Cost Effective Small Scale Orc Systems for Power Recovery from Low Grade Heat Sources

The growing need to recover power from low grade heat sources, has led to a review of the possibilities for producing systems for cost effective power production at outputs as little as 20-50kWe. It is shown that by utilizing the full potential of screw expanders instead of turbines, it is possible to produce Organic Rankine Cycle (ORC) systems at these outputs, which can be installed for a cost in the range of $1500 to $2000 /kWe of net output. This low capacity cost combined with the ORC’s fuel-free specification results in a very favorable value proposition. INTRODUCTION The idea of using organic working fluids, to replace steam, in power generation systems, has been traced back as far as 1823 [1] and the technology for power recovery from low grade heat sources, such as geothermal and waste heat streams, using Organic Rankine Cycle (ORC) systems, is now well established. However, most systems of this type have only been cost effective for power outputs of the order of 1 MWe or more. More recently, Brasz [2] has shown that centrifugal compressor driven air conditioning chiller units can be converted to ORC systems at a relatively low cost, mainly, by adapting the compressor to operate, in reverse, as a radial inflow turbine. By this means it is claimed that units of as low as 200kWe power output can be built and installed cost effectively. Since the thermodynamic efficiencies of these systems are generally only of the order of 10%, this implies that at least 2 MW of heat must be recoverable for such systems to be worthwhile. Low grade heat sources of this magnitude, which cannot be used for heating purposes more cost effectively, are relatively rare. Rising fuel prices and the wide availability of heat sources of the order of only 250 kW, mainly as waste heat from small IC engine generator sets, have opened a considerable market for units of the order of 20-50 kWe output, if these could be built and installed for an economic price. At such outputs, the use of turbines as expanders has many disadvantages, since they rotate at very high speeds, and hence require high ratio reduction gearboxes as well as relatively expensive lubrication systems. A closer examination was therefore made of alternative types of expanders and the most promising of these was found to be the twin screw type. The use of screw expanders in ORC systems has been proposed before [3] and a small number of these have been constructed and operated. One of the most widely publicized of these units was a geothermal plant in Birdsville, Australia [4]. In those cases, the plant designers lacked expertise in the engineering science required for the optimum design of such machines. Consequently, many of the potential advantages in their use were not realized. This paper describes the combined efforts of a US company that recognized the potential for small scale ORC units, and a UK university, where research and development on screw expanders and compressors has proceeded for over 25 years, to produce a low cost ORC system.