Nanotechnologies in heat transfer and energetics

A.A.Khalatov

Institute of Engineering Thermophysics, National Academy of Sciences of Ukraine

The humanity stands in front of the era when the share of natural fuel consumption constantly drops down due to reduction in the fuel extraction, exhaustion of opened deposits, and decrease in the opening of new energy sources. In 1983 for the first time the world oil consumption exceeded the opening of new supplies, in 2004 - 2006 the world oil prices increased almost in two times, while the natural gas export prices elevated by more than two times in 2003 - 2006. The natural gas price in USA increased by eight times for the last years. Also the downfall in the world natural gas production will begin only from 2040, but according to the U.S. Geological Service predictions the downfall in the world total oil and natural gas extraction will be started in 2013.

The growth in world prices of natural energy sources, reduction in their extraction, the fast growth of the oil consumption in Asia-Pacific region will affect on all aspects of the world economy. As a result, to reduce the the new and improved energy technologies, based on organic fuels and nuclear engineering, will be developed in the nearest future. However, only improvement of the traditional energy technologies is not enough to fill up the coming growth in the world energy consumption. According to the Russian Heat Engineering Institute (Russia) forecast the possibilities in the efficiency growth for the mostly demanded energy blocks of 300 megawatts power is only 2 - 3% for the coal stations and 6 - 8% for the stations using the natural gas. Thus, the new technologies are now required, based on the nano- and other advanced technologies.

The heat transfer is the theoretical basis of all modern and new energy technologies. The present paper considers some potential energy nanotechnologies and shows the role of heat transfer, as the science, in study and improvement of such technologies.

Historical information.

The nanotechnologies will discover the boundless prospects for the humanity.љ In the nearest future they will improve many technological processes in the energetics, aviation, airspace and solar engineering, cryogenics, and other important applications. The crushing of matter up to nanosize radically changes its physical and chemical properties. The substance atoms on the nanoparticle surface requires very unusual properties, they have the high chemical activity differing them from . Just the surface atoms in many cases define principally new properties of the nano-substance.

Appearance of nanotechnologies is associated with discovery of nanotubes made in 1991. The nanotube is the extended structure of hexagonal nets with carbon atoms in knots, rolled up into the cylinder. The nanotube size is a few tens of nanometers, while the length is up to a few centimeters. The nanotubes are widely employed in the airspace engineering, automobile industry, electronics, optoelectronics, in production of super strong treads, composite materials, fuel cells, light diodes, micro-sensors.

Fullerenes are the molecular combinations in form of the convex closed polyhedron, including the even number of three-coordinated carbon atoms located at the tops of regular six- or five pentagons, of which the sphere or ellipsoid surface is formed. They widely used in semiconductor area, in optic locks and photo-resistors, in production of nanolubricating materials.

The first nanotube was produced by means of graphite pulverizing in the electrical arc. Later on the gas-phase synthesis, in-plasma material evaporation, solid state chemical reactors, mechanical-chemical and shock-wave synthesis, electric-explosion method (nano-clasters), amorphous structure crystallization (nanoclaster) were successfully employed for nanotube and nanoclaster production.

Over the last years new industrial technology was developed in China to produce nanopowders of CaCO₃, SiO₂, TiO₂, ZnO, ZnS, SrCO₃ in the field of centrifugal forces (hundred and thousand per minute rotor rotation), where the mass transfer rate in hundreds and thousands time greater than that in the gravitation field. The higher rate of solution over-saturation, the uniform solution volume concentration and approximately identical crystal growth time provide sufficiently <narrow> curve of the particle-size distribution.

Currently the nanotechnology world market size is around 3ћ10¹¹ US dollars with a tendency to grow in ten times by 2015, that is close to the current world energy market. The leading trends of the world nanotechnology market are material science, biotechnology, photonics, electronics, computer and software science. The mostly large consumers of the nanotechnology market are environment (56%), electronics (20.8%), power engineering (14.1%).

According to publications the leadership belongs to USA (15000 papers in 2007), the European countries (around 12000 papers in 2007), and China (over 10000 papers in 2007). The United States hold around 40% of the world patents in the nanotechnology field. Russia is behind of the Western countries by 7 - 10 years, currently the share of the world technological sector of Russia is only 0.3%, while the nanotechnology market share is 0.04%. However by 2015 Russia plans to invest large finances into nanotechnology field with a big output in the nearest future.

Nanofluids and microchannels.

Already first investigations have shown even small addition of nanoparticles to a fluid (less than 1.0 volume percent) leads to the growth of heat conductivity by 60%, heat transfer - by 60% and critical heat flux - by 300%. However additional pressure losses are actually absent at these conditions. The materials for nanoparticles production are ceramics (SiN), metal oxides (Al₂O₃, Fe₃O₄, CuO), carbides (SiC, TiC), metals (Ag, Au, Cu, Fe), semi-conductors (TiO₂), carbon nano-tubes. The mostly spread areas of nanofluid application are thermal engineering, nuclear engineering, radio-electronics, transport, electronics, defense industry, medicine, chemical analysis.

The additional heat transport in nanofluid occurs due to nanoparticle movement under the Van der Vaals force (small distance between nanoparticles), electro-static force (small nanoparticle size), stochastic force (Brown particle movement), and hydrodynamic forces.

The nanofluid viscosity slightly changes with a fluid temperature and nanoparticle concentration that is the primary factor providing absence of additional pressure losses.

The volume concentration, nanoparticle nature, size and shape, nanosuspension temperature, as well as various impurities (acid, for example) are the primary factors affecting nanosuspension heat conductivity. The anomalous high heat conductivity of the organic oil and nanotubes blend (up to 260%) is due to very high nanotube heat conductivity (~ 3000 W/mћл) and large length-to-diameter ratio (~ 2000).

The number of different factors, such as a fluid speed, heat conductivity, nanoparticle concentration, heat capacity, nanoparticle nature, shape and size influence on the convective heat transfer at in-channel nanofluid flow. Despite increase in the nano-suspension heat conductivity, deterioration in heat transfer rate occurs at the nanofluid boiling compared with a pure fluid boiling. It is due to the small size nanoparticles fill in the roughness elements of boiling surface and deteriorate conditions for separation bubble formation. On the roughed surface this effect appears more remarkably. As far as the critical heat flux is concerned, it increases by 300%, moreover more remarkably in the area of greater nanosuspension mass flow rate.

The heat transfer and hydrodynamics in the microchannels (diameter below of 100 micron) is very important subject for new applications (micro-energy systems, radio-electronics, medicine, others). For the fluid pumping in such channels very high pressure should be employed, therefore more preferable is to provide fluid flow by means of joint action of pressure gradient and external electrical field, <helping> or counteracting pressure gradient. It is known, the in-channel temperature and velocity fields depend on the channel radius to Debye length ratio, external electrical field power and parameter, which is proportional to the ratio of acting forces (dP/dx)/(dж/dx).

As a whole, investigations of heat transfer and hydrodynamics at the in-channel nano-fluid flow are only in the initial stage, therefore in some cases the published results have contradictories. The specific attention in the future should be taken to the further study of thermophysical properties of nanosuspensions, the mechanism of heat transfer at the boiling, forced and natural convection studies, kinetics and thermodynamics of the phase transition in nanostructural materials and objects, as well as gas flow in microchannels, including the supersonic area.

Nanotechnologies in energetics.

Application of micron coal (5 - 20 micron) with mechanical activation is one of the potential subject of the power engineering. Smaller size and higher contact surface of coal particles promote greater combustion speed, while mechanical activation effect (growth of the chemical activity of coal particles) leads to the reduction in the ignition temperature. The mechanical-chemical synthesis can be carried out in various grinders, moreover the vibro-centrifugal grinders and des-integrators have low energy consumption (around 25 kilowatt per coal tons, as the <ball> and <drum> mills.

The reactionary properties of the micro-coal are close to the properties of the black oil and natural gas, moreover mechanical-chemical reactions lead to appearance of new combinations, which are impossible to obtain in reactions stipulated by the temperature. The high reactionary micro-coal ability enables to use it as the primary fuel in small boilers, at the of powder-coal energy blocks and direct combustion of small coal particles (5 - 10 micron) in gas turbines. Application of micro-coal enables to reach the higher efficiency of gas turbine plant, which is close to the efficiency of gas and steam turbine plants.

The study of a micro-coal combustion mechanism is a very complex problem of heat transfer engineering. Such investigations were carried out in United States, Germany, over the last years they conduct in the Siberian Branch of Russian Academy of Sciences. In the Institute of Engineering Thermophysics (Kiev, Ukraine) theoretical investigations of micro-coal combustion are carried out using the commercial software (Great Britain) and modified combustion models. The thermodynamics, the nitrogen, sulfur and carbon oxides formations are studied at the burning of mechanically and chemically activated coal of Ukrainian deposits in boilers of various power sizes. The novel energy saving technology of the Ukrainian micro-coal grinding up to 5 - 15 micron in a vortex chamber was developed protected by patents of Russia and Ukraine.

The water-coal fuel (WCF) seems to be very perspective fuel for power stations and industrial boilers, application of which is associated with coal micro grinding and stable water-fuel suspension obtaining with high rheological properties. Over the last years the WCF technology received high development in China, where around 100 million tons of WCF will annually be produced in the nearest future for the domestic applications and export to Japan. The WCF is the disperse system (water - 30-40%; coal - 60-70%; chemical components - 1%), in which the energetic and non-energetic coal can be employed as the fuel substance. The WCF can be produced from the anthracite, pure and brown coal of different trade-mark and ash content. The water of various quality, as the mine and industrial waters, oil <flowings> can be used for this purposes. To exclude the WCF freezing in the winter time some water portion can be substituted by spirits or blend of spirits and hydrocarbons. The areas of WCF application are small energy systems, such as municipal and domestic boilers. The important advantages of WCF are the low ignition (450...650ºу) and combustion temperature (950...1050ºу), high rate of burning out (up to 99.5%). It substantially reduces the content of nitrogen oxides (1.5 - 2 times), carbon oxide (in 2 times) and harmful (cancer) products (in 5 times) in combustion products. The other WCF advantage is a lower soiling of heating surfaces - the thermal effectiveness is almost 50% higher than that at the powder-coal blend burning. The increase of three-atom gases content in combustion products (water molecules) leads to the growth in radiant heat transfer, that compensates combustion products temperature reduction, associated with heat losses for water evaporation.

The important thermophysical problems, which are necessary to solve for wider WCF implementation into the practice are: providing of higher WCF stability (up to a few months), low viscosity and small pressure losses. Very important problems are to study some peculiarities of WCF combustion, as well as physical and chemical processes at the mechanical coal and water activation due to capitation treatment.

Quantum polyresonance activation (QPRA) enables to control the system entropy on the molecular level, thus to enhance combustion processes. For appearance of QPRA one necessary the average movement energy of particles in the surrounding space was higher of the pseudo stable level of the particles, inducing a QPRA. For this, the heat due to fuel combustion is used. As initiator of QPRA the nanoparticles with low level of are used therefore the system continuously receives the energy from combustion heat. To initiate QPRA very small number of quantum nano-activators are necessary, usually to invert into condition with only one particle is required for one billion of passive medium molecules.

For the liquid hydrocarbon fuels the well dissolved nano-activators are usually used. The adding of small amount of nano-activator to the diesel fuel (around 100 milligram per ton) provides reduction in the specific fuel consumption by 10 - 15%. Application of the black oil nano-activator reduces its viscosity, while combustion products activation leads to the growth in flame temperature by 100 - 150⁰C and reduction in harmful products production.

The power stations running the natural gas and coal can use the nano-activator water solution for injection into the furnace or into the secondary air. The amount of nano-activator is around 0.5 gram per coal ton or per 1000 cubic meters of the natural gas. The nano-activator can also be dissolved in the water heated in the combustion chamber.

Reduction of harmful products into ambient is very important problem of the coal power engineering. Based on the nanotechnology, the U.S. Hydrocarbon Technologies Company has developed a novel technology of the preliminary coal treatment on the molecular level, thus producing the environmentally clean fuel. The clean technology of non-traditional fuel burning using the catalytic soot properties was developed in Russia.

The future of energetics is closely associated with solid fuel cells using nanoceramic electrolytes with ion conductivity, which are the basis of energy plants of the direct transformation of chemical energy into electricity. Such fuel cells operate at temperatures of 800:1000⁰у, they use hydrogen, natural gas, as well as sin-gas, obtained from the natural gas or by coal gasification or conversion.

Conversion of hydrocarbon raw material into the hydrogen is the complex and many-stage chemical process therefore traditional chemical reactors are of a big size and very difficult in a control. The principal solution of this problem is application of mini-hydrogen generators with micron channels covered with nanostructure catalysts.

The nanotechnologies enable to develop catalysts with optimal characteristics to elevate their chemical activity, selectivity and production of mini-generators. The nanocatalyst of 10 micron size, based on the noble metals having the higher surface activity and absorption capacity was developed in the Institute of Thermal Engineering (Siberian Branch of Russian Academy of Sciences). Such catalysts were used for syn-gas production at the methane partcial oxidation and steam conversion. The optimal conditions of syn-gas production were determined providing higher hydrogen and carbon oxide selectivity. Significant prospects for the direct hydrocarbon fuel from syn-gas production, as well as oil refinement has application of nanocatalysts on the base of nanopowders of Fe, Ni and Fe-Cп.

Despite the high cost (up to 3000 US. dollars per 1 kW of installed power), the world solar energy market grows by 40% every year, while the cost of solar energy becomes cheaper by 20% annually. As supposed, the world market of solar batteries and solar panels will reach around 20 US billion dollars by 2013. Currently for production of thin-film solar elements the microcrystal silicon is widely employed. However the economically beneficial ratio of cost and efficiency is unreachable for photoelectric manufacturers, so far. Application of nanotechnologies will enable to develop more efficient than silicon ones solar elements with lower cost and higher efficiency. For example, application of CIGS films enables realizing the of nanofluids on the base surface. Such solar elements can be placed even onto the flexible basis.

Solution of the direct transformation of nuclear reaction thermal energy into electricity problem will enable to change radically the nuclear power station operation. Based on the nanotechnology, the new material was developed in USA, exceeding in few times transformation of radioactive emanation into electricity. The new material is a set of carbon nanotube layers filled in with a gold surrounded with lithium hydride. The radioactive emanation affects electrons in gold atoms and forces them to leave their orbits. The electrons come through the nanotubes, hit against the lithium hydride and move toward the electrode forming the electricity flow. The right nanotube orientation enables to use the radioactive energy in the best way.

The other important direction of a direct transformation of thermal energy into electricity is thermal electricity. Recently, the new technology was developed in USA enabling to transform the heat into electricity using metallic nanoparticles connected with organic molecules. For each 1⁰C of the temperature difference from 8.7 to 14.2 microvolt of the potential difference was obtained using different organic molecules. This direction can derive development of new generation of the electricity nanogenerators.

The great prospects are associated with application of nanoadding to lubricators in the power engineering and power machine engineering. The polymeric adding at the temperature of 150:200⁰у forms the nanostructure films with optimal roughness on the operating surface. The antifriction and anti-ware layer nanoadding include elements and combinations with lower share force between layers. Lubricants with ultra-disperse nanosize graphite reduces significantly friction and operation ware of friction details. When introducing spherical fullerenes into lubrication (oil), the ware-ness and operational time of the equipment is increased, while application of un-roughed nanodiamonds (4 - 6 micron) and cluster carbon with chemical impurities enables of friction details.

The great interest presents development of the high capacity accumulators of thermal and electric energy. The U.S. Altair Nanotechnologies Company developed novel nano-technology material for electrodes of the lithium-ion accumulators. Accumulators with Li4Ti5O12-electrodes have only 10 - 15 minutes of the charge time. In 2006 this company started production of accumulators in its plant located in the Indiana state. The small-size thermal energy accumulators of a higher capacity are very important for application in the municipal sector, where the reduced electricity tariff is applied in the night time (from 23: 00 to 7:00). Especially important is development of thermal accumulators of the isothermal type using the phase transition, chemical reactions, capillary-porous effects, enabling the very low loss of the thermal energy into the ambient for a long period of time.

Recently scientists in USA developed the new technology enabling to manufacture nanotubes of 50 micron thick with actually unlimited length. Therefore, the new electricity transmission lines using the carbon nanotubes can be developed in the future having lower electricity losses than that in the present time.

In ordinary conditions the bacteria behavior is very chaotic in space or volume. The scientists in USA have found conditions to enforce bacteria colony move orderly, in the right direction and with necessary speed, thus to rotate small size gear (380 micron in diameter), the mass of which is million times greater of the bacteria mass. This phenomenon can successfully be used in small size devices of the electricity production.

Measurement systems.

The development of nano- micro- and small size sensors will lead to the radical changes in control systems for energy and power engineering. This will enable to introduce the continuous monitoring of complex technical systems - to determine local parameters, to detect appearance of cracks, to control the current machine condition and to react instantly on appearance of dangerous operating regimes. Recently in USA was developed and tested the temperature sensor 100 micron in size, operating in the flow temperature of 870⁰у. The temperature sensor operating in flow temperature of 1200⁰у and pressure sensor operating in flow temperature of 500⁰у is now in the development.

The one-wall nanotubes have the ultra-high sensitivity and can be used in detection of certain type molecules in the gas medium or solutions. The nanosensor for detection of NO₂, CO, CO₂ and H₂ molecules, as well as hydrocarbons was recently developed in USA, in which the optic properties of gold nanoparticles incorporated into nanoaggregate of metal oxide were used.

Nanotechnologies and humanity.

The nanoparticles have the very high penetrating ability, therefore nanoparticles (nanomolecules) of heavy metals or other toxic matters are able to penetrate into the human or animals bode without obstacle. Afterwards they can be accumulated inside the human body, to form durable associations with bio-molecules (albumens), to damage or , as well as activate them. Moreover, nanoparticles can lead to the catalysis of harmful for human body chemical reactions.

The carbon nanotubes are longer of a certain size are very dangerous for the human health, as they can provoke illness of respiratory paths, similar to that appeared at the asbestos poisoning. In Great Britain the products containing artificial nanoparticles are not certificated, the materials containing particles less than 125 micron, as well as materials with nanoparticles below 200 micron in size are forbidden for application.

The nanotechnologies will widely be used to improve ecological characteristics of various energy technologies. Recently U.S. Company Nanokinetix developed new catalysts technology for the organic residual catching up of the car outgoing gases. The ecologically clean combustion technology to burn alternative (non-conditional) fuels was developed in Russia, based on the soot catalytic properties.

Development of nanotechnologies can lead to undesirable sequences for the civilization. The microscopic nanobombs and nanoexplosion devices are difficult for identification, they can easily be delivered to any place and to use for terrorist attacks.

nanotechnologies in energetics.

The above analysis shows that significant in the energetics can be reached due to implementation of the following primary nanotechnologies.

-         New materials with improved durability, thermal and corrosion firmness (steam generators, steam over-heaters, steam and power turbines), nano-structural ceramic and metal-ceramic products.

-         Nanocoverings protecting various objects from erosion and corrosion (combustion chambers, blades and vanes, steam generator tubes).

-         Nanofluids; new working fluids with high heat transfer enhancement rate: channels of energy and power systems, new cooling technologies in nuclear engineering and gas turbine engineering.

-         Nanolubricating materials and nanoadditions, reducing wearies of machine details, friction and vibration losses, providing detail repair and recovery according to the technology (intellectual recovery).

-         Micron coal and water-coal fuel application.

-         Nano-activators in combustion, new technologies of the coal treatment on the molecular level to produce the clean and ecology friendly fuel.

-         Selective nanocatalysts and dividing nanomembranes protecting the environment, new technologies of CO₂ , new technologies of water and air clean up.

-         Reformers with selective hydrogen and carbon oxide production.

-         Nanoceramic electrolytes with ion conduction for the solid oxide fuel cells of a direct electricity from chemical energy production.

-         Thermoelectric and nuclear batteries for direct transformation of radioactive emanation and thermal energy into the electricity.

-         Nano- and microsensors for pressure, temperature, concentration and other parameter measurements and providing continuous monitoring and optimization of the energy and power systems.

The nearest future problems.

The formulated above the primary trends of the nanotechnology for energy and power engineering cover a wide time period. As far as the nearest future is concerned, the following primary directions can be noted.

1.           Investigation of thermophysical properties of nanomaterials, kinetics and phase transition in nanostructure materials and objects, heat and mass transfer in nanofluids and microchannels.

2.          Industrial production of stable nanofluids, carbon nanotubes and nanofirers, synthesis of nanotubes with curve of particle to size distribution. The one-layer nanotubes are manufactured by only ten world companies in amount not exceeding a few grams a day. Currently their price is from 500 to 750 U.S. dollars per gram. A few companies provide industrial production of nanofibers. Russia, Ukraine and Belarus synthesize the carbon nanotubes and nanofibers only in laboratory conditions.

3. љљљљ Providing the minimal risks at the production of nanoproducts and nanotechnologies applications.

Artem Artemovich Khalatov, Corresponding member of the National Academy of Sciences, Doctor of sciences, Professor. Head of the High Temperature Thermogasdynamics Department, Institute of Engineering Thermophysics. The scientific interests: vortex and swirling flows, heat transfer augmentation, cooled gas turbines, nano-technologies.