Storing solar energy principle of zeolites
Experimental research and results analysis
Jilin Agricultural University
Changchun, Jilin Province, 130118, China
Utilizing 13X synthetic zeolite to store solar energy has been successful. In this paper, the storing solar energy principle of zeolites is discussed, the contrast study of natural zeolites to the 13X synthetic zeolite was made, and the conclusion showed that natural zeolites can be used as storing solar energy material completely instead of the 13x synthetic zeolite below 100°C.
Solar energy intermittence and its varying radiation intensity are the key factors affecting solar energy efficiency. For example, radiation intensity of solar energy in the same place varies with season, day and night, and weather. In general, it is difficult to match the energy demand of some users with variety of solar energy. Therefore, solar energy storage is one of the critical problems of present solar energy application research. Ways of storing solar energy can be roughly classified into three types.
1. Sensible Heat Storage
In middle and low temperature solar energy systems, water and stones are the best and cheapest storing energy medium. The heat energy collected by solar energy collectors increases the temperature of the medium, so the heat energy is stored in the medium. When needed, the heat energy is desorbed for use.
The density of sensible heat storing energy is low, and the period of it is short.
2. Latent Heat Storage
For this type of storage, the storing energy medium is required to have the features of big energy absortivity, small variety of volume, little corrosion, and good repeatability. Presently, sulphate, nitrate, and hydrate carbonates are usually used as the medium.
The latent heat storage from phase transformation is characterized by high energy storing density and long storage period. Nevertheless, the medium is easily cooled. Once this happens, crystallization of the medium is made difficult.
3. Chemical Reaction Energy Storage.
In this type, the endothermic reaction of chemical material is used to store heat energy. When the inverse process takes place, the heat can be released. Some inorganic oxides are usually used as the medium.
The merits of this type are high capacity for storing heat and long storage period. Nevertheless, the high temperature required by endothermic reactions, results in high equipment costs.
Zeolite is a aluminosilicate mineral of alkali or alkaline earth metal which contains crystal water. Its general chemical formula is
AmXpO2p · nH2O
Where A represents Ca, Na, K, Ba, and Sr; X represents Al and Si. Zeolites consist of three dimensional networks of AlO4 and SiO4 tetrahedra linked by sharing of all oxygen atoms. The aluminosilicate frameworks of zeolites are remarkably open and contain channels, and interconnected voids partially filled with cation and water molecules. The intracrystalline voids make up from 20 to 50% of the total crystal volume of most zeolites. When zeolites are heated, water molecules in the intracrystalline voids are partially or wholly desorbed. After being desorb, the water molecules can be adsorbed in the air or water again, and crystal lattice construction of zeolite are not damaged.
Zeolites have extremely nonlinear adsorption isotherms to water. The feature of adsorbing and desorbing water makes zeolites a new type of material for storing solar energy and to be showed off. When zeolites are heated, water molecules in it escape, and heat energy is stored in it in the meantime; when water molecules are adsorbed again, the heat energy in zeolites is released. These two process can be shown by chemical equation as follow:
AmXpO2p · nh2O = AmXpO2p+nH2O (endothermic)
AmXpO2p · nh2O = AmXpO2p+nH2O (endothermic)
When zeolites absorb heat and desorb crystal water molecules, the temperature of it does not vary, therefore, this process belongs to the one of latent heat storage. So long as zeolites, of which water molecules are desorbed, keep apart water, the heat energy of it can be stored as long as you like. The energy storing density of zeolites is higher than the aforementioned three types of ways of storing energy. When the heat energy in zeolites is desorbed, we can control the speed of desorption by controlling the speed of water absorption. Therefore, zeolites have better merits than the aforementioned ways of storing energy.
Usually, the 13X synthetic zeolite is chosen as storing energy material.
At present, the price of 13X synthetic zeolite is too high in China and we may not use the 13X synthetic zeolite in quantity in the near future. For these reasons, we attempted to use natural zeolites instead of 13X synthetic zeolite, and the contrast experiment was made in respect of desorbing water, adsorbing heat and redesorbing water, desorbing heat. The experimental process is as follows:
1. Sample natural zeolites of having representatives, high purity, and bigger reserves from different mines, and work these samples for different grain sizes.
2. Sort natural zeolites samples and 13X synthetic zeolite in a fixed quantity, then put into metal boxes, mark and weigh the metal boxes separately.
3. Put these samples into the thermostat to heat for one hour at 50 °C, 100°C, 150°C, and 200°C respectively, for the common solar collector works at about 50--200°C.
4. Take out boxes from the thermostat after heated, weigh them separately, work out water loss of each box, and seal them up.
5. Mix samples rapidly with water of fixed quantity separately, until they are cooled down to the ambient temperature. Stir the mixture of water and zeolites, take measure of temperature variation.
We carried out the experiment with 12 type of natural zeolite samples, which are B1, B2, B3, B4, C2, C3, D2, D3, El, Fl, G1, HI, and 13X synthetic zeolite. Where subscript 1, 2, 3, and 4 represent the grain size of natural zeolites samples from little to big, letter B, C, D, E, F, G, and H represent the different natural zeolites samples.
Fig.1 shows the experiment curves of the heating temperature of typical natural zeolites samples B1, B2, G1 and 13X synthetic zeolite sample A as water loss of it varies.
Fig. 1 Relational curves of heated temperature and water loss
From the relation curves of heating temperature and water loss, we can see:
1. The water loss of each type of sample rises as the heating temperature goes up.
2. At the same temperature, sample B1 's water loss is greater than sample B2's. At this point, the smaller the grain size of natural zeolites is, the bigger the water loss is, with the same type of natural zeolites. The smaller the grain size of the sample, the bigger the voids of it.
3. Sample A's water loss is least at the heated temperature of 100°C. The others are bigger than 13X synthetic zeolite's.
Fig. 2 shows the experiment curves of the heated temperature of representative natural zeolites samples B1, B2, G1 and 13X synthetic zeolite as the temperature variation, which is the difference of the temperature after the sample is mixed with water and ambient temperature changes.
From Fig. 2 we can see:
1. The temperature variation of every type of sample rises with increased temperature. It is the reason that the water loss of samples is big at the high heated temperature, and the stored energy is also big and vice versa.
2. In 50--100°C, the low heated temperature range, the difference of sample A's temperature variation and the others is less. The reason for that is that the water loss of sample A and the others is less.
3. In 100--200°C, the high heated temperature range, the difference of sample A's temperature variation and the others rises gradually as the heated temperature does. At 200°C, the difference is biggest due to the water loss.
Fig. 2 Relational curves of the heated temperature and temperature variation
1. When the heating temperature is below 100°C, the effect of natural zeolites' storing energy is roughly the same as 13X synthetic zeolite's. So we can use these natural zeolites as the storing materials instead of 13X synthetic zeolite below 100 °C . The operating temperature of the conventional flat-plate solar collectors generally is below 100°C, so this aforementioned conclusion has important significance.
2. In 100--200°C, the heated temperature range, 13X synthetic zeolite has obvious advantages, which because that (1) natural zeolites samples contain more impurity than the 13X synthetic zeolite, which has an effect on the storing heat energy of zeolites. (2) the structure of natural zeolites' samples is not as good as 13% zeolite's.
3. The tentative idea of utilizing natural zeolites to store solar heat energy is feasible. For the next research, concentration should be paid to the option of natural zeolites type and grain size, forming method, and the storing energy device structure.
1. XU Ruren, etc. Structure and Synthesis of Zeolites Molecular Sieve, Published by Jilin Universityi 1987.8.
2. WU Xianggan, etc. Rural Energy Source, Published by Agriculture Publishing House, 1988.6.