CSP trough - Molten salt annual energy compared to diathermic oil

Hello,

I'm running SAM for a feasibility study regarding a possible CSP trough power plant in Sicily, Italy. For the study I considered two options as HTF, molten-salts and Dowtherm A (as used in Andasol power plant in Spain).

Dowtherm A-HTF results could be considered appropriate, especially by looking at NREL case study about the sam power plant, while for molten-salt case the annual production is quite lower (127 GWh/year versus 182 GWh/year). Inputs parameters have been carefully selected using parametrics to optimize annual energy production, moreover for molten-salts case I used inputs as suggested in the NREL web-seminar for molten salt CSP trough.

Is is possible that molten salt with higher operating temperature produce so much less energy than Dowtherm A?. Also, regarding the dumped thermal energy I was expecting to have high values during summer (when collectors have to defocus due to high temperatures), instead dumped energy occur only during winter and mid-season for both case. Is there an explanation for this behaviour?

I've attached the SAM file I've been running together with Dowtherm A properties table and the hourly irradiation data for the specific location.

Dear Carlo,
Increasing the outlet temperature of a trough plant both increases the thermal losses from the receiver and improves the power cycle efficiency. This trade-off governs whether a high-temperature design produces more electric output than a lower-temperature design.
Because radiative losses are proportional to the fourth power of temperature, at some temperature these losses begin to dominate. Various companies are investigating designs to reduce trough thermal losses at higher temperatures. A comparison of the electric output between the high and low temperature plant designs is important, but perhaps the most significant proposed benefit of a molten salt trough is the potential reduction in storage and piping volume and the associated costs.
We found a few problems with the inputs in your SAM file:

1. 
   Sequential defocusing occasionally can result in model convergence problems. It looks like that is happening in the Dow case. We would recommend using Simultaneous defocusing to avoid this issue.
2. 
   For now, SAM only allows electric freeze protection, which results in a large parasitic load for the molten salt case. An actual plant would likely recirculate flow from the cold storage tank. This strategy still consumes some amount of thermal energy that could eventually be used to produce electricity, but the impact on net plant electric output is significantly reduced. We recommend either post-processing the results to reduce the freeze protection parasitic losses or approximating the effect by decreasing the freeze protection temperature input to one below the practical limit for molten salt.
3. 
   The difference in power cycle efficiencies between the high and low temperature cases appears to be less than expected. We recommend using Carnot scaling between the hot and cold designs. You should also probably use the same cooling system 'Ambient Temperature at Design' value for both cases unless you have a reason for doing otherwise.
4. 
   Please be sure that the variant weighting fractions on the Receiver page add up to 1.0. (We will add a check on this in the performance code in future versions.)
5. 
   The irradiation data you attached is not in one of the weather file formats SAM recognizes, so we reviewed your file using different solar resource data.

Best regards,

Paul.

Thank you Paul!