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density and increases in viscosity that occur above the dome as well, generally along the pseudo critical line shown as the blue dashed line in Figure 2-9. These increases are large. The heat capacity increases by over a factor of 10-30 depending on the pressure. This effect also increases the heat transfer coefficient because the heat transfer near the critical point is nearly a two-phase vaporization/condensation process and hence occurs very nearly at a constant temperature. This therefore causes a very low dT in the heat rejection gas-chiller. Because this small temperature change (dT) makes the cycle more “Carnot” like, it also results in and efficiency increase. The effect of the pseudo-critical point is clearly seen in the operating state point values shown in Figure 2-2. Here the temperature drop in the gas chiller (which is rejecting 50.2 kW) is only 0.7 K due to the large increase in heat capacity. Again this illustrates the very high power density available in S-CO2 systems. These phenomena reveal two important practical control issues: 1. a very large amount of heat transfer is required to change the temperature, even by a little as 1 K (in the gas chiller) when the CO2 is near the critical point, and 2. it will be very difficult to measure or to know the real fluid temperature to within a few tenths of a degree, let alone control it to within a few tenths of a degree K. There are other factors that one wishes to control or manage to avoid potential operational difficulties. A control system is desired that will avoid allowing the compressor inlet conditions from entering the liquid–vapor dome of the T-S diagram. It is generally thought that pumping a two phase mixture will be more difficult for axial compressors, which is one of the reasons why the SNL compression loops use radial compressors. Even though the average density changes in the compressor are not large, it is recognized that entering the dome will result in phase changes that may cause unanticipated behavior. One goal of the test-loop is to have sufficient control of the compressor inlet temperature and inlet pressure to avoid entering the liquid-vapor dome or to purposely enter it, when desired. This will be done primarily by controlling the CO2 inventory in the loop and by controlling the compressor inlet temperature. Some tests were intentionally operated in the two phase region and the results are described later in this report. The Sandia S-CO2 compression loop is being fabricated with an inventory control system that can vary the fill gas pressure so that a relatively larger range of compressor inlet pressures (5.0-9.2 MPa) can be explored. We will fill the loop from a series of gas bottles connected through a booster pump. Solenoid valves will be used to bleed gas into or from the loop to control the fill inventory. The control points 3 & 4 (see red numbers in Figure 2-2) indicate the fill and bleed control points. Two mechanisms are used to control the compressor inlet temperature that we wish to vary from 290- 318K. One is the speed at which the cooling water flows (control point 5 in Figure 2-2), and the second is the recirculation of warm water through the water control valve 6. In addition a heater to preheat the water has only been recently added to the loop. It has not been fully tested yet. It will be used to coarsely control the feed water temperature from the evaporator cooler. As already indicated, we expect that small changes in CO2 temperature will require large changes in heat removal capability. Heat removal and temperature control is further complicated by the fact that at Sandia the waste heat removal system uses a large evaporative cooler, with a large amount of coolant volume (400 gallons) at 21PDF Image | Operation and Analysis of a Supercritical CO2 Brayton Cycle
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