Energy economic evaluation of process heat supply by solar tower and high temperature reactor based on the ammonia production process

Applied Energy 212 (2018) 622–639

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Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Energy economic evaluation of process heat supply by solar tower and high temperature reactor based on the ammonia production process

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Sarah Schrödersa, , Hans-Josef Alleleina,b a b

RWTH Aachen University, Institute for Reactor Safety and Reactor Technology, Kackertstrasse 9, 52072 Aachen, Germany Forschungszentrum Jülich, IEK-6, 52425 Jülich, Germany

H I G H L I G H T S of the high temperature heat supply system of an industrial process. • Optimization cost analysis of solar tower and high temperature reactor (HTR). • Detailed and nuclear heat supply systems should include a fossil backup heater. • Solar the solar tower nor the HTR can compete to the fossil heat supply. • Neither • A gas price rise of 250–300% is necessary for economic competitiveness.

A R T I C L E I N F O

A B S T R A C T

Keywords: Solar tower High temperature reactor Process heat supply Economic analysis Optimization

Changing the heat supply of energy intensive industries from today’s mostly fossil sources to nuclear or renewable sources offers the opportunity of reducing greenhouse gas emissions. Many industrial processes require heat at a high temperature level, which restricts the selection of the heat source. The high temperature reactor (HTR) as a nuclear and the solar tower as a renewable technology are technically capable of supplying high temperature process heat, but their implementation on an industrial scale depends not only on the technical feasibility but also on the economic competitiveness to the conventional basically fossil heat supply. In this paper, the economics of the high temperature process heat supply by HTR and solar tower are analyzed and the question whether these alternative systems can compete to the fossil heat supply is answered. The analyses focus on the example process of ammonia production. A new mathematical optimization model is applied which determines the optimal facilities sizes and the optimal facilities operation modes for different energy supply systems. The results reveal that none of the energy supply systems containing a solar tower or an HTR can compete to the heat supply of a heater fired by natural gas. The pure solar heat supply of a constantly operated ammonia plant turns out to be particularly disadvantageous. Moreover, the coupling of the HTR and the solar tower as two capital-intensive technologies is not a sensible option. Under good solar conditions, the heat supply of a constantly operated ammonia plant by an HTR and a fossil backup system turns out to be the best alternative to the fossil heat supply. Only under excellent solar conditions, which occur for example in South Africa, the fossil supported basically solar heat supply is the best alternative system. Nevertheless, even those best alternative heat supply systems can only compete to the fossil heat supply if the gas price rises significantly, in a scope which cannot be expected in near future.

1. Introduction In Europe, the industrial process heat supply accounts for 27% of the whole end energy consumption [1]. The biggest share of the industrial process heat demand is required at high temperatures above 400 °C [1]. On a world-wide level, these proportions are similar [2].

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Ninety percent of the world’s industrial heat demand is supplied by the combustion of fossil fuels accompanied by massive greenhouse gas emissions [2]. Changing the basically fossil heat supply by applying renewable or nuclear heat sources offers a huge potential to reduce greenhouse gas emissions. Further advantages offered by the application of alternative heat supply systems lie in a reduced dependency on

Corresponding author. E-mail address: [email protected] (S. Schröders).

https://doi.org/10.1016/j.apenergy.2017.12.063 Received 5 October 2017; Received in revised form 1 December 2017; Accepted 10 December 2017 0306-2619/ © 2017 Elsevier Ltd. All rights reserved.

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Department of Energy (DOE) is used for the economic analysis. INL estimates the hydrogen generation costs and conducts a parameter variation, whereby the expected rate of return is identified as main driver on the economics. Furthermore, the process of nuclear integrated ammonia production is investigated by INL in [14]. A preliminary economic evaluation is conducted calculating different urea generation costs by given rates of return and different rates of return by given urea generation costs. Moreover, INL investigates the impact of changing the hydrogen production process on the economics of the ammonia production process and comes to the conclusion that the hydrogen production by HTSE is at present not economic competitive. General Atomics (GA) investigates hydrogen production via the nuclear powered sulfur iodine (SI) process and via the nuclear powered HTSE. In contrast to INL’s results, the parameter variations conducted in [15,16] by GA indicate that both, the SI-process powered by HTR as well as the HTSE powered by HTR can compete to the fossil fired steam methane reforming process. The U.S. Electric Power Research Institute (EPRI) investigates in [17] the economic performance of different nuclear heated hydrogen production processes. The results of the economic evaluation conducted by EPRI indicate that the steam methane reforming process heated by HTR is already cheaper than the fossil heated steam methane reforming process. Herd et al. investigate in [17] the availability of a steam supply system heated by HTR. They calculate the necessary backup capacities and their impact on the systems economics. The analysis includes a cost comparison of different heat supply systems. The authors conclude that fossil fired backup capacities are by far cheaper than nuclear backup capacities.

fuel prices, in the diversification of the energy supply and in savings of fossil resources. Just a few non-fossil technologies are capable of supplying heat at the required high temperature level. Among the renewable sources, the solar tower is a promising technology as it is able to reach temperatures of about 1000 °C and it can be built in power ranges suitable for industrial application [3]. Among the nuclear technologies, the high temperature reactor (HTR) is capable of supplying high temperature heat for industrial processes due to its high reactor outlet temperature in the range of 700–950 °C [4]. However, the implementation of solar or nuclear heat supply systems on an industrial scale depends on the economic competitiveness to fossil sources. In this paper, energy economic evaluations are conducted in order to analyze the economic competitiveness of the high temperature process heat supply by solar tower and HTR. Besides solar and nuclear heat sources, bioenergy could be used to provide industrial non-fossil heat. Thereby the ecologic effects of bioenergy such as land use and CO2 emissions must be considered. A good overview is given by Muench and Guenther [5]. The consideration of bioenergy is not scope of this paper but could be analyzed in further investigations. 1.1. Current state of research The analysis and choice of suitable industrial processes which can be supplied by heat coming from solar tower or HTR is subject of current research. A few high temperature applications (for example the solar and nuclear hydrogen production) have been tested successfully in pilot scale. Nevertheless, they have not yet been applied in industrial scale. The International Atomic Energy Agency (IAEA) and Idaho National Laboratory (INL) provide an overview of industrial processes whose heat demand can be supplied by the HTR [6,7]. Possible areas of application lay in the oil industry, the petrochemical industry, the metal production, the textile industry and in the food industry [6]. One famous and often discussed application is the production of hydrogen. Numerous papers focus on the economic evaluation of the solar or nuclear electricity production. Moreover, numerous research papers analyze technical aspects of the high temperature process heat supply by solar tower or HTR. Nevertheless, only little attention has been paid to the economics of high temperature process heat supply by solar tower or HTR. The few research activities in this field will be presented first. Subsequent, the limits of those research activities will be discussed and the new approach presented in this paper will be introduced. The German Aerospace Center (DLR) presents in [8] an overview of different solar thermal hydrogen production processes. For each process a cost calculation is given. Afterwards, the hydrogen generation costs are estimated and compared to those of conventional hydrogen production processes. As a result the DLR claims that the solar heated steam methane reforming process is close to the economic competitiveness to the fossil fired steam methane reforming process. In [9] Möller et al. investigate the process of solar steam methane reforming in detail. Thereby, the reforming step takes place in the directly solar radiated receiver-reactor without the supply of fossil heat. The economic evaluation of that process bases on the assumption of 2.000 full load hours. Under that assumption Möller et al. find out that this discontinuously running solar heated steam methane reforming process becomes competitive to the fossil fired process if gas prices double. On the nuclear side numerous studies focus on hydrogen production processes powered by HTR. INL investigates in [10] the economic performance of the nuclear integrated steam methane reforming process. The hydrogen generation costs are calculated for the fossil energy supply case and for the HTR integrated energy supply case. INL estimates the gas and CO2-prices which are necessary for the nuclear integrated system to break even. Besides the nuclear integrated steam methane reforming process, the nuclear powered hydrogen production via high temperature steam electrolysis (HTSE) is analyzed by INL in [11–13]. The Hydrogen Analysis Project (H2A) developed by the U.S.

1.2. Limits of previous research activities and classification of the new approach Until now, neither the solar nor the nuclear process heat supply has been investigated with the help of an optimization model. But only by applying an optimization model the economic optimal combination of facilities sizes and facilities operation modes can be determined which is essential for an objective comparison. Especially, the optimal dimensioning of the solar components like reflection area and storage capacity is very complex and depends on several economic and technical parameters [18]. In the here defined optimization model those complex economic and technical interdependencies are considered in the constraint matrices of the optimization problem, whereas in previous investigations simplified assumptions (e.g. the dimensioning of the solar system based on nominal conditions) have been made. Instead of using the simplifying assumption of full load hours an hour-sharp calculation of a whole year is used to calculate the facilities operation modes. Apart from the work of Herd et al. [19] none of the in Section 1.1 mentioned economic analyses considers the requirement of industrial applications for a high security of supply, even though this topic is essential for the economic performance of capital intensive heat supply systems such as nuclear and solar. Consequently, the results of the above mentioned economic analyses must be examined critically as backup capacities are not part of their cost estimations. In this paper, a special emphasis is put on plants availabilities, backup capacities and their influence on the economic performance of each heat supply system. Realistic cost data is a fundamental requirement for a profound economic evaluation of the HTR and the solar tower. As neither the aircooled solar tower nor the HTR are established technologies, real cost and price data is not available. The discrepancy between cost assumptions of the economic evaluations summarized in Section 1.1 indicates that at least some of the investigations do not base on wellfounded data analyses. Moreover, most assumptions are not justified or even not mentioned. The input data used in this paper base on a 623

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duration of downtimes can be reduced [4]. The low power density of 2–6 MW/m3, the disperse arrangement of fuel particles in very heat conductive graphite, the small core diameter and the empty spaces between the fuel pebbles allow a passive heat removal during accidents [4]. Coolant outlet temperatures of 750–950 °C are ideally suited for a wide spectrum of high temperature process heat applications in various industries. When applying a HTR to heat a chemical process, the nuclear part can be decoupled from the chemical part by an intermediate heat exchanger (IHX). Primary helium exiting the reactor transfers heat to the secondary helium via the IHX, which is used to heat the chemical plant. Under normal operating conditions, the IHX prevents the primary helium from accessing the process plant limiting or excluding a potential radioactive contamination of the product. Vice versa, the IHX prevents processing gases from being routed through the reactor containment. The clear separation between nuclear plant and heat application is a necessary precondition to certificate the chemical plant under conventional, non-nuclear conditions. Disadvantages of the IHX are the exergy losses due to the additional heat exchange step [21]. HTR operational experience could be gathered, inter alia, through test facilities like the German AVR, the Chinese HTR-10 and the Japanese HTTR [23]. A demonstration plant is currently under construction in China. This HTR-PM (High Temperature Reactor – Pebble Bed Module) is a double block plant with a thermal power of 2 × 250 MW used for electricity generation [23]. The nuclear steam methane reforming, which is the central part of the ammonia plant, was investigated in the German test facilities EVA I and EVA II. Thereby, electrically heated helium could heat the reformer tubes up to a temperature of 800 °C reaching a methane conversion rate of 85% [21].

profound literature research. Thereby, published data is summarized, compared and critically examined. Moreover, a special attention has been given to the consistency of the solar, nuclear and fossil data, facilitating economic evaluations under uniform conditions and with a uniform evaluation method. Until now, no direct comparison between the economics of the solar tower and the economics of the HTR regarding high temperature heat supply has been drawn. Furthermore, the coupling of solar towers and HTRs has not yet been investigated. Applying the optimization model for different heat supply cases a direct comparison between solar and nuclear process heat supply systems will be drawn in this paper. Furthermore, the coupling of nuclear, solar and also fossil heat sources will be investigated in detail. Due to the high variety of energy demand structures of different industrial processes, it is necessary to focus on an example process. Here, the production of ammonia is chosen as a highly relevant example process with a high temperature heat demand. 2. System descriptions 2.1. Ammonia synthesis The Ammonia Synthesis requires a syngas stream consisting of hydrogen and nitrogen. Today’s ammonia plants basically provide this syngas by applying the process of steam methane reforming. In the first reforming step, the educts steam and methane are catalytically reformed into a mixture of carbon monoxide, carbon dioxide and hydrogen. In a conventional ammonia plant, the required nitrogen is provided by burning a part of the produced hydrogen with air in a second reforming step. In this reforming step, a temperature of about 1200 °C is reached. After the two reforming steps, the carbon monoxide shift conversion is applied to convert carbon monoxide into carbon dioxide, which can be extracted physically or chemically by a carbon dioxide removal unit. A drying unit removes water from syngas. The final purification of the syngas stream can be achieved by Methanation and by a purifier. After final purification, the syngas stream is compressed and enters the Haber-Bosch-Reactor where it is partly converted into ammonia [20]. In this paper, a different process configuration is investigated. Analogous to the conventional ammonia plant, hydrogen is provided by applying a steam methane reforming process. Instead of supplying the nitrogen by burning a part of the produced hydrogen in a second reforming step, the required nitrogen is supplied by an air separation unit. The maximum reforming temperature is lower compared to the conventional ammonia plant leading to a reduced methane conversion. The carbon monoxide shift conversion, the carbon dioxide removal unit and the drying unit are applied analogues to the conventional process. Final purification can be achieved by a pressure swing adsorption (PSA). The ammonia synthesis is similar in both processes. The educts nitrogen and hydrogen are compressed to about 200–300 bar and then converted into ammonia through an endothermic equilibrium reaction in the Haber-Bosch-Reactor. The significant high temperature heat demand of the ammonia plant is required in the reforming step. Instead of heating the reformer tubes with a fossil heater through the combustion of natural gas, the so called allothermal reforming can be applied in which the reformer tubes are heated convectively through a solar or a nuclear heated transfer medium [8,21].

2.3. Solar tower and thermal storage Amongst the solar thermal facilities, just point-focusing solar towers and dish-collectors reach the required high temperatures above 600 °C [1]. Due to their small power range of about 10–40 kW, dish-collectors are not suitable for industrial application [24]. As solar towers are scalable into the high MW-range, they represent the most promising technology for solar high temperature process heat supply [24]. A solar tower consists basically of the components heliostat field, receiver and tower [25].The biaxial tracked heliostats reflect the direct part of the solar radiation onto the central receiver which is installed on top of the tower. Concentration factors of 600–1000 are reached [25]. Inside the receiver, solar radiation is transformed into heat which is transferred to a heat carrier fluid. Several solar tower concepts have been developed, which basically differ in the receiver design and the heat carrier applied [26]. High temperatures of around 1000 °C can be reached if air is used as heat carrier fluid [27]. In volumetric receivers, the absorber material is cooled by passing air [27]. Steel mesh or porous ceramics can be used as absorber material [25,27]. The big inner surface compensates for the disadvantage of the bad heat transfer properties of the heat carrier air. The transformation of radiation into heat takes place ideally inside the absorber material and not at the absorber front, so that the absorber front temperature, which is decisive for the thermal losses, is reduced [28]. Currently, two different designs of the volumetric receiver are pursued: the open and the closed volumetric receiver. In closed volumetric receivers, a dome shaped window is installed in front of the receiver. The air is pressurized up to 7–15 bar [29]. The window surface is limited due to the pressure. Therefore, a secondary concentrator is installed in front of the window to concentrate the radiation coming from the heliostats onto the limited receiver surface [30]. This secondary concentrator reduces the optical efficiency. The open volumetric receiver is operated at ambient pressure. After the hot air transferred its heat to the heat application, it is returned via air channels back to the receiver front where it can be partly sucked back into the receiver. Hereby, around 60% of the still warm returned air can be

2.2. High temperature reactor According to the Gen-IV International Forum, the HTR is one of the most promising nuclear reactor concepts of the next generation [22]. The HTR is a helium-cooled, graphite-moderated reactor using graphite-embedded coated particles as fuel [4]. In a pebble bed HTR, the fuel pebbles are changed continuously during operation, whereby the 624

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optimization model.

reused [25]. The rest of the sucked air has ambient conditions. Experience with the open volumetric air-cooled receiver could be gathered with the German 1.5 MWel solar tower in Jülich [27]. This solar tower reaches air outlet temperatures of up to 700 °C [31]. At very high temperatures, the closed volumetric receiver reaches better efficiencies than the open volumetric receiver, as thermal losses are lower in the closed receiver, but the secondary concentrator causes temperature independent optical losses. Both the open and the closed receiver concept, achieve the same level of efficiency at the investigated temperature of about 850 °C. Li et al. showed in [32] that the open volumetric receiver is far more developed than the closed one. As we focus on technologies which can be applied in an industrial scale soon, the open volumetric receiver will be investigated in this paper. Besides the advantage of a higher development stage and the existing experience with a pilot plant (solar tower in Jülich) the open volumetric receiver causes lower investment costs. The solar tower can be complemented by a thermal storage in order to decouple the industrial process from the fluctuating solar radiation. It can be used to bridge clouds and to balance daily and even seasonal radiation fluctuations enabling a flexible heat supply in accordance to the heat demand [33]. A solid particle storage is ideally suited to be coupled with an open volumetric air cooled receiver. This storage technology is, for example, applied in the solar tower in Jülich [34]. Compared to a thermochemical storage, which would be also suitable for high temperature applications, the solid particle storage, as sensible heat storage, is convincing because of the higher development stage and lower investment costs [33]. To charge a solid particle storage, hot air coming from the receiver flows through the particle filled chambers of the storage from the top to the bottom whereby the particles are heated up [35]. To discharge the storage air flows through the storage reversely. Thereby, the hot air temperature can be kept approximately constant until it drops just before the storage is completely emptied [34]. There are different possibilities to operate the solar steam-methanereforming. The first possibility is to irradiate the reformer directly, for example, in a so called receiver-reactor. An advantage of this technology is the low number of heat exchange steps reducing exergetic losses [8]. The reforming reactions take place directly in the receiver, which is the hottest place, achieving relatively high methane conversion rates [36]. A disadvantage of a direct radiated reformer is the intermittent operation mode which may have negative impact on reliability, lifetime and product quality [8]. In an indirectly heated reformer, the reformer tubes are heated convectively through hot air. This concept has been successfully tested at the Plataforma Solar de Almeria within the ASTERIX project [37,38]. The decoupling of the reformer and the receiver facilitates the integration of a thermal storage and the hybrid operation mode of the solar tower together with a nuclear or fossil heat source [8,38]. Thus, the reforming process can be operated constantly despite fluctuating solar radiation [38]. Moreover, this reformer concept allows the application of the further developed open volumetric receiver [39].

3.1. Model overview Within an optimization model, an objective function is optimized under several constraints. In the here defined optimization model, the Mixed Integer Linear Programming (MILP) is applied. An advantage of the MILP is that the optimality of the solution, which can be found through standardized solution methods, is guaranteed [40]. Disadvantages are the simplifications which are necessary to formulate the objective function and all constraints linearly. The model optimizes the size and operation mode of each facility for one year. Thereby, an hourly view is chosen. All cash flows arising from this one-year calculation are extrapolated for the observation period (72 years) using the discounted cash flow method. By this means, the net present value of the whole system can be determined taking into account interest effects. The maximization of the so derived net present value serves as objective function. The ammonia generation costs, which serve as the main assessment parameter, are derived by dividing the net present value by the discounted ammonia production quantity. Mass and energy balances, limits of the flexible operation mode, technical availabilities, the availability of the solar radiation and the fulfillment of ammonia delivery obligations are considered within the constraints. The facility structure determines which facilities are considered within the optimization of the energy supply system. By varying the facility structure, different supply cases (solar, nuclear, fossil and hybrid cases) can be investigated. This structure is determined before the optimization starts. For each facility allowed in the facility structure, the size and operation mode are optimized simultaneously. The facility structure and the facilities sizes are kept constant during the whole observation period. Time dependent variables define the operation mode. An overview of the necessary input data, the characteristics of the optimization program, the software and the program flow is given in Fig. 1. 3.1.1. Facilities structure The ammonia plant requires heat and electricity. The heat demand can be supplied directly through one of the three implemented heat supply facilities: solar tower, HTR or fossil heater. Moreover, the heat can be supplied by discharging the thermal storage. The thermal storage can be charged by each heat supply facility. The electricity demand of the process can be fulfilled either externally by purchasing electricity or internally by a steam turbine. Here and in the following, the term “turbine” includes all facilities necessary for electricity production in a Rankine cycle, like feed water pumps, the steam turbine, the condenser and the generator. The heat supply facility is not included in this definition. The necessary heat to run the turbine can be supplied by solar tower, HTR, fossil heater or thermal storage. Surplus electricity can be sold at the electricity market. Fig. 2 shows the implemented facility structure of the optimization model. In order to investigate different supply cases, the facility structure can be adopted by excluding certain facilities. One solar tower and one HTR-module underlie certain power limits. To facilitate higher power levels, the solar tower and the HTR is designed modularly. In this paper, it is assumed that the ammonia plant must fulfill constant delivery obligations. In most investigations, which are presented in Section 4, it is assumed that the ammonia plant runs continuously at full load every time it is not shut down for maintenance. Therefore, the continuous heat supply must be ensured. Besides, the discontinuous operation mode of the ammonia plant will be investigated in Section 4.3.2. In order to fulfill the constant ammonia delivery obligations, the system can be complemented by an ammonia storage. For each facility implemented in the optimization model, individual

3. Methodical approaches For each heat supply system (solar, nuclear, fossil or hybrid), the economic optimal energy supply system should be considered to enable an objective evaluation of their economic competitiveness. Due to fluctuating solar conditions, the determination of the optimal thermal power of the solar towers in relation to the optimal storage capacity is not trivial and depends inter alia on the operation mode of all involved facilities. The consideration of variable electricity prices, time dependent availabilities of each facility and restrictions concerning the flexible operation mode of the facilities further increases the complexity. To ensure that for each heat supply technology the optimal energy supply system is considered for the economic assessment, each facility’s size and each facility’s operation mode are optimized within an 625

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Program flow Input

Microsoft Excel Economical data Interest rate Tax rate Depreciation time Life time Construction time Prices CO2 Gas Electricity Costs of all facilities Investment O&M Decommissioning Fuel / CO2 Start up costs Technical data of all facilities Temperatures Parameter limiting the flexible operation Heat media data Efficiencies Downtimes Weather Data DNI Ambient temperature Angle of solar radiation

Defining the optimization problem

Solving the optimization problem

Output

Matlab

IBM CPLEX

Matlab / Microsoft Excel

Linear objective function Maximization of the net present value

Simultaneous optimization of all variables

Linear constraints Mass balances Heat balances Electricity balances Compensation of heat losses (storage) Limits of flexible operation modes Availability of solar radiation Technical availability of each facility Fulfillment of ammonia delivery obligation

Net present value Ammonia generation costs Number and sizes of all facilities Operation modes of all facilities Electricity purchase and sales amounts

Variables (binary, integer and continuous) Number and sizes of all facilities Operation modes of all facilities Electricity purchase and sales amounts

Fig. 1. Overview of the program flow and software.

a few towers with a high thermal power or many towers with a small thermal power. The nominal electric power of the turbine, the nominal thermal power of the fossil heater and the maximum energy content of the thermal storage serve as size variables for those three components. The size of the ammonia plant is determined through the maximum ammonia production rate. The size variable of the ammonia storage is determined by its maximum capacity. The simultaneous optimization of mass flow and temperature would induce quadratic constraints. Since the model is defined as a linear problem, the inlet and outlet temperatures of each facility are fixed and assumed to stay constant. Power changes are performed by mass flow adaption. For every heat stream shown in Fig. 2, operation mode variables for mass and heat flow are defined for every hour. How the generated heat of a heat supply facility is divided between ammonia plant, thermal storage and turbine is consequently a result of the optimization. Furthermore, an operation mode variable is defined for the electricity demand of each electricity-requiring facility for each time step. The electricity sales and production quantities and the electricity generated by the turbine reflect further operation mode variables. Moreover, for every time step, the storage levels of the thermal and the

downtimes are defined. This applies also for every solar tower and for every HTR-module. Downtimes are specified through shutdown duration and the shutdown point in time. The downtimes are differentiated between planned and unplanned downtimes. While the planned downtimes are determined via pre investigations in a way which is optimal for the energy supply system, the unplanned downtimes are generated randomly.

3.1.2. Size and operation mode optimization The size and the operation mode of each facility is optimized simultaneously, therefore suitable size and operation mode variables need to be determined. The most important optimization variables are introduced to show the degrees of freedom within the optimization. The thermal power of one HTR-module is fixed before the optimization and is set to 250 MW. The number of built HTR-modules is a size variable and is therefore optimized. In contrast, the solar tower size is scalable in certain boundaries. The size of the solar towers is determined through three size variables: the thermal power of one receiver, the size of the heliostat field and the number of built towers. As no specific power is determined, the solver can decide between building 626

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electricity sale turbine

electricity market

solar tower

electricity purchase electricity demand thermal storage

HTR

heating

fossil burner ammonia plant heat

electricity

ammonia storage

facility with electricity demand

ammonia

Fig. 2. Facilities structure of the optimization model.

required for every further tower and one part which is depending on the solar tower size (see Section 3.2.3.2). In a linear optimization model, the modeling of the thermal storage also requires special facilitations. It is assumed that thermal losses occurring due to self-discharging are reheated every hour by an electric heater (the investment costs of the electric heater are considered). Through this assumption, the discharge temperature is not correlated to the storage duration and is therefore time independent. Since the amount of heat losses depends on the temperature, heat losses are higher in this configuration than they would be in reality. Nevertheless, previous calculations have shown that this has only a minor influence on the economics of the solar system. If heat losses would be reheated by natural gas the economics could be improved slightly (reduction of the ammonia generation costs of 1.8% in the pure solar and of 1.1% in the solar-fossil-hybrid case). Furthermore, exergetic losses need to be taken into account. While discharging the storage, cold air flows from the bottom to the top. The air exiting the storage at the top is not as hot as the air which charged the storage before. If this temperature is too low to heat the ammonia plant a second electric heater is needed which raises the air temperature back to the temperature level required by the ammonia plant.

ammonia storage are optimized. The size and operation mode variables are correlated. These correlations are defined through equality and inequality constraints. Generally, the size variable is determined by the maximum value of the corresponding operation mode variable. Within an optimization problem, the modeling of the solar heat supply system is due to the dependency on the fluctuating solar conditions being more complex than the modeling of the heat supply by fossil heater or HTR. Therefore, the modeling of the solar system is explained in more detail within the next chapter. 3.1.3. Modeling of the solar system Numerous optical and thermal losses occur during the transformation of the radiation energy reaching the heliostats into the thermal energy of the receiver’s heat transfer fluid. Those losses depend inter alia on the direction of the solar beams, the arrangement and size of the heliostats and the tower and the temperature and surface of the absorber front of the receiver. Due to limited computing capacities, all these aspects cannot be modeled in the optimization tool itself. Consequently, a two-step approach is chosen. In a first step, the solar tower efficiency is determined in an upstream self-developed heliostat field arrangement tool. Thereby, the heliostat field layout is designed considering optical and thermal losses. (Losses which occur due to the incomplete air return are considered in the optimization tool itself.) As soon as the heliostat field layout is fixed, the hourly time series of the solar tower efficiency can be generated. Hourly time series of the direct normal irradiance (DNI), the position of the sun and ambient conditions are taken as input data for this first step. The second step is the application of the optimization tool, in which the solar tower efficiency time series generated in the first step are used as input data. Together with the hourly DNI time series, the hourly available solar energy is determined. Therefore, the fluctuating nature for the solar radiation is considered (daily and yearly fluctuations). Generally, investment cost, personnel need and the efficiency of a solar tower all depend on the solar tower size. A direct consideration of this relation cannot be modeled in a linear optimization tool. Therefore, the solar towers are discretized into three ranges: small, medium and large. For each power range, an own solar tower efficiency time series is generated applying the heliostat field arrangement tool. Different specific investment costs for each power range are considered through scale factors. The personnel requirement is separated into three parts: one part that is required as soon as one tower is build, a part which is

3.1.4. Objective function All cash flows arising from the year prospective of the size and operation mode optimization are extrapolated for the whole observation period (sum of construction time, lifetime and decommissioning time) and are discounted to the year of commissioning which serve as reference year. The so derived net present value is implemented in the objective function. By maximization of the net present value, the lowest possible ammonia generation costs can be derived, as the annual ammonia production quantity is fixed. The amount and the time structure of all cash flows are considered. The cash flows include the investment costs, fixed and variable operation costs, decommissioning costs as well as costs and benefits for electricity purchase and sale. Fixed operation costs include costs for operation and maintenance, insurance as well as personnel. Variable operation costs basically consist of fuel costs and start-up costs. To finance the decommissioning of a facility, constant provisions are saved during the facility’s operation time. Income taxes are considered. A linear depreciation method is assumed. The depreciation time is set to 20 years [12,17,41,42]. The lifetime of a solar tower and a thermal storage is assumed to 627

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The prices and costs used as input data are derived by an extensive literature research. In order to make cost assumptions from different literature sources comparable, they are inflation adopted and converted into Euro. As a reference year for prices and costs, the year 2015 is taken. Investment costs are inflation adopted by the Chemical Plant Cost Index (CEPCI). All other cost components are adapted to the 2015 price level via a constant inflation rate of 1.9%. The exchange rate between US-Dollar and Euro is assumed to amount to 0.9 €/$.

amount to 30 years [9,43]. The lifetime of all other facilities is assumed to amount to 60 years [23,44–47]. Consequently, the solar components must be rebuilt after 30 years in order to guarantee the heat supply of the ammonia plant for 60 years. The investment costs and the decommissioning costs arising from the rebuilding of the solar facilities are considered in the objective function. It is assumed that an HTR can be built within 6 years [48]. During this period the investment costs are distributed according to an S-curve [49]. The construction time of a solar tower and a thermal storage is set to two years [50]. Further, it is assumed that the ammonia plant can be built within two years, whereas the turbine and the fossil heater can be built within one year. Real (inflation adjusted) cash flows are discounted with a real (inflation adjusted) interest rate. When calculating the interest rate it is important to bear in mind that the interest rate depends on the risk of an investment. Due to the lower development stage and the higher technology complexity, investments in alternative heat supply systems using HTRs or solar towers are riskier than an investment in the conventional fossil supplied ammonia plant. Thus, the interest rate for the alternative heat supply systems is higher. Furthermore, for a nuclear plant a risk surcharge is applied [51,52]. The interest rate is calculated using the WACC approach. Data provided by the International Renewable Energy Agency (IRENA) [53], Fraunhofer ISE [54], INL [7,10,11], NGNP [46] and KPMG [55] is used to calculate the nominal interest rates. An inflation rate of 2% is assumed to derive the real interest rate [56]. For a heat supply system without an HTR or a solar tower, a real interest rate of 5% is assumed. If the heat supply system includes a solar tower, a real interest rate of 5.5% is assumed. If an HTR is included in the heat supply system, the real interest rate amounts to 6%.

3.2.1. Site conditions Solar radiation conditions, prices for electricity, natural gas, water and CO2-Emissions, the composition of natural gas and the tax rate all depend on the location under investigation. It should be noted, that the wage level and therefore all cost components also depend more or less on the location. But, as the cost data base is already vague this aspect is not considered. The south of Spain gets natural gas from Algeria via the two import stations: Almeria and Tarifa [58]. Taking the gas consumption of these import stations, the specific CO2-Emissions can be calculated to 2.76 kg CO2 per kg natural gas [59]. The heating value of the natural gas amounts to 14.53 kWh/kg [58]. The natural gas price refers to the price which an industrial company in Spain has to pay depending on its annual gas demand. Eurostat is used as the data source [60].The gas demand of the ammonia plant itself is already high enough, so that the last price band with the lowest gas price is considered. It amounts to 27.05 €/MWh [60]. The electricity purchase price of a Spanish industrial company is also determined using data from Eurostat [60]. Again, the electricity demand of the ammonia plant itself is so high that the last price band is considered. In 2015, the corresponding electricity purchase price amounted to 70.95 €/MWh [60]. The electricity sale price corresponds to the spot market price, which varies. Therefore, an hourly time series of the spot market price in 2015 is taken as input data. In this year, the spot market price lay between 4.00 and 85.05 €/MWh with an average price of 50.32 €/MWh [61]. The price for CO2-Emissions that an industrial company in Spain has to pay corresponds to the auction price for CO2-certificates. In 2015, the average CO2-certificate price was 8 €/t CO2 [62]. The water price is set to 0.763 €/m3 [63]. In 2015, the Spanish income tax rate for companies amounted to 28% [64]. The hourly DNI time series was provided by DLR.

3.1.5. Plausibility check The presented optimization model has successfully been checked for plausibility in three steps. The first two steps (a benchmark with the IAEA software HEEP and a comparison to published economic evaluations) can be looked up in [57]. These two steps could verify the correct implementation of the net present value criteria in the objective function. In a third step the optimization itself has been plausibilized successfully by manipulating input data in a way to derive expected results. 3.2. Definition of base cases

3.2.2. Technical parameter 3.2.2.1. Shutdowns. As a deterministic approach is pursued in this paper, the shutdown times and shutdown durations are fixed before the optimization. How the shutdown time and the shutdown duration are determined for each facility respectively for each module is shown in this paragraph. INL and GA estimate that the availability of an HTR amounts to 92% [7,65]. This estimation is taken as input data. By analyzing the information provided by GA [15], it is assumed that 60% of the unavailability occurs due to one planned shutdown, whereas 40% occurs due to unplanned shutdowns. In accordance to the experience gathered during the last two operation years of the AVR, two unplanned shutdowns per year are assumed [66]. For the solar tower, the planned shutdown time is assumed to amount to 240 h a year (estimation of DLR). The unplanned shutdown time is set to 175 h a year in accordance to the availability of the solar tower SEGS VI in 1999 [67]. Here again, it is assumed that two unplanned shutdowns occur each year. In total, this leads to a solar tower availability of 95.3%. The availability of the thermal storage is assumed to be higher [68]. It is set to 99%. The availably of the turbine is assumed to amount to 97% with the information provided by a German turbine supplier. The availability of the ammonia plant amounts to 96%. This information was provided by a German chemical company. As the fossil heater is part of a conventional ammonia plant, its availability must be at least as high as the

Different energy supply systems of an ammonia plant are investigated. Besides the pure solar and the pure nuclear heat supply, the hybrid cases solar + fossil, nuclear + fossil and solar + nuclear are also analyzed. A pure fossil case is also investigated in order to evaluate the economic competitiveness of the alternative energy supply systems to the status quo system. In all base cases, the turbine and the thermal storage can be built. Furthermore, the coupling to the electricity market is given. As the location, the South Spanish province Huelva is chosen with good solar conditions (annual DNI: ∼2000 kWh/m2). Pre-investigations revealed that the alternative ammonia process configuration (with air separation unit) reaches better results than the conventional ammonia process configuration (with a second reformer step) if the heat is supplied by solar tower or HTR. Concerning the fossil heat supply, the two ammonia process configurations nearly reach the same ammonia generation costs. Thus, the alternative ammonia process configuration with air separation unit is considered in the base cases. Within the base case investigations, it is assumed that the ammonia plant runs continuously at full load to fulfill delivery obligations. The required heat demand of the ammonia plant is set to 250 MW which is equivalent to the thermal power of one HTR-module. Using this heat demand, the ammonia quantity, which must be supplied every hour, is derived. 628

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through an extensive literature research. The HTR and the air-cooled solar tower are both technologies which are still in the development stage. Both technologies have not prevailed the heat market yet. Worldwide, just a few test facilities have been built, so it is important to mention that especially cost data is uncertain. Therefore, the determination of costs is a big challenge as values vary strongly in literature. The next paragraphs show which publications are analyzed and compared and which assumptions are made to derive reasonable input parameters for the base cases.

availability of the ammonia plant. Here, it is assumed that it has the same availability. Pre-investigations have shown that a higher availability has no influence on the economics of the fossil supplied ammonia plant. For the four facilities; fossil heater, turbine, thermal storage and ammonia plant; only planed shutdowns are considered. The unplanned shutdown times are stochastically generated. Through pre-investigation with different unplanned shutdown times, it can be ensured that no extreme scenario is considered in the base cases. The planned shutdown times are defined based on pre investigations. These pre investigations revealed that for the HTR, it is optimal if one HTR-module shuts down together with the ammonia plant. If there is more than one HTR-module, it is optimal if the second HTR-module shuts down together with the turbine. Apart from that, the time when the HTR is shut down is not important. In contrast, it is very important that HTR-modules are not in maintenance at the same time, otherwise more backup capacity is needed. In contrast, for the solar system the maintenance time of the solar towers is much more important for the economics than the simultaneity of maintenances of different solar towers. The pre-investigations revealed that planned shutdowns should be scheduled in summer rather than in winter. As the heat supply of an ammonia plant must be supplied continuously, missing capacity can easier be compensated in summer, as there is more surplus energy. Also in winter, the heat supply must be ensured. Therefore, the most critical time is decisive for the solar tower dimensioning. If there are solar towers unavailable in winter, more solar tower capacity is needed to compensate the unavailability of one tower in that more critical time.

3.2.3.1. HTR. The assumed specific investment cost of a 250 MWth HTR-module with a reactor outlet temperature of 850 °C amount to 2400 €/kWth. This investment cost assumption is based on two sources: the HTR cost model of INL [71] and a budget assumption for the HTRPM given by INL in 2016 [72]. Applying INL’s cost model, the development stage “Nth of a kind” was assumed. The specific investment costs represent the entire overnight costs of the first HTRmodule built on one site including indirect costs, owner’s costs and a contingency surcharge. Financing costs are not included as they are considered in the determination of the objective function. Building more than one HTR-module on one site lowers the specific investment cost. This is called co-siting effect [73]. Here, it is conservatively assumed that only the licencing costs occur just ones for the whole site. All other parts of the investment cost occur again for every HTRmodule. According to the data provided by INL [71], the licencing costs cause 12% of the specific investment cost. Consequently, it is assumed that the investment cost of the 2nd to the nth module amounts to 2112 €/kW. According to INL [71], the personnel requirement for the first module exceeds by far the personnel requirement of every following module, as most staff members are only needed once on a site. Taking the average value of the data provided by INL, the personnel requirement amounts to 275 for the first module and to 50 for every following module [71]. The annual labor costs per person are also estimated with the help of INL [11,12,71]. It is assumed to amount to 100,000 € per person and year. The operation and maintenance costs of an HTR are assumed to amount to 5% of the investment cost per year according to INL [11,12], EPRI [17] and ARGE KT [42]. Insurance costs amount to 2% of the investment cost per year according to Kugler et al. [4], INL [11,12] and EPRI [17]. Khamis et al. [44], Brinkmann et al. [74], ARGE KT [42], STL [75] and E.ON [47] provide fuel cost assumptions for fuel pebbles which lay in the range of 5.6–9 €/MWh. The average value, which amounts to 7 €/MWh, is taken as input value. Discussions about HTR decommissioning show a large range of opinions about its costs. Decommissioning costs assumptions lay in the range of 10% [11] to 130% [75] of the investment cost. The investigations will show that this cost component has only very limited effect on the economics of an HTR. Following a conservative approach, 100% of the investment costs are assumed for decommissioning.

3.2.2.2. Temperatures. It is assumed that the HTR can supply heat reliably at a temperature of 850 °C. Pre investigations revealed that a higher reactor outlet temperature of 950 °C is not sensible as investment costs rise significantly at higher temperature levels due to raised material requirements. Also for the solar tower, a receiver outlet temperature of 850 °C is assumed. Pre investigations with an outlet temperature of 950 °C showed that the disadvantage of higher thermal receiver losses outweigh the slightly higher reformer efficiency. Considering exergetic losses occurring during the heat transfer steps, the maximum reformer temperature is set to 800 °C. 3.2.2.3. Heat and electricity demands. Solar towers and HTR-modules both require electric power for operation. This auxiliary power requirement is related to the thermal power and is set to 3% for a solar tower [69] and to 4% for an HTR-module (estimation of G. Brinkmann, BriVaTech). For the turbine, an electric efficiency of 42% is assumed. The solar tower ISEGS as well as the Chinese HTR-PM is supposed to achieve this electric efficiency [23,70]. The self-discharge rate of the thermal storage is assumed to amount 0.5% per hour [35]. The ammonia storage requires for every ton of stored ammonia 80 kWh of electricity for compression [20]. No time-dependent self-discharging of the ammonia storage is considered. The heat and electricity requirements of the ammonia plant are derived based on Möller et al. [9] and based on an internal report provided by DLR [69]. Assuming a maximum reformer temperature of 800 °C, the ammonia plant needs 1.43 MWh heat and 1.01 MWh electricity to produce one ton of ammonia. Thereby, the alternative process configuration with an air separation unit is taken as basis. CO2 which is separated in the steam methane reforming process is compressed and stored. Therefore, the ammonia process itself does not cause any CO2emissions. In the following investigations, it is assumed that after the hot helium, respectively the hot air has provided heat to the ammonia plant at 850 °C, it is cooled down to 275 °C. This assumption bases on an internal report of DLR [69].

3.2.3.2. Solar tower. As the heliostat field surface and the thermal receiver power are both optimization variables, both components must be included in the objective function and therefore require specific investment costs. Consequently, the solar tower investment costs are split into the investment costs for the heliostat field, the receiver and the tower plus a surcharge for further direct costs, indirect costs, owner’s cost and contingency. This surcharge is estimated analyzing data provided by Weinrebe [25], IRENA [53], Moeini [76] and Fichtner [77]. It is set to 60%. According to Sandia National Laboratories [70], the National Renewable Energy Laboratory (NREL) [50] and internal DLR-reports the specific investment cost of the heliostat field are set to 150 €/m2. The specific investment cost of an air-cooled open volumetric receiver is estimated with the help of Weinrebe [25] and DLR [43,69]. It amounts to 110 €/kWth. A calculation rule given by DLR [69] is applied to estimate the investment cost of the tower, which amounts to 2.2 Mio. €. Thereby, a tower height of 130 m is assumed.

3.2.3. Cost assumptions The different cost components serving as input data are estimated 629

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storage amount to 150 €/kWth [81]. The investment costs of the turbine are estimated by analyzing data of INL [71] and Fichtner [77]. This parameter is set to 1.000 €/kWel. The investigations show a minor influence of the turbine costs; therefore, economies of scale are neglected. The investment costs of the ammonia plant are estimated using an internal DLR report [69]. These investment costs are related to the maximum production rate to estimate the specific investment costs, which amount to 9050 €/(kg/h). The comparison of that value to the investment cost of a real ammonia plant (Louisiana ammonia plant 8400 €/(kg/h) [82]) shows good agreement. The specific investment costs of a liquid ammonia storage operated at ambient pressure amount to 435 €/t [83]. It is assumed that one person is necessary to operate the turbine. Assuming five-shift operation the turbine has a personnel requirement of five persons. To operate the ammonia plant six persons are needed every time (estimation of B. Geis, BASF). This leads to a personnel requirement of 30 persons. For both facilities annual labor costs of 80.000 € per person are assumed. Furthermore, it is assumed that the thermal storage is operated by the solar tower staff, whereas the fossil heater is operated by the ammonia plant staff. Therefore, no further staff is required for these two facilities. The operation and maintenance cost of the fossil heater and the turbine is estimated to 1% of the investment cost per year. For the thermal storage, the operation and maintenance cost of the solar tower are assumed amounting to 2% of the investment cost per year. Operation and maintenance costs of the ammonia plant amount to 3% of the investment cost per year (estimation of B. Geis, BASF). The insurance costs of the fossil heater, the thermal storage, the turbine and the ammonia plant are estimated to 1% of the investment cost per year. The variable operation costs of the fossil heater consist of costs for natural gas and cost for CO2 emissions. Assuming the Spanish price levels introduced in Section 3.2.1, the variable operation costs of the fossil heater amount to 28.6 €/MWh. The variable operation costs of the ammonia plant include costs for natural gas (only the part which is reformed), CO2 storage costs (1.8 €/t CO2 [84]), costs for water and for the CO2-solvent (45 €/t CO2-solvent [69]) as well as a revenue for oxygen (18 €/t oxygen [12]) produced in the air separation unit. In total this leads to variable operation costs of 157.4 €/t ammonia. The variable operation costs of the facilities solar tower, thermal storage and turbine can be neglected [25]. Moreover, it is assumed that costs occurring for the decommissioning of the facilities solar tower, fossil heater, turbine, thermal storage and ammonia plant are compensated through revenues achieved by scrap sales. All cost assumptions are summed up in Table 1.

Land costs are partly allocated to the heliostat field and partly allocated to the tower. As solar towers are basically installed in remote desert areas, land costs are quite low amounting to 2 €/m2 (estimation of DLR). According to Weinrebe [25], 1.3 times the heliostat field size is needed for land. Furthermore, 0.18 km2 are needed for buildings. In conclusion, the investment cost of a solar tower can be calculated applying formula (1). The total investment costs (KInv,CSP) depend on the number of build towers (n CSP) , the receiver size (Q̇ CSP,max ) and the size of the heliostat field (A CSP,field) . The economies of scale are quantified with the help of [78] and information provided by DLR. Small towers in the power range of 0–100 MWth serve as reference (scale factor of 100%). For the medium power range from 100 to 200 MWth, a scale factor of 90% is applied. Whereas, for the big power range from 200 to 300 MWth, a scale factor of 86% is applied. Thereby, it is assumed that the economies of scale only affect the costs of the heliostat field and the receiver. The height of the tower and therefore the tower costs are simplistically assumed to be size independent. Pre investigations revealed that larger towers do not need to be taken into account, as the optical losses grow significantly with a rising heliostat field so that the application of larger towers is not economic (for this application und investigation). The co-siting effect is neglected for solar towers as the licencing costs are of minor importance. The solar tower size influences the personnel requirement, because with a growing heliostat field, more staff is needed for mirror cleaning. By analyzing the personnel requirement of the solar tower facility ISEGS and data provided by Weinrebe [25] and DLR [43], formula (2) is derived showing that the total personnel requirement (Npers,CSP) of a solar tower depends on the number of built towers (n CSP) and the receiver size (Q̇ Rec,Nenn) . The annual labor costs per person are set to 60,000 € according to DLR [43]. It is remarkable that the wage level of the solar tower staff is much lower than the wage level of the HTR. One possible explanation for that discrepancy can be suspected in different skills requirements as for mirror cleaning lower paid workers are employed, whereas in the nuclear field higher paid qualified personnel are needed. Operation and maintenance cost are set to 2% of the investment cost per year by averaging data provided by DLR [8,43], Möller et al. [9] and Weinrebe [25]. The average insurance costs provided by DLR [43], Möller et al. [9], Weinrebe [25], Moeini [76] and IRENA [53] amount to 1% of the investment costs per year. This value is taken as input value.

̇ ,Nenn Npers,CSP = 30 + 2·(nCSP −1) + 0.045·QRec

(1)

€ ̇ ,max + 150 € ·ACSP,field ⎞ ·QCSP KInv,CSP = 1.6·⎛2,560,000 €·nCSP + 110 kWth m2 ⎝ ⎠ (2) ⎜

⎟

4. Results of the energy economic evaluations 4.1. Results base cases

3.2.3.3. Fossil heater, thermal storage, turbine and ammonia plant. The investment cost of the fossil heater are assumed to amount to 80 €/kWth [79]. Moeini [76], the Solar Institute Jülich [80] and DLR [43] provide cost estimations for a thermal storage using solid particles. The average of their data amounts to 60 €/kWh, which is taken as input value. The investment cost of the required electric heaters inside and behind the

4.1.1. Fossil heat supply system In the fossil base case, the optimal energy supply system just consists of one fossil heater with an installed thermal power of 261 MW. The installed capacity of the fossil heater must be slightly higher than

Table 1 Cost assumptions. HTR

Solar tower

Fossil heater

Turbine

Thermal storage

Ammonia plant

Investment costs Maintenance costs Insurance costs

2400 €/kWth 5%-KInv 2%-KInv

Formula (2) 2%-KInv 1%-KInv

80 €/kWth 1%-KInv 1%-KInv

1000 €/kWel 1%-KInv 1%-KInv

60 €/kWh 2%-KInv 1%-KInv

9050 €/(kg/h) 3%-KInv 1%-KInv

Personnel requirement Labor costs per person Variable operation costs Decommissioning costs

275/50 100,000 €/a 7 €/MWh 100%-KInv

Formula (1) 60,000 €/a – –

– – 28.6 €/MWh –

5 80,000 €/a – –

– – – –

30 80,000 €/a 157.4 €/t –

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35

storage level [GWh]

storage level [GWh]

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30 25 20 15 10

Fig. 3. Annual course of the storage level for the solar (left) and solar + fossil (right) base case.

12 10 8 6 4 2

5

0

0 1

2001

4001

6001

8001 time [h]

1

2001

4001

6001

8001 time [h]

are caused for electricity purchase. In the nuclear case, the turbine causes just 1.4% of the AGC. With a share of 56%, the biggest share of the AGC is still needed for the ammonia plant. Overall, it becomes evident that the nuclear system cannot compete to the fossil system.

the heat demand of the process, which amounts to 250 MW, because the flue gas, which is returned at a temperature of 275 °C after the main part of its heat has been supplied to the ammonia plant, cannot be used entirely to preheat the combustion air. A small fraction of the remaining heat is lost. The fossil heater can supply the heat demand of the process at any time; therefore, no thermal storage is needed. A turbine is also not built, as the electricity generation costs of a natural gas fired steam turbine are higher than the electricity purchase price. Thus, the entire electricity demand, which consists just of the electricity demand of the process (1485 GWh per year), is supplied by external electricity purchase. The ammonia generation cost (AGC) of the fossil supplied ammonia plant amount to 391 €/t. Compared to the spot market prices for ammonia, which lay between 230 and 460 €/t in the period from February 2015 to February 2017 [85], the calculated AGC seem to be in the correct range. Analyzing the composition of the AGC, it becomes evident that nearly 70% of the costs arise for the ammonia plant itself (see Fig. 5). The electricity purchase accounts for 18% of the AGC while just 11% of the AGC are caused by the fossil heater, whereof the biggest share is variable operation costs (natural gas and CO2-certificates). The investment costs of the fossil heater only cause 0.24% of the AGC. Consequently, if a backup fossil heater would be necessary, the AGC would rise less than half a percent. As today most ammonia plants are heated by natural gas, the fossil heat supply system is the reference case with which all alternative heat supply systems have to compete.

4.1.3. Solar heat supply system If the entire heat demand of the ammonia plant is supplied by solar energy, the optimal heat supply system consists of 12 solar towers and a thermal storage. Each solar tower has a thermal power in the range of 192–199 MW. The thermal power of all solar towers together amounts to 2363 MW. This installed capacity exceeds by far the installed capacity of the nuclear and fossil base cases. The reason for the necessity of a higher installed capacity is the fluctuating nature of solar radiation which leads to low utilization rates of the solar towers. The solar towers achieve full load hours in the range of 1794–1832 h a year. To ensure an uninterrupted heat supply of the ammonia plant, a huge thermal storage with a storage capacity of 32.6 GWh is necessary (the number of solar towers, the thermal power and the storage capacity are all results of the optimization and have been optimized simultaneously). If completely filled, this storage would be capable of supplying the heat demand of the ammonia plant for 130 h. The annual curve of the thermal storage level is shown in Fig. 3 (left). It becomes obvious that the whole storage capacity is needed only for two hours a year. For most of the time of the year (6550 h), the storage level lies between 0 and 30%. Nevertheless, in times of low solar radiation, the heat supply of the ammonia plant must be ensured as well. Consequently, the storage must be sized according to the most critical times, which occur in autumn and winter. The solar overcapacity, which is available especially during summer, is used to generate electricity. During summer days, the turbine, which has an installed capacity of 190 MW, is basically operated at full load. Nevertheless, the produced electricity is not enough to supply the entire electricity demand, which consist of annually 1485 GWh for the ammonia plant, 375 GWh for electric heating of the thermal storage and 128 GWh for the solar tower. Still, 1374 GWh of electricity must be purchased at the market. Due to the necessity of huge storage and huge solar tower capacities, the AGC of the solar base case are high. They amount to 664 €/t. The overcapacity of the storage results in storage costs which even exceed the entire costs for the 12 solar towers. The costs of the solar towers and the cost of the thermal storage are both dominated by capital costs. The necessity of building new solar systems after the old systems reach the end of their lifetime of 30 years of operation is due to interest effects less important. Analyzing the capital costs of the solar towers reveals that with a share of 75%, the heliostat field causes the biggest share of the capital costs. Twenty-two percent of the solar tower capital costs are spent for the receiver, whereas just 3% are needed for the tower. As the heliostat field and the receiver sizes are known after the optimization, the specific investment costs of one solar tower can be calculated. The specific investment costs of one tower lay between 720 and 741 €/kWth (excluding storage costs). It becomes evident, that despite the fact that the specific investment costs of a solar tower are far lower than those of an HTR (2400 €/kWth), the nuclear system still reaches lower AGC than the solar system. This is due to the fluctuating nature of solar radiation,

4.1.2. Nuclear heat supply system As the availability of one HTR-module is lower than the availability of the ammonia plant, one backup HTR-module is necessary. Consequently, two 250 MWth HTR-modules are necessary to ensure a constant heat supply of the ammonia plant in the nuclear base case. In those times the backup HTR-module is not needed for the heat supply of the ammonia plant, it is used to generate electricity. Therefore, the optimal nuclear energy supply system includes a turbine. According to the thermal power of one HTR module and according to the electric efficiency of the turbine, it has an installed electric power of 105 MW. This power is not enough to supply the total electricity demand of the system, which includes the electricity demand of the ammonia plant and the electricity demand of the HTR-modules. Consequently, 52% (870 GWh per year) of the electricity demand still needs to be purchased at the electricity market. During the planned shutdown of the ammonia plant, the generated electricity exceeds the electricity demand, so this surplus electricity (33 GWh per year) is sold. Apart from the downtimes caused by maintenance and unplanned shutdowns, both HTR modules run constantly at full load achieving 8060 full load hours a year. Accordingly, the turbine also runs at full load any time both modules are available. In the nuclear base case, no storage is built. The nuclear energy supply system achieves AGC of 519 €/t. 35% of these AGC are caused by the HTR-modules. Thereof, the investment costs of the HTR-modules have the major influence. Fuel and decommissioning costs are less important. Compared to the fossil base case, the electricity purchase costs can be reduced, as only 7.8% of the AGC 631

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ammonia generation cost [€/t]

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700

base cases. With such a low ratio, both hybrid systems are still strongly dominated by the solar or rather nuclear heat source. In the nuclearfossil-hybrid case a further raise of the fossil ratio brings no benefit, whereas in the solar-fossil-hybrid case the main cost advantages of the hybrid concept can be used.

650 600 550 500

4.1.4.1. Base case “nuclear + fossil”. In the nuclear-fossil-hybrid case, just one HTR-module is needed. The fossil heater takes the task of the backup system so that no further HTR-module is needed. The fossil heater has an installed thermal power of 261 MW so that it is capable of supplying the entire heat demand of the ammonia plant when the HTR is unavailable. In contrast to the pure nuclear heat supply case, the backup system in the nuclear-fossil-hybrid case is not used for electricity production due to the higher fuel costs of the fossil heater. Consequently, no turbine is built. The entire electricity demand for the ammonia plant and the HTR-module is supplied by external electricity purchase. Analogous to the pure nuclear case, the HTR runs every time it is available at full load, reaching 8060 full load hours a year. The fossil heater only runs to heat the ammonia plant when the HTR is unavailable, reaching just 350 full load hours. Compared to the pure nuclear case, the AGC can be significantly reduced through changing the backup system from one HTR-module to the fossil heater. The AGC amounting to 472 €/t still exceed the AGC of the fossil reference case by 20%. The composition of the AGC can be extracted from Fig. 5.

450 400

0

20

solar + fossil

40

nuklear + fossil

60 maximum share fossil heat [%]

Fig. 4. influence of the maximum share of fossil supplied heat to solar respectively nuclear supplied heat on the AGC of the soar-fossil-hybrid and the nuclear-fossil-hybrid supply systems.

which makes the constant heat supply of the ammonia plant very expensive.

ammonia generation costs [€/t]

4.1.4. Hybrid heat supply system In a next step four, different hybrid systems are investigated. First the fossil support of the solar and nuclear systems shall be analyzed. Subsequently, the coupling of the solar towers with an HTR with and without a fossil support is under investigation. As the fossil heat supply is cheaper than the nuclear or solar heat supply, the fossil heat source must be restricted before the optimization in order to analyze hybrid systems. This is achieved by limiting the ratio of fossil supplied heat to solar or nuclear supplied heat. This limit is set by pre-investigations. Thereby, the maximum ratio of fossil supplied heat is varied in both hybrid systems, the nuclear-fossil-hybrid system and the solar-fossil-hybrid system. The result of that pre-investigation is shown in Fig. 4. In the nuclear-fossil-hybrid system, the AGC fall abruptly if the ratio of fossil supplied heat to nuclear supplied heat exceeds a value of 4.5%. If the restriction is tighter, the fossil heat is not sufficient to ensure the heat supply of the ammonia plant in those times the HTR is not available. A further raise of the fossil limitation above 4.5% brings no benefit, because the fossil heater is due to its higher fuel costs just operated if the HTR is unavailable. In the solar-fossil-hybrid heat supply system, even a small amount of fossil heat reduces the AGC significantly. The main reason for the cost reduction, even at small ratios of fossil heat, is the significant reduction of the required storage capacity. If just one percent of the entire supplied heat is provided by the fossil heat source, the storage capacity can be divided in half reducing the AGC around 12%. Raising the share of fossil supplied heat causes the AGC to fall degressively. In the solarfossil-hybrid and in the nuclear-fossil-hybrid base cases, the maximum ratio of fossil supplied heat is set to 4.5% in order to define comparable

4.1.4.2. Base case “solar + fossil”. If the solar heat supply system is supplemented by a fossil heater, the solar tower capacity can be reduced from 2359 MWth to 2000 MWth. This thermal capacity is provided by ten solar towers. The installed capacity of the fossil heater amounts to 203 MWth. Due to the possibility to use dispatchable fossil heat, neither the solar tower nor the thermal storage must be sized according to the critical times of low solar radiation. The fossil heater is basically used to supply the heat demand of the ammonia plant during autumn and winter times when the solar radiation is low. Therefore, the storage capacity can be reduced around 70% compared to the pure solar case amounting to 9.7 GWh. The curve of the storage level shown in Fig. 3 (right) reveals that in the solarfossil-hybrid case, the existing storage capacity is better used compared to the solar case. Despite the much better utilization level, in the solarfossil-hybrid case the summer is also characterized by overcapacity which is used in a 172 MWel turbine to generate electricity. The solarfossil-hybrid heat supply system reaches a better efficiency than the pure solar heat supply system, as especially storage losses are reduced. The electricity requirement of the entire system can be reduced around 10% compared to the solar base case. Therefore, the amount of purchased electricity can be reduced to 1264 GWh. Overall, the

700

Fig. 5. Ammonia generation costs of the 7 base cases.

600 500 400 300 200 100 0

fossil

ammonia plant

nuklear

solar tower

nuklear + fossil

solar

solar + fossil

thermal storage

HTR

fossil heater

solar + nuklear turbine 632

solar + nuklear + fossil electricity trade

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4.2. Parameter variation

supplementation of the solar system by a fossil heater not only reduces the necessary solar tower and storage capacity, but also reduces the heat and electricity demand of the entire system. These aspects lead to an improved economic competitiveness. The solar-fossil-hybrid system reaches AGC of 536 €/t. Compared to the solar system, the AGC are reduced by 19%. Main reason for the cost reduction can be seen in reduced storage costs (compare Fig. 5). Nonetheless, the solar-fossil-hybrid system cannot compete with the fossil reference system.

The results presented so far apply under the assumption of the input data summarized in Section 3. Though, this input data is uncertain especially concerning cost assumptions. In order to consider this uncertainty parameter variations are conducted. Thereby, the coupling of the solar towers and the HTR is not further investigated. In a first step, the cost assumptions of the heat supply facilities and the economic frame conditions are varied. According to the AACE International project class four, the cost assumptions are varied by −30% and +50% [86]. Supplementary, a cost raise of 100% is investigated as recent mega projects were frequently characterized by massive cost overruns [87] so that significant higher costs seem possible. Moreover, the interest rates are varied. If a project is supported or even conducted by government, a significant reduction of the assumed interest rates seems plausible. This is because financing conditions are better due to reduced risk and the expected return is generally lower compared to a profit orientated company. In contrast, the interest rates could also rise significantly as the interest rates highly depend on the individual time- and risk preferences of the investor. Within the parameter variation, an interest rate change of ± 50% is investigated. Furthermore, the influence of the construction times and the lifetimes on the economic performances shall be investigated. Both parameters are varied in the range of ± 50%. The results of the parameter variations are shown in Fig. 6. Thereby three aspects need to be remarked:

4.1.4.3. Base case “solar + nuclear”. Subsequent, the coupling of the solar tower and the HTR technology is investigated, first without a fossil support. If neither the solar tower nor the HTR capacity is limited, the optimization would lead to a pure nuclear heat supply system consisting of two HTR-modules, as this system has the lowest AGC. Therefore, the maximum number of HTR-modules is limited to one in order to analyze a solar-nuclear-hybrid heat supply system. As the uninterrupted heat supply of the ammonia plant cannot be fulfilled by one HTR-module, six solar towers with a total thermal power of 1200 MW are needed. Supplementary, a thermal storage with a storage capacity of 12.9 GWh is necessary. In those times the HTR is available, the heat supply system is characterized by massive overcapacity, which is used in a 143 MWel turbine to generate electricity. The HTR-module is basically operated at full load, reaching 7796 full load hours a year. In a few hours, the available solar heat is sufficient to operate both the ammonia plant and the turbine at full load. In these hours, the HTR is shut down. The heat demand of the ammonia plant is basically supplied by the HTR, whereas the turbine is mainly supplied by the solar system. The AGC of the solar-nuclear-hybrid base case amount to 595 €/t. Therefore, this system is just slightly better than the pure solar system, but it cannot compete to the pure nuclear system. If a heat supply system which operates completely without fossil heat is desired, the pure nuclear system would be preferred from an economic perspective. Changing one HTR-module by a solar heat supply system offers no advantages. Moreover, the coupling of two completely different and already complex technologies would even raise complexity.

1. As in the solar and in the solar-fossil-hybrid cases, the storage costs are decisive for the AGC, their investment costs (“TS”) are varied in addition to the investment costs of the solar tower (“CSP”) 2. In the hybrid cases, only the cost components of the HTR respectively of the solar tower are varied. The cost components of the fossil heater are kept constant. 3. All cost components of the ammonia plant and of the turbine are kept constant. Even though the fuel costs of the fossil heater are varied in the fossil heat supply case, the variable costs of the ammonia plant are not varied, even though they basically consist of the costs for natural gas.

4.1.4.4. Base case “solar + nuclear + fossil”. If the maximum number of built HTR-modules is limited to one and if the maximum ratio of fossil supplied heat to nuclear and solar supplied heat is limited again to 4.5%, the optimized solar-nuclear-fossil-hybrid heat supply system equals exactly to the nuclear-fossil-hybrid system. No solar tower is built. As the results can be taken from the base case “nuclear + fossil”, this case is not further discussed.

The results of the parameter variation reveal that for every heat supply system, the interest rate has the biggest impact on the AGC. The comparison of the different cases shows that the influence of the interest rate on the AGC is the highest in the solar case and the lowest in the fossil case. The influence is lower in the hybrid cases compared to the pure solar respectively the pure nuclear case. Furthermore, it is notable that the negative effect of an interest rate increase of 50% is higher than the positive effect of an interest rate decrease of 50%, according to amount. In the fossil heat supply case, the fuel costs have a strong impact on the AGC. In contrast, the influence of the investment, operation and maintenance and insurance costs as well as the influence of the construction time on the AGC is negligibly low. The economics of the four alternative heat supply systems show a strong dependency on the investment costs. The influence of the HTR investment costs on the nuclear case turns out to be higher than the influence of the solar tower investment cost on the solar case. The influence of the investment costs is higher in the pure solar respectively the pure nuclear case than in the hybrid cases. It is remarkable that in the solar-fossil-hybrid case an increase of the storage costs does not affect the AGC as strong as in the solar heat supply system. In contrast, the decrease of the storage costs leads to bigger savings in the solarfossil-hybrid case than in the solar case. The reason for that is the higher flexibility provided by the fossil heater. In the solar case the storage capacity is nearly not influenced by the parameter variations (changes in the range of −0.2 to 1%), whereas in the solar-fossil-hybrid case the

4.1.5. Evaluation of base case results The comparison of the AGC of the different base cases reveals that the conventional fossil heat supply system is currently the cheapest option. No alternative heat supply system is competitive. Within the direct comparison of the two alternative heat supply technologies, the HTR turns out to be economic favorable compared to the solar tower. This result becomes evident while comparing the pure nuclear case to the pure solar case and while comparing the nuclear-fossil-hybrid case to the solar-fossil-hybrid case. Furthermore, the base case results reveal that the HTR and, even more, the solar towers both benefit from a supporting fossil heater. Even though the ratio of fossil supplied heat to nuclear respectively solar supplied heat is relatively low (4.5%), the AGC can be significantly reduced. Moreover, the base cases indicate that the coupling of the HTR and the solar tower is no sensible option. One main reason is the similar cost structure of both technologies, characterized by high capital costs and low variable operation costs. Therefore, both systems need high utilization rates for amortization, so neither of these two technologies is suitable as backup system. 633

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abscissa: relative deviation of AGC compared to base case [%]

Fig. 6. Results parameter variation.

fossil interest rate fuel costs lifetime investment costs O&M costs insurance costs construction time

-50 % -30 % +50 % +100 % -20 -15 -10 -5 0 5 10 15 20 25 30 nuclear

nuclear + fossil

interest rate investment costs lifetime O&M costs personnel costs fuel costs construction time decommissioning time insurance costs

interest rate investment costs TS investment costs CSP lifetime O&M costs insurance costs construction time personnel costs

-20 -15 -10 -5 0 5 10 15 20 25 30 -20 -15 -10 -5 0 5 10 15 20 25 30 solar solar + fossil

-20 -15 -10 -5 0

5 10 15 20 25 30 -20 -15 -10 -5 0

5 10 15 20 25 30

In the next step, the influence of the electricity purchase price is investigated. Because in all cases, nearly no electricity is sold, the influence of the electricity sell price on the AGC is negligible and is no further investigated. The electricity purchase price is varied in the range of 35.5 €/MWh (50% of the base case value) to 212.9 €/MWh (300% of the base case value) in five steps. The results shown in Fig. 7 (middle) reveal that these price variations do not lead to the economic competitiveness of the nuclear or solar heat supply systems. The higher the electricity purchase price, the higher the incentive to produce electricity internally via turbine. In the fossil, the nuclear and the nuclear-fossil-hybrid cases a certain price limit is observable. If the electricity purchase price exceeds this price limit, all electricity is generated internally. In the solar and the solar-fossil-hybrid case this certain price limit is not observable. The higher the electricity price, the more electricity is generated internally via turbine. Due to the fluctuating nature of the solar radiation, the complete internal electricity supply would be extremely expensive. Therefore, even at high prices the electricity demand is partly fulfilled externally. Finally, the natural gas price is varied in four steps from 27.05 €/ MWh (base case value) to 125.25 €/MWh (500% of the base case value). Due to the natural gas demand of the ammonia plant, the AGC of all heat supply cases rise with a rising gas price. In the fossil and the two hybrid cases, AGC rise additionally because of the higher fuel costs of the fossil heater. The biggest AGC increase can be observed in the fossil case. Fig. 7 (bottom) shows that the nuclear-fossil-hybrid system becomes economically competitive to the fossil systems if the gas price rises by the factor three. A gas price rise of nearly 400% is necessary so that the pure nuclear heat supply system becomes economically competitive. The solar-fossil-hybrid system needs a gas price rise of nearly 500%. Moreover, the results show that even a gas price rise of 500% is not enough for the solar system to reach competitiveness. Calculations with even higher gas prices reveal that a gas price rise of 700% is

storage capacity can be adjusted to the economic conditions. Here, an adjustment of the storage capacity by 13% is observable. The AGC of all five investigated systems react sensitive to a lifetime decrease. In contrast, an increase of the lifetime leads just to a very limited cost reduction. The parameter variations further reveals that the fixed operations costs, the fuel costs and the decommissioning costs are less important for the economics of the alternative heat supply systems. Among these cost parameters, the operation and maintenance cost have the biggest impact on the nuclear and on the nuclear-fossil-hybrid case’s economics. Moreover, the construction time seems to be of minor importance for the economics. But, here it is important to remark, that only interest effects due to the extended construction time are considered. Actually, an extended construction time would lead to an increase of investment costs as, for example, cost for crane rental, etc. would increase. Due to limited data available, this aspect is not considered. Last, it is important to mention that none of the parameter variations led to the economic competitiveness of the alternative heat supply systems. In order to find out whether increased gas-, CO2-certificate, or electricity prices lead to the economic competitiveness of the solar or nuclear heat supply systems, these parameters are varied as well. In order to determine break-even points, the three prices are varied in an extended range. First, the CO2-certificate price, which was set to 8 €/t in the base cases, is increased in five steps to 48 €/t (600% of base case value). This variation reveals that the CO2-certificate price has nearly no influence on the AGC (see Fig. 7 (top)). Even in the fossil case, the increase of CO2-certificate prices by the factor six just leads to a raise of the AGC of 3%. Even this radical increase of CO2-certificate price does not lead to the economic competitiveness of the solar or nuclear heated ammonia plant.

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700

Fig. 7. Variation of CO2-certificiate-, electricity- and gas price.

650 600 550 500 450 400 350 100%

200%

300%

400%

500%

600%

ammonia generation costs [€/t]

CO2-certificate price relative to base case 800 750 700 650 600 550 500 450 400 350 300 50%

100%

150%

200%

250%

300%

ammonia generation costs [€/t]

electricity price relative to bace case 1.300 1.200 1.100 1.000 900 800 700 600 500 400 300 100%

200%

300%

400%

500%

gas price relative to base case fossil

nuklear

nuklear + fossil

solar

4.3. Scenario analysis

Table 2 Solar conditions of the five investigated locations.

Annual DNI Duration of the longest period in which the product of DNI and efficiency is below 10 W/m2

solar + fossil

Unit

Huelva

Sevilla

Granada

Faro

Upington

[kWh/m2] [h]

2007 70

2012 185

1941 87

2170 94

2793 63

4.3.1. Varied locations In the first scenario analysis, the influence of the solar conditions on the economics of the solar and the solar-fossil-hybrid systems is investigated. Thereby, the solar conditions of different locations are assumed. All other parameters which are not affected by the solar conditions like the electricity- and the gas price stay constant. The radiation data, which is necessary for the calculation of the solar tower efficiency time series and the subsequent optimization of the energy supply system, are provided by DLR for five different locations: Beside Huelva, which was analyzed in the base cases, two different South Spanish locations: Sevilla and Granada, Faro, a Portuguese location, and Upington, a South African location, are investigated. The annual DNI of the five different locations are listed in Table 2. Fig. 8 shows the AGC, the optimal storage capacity and the optimal

necessary to reach the economic competitiveness of the solar system. The variation of technical parameters such as temperature levels or efficiencies would exceed the scope of this paper and will be subject of further investigations.

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1.200

Fig. 8. AGC, optimal storage capacity and optimal thermal power of all solar towers assuming the solar conditions of five different locations.

1.065

1.000

732

664

662

600

511

536

Huelva

800

Upington

ammonia generation costs [€/t]

S. Schröders, H.-J. Allelein

583

565

521

448

400 200

storage capacity [GWh]

0 100 80 60 40 20

4000 3500 3000 2500 2000 1500 1000 500

solar

Upington

Faro

Granada

Sevilla

Faro

Granada

Sevilla

0

Huelva

thermal power solar tower [MWth]

0

solar + fossil

Consequently, if a continuous heat supply of the ammonia plant is needed, the solar heat supply system should be complemented by a supporting fossil heater in any case. The fossil heater can guarantee the heat supply in times of low solar radiation. The results of the varied solar conditions of the solar-fossil-hybrid case show that with a fossil backup system the AGC cost just rise around 9% if the solar conditions of Sevilla are assumed. Furthermore, the results of the varied solar conditions reveal that the economics of the solar systems can be significantly improved if the solar conditions in Upington are assumed. Due to the improved solar conditions, the solar tower capacity and the storage capacity can be significantly reduced lowering the investment costs of the entire system. The AGC in Upington amounts to 511 €/t in the solar respectively to 448 €/t in the solar-fossil-hybrid case. Therefore, assuming the excellent South African solar conditions, the solar systems are economically favorable compared to the nuclear systems. Nevertheless, the solar systems are still not competitive to the fossil reference system.

thermal power of all solar towers assuming the solar conditions of the five different locations. First, it is important to compare the results for the four locations on the Iberian Peninsula. The results reveal that, despite the fact that in Sevilla the annual DNI is as high as in Huelva, the AGC of Sevilla exceed those of Huelva by far. The results reveal that the duration of the longest period with low solar radiation is decisive for the economics of the solar system. In Sevilla, the longest period in which the product of DNI and solar tower efficiency is below 10 W/m2 amounts to 185 h (see Table 2). In all other locations, this period is much shorter. Consequently, in Sevilla the solar storage has to be large enough to compensate the nearly eight days of insufficient solar radiation. Therefore, the thermal storage in Sevilla is nearly three times as large as the storage in Huelva. Furthermore, in Sevilla more solar tower capacity is needed in order to charge the storage before the critical period. The optimization tool only considers the solar conditions of one year. But, it is highly likely that the long period of insufficient solar radiation, which occurred in 2015 in Sevilla, will also occur another year in the other locations. In order to guarantee the uninterrupted heat supply of the ammonia plant, the solar heat supply system must be sized according to the worst solar conditions occurring in 30 years of operation. Such an over dimensioning of the solar capacities is not advisable.

4.3.2. Ammonia storage for discontinuous operation of ammonia plant In all previous investigations, the ammonia plant was operated constantly at full load (apart from a shutdown due to maintenance). In a second scenario analysis, it is assumed that the ammonia plant can be operated discontinuously according to the available heat. Thereby, it is 636

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511 €/t to 461 €/t by implementing an ammonia storage assuming the solar radiation data of Upington. The solar-fossil-hybrid heat supply system does not benefit from the ammonia storage assuming the solar radiation data of Upington. Consequently, even under the excellent solar conditions occurring in South Africa the solar systems with ammonia storage cannot compete to the fossil system.

assumed that despite the discontinuous operation of the ammonia plant, the delivery obligations are the same. Through the possibility to build an ammonia storage, these delivery obligations can be fulfilled. The influence to build the ammonia storage on the AGC is investigated for the heat supply cases fossil, solar, nuclear, solar-fossilhybrid and nuclear-fossil-hybrid. This investigation reveals that the fossil and the nuclear-fossil-hybrid heat supply systems do not benefit from the possibility to build an ammonia storage. In these cases, the fossil heater provides cheap flexibility, so that the flexible operation mode of the ammonia is not economic. The nuclear heat supply system benefits from the ammonia storage if the annual ammonia production rate is reduced so that the entire heat demand can be supplied by just one HTR-module. The annual ammonia production quantity has to be reduced from the base case quantity of 1,470,000 tons to 1,410,000 tons. In those times when ammonia has to be delivered but the HTR is unavailable, the delivery obligation can be fulfilled by discharging the ammonia storage. Accordingly, more ammonia must be produced when the HTR is available, so the capital cost of the ammonia plant rise from 88 €/t in the base case to 92 €/t of delivered ammonia in the case with ammonia storage. The costs of the ammonia storage are relatively low (1.3 €/t of delivered ammonia). As no second HTR-module is needed, the AGC decrease in total from 519 €/t in the base case to 480 €/t in the case with ammonia storage. The pure solar heat supply system benefits the most from the ammonia storage. The AGC can be reduced from 664 €/t in the base case to 540 €/t in the case with ammonia storage. This massive cost saving is achievable, as the solar heat supply system must not be sized according to the most critical time. Since no uninterrupted heat supply must be ensured, the solar tower capacity can be reduced from 2363 MW to 1600 MW provided by eight solar towers. Furthermore, the much cheaper ammonia storage allows reducing the thermal storage capacity from 32.6 GWh in the base case to 6.7 GWh. The ammonia storage has a capacity of 163,000 tons. The ammonia plant is shut down during longer periods of insufficient solar radiation. In those times, the ammonia storage is discharged to fulfill the delivery obligations. Accordingly, more ammonia must be produced during times of good solar radiation so that the production rate of the ammonia plant must be 30% higher than in the base case. Consequently, the costs of the ammonia plant rise. Nevertheless, the thermal storage still enables a relative constant operation of the ammonia plant, which reaches 6484 full load hours a year. The cost reductions achievable by the flexible operation mode of the ammonia plant are much lower in the solar-fossil-hybrid system than in the pure solar system, because the fossil heater already provides system flexibility. The AGC can be reduced from 536 €/t in the base case to 523 €/t in the case with ammonia storage. The thermal storage capacity is reduced from 9.7 GWh to 6.6 GWh by implementing an ammonia storage with a capacity of 55,000 tons of ammonia. Furthermore, the solar tower capacity is reduced from 2000 MW to 1600 MW. As the ammonia storage is much cheaper than the thermal storage, economic benefits occur. The production rate of the ammonia plant has to be 17% higher than in the solar-fossil-hybrid base case in order to charge the ammonia storage so that the ammonia plant can shut down during longer periods of insufficient solar radiation. The ammonia plant achieves 7.164 full load hours a year. Negative influences of a very discontinuous operation mode on the product quality or on the efficiency of the ammonia plant are not considered within the optimization. Nevertheless, since the operation mode of the ammonia plant is still relatively constant in the solar and even more constant in the solarfossil-hybrid case, this negative effect should not play a significant role. Even with the possibility to operate the ammonia plant discontinuously, no alternative heat supply system becomes competitive to the fossil system. In order to check whether the solar systems reach the economic competitiveness assuming the much better South African solar conditions, two further calculations are conducted with ammonia storages. In the solar heat supply system, the AGC can be reduced from

4.4. Discussion of the results Taking into account the site-dependent solar conditions and the possibility to operate the ammonia plant flexibly, the most economical solar and the most economical nuclear heat supply system can be identified and evaluated. The lowest AGC of the considered solar heat supply systems can be achieved under South African solar conditions. The energy supply system consists of six solar towers, a thermal storage, a supplementing fossil heater and a turbine. The ammonia plant is constantly operated at full load. The AGC amount to 448 €/t. This is the best solar heat supply system. Assuming the solar conditions of Huelva, it is better to implement an ammonia storage to operate the ammonia plant flexibly. In contrast to Upington, more solar towers and a bigger thermal storage are needed in Huelva. A supplementing fossil heater and a turbine are also part of the optimal heat supply system. The AGC amount to 523 €/t. The most economical nuclear heat supply system consist of one HTR-module and a fossil backup heater, which guarantees the continuous heat supply of the ammonia plant when the HTR is unavailable (base case nuclear + fossil). No ammonia storage is built. The AGC amount to 472 €/t. Even the most economical alternative heat supply systems cannot compete to the conventional fossil heated ammonia plant whose AGC amount to 391 €/t. The parameter variation in Section 4.2 reveled that gas price rises can improve the economic situation of the nuclear and solar heat supply systems. In order to compete to the fossil system, a gas price rise of the factor 2.5 is necessary for the best solar heat supply system. As mentioned in Section 4.2, the best nuclear heat supply system needs a gas price rise of the factor 3 for economic competitiveness. Since 2007, the half-annual average gas price for industrial customers in Spain has varied in the range of −30% and +20% referred to the base case natural gas price of 27 €/MWh. With regard to these historic price data, it seems unlikely that gas price rises of the factor 2.5–3 will occur in near future. Therefore, from an economic point of view, the integration of solar towers or HTR-modules into the industrial high temperature heat supply sector cannot be expected. Within investment decisions, non-monetary aspects, like the reduction of greenhouse gas emissions or savings of fossil resources, can also play a significant role. Applying the best solar or the best nuclear heat supply system instead of the fossil heat supply system can save nearly 21% of the natural gas demand. Still, the biggest amount of natural gas is needed for nonenergy purpose in the reforming step of the ammonia plant (7690 GWh/ a), independent of the heat supply system. For heat supply purpose, the natural gas demand can be reduced from 2194 GWh/a in the fossil heat supply system to 91 GWh/a in the best nuclear respectively to 122 GWh/a in the best solar heat supply system. In the ammonia plant, 1470 ktons of CO2 are captured and stored independent of the heat supply system. Applying the conventional fossil heat supply system, additionally 417 ktons of CO2 are emitted. Replacing the fossil heat supply system by the best nuclear heat supply system reduces the CO2-emissions by 96% amounting to 17 ktons. Applying the best solar heat supply system instead of the fossil heat supply system reduces the CO2-emissions by 94.5% amounting to 23 ktons of CO2. Nevertheless, the net electricity demand rises if the alternative heat supply systems are applied due to the electricity demand of the HTR, the solar tower and the electric heating of the thermal storage. The 637

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heat supply system containing a solar tower or an HTR can compete economically to the fossil heat supply system. Different parameter variations showed that the interest rate and the investment costs of HTR, solar tower and thermal storage are the most decisive parameters for the economic performance of the nuclear and solar systems. But, neither a cost reduction of 30% nor an interest rate reduction of 50% was enough to reach the breakeven point of the alternative systems. Furthermore, the CO2-certificate price has nearly no influence on the economic performance of the fossil heat supply system, thus even a massive CO2-certificate price rise would not lead to the competitiveness of the solar or nuclear systems. Moreover, changes concerning the electricity price do not improve the economic situation of the alternative heat supply systems. The alternative heat supply systems can only compete to the fossil system if gas price rise massively. The gas price must rise from 27 €/MWh (base case assumption) to 81 €/MWh (trebled gas price) in order to make the best nuclear system competitive. Assuming South African solar conditions, the best solar heat supply system can compete to the fossil system if the gas price rises by the factor 2.5. In order to establish solar or nuclear heat supply systems in real industrial applications strong political incentives like subsidies are necessary to compensate the economic disadvantages. Otherwise, the today fossil dominated industrial process heat supply won’t change.

conventional fossil heated ammonia plant system requires 1485 GWh of electricity. The best nuclear system needs 5% more electricity amounting to 1565 GWh. Despite the partially internal electricity production, the best solar heat supply system requires 3% more electricity than the fossil system amounting to 1534 GWh. If the purchased electricity is basically supplied by conventional fossil heated power pants, the application of the alternative heat supply systems would induce further usage of fossil resources and further CO2 emissions to supply the higher electricity demand, which would relativize the savings mentioned above. For a final evaluation of the saving amounts of CO2 emissions and natural gas, the electricity mix of the location under investigation must be analyzed in detail. In a certain investment decision, the advantages offered by the alternative heat supply systems must be opposed to the economic disadvantages. If the advantages of reduced CO2-emissions, savings of fossil resources, the diversification of energy supply and the raised independency on fuel imports and fuel prices is added a high value, investments in nuclear or solar heat supply systems are possible despite missing economic competitiveness. To foster CO2-neutral heat supply systems in the industrial sector political incentives like subsidies or tax reliefs are necessary. Whether in this case the HTR or the solar tower should be preferred, depends on the location. The solar system supplemented by a fossil heater is only under excellent solar conditions, occurring for example in South Africa, the best alternative. If the solar conditions are just good, assuming for example solar conditions which occur in the South of Spain, the HTR-module supplemented by a fossil heater should be preferred.

Acknowledgements The authors gratefully acknowledge the computing time grant of the compute cluster of RWTH Aachen.

5. Summary and conclusions References The emissions of climate affecting greenhouse gases can be reduced by changing the high temperature heat supply of energy-intensive industries from today’s mostly fossil sources to nuclear or solar sources. Energy economic evaluations have been conducted applying a self-developed optimization tool in order to evaluate the economic competitiveness of an ammonia plant heated by solar tower or HTR to an ammonia plant supplied by natural gas. The energy economic evaluations revealed that the heat supply of a constantly operated ammonia plant by solar towers and a thermal storage only is not a sensible option. The solar tower system should be supplemented by a fossil backup system, which can ensure the heat supply during periods of low solar radiation. Under South African solar conditions, the best solar system consisting of six solar towers, a thermal storage, a supplementing fossil heater and a turbine achieves AGC of 448 €/t. In this case, the ammonia plant is operated continuously. Assuming South Spanish solar conditions, the best solar system achieves AGC of 523 €/t. Here, the ammonia plant is operated discontinuously. The constant ammonia delivery obligation can be fulfilled by an ammonia storage. In the nuclear heat supply case, two HTR-modules are necessary to run the ammonia plant continuously. The second HTR-module serves as a backup system. If both HTR-modules are available, the second module is used to produce electricity. If an ammonia storage is implemented to run the ammonia plant discontinuously, the ammonia storage can take the task of the backup system so that just one HTR-module is needed. The most economic nuclear heat supply system consists of one HTRmodule and a fossil backup system. The entire electricity demand is fulfilled by electricity purchase. The ammonia plant is operated continuously. The AGC amount to 472 €/t. The coupling of the two capital-intensive heat supply facilities solar tower and HTR is not advisable. If a heat supply system without any fossil heat is desired, the pure nuclear heat supply system would be the cheaper option. The lowest AGC are achieved if a conventional fossil heat supply system is applied. In this case the AGC amount to 391 €/t. Overall, no

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