What is the Organic Rankine Cycle (ORC)?
An ORÇ technology (Figure 1) based on the system that generates electricity from heat uses the heat from the hot source (red line in Figure 2) to evaporate the organic working fluid (green line) in the evaporator (1). The working fluid of choice can be silicone-based fluids or hydrocarbon or coolant-based fluids can be used for low temperatures. Pressurized steam is then sent to turbines (# 2) and when combined with the generator, it generates electricity. The steam is condensed back to a liquid state (# 3) in the condenser. Here, either the cooling tower (No. 5), groundwater or river water (blue line) are used as cooling means. The air cooling system can also be used as an alternative. Then the coolant pump (No. 4) pumps the working fluid back to the evaporator and this closed loop process repeats. Fuel cost is zero since it is used as a hot liquid fuel source. In addition, since no combustion occurs, no emission to the atmosphere occurs in the ORÇ energy system. Hot springs are generally in hot liquid or gaseous state. The heat from such sources is transferred to the ORÇ working fluid directly or indirectly through an intermediary vehicle, depending on the characteristics of the waste heat source or other restrictions. The liquid waste heat sources are generally combined directly with the ORÇ unit. Gaseous heat sources are combined indirectly.
The thermodynamic cycle of ORÇ is shown schematically in Figure 2. Same to the left ring
It is also called the Temperature-Entropy or T-S diagram. The turbocharger uses the heat generated from the hot source to preheating and evaporates the appropriate organic working fluid in the evaporator (2 → 3 → 4). The organic liquid vapor energizes the electric turbine, which can be combined with the electric generator directly or through the reduction gear (4 → 5). The steam is then cooled with water or air and condensed in the condenser (5 → 1). The organic liquid is finally pumped into the preheater and evaporator (1 → 2); Thus, the sequence of operations in the closed loop circuit ends.
Organic Rankine Cycle (ORC) Advantages
Compared to steam turbine systems, ORÇ has some special advantages. Although the steam turbine is generally more efficient as it is used at high temperatures and pressures, ORÇ is preferred in many applications due to its specific features below.
1) In ORÇ, it is not uncommon for the turbine to operate without problems for more than 20 years, since the steam that comes into contact with the turbine blades is the steam of the clean working fluid.
2) In addition, the organic working fluid is a high molecular weight substance while in the steam turbine. Smaller high velocity vapor molecules hit the turbine at high velocity, thus causing blade damage. This increases the maintenance and replacement frequency of the turbine and the total cost.
3) Since ORÇ operates at lower temperatures and pressures, the mechanical and thermal stress in the components is lower compared to the steam turbine. This increases the life of the components.
4) Since ORÇ operates at lower temperatures and pressures, there is no need for an operator in the work area. ORÇ can be controlled and operated remotely without the need for personnel. This significantly reduces the operating cost.
5) An important advantage of ORÇ is that it can operate with relatively higher efficiency even at partial loads such as 10% of nominal energy. This is particularly advantageous in some industrial applications where the input heat is in frequent motion and in Solar-Thermal applications where ORÇ can operate longer than steam turbines. Because in such applications, it can start working in the early hours of the morning when the sun rises and work until late in the evening when the sun sets.
6) Water use is not required as the working fluid is used instead of water / steam. In addition, there is no need for non-mineral water or other auxiliary systems that are usually required in steam systems. Therefore, the operation and control of the entire system is easier.
7) In ORÇ, the turbine rotates at a lower speed compared to the steam turbine, which reduces the mechanical stress. It also allows a direct link between the turbine and the generator through the reduction gear system and its associated parts as an intermediary.
8) High turbine efficiency (up to 85%) can be achieved with ORÇ.
9) In addition, ease of start-stop operations, automatic / continuous start-up, safe and quiet operation, high field compatibility (98% and more are not uncommon), and long life (20+ years) without serious maintenance are among other advantages.
10) In special cases where the generated energy is in the low MW range, ORÇ has a special economic advantage over steam turbines. Because the steam turbine requires an additional environmental system; This system is added to the cost and this is not small in economic terms.