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Experiments

Temperature, flow, and pressure data from the HTF and water loops are being used in the unsteady state energy balance and the heat transfer relations making use of a multivariable correlation program to estimate

  • the optical efficiency of the receivers, the fraction of the incident normal solar on the reflector reaching the optically coated pipe absorber, α.
  • the effective thermal capacity of the receiver, MCp.
  • the two coefficients empirically relating thermal loss from the receivers to the temperature difference between the transfer fluid and the ambient and in addtion the square of that difference, b and c.
  • the heat transfer UA values for the balance of the loop and for the heat recovery and the load test exchangers.

These are the parameters in the unsteady state energy balance equation for the receivers, and they are key parameters in both fundamental and Trnsys modeling of the system’s performance in both heating and cooling.  Their values are now being estimated by multiple variable correlation based on the numerous measurements of the incident normal solar radiation, I; the incident angle cosΘ; and the heat collected in the circulating transfer fluid, mcp*ΔT.

Previously, a significant amount of scatter was observed in the values of the various parameters determined by the data correlation program.  And their values did not seem reasonable.  But when the loop data obtained when incident solar radiation was set to zero by reflector defocus were excluded from the correlation program the values of the parameters were consistent and reasonable.  We have concluded that the diversion of incident solar radiation results in continued heating of the flowing solution by thermal energy stored in the structure of the receiver.  This heating was not accounted for in the energy balance model used in the correlation program.  Further thought will be given this matter.

The values of the optical efficiency and the heat coefficients from the correlation program making use of data for obtained with positive input incident solar radiation are in reasonable agreement with the calculations of the solar thermal receiver model previously programmed.  But the overall heat capacity of the receiver, estimated from the unsteady state performance of the receiver in heat up is significantly larger, by a factor of 3 or more, than the estimated value.  This anomaly is being investigated.