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Better heat exchangers for concentrated solar power

Solar thermal systems are a good example of the particle-wave dualism expressed in Planck’s constant h: E = hf. Where h is the Planck constant, f is the frequency of the light and E is the resulting energy. Thus, the higher the frequency of the light, the higher the amount of energy. Solar thermal metal collectors transform the energy of high-frequency light by generating them to an abundance of low-frequencies through Compton shifts. Glass or ceramic coatings with high visible and UV transmittance absorb the low frequency light generated by the metal because they effectively absorb infrared light (so-called heat blockers). The efficiency of the solar thermal system improves significantly with increasing size, which is also the biggest advantage of such systems compared to photovoltaic generators. One disadvantage, however, is the downstream transformation of heat into electricity with the help of heat exchangers and turbines − a problem not only in solar thermal systems.

To provide the hot gas (supercritical CO2) to the turbines, heat exchangers are necessary. These heat exchangers transfer the heat energy generated by a power plant to the working fluid in a heat engine (usually a steam turbine) that converts the heat into mechanical energy. Then, the mechanical energy is used to generate electricity. These heat exchangers are operated at ~800 Kelvin and could be more efficient if the temperature were at >1,000 Kelvin. The entire process of converting heat into electricity is called a power cycle and is a critical process in power generation by solar thermal plants. Obviously, heat exchangers are pivotal elements in this process.

Ceramics are a great material material for heat exchanger because they can withstand extreme temperature fluctuations. However, unlike metals, ceramics are not easy to shape. Relatively coarse shapes, in turn, are made quickly and easily. In contrast, metals can be easily formed and have a high mechanical strength. Metals and ceramics have been valued for centuries for their distinctive properties. For example, bronze and iron have good impact resistance and are so malleable that they have been made into complex shapes such as weapons and locks. Ceramics, like those used to make pottery, have been formed into simpler shapes. Their resistance to heat and corrosion made ceramics a valued material. A new composite of metal and ceramic (a so-called cermet) combines these properties in amazing ways. A research group led by Mario Caccia reported now in the prestigious journal Nature about a cermet with properties that makes it usable for heat exchangers in solar thermal systems.

The history of such composites goes back to the middle of the 20th century. The advent of jet engines has created a need for materials with high resistance to heat and oxidation. On top of that, they had to deal with rapid temperature changes. Their excellent mechanical strength, which often surpassed that of existing metals, was highly appreciated by the newly created aerospace industry. Not surprisingly, the US Air Force funded more research into the production of cermets. Cermets have since been developed for multiple applications, but in most cases have been used for small parts or surfaces. The newly released composite withstands extreme temperatures, high pressures and rapid temperature changes. It could increase the efficiency of heat exchangers in solar thermal systems by 20%.

To produce the composite, the authors first produced a precursor, which was subject to further processing, comparable to potting the unfired version of a clay pot. The authors compacted tungsten carbide powder into the approximate shape of the desired article (the heat exchanger) and heated it at 1,400 °C for 2 minutes to bond the parts together. They then further processed this porous preform to produce the desired final shape.

Next, the authors heated the preform in a chemically reducing atmosphere (a mixture of 4% hydrogen in argon) at 1,100 °C. At the same temperature, they immersed the preform in a tank of liquid zirconium and copper (Zr2Cu). Finally, the preform was removed by heating to 1,350 °C. In this process, the zirconium displaces the tungsten from the tungsten carbide, producing zirconium carbide (ZrC) as well as tungsten and copper. The liquid copper is displaced from the ZrC matrix as the material solidifies. The final object consists of ~58% ZrC ceramic and ~36% tungsten metal with small amounts of tungsten carbide and copper. The beauty of the method is that the porous preform is converted into a non-porous ZrC / tungsten composite of the same dimensions. The total volume change is about 1-2%.

The elegant manufacturing process is complemented by the robustness of the final product. At 800 °C, the ZrC / tungsten cermet conducts heat 2 to 3 times better than nickel based iron alloys. Such alloys are currently used in high-temperature heat exchangers. In addition to the improved thermal conductivity, the mechanical strength of the ZrC / tungsten composite is also higher than that of nickel alloys. The mechanical properties are not affected by temperatures of up to 800 ° C, even if the material has previously been subjected to heating, e.g. for cooling cycles between room temperature and 800 °C. In contrast, iron alloys, e.g. stainless steels, and nickel alloys loose at least 80% of their strength.

(Photo: Wikipedia)

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Ammonia energy storage #1

The ancient, arid landscapes of Australia are not only fertile soil for huge forests and arable land. The sun shines more than in any other country. Strong winds hit the south and west coast. All in all, Australia has a renewable energy capacity of 25 terawatts, one of the highest in the world and about four times higher than the world’s installed power generation capacity. The low population density allows only little energy storage and electricity export is difficult due to the isolated location.

So far, we thought the cheapest way to store large amounts of energy was power-to-gas. But there is another way to produce carbon-free fuel: ammonia. Nitrogen gas and water are enough to make the gas. The conversion of renewable electricity into the high-energy gas, which can also be easily cooled and converted into a liquid fuel, produces a formidable carrier for hydrogen. Either ammonia or hydrogen can be used in fuel cells.

The volumetric energy density of ammonia is almost twice as high than that of liquid hydrogen. At the same time ammonia can be transported and stored easier and faster. Researchers around the world are pursuing the same vision of an “ammonia economy.” In Australia, which has long been exporting coal and natural gas, this is particularly important. This year, Australia’s Renewable Energy Agency is providing 20 million Australian dollars in funding.

Last year, an international consortium announced plans to build a $10 billion combined wind and solar plant. Although most of the 9 terawatts in the project would go through a submarine cable, part of this energy could be used to produce ammonia for long-haul transport. The process could replace the Haber-Bosch process.

Such an ammonia factories are cities of pipes and tanks and are usually situated where natural gas is available. In the Western Australian Pilbara Desert, where ferruginous rocks and the ocean meet, there is such an ammonia city. It is one of the largest and most modern ammonia plants in the world. But at the core, it’s still the same steel reactors that work after the 100 years-old ammonia recipe.

By 1909, nitrogen-fixing bacteria produced most of the ammonia on Earth. In the same year, the German scientist Fritz Haber discovered a reaction that could split the strong chemical bond of the nitrogen, (N2) with the aid of iron catalysts (magnetite) and subsequently bond the atoms with hydrogen to form ammonia. In the large, narrow steel reactors, the reaction produces 250 times the atmospheric pressure. The process was first industrialized by the German chemist Carl Bosch at BASF. It has become more efficient over time. About 60% of the introduced energy is stored in the ammonia bonds. Today, a single plant produces and delivers up to 1 million tons of ammonia per year.

Most of it is used as fertilizer. Plants use nitrogen, which is used to build up proteins and DNA, and ammonia delivers it in a bioavailable form. It is estimated that at least half of the nitrogen in the human body is synthetic ammonia.

Haber-Bosch led to a green revolution, but the process is anything but green. It requires hydrogen gas (H2), which is obtained from pressurized, heated steam from natural gas or coal. Carbon dioxide (CO2) remains behind and accounts for about half of the emissions. The second source material, N2, is recovered from the air. But the pressure needed to fuse hydrogen and nitrogen in the reactors is energy intensive, which in turn means more CO2. The emissions add up: global ammonia production consumes about 2% of energy and produces 1% of our CO2 emissions.

Our microbial electrolysis reactors convert the ammonia directly into methane gas − without the detour via hydrogen. The patent pending process is particularly suitable for removing ammonia from wastewater. Microbes living in wastewater directly oxidize the ammonia dissolved in ammonia and feed the released electrons into an electric circuit. The electricity can be collected directly, but it is more economical to produce methane gas from CO2. Using our technology, part of the CO2 is returned to the carbon cycle and contaminated wastewater is purified:

NH3 + CO2 → N2 + CH4