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Space power systems: Producing transportation (and other chemical) fuels as an alternative to electricity generation
Acta Astronautica 65 (2009) 1261 – 1271 www.elsevier.com/locate/actaastro
Space power systems: Producing transportation (and other chemical) fuels as an alternative to electricity generation Robert S. Wegenga,∗ , John C. Mankinsb a Pacific Northwest National Laboratory, Battelle Memorial Institute, USA b Artemis Innovation Management Solutions, USA
Received 5 December 2007; accepted 10 March 2009 Available online 5 August 2009
Abstract While most studies on space power systems target electricity generation as the energy product, industrialized nations also have a need for chemicals to support transportation and other purposes. This paper therefore describes an alternative target for the application of space power systems: the production of chemical fuels based on radiant energy beamed or reflected from orbiting platforms. If cost and efficiency targets can be achieved, Solar Thermochemical Plants—occupying a few square kilometers each—can potentially generate substantial quantities of transportation fuels, therefore enabling reductions in the consumption of petroleum and the emission of carbon dioxide. The specifics of the approach that are described in this paper include the concentration of radiant energy within ground-based systems so that high temperature heat is provided for thermochemical process networks. This scoping study includes the evaluation of various feedstock chemicals as input to the Solar Thermochemical Plant: natural gas, biomass and zero-energy chemicals (water and carbon dioxide); and the production of either hydrogen or long-chain hydrocarbons (i.e., Fischer–Tropsch fuels) as the Solar Fuel product of the plant. © 2009 Published by Elsevier Ltd.
1. Introduction Historically, space power systems (SPS) that have been described by others comprise orbiting systems—that collect and transform solar energy into other radiant energy (e.g., microwaves)—and receiving stations on the Earth’s surface that convert the incoming power beam into electricity and deliver it to an electricity grid. In this paper, we describe an alternative conversion system, for application on the ground, that employs thermochemical processing methods to generate Solar
∗ Corresponding author.
E-mail addresses: [email protected] (R.S. Wegeng), [email protected] (J.C. Mankins). 0094-5765/$ - see front matter © 2009 Published by Elsevier Ltd. doi:10.1016/j.actaastro.2009.03.054
Fuels and other chemicals of interest. The system preferably tracks the sun (when it is available) and the orbiting satellite (when the sun is not available). It is our contention that thermochemical processing methods are generally more energy-efficient than current biochemistry methods for fuels productions (e.g., corn-to-ethanol); however, they require substantial capital investments in order to generate chemical fuels in significant volumes. 2. Assumptions and definitions In this paper, we present the preliminary results of scoping/productivity studies for various thermochemical systems and configurations that may be employed to produce Solar Fuels. Accordingly, a number of assumptions and definitions are important to note.
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Energy Choices Considered • Direct Solar Only • Direct Solar + Natural Gas (combustion of) • Direct Solar + Space Solar (beamed)
Feedstocks
Solar Fuels
Considered
Considered Solar
• Natural Gas (CH4)
Thermochemical
• Biomass (CH4 + CO2)
Plant
• Zero-Energy Chemicals
• Hydrogen (H2) • Synthetic Fuels (CnH2n+2)
(H2O + CO2) Fig. 1. Scope of evaluation/study variables.
2.1. Solar Fuels We define Solar Fuels to be chemicals where solar energy is an important, but not necessarily the only, source of chemical energy in the fuel. As an energy source, solar energy is ubiquitous but diffuse, requiring significant capital investment in collection systems that cover large areas in order to accumulate a substantial amount of energy. Fig. 1 includes a partial listing of fuels that may be produced using solar energy. Chemistries for producing these fuels are well known and/or are receiving considerable development funding and, in each case, the fuels have properties that make them desirable for various applications, including transportation. Hydrogen (H2 ) is of great interest as a future transportation fuel and for its potential role in establishing an overall “hydrogen economy”. Notably, hydrogen is the fuel of choice for most types of fuel cells. The chief technical challenges associated with hydrogen involve storage and transportation and are to some degree related to the overall low energy density of hydrogen compared to other chemical fuels. If these challenges can be overcome, hydrogen may replace long-chain hydrocarbons as the fuel of choice for commercial transportation. Long-chain hydrocarbon fuels, listed in the figure as synthetic fuels (which we also represent as Cn H2n+2 , where n is a variable from about 5 to about 12, or higher), have been the fuel of choice for terrestrial transportation for over 100 years. Reasons include global availability, ease and relative safety
of storage and transportation, and very high energy densities. Various alcohols (Cn H2n+1 OH) have also been proposed and are in use today, although to a lesser extent than gasoline and diesel fuel. At least one Nobel Prize winner is currently a proponent of methanol (CH3 OH) over hydrogen [19] and it is well known that Henry Ford was an early proponent of ethanol (C2 H5 OH) for automotive transportation. However, we did not include alcohol fuels in our calculations. 2.2. Chemical feedstocks In general, feedstocks for the production of Solar Fuels can include a variety of chemicals. As Fig. 1 shows, for this study we have investigated a number of feedstocks, including some that bring chemical energy and some that do not. Natural gas (e.g., CH4 ) is considered an obvious potential feedstock, and chemical processes that produce hydrogen or hydrocarbon fuels from natural gas are well known. Biomass is another category of potential feedstocks that bring chemical energy, one which we note is carbon neutral (since the atmosphere is the original source of carbon for biomass feedstocks). Although several biomass feedstock mixes are possible, for our study we consider methane and carbon dioxide (CO2 ), such as can be obtained through the anaerobic decomposition of biomass materials or through other methods. As zero chemical energy sources, we consider water (H2 O) and carbon dioxide. Efficient, thermochemical processes for splitting water and carbon dioxide are
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not well developed; however, considerable research is underway on solar thermochemical water-splitting and there is significant potential for this to be supportive of the proposed hydrogen economy. Alternately, if a hydrocarbon fuel is the desired product, a source of carbon is also needed; therefore we also consider carbon dioxide. 2.3. Energy sources Although our study is primarily focused on radiant energy as the input energy source, we also consider other supplemental energy sources in order to increase the overall daily/annual productivity of the Solar Thermochemical Plant. As Fig. 1 indicates, we considered (1) direct solar energy only, which provides a capacity factor of only about 25% (or slightly less); (2) direct solar energy plus the supplemental combustion of natural gas, so that the capacity factor increases to 90% and (3) direct solar energy plus the supplemental use of space solar energy, which consists of beamed, radiant energy (visible laser or microwave) from an orbiting platform, also increasing the capacity factor to 90%. These three alternatives allow us to compare the productivity of various hypothetical Solar Thermochemical Plants in terms of the approximate quantities of transportation fuel produced per day (or per year) and the reductions in carbon dioxide emissions based on displacing gasoline as a transportation fuel. 3. Technical approach to solar thermochemical processing The technical approach for using solar energy to produce chemical fuels generally consists of concentrating solar and/or beamed energy to support one or more high-temperature, endothermic chemical reactions, which increases the chemical energy content of a reacting stream, then performing additional unit operations (including one or more low- to moderate-temperature endothermic reactions) to create the chemical fuel of interest. Fig. 2 presents a generic chemical process flowsheet including additional steps for purification of the product and for recycle, plus heat exchangers that recuperate thermal energy and/or help to control the chemical reactions and separations steps. Fig. 2 is meant to illustrate the overall approach for chemical fuels production from solar energy. Specific chemical process flowsheets will vary from case to case, depending upon feedstocks, chemical products, methods of chemical separations, and the overall need to
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thermally integrate and otherwise optimize the process into an energy-efficient, economic operation. 3.1. Solar collectors One of the technical challenges associated with the production of Solar Fuels is the integration of solar/radiant energy concentrators with endothermic unit operations. These include: (1) Performing high-temperature endothermic reactions with thermal energy collected at the focal point of parabolic dish or central receiver concentrators. (2) Utilizing moderate-temperature heat from lessexpensive, trough concentrators for moderate- to low-temperature endothermic unit operations. (3) Performing additional unit chemical operations to produce a final chemical fuel product. A central receiver concentrator system consists of a field of heliostat mirrors, which track the sun and reflect solar energy to a single receiver atop a tower structure, and are capable of providing heat at temperatures between 1000 and 2000 ◦ C. Central receiver units, designated Solar One and Solar Two, have been demonstrated in the USA for electrical power generation applications in Barstow, California. Solar Two, which is shown in Fig. 3, consisted of a field of 1926 heliostats, providing a total collector area of 81, 000 m2 and producing steam for a 10 MWe electrical powerplant [20]. Parabolic dish concentrators are able to provide process heat at a temperature of about 1000 ◦ C, which is sufficiently high for most of the endothermic reactions of interest. Dish concentrators at this scale already exist and are being readied for commercialization in the American Southwest for electrical power generation. Fig. 4 shows a parabolic dish concentrator with a 25 kWe kinematic Stirling cycle heat engine at the focal point. Current plans call for tens of thousands of similar dish structures to be assembled in the USA’s Mojave Desert, generating several hundred megawatts of electrical power for the California electrical grid. Investors anticipate achieving economies of mass production in manufacturing large numbers of these concentrators and heat engines. One of the principals, Stirling Energy Systems, has reported that they hope to reduce the overall capital unit costs to about $50,000 per dish, or about $50,000 per 100 kWs input. Central receiver and dish systems address economics in somewhat different ways. Central receiver systems are expected to exploit economies of mass production
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Highly Concentrated Solar Energy from Parabolic Dish or Central Receiver Concentrators
High Temp Endothermic Reactor Product Stream “F”
Separations
Moderately Concentrated Solar Energy from Trough Concentrators
Reactant “A”
Moderately Concentrated Solar Energy from Trough Concentrators HXR Low-Mod Temp Exothermic Reactor
Preheater/ Vaporizer
Product Stream “G” Separations
Recycle “C”
Reactant “B”
Recycle “D” Recycle “E”
Fig. 2. Generic chemical process flowsheet.
Fig. 3. Solar Two central receiver with heliostats. Photo courtesy of Sandia National Laboratory.
with regard to the heliostats while also attempting to achieve economies of scale for the receiver system and the chemical plant. Dish systems are expected to ex-
ploit economies of mass production for the dishes as well as for the portion of the chemical process plant that is associated with each dish. Some elements of the
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than will high temperature heat from a dish or central receiver concentrator. Since central receivers and dish concentrators are more expensive than trough concentrators, it makes financial sense to considering allocating heat from trough concentrators to low- to moderate-temperature unit operations and heat from dish and central receiver concentrators to hightemperature endothermic reactions. 3.2. Chemical operations
Fig. 4. Parabolic dish with 25 kWe kinematic Stirling cycle heat engine. Photo courtesy of Sandia National Laboratory.
chemical plant, such as some of the unit operations that are downstream of the high temperature, endothermic reactor may also be centralized, to some degree, providing an opportunity for economies of scale. Trough concentrators, which are less expensive than dish concentrators, are capable of delivering heat at up to about 300.500◦ C [20]. For this reason, trough concentrators are preferred over dish concentrators for moderate- to low-temperature endothermic unit operations within a thermochemical process network. For example, if water is one of the reactants for an endothermic chemical reaction, such as the methane steam reforming reaction, heat from a trough concentrator can be used to provide vaporization of the water. Additionally, decomposition of some biomass materials can be accomplished with heat from trough concentrator units. Low- to moderate-temperature heat from a trough concentrator will have a lesser exergy1 proportion 1 Equivalent to “availability”, the thermodynamic parameter, “exergy”, is defined to be the maximum theoretical work that can be obtained from an energy source as it is brought into equilibrium with the environment. High temperature heat has a greater exergy content than low temperature heat; high temperature heat can therefore provide a greater increase in the chemical energy content of the products of an endothermic chemical reaction.
At least one high-temperature, endothermic chemical reactor is a central element of the thermochemical approach to producing Solar Fuels. With a central receiver concentrator system, this reactor may be located at the receiving point, or at the base underneath the receiving point, of the central receiver tower. Alternately, with a dish-based system, the endothermic chemical reactor may preferentially be located at or in close proximity to the focal point of each dish. This reactor changes the chemical form of the feedstock and increases its chemical energy content, using concentrated solar energy or radiant energy from the orbiting platform. At least one moderate-temperature, exothermic chemical reactor is also among the additional unit chemical operations shown in Fig. 2. This operation is needed for each of the Solar Fuels discussed in this paper. For example, for the production of hydrogen using methane as a feedstock chemical, the endothermic, steam reforming reaction is followed by an exothermic, water–gas shift reaction. The latter reaction increases the hydrogen content of the product stream while producing heat that can be used elsewhere in the overall chemical process. Alternately, if long-chain hydrocarbons are the desired product, the endothermic steam reformer is followed by an exothermic Fischer–Tropsch reactor. The separations and purification operations, also identified in Fig. 2, may additionally produce heat that can be used elsewhere in the chemical process or which must be rejected to the environment. For example, thermal-swing sorption processes and distillation processes each require heat from a moderate temperature source and reject heat at a lower temperature. Table 1 lists the overall net chemical reactions for many of the feedstocks and Solar Fuels discussed in this paper. However, numerous details have been omitted in this summary table. For example, while the idealized net reaction for producing Fischer–Tropsch hydrocarbons from methane does not show the consumption of water, readers familiar with these processes are aware that some makeup water will be required since it is
Net thermochemical process (idealized)
CH4 + 2H2 O = 4H2 + CO2 nCH4 = Cn H2n+2 + (n − 1)H2 (3n + 1)/4CH4 + (n − 1)/4CO2 = Cn H2n+2 + (n − 1)/2H2 O H2 O = H2 + 21 O2 nCO2 + (n + 1)H2 O = CnH2n+2 + (3n + 1)O2
Primary reactions
Methane reforming plus water–gas shift Methane reforming plus Fischer–Tropsch Methane reforming plus Fischer–Tropsch Water splitting processes Water splitting processes plus reverse water–gas shift and Fischer–Tropsch
Reactants
CH4 + H2 O CH4 + H2 O CH4 + CO2 H2 O CO2 + H2 0
Table 1 List of chemical reactions.
H2 Cn H2n+2 + H2 Cn H2n+2 H2 Cn H2n+2
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Solar Fuels
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extremely difficult to completely separate and recycle all water from the product stream. 3.3. Hybrid systems If direct solar energy only is used by a Solar Thermochemical Plant, the plant capacity factor is likely to be about 25% or lower. A higher capacity factor is beneficial, however, both in terms of the productivity and the profitability of the plant. For this reason, supplemental methods of providing thermal energy are considered as a means to increase the plant’s capacity factor. Dish systems and/or heliostat units that can track and follow the sun can also track an orbiting satellite. Accordingly, we consider an alternate, hybrid system that uses beamed energy (e.g., microwaves) from the orbiting platform in order to increase the daily productivity of the Solar Thermochemical Plant. In this case, the overall product output of the plant will be a function of both the solar flux and the beamed energy flux. Alternately, another method of increasing the plant capacity factor is to combust natural gas or utilize another energy source (e.g., stored solar thermal energy) to provide heat for the endothermic unit operations of the chemical process. Hybrid systems of these types allow the capital and operating costs of the chemical plant to be amortized over a larger quantity of chemical product than has previously been contemplated by others for solar thermochemical processing. Disadvantages include the inclusion of additional capital and/or fuel costs (and associated increases in carbon dioxide emissions if combustion is the source of supplemental energy). 3.4. Previous solar thermochemical work A number of lab-scale and prototype solar thermochemical demonstrations have been performed in the USA and abroad. For example, in the late 1980s, CO2 reforming was conducted at the Sandia National Laboratory in a direct catalytic absorption reactor (with solar energy being absorbed by catalyst particles after passing through a quartz window in the reactor). This demonstration, which took place at the 3.5 kW scale, used a non-tracking, 6.7 m diameter concentrator, with illumination by a tracking flat-plate heliostat. Maximum thermal efficiencies and chemical efficiencies2 of 79.8% and 55.5% were reported [9]. Other small-scale demonstra2 Chemical efficiency is defined to be the percent of incident solar (or other thermal energy) that is converted into an increase in the chemical energy of the reacting stream.
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tion tests have been reported, including an indirectly heated, inconel reactor that was tested at 6.4 kW using the Schaeffer Solar Furnace of the Weizmann Institute of Science in Israel [12]. A larger scale demonstration of solar CO2 reforming of methane was performed in the 1990s via a joint effort by the USA and Germany, with the reaction system being provided by Sandia National Laboratory and testing conducted at the German Aerospace Research Center (Deutsches Zentrum für Luft- und Raumfahrt [DLR]) in Lampoldschausen, Federal Republic of Germany. This demonstration used a 17-m, parabolic dish capable of 150 kWs power in a number of tests, ultimately achieving a chemical efficiency of 76.1%. Researchers reported that the chemical efficiency had been improved through the addition of a counterflow, recuperative heat exchanger [15,2,16]. In another example, solar steam reforming of methane was performed at a central receiver demonstration system at the Weizmann Institute of Science, utilizing a DLR-provided, 400 kWs reactor, at 800 ◦ C. Reporting on this and cost studies, DLR researchers reported that, “in the context of a preliminary cost study, it was shown that it is possible to produce hydrogen from natural gas in a large 50 MW (thermal) production plant for five cents per kilowatt-hour (calorific value of hydrogen)” [14]. Yet another demonstration of solar methane reforming was achieved through the use of the gas-cooled solar tower (GAST) system at the Plataforma Solar de Almeria through a joint effort by DLR and the Spanish Centro de Investigaciones Energe ticas, Medioambientales y Tecnologicas (CIEMAT), using an indirectly heated steam reformer. For the GAST demonstration, air was heated in a central receiver system to about 1000 ◦ C, providing heat for a 170 kW (nominal) steam reforming reactor. Reactor temperatures ranged from about 700 to about 800 ◦ C, with the highest methane conversions (93%) being accomplished at the highest temperatures [1,25].
4. Preliminary production estimates In this section, we present preliminary estimates of the productivity of a Solar Thermochemical Plant. Calculations have been performed based upon limiting features of the various chemical process flowsheets, such as the amount of highly concentrated solar energy (for endothermic chemical reactions) and the conversion and selectivity of the low- to moderate-temperature exothermic chemical reactions.
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We hypothesize a Solar Thermochemical Plant with sufficient numbers of solar concentrators such that, during periods of bright sunlight, cumulative solar energy rates of 1.0 GWs would be used to drive hightemperature, endothermical chemical reactions. For example, a system based on parabolic dish concentrators at 100 kWs each would require 10,000 dishes. Likewise, a system based on central receiver concentrators at 50,000 kWs each would require 20 tower systems. For our productivity calculations, we assume that the chemical efficiency of the high temperature reaction is 40% (except for thermochemical water-splitting, where we selected a range of 30–50% based on the various chemical processes under investigation [26,27]. This is conservatively low compared to values reported in the literature for the solar reforming of methane and may justify a re-examination in the near future: if high operating temperatures are obtained, and if highly effective recuperative heat exchange is provided for the reacting stream, the actual chemical efficiency may be considerably higher. Also as previously described, we have assumed that the thermal energy for low- to moderate- temperature operations such as water vaporization, thermal-swing separations, and distillation, will be provided in part through thermal integration with exothermic unit operations and in part through the utilization of parabolic trough concentrators. The parabolic trough collection system will be oversized and operated in conjunction with thermal energy storage containing suitable materials (such as hydrated salts) so that heat is available during periods of darkness. In Column (A) of Table 2, we consider the Solar Thermochemical Plant when operated with direct solar energy only (based on the diurnal cycle of the Earth) with an overall capacity factor of 25%. This is equivalent to full production for six hours per day, 365 days per year. In terms of the productivity of the plant, our preliminary estimate is that, if natural gas (methane) is used as a feedstock, the output of the plant will be approximately 390,000–430,000 gallons of gasoline equivalent (gge) per day. Note that if zero-chemical energy feedstocks such as water and/or carbon dioxide are used, the productivity of the plant is reduced to about 47,000–79,000 gge/day. This calculation clearly shows an advantage if methane is used as a feedstock for a Solar Thermochemical Plant. To maximize the profitability of their capital investment, investors will want to maximize its productivity. Along with using methane as a feedstock chemical, productivity can be improved if the plant’s capacity factor can be increased. Accordingly, in Columns (B) and (C) of Table 2, we evaluate the performance of the plant
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Table 2 Solar Thermochemical Plant performance 10,000 dishes (at 100 kWs per dish) or 10 central receivers (at 100,000 kWs per central receiver). Capacity factor
Input thermal energy
Solar Fuel products
(A) Solar only 25%
Solar + Natural gas 90%
(C) Solar + Beamed power 90%
Natural gas (CH4 ) + H2 O
Production (gge/dish/day) Production (gge/receiver/day) System Production (gge/day) CO2 Emission reductions (metric tons/year)
39–43 39,000–43,000 390,000–430,000 220,000–350,000
140–160 140,000–160,000 1,400,000–1,600,000 −280, 000–180,000
140–160 140,000–160,000 1,400,000–1,600,000 810,000–1,300,000
Hydrogen and/or long-chain hydrocarbons
Biomass (CH4 CO2 ) + H2 O
Production (gge/dish/day) Production (gge/receiver/day) System production (gge/day) CO2 emission reductions (metric tons/year)
29 29,000 290,000 920,000
100 100,000 1,000,000 2,200,000
100 100,000 1,000,000 3,300,000
Hydrogen and/or long-chain hydrocarbons
H2 O
Production (gge/dish/day) Production (gge/receiver/day) System production (gge/day) CO2 emission reductions (metric tons/year)
4.7–7.9 4700–7900 47,000–79,000 150,000–250,000
17–28 17,000–28,000 170,000–280,000 −540, 000 to −170, 000
17–28 17,000–28,000 170,000–280,000 550,000–910,000
Hydrogen
CO2 + H2 O
Production (gge/dish/day) Production (gge/receiver/day) System Production (gge/day) CO2 emission reductions (metric tons/year)
7.3 7300 73,000 230,000
26 26,000 260,000 840,000
Long-chain hydrocarbons
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Input chemicals
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based on using beamed energy from an orbiting platform or the combustion of natural gas as supplemental heating sources, to improve operations when the sun is partially obscured by clouds and to allow the plant to operate through the nighttime. Accordingly, if the plant is operated with a capacity factor of 90%, the productivity of the plant increases. Without methane as a chemical feedstock, the daily production rate improves from about 47,000–79,000 to about 170,000–280,000 gge/day. This is nearly a fourfold improvement. However, if methane is used as a chemical feedstock, the plant’s daily production rate increases to 1,400,000–1,600,000 gge/day. The combustion of one gallon of gasoline results in the release of 8.82 kg of carbon dioxide to the atmosphere. For the same net chemical energy production (based on the higher heating values of gasoline and methane), the combustion of methane only generates about 6.67 kg of carbon dioxide; accordingly, displacing gasoline with methane-derived Solar Fuels should generally reduce carbon dioxide emissions. However, the actual Greenhouse Gas emissions will depend upon the source of the energy that is used to produce Solar Fuels. Table 2 additionally presents preliminary estimates of the Greenhouse Gas emissions (increases and reductions) associated with operation of the Solar Thermochemical Plant. For the cases where only solar energy is used to support the endothermic chemical reactions, for example, Column (A), net Greenhouse Gas emissions are reduced (compared to using gasoline as a transportation fuel). However, where natural gas is burned to support the endothermic chemical reactions, mixed results occur. If methane is the feedstock for the reaction, then net carbon dioxide emissions are reduced. But in many cases where methane is used as a combustion fuel, for supplemental heating of the endothermic reaction, net carbon dioxide emissions increase. The best case for emission reductions occurs when biomass feedstocks are combined with solar energy and beamed energy from an orbiting platform. In this case, the biomass feedstock brings carbon-neutral, chemical energy content and the orbiting platform supports a capacity factor of 90%. The net reductions in carbon dioxide emissions are estimated to be about 3,300,000 metric tons of carbon dioxide per year. This version of the Solar Thermochemical Plant additionally produces 1,000,000 gge per day, an amount equal to about 0.24% of the USA’s annual oil imports. Forty such Solar Thermochemical Plants, each occupying a few square kilometers, could reduce US oil imports by nearly 10%.
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5. Additional considerations This paper presents a variety of concepts and configurations for producing Solar Fuels via thermochemical processing methods. Benefits of the concept include the option to use Solar Fuels to reduce oil consumption and Greenhouse Gas emissions. There should be little doubt as to the technical viability of using solar energy to drive thermochemical process systems. However, there are important technology development, financial and economic issues to consider. 5.1. Technology development issues The technologies that are needed in order to realize Solar Fuels, via a thermochemical approach, vary in their readiness for commercialization. With regard to solar concentrators, we note that much work has been performed, in the USA and abroad, over the last few decades, and solar reforming reactors have been demonstrated both in dish and central receiver configurations. More work is needed, however, in order to realize either economies of mass production or economies of scale with solar concentrators. Accordingly, we consider solar concentrators to be of moderate to high technology readiness. A significant technical question is the chemical efficiency of the overall system. As noted earlier, our assumption of 40% may be conservatively low; if higher efficiencies can be achieved/maintained, such as has been suggested from previous work, the productivity and financial viability of solar thermochemical processing is enhanced beyond that suggested by our productivity calculations. For systems that utilize methane as a feedstock, the chemistry is well known; in fact, steam reforming is currently the industrially-preferred method of producing hydrogen. For application within dish concentrators, we note that compact chemical reactors and heat exchangers have been demonstrated for other applications, using process intensive hardware such as microchannel reactors and heat exchangers which also can achieve high energy efficiencies. Microchannel reactors and heat exchangers have been previously demonstrated for unit operations such as steam production, recuperative heat transfer, the methane steam reforming reaction, the water–gas-shift and reverse-water–gas shift reactions, and for the Fischer–Tropsch reaction. These systems are of moderate to high technology readiness; work is still needed, in particular, to develop mass production methods in order to reduce hardware capital costs.
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Thermochemical technologies and systems that utilize feedstocks that bring no chemical energy, such as thermochemical water-splitting processes, are currently of low technology readiness. However, significant work is currently underway within the Department of Energy’s Hydrogen Program to understand the chemistries and the chemical process flowsheets. The effort has included the screening of over 300 potential water-splitting cycles [22–24] and work is progressing on several that were deemed to be most promising. At the current time, the chemistries and process flowsheets appear to be quite challenging; however, worthwhile technology demonstrations will soon be underway. For example, a dish system, using one of the metal oxide processes, is currently being assembled at Sandia National Laboratory and is scheduled to begin testing later this year [4]. Technologies and systems that provide supplemental heat through the use of one or more orbiting platforms are also of low technology readiness, despite having initially been proposed decades ago. After its initial proposal in the 1960s [8,11] development efforts were initiated, then halted, in the 1970s [17] and again in the 1990s [6,7,13,18]. For Space Solar Power to be developed to the point where it can be used to produce transportation fuels, critical areas include the design of, and assembly approach to, the orbiting platform that intercepts solar energy and converts it beamed power directed at the ground receiver as well as the launch systems that would place hardware in orbit at reasonable costs. 5.2. Economic issues While financial analyses will consider the costs and benefits of Solar Thermochemical Plants based on the perspective of owners and potential investors, economists will additionally consider other real costs and benefits associated with producing Solar Fuels. Macroeconomic analyses consider both financial costs and benefits as well as national and societal costs and benefits, such as: (1) Environmental costs and benefits, such as may be obtained through reductions in carbon dioxide emissions. (2) National and international costs and benefits, such as may be obtained through reductions in oil imports from politically-unstable regions, as well as the impact of shifting a portion of the transportation energy burden from petroleum to natural gas (which would increase the demand for and consumption of natural gas).
(3) Regional or national costs and benefits (e.g., producing Solar Fuels domestically can provide local economic benefits and reduce the magnitude of potential trade deficits that are otherwise incurred through purchases of foreign petroleum). Other than estimates for reductions in carbon dioxide emissions, we have not attempted to quantify inputs for an economic analysis of the Solar Fuels concept. Here we note that our calculations have assumed no efforts to sequester carbon dioxide. If sequestration is taken into account, the Solar Thermochemical Plant can also be thought of as a decarbonization system for cases such as when hydrogen is the Solar Fuel product. Accordingly, significantly higher reductions in carbon dioxide emissions would be calculated for those cases. We are currently working to develop the concepts for Solar Fuels into sufficient detail so that a comprehensive financial analysis using lifecycle cost analyses techniques can be performed. By “financial analysis,” we mean that we intend to consider the costs (capital costs, operating costs, etc) and income (gross and net) of potential Solar Thermochemical Plants. 6. Conclusions This paper has presented a number of options for the solar thermochemical production of transportation and other fuels. Both dish-based and central receiver systems appear to be technically viable, although some of the chemistry options require substantial technology development (such as thermochemical water-splitting) before financial risks are acceptable. However, some of the highest productivity systems, such as those that employ methane (from natural gas or biomass) and steam reforming reactions already have a moderate to high degree of technology readiness. Financial viability depends heavily upon three elements: (1) thermochemical efficiency, (2) plant capacity factor and (3) capital costs. For systems based on methane steam reforming, there is high confidence that reasonable efficiencies can be obtained, whereas for thermochemical water-splitting processes, high confidence is not yet available. High plant capacity factors appear to be possible. If the combustion of natural gas is used as a supplemental source of high temperature heat, technology readiness is high, with the disadvantage being increased usage of natural gas and associated increases in carbon dioxide emissions. One of the most attractive cases evaluated involves the use of biomass as a feedstock chemical. Biomass feedstocks can be used to produce a carbon-neutral form of synthetic gasoline. If the combustion of
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natural gas is used as a supplemental heat source for this system, so that a capacity factor of 90% is achieved, the net productivity of the system evaluated is high, at 1,000,000 gge/day, and compared to burning gasoline for transportation, carbon dioxide emissions are reduced by 2.2 metric tons/year. Alternately, if space solar power is used as the supplemental heating source, natural gas demand is reduced, high plant capacity factors are maintained, and carbon dioxide emissions are further reduced by an additional 1.1 metric tons/year; however, the technology readiness of the orbiting platform is currently very low and capital and operating costs are unknown. Despite this, space solar power, especially combined with biomass feedstocks, may be very attractive due to the combination of high productivity and substantial reductions in carbon dioxide emissions. Detailed financial analyses are still required. Additional work is needed, including technical development/evaluation and lifecycle cost analyses, in order to set capital cost targets and provide a thorough evaluation of the financial viability of the concepts presented in this paper. In particular, there is substantial uncertainty regarding the costs (capital and operating) associated with an orbiting space solar power satellite. Despite uncertainties, there are substantial incentives associated with pursuing the Solar Fuels concepts. These include reductions in oil consumption and carbon dioxide emissions. Given current political and technical uncertainties, including the expectation that petroleum production capacities will decline, and increasing concerns about carbon dioxide emissions, there is a great need to pursue renewable energy sources as an alternative source of transportation fuels. References [1] M. Bohmer, U. Langnickel, M. Sanchez, Solar steam reforming of methane, Solar Energy Materials 24 (1991) 441–448. [2] R. Buck, J.F. Muir, R.E. Hogan, R.D. Skocypec, Carbon dioxide reforming of methane in a solar volumetric receiver/reactor: the CAESAR project, Solar Energy Materials 24 (1991) 449–463. [4] R.B. Diver, J.E. Miller, M.D. Allendorf, N.P. Siegel, R.E. Hogan, Solar themochemical water-splitting ferrite-cycle heat engines, in: ASME International Solar Energy Conference, Proceedings of ISEC2006, July 2006. [6] H. Feingold, M. Stancata, A. Freidlander, M. Jacobs, D. Comstock, C. Christensen, G. Maryniak, S. Rix, J. Mankins, Space solar power: a fresh look at the feasibility of generating solar power in space for use on earth, in: SAIC-97/1005, 1997. [7] P. Glaser, Power from the sun: its future, Science 162 (3856) (1968) 857–866. [8] P.E. Glaser, F.P. Davidson, K. Csigi (Eds.), Solar Power Satellites, Wiley and Praxis Publishing, 1998. [9] R.E. Hogan, R.D. Skocypec, R.B. Diver, J.D. Fish, A direct absorber reactor/receiver for solar thermal applications, Chemical Engineering Science 45 (8) (1990) 2751–2758.
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[11] C.C. Kraft, The solar power satellite concept, in: The Von Karman Lecture, 15th Annual Meeting of the American Institute of Aeronautics and Astronautics, 1979. [12] R. Levitan, M. Levy, H. Rosin, R. Rubin, Closed-loop operation of a solar chemical heat pipe at the weizmann institute solar furnace, Solar Energy Materials 24 (1991) 464–477. [13] J.C. Mankins, Space solar power: a perspective future energy source?, Space Energy and Transportation 4 (3–4) (1999) 97–114. [14] S. Moeller, M. Epstein, Solar fuel production, DLR Nachrichten 109—Special Issue Solar Research, January 2005, accessed April 2007 at http://www.dlr.de/en/desktopdefault. aspx/tabid-618/1034_read-1412/ . [15] J.F. Muir, R.E. Hogan, R.D. Skocypec, R. Buck, Solar reforming of Methane in a direct absorption catalytic reactor on a parabolic dish: i—test and analysis, SAND-90-2674C, 1990. [16] J.F. Muir, R.E. Hogan Jr., R.D. Skccypec, R. Buck, The CAESAR project: experimental and modeling investigations of methane reforming in a catalytically enhanced solar absorption receiver on a parabolic dish, SAND92-2131, July 1993. [17] National Resource Council, Electric Power from Orbit: A Critique of a Satellite Power System, National Academy Press, 1981. [18] National Resource Council, Laying the Foundation for Space Solar Power, National Academy Press, 2001. [19] G.A. Olah, G.K. Surya Prakash, A. Goeppert, Beyond Oil and Gas: The Methanol Economy, Wiley, New York, March 2006. [20] M.R. Patel, Wind and Solar Power Systems, CRC Press, Boca Raton, FL, 1999. [22] C. Perkins, Development of solar powered thermochemical production of hydrogen from water, (Presentation) 2007 DOE Hydrogen Program Merit Review and Peer Evaluation Meeting, US Department of Energy, May 2007. [23] R. Perret, W. Culbreth, G. Besenbruch, R. Diver, A. Weimer, A. Lewandowski, E. Miller, Solar Hydrogen Generation Research, 2005. [24] R. Perret, A. Weimer, G. Besenbruch, R. Diver, M. Lewis, Y. Chen, Development of solar-powered thermochemical production of hydrogen from water, DOE Hydrogen Program, FY2006 Annual Progress Report, 2006. [25] I. Spiewak, C.E. Tyner, U. Langnickel, Applications of solar reforming technology, SAND93-1959, November 1993. [26] A. Steinfeld, R. Palumbo, Solar thermochemical process technology, in: R.A. Meyers (Ed.), Encyclopedia of Physical Science & Technology, vol. 15, Academic Press, 2001, pp. 237–256. [27] A. Steinfeld, Solar thermochemical production of hydrogen—a review, Solar Energy 78 (2005) 603–615.
Further reading [3] P. Davis, Program overview—2007 DOE Hydrogen Program Merit Review and Peer Evaluation Meeting, (Presentation) 2007 DOE Hydrogen Program Merit Review and Peer Evaluation Meeting, US Department of Energy, May 2007. [5] B.C. Esty, Modern Project Finance, Wiley, NJ, 2004. [10] H. Khatib, Economic Evaluation of Projects in the Electricity Supply Industry, The Institution of Electrical Engineers, London, UK, 2003. [21] M. Paster, Hydrogen production and delivery program element, (Presentation) 2007 DOE Hydrogen Program Merit Review and Peer Evaluation Meeting, US Department of Energy, May 2007.
Space power systems: Producing transportation (and other chemical) fuels as an alternative to electricity generation Robert S. Wegenga,∗ , John C. Mankinsb a Pacific Northwest National Laboratory, Battelle Memorial Institute, USA b Artemis Innovation Management Solutions, USA
Received 5 December 2007; accepted 10 March 2009 Available online 5 August 2009
Abstract While most studies on space power systems target electricity generation as the energy product, industrialized nations also have a need for chemicals to support transportation and other purposes. This paper therefore describes an alternative target for the application of space power systems: the production of chemical fuels based on radiant energy beamed or reflected from orbiting platforms. If cost and efficiency targets can be achieved, Solar Thermochemical Plants—occupying a few square kilometers each—can potentially generate substantial quantities of transportation fuels, therefore enabling reductions in the consumption of petroleum and the emission of carbon dioxide. The specifics of the approach that are described in this paper include the concentration of radiant energy within ground-based systems so that high temperature heat is provided for thermochemical process networks. This scoping study includes the evaluation of various feedstock chemicals as input to the Solar Thermochemical Plant: natural gas, biomass and zero-energy chemicals (water and carbon dioxide); and the production of either hydrogen or long-chain hydrocarbons (i.e., Fischer–Tropsch fuels) as the Solar Fuel product of the plant. © 2009 Published by Elsevier Ltd.
1. Introduction Historically, space power systems (SPS) that have been described by others comprise orbiting systems—that collect and transform solar energy into other radiant energy (e.g., microwaves)—and receiving stations on the Earth’s surface that convert the incoming power beam into electricity and deliver it to an electricity grid. In this paper, we describe an alternative conversion system, for application on the ground, that employs thermochemical processing methods to generate Solar
∗ Corresponding author.
E-mail addresses: [email protected] (R.S. Wegeng), [email protected] (J.C. Mankins). 0094-5765/$ - see front matter © 2009 Published by Elsevier Ltd. doi:10.1016/j.actaastro.2009.03.054
Fuels and other chemicals of interest. The system preferably tracks the sun (when it is available) and the orbiting satellite (when the sun is not available). It is our contention that thermochemical processing methods are generally more energy-efficient than current biochemistry methods for fuels productions (e.g., corn-to-ethanol); however, they require substantial capital investments in order to generate chemical fuels in significant volumes. 2. Assumptions and definitions In this paper, we present the preliminary results of scoping/productivity studies for various thermochemical systems and configurations that may be employed to produce Solar Fuels. Accordingly, a number of assumptions and definitions are important to note.
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Energy Choices Considered • Direct Solar Only • Direct Solar + Natural Gas (combustion of) • Direct Solar + Space Solar (beamed)
Feedstocks
Solar Fuels
Considered
Considered Solar
• Natural Gas (CH4)
Thermochemical
• Biomass (CH4 + CO2)
Plant
• Zero-Energy Chemicals
• Hydrogen (H2) • Synthetic Fuels (CnH2n+2)
(H2O + CO2) Fig. 1. Scope of evaluation/study variables.
2.1. Solar Fuels We define Solar Fuels to be chemicals where solar energy is an important, but not necessarily the only, source of chemical energy in the fuel. As an energy source, solar energy is ubiquitous but diffuse, requiring significant capital investment in collection systems that cover large areas in order to accumulate a substantial amount of energy. Fig. 1 includes a partial listing of fuels that may be produced using solar energy. Chemistries for producing these fuels are well known and/or are receiving considerable development funding and, in each case, the fuels have properties that make them desirable for various applications, including transportation. Hydrogen (H2 ) is of great interest as a future transportation fuel and for its potential role in establishing an overall “hydrogen economy”. Notably, hydrogen is the fuel of choice for most types of fuel cells. The chief technical challenges associated with hydrogen involve storage and transportation and are to some degree related to the overall low energy density of hydrogen compared to other chemical fuels. If these challenges can be overcome, hydrogen may replace long-chain hydrocarbons as the fuel of choice for commercial transportation. Long-chain hydrocarbon fuels, listed in the figure as synthetic fuels (which we also represent as Cn H2n+2 , where n is a variable from about 5 to about 12, or higher), have been the fuel of choice for terrestrial transportation for over 100 years. Reasons include global availability, ease and relative safety
of storage and transportation, and very high energy densities. Various alcohols (Cn H2n+1 OH) have also been proposed and are in use today, although to a lesser extent than gasoline and diesel fuel. At least one Nobel Prize winner is currently a proponent of methanol (CH3 OH) over hydrogen [19] and it is well known that Henry Ford was an early proponent of ethanol (C2 H5 OH) for automotive transportation. However, we did not include alcohol fuels in our calculations. 2.2. Chemical feedstocks In general, feedstocks for the production of Solar Fuels can include a variety of chemicals. As Fig. 1 shows, for this study we have investigated a number of feedstocks, including some that bring chemical energy and some that do not. Natural gas (e.g., CH4 ) is considered an obvious potential feedstock, and chemical processes that produce hydrogen or hydrocarbon fuels from natural gas are well known. Biomass is another category of potential feedstocks that bring chemical energy, one which we note is carbon neutral (since the atmosphere is the original source of carbon for biomass feedstocks). Although several biomass feedstock mixes are possible, for our study we consider methane and carbon dioxide (CO2 ), such as can be obtained through the anaerobic decomposition of biomass materials or through other methods. As zero chemical energy sources, we consider water (H2 O) and carbon dioxide. Efficient, thermochemical processes for splitting water and carbon dioxide are
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not well developed; however, considerable research is underway on solar thermochemical water-splitting and there is significant potential for this to be supportive of the proposed hydrogen economy. Alternately, if a hydrocarbon fuel is the desired product, a source of carbon is also needed; therefore we also consider carbon dioxide. 2.3. Energy sources Although our study is primarily focused on radiant energy as the input energy source, we also consider other supplemental energy sources in order to increase the overall daily/annual productivity of the Solar Thermochemical Plant. As Fig. 1 indicates, we considered (1) direct solar energy only, which provides a capacity factor of only about 25% (or slightly less); (2) direct solar energy plus the supplemental combustion of natural gas, so that the capacity factor increases to 90% and (3) direct solar energy plus the supplemental use of space solar energy, which consists of beamed, radiant energy (visible laser or microwave) from an orbiting platform, also increasing the capacity factor to 90%. These three alternatives allow us to compare the productivity of various hypothetical Solar Thermochemical Plants in terms of the approximate quantities of transportation fuel produced per day (or per year) and the reductions in carbon dioxide emissions based on displacing gasoline as a transportation fuel. 3. Technical approach to solar thermochemical processing The technical approach for using solar energy to produce chemical fuels generally consists of concentrating solar and/or beamed energy to support one or more high-temperature, endothermic chemical reactions, which increases the chemical energy content of a reacting stream, then performing additional unit operations (including one or more low- to moderate-temperature endothermic reactions) to create the chemical fuel of interest. Fig. 2 presents a generic chemical process flowsheet including additional steps for purification of the product and for recycle, plus heat exchangers that recuperate thermal energy and/or help to control the chemical reactions and separations steps. Fig. 2 is meant to illustrate the overall approach for chemical fuels production from solar energy. Specific chemical process flowsheets will vary from case to case, depending upon feedstocks, chemical products, methods of chemical separations, and the overall need to
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thermally integrate and otherwise optimize the process into an energy-efficient, economic operation. 3.1. Solar collectors One of the technical challenges associated with the production of Solar Fuels is the integration of solar/radiant energy concentrators with endothermic unit operations. These include: (1) Performing high-temperature endothermic reactions with thermal energy collected at the focal point of parabolic dish or central receiver concentrators. (2) Utilizing moderate-temperature heat from lessexpensive, trough concentrators for moderate- to low-temperature endothermic unit operations. (3) Performing additional unit chemical operations to produce a final chemical fuel product. A central receiver concentrator system consists of a field of heliostat mirrors, which track the sun and reflect solar energy to a single receiver atop a tower structure, and are capable of providing heat at temperatures between 1000 and 2000 ◦ C. Central receiver units, designated Solar One and Solar Two, have been demonstrated in the USA for electrical power generation applications in Barstow, California. Solar Two, which is shown in Fig. 3, consisted of a field of 1926 heliostats, providing a total collector area of 81, 000 m2 and producing steam for a 10 MWe electrical powerplant [20]. Parabolic dish concentrators are able to provide process heat at a temperature of about 1000 ◦ C, which is sufficiently high for most of the endothermic reactions of interest. Dish concentrators at this scale already exist and are being readied for commercialization in the American Southwest for electrical power generation. Fig. 4 shows a parabolic dish concentrator with a 25 kWe kinematic Stirling cycle heat engine at the focal point. Current plans call for tens of thousands of similar dish structures to be assembled in the USA’s Mojave Desert, generating several hundred megawatts of electrical power for the California electrical grid. Investors anticipate achieving economies of mass production in manufacturing large numbers of these concentrators and heat engines. One of the principals, Stirling Energy Systems, has reported that they hope to reduce the overall capital unit costs to about $50,000 per dish, or about $50,000 per 100 kWs input. Central receiver and dish systems address economics in somewhat different ways. Central receiver systems are expected to exploit economies of mass production
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Highly Concentrated Solar Energy from Parabolic Dish or Central Receiver Concentrators
High Temp Endothermic Reactor Product Stream “F”
Separations
Moderately Concentrated Solar Energy from Trough Concentrators
Reactant “A”
Moderately Concentrated Solar Energy from Trough Concentrators HXR Low-Mod Temp Exothermic Reactor
Preheater/ Vaporizer
Product Stream “G” Separations
Recycle “C”
Reactant “B”
Recycle “D” Recycle “E”
Fig. 2. Generic chemical process flowsheet.
Fig. 3. Solar Two central receiver with heliostats. Photo courtesy of Sandia National Laboratory.
with regard to the heliostats while also attempting to achieve economies of scale for the receiver system and the chemical plant. Dish systems are expected to ex-
ploit economies of mass production for the dishes as well as for the portion of the chemical process plant that is associated with each dish. Some elements of the
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than will high temperature heat from a dish or central receiver concentrator. Since central receivers and dish concentrators are more expensive than trough concentrators, it makes financial sense to considering allocating heat from trough concentrators to low- to moderate-temperature unit operations and heat from dish and central receiver concentrators to hightemperature endothermic reactions. 3.2. Chemical operations
Fig. 4. Parabolic dish with 25 kWe kinematic Stirling cycle heat engine. Photo courtesy of Sandia National Laboratory.
chemical plant, such as some of the unit operations that are downstream of the high temperature, endothermic reactor may also be centralized, to some degree, providing an opportunity for economies of scale. Trough concentrators, which are less expensive than dish concentrators, are capable of delivering heat at up to about 300.500◦ C [20]. For this reason, trough concentrators are preferred over dish concentrators for moderate- to low-temperature endothermic unit operations within a thermochemical process network. For example, if water is one of the reactants for an endothermic chemical reaction, such as the methane steam reforming reaction, heat from a trough concentrator can be used to provide vaporization of the water. Additionally, decomposition of some biomass materials can be accomplished with heat from trough concentrator units. Low- to moderate-temperature heat from a trough concentrator will have a lesser exergy1 proportion 1 Equivalent to “availability”, the thermodynamic parameter, “exergy”, is defined to be the maximum theoretical work that can be obtained from an energy source as it is brought into equilibrium with the environment. High temperature heat has a greater exergy content than low temperature heat; high temperature heat can therefore provide a greater increase in the chemical energy content of the products of an endothermic chemical reaction.
At least one high-temperature, endothermic chemical reactor is a central element of the thermochemical approach to producing Solar Fuels. With a central receiver concentrator system, this reactor may be located at the receiving point, or at the base underneath the receiving point, of the central receiver tower. Alternately, with a dish-based system, the endothermic chemical reactor may preferentially be located at or in close proximity to the focal point of each dish. This reactor changes the chemical form of the feedstock and increases its chemical energy content, using concentrated solar energy or radiant energy from the orbiting platform. At least one moderate-temperature, exothermic chemical reactor is also among the additional unit chemical operations shown in Fig. 2. This operation is needed for each of the Solar Fuels discussed in this paper. For example, for the production of hydrogen using methane as a feedstock chemical, the endothermic, steam reforming reaction is followed by an exothermic, water–gas shift reaction. The latter reaction increases the hydrogen content of the product stream while producing heat that can be used elsewhere in the overall chemical process. Alternately, if long-chain hydrocarbons are the desired product, the endothermic steam reformer is followed by an exothermic Fischer–Tropsch reactor. The separations and purification operations, also identified in Fig. 2, may additionally produce heat that can be used elsewhere in the chemical process or which must be rejected to the environment. For example, thermal-swing sorption processes and distillation processes each require heat from a moderate temperature source and reject heat at a lower temperature. Table 1 lists the overall net chemical reactions for many of the feedstocks and Solar Fuels discussed in this paper. However, numerous details have been omitted in this summary table. For example, while the idealized net reaction for producing Fischer–Tropsch hydrocarbons from methane does not show the consumption of water, readers familiar with these processes are aware that some makeup water will be required since it is
Net thermochemical process (idealized)
CH4 + 2H2 O = 4H2 + CO2 nCH4 = Cn H2n+2 + (n − 1)H2 (3n + 1)/4CH4 + (n − 1)/4CO2 = Cn H2n+2 + (n − 1)/2H2 O H2 O = H2 + 21 O2 nCO2 + (n + 1)H2 O = CnH2n+2 + (3n + 1)O2
Primary reactions
Methane reforming plus water–gas shift Methane reforming plus Fischer–Tropsch Methane reforming plus Fischer–Tropsch Water splitting processes Water splitting processes plus reverse water–gas shift and Fischer–Tropsch
Reactants
CH4 + H2 O CH4 + H2 O CH4 + CO2 H2 O CO2 + H2 0
Table 1 List of chemical reactions.
H2 Cn H2n+2 + H2 Cn H2n+2 H2 Cn H2n+2
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Solar Fuels
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extremely difficult to completely separate and recycle all water from the product stream. 3.3. Hybrid systems If direct solar energy only is used by a Solar Thermochemical Plant, the plant capacity factor is likely to be about 25% or lower. A higher capacity factor is beneficial, however, both in terms of the productivity and the profitability of the plant. For this reason, supplemental methods of providing thermal energy are considered as a means to increase the plant’s capacity factor. Dish systems and/or heliostat units that can track and follow the sun can also track an orbiting satellite. Accordingly, we consider an alternate, hybrid system that uses beamed energy (e.g., microwaves) from the orbiting platform in order to increase the daily productivity of the Solar Thermochemical Plant. In this case, the overall product output of the plant will be a function of both the solar flux and the beamed energy flux. Alternately, another method of increasing the plant capacity factor is to combust natural gas or utilize another energy source (e.g., stored solar thermal energy) to provide heat for the endothermic unit operations of the chemical process. Hybrid systems of these types allow the capital and operating costs of the chemical plant to be amortized over a larger quantity of chemical product than has previously been contemplated by others for solar thermochemical processing. Disadvantages include the inclusion of additional capital and/or fuel costs (and associated increases in carbon dioxide emissions if combustion is the source of supplemental energy). 3.4. Previous solar thermochemical work A number of lab-scale and prototype solar thermochemical demonstrations have been performed in the USA and abroad. For example, in the late 1980s, CO2 reforming was conducted at the Sandia National Laboratory in a direct catalytic absorption reactor (with solar energy being absorbed by catalyst particles after passing through a quartz window in the reactor). This demonstration, which took place at the 3.5 kW scale, used a non-tracking, 6.7 m diameter concentrator, with illumination by a tracking flat-plate heliostat. Maximum thermal efficiencies and chemical efficiencies2 of 79.8% and 55.5% were reported [9]. Other small-scale demonstra2 Chemical efficiency is defined to be the percent of incident solar (or other thermal energy) that is converted into an increase in the chemical energy of the reacting stream.
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tion tests have been reported, including an indirectly heated, inconel reactor that was tested at 6.4 kW using the Schaeffer Solar Furnace of the Weizmann Institute of Science in Israel [12]. A larger scale demonstration of solar CO2 reforming of methane was performed in the 1990s via a joint effort by the USA and Germany, with the reaction system being provided by Sandia National Laboratory and testing conducted at the German Aerospace Research Center (Deutsches Zentrum für Luft- und Raumfahrt [DLR]) in Lampoldschausen, Federal Republic of Germany. This demonstration used a 17-m, parabolic dish capable of 150 kWs power in a number of tests, ultimately achieving a chemical efficiency of 76.1%. Researchers reported that the chemical efficiency had been improved through the addition of a counterflow, recuperative heat exchanger [15,2,16]. In another example, solar steam reforming of methane was performed at a central receiver demonstration system at the Weizmann Institute of Science, utilizing a DLR-provided, 400 kWs reactor, at 800 ◦ C. Reporting on this and cost studies, DLR researchers reported that, “in the context of a preliminary cost study, it was shown that it is possible to produce hydrogen from natural gas in a large 50 MW (thermal) production plant for five cents per kilowatt-hour (calorific value of hydrogen)” [14]. Yet another demonstration of solar methane reforming was achieved through the use of the gas-cooled solar tower (GAST) system at the Plataforma Solar de Almeria through a joint effort by DLR and the Spanish Centro de Investigaciones Energe ticas, Medioambientales y Tecnologicas (CIEMAT), using an indirectly heated steam reformer. For the GAST demonstration, air was heated in a central receiver system to about 1000 ◦ C, providing heat for a 170 kW (nominal) steam reforming reactor. Reactor temperatures ranged from about 700 to about 800 ◦ C, with the highest methane conversions (93%) being accomplished at the highest temperatures [1,25].
4. Preliminary production estimates In this section, we present preliminary estimates of the productivity of a Solar Thermochemical Plant. Calculations have been performed based upon limiting features of the various chemical process flowsheets, such as the amount of highly concentrated solar energy (for endothermic chemical reactions) and the conversion and selectivity of the low- to moderate-temperature exothermic chemical reactions.
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We hypothesize a Solar Thermochemical Plant with sufficient numbers of solar concentrators such that, during periods of bright sunlight, cumulative solar energy rates of 1.0 GWs would be used to drive hightemperature, endothermical chemical reactions. For example, a system based on parabolic dish concentrators at 100 kWs each would require 10,000 dishes. Likewise, a system based on central receiver concentrators at 50,000 kWs each would require 20 tower systems. For our productivity calculations, we assume that the chemical efficiency of the high temperature reaction is 40% (except for thermochemical water-splitting, where we selected a range of 30–50% based on the various chemical processes under investigation [26,27]. This is conservatively low compared to values reported in the literature for the solar reforming of methane and may justify a re-examination in the near future: if high operating temperatures are obtained, and if highly effective recuperative heat exchange is provided for the reacting stream, the actual chemical efficiency may be considerably higher. Also as previously described, we have assumed that the thermal energy for low- to moderate- temperature operations such as water vaporization, thermal-swing separations, and distillation, will be provided in part through thermal integration with exothermic unit operations and in part through the utilization of parabolic trough concentrators. The parabolic trough collection system will be oversized and operated in conjunction with thermal energy storage containing suitable materials (such as hydrated salts) so that heat is available during periods of darkness. In Column (A) of Table 2, we consider the Solar Thermochemical Plant when operated with direct solar energy only (based on the diurnal cycle of the Earth) with an overall capacity factor of 25%. This is equivalent to full production for six hours per day, 365 days per year. In terms of the productivity of the plant, our preliminary estimate is that, if natural gas (methane) is used as a feedstock, the output of the plant will be approximately 390,000–430,000 gallons of gasoline equivalent (gge) per day. Note that if zero-chemical energy feedstocks such as water and/or carbon dioxide are used, the productivity of the plant is reduced to about 47,000–79,000 gge/day. This calculation clearly shows an advantage if methane is used as a feedstock for a Solar Thermochemical Plant. To maximize the profitability of their capital investment, investors will want to maximize its productivity. Along with using methane as a feedstock chemical, productivity can be improved if the plant’s capacity factor can be increased. Accordingly, in Columns (B) and (C) of Table 2, we evaluate the performance of the plant
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Table 2 Solar Thermochemical Plant performance 10,000 dishes (at 100 kWs per dish) or 10 central receivers (at 100,000 kWs per central receiver). Capacity factor
Input thermal energy
Solar Fuel products
(A) Solar only 25%
Solar + Natural gas 90%
(C) Solar + Beamed power 90%
Natural gas (CH4 ) + H2 O
Production (gge/dish/day) Production (gge/receiver/day) System Production (gge/day) CO2 Emission reductions (metric tons/year)
39–43 39,000–43,000 390,000–430,000 220,000–350,000
140–160 140,000–160,000 1,400,000–1,600,000 −280, 000–180,000
140–160 140,000–160,000 1,400,000–1,600,000 810,000–1,300,000
Hydrogen and/or long-chain hydrocarbons
Biomass (CH4 CO2 ) + H2 O
Production (gge/dish/day) Production (gge/receiver/day) System production (gge/day) CO2 emission reductions (metric tons/year)
29 29,000 290,000 920,000
100 100,000 1,000,000 2,200,000
100 100,000 1,000,000 3,300,000
Hydrogen and/or long-chain hydrocarbons
H2 O
Production (gge/dish/day) Production (gge/receiver/day) System production (gge/day) CO2 emission reductions (metric tons/year)
4.7–7.9 4700–7900 47,000–79,000 150,000–250,000
17–28 17,000–28,000 170,000–280,000 −540, 000 to −170, 000
17–28 17,000–28,000 170,000–280,000 550,000–910,000
Hydrogen
CO2 + H2 O
Production (gge/dish/day) Production (gge/receiver/day) System Production (gge/day) CO2 emission reductions (metric tons/year)
7.3 7300 73,000 230,000
26 26,000 260,000 840,000
Long-chain hydrocarbons
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Input chemicals
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based on using beamed energy from an orbiting platform or the combustion of natural gas as supplemental heating sources, to improve operations when the sun is partially obscured by clouds and to allow the plant to operate through the nighttime. Accordingly, if the plant is operated with a capacity factor of 90%, the productivity of the plant increases. Without methane as a chemical feedstock, the daily production rate improves from about 47,000–79,000 to about 170,000–280,000 gge/day. This is nearly a fourfold improvement. However, if methane is used as a chemical feedstock, the plant’s daily production rate increases to 1,400,000–1,600,000 gge/day. The combustion of one gallon of gasoline results in the release of 8.82 kg of carbon dioxide to the atmosphere. For the same net chemical energy production (based on the higher heating values of gasoline and methane), the combustion of methane only generates about 6.67 kg of carbon dioxide; accordingly, displacing gasoline with methane-derived Solar Fuels should generally reduce carbon dioxide emissions. However, the actual Greenhouse Gas emissions will depend upon the source of the energy that is used to produce Solar Fuels. Table 2 additionally presents preliminary estimates of the Greenhouse Gas emissions (increases and reductions) associated with operation of the Solar Thermochemical Plant. For the cases where only solar energy is used to support the endothermic chemical reactions, for example, Column (A), net Greenhouse Gas emissions are reduced (compared to using gasoline as a transportation fuel). However, where natural gas is burned to support the endothermic chemical reactions, mixed results occur. If methane is the feedstock for the reaction, then net carbon dioxide emissions are reduced. But in many cases where methane is used as a combustion fuel, for supplemental heating of the endothermic reaction, net carbon dioxide emissions increase. The best case for emission reductions occurs when biomass feedstocks are combined with solar energy and beamed energy from an orbiting platform. In this case, the biomass feedstock brings carbon-neutral, chemical energy content and the orbiting platform supports a capacity factor of 90%. The net reductions in carbon dioxide emissions are estimated to be about 3,300,000 metric tons of carbon dioxide per year. This version of the Solar Thermochemical Plant additionally produces 1,000,000 gge per day, an amount equal to about 0.24% of the USA’s annual oil imports. Forty such Solar Thermochemical Plants, each occupying a few square kilometers, could reduce US oil imports by nearly 10%.
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5. Additional considerations This paper presents a variety of concepts and configurations for producing Solar Fuels via thermochemical processing methods. Benefits of the concept include the option to use Solar Fuels to reduce oil consumption and Greenhouse Gas emissions. There should be little doubt as to the technical viability of using solar energy to drive thermochemical process systems. However, there are important technology development, financial and economic issues to consider. 5.1. Technology development issues The technologies that are needed in order to realize Solar Fuels, via a thermochemical approach, vary in their readiness for commercialization. With regard to solar concentrators, we note that much work has been performed, in the USA and abroad, over the last few decades, and solar reforming reactors have been demonstrated both in dish and central receiver configurations. More work is needed, however, in order to realize either economies of mass production or economies of scale with solar concentrators. Accordingly, we consider solar concentrators to be of moderate to high technology readiness. A significant technical question is the chemical efficiency of the overall system. As noted earlier, our assumption of 40% may be conservatively low; if higher efficiencies can be achieved/maintained, such as has been suggested from previous work, the productivity and financial viability of solar thermochemical processing is enhanced beyond that suggested by our productivity calculations. For systems that utilize methane as a feedstock, the chemistry is well known; in fact, steam reforming is currently the industrially-preferred method of producing hydrogen. For application within dish concentrators, we note that compact chemical reactors and heat exchangers have been demonstrated for other applications, using process intensive hardware such as microchannel reactors and heat exchangers which also can achieve high energy efficiencies. Microchannel reactors and heat exchangers have been previously demonstrated for unit operations such as steam production, recuperative heat transfer, the methane steam reforming reaction, the water–gas-shift and reverse-water–gas shift reactions, and for the Fischer–Tropsch reaction. These systems are of moderate to high technology readiness; work is still needed, in particular, to develop mass production methods in order to reduce hardware capital costs.
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Thermochemical technologies and systems that utilize feedstocks that bring no chemical energy, such as thermochemical water-splitting processes, are currently of low technology readiness. However, significant work is currently underway within the Department of Energy’s Hydrogen Program to understand the chemistries and the chemical process flowsheets. The effort has included the screening of over 300 potential water-splitting cycles [22–24] and work is progressing on several that were deemed to be most promising. At the current time, the chemistries and process flowsheets appear to be quite challenging; however, worthwhile technology demonstrations will soon be underway. For example, a dish system, using one of the metal oxide processes, is currently being assembled at Sandia National Laboratory and is scheduled to begin testing later this year [4]. Technologies and systems that provide supplemental heat through the use of one or more orbiting platforms are also of low technology readiness, despite having initially been proposed decades ago. After its initial proposal in the 1960s [8,11] development efforts were initiated, then halted, in the 1970s [17] and again in the 1990s [6,7,13,18]. For Space Solar Power to be developed to the point where it can be used to produce transportation fuels, critical areas include the design of, and assembly approach to, the orbiting platform that intercepts solar energy and converts it beamed power directed at the ground receiver as well as the launch systems that would place hardware in orbit at reasonable costs. 5.2. Economic issues While financial analyses will consider the costs and benefits of Solar Thermochemical Plants based on the perspective of owners and potential investors, economists will additionally consider other real costs and benefits associated with producing Solar Fuels. Macroeconomic analyses consider both financial costs and benefits as well as national and societal costs and benefits, such as: (1) Environmental costs and benefits, such as may be obtained through reductions in carbon dioxide emissions. (2) National and international costs and benefits, such as may be obtained through reductions in oil imports from politically-unstable regions, as well as the impact of shifting a portion of the transportation energy burden from petroleum to natural gas (which would increase the demand for and consumption of natural gas).
(3) Regional or national costs and benefits (e.g., producing Solar Fuels domestically can provide local economic benefits and reduce the magnitude of potential trade deficits that are otherwise incurred through purchases of foreign petroleum). Other than estimates for reductions in carbon dioxide emissions, we have not attempted to quantify inputs for an economic analysis of the Solar Fuels concept. Here we note that our calculations have assumed no efforts to sequester carbon dioxide. If sequestration is taken into account, the Solar Thermochemical Plant can also be thought of as a decarbonization system for cases such as when hydrogen is the Solar Fuel product. Accordingly, significantly higher reductions in carbon dioxide emissions would be calculated for those cases. We are currently working to develop the concepts for Solar Fuels into sufficient detail so that a comprehensive financial analysis using lifecycle cost analyses techniques can be performed. By “financial analysis,” we mean that we intend to consider the costs (capital costs, operating costs, etc) and income (gross and net) of potential Solar Thermochemical Plants. 6. Conclusions This paper has presented a number of options for the solar thermochemical production of transportation and other fuels. Both dish-based and central receiver systems appear to be technically viable, although some of the chemistry options require substantial technology development (such as thermochemical water-splitting) before financial risks are acceptable. However, some of the highest productivity systems, such as those that employ methane (from natural gas or biomass) and steam reforming reactions already have a moderate to high degree of technology readiness. Financial viability depends heavily upon three elements: (1) thermochemical efficiency, (2) plant capacity factor and (3) capital costs. For systems based on methane steam reforming, there is high confidence that reasonable efficiencies can be obtained, whereas for thermochemical water-splitting processes, high confidence is not yet available. High plant capacity factors appear to be possible. If the combustion of natural gas is used as a supplemental source of high temperature heat, technology readiness is high, with the disadvantage being increased usage of natural gas and associated increases in carbon dioxide emissions. One of the most attractive cases evaluated involves the use of biomass as a feedstock chemical. Biomass feedstocks can be used to produce a carbon-neutral form of synthetic gasoline. If the combustion of
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natural gas is used as a supplemental heat source for this system, so that a capacity factor of 90% is achieved, the net productivity of the system evaluated is high, at 1,000,000 gge/day, and compared to burning gasoline for transportation, carbon dioxide emissions are reduced by 2.2 metric tons/year. Alternately, if space solar power is used as the supplemental heating source, natural gas demand is reduced, high plant capacity factors are maintained, and carbon dioxide emissions are further reduced by an additional 1.1 metric tons/year; however, the technology readiness of the orbiting platform is currently very low and capital and operating costs are unknown. Despite this, space solar power, especially combined with biomass feedstocks, may be very attractive due to the combination of high productivity and substantial reductions in carbon dioxide emissions. Detailed financial analyses are still required. Additional work is needed, including technical development/evaluation and lifecycle cost analyses, in order to set capital cost targets and provide a thorough evaluation of the financial viability of the concepts presented in this paper. In particular, there is substantial uncertainty regarding the costs (capital and operating) associated with an orbiting space solar power satellite. Despite uncertainties, there are substantial incentives associated with pursuing the Solar Fuels concepts. These include reductions in oil consumption and carbon dioxide emissions. Given current political and technical uncertainties, including the expectation that petroleum production capacities will decline, and increasing concerns about carbon dioxide emissions, there is a great need to pursue renewable energy sources as an alternative source of transportation fuels. References [1] M. Bohmer, U. Langnickel, M. Sanchez, Solar steam reforming of methane, Solar Energy Materials 24 (1991) 441–448. [2] R. Buck, J.F. Muir, R.E. Hogan, R.D. Skocypec, Carbon dioxide reforming of methane in a solar volumetric receiver/reactor: the CAESAR project, Solar Energy Materials 24 (1991) 449–463. [4] R.B. Diver, J.E. Miller, M.D. Allendorf, N.P. Siegel, R.E. Hogan, Solar themochemical water-splitting ferrite-cycle heat engines, in: ASME International Solar Energy Conference, Proceedings of ISEC2006, July 2006. [6] H. Feingold, M. Stancata, A. Freidlander, M. Jacobs, D. Comstock, C. Christensen, G. Maryniak, S. Rix, J. Mankins, Space solar power: a fresh look at the feasibility of generating solar power in space for use on earth, in: SAIC-97/1005, 1997. [7] P. Glaser, Power from the sun: its future, Science 162 (3856) (1968) 857–866. [8] P.E. Glaser, F.P. Davidson, K. Csigi (Eds.), Solar Power Satellites, Wiley and Praxis Publishing, 1998. [9] R.E. Hogan, R.D. Skocypec, R.B. Diver, J.D. Fish, A direct absorber reactor/receiver for solar thermal applications, Chemical Engineering Science 45 (8) (1990) 2751–2758.
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[11] C.C. Kraft, The solar power satellite concept, in: The Von Karman Lecture, 15th Annual Meeting of the American Institute of Aeronautics and Astronautics, 1979. [12] R. Levitan, M. Levy, H. Rosin, R. Rubin, Closed-loop operation of a solar chemical heat pipe at the weizmann institute solar furnace, Solar Energy Materials 24 (1991) 464–477. [13] J.C. Mankins, Space solar power: a perspective future energy source?, Space Energy and Transportation 4 (3–4) (1999) 97–114. [14] S. Moeller, M. Epstein, Solar fuel production, DLR Nachrichten 109—Special Issue Solar Research, January 2005, accessed April 2007 at http://www.dlr.de/en/desktopdefault. aspx/tabid-618/1034_read-1412/ . [15] J.F. Muir, R.E. Hogan, R.D. Skocypec, R. Buck, Solar reforming of Methane in a direct absorption catalytic reactor on a parabolic dish: i—test and analysis, SAND-90-2674C, 1990. [16] J.F. Muir, R.E. Hogan Jr., R.D. Skccypec, R. Buck, The CAESAR project: experimental and modeling investigations of methane reforming in a catalytically enhanced solar absorption receiver on a parabolic dish, SAND92-2131, July 1993. [17] National Resource Council, Electric Power from Orbit: A Critique of a Satellite Power System, National Academy Press, 1981. [18] National Resource Council, Laying the Foundation for Space Solar Power, National Academy Press, 2001. [19] G.A. Olah, G.K. Surya Prakash, A. Goeppert, Beyond Oil and Gas: The Methanol Economy, Wiley, New York, March 2006. [20] M.R. Patel, Wind and Solar Power Systems, CRC Press, Boca Raton, FL, 1999. [22] C. Perkins, Development of solar powered thermochemical production of hydrogen from water, (Presentation) 2007 DOE Hydrogen Program Merit Review and Peer Evaluation Meeting, US Department of Energy, May 2007. [23] R. Perret, W. Culbreth, G. Besenbruch, R. Diver, A. Weimer, A. Lewandowski, E. Miller, Solar Hydrogen Generation Research, 2005. [24] R. Perret, A. Weimer, G. Besenbruch, R. Diver, M. Lewis, Y. Chen, Development of solar-powered thermochemical production of hydrogen from water, DOE Hydrogen Program, FY2006 Annual Progress Report, 2006. [25] I. Spiewak, C.E. Tyner, U. Langnickel, Applications of solar reforming technology, SAND93-1959, November 1993. [26] A. Steinfeld, R. Palumbo, Solar thermochemical process technology, in: R.A. Meyers (Ed.), Encyclopedia of Physical Science & Technology, vol. 15, Academic Press, 2001, pp. 237–256. [27] A. Steinfeld, Solar thermochemical production of hydrogen—a review, Solar Energy 78 (2005) 603–615.
Further reading [3] P. Davis, Program overview—2007 DOE Hydrogen Program Merit Review and Peer Evaluation Meeting, (Presentation) 2007 DOE Hydrogen Program Merit Review and Peer Evaluation Meeting, US Department of Energy, May 2007. [5] B.C. Esty, Modern Project Finance, Wiley, NJ, 2004. [10] H. Khatib, Economic Evaluation of Projects in the Electricity Supply Industry, The Institution of Electrical Engineers, London, UK, 2003. [21] M. Paster, Hydrogen production and delivery program element, (Presentation) 2007 DOE Hydrogen Program Merit Review and Peer Evaluation Meeting, US Department of Energy, May 2007.