Direct Solar Energy
a 100-kWth monolithic dual-chamber solar reactor for a mixed-iron-oxide cycle, demonstrated within the European R&D project HYDROSOL-2 (Roeb et al., 2009); a rotary solar reactor for the ZnO/Zn process being scaled up to 100 kWth (Schunk et al., 2009); the Tokyo Tech rotary-type solar reactor (Kaneko et al., 2007); and the Counter-Rotating-Ring Receiver/Reactor/ Recuperator, a device using recuperation of sensible heat to efficiently produce H2 in a two-step thermochemical process (Miller et al., 2008).
High temperatures demanded by the thermodynamics of the thermo- chemical processes pose considerable material challenges and also increase re-radiation losses from the reactor, thereby lowering the absorp- tion efficiency (Steinfeld and Meier, 2004). The overall energy conversion efficiency is improved by reducing thermal losses at high temperatures through improved mirror optics and cavity-receiver design, and by recov- ering part of the sensible heat from the thermochemical processes.
High-temperature thermochemical processes require thermally and chemically stable reactor-wall materials that can withstand the extreme operating conditions of the various solar fuel production processes. For many lower-temperature processes (e.g., sulphur-based thermochemical cycles), the major issue is corrosion. For very high-temperature metal- oxide cycles, the challenge is the thermal shock resistance of the ceramic wall materials. Near-term solutions include surface modification of ther- mally compatible refractory materials such as graphite and silicon carbide. Longer-term solutions include modifications of bulk materials. Novel reac- tor designs may prevent wall reactions.
A key aspect is integrating the chemical process into the solar concen- trating system. The concentrating optics—consisting of heliostats and secondary concentrators (compound parabolic concentrator)—need to be further developed and specifically optimized to obtain high solar-flux intensities and high temperatures in solar chemical reactors for producing fuels.
Photochemical and photobiological processes are other strong can- didates for solar fuel conversion. Innovative technologies are being developed for producing biofuels from modified photosynthetic micro- organisms and photocatalytic cells for fuel production. Both approaches have the potential to provide fuels with solar energy conversion effi- ciencies far greater than those based on field crops (Turner et al., 2008). Solar-driven fuel production requires biomimetic nanotechnology, where scientists must develop a series of fundamental and technologi- cally advanced multi-electron redox catalysts coupled to photochemical elements. Hydrogen production by these methods at scale has vast tech- nical potential and promising avenues are being vigorously pursued.
A combination of all three forms is found in the synthesis of biogas, a mixture of methane and CO2, with solar-derived hydrogen. Solar hydrogen is added by electrochemical water-splitting. Bio-CO2 reacts with hydrogen in a thermochemical process to generate hydrocarbons such as synthetic natural gas or liquid solar fuels (Sterner, 2009). These
approaches are still nascent, but could become viable in the future as energy market prices increase and solar power generation costs con- tinue to decrease.
Other potential future applications
There are also methods for producing electricity from solar thermal energy without the need for an intermediate thermodynamic cycle. This direct solar thermal power generation includes such concepts as thermoelectric, thermionic, magnetohydrodynamic and alkali-metal methods. The thermoelectric concept is the most investigated to date, and all have the attraction that the absence of a heat engine should mean a quieter and theoretically more efficient method of producing electricity, with suitability for distributed generation. Specialized appli- cations include military and space power.
Space-based solar power (SSP) is the concept of collecting vast quanti- ties of solar power in space using large satellites in Earth orbit, then sending that power to receiving antennae (rectennae) on Earth via microwave power beaming. The concept was first introduced in 1968 by Peter Glaser. NASA and the US Department of Energy (US DOE) studied SSP extensively in the 1970s as a possible solution to the energy crisis of that time. Scientists studied system concepts for satellites large enough to send GW of power to Earth and concluded that the concept seemed technically feasible and environmentally safe, but the state of enabling technologies was insufficient to make SSP economically competitive. Since the 1970s, however, great advances have been made in these technologies, such as high-efficiency PV cells, highly efficient solid-state microwave power electronics, and lower-cost space launch vehicles (Mankins, 1997, 2002, 2009; Kaya et al., 2001; Hoffert et al., 2002). Still, significant breakthroughs will be required to achieve cost-competitive terrestrial base-load power (NAS, 2004).
Passive solar and daylighting technologies
High-performance building envelopes entail greater upfront construction costs, but lower energy-related costs during the lifetime of the building (Harvey, 2006). The total investment cost of the building may or may not be higher, depending on the extent to which heating and cooling systems can be downsized, simplified or eliminated altogether as a result of the high- performance envelope.Any additional investment cost will be compensated for, to some extent, by reduced energy costs over the lifetime of the building.
Discussion of costs in this section is largely limited to the perspective of private investors. Chapters 1 and 8 to 11 offer complementary perspectives on cost issues covering, for example, costs of integration, external costs and benefits, economy- wide costs and costs of policies.