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Direct Solar Energy

line with technology development, regulations and investment capital. Multiple dishes have also been proposed as a source of aggregated heat, rather than distributed-generation Stirling or Brayton units.

CSP or PV electricity can also be used to power reverse-osmosis plants for desalination. Dedicated CSP desalination cycles based on pres- sure and temperature are also being developed for desalination (see Section 3.3.2).

3.3.5

Solar fuel production

Solar fuel technologies convert solar energy into chemical fuels, which can be a desirable method of storing and transporting solar energy.They can be used in a much wider variety of higher-efficiency applications than just electricity generation cycles. Solar fuels can be processed into liquid transportation fuels or used directly to generate electricity in fuel cells; they can be employed as fuels for high-efficiency gas-turbine cycles or internal combustion engines; and they can serve for upgrading fossil fuels, CO2 synthesis, or for producing industrial or domestic heat. The challenge is to produce large amounts of chemical fuels directly from sunlight in cost-effective ways and to minimize adverse effects on the environment (Steinfeld and Meier, 2004).

Solar fuels that can be produced include synthesis gas (syngas, i.e., mixed gases of carbon monoxide and hydrogen), pure hydrogen (H2) gas, dimethyl ether (DME) and liquids such as methanol and diesel. The high energy density of H2 (on a mass basis) and clean conversion give it attractive properties as a future fuel and it is also used as a feedstock for many industrial processes. H2 has a higher energy density than batteries, although batteries have a higher round-trip efficiency. However, its very low energy density on a volumetric basis poses economic challenges associated with its storage and transport. It will require significant new distribution infrastructure and either new designs of internal combustion engine or a move to fuel cells. Additionally, the synthesis of hydrogen with CO2 can produce hydrocarbon fuels that are compatible with exist- ing infrastructures. DME gas is similar to liquefied petroleum gas (LPG) and easily stored. Methanol is liquid and can replace gasoline without significant changes to the engine or the fuel distribution infrastructure. Methanol and DME can be used for fuel cells after reforming, and DME can also be used in place of LPG. Fischer-Tropsch processes can produce hydrocarbon fuels and electricity (see Sections 2.6 and 8.2.4).

There are three basic routes, alone or in combination, for producing storable and transportable fuels from solar energy: 1) the electrochemi- cal route uses solar electricity from PV or CSP systems followed by an electrolytic process; 2) the photochemical/photobiological route makes direct use of solar photon energy for photochemical and photobiological processes; and 3) the thermochemical route uses solar heat at moderate and/or high temperatures followed by an endothermic thermochemical process (Steinfeld and Meier, 2004). Note that the electrochemical and thermochemical routes apply to any RE technology, not exclusively to solar technologies.

358

Chapter 3

Figure 3.8 illustrates possible pathways to produce H2 or syngas from water and/or fossil fuels using concentrated solar energy as the source of high-temperature process heat. Feedstocks include inorganic com- pounds such as water and CO2, and organic sources such as coal, biomass and natural gas (NG). See Chapter 2 for parallels with bio- mass-derived syngas.

Electrolysis of water can use solar electricity generated by PV or CSP technology in a conventional (alkaline) electrolyzer, considered a benchmark for producing solar hydrogen. With current technologies, the overall solar-to-hydrogen energy conversion efficiency ranges between 10 and 14%, assuming electrolyzers working at 70% effi- ciency and solar electricity being produced at 15% (PV) and 20% (CSP) annual efficiency. The electricity demand for electrolysis can be significantly reduced if the electrolysis of water proceeds at higher temperatures (800° to 1,000°C) via solid-oxide electrolyzer cells (Jensen et al., 2007). In this case, concentrated solar energy can be applied to provide both the high-temperature process heat and the electricity needed for the high-temperature electrolysis.

Thermolysis and thermochemical cycles are a long-term sustainable and carbon-neutral approach for hydrogen production from water. This route involves energy-consuming (endothermic) reactions that make use of concentrated solar irradiance as the energy source for high- temperature process heat (Abanades et al., 2006). Solar thermolysis requires temperatures above 2,200°C and raises difficult challenges for reactor materials and gas separation.Water-splitting thermochemi- cal cycles allow operation at lower temperature, but require several chemical reaction steps and also raise challenges because of ineffi- ciencies associated with heat transfer and product separation at each step.

Decarbonization of fossil fuels is a near- to mid-term transition path- way to solar hydrogen that encompasses the carbothermal reduction of metal oxides (Epstein et al., 2008) and the decarbonization of fossil fuels via solar cracking (Spath and Amos, 2003; Rodat et al., 2009), reforming (Möller et al., 2006) and gasification (Z’Graggen and Steinfeld, 2008; Piatkowski et al., 2009). These routes are being pur- sued by European, Australian and US academic and industrial research consortia. Their technical feasibility has been demonstrated in concen- trating solar chemical pilot plants at the power level of 100 to 500 kWth. Solar hybrid fuel can be produced by supplying concentrated solar thermal energy to the endothermic processes of methane and biomass reforming—that is, solar heat is used for process energy only, and fossil fuels are still a required input. Some countries having vast solar and natural gas resources, but a relatively small domestic energy market (e.g., the Middle East and Australia) are in a position to pro- duce and export solar energy in the form of liquid fuels.

S o l a r f u e l s y n t h e s i s f r o m s o l a r h y d r o g e n a n d C O 2 p r o d u c e s h y d r o c a r bons that are compatible with existing energy infrastructures such as the natural gas network or existing fuel supply structures. The renew- able methane process combines solar hydrogen with CO2 from the -

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