Kalina cycle
Under the Kalina cycle or Kalina cycle process is meant a in the 1970s by the Soviet Engineering Aleksandr Kalina developed heat transfer process for ammonia-water -Dampferzeugung at a lower temperature level than with conventional steam systems. Conventional steam turbines require steam temperatures of several hundred degrees Celsius in order to be able to operate economically at an expansion temperature of over 100 ° C. The maximum possible efficiency ( Carnot efficiency ) is determined by the distance to each other and the ratio of the upper and lower working gas temperature .
General
In geothermal power plants with steam turbines, high steam temperatures can only be tapped through costly deep drilling . The geothermal gradient is around 3 ° C per 100 m, so that several kilometers deep boreholes are necessary in order to achieve sufficient efficiency with water as the heat transfer medium.
In order to be able to use geothermal water with low temperatures, possibly even below 100 degrees Celsius, Kalina developed an efficient heat transfer to an ammonia-water mixture. The water (steam) is primarily the heat exchanger for the ammonia gas in the same circuit. The steam mixture produced at significantly lower temperatures is then used to drive turbines. The Kalina process could possibly be attached to a normal steam process.
Compared to an Organic Rankine Cycle (ORC), a Clausius-Rankine cycle based on organic substances such as isopentane, the efficiency should be 10–60% higher. As a result, a geothermal power plant can be operated even at shallower drilling depths, or the current yield can be increased at the same drilling depth / temperature.
One problem is to prevent the toxic and strongly smelling ammonia from escaping into the environment during operation and maintenance work.
Few geothermal power plants worldwide are currently working according to this operating principle, the best known is the Húsavík geothermal power plant in Iceland .
The process experienced a renaissance due to the high energy prices in 2008, which led to the second plant of its kind in Europe in Unterhaching . In the meantime, electricity generation is no longer considered profitable there due to numerous technical problems. The plant was shut down in 2018. Another Kalina power plant went into operation in Taufkirchen near Munich in 2018.
The process is protected by various patents held by the Californian company Exergy . Siemens Industrial Solutions and Services has secured the European licenses for a type of process, the so-called SG1 Cycle, and the SG2 Cycle, which has been further developed from this, is held by the plant manufacturer Exyte . The various types of processes differ in terms of the equipment required and the effectiveness that can be achieved with them.
technical description
In the evaporator , the liquid “working solution” made up of ammonia and water (commonly known as salmiakgeist) is evaporated into a gaseous mixture of ammonia and water vapor with an increase in volume. The gas mixture is fed polytropically into a turbine to a temperature of z. B. 70 ° C relaxed. Kalina makes use of the property of the two-component system NH 3 + H 2 O, in which the boiling pressure drops due to the decrease in the total ammonia concentration from the liquid and vapor phase (at constant temperature) or the boiling temperature required for boiling at constant pressure gets higher. The change in concentration takes place in the recuperator by adding a “poor” ammonia solution from the expeller to the turbine steam. The decrease in concentration increases the pressure gradient for the turbine. For this, the multiple turbine mass flow must be circulated in the absorption section. The heat of absorption and condensation is dissipated to the cooling water.
The resulting “basic solution” is brought to the necessary condensation pressure of the working solution by means of a pump and the larger partial flow of it is conveyed into the expeller. There, with the help of waste heat, almost pure ammonia is expelled from the turbine exhaust steam. The remaining poor solution flows back to the condenser via a throttle valve. The ammonia vapor is now brought together in the absorber / condenser with the other partial flow of the basic solution and can finally condense there as the working solution which is now available again at the boiling pressure required for this, giving off heat to the cooling water. After the pressure has increased, the working solution is fed back into the waste heat steam generator.
advantages
The particular advantage of the Kalina circuit is essentially the more favorable heat transfer conditions in the steam generator and condenser. The property of the mixtures is used to bring about temperature changes through changes in concentration. This is done here by changing the concentration of the individual phases of vapor and liquid at a constant total concentration and constant pressure. The mixture evaporates at steadily increasing temperatures or condenses at steadily decreasing temperatures. Due to the non- isothermal evaporation of the mixture, the evaporation temperatures are closer to the ideal line of the heat source than those of the water, which evaporates at a constant temperature. Another effect is that more liquid and overheating heat can be transferred.
The losses in the heat transfer are thereby lower or the mean temperature of the heat supply is raised , which according to Carnot means an improvement in the process efficiency . Conversely, in the case of heat dissipation, the mean temperature of the heat dissipation is lowered in a similar manner due to the falling boiling temperatures of the mixture during condensation , with the same positive effect on the efficiency.
The thermodynamic advantage of small temperature differences in the heat transfer, however, comes at the price of large heating surfaces of the heat exchangers, which are additionally burdened by poor heat transfer due to diffusion and absorption processes.
The actually possible gain in efficiency compared to a simple Rankine process is stated differently in the literature. While Gajewski et al. indicate it with approx. 5%, however, H. M. Leibowitz and D. W. Markus (Energy Inc., Hayward, California) speak of a possible efficiency gain of up to 50%. This widely spaced information is also a characteristic sign of the early stage of development of this technology. Critical for the Kalina process are, in addition to the only limited controllable decomposition problems of ammonia, in particular the considerably larger heat exchanger surfaces required by the process. This is all the more important as the area required for heat transport increases sharply with falling source temperature (temperature difference). In a simple ORC process, the heat exchangers account for around 20% of the system costs. Gajewski et al. have determined the minimal additional costs for a Kalina process with approx. 40% compared to a Rankine process.
disadvantage
Maintenance and operation of Kalina plants is technically complex in terms of safety, since ammonia has a strongly alkaline effect in both phases, it burns the respiratory tract and skin when inhaled and it is a strong metabolic toxin. It has a strong smell and is flammable in air and explosive when mixed with air.
swell
- ↑ Notes from the North: a Report on the Debut Year of the 2 MW Kalina Cycle® Geothermal Power Plant in Húsavík, Iceland ( Memento of the original from October 19, 2013 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked . Please check the original and archive link according to the instructions and then remove this notice. (PDF; 65 kB)
- ↑ Geothermie Unterhaching GmbH & Co KG: What is geothermal energy? - Procedure and use
- ↑ Pro-Physics: Geothermal power plant generates electricity with new technology , May 14, 2008
- ↑ Pilot project failed . In: https://www.merkur.de . December 13, 2017 ( merkur.de [accessed January 4, 2018]).
- ^ Süddeutsche Zeitung: Energy supply in Taufkirchen - Under high voltage April 10, 2018
