Direct Solar Thermal Hydrogen Processes

The problems with materials and separations at such a high temperature make direct decomposition not attractive at this time. The production of hydrogen by direct thermal splitting of water generated a considerable amount of research during the period 1975-1985. Fletcher and co-workers stressed the thermodynamic advantages of a one-step process with heat input at as high a temperature as possible.25-27 The theoretical and practical aspects were examined by Olalde,28 Lede,29-31 Ounalli,32 Bilgen33,34 and by Ihara.35,36 However, no adequate solution to the crucial problem of separation of the products of water splitting has been worked out so far. Effort was spent to demonstrate the possibility of product separation at low temperature after quenching the hot gas mixtures by heat exchange cooling,8 by immersion of the irradiated, heated target in a reactor of water liquid,910 by rapid turbulent gas jets,29,30 or rapid quench by injecting a cold gas.31 Based on a theoretical evaluation, Lapique18 concluded that by quenching under optimal conditions it should be possible to recover up to 90% of the hydrogen formed by thermal water splitting. However, the quench introduces a significant drop in the efficiency and produces an explosive gas mix-ture.2

To attain efficient collection of solar radiation in a solar reactor operating at the requisite 2500 K, it is necessary to reach a radiation concentration of the order of 10000 suns. This is a rather stringent requirement. By way of example, a 3-MW solar tower facility, consisting of a field of 64 slightly curved heliostats, has each heliostat capable of concentrating solar radiation approximately by a factor of 50. Even by directing all the heliostats to reflect the sun rays towards a common target, a concentration ratio of only 3000 may be obtained.37 It is possible, however, to enhance the concentration ratio of an individual heliostat by the use of a secondary concentration optical system, and such systems have been explored.37,38

Ordinary steels cannot resist temperatures above a few hundred degrees centigrade, while the various stainless steels, including the more exotic ones, fail at less than 1300 K. In the range 3000-1800 C alumina, mullite or fused silica may be used. A temperature range of about 2500 K requires use of special materials for the solar reactor. However, higher melting point materials can have additional challenges; carbide or nitride composites are likely to react with water splitting products at the high temperatures needed for the reaction. A list of candidate materials of high temperature oxide, carbide and nitride ceramics, is presented in Table 2.

Separation of the generated hydrogen from the mixture of the water splitting products, to prevent explosive recombination, is another challenge for thermochemi-cally generated water splitting processes. Separation of the thermochemically generated hydrogen from the mixture of the water splitting products by gas diffusion through a porous ceramic membrane can be relatively effective. Membranes that have been considered include commercial and specially prepared porous zironias, although sintering was observed to occur under thermal water splitting conditions,1,39 and ZrO2-TiO2-Y2O3 oxides.40 In such membranes, it is necessary to maintain a Knudsen flow regime across the porous wall.24 The molecular mean free path X in

Table 2. Melting points of refractory materials, modified from Ref. 1.

Material

Type

Melting Point (°C)

SiO2

oxide

1720

Quartz

oxide

1610

TiO2

oxide

1840

Cr2O3

oxide

1990-2200

Al2O3

oxide

2050

UO2

oxide

2280

Y2O3

oxide

2410

BeO

oxide

2550

CeO2

oxide

2660-2800

Z1O2

oxide

2715

MgO

oxide

2800

HfO2

oxide

2810

ThO2

oxide

3050

SiC

carbide

2200(decomp)

B4C

carbide

2450

WC

carbide

2600(decomp)

TiC

carbide

3400-3500

Electrolytic graphite

carbide

3650(subl)

HfC

carbide

4160

Si3N4

nitride

1900

BN

nitride

3000(decomp)

the gas must be greater than the average pore diameter, By kinetic theory, X = Vnvd2; where d = molecular diameter (cm), v = molecular density = 273.15pvo/T, p = pressure (bar), T = temperature (K), and vo = 2.685x1019 molecules/cm3.1 A double-membrane configuration has been suggested as superior to a singlemembrane reactor.41

In recent times, there have been relatively few studies on the direct thermochemical generation of hydrogen by water splitting1,24,39-45 due to continuing high temperature material limitations. Recent experimental work has been performed by Kogan and associates1,37,40-41 and the cross section of one of their solar reactors is shown in Fig. 3. This reactor consists of a cylindrical zirconia housing of 10-cm inside diameter and 20-cm length, and insulated by 2-in thickness of Zircar felt and board. One end of the housing is closed by a circular disc with a central aperture 3 cm in diameter. A zirconia crucible having a porous wall is installed at the opposite end of the housing, and sintering of the crucible considerably limited performance. In 2004, Bay ara reiterated that conversion rates in direct thermochemical processes are still quite low, and new reactor designs, operation schemes and materials are needed for new breakthroughs in this field.45

Fig. 3. Example of a solar reactor for direct thermochemical water splitting and solar hydrogen generation. Reprinted with permission from Ref. 1. Copyright (1998) International Journal of

Hydrogen Energy.

Fig. 3. Example of a solar reactor for direct thermochemical water splitting and solar hydrogen generation. Reprinted with permission from Ref. 1. Copyright (1998) International Journal of

Hydrogen Energy.

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