Low Temperature Power Conversion Technologies:
Electricity generation from low enthalpy resources is most efficient in binary plants. The heat of the brine is transferred via heat exchangers to a working fluid, which evaporates at low temperatures..
Currently two binary cycles suitable for geothermal power plants utilizing low enthalpy resources are available on the market:
1. Organic Rankine Cycle (ORC) (i.e. a conventional Rankine cycle running with a specific working fluid evaporating below -22 °C)
2. Kalina cycle using working fluid consisting of a mixture of ammonia and water).
The use of a mixture as working fluid has the advantage that the heat can be transferred more efficiently between the
brine and the power plant 's working fluid; with improved efficiency. The Kalina cycle offers higher efficiency in the geothermal temperature range of 70-200oC but is only possible in a more complex plant. Additionally, Kalina cycle's corrosive working fluid requires special alloys throughout the plant and special bearings and seals within the turbine section, both of which add substantially to the cost. . This is one of the reasons why there are very few Kalina cycle plants in operation thus far.
In contrast there are numerous Organic Rankine Cycle(ORC) plants in operation worldwide. Plants using these cycles have more in common than not. Each plant's cycle can be optimized for the conditions prevailing there via design of the working fluid, the cycle and turbine designs, as well as the cooling system.
The Institute chose to develop a bi-modal ORC to gain the efficiencies of the Kalina cycle, using components readily available. Doing so reduced the delivery time for our turbogenerators from 36 to 14 months. Time is money.
The turbine used is in most plants, an axial inflow type. This is derived from the conventional steam turbine industry where axial turbines have been in use for many years, and their development has progressed very far, via inclusion of state of the art technology. Design parameters for turbines in cycles driven by low enthalpy resources vary decisively from those experienced by “classic” turbines in steam cycles (e.g. enthalpy drop, stream and rotor velocity).
Studies have shown that radial inflow turbines are superior under specific conditions, giving higher
efficiencies. Radial inflow turbines were originally developed for the petrochemical and refrigeration industries, with millions in service. Their reliability and characteristics are known. The company has chosen to utilize custom radial inflow turbines developed for it by Cryostar to maximize plant efficiency and output..
The working fluid of a power plant must be cooled to reduce back pressure on the turbine, improving power output, and reducing pumping losses. Cooling with air reduces plant output by 10% and increases costs by 15% compared to water cooling. Additionally, air cooled plants can often not guarantee maximum power from the plant throughout the year, particularly during the summer, and air cooling involves running fans which consume electricity, take space; and emit noise.
In contrast, water cooled plants have lower and more constant condensation temperatures and pressures. They have larger enthalpy drops in the turbine and thus higher efficiencies. The Institute's Solar Gradient Pond Plant design uses the top freshwater layer as a spray cooling pond, while the bottom hot brine layer drives the plant. Using a model from Consolidated Edison of Illinois, in which each of their plants is adjacent to a cooling water reservoir, the Institute recommends construction of reservoirs adjacent to each plant, to capture rainy season runoff for plant makeup and cooling water. The Institute recommends developers arrange with their local authorities, for administration of these as multi-use reservoirs, supplying not only their plants, but providing recreation, or water for an adjacent irrigation scheme..
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Public Research Institute, (PRI), is a state of the art provider of engineering services.
Wind Power Conversion Technologies:
The Institute commissioned a rotor with an efficiency of 96% of the theoretical maximum, and teamed this with a very efficient 3 phase permanent magnet alternator (PMA), in direct drive configuration. This eliminates the gear box and it's problems and expense, eliminates the need to synchronize generator speed to the grid frequency, and permits use of a simple stall controlled design. The Inverter is at ground level which minimizes maintenance at the top of the tower. The following chart compares the output of the PRI 32-100 turbine to the well known Enercon V33 turbine: