A thermal energy storage system (TES) stores heat from the solar field in a liquid medium. Heat from the storage system can drive the power block turbine during periods of low or no sunlight. A thermal storage system is beneficial in many locations where the peak demand for power occurs after the sun has set. Adding thermal storage to a parabolic trough system allows the collection of solar energy to be separated from the operation of the power block. For example, a system might be able to collect energy in the morning and use it to generate electricity late into the evening.

In direct storage systems, the solar field's heat transfer fluid itself serves as the storage medium. In indirect systems, a separate fluid is the storage fluid, and heat is transferred from the solar field's heat transfer fluid to the storage fluid through heat exchangers. The thermal storage system consists of one or more tank pairs, pumps to circulate the liquids, and depending on the design, heat exchangers. Each tank pair consists of a hot tank to store heat from the solar field, and a cold tank to store the cooled storage medium after the power block has extracted its energy.

Note. For more information on thermal energy storage systems for parabolic trough systems, see http://www.nrel.gov/csp/troughnet/thermal_energy_storage.html.

The storage system variables describe the thermal energy storage system. The thermal storage dispatch control variables determine when the system dispatches energy from the storage system, and from a fossil-fired backup system if the system includes one.

Storage System

Full Load Hours of TES (hours)

The thermal storage capacity expressed in number of hours of thermal energy delivered at the power block's design thermal input level. The physical capacity is the number of hours of storage multiplied by the power cycle design thermal input. Used to calculate the system's maximum storage capacity.

Storage volume (m3)

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.

TES Thermal capacity (MWt)

The equivalent thermal capacity of the storage tanks, assuming the thermal storage system is fully charged. This value does not account for losses incurred through the heat exchanger for indirect storage systems.

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.

Tank height (m)

The height of the cylindrical volume of heat transfer fluid in each tank.

Tank fluid min height (m)

The minimum allowable height of fluid in the storage tank(s). The mechanical limits of the tank determine this value.

Tank diameter (m)

The diameter of a storage tank, assuming that all tanks have the same dimensions. Solar Advisor calculates this value based on the specified height and storage volume of a single tank, assuming that all tanks have the same dimensions.

Min fluid volume (m3)

The volume of fluid in a tank that corresponds to the tank's minimum fluid height specified above.

Tank loss coeff (W/m2-K)

The thermal loss coefficient for the storage tanks. This value specifies the number of thermal watts lost from the tanks per square meter of tank surface area and temperature difference between the storage fluid bulk temperature and the ambient dry bulb temperature.

Estimated heat loss (MWt)

The estimated value of heat loss from all storage tanks. The estimate assumes that the tanks are 50% charged, so that the storage fluid is evenly distributed among the cold and hot tanks, and that the hot tank temperature is equal to the solar field hot (outlet) temperature, and the cold tank temperature is equal to the solar field cold (inlet) temperature.

Tank heater set point (ºC)

The minimum allowable storage fluid temperature in the storage tanks. If the fluid temperature falls below the set point, the auxiliary heaters deliver energy to the tanks, attempting to increase the temperature to the set point.

Aux heater outlet set temp (ºC)

The temperature set point for the auxiliary heaters, assumed to be electric heaters.

Tank heater capacity (MWt)

The maximum rate at which heat can be added by the auxiliary electric tank heaters to the storage fluid in the tanks.

Tank heater efficiency

The electrical to thermal conversion efficiency of the auxiliary electric tank heaters.

Hot side HX approach temp (ºC)

Applies to systems with a heat exchanger only (indicated by a heat exchanger derate value of less than one). The temperature difference on the hot side of the solar-field-to-thermal-storage heat exchanger. During charge cycles, the temperature is the solar field hot outlet temperature minus the storage hot tank inlet temperature. During discharge cycles, it is defined as the storage hot tank temperature minus the power cycle hot inlet temperature.

Cold side HX approach temp (ºC)

Applies to systems with a heat exchanger only (indicated by a heat exchanger derate value less than one). The temperature difference on the cold side of the solar field-to-thermal-storage heat exchanger. During charge cycles, the temperature is the storage cold temperature (storage outlet) minus the heat exchanger cold temperature. During discharge cycles, it is the heat exchanger cold temperature minus the storage cold temperature (storage inlet).

Heat exchanger derate

A calculated value indicating the temperature derate caused by the heat exchanger approach temperatures. The derate factor is for reference only and not used in performance calculations. The derate is defined as the temperature difference between the hot and the cold field design temperatures minus the heat exchanger approach temperatures all divided by the difference between the hot and cold field design temperatures. A derate of one indicates a system that uses the same fluid for the solar field heat transfer fluid and for the storage fluid and therefore does not require a heat exchanger between the solar field and storage system.

Initial TES Fluid temp (ºC)

The temperature of the storage fluid in the thermal energy storage system in the first time step of the simulation.

Storage HTF fluid

The storage fluid used in the thermal energy storage system. When the storage fluid and solar field heat transfer fluid (HTF) are different, the system is an indirect system with a heat exchanger (heat exchanger derate is less than one). When the storage fluid and solar field HTF are the same, the system is a direct system that uses the solar field HTF as the storage medium (heat exchanger derate equals one).

User-defined HTF fluid

When you choose user-defined from the Storage HTF fluid list, you can specify a table of material properties of a storage fluid. You must provide values for two temperatures (two rows of data) of specific heat, density, viscosity, and conductivity values. See Specifying a Custom Heat Transfer Fluid for details.

Fluid Temperature (ºC)

A reference value indicating the temperature at which the substance properties are evaluated for thermal storage.

TES fluid density (kg/m3)

The density of the storage fluid at the fluid temperature, used to calculate the total mass of thermal fluid required in the storage system.

TES specific heat (kJ/kg-K)

The specific heat of the storage fluid at the fluid temperature, used to calculate the total energy content of the fluid in the storage system.

Thermal Storage Dispatch Control

The storage dispatch control variables each have six values, one for each of six possible dispatch periods. They determine how Solar Advisor 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.

Storage Dispatch Fraction with Solar

The fraction of the maximum storage capacity (TES thermal capacity) required for the system to start when the solar field energy is greater than zero. A value of zero will always dispatch stored energy in any hour assigned to the given dispatch period; a value of one will never dispatch energy from storage. Used to calculate the storage dispatch levels.

Storage Dispatch Fraction without Solar

The fraction of the maximum storage capacity (TES thermal capacity) required for the system to start when the solar field energy is equal to zero. A value of zero will always dispatch stored energy in any hour assigned to the given dispatch period; a value of one will never dispatch energy from storage. Used to calculate the storage dispatch levels.

Turbine Output Fraction

The fraction of the power cycle design gross output from the Solar Field page at which energy from the storage system can drive the power cycle. See Storage and Fossil Backup Dispatch Controls for details.

Fossil Fill Fraction

Determines how much energy the backup boiler delivers during hours when there is insufficient energy from the solar field (and storage system, if available) to drive the power cycle at its design output capacity.  A value of one for a given dispatch period ensures that the power cycle operates at its design output for all hours in the period: The boiler "fills in" the energy not delivered by the solar field or storage system. For a fossil fill fraction less than one, the boiler supplies enough energy to drive the power cycle at a fraction of its design point. To define a system with no fossil backup, use a value of zero for all six dispatch periods. See Storage and Fossil Backup Dispatch Controls for details.

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 cycle in each hour of the simulation based on how much energy is available in storage, how much energy is delivered by the solar field, and the values of the thermal storage dispatch control parameters. You can define a different dispatch strategy for each of six dispatch periods for weekdays and weekends. See Defining Dispatch Schedules for details.

For each hour in the simulation, Solar Advisor looks at the amount of energy in storage at the beginning of the hour and decides whether or not to operate the power cycle in that hour. For each dispatch period, there are two dispatch targets for starting or continuing to run the power cycle: one for periods of sunshine (storage dispatch fraction w/solar), and one for periods of no sunshine (storage dispatch fraction w/o solar). The dispatch target  for each dispatch period is the product of the storage dispatch fraction for that period and the thermal storage capacity defined by the TES thermal capacity input variable.

During periods of sunshine when there is insufficient energy from the solar field to drive the power cycle at its load requirement, the system dispatches energy from storage only when energy in storage is greater than or equal to the dispatch target.

During periods of no sunshine, the power cycle will not run unless energy in storage is greater than or equal to the dispatch target.

The turbine output fraction for each dispatch period determines the power cycle output requirement for hours that fall within the dispatch period. A turbine output fraction of one defines an output requirement equivalent to the power cycle's design gross output defined on the Power Cycle page. For hours when the solar field energy is insufficient to drive the power cycle at the output requirement, the power cycle runs on energy from both the solar field and storage system. For hours when the solar field energy exceeds the output requirement, the power block runs at the required output level, and any excess energy goes to storage. If the storage system is at capacity, the collectors in the field defocus as specified on the Solar Field page to reduce the field's thermal output.

By setting the thermal storage dispatch control parameters, you can simulate a dispatch strategy for clear days when storage is at capacity that allows the operator to start the plant earlier in the day to avoid defocusing collectors in the field, for cloudy days that allows the operator to store energy for later use in a time period when the value of power is higher.

Fossil Backup Dispatch

When the fossil fill fraction is greater than zero for any dispatch period, the system is considered to include a fossil-fired boiler that heats the heat transfer fluid before it is delivered to the power cycle. The fossil fill fraction defines the backup boiler output as a function of the thermal energy from the solar field (and storage, if applicable) in a given hour and the power cycle design gross output defined on the Power Cycle page. For example, for an hour with a fossil fill fraction of 1.0 when solar energy delivered to the power cycle is less than that needed to run at the power cycle design gross output, the backup boiler would supply enough energy to "fill" the missing heat, and the power cycle would operate at the design gross output. If, in that scenario, solar energy (from either the field or storage system) is driving the power cycle at full load, the boiler would not operate. For a fossil fill fraction of 0.75, the boiler would only be fired  when solar output drops below 75% of the power cycle's design gross output.

The boiler LHV efficiency value on the Power Cycle page determines the quantity of fuel used by the backup boiler. A value of 0.9 is reasonable for a natural gas-fired backup boiler. 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 boiler is defined on the Trough System Costs page.

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

The dispatch schedule library only assigns period numbers to the weekday and weekend schedule matrices. The dispatch fractions that you specify are not stored in the library.

To choose a schedule from the library:

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

To define a dispatch schedule:

This section will describe equations for the calculated values on the Thermal Storage page. It is currently under development. For general descriptions of the variables, see Input Variable Reference.

Storage volume

TES thermal capacity

Tank diameter

Min fluid volume

Estimated heat loss

Heat exchanger derate

Fluid temperature

TES fluid density

TES specific heat