation [DALY] Climate change [DALY] Resp. Inorganics [DALY] Resp. Organics [DALY] Carcinogens [DALY] 85

90

95

100

Damage Percentage (%) Fig. 6. Impact categories for both end-of-life scenarios: Case A and Case B considering the endpoint approach.

Resources and Ecosystem. However, the most important reduction is found in the Human Health indicator, especially, in respiratory effects (inorganics), climate change and carcinogens categories. These categories are significantly affected by emissions produced during the steel production process. In fact, it has been reported that several common pollutants present in these emissions have an impact on human health in terms of DALY in which respiratory disorders, climate change and carcinogens effects are included (Hofstetter, 1998; Zhang et al., 2009). Nitrogen oxides, sulphur dioxide, particulates ammonia and sulphate were the most representative pollutants considered into the respiratory effects to calculate the impact of these emissions on human health. With regard to the climate change category, fossil carbon dioxide, nitrogen monoxide, methane fossil, carbon monoxide fossil and sulphur hexafluoride are the most representative contribution. Finally, carcinogens effects included some metals, such as arsenic, cadmium, chromium and other compounds such as particles or benzene. All those compounds, present in the emissions of the steel production process, had a significant reduction when Case B is analysed. This is because of the effect of recovering steel, since new raw materials extraction and processes for primary steel manufacturing are avoided. Therefore, this leads to a decrease of the emissions and consequently a remarkable reduction of the Human Health indicator observed in Case B. Additionally, Table 6 shows the main values of damage for both Cases A and B considering the aggregated endpoint indicators. In the scenario with steel recovery there is an implicit environmental benefit in the end-of-life-stage in Human Health (3.01E− 4 DALY) and Ecosystem

Table 6 Damage indicators for both end-of-life scenarios (Case A and Case B) and the main stages. Damage category

Human Health ×103

Ecosystem Quality ×10−2

Resources ×10−3

Unit

DALY

PDF*m2yr

MJ surplus

Total with recycling End-of-life with recycling TES tank Total w/o recycling End-of-life w/o recycling TES tank

4.010 −0.301 4.310 4.340 0.026 4.310

3.560 −0.020 3.580 3.630 0.050 3.580

4.040 0.070 4.034 4.100 0.007 4.030

Quality (2.05 PDF*m2yr) that causes a global decrease in the damage value of these indicators. This is especially important since some emissions mentioned previously are reported to affect the state of human health (Aunan et al., 1998). In fact, it was reported in 1989 that from epidemiological studies of mortality rates and air pollution, death rates due to respiratory and cardiovascular failure, increase relatively more than the total rate (Aunan et al., 1998; Derriennic et al., 1989). This is because of air pollution exposure, which enhances the probability of premature death of individuals in advanced stages of several common diseases. Therefore, reduction of emissions due to the recovery of the steel can be considered as an implicit environmental benefit in the end-of-life stage in the case of Human Health. Similarly an environmental benefit is associated to the Ecosystem Quality. A decrease in this indicator, especially in the categories land use and eco-toxicity, contributes to generate large benefits for the society, since environmental problems, such as endangerment of species, nitrification of lakes and rivers, loss of fertile soil, impact on landscapes' aesthetics and toxic chemicals on ecosystem health, are reduced. This is reflected in the decrease of the damage value of the Human Health and Ecosystem Quality indicators and therefore in important environmental benefits. Although globally this effect is positive, both disposals, the PCM and HTF, have a significant negative effect, which reduce the global environmental benefits or even cancel it as is depicted in Table 6 in the case of Resources category. On the other hand, when the scenario without material recovery is analysed, the highest contribution to the three damage categories is the manufacture of the TES tank accounting for the 99.4% of the total damage in Human Health, 98.62% in Ecosystem Quality and 98.21% in Resources. In addition, the disposals of PCM and HTF again have a higher negative impact in the final results, especially on respiratory effects and fossil fuels which agrees with the analyses of manufacturing/disposal impact of PCM carried out by Menoufi et al. (Menoufi et al., 2013). However, taking into account results in relative values, the total impact in the main indicators could be considered as not remarkable. In order to obtain an overall impact index, the mixing triangle technique is used (Hofstetter et al., 1999) (see Fig. 7). This triangle can be used to graphically depict the outcome of cases comparisons for all possible weighting sets by displaying y the result of an LCA using Ecoindicator 99. In the mixing triangle, each corner represents a weight of 100% for one damage category. For example, the top corner is the weighting combination where Ecosystem Quality is weighted 100%, and 0% weight is given to both Human Health and Resources. In this

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Fig. 7. Comparison of the global damage of the Case A and Case B by using the mixing triangle approach.

case, the default weighting set used for the Eco-indicator method has been applied that means Human Health 40%, Ecosystems 40% and Resources 20%. Although typically there are two areas that limit the conditions where both case studies are more advantageous with respect to the other, in the analysis depicted in Fig. 7 there is only one valuable option from an environmental point of view. Therefore, it could be noted that the case study where a steel recovery end-of-life scenario is considered is the suitable option in all cases. 3.3. Environmental approach considering the use phase and Net Zero Environmental Impact Times In order to analyse the time period needed for the benefits obtained, Net Zero Environmental Impact Times (NZEIT) were calculated in terms of damage impact modelling and including the use phase of the energy recovered by the TES system. To do this, the energy storage in the LH systems, which can be calculated by Eq (1), can be applied to evaporating water instead of burning natural gas in a low NOx emission condensing boiler. Therefore, the system avoids the loss of 840 GJ of thermal energy. Considering the entire life cycle (including the use phase), it is possible to determine whether the energy savings of conventional fuels during the operation stage are large enough to balance the environmental impact that originated in an industrial TES including manufacture, use and disposal phases. The calculation of NZEIT is a recent practice. Some authors, such us, Battisti et al. (Battisti and Corrado, 2005) denoted this parameter as pay back times from an environmental point of view, in contrast to economic payback time. The authors developed an extended environmental assessment of solar thermal collectors with integrated water storage. In addition to the LCA of the system, NZEIT were also calculated for both CO2 emissions and primary energy consumption. Given that the expected lifetime of the solar system was established at 20 years, very

short payback results from 0.5 to 1.5 years approximately were obtained. Although the considered systems are quite different in the research by Battisti (Battisti and Corrado, 2005) and this study, the expected lifetime of the system is also 20 years and payback times obtained are also quite short. As can be seen in Fig. 8, different NZEIT are found depending on the damage category and case study, although in all the cases the environmental benefits are lower in the case with steel recovery in the endof-life stage (Case B). In addition, Table 7 shows the specific NZEIT in terms of the environmental damage obtained. Considering that the energy saved by the LH is applied directly to avoid fossil fuel consumption, the Resources indicator shows the lowest NZEIT, with around 0.6 years. In Human Health, the payback time is slightly higher close to 4 years and finally, the Ecosystem Quality whose NZEIT is around 10 years. 3.4. Technical solutions for improving the environmental performance In line with the results obtained in this study, a matrix of possible technical solutions is displayed in Fig. 9 to improve the environmental performance of the TES system. Processes and compounds evaluated have a specific impact in the environmental assessment. The analysed aspects with the most impact are the maritime transport media, the suitable selection of the end-oflife stage and the use phase. In addition, compounds such as the diphenylether used as HTF or steel to produce the tubes and vessel are also particularly relevant in the damage results. Therefore, it is possible to establish a list of alternative options or decision rules to improve the design and management of the TES from an environmental standpoint. Thus, as observed in Fig. 9, possible options can be to change the transport media, search for a new location for the production of sodium nitrate, change the materials used in the TES system, increase the material recovery ratio and also consider other heat generation processes based on fossil fuels where the energy storage by the PCM could be applied, among others.

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preliminary environmental or technical evaluation to be classified as a benefit from an environmental point of view. This is the case for changing the materials used in the manufacture of the TES. The selection of another HTF should be subjected to the physical and thermal properties of the new material and its environmental performance compared to the current option. 4. Conclusions

Fig. 8. Representation of the damage indicator values as a function of the time a) Human Health, b) Ecosystem Quality and c) Resources.

Some of these alternatives directly represent an advantage from the an environmental point of view, such as higher material recovery ratios in the end-of-life stage or using the energy saved in thermal processes based on fossil fuel consumption. However, other cases require a

Table 7 Time period needed for the benefits obtained from an environmental point of view.

Case A —NZEIT Case B —NZEIT

Unit

Human Health

Ecosystem Quality

Resources

Years Years

3.91 3.64

10.05 9.85

0.59 0.58

In this research, a TES system including the manufacture, use and end-of-life stages was environmentally evaluated using LCA methodology with CML 2001 and Eco-indicator 99 methods to assess midpoint and endpoint approaches, respectively. In addition, two end-of-life scenarios were considered, with and w/o material recovery. Considering only the TES manufacture stage, although PCM entails the highest impact in almost all the midpoint categories, HTF showed to have a greater impact in human toxicity, being around 64.56% of total impact contribution in human toxicity. However, particularly in the global warning category, PCM revealed the highest contribution with approximately 76% of the total CO2 equivalent released. This is because of the high CO2 emissions associated to both maritime transport and the electric energy consumed in the material production. On the other hand, the main midpoint categories affected by environmental benefits associated with recycling of steel components (Case B) are global warming, abiotic depletion and photochemical oxidation, being the highest reduction in photochemical oxidation (around 13% respects to Case A). The analysis of the endpoint approach assessment showed that the disposal of all components in the landfill (end-of-life scenario without recycling) has a negative impact on the final results. The manufacture of the TES tank has highest contribution to the damage categories (around 98% for Ecosystem Quality and Resources and 99% for Human Health). Nevertheless, environmental benefits in the disposal stage were found when 50% of the steel used is recovered in Human Health an Ecosystem Quality although they were not enough to compensate TES manufacture. This fact was supported by using the mixing triangle technique, which concluded that in all possible combinations of the three damage indicators, the best behaviour is obtained when the material recovery is included. Additionally, it was found that the use of energy storage in the LH systems prevented the loss of 840 GJ of thermal energy and analyse the use phase of the TES. In this regard, the NZEIT of the Human Health and Resources are less than 50% of the lifetime of the TES system, whereas Ecosystem Quality requires approximately 50% of the lifetime to compensate the environmental impacts. These results indicate that the energy saving of conventional fuels during the operation stage is large enough to balance the environmental impact stemming from the cradle-to-grave perspective of the TES concept. Finally, a matrix of possible technical and environmental improvements to the TES system was designed, considering the most relevant processes or compounds affecting to the global environmental damage. The manufacture of the PCM was the most remarkable process together with other aspects such as, the maritime transport involved in that stage, the fuel oil associated consumption, and the use of diphenylether as HTF. The main improvements included the change of materials in the manufacture of the TES or the rise in the material recovery ratios and the selection of another HTF to increase the positive impact of the end-of-life stage. Acknowledgement This paper has been developed in the framework of the ‘New Designs of Ecological Furnaces’-EDEFU project (grant agreement 246335) co-financed by the European Commission under the Seventh Framework Programme for Research (FP7).

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Relevant processes or compound Possible alternatives

Maritime transport

Change the transport media

EER

Search new location where sodium nitrate is produced

EER / TER

Change the material

EER / TER

HTF

End-of-life stage

EER / TER

Increase the ratio of material recovery Heat used in thermal processes based on other fossil fuels

EE - Environmental Evaluation Required

Steel material

Use phase

EER / TER improving improving

TE - Technical Evaluation Required

Fig. 9. Matrix of possible technical and environmental improvements in the TES system design.

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