Organic Rankine cycle

The organic Rankine cycle (ORC) is named for its use of an organic, high molecular mass fluid with a liquid-vapor phase change, or boiling point, occurring at a lower temperature than the water-steam phase change. The fluid allows Rankine cycle heat recovery from lower temperature sources such as biomass combustion, industrial waste heat, geothermal heat, solar ponds etc. The low-temperature heat is converted into useful work, that can itself be converted into electricity.

Naphtha engines, similar in principle to ORC but developed for other applications, had been in use since the 1890s.

Working principle of the ORC

T-s diagram for the ideal/real ORC

The working principle of the organic Rankine cycle is the same as that of the Rankine cycle: the working fluid is pumped to a boiler where it is evaporated, passed through an expansion device (turbine or other expander), and then through a condenser heat exchanger where it is finally re-condensed.

In the ideal cycle described by the engine's theoretical model, the expansion is isentropic and the evaporation and condensation processes are isobaric.

In any real cycle, the presence of irreversibilities lowers the cycle efficiency. Those irreversibilities mainly occur:[1]

Improvement of the organic Rankine cycle

ORC with Regenerator

In the case of a "dry fluid", the cycle can be improved by the use of a regenerator: since the fluid has not reached the two-phase state at the end of the expansion, its temperature at this point is higher than the condensing temperature. This higher temperature fluid can be used to preheat the liquid before it enters the evaporator.

A counter-current heat exchanger (gas to liquid) is thus installed between the expander outlet and the evaporator inlet. The power required from the heat source is therefore reduced and the efficiency is increased.

Applications for the ORC

75-kilowatt ORC turbogenerator originally used at an experimental power plant in Kuopio, Finland.

The organic Rankine cycle technology has many possible applications, and counts more than 2.7 GW of installed capacity and 698 identified power plants worldwide.[2] Among them, the most widespread and promising fields are the following:[3]

Waste heat recovery

Waste heat recovery is one of the most important development fields for the organic Rankine cycle (ORC). It can be applied to heat and power plants (for example a small scale cogeneration plant on a domestic water heater), or to industrial and farming processes such as organic products fermentation, hot exhausts from ovens or furnaces (e.g. lime and cement kilns), flue-gas condensation, exhaust gases from vehicles, intercooling of a compressor, condenser of a power cycle, etc.

Biomass power plant

Biomass is available all over the world and can be used for the production of electricity on small to medium size scaled power plants. The problem of high specific investment costs for machinery, such as steam boilers, are overcome due to the low working pressures in ORC power plants. Another advantage is the long operational life of the machine due to the characteristics of the working fluid, that unlike steam is non eroding and non corroding for valve seats tubing and turbine blades. The ORC process also helps to overcome the relatively small amount of input fuel available in many regions because an efficient ORC power plant is possible for smaller sized plants.

Geothermal plants

Geothermic heat sources vary in temperature from 50 to 350 °C. The ORC is therefore perfectly adapted for this kind of application. However, it is important to keep in mind that for low-temperature geothermal sources (typically less than 100 °C), the efficiency is very low and depends strongly on heat sink temperature (defined by the ambient temperature).

Solar thermal power

The organic Rankine cycle can be used in the solar parabolic trough technology in place of the usual steam Rankine cycle. The ORC allows power generation at lower capacities and with a lower collector temperature, and hence the possibility for low-cost, small scale decentralized CSP units.[4]

Choice of the working fluid

The selection of the working fluid is of key importance in low temperature Rankine Cycles. Because of the low temperature, heat transfer inefficiencies are highly prejudicial. These inefficiencies depend very strongly on the thermodynamic characteristics of the fluid and on the operating conditions.

In order to recover low-grade heat, the fluid generally has a lower boiling temperature than water. Refrigerants and hydrocarbons are two commonly used components.

Optimal characteristics of the working fluid :

Since the purpose of the ORC focuses on the recovery of low grade heat power, a superheated approach like the traditional Rankine cycle is not appropriate. Therefore, a small superheating at the exhaust of the evaporator will always be preferred, which disadvantages "wet" fluids (that are in two-phase state at the end of the expansion). In the case of dry fluids, a regenerator should be used.

Unlike water, organic fluids usually suffer chemical deteriorations and decomposition at high temperatures. The maximum hot source temperature is thus limited by the chemical stability of the working fluid. The freezing point should be lower than the lowest temperature in the cycle.

A fluid with a high latent heat and density will absorb more energy from the source in the evaporator and thus reduce the required flow rate, the size of the facility, and the pump consumption.

The main parameters taken into account are the Ozone depletion potential (ODP) and the global warming potential (GWP).

The fluid should be non-corrosive, non-flammable, and non-toxic. The ASHRAE safety classification of refrigerants can be used as an indicator of the fluid dangerousness level.

Examples of working fluids

Modeling ORC systems

Simulating ORC cycles requires a numerical solver in which the equations of mass and energy balance, heat transfer, pressure drops, mechanical losses, leakages, etc. are implemented. ORC models can be subdivided into two main types: steady-state and dynamic. Steady-state models are required both for design (or sizing) purpose, and for part-load simulation. Dynamic models, on the other hand, also account for energy and mass accumulation in the different components. They are particularly useful to implement and simulate control strategies, e.g. during transients or during start & stop.

Another key aspects of ORC modeling is the computation of the organic fluid thermodynamic properties. Simple equation of states (EOS) such as Peng–Robinson should be avoided since their accuracy is low. Multiparameter EOS should be preferred, using e.g. state-of-the-art thermophysical and transport properties databases.

Various tools are available for the above purposes, each presenting advantages and drawbacks. The most common ones are reported hereunder.

Tool Causality Distribution Examples available online Description
Steady-state Modeling tools:
Engineering Equation Solver Acausal Non-free Simple ORC Model in EES Popular equation-based solver that includes a database of fluid thermodynamic and transport properties.
AxCYCLE Software Platform Acausal Non-free Software Details For the thermodynamic simulation and heat balance calculations of heat production and electric energy cycles, the AxCYCLE™ software platform allows users to design, analyze and optimize thermodynamic systems quickly and efficiently.
MATLAB Causal Non-free High-level language and interactive environment for numerical computation, visualization, and programming
LMS Imagine.Lab Amesim Causal Non-free Graphical development environment and validated, packaged physical libraries for system simulation
Scilab Acausal Open-source Simple ORC model Open-source alternative to Matlab.
Cycle-Tempo Causal Non-free Tool for the thermodynamic analysis and optimization of systems for the production of electricity, heat and refrigeration
Dynamic modeling tools:
Modelica Acausal Open-source Dynamic model of a waste heat recovery system Object-oriented, declarative, multi-domain modeling language for component-oriented modeling of complex systems
Simulink Causal Non-free A block diagram environment for multidomain simulation and Model-Based Design
LMS Imagine.Lab Amesim Causal Non-free Graphical development environment and validated, packaged physical libraries for system simulation
Thermophysical and transport properties of organic fluids:
CoolProp / Open-source Cross-platform, free property database based in C++ that includes pure fluids, pseudo-pure fluids, and humid air properties.
Refprop / Non-free Reference Fluid Thermodynamic and Transport Properties Database
FluidProp / Free Software for the calculation of thermophysical properties of fluids
AspenProp / Non-free Software for the calculation of thermophysical properties of fluids

See also

External links

References

  1. ↑ "Sustainable energy conversion through the use of Organic Rankine Cycles for waste heat recovery and solar applications" (PDF). Retrieved 2011-10-31.
  2. ↑ T. Tartiere. "ORC World Map". Retrieved 16 August 2016.
  3. ↑ "Techno-economic survey of Organic Rankine Cycle (ORC) systems" (PDF). 2013. Retrieved 2013-03-02.
  4. ↑ "Solar micro-generator". Stginternational.org. Retrieved 2010-09-15.
  5. ↑ http://www.turboden.eu/en/public/downloads/ORC_fluid_selection.pdf
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