4. MODELLING AND RESULTS
4.2 Intra-technology simulation results
4.2.8 CLFR simulation
The CLFR plant system modelled in this section is described in Figure 4-7. This is a standard DSG CLFR configuration similar to the Novatec Biosol PE-1 plant in the Spanish town of Murcia, the variances being that this plant does not have steam storage and does use a backup fossil fuel boiler (Selig, 2009).
Figure 4-7: CLFR plant schematic
Version 2011.12.2 of SAM is the first to include a linear Fresnel model, and certain errors in the simulation were noted. For example the annual washing water result is zero despite a mirror washing regime being specified in the input. This result was calculated manually and included in the results table. The following sections describe the input parameters used to model a linear Fresnel plant under Upington’s weather conditions.
4.2.8.1 The solar field
The linear Fresnel model also allows the solar field to be sized either by solar multiple (option 1) or solar field aperture (option 2). Option 1 was chosen for this case as this is a new plant being modelled. The default solar multiple of 1.79 was selected to provide an aperture area only 0.7% larger than that of the parabolic tough plant. This provides for ease of comparison with both parabolic trough and CRS.
The number of collector and receiver modules supplying the boiler and superheater were left at the default values in order not to change the solar field outlet temperature from the design value.
The energy required to heat the HTF, receiver components, piping, fittings and insulation during startup is accounted for by SAM with the “Thermal inertia per unit of solar field”
parameter. This was set at the default 2.7 kJ/K-m2. The steam conditions at design DNI and temperature were left at the linear Fresnel model default values.
The linear Fresnel model has a default non-solar field land area multiplier of 1.6 yet the total land area of the plant still works out to less than half of the land area required for a parabolic trough plant. This is due to two reasons. Firstly the aperture area of a Fresnel collector is approximately the same as its collector area whereas the aperture area of a parabolic trough is only the projection of its collector area (Häberle et al., 2002). Secondly the spacing required between parabolic trough collectors (15 m centre to centre in the SAM simulation) leaves a 9.25 m gap between collector rows, whereas the linear Fresnel reflectors are more densely packed. Häberle et al. (2002) provide a diagram comparing parabolic trough and linear Fresnel collector aperture area (A) and the gross land area (B). In Figure 4-8, the aperture area of the linear Fresnel collector is ∑ and the gross land area ∑ ∑ . With being the space between primary collector mirrors.
Figure 4-8: Parabolic trough (above) and linear Fresnel (below) collector aperture area and gross land area (Häberle et al., 2002)
The mirror washing water usage in the solar field component model uses a default of 0.2 litres per square meter of solar field aperture at a washing cycle of 120 washes per annum. This value was changed to 0.7 litres per square meter of solar field aperture at a washing cycle of 63 washes per annum to bring it in line with the washing cycles used for the parabolic trough and central receiver plant models. Considering the approximate average field aperture of 862,848 m2, this results in an estimated annual water usage of over 38 million litres or 38,052
m3 for this activity. The field control parameters were set to the linear Fresnel model default values.
4.2.8.2 Collector and receiver
The collector and receiver component model allows differing boiler and superheater collector geometry. It was decided to keep the geometry the same for ease of installation and maintenance.
The collector geometry and optical performance element in the collector and receiver component model allows three methods of optical characterization, namely the solar position table method, the collector incidence angle table method or the incident angle modifiers method. The solar position table method with the default solar position/collector incidence angle table was selected as it defines the performance of the solar field at all sun positions for which the plant will operate. A polynomial fit heat loss model with SAM default coefficients was selected for the receiver.
4.2.8.3 Power cycle
The linear Fresnel power cycle component model, while similar to the physical trough and power tower power cycle component models, has a higher estimated gross to net electrical power conversion factor. The design gross output of the power plant to achieve net output of 100.58 MWe is 107 MWe due to a gross to net conversion factor of 0.94. As with the parabolic trough and central receiver options, a fossil fuel fired backup boiler was selected for this simulation.
The startup time of the power block in this model is less than the default time settings selected for the physical trough and power tower models. SAM suggests that 0.35 hours (21 minutes) are required for this plant while 0.5 hours (30 minutes) are required for the parabolic trough and central receiver options. This is due to the flat primary reflectors requiring less complicated solar tracking mechanisms and smaller physical movements to focus DNR onto the collector. This setting was thus left as is to observe how this benefit of the CLFR system impacts its net annual electric output.
A dispatch control component is included in the power cycle component of the linear Fresnel model as it does not have a TES component model. The dispatch schedule here was made identical to that shown in Figure 4-1 in section 4.2.2.5 for the fossil fill aspect.
4.2.8.4 Parasitic power consumption
The linear Fresnel parasitic losses component model calculates the total tracking power loss and the fixed parasitic loss. It also accounts for the auxiliary heater and boiler parasitic power consumption. The default values were selected for this simulation.