Kalina cycle

Under the Kalina cycle or Kalina cycle process is meant in the 1970s developed by Russian engineer Alexander Kalina heat transfer process for ammonia - water - steam generation at a lower temperature level than in traditional steam systems. Conventional steam turbines require steam temperatures of several hundred degrees Celsius in order to operate economically at a relaxation temperature of above 100 ° C can. The maximum possible efficiency ( Carnot efficiency) is limited by the upper and lower working gas temperature.

General

In geothermal power plants with steam turbines, high steam temperatures to tap only by costly deep drilling. The geothermal gradient is about 3 ° C per 100 m, so that several kilometers deep drilling will be required to reach with water as heat transfer sufficient efficiencies.

In order to make use of geothermal water at low temperatures, possibly even below 100 degrees Celsius, Kalina developed an efficient heat transfer to an ammonia - water mixture. The water ( water vapor) is primarily the heat exchanger for the ammonia gas in the same cycle. The resulting vapor at significantly lower temperatures mixture is then used to drive turbines. The Kalina could possibly be attached to a normal steam process.

Compared to an Organic Rankine Cycle (ORC ), a Rankine cycle based on organic substances such as isopentane, the efficiency should be 10-60 % higher. Thereby, the current yield of a geothermal power plant can be operated even with small drilling depths, and at the same depth / temperature can be increased.

One problem is to prevent the toxic and strong smelling ammonia in operation and maintenance from escaping into the environment.

At present, few geothermal power plants around the world work under this mode of action, the most famous is the Húsavík geothermal power plant in Iceland.

However, the process experienced because of high energy prices currently experiencing a renaissance, resulting in the second of its kind in Europe in Unterhaching.

The procedure is covered by several patents, which holds the Californian company Exergy. The European licenses for a process type, so called SG1 Cycle has, Siemens Industrial Solutions and Services secured, the resulting advanced SG2 Cycle is held by the engineering company M W. The different models that differ in expenditure on equipment and the effectiveness that can be achieved.

Technical Description

In the evaporator, the " working solution ", is a mixture of ammonia and water is evaporated, so that an increase in volume occurs. It is passed to a turbine to a temperature of, for example polytropic 70 ° C relaxed. Kalina makes here the property of the mixture of NH3 H2O use, decreases as the reduction of the total ammonia concentration in the liquid and vapor phases (at constant temperature) of the boiling pressure or at a constant pressure necessary for boiling the boiling temperature is higher. The concentration change occurs in the recuperator by admixing a "poor" ammonia solution from the desorber to the turbine steam. By focusing lowering the pressure gradient for the turbine increases. For the multiple turbine mass flow has to be circulated in the absorption part. Absorption and condensation heat can be dissipated to the cooling water.

The resulting " basic solution " is brought by a pump to the required condensing pressure of the working solution and which promoted the greater part of current in the expeller. There is expelled by means of waste heat from the turbine exhaust almost pure ammonia. The remaining weak solution flows back via a throttle valve to the condenser. The ammonia vapor is now merged in the absorber / condenser to the other part of the current basic solution and can, as once again condense final working solution when necessary boiling pressure of heat transfer to the cooling water. After pressure increase the working solution is conveyed back into the heat recovery steam generator.

Benefits

The particular advantage of the Kalina circuit is primarily due to the favorable heat transfer conditions in the steam generator and condenser. The property of the mixture is used to cause changes of concentration by temperature changes. Here this is done by changing the concentration of the individual phases of vapor and liquid at a constant total concentration and constant pressure. Here, the mixture evaporated under steadily increasing temperatures and condensed under steadily falling temperatures. By the non-isothermal vaporization of the mixture, the evaporation temperatures are closer to the ideal line of the heat source than that of water, which evaporates at constant temperature. Another effect is that more liquid and super heat can be transferred.

The losses in heat transfer are thus low and the mean temperature of the heat supply is increased, which is an improvement of the process efficiency according to Carnot. Conversely, even when the heat dissipation in a similar way, the average temperature of the heat dissipation is lowered 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 is achieved at the expense of large heating surfaces of the heat exchangers, which are additionally burdened by poor heat transfer due to diffusion and absorption phenomena.

The actual possible efficiency gain over a simple Rankine cycle is specified differently in the literature. During Gajewski et al. specify it with about 5%, H. M. Leibowitz and D. W. Markus ( Energy Inc., Hayward, California ) militate against a possible gain in efficiency of up to 50 %. These widely spaced data are also a characteristic sign of the early development of this technique. Critical to the Kalina process are particular due to the process required much larger heat transfer surfaces in addition to the limited controllable decomposition of ammonia problems. This is all the more weight than the area required for the heat transport with decreasing source temperature ( temperature difference ) increases strongly. In a simple ORC process accounts for the heat exchanger about 20 % of the investment costs. Gajewski et al. have identified over a Rankine cycle, the minimal additional cost of a Kalina process with about 40 %.

Disadvantages

In particular, the maintenance of Kalinaanlagen is complicated, since the ammonia is highly alkaline, corrosive to the respiratory tract and skin in Einatmem and it is a strong metabolic poison. It has a strong smell and is flammable and explosive.

Swell

• Thermodynamic cycle
• Geothermal