SunHydrogen Inc (HYSR)

[Consiglieri]

WHITEPAPER: SOLAR HYDROGEN AND NANOTECHNOLOGY

http://samples.sainsburysebooks.co.uk/9780470823989_sample_384272.pdf

1.3.1 The Solar Resource

In discussing the world energy situation of the early twentieth century, Thomas Edison once said:

“I’d put my money on the sun and solar energy. What a source of power! I hope we don’t have to wait ’til oil and coal run out before we tackle that”

[11]. It’s almost 100 years later, and we are still hoping, perhaps now with a little more urgency. The sun is, in fact, the ultimate renewable energy resource, continuously bombarding earth with about 180000000000000000W (or 180 000 TW) of radiant power, enough to power 3 quadrillion 60 W light bulbs [12,13]! About 50 000 TW of this is directly reflected back to space, and 82 000 TW is absorbed by earth and re-emitted as heat. Of this, 36000 TW is absorbed at the earth’s land masses, where terrestrial-based solar-energy conversion plants could be installed practically.
To put this in perspective, our society on average consumes 13–15 TW, with some predictions doubling this consumption rate by the year 2060 [14]. Although these numbers are staggering, they still represent a small fraction of the sun’s influx of radiant power. It might seem that solar energy alone could satisfy our insatiable hunger for energy. Of course, it is not that simple. The planet relies on the sun for many things, including sustaining plant-life and driving its weather patterns, and our voracious energy demands are relatively low on nature’s priority list. Still, despite the abundance of spare solar energy at our disposal, large-scale conversion is currently quite costly and somewhat problematic. At peak times of daylight, the solar intensity available for terrestrial conversion scales to approximately 1000 W m
2. Large collection areas and significant landmass would therefore be needed for commercial-scale power production. Such expansive commercial deployment requires an enormous capital investment. For example, commercial photovoltaic technologies today can convert sunlight to electricity at efficiencies between 10 and 20%, at $2–5 per installed watt [15]; a single Gigawatt plant would cost billions of dollars and span over 2500 acres! Even worse, this gargantuan installation would be a sleeping giant at night and under severe cloud cover.
There are certainly practical difficulties and challenges, yet our sun is still the most generous renewable resource, and the most underutilized in modern society. Currently, less than 0.05% of the world energy production is from solar-energy plants, though this number is on the rise of late [16]. Encouragingly, improved technologies for solar-energy conversion, storage and utilization are emerging to make their impact on the world energy scene. New and improved solar-to-electric and solar-to-hydrogen conversion technologies are all poised to be part of the new energy mix.

1.3.2 Converting Sunlight
In converting sunlight, whether to electricity or to hydrogen, fundamental thermodynamic principles govern the energy-conversion process. As illustrated in Figure 1.1, the sun can be viewed as a black body radiating at a temperature of 5780 K, while the earth, as a black body, radiates at 300 K. The Carnot limit between these source and sink temperatures is readily calculated to 95%, representing the amount of radiant energy that can be converted into other more useable energy forms. This is very encouraging! A lot of solar energy available, and in theory most of it can be converted for practical end-uses. Unfortunately, however, actually converting sunlight is always further limited by unavoidable losses associated with available energy-conversion routes. Thermodynamically, efficiency is lost with every added conversion step in the process.
The sun transmits energy radiatively via photons, quantum particles of light with discrete energy content. Figure 1.2 shows the standard AM1.5global atmosphere-filtered solar spec- trum [17] indicating the range of photon energies comprising sunlight, and the distribution of energy transmitted by these photons. The solar photons (g) reaching earth readily interact with electrons, energizing them to excited states (e
), as illustrated in Figure 1.1. Two basic routes for energy conversion of the photoexcited electrons are also depicted. In the solar-thermal route, the energized electrons thermalize to their surroundings, converting the energy to heat (n). This thermal energy can be converted further, for example, using heat-engines to produce work, though now restricted by a lower Carnot limit based on an intermediate source temperature.

Alternatively, in the solar-potential route, the elevated electrochemical potential of the energized electrons can directly drive further conversion processes, for example, producing electricity or chemical products. Thermal energy is not being converted, so no additional Carnot limits are imposed.
1.3.3 Solar-Thermal Conversion
In solar-thermal conversion systems, concentrated sunlight produces high temperatures to drive heat-engines for generating mechanical work, electrical energy, or chemical products.

A good example of this route is the solar-thermal production of electricity. Concentrating solar thermal (CST) systems employ mirrored troughs, dishes or heliostats for focusing sunlight to heat working fluids of a gas- or liquid-phase turbine cycle. Solar concentration up to 1000 and working-fluid temperatures in the 250–1100 C range are common. Conversion efficiencies, governed by the Carnot limit, can be quite high for the highest operational temperatures, but significant materials issues arise. Exotic refractory materials are needed, adding significant cost to plant production, operations and maintenance. One important advantage of solar- thermal production of electricity is that conventional generators and infrastructure can be used, facilitating plant design and implementation. Thermal storage of the energy via storage of the heated fluid can also be an advantage, especially in the higher-temperature systems. Recently, an experimental CST 25 kW system installed at Sandia National Laboratories has reported solar-to-electric efficiencies as high as 31% [18]. Larger-scale installations, such as the 50 MW AndaSol plant in Spain operate at gross efficiencies closer to 3% [19].

Another example of solar-thermal energy conversion is the production of hydrogen as a chemical by-product of solar-thermochemical cycles (STC). Concentrated sunlight provides the net heat for driving a multistep thermochemical process involving the splitting of water into hydrogen and oxygen gases. Many STC chemical cycles are known, including the sulfur– iodine [20,21] and copper–chlorine [22] cycles with reaction temperatures ranging up to 1200K. High solar-to-hydrogen conversion efficiencies are possible, reported between 42–57% in the sulfur–iodine cycles, with a high-temperature step at 1123K. The high- temperature, corrosive operating environment of all STC reactors, however, can be problem- atic, requiring specialized, and usually expensive materials, components and maintenance.

1.3.4 Solar-Potential Conversion


In the solar-potential route, photons in the incident solar light energize electrons, which can be converted directly to electrical or electrochemical energy. The primary example of the solar- potential conversion process is the photovoltaic (PV) production of electricity. Photons are absorbed in semiconductor materials where they excite electrons from the valence into the conduction band. These excited electrons, at elevated electrochemical potentials, can be extracted into an external circuit, directly converting the photon energy into electric energy. Though direct, the conversion is not without loss. Some of the excited electrons thermalize to their surroundings, causing waste heat. In efficient PV cells, however, this waste is minimal, resulting in moderate temperature rises. Operating temperatures in PV installations without concentration can be quite low, typically ranging from 30 to 80C. This is a particularly attractive feature, since low-temperature plants do not require specialized materials, and are easy to operate and maintain. Another attractive feature of PV-generated electricity is the absence of mechanical “moving parts” common to the turbine systems in CST generation. Large-scale power electronics such as power inverters are needed, but these systems have become more efficient and robust in recent years. On the down side, PV semiconductor materials and systems are still relatively expensive. Although cumulative global installations of PV generation has reached over 1500 MW [15], the per-installed-watt price still exceeding $3 is somewhat prohibitive in may economic sectors.
Other examples of solar-potential conversion include photoelectrochemical processes such as waste-water remediation, and the industrial synthesis of chemicals and synthetic fuels.

(STC stands for solar-thermochemical, CST for concentrating solar-thermal, and PEC for photoelectrochemcial).

Hydrogen production by PEC water splitting, an attractive low-temperature alternative to solar-to-hydrogen water splitting, falls into this category.

1.3.5 Pathways to Hydrogen

Using sunlight to split water for hydrogen production can follow several different conversion routes, as shown in Figure 1.3. The solar-thermal route is essentially a two-step process, with a photon-to-thermal energy-conversion step followed by a thermal-to-chemical conversion step. The other two-step process shown in the figure represents PV-electrolysis, where a photon-to-electric conversion step is followed by an electric-to-chemical conversion process. The three-step process represents a CST-electrolysis route, involving photon-to-heat, heat-to- electricity and electricity-to-chemical conversion steps. The final pathway depicted, repre- senting a single-step direct conversion from photon-to-chemical energy, is the PEC water- splitting process. Other solar-to-hydrogen pathways are possible, including photobiological routes [23,24] and the ultra-high-temperature thermolysis route [25]. All pathways can contribute to renewable hydrogen production for future “green economies,” but economics will determine which will predominate.
From an economic viewpoint, it is important to remember that both hydrogen and electricity will be valuable as renewable-energy carriers in the future. Processes capable of producing both, such as PV-electrolysis and CST-electrolysis, could be advantageous. In fact, PV- and CST-electrolysis systems can be assembled today using off-the-shelf components. The electricity and the hydrogen produced would not be inexpensive, but this will change with further maturing of the technologies. It will remain vital to keep an eye on the alternative, even less-mature approaches. The solar-electrolysis routes comprise multiple conversion steps, with efficiency loss at each step. In terms of hydrogen production, the most direct conversion processes, such as PEC water splitting, could have some inherent performance advantages. PEC hydrogen production as a low-temperature single-stage process remains one of the front- running alternatives.