03-09-2012, 11:49 AM
Recent advances in direct solar thermal power generation
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ABSTRACT
The recent energy crisis and environmental burden are becoming increasingly urgent
and drawing enormous attention to solar-energy utilization. Direct solar thermal
power generation technologies, such as thermoelectric, thermionic, magnetohydrodynamic,
and alkali-metal thermoelectric methods, are among the most
attractive ways to provide electric energy from solar heat. On the one hand, these
methods have the potential to be more efficient than traditional ways since they can
convert heat to electricity directly without experiencing the conventional intermediate
mechanical energy conversion process; on the other hand, these electricity
generators are generally silent, reliable, and scalable, making them very suitable to
serve as a distributed power generation system for certain specialized fields, such as
military and space applications. A lot of effort has been devoted to investigate the
energy conversion theory and practical applications thus far. This paper is intended
to present a thorough review on recent advances in developing the thermoelectric,
thermionic, magnetohydrodynamic, and alkali-metal thermoelectric technologies
for direct solar thermal power generation.
INTRODUCTION
Direct solar thermal power generation has been an attractive electricity generation technology
using a concentrator to gather solar radiation on a heat collector and then directly converting heat
to electricity through a thermal electric conversion element. Compared with the traditional indirect
solar thermal power technology utilizing a steam-turbine generator, the direct conversion technology
can realize the thermal to electricity conversion without the conventional intermediate mechanical
conversion process. The power system is, thus, easy to extend, stable to operate, reliable,
and silent, making the method especially suitable for some small-scale distributed energy supply
areas. Also, at some occasions that have high requirements on system stability, long service life,
and noiselessness demand, such as military and deep-space exploration areas, direct solar thermal
power generation has very attractive merit in practice. At present, the realistic conversion efficiency
of direct solar thermal power technology is still not very high, mainly due to material
restriction and inconvenient design. However, from the energy conversion aspect, there is no
conventional intermediate mechanical conversion process in direct thermal power conversion,
which therefore guarantees the enormous potential of thermal power efficiency when compared
with traditional indirect solar thermal power technology.
Thermionic technology
Thermionic conversion is based on thermionic emission phenomenon which means under high
temperature electrons will evaporate from the metal surface. A simple thermionic converter consists
of an emitter and a collector close to each other. When the emitter cathode with high work
function is heated, the electrons will emit from the metal surface and be received by the collector
anode, so current can be generated when using electrical load to connect the emitter and collector.
The schematic view of a thermionic converter is shown in Fig. 2.
The thermionic conversion efficiency is much higher than thermoelectric conversion and can
exceed 30%.8,9 However, it should work at relatively high temperatures. Generally, the emitter
temperature could work around 2000 K.
Magnetohydrodynamic technology
MHD power generation is based on Faraday’s law of electromagnetic induction. When conductive
fluid flows through a magnetic field that is perpendicular to the flowing direction, it will
cut the magnetic lines and then the electromotive force is induced in the direction perpendicular to
both magnetic field direction and flow direction,12 as shown in Fig. 3.
MHD power generation can be classified on the basis of working fluid as gaseous plasma
MHD power generation and liquid metal MHD power generation. Gaseous plasma-based MHD
generator generally requires a relatively high operating temperature to ensure the gas has a reasonable
value of electrical conductivity.
Alkali-metal thermoelectric technology
Alkali-metal thermoelectric converter AMTEC is a kind of direct thermal electric energy
conversion element, using BASE as ion selective permeation layer and alkali metal as working
fluid, when the working temperature range is about 1000–1200 K at the hot side. Theoretically, the
thermoelectric conversion efficiency can reach 20%–40%.18
The schematic view of AMTEC is shown in Fig. 5. The AMTEC is divided into two parts with
different pressures by BASE. The high-pressure side is filled with sodium metal under the temperature
maintaining at 900–1300 K, and the low-pressure side keeps it at 400–700 K. The surface
of the low-pressure side of the BASE is covered with porous metal electrode cathode while the
other side in the high pressure-temperature region is covered by the anode surface. External load
is connected between the cathode and anode at the two sides of BASE which therefore enables the
circulation of electrons.19
When the system is operating, sodium would be ionized in the high pressure-temperature area.
While the electrons cannot pass through an alumina solid electrolyte, the sodium ions can go
through the BASE and meet the electrons circulated through the external loads to recombine the
neutralized sodium in the low-pressure side of BASE. After that, the neutralized sodium would be
cooled by the condenser and returned to the high-pressure evaporator region through an electromagnetic
pump or a porous capillary wick to circulate again.