ABSTRACT: In this paper hybridization and cogeneration with
concentrated solar radiation (CSR) technology coupled a) with alkali metal
thermoelectric conversion (AMTEC) and b) with combined AMTEC-steam power
cycles (AMTEC/SPC) has been studied. Thermodynamic models of combined CSR
- AMTEC system and CSR - AMTEC/SPC for the cogeneration of electric and
thermal energy, using hybridization of solar and fossil fuels, has been
analyzed. Parametric system analysis has been performed taking in account
radiative - reflective losses and thermal energy losses by thermal radiation,
convection and conduction. Finally, a program, defined to evaluate the
commercial applications of these emerging hybrid solar technologies is
presented. Program includes the evaluation of case studies as possible
sites for commercial hybrid solar energy utilization.
Solar technologies are maturing and new opportunities are emerging due to continuing improvements in designs. Concentrated solar radiation (CSR) technologies, combining less-expensive optical components with small area, highly efficient, and somewhat more expensive devices to achieve low cost, in general, are on the threshold of significant development The stretched membrane concentrator cluster and innovative cavity - heart type high temperature receiver focally positioned present attractive solution for thermal power and photothermal applications. Concentrated solar radiation transmitted by the receiver's cover and adjacent liquid is absorbed by the absorber which has a black nonselective coating and is with its integrated liquid passages immersed directly inside the circulating stream of liquid (4, 5). Highly specularly reflective internal skin of receiver walls reduces the complex of radiative, conductive and convective heat losses and results in a more efficient alternative to highly insulated receiver enclosures (4). The receiver's instantaneous thermal efficiency is equal to the ratio
of heat supplied to the receiver's working fluid and used for the sodium
evaporation and its isothermal expansion q , (1) where q The concentrated solar radiation energy flux incident in the receiver's
plane is a function of the concentrator field area A q = I where I Steady-state energy flux losses can be expressed as a sum of reflective radiation losses and heat-transfer losses as follows: (3) Radiative losses are given as. (W) (4) Convective q _{kon}+q_{pr} = A_{z} (h+k_{z}/d_{z})·(T_{z}-T_{a})
(5) T_{a} , T_{z},
, A_{z} , d_{z}, , k_{z,
}andhrespectivelly.
Thus total heat transfer losses are (6) The influence of the solar radiation concentration factor and the temperature
on receiver's simultaneous short and long wave radiative as well as convective
and conductive heat transfer losses is presented on diagrams in Figure
1. More directly, the kind of efficiency dependence on the solar radiation
concentration factor illustrates the diagram given in the Figure 2.
concentration factor and receivers wall surface temperature.
The Alkali Metal Thermo-Electric Conversion (AMTEC) (1, 6) a very prospective
(2-5) solution for a high performance power generation became recently
a subject of our interest (4, 5). The key element of an AMTEC device is
the b alumina solid electrolytte (BASE) which
conducts positive sodium ions much better than sodium atoms or electrons
(Figure 3). A sodium pressure difference across a thin BASE sheet drives
sodium ions from the high pressure side to the low pressure side. Thus
positive sodium ions accumulate on the low pressure side, and electrons
collect on the high pressure side, resulting in an electrical potential.
By the appropriate electrode use, this electrical potential can be utilized
and an electrical current can be driven through a load. Liquid sodium in
upper part is maintained at the temperature T
Figure 3 Scheme of alcali-metal-thermo-electric conversion system: A
- Heat source T Thus, BASE presents a mean of converting mechanical energy, related
to the established pressure difference, into electrical energy - equivalent
to the chemical potential conversion in an electrical potential difference.
More accurate study shows that the AMTEC process is more complex interaction
of a variety of irreversible transport processes, kinetically governed
at the electrode interfaces by the BASE material's specific features. The
BASE process described as an isothermal expansion of sodium from pressure
p Assuming that the sodium vapor can be represented as an ideal gas and employing the Clausius-Clapeyron equation, the instantaneous efficiency of the reversible AMTEC process can be expresses as follows: (7) where is W
Fig. 4. The thermodynamic cycle equivalent to the reversible AMTEC process and binary AMTEC/SPC.cycle.
The combined CSR-AMTEC system thermal efficiency can be expressed by
the product of CSR receivers thermal efficiency and the reversible AMTEC
process efficiency ( (8) The results of performed analysis of the instantaneous thermal efficiency of the thermodynamic model of the combined system CSR - AMTEC (coupled concentrated solar radiation technology CSR and AMTEC process) are presented in Figure 5. Fig. 5. The dependence of the combined CSR - AMTEC cycle thermal efficiency on the effective solar radiation concentration factor, vapor temperatures and condenser temperature. Further thermodynamically has been studied binary cycle - combination of AMTEC process and steam power cycle (a scheme of system given in Fig.6). Fig. 6. Scheme of the hybrid binary CSR - AMTEC/SPC system for the cogeneration
of electrical and thermal energy.
The instantaneous thermal efficiency of the binary CSR - AMTEC/SPC cycle, which has been defined as the combination of the CSR - AMTEC cycle and the steam power cycle can be determined as below: (9) (10) where the steam power cycle efficiency, steam power cycle technical
work, AMTEC cycle technical work, and the heat rejected by AMTEC are respectively
denoted by h The results of conducted thermodynamic analysis are very impressive.
The obtained values of the relevant thermal efficiencies of CSR - AMTEC
and CSR - AMTEC/SPC processes given in Figure 7 are significantly higher
than the corresponding values of any conventional power plant in existence
today (calculated for the steam power reheat cycle - superheated vapor
temperature of 450 Fig. 7. The dependence of the thermal efficiencies of CSR - AMTEC and
CSR - AMTEC/SPC processes on the temperature T
For spreading the use of solar energy today is mandatory to have fully
dispatchable systems. The best way to do this nowadays is by hybridization
with natural gas or oil. In particular both near- and mid-term applications
are foreseen for different hybrid solar systems. Such systems, especially
those based on concentrated solar radiation conversion technologies are
much more competitive and better suited for many diverse applications and
emerging international markets than mono-type technologies. Scheme of the
hybrid combined. CSR - AMTEC/SPC system given on Figure 6 is ideal for
the cogeneration of electrical and thermal energy. For example the binary
cycle efficiencies given in Figure 7 are obtained for the steam power cycle
condenser temperature of 150
A review of current local conditions in Yugoslavia confirms that the
technical expertise is appropriate and partly necessary technologies (including
concentrated radiation technologies and PV) are available but financial
situation and economy status present serious barriers and do not favor
promotion of new programs neither in manufacturing and energy engineering
nor investment generally. However demand side menaged decentralized energy
production and integration of NRES in local energy structure shall be seen
as an option which can contribute to improve basic energy supply and living
conditions coupled with the creation of small-scale industry providing
labour and hindering migration to urban centers. In addition to the possible
impact on decentralized energy production development, combined hybrid
solar CSR - AMTEC systems applications include as very prospective generally
autonomous systems (for example electricity for the self powered convective
solar dryers - Figure 8).
Fig. 8. Schemes of convective solar dryers which become self powered and controlled - by the addition of CSR - AMTEC units. C There is a pre-proposal for a two-year program to conduct and evaluate
case studies as possible demonstration sites for hybrid solar-thermal energy
utilization in industrial and commercial sectors in Yugoslavia (a few typical
applications are referred in Table 1). For the program further development
and its effective technical feasibility - reliable and cost-effective solutions
have to be searched encompassing as more as possible accurate investigation
of demand diversity and storage capacities. The program is focused on modular
units from small size engine capacities to, and around 1 MW capacity. .
Table 1.
As the final conclusions of conducted study otlined can be following needs for furture work: - Thermodynamic study of innovative combined cycles.
- Fundamental research on different AMTEC systems and relevant materials an structures.
- Engineering investigation,development and standardization of relevant technical systems
- Case studies: loads profiles, storage, cogeneration and hybridization relation dynamics.
- Optimization under various policy/rate scenarios including approach to the dispatch optimization and generalized procedure.
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