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Rapport 2414173 - October 2007 __________________________________________________________________ In such a system, the pressure is therefore fixed by a control device so that the cycle operates under optimum conditions, yielding the best energy efficiency. Energy efficiency is defined as the amount of useful energy divided by the amount of input energy. Hence, in heating mode, the energy efficiency, or Coefficient of Performance is defined as : COP = HeatingDuty(kW ) Electric Power(kW ) Figure 7a : Duty vs. Electric power Figure 7b : Performance vs. Discharge pressure Figure 7a shows a cycle operated at a given value of high pressure. At this value, the system requires a heat input q at the evaporator and a work input w to the compressor. The delivered heat duty at the gas cooler is therefore q+w. Under these conditions, the R744 refrigerant at exit of the gas cooler reaches a temperature T. If the cycle is operated at a different values of high pressures and a constant value of temperature T, both q and w will vary, and so will the value of COP. Figure 7b shows the variation of COP when the high pressure is allowed to vary. It can observed that there exists a value of pressure (called "optimal pressure") for which the COP is highest. Thus, the most efficient heat pumps will use control schemes that enable high pressure control during operation. However, some systems also use simpler methods such as expansion through a capillary tube, or some simple control scheme that enables operation at fixed high pressure. As a rule of thumb, the optimal pressure is approximately given by the following equation : P =2.6T +8 opt exit __________________________________________________________________________________________ SHERPHA PROJECT - Transcritical R744 Heat Pumps – Technician's Manual 16/63PDF Image | Transcritical R744 (CO2) Heat Pumps 2017
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