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 TES is beneficial in many places where the peak demand for power occurs after the sun has set. Adding TES 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 systems, the heat transfer fluid itself serves as the storage medium. In indirect systems, a separate fluid is the storage medium, and heat is transferred from the HTF to the storage medium through heat exchangers. The TES system consists of one or two tanks, pumps to circulate the liquids, and depending on the design, heat exchangers. A thermocline system stores both the hot and cold storage medium in one tank. The zone in the tank where hot and cold fluids meet is called a thermocline. The storage tank in a thermocline system contains low-cost filler materials such as sand and rock. A two-tank system 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 user inputs on the Storage page are divided into two groups. The thermal energy storage (TES) group defines the thermal energy storage capacity and type along with some efficiency parameters. The thermal storage dispatch controls group variables determine the the operation of the storage and fossil back up systems.

For a more detailed description of the model, please download the CSP trough reference manual from the Solar Advisor website's support page: https://www.nrel.gov/analysis/sam/support.html.

Input Variable Reference

Estimating Tank Heat Loss Values

An increase in the hours of thermal storage requires a both an increase in the solar field size to minimize the levelized cost of energy for the system, and an increase in the tank heat losses to account for the larger tank. The "100 MW Baseline - Parameterized Storage" case in Sample Parabolic Trough Systems.zsam illustrates these relationships. See Solar Multiple Optimization for a description of the case.

The following table shows suggested tank heat loss values for three sample systems over a range of thermal storage capacities. The relationship between tank heat losses and hours of thermal storage is linear, so you can extrapolate to estimate values for storage capacity values not on the table.

Table 24. Suggested Tank Heat Losses (MWt) values for different thermal storage capacities (hours).

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 Block 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 boiler LHV efficiency value on the Power Block page determines the quantity of fuel used by the fossil backup system. 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 system is defined on the Trough 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:

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:

Equations for Calculated Values

Maximum Energy Storage

Where,

Design Turbine Thermal Input

The thermal energy required by the power block to operate at its rated capacity, described on the Power Block page.

Maximum Power To and From Storage

The maximum power to and from storage depends on whether the TES includes a heat exchanger. When the TES fluid is different from the solar field fluid, the TES includes a heat exchanger. When the fluids are the same, there is no heat exchanger.

For a TES with heat exchanger (TES fluid and solar field fluid are different):

For a TES with no heat exchanger (TES fluid and solar field fluid are the same):

Where,

Heat Exchanger Duty

The heat exchanger duty depends on the value of the solar multiple. When the solar multiple is greater than one:

When the solar multiple is equal to or less than one:

Where,