dsc測量比熱容.ppt

1、a,1,New proposed methodology for specific heat capacity determination of materials,for thermal energy storage (TES) by DSC,Abstract,1.Introduction,2.Materials,3.Methodology,4.Results,5.Conlusions,abstract,This study presents a methodology to determine the specific heat capacity (Cp) of materials for

2、 thermal energy storage (TES) by DSC.Three different DSC methods are considered to be applied in the methodology, and are explained and compared in this study in order to select the most proper one for Cp determination. To perform this study, the Cp of three materials commonly used in sensible TES s

3、ystems, slate, water, and potassium nitrate (KNO3), is determined.,一、introduction,lack of fossil energetic resources its price need to decrease CO2 emissions lead to dedevelop new energy systems able to take profit of renewable energy and be environmentally friendly and less expensive,TES systems ar

4、e a possible solution Solar energy abundant clean and easy accessible. problem:as the hours of maximum energy demand match with the hours of no solar irradiation,with the aim to fill this existing energy supply gap, TES systems are presented as the solution to store the energy and use it anytime,TES

5、 technologies sensible heat storage the process by which the heat is accumulated due to the increase of the material temperature without experimenting structural changes, no phase change latent heat storage phase change , thermochemical energy material selection A high storage capacity Therefore,nee

6、d a high specific heat capacity.,(1),定義：在程序控制溫度下，測量物質(zhì)與參比物之間的能量差隨溫度變化關(guān)系的一種技術(shù)。 DSC曲線峰包圍的面積正比于熱焓的變化 可以測定多種熱力學(xué)和動力學(xué)參數(shù)，例如比熱容、反應(yīng)熱、轉(zhuǎn)變熱、相圖、反應(yīng)速率、結(jié)晶速率、高聚物結(jié)晶度、樣品純度等。該法使用溫度范圍寬（-175725）、分辨率高、試樣用量少。適用于無機物、有機化合物及藥物分析。,差示掃描量熱法（DSC） （Differential Scanning Calorimetry）,采用試樣和一標(biāo)準(zhǔn)物質(zhì)在相同條件下進行掃描，然后量出二者的縱坐標(biāo)進行計算。要求標(biāo)準(zhǔn)物質(zhì)在所測溫度范圍

7、內(nèi)沒有發(fā)生物理化學(xué)變化，且比熱容已知。常用的標(biāo)準(zhǔn)物質(zhì)是藍寶石。具體作法是在DSC內(nèi)先用兩個空樣品皿，以一定的升溫速度作一條基線，然后放入藍寶石樣品在相同條件下作一條DSC曲線，最后在同樣條件下作未知試樣的DSC曲線，如圖,three DSC methods for Cp measurement The dynamic method has traditionally been used to measure PCM latent heat but also for Cp measurements applying high heating rates in the temperature ra

8、nge of study The areas method is based on the Cp value, thus, how much heat flux is needed to heat up a material for temperature increase, and consists of consecutive isothermal stages differing 1 with no heating segments amid The isostep method is an intermediate between these two, as it is a succe

9、ssion of dynamic methods applied to heat up the material just 1 between isothermal segments. This method has been tested in glass transition pharmaceutical studies with good results obtained , but it has not been used for TES materials Cp determination, thereby its selection for this study,a,9,二、Mat

10、erials,To ensure the performance of the methodology in a representative variety of material phase forms and chemical structures, three different materials widely used in sensible heat storage systems have been chosen to perform this study. Water. Its thermal properties are well known, and therefore,

11、 these values can be used as a reference to see the approach on the measurements each method has. Commercial Bi-distillated water from Panreac has been used to perform the experiments Slate. It is a widely used construction material and its performance depends on its specific heat capacity, therefor

12、e authors find it an interesting material to be tested. The slate samples used in this study were taken from a quarry in the Catalan Pyrenees. Potassium nitrate. It is an inorganic salt mainly used as molten salt in concentrated solar power plants (CSP Plants). It has high melting point (320) and, a

13、s all inorganic salts, a complex chemical structure, and that is why it is also found to be useful for this papers purpose. It is important to remark that this material selection responds to the need to test the methodology and not to tie it to a concrete material type or a specific TES application.

14、 Therefore, materials used in different TES systems and with which authors commonly work have been selected.,determined by differential scanning calorimetry (DSC) Standard 40 uL aluminium crucibles weigh with a analytical balance with a precision of 0.01 mg. The procedure to determine the specific h

15、eat capacity of a material consists of three different measurements, all done under the same conditions, thus, using the same DSC method: Blank measurement. It is necessary to run an experiment with an empty crucible to measure the heat flux that corresponds to the crucible material in order to subt

16、ract this signal and consider only the sapphire and material sample ones. Sapphire measurement. Sapphire is the material used as reference as its specific heat capacity is well known at every temperature and its signal is hugely stable over temperature, data necessary for the materials Cp calculatio

17、n. Material measurement. Needed to determine the Cp value of the material under study.,三、Methodology,mW,K/S,(2),The sensitivity of the analysis is tied to the heating rate and the sample mass. literature studies found high heating rates of about 1020 K/min are desired Eq. (2) shows, minor sample mas

18、ses are needed. The Cp was measured between 15 and 16 , 2526 and 35 36 for all the materials under study. These measurement ranges were selected in order to ensure that water was in liquid state and to be able to take the certain known water 4.18 J/gCp value at 25 as a reference to evaluate each met

19、hod performance.,3.1. DSC measurement methods 3.1.1.Dynamic method It is a temperature controlled method that achieves the thermal equilibrium before and after a controlled heating segment ,it consists of three segments. 1015 min to homogenize the material temperature high heating rate another isoth

20、ermal stage again for 1015 min the lower and higher temperatures of the method can be 150 ,（3）,a,12,Similar to the previous explained dynamic method, the isostep method This method uses Eq. (3) to calculate the Cp of the material along each heating segment.,3.1.2.isostep method,3.1.3.Areas method Th

21、e areas method consists of consecutive isothermal segments with no heating stages amid.Each of the peaks corresponds to every temperature step of the method. The temperature difference between isotherms is of 1 .,where As J/g is the integrated peak area for the sapphire curve,In this study, results

22、are given with an error of 3%, including both equipment and calculation errors. The dynamic method gives a continuous signal along the whole temperature study range, as shown in red Three different continuous signals are obtained with the isostep method, one for each heating segment for the 1516 C,

23、25 26 C and 3536 C steps. However, as the measurement temperature range is of just 1 C and the material is heated at a really fast rate,四、results,the three signals are really steep and no clear tendency can be seen in the results, fact that already shows the low sensitivity this method has for Cp ca

24、lculation. Nevertheless, and in order to compare the methods, an average value of the most constant parts of these three signals is presented in the graphics. Contrary, the areas method provides just three points, one for each temperature increment between the isothermal segments, hence, one at 16 C

25、, one at 26 C and one at 36 C.,4.1water,areas method gets closer values to the theoreticai values The dynamic method inconsistent values are found in the begining and the end,that indicates sensitivity problems in these two measurement parts. the isostep method are the average of the most constant p

26、arts of the obtained signal,being the standard deviation for water of 0.08 J/g .,The sensitivity problems. Due to the abrupt temperature change on both initial and end points, the DSC temperature sensor cannot react fast enough to read the real temperature.This fact has greater significance in the i

27、sostep method results due to the higher relation between heating rate and temperature increase.,a,16,results show that the areas method is the method that provides Cp close to the theoretical range dynamic method the isostep method, with a standard deviation of 0.03 J/g .most inconsistent,4.2.slate,4.3. potassium nitrate,same condition same reason,both dynamic and isostep method measurements are limited by the high heating rates applied, that low down the sensitivity of the analyses and increase the error of the results. comparing the three material results analysis