Experimental Study of Thermal Storage System Molten Salt Characteristics
in Solar Thermal Power Plant
[DU Jinglong, SPIC Science and Technology Research Institute, 8610 56681937, [email protected]]
[ZONG Jun, SPIC Science and Technology Research Institute, 8610 56681170, [email protected]]
[FAN Yufeng, SPIC Science and Technology Research Institute, 8610 56681195, [email protected]]
[LI Yang, SPIC Science and Technology Research Institute, 8610 56681951, [email protected]]
[LI Jihong, SPIC Science and Technology Research Institute, 8610 56682266, [email protected]]
[WANG Jianqiang, Shanghai Institute of Applied Physics 8621 39194051, [email protected]]
ABSTRACT
Nitrate is commonly used as absorption, transfer and thermal storage medium in solar thermal power plant,
its performance directly affects the overall performance of the power plant itself. The melting temperature, the
molten salt thermal conductivity and specific heat capacity were generated with different component changes,
on the basis of the traditional Two element molten salt, in different proportion gradually joined the LiNO 3,
CsNO3, Ca (NO3)
2
three kinds of nitrate, the melting temperature, specific heat capacity and the thermal
conductivity test results showed that with the content of certain specific heat values change, but the change in
the operating temperature range is very small, the biggest change is only 17%. Component change on the
melting temperature is larger which the biggest drop is about 160℃. The thermal conductivity of phase change
is still at the level of an order of magnitude, it does not impact the molten salt heat transfer performance
significantly.
According to the analysis and comparison of the experimental data, some specific heat capacity and thermal
conductivity of the molten salt are needed at the same time. In the case of a certain amount of storage capacity,
the amount of working fluid is larger, and the investment of the molten salt part is improved. So it is necessary to
consider the melting temperature of molten salt, heat capacity, cost and other factors, take comparative method
to obtain the best molten salt type.
KEYWORDS: Solar Thermal Power Plant, Thermal Storage System, Molten Salt, Physical Property
1. INTRODUCTION
Solar thermal power generation is the most promising renewable energy power generation in the future,
in the United States and Spain and other countries have been the scale of development. Along with the
incentive policies, solar thermal power generation will be developed rapidly in China, but the suitable
areas for solar thermal power generation are located at the northwest, although the solar resource is better,
but the local temperature lower, minimum can reach 30 degrees below zero, so thermal insulation systems
require a higher, especially in molten salt for refrigerant tower solar power system, power plant cold
insulation requirements and higher cost.
Using temperature range, low viscosity, low volatile and thermal conductivity of large advantages with
inorganic molten salts (molten salt) is a good heat storage heat transfer medium, which has been widely applied
in solar thermal power generation industry. In order to further reduce the cost of construction and operation of
solar thermal power station, the molten salt composition and ratio were changed in this paper, and carried out a
variety of physical properties of test, laid the foundation for chosen heat storage material with low solidifying
point and high specific heat.
2. MAIN CONTENTS
2.1 Experimental principle
On the basis of traditional binary molten nitrate salt, increase different other chemical, molten salt material
component ratio change, found the variation of molten salt melting point, heat capacity, thermal conductivity, in
different molten salts. It has laid a foundation for the development of solar thermal power plant in the northwest
of China, which has the characteristics of freezing point and specific heat.
In the experiment, three kinds of main raw materials, namely LiNO3, CsNO3, Ca (NO3) 2, were added to
complete different types of molten salt according to a certain weight proportion. The thermal properties and
specific heat values of various mixed salts were studied by differential scanning calorimetry and sapphire
method.
2.2 Experimental process
2.2.1
New type of molten saltⅠ
The traditional two element nitrate is the mixture of 60% sodium nitrate and 40% potassium nitrate. The
freezing point of the molten salt is higher, about 220℃, and the specific heat capacity is about 1.52J/g.K. In
experiment molten salt I, the sodium nitrate and potassium nitrate composition were reduced in the traditional
nitrate, and increased the proportion of LiNO 3 , which nearly1/3, physical test results as shown in table 1. Under
this condition, the freezing point temperature of molten salt is decreased, the heat value is slightly increased, the
thermal conductivity is decreased, and the solidification temperature of the molten salt has a certain
improvement.
2.2.2
New type of molten saltⅡ
On the basis of the experiment molten salt I, the proportion of molten salt component is further changed, a
large number of CsNO3 components is added in the new molten salt typeⅡ,which the CsNO3 content is about
45%. The molten salt results after the test are shown in table1. The melting temperature of molten salt II is
reduced to below 100℃, and the specific heat capacity value is about 20% less than molten saltⅠ. The thermal
conductivity coefficient has a little change.
2.2.3
New type of molten salt III
On the basis of the experimental molten saltⅡ’s results , the Ca(NO3)
2
content is increased by 20% in the
same component of CsNO3 situation, and the molten salt type III is formed. The tested results of type III molten
salt are also shown in table 1. Melting point temperature of molten salt III is further reduced to 65℃, in addition
to good low temperature performance, the specific heat capacity value is reduced to 1.25J/g.K, thermal
conductivity is similar to molten saltⅡ.
Table 1. Measurements of three type molten salt
Parameter
Measured Value
M-salt
TypeⅠ
TypeⅡ
TypeⅢ
Melting Temperature(℃)
220
120
94.3
65
Specific Heat Capacity(J/gK)
1.52
1.65
1.32
1.25
Thermal Conductivity (W/mK)
0.45
0.38
0.309
0.315
2.3 Results
2.3.1 The melting temperature of four type salt
It can be seen from figure 1, along with the changes in the composition of the molten salt, melting
temperature decreased gradually, from the previous 220℃ down to a minimum of 65℃. It shows that LiNO3,
CsNO3 and Ca (NO3) 2have taken an important role in molten salt freezing point reduction. In the cold weather
in the northwest China, it is recommended with the components of nitrate to reduce molten salt system cost of
insulation.
Melting Temperature(℃)
250
200
150
100
50
0
Type Ⅰ
M-salt
Type Ⅱ
Type Ⅲ
Figure1. The melting temperature of four type salt
2.3.2 The melting temperature of four type salt
From figure2, the thermal conductivity of the four kinds of molten salts varies little with the composition. The
internal heat transfer effect of molten salt has no influence, and the components can form a good heat exchange
carrier, which is beneficial to the uniformity of temperature distribution in the heat storage and heat transfer
system. At the same time, it is suggested that the composition of the molten salt is recommended to select the
components with similar thermal conductivity.
Thermal Conductivity(W/(m.K))
0.5
0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
M-salt
Type Ⅰ
Type Ⅱ
Type Ⅲ
Figure2. The thermal conductivity of four type salt
2.3.3 The melting temperature of four type salt
It can be seen from figure 3, after the addition of LiNO3, the specific heat capacity of molten salt has a small
increase of about 10%, indicating that the specific heat capacity of LiNO 3 is higher than NaNO3 and KNO3.
With the addition of CsNO3 and Ca (NO3) that with low specific heat capacity components, the specific heat
capacity of the new mixed salt was reduced to about 18% under the conventional nitrate level. On the whole, the
specific heat capacity changes between these new molten salts are not as big as the melting point.
Specific Heat Capacity(J/(g.K))
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
M-salt
Type Ⅰ
Type Ⅱ
Type Ⅲ
Figure3. The Specific heat capacity of four type salt
3. CONCLUSION
The properties of molten salt have great influence on the whole performance and investment of the solar
thermal power plant. The physical properties of the three different types of molten salts are analyzed, from the
tested results, we have get the following conclusions:
a. The specific heat capacity of molten salt decreases with the decrease of melting temperature, and the low
melting temperature performance and storage capacity cannot be obtained simultaneously. In the future, it is
necessary to develop new molten salt to strengthen the heat storage properties and melting temperature.
b. The difference of the thermal conductivity of the three new types molten salts is not too large, which
proves that the heat transfer performance of the mixture is similar to that of the molten salt.
c. In the process of selecting the type of molten salt in the solar thermal power station, the melting salt type is
determined by taking the economy as the premise, taking into account the melting temperature and specific heat
capacity.
REFERENCES
[1] Roberts B, McDowall J. Commercial successes in power storage[J].IEEE Power and Energy Magazine,2005,
3(2):24-30.
[2] Gyuk I, Kulkarni P, Sayer J H, etc al. The United States of storage [electric energy storage] [J]. IEEE Power
and Energy Magazine,2005,3(2):31-39
[3] Goods, S. H., Bradshaw, R. W., Prairie, M. R., and Chavez, J. M., 1994,Corrosion of Stainless and Carbon
Steels in Molten Mixtures of Industrial Nitrates, Sandia National Laboratories Report, SAND94-8211.
[4]Bradshaw, R. W., and Carling, R. W., 1987, A Review of Chemical and Physical Properties of Molten Alkali
Nitrate Salts and Their Effect on Materials Used for Solar Central Receivers, Sandia National Laboratories
Report,SAND87-8005.
[5] Pacheco, J. E., and Gilbert, R., 1999, ‘‘Overview of the Recent Results of the Solar Two Test and
Evaluations Program,’’ Proce. of ASME Int. Solar Energy Conf. Renewable and Advanced Energy Systems for
the 21st Century, Maui,HI.
[6] Smith, D. C., Rush, E. E., Matthews, C. W., Chavez, J. M., and Bator, P. A., 1992, Report on the Test of the
Molten-Salt Pump and Valve Loops, Sandia National Laboratories Report, SAND91-1747.
[7] Nexant Inc., 2001, ‘‘USA Trough Initiative: Nitrate Salt HTF Rankine Cycle, Steam Generator, and Thermal
Storage Analyses,’’ prepared for NREL. Pacheco, J. E., Showalter, S., and Kolb, W., 2001, ‘‘Development of a
MoltenSalt Thermocline Thermal Storage System for Parabolic Trough Plants,’’ Solar Forum 2001, Washington,
DC.
[8] E. Zarza, L. Valenzuela, J. Leon, K. Hennecke, M. Eck, H.-D. Weyers, M. Eickhoff, Direct steam generation
in parabolic troughs: final results and conclusions of the DISS project, Energy 29 (2004) 635–644.
[9] U. Herrmann,D.W.Kearney, Survey of thermal energy storage for parabolic trough power plants, J. Solar
Energy Eng. 124 (2002) 124–132.
[10] X. Py, R. Olives, S. Mauran, Paraffin/porous-graphite-matrix composite as a high and constant power
thermal storage material, Int. J. Heat Mass Transfer 44 (2001) 2727–2737.
[11] Dinter, F., Geyer, M., and Tamme, R., 1990, ‘‘Thermal Energy Storage for Commercial Applications,’’
Springer-Verlag.
[12] Klaiss H, Kohne R, Nitsch J, Sprengel U. Solar thermal power plants for solar countries - technology,
economics and market potential. Appl Energy 1995;52:165–83.