The Plant Design

It is debatable how much climate change we can live with. It is debatable whether we are close to the tipping point that leads to catastrophe or not. It is also debatable how much of our economic resources should be devoted to stabilizing or reversing humankind’s growing carbon footprint. What is not debatable is that the time for debating is over. We must start dealing with facts. The time for action has arrived. If we care about the future of our children and grandchildren, we must stop debating and start building.

1% to 2% of the gross world product (GWP) would be sufficient to completely convert our energy economy to renewable energy sources (such as solar–hydrogen) by the end of the century. What is proposed in this book is to build the first such demonstration plant and thereby prove its feasibility, determine its performance, and evaluate its initial and generating costs.

The proposed solar–hydrogen demonstration plant is sized to generate electricity at a yearly average rate of 1 gW, which, at peak insolation on a summer day, will result in about 5 gW. In contrast, a 1 gW nuclear or fossil power plant cannot generate at a rate more than 1 gW, and on a yearly average generates only at a rate of 0.6–0.8 gW. Because the solar–hydrogen plant generates five to six times that during peak demand periods, and because electricity is more expensive during these peak periods, the plant’s profitability greatly exceeds that of the fossil power plants even if one disregards the “cap-and-trade” carbon emission income of renewable energy plants.

The figure below describes a 1 gW power plant that can be located in either the equatorial, subtropical, or temperate zones. This power plant, which generates an average of 1 gW power during the year, will generate about 5 gW at peak insolation, and the energy not sent to the grid in the form of electricity can be put into storage in the form of hot oil or hydrogen (H2).

In order to determine the solar plant area requirement for the above described power plant if it is located in southern California, one can assume that Figures 1.28 and 1.29 accurately give the average yearly insolation as about 2,600 kWh/m2/year. If the efficiency of the solar collectors is 20%, the collectors could be arranged on a square with 2.5 mi sides (6.5 mi2 or 16.6 km2). Naturally, if the plant is located in the equatorial zone (such as the Sahara or on a floating island), the area requirement would be less. For example, if the insolation in the selected area is 3,000 kWh/m2/yr and the solar plant efficiency is 30%, the radius of a circular plant would be about 1 mile, while in the temperate zone, with 1,500 kWh/m2/yr insolation and a 15% efficient system the required radius would be about 2.0 miles.

If we assume that there is no electric grid in the area and therefore all the solar energy will have to be converted, stored, and transported in the form of chemical energy (hydrogen), we can determine the yearly average hydrogen production of the plant by considering the energy efficiencies of the electrolyzers, compressors, liquefiers, and related equipment. Table 1.46 estimated these efficiencies and also gave the total energy cost of generating a kilogram of liquid hydrogen (LH2) as 66 kWh2. If that estimate is correct, the average LH2 production rate would be about 15 tons/hour. This being a yearly average, the peak production would be 4–5 times higher during periods of maximum insolation and naturally would drop to zero at night.

The main features of this demonstration power plant are the following: (1) integration of several technologies that include not only solar and hydrogen, but also geothermal, methanol, and dual-function regenerative fuel cell technologies, (2) provision of multiple forms of energy storage through the capability of selling the excess generation to the grid, or storing it in hot oil, H2, or methanol, and (3) full integration and optimization of the components of the plant to maximize profitability of the operation, while responding to variations in the electricity or hydrogen cost and other market conditions.

The Process

In this design, sun-tracking and concentrating thermal solar collectors are assumed to be used. They generate 400°C (752°F) hot oil. It should be noted that all equipment for this demonstration plant (including the solar collectors) will be purchased on the basis of competitive bidding.

Assuming that thermal solar collectors are installed, they will be operated by sending the hot oil either to a boiler to generate steam or to storage for use at night or during periods of low insolation. The steam generated by the boiler will drive the steam turbine generators, which produce electricity. If an electric grid exists in the area, and if that is the most profitable option, the generated electricity is sent to the grid during periods when electric energy is most valuable. During off-peak periods, the energy can be stored either as heat (hot oil) or as chemical (H2 or methanol) energy. The generated H2 can either be sold as transportation fuel or converted back into electricity during the next peak period, whichever is more profitable.

The heat input into the boilers can be supplemented by geothermal heat. Depending on the temperature of the groundwater in the selected location, the geothermal heat is either used directly or as preheat for the boiler feedwater. If substantial geothermal potential exists in the area, the integration of the solar and geothermal heat sources will increase the continuous energy availability and reduce the need for storage.

When it is more profitable to use the generated electricity to make H2, the electricity is sent to electrolyzers, which convert it to chemical energy by splitting water into H2 and O2. The H2 will be collected at about 3 bar (45 psig) pressure, and will be either liquefied and sent to storage or compressed to up to 1,000 bar (15,000 psig) and sent to high-pressure gas storage.

When, during peak periods, electricity is more valuable than H2 or when solar energy is unavailable (such as at night), and it is profitable to reconvert the stored hydrogen back into electricity, this will be done in either conventional fuel cells or if available by that time in my reversible fuel cells (RFCs) that were operating as electrolyzers when hydrogen was generated and now will be switched into their fuel cell mode of operation so that using the stored hydrogen as a fuel they will generate electricity for the grid.

If there is a fossil power plant in the area, or if carbon dioxide (CO2) is available from other sources, the generated H2 can also be used to produce methanol from CO2. Assuming that the CO2-capturing and methanol conversion technologies have matured by the time of building this demonstration plant, the plant’s flexibility will further increase because income can be generated by both the recapturing of CO2 and also from the sale of methanol. An added benefit to this dual income is that storage and transportation of methanol is less expensive than that of H2.

A simplified block diagram of the plant and a schematic of the operation of the solar-hydrogen energy cycle are shown below:

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