A Comparative Environmental Assessment of Different Thermal Solar Energy System

Transcription

1 A Comparative Environmental Assessment of Different Thermal Solar Energy System Adrian BADEA, Cristian DINCĂ Abstract The paper objectives consists to evaluate the environmental impacts of the electricity produced in a two solar thermal power plants with central tower technology and with parabolic trough technology, to identify the opportunities to improve the systems in order to reduce their environmental impacts, and to evaluate the environmental global impact. The global emissions are provided from all stages of the solar thermal power plant life cycle. The energy payback time was around 1 year for both power plants. The global warming impacts along the whole life cycle of the power plants were around 200 g/kwh generated. Keywords: electrical energy, thermal solar energy, greenhouse gas emissions, life cycle assessment, impact analysis, collector parabolic 1. Introduction The limited supply of fossil fuels and their environmental impacts have dictated the increasing usage of renewable energy sources. Although, there is an increase in the utilization of renewable energy sources there is still much to be done. To date, petroleum and natural gas remain the dominant energy sources, close to 65 % of the consumed energy, while the renewable share is close to 7 % [1]. Renewable electricity generation capacity reached an estimated 240 GW worldwide in 2007, an increase of 50 % over 2004, representing a 3.4 % of global power generation [1]. Even though there is an annual increase of the solar energy utilization due to the thermal solar systems, hydro power is the most popular renewable energy source for electricity generation. In future, concentrated solar power (CSP) is the most likely candidate for providing the majority of this renewable energy produced electricity. CSP t echnology is a proven technology for energy production, with a potential market increase, and significant cost reductions [2,3]. Three main CPS technologies have been identified during the past decades for generating electricity: Dish/engine technology, which can directly generate electricity, Adrian BADEA, professor; Cristian DINCĂ, lecturer University Politehnica of Bucharest. Parabolic trough technology producing high pressure superheated steam Solar tower technology. Concentrating solar thermal energy has become one of the most promising sources of energy in the past years. Its renewable and almost carbon neutral nature, the fact that it can provide energy on a large scale and its near-to-commercial technological development, has created a growing interest in this technology. In this study, we have compared the parabolic trough technology and solar tower technology which are described in the next paragraph. This study enlarges the current body of knowledge by analyzing the construction and operational environmental impacts of thermal power plant projects, and comparing them to those of other competing energy options. The methodology used to evaluate the environmental impact of solar thermal power plants is the so called LCA, which is a method for systematic analysis of environmental performance from a cradle to a grave perspective. This analytic tool systematically describes and assesses all flows that enter into the studied systems from nature and all those flows that go out from the systems to nature, all over the life cycle. The practice of LCA is normalized by the series of ISO Standards Life cycle assessment presentation In this paper, we used the Life Cycle

2 48 ELECTROTEHNICA, ELECTRONICA, AUTOMATICA, 59 (2011), Nr. 2 Assessment (LCA) methodology in order to evaluate the environmental global impact of the coal channel. The evaluation of the environmental global impact takes into consideration the utilization of the coal in a power plant in order to produce the electricity. The methodology has been performed in order to examine the net emissions of greenhouse gases, as well as other major environmental consequences for all the stages of the coal channel such as extraction, treatment, transport, storage and combustion. LCA is a systematic analytical method that helps identify and evaluate the environmental impact of a specific process or of competing processes. In order to quantify emissions, resource consumption, and energy use (i.e., environmental stressors), material and energy balances are performed in a cradle-to-grave manner on the operations required to transform raw materials into useful products. The purpose of this paper is to choose the best (optimal) energy solution form the environmental, technical and economical points of view. The Life cycle assessment study consists of four steps: definition of the goal and scope of the study; life cycle inventory (LCI) phase: collection of all the environmental inflows and outflows; life cycle impact assessment (LCIA) phase and interpretation of the study results. The first step is then the definition of the objectives of this study. These objectives are as follows: to evaluate the environmental impacts derived from the electricity production of a 20 MW solar thermal plant with central tower technology and a 60 MW solar thermal plant with parabolic trough technology, both of them hybrid operation power plants; to identify the opportunities to improve the systems in order to reduce those environmental impacts; to evaluate the environmental global impacts. The functional unit used is 1 kwh produced at the power plant. The processes modeled are the manufacturing of materials and components of the power plant, construction activities, operation and maintenance, as well as decommissioning of the power plant and the disposal of all the waste materials. Processes modeled in the life cycle of the solar thermal power plants are presented in Figure 1. Solar thermal power plant Energy Resources Manufacturing of materials and components Solar field Tower Storage system Buildings Power block Construction activities Cranes Transports Operation and maintenance Electricity Natural gas Water Dismantiling activities Dismantling cranes Transport to landfill Land filling Emissions Electricity to the grid 1 kwh Figure 1. The system boundaries of the solar system Data needed to perform the LCI were obtained from the most up-to-date LCA databases mainly from ECOINVENT V At the time of performing this LCI, some data related to the weight of materials of some parts of the power plants were missing. Consequently, the following assumptions were made. Weight of steel in the steam generator. Data for our plants were missing. Therefore, data from literature [5] were considered for our analysis ( kg/kw e for the central tower power plant and 9500 kg/kw e for the parabolic trough power plant); Weight of steel in the steam turbine of

3 ELECTROTEHNICĂ, ELECTRONICĂ, AUTOMATICĂ, 59 (2011), nr the parabolic trough power plant. There were no data available, and therefore data from the literature were used [5]. Weight of molten salts in the storage system of the central receiver power plant. Only the cost of the salts was available. Based on that, and considering a unit cost of 0,2 /kg, the weight of salts was estimated. Weight of steel in the storage system tanks. Data were missing as well. For this analysis, it was considered that, in both plants, the salts are stored in two tanks of diameter 40 m. Based on that, a total weight of steel was then estimated. There were no data available for the natural gas boilers that exist in both plants. Given this limitation, the manufacturing of this component was excluded from the analysis. 3. Description of solar energy systems In the Life Cycle Impact Assessment (LCIA), the impact assessment method developed by the Leiden University Institute of Environmental Sciences (CML) was selected. In this method, the following impact categories are assessed in the table 1. Table 1. Impact categories assessed Impact category Impact indicator Global warming kg CO 2eq Abiotic depletion potential kg Sb eq Ozone layer depletion kg CFC-11 eq Human toxicity kg 1,4-DCB eq Fresh water aquatic ecotoxicity kg 1,4-DCB eq Marine aquatic ecotoxicity kg 1,4-DCB eq Terrestrial ecotoxicity kg 1,4-DCB eq Photochemical oxidation kg C 2H 4 Acidification kg SO 2eq Eutrophication kg PO 4eq Furthermore, the cumulative energy consumption or demand in MJ has been assessed. Based on this parameter, the Energy Payback Time (EPT) has been calculated. EPT is considered the time in which an energy system produces the same amount of energy as consumed for its production, operation and dismantling. The EPT is calculated using the following formula: CEDc EPT = Enet CED 0 g (1) where: CED c is the cumulative energy demand for construction of the power plant; E net is the yearly produced net energy (MJ/y); g is the utilization grade of primary energy source to produce electricity. Since most products are assumed to be produced in European Union, this utilization grade is taken as 46 %; CED o is the annual energy demand for maintenance (MJ/y). The characteristics of the power plants analyzed are summarized below. Details can be seen in table 2. Table 2. The technical characteristics of the studied solar energy systems Energy system Central tower Parabolic trough receivers Installed capacity, in MW Direct normal 2 irradiation, in kwh/m Number of heliostats or receivers Load factor, in % Life time, in years Energy produced in the life time, in GWh Storage capacity, in hours 15 8 Storage medium Molten salts Molten salts (calcium nitrate, (sodium nitrate sodium nitrate and potasium and potasium nitrate nitrate) Natural gas consumption, in MWh/year Efficiency of solar field, in % A 20 MW solar termal power plant with central tower technology and 3000 heliostates. The plant uses molten salts as heat transfer fluid. The storage system also uses molten salts and provides 12 hours of stored energy. The power plant occupies a total area of 120 has. The power plant is hybridized with natural gas up to a 15 % of the energy generated. A 60 MW solar thermal power plant with parabolic trough technology consisting of 650 parabolic troughs collectors. This plant uses synthetic oil as transfer fluid and molten salts to create a seven hours storage system. Total area occupied by the plant is 250 has. As the former plant, 15 % of total output is generated by natural gas. The spent steam is then condensed and pumped again through the heat exchangers to be

4 50 ELECTROTEHNICA, ELECTRONICA, AUTOMATICA, 59 (2011), Nr. 2 superheated again, while the HTF is recirculated through the solar field. The collectors have the potential to track the sun from east to west during the day, to ensure that the sun is continuously focused on the linear receiver tube [2]. Parabolic trough technology is currently the most proven of solar thermal technologies and the only one commercially available. dismantling of the power plant range from 17 to 35 g/kwh, which are similar to the values reported in the literature. In Figure 2, the relative contribution of the different parts of the operational stage is presented. 4. Results ans discussion The estimated cumulative fossil energy demand for the life cycle of the solar energy systems analyzed is shown in table 3. Table 3. The energy necessary in the life cycle of solar system Energy consumed Central tower Parabolic trough receivers Solar field 0,09 0,26 Power block 0,01 0 Storage system 0,05 0,07 Tower 0, n.a. Buildings 0,02 0,01 Construction 0, ,0101 Decommissioning 0, ,00014 Total 0,18 0,36 Most of the fossil energy required comes from the operational stage and it is mainly due to the natural gas and electricity consumption. Without taking into account these energy expenses, the energy required to build and dismantle the power plants is 0,18 MJ/kWh for the central tower technology and 0,36 MJ/kWh for the parabolic trough technology. Results from other life cycle assessments give values of 0,14-0,16 MJ/kWh for a SEGS type plant. Global warming emissions produced during the life cycle of the analyzed solar thermal power plants are shown in Table 4. Table 4. The values for global warming potential for each solar energy systems [6] CO 2 equivalent Parabolic Central tower emissions (g/kwh) trough receivers Solar field 5,6 18,6 Power block 0,64 0,46 Storage system 9,49 14,9 Tower 0,04 n.a. Buildings 1,03 0,47 Construction 0,09 0,37 Decommissioning 0,0028 0,071 Sub-total Operation Total Values are around 200 g/kwh, and most of them come from the operational stage. Emissions due to the construction and Figure 2. The global warming indicator (g/kwh) for the parabolic trough solar thermal plant. It can be observed that most of the operational stage emissions come from: natural gas combustion: 95 g/kwh for the parabolic trough receivers power plant and 114 g/kwh for the central tower power plant; natural gas provision: 16,6 g/kwh for the parabolic trough receivers power plant and19,9 g/kwh for the central tower power plant; electricity consumption from the grid: 48,5 g/kwh for the parabolic trough receivers power plant and 50,1 g/kwh for the central tower power plant. It is worthy to note the relevant contribution to the global warming emissions of the electricity consumption of the plants. This contribution is mainly due to the fact that an important part of the electricity generation in European Union is produced in coal power plants with very high associated greenhouse gas emissions. If the selfproduced electricity was consumed to meet the electricity requirements of the plants during the operational stage, instead of taking it from the grid, the associated global warming emissions would have been much more reduced. Results obtained in the other impact categories analyzed are shown in Table 5. For some of these impact categories, values reported in the literature are lower than the values obtained in this study.

5 ELECTROTEHNICĂ, ELECTRONICĂ, AUTOMATICĂ, 59 (2011), nr Table 5. Impact analysis results Impact category Abiotic depletion potential, in g Sb eq Central tower Parabolic trough receivers 1,58 1,46 Ozone layer depletion, mg CFC-11 eq 0,01 0,01 Human toxicity, g 1,4-DB eq 87,9 288,93 Fresh water aquatic ecotoxicity, g 1,4-DB eq 8,67 39,81 Marine aquatic ecotoxicity, kg 1,4-DB eq 114,7 139,39 Terrestrial ecotoxicity, mg 1,4- DB eq ,15 Photochemical oxidation, mg C 2H 6 eq 28,12 29,55 Acidification, mg SO 2 eq 608,9 654,16 Eutrophication, mg PO 4 eq 49,11 57,68 Acidification values reported by [4] are 69,28 mg SO 2 equiv/kwh for a parabolic trough plant, and eutrophication impacts are 5,69 mg PO 4 equiv. However, it is important to acknowledge that most of the impacts are produced in the operation of the power plant due to the consumption of natural gas and external electricity (as can be observed in Figure 3 for the central tower power plant). Figure 3. Impact analysis results for the central tower solar thermal power plant The behavior observed in the parabolic trough power plant is shown in Figure 4. Figure 4. Impact analysis results for the parabolic trough solar thermal power plant In this case, the solar field associated impacts have greater importance in some impact categories such as human toxicity and fresh water aquatic eco-toxicity. When tracing back the origin of these impacts, it was found that they were due to the use of chromium steel in the absorber tube of the receiver. In fact, [2] reported similar figures for hybrid operation of solar thermal power plants: 370 to 510 mg SO 2 eq/kwh for acidification and 40 to 56 mg PO 4 eq/kwh for eutrophization. Based on these objectives, the installed capacity in 2010 would reach 500 MW. In the considered scenario, it has been assumed that 80 % of such capacity would be met by parabolic trough plants while 20% would be met by solar tower power plants. Therefore, central tower power plant technology would have a 100 MW installed capacity and parabolic trough power plant technology would have 400 MW installed. An analysis of the global impact on the environmental of both studied technologies has lead to achieve results for all impact classes considered. As result of this study we conclude that there is no clear picture about technology preferences, but we can say that for most classes of impact, the central tower shows a lower impact on the environment (see Table 5). Avoided impacts are computed considering that the electricity produced in these plants would replace electricity produced by the European Union electricity generation mix (year 2004). 5. Conclusions From the LCA performed of these two hybrid operation solar thermal power plants, some important conclusion can be drawn: First at all, both technologies show an environmental profile much better than the current mix of technologies used to produce electricity in European Union. The cumulative energy demand of the life cycle of both plants is lower than the energy produced and the energy payback time calculated range from 1 to 2 years. The global warming emissions are around 200 g/kwh, which are much lower than competing fossil technologies. However, these emissions are mainly due to the use of fossil fuels in the operation of the plant (natural gas consumption and external

6 52 ELECTROTEHNICA, ELECTRONICA, AUTOMATICA, 59 (2011), Nr. 2 electricity consumption). The fact that the electricity consumed in the plant is taken from the grid has important environmental consequences due to the high fossil share of electricity generation in European Union. These imported impacts could be reduced if the electricity needed in the operation of the plant were taken from the self-produced electricity. If neither natural gas nor external electricity were used, the greenhouse gas emissions would be much lower Other impacts calculated are lower than those produced by the current European Union electricity mix, and most of them are produced in the operation of the plant due to the consumption of natural gas and electricity from the grid. Impacts that would be avoided by the implementation of the Renewable Energies Plan are important. Regarding global warming emissions, 618 kt of CO 2 equivalent would be avoided, which represent a 0,2% of the emissions of the energy sector in European Union in year Once again, if neither natural gas nor external electricity were used, the CO 2 avoided emissions would be much higher. Acknowledgements This work was supported by the European project POSDRU /89/1.5/S with the number ID References [1] WEINREBE G., Life cycle assessment of an 80 MW SEGs plant and a 30 MW Phoebus power tower, în Solar Engineering ASME, [2] VIEGAHN P., Integration of DSG Technology for Electricity production. WP 4.3. Impact Assessment. Life Cycle Assessment of construction materials, energy demand and emissions of DSG, Final report, [3] UCHIYAMA Y., Energy technology life cycle analysis that takes CO 2 emission reduction into consideration, CRIEPI [4] KREITH F., NORTON, P., BROWN, D., A comparison of CO2 emissions from fossil and solar power plants in the United States, în Energy, Vol 15 pp, , [5] VANT-HULL D., Solar thermal electricity: an environmental benign and viable alternative. Perspectives, în Energy Vol 2 pp [6] NORTON, B. AND LAWSON, W.R., Full energy chain analysis of greenhouse gas emissions for solar thermal electric power generation systems, în IAEA, Technical press, 1996.