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Solar Thermal Storage System

One big shining point of Parabolic Trough Power Plant (PTPP), the so-called dispatchability, is its potential to provide power 24 hours a day, by storing the heat energy in a thermal storage unit for later use during peak hours, in the evening or on a cloudy day. It enhances the annual capacity of a plant by 50 % over one without a thermal energy storage system (TES). Within current technology, heat is much cheaper to store than electricity. Nearly all current existing solar thermal plants that have back-up systems are supported by fossil fuels, but a TES completely hoisted by the power the plant generates itself is within reach. Several storage mechanisms have been put in place while other proposals are still in lab-scale. Progress is being achieved by improvements on old systems and alternative designs. ,

Two-tank direct storage system
The early two-tank direct system was used in the first Luz mirror plant, the ¡°Solar Energy Generating System I (SEGS I)¡±in California. It has two tanks, one of low and one of high temperature. Only one heat transfer fluid (HTF), in this case mineral oil (Caloria), circulates from the low-temperature tank through the solar collectors picking up the heat. Part of the heat goes to generate the steam to run the turbine and the excessive heat goes back to the high-temperature tank for storage. After passing through a heat exchanger, the cooled fluid flows back to the low-temperature tank to be reused. The Solar Two power tower in California also uses this system, only with molten salt as the HTF.



Two-tank sensible heat storage


But as later SEGSs moved to synthetic oil (a eutectic mixture of biphenyl-diphenyl oxide) to achieve a higher operating temperature and hence a higher efficiency, the two-tank direct was no longer suitable. The old mineral oil has a high vapor pressure so it cannot be used in the large unpressurized storage tank system as the one adopted for SEGS I. Pressurized storage tanks are very expensive. In addition, the HTF in some places is too expensive or not suitable to also serve as a storage fluid. It takes the freezing point and local temperature (day and night) into consideration in terms of choosing the transfer medium.

Two-tank indirect storage system
The subsequently developed two-tank indirect storage system has not only a HTF but also a storage fluid (ST) and an extra heat exchanger. The storage fluid coming out of the low-temperature tank absorbs the heat energy of the high-temperature HTF in the extra heat exchanger. The now high-temperature ST flows back to a high-temperature storage tank and the now low-temperature HTF moves on to the solar collector to start the power cycle again. Despite the extra cost resulting from a second heat exchangerand smaller temperature difference between the two tanks, the two-tank indirect system with molten salt as the ST is still dominant in most of the PTPPs around the world. The technology originated from the experiment of Solar Two power tower in California. Two PTPP in plan, the 50MW AndaSol project in Granada, Spain and the 280MW Solana, in Gila Bend, Arizona, will both adopt the molten salt thermal storage system.1,2, Andasol, for example, aims at a capacity of 1,010 MWh, equivalent to 7.5 hours of full load operation.

Two-tank indirect thermal energy storage system for Andasol 1 and 2. The storage tank is 10m in height and 37m in diameter. The storage fluid is a mixture of 60% NaNO3 and 40% KNO3 Credit: Flagsol

For high temperature thermal storage, above 400¡ãC, organic HTFs tend to thermally decompose, while molten-salt or liquid metal is still generally stable. It is also non-flammable and nontoxic and has been used in other industries . But problem with molten salt is its relatively high freezing temperature 120 to 220¡ãC (250-430¡ãF). Special operating maintenance needs to be done to make sure it doesn¡¯t freeze during cold night, especially in deserts.

Single-Tank Thermocline
To further reduce the cost of the storage fluid and the storage tanks, researchers moved forward to a single tank called thermocline. Energy is stored in a tank made of solid storage medium--commonly concrete or silica sand¡ªinstead of a storage fluid. High-temperature fluid flows into in the tank from the top, all the way down through to the bottom and cools. It creates two different temperature regions from high to low, between which there is a space called temperature gradient or thermocline. When the stored-up thermal energy is needed, the flow reverses taking up the heat on its way up. Buoyancy effects make sure that hot, less dense materials stay on top of cool, dense materials at the bottom, creating thermal stratification of the fluid.

Sandia National Laboratories in New Mexico has tested a 2.5 MWhr, backed-bed thermocline storage system with binary molten-salt fluid, and quartzite rock and sand for the filler material. The cost for a TSE system is reduced substantially by replacing most of the storage fluid and cheap filling material for the tank.

Thermocline test at Sandia National Laboratories. Credit: Sandia National Laboratories
The research goals now directing current R&D in solar thermal storage encompass finding heat-transfer fluid that can operate at higher temperature with low freezing point, hence a higher overall heat transfer efficiency. Another goal is to develop a storage fluid that has high heat capacity so that less amount of fluid is needed in the system.


Although these above-mentioned systems are very reliable technically, they still pose a high overall cost. Other concepts for a cheaper cost are being explored and investigated too. Some research is under way to find more efficient and less costly filler materials for the one-tank system which possesses high potentiality for cost reduction.

Phase-Change Materials
Although using concrete as the filler materials is very cost efficient(it is much cheaper to hold the same amount of energy than molten salt), easy to handle and has higher strength, it faces problems such as maintaining good contact between the concrete and pipelines and low efficiency of heat transfer from the concrete to the HTF.

Another rather promising solution is phase-change materials (PCMs), use d in high temperature latent heat thermal energy storage system (HTLTTES) for direct steam generation (DSG).Its primary advantage resides in its ability to hold up large amounts of energy in relatively small volumes, at one of lowest costs among other storage materials. It utilized different PCM¡¯s different latent heat of fusion (melting), which should be matched to the temperature of the incoming sensible HTF. The PCMs are cascaded from low melting temperature at the bottom of the tank to high temperature at the top (maximum operating temperature around 390¡ãC).

 

The HTF flows downward when charging (melting the PCMs) and upward when discharging providing heat to generate steam (solidifying the PCMs). Current researches propose nitrate/nitrite salts and eutectic mixtures of these salts, such as lithium nitrate and potassium nitrate as the PCMs for HTLHTES, for their enthalpy and economic feasibility.

Despite its encouraging prospect, however, PCMs is challenged by the complexity of the system itself, unstable lifespan of the PCMs and low heat conductivity. Researchers are looking for other material sources that possess more sufficient heat of fusion, corrosiveness and high heat conductivity (at least 2 W/(m K)). Or it can also be improved by developing proper heat transfer techniques to offset the low conductivity of PCMs. ,

cost of storage concepts.bmp
A cost comparison of the three storage concepts in different parts including 2-tank direct liquid salt, thermocline (concrete, solid salt and liquid salt), and PCM. The latter two are only in testing phrase.


"Thermal energy storage is the killer app of concentrating solar power technology," said Andrew McMahan, vice president of SkyFuel, New Mexico, told a packed solar technology conference last month held in conjunction with Semicon West. This month, the U.S. Department of Energy (DOE) just announced a funding of $35 million to facilitate developing lower-cost energy storage for CSP technology. An increasing number of major venture capital also flows into researches that focus on more cost efficient solar thermal storage technologies.

Bibliography


Nava P, Herrmann, U. Trough Thermal Storage Status Spring 2007 NREL/DLR Trough workshop -Denver Mar 2007

Leopold, G., Solar thermal technology heats up Electronic Engineering Times August 2008 4 Pg 38

¡° DOE to invest $35 million in concentrating solar plant projects¡± National Renewable Energy Lab, Sep 19, 2008 http://www.nrel.gov/csp/news/2008/634.html


Michels, H., Pitz-Paal, R., Cascaded Latent Heat Storage For Parabolic Trough Solar Power Plants Solar Energy 81 (2007) 829¨C837

Guo, C., Zhang, W. Numerical simulation and parametric study on new type of high temperature latent heat thermal energy storage system Energy Conversion and Management Volume 49, Issue 5, May 2008, Pg 919-927


Michels, H., Pitz-Paal, R., Cascaded Latent Heat Storage For Parabolic Trough Solar Power Plants Solar Energy 81 (2007) 829¨C837

Guo, C., Zhang, W. Numerical simulation and parametric study on new type of high temperature latent heat thermal energy storage system Energy Conversion and Management Volume 49, Issue 5, May 2008, Pg 919-927


¡°Solar Storage And Research Development¡±, U.S Department of Energy Efficiency and Renewable Energy http://www1.eere.energy.gov/solar/thermal_storage_rnd.html#storage_systems

Solar Power; Sunny Future For Parabolics In Granada And Nevada Modern Power System February 14, 2007

¡°National solar thermal testing facilities¡± Sandia National Laboratories http://www.sandia.gov/Renewable_Energy/solarthermal/NSTTF/salt.htm

Taggard, S., Parabolic troughs: CSP¡¯s quiet achieverRenewable Energy Focus Volume 9, Issue 2, March-April 2008, Pages 46-48, 50


¡°Parabolic Trough Thermal Energy Storage Technology¡±NREL
http://www.nrel.gov/csp/troughnet/thermal_energy_storage.html#direct

¡°Thermal Storage¡± U.S Department of Energy Efficiency and Renewable Energy http://www1.eere.energy.gov/solar/thermal_storage.html

Stine, W.B., Harrigan, R.W. an online update version of the book "Power From The Sun" http://www.powerfromthesun.net/Chapter11/Chapter11.htm

 

Copyright Yiting Wang 2008