The Rankine cycle is one of the most important ways to transform on large scale thermal energy into mechanical or electrical energy. Basically, the cycle consists of a steam boiler (1) into which the thermal energy is supplied, a turbine (3), a condenser (6) and a feed pump (8). In this closed circuit circulates the working medium, usually water/steam is used in large power plants. (Source : Electrabel) |
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To understand how it works, the Rankine cycle can be presented in a T-s diagram. On this diagram, the different states of the working fluid are represented, these are : the liquid region, coexistence region (wet steam) and superheated steam region. An important parameter used in thermodynamic calculations and characterising the fluid, is entropy s. The change of entropy is a measure for the heat exchange and can be calculated as: |
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So, an entropy rise means a heat input and vice versa. Ideally, the expansion in the steamturbine should be a reversible adiabatic process (no heat is exchanged). Therefore the entropy theoretically remains constant during expansion. In the T-s diagram the expansion is represented by a vertical line (4-5).
The working fluid Normally water is used as working fluid in the Rankine cycle. The main benefits to using water are, that water is chemically very stable and hence there are almost no special requirements to the construction materials that can be used. Moreover, water has a very low viscosity so that the energy needed for the transportation through the various components, such as heat exchangers and pipes, is limited. The specific heat of water is relatively high, this makes it a good medium for energy transportation. Water is also non-toxic and is available worldwide in large quantities. This makes it much cheaper than any other possible working fluids. Water has also several drawbacks. In order to achieve the highest cycle efficiency and increase the electricity production, the steam is expanded at a temperature close to ambient temperature. At this low temperature the vapour pressure, and therefore also the density, is very low. This results in large steam volumes at the turbine exit. Consequently, large turbines and condensers are needed. Furthermore, the pressure drop in the steam turbine is very large. To expand steam from 30 to 0,1 bar a multistage turbine is needed because of limited pressure ratio of a single stage. This makes steam turbines relatively complex and expensive. If saturated steam (3) is expanded directly (dashed line), the steam will partially condense immediately and so it will contain liquid droplets which, considering the high rotation speed of the turbine, cause erosion on the turbine blades. To avoid this premature condensation, the wet steam is normally superheated (3-4) to higher temperatures so that there won't be any condens formed in the sucsessive turbine stages. The amount of heat needed for superheating is relatively small, compared to the first two stages, but it demands high temperature levels in the superheater. These high temperatures can normally be reached in the exhaust gases of a combustion proces, so these flue gases can be used as heat source for superheating. But the flue gases produced by industrial combustion processes frequently contain contaminations and dust particals that stick onto the surface of the superheater. Hence the superheater is one of the most critical elements and its construction materials must withstand these extreme conditions. The biggest part of the thermal energy supplied by the heat source is used to evaporate the water to steam. Only a small fraction of the thermal energy is required to heat up the water to boiling temperature. Hence low temperature heat sources can be rarely applied because of the limited heat recovery. |
Organic Rankine Cycle |
On the figure the highest temperature of the heat source is about 280°C, the pressure of the working fluid is 10 bar. Condensation occures at 100°C (0,2 bar), what makes the condenser heat still usable for heating purposes. All general thermodynamic laws remain always applicable : the bigger the temperature difference evaporator-condenser, the higher the cycle efficiency. Advantages ORC
Since the temperature after expansion in the turbine (4-5) remains for most ORC fluids higher than the fluid's condensing temperature (7-8), one must extract this additional heat in the condensor. However this is at the expense of the thermal efficiency. This loss can be minimalised by integrating an additional heat exchanger in the ORC installation. This recuperated heat (5-6) is then used to preheat the ORC fluid that is pumped to the boiler for further evaporation. This additional heat exchanger is usually called the recuperator or the regenerator. The efficiency of an ORC unit varies from 10 to 20%, depending on the temperature levels and the use of a regenerator. The selection of a suitable working fluid is not that easy. For most of the organic fluids the vapor tables and saturation curves are unknown. Without the knowledge of the saturation pressures and temperatures it is not possible to evaluate the suitability of a fluid in any given application. Each (organic) fluid has it's own specific properties. Hence not every fluid can be used in a certain application. Depending on the type of heat source (hot water, exhaust gases, ...) and it's temperature level, a suitable working fluid with appropriate evaporation and condensing temperatures has to be selected. Each ORC is constructed for a given working fluid, so there isn't always een optimal ORC unit available on the market for each type of heat source. Usually, constructors offer only a few standard units with their own organic fluid. This way, the energy of the heat source isn't always recovered optimally. Some suppliers also develop customer tailored ORC units, but usually these units are very expensive. However, the technology is still in development and in many cases still isn't optimised. Also following aspects of the working fluid should be taken into account : toxicity, safety, explosivity, flammability, environmental issues,... Herefore, all safety presciptions and legislations should be consulted. Application domain A substantial part of the produced energy in the industry is wasted as mechanical losses (friction) and as thermal losses (heat losses). Heat losses can be found in dryers, incinerators, exhaust gases, gasengines,... When the waste heat cann't be used as a useful heat source in the production process, it usually will still be posible to recover a lot of energy with an ORC unit, so losses can be minimised. In this way waste heat can be transformed to a limited amount of mechanical energy, that can be used for generating electricity or as a driving power for some equipment. So no additional primary energy will be consumed by the latter. Re-usable heat sources :
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The foregoing problems can be partially solved by selecting an appropriate fluid. When the water is replaced by an organic fluid, then the cycle is called an Organic Rankine Cycle (ORC). Most of the organic fluids are so called dry fluids. These dry fluids have the advantage that they remain superheated after expansion (4-5), so condensation of the fluid in the turbine can be avoided. Some commonly used fluids are pentane, propane, toluene, ammonia and some coolants.
By selecting a working fluid with a relatively high density at condens temperature, the use of large turbines and condensors can be avoided. The smaller temperature difference between evaporation and condensation, also means that the pressure drop/ratio will be much smaller, although this depends on the used fluids. In this case the use of multistage turbines is no longer needed. Moreover the lifetime of the turbine is much longer, because no liquid droplets are formed during expansion, so erosion of the turbine blades will not occur.
Disadvantages ORC
