MODELLING AND SIMULATION THERMOCLINE STORAGE FOR SOLAR THERMAL POWER PLANT

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1 International Journal of Mechanical Engineering and Technology (IJMET) Volume 9, Issue 13, December 2018, pp , Article ID: IJMET_09_13_0311 Available online at aeme.com/ijmet/issues.asp?jtype=ijmet&vtype= =9&IType=13 ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed MODELLING AND SIMULATION OF THERMOCLINE STORAGE FOR SOLAR THERMAL POWER PLANT Dr. R. N. Patil Principal, Bharati Vidyapeeth College of Engineering, Lavale Pune University, Pune, Maharashtra, India Prof. Jadhav Sandhya Dilip Research Scholar, Department of Mechanical Engineering, Bharati Vidyapeeth Deemed University, Pune, Maharashtra, India ABSTRACT Storage of thermal energy increases the capacity factor of a solar plant. Thermocline storage tanks are preferred option of thermal energy storage in central receiver solar thermal power plant. It is difficult to conduct the experiment due to large size of storage tank. Code is developed using Visual Basic for studying performance of plant. Code is first validated from experiment data in available literature. The effect of inclusion of storage tank on electrical power generation is studied by simulation. Then simulation is done for thermocline tank with Molten salt Hitec XL and quartzite rocks as storage media. Heat exchange between molten salt and quartzite rock and their temperature profiles are studied. Keywords: Central Receiver Solar Thermal Power Plant, Modelling and Simulation, Thermocline Storage Tank, Molten Salt. Cite this Article: Dr. R. N. Patil and Prof. Jadhav Sandhya Dilip, Modelling and Simulation of Thermocline Storage For Solar Thermal Power Plant, International Journal of Mechanical Engineering and Technology, 9(13), 2018, pp et/issues.asp?jtype=ijmet&vtype=9&itype e=13 1. INTRODUCTION In the present solar power generation technologies available, central receiver solar thermal power plant is the most efficient technology therefore development work is going in this field all over the world. In photovoltaic plants, energy storage is too costly and hence plant has less efficiency. In thermal power plants it is easy and cheaper to store thermal energy as compared to electrical energy in photovoltaic plants. In central receiver thermal plants, a large array of mirrors called heliostats receive the solar radiation and focus it on a large receiver placed on IJMET/index.asp 284 editor@iaeme.com

2 Dr. R. N. Patil and Prof. Jadhav Sandhya Dilip the top of a tower. Heat transport flows through the tubes of receiver and absorbs thermal energy and delivers the thermal energy to power block. The central receiver solar thermal power plant must satisfy the energy demand like any other power plant. The electrical energy of a central receiver solar thermal power plant is affected by the variation in weather. It is unavailable in night and every time a shadow is cast on collection area, the power output is either reduced or plant stops operating. In order to cope with these fluctuations and to utilize the maximum solar radiation, excess thermal energy is produced during day time and stored for cloudy weather conditions or later use in night. Hence, solar thermal power plant can be operated in absence of solar radiation without burning fossil fuel. Similarly gap between the amount of solar radiation supplied by the sun and required electrical energy is reduced. All thermal power generated from the receiver is delivered to the thermal storage tank, provided the thermal storage tank can accept it (i.e. tank is not full). The turbine operates, after a minimum amount of thermal energy required to run the turbine is accumulated in the storage tank. The extra thermal energy remains in the storage tank. This process continues till the availability of sunshine hours. The stored thermal energy is used to run the plant after the sunshine hours. The daily variation in solar radiation and electricity requirement profile decides the use of thermal storage system. The earliest storage system was incorporated in Solar One central receiver solar thermal pilot power plant (from 1982 to 1986). Fig.1 Central receiver solar thermal power plant 2. THERMAL ENERGY STORAGE The most developed concept is the sensible heat storage concept due to its commercial use especially in chemical plants. Two options for thermal energy storage are available, namely two tank storage system and single tank thermocline system. Although the two-tank model for thermal storage has been more commonly adopted in CSP plant designs, the prominence of thermal storage with stratification in solar applications has prompted the development of a single tank plant model. The potential benefits of using a single tank with stratification are reduction in required total tank volume, reduced construction costs due to consolidation from two tanks to one, a simplified control scheme, reduced piping, and reduced pumping requirements. However, there are some considerable drawbacks with stratification. The average temperature of the tank approximates the thermal inventory of the tank. When the average temperature in the tank approaches the cold outlet temperature, the thermal storage is nearly fully discharged, and when the average temperature is near the hot outlet temperature, the opposite is true. IJMET/index.asp 285 editor@iaeme.com

3 Modelling and Simulation of Thermocline Storage For Solar Thermal Power Plant The cost of a thermal energy storage system mainly depends on the storage material used for storage of thermal energy, the heat exchanger for charging and discharging the system and the cost for the vessel or tank for the thermal energy storage system. The synthetic oils used for heat transport fluid are very expensive as well as have pressures at high temperatures, therefore molten salts namely, Hitec, Hitec XL, solar salts are preferred as fluid for high temperature thermal energy storage. Molten salts are stable up to higher temperature range of C C with very low vapour pressure. Their operating temperatures are compatible with todays high pressure and high temperature steam turbines. Molten salts are efficient low cost medium to store thermal energy. It is non-flammable and non toxic. At present, molten salts are widely used in chemical and metal industries as a heat transport fluid. At high temperatures, the heat of fusion of molten salts is preferred. Many salts are being used because of their high mass and volumetric heat storage capabilities, their abudance in nature and their low cost per unit storage capability. By utilizing the heat of fusion (liquid-solid transition) of various salts, large amounts of thermal energy can be stored and subsequently released at nearly constant temperature. 3. THERMOCLINE STORAGE The performance of thermocline system is of significant interest to future central receiver solar thermal power plants. The intent of this article is to review thermocline tank storage studies and also study temperature profile in the tank. It is difficult to conduct physical experiments due to large size of thermal energy storage tanks. Therefore, numerical solutions are preferred to study performance of thermal energy storage tanks. Kolb simulated a 1 MW e Sagurao parabolic trough power plant with addition of 30 MWh t thermocline tank and found that capacity factor of plant was increased by 23% to 42%. Van Lew used Schumann equations and proved that a small increase in thermal storage efficiency due to larger aspect ratios of tank height to diameter. This gain is due to improved forced convection between solid material and fluid. Increased fluid due to larger tank height increases the convection. Molten salts are expensive, therefore molten salts are substituted partially by low cost storage solid materials, thus reducing the total cost of required storage material. Packed bed storage uses the tank filled with rocks, through which hot fluid is passed from top to bottom of the tank. The filler material used should have compatibility with molten salt (the heat transport fluid used in tank) and also high heat capacity. On a volumetric basis, the specific heat of the rock is about five percent larger than the specific heat of the nitrate salt. The rocks widely available are quartzite, taconite, marble and limestone. Out of these quartzite rock and silica sand can withstand high temperature molten salt without deterioration. Fig.2 Packed bed storage tank IJMET/index.asp 286 editor@iaeme.com

4 Dr. R. N. Patil and Prof. Jadhav Sandhya Dilip 4. MODELLING OF THERMOCLINE TANK The temperature distribution of solid and heat transport fluid in thermocline tank varies along the distance in bed. It is also a function of time. The transient heat transfer analysis is given by Schumann. The height of packed bed unit is L and diameter D packed with solid (filler material) having an equivalent spherical diameter d and a void fraction ε. The mass flow rate of heat transport fluid is m and it enters with a constant temperature T fi. For the modeling, it is assumed that, the bed material has infinite thermal conductivity in the radial direction and zero conductivity in the axial flow direction. The heat transfer does not vary with time and place inside the bed. Heat losses to the surroundings are negligible. The energy equation is applied separately to the hot fluid and solid material as both have different thermal conductivities and heat capacities. Let the temperature of the solid change from T s to (T s + dt s ) and the temperature of the fluid change from T f to (T f + dt f ). The density of solid and fluid are represented by ρ s and ρ f, and C ps and C pf are respective specific heats. Heat exchange between HTF and solid material is accounted with volumetric heat transfer coefficient h v defined per unit volume of the bed. The energy equations for solid and fluid is given by equation T 1 ερ C t =h T T T ερ C +4mC T t πd x =h T T Lof and Hawley correlation for Volumetric heat transfer coefficient h =650! " #.% W/m 3 K G is mass velocity and is given by 4& '( ) " in kg/sm 2 Defining dimensionless time τ and dimensionless distance X, for a time t seconds and distance x meters is as follows, += h t ρ C 1 ε -= πd h x 4 m C Corresponding to values of τ and X, and neglecting other terms ερ C in comparison.1 to other two terms in equation the dimensionless temperature distributions are given by = : and 23 ; 2< = Equations are solved by Schumann to obtain a non dimensional temperature distribution. The solid is assumed to be at initial temperature T i and hot fluid at initial temperature of T fi. The solution, as a function of non dimensional X and τ, are given by 3 4 =3 > G =1? =<@5 3 ;> =3 BH# - B > 3 ; =3 > D G < C EH# E!E@B! G =1? =<@5 3 ;> =3 BHI - B > < D G C EH# E!E@B!./ 0 IJMET/index.asp 287 editor@iaeme.com

5 Modelling and Simulation of Thermocline Storage For Solar Thermal Power Plant Where T s is temperature of bed material in Kelvin, T i is initial temperature of bed material in Kelvin and T fi is initial temperature of fluid in Kelvin which can be read by interpolation from Tables for 0 X 20 and 0 τ 30 and the values of T s and T f can be calculated. These equations are valid regardless of whether T i <T fi or T i >T fi. In the first case, bed heats up and energy is stored, where in the second case, energy is given and bed is cooled. The maximum amount of energy which can be stored =1 J K 4 L MN 9 O P9 7 8Q 7 Q 5. SIMULATION OF THERMOCLINE TANK The code is developed for the entire central receiver solar thermal power plant. The code developed is validated with the data available for energy storage in Solar One power plant. For simulation 10 MW e power plant with 8 hours of storage is studied. Hourly weather data (incident solar radiation, wind velocity and ambient temperature) of Jodhpur city of Rajasthan, India is taken from website of National Renewable Energy Laboratory NREL, USA as input. In the thermocline tank, Hitec XL molten salt is considered as heat transport fluid and quartzite rock a solid material. The specific heat and density of the fluid are dependent on the temperature. The System Advisor Model (SAM) software provided by NREL gives the table of properties for four heat transport fluids, namely Solar salt, Hitec, Hitec XL and liquid sodium. Following equation is obtained for the properties (specific heat and density) of Hitec XL by applying multiple regression analysis to the values already known at specific temperatures, given by SAM. C p = ( * t ) ρ = ( * t ) Fig. 3(a) Screenshot of input to plant Fig. 3(b) Screenshot of input and volume & size calculation for thermocline storage tank simulations IJMET/index.asp 288 editor@iaeme.com

6 Dr. R. N. Patil and Prof. Jadhav Sandhya Dilip Fig.3(c) Screenshot of temperature calculation for Hitec XL and quartzite rock along length of thermocline storage tank Fig 3(a) to 3(c) gives some screenshots of the code developed for plant. The specific heat and density of quartzite rock are treated as constant and are taken as 900 J/kgK and 2800 kg/m 3 respectively. Initial temperature of the solid is taken as C and heat transport fluid is taken as C. The hot fluid to the storage tank comes from receiver. Its mass flow rate in receiver is controlled to obtain required output temperature. Therefore mass flow rate of hot fluid to storage tank also keeps on changing. Therefore mass flow rate of hot fluid to storage tank also keeps on changing. Here for sample calculation, mass flow rate of Hitec XL is taken as 125 kg/s for power plant of 10 MW e. Also porosity is assumed to be 0.35 and diameter of quartzite rock is taken as 0.5 m. Temperatures of both can be determined at any height of storage tank. Here, total height of tank taken is 13.75m. 16th Jan 2014 observes cloudy weather in afternoon with absence of solar radiation, therefore it is considered for study of the advantage of thermal energy storage on a cloudy day. 6. RESULTS & DISCUSSIONS The use of storage tank increases the thermal energy by almost 76% for storage hour of 8 hrs. On 16th Jan 2014, as shown in Fig.4, cloud appears at p.m. to 1.30 p.m., but the turbine continues to run using the thermal energy from storage tank. Values of T s and T f are determined at regular interval of 3.44 m in storage tank, i.e. dividing the tank in 4 equal parts in height. Similarly, the temperature of solid bed material and hot fluid are calculated after 60 min time interval, i.e. at 60 min, 120 min, 180 min and 240 min. These temperatures are given in Table No.1 & 2 respectively. Table No 1. Temperature of solid bed along axial distance of tank at time intervals during charging Distance in m Temperature of solid bed in tank in 0 C (from top of tank) 60 min 120 min 180 min 240 min IJMET/index.asp 289 editor@iaeme.com

7 Modelling and Simulation of Thermocline Storage For Solar Thermal Power Plant Table No 2. Temperature of hot fluid along axial distance of tank at time intervals during charging Distance in m Temperature of hot fluid in tank in 0 C (from top of tank) 60 min 120 min 180 min 240 min Fig. 4 Effect of storage of thermal energy in power generation on a cloudy day At 0 minutes, temperature of solid is initial temperature i.e C. As hot fluid is introduced from top of tank with mass flow of 125 kg/sec, it travels down the tank height. 650 Hot fluid Solid 60 min 650 Hot fluid Solid 120 min Temperature in 0C Distance in bed in metres Temperature in 0C Distance in bed in metres Fig. 5(a) Fig. 5(b) Temperature in 0C Hot fluid 180 min Distance in bed in metres Temperature in 0C Hot fluid 240 min Distance in bed in metres Fig. 5(c) Fig. 5(d) IJMET/index.asp 290 editor@iaeme.com

8 Dr. R. N. Patil and Prof. Jadhav Sandhya Dilip In Fig, 5(b), rise in temperature of solid can be observed. This increase in temperature of both continues till equilibrium occurs. The equilibrium reaches after 240 min, as shown in Fig. 5(d). Fig.No.6 shows the variation of temperature of solid bed with time, along the height of tank. It can be seen, that, after 150 minutes (1.5 hrs), temperature of hot fluid and solid become almost equal at a distance of 3.44m from top of tank. After 180 minutes (3 hrs), it reaches equilibrium till 6.88m, i.e. only till mid of tank, whereas it takes 210 minutes (3.5 hrs) to reach equilibrium at a distance of m from top of tank as shown in Fig.No.5(d). It takes 240 min (4 hrs) for the temperature of solid and hot fluid to be equal at the bottom of tank, i.e. at m. Fig CONCLUSION The effect of thermal storage on plant performance is studied by the developed code. The storage system helps the plant to operate in intermittent cloudy weather conditions. Thermocline tank with Hitec XL as heat transport fluid and quartzite rock as solid bed material is studied. Thermal characteristics, including temperature profiles are explored and it is observed that it almost takes 4 hrs for the entire temperature in tank to reach C. The efficiency increases with tank height. However, it can be expensive in terms of storage material. The smaller size of rocks also increases efficiency as area of contact between Hitec XL and quartzite rock is increased. Comparing the different storage materials, it is observed that density of liquid sodium is very less; therefore volume of liquid sodium required is more as compared to other storage materials. Hitec XL is having the highest density, therefore volume required is least among all the storage materials. The use of storage of thermal energy reduces the cost of power generation. REFERENCES [1] Herrmann Ulf, David W. Kearney, Survey of Thermal Energy Storage for Parabolic Trough Power Plants, Journal of Solar Energy Engineering, Vol. 124, pp , May [2] L.J.Yebra, M. Berenguel, S. Dorando and M. Romero, Modelling and Simulation of Central Receiver Solar Thermal Power Plants in Proceedings of the 44th IEEE Conference on Decision and Control and the European Control Conference,Spain Dec 12-15, [3] M. C. Stoddard, S. E. Faas, C. J. Chiang & J. A. Dirks, SOLERGY A Computer Code for Calculating the Annual Energy from Central receiver Power Plants, Sandia Report, May [4] Magal, B. S., 1994, Solar Power Engineering, Tata McGraw Hill Publishing Company Limited, Second Print. IJMET/index.asp 291 editor@iaeme.com

9 Modelling and Simulation of Thermocline Storage For Solar Thermal Power Plant [5] Manuel Romero, Reiner Buck & James E. Pacheco, An Update on Solar Central Receiver Systems, Projects, and Technologies, Journal of Solar Energy Engineering May 2002, Vol. 124, pp [6] Mary Jane Hale, Survey of Thermal Storage for Parabolic trough Power Plants, NREL, September 2000, NREL/SR [7] Ortega, J., J. Burgaleta, M. Tellez, May 2008, Central Receiver System (CRS) Solar power plant using molten SALT as heat transfer fluid, Journal of Solar Energy Engineering, Vol. 130, No. 2 [8] S. P. Sukhatme, Solar Energy Principles of thermal collection and Storage, Second Edition, Tata McGraw-Hill, 1996 [9] Winter, C. J., R. L. Sizmann, L. L. Vant, - Hull (Eds.), Solar Power Plants Fundamentals, Technology, Systems, Economics, 2005 [10] Wang K. Y., West R. E., Kreith and Lynn P., High-temperature Sensible-Heat Storage Options, Energy Vol. 10, No. 10, pp , [11] May IJMET/index.asp 292 editor@iaeme.com