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accelerates a 50 kW (460 V) asynchronous induction generator (alternator) connected to the local electrical grid. Control of the evaporator exit dryness fraction is achieved with a microprocessor that generates signals from the pressure and temperature inputs to vary the feed pump speed and thus provide proper superheat levels [14]. Inside the screw expander, the working fluid expands creating pressure and temperature drops at the exit port, thus converting the thermal energy of the high pressure vapour into mechanical work. Without any liquid injection, the organic fluid would leave the expander at state 6 but, because of this process, it actually leaves the expander at state 6a, closer to the vapour saturated wet curve. Fig. 5 shows that the increased wetness of the organic fluid at the expander inlet reduces the superheat at its exit port. As the vapour wetness at the expander inlet increases, it even allows the working fluid to leave the expander as wet vapour. Since less heat is required per unit mass flow to evaporate the working fluid only partially, given the same heat supply, the mass flow rate of the organic fluid is thus increased. Also, reducing the need for desu- perheat after expansion tends to increase the specific enthalpy drop in the expander. Both these effects tend to increase power output. The actual expansion process (5e6) in the expander is a non- isentropic, non-reversible process which allows for dry expan- sion, thus preventing the vapour from condensing or forming droplets. The entropy increases, thus decreasing the power gener- ated when compared with the ideal (isentropic) process 5e6s. This means that the efficiency of the heat-to-electricity transformation process in the expander could never reach its full value (100%). During machine start-up, the asynchronous induction generator is first mechanically brought to near its synchronous speed (1800 rpm for 60 Hz) via the expander. For less than 100 ms there is a small inrush current spike at grid connection, but not higher than the generator’s Locked Rotor Amps. Then, the generator is con- nected to the grid being driven slightly above the synchronous speed (i.e. at about 1840 rpm for 60 Hz). Integral power factor correction capacitors are used to improve the inherently low power factor of the induction alternator. In the event of a grid loss, the unit will automatically shut down, and cannot be re-started until line conditions return to normal [8]. The power generated by the twin screw expander can be calculated with one of the following formulas: (3) where Wexp,ideal is the expander ideal power, hs 1⁄4 ðh5 h6Þ=ðh5 h6sÞ e the expander isentropic efficiency (see Fig. 5), hm e the mechanical efficiency, h5 and h6s e the working fluid enthalpies at the inlet and outlet ports of the expander in the ideal case, respectively (kJ/kg), cp e the isobaric specific heat (kJ/ kgK), T5 e the expander inlet temperature (C), p e the expander pressure ratio (p5/p6), n e the rotation speed of the expander shaft, g e the polytropic exponent, and G e the expander torque. Generally, the isentropic efficiency of turbines/expanders can reach 80% [15,16], but for such a relatively small twin screw expander, a design isentropic efficiency (hs) of 70% has been considered reasonable [12]. The 40 HP/460 V asynchronous alternator of the ORC-50 machine is linked to the 25 kV Hydro-Québec electrical grid via a 25 kV/600 V transformer with a nominal power of 2.5 MVA. The working fluid enters the condenser at state 6a, closer to its saturated state, where it is condensed at constant pressure and temperature (process 6a-1) to become a saturated or sub-cooled liquid (state 1). The condensing (latent) heat (enthalpy) is trans- ferred from the vapour to the cooling fluid circulating within a water/glycol (50% by weight) closed loop linked to an air-cooled liquid cooler (see Fig. 1). The condenser thermal power is expressed by the following equation: Q_cd1⁄4m_OFðh6ah1Þ (4) where h6a and h1 are the organic fluid mass enthalpies entering and leaving the condenser (kJ/kg). The cooling fluid flow rate in the condenser varies between 11.68 and 14.28 l/s (180e220 GPM) in order to keep the pre-set inlet temperatures constant between 15 C and 30 C. The condensed organic fluid at state 1 is stored inside the liquid receiver, pumped back to the pre-heater/ evaporator assembly, and then a new cycle begins. The organic fluid pressure drops within the condenser, pre-heater and evapo- rator to about 30, 10 and 20 kPa, respectively. 2.2. Working fluid Depending on the operating temperature ranges of ORC ma- chines, the proper selection of the organic fluid may help reduce cycle thermodynamic inefficiencies and achieve higher energy conversion efficiency rates and lower capital costs [14]. Several fluids such as HCFC and HFC pure and azeotropic mixture refrigerants [16,17], benzene and toluene [18], were considered in the past as working fluids for low- and medium- temperature ORC cycles. For the ORC-50 machine under study, the HFC-245fa refrigerant, which has a lower boiling point than water, a relatively high molecular mass, and a high enough critical temperature (154 C), was selected as a dry organic fluid. The main characteristic of such a fluid is that the slope of its saturation vapour curve is positive, which allows for dry expansion, thus preventing the vapour from condensing or forming droplets. HFC- 245fa is also a low-pressure, high-temperature, non-corrosive, non-flammable, low-toxicity, and environmentally safe fluid. Its Ozone Depletion Potential (ODP) and Global Warming Potential (GWP) are 0 and 1900, respectively, and it has a B1 ASHRAE safety classification [19]. It also meets other operational, thermo-physical and environ- mental requirements, as a high stability, to avoid chemical deteri- oration and decomposition at the highest operating temperature, and low toxicity, explosion and flammability characteristics [20]. It must be safe (for human health), non-corrosive, compatible with common engine materials and lubricating oils, and available at affordable costs [21]. Organic fluids must also have a high density and latent (vaporization) heat to absorb more energy from low- grade waste heat sources, and thus provide higher power out- puts. Another important parameter is the fluid critical point, which has to be above 300 K (w27 C) in order to be able to reject heat into the ambient air during the condensation process. 2.3. Expander Since standard turbines may rotate at very high speeds at low power output levels, requiring high ratio reduction gearboxes and relatively expensive lubrication systems [22,23], the ORC-50 pro- totype was equipped with a patented twin screw expander with simple bearing lubrication [8]. It operates at low speeds and is directly coupled via a belt to an asynchronous alternator (gener- ator) without any intermediate reduction gearbox, which provides efficiency advantages over conventional turbines [13,14,24,25]. The patented twin screw expander consists of a pair of meshing helical rotors located inside a casing. The spaces between the lobes and the casing form together a series of rotating chambers. When the organic fluid is admitted, its volume will increase, while power is transferred between the fluid and the rotor shafts from the tor- que created by the forces acting on the rotor surfaces due to the Wexp 1⁄4 Wexp;ideal *hs hm 1⁄4 m_ OF ðh5 h6s Þhs hm h 1gi 2pn 1⁄4m_OFcphsT51pg 1⁄460G V. Minea / Applied Thermal Engineering 69 (2014) 143e154 145PDF Image | Power generation with ORC machines low-grade waste heat
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