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Since the FPSHP operates at essentially the suction pressure of the heat pump (Fig. 1), the engine section must be able to offer satisfactory power and efficiency over all possible suction pressures. The two pressures representing likely limits of the suction pressure are estimated from the ISO 13256-1 standard for winter and summer operation using CO2 as the working fluid. These are 36.7 bar (abs) for winter (heating) and 42.8 bar (abs) for summer (cooling) as shown in Table 2. Operation over such a broad pressure range results in an inevitable frequency change of about 5 Hz over the nominal 60 Hz. In order to retain engine tuning over this range of pressure, the displacer is resonated by a gas spring rather than the mechanical springs typically used. As the engine starts up, its working gas may be more dominantly CO2. After a short while, the working gas mixture settles out to about a 50/50 mole mixture of CO2 and He. In addition to this, the power must be modulated in order to meet the requirements of the compressor. It is therefore important to understand how the engine’s performance and dynamic operation changes over these various conditions. A simulation procedure was used to analyze the different operating conditions and presents them in a performance map from which an arbitrary operating condition can be inferred. Two maps for heating and cooling operation are shown in Fig. 5. From these figures it can be seen that the working gas mixture and piston amplitude define both power and efficiency for particular charge pressure and temperature conditions. Note that temperature is not used as a primary control for power. As the piston stroke changes, so does the power and as a consequence, the heat input. The burner fuel input is then separately modulated to always keep the head temperature constant within a band of 10 to 20°C. 35 30 25 20 15 10 Heating Mode 50/50 60/40 70/30 100/0 6mm 8mm 10mm 4mm 2mm 0 500 1000 1500 2000 Power [W] 35 30 25 20 15 10 Cooling Mode 50/50 60/40 70/30 4mm 6mm 8mm 10mm 100/0 2mm 0 500 1000 1500 2000 Power [W] Figure 5: FPSE performance simulation for gas mixture (CO2/He) and piston amplitude (mm). 3.2 CO2 Cycle Heat Pump Compressor efficiency and He separation are discussed for the design condition described in Table 2. An experiment for He separation was conducted using a demonstrative R134a cycle. 3.2.1 Compressor calculation Compressor swept volume is determined by the requirement of 8 kW evaporation during the heating mode while piston stroke is set by the FPSE operation. This leads to a piston diameter for a given volumetric flow rate. As discussed above, the FPSE frequency varies between 60 and 65 Hz due to the different suction pressure for the heating and cooling mode and during capacity modulation. Compressor performance is generally set by volumetric efficiency, isentropic compression efficiency, and motor efficiency (Rasmussen and Jakobsen 2002). Since the compressor utilizes power directly from the FPSE, the motor efficiency is no longer a concern since it is not an intermediary. However, gas leakage must be accounted for because the dry gas bearings that support the compressor piston are subject to a certain degree of CO2 leakage across the piston. Since the He–CO2 mixture of the engine is returned to the compressor buffer space through the return device, He will enter the heat pump cycle and affects the 8 Efficiency EfficiencyPDF Image | HERMETIC GAS FIRED RESIDENTIAL HEAT PUMP
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CO2 Organic Rankine Cycle Experimenter Platform The supercritical CO2 phase change system is both a heat pump and organic rankine cycle which can be used for those purposes and as a supercritical extractor for advanced subcritical and supercritical extraction technology. Uses include producing nanoparticles, precious metal CO2 extraction, lithium battery recycling, and other applications... More Info
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