Candidate of Physico-Mathematical Sciences, thermophysic, advisor
Development of thermal energy conversion technologies and devices
The first kea idea (let's call it "the fragmentation principle") is to organize the working process of energy converting device in long time cycles, when a certain operation is carried out over a relatively large portion of the working medium in each cycle. This approach is fundamentally different from the traditional principle of working process organization, when the state of the transforming device is constant (despite the fact that the state of the working medium is constantly changing). Thus, a substance goes through several phases of the process during the steam-power cycle - an evaporation, an expansion, a condensation and a pressure increase, but different portions of a substance pass these phases simultaneously. Due to this, the working parameters of the process - for example, a temperature and an evaporation pressure, or a condensing pressure and a temperature - remain constant in time. As a result, the installation is obliged to have a pump to increase the pressure of the working substance after its condensation, which creates significant difficulties in implementing small steam-power cycle plants, using organic working substances.
It enables us to get rid of the liquid pump in the conversion device, if we divide the working substance into portions and operate with theses portions. While heating a portion of the working substance in an enclosed volume, the temperature rise is necessarily accompanied by a rise of pressure. If the part of the working substance in an enclosed volume is in the vapor state, then the pressure will be equal to the pressure of saturated vapor at the current temperature during a heating process at each time period. Thus, it becomes possible to increase the working medium pressure solely by supplying heat. Technical implementation of the principle is described in the patent.
The second idea (let's call it the "principle of isotherm") –is a method of providing an intensive heat exchange between the working medium and walls of an expansion device during the expansion process. Expansion of the working medium is the process where a mechanical work is performed (which converts to electricity later on). The expansion is accompanied by a pressure drop and, we may say that all the other processes of the device working cycle that converts energy, are intended to restore the initial state of the working medium before the expansion.
The expansion of the steam-power cycle happens without heat input and it is accompanied by a temperature fall. Heat exchange with the walls and other elements of the expansion device is a detrimental factor. The expansion process may instead be useful if we use a different workflow of the heat exchange cycle. Moreover, if a heat exchanging process is intensive, the expansion will take place at almost constant temperature value.
It can show bigger conversion efficiency coefficient values than in the steam cycle with a proper workflow organization. Ensuring constancy of gas temperature during its expansion and contraction in the Stirling engine, increases the efficiency coefficient up to similar values of the efficiency coefficient in the Carnot cycle. For example, to achieve the Carnot cycle efficiency of 40%, we need to heat a working gas up to 230 ° C, whereas in the steam-power cycle, the required temperature should be above 550 ° C.
An implementation of heat exchange between working medium and the walls of the expansion device gives an opportunity to organize an open air cycle. One of the options is a four-stroke-cycle consisting of the following processes: filling cylinder with the air, its compression, extension and release into atmosphere. Heat exchange between the air and the walls of the cylinder and a piston causes the higher air temperature during expansion than during compression step, and therefore it requires bigger energy for expansion than for compressing it. Heat must be supplied from an external source to the cylinder walls and the residual heat, which must be removed from the transforming device in accordance to the Second Law of Thermodynamics, is discharged together with a released air. Thus, it eliminates the necessity of working medium cooling device, and enables to create a sufficiently compact heat converter for self-generated power supply purposes as well as for utilization of waste heat from the aluminum production.
One of the key groups of human civilization technology are the methods of converting thermal energy into mechanical and electrical energy. To solve the acute problem of our time - bringing the quality of life on Earth to an acceptable level - requires the usage of all possible sources of renewable energy. Among the most promising energy sources, whose widespread development is a matter of the nearest future, you can select the following sources:
- Biofuels; - The heat of solar radiation; - Geothermal heat; - Enterprise's rejected heat.
Despite the variety of heating energy sources, the methods of converting thermal energy into mechanical and electrical energy in practice are extremely scarce. Why don't we leave beyond the discussion the methods based on internal fuel combustion, which requires a specifically manufactured, expensive fuel and should be only applied in vehicles or to solve an emergency power supply problems. A situation, when the consumer is provided by a stationary power supply from a diesel generator on a permanent basis, is abnormal. Universal methods of converting thermal energy are those, where obtaining of thermal energy and its conversion are carried out in two different, but co-operating units. There are only two methods among all that are used in practice: steam-engines and the Stirling engine.
Steam-power plants represent the most common type of device that converts heat into mechanical energy. Devices of such type are used on thermal power plants (TPP, HEP), nuclear and geothermal power plants, on solar power plants, where the principle of solar radiation concentration is used along with its conversion into heat. The major amount of energy is produced by steam power plants running on moisture vapor. There are also plants that work on organic substances vapors (Freon or hydrocarbons). Their application field is in an autonomous power supply with small power requirements (100 kW or less), or a low-temperature transformation heat (50-150 ° C).
A steam power cycle usage experience suggests that this method of converting thermal energy is very effective in "big" energy where there is practically no alternative, but it is rather unsuitable in small and medium power plants (100 kW and below). The main problem is how to raise the pressure of the working medium before it enters the evaporator. A pump is required for these purposes and it should be able to handle organic liquids, creating a large pressure drop (about 10 atmospheres), but with a low liquid flow rate, and it should consume relatively little power. The selection of such devices is very limited, the price (in relation to the power setting) is high enough, compared to the water feed pumps of large steam power plants.
The Stirling engines are the devices with relatively small size and capacity (20-50 kW), which allow to convert heat from high-temperature level with high efficiency (300 Cesium degrees and above). A great advantage of the Stirling engine is a long service life, reliability, ease of use. In fact, the Stirling engine is a kind of a "black box" where the high-grade heat is supplied and a low-temperature heat along with the mechanical energy are discharged. A common assertion, that the Stirling engine is different from other energy conversion methods due to higher efficiency, is fair. The Stirling engine is truly superior in efficiency then the majority of internal combustion engines, but it's more suitable to compare the Sterling engine with the other heat converting devices, where the heat is supplied from the outside. In comparison with the same steam-power cycle on identical maximum working temperature of the substance, the Stirling engine loses in efficiency. Yet the assertion about high performance of the Stirling engine is not entirely baseless. In fact, the Stirling engine cycle is potentially able to implement a cycle that is sufficiently close to the generalized Carnot cycle with the greatest possible efficiency. This requires the gas, during the expansion process, to have the closest temperature value to the heat source temperature, and during the compression process, to have a temperature as close as possible to the ambient temperature. Assuming, this can be achieved by an intensive heat exchange between the working medium and hot cylinder walls, where the expansion takes place and between the working medium and cold cylinder walls, wherein the compression occurs. In practice, the requirement of gas temperature conservation during its compression and expansion contradicts the requirement to make these processes fast enough in order to obtain acceptable values of engine power. Therefore, the real cycle of the Stirling engine in commercially available models is quite far from the idealized, approximated to the Carnot cycle, version. The Stirling engines are very high-tech and expensive in production. Devices with combination of the Stirling engine and parabolic solar radiation concentrator, which creates in its focus a temperature up to 1000 ° C on a heat-absorbing surface, are widely used. During last years, these installations have been losing the competition to various models of solar heat converters based on steam power cycle.
At the same time there is a significant technological and scientifical potential of the Stirling engine, and if new approaches to considerably improve the engine's performance are found (such as the use of regenerative heat exchanger proposed in the 1930s), these devices can again become competitive in a wide application area.
It all together make new methods and thermal energy conversion devices, as well as options for improving existing traditional methods, very popular and widely demanded nowadays. Particular attention should be paid to respond to the needs of settlements and enterprises located away from the main networks, requiring a small self-contained sources of electrical power (from 1-100 kW); of heating (from a few kW to 1 MW), e.g. small towns, villages, agricultural enterprises, forestry, etc. As a rule, they have a primary energy source, such as biofuels (agricultural wastes and wood processing wastes, peat), or geothermal heat. In Russian conditions, the need of autonomous power source is combined with the need of heat, and the proportion of the required heat and electrical power is typically in the range from 10: 1 to 20: 1.
Thus, the actual task is the development of power plants that are quite cheap and able to convert into electricity up to 5-10% of the initial heat, releasing the rest to the heating system. The waste heat of enterprises is an insufficiently assimilated resource. As a rule, the heat, generated in various industrial processes, is usually released through the water recycling system. The temperature of circulating water is quite low (30-40 ° C and lower), which prevents the use of this heat to generate electricity. However, there are other situations, particularly in the aluminum production, where the heat from the electrolytic cells and gas removal systems can be discharged at temperatures above 100 ° C (theoretically, up to 800-900 ° C). Existing methods are not suitable for converting heat which is generated in aluminum electrolysis, for several reasons, particularly due to the necessity to accommodate converting devices in the immediate vicinity of the cooled object, the inability to use water (and, presumably, any other liquid coolants), dimensional restrictions. To solve this problem we need fundamentally new approaches. Moreover, its successful solution can create a significant source of energy. For instance, conversion of only 10% of heat generated by the Krasnoyarsk aluminum plant into electricity, will give 750 million KWh per year, which is enough to power half of the housing stock in Krasnoyarsk.
The company was involved in work on new methods and heat conversion devices development due to two key ideas, proposed and patented by its employees. These ideas can build the foundation for a technological breakthrough in energy field in the future. We would like to discuss these ideas, examples of application and advantages.
Let's look at some possible options of heat converting devices that use principles of fragmentation and isotherms together or separately. 1. The conversion device according to patent [?], which has separate chambers for heating the portions of the working substance and a piston expander with a heat supply to the expandable working substance. After the piston expander, when the energy is produced, the material in the vapor state retains high temperature (heat); its cooling to the condensation point is made at the expense of its heat exchange with the heated portions of the substance in the chambers. Here is an example of the working characteristics.
Steam power cycle efficiency is 18 % with the same temperature value of the heat source and cooling water, and the same working medium. There is a significant complexity in technical implementation of working substance pressure increase before it enters the evaporator in the small-scale steam power installation. An increase in pressure occurs along with the heating of the substance portions in the chambers in this device.
Devices of this type are suitable for high-efficiency heat conversion from high-temperature sources. Their potential advantages over steam power plants are the following: there are no expensive components (all components can be manufactured using standard equipment), they have bigger efficiency coefficient.
2. The difference between this device and the one, described above, is that the heat is not supplied to the converter, and the substance condenses without prior cooling after the converter, as well as heating of the substance portions in chambers is performed by an external heating source. Here is an example of the working characteristics:
Steam power cycle efficiency is 16% for similar temperature of the heat source and cooling water, and for the same working medium, Devices of this type are optimal for converting heat from a relatively low temperature heat source (in particular, geothermal heat) and have the same advantages over steam power plants, as the devices in 1.
3. A device, based and on one or more cylinders and on system of heat input, working on four-stroke cycle consisting of the following processes: filling the cylinder with atmospheric air, its compression, expansion and release into the atmosphere, when all processes are accompanied by intense heat exchange between the air and cylinder walls.
Devices of this type are optimal for the heat conversion when a heat discharge from the conversion device is difficult to arrange. Their potential advantages are: simplicity of construction, which resembles the design of internal combustion engines without fuel systems, ignition and cooling, and with extra system of heat input to the walls of the cylinder system, but there is no need for heat discharge.