Thermal storage in mechanical engineering

Thermal storage in mechanical engineering

1. Thermal Energy Storage Solar Energy 1

2. Sensible heat storage Latent heat storage Thermochemical storage 2

3. Need of Energy Storage To supply energy reliably, efficiently, economically. To meet the peak demand. To offset the adverse effect of fluctuating demand of electricity . To assure a steady output from power plants. To supply energy at demand period for intermittent generation technologies like wind and solar energy where production varies with demand. To provide renewables with a zero GHG emission backup. Fig. Power supply and demand curve for a day 3

4. Fig. Diurnal variation of power produced by renewable sources over a typical sunny day Curve 1: Solar power, curve 2: wind power, curve 3: hydroelectric power and other miscellaneous sources Curve 1: Production, Curve 2: demand Fig. Comparison of diurnal variation over a typical sunny day Diurnal variation of power production 4

5. Thermal Energy Storage Electrochemical Energy Storage Pumped Hydro Compressed air Energy storage Energy storage by flywheels Magnetic Energy Storage Chemical Energy Storage Hydrogen storage Important factors and schemes for energy storage systems 5

6. What is the need??? The intermittent, variable and unpredictable nature of solar radiation generally leads to a mismatch between the rate and the time of collection of solar energy and the load needs of a thermal application. It is often required to put energy storage system in between . How it works??? The storage system stores energy when the collected amount is in excess of the requirement of the application and discharges energy when the collected amount is inadequate. What is the size??? The size of a storage system is largely determined by the specific purpose for which it is used. Thermal Energy Storage 6

7. Diurnal storage : load demand is 24 hours, whereas collection takes place only during the sunshine hours. A system larger than a buffer storage having the capacity to store energy for a day or two is required. Annual Storage : storage system stores energy during the summer when the collection is in excess of the demand, and delivers the excess energy in winter when the collection is less than the demand. A large long-term storage system is required. Fig. Energy storage system Fig. Different situation of storing thermal energy Various situations for using a thermal energy storage 1 Energy collected, 2 Load requirement Buffer Storage : Energy requirement is essentially the same as the collection; store energy for short intervals of time and small in size. 7

8. Sensible heat storage (heating a liquid or a solid which does not change phase): The amount of energy stored is dependent on the temperature change of the material . T 1 -T 2 = temperature swing Latent heat storage (heating a liquid or a solid which undergoes a phase change): The amount of energy stored is depends upon the mass and the latent heat of fusion of the material. Storage operates isothermally at melting point of the material. If isothermal operation at the phase change temperature is difficult, the system operates over a range of temperatures which includes melting point. Thermo chemical storage: Using heat to induce a certain chemical reaction and then storing the product, the heat is released when the reverse reaction is made to occur. (a) (b) (c) Basic methods for storing thermal energy 8

9. Different methods and materials for Thermal Energy Storage Other compounds Ice, Sodium nitrate, sodium hydroxide etc. Water, 9

10. Design considerations The temperature range over which the storage has to operate. The capacity of the storage has a significant effect on the collectors. A smaller storage unit operate at a higher mean temperature. This results in a reduced collector output as compares to a system having a larger storage unit. Heat losses from the storage have to be kept a minimum. Cost of the storage unit (includes cost of storage medium, the containers, insulation and operating cost). 10

11. Availability of solar radiation Nature of thermal process Economic assessment of solar vs. auxiliary energy Physical and chemical properties of the storage medium employed Storage capacity of solar thermal storage systems 11

12. An important criterion is selecting a material for sensible heat storage is its value. 75 – 100 liters of storage per square meter of collector area. Sensible heat storage Various substances used: water Heat transfer oil Inorganic molten salts Rock, pebbles and refractories Water being used for temperature below 100 o C Refractory bricks being used for temperature around 1000 o C 12

13. The choice of storage media depends on the nature of solar thermal process Water storage Air based thermal storage (packed bed storage) Storage walls and floors Buried earth thermal storage Materials Specific heat kJ/ kg. K Density kg/ m 3 Volumetric specific heat kJ/ m 3 . K Adobe 1.0 1700 1700 Aluminum 0.896 2700 2420 Brick 0.84 1920 1600 Concrete 0.92 2240 2100 Fiberglass Batt insulation 0.71-0.96 5-30 4-30 Polyurethane Board insulation 1.6 24 38 Rock pebbles 0.88 1600 1410 Steel 0.48 7850 3800 Stone(granite) 0.88 2720 2400 Water 4.18 1000 4180 Wood 2.5 510 1300 Table : Properties of sensible heat storage materials Water has three times more heat capacity than rock on a volume basis , it means rock requires three times more volume than water to store the same amount of sensible heat. Sensible heat storage media 13

14. Fig. Solar space heating with water storage tank Fig. Solar space heating Water is the ideal material to store useable heat because: It is low in cost It has a high specific heat The use of water is particularly convenient. Water is used as The mass and heat transfer medium in the solar collector A solar space heating system can also use water as the storage as well as the transport medium. Water Storage 14

15. Storage in solids Fig. Application of solid as sensible heat storage medium Approximate thumb rule followed for sizing : 300 to 500 kg rock per square meter of collector area for space heating. 15

16. Example 1: Calculate the energy required to heat 270 litres of water from 15°C to 55°C. Assume that no heat loss is taking place from the tank where water is kept. We have the data: The density of water is 993, C P = 4.18 The price of electricity is Rs. 6/kWh 1 Joule is equal to a watt second ∆T=40 °C Q = 270 * 993kg/cum * 4.18 kJ/ kg ° C * 40°C = 44,827.99 kJ (or 44.8 MJ) = 44,827.99 kWs = 12.45 kWh At an electrical energy cost of Rs. 6/ kWh, this energy costs: = Rs. 6/ kWh * 12.45 kWh = Rs. 74.7 Sensible heat storage (problem) 16

17. A melting point in the temperature range of the application for which it is being considered. A high value of the latent heat of fusion. A small volume change during the phase change. A negligible amount of super cooling or super heating for phase change to occur. Phase change material(PCM) Properties should be stable and should not degrade after repeated cycle (involving melting and solidification). High thermal conductivity Low vapor pressure Non corrosive. Fig. Solid to liquid phase transition Fig. Constant temperature range for PCM application Properties required for a PCM 17

18. ( Pasupathy , 2008) Benefits and Drawbacks of PCM Higher storage density than sensible heat Smaller volume Smaller temperature change between storing and releasing energy High cost Corrosiveness Density change Low thermal conductivity Phase separation Incongruent melting Super cooling Benefits Drawbacks 18

19. Type Melting point (°C) Heat of fusion (KJ/ kg) Butyl stearate 19 140 Capric – Launic acid (45-55%) 21 153 Hexadecane 18 236 Heptadecane 22 214 Propyl palmitate 19 186 PCMs for space heating applications in buildings Properties of PCM used for buildings Fig. Design of sealed tube containing PCM Fig. Design of latent heat storage for buildings 19

20. PCM Options (Organic) Metallic fillers Metal matrix structures Finned tubes Encapsulation Melts congruently Chemically and physically stable High heat of fusion Preparation of form- stable material using porous supporting matrix Drawbacks More expensive and flammable Low thermal conductivity in solid state Lower heat storage capacity per volume Leakage during phase transition Solutions Benefits 20

21. Prevents reactivity towards environment Compatible with stainless steel, polypropylene, and polyolefin Controls volume when phase change occurs A large improvement in heat transfer rates (Farid, 2004) Encapsulation Fig. Encapsulation applied to PCM ( Farid , 2004) Drawbacks High pressure drop of HTF through the bed High initial cost 21

22. The solar energy to be stored is used to produce a certain endothermic chemical reaction and the products of the reactions are stored . When the energy is required to be released , the reverse exothermic reaction is made to take place. Suitable for medium or high temperature applications only. Reaction Temperature of forward reaction ( o C) Temperature of reverse reaction ( o C) Energy stored per unit volume of storage (kJ/m 3 ) 780 610 209.4 x 10 3 1028 590 460.6 x 10 3 498 135 2143.7 x 10 3 Thermochemical storage reactions Thermochemical storage 22

23. Reactor (A + B) (X + Y) Storage tanks Storage tanks (X + Y) Reactor (A + B) Heat from collectors Heat released for application Forward reaction Reverse reaction Fig. Schematic representation of thermochemical storage reactions The products of the forward reaction store thermal energy as chemical energy which can be recovered as thermal energy when the conditions are changed to permit the reverse reaction to occur. Example: Reversible Chemical Reactions 23

24. Criteria for selection of thermochemical reactions for solar applications 24

25. Solar Pond – Salt Gradient Solar Pond 25 Economical way of colleting and storing of solar energy requiring low temperature process (70-80 o C)

26. Solar Pond A  solar pond  is a pool of saltwater which collects and stores solar thermal energy. The saltwater naturally forms a vertical salinity gradient, in which low-salinity water floats on top of high-salinity water. The layers of salt solutions increase in concentration (and therefore density) with depth. Below a certain depth, the solution has a uniformly high salt concentration.   26

27. Some facts related to Solar Pond First solar ponds were constructed in Israel in the early sixties by Tabor and his co- workers. A maximum temperature of 100°C were obtained at the bottom, many practical difficulties were encountered and the work was abandoned. Number of solar ponds have been built all around the world to utilize the stored heat for providing process heat and generating power . Largest solar pond : Installed at Beit Ha’aravah in Israel (area -250000 m 2 ). Heat is used to generate electricity using an ORC. Applications: Desalination and brine management Australia: used to supply heat in salt production process (Pyramid Hill) India: Largest pond about 6000 m 2 built at Bhuj , Gujarat (used to supply process heat to a dairy farm) 27

28. Principle of working of Solar Pond 28 Temperature, T T 1 T 2 Density C 1 C 2 L x C 2 C 1 Bottom Top: T 1 ; ; C 1 : T 2 ; ; C 2 No convection will occur so long as the slope of the curve PQ is positive P Q

29. Stability criteria 29 Minimum concentration gradient required for maintaining a given concentration gradient at a particular level in a solar pond.

30. Ex.1: Sodium chloride is used as the salt in a solar pond. Estimate the minimum concentration (kg of salt per kg of water) required at the bottom if the concentration at the top is 0.02 and a temperature difference of 65 o C is to be maintained. Assume that the concentration and temperature profiles are straight lines and take the average values of to be -0.5kg/m 3 - o C and 650 kg/m 3 respectively. 30

31. Working principle Surface convective zone, SCZ 10-20 cm thickness Uniform concentration Uniform temperature Concentration gradient zone (Non convective zone-NCZ) Half of the depth of the pond Temperature and concentration increase with depth Act as insulating layer, reduces heat losses in the upward direction Lower convective zone, LCZ Temperature and concentration constant Serves as the main heat collection as well as thermal storage medium 31

32. Working principle 32 Typically 2-3 m deep Thick durable liner (low density polyethylene-LDPE, High density polyethylene-HDPE, Woven polysestern yarn) at the bottom. Slats: magnesium chloride, sodium chlorise Concentration varies from – 20-30% at the bottom to zero at the top the temperature of the lower layer may rise to as much as 95°C Salt required is about 50 g/m 2 -day The annual collection efficiency varies between 15 -25 % which is less than flat-plate collector Cost per square meter is much less that that for a LFPC

33. Solar Pond Power Generation 33

34. Transmissivity based on reflection and refraction at the air-water interface of a solar pond For angle of incidence from 0 to 60 o the loss due to reflection is small i.e. 2-6% For large angles, the loss is large (not intersected because these are associated with low values of radiation) Angle of incidence Ɵ 1 (degree) Angle of refraction Ɵ 2 (degree) ρ 1 ρ II ρ = ½( ρ 1 + ρ II ) τ r = (1—p) 0.020 0.020 0.020 0.980 15 11.32 0.022 0.018 0.020 0.980 30 22.08 0.030 0.012 0.021 0.979 45 32.12 0.052 0.003 0.027 0.973 60 40.63 0.114 0.004 0.059 0.941 75 46.57 0.312 0.111 0.211 0.789 90 48.75 1 1 1 34

35. A 1 = 0.237, K 1 = 0.032 m -1 for 0.2 < λ < 0.6 µm A 2 = 0.193, K 2 = 0.45 m -1 for 0.6 < λ < 0.75 µm A 3 = 0.167, K 3 = 3 m -1 for 0.75 < λ < 0.9 µm A 4 = 0.179, K 4 = 35 m -1 for 0.9 < λ < 1.2 µm Trasmisivity based on Absorption Extinction coefficient is a strong function of wavelength Rebl and Nielsen: x = depth of water 77.6 % of radiation is accounted (corresponding to wavelength 02-1.2 µm) Balance 22.4 % corresponding to the radiation wavelengths grater than 1.2 µm – absorbed near the surface (1-2 cm) Bryant and Colbeck : x = depth of water in meter, valid for x >0.01 m 35 If the radiation is not incident normally, Bouger’s law:

36. Example-2: A 2 m deep solar pond is built in Guwahati (26 o 8’). The values of global and diffuse radiation measured on a horizontal surface on 15 th May at 1300 hr (LAT) are 900 W/m 2 and 200 W/ m 2 respectively. Calculate (1) flux reflected from the water surface, (2) Flux entering the water and (3) solar flux at a depth of , 0.01 m, 0.5 m, 1 m and 2 m. 36 Angle of incidence Ɵ 1 (degree) Angle of refraction Ɵ 2 (degree) ρ 1 ρ II ρ = ½( ρ 1 + ρ II ) τ r = (1—p) 0.020 0.020 0.020 0.980 15 11.32 0.022 0.018 0.020 0.980 30 22.08 0.030 0.012 0.021 0.979 45 32.12 0.052 0.003 0.027 0.973 60 40.63 0.114 0.004 0.059 0.941 75 46.57 0.312 0.111 0.211 0.789 90 48.75 1 1 1 On May 15, n = 135

37. Flux reflected from the water surface: Flux entering the water : 900 – 25.8 = 874.2 W/m 2 Transmissivity based on the absorption: 37 If the radiation is not incident normally, Solar flux at various depth = Depth (m) Solar Flux 0.01 631.44 0.5 357.81 1 309.38 2 260.86 Depth (m) 0.01 631.44 0.5 357.81 1 309.38 2 260.86 Transmissivity At x = 0.01 m X = 0.5 X = 1 X = 2 For beam, 0.7267 0.4137 0.3583 0.3028 For diffuse, 0.7063 0.3933 0.3379 0.2824

38. Reflection and absorption of a solar radiation in a solar pond SCZ: 10 cm, radiation absorbed in the wavelengths: 1- 2 µm. 269 W/m 2 ~30% of the incident energy is absorbed in SCZ. This energy is almost entirely lost to the surroundings – reason for low collection efficiency. Flux penetrating to the bottom of the pond is ~261 W/m 2 ~ 31% of the incident energy. Fig. Variation of solar radiation flux with depth 38

39. Temperature distribution and collection efficiency For an exact solution, one has to solve the appropriate differential equation for each zone. Matching condition has to be used at the interfaces between the zones and satisfy the boundary conditions at the top and bottom surfaces of the pond. Assumption: (a) the upper convective zone and the lower convective zone are assumed to be perfectly-mixed layers at uniform temperatures which change only with time, (b) lateral dimensions of the pond are large compared to its depth L (temperature varies only in the vertical direction), properties are constant. 39 Energy flow diagram

40. Differential equation for the non-convective zone is the heat conduction equation of the form: 40 Solar radiation absorbed in the pond Energy Balance For the surface Convective Zone: Rate of change of energy contained in the surface convective zone of thickness, Rate at which heat is conducted in from the non-convective zone Solar radiation absorbed in the thickness, Rate at which heat is lost from the top surface by convection, evaporation and radiation For the Lower Convective Zone : Rate of change of energy contained in the lower convective zone of thickness, Rate at which heat is conducted in from the non-convective zone Solar radiation absorbed in the thickness , Rate at which heat conducted out to the ground underneath Rate at useful heat extraction

41. Annual collection efficiency and extraction temperature as a function of pond depth 41

42. Applications of Solar ponds Combined system of thermosiphon and thermoelectric modules to generate electricity from solar ponds. 42

43. Electric power generation from solar pond using Organic Rankine cycle Active solar distillation systems integrated with solar ponds. 43

44. Operational shortcomings of Solar Pond Wind-induced waves Effect of rain Biological Growth Fouling due to dirt and leaves Effect of bottom reflectivity 44

45. Solar Gel Pond A thick layer of a polymer gel floats on the lower convection zone and act as non-convective zone. Gel (98.3% water and 1.7 % polyacrylamid ) has good optical and thermal insulating properties. Project demonstration at New Mexico: Surface area: 400 m 2 , and 5 m deep. Small concentration is necessary to float gel on top of LCZ. Gel was kept in thin transparent plastic bags made from Tedlar and floated on the salt solution. Thickness of the gel: 0.6 m, Designed to supply a minimum of 1 GJ per day at 70 ◦ C . 45 Evaporation loss from the surface are eliminated. Maintenance requirement reduces. The environment hazards associated with handling salt are eliminated .

46. Solar ponds across the globe Israel's 150kw Solar Pond Solar Pond in Gujarat India ORC operated Solar Pond of Alice springs in Australia 46

47. Thank you 47