Interest in low-grade heat recovery has grown in recent years. A number of approaches have been proposed to capture and convert low-grade heat energy, traditionally viewed as not commercially useful, into productive electric power. Among the proposed solutions is the use of ORC technology.
ORC is a vapor power cycle named for its use of an organic, high molecular mass fluid with a liquid-vapor phase change taking place at a lower temperature than the traditional fluid of water-steam. This lower temperature phase change is what allows for the capture and conversion of low-grade heat from wasted thermal energy to usable electrical energy.
Besides the use of an organic compound as the working fluid, ORC functionally resembles the steam cycle power plant:
a pump increases the pressure of condensed liquid;
the high-pressure liquid is pre-heated via a recuperator exchanger;
the liquid is then vaporized by extracting waste heat from the heat source through a heat exchanger;
the high-pressure vapor expands in a turbine that drives a generator thus producing power;
and the low-pressure vapor leaving the turbine is de-superheated through the recuperator and condensed before being sent back to the pump to restart the cycle.
A key component of the cycle is the vapor expansion turbine. The maturity of this turbomachinery, in large measure, defines the commercial viability of an ORC solution. Today, small-scale positive displacement units in the tens or few hundreds of kW capacity are available to function as the ORC driver.
Additionally, oil and gas providers can, with little modification, supply commercially viable turboexpanders suitable for ORC applications at the larger sizes (>5-7MW). However, many of the available heat sources do not line up with this supply, and as a result, ORC applications that are in the middle scale of the above ranges have struggled to find their footing in the market place.
To satisfy this market need and supply ORC solutions in the 500kW-to-5MW range, an axial turbine has been developed that is capable of supporting applications with low grade heat sources found in geothermal projects, as well as moderately higher temperatures found in industrial waste heat segments.
More in March/April 2013 issue of Turbomachinery International