Abstract
This paper analyzes the performances of an evaporator for small scale waste heat recovery applications based on bottoming Organic Rankine Cycles with net output power in the range 2-5 kW. The heat recovery steam generator is a plate heat exchanger with oil as hot stream and an organic fluid on the cold side. An experimental characterization of the heat exchanger was carried out at different operating points measuring temperatures, pressures and flow rates on both sides. The measurement data further allowed to validate a numerical model of the evaporator whereas heat transfer coefficients were evaluated comparing several literature correlations, especially for the phase-change of the organic fluid. With reference to a waste heat recovery application in industrial compressed air systems, multiple off-design conditions were simulated considering the effects of oil mass flow rate and temperature on the superheating of the organic fluid, a key parameter to ensure a proper operation of the expansion machine, thus of the energy recovery process.
Highlights
Medium and low grade energy recovery might be accomplished in multiple contexts, such as cement and glass industry, heavy duty internal combustion engines as well as solar and geothermal power plants [1]
The evaporator is a key component of the Organic Rankine Cycle (ORC)-based energy recovery system since it has to guarantee a correct heat transfer during the pre-heating, vaporization and superheating of the high pressure organic fluid
An even more challenging situation occurs whether the energy recovery system worked at off-design conditions since vaporization and slight superheating of the organic working fluid must be anyway guaranteed for a proper mechanical energy recovery through the expansion machine
Summary
Medium and low grade energy recovery might be accomplished in multiple contexts, such as cement and glass industry, heavy duty internal combustion engines as well as solar and geothermal power plants [1]. A plate heat exchanger used as evaporator of an ORC system for waste heat recovery on an industrial air compressor was tested at different operating conditions using oil as hot stream and R236fa as cold stream. In order to maximize the heat exchanger efficiency, the device was thermally insulated such that the R236fa mass flow rate was calculated from an energy balance This indirect measurement led to a higher uncertainty on this quantity (Table 2). 1 2 3 1 4 2 5 3 6 4 7 5 8 6 9 710811912101311 12 13 test casetest case (b) cold side
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