Now that we set up our solar field and selected a solar reflector and receiver tube, we can jump to the next menu in SAM, which deals with the power block.

In the upper field, named plant characteristic, the power of the turbine and the power cycle is specified. The design gross output describes the electrical power output of the turbine without deduction of parasitic losses. In practice, a part of the produced electrical energy is needed for the operation of the plant itself, for instance for the tracking of the PTC collectors.

The estimated gross-to-net conversion factor accounts for these parasitic losses and by multiplying it with the design gross output, we receive the estimated net output at design. This is the nominal electrical power that is delivered to the electricity grid. For our PTC plant, we assume a gross output of 55.5 MW and a conversion factor of 0.9 MW.

The nominal capacity is then exactly 50 MW. The gross output we specify here is also decisive for the size of the solar field, as it is scaled accordingly to the power cycle with the solar multiple we defined on a previous subpage in SAM.

Here on the right side, SAM provides the option to set losses for the power output due to system outages, repair and maintenance works or other events preventing the power cycle from operating. Constant losses, hourly losses or user-defined loss values can be specified after clicking on the button added losses here.

The default setting of 4% constant losses is appropriate to account for regular maintenance work and unexpected outages, so we can adopt the value for our project file. In the table here, different reference power cycles are available for selection.

In the previous field, we already stated our gross output with 55.5 MW. The table does not provide a selection for a power cycle with this exact gross output, but it is actually not necessary to select the perfect match. SAM only uses the reference values, for example the part load behavior from the reference power cycle in the list

and then scales the power output of the power block accordingly to the value we specified in the first section. The default dry-cooled SEGS 80 MW turbine is suggested in SAM's documentation for a typical power cycle with water steam as working medium and therefore it is also an appropriate choice for our project.

Here, below the table, the properties of the selected power cycle are listed. The option Use Library Values can be left activated so that all library values are prefilled in the text boxes. Besides the thermal input at design point, startup, maximum load and minimum load, the

rated cycle conversion efficiency and a boiler efficiency is indicated here on the right side. This describes the efficiency of a backup boiler for the power cycle. The purpose of this fossil fuel backup boiler is to deliver heat to the power cycle and to extend its operating hours when there is no energy provided by the solar field and thermal storage.

The boiler efficiency can be kept at 90% here. The operating strategy and boundary conditions for the actual use of the backup boiler can be defined later on the subpage Thermal Storage. The part load behavior of the power cycle is described with several factors.

In the first row, the thermal to electrical efficiency of the turbine is defined with a polynomial function. In the second row, another polynomial function defines the electrical to thermal efficiency of the turbine. This is strange at first look, since the turbine is supposed to convert thermal energy into electrical energy. But SAM actually only uses this correlation to calculate the equivalent natural gas demand

for the fossil fueled backup burner for time periods when the backup burner is operating. A third polynomial function accounts for losses of the cooling tower. A temperature correction factor is calculated in SAM, which is a function of the ambient temperature for dry-cooled power cycles.

The temperature correction mode can be set either to dry bulb or wet bulb mode and determines whether the ambient temperature for dry-cooled systems or the wet bulb temperature for evaporative cooling towers is used. The wet bulb temperature is lower than the dry bulb temperature when the ambient relative humidity is less than 100% due to the effect of evaporative cooling.

As we chose a dry-cooled power cycle in the table above, we can keep the prefilled selection dry bulb basis in this field here. The part load behavior of the power cycle is a function of the ratio of the actual thermal energy input and the designed thermal energy input.

It is visible here in the graph that the power block efficiency generally decreases with declining load factors. The optimum efficiency is reached at nominal load. In SAM, the part load behavior of the power cycle is calculated with a polynomial function.

First, the non-dimensional thermal energy of the reference power cycle QPb is calculated. The other QPb here stands for the actual thermal power input to the reference power cycle and Qdesign stands for the nominal thermal power of the reference power cycle at the design point.

When I am talking about the reference power cycle, I mean the one we selected from the list in SAM, the dry-cooled SEGS 80 MW turbine. As our defined turbine gross output is only 55.5 MW electrical, the absolute cycle output must be scaled to our plant.

QPb is then used for the calculation of the non-dimensional gross cycle output Wgr. F0 to F4 are the factors of the polynomial function which characterizes the part load behavior of the reference power cycle. The dimensional gross output Wgr is then calculated by multiplying the non-dimensional gross cycle output

with the design point gross cycle output which is 55.5 MW for our power cycle. With Wgr, we now have the gross output of our power cycle at a certain load factor. In the next video, we will talk about the operating strategy of the thermal storage in the PTC plant and create a schedule for the plant operation in SAM.

So thank you for listening and I hope to see you in the next video.