Thermal Storage

The parameters on the Storage page describe the properties thermal energy storage system and the storage dispatch controls.

The variables on the Storage page are similar to the layout and control scheme implemented in the parabolic trough model, with the following differences:

•	The power tower storage model uses storage tank geometry, which requires that the heat transfer fluid volume, tank loss coefficients, and tank temperatures be specified.

•	Solar Advisor calculates the storage tank geometry to ensure that the storage system can supply energy to the power block at its design thermal input capacity for the number of hours specified by the Full Load TS Hours variable.

Note. Because the storage capacity is not tied to the solar multiple on the Heliostat Field page, be careful to choose a storage capacity that is reasonable given the system's thermal capacity. Mismatched storage and solar thermal capacities will result in high levelized cost of energy values.

Contents

►	Input Variable Reference describes the input variables on the Heliostat Field page.

►	Storage and Fossil Backup Dispatch Controls describes the dispatch controls that determine the timing of energy releases from the storage and fossil back up systems.

►	Defining Dispatch Schedules explains how to assign dispatch periods to weekday and weekend schedules.

Input Variable Reference

Storage System

Variable	Description	Units
Storage Type	Solar Advisor models only two-tank storage systems for power towers. A two-tank system consists of separate hot and cold storage tanks.	--
Full Load Thermal Storage Hours	The storage capacity expressed in hours at full load: The number of hours that the storage system can supply energy at the power block design turbine input capacity. Note that Solar Advisor displays the equivalent storage capacity in MWht on the Tower System Costs page.	hours
Storage HTF Volume	Solar Advisor calculates the total heat transfer fluid volume in storage based on the storage hours at full load and the power block design turbine thermal input capacity. The total heat transfer fluid volume is divided among the total number of tanks so that all hot tanks contain the same volume of fluid, and all cold tanks contain the same volume of fluid.	m3
Tank Diameter	The diameter of the cylinder-shaped heat transfer fluid volume in each storage tank.	m
Tank Height	The height of the cylinder-shaped heat transfer fluid volume in each tank. Solar Advisor calculates the height based on the diameter and storage volume of a single tank.	m
Parallel Tank Pairs	The number of parallel hot-cold storage tank pairs. Increasing the number of tank-pairs also increases the volume of the heat transfer fluid exposed to the tank surface, which increases the total tank thermal losses. Solar Advisor divides the total heat transfer fluid volume among all of the tanks, and assumes that each hot tank contains an equal volume of fluid, and each cold tank contains and equal volume.	--
Min Fluid Volume	The minimum storage heat transfer fluid volume allowed in each storage tank. The usable fluid volume is equal to the total volume minus the minimum fluid volume. Calculated based on the minimum tank volume fraction, the total volume, and the number of parallel tank pairs.	m3
Min Tank Volume Fraction	The minimum allowed fraction of the total storage heat-transfer fluid volume of each storage tank.	--
Max Fluid Volume	The maximum usable heat transfer fluid volume allowed in each storage tank. The maximum volume is less than the total volume when the minimum tank volume is greater than zero, or the number of parallel tank pairs is greater than 1.	m3
Wetted Loss Coefficient	The thermal loss coefficient that applies to the portion of the storage tank holding the storage heat transfer fluid.	W/m2-K
Dry Loss Coefficient	The thermal loss coefficient that applies to the portion of the storage tank that contains storage heat transfer fluid.	W/m2-K
Initial Hot HTF Temp	The temperature of the storage heat transfer fluid in the hot storage tank at the beginning of the simulation.	°C
Initial Cold HTF Temp	The temperature of the storage heat transfer fluid in the cold storage tank at the beginning of the simulation.	°C
Initial Hot HTF Percent	The fraction of the storage heat transfer fluid in the hot storage tank at the beginning of the simulation.	%
Initial Hot HTF Volume	The volume of the storage heat transfer fluid in the hot storage tank at the beginning of the simulation.	m3
Initial Cold HTF Volume	The volume of the storage heat transfer fluid in the cold storage tank at the beginning of the simulation.	m3
Cold Tank Heater Temp Set-Point	The minimum allowed cold tank temperature. Whenever the heat transfer fluid temperature in storage drops below the set-point value, the system adds sufficient thermal energy from an electric heater to storage to reach the set-point.	°C
Cold Tank Heater Max Load	The maximum electric load of the cold tank electric heater.	MWe
Hot Tank Heater Temp Set-Point	The minimum allowed hot tank temperature. Whenever the heat transfer fluid temperature in storage drops below the set-point value, the system adds sufficient thermal energy from an electric heater to storage to reach the set-point.	°C
Hot Tank Heater Max Load	The maximum electric load of the hot tank electric heater.	MWe
Tank Heater Efficiency	The electric-to-thermal conversion efficiency of the hot- and cold-tank heaters.	--

Thermal Storage Dispatch Control

The storage dispatch control variables each have six values, one for each of six possible dispatch periods. They determine how SAM calculates the energy flows between the solar field, thermal energy storage system, and power block. The fossil-fill fraction is used to calculate the energy from a backup boiler.

Name	Description
Storage Dispatch Fraction with Solar	The fraction of the TES maximum storage capacity (see table above) required for the system to start when the solar field energy is greater than zero. A value of zero will always dispatch the TES in any hour assigned to the given dispatch period; a value of one will never dispatch the TES. Used to calculate the storage dispatch levels.
Storage Dispatch Fraction without Solar	The fraction of the TES maximum storage capacity (see table above) required for the system to start when the solar field energy is equal to zero. A value of zero will always dispatch the TES in any hour assigned to the given dispatch period; a value of one will never dispatch the TES. Used to calculate the storage dispatch levels.
Turbine Output Fraction	A fraction of the design turbine thermal input adjusted by the turbine part load electric-to-thermal efficiency factors. Used to calculate the power block load requirement.
Fossil Fill Fraction	A fraction of the power block design turbine gross output from the Power Block page that can be met by the backup boiler. Used by the power block module to calculate the energy from the backup boiler.

Storage and Fossil Backup Dispatch Controls

The thermal storage dispatch controls determine the timing of releases of energy from the thermal energy storage and fossil backup systems to the power block. When the system includes thermal energy storage or fossil backup, Solar Advisor can use a different dispatch strategy for up to six different dispatch periods.

Storage Dispatch

Solar Advisor decides whether or not to operate the power block in each hour of the simulation based on how much energy is stored in the TES, how much energy is provided by the solar field, and the values of the thermal storage dispatch controls parameters. You can define when the power block operates for each of the six dispatch periods. For each hour in the simulation, if the power block is not already operating, Solar Advisor looks at the amount of energy that is in thermal energy storage at the beginning of the hour and decides whether it should start the power block. For each period, there are two targets for starting the power block: one for periods of sunshine (w/solar), and one for period of no sunshine (w/o solar).

The turbine output fraction for each dispatch period determines at what load level the power block runs using energy from storage during that period. The load level is a function of the turbine output fraction, design turbine thermal input, and the five turbine part load electric to thermal factors on the Power Cycle page.

For each dispatch period during periods of sunshine, thermal storage is dispatched to meet the power block load level for that period only when the thermal power from the solar field is insufficient and available storage is equal to or greater than the product of the storage dispatch fraction (with solar) and maximum energy in storage. Similarly, during periods of no sunshine when no thermal power is produced by the solar field, the power block will not run except when the energy available in storage is equal to or greater than the product of storage dispatch fraction (without solar) and maximum energy in storage.

By setting the thermal storage dispatch controls parameters, you can simulate the effect of a clear day when the operator may need to start the plant earlier in the day to make sure that the storage is not filled to capacity and solar energy is dumped, or of a cloudy day when the operator may want to store energy for later use in a higher value period.

Fossil Backup Dispatch

When the fossil fill fraction is greater than zero for any dispatch period, the system is considered to include fossil backup. The fossil fill fraction defines the solar output level at which the backup system runs during each hour of a specific dispatch period. For example, a fossil fill fraction of 1.0 would require that the fossil backup operate to fill in every hour during a specified period to 100% of design output. In that case, during periods when solar is providing 100% output, no fossil energy would be used. When solar is providing less than 100% output, the fossil backup operates to fill in the remaining energy so that the system achieves 100% output. For a fossil fill fraction of 0.5, the system would use energy from the fossil backup only when solar output drops below 50%.

The tank heater efficiency determines the quantity of fuel used by the fossil backup system. Solar Advisor includes the cost of fuel for the backup system in the levelized cost of energy and other metrics reported in the results, and reports the energy equivalent of the hourly fuel consumption in the hourly results. The cost of fuel for the backup system is defined on the Tower System Costs page.

Defining Dispatch Schedules

The storage dispatch schedules determine when each of the six periods apply during weekdays and weekends throughout the year. You can either choose an existing schedule from one of the schedules in the CSP trough TES dispatch library or define a custom schedule. For information about libraries, see Working with Libraries.

The TES dispatch library only assigns period numbers to the weekday and weekend schedule matrices. The dispatch fractions assigned to each of the six periods are not stored in the library.

To choose a schedule from the library:

1.	Click Dispatch schedule library.

2.	Choose a schedule from the list of four schedules. The schedules are based on time-of-use pricing schedules from four California utilities.

Click OK.

You can modify a schedule using the steps described below. Modifying a schedule does not affect the schedule stored in the library.

4.	For each of the up to six periods used in the schedule, enter values for the dispatch fractions described above. Use the period number and color to identify the times in the schedule that each period applies.

To define a dispatch schedule:

1.	In the weekday schedule, select the times to which Period 1 applies.

2.	Type the number 1.

3.	Repeat Steps 1 and 2 for each of the up to six dispatch periods that you want to define, typing the period number to assign each period to times in the schedule.

4.	Repeat Setps 1 through 3 for the weekend schedule.

5.	For each of the up to six periods used in the schedule, enter values for the dispatch fractions described above. Use the period number and color to identify the times in the schedule that each period applies.