Abstract
A novel method proposed to choose the optimal working fluid—solely from the point of view of expansion route—for a given heat source and heat sink (characterized by a maximum and minimum temperature). The basis of this method is the novel classification of working fluids using the sequences of their characteristic points on temperature-entropy space. The most suitable existing working fluid can be selected, where an ideal adiabatic (isentropic) expansion step between a given upper and lower temperature is possible in a way, that the initial and final states are both saturated vapour states and the ideal (isentropic) expansion line runs in the superheated (dry) vapour region all along the expansion. Problems related to the presence of droplets or superheated dry steam in the final expansion state can be avoided or minimized by using the working fluid chosen with this method. Results obtained with real materials are compared with those gained with model (van der Waals) fluids; based on the results obtained with model fluids, erroneous experimental data-sets can be pinpointed. Since most of the known working fluids have optimal expansion routes at low temperatures, presently the method is most suitable to choose working fluids for cryogenic cycles, applied for example for heat recovery during LNG-regasification. Some of the materials, however, can be applied in ranges located at relatively higher temperatures, therefore the method can also be applied in some limited manner for the utilization of other low temperature heat sources (like geothermal or waste heat) as well.
Highlights
For converting the heat of low-temperature heat sources into electricity, the Organic RankineCycle (ORC) is often applied [1,2]
For a given heat source—heat sink pair one can select a real, one-component working fluid from a database [14], with an ideal adiabatic expansion process starting from a saturated vapour state and terminating in a saturated vapour state, utilizing the “belly” of the reverse S-shaped saturated vapour branches of various materials
Due to the temperature range covered by the T-s data, these fluids should be proper candidates mostly for cryogenic cycles, but after proper expansion, the database can be used for other temperature ranges as well
Summary
For converting the heat of low-temperature heat sources into electricity, the Organic Rankine. For a given heat source—heat sink pair one can select a real, one-component working fluid from a database [14], with an ideal adiabatic (isentropic) expansion process starting from a saturated vapour state and terminating in a saturated vapour state (or at least in the vicinity of this state), utilizing the “belly” of the reverse S-shaped saturated vapour branches of various materials In this way, one can use the simplest ORC layout, consisting of only a pump, two heat exchangers (evaporator, condenser) and an expander, avoiding the use of superheater or droplet separator and a recuperative heat exchanger (recuperator). Due to the temperature range covered by the T-s data, these fluids should be proper candidates mostly for cryogenic cycles, but after proper expansion, the database can be used for other temperature ranges (like geothermal, solar or waste heat applications) as well
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