6.H3 VI.H.2: DESCRIPTION OF BRAYTON HEAT PUMP CYCLE The temperature (or enthalpy) versus entropy diagram for the ideal Brayton heat pump cycle is shown in Figure 6.Hl. The cycle consists of four processes applied to a gas. At the stages between the processes, the state of the gas can be described by its pressure and temperature. In state 1, the gas is at pressure p 1 , and has temperature T 1 . It is first isentropically compressed to state 2: pressure p 2 , temperature T 2 . It is then cooled at constant pressure, leading to state 3: pressure p 2 , temperature T 3 . The gas then passes through a turbine and is isentropically expanded to pressure p 1 ; the temperature drops to T 4 , lower than the starting temperature T 1 . This is state 4. The cold gas is used to absorb energy from the object or volume to be cooled, bringing the qas back to state 1. The power produced by the turbine when the gas expands through it helps an external power input to run the compressor. Figure 6.H2 is a schematic of the actual Brayton heat pump system proposed for the colony. The cycle uses air from the hull as its gas. This air, at phull and T 1 (state 1) is compressed to prad and T 2 (state 2). It is then piped into the external radiator and cooled to state 3: (prad'T 3 ). The air then expands to phull and T4 (state 4) in the turbine. The power Pturb produced helps Pthermal run the compressor. The cold air is returned to the hull and warmed there to state 1 again. Several ineffficiencies affect the actual cycle's 9erformance. The compressor cannot convert all its power input from Pthermal and Pturb into compressive work on the gas. The difference between this power input and the rate of work done on the air is called Pcomploss' and adds to the heat released into the hull. Similarly, the turbine cannot transfer all the power available from gas expansion to the compress~r- The difference Pturbloss also adds to the heat released into the hull. Finally, the pressure prad drops slightly between states 2 and 3; it is that small pressure difference between the inlet and outlet of the radiator which keeps the air flowing. However, careful radiator design can keep this difference small, and for the purposes of this analysis prad is considered constant.
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