Dish-Stirling technology for power generation. Environmental evaluation.

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1 Dish-Stirling technology for power generation. Environmental evaluation. Bravo Y., Carvalho M., Serra L.M., Monné C., Alonso S., Moreno F., Muñoz M. Aragon Institute of Engineering Research (I3A) - Department of Mechanical Engineering Universidad de Zaragoza, Spain. ybravo@unizar.es, mcarvalho@unizar.es, serra@unizar.es, cmmb@unizar.es, sealonso@unizar.es fmoreno@unizar.es, mmunoz@unizar.es ABSTRACT The dish-stirling technology for power generation, using a Stirling engine fed with a renewable energy such as solar, means a promising development regarding electricity generation. The efficiency value, around 30% of normal direct solar radiation converted to electricity, is the highest when compared with other solar energy generation systems. As for comparison with other solar energy exploitation alternatives, the environmental evaluation of the dish-stirling technology must be taken into account. The aim of this paper is to provide a comparative environmental assessment of dish-stirling technology with respect to a similar photovoltaic facility. Life Cycle Assessment procedure has been used. The results have been analysed in terms of CO 2 emissions and using two impact evaluation methods: Ecoindicator- 99 and CML2. It has been obtained that the level of environmental impact is similar for both technologies. INTRODUCTION The so-called renewable energy sources play an important role regarding future long-term sustainability for energy production and consumption. Nowadays, an increasing development of the technologies associated to exploitation of renewable energy sources is occurring, due to conventional fossil fuel energy sources depletion, together with environmental degradation caused by their use. In particular, the electric energy generation represents one of the highest consumption of fossil fuels and emissions generation. If this fact is considered together with the rising trend for electric energy consumption for the coming years [1], a major concern for policymaking regarding energy issues is the evaluation of risk to the environment connected to energy production. Thus, one of the key tools for policymaking is the environmental evaluation, in such a way that this evaluation is used to promote environmental friendly energy sources. When selecting and planning the energy technologies to be implemented and promoted, not only local inputs should be considered but also global environmental loads with a broad perspective. In this respect, the environmental evaluation must take into account all the life phases for energy production and consumption and, particularly in the systems for renewable sources exploitation, the design of the facility itself will be a major concern. According to literature review, the environmental evaluation for electricity production is favourable for renewable energies when compared to conventional sources (fossil and nuclear), as it is stated in several studies carried out where the emissions of CO 2 per kwh of produced energy are analysed [2]. The nuclear energy shows also adequate values of CO 2 emissions per kwh, though other issues must be taken into account regarding this energy source, such as security Corresponding author

2 and residues treatment. Generally speaking, the emissions associated to conventional energy sources occur mainly in the production phase, while systems based on renewable energies do not cause impact during operation. In these facilities, the most significant environmental impact is associated to construction, transportation and dismantling of power stations and equipments. Regarding renewable energies, the environmental evaluation, expressed as CO 2 emissions per kwh, coming from existing data in literature is, in order from the lowest environmental impact, wind energy, hydraulic and solar thermal, followed by biomass and, at last, solar photovoltaic [2]. Thus, it is assessed that solar thermal energy should be priorized in relation to solar photovoltaic for locations where sun energy has a high potential for use for electricity generation. The dish-stirling technology is one of the solar thermal technologies of high concentration, together with central receiver and parabolic trough technologies. The three of them use the concentrated power of sun to produce energy by means of different power cycles. In the case of dish-stirling a dish receiver is used to collect and direct the sun rays in such a way that the concentrated sun energy is transferred to the fluid inside a Stirling engine. This engine is the responsible for producing the power that will be converted to electricity. In spite of the fact that the dish-stirling technology is not a long-term proven and mature technology [3], the high potential comes from its high efficiency of around 30% of normal direct isolation into electric power and its modularity consisting on units varying from 7 to 25 kw [4]. Thus, it could play an important role for future systems of distributed energy where it could be competitive with current commercial systems, such as photovoltaic and diesel generation [5]. The use of the dish-stirling technology for big scale applications is also an objective for some industrial companies involved in its development, whose objective is related to cost decrease for energy production up to levels around 0.08 /kwh [6], that are in line with objective values around 0.06 /kwh of energy prices fixed by DOE (Department of Energy) of USA [7] for In addition to these issues, the environmental evaluation is a relevant information, particularly when comparing the dish-stirling technology with other renewable energy based technologies potentially used in the same conditions, such as photovoltaic energy. Though there is already information assessing this comparison [2], these studies have not been carried out under the same perspectives and considerations, such as location and size of the facilities compared. In addition to it, the environmental evaluation has been presented focused on CO 2 emissions. Of course, this is a major environmental criterium due to its contribution to global warming, one of the main environmental problems and threats for humankind. However, a complete environmental evaluation must take into account further aspects, so in this work tools such as Eco-indicator 99 and CML 2 impact evaluation methods have been considered, where 11 and 10 impact categories respectively are calculated covering a variety of environmental aspects further than only CO 2 emissions. Thus, the objective of the current study is to evaluate the environmental impact of a dish-stirling facility of 10 kw used for distributed energy, and to compare it with a similar photovoltaic facility by using a broad variety of environmental aspects. The same procedure and database for systems characterization are used for both technologies, so the results are useful to establish a reliable comparison between them by using the same environmental damage indicators. The main restriction for the analysis carried out is the systems description, since the availability of data for the dish-stirling system is limited due to confidentiality reasons. To achieve the afore-mentioned objective, the Life Cycle Assessment (LCA) procedure has been used, in order to analyse the whole range of environmental and damages associated to products and services. In general, the LCA covers the whole process of production and use of the product or service providing a comprehensive view of the environmental impact. It is used to determine the input flows of material and energy, through the definition of needed

3 components and technical processes, from the raw material extraction, intermediate and final manufacturing processes, packaging, transport and use, to disposal. The LCA procedure has been standardized by the International Organization for Standardization (ISO) in the ISO series [8, 9]. It consists of four phases: goal and scope definition, Life Cycle Inventory (LCI) analysis, Life Cycle Impact Assessment (LCIA) and interpretation of results. The inventory analysis consists of collecting the whole information regarding inputs and outputs of material and processes, in such a way that it is implemented in a model to obtain the environmental impact. A result of the inventory is the evaluation of CO 2 emissions that has been used also for comparison of the technologies. The impact on a whole variety of environmental aspects is valued with a specific impact evaluation method. The units where impact is valued depend on the method used, that result in different impact categories. In the case of study, the Eco-indicator 99 and CML 2 methods have been used. In the interpretation, the most important contributions are studied, together with sensitivity analysis to qualify the results and conclusions. This paper is organised in two main sections: literature review and the comparative Life Cycle Assessment (LCA) of dish-stirling versus photovoltaic facility. The first section is useful to analyse methods for environmental evaluation and results obtained so far, so the current study intends to overcome some limitations found. It is also useful to establish comparison with the current state-of-the-art of environmental evaluation for renewable energy technologies. The second section describes the LCA procedure, including data about modellisation of the systems and finally focusing on results obtained and its interpretation. LITERATURE REVIEW Previuosly to Lyfe Cycle Assessment, a literature review has been carried out in order to know current status of analysis performed for the facilities of this study, dish-stirling and photovoltaic systems. The main objective is to get information related to these technologies about description of the systems, tools and results of LCA that come from similar studies done from a variety of researchers. Most of the information found is related to photovoltaic energy, mainly for silicon technology that is the one more used nowadays. There are also studies including new technologies for photovoltaic systems, such as thin film or alternative materials to silicon. The information related to dish-stirling systems is more limited (only one study has been identified) probably due to the current status of this technology, closer to demonstration than to serial production. The most relevant information found is detailed next. The most general work found related to energy sources environmental evaluation is a review for the whole range of power generation technologies that presents the values for CO 2 emissiones per kwh produced (units: g-co 2 /kwh) [2], including both solar thermal and photovoltaic technologies. This article gathers information for different technologies in studies carried out by different researches. Thus, there is a limited reliability in values comparison, since these studies are not made with the same procedures and do not take into account the same considerations. However, having a magnitude for all the power generation technologies is an interesting starting point for any further analysis. For example, it enables to establish a relative order between conventional fossil fuels and renewable energies. One first result is that for renewable energies, there is a very low contribution of the operation phase and the emissions are produced mainly during the phase of equipment construction and installation. This is just the opposite of what happens with conventional fossil fuels in which the emissions and environmental loads provoked during the phase of equipment construction and installation is about one order of magnitude lower than the environmental burden associated to the operation phase. This is one of the assumptions that have been considered for the present study, in order to assess that the operation phase is negligible in relation to construction. The table 1 shows the summary values for all the systems considered in this

4 review [2]. The general trend shows favourable values for renewable energies, though the ranges are quite wide for some of these technologies. It depends on different factors. In the case of wind energy, for example, the minimum values correspond to small installations without specific information about utilization factor. As for photovoltaics, it depends on the technology used and degree of advancement of this technology, and also on the location, since it has a strong relationship with irradiation values and therefore with system efficiency. From the values of this review, it can be noticed that the constant evolution of photovoltaics is leading to lower environmental impact. In particular, the minimum value presented for this technology (53.4 g-co 2 /kwh is for amorphous silicon technology in a study performed at Japan [10]. Conventional systems g CO 2 / kwh Coal fired Oil fired Gas fired Nuclear 24.2 Renewable systems Wind Hydro Biomass Solar PV Solar thermal Systems: Paraboloidal dish 13.6 Central receiver 43 Parabolic trough 196 Central tower 202 Table 1. Comparison of power generation technologies, conventional and renewable systems (unit: g-co 2 /kwh) [2]. The dish-stirling technology is included in the solar thermal systems. A detailed attachment is presented for solar thermal systems in table 1. The minimum value of the range for solar thermal technologies corresponds to dish-stirling technology for a facility of 1 MW (13.6 g- CO 2 /kwh). So, this review enables to foresee the potencial of dish-stirling technology to minimize environmental impact in relation to photovoltaics. As previously mentioned, this review is useful for first analysis, though it must be taken into account that the values collected correspond to different facility characteristics and sizes that can be determinant for the environmental evaluation result. Additionally to this review, a more detailed literature analysis has been made both for photovoltaic energy and dish-stirling technology. As for photovoltaic energy there are studies where specific information is presented specially regarding the different technologies. However, for dish-stirling technology only one study has been found, corresponding to that already referred in the previous review. The studies concerning photovoltaic systems have been useful to gather information about softwares, databases, impact evaluation methods and types of facilities analysed. In the work of Mohr et al. [11] a comparison is established for multicristalline silicon and thin film technologies (GaInP / GaAs gallium indium phosphide / gallium arsenide). The tools used for this study are SimaPro software, Ecoinvent 2.01 database, and CML 2001 impact evaluation method. The functional unit deals with a facility of 1 kwp with an irradiation of 1000 kwh/m 2 in Western Europe and a lifetime of 30 years. The phases included in the

5 analyses are production, maintenance and disposal. The results of this study are presented in the 10 impact categories associated to CML 2001 method: abiotic depletion, acidification, eutrophication, global warming, ozone layer depletion, photochemical oxidation, human toxicity, freshwater aquatic ecotoxicity, marine aquatic ecotoxicity and terrestrial ecotoxicity. The results obtained for both technologies are quite similar. Besides, it is also analysed the use of electricity coming from PV modules for life cycle of the photovoltaic energy, instead of using conventional electricity. It means a favourable impact regarding the following impact categories: global warming, abiotic depletion, acidification and photochemical oxidation. However, the reductions in toxicity are negligible. Alsema et al. [12] compare also silicon and thin film technology (CdTe cadmium telluride) at present and future expectations. The environmental impacts results were obtained by a full Life Cycle Assessment, using the software SimaPro 7 and the database Ecoinvent 1.2. The greenhouse gas emissions were evaluated by means of the IPCC 2001 GWP 100a method (v. 1.02). The photovoltaic systems are located at middle and south Europe (irradiation values varying from 1000 to 1700 kwh/m 2 /year), with a lifetime of 30 years and a system performance ratio of The results are presented as greenhouse emissions (g-co 2 /kwh) and energy pay-back time (EPBT). A comparison is also included for conventional power generation systems regarding greenhouse emissions. As for photovoltaics, the greenhouse gas emissions are now in the range of g-co 2 /kwh (25 for thin film technology and 32 for crystalline silicon) and this value could decrease to 15 g-co 2 /kwh in the future for both technologies whereas the values for conventional power generation systems varies in the range of 120 to 400 g-co 2 /kwh [12]. The emissions associated to photovoltaic systems are mainly due to construction processes. Therefore PV energy systems have a very good potential as a low-carbon energy supply technology, though further efforts should be made for reduction of energy required for silicon processes, that is in production of the solar modules. The development of thin film technologies represents the major potential of photovoltaic systems for the future. As for EPBT, crystalline silicon PV systems presently have energy pay-back times of years for South-European locations and years for Middle-European locations [11]. For silicon technology clear prospects for a reduction of energy input exist, and an energy pay-back of 1 year may be possible within a few years. Thin film technologies have energy pay-back times in the range of years (South Europe) [12]. The paper of Fthenakis and Kim [13] presents the most updated review of information regarding environmental impact for photovoltaic energy, with a comprehensive overview of systems description and studies carried out so far. The results analysed are mainly expressed in terms of energy pay-back time (EPBT) and CO 2 emissions, and show comparison for the different photovoltaic technologies, at present and future expectations. The results have in common the following considerations: Southern European isolation of 1700 kwh/m 2 /year, a performance ratio of 0.75 and a lifetime of 30 years, with the exception of thin film technology (CdTe) that considers US isolation of 1800 kwh/m 2 /year and a performance ratio of 0.8. The results show that at present the values of EPBT and CO 2 emissions are favourable for thin film technology, with values around 0.7 years of EPBT and 18 g-co 2 /kwh, in relation to values around 2.4 years of EPBT and 37 g- CO 2 /kwh for monocrystalline silicon technology, the most used nowadays. Multicrystalline and ribbon silicon present medium values in relation to these two technologies [13]. However, future evolutions of photovoltaic technologies show that both the values of EPBT and CO 2 emissions could decrease significantly and, what is more, the different technologies, both silicon and thin film, will present similar values around 0.5 years of EPBT and 10 g-co 2 /kwh [13]. Apparently, the PV industry is developing cost savings simultaneously with advanced performance, which translates into decrease of environmental impact. In fact, the conversion efficiency, material usage, and production energy efficiency of both Si and CdTe PVs are improving rapidly [13]. So, the environmental evaluation of these systems should be

6 frequently updated in order to follow this evolution. Other studies are focused on analysing the phases with higher impact on PV technology, such as the work of Stoppato [14], where an analysis is made for multicristalline silicon technology. The LCA software used in this case is Boustead Model V5.0. In this study, the environmental impact for the construction of a photovoltaic panel of surface 0.65m 2 is analysed. The paper includes detailed information about the phases of construction. The impact associated to transport, installation and operation are not taken into account. The lifetime considered is 28 years. The environmental impact has been calculated for the following factors: energy necessary for construction (1494 MJ/panel, GER Gross Energy Requirement ), greenhouse effect or global warming potencial (80 kg- CO 2 per panel, GWP Global Warming Potencial ), and energy pay-back time (6.5 years, EPBT Energy Pay-Back Time ). An operation analysis has been performed, in order to show the most significant phases for GER and GWP values, resulting in conversion to solar silicon and panel assembling. Sensitivity analysis can be performed to analyse construction and operation alternatives for photovoltaic energy, as in the work of Kannan et al. [15]. This analysis uses a functional unit of 2.7 kwp monocristalline silicon system located at Singapore. The results are given in terms of energy required, energy pay-back time (EPBT) and CO 2 emissions. The alternatives analysed are: reduction by 50% of energy required for construction, replacement of metallic support by concrete, and increase of efficiency. The most significant reductions are for the first alternative, that is energy use for manufacturing of solar PV reduced by 50%, resulting in reductions of energy, EPBT and CO 2 emissions from 2.91 to 1.72 MJ t /kwh e, from 5.87 to 3.48 years and from from 217 to 129 g/kwh e respectively. Of course, the implementation of the three alternatives means a further reduction in the three results analysed. Only one study has been found for specific analysis of dish-stirling technology [16]. A facility for power mass production of 1 MW, located at Italy, consisting of 17 solar dishes generating steam to drive 5 Stirling engines to produce energy, with a rate of conversion efficiency from solar to electrical energy of 18%. So, in this case the facility is different from the one analysed in this paper, where each dish is coupled to a Stirling engine forming an isolated unit with conversion energy efficiency around 30%. The tools used in this study for dish-stirling evaluation [16] are SimaPro software, ETH-ESU 1996, IDEMAT 2001 and BUWAL 1996 databases, and Eco-indicador 99 (EI-99) impact evaluation method. The results presented correspond to the impact categories: carcinogens, respiratory organics, respiratory inorganics, climate change, radiation, ozone layer, ecotoxicity, acidification, land use, minerals and fossil fuels. The analysis is made considering the facility construction with and without recycling. The phases included are related to: the construction of the plant, the manufacture of the big dish solar collectors and the Stirling engines, the extraction and supply of consumables, transport, and the decommissioning and recycling of the power plant. The environmental effects are valued in terms of CO 2, SO 2 (acidification), CFC11 (ozone layer), PO 4 (eutrophication), SPM (winter smog), C 2 H 4 (summer smog) and solid waste. As a comparison with results associated to similar estudies, the g-co 2 /kwh must be pointed out: 7.3 and 13.6 with and without recycling. The damage evaluation is presented in the three categories considered in the EI-99 method: human health, ecosystem quality and natural resources consumption. In the contribution of the individual process stages, the dish-plant construction means the most significant phase with a major difference with other phases. The recycling means a lower contribution, but it presents also a relatively important value. As a general conclusion, it is remarked that the environmental impact associated to the whole life cycle is negligible in relation to traditional fossil fuels power stations. It is also pointed out that the results obtained are rather encouraging and deserve to be studied further in particular to develop a comparative study with other thermodynamic solar technologies that use solar concentrators.

7 DESCRIPTION OF THE ANALYSED FACILITIES As previously presented, the dish-stirling system is one of the solar thermal technologies for power generation. In this system, the sun rays supply the energy that makes the Stirling engine works. A parabolically curved concentrator reflects the parallel solar rays and gathers them into one focal point. The dish moves along the day in order to follow the sun trajectory, so the tracking system is one of the key elements in the dish-stirling technology. A heat exchanger located at the focal point of sun concentration absorbs the radiation, thus heating the thermal transfer fluid of the Stirling engine. The Stirling engine transforms the heat into rotational energy and then into electrical power through a generator attached directly to the crankshaft of the motor. The essential components of a Dish-Stirling system are the concentrator, the Stirling engine with a solar receiver and the solar tracking device. The concentrator, that is the paraboloidal dish, is supported by a metallic structure fixed at the ground with concrete to provide the system with the necessary stability to guarantee a good focus of concentrated radiation. The dish-stirling technology has been used in several research and development projects with a number of demonstration facilities supported by different companies and research bodies around the world [17]. The systems vary from 3 to 50 kw per unit. For the current study, the system of the company SBP of 10 kw (Schlaich Bergermann und Partner) has been selected, since there were availability of information for its description for Life Cycle Assessment procedure. A system of SBP is presented in figure 1. The main characteristics of the different components of this sytem are described next [3]. The concentrator, also called dish receiver, consists of twelve independent segments of glass fibre. The segments are assembled in a paraboloidal shape externally tensioned to guarantee the proper shape. A layer of thin glasses is added in order to obtain a high reflectivity of around 94%. The maintenance of the dish-sitrling facility includes the cleaning of the concentrator to guarantee the maximum efficiency. The Stirling engine together with the alternator is the unit which converts the concentrated solar energy into electricity. The engine efficiency is around 30%. The SBP facility uses hydrogen as working fluid for the Stirling engine. The engine used for this facility is SOLO-161 of the german company SOLO KLEINMOTOREN GMBH. A current concern for this facility is the leakage level that makes necessary a periodic replacement of the working fluid. However, the big scale application must be designed in order to minimise these leaks. The solar receiver is the heat exchanger which is mounted on the Stirling engine to transfer energy from the sun concentration to the working fluid of the Stirling engine. The concentration of the sun is made through a ceramic cavity to receive the sun rays reflected from the concentrator. The heat exchanger can be of direct illuminated tubes or reflux type using a liquid metal as intermediate fluid for heat transfer (heat pipe). In particular, the SBP facility uses a direct illuminated tubes type where the sun radiation impinges on a tubes bundle where the working fluid of the Stirling engine flows inside. The design of the heat exchanger consists of 78 tubes of Inconel 625 with an external diameter of 3 mm joined by brazing process. The system is supported by a metallic structure made of steel bars with an approximate weight of 3700 kg. The concrete settlement where the system is installed consists of around 150 ton.

8 Figure 1. SBP system [6] As for the photovoltaic system, a facility of 10 kw of monocrystalline silicon technology has been selected. It has been considered a typical installation at roof. The system analysed includes the photovoltaic panels, the electric installation and the AC/DC converter. The definition of this facility is the one corresponding to the description found in the Ecoinvent database, version 2.04 [18]. LIFE CYCLE ASSESSMENT Through the Life Cycle Assessment analysis, the embodied energy and emissions are calculated and translated into environmental and social effects. These relevant aspects of renewable energy development are valued in terms of the results provided by the software SimaPro v.7.3 [19], a specialized LCA tool. The analysis consists of a whole inventory of products and processes involved that are valued in terms of emissions and energy, and a later impact evaluation that translates environmental effect into valuable impact categories. The present study is mainly focused on comparison of these impact categories. SimaPro was used to calculate the impact associated with the production and final disposal of the dish-stirling and photovoltaic facilities.the software includes several inventory databases with thousands of processes, plus the most important impact assessment methods. A former step for LCA is the selection of the database and the impact assessment method because results are dependant on this decision. As for the database, it is important to select an updated version of a database containing the more accurate and representative information possible concerning both material and processes. If the inventory of the system is all referred to the same database the results obtained are more reliable, since it means coherence and uniformity in the criteria for emissions and resources calculation. So, a preliminary analysis of the database is needed in order to verify if it is complete enough to define the systems under study, in this case the dish- Stirling and photovoltaic systems. The Ecoinvent database [18], version 2.07, has been used, since it is considered one of the most complete and updated commercially available databases for LCA. So, the inventory results (emissions and resources employed) are referred to Ecoinvent database. In particular, the CO 2 emissions will be analysed in order to compare with the literature review presented in the previous chapter. As for the impact assessment method, it must be pointed out that the results of LCA are given in function of indicators corresponding to the impact assessment method used. These methods utilize different environmental criteria and therefore evaluate and assess different environmental aspects. The methods differ in quantifying the relationship cause-effect from

9 emissions obtained from the inventory phase to environmental impact. For this study, the Eco Indicator-99 (EI-99) [20] was firstly selected because it is widely used in LCA, incorporating relevant environmental burdens into different impact categories, which in turn allow the evaluation of damages to human health, ecosystem quality, and resources. Furthermore, the LCA results of EI-99 can be aggregated into an easily understandable number, the Single Score. According to ISO [21] terminology, the EI-99 is considered as an end-point level method, since it calculates the damage categories (human health, ecosystem quality and resources) by means of a damage model from the impact categories. The EI-99 method considers the values of eleven impact categories at intermediate level in the environmental evaluation, a phase known as characterization. Later implementation of damage models enable to aggregate the eleven impact categories into three damage categories (human health, ecosystem quality and resources figure 2-). The damage models include different analysis for calculation: fate, exposure, effect, resource and damage analysis, evaluating the impact caused by emissions and resources depletion. The normalization of the values obtained for the three damage categories means obtaining isodimensional values for these categories, in such a way that it is possible to establish comparisons. The normalized values are expressed in function of Eco Indicator points that can be interpreted as one thousandth of the annual environmental load of one average European inhabitant [20]. Furthermore, these values can be weighted adding social perspectives. This method enables also the aggregation of the damage categories into an index, the Single Score, which represents the overall environmental load in points. In order to account for the subjectivity of the impact assessment procedure, EI-99 presents three different perspectives (Egalitarian, Individualist and Hierarchist versions), that differ mainly in the assumptions considered for the models and in the time perspective. They have different impact perceptions, different normalizing factors and weights and thus, lead to different results. For the present study, the Hierarchist version of the damage model was selected since it is assumed that it is the closest to scientific criteria, with a balanced perspective in time [20]. Figure 2. Model of impact evaluation method Eco-indicator 99 [22] In addition to Eco Indicator-99, also the CML 2 impact assessment method has been used, also widely used for LCA. This method was developed by the Center for Environmental Studies of Leiden University, Holland. The CML 2 is considered as a mid-point level method, since it uses only impact categories, without applying any further calculation to obtain damage categories. The CML 2 method considers 10 impact categories, presented in

10 table 2 (different from EI 99 method), but there is no further calculation for damage categories ( mid-point level method), so it ends at characterization step. In this case, the normalization can be applied at impact category level, thus enabling also comparison between the different categories. The reference for this normalization is the damage caused by a region at a given year. In the software used, four normalization alternatives are presented: Holland 1997, Western Europe 1995, World 1990 and World The latter has been selected for this work, since it presents a more global approach. The CML 2 method does not establish dependence on social perspectives. Abiotic resources depletion Global warming Stratospheric ozone depletion Human toxicity Freshwater aquatic ecotoxicity Marine ecotoxicity Terrestrial ecotoxicity Photo-oxidant formation Acidification Eutrophication Table 2. Impact categories of CML 2 impact evaluation method The use of two different impact assessment methods, Eco Indicator-99 [20] and CML 2 [23] has enabled to assess the conclusions for the current study valued according to different evaluation criteria. Modelling One of the most important issues in LCA is the considerations made about the facilities objective of the study, in this comparative analysis are considered a dish-stirling facility of 10 kw for distributed generation and a 10 kw photovoltaic installation. The dish-stirling facility [6] has been defined in the software through its main equipments, in such a way that a complete inventory of material and processes has been collected. These data have been collected from information available in the different documentation analysed and also from the SBP (Schlaich Bergermann und Partner) [6] demonstration facility at Odiello (France). However, it has not been possible to get this information from constructors due to confidentiality reasons. That is also the reason why there are not data available about operation and maintenance. Thus, only construction and disposal (including assembling and transport) have been considered for life cycle phases. This consideration is supported by results obtained in the literature review, where it is stated that construction is the most significant phase, together with disposal [15]. Nevertheless it must be taken into account that the novelty of this technology includes an uncertainty factor in relation to reliability and maintenance of the Stirling engine [3]. As for photovoltaic energy, the information has been taken from the database selected, Ecoinvent [18]. The technology selected has been monocrystallyne silicon. The silicon technology is the most used currently, and the monocrystallyne silicon has a good efficiency together with a reasonable cost, also in continuous decrease due to production issues. It is also assumed that operation impact is negligible. The lifetime for both facilities has been considered of 30 years [14], and all the materials are recycled at disposal to foresee future use of the components. This is due to the fact that previous studies also include recycling [14]. In any case, the analysis performed

11 shows that there is not a significant impact if recycling is not included regarding comparison of the two facilities, showing similar values lower than 15% of impact for both cases. As for modellisation, the basic elements for dish-stirling facility definition are: engine, dish receiver (made from glass fibre), metallic structure (construction steel) and concrete for settlement. The concrete where the metallic structure is supported has a strong implication concerning the operation of the dish-stirling unit due to stability. This is a significant issue to guarantee the proper operation of the tracking system that follows the sun rays to provide the proper shape and position of the concentrated sun energy. For the elements of the most complex component, the engine, the description of SOLO-161 engine [24] has been considered, that is the one used in the SBP system. The Stirling engine consists basically of the following subcomponents: heat exchanger, expansion cylinders and pistons, connecting rods, electric generator and regenerator. The materials used in the Stirling engine include different types of steel, copper and nickel alloys. The heat exchanger is also joined to a ceramic cavity for sun rays caption. In addition to material, the production processes used include cold and hot work for metallic materials, joining techniques (welding and brazing) and general product manufacturing technologies. The transport has been included for each material, but not for the whole facility since it is considered that the assembly is done at the site itself. The disposal has been defined for metallic, glass, ceramic and plastic materials. The table 3 presents a summary of materials and manufacturing processes for the materials of the components of the dish-stirling facility. Material (s) Quantity (kg) Process (es) Concrete settlement Concrete 150,000 Metallic structure Steel 3700 Cold working Dish receiver Glass fibre Engine Sub-component Cold working Casting Electic welding Heat exchanger Nickel alloy 12 Brazing Cold working Forging Cylinder / piston Steel 200 Machining Connecting rods Steel 100 Cold working Steel 50 Cold working Electric generator Copper 50 Casting Regenerator Steel 30 Cold working Ceramic cavity Ceramics 15 Table 3. Summary of definition of materials and processes for the dish-stirling system. As previously mentioned, the photovoltaic system has been characterized through the data available in the Ecoinvent database for monocrystalline silicon technology. The inventory of the system includes cells construction, photovoltaic panel, electric installation and AC/DC converter, in addition to a typical installation at roof. The transport and energy necessary for processes are also included. The corresponding data have been taken to define a facility of 10 kw. The data taken from Ecoinvent correspond to the system of a photovoltaic facility of 3 kwp, facade installation, single-si, laminated, integrated, at building. Thus, the components have been taken to build a facility of 10 kw: inverter, electric installation, facade construction, photovoltaic panel single-si, and transport.

12 Life Cycle Assessment results The results of LCA are inventory and impact evaluation results. The calculation conditions that have been considered for these results are the following: functional units of dish-stirling and photovoltaic facilities of 10 kw useful power of electricity with a lifetime of 30 years, taking into account the construction of the facility but not the operation phase that is considered as negligible. For both facilities, recycling of materials used for construction is considered. Life Cycle Inventory (LCI). The inventory results are calculated through addition of effects (emissions and resources) caused by all the materials and processes involved in the definition of the facility. These effects represent the environmental outputs, corresponding to different categories, such as gaseous, liquid or solid emissions. The values obtained depend on the database used in characterization of material and processes. In the case of study, four gaseous emissions have been considered: CO 2, SO x, NO x and volatile organic compounds different from methane (NMVOC). As previously verified in literature, CO 2 emissions are a major concern due to its greenhouse effect. These emissions are used frequently as a basis for environmental evaluation. So, this value will be used for comparison with other studies that analyse dish-stirling and photovoltaic facilities. The rest of gaseous emissions have been selected due to their implication in different environmental effects. The SO x emissions are responsible for acid rain, a complex chemical phenomenon that takes place when sulphur and nitrogen compounds emissions react with other substances in the atmosphere. This phenomenon happens normally far away from the sources and lay down on the ground as wet (rain, snow or mist) or dry (acid gases or particles) depositions. The NO x is the main component of the photochemical smog, related to ozone layer degradation, also responsible for acid rain. The NMVOC are compounds that are formed mainly by hydrocarbons and some chemical groups, such as alcohol, aldehides, alkanes, aromatics, cetones and halogen derivates. They can be extremely dangerous for human health depending on the composition and toxicity. Besides, when they are mixed with other atmospheric pollutants, such as NO x, react with the sun and form ozone at ground level, what is toxic for human health, contributing to photochemical smog. The results associated to inventory, that is, the environmental outputs are presented in the table 4. System STIRLING PHOTOVOLTAIC Carbon dioxide g CO 2 /kwh Sulfur oxides g SO x /kwh Nitrogen oxides g NO x /kwh Volatile organic compouns different from methane g NMVOC/kWh Table 4. Results of Life Cycle Inventory. The values of the gaseous emissions selected are presented in terms of emissions per kwh produced per each facility. The values of kwh take into account the lifetime assumed for each facility, 30 years for both cases. The order of magnitude of these emissions is the same for both facilities. The value of emissions of CO 2 is favourable for the photovoltaic facility, though the difference is not

13 clearly significant, especially if we take into account the uncertainties in the definition of the systems. So, modifications in these definitions could alter this favourable balance. As for the rest of emissions, it is generally favourable for the dish-stirling facility but also without a significant difference. The results of literature show values also with the same order of magnitude, though the difference is higher for dish-stirling facility. For example, for photovoltaic facility, the current value of CO 2 emissions per kwh for monocrystalline photovoltaic technology presented in [13] is 37, with a foreseen evolution to future values around 10. So, the value obtained of g-co 2 /kwh is in good accordance. The difference is higher for the dish- Stirling facility, with a value presented in [16] of 7.3 in comparison with the value of obtained in the present work. It must be pointed out that, apart from the differences in tools used, the definition of the system is significantly different, especially in size (1 MW with 17 solar dishes and 5 Stirling engines driven by steam in relation to the facility of 10 kw wih 1 solar dish that feed directly to 1 Stirling engine used in this work). In fact, it compares a facility used for big-scale application with a facility for distributed generation. Also the evaluation of embodied energy for the construction of both falicities can be obtained from inventory results. These results show values of 59 GJ for the dish-stirling facility versus 125 GJ for the photovoltaic facility. These values have been calculated by adding the energy coming from different sources that is needed for the production of the different components of the facilities. There are no data in literature review concerning the dish-stirling technology so no comparison can be established with previous studies. However, the photovoltaic facility has been evaluated in previous studies in MJ/m 2 units. In the present study, the surface for this facility is 73.2 m 2 so a value of 1708 MJ/m 2 for the photovoltaic facility has been obtained resulting in a good comparison with values previously reported in literature [13]. In the case of the dish Stirling facility it is obtained a value of 1040 MJ/m 2 (corresponding to 56.7 m 2 of area for this facility), which is significantly lower than in the case of the photovoltaic facility. From these data the energy payback period for the dish Stirling is about 0.6 years being 1.3 years for the photovoltaic facility, considering the same operation time per year for both facilities of 10 kw. Impact evaluation results. The impact evaluation results come from conversion of the whole collection of outputs of the inventory results by means of damage models into a limited number of data known as impact categories. The EI-99 impact evaluation method converts these impact categories into damage categories. The calculation includes different considerations, such as the perspectives, that make the values be dependant on them. It is considered that the presentation of environmental loads into these damage categories help to understanding and interpretation of results. In the case of study, the impact evaluation results are presented in the three damage categories corresponding to EI-99 method: human health, ecosystem quality and resources. In addition to it, the EI-99 enables aggregation of these three categories into a single score value, used basically to simplify comparisons between systems. This single score value is also used in this study [18]. Additionally, the environmental loads have also been evaluated with the CML 2 method in order to verify validity of results not dependant on the impact evaluation method used. The impact evaluation results are focused on the comparison of damage evaluation of the dish-stirling facility of 10 kw with the photovoltaic system. As presented, the impact evaluation method considered is EI-99, with Hierarchist (H) perspective and the CML 2 method which is used to verify results. The comparison between the dish-stirling and photovoltaic system is presented in the three damage categories (human health, ecosystem quality and resources) and single score comparison. For both systems it is considered that most of materials are recycled at disposal.

14 The figure 3 shows the environmental evaluation in relation to the three damage categories compared for the systems. Figure 3. Normalised damage categories Eco-indicator 99 H/H for comparison of Stirling and photovoltaic units. In spite of the fact that the final result for Eco-indicator 99 H/H are damage categories, the mid-point evaluation regarding impact categories corresponding to this method can be also evaluated, as presented in figure 4. Figure 4 Normalised impact categories Eco-indicator 99 H/H for comparison of Stirling and photovoltaic units. As for the damage categories, the values show that the level of damage evaluation is quite similar for the systems considered. The order of impact in damage categories (from highest to lowest) is: resources, human health and ecosystem quality. As for the impact categories, there are different favourable balances for Stirling and photovoltaic facility in different categories. The presentation of the analysis aggregated into the single score value enables the direct

15 comparison between the systems, as presented in figure 5. So, for the single score the analysis show the same consideration, a balance favourable to the photovoltaic system but without a significant difference in values. Figure 5. Single score Eco-indicator 99 H/H for comparison of Stirling and photovoltaic unit. As for the analysis of the dish-stirling unit, one of the most significant results is the contribution analysis. In this analysis, the element and processes that represent a highest environmental impact are stressed. The contribution analysis is presented as the contribution tree, with the contribution expressed as a percentage, and as a graphic, presenting the contribution in Eco-points for the single score value. The cut-off value for graphic evaluation considered has been 5% of contribution value. It is presented in figure 6.

16 Figure 6. Contribution tree Eco-indicator 99 H/H of Stirling unit. So, the impact of the different elements is valued. The metallic structure shows the highest impact, 47.48%, followed by the engine, 37.09%, and the concrete settlement, 27.32%. The weight of the impact is mainly due to contribution of materials used, such as steel in the metallic structure, rather than for processes considered. This contribution analysis enables to consider modifications or future developments to reduce the environmental impact. It is done through the sensitivity analysis, where the characterisitics of the systems are modified in order to verify the evolution of the environmental impact. In the case of the dish-stirling facility, a feasible modification would be the quantity of concrete used for the settlement. Though the percentage contribution is not the highest (around 26% in relation to 37% of the engine and 47% of the metallic structure), it seems to be the most over-dimensioned element used currently in the facility, according to the data provided for the facility located at Odiello [25]. Of course, the impact of modifying the elements with higher percentage would be interesting but the data are not available to foresee future evolutions. The following figure 7 shows the evolution of the dish-stirling facility from the current value considered for concrete (150 ton) to 0 ton, taking 75 ton as medium value. In figure 6 it is shown that the favourable balance in relation to the photovoltaic system is inverted when the quantity of concrete is reduced.