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control volumes or for thermal losses through the walls of the tank; the TRNSYS Type 10 includes these effects. However during validation of this model with performance data obtained from the Solar One system it became apparent that a few modifications to the FORTRAN code were necessary to obtain agreement with the data. In particular, it was necessary to add thermal losses from the roof and floor of the tank, as well as the thermal inertia caused by the massive concrete foundation. These changes led to generally good agreement with Solar One data recorded during a discharge test [11] and during a multi-day cool down of the tank [12], as depicted in Figure 7. The thermocline within the storage tank is clearly visible, hot zone on top of cold zone with a thermal gradient in between. It can be seen that the slope of the thermal gradient predicted by TRNSYS during the discharge test is not quite as steep as the actual data. The reason for this difference is unknown. The validated model, labeled Type 502 in Figure 5, was scaled down to the size defined by Nexant and integrated within a TRNSYS model of the entire power plant. Discharge Test Distance from tank bottom (m) 16.5 Day Cooldown of Solar One Thermocline Storage Distance from tank bottom (m) Fig. 7 The cool down test was performed in November 1982. There was no flow into or out of the tank during the test. The TRNSYS model was given the Solar One initial conditions (Start). After 16.5 days we compare the temperature profiles. The discharge test was performed June 28, 1983. The flow remained constant during the 8 hr test. The TRNSYS model was given the Solar One initial conditions (Midnight). We compare the temperature profiles 4 and 8 hrs later. Test data has been corrected for flowmeter errors identified during the test. SIMULATION OF ANNUAL PERFORMANCE The TRNSYS model was used to estimate the annual performance of Saguaro given the Phoenix TMY2 hourly insolation and weather file4. Simulations of the plant with and without storage were performed. Since Saguaro will initially be operated without storage, a model of this configuration has near-term relevance. The TRNSYS model without storage is a subset of the components listed in Figure 5. The tank model (Type 502), storage control (STOCO), and the energy dispatch controller are removed. In addition, the solar trough model (Type 197) is modified to allow constant-flow operation rather than constant outlet temperature; with storage included in the design, constant temperature is needed to maintain the tank thermocline, but without storage, a simpler control strategy is warranted in which the HTF pumps run at a constant flow and outlet temperature is allowed to float. The annual turbine output predicted by TRNSYS is 2000 MWh; this is equivalent to an annual solar-to-electric efficiency of 7.8% and capacity factor of 23%5. This estimate is very close to the independent estimate by the plant builder, SolarGenix. An insight gleaned from the analysis is that annual output can be improved through monthly or seasonal changes in the HTF flow rate. As stated above, the plant operates at constant flow throughout the year (42300 kg/hr). On winter days, with relatively poor solar intercept by the troughs, this can lead to HTF temperatures that are much lower than the design point temperature of 300oC. Low temperatures delay startup and cause early trips of the ORC (need 190 oC) and reduces the output from the turbine after startup. The problem is depicted in Figure 8. The problem can be mitigated by changing the flow rate to more closely match the solar power intercepted by the troughs such that the peak operating temperature during the day achieves the 300 oC design point required by the ORC. Thus, flows in winter months would be set to a lower value than flows in the summer. 290 270 250 230 210 190 170 150 0.00 2.00 4.00 6.00 8.00 10.00 12.00 Sol 1 Midnight T (deg C) Sol 1 at 4 am TRNSYS at 4 am Sol 1 at 8 am TRNSYS at 8am 330.0 310.0 290.0 270.0 250.0 230.0 210.0 190.0 170.0 150.0 0.00 2.00 4.00 6.00 8.00 10.00 12.00 Start Solar 1 TRNSYS 1000 900 800 700 600 500 400 300 200 100 0 DNI (W/m2) HTF Temp (oC) 30 32 34 36 38 40 42 hr Fig. 8 TRNSYS prediction of solar field outlet temperature on January 2nd given constant-flow operation. The outlet temperature is higher than the ORC startup temperature of 190 oC for only a few hours. 4 Typical Meteorological Year (TMY) 2 files can be downloaded from the NREL website. The annual direct normal insolation (DNI) for Phoenix is 2.5 MWh/m2. 5 Annual efficiencies and capacity factors do not include losses due to plant parasitics or equipment unavailability. 7.8% = 2000 MWh/(2.5 MWh/m2 * 10300 m2). Annual capacity factor: 23% = 2000 MWh/(1 MW * 8760h). Saguaro’s efficiency (7.8%) is lower than standard SEGS plants (14%) because the efficiency of the power block is less, i.e., 20% for ORC vs. 37% for steam Rankine. 4 Copyright © 2006 by ASME Temperature (Co) Temperature (Co)PDF Image | PERFORMANCE ANALYSIS OF THERMOCLINE ENERGY STORAGE
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