In the last lesson, we set the gross power and conversion factor for our power cycle and choose a reference power cycle from SAM's library. In this video, we take a look at another central component in our PTC plant, which is the thermal storage.

Before we go to the menu in SAM, I want to show you some of the basics of the two-tank molten salt storage system and talk about the operating strategies and the sizing of the thermal storage in the PTC power plant. On this slide, the schematic structure and components of a two-tank molten salt system are illustrated.

It consists of a tank with cold molten salt and a tank with hot molten salt as well as a circulation pump and a heat exchanger. During the day, while the solar field is in operation, cold molten salt is sent through the oil-salt heat exchanger here and pumped into the hot salt tank.

Thermal energy is transferred from the thermal oil to the molten salt in the heat exchanger and the molten salt is heated up. In periods with no or less solar radiation, the heat storage is discharged and the direction of the salt mass flow is reversed. Hot salt flows through the oil-salt heat exchanger into the cold tank.

The thermal energy stored in the salt is transferred to the thermal oil while flowing through the heat exchanger. The hot thermal oil can then be used to supply heat to the power cycle in time periods when the solar field is not providing enough thermal energy. In a previous lesson we discussed that the solar field is dimensioned with a solar multiple depending on the size of the power cycle.

During operation of a solar thermal power plant, the condition of excess energy from the solar field can occur temporarily. For a PTC plant with thermal storage, this is the case when the solar field delivers thermal energy that cannot be utilized as the thermal storage is fully charged and the power cycle is operated at nominal load.

In practice, the excess energy is dumped by slightly defocusing the collectors in the solar field. This way the solar field provides only as much energy as is usable for the power cycle and thermal storage in this moment. On the next slide, the distribution of the provided power of the solar field, thermal storage and backup burner is illustrated.

The fixed gross power of the power cycle is represented by the green line. The yellow area shows the amount of energy provided by the solar field during the day. Between 6 am and 6 pm, the gross power of the power cycle is solely provided by the solar field.

The excess heat of the solar field, here shown in red color, is stored in the thermal storage system. In the evening, the heat supply from the solar field decreases and the thermal storage system compensates the power drop of the solar field. The red area in the diagram here shows that the nominal power for the power cycle process is covered by the thermal storage for about 4 hours after sunset.

Shortly after 11 pm, the thermal storage is fully discharged and a backup burner is activated to supply thermal energy to the power cycle. The backup burner runs between about 11 pm to 6 am. In the morning hours, starting at about 5 am, the heat supply is again gradually

taken over by the solar field and the output of the backup burner is gradually reduced. With the power profile shown in this diagram, it is possible to operate the power block continuously for 24 hours a day and 7 days a week.

The dimensioning of the thermal storage and gross output of the power cycle for a given solar field size is shown in these pictures. Depending on the boundary conditions and desired electricity production profile of the PTC power plant, a suitable sizing of the storage and power cycle gross output is chosen. In the top right picture, the so-called intermediate load configuration is shown.

It is designed to produce electricity to cover peak and shoulder loads during the day. It has a 250 MW turbine and requires only a small sized storage. It has the smallest investment costs and the least expensive electricity production costs compared to the other systems.

The delayed intermediate load design collects solar energy all day and produces electricity from noon on to after sunset to cover peak and shoulder loads. It has the same sized turbine as the intermediate load plant but requires a larger storage system to store the thermal energy for the electricity production after sunset.

The base load configuration, shown in the top right picture here, runs 24 hours a day for most of the year. It is designed to supply a constant power to the electricity grid. It requires a large thermal storage and smaller turbine. If the cost for the storage capacity is lower compared to the capacity of a larger turbine, the

electricity from the base load plant can be slightly cheaper compared to that of the delayed intermediate load plant. This is likely the case with higher working temperatures, which allow less expensive storage but require more sophisticated and costly turbines. The peak load plant, shown on the bottom right side, is designed to provide electricity

for only a few hours per day to meet a large demand of peak load. It requires a large turbine and a big thermal storage system. Out of the four systems, this constellation produces the most expensive but also the most valuable electricity. Now that we addressed the basics of the molten salt thermal storage system, we go back to SAM and open the next subpage, Thermal Storage.

In the first section of the page, the dimensions, heat storage medium and heat losses of the thermal storage are set. The size of the thermal storage is entered here in this first text field in the unit of equivalent full load hours. One full load hour corresponds to the storage capacity required to provide the gross thermal input power to the power cycle for one hour.

For our PTC project, we select the default value of six hours here. So our storage can run the power cycle alone for six hours, provided that it is fully charged before the start of operation.

Our PTC system now corresponds to an intermediate load configuration as shown on the previous slide. For the storage system configuration, the default option to tank is the right one for our molten salt thermal storage. In the field storage fluid type, the thermal storage medium is defined. Besides different types of molten salt, also various thermal oils can be selected.

We choose the molten salt, high-tech solar salt for our project. The parameters Turbine Test Adjusted Efficiency and Turbine Test Adjusted Gross Output take into account the temperature loss due to the heat exchanger efficiency of the thermal storage system and adjust the turbine efficiency accordingly with respect to lower steam temperatures in the power cycle.

The default values are suitable for our project and we can adopt them. The initial energy specifies the thermal energy amount inside the thermal storage system at the start of simulation on 1st January. The standard value is zero and represents a fully discharged storage.

We assume that there is no energy stored in our salt storage in the beginning, so we can keep the value of zero here. The thermal storage in the PTC plant constantly loses energy to the environment due to the temperature difference between molten salt and ambient air. Both the hot and cold molten salt tank are insulated to reduce the heat losses, but still a fraction of the thermal energy is lost.

In SAM, the losses are implemented with a constant loss value. Each simulation hour, the losses are deducted from the energy capacity in the cold and hot molten salt tank. In the SAM help menu, different guideline values are suggested for the heat losses depending on the number of equivalent full-load hours and the storage configuration.

In our case, the molten salt storage is dimensioned to 6 equivalent full-load hours and has a discharge rate of around 150 MW. With the help of the guideline table in the SAM help menu, we can interpolate a constant loss value of 1.28 MW thermal.

This value we can add in the field named tank heat losses. Here on the right side we have a set of automatically calculated parameters for the thermal storage. The maximum thermal energy content of the molten salt storage is close to 1 GWh. The maximum discharge and charge rate of the storage are indicated in these fields.

The discharge rate exceeds the designed thermal input of the power cycle, which means that the storage can provide enough thermal energy to run the turbine at nominal load. The heat exchanger duty applies for indirect thermal storage systems which use different media in the solar field and heat storage and therefore require a heat exchanger.

The value is determined by subtracting the number of 1 from the solar multiple. The maximum charge rate of the thermal storage is calculated by multiplying the heat exchanger duty with the designed turbine thermal input power. In the bottom two fields here, the minimum and maximum operating temperature of the molten salt is stated.

The values are set by SAM according to the selected storage fluid type. Here in the second part of the page, the dispatch control of the thermal storage is specified. The dispatch control can be specified with hourly resolution for each month of the year. The settings for weekend days can be changed separately from working days.

On the left side, a total of 9 periods with different operating strategies for the thermal storage can be defined. The periods are assigned to the schedules on the right side by simply clicking on a field and then pressing the corresponding number of the period to be assigned. Multiple fields can be selected by clicking on a field and dragging an area with the mouse.

All selected fields are marked in blue and by pressing a number between 1 and 9 on the keyboard, the period is assigned to all selected fields. The thermal storage dispatch control, integrated in SAM, determines for every simulation hour whether or not the thermal storage is used to supply energy to the power cycle.

The operation of the thermal storage can be tied to various conditions, which we will go through together. In the first two columns there are two dispatch limits which define the minimum charge level to which the thermal storage can be discharged. The charge limits are defined once for periods when the solar field provides power, indicated with the parameter storage dispatch with solar,

and once for periods when no solar radiation is available and the solar field is not providing power, indicated with the parameter storage dispatch without solar. The absolute value of the minimum charge level is calculated by multiplying the maximum energy storage capacity

with the number between 0 and 1 entered in the fields in the first two columns here. If for example a storage dispatch factor of 0.5 is set, then the thermal storage is only discharged in the corresponding hour if the charge level of the thermal storage is larger or equal to 50%.

In the third column the turbine output fraction is specified. The parameter specifies the maximum value of the supplied thermal energy to the power cycle up to which the thermal storage is discharged and optionally up to which the backup burner is operated. If for example a value of 0.7 is set here, the thermal storage and backup burner are only

operated up to the point when the power cycle is running at 70% of its designed thermal input, provided that none of the other conditions stated in the other columns are met first. In the last column fossil fill fraction, a factor indicates the relative fraction of gross turbine output that may be met by the fossil backup burner in the corresponding time period.

For example a value of 0.2 would mean that the backup burner is allowed to provide thermal energy to the power cycle to cover a fraction of up to 20% of the power cycle's gross input. The backup burner is only used if the solar field and thermal storage cannot provide sufficient thermal energy to run the power cycle at a desired load point.

The first time going through this dispatch control section can be a bit confusing. The best way to get familiar to it is to enter values yourself and compare the behavior of the storage, the backup burner and the power cycle in the simulation results. And to mention it again, the help menu in SAM is also a very helpful guide if you get stuck.

Now we want to set the dispatch control settings for our project file. The values to enter are shown in the screenshot on the right. For the first period we enter a storage dispatch factor of 1 in both columns. For the turbine output fraction and the fossil fill fraction we set a value of 0.

By setting both storage dispatch factors to 1, the thermal storage is never discharged regardless of whether the solar field is supplying heat or not. The turbine output fraction of 0 means that no thermal energy is provided by the thermal storage or backup burner to the power cycle. And finally, the fossil fill fraction of 0 indicates that the backup burner is not active.

We assign the first period to the field as shown here in the screenshot, between 7pm and 5pm, or 6pm to 4pm for the winter months. We choose the same setting for working days and weekends. The priority in this time period is to charge the thermal storage system to make sure the power cycle can be operated with the stored thermal energy after sunset.

For the operating periods 2 and 3, the turbine output fraction is set to 1 and the storage dispatch factors are reduced to 0.3 and 0. In period 2, the thermal storage can be discharged if the storage level is equal to or larger than 30% and a full thermal design load can be supplied to the power cycle if necessary.

This way, the thermal storage can compensate the decreasing power supply of the solar field around the sunset hours. In period 3, the threshold for discharging the thermal storage is reduced to 0. This allows the complete discharging of the stored energy in the molten salt so the

power cycle can be operated independently from the solar field during evening and nighttime operation. The fossil fill fraction is kept at a value of 0 for period 2 and 3 and also for all following periods as our PTC system should operate solar only without the need for an auxiliary fossil fuel firing. We assign period 2 to the hours around sunset and period 3 to the evening and nighttime hours as it is shown in the screenshot on the right side.

For the winter months, period 2 and 3 start one hour earlier because the sunset occurs earlier compared to summer. In period 4, the turbine output fraction as well as storage dispatch factors are adjusted to 0.

Period 4 is assigned to the nighttime and early morning hours. Here, the power cycle is not running and the thermal storage is not discharged. In period 5, the storage dispatch factors are set to 0.5 and the turbine output fraction is set to 40%. The thermal storage can provide energy to the power cycle until the power cycle reaches the threshold of 40

% of its design thermal input power provided that the thermal storage has a charge level of at least 50%. Period 5 is assigned to the operating hours around sunrise as shown here in the picture. In the last 4 rows, period 6 to period 9, no values need to be entered.

As the periods 6 to 9 are not assigned in the schedule on the right, they are not considered for the operating strategy by SAM. So, this was a rather long and challenging section in SAM. I hope that you got an insight into the thermal storage dispatch strategy. Practicing the steps yourself in the software is the best way to get used to SAM and to understand the underlying calculation methods.

Our storage dispatch strategy is now set up and we are ready to go to the next page in SAM, which is about parasitic losses of our PTC plant. Thank you for listening to this lecture and I hope to meet you again in the next one.