Microsoft Word - B_01_Arpad_R.doc HUNGARIAN JOURNAL OF INDUSTRIAL CHEMISTRY VESZPRÉM Vol. 39(2) pp. 163-167 (2011) INVESTIGATION OF SENSIBLE HEAT STORAGE AND HEAT INSULATION IN THE EXPLOITATION OF CONCENTRATED SOLAR ENERGY I. ÁRPÁD University of Pannonia, Doctoral School of Chemical Engineering and Material Sciences 10, Egyetem Street, H-8201 Veszprém, HUNGARY E-mail: arpad.istvan@hotmail.com MVM ERBE Power Engineering & Consulting Ltd. 95, Budafoki Road, H-1117 Budapest, HUNGARY This paper analyses the exploitation of solar energy by wholly relying on it to heat homes by 100% solar heating in Hungary. It determines the necessary amount of heat and the heat storage capacity and considers the time sequence between charge and discharge. Further, it provides a feasible technology for sensible heat storage and it determines the sizes of heat store- building, the thickness of heat insulation and it calculates the heat losses of heat storage. On the basis of the results, the paper provides proposals for the method of heat storage and for the technical parameters to be considered for the heat insulation. It describes the application of the heat storage method for district heating and electricity generation. Keywords: solar energy, sensible heat storage, heat insulation, 100% solar heating of home, electrical energy generation Introduction We have been striving for a long time to be capable of collecting the energy of solar radiation and of storing it. Certainly, we would like to store and use solar energy for a long time without suffering any great losses. The question is, whether the collected and stored energy could provide 100% of the homes’ heatingthe whole year round or whether it could generate electricity over several months? To achieve that several problems need to be solved: - The flux of solar energy is low. - There is no harmony between energy generation and consumption and it is incalculable as a function of time. Because of that big energy storage is needed. - Efficient and economic energy storage for a long time is an unsolved problem. This paper analyses this issue and presents a feasible technological solution of how the buildings could continuously be supplied with heat energy from the direct solar radiation and how the energy must be stored as sensible heat storage and how the heat insulation of the heat storage facilities must be planned [1]. Calculations for the size of solar radiation field and for the heat storage capacity The solar radiation, that passes directly through the atmosphere to the Earth’s surface, is called direct solar radiation. The period, when the direct radiation is more than 210 W/m2, is called sunny hours. In Hungary, the number of sunny hours varies between 1900–2200 hours per year, which is quite long compared to that in the world. The indicated data are based on statistics of several years [2, 3, 4]. The direct radiation is approx. 1000 W/m2 on the surface of the Earth in fine weather. This value is lower under cloudy weather conditions and during air pollution. We use in the following calculations the average sunny hours of 400 W/m2 (Fig. 1). In a year’s time period, perpendicularly to the direction of solar radiation, we can estimate the amount of the collectable direct solar energy as below: 2 2 2000 3600 400 2880 ( ) h s W year h m MJ m year ⋅ ⋅ = = ⋅ ⋅ (1) Figure 1: Intensities of direct solar radiation 164 Heat energy consumption of a family house In a dwelling-house. heat energy is used for heating and hot water production. Without detailed explanation of the calculations, we have estimated alltogether 80000 MJ heat energy consumption per year for five persons and an average house of cc. 100 m2. That number is equivalent to approx. 2350 m3 natural gas (34 MJ/m3). Table 1 shows the energy consumption (heating and hot water generation) in each month of a year. As a matter of fact, new houses and block houses have lower energy requirements. The task is to collect the above indicated 80000 MJ heat energy and the heat losses of the heat storage “tank”. Sizes of the solar field We can collect energy of approx. 400 W on a surface area of 1 m2. This energy can be collected by a surface right-angle to solar radiation in sunny hours and if there are approx. 2000 sunny hours/year in Hungary. We can calculate this surface area of solar radiation by using the following formula (the calculation relates to the demand of 80000 MJ energy): 2 2 2 80000 400 2000 3600 ( ) 27.75 28 MJ year A J s h s m h m m = = ⋅ ⋅ ⋅ = ≈ (2) This 28 m2 does not include the heat losses. Table 1: Energy consumption of a dwelling-house per year period (days) sunny hours collectable direct solar energy per m2 [MJ/m2] Charge [MJ] collected solar energy on 27.75 m2 heating [MJ] hot water production [MJ] Discharge [MJ] total heat energy requirement Amount of energy to be stored [MJ] Apr (30) 187 269 7470 1840 1200 3040 4430 May (31) 253 364 10110 900 1240 2140 12400 June (30) 267 384 10670 0 1200 1200 21870 July (31) 297 428 11870 0 1240 1240 32500 Aug (31) 278 400 11110 0 1240 1240 42370 Sept (30) 202 291 8070 260 1200 1460 48980 Oct (31) 139 200 5550 1900 1240 3140 51390 ≈52000 MJ Nov (30) 63 91 2520 9470 1200 10670 43240 Dec (31) 40 58 1600 14730 1240 15970 28870 Jan (31) 57 82 2280 16830 1240 18070 13080 Feb (28) 83 120 3320 12100 1120 13220 3180 Mar (31) 136 196 5430 7360 1240 8600 10 year (365) 2002 2883 80000 65390 14600 79990 Capacity of heat storage A heat storage “tank” shall be used due to the sequence of time between charge and discharge. Table 1 shows the calculating of the capacity of heat storage “tank”. The heat capacity depends on the charge and the discharge. The total capacity of the heat storage “tank” amounts to 52000 MJ. This size of heat storage “tank” can ensure heat energy supply for a house all the year round. Method of heat storage and sizes of the heat storage “tank” The method of sensible heat storage is the simplest one. We have surveyed many heat storage materials and have chosen magnesite brick. Calculations with magnesite brick showed the best results. Table 2 shows the properties of the magnesite brick. Corundum (95% Al2O3) brick is also a very good heat storage material: its density of energy amounts to 3.3 MJ/(m3K) and its melting point is 2020 °C. Table 2: Properties of magnesite brick [5, 6] Content Application range of temperature ΔT Specific heat J/(kgK) Density kg/m3 Density of energy MJ/(m3K) Heat conductivity W/(mK) Price $/ton 37–98 % MgO 1–60 % CaO and/or Cr2O3 65–500 °C (melting point: 2852 °C) 1172 3020 3.54 8.4 (on 500 °C) 100–500 165 We can calculate the mass and volume of magnesite brick from the energy capacity of heat storage “tank” (≈52000 MJ), from the planned range of temperature (∆T = 500 – 65 °C = 435 °C) and from its specific heat and density. The following calculation is applicable: Q Q c m T m c T = ⋅ ⋅Δ → = ⋅Δ (3.a) 952 10 101997 1172 435 102 J m kg J K kg K m tons ⋅ = = ⋅ ⋅ ≈ (3.b) 3 3 3 102 33, 7 3, 02 34 m tons V m tons m V m ρ = = = ≈ (4) This size seems to be a normal value and normal scale. If the end point of maximum temperature were just 430 °C, the size of heat receiver would be 40 m3. However, the 500 °C of maximum temperature is real too, scilicet the thermooils (heat transfer fluids) work on 580 °C (1060 °F) in the existing concentrated solar power plants. The temperature difference, needed to the heat exchange, is ensured. Heat insulation and heat losses Heat store-building made of bricks Hereinafter, the heat storage “tank” will be named as heat store-building because there is no tank in the construction. We analyise here only a cubic shaped heat store-building. The construction is shown in Fig. 2. We make a difference between the bottom and the upper parts of the store-building as follows [7]: Figure 2 1 − external wall, 2 − coat of heat insulation from rock wool, 3 − magnesite bricks, 4 − concrete pad, 5 − gravel bed, 6 − pipe of heat transfer fluid The bottom part is in contact with the soil and the upper parts of the store-building are in contactwith the the ambient air. The upper parts are built up from lateral- walls and from the roof. We calculated these parts (wall and roof) in the same way. The thermal resistance of the upper part and of the bottom part (Rcond = δ/λ, respectively Rconv = 1/α) are indicated below: joint 1 1 conv rad R α α α = = + (5) Ruppers = Rcond + Rjoint (6) Ruppers = Rins + Rbrick + Rjoint (7) Rbottom = Rconcret + Rgravel + Rsoil (8) The value of the heat transfer coefficient between the external side of the wall and the ambient air is α = 24 W/(m2K). This value has been derived from a Hungarian architectural standard (MSZ 04-140-02). The foundation of the store-building would be constructed from cellular concrete: its density is 700 kg/m3 and its bearing strength is more than 150 N/m2. Table 3 shows the material properties of the store- building, which we have used in the calculation process [6, 7, 8,]. Table 3: Applied value of λ thermal conductivities and δ coating thickness rock woll brick/barge stone cellular concrete gravel soil °C W/(mK) 500−400 0.180 400−300 0.100 300−200 0.070 200−100 0.049 <100 0.038 0.64 W/(mK) < 0.17 W/(mK) 0.35 W/(mK) 1.3 W/(mK) δins= to be determined δbrick = 0.12 m δconcrete = 0.6 m δgravel = 0.3 m δsoil = 0.4 m 166 The thermal conductivity of the heat insulation (rock wool) increases significantly with the rise of temperature λ(T). The curve can be seen in Fig. 3. The above mentioned function λ(T) has been considered in the calculation process. Actually, in every month we experienced various heat resistances. Table 4 shows the values of the ambient temperature and the soil temperature at a dept of 1 m. Figure 3: Thermal conductivity of rock wool versus temperature Heat losses of the heat store-building We calculated the heat current as heat conduction through the flat wall. The heat transfer between the external surface of the wall and the ambient air (at the joint) equals to the conductive heat current in the wall. If we know the temperature of the external surface of the wall, the internal temperature ΔTconductiv = Tmagnesite brick – Toutside wall (9) ΔTconductiv = Tint – Text (10) and the thickness of heat insulating material δins, we are able to calculate the heat current q& [W/m2]: transfer cond transfer cond T T q és q R R Δ Δ = =& & (11,12) ;cond transfercond ins transfer T R R q T δ Δ ⋅ = → Δ & (13) Q q A= ⋅& (14) We designed the maximum internal temperature (Tint) and the maximum external temperature of the wall’s surface area (Text) to be 16 °C under conditions in October. Further, we calculated 40 cm thick insulating material (rock wool), which represents a realistic value. Table 4 shows the heat losses suffered in each month and all the year round. The calculations were performed on one house (with a store building volume of 34 m3), on 50 houses (with a store building volume of 1700 m3) and on 100 houses (with a store building volume of 3400 m3). It is a remarkable result that the specific heat losses fall with increasing store-building size (m³). The amount of decrease is remarkable. The cause of that is that the specific surface “A/V – surface/volume” decreased. Further, we calculated the following values: the specific heat loss is as high as 14% in the case of store-building of 500 dwelling-houses ( with size of 17000 m3) and it is as high as 11% in the case of 1000 dwelling-houses (34000 m3)! We analysed the dependence of specific surface area “A/V” on the volume “V”. We performed the analysis by using a cube. Table 5 shows the results of the volume (V [m3]) and the specific surface (A/V [m2/m3]) with different lengths of the edge of the cube. Then we graphed them in Fig. 4. Table 4: The heat losses of different sized heat store-buildings with 40 cm thickness of the rock wool 34 m3 1700 m3 3400 m3 Tint [°C] Text [°C] Tamb [°C] qupper [W/m²] Tsoil [°C] qbottom [W/m²] Qtotal [GJ] Qtotal [GJ] Qtotal [GJ] Apr (30) 102 12 12 8 10 20 1.6 22 35 May (31) 168 18 17 16 14 33 3.2 43 68 June (30) 247 21 20 28 18 49 5.1 70 111 July (31) 335 24 22 45 20 67 8.2 111 177 Aug (31) 417 24 21 69 21 84 12.1 164 260 Sept (30) 472 21 17 94 19 97 15.4 209 332 Oct (31) 492 15 11 104 14 102 17.5 237 377 Nov (30) 424 9 6 74 10 88 12.5 169 268 Dec (31) 305 4 2 40 7 63 7.4 100 159 Jan (31) 174 1 0 18 5 36 3.5 48 76 Feb (28) 91 2 2 8 4 19 1.5 20 32 Marc (31) 65 6 6 5 5 13 1.1 14 23 total heat losses of a year [GJ] 89.1 1209 1919 total heat consumption of a year [GJ] 80.0 4000 8000 heat losses versus heat consumption per cent [%] 111% 30% 24% 167 Table 5: The specific surface area of a cube versus its size a [m] 1 2 3 4 5 6 7 8 9 10 20 30 40 50 A [m²] 6 24 54 96 150 216 294 384 486 600 2400 5400 9600 15000 V [m³] 1 8 27 64 125 216 343 512 729 1000 8000 27000 64000 125000 A/V [m²/m³] 6.00 3.00 2.00 1.50 1.20 1.00 0.86 0.75 0.67 0.60 0.30 0.20 0.15 0.12 The next algebraic formula describes the function of Fig. 4: 3 6 y x = (15) Figure 4: Specific surface versus volume of cube Conclusions The paper sets out that it is possible to store solar energy all the year round or for a long period. The stored heat energy stored can meet the total heating demand of the houses or can also generate electricity in Hungary. We can keep the heat losses at low level (<20%). Certainly we must consider some technical facts. Heat energy shall be stored as below: - at high temperature: the higher the better, - using materials with high energy density [MJ/(m3K)] (using one of solid materials, for example magnesite brick) and - in store-building whose size is big enough, because the heat losses shall be low. We would emphasize here that the increase of size, up to a certain size, is one of the best heat insulation technique. The heat storage in solid material is easy and safe. No steel tank is used and the brick isn’t flammable and explosive. In my opinion this method should be used for district heating and for electricity generation. Our next goals are: - to determine the optimum size of the heat store- building and the thickness of heat insulation coat [9, 10] and - to investigate different heat transfer materials. We would like to achieve higher temperature in the store-building. Perhaps gaseous materials would be good heat transfer materials from the solar trough to the store building. REFERENCES 1. I. ÁRPÁD: Investigation of the Sensible Heat Storage and the Heat Insulation in the Exploitation of Solar Energy (in Hungarian). 19th International Conference on Mechanical Engineering April 28 – May 1 2011. OGET 2011. p. 31–34, Sumuleu Ciuc, Romania 2. GY. MAJOR, A. V. MORVAY, F. WEINGARTNER, O. FARKASNÉ TAKÁCS, ZS. ZEMPLÉNYINÉ TÁRKÁNYI (eds.): Solar radiation in Hungary (in Hungarian). Official Publication of Hungarian Meteorological Service, No. 10, Budapest, Hungary, 1976, ISBN 9637701052 3. Homepage of Hungarian Meteorological Service, Data of Climate, www.met.hu 4. I. BARÓTFI: Exploitation of Solar Energy (in Hungarian). Handbook for Users of Energy. Környezettechnikai Szolgáltató Kft., Budapest, Hungary, 1994 5. I. SZŰCS, Á. B. PALOTÁS, N. HEGMAN: Effect of Inhomogeneous Radiation Coefficient on the Surface Temperature Field of Refractory Lining Using Thermovision (in Hungarian). Sciences of Material and Metallurgy, Research report, Miskolc, Hungary, 2000 6. F. TAMÁS (ed.): Handbook of Silicate Industry (in Hungarian). Műszaki Könyvkiadó, Budapest, Hungary, 1982 7. K. C. KWON: Engineering Model of Liquid Storage Utility Tank for Heat Transfer Analysis. International Joint Power Generation Conference, Minneapolis, 1995 8. Brochures of Rockwool. Insulation of high temperature applications. Rockwool Hungary Kft., Budapest 9. I. TIMÁR, I. ÁRPÁD: Optimization of Pipes’ Insulation. (in Hungarian). Energiagazdálkodás 27(10), (1986), 449–459, Budapest, Hungary 10. I. TIMÁR: Optimierung ebener Fachwerke mit mehreren Zielfunktionen. 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