Optimal upper solar field temperature

Looking at the sensitivities in Fig. 16.7, it is obvious that an increase tem­perature up to the assumed limit of 393°C is beneficial. The upper tempera­ture limit was chosen according to material stability restrictions for the thermal oil used (Solutia, 2008). Disregarding material stability issues, an interesting question is up to what temperature this LCOE trend can be extrapolated until the heat loss in the collector compensates gains on the power block side from further temperature increase. This can give indica­tions for the optimal operating temperatures when using other heat transfer fluids such as molten salt or direct steam generation. Often, it is claimed that an increase in operating temperatures increases plant efficiency and plant economics. This assessment shows that this statement is only valid for temperatures up to 500°C in a state-of-the-art parabolic trough plant. However, operating temperatures of 540°C and above which are usually applied in fossil steam plants are not beneficial here.

Figure 16.12 shows the influence of the solar field operating temperature on energy production and LCOE. Again, the starting point was the opti­mised design from optimisation 1 (cf. Fig. 16.7). For better visual compara­bility, 390°C instead of 393°C was chosen as the reference temperature in Fig. 16.12. Both LCOE and net plant efficiency (in this context equivalent to Eel, net) show their optimum for 500°C upper fluid temperature. Whereas power block efficiency increases in the assessed temperature range up to 640°C, the efficiency of the collector (equivalent to Qth, sol. pot.) decreases progressively because radiative thermal losses become significant for high

temperatures. When looking at the two dotted functions, it is apparent that not only collector and power block influence the optimal operating tem­perature: the difference between the collector efficiency (equivalent to Qth, sol. pot.) and QPB is attributed to the storage. Storage to some extent com­pensates the collector efficiency decrease because its energetic capacity increases (physical storage size assumed constant). The energy stored in 1 kg of storage medium increases if it is not only heated by 100 K (up to 380°C) but by 150 K and more. This leverage effect of storage in favour of higher temperatures can also be noted when comparing Eel, net and the product of collector efficiency and power block efficiency (dotted line with ‘+’), the latter corresponding to the gross electricity production of a PTC plant without storage. For both, the optimum is 500°C, but the storage configuration benefits more from higher temperature: increasing the upper solar field temperature from 390°C to 500°C will lead to an LCOE improve­ment of 6.3% (with storage). Without storage, this benefit is only 4.0%.

The effect of increased solar field outlet temperature on storage is assessed in Fig. 16.13. Assuming the same fixed storage size in tons of storage medium, the energetic storage capacity increases linearly with the upper solar field temperature because a higher AT between hot and cold storage medium increases energetic storage capacity. According to the assumed temperature model, storage design temperatures scale directly with solar field temperature. The increase in energetic storage capacity also

implies reduced storage investment in €/MWh and reduced dumping of solar thermal energy due to full storage from initially 4.2% for 390°C down to 0% for HTF temperatures above 600°C.

The calculated optimal temperature of 500°C will be lower:

• for sites with lower DNI (e. g. for European sites) because relative heat loss will be higher

• for higher receiver heat loss than assumed (PTR70 from the company Schott; Burkholder and Kutscher, 2009), e. g. with part of the receivers being degraded or with other (older) receivers showing higher heat loss.

• for plants without storage.

To summarise, in a temperature range where costs can be assumed inde­pendent of temperature, the optimal operating temperature is an efficiency optimisation problem between power block efficiency and solar field effi­ciency. Other factors that show an influence on optimal operating tempera­ture are energetic storage capacity and solar field parasitics.