Organic Rankine Cycle Plant

 

The developed and patented innovative screw expander can be used to replace the conventional turbines improving the isentropic efficiency up to almost 90% ensuring a leadership role in waste heat recovery systems and waste pressure recovery systems at low temperature and pressure. The system has a short payback period on the client’s investment and reduces CO2 emissions and heat to the atmosphere also ensuring an environmental success. The oil free screw expander generates power expanding steam or gases. Using this technology has led to the recovery of near to 100MW around the world.

Steam expander in operation

Our research trips covered six different countries and during that time we also had the opportunity to view 3 steam expanders systems of different capacities installed in three different plants. One of these plants actually had three units operating at different pressures (High pressure unit was not operating on that day due to no high pressure steam) At a chemical plant there was a 450kW gross (250 nett) steam expander being used as a let-down station (pressure reducing station).
The steam expanders can only operate with a maximum pressure of 30bar. This technology has the same characteristics of a steam turbine system, except the screw expander can operate with variable pressures while on line. The reduction in pressure translates to a reduction in electrical output, but the expander does not have to shut down due to any decrease in steam pressure.
The expander can accept saturated, dry or overheated steam as well any kind of gas with exception to O2 and H2.


Direct Expansion Applications.

  • Drop and control the pressure in steam and gas pipelines
  • Steam Thermal plants burning any kind of fuel
  • Flash steam expansion from Geothermal wells
  • Expansion and/or Condensate of wasted steam to recirculate hot water to a boiler.
  • Take advantage of any differential pressure in steam pipelines

 


Skid mounted screw steam expanders units in assembly

 

Steam expander housings

 

The ORC is used for the transformation of heat into clean electricity. We witnessed two different systems where waste gas from steel mills was being used to produce electricity and in one case heating for the staff showers.
The inclusion of a shell and tube heat exchanger within a waste heat flow was used as an evaporator. There a refrigerant passed through on the other side of the evaporator thereby evaporating into its gas state by the absorption of the waste heat. With the evaporation complete the gas pressure increased. Once the gas expands into the lubricated screw expander it generates movement and at the same time electricity.
The refrigerant is condensed and circulated via a fluid pump to flow back to the evaporator to continue absorbing waste heat and then generating electricity continuously.


Expansion process



Some advantages of a Screw expander are:

  • Positive displacement
  • Simple structure, less moving parts, reliable and cost effective
  • Wide application ranges

 

 

ORC unit



ORC Applications.
Power generation applications from hot fluids, liquids or gases;

  • Hot water of low temperature (800C and above) of cooling circuits or engines, machines or process
  • Geothermal hot water
  • Process hot fluids: Ethanol, gasoline, water, low pressure to condensate steam, thermal oil, chemicals etc.
  • Hot exhaust gases from combustion in gas turbines or internal combustion engines burning gas or diesel.
  • Hot gases from kilns and furnaces in industries like cement, glass, steel etc.
  • Solar thermal energy generated by hot water or steam
  • Any application where heat can be transferred to a fluid and is taken to the ORC




With both the steam and ORC systems various design layouts are available to produce the maximum electricity output from the waste heat. With Geothermal flash steam and steam applications a single plant can be installed or multiple units. A cascade system that allows for a steam expander followed by an ORC or ORCs.
Cascade systems are also available if the temperatures allow for it when a steam expander is being used as a pressure reducing valve. The condensing water can be cascaded to an ORC system and heat transferred through the ORC evaporator from the condensing water to the refrigerant.


Expander output

 


Expander housing


Tejinder Singh, Kevin Malpas and Ngaire Neal