Power Tower Questions

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Paul Gilman
Power Tower Questions

1. For the molten salt model, does the turbine inlet temperature vary as a function of the thermal input to the power block? In the Wagner thesis, it was not clear if the temperature was updated at the end of his iteration loops. In the DSG model, it seems that we can set the superheat.

In the molten salt power tower, the tower is operated to always meet its target outlet temperature by decreasing the mass flow rate. The the power cycle inlet temperature can be less than the receiver outlet depending on TES thermal losses. For most practical designs those losses should be minimal, and the power cycle inlet HTF temperature should be nearly constant. The direct steam receiver is controlled in the same manner. Because there is no storage, the turbine inlet temperature is exactly equal to the tower outlet temperature. 

2. For the molten salt and direct steam tower models, we note that the same default power block efficiency is used: 41.2 %, even though the turbine inlet pressure and temperature are different. Is there a reason for that? In comparing these systems, we assume that the design efficiency will vary with the turbine inlet pressure and temperature and plan to correct for that with a thermodynamic calculation, but assume the same nominal turbine efficiency – that is, each system has a turbine specifically designed for it. Does this seem reasonable in your experience?

 Yes, your approach seems reasonable. The power cycle efficiency should represent the efficiency expected under design conditions: hot-side temperature(s), upper pressure, ambient temperature, cooling temperature difference, and thermal input. The assumption is that the cycle can be designed to achieve this efficiency (by varying turbine efficiency, for example). The model then considers the hot-side temperature and mass flow rate and ambient temperature to calculate the cycle performance relative to the design performance – the power cycle performance model is non-dimensional.

3. We note that in the direct steam model, the balance of plant cost is zero, while in the molten salt tower model, the BOP cost is nonzero. Can you confirm that this is because no storage is included for DSG and that all the balance of plant components in your cost model relate to the storage portion?

The balance-of-plant cost represents the cost of the steam generation train, which is the heat exchanger that transfers thermal power from the hot molten salt to the steam power cycle. In the direct-steam tower model, the direct-steam receiver essentially provides this functionality.

Best regards,
Paul.

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