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144 V. Minea / Applied Thermal Engineering 69 (2014) 143e154 2. Experimental set-up A 50 kW beta-prototype ORC machine was designed, built [9], and installed on a laboratory test bench [10e12]. The prototype includes a single-stage twin screw expander, a stainless steel condenser and pre-heater/evaporator heat exchanger assembly, a liquid receiver, and a working fluid variable speed circulation pump (WFP) (Fig. 1). The machine is scalable, allowing for the connection of multiple similar units to a single heat source if enough energy is available. If these units are installed in parallel, enough hot water flow rate for each machine is needed. If they are connected in series with one using hot water from the pre-heater outlet of the other, the heat source entering temperature will need to be high enough, as well. Routine maintenance is easy, and with a basic set-up, techni- cians with an HVAC and mechanical background can handle required maintenance. Additionally, the control system is fully automated, allowing for remote control and monitoring via an Internet connection, as well as off site maintenance. The small footprint, skid-mounted ORC-50 machine has been connected to a 700 kW electrical boiler, simulating the waste (or renewable) heat source, to an air-cooled liquid cooler, and to the Hydro-Québec electrical grid. The machine converts into electrical power the thermal energy recovered from the waste (or renewable) heat entering the machine in a liquid form (water) at temperatures varying from 85 C up to 125 C. The ORC-50 machine’s output power ranges from 20 to 50 electrical kW, depending on the inlet temperatures and flow rates of the source and sink heat sources, respectively. The system was comprehensively instrumented with thermo- couples, power and pressure transducers, flow meters for the working fluid, and both organic fluid, heat source, and sink thermal carriers. A data transmission system and associated analysis soft- ware were set up to monitor system operation. All parameters were scanned at 15 s intervals, then averaged and saved every minute, to help determine the cycle’s instantaneous and overall thermody- namic performance. 2.1. Thermodynamic cycle The low-pressure (p1) organic fluid leaving the condenser as a saturated or sub-cooled liquid (state 1) accumulates inside the receiver in equilibrium with its vapour phase (see Figs. 1 and 5). Then it enters the working fluid feed pump (WFP) where its pres- sure is adiabatically raised to the saturation (evaporating) pressure (p2) (state 2s), prior to entering the pre-heater/evaporator heat exchanger assembly. The multistage feed pump is driven by a var- iable frequency drive (from 0 to 60 Hz) in order to supply the flow rate required to achieve a relatively small superheating effect (i.e. below 5 C) at the evaporator outlet [13]. Also, between 10% and 15% of the organic fluid flow rate leaving the feed pump is injected into the expander at both the inlet and outlet ports. Since the exergy destruction rate in the feed pump is relatively small, the 1e2s process is considered as isentropic (adiabatic), and the power input is expressed as Wp 1⁄4 Wp;ideal 1⁄4 m_ OFðh2s h1Þ 1⁄4 m_ OFðp2 p1Þ (1) hp hp rhp where Wp,ideal is the feed pump ideal power input (kW), hp e the feed pump isentropic efficiency (90%), m_ OF e the organic fluid mass flow rate (kg/s), h1 and h2s e the organic fluid mass enthalpies at the inlet and outlet ports of the feed pump in the ideal case, respectively (kJ/kg), p1 and p2 e the organic fluid pressures entering and leaving the feed pump, respectively, and r e the organic fluid average mass density (kg/m3). On the other hand, the waste heat fluid carrier (water) enters the evaporator where it transfers heat to the working fluid during the evaporation and superheating pro- cesses (3e4e5). At the evaporator outlet, approximately 20% of the waste heat fluid (water) flow rate enters the pre-heater to partially or completely preheat the organic fluid from the sub-cooled liquid state (2s) up to (near) the saturated state (2a). The preheated organic fluid enters the evaporator at state 2a where it will vaporise and superheat at a constant pressure (process 2ae3e4e5). The pre- heater/evaporator thermal power recovered from the waste heat source is given by Q_ preheat=ev 1⁄4 m_ OFðh5 h2aÞ (2) where h5 and h2a are the organic fluid mass enthalpy leaving and entering the pre-heater/evaporator, respectively (kJ/kg). The pre- heater/evaporator assembly was designed and selected to opti- mally work with hot water flow rates in the range of 7.6e12 l/s (120e200 GPM). Its thermal efficiency versus size and cost was carefully analysed in order to achieve optimum pinch points defined as the differences between the corresponding waste heat (source) temperature and the temperature at which the organic fluid begins to vaporize [9]. The high-pressure saturated (state 4) or superheated vapour (state 5) leaving the evaporator enters the twin screw expander. It P; T C Hydro-Québec electrical grid Expander Outdoor air-cooled liquid cooler Cooling loop MV FM Evaporator 5 3 2a 4 SV Condenser Alternator 6a GP WP Waste heat loop FM 1 ORC-50 machine 700 kW eletrical boiler Preheater 2 (2s) Liquid injection WFP Receiver FM Fig. 1. Schematic representation of the experimental setup [9e11]. C e controller; FM e flow meter; GP e cooling fluid (water/glycol) pump; MV e mixing valve; P e pressure; T e temperature; WP e waste heat thermal carrier (water) pump; WFP e working fluid pump; SV e solenoid valve. 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