National Renewable Energy Laboratory
Concentrating Solar Power
One advantage of parabolic trough power plants is their potential for storing solar thermal energy to use during non-solar periods and to dispatch when it’s needed most. As a result, thermal energy storage (TES) allows parabolic trough power plants to achieve higher annual capacity factors—from 25% without thermal storage up to 70% or more with it.
- Storage systems
- Molten-salt heat transfer fluid
- Storage media
Thermal Energy Storage Systems
The first Luz trough plant, SEGS I, included a direct two-tank thermal energy storage system with 3 hours of full-load storage capacity. This system simply used the mineral oil (Caloria) heat transfer fluid (HTF) to store energy for later use. It operated between 1985 and 1999 and was used to dispatch solar power to meet the Southern California Edison winter evening peak demand period (weekdays between 5-10 p.m.).
Because power plants later moved to higher operating temperatures for improving power cycle efficiency, they also switched to a new higher temperature heat transfer fluid—a eutectic mixture of biphenyl-diphenyl oxide (Therminol VP-1 or Dowtherm A). Unfortunately, this fluid has a high vapor pressure. Therefore, it cannot be used in the same type of large unpressurized storage tank system similar to the one used for SEGS I.
Pressurized storage tanks are very expensive. They cannot be manufactured at the large sizes needed for parabolic trough plants.
Figure 1. Two-tank indirect thermal energy storage system for Andasol 1 and 2. Credit: Flagsol
In recent years, a new indirect thermal energy storage (TES) approach has been developed. This approach takes advantage of the experience with the storage system used in the Solar Two— a molten-salt power tower demonstration project—and integrates it into a parabolic trough plant with the conventional heat transfer fluid through a series of heat exchangers.
The thermal energy storage system is charged by taking hot, heat transfer fluid (HTF) from the solar field and running it through the heat exchangers. Cold molten-salt is taken from the cold storage tank and run counter currently through the heat exchangers. It’s heated and stored in the hot storage tank for later use. Later, when the energy in storage is needed, the system simply operates in reverse to reheat the solar heat transfer fluid, which generates steam to run the power plant. It’s referred to as an indirect system because it uses a fluid for the storage medium that’s different from what’s circulated in the solar field.
Several parabolic trough power plants under development in Spain plan to use this thermal energy storage concept. For future parabolic trough power plants, a number of alternative approaches are being considered for reducing the cost of the thermal energy systems.
A two-tank indirect thermal energy storage system is relatively expensive—its primary disadvantage. The expense is due to the heat exchangers and the relatively small temperature difference between the cold and hot fluid in the storage system.
For more information, see our publications on two-tank indirect thermal energy storage systems.
A single tank for storing both the hot and cold fluid provides one possibility for further reducing the cost of a direct two-tank storage system. This thermocline storage system features the hot fluid on top and the cold fluid on the bottom. The zone between the hot and cold fluids is called the thermocline.
A thermocline storage system has an additional advantage—most of the storage fluid can be replaced with a low-cost filler material. Sandia National Laboratories has demonstrated a 2.5-MWhr, backed-bed thermocline storage system with binary molten-salt fluid, and quartzite rock and sand for the filler material.
Depending on the cost of the storage fluid, the thermocline can result in a substantially lower cost storage system. However, the thermocline storage system must maintain the thermocline zone in the tank, so that it does not expand to occupy the entire tank.
Figure 2. Thermocline test at Sandia National Laboratories. Credit: Sandia National Laboratories
For more information, see our publications on thermocline systems.
Direct Molten-Salt Heat Transfer Fluid
Using molten-salt in both the solar field and thermal energy storage system eliminates the need for expensive heat exchangers. It allows the solar field to be operated at higher temperatures than current heat transfer fluids allow. This combination also allows for a substantial reduction in the cost of the thermal energy storage (TES) system.
Unfortunately, molten-salts freeze at relatively high temperatures 120 to 220°C (250-430°F). This means that special care must be taken to ensure that the salt does not freeze in the solar field piping during the night.
The Italian research laboratory, ENEA, has proven the technical feasibility of using molten-salt in a parabolic trough solar field with a salt mixture that freezes at 220°C (430°F). And Sandia National Laboratories are developing new salt mixtures with the potential for freeze points below 100°C (212°F). At 100°C the freeze problem is expected to be much more manageable.
For more information, see our publications about molten-salt heat transfer fluid.
Thermal Energy Storage Media
The German Aerospace Center constructed a facility at the University of Stuttgart for testing a concrete, thermal energy storage system.
The German Aerospace Center (DLR) is examining the performance, durability and cost of using solid, thermal energy storage media (high-temperature concrete or castable ceramic materials) in parabolic trough power plants.
This system uses the standard heat transfer fluid (HTF) in the solar field. The heat transfer fluid passes through an array of pipes imbedded in the solid medium to transfer the thermal energy to and from the media during plant operation.
The primary advantage of this approach is the low cost of the solid media. Primary issues include maintaining good contact between the concrete and piping, and the heat transfer rates into and out of the solid medium.
At the Plataforma Solar de Almeria in Southern Spain, Ciemat and DLR performed initial testing that found both the castable ceramic and high-temperature concrete suitable for solid media, sensible heat storage systems. However, the high-temperature concrete is favored because of lower costs, higher material strength, and easier handling. There is no sign of degradation between the heat exchanger pipes and storage material.
DLR has also developed a design tool that helps optimize the storage layout, including the geometric dimensions and piping and module arrangement to minimize pressure losses and optimize manufacturing aspects and costs.
Because of the modular nature of concrete storage, DLR has identified approaches that allow the storage system to better integrate with the solar field and power cycle. This allows for improved overall utilization of the concrete storage system. DLR is also testing a new, more optimized concrete storage module at the University of Stuttgart.
Phase-change materials (PCMs) allow large amounts of energy to be stored in relatively small volumes, resulting in some of the lowest storage media costs of any storage concepts.
Initially phase-change materials were considered for use in conjunction with parabolic trough plants that used Therminol VP-1 in the solar field. Luz, and later ZSW, proposed an approach that used a cascading set of phase-change materials to transfer heat from the heat transfer fluid (HTF). In this approach, thermal energy transfers to a series of heat exchangers containing phase-change material that melt at slightly different temperatures. To discharge the storage, the heat transfer fluid flow is reversed. This results in reheating of the heat transfer fluid.
Although testing proved the technical feasibility of this system, further development of the concept was hindered because of the:
- Complexity of the system
- Thermodynamic penalty of going from sensible heat to latent heat and back to sensible heat
- Uncertainty over the lifetime of phase-change materials.
More recently DLR is evaluating phase-change thermal energy storage for application with direct steam generation in the parabolic trough solar field. This allows for a better thermodynamic match between the phase-change material and the phase-change of steam used in the solar field. In this approach a single phase-change material can be used to preheat, boil, and superheat steam. DLR has found that the cost of the system is driven not only by the cost of phase-change storage material, but also by the rate at which energy will be charged or discharged from the material.
Also, DLR has developed a graphite foil that it uses to sandwich the phase-change material for increasing heat transfer rates. Lab scale tests of this approach have demonstrated its feasibility. And future tests will be integrated into the DISS facility at the Plataforma Solar de Almeria.
For more information, see our publications about phase-change materials.