1 UNIVERSITEIT GENT FACULTEIT ECONOMIE EN BEDRIJFSKUNDE ACADEMIEJAAR Defining the techno economic optimal configuration of hybrid solar plants Masterproef voorgedragen tot het bekomen van de graad van Master in de Bedrijfseconomie Bosschem Siemon Debacker Alice onder leiding van Prof. Johan Albrecht
3 UNIVERSITEIT GENT FACULTEIT ECONOMIE EN BEDRIJFSKUNDE ACADEMIEJAAR Defining the techno economic optimal configuration of hybrid solar plants Masterproef voorgedragen tot het bekomen van de graad van Master in de Bedrijfseconomie Bosschem Siemon Debacker Alice onder leiding van Prof. Johan Albrecht
4 Defining the techno economic optimal configuration of hybrid solar plants 2009 IV PERMISSION The undersigned certifies that the contents of this master thesis can be consulted and/or reproduced, if source acknowledged. Bosschem Siemon & Alice Debacker
5 Defining the techno economic optimal configuration of hybrid solar plants 2009 V FOREWORD We want to thank several people without whom we would not have been able to complete this project so smoothly. First of all, we would like to thank our supervisor, Prof Johan Albrecht for the time and advice he has given us. We also want to thank Jonas Verhaeghe for his availability and the time he spent answering our numerous questions. In addition, we thank CEG for all the information set at our disposal which helped us getting started easily. Lastly, we thank all the people who helped us find information, supported us all along and helped in any way.
6 Defining the techno economic optimal configuration of hybrid solar plants 2009 VI TABLE OF CONTENTS PERMISSION... IV FOREWORD... V TABLE OF CONTENTS... VI LIST OF TABLES... VIII LIST OF FIGURES... IX ABBREVIATIONS... XI 1 INTRODUCTION HYBRID SOLAR POWER HYBRID SOLAR POWER Solar Power Technologies Conventional Thermal Power ISCC CURRENT AND FUTURE PROJECTS ENERGY TRANSPORTATION NETWORK ECONOMIC ANALYSIS INTRODUCTION REFERENCE PLANT PLANT SCALE UP TECHNOLOGY, COST AND BENEFIT Parabolic Trough Central receiver systems (CRS) Investment costs and LEC Sensitivity on LEC Conclusion THERMAL ENERGY STORAGE Thermal Storage Technologies Impact on the costs of the power plant EXTRA BURNER OPERATION AND MAINTENANCE FINANCIAL INCENTIVES, GRANTS... 31
7 Defining the techno economic optimal configuration of hybrid solar plants 2009 VII Feed in Tariffs Other National incentives Other International Support Mechanisms SITE SOLAR RESOURCES, DNI NATURAL GAS AND ELECTRICITY PRICES CONCLUSION BIBLIOGRAPHY ANNEXES ANNEX 1 : LIFE CYCLE ASSESSMENT OF GREENHOUSE GAS EMISSIONS  ANNEX 2 : INCENTIVE SYSTEMS BY COUNTRY IN EUROPE... 48
8 Defining the techno economic optimal configuration of hybrid solar plants 2009 VIII LIST OF TABLES Table 2 1. List of planned hybrid solar plants  ... 9 Table 3 1. ISCC Reference plant properties Table 3 2. Investment costs of different ISCC technologies  Table 3 3. Investement costs of thermal storage for different solar technologies  Table 3 4. Operation and Maintenance costs of different ISCC Technologies and CC Table 3 5. Operation and Maintenance costs selected to calculate the LEC  Table 3 6. Feed in tariffs in Algeria  Table 3 7. Feed in laws in several countries ... 32
9 Defining the techno economic optimal configuration of hybrid solar plants 2009 IX LIST OF FIGURES Figure 2 1. Electric energy generation from solar power ... 3 Figure 2 2. Concentrated Solar Power, types of solar receivers ... 4 Figure 2 3. Combined Cycle Power Plant ... 5 Figure 2 4. Net efficiency of different technologies in maximum capacity factor ... 5 Figure 2 5. Integrated Solar Combined Cycle plant with PT ... 7 Figure 3 1. LEC and Investment costs of the ISCC reference plant Figure 3 2. Scale up effect : LEC vs Total capacity of the power plant Figure 3 3. Scale up effect: Specific investment cost vs Total capacity of the power plant Figure 3 4. Levelized Electricity Cost of different ISCC technology Figure 3 5. Investment costs of different ISCC technology Figure 3 6. Levelized Electricity Cost with reduction of the solar field Figure 3 7. Solar Tower power plant using two tanks molten salt storage  Figure 3 8. Growth factor of the solar field with the hours of thermal storage in two different locations   Figure 3 9 CSP Investment Cost of 3h storage in Barstow and Seville compared with no storage Figure Evolution of the LEC with the thermal storage time for two sites with different DNI Figure Evolution of the annual solar contribution with the thermal storage time for two sites with different DNI Figure Evolution of the CO 2 emission with the thermal storage time for two sites with different DNI Figure Annual electric production and LEC of ISCC power plants with or without extra burner 28 Figure Comparison of the CO 2 emissions of ISCC plants with or without extra burner and a CC plant Figure Direct Normal Irradiance map Figure Levelized Electricity Cost of various DNI levels and different solar shares Figure Carbon Dioxide Emissions for various DNI levels and different solar shares Figure Oil, coal and liquefied natural gas prices from1970 to Figure Gas prices for medium size industries in Europe and Spain  Figure Evolution of the LEC with the gas price for different ISCC Technologies and CC Figure Electricity prices in Spain from 1998 till 2008 ... 39
10 Defining the techno economic optimal configuration of hybrid solar plants 2009 X Figure 4 1 LEC vs CO 2 emission for different evolutions of the solar share (green), thermal storage (purple), DNI (dark blue), plant size (red) and extra burner (light blue) Figure 4 2. EUA prices from January 2008 till May 2009  Figure 4 3 LEC vs annual green energy production for different evolutions of the solar share (green), thermal storage (purple), DNI (dark blue), plant size (red) and extra burner (light blue)... 43
11 Defining the techno economic optimal configuration of hybrid solar plants 2009 XI ABBREVIATIONS CC CLFR CSP CRS DNI DSG EUMENA GEF GT GW HRSG HTF HVAC HVDC ISCC LEC MENA MW MW e MWh e MWh th PCM PT RTIL SEGS ST TES Combined Cycle Compact Linear Fresnel Reflector Concentrated Solar Power Central Receiver System Direct Normal Irradiance Direct Steam Generation Europe (EU), the Middle East (ME) and North Africa (NA) Global Environment Facility Gas Turbine Gigawatt (10 9 watt) Heat Recovery Steam Generator Heat Transfer Fluid High Voltage Alternative Current High Voltage Direct Current Integrated Solar Combined Cycle Levelized Electricity Cost Middle East and North American Countries Megawatt (10 6 watt) Megawatt electric Megawatt hour electric Megawatt hour thermal Phase Changing Materials Parabolic Trough Room Temperature Ionic Liquids Solar Energy Generating System Solar Tower Thermal Energy Storage
12 Defining the techno economic optimal configuration of hybrid solar plants INTRODUCTION The world s resources are diminishing day by day. The worst predictions plan the depletion of main resources like oil, natural gas and coal in the next 100 years. Besides, the climate changes due to global warming are pushing energy producers to think of new possibilities. Solar power is the most powerful natural resource on earth but we cannot take full advantage of it. The first problem resides in turning this energy into electricity or heat usable in everyday life. The second problem is linked to the fluctuating and unpredictable nature of solar power. Actual solar plants are developed and solutions are thought of to reduce the issue of partial production. Unfortunately these projects are not profitable and would never be brought to life without the financial help of governments and environmentally concerned organizations. One promising solution is the hybrid solar thermal power plant. Instead of producing solar power only, the energy coming from the solar field is used to improve the efficiency and to lower the CO 2 emissions of a common thermal power plant. If solar power is maturating, ISCC is still young. In the literature, a few studies can be found on the feasibility of a ISCC power plant. However, these studies are usually conveyed to determine the viability of a certain project, in a defined place, with a defined technology This project aims to define the optimal configuration of hybrid solar plants. The results presented in this master thesis are based on the work of Jonas Verhaeghe and Bram Van Eeckhout, for Clean Energy Generation . The first section describes what a hybrid solar plant is and how it works. It also describes the main technologies that are used to produce solar based energy as well as how it can be combined with a conventional thermal power plant. It follows the choice of Integrated Solar Combined Cycle. The second section analyses the impact of the main parameters on the green production, plant costs and CO 2 emissions of the ISCC power plant. Among others, the type of solar technology, the use of
13 Defining the techno economic optimal configuration of hybrid solar plants thermal energy storage, the different incentives and grants systems of several countries and the importance of the site are studied. Finally, optimal configurations are presented for the corresponding priorities and personal choices of the investors.
14 Defining the techno economic optimal configuration of hybrid solar plants HYBRID SOLAR POWER 2.1 HYBRID SOLAR POWER For ages, mankind has tried to tame the energy of the sun. Many different technologies have been born, some efficient, others not. To increase the efficiency of solar power and make it competitive, the concept of hybrid power plant has been developed. By combining solar thermal energy with conventional thermal energy, a basic electric load can be assured at all times while solar power can be used to reduce the consumption of classic fuel and decrease greenhouse gas emissions SOLAR POWER TECHNOLOGIES In the large scale production of electricity, the most developed technology is CSP, Concentrated Solar Power. The sunlight is concentrated on a focal point by reflecting surfaces. Solar radiation is concentrated and then converted into thermal energy. This thermal energy can be converted into electricity by means of a thermodynamic cycle. Solar power can be converted to electricity directly if the HTF is steam which drives a steam turbine. To reach higher temperatures with liquid mediums, oil or high phase change temperature fluids can be used as HTF. Then, a heat exchanger is needed to warm up the steam driving the turbine. Figure 2 1. Electric energy generation from solar power 
15 Defining the techno economic optimal configuration of hybrid solar plants The most common CSP technologies are the parabolic trough, central solar receiver or solar tower and the parabolic dish or dish Stirling. Parabolic troughs and solar towers can be developed in large fields with a power block quite similar to those of conventional power plants. These are used for large scale production of energy. The power block of parabolic dishess is situated at the focal point of the dish. Therefore, the electricity that can be produced is greatly limited. Receiver Figure 2 2. Concentrated Solar Power, types of solar receivers  CONVENTIONAL THERMAL POWER Solar power can be combined with different types of conventional thermal power generation. In Australia, solar power is used to enhance the efficiency of existing coal power plants . Depending on the availability different kinds of fuel are chosen to power new plants. Most of the new hybrid power plants are based on combined cycles. A combined cycle power plant uses the waste heat from the exhaust of a gas turbine to generate steam by passing it through a heat recovery steam generator (HRSG). Then, the Steam of the HRSG feeds a steam turbine from a Rankine cycle.
16 Defining the techno economic optimal configuration of hybrid solar plants Figure 2 3. Combined Cycle Power Plant  Such a thermal cycle allows the plant efficiency to grow from 30 40% to 60% for the production of electricity. If the combined cycle is used for cogenerationn of electricity and heat, the overall efficiency of the plant can add up to 85% . Combined cycle power plants can assure base load as well as peak production. Besides their high efficiency, they have relatively low investment costs, long life cycle and low greenhouse gases emissions (see figure 2 4). The emission of toxic gases like SO 2 and NO x is also much lower than diesel, heavy oil or bituminous coal 1. Figure 2 4. Net efficiency of different technologies in maximumm capacity factor  1 Annex 1 : Life cycle assessment of greenhouse gas emissions 
17 Defining the techno economic optimal configuration of hybrid solar plants The integration of CSP technology with a combined cycle power plant is a very interesting hybrid power plant configuration. This configuration is referred to as integrated solar combined cycle systems (ISCCS). The net efficiency of ISCC is higher than that of SEGS but also higher than a Combined Cycle plant (see figure2 4). Therefore in this project, the type of hybrid thermal solar power studied, is the Integrated Solar Combined Cycle. The key question is how to design and optimize the integration of the solar field and the power cycle.
18 Defining the techno economic optimal configuration of hybrid solar plants ISCC Integrated solar combined cycle (ISCC) are modern combined cycle power plants with gas and steam turbines and additional thermal input of energy from a solar field . The plant concept was initially proposed by Luz Solar International . Figure 2 5. Integrated Solar Combined Cycle plant with PT  Solar thermal energy can be used in two different ways. The first use is presented in figure 2 5. In this schematic power plant, the heat of the HTF is transferred in the solar steam generator to produce steam to drive the steam turbine. In case the steam cannot be warmed up enough, because of lack of sunlight, the duct burner produces the additional heat by burning gas. In other designs, the solar field produces an additional volume of steam, directly as HTF or through a heat exchanger, to drive the steam turbine. This design requires the steam turbine to be oversized and work at a partial load when the sun is not shining.
19 Defining the techno economic optimal configuration of hybrid solar plants CURRENT AND FUTURE PROJECTS ISCC is a very young technology and the investments in these projects are still risky. However, a few projects are already in construction phase and they will soon be finished. Six countries are now constructing an ISCC plant: Algeria, Egypt, Iran, Italy, Morocco and the U.S. In Australia a Compact Linear Fresnel Reflector field has already been finished and added to an old coal fired power plant    . One of the first ISCC plants to be built is Yazd Solar Thermal Power Plant, in Iran. Since 1997, the government of Iran has been interested in the implementation of a m² parabolic trough field into a 300MW natural gas fired combined cycle plant in the Luth desert in the area of Yazd . Later on they raised up the total capacity to 430MW with 67MW solar field plant . To finance the incremental cost of the solar field, Iran approached GEF with a request for a $50 million grant. But as GEF was not in the position to hand out any grants, in 2005, Iran changed the plant configuration and now intends to build a solar field equivalent to about 17MW. The total plant capacity will be 467MW . In Ain Beni Mathar, Morocco, an ISCC project of 472MW, supported by GEF is being built. The plant includes a parabolic trough solar component of 20MW ( m 2 ) with an expected annual net production of GWh per year. The solar output is estimated at 1,13% of the annual production representing 40GWh per year . According to the constructors (Abener), they started the works on the 28th of March 2008 and plan to be finished in August 2010 . Abener is currently building the second ISCC Power Plant in Hassi Mel, Algeria . The complex will comprise a 130MW combined cycle, with a gas turbine power of the order of 80MW and a 75MW steam turbine. A 25MW solar field, requiring a surface of around m 2 of parabolic mirrors, will be the source of non fossil energy. The investment will be nearly 140 million dollars and is the first privately financed solar thermal plant in North Africa, based on the feed in law of Algeria . The construction of the ISCC is planned to finish in August 2010.
20 Defining the techno economic optimal configuration of hybrid solar plants In Egypt, there is a project in building phase with a total capacity of 140 MW. Also it has a large solar contribution of 30MW and is supported by GEF with a $50 million grant. In Italy a a solar field of 30MW is being added to an existing power plant of 700MW. The U.S. is in the process of building an ISCC plant in Victorville, CA. Three others are planned in California and Florida. In Mexico there is an ISCC project approved by GEF in 2006 and in India a 150MW ISCC plant is being planned with a solar contribution of 30MW. But this project is not yet approved by GEF. Country Technology Capacity (MWe) Australia, Lake Lidde Iran, Yazd Algeria, Hassi R mel Morocco, Ain Beni Mathar Egypt, Kuraymat U.S., Victorville, CA U.S., Indiantown, FL Italy, Siracusa U.S., Fresno County, CA U.S., Palmdale, CA Mexico, Sonora State India, Mathania Coal CLFR ISCCS PT ISCCS PT ISCCS PT ISCCS PT ISCCS PT ISCCS PT ISCCS PT Biomass PT ISCCS PT ISCCS PT ISCCS PT Solar Capacity Solar Share (MWe) 2004,4 4,4 0,2% 2300 DNI Phase Online date Finshed ,6% 2500 Under 2010 construction ,7% 2300 Under 2010 Construction ,2% 2300 Under 2010 Construction ,6% 2400 Under 2010 Construction ,9% 2200 Under Construction ,7% ,1% 2100 Under 2010 construction ,2% ,8% Planned ,0% 2600 Approved by World Bank/GEF ,0% 2250 Table 2 1. List of planned hybrid solar plants   Table 2 1 above shows that most of the projects contain a small solar share. This is because of the high equipment cost of the solar field and the scanty support by incentives for ISCC projects. Only in Morocco, Egypt and Mexico will the projects be supported by GEF. However several ISCC projects are supported by private investments. This indicates that ISCC can be competitive without large grants.
21 Defining the techno economic optimal configuration of hybrid solar plants ENERGY TRANSPORTATION NETWORK Many highly populated areas in the world don t have the ability to produce competitive solar energy, although there is a great potential for solar energy on this planet. By building a well functioning electricity network over big distances, solar power can be transferred from thousands of kilometers. With such a large electric infrastructure, all types of renewable energy sources can provide electricity over huge distances. Europe (which has little solar potential) and the MENA (high solar potential) have plans to build a large electricity network which will interconnect the greatest power plants over the EUMENA. This project fits into a major concept, DESERTEC. This concept describes the perspective of a sustainable supply of electricity for Europe, the Middle East and North Africa up to the year According this scenario, several GW of solar energy produced in the deserts of MENA can be transported towards the less sunny regions in Europe. This electricity network won't be operative before 2020, but it will be necessary for the redundancy and stability of the future power supply system. The currently used technology (HVAC) is not sufficient to create such a large scale network without having huge energy losses. Therefore a technology, called HVDC, can be used. These HVDC wires have less electricity losses than the currently used AC grid (HVAC), particularly in the case of overseas connections. Over smaller distances, AC grid can be used, which is more useful for small distances.
22 Defining the techno economic optimal configuration of hybrid solar plants ECONOMIC ANALYSIS 3.1 INTRODUCTION The objective of this economic analysis is to assess the cost efficiency of ISCCS power plants, to determine the economics of plants with different specifications and to compare it with the conventional power generation system, combined cycle. The specifications that will be studied in this analysis are the type of solar thermal technology, the number of storage hours, the use of an extra burner, the level of DNI, the plant scale, gas prices For the comparative assessment, the Levelized Energy Cost (LEC) is used as the figures of merit. The LEC is the present value of the life cycle costs converted into a stream of equal yearly payments. As an advantage, the LEC figure allows an economic evaluation of different power generating technologies with varying capacities, full load hours, lifetime, etc . The LEC values for power generation systems are computed by the following methodology: ( /MWhe) Total annual capital Cost Total annual Operational & Maintenance Cost no fuel expenses Total annual fuel expenses Annual electricity production MWh e
23 Defining the techno economic optimal configuration of hybrid solar plants REFERENCE PLANT As reference plant for this study, a 265 MW ISCC plant is chosen with a solar contribution of 36MW. The plant has 3 solar towers of 12 MW e peak capacity each. The Heat Transfer fluid is steam. Values for O&M cost, solar equipment cost and efficiencies are used from CEG  and ECOSTAR . Item Parameter Units Solar Technology Solar Tower (CRS) Fuel type Natural gas Nominal power 265 MW e Gas turbine power 146,7 MW e Steam turbine power 109,3 MW e Solar contribution 36 (3 x 12) MW e Plant Capacity factor ISCC 63 % Efficiency CC 52 % DNI annual 2100 kwh/m²/y DNI peak 850 W/m² Thermal storage 0 hours Solar to thermal efficiency (%) 50 % Extra burner no Depreciation time 20 years Mortgage repayment time 20 years Debt capital/total capital 80 % Debt capital interest rate 6 % Capital cost venture capital 12 % Inflation 2 % Taxes 0 % Fuel price Gas 20 /MWh th Investment CSP 57,42 mio Investment Power block (CC) 94,03 mio Investment Civil and structural work 4,51 mio Investment Indirect costs 43,00 mio Investment ISCC (total) 198,97 mio Annual production 1411,18 GWh/y Annual solar production 59,2 GWh/y Emissions 335,8 kg/mwh e LEC (min) 2 58,3 /MWh e LEC (max) 73,1 /MWhe Table 3 1. ISCC Reference plant properties 2 In further calculations, the minimum LEC is always shown in graphs and texts.
24 Defining the techno economic optimal configuration of hybrid solar plants The biggest part of the investment cost is attributed to the power block which contains the gas and steam turbine. The second part goes to the solar contribution (CSP), which contains costs for the solar field, tower infrastructure, receivers, The segment indirect costs includes engineering, contingencies and service during implementation. 64% 21% 15% LEC Total Capital Cost Total Operational Cost Fuel Expenses 22% 47% 29% 2% Investment cost Power block Civil and structural work CSP Indirect costs Figure 3 1. LEC and Investment costs of the ISCC reference plant The Levelized Electricity Cost of the reference plant consists of 15% operational and maintenance cost, 21% capital cost and 64% fuel expenses. Regarding the LEC, the fuel expenses are very high and the capital cost rather low, because of the small solar share of the reference plant. The LEC (min) is the cost of the ISCC plant in the first year of operation. The LEC (max) is the cost of plant in the 20 th year of operation. The LEC (max) is much higher, due to increasing operational costs by inflation and increasing gas prices. An increase of the gas prices by 1,34% per year has been taken into account for calculating the LEC (max). The solar output is estimated at 4,2% of the annual production representing 59,2GWh green electricity per year. The annual avoided CO 2 emission of the reference plant is tonne.
25 Defining the techno economic optimal configuration of hybrid solar plants PLANT SCALE UP One of the primary opportunities to reduce costs is to increase the size of the power plant. In general, power plant equipment costs decrease with the size of the plant. Looking at the specific investment cost of several Combined Cycle plants, the costs drop significantly with the net plant output. This is also the case for the ISCCS plants where more than 50% of the equipment cost of the plant (14% solar share) goes to the Combined Cycle installation (power block). Huge cost reductions would ensue if the ISCC plant capacity doubled (figures 3 2 and 3 3). A big plant however, implies great investment costs. It can be difficult to find enough financial resources, especially for the ISCC technology which is in a premature phase. 59,0 58,5 58,0 57,5 57,0 56,5 56,0 55,5 55,0 54,5 54,0 53,5 LEC ( /MWh e ) 58,3 56,6 55,9 55,5 55, Total capacitiy of plant (MW) Figure 3 2. Scale up effect : LEC vs Total capacity of the power plant Specific investment cost ( /W) 0,7 0,6 0,5 0,4 0,3 CIVIL STR WORK 0,2 CSP 0,1 POWER BLOCK Total capacitiy of plant (MW) Figure 3 3. Scale up effect: Specific investment cost vs Total capacity of the power plant
26 Defining the techno economic optimal configuration of hybrid solar plants TECHNOLOGY, COST AND BENEFIT To capture solar energy, there are several technologies existing today. However it cannot be predicted which of the technologies may finally achieve what market share or which options may eventually drop. For ISCC two interesting options have been developed: Parabolic Trough (PT) and Solar Tower (CRS)  PARABOLIC TROUGH Today all ISCC projects are planned using the Parabolic Trough technology. One of the possible reasons is because the PT technology is more commercially developed. A Trough is constructed as a long parabolic mirror (usually coated silver or polished aluminum) with a tube running its length at the focal point. Sunlight is reflected by the mirror and concentrated on the tube. The trough is usually aligned on a north south axis, and rotated to track the sun as it moves across the sky each day. Parabolic trough technology can only be deployed in very flat area with slope below 3%. The collector as the dominant cost fraction of the whole plant is estimated (by ECOSTAR ) between / f, depending on the type of heat transfer fluid (HTF) running through the tube. In spite of the high maturity, PT still has a potential for slight performance improvement and significant cost reduction. ECOSTAR  predicts a cost drop of 10% due to technological improvements. Sargent & Llundy  predicts a drop of the solar field costs around 20% between 2004 and The parabolic trough can use two types of heat transfer fluids, Thermal Oil or DSG (Direct Steam Generation). Trough systems using thermal oil can be considered as the most mature CSP technology. Major limitations of today s trough systems are caused by synthetic thermal oil, which is costly, may raise environmental concerns and is limited in its application temperature. DSG or steam collectors do not face the limits of the thermal oil. Also, the direct superheating of the steam increases the efficiency. This saves costs, reduces heat losses, pumping parasitic and eliminates the temperature limit.
27 Defining the techno economic optimal configuration of hybrid solar plants CENTRAL RECEIVER SYSTEMS (CRS) Central receiver systems, or solar tower, use a circular array of large, individually tracking mirrors (heliostats) to concentrate sunlight onto a central receiver mounted on top of a tower. Heat is then transferred for power generation through a choice of transfer media. There are three types of transfer media: molten salt 3, steam and atmospheric air . Today there are no planned ISCCS with a Solar Tower. The CRS technology needs 2 axis tracking, instead of 1 axis tracking like PT. In the past, 2 axis tracking was very expensive and hard to produce. Therefore PT was more commercially developed and is nowadays cheaper. Nevertheless the CRS has interesting prospects. ECOSTAR  predicts a 20% drop of solar field cost, due to very large heliostats or ganged heliostat concepts. Sargent & Llundy  estimate the cost reduction even higher, up to a maximum of 70%. Molten salt With respect to Central Receiver Systems, molten salt technology is the most developed. This is mainly attributed to very attractive costs for the thermal energy storage that benefits from a temperature rise in the three times greater than in the parabolic trough system. Additionally a higher annual capacity factor is possible for CRS due the smaller difference between summer and winter performance compared to parabolic trough systems . Saturated steam Steam receivers that have been built in several demonstration plants showed operational difficulties in the past, mainly attributed to the superheating of steam. This means it doesn t benefit from the high temperatures of the molten salt, which leads to a more expensive storage option. Saturated steam is considered as a low risk approach. Design concepts are based on experience in steam generator technology. This leads to relatively low investment costs for the receiver and combined with the low temperature, to a high receiver performance . Atmospheric air The benefit of this technology is mainly regarded for its simple design concept based on atmospheric air as heat transfer medium compared to synthetic oil or molten salt systems. The CRS with 3 Molten Salt is a nitrate mixture mainly of Sodium and Potassium. It has a relatively high melting point between 120 and 220 C .
28 Defining the techno economic optimal configuration of hybrid solar plants atmospheric air receiver technology may benefit from its simple design that promises quick start ups. However, this technology is still in R&D phase and it is only being tested in pilot plants. Further improvements are necessary to achieve cost figures similar to the other technologies presented here . Technology PT Oil PT DSG CRS M.Salt CRS Steam CRS Air Solar field ( /m²) Receiver & piping ( /kw th ) Civil works + tower ( /tower) 2% 4 2% Thermal storage ( /kwh th ) Indirect costs 20% 20% 20% 20% 20% Land use factor 30% 30% 35% 35% 35% Solar to thermal eff. 46,2% 48,4% 52% 50% 47,7% HTF Temperature 5 ( C) Table 3 2. Investment costs of different ISCC technologies  INVESTMENT COSTS AND LEC The trough option with steam has the lowest LEC (57,5 /MWh e ). The differences in LEC between the technologies are not large, partially due to the modest solar fraction. The larger the solar fraction, the larger the differences will be. The slight differences in LEC prove that the 5 technologies are very competitive nowadays. 61,0 Levelized Electricity Cost ( /MWh e ) 59,0 57,0 55,0 53,0 51,0 49,0 58,3 57,5 58,3 58,3 58,9 ISSC PT Oil ISCC PT DSGISSC CRS M.SALT ISSC CRS STEAM ISCC CRS AIR Figure 3 4. Levelized Electricity Cost of different ISCC technology LEC CC 4 For parabolic trough, 2% of the investment cost is charged for the civil works. 5 Temperature at field exit 
29 Defining the techno economic optimal configuration of hybrid solar plants SENSITIVITY ON LEC We can assume that the cost of the solar field and heliostats will decrease over time because of scale effects and technological improvements. The cost fraction of the solar field for a Trough field is a lot higher than the CRS option. This leads to a more sensitive LEC when the solar field or heliostat field cost decreases. The second biggest cost of the CRS technology is the receiver (30 40% of the CSP cost). 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% ISSC PT Oil ISCC PT DSGISSC CRS M.SALT Investment Cost of CSP ISSC CRS STEAM ISCC CRS AIR Figure 3 5. Investment costs of different ISCC technology Land Civil works + tower Receiver & piping Solar field In the long run cost drops of more than 70% are been predicted by Sergeant & Llundy for the CRS technology. The trough technology has less reduction prospects (20%) . The figure 3 6 below indicates the interesting future for CRS, in particular for Molten Salt and DSG. CRS with Saturated air is more expensive now but will benefit in the long run from the same cost reductions as DSG.
30 Defining the techno economic optimal configuration of hybrid solar plants Levelized Electricity Cost ( /MWh e ) 60 Parabolic trough Oil CRS M.Salt CRS Steam (20% reduction reciever) CRS Air Parabolic trough DSG CRS Steam CRS Steam (50% reduction reciever) % 10% 20% 30% 40% 50% 60% Reduction solar field/heliostat field Figure 3 6. Levelized Electricity Cost with reduction of the solar field CONCLUSION If we compare the LEC now for an ISCC with Parabolic Trough and an ISCC with Solar Tower, we can see there are slight differences. The key difference has to be sought in the potential cost reduction of the solar field, due to scale effects and technological improvements. Also the low prices of thermal storage for CRS with Molten Salt can result in very low costs. According to the predictions of Sargent & Llundy, the CRS technology with DSG will become the cheapest solution.
31 Defining the techno economic optimal configuration of hybrid solar plants THERMAL ENERGY STORAGE The main problem associated with solar power is its irregularity. The sun only shines for a limited period of the day and can be obscured by clouds or others things. Therefore, solar power has mainly been used to provide peak power. The use of thermal storage can lengthen the working hours of a solar plant. This allows furnishing base load instead of peak and reduces the inconveniences linked to the daily starting of the turbine. There are two kinds of thermal storage. Short term thermal storage (a few minutes to one hour) can prevent inefficiency of the power plant in case clouds hide the sun for some time. Long term storage (up to 15 hours) is used to assure constant production of electricity even during night time. A part of the heat generated by the solar field goes into the heat recovery steam generator while the rest is stored for later use. Some systems use the heat transfer fluid to store heat, others make use of a heat exchanger between two different fluids. Figure 3 7. Solar Tower power plant using two tanks molten salt storage 
32 Defining the techno economic optimal configuration of hybrid solar plants THERMAL STORAGE TECHNOLOGIES Thermal storage media can be solid, liquid or gaseous. The most common types of storage are  Molten salt storage and Room Temperature Ionic Liquids (RTILs) Concrete Storage Phase Changing Materials (PCM) Storage using solid materials Storage for saturated water/steam Molten salt storage and RTILs A state of the art storage type is the 2 tank molten salt storage tested in the Solar Two demonstration project in combination with a Central Receiver Solar Power Plant using solar salt as heat transfer fluid. This 2 tank molten salt storage was also proposed for parabolic trough solar power plants with synthetic oil as heat transfer fluid. Therefore it is necessary to have a heat exchanger for the heat transfer from oil to salt. The heat exchanger between molten salt and oil leads to security issues from possible chemical reactions and explosions in case of leaks . Pacheco et al.  published experimental results and theoretical investigations on the usage of a thermocline molten salt storage with a filler material in a parabolic trough power plant. The general idea is to reduce costs through the replacement of expensive salt by cheaper materials. The authors are nominating a cost reduction of about one third compared to a 2 tank molten salt storage. Therefore the 1 tank thermocline storage for parabolic trough plants, the selection of a durable filler material and the optimization of charging and discharging methods and devices are the main items. The development risk for them is low. And in the short term the technology can be implemented. The usage of new storage materials, so called Room Temperature Ionic Liquids (RTILs), may overcome this general drawback since these materials are liquid even at low temperatures. RTILs are organic salts with negligible vapour pressure in the relevant temperature range and a melting temperature below 25 C . Room temperature ionic liquids are quite new materials and it is rather uncertain, whether they are stable up to the temperature level required for CSP and also whether they may be produced at reasonable cost . The two tanks technology is already well used. The time required for full development and commercial implementation is estimated at less than 5 years. The 1 tank thermocline meanwhile will need 5 to 10 years to be commercially interesting. As for the RTILs, they will still need more than 10 years .
33 Defining the techno economic optimal configuration of hybrid solar plants Concrete storage The concept of using concrete or castable ceramics to store sensible heat in parabolic trough power plants with synthetic oil as heat transfer fluid (HTF) has been investigated. Since the steel tube register inside the storage material are rather expensive, a tubeless storage could lead to lower specific costs, but there are still some investigations needed for this design. The costs for the tubing are about 45 55% of the total storage costs. Advanced charging/discharging modes need additional investment in tubes and valves, but they may considerably increase the storage capacity for a given size and material. The basic idea of modular storage charging and discharging is to increase storage capacity by raising the temperature variation between both operating modes. Computer simulations from Tamme et al.  showed that the capacity of a given storage size could be increased by about 200% compared to the base case operation. The implementation of a concrete storage system can be realized within less than 5 years. The uncertainties and risks are for both cases (with or without tubes) in a medium range. And in addition the charging/discharging modes are promising . Storage with Phase Change Materials (PCM) Phase change materials (PCM) are potential candidates for latent heat storage, which is of particular importance for systems which have to deal with large fractions of latent heat, such as direct steam generating systems. PCM storages are not restricted to the solid liquid transition, they could also use solid solid or liquid vapour transition, but actually the solid liquid transition has some advantages compared to the other phase transitions. At present, two principle measures are being investigated: encapsulation of small amounts of PCM embedding of PCM in a matrix made of another solid material with high heat conduction. The first measure is based on the reduction of distances inside the PCM and the second one uses the enhancement of heat conduction by other materials. Storages based on PCM are in an early stage of development and many of the proposed systems are only theoretical or laboratory scale experimental work. Therefore cost estimation is difficult, but the cost target is to stay below 20 /kwh based on the thermal capacity. Even the uncertainties and risks of the PCM storage technology are in a medium range. The technology time required for full development and commercial implementation is more than 10years. PMC storage can be used for PT as well as ST power plants.
34 Defining the techno economic optimal configuration of hybrid solar plants Storage for air receivers using solid materials Storage types using solid material for sensible heat are normally used together with volumetric atmospheric or pressurized air systems. The heat has to be transferred to another medium, which may be any kind of solid with high density and heat capacity. Other parameters for a solid material storage are size and shape of the solids which may be chosen in order to minimize pressure loss (high pressure loss cause high parasitic). Beside fixed solid material as storage medium a new concept using silica sand as intermediate heat transfer medium was developed by DLR to avoid the disadvantages of storage vessels filed with fixed solid material in CSR with open volumetric air technology. The fixed solid storage medium technology is realizable within a shorter term (less than 5 years) than the moving solid storage medium technology (5 to 10 years) also the uncertainties and risks are in a medium range for solid medium and in a high range for the moving storage material system. Another innovation is to develop for pressurized closed air receivers a storage container that has to be pressure resistant up to about bar depending on the gas turbine pressure ratio. The receiver and the solar field for such a system would be able to deliver thermal power in excess of the power needed by the gas turbine during high insolation periods. This excess power is utilized to charge the thermal storage using a second air cycle driven by an additional blower. In the discharging mode, during non sunshine hours, the receiver is bypassed and the flow direction through the storage is reversed. In addition it would be possible to split up the compressor air flow during low insolation periods, in order to use thermal energy from the receiver and from the storage. For this case the time for development and implementation is 10 year and the risks and uncertainties are in a medium range. Storage for saturated water/steam In principle the steam drum, which is a common part in many steam generators, is a certain kind of storage because it contains an amount of pressurized boiling water. Steam could be produced from this component solely by lowering the pressure. This storage type has been built several times as process heat storage in industries thus the time required for full development and commercial implementation is rather low. The main problem is the size of the steam vessel for larger storage capacity and the degradation of steam quality during discharge.
35 Defining the techno economic optimal configuration of hybrid solar plants IMPACT ON THE COSTS OF THE POWER PLANT The investment costs for thermal storage that can be found in ECOSTAR  show that the cheaper technology with the longer storage possibilities is the 2 tank molten salt (Table 3 3). It is more profitable to use molten salt also as heat transfer fluid. It would reduce the losses due to the heat exchanger between the HTF and the storage medium. Besides, a molten salt cycle can reach higher temperatures than steam cycles. Plant Technology HTF Plant Capacity Thermal Storage Technology Storage Capacity Thermal Capacity of the Storage Spec. Investment Cost for Storage Investment Storage (% of total investment) PT thermal oil 50MW 2 tank molten 3h MWh 31 /kwh th 7.64% CSR molten salt 17MW 2 tank molten 3h MWh 14 /kwh th 3.42% CSR molten salt 50MW 2 tank molten 3h MWh 13 /kwh th 3.38% CSR saturated steam 11MW Water/steam 50min 15MWh 100 /kwh th 4.03% CSRatmospheric air 10MW Ceramic thermocline 3h 94MWh 60 /kwh th 12.88% Table 3 3. Investement costs of thermal storage for different solar technologies  The 17MW Solar Tres will be the first commercial molten salt central receiver plant in the world. With a 15h molten salt storage system it will be able to furnish electricity almost constantly. One of the other costs associated with thermal storage is the extra solar field needed to secure the same peak production while storing heat for later use. The following figure (3 8) compares the growth factor of the solar field in two different locations. The DNI influences greatly the need in extra solar field. For a plant in Barstow (DNI 2700) the solar field has to be doubled up to implement a 15h storage (figure 3 8). For the plant in Seville (DNI ) the solar field has to be tripled to add a thermal storage of 15 hours 6. 6 The growth factor for Seville is an estimation, based on data of Barstow from the document Two tank molten salt storage for parabolic trough solar power plants .
36 Defining the techno economic optimal configuration of hybrid solar plants ,50 3,00 2,50 2,00 1,50 1,00 0,50 0,00 Growth factor of the solar field Barstow (DNI=2700) Seville (DNI=2014) Storage size (h) Figure 3 8. Growth factor of the solar field with the hours of thermal storage in two different locations   Calculations of the impact of thermal storage on the cost of an ISCC power plant are based on two solar tower plants using molten salt as heat transfer fluid . The two sites investigated are Barstow, in the Mojave Desert, California where the plant Solar Two  was built, and Seville, Spain where Solar Tres is planned to be built. CSP Investment Cost ( ) Civil Works Solar field Extra solar field Land Reciever & Piping Storage 0h Storage (Seville, DSG) 3h Storage (Barstow, M.Salt) 3h Storage (Seville, M.Salt) Millions Figure 3 9 CSP Investment Cost of 3h storage in Barstow and Seville compared with no storage. As shown in figure 3 9, adding a storage of 3h implies increasing investment costs for the CSP installation. The biggest rise in cost of the thermal storage is the extra solar field. This cost is much higher for Seville due to the higher growth factor. The second main extra cost is the equipment cost for storage. The receiver and the land costs also increase.
37 Defining the techno economic optimal configuration of hybrid solar plants Figure 3 10 shows that a longer storage implies a higher LEC. This is mainly due to the increasing size of the solar field and the equipment cost of thermal storage. The lower the DNI, the higher the solar field growth and thus the higher the LEC. LEC ( /MWh e ) DNI 2000 (Seville) DNI 2700 (Barstow) Hours storage (h) Figure Evolution of the LEC with the thermal storage time for two sites with different DNI The figures 3 11 and 3 12 show that the solar contribution and the carbon dioxide emission evolve in desired direction as thermal storage increases. For high storage capacity (6h or more), the plant with the smallest DNI gives better results. This can be explained by the overrated growth factor of the solar field of Seville.
38 Defining the techno economic optimal configuration of hybrid solar plants Annual solar contribution DNI 2000 (Seville) DNI 2700 (Barstow) 12% 10% 8% 6% 4% 2% 0% Hours storage (h) Figure Evolution of the annual solar contribution with the thermal storage time for two sites with different DNI Carbon dioxide emissions (kg/mwh e ) DNI 2000 (Seville) DNI 2700 (Barstow) Hours storage (h) Figure Evolution of the CO 2 emission with the thermal storage time for two sites with different DNI
39 Defining the techno economic optimal configuration of hybrid solar plants EXTRA BURNER To increase the efficiency of the steam cycle of a common combined cycle an extra burner is usually added to super heat the steam already heated by the exhaust gases from the gas turbine. As these exhaust gases still contain a sufficient level of oxygen, the added fuel can burn. The same system can be installed in ISCC plants. However, the goal of ISCC technology being to reduce non renewable resources consumption and lowering greenhouse gases emissions, we can question the merits of an extra burner. Anual production (GWh e /y) Levelised Electricity Cost ( /MWh e ) , , , NO EXTRA BURNER WITH EXTRA BURNER 58,0 57,5 58,3 59,4 NO EXTRA BURNER WITH EXTRA BURNER Figure Annual electric production and LEC of ISCC power plants with or without extra burner 350,0 345,0 340,0 335,0 330,0 325,0 Carbon dioxide emissions (kg/mwh e ) 335,8 347,2 348,2 NO EXTRA BURNER WITH EXTRA BURNER CC Figure Comparison of the CO 2 emissions of ISCC plants with or without extra burner and a CC plant Figure 3 13 shows an increased annual production for the plant with extra burner, as anticipated. Also the LEC is slightly higher because of extra expenses of fuel for the duct burner. The CO 2 emission per MWh e on the other side is almost the same as emitted by a combined cycle (figure 3 14).
40 Defining the techno economic optimal configuration of hybrid solar plants OPERATION AND MAINTENANCE The operation and maintenance (O&M) of a parabolic trough power plant is very similar to conventional steam power plants that cycle on a daily basis . Parabolic trough power plants typically require the same staffing and labour skills to operate and maintain them 24 hours per day. However, they require additional O&M requirements to maintain the solar fields. Initial plants required a substantial number of mechanics, welders, and electricians to maintain immature solar technology. Modern parabolic trough solar technology is much more robust and requires minimal preventive or corrective maintenance. The one exception is mirror washing. The high pressure demineralised water system (called Mr. Twister) has sprayers that spin as they move down when washing the mirrors. Experience has shown that solar field mirrors must be washed frequently during the summer. But the increase in solar output pays for the cost of labour and water. Current power plants may wash mirrors weekly during the peak solar times of the year. It's usually only necessary every few months during the winter. The reduction of O&M cost is primarily a result of the increase in annual plant capacity factor . The plant capacity increases as a result of the increase in thermal storage. However, increasing the size (MW e ) and utilization (capacity factor) of the power plant incurs very little increase in O&M expenses ($/year). This is because the quantity and complexity of the equipment remain constant and staffing remains fairly constant. The following table gives a comparison of O&M costs for a parabolic trough ISCC, a solar tower ISCC and a combined cycle plant. As expected, the fixed O&M costs are much lower for a CC plant than for solar technology while the variable costs are higher . Unit HTF trough Air Tower Reference CC Fixed O&M cost $/kw/a Variable O&M cost /kwh Total O&M cost /kwh Table 3 4. Operation and Maintenance costs of different ISCC Technologies and CC