_{3}and KNO

_{3}in the Thermal Storage System

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The experiment is studied on thermal distribution in the thermal energy
storage system with non-phase change materials (NPCM): NaNO_{3}, KNO_{3 }and NaCl in the range of 25°C - 250°C. The cylindrical storage
system was made of stainless steel with 25.6 cm-diameter and 26.8 cm-height
that was contained of these NPCM. There was one pipe for heat transfer fluid
(HTF) with 1.27 cm-diameter that manipulates in the storage tank and submerges
to NPCM. The inner pipe was connected to the 2.27 cm-diameter outer HTF tube.
The tube was further connected to the thermal pump, heater and load. The pump
circulates the synthetic oil (Thermia oil) within the pipe for heat
transferring purposes (charging and discharging). An electric heater is used as
the heat source. The limitation of the charging oil temperature is maintained
at 250°C with the flow rates in the range of 0.58 to 1.45 kg/s whereas the
inlet temperature of the discharge oil is maintained at 25°C. Thermal
performances of TES (thermal energy storage) such as charging and discharging
times, radial thermal distribution, energy storage capacity and energy efficiency
have been evaluated. The experimental results show that the radial thermal
distribution of NaCl for TR inside, TR middle and TR outside was optimum of
temperature down to NaNO_{3} and KNO_{3} respectively.
Comparison of NPCMs with oil, flow rates for NaCl were charging and discharging
heat transfer than KNO_{3} and NaNO_{3}. The thermal stored
NaCl ranged from 5712 - 5912 J; KNO_{3} ranged from 7350 - 7939 J and
NaNO_{3} ranged from 6623 - 6930 J respectively. The thermal energy
stored for experimental results got with along the KNO_{3}, NaNO_{3} and
NaCl respectively. The thermal energy efficiency of NaCl, KNO_{3} and
NaNO_{3} was in the range 66% - 70%.

Solar energy is the most abundant energy source compared with other energies and supply’s unlimited clean energy. Solar energy can be transferred to thermal energy via a collector and sent to the equipment by heat transfer fluid (HTF). The most important application of solar energy is to transfer its thermal energy and then to electricity. Solar thermal power plant is one of the available applications of solar energy that its energy source depends on time, weather condition and electricity demand. Solar energy source of solar power plant is needed to be stored for smooth electric generation. The thermal storage system of the solar thermal power plant is necessary for the power plant stability and reducing rate of mismatch between energy demand and supply [

A spiral heat exchanger, commonly used in stainless steel, seems to be a very good candidate for this task. Compactness, it enhanced heat transfer due to centrifugal forces and easy sealing. Large heat transfer surface and a shorter undisturbed flow length are the most appealing features of such a choice. To transform this device into a spiral thermal storage energy unit (STES), one of its two working fluids is substituted by a material that is capable of storing energy due to both the sensible [

Curvilinear spiral geometry has not yet been studied in the context of the heat storage energy systems. Indeed, so far the classical double-pipe or shell-and-tube heat exchangers have been applied in the storage energy systems and a relevant theoretical and experimental analysis of the phase-change phenomenon has been restricted to geometry of a cylindrical capsule [

Therefore, an experimental computerized stand has been constructed, where the one pipe stainless steel heat exchanger of spiral thermal distribution has been studied to determine overall characteristics of the temporal behavior of the NPCMs and of the performance of the TES unit. The empirical findings were presented in terms of temperature changes of both media. Moreover, the thermal analysis of the storage energy unit is carried out where temporal energy stored, charging and discharging time, energy storage capacity, and energy efficiency of melted and solid phases of the storage medium were estimated.

The storage system is the sensible heat in solid storage. Assuming that the storage tank has a uniform temperature so the energy balance on the storage tank gives if the cumulative heat energy in the storage tank is not constant so we can re-write into the new equation as;

where M: the medium mass in storage tank (kg),^{2}K),^{2}),

The total amount of heat transfer between the HTF to storage medium was calculated based on the mass flow rate_{p} and the difference in inlet and outlet temperature

The heat balance from the convection of the HTF to the storage medium is used to calculate the outlet HTF temperature from the pipe by

where k is the material thermal conductivity (W/m-K), ^{2}∙K) and r is radial coordinate from heat pipe (m) respectively.

The inlet temperature of HTF is changed when it flows out of the pipe and is heated by a heater for the next round of charging experiment, HTF temperature is increased and flowed back to the pipe again after heated by the heater, the inlet HTF temperature is given by Equation (4).

where

The outlet HTF temperature after HTF transfer heat to the pipe and the storage medium is decreased from the calculation that depends on the storage temperature, flow rate of HTF, heat capacity, and the mathematical model will be explained as in the Equation (5).

The amount of thermal energy stored in the different storage materials at their respective charging times is calculated using Equation (6).

where, ^{3}), specific heat (J/kg∙K) Storage volume required (m^{3}) and inlet of charging temperature range (˚C) respectively.

The amount of thermal energy recovered from storage bed of different materials at their respective discharging time is calculated using Equation (7).

where,

Energy efficiency is the ratio of energy recovered from the storage bed during discharging cycle to the total energy input to storage bed which is given by the Equation (8). The total energy input is the sum of energy supplied to heat the storage bed from atmospheric temperature

Based on thermodynamic data, corrosiveness and handling characteristics, three non-phase change materials (NPCM) were selected for experimental investigations: sodium nitrate (NaNO_{3}), potassium nitrate (KNO_{3}) and sodium chloride (NaCl). The most important properties of these Three NPCMs and lubricant oil (LO) are given in

A two co-axial cylindrical storage tank was used to house the charging and discharging pipe. The inner tank has a diameter and height of 25.8 cm. and 26.8 cm, respectively. The storage tank stranded vertically in ambient air during the experiment and the storage temperature was reduced from heat loss to the air through the insulator as shown in

The vertical charging straight pipe was made of stainless steel of 95 cm length, 1.27 cm diameter and 0.1 cm thickness. The fluid was pumped through the pipe using a positive displacement pump connected to a variable

. Properties of three NPCMs and lubricant oil [6] [13]

Properties | NaCl | KNO_{3} | NANO_{3} | LO [13] |
---|---|---|---|---|

Melting temperature (˚C) | 800 | 335 | 306 | 340 |

Heat of fusion (melting) (kJ/kg) | 492 | 74 | 172 | - |

Density (kg/m^{3}) | 2160 | 2100 | 2261 | 863 |

Heat capacity of heat transfer solid (kJ/kg-K) | 0.85 | 1.21 | 1.10 | 1.882 |

Thermal conductivity (W/mK) | 7 | 0.5 | 0.5 | 0.133 |

The model of experimental set up

speed motor. The speed of motor was adjusted to obtain the flow rates of 0.58, 0.87, 1.16 and 1.45 kg/s, respectively. Three non-phase change materials (NPCM) were selected for experimental investigations: sodium nitrate (NaNO_{3}), potassium nitrate (KNO_{3}) and sodium chloride (NaCl). These correspond to laminar and turbulent flow regimes that depend on the HTF viscosity from the Reynolds numbers. The purpose of using different flow rates was to observe the rate of heat transfer from HTF to storage medium. Lubricant oil (LO) (from Shell Company) at an average room temperature (25˚C) was used as the inlet HTF and flowed from the inner tank. The performance parameters are charging time, energy stored, discharging time, energy recovered and energy efficiency.

For charging process, HTF brings heat from heater 3 kW to the storage tank. The pump is used to circulate HTF in the system. For the discharging experiment, thermal oil was used as HTF for drawing the heat from the storage tank. The load was made by a boiler vessel that was filled with water of 10 kg for absorbing heat. The water tank has a diameter and height of 11.4 cm. and 10.2 cm, respectively. The storage temperature decreased from 250˚C with thermal loss to ambient during the time HTF flowed through the pipe that was submersed in the storage tank. The HTF temperature increased for around 50 min and went down to 100˚C a minimum temperature of load.

The temperature distribution of the NPCMs, during charging was taken at four different mass flow rates at 0.58, 0.87, 1.16 and 1.45 kg/s, respectively. For each mass flow rates, curve was plotted for variation of temperature at each point in the wax against time elapsed, to get melting curve in case of charging. Storage and outlet HTF temperatures are presented for comparing the temperatures between bulk HTF temperature and mixed storage temperature of charging experiment in _{3} and NaNO_{3}. The temperature of storage bed is averaged over the entire bed volume thus it is the function of time only. Variation of average bed temperature with time is shown in _{3} and NaNO_{3} are more than that NaCl due to the high heat capacity and low thermal conductivity. The KNO_{3} and NaNO_{3} bed took 620 minutes for complete charging whereas NaCl gets completely charged within 500 minutes respectively. In _{3} and NaNO_{3} bed took 500 minutes for complete charging whereas NaCl gets completely charged within 300 minutes respectively. For

Heat transfer oil temperature over time for three different NPCM configurations and temperature of HTF and storage medium in charging experiment at 0.58, 0.87, 1.16, 1.45 kg/s

stored thermal energy as a function of time is depicted for oil flow rate of 1.16 kg/s. The KNO_{3} and NaNO_{3} bed took 500 minutes for complete charging whereas NaCl gets completely charged within 270 minutes, respectively. The finally _{3} and NaNO_{3} bed took 450 minutes for complete charging whereas NaCl gets completely charged within 300 minutes, respectively. The temperature distribution of the NPCM during charging was taken at four different mass flow rates at 0.58, 0.87, 1.16 and 1.45 kg/s, respectively. Comparing of NPCMs with oil flow rates for NaCl are more charging heat transfer than KNO_{3} and NaNO_{3} due to the high heat capacity and low thermal conductivity of NaCl compare with KNO_{3} and NaNO_{3}, which is consistent with the research of Horst Michels and. Robert Pitz-Paal [

For discharging experiment, thermal oil was used as HTF for drawing the heat from the storage tank. This load power was adjusted for a long period of discharge. The storage temperature started at 250˚C with thermal loss to ambient as HTF flows through the pipe submersed in the storage tank. The extracted heat from the storage tank is affected on its increasing temperature. The HTF and storage temperatures are parallel decreased after the HTF temperatures reach the turning point, the beginning HTF temperature for discharging from storage medium to the water heat exchange. The period of discharge was in the range of 450 - 600 minutes. The initial temperature of HTF average room temperature at 25˚C, as shown in _{3} discharging heat transfer was slower than KNO_{3} and NaCl due to the low heat capacity and low thermal conductivity. The discharging time of NaCl was 260 minutes, NaNO_{3} was 200 minutes and KNO_{3} was 370 minutes respectively. For _{3} was discharging heat transfer slower than KNO_{3} and NaCl due to the low heat capacity and low thermal conductivity. The discharging time of NaCl was 210 minutes, NaNO_{3} was 275 minutes and KNO_{3} was 170

Storage discharging fluid temperatures decreasing at 0.58, 0.87, 1.16, 1.45 kg/s

minutes respectively. In _{3} was discharging heat transfer slower than KNO_{3} and NaCl due to the low heat capacity and low thermal conductivity. The discharging time of NaCl was 140 minutes, NaNO_{3} was 205 minutes and KNO_{3} was 165 minutes, respectively. The finally _{3} was discharging heat transfer slower than KNO_{3} and NaCl due to the low heat capacity and low thermal conductivity. The discharging time of NaCl was 165 minutes, NaNO_{3} is 195 minutes and KNO_{3} was 155 minutes, respectively. Comparing substances (NPCM) with oil flow rate. NaNO_{3} discharging heat was slower than KNO_{3} and NaNO_{3}, so NaNO_{3} is appropriate to store thermal energy that corresponding to the literature of Horst Michels and Robert Pitz-Paal [

The thermal energy storage rates at the bed of the system for the sodium nitrate (NaNO_{3}), potassium nitrate (KNO_{3}) and sodium chloride (NaCl) was shown in

Flow rates of energy stored in NaCl, KNO_{3} and NaNO_{3}

respectively for charging at 400 minutes. The Energy Stored the flow rate of 1.45 kg/s was 6930 J, the flow rate of 1.16 kg/s was 7117 J, the flow rate of 0.87 kg/s was 6778 J and a flow rate of 0.58 kg/s was 6623 J respectively. For the thermal stored of NPCMs that compared for flow rate 1.45 were summarized as follows: flow rate, faster charging are optimum of flow rates 1.16, 0.87 and 0.58 kg/s respectively. The thermal stored of NaCl was lain from 5712 - 5912 J, KNO_{3} was lain from 7350 - 7939 J and NaNO_{3} was lain from 6623 - 6930 J respectively. The thermal energy stored for experimental results were get with along the KNO_{3}, NaNO_{3 }and NaCl respectively.

The energy was degraded in the process of storage since it was extracted at a temperature lower than that it was previously stored. The energy efficiency of storage beds was evaluated using Equation (7). Energy efficiency variation with bed temperature difference during discharging cycle was shown in _{3} for a flow rate of 1.45 kg/s reached to time of energy efficiency faster than the rates in 1.16, 0.87 and 0.58 kg/s for discharging at a temperature of 100˚C. Energy efficiency of flow rate 1.45 kg/s was 66% at 40 minutes, the flow rate of 1.16 kg/s was 70% at 45 minutes, flow rate 0.87 kg/s was 68% at 105 minutes and a flow rate of 0.58 kg/s was 67% at 60 minutes, respectively. _{3} for a flow rate of 1.45 kg/s reached to time of energy efficiency faster than the rates in 1.16, 0.87 and 0.58 kg/s for discharging at a temperature of 100˚C.

Variation of energy efficiency with bed temperature difference during discharging cycle in NaCl, KNO_{3} and NaNO_{3}

Energy efficiency of flow rate 1.45 kg/s was 66% at 40 minutes, the flow rate of 1.16 kg/s was 70% at 40 minutes, flow rate 0.87 kg/s was 68% at 40 minutes and a flow rate of 0.58 kg/s was 67% at 40 minutes respectively. For the thermal energy efficiency of NPCMs that compared for flow rate 1.45 were summarized as follows: flow rate, faster charging were optimum of flow rates 1.16, 0.87 and 0.58 kg/s respectively. The thermal energy efficiency of NaCl, KNO_{3} and NaNO_{3} were in the range of 66% - 70% for discharging at a temperature of 100˚C.

The study of the radial thermal distribution for thermal energy storage was using the three non-phase change materials that were selected for experimental investigations: NaNO, KNO_{3} and NaCl. The motor speed was adjusted to obtain the oil flow rates of 0.58, 0.87, 1.16 and 1.45 kg/s respectively. The radial flow rate of 1.45 kg/s was faster than of 1.16, 0.87 and 0.58 kg/s respectively, as shown in _{3} for TR inside the thermal distribution of temperature in the range of 150˚C - 200˚C, TR middle the thermal distribution of temperature in the range 55˚C - 125˚C and TR outside the thermal distribution of temperature in the range of 25˚C - 45˚C for experimental results comparison at a time 500 min. _{3} for TR inside the thermal distribution of temperature in the range of 125˚C - 200˚C, TR middle the thermal distribution of temperature in the range 65˚C - 25˚C and TR

Flow rates and radial thermal distribution for thermal energy storage (a) NaCl (b) KNO_{3} (c) NaNO_{3}

outside the thermal distribution of temperature in the range of 25˚C - 40˚C for experimental results compared at a time 500 min. The thermal distribution of radial for thermal energy storage as follows: Thermal distribution radial of NaCl for TR inside, TR middle and TR outside were optimum of temperature down to NaNO_{3} and KNO_{3} respectively, due to the higher heat capacity and low thermal conductivity. Oil flow rates have resulted in the heat transfer for thermal distribution radial and physics properties of NPCMs.

The high solar thermal energy storage was presented in terms of thermal distribution as NPCMs experiments. For charging experiment, the increasing storage temperature depends on HTF temperature, flow rates, and initial temperature. The high heat transfer rate to storage medium was caused by a small flow rate but the storage temperature was closely increasing with the same temperature of other flow rates. The design of thermal storage from this conclusion is that the thermal storage with heat exchanger, appropriate HTF, motor pump for a slow charge, and the preheating for HTF discharge was used before discharge to heat HTF to turning point for rapid discharge process. The radial thermal distribution of NaCl for TR inside, TR middle and TR outside was optimum of temperature down to NaNO_{3} and KNO_{3} respectively, due to the higher heat capacity and low thermal conductivity. Oil flow rates have resulted in the heat transfer for radial thermal distribution and physics properties of NPCMs. Comparison of NPCMs with oil flow rates for NaCl was charging and discharging heat transfer than KNO_{3} and NaNO_{3} due to the high heat capacity and low thermal conductivity. The thermal stored NaCl ranged from 5712 - 5912 J; KNO_{3} ranged from 7350 - 7939 J and NaNO_{3} ranged from 6623 - 6930 J respectively. The thermal energy recovery of NaCl ranged from 4332 - 6028 J; KNO_{3} ranged from 6367 - 6887 J and NaNO_{3} ranged from 5453 - 7620 J respectively. The thermal energy stored for experimental results got with along the KNO_{3}, NaNO_{3 }and NaCl respectively. The thermal energy efficiency of NaCl, KNO_{3} and NaNO_{3} was in the range 66% - 70%.

The success of this thesis can be attributed to extensive support and assistance of the thesis advisor; Assistant Professor Dr. Sarayooth Vaivudh, the co-advisors; Associate Professor Dr. Wattanapong Rakwichian and Dr. Sukrudee Sukchai. I deeply thank for valuable advices and warm guidance, suggestions and encouragements throughout this study.

I would also like to thank: Nakhonratchasima Rajabhat University and Energy Policy and Planning Office (EPPO), Ministry of Energy Thailand, for sponsoring a Ph.D. scholarship, staffs of School of Renewable Energy Technology (SERT), Naresuan University for helps and useful suggestions, my dedicated friends for their warm supports and friendships. Thank you for research assist form Mr. Pongsak Jittabut and Dr. Pattanapong Jumrusprasert for revising English.