The Benefit of Variable-Speed Turbine Operation for Low Temperature Thermal Energy Power Recovery

This paper analyzes, given the large variation in turbine discharge pressure with changing ambient temperatures, whether variable-speed radial-inflow turbine operation has a similar benefit for Organic Rankine Cycle (ORC) power recovery systems as variable-speed centrifugal compression has for chiller applications. The benefit of variable-speed centrifugal compression over fixed-speed operation is a reduction in annual electricity consumption of almost 40 %. Air-conditioning systems are by necessity always designed for the highest possible ambient temperature. This is not necessary for ORC systems. Lower cost ORC systems are obtained when the design point is selected at moderate ambient temperatures. However, these systems show reduced power output at higher ambient temperatures. The more expensive ORC systems designed to achieve full power at higher ambient temperatures will produce constant power independent of ambient temperature but consume more heat and require a control mechanism to prevent overloading the turbine at lower ambient temperatures. The benefit of variable-speed ORC turbine operation over fixed-speed is an increase in annually averaged ORC power output and/or a reduction in annually averaged thermal heat input demand. However, the variable-speed benefit is as, will be explained in the paper, inherently smaller for ORC systems than for centrifugal chillers

Increasing the Stable Operating Range of a Fixed-Geometry Variable-Speed Centrifugal Compressor

Centrifugal compressors used on water-cooled chillers require stable operation over a wide range of flows at greatly varying pressure ratios. These operational requirements are dictated by variations in cooling demand and ambient conditions. Variable-speed centrifugal compressors are known to maintain their peak efficiency at the varying operating conditions much better than fixed-speed compressors. Replacing a fixed-speed centrifugal compressor with a variable speed one can reduce the annual energy consumption of a chiller by 40-45%. The majority of centrifugal chillers sold today are therefore inverter driven. Lower speed operation maintains and sometimes even increases compressor efficiency along a wide band of capacity and head combinations which fits quite naturally with most of the chiller operating requirements. However, the variable speed compressor will eventually surge when forced to operate at lower capacity while maintaining head. Some variable-geometry compressor features are necessary to enable stable compressor operation at these conditions. Variable-geometry inlet-guide-vanes and/or variable-geometry diffusers have to be added to variable speed centrifugal compressors to allow stable operation at all possible centrifugal chiller operating conditions. The inherent mechanical complexity of variable-geometry hardware has a negative effect on compressor cost and reliability. What is less appreciated is that compressor efficiency also suffers from variable geometry hardware. The inlet guide vanes introduce additional flow blockage and frictional losses at compressor inlet while the clearances needed for the movement of the variable geometry diffuser hardware introduce flow leakage passages resulting in parasitic flow leakage losses. Moreover, these losses affect compressor performance under all operating conditions, even those where variable speed control without variable geometry flow passage reduction results in stable compressor operation. This paper describes the application of the newly developed IntraFlowTM technology on a recently introduced two-stage variable-speed centrifugal refrigeration compressor. The concept will be explained in detail and test results will be shown. The compressor is stabilized and surge is postposed by injecting a small amount of flow upstream of the throat area of the vaned diffuser of the first stage compressor. The increase in stable operating range using this technique is substantially larger than what can be obtained with variable geometry inlet guide vanes. Using this technology the compressor also achieves higher efficiency due to the elimination of the blockage, friction and leakage losses that accompany the variable mechanical geometry surge/capacity control concepts. The amount of flow to be injected is controlled by an externally mounted flow control valve which increases reliability and serviceability

Geothermal ORC Systems Using Large Screw Expanders

Geothermal ORC Systems using Large Screw Expanders Tim Biederman Cyrq Energy [email protected] Abstract This paper describes a low-temperature Organic Rankine Cycle Power Recovery system with a screw expander a derivative of developed of Kaishan\u27s line of screw compressors, as its power unit. The screw expander design is a modified version of its existing refrigeration compressor used on water-cooled chillers. Starting the ORC development program with existing refrigeration screw compressor hardware has resulted in reduced development time and lower development cost. Lower equipment cost has been realized by assembling the screw expanders in parallel with the higher volume screw compressors. The net electrical output power of the Kaishan ORC screw expander varies from 5kW to 950 kWel which is an order of magnitude larger than the output power of currently available ORC screw expanders. A 300 kWel unit, using R245fa as its working fluid, has been installed and commissioned at Chena Hot Springs near Fairbanks, AK and 4 950 kWel units using R245fa have been started at Lightning Dock in New Mexico. Initial operating experience with these ORC systems will be presented. The paper will conclude with a comparison between screw and turbine driven ORC systems for low temperature waste heat power recovery

Comparing R1233zd And R245fa For Low Temperature ORC Applications

The majority of recently introduced ORC systems use R245fa as working fluid. The lower saturation pressures of R245fa versus R134a allowed the use of existing HVAC electric motors, compressors, evaporators and condensers after minor modifications as ORC electric generators, turbines/expanders, boilers and condensers. At elevated saturation temperatures R245fa turbine/expander equipment matches the power density of R134a HVAC compressors equipment. Refrigerants with still lower saturation pressures such as R123 and the newly developed low GWP fluid DR2 are excellent ORC working fluids but lack the synergy that exists in terms of power density between existing R134a compressors and their R245fa turbine derivatives. The use of these fluids in ORC applications prevents the use of existing HVAC compressor/motor hardware. Recently, a new refrigerant R1233zd has been introduced as a low GWP alternative for R245fa. This paper analyses the effect of this new fluid as a drop-in for R245fa into an existing 75 kW variable-speed, oil-free low temperature ORC system. The somewhat lower saturation pressure and vapor density of R1233zd allows a somewhat higher boiling temperature at maximum operating pressure, resulting in 8.7% higher cycle efficiency at equal capacity