The Organic Rankine Cycle (ORC) was invented by the Scottish Professor William Rankine in the mid 1800's. The ORC cycle is a simple thermodynamic cycle with an organic fluid as a working media.
Entrans´ products are based on the ORC technology for waste heat recovery at low temperatures (from 55oC to 220oC) for production of electric and mechanical power.
The ORC process
The ORC cycle works in the same way as a conventional power plant (which uses the Rankine cycle) with a boiler, turbine, condenser and feed water pump. The major differences between the systems are the working medium and the fact that the ORC system can technically be much simpler built, due to lower operation temperature and the fact that the working fluid is not corrosive.
In the ORC circuit an advanced boiler can be replaced by a simple heat exchanger. The figure below shows the ORC cycle principle.
1. The waste heat for preheating, from about 30°C and above
2. Preheating of the working medium, heat sources at low temperature can be used
3. Waste heat for preheating and evaporation, from 60°C and above
4. Evaporation of the working medium at high pressure
5. Expansion of the working medium in the expander (turbine) to mechanical power
6. Condensation of the working medium at low pressure
7. Heat sink, e.g. sea water
8. Pump, the working medium is pumped from the condenser to the evaporator
Using an organic working medium which has a low boiling point makes it possible to use low temperature heat for electricity generation. During the phase change from liquid to gas, large amounts of heat is absorbed by the working medium. During evaporation the pressure is high and the steam volume per absorbed kJ of heat is small, which means that the bulk volume of the ORC units can be kept down. Production plants that use water as working medium at low temperature must be large, because the steam volume is large and the pressure is low. Some examples are the condenser in a condensing power plant or absorption chillers (Li-Br).
Power output
The power output of the ORC system is mainly a consequence of the temperature difference between the condensation and evaporation temperature. Other important factors are the working medium used and how the expander is designed. For small temperature differences the power output is low, about 5-10%. The fuel is waste heat and usually free.
The chart below shows the electric power output in relation to evaporation- and condensation temperature. If the heat sink and the heat source are liquids, then the evaporation temperature is about 10-13°C lower than that of the heat source and the condensation temperature is about 7-12°C higher than that of the heat sink.
Heat sink - more important than the waste heat temperature
In the ORC process the temperature of the heat sink is very important. The lower the temperature of the heat sink (cooling water or other medium), the better the efficiency. If there is access to a heat sink of e.g. -60°C, then water at 0°C and ice formation heat can be used as heat source. Heat sinks that are colder than their surrounding is an inaccessible resource that is equally or more valuable than the waste heat source.
Preheating of the working medium in the ORC system
Of the supplied heat to the ORC cycle, a large portion of it goes to the preheating phase of the working medium, approximately 30-60%. The working medium is preheated from condensation temperature to evaporation temperature, typically from about 20-30 °C to about 70-100°C depending on current conditions. For preheating, heat sources at very low temperature can be used, from about 30°C and above. It is therefore important for Entrans to take note of all available waste heat sources during design of each system.
The log p-h diagram below shows the ORC cycle using refrigerant R134a. Heat is supplied from point 1-4. Between points 1-3 preheating of the working medium in liquid phase occurs.
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