Technology in Design of Heat Exchangers for Thermal Energy Storage

Abstract

In today’s world, the energy requirement has full attention in the development of any country for which it requires an effective and sustainable potential to meet the country’s needs. Thermal energy storage has a complete advantage to satisfy the future requirement of energy. Heat exchangers exchange heat in the thermal storage which is stored and retrieved later or can be used as a pre-heating or post-heating devices to save energy. Criteria of design of heat exchangers for various thermal energy storage applications along with their various components are being elaborated. The latent heat thermal energy storage in a mass application has got many advantages over the sensible heat storage. The existing approaches in the design, integration and application of phase change materials (PCMs) are explored by experimenting on a prototype of a single heat exchanger module and analysing all the design aspects to get a significant idea and the analysis needed while charging and discharging the heat exchanger.

1. Introduction

Thermal energy storage (TES) systems can be employed for both heating and cooling applications. TES is a process of storing heat from various sources like waste heat or solar thermal applications or electricity used at off-peak rates or can also be used in cooling applications. The heat transfer fluid (HTF) at low temperature is stored and used in peak hours of heating TES. The estimated market by the research study of the global thermal energy storage market was 4,281.6 Million USD in 2019 and is anticipated to grow upto USD 8558.34 Million by 2026. An annual compound rate of growth of the thermal energy storage market is going to be 10.4% from 2019 to 2027 as reported.

Various types of new and innovative TES are available and used for various purposes as follows:

1. Cooking and Greenhouse

2. Building passive heating and cooling from stable PCMs

3. Performance enhancement techniques

4. Steam accumulators in solar parabolic trough

5. Power generating fields

6. Desalination and still water

7. Production/crop drying

The TES can be combined with waste heat recovery or solar thermal heating or cooling applications. In heating, electric-resistance efficiencies (near 100% on an energy basis) combined with lower off-peak electric rates can produce heating at a fraction of the cost of conventional systems. One of the applications in cold countries is to produce heat at night with air heating using electrical resistance heater and stored in such storage media like ceramic bricks in insulated containers. The heat produced due to electric off-peak rates in the night is advantageous and generally 33–75% cheaper than the peak rates. On demand of the space heating, the stored heat is transferred to the room in the peak hours.

Another important application is the Concentrating Solar Power (CSP) plants which are important solar energy technologies where sunlight is converted to high-temperature heat by concentrating it in mirrors and lenses from 25 to 3000 times of the solar light intensity. If heat transfer fluids like sodium are used, heat at high temperatures (3000⁰C) can be retrieved. Sunlight being an intermittent source of energy that can produce heat helps to fill the gap between the energy demand and supply. A thermal energy storage is designed based on the diurnal, seasonal or annual storage needs and integrated into the system to sustain full or partial load operation.

This thermal energy can be stored at any particular temperature by using sensible or latent heat or both sensible and latent heat and can also be designed based on the application which can always be used in tropical countries due to the availability of solar radiation. A line diagram of a CSP with a two-tank TES taken from Ref. [1] is depicted in Figure 1 where the line-concentrating solar panels concentrate the light and generate high-quality heat, which heats the heat transfer fluid. The heat transfer fluid in turn heats the incoming steam in the super heater. This two-tank molten salt TES system used in CSP is a technology commercially employed.

Figure 1.

Line diagram for a two-tank thermal energy storage CSP system.

1.1 Objectives of thermal storage

The main objective of thermal storage is to deal with the amount of heat being stored at a particular temperature based on the application. The disparity of energy when supplied and consumed along with the necessity to store the surplus energy which would or otherwise go to waste including the shifting peak energy or power demand suggests the use of thermal energy storage for various areas of application such as space heating, hot water and air conditioning. Thermal energy storage facilitates superior and efficient use of the sources of fluctuating energies by harmonising the supply and demand of energy.

The methods used for thermal energy storage apply for less than 8 hours in northern winters, and the economical supply of the required energy becomes limited when compared to the costs of the combined solar collector and thermal energy storage system in these winters. An alternate heating source is taken as a backup for extended sunless periods but it limits [2] the use of solar energy further. However, an economically acceptable storage system will take advantage of low off-peak electricity prices, which enhances the benefits in thermal energy storage.

1.2 Types of thermal energy storage

Three types of TES systems are categorised as sensible heat thermal energy storage, latent heat thermal energy storage and thermochemical energy storage [1, 3].

1.2.1 Sensible thermal energy storage

Storing of sensible energy due to the virtue of increase or decrease of temperature for a storage material is called sensible thermal energy storage. Air, water, rock, brick and concrete are a few sensible heat storage materials. Based on each material’s advantages and disadvantages, it is preferred subject to the specific heat capacity including volume occupied by the TES [3]. To quantify sensible thermal energy stored, the following equation is given as follows:

$$Q=m.\mathit{Cp}.∆T$$E1

Q represents the total quantity of heat stored or saved in the sensible heat material (J), m represents the mass of the sensible heat material used (kg), Cp represents the specific heat of the stored material used (J/ kg K) and ΔT means the rise or fall of temperature difference measured (K). All mediums like solid liquid and gas can store sensible heat.

The TES materials are listed in Table 1, referred from Ref. [4], they have a high thermal capacity, are obtainable in profuse and found to be low priced. To select a suitable sensible heat storage material, the properties required have higher thermal conductivity, higher specific heat, lower density, lower vapour pressure, higher diffusivity, complementary to the storage tank materials and chemical stability. Sometimes sensible heat also needs high fluidity to carry the heat from one place to another, such as water and oil, which is also mentioned as the HTF.

Material	Density (kg/m^3)	Specific heat (J/kg K)	Volumetric thermal capacity(106 J/m^3K)
Brick	1,800	837	1.51
Clay	1,458	879	1.28
Sandstone	2,200	712	1.57
Concrete	2,000	880	1.76
Wood	700	2,390	1.67
Glass	2,710	837	2.27
Aluminium	2,710	896	2.43
Steel	7,840	465	3.68
Iron	7,900	452	3.57
Magnetite	5,177	752	3.89
Water	988	4,182	4.17
Gravelly earth	2,050	1,840	3.77

Table 1.

Typical materials used in sensible heat TES storage.

1.2.2 Latent heat thermal energy storage

Latent heat TES utilises the change of phase of latent heat TES material. The transition of phase change is utilised to heat up or retrieve heat from the latent heat material by melting or solidification from solid to liquid or vice versa respectively. On melting, large amounts of latent heat are transferred at a consistently constant temperature to the latent heat material; on solidification of the material the stored heat is released. The materials utilised here are called latent heat thermal energy storage (LHTES) materials and are named as phase change materials (PCM). The quantity of heat stored can be quantified in phase change material, as shown in Eq. (2)

$$Q=m.L$$E2

Q represents the heat stored for the latent heat storage material used (J), m represents the mass of latent heat material used (kg), whereas L represents the enthalpy of the phase change (J/kg). Water is being utilised most commonly as ice for cold storage. There are other materials which are listed in Table 2 are referred from Ref. [4]

Material	Melting temperature (°C)	Range of melting enthalpy (MJ/m^3)
Water	0	330
Water-salt solutions	−100 to 0	200–300
Paraffins	−20 to100	150–250
Clathrates	−50 to 0	200–300
Salt hydrates	−20 to 93	200–600
Sugar alcohols	20 to 450	200–450
Nitrates	120 to 200	200–700
Hydroxides	150 to 400	500–700
Chlorides	350 to 750	550–800
Carbonates	400 to 800	600–1,000
Fluorides	700 to 900	>1,000

Table 2.

The melting temperatures and latent heat enthalpies of the different materials.

Though numerous materials have been reviewed for PCM, only a small number of latent heat materials have been practically used, mostly due to corrosion, subcooling, separation of phase, and low conductivity of heat, longer-term cyclic stability. So generally, PCMs are chosen depending upon the suitable temperature, enthalpy required for melting, accessibility and price.

The comparison between the different hot storage media as shown in Ref. [4], the operating temperatures can be in different ranges. The specific heat and cost are less for the sensible heat storage materials, however, the latent heat observed in PCM’s is higher and is available at constant temperature.

1.2.3 Thermochemical energy storage

The thermochemical form of energy is stored and generated once a high-energy thermochemical reaction is utilised to store up energy. Products of the resultant reactions and the heat are separately stored during the forward reaction. In the reversed reaction, the stored heat is retrieved. Thus, a reversible reaction only could be utilised for this method of heat stored.

There are two types of thermochemical energy storage, mainly bifurcated into thermochemical reactions and sorption systems. Preferred high intensity of thermal energy storage density, along with cyclic reversibility, is the prerequisite of any chemical reaction. The thermochemical energy transformation has improved the efficiency of performance than the methods adopted by physical nature. It is difficult to search for the proper chemical reaction, which is reversible, for the energy source utilised. In chemical reactions, reversibility is an important behaviour of materials required.

Higher temperatures of greater than 400°C and with higher enthalpy in the range of 80–180 kJ/mol are the properties of thermochemical materials (TCM) used for the thermochemical reactions. The reactants– products are stored separately from the heat of reactions; the TCM systems are useful as seasonal storage systems.

For example, Zeolites and Silica gels are commonly used adsorbents. To characterise these storage materials, the probable temperature lifts, the breakthrough curves, energy density and the thermal coefficient of the absorbent volume performance are to be understood as also illustrated in Figure 2 referred from Ref. [4].

Figure 2.

Curves of thermal breakthrough (adsorption) of zeolite and silica gel.

The volumetric storage capacities are illustrated in Figure 3 in addition to a schematic comparison. Water being easily available is selected as a representation of the sensible heat storage materials taken from [1]. There are no strict limitations on the coloured region boundaries of PCMs and TCMs. It is a gross illustration of the overall trend, and appreciably other TCMs and PCMs are available at high temperatures. A broader understanding of the available PCM with higher temperatures of melting is visualised in the following Figure 4 taken from Ref. [5].

Figure 3.

Storage capacities of various PCM.

Figure 4.

Existing PCMs melting temperature vs. phase change enthalpy.

2. Design criteria

The thermal energy storage system consists of multiple components like the heat exchanger based on the phase change materials, the pumps, solar panels, insulations, storage tanks, etc. Each component has a different design criteria based on the aspects such as available heat content, flow rates, and applications. The aim of the design criteria is to maximise and optimise the heat transfer with the most effective costing using available resources and components designed for it. The established design criteria for the heat exchanger components to achieve their targets are discussed in the next section is referred from Ref. [2].

2.1 Selection criteria for thermal energy storage system

In CSP plants or any process industries, the TES system depicts an important part in the stability of generation and power supply to be met with energy demand; nevertheless there are only few TES plants with high temperature, tested using thermal energy storage and have a lot of scope for research. Thermal energy storage systems must meet the following criteria:

• High storage capacity or energy density

• Chemical and mechanical stability

• Efficient heat transfer between HTF and the storage medium

• Reversibility (maximum amount of energy must be extracted from the storage medium using HTF during charging and release this energy during discharging then return to the initial state)

• Minimum thermal losses

• Enhancement of Heat transfer can be done to the TES, if it is compatible along with the implementation of the system complexities.

2.2 PCM selection criteria

Melting point becomes the main criteria for the selection of the phase change material which in turn depends on the range of operating temperatures of the HTF. However heat transfer losses and enhanced effectiveness of the system are also necessary. The design criteria properties are differentiated into thermodynamic, chemical, economic and nucleation properties. The significant properties are listed as follows:

• High latent heat or enthalpy is produced to change the material phase at constant pressure and temperature.

• Corrosion resistance helps the material not to damage other contacted substances during high temperature and pressure working cycles.

• Environmental pollution to the society and the organisms has to be avoided by using non-toxic substances.

• High capacity of heat storage shows the material’s ability to store a high quantity of energy per mass and volume.

• High thermal conductivity property ensures faster rate of heat exchange with the surroundings.

• Inflammable material ensures no burning or combustion of the material.

• The criteria also involve to avoid sub-cooling or liquefying of the PCM below its boiling point and super-cooling or solidifying of the PCM above its melting point.

The material degrading should be avoided during the process of phase change.

Considering the other important criteria required for a suitable design of PCM material are high heat density, high thermal conductivity, non-corrosiveness and economic price. In addition, the range of melting point and the availability of the PCM have to be probed.

2.3 Heat transfer fluid selection criteria

The selection of heat transfer fluid will be based on the following criteria:

• The commercial availability of the HTF and the stability of the HTF are primarily important criteria.

• The criteria of handling complexity detect the possibility of handling the HTF in the TES system.

• The criterion of phase change temperatures and vapour pressure detects whether the HTF is suitable for the TES system operating temperature and pressure.

2.4 Heat exchanger selection criteria

Selection of design criteria for the heat exchanger type is mainly prioritised as

Performance efficiency is an important criterion and it studies the affects of heat exchanger factors and parameters.

The availability and the compatibility of the components of the heat exchanger without corrosion or contamination along with the cost-effectiveness are the important criteria.

Implementation complexity criteria detect the implementation of the system complexities and handling issues.

2.5 Criteria for heat exchanger components material

In general, commercial availability, compatibility, stability and cost-effectiveness of the components investigation are few important common criteria considered for all the components and their material.

• Density is a criterion that indicates the size and portability of the design.

• High thermal conductivity criterion illustrates the heat conduction in the material preferable.

2.6 Criterion for insulating material

• As mentioned earlier, the criteria such as commercial availability, least thermal conductivity and withstand maximum operating temperature along with the cost-effectiveness for the insulating material help to appropriately choose the insulating material.

For an efficient thermal energy storage system, the selection of suitable thermal energy storage media based on different criteria and systems is essential. Basically, both sensible and latent heat storage systems are feasible. The heat transfer media for sensible thermal energy storage are selected based on the temperature at which the heat has to be supplied, for example: thermal oils, molten salts, liquid metals, concrete and sand. For latent thermal energy storage systems, water/steam and phase change materials were considered. Liquid metals as heat transfer fluid have been eliminated because of their high density and therefore their bad pump ability. Thermal oils are problematic due to their flammability. If the temperature is less than 100°C, water can be used as a heat transfer fluid.

3. Heat exchanger design

There are many types of heat exchangers that can be implemented as a TES system with PCMs but few types have been tested such as screw heat exchanger, shell and tube heat exchanger and module PCM heat exchanger. Thus the three types of heat exchangers have been discussed in the next section.

3.1 Screw heat exchanger

Screw heat exchangers (SHE) are mostly used for industrial purposes as shown in Figure 5. In general, it consists of a drum and rotating hollow shafts (screws) arranged parallel to each other, and fluid passes through them. A prototype of SHE with PCM was implemented and tested to be integrated into the CSP plant as a TES system. The PCM is placed inside SHE; the phase change takes place when HTF flows through the hollow shaft during the charging/discharging process. In this prototype, the storage system type is two storage tanks, where one is for molten PCM and the other is for solid PCM. The continuous rotation of the hollow shafts will decrease the solidification of the PCM during the discharge process. However, during the discharging process, the PCM will be solidified on the shaft, then it will be crushed due to the shaft rotation. Furthermore, its size will be large to be reused for the charging process. Therefore, a granulate crusher is required in order to decrease the size of the collected PCM, and SHE is compatible with the usage of PCMs and HTF. While the implementation is kind of complex due to the two-tank storage system, PCM granulates and the requirement of a granulate crusher. From a cost-effectiveness point of view, SHE cost depends on the size of a small prototype. The benefit of it is the alternative usage in the charging/discharging process, self-shaft cleaning due to the continuous rotation and the less solidification of PCM effects on heat transfer. However, the cost of transportation of SHE, supplied power, two-tank storage system and the crushed granular is much higher than the obtained benefits. Finally, there are no data mentioned about the efficiency of using SHE with PCMs but in general heat exchangers have a storage density of 95% of volume.

Figure 5.

Screw heat exchanger schematic.

3.2 Shell and tube heat exchanger

Shell and tube heat exchanger (STHE) generally consists of one or more round tubes mounted in a parallel configuration to a cylindrical shell shown in Figure 6. It is the most investigated type of heat exchanger with PCM. Also, it has a simple structure and can be easily manufactured beside the variety of configurations that can be made. Additionally, PCM can be easily replaced at the end of its lifetime. STHE exchanges heat through the pipe walls between flowing HTF in pipes and the PCM contained in the shell. As HTF passes through the pipes, phase change in PCMs will occur during the charging/discharging process. In discharging, the solidification of PCM will start at the pipe walls so it will act as a thermal resistor.

Figure 6.

Shell and tube heat exchanger.

However, the thermal energy storage system with shell and tube heat exchangers is one of the most promising and cost-effective heat exchangers for latent heat storage. Moreover, its performance was investigated in different heat transfer enhancement techniques such as fins and cascaded PCM. Therefore, available data can be used. Additionally, this design has been recognised to be the promising configuration of a latent storage system acquiring high efficiency and minimum volume. STHE can be manufactured based on design requirements. Moreover, it is compatible with the usage of HTF and PCMs as well as being integrated as a TES system. Furthermore, it has a simple structure that can be easily implemented and handled. Its cost is a function of the outer surface area. In terms of cost-effectiveness, STHE can be alternatively used for charging/discharging, heat transfer enhancement techniques can be integrated with it, and it stands as a TES system. Therefore, an extra storage tank is not needed as the space required is designed properly, and the benefit of thermal stratification can be taken. The only additional cost required is for a pumping system for fluid circulation. Finally, from performance efficiency perspective heat exchangers when used with PCM has a storage density of 95% of volume.

3.3 Heat exchanger module (HEM)

This type is similar to any heat exchanger in construction. The only difference is that Macro encapsulated PCMs are placed in modules (cylindrical or spherical or any container) where the HTF flows over the PCM modules through the tank as shown in Figure 7. This type of design solves issues related to volume change, heat transfer and material compatibility besides availing flexibility and high package density since it can be fabricated in different sizes and shapes. Also, modules are easily handled and shipped. The encapsulation salt particles are more effective than heat exchangers with lower possibilities of success. However, this technology is researched a lot with great potential of energy storage for the purpose of high and medium-temperature storage systems. Additionally, the coating material of the encapsulation may cause contamination of HTF. In most of the cases, the replacement of PCM at the end of the life cycle is not possible, HEM is not commercial yet, therefore, they have to be individually designed. Moreover, it is not fully compatible with HTF, since the coating material may contaminate it and cause degradation. Implementation complexity is not a big issue since it is a similar and common type of heat exchanger.

Figure 7.

Heat exchanger module (HEM).

In terms of cost-effectiveness, there are no common available data that estimate the cost of it but it is more cost-effective compared to heat exchangers. From a performance efficiency perspective, it has a storage density of 74%. However, heat transfer enhancement methods are being researched.

3.3.1 Design considerations for stratified HEM tanks

The following design criteria help in the design of an efficient TES tank with enhanced thermal stratification [1].

Geometrical considerations: A deep water-storage container is desirable to improve thermal stratification. The water inlet and outlet should be installed to produce a consistent water flow to evade mixing. The location of inlet and outlet openings should be placed as close as possible to the top and bottom respectively of the stored volume to minimise the dead water volume and the surface area in contact with the water.

Operating considerations: The difference of temperature linking the top and bottom parts of the tank should at the least be 5°C to 10°C. Controls can be used to maintain fixed water temperatures in the tank’s upper and lower parts if desired. The velocity of the water flowing into and out of the tank should be low.

Other considerations: The insulating and water-proofing characteristics of the tank should be designed to meet appropriate specifications.

For proper installation and control, using state-of-the-art equipment requires less energy for heating and cooling with substantial potential to substitute with low initial costs. More sophisticated thermal-design calculations are required to get the best design for better operating conditions.

3.3.2 Degradation of thermal stratification

Degradation of thermal stratification or mixing in a thermal storage tank can be essentially attributed to the following physical mechanisms:

• Diffusion of heat to the bottom of the storage tank due to the temperature difference between the fluids in the top and bottom regions.

• Heat loss to the surroundings due to improper insulation.

• Effect of flow turbulence due to incoming /exiting flow rate.

• Thermal conduction along with the height of the storage tank through the walls of the tank.

The medium used for heat transfer interchanges heat through direct and indirect contact with the medium used for storage which forms a thermocline in HTF. The dual medium concept has a temperature drop as a drawback when compared to the single medium hot or cold tank while charging and discharging. The single medium maintains a constant outlet temperature while charging and discharging till the heat is removed in the tank, however, the HTF outlet temperature increases as it is discharged and decreases the more it is discharged for a dual medium storage system which leads to unfeasible storage capacity.

During the charging and discharging periods, mixing forms the main cause of stratification loss in general and major mixing losses occur during the lengthy storage periods in general. Enhancing the stratification significantly increases the efficiency of TES compared to a thermally mixed-storage tank. Hence the TES tank has to be evaluated quantitatively and comprehensively to clearly analyse the effects of stratification on the performance of a TES system.

3.3.3 Parameters to measure the stratification

Different methods for the stratification characterisation in TES system are being recommended over the years. A density method and a temperature approach are two fundamentally distinct approaches to stratification. In general, only temperature-based techniques are presented. Furthermore, it is critical to distinguish between elements that effect stratification (e.g., Richardson number, H/D ratio, etc.) and figures that evaluate the degree at different time intervals. Furthermore, the thermocline thickness, which divides the hot and cold zones inside the storage, may be used to analyse indicators of the degree of stratification.

Numerical figures given in terms of efficiency or ratio, on the other hand, are typically used for comparing the experimental storage process to a hypothetical storage process and thus include information about the history of the storage process. The most commonly used dimensionless numbers, such as MIX number, Richardson number, ratio H/D, discharging efficiency ratio, Peclet number and Reynolds number, to characterise stratification in water tanks and studied their suitability.

According to their findings, Peclet and Reynolds values do not explain stratification inside the tank, and it is unclear if discharging efficiency can define stratification inside the storage tank at low flow rates.

Richardson number is used to evaluate the stratification by comparison, for example, between PCM-filled storage against a reference case without PCM. The Richardson number is a popular way to describe stratification. It is a ratio of buoyancy forces to mixing forces; a low Richardson number implies a mixed storage tank, whereas a high Richardson number indicates a stratified storage tank.

The MIX number is used to analyse stratification in a water storage tank at a specific time, with a figure ranging from 0 to 1 indicating the degree of stratification; nevertheless, it is sensitive to slight variations in the temperature profile. Because the stratification degree is of considerable importance in this work, several stratification efficiencies such as energy efficiency (first law of thermodynamics), exergy efficiency (second law of thermodynamics), stratification efficiency and MIX number were utilised. The MIX number ranges from 0 to 1, with MIX = 0 representing a perfectly stratified tank and MIX = 1 for a fully mixed tank.

Stratification efficiencies based on the first law of thermodynamics most commonly compute a proportion of energy recovered with a certain charging and discharging operation with a set intake temperature and mass flow. The charging energy efficiency is defined by Ref. [4] and is based on the first rule of thermodynamics.

Methods for evaluating exergy efficiency based on the second law of thermodynamics are especially relevant when the energy will be utilised to do work, but they are not the only ones. The exergy stored in this example represents the thermodynamic limit of the work that may be generated. To calculate values based on exergy, a reference' dead state' that corresponds to the thermodynamic condition of the environment must be determined. Exergy is always greater in storage with a more significant temperature gradient and better stratification than in storage with an identical energy content but less pronounced stratification. As a result, numbers based on the second law of thermodynamics may be utilised to provide information regarding stratification efficiency.

Oro et al. [6] and Cabeza et al. [7] reported that the various parameters such as degree of stratification first law efficiencies, second law efficiencies, mix number, stratification number and Richardson number were used to characterise the storage system and they reported that energy and exergy efficiencies have no relationship with thermal stratification.

3.3.4 Advantages of stratification

Several studies have illustrated that the stratified STES system will give better results in comparison to a mixed tank of STES. In a STES system when connected to a solar panel, the heat extracted from a stratified charged tank is greater and for a longer duration in contrast to a fully mixed STES system. This stratification causes the low temperature at the bottom of the fluid of the tank returning to the solar collector and this creates a higher temperature difference between the fluid and the collector. It is concluded that the use of stratification in thermal storage designs should be considered as it increases the exergy storage capacity of thermal storage.

Developments in Stratification Analysis Stratified TES tanks often exhibit superior performance to non-stratified tanks, especially in low HTF flow rate heating systems. The level of TES stratification is decreased by HTF mixing, due to natural or forced convection, or by heat diffusion by conduction. Haller et al. [3], reviewed and compared methods to determine the stratification efficiency of a TES during a complete thermal cycle. Only one method considers the entropy generation, which provides a more meaningful interpretation of efficiency than energy analysis. Some of the advantages of stratified thermal storage systems in construction are as follows:

• They can often incorporate inexpensive above-ground steel tank construction.

• They provide a less expensive option than adding more chillers for meeting peak cooling loads.

• They are normally compatible with any water chiller technology.

• They can be incorporated into air conditioning systems, process cooling or heating and district cooling or heating loads.

Research activities on TES are ongoing at various national laboratories, universities, and research centres worldwide. The TES system can utilise thermal stratification. Such stratification separates lower-density warm water above higher-density cool water and can be applied in a single economic storage tank to provide demand-side management of cooling or heating loads. Stratified thermal storage is expected to minimise the tank’s costs, insulation, system integration, piping, and controls. Consider the example of cool supply water being withdrawn from the storage tank’s bottom during peak cooling periods and used directly in the cooling loop and then sent back to the top of the tank. However in the off-peak periods, warm water is taken from the tank’s top and cooled with the help of low-cost off-peak energy, and sent back to the tank’s bottom.

4. Experimentation on the heat exchange module

An experimental project was conducted using a HEM and its experimental setup [8, 9], along with all the design data is being discussed in the following sections.

4.1 Experimental set-up

The experiment represented in Figure 8 has thermal energy storage with a heat exchanger module or storage tank, a circulating hot water bath, a electric heater to simulate a solar heat situation, components of a heat transfer loop along with a data acquisition system. A cylindrical storage tank was fabricated with a height of 300 mm and a diameter of 320 mm as shown in Figure 8a. A 50 mm space on the top and a 25 mm space on the bottom is the working space left free and in between these two regions a packed bed of PCM balls is arranged in four layers and each layer was distinguished using a steel mesh and had 11 PCM balls in each layer which were dispersed uniformly within the layers of the storage HEM module. The PCM HS89 was a phase change storage medium and is a hydrated salt having a melting temperature of 89 °C which was procured from Pluss advanced technologies Pvt. Ltd., New Delhi. The properties such as specific heat, phase transition temperature, liquid and solid densities and the latent heat were got from the technical data sheets of the company and are presented in Table 3. The 2 mm thick stainless steel encapsulated spherical ball has an average outer diameter of 80 mm. A 40 mm thick insulation of polyurethane foam for the storage tank was used to minimise the heat loss.

Figure 8.

Charging and discharging of a HEM and its circuit illustration (a) charging experiment with storage tank (b) discharging experiment.

S. No	Property	Value
1	Melting temp (°C)	87–89
2	Liquid bath latent heat (kJ/kg)	125
3	Liquid bath freezing temp (°C)	89
4	Solid density (kg/m^3)	1630
5	Solid specific heat (kJ/kgK)	2.65
6	Liquid density (kg/m^3)	1540
7	Melting temp (°C)	2.65

Table 3.

Technical specification for HS (Hydrated Salts) 89 PCM.

The 30 liter HTF hot water bath is simulated with two immersion heaters of 1.5 kW capacities. The temperature of the inlet water to the storage tank from the heater gradually increases until it reaches 95°C, and this temperature was controlled with the help of a thermostat. To observe the water level as lot of steam evaporates the water in the heater tank, a glass tube was parallelly attached to the HEM as shown in Figure 8. The steam produced while heating the HTF or water reduces the level of the water and as the level reduces below a required liquid level, makeup water is added. A quarter-watt centrifugal pump is used to circulate the HTF from the storage tank to the hot water bath and if the water level was increased beyond the brim of the hot water bath it would flow to the HEM through gravity. A rotameter is placed in the reverse flow from the storage tank to the hot water bath to measure the flow rate and maintain it to 1 liter per minute using a valve placed at the bottom of the HEM tank. The inlet and outlet pipes are placed separately in HEM and used to circulate the water based on the charging or discharging applications as illustrated in Figure 8. The temperature in the PCM encapsulation in each layer was measured using a thermocouples located inside any one of the PCM balls itself. The thermocouples were inserted in the head of the PCM balls by drilling a hole in the nut of the encapsulation, inserting the thermocouple, and then applying an adhesive (m-seal-curing epoxy compound) to avoid leakage. Four thermocouples are placed equally among the layers of water to measure and record the layer temperatures. Four thermocouples are also placed in the encapsulated PCM balls to record the PCM temperature inside the ball to identify whether the PCM has melted on heating with HTF. A data acquisition system NI 9213 was used to connect the thermocouples to the laptop using LABVIEW interface. The data is recorded in MS-EXCELL sheets for further analysis.

4.2 Experimental procedure

In the present experiment, initially after checking all the connections of the circuit charging of the HEM was conducted by allowing the heated water to flow from the hot water bath to the HEM across all the balls so that the PCM capsules get heated and reach the temperature of 89°C. As the temperature of the water increased beyond 90°C, the PCM in the spherical capsules is melted, and while melting it is simultaneously ensured that the hot water bath did not go beyond 95°C using thermostat. Using a centrifugal pump the water from the bottom of the HEM is pumped back at a rate of 1 L/min to flow to the hot water bath, so the centrifugal pump was switched on. After reaching the melting temperature, the temperature of the balls remains constant due to phase change taking place in the HS89 salts in the PCM balls. The data acquisition system NI 9213 is used to monitor and record continuously the increase in temperatures with respect to time. As the temperature of the balls reaches 89°C and remains constant, salts in the PCM balls will melt. This constant temperature indicates the charging of the balls and the HEM is completely charged. After recording the temperatures of balls and the layers of the charging experiment using thermocouples and DAQ as shown in Figure 8a, the data is stored and the charging experiment is complete. After ensuring the finish of charging of the experiment as the complete phase change takes place, the discharging experiment was initiated. In the discharging experiment, the centrifugal pump was stopped and the hot water supply was cut off. Room temperature water at 30°C is circulated from the cold water inlet at a rate of 1 L/min into the hot-charged HEM. As the thermocouples placed inside the balls and within the layers start recording the temperatures at various positions while discharging. The outlet temperature of the water coming out from the storage tank also was recorded separately during this process of discharging. Instantaneous and cumulative heat transfer, charging and discharging efficiencies and the analysis of stratification characteristics for the storage tank were deduced from the transient temperature variation obtained in the PCM and the HTF of the HEM.

4.3 Data analysis

The various equations used to evaluate the performance of the storage tank such as Q__ins, Q__cumm, Q__loss, Q__stored, charging efficiency, stratification number and Richardson number are presented in this section. The analysis of the experiments can be done by deriving these parameters which help us to identify the different aspects of the heat flow and the stratification behaviour across the storage tanks. The analysis is drawn based on the values of these parameters also helps to conclude which storage tank is better for which purpose.

4.3.1 Overall heat loss coefficient (U_overall)

This parameter is evaluated from the drop in temperature of the water in the storage tank over a long duration of time when the storage tank is under idle condition (without the PCM balls) using Eqs. (3) to (6). This parameter helps to calculate the heat loss and becomes important to understand the loss of heat while charging and discharging experiments are conducted:

$$Q=U_{\mathit{\text{overall}}}.A.T_{\mathit{\text{LMTD}}}$$E3

$$U_{\mathit{\text{overall}}}=\frac{Q}{A.T_{\mathit{\text{LMTD}}}}$$E4

$$\mathrm{Q}=m{C}_p\left(T_{\mathit{ini}}-T_{\mathit{\text{final}}}\right)$$E5

$$T_{\mathit{\text{LMTD}}}=\frac{\left(T_{\mathit{ini}}-T_{\infty }\right)-\left(T_{\mathit{\text{final}}}-T_{\infty }\right)}{\mathit{\text{lnln}}\frac{\left(T_{\mathit{\text{in}}}-T_{\infty }\right)}{\left(T_{\mathit{\text{final}}}-T_{\infty }\right)}}$$E6

Q = Total heat lost to the ambient, W

T_ini = Initial temperature in the tank, °C

T_final = Final temperature in the tank, °C

T_∞ = Ambient Temperature, °C

Cp = Specific heat of water, kJ/kgK

m = mass of water in the idle tank, kg

A = outside area of the idle tank, m²

The overall heat loss coefficient (U_overall) is evaluated for all three storage tanks and the average value is considered for further analysis.

Separate experiment was conducted under idle conditions to evaluate the average heat loss coefficient. The temperature of the water in the storage tank was averaged out for all the layers of water with respect to time. The initial and final temperatures of the layers while discharging experiments under idle conditions were taken to evaluate the LMTD as given in Eq. 6. The overall heat loss coefficient was calculated with Eq. 4 and an average value of 5.056 W/m²K was being evaluated for the storage tank and used for calculations of both charging and discharging of the storage tank with PCM balls

4.3.2 Instantaneous heat transfer (Q_i)

The instantaneous heat transfer represents the amount of heat transferred into the storage tank at any instant of time during the charging process as given in Eq. (7). It shows the effect of heat based on the inlet and outlet temperature and is directly proportional to the stratification of the tank.

$$Q_i=\dot{m}{C}_p\left(T_{H-\mathit{\text{in}}}-T_{H-\mathit{\text{out}}}\right),\mathrm{W}$$E7

$T_{H-\mathit{\text{in}}}$ = Inlet HTF temperature to the tank, °C

$T_{H-\mathit{\text{out}}}$ = Outgoing HTF temperature from the tank, °C

$\dot{m}$ = Mass flow rate of the HTF, kg/s

4.3.3 Cumulative heat transfer (Q_c)

The cumulative heat transfer represents the total amount of heat stored at any instant from the start of the charging process as Eq. (8) given below shows. It adds on to the total heat from the starting of the experiment and this cumulative heat indicates the total amount of heat either stored while charging or retrieved while discharging of the storage tank

$$Q_c=\stackrel{̇}{\sum _0^t\dot{m}{C}_p{\left(T_{H-\mathit{\text{in}}}-T_{H-\mathit{\text{out}}}\right)}_t\mathit{\Delta t}},\mathrm{W}$$E8

t = Instantaneous time

4.3.4 Heat lost (Q_loss)

It is the amount of heat lost from the external surface of the storage tank during the charging process. It is evaluated using Eq. (9).

$$Q_l=U_{\mathit{\text{overall}}}A\mathit{dT}\mathrm{W}$$E9

dT = T_avg - T_∞

T_avg = Average temperature of all the layers of water in the storage tank, °C

U_{overal l} = Overall heat loss coefficient of the storage tank, W/m²K

A = Area of the storage tank, m²

4.3.5 Heat stored (Qs)

It is the actual amount of heat retained in the storage tank during the charging or discharging process after subtracting the heat lost to the surrounding, from the storage tank. It gives an idea as to what amount of heat is stored and is calculated using Eq. (10).

$$Q_{\mathit{\text{stored}}}=Q_{\mathit{cum}}-Q_{\mathit{\text{loss}}}$$E10

4.3.6 Charging efficiency

The charging efficiency of the storage tank is the ratio of the instantaneous heat transfer to the maximum heat transfer at a given inlet temperature at any instant, keeping constant the flowrate of the HTF at the inlet. It illustrates how the efficiency varies with respect to time for the charging done to the storage tanks without and with PCM. It is determined by the following Eq. (11)

$$`{\eta }_{\mathit{ch}}=\frac{{\left[T_{H-\mathit{\text{in}}}-\mathit{TLb}\right]}_t}{T_{H-\mathit{\text{in}}}-T_{\mathit{ini}}}$$E11

T_ini = Initial temperature of the tank

4.3.7 Discharging efficiency

The discharging efficiency of the storage tank is the ratio of the instantaneous heat transfer to the maximum heat transfer at a given inlet temperature at any instant keeping constant flowrate of the HTF at the inlet as given by Eq. (12). The cold water inlet at 30°C (TC-in) is made to enter the storage tank till the end of the experiment.

$${\eta }_{\mathit{\text{disch}}}\left(\mathrm{t}\right)=\frac{{\left(T_{C-\mathit{\text{out}}}-T_{C-\mathit{\text{in}}}\right)}_t}{{\left(T_{C-\mathit{\text{out}}}\right)}_{\mathit{ini}}-T_{C-\mathit{\text{in}}}}$$E12

$T_{\mathit{Cin}}$ = Cold inlet water temperature to the tank at the bottom, °C

T_Cout = Outgoing water temperature at the top, °C

4.3.8 Stratification number (Str)

It is the ratio of the average temperature gradients at each interval of time to the temperature gradient at the initial charging process (t = 0). Eq. (13) shows the evaluation of stratification number, depending on the temperatures acquired at equidistant points in the thermal energy storage tanks

$$\mathit{Str}=\frac{\left(\partial T/\partial y\right)t}{\left(\partial T/\partial y\right)t=0}$$E13

where

$$\left[\frac{\partial T}{\partial y}\right]=\frac{1}{N-1}\left[\sum _{k=1}^{N-1}\frac{\left(T_k-T_{k+1}\right)}{∆Z}\right]$$

$${\left[\frac{\partial T}{\partial y}\right]}_{t=0}=\frac{T_{H-\mathit{\text{in}}}-T_{\mathit{ini}}}{\left(N-1\right)∆Z}$$

where k is the nodal points where the temperature measurements are made, N is the number of nodal points and ∆Z is the distance between the nodal points T_{_(H-in)} and T_{_ini} which shows the hot water inlet and initial temperatures of the HTF, respectively.

4.3.9 Richardson number (Ri)

It is an effective indicator of stratification performance that considers the various aspects of storage systems such as the geometry of the storage tank, specific velocity, and the top and bottom temperature of the layers within the tank. It is given by the ratio of the buoyancy forces to the mixing forces and is estimated by Eq. (14).

$$\mathrm{Ri}=\frac{\mathit{g\beta H}{\left[\left(T_{H-\mathit{\text{in}}}\right)-\left(T_{H-\mathit{\text{out}}}\right)\right]}_t}{v_{\mathit{sf}}^2}$$E14

$${\nu }_{\mathit{sf}}=\frac{V}{{\mathit{\pi r}}^2}$$

where T_H-in and T_H-out represent the top and bottom temperature of the HTF measured in the TES tank and H is the distance between these locations. V and ν_sf represent the volumetric flow rate and superficial velocity of HTF entering the TES tank, respectively and r is the radius of the inlet pipe which is ¼^th of an inch in the present experiment.

4.3.10 Mix number

Mix number is the level of energy in the TES tank weighted by its vertical distance from the bottom of the storage tank. The vertical moment of energy in a storage tank was shown in Eq. (15).

$$M_E={\int }_0^H\mathit{ydE}$$E15

This equation was further deciphered as shown in Eq. (16), as follows

$$M_E=\sum _{i=1}^N{y}_i{E}_i$$E16

where N = the number of layers used

y_i = the distance measured from the bottom of the tank.

$E_i={\left(\rho C_p{V}_i{T}_i\right)}_{\mathit{\text{water}}}+{\left(\rho C_p{V}_i{T}_i\right)}_{\mathit{pcm}}$, which is the energy stored at a particular level i

On calculating the energy stored for PCM, the sensible heat is accounted as ${\left(m{C}_p{V}_i{T}_i\right)}_{\mathit{pcm}}$and the latent heat is accounted as ${\left(\rho V_{i}L\right)}_{\mathit{pcm}}$ cumulatively based on the temperature which the PCM attained on heating or cooling and when the PCM reached the phase change temperature. The term V_i represents the volume of the PCM or the water at a particular level i and L here represents the latent heat of the PCM when the PCM reaches the phase transition temperature.

When considering mixing as a function of time of the day, the moment of energy increases with increase in temperature of the heated water and decreases when hot water is withdrawn from the tank. The temperature profiles were determined experimentally in the fully mixed and the unmixed storage tank. Hence the moments of energy for the unmixed or fully stratified (M_str) and the fully mixed (M_mix) are determined and substituted in the MIX number as shown in Eq. (17) given as follows:

$$\mathit{MIX}=\frac{\left(M_{\mathit{str}}-M_{\mathit{\text{actual}}}\right)}{\left(M_{\mathit{str}}-M_{\mathit{mix}}\right)}$$E17

For a given set of inlet conditions, the M_str and M_mix represent the largest and the smallest values of the moment of energy, respectively.

4.4 Error analysis

The error associated with various primary experimental measurements and the calculation of estimated uncertainties for the performance parameters are given in Table 4.

Fundamental parameters	Uncertainty (%)
Mass (m)	1
Length (L)	0.253
Diameter (D)	0.4167
Volume (v)	1
Temperature (T)	0.334
Time (t)	0.033
Derived parameters
Mass flow rate (ṁ)	1.054
Volume flow rate (v ̇)	1.054
Instantaneous heat rate (Qinst)	1.053
Cummulative heat rate (Q cumm)	1.0548
Overall heat loss coefficient (Uoverall)	1.3853
Heat lost (Qlost)	1.6173
Heat stored (Q stored)	1.9309
Charging efficiency (ηch)	0.4723
Stratification number (Str)	0.5927
Richardson Number (Ri)	1.7268

Table 4.

Summary of the estimated uncertainties.

4.5 Results

The temperatures being measured across the HEM tank along with the PCM while carrying out the experiment are plotted under various dimension with respect to time and analysed in the following section. The temperature variation is initially noted with respect to time and later on various types of data like the amount of heat transferred is quantified along with the different stratification analysis.

4.5.1 Comparing the charging and discharging characteristics of the storage tanks filled with PCM for all the HEM

Figure 9 shows the temperature variations of the heat transfer fluid in the top and bottom layers of the storage tank along with the variation of the HTF inlet temperature during the charging process. Figures 9a and b represent the temperature variations of the HTF in the storage tank with PCM balls while charging and discharging respectively. In both the figures, the shaded portion represents the temperature variations of the later of HTF along the height of the storage tank at any instant of time. The thick line on the top for Figures 9a and b represents the incoming and outgoing HTF temperature of the storage tank on charging and discharging, respectively.

Figure 9.

Temperature variation in the top and bottom layers of the storage tank with PCM balls in the HEM while a) charging b) discharging.

In the case of storage tank with PCM balls, while charging as shown in Figure 9a, the incoming hot water referred by the dark line on the top of the storage tank is slowly heated and the temperature of it rises until it reaches around 95 °C after which the thermostat stops heating the incoming water temperature to rise, as it forms steam at 100 °C. As the cold water is circulated from the bottom back to the heater tank, the HTF temperature keeps rising due to the mixing of incoming hot water and the layers temperature slowly rises. Hence the stratification or the temperature difference for the HEM continuously increases to 16.68°C till the end of charging upto 270 minutes. This temperature difference of more than 15°C is maintained for more than 125 minutes at the end of charging.

However in discharging we find that a maximum temperature difference above 20°C is available for 25 minutes and it diminishes to 7°C at the end of discharging upto 66 min. The T_c-out temperature of water is the outgoing hot water temperature and more than 80°C of water can be retrieved for around 24 minutes and later on the temperature of the outgoing water reduces gradually. The outgoing water temperature runs through the center of the shaded portion as mixing of the HTF layers are predominant. Substantial number of balls present with high heat capacity is facing the incoming hot water in the layers of HEM, and due to this arrangement of the PCM in the storage tank, it helps to achieve the required type of stratification.

Figure 10 shows the instantaneous and cumulative heat transfer during the charging and discharging process which are calculated using the temperature data. It is seen from the figure that the instantaneous heat transfer continuously rises and the cumulative heat stored in the HEM at the end of the charging process is around 14,000 kJ. After deducting the heat lost during the charging process from the cumulative heat transfer, the heat stored is also calculated. From the graph, the heat lost is found to be significant at the end of the charging process.

Figure 10.

Instantaneous and cumulative heat transfer during the charging and discharging process.

During the discharging process, the instantaneous heat transfer is initially very high in the range of 4 to 4.5 kW. This higher instantaneous heat transfer is maintained for the first 10 min duration. Even at the end of the discharging, the instantaneous heat transfer is maintained consistently higher till the heat is discharged in the HEM. The gradual fall in the instantaneous heat transfer could be due to the slow retrieval of heat in the PCM at the bottom of the storage tank, due to high stratification created as the cold water enters from the bottom and cannot move up quickly due to high density. Hence in this case of the HEM, almost all the heat available in the storage tank of 14000 kJ is removed within the duration of 65 min. Since the instantaneous heat transfer rate decreases, further removal of heat from the storage tank appreciably will be a prolonged process.

Charging efficiency and discharging efficiency have been plotted in Figure 11 for the HEM considered in the present investigation. It is seen from Figure 11a that charging efficiency is high initially as the temperature difference between the incoming hot water and the room temperature is high and hence stratification effect increases. Also during the charging process, the presence of reasonable quantity of PCM balls in the top layer retains more amount of heat at the top layer of HEM and hence the charging efficiency remains steady up to 38 % till the end and does not drop further.

Figure 11.

a) Charging efficiency and b) Discharging efficiency for the HEM.

During the discharging also the discharging efficiencies gradually drops to 20% for a time span of 66 min as the convection heat gradually flow from the top and bottom layers. Initially, the heat transfer is arrested as the PCM balls melt upto the first 23 min and the temperatures also remain high. This temperature pattern of discharging provides favourable heat transfer during the entire discharge process.

Figure 12 shows the variation of the stratification number in the HEM during the charging and discharging process. It is observed that in this module there is a gradual increase in the stratification that favoured the rate of charging and hence the charging efficiency is upto 40%. The stratification increases upto 160 min as there is continuous difference of temperature which causes the convection of heat from top to bottom layers of HTF and later on stabilises once the temperature of HTF is around the melting point of PCM 89°C.

Figure 12.

The variation of stratification number for a) charging and b) discharging of HEM.

The stratification behaviour during the discharging process shows a steep increase after 10 minutes and stratification number reaches its maximum of 5.58 till 38 min. Then it decreases slowly and reaches a level around 2 after a period of 63 min. This infers that the maximum amount of heat can be discharged upto 40 min due to better stratification and also though stratification is reduced but significant amount of heat can be retrieved in this HEM.

Figure 13 shows the variation of the Richardson number in the storage tank during the charging and discharging process. Richardson number is a more accurate method to calculate the stratification as it includes the buoyant forces also. When the Richardson number increases it shows greater stratification whereas when the Richardson number decreases, it represents mixing forces dominate. Continuous rise in the Richardson number shows stratification found till the end and while discharging also it supports the retrieval of heat by rising gradually and then decreasing gradually after 40 minutes. The gradual increase and decrease of stratification give us an idea as to how the increase of heat charging can effectively retain and discharge the heat effectively.

Figure 13.

The variation of the Richardson number in the storage tank during the charging and discharging process.

Figure 14 illustrates the MIX number variation while charging and discharging. The stratification remains long as the MIX number ends reduce from 1 to 0 while charging. The MIX = 1 represents a completely mixed condition and reaches MIX = 0 for a totally unmixed condition due to charging. The reduction of MIX number is gradual due to gradual increase in stratification.

Figure 14.

Variation of MIX number during the a) charging process and the b) discharging process.

On discharging, it is observed that there is a slow and steady increase and the mix number only increases after 10 minutes so the stratification initiates. The gradual rise in MIX number shows that there is a gradual increase in stratification and represents consistent discharging of uniform heat which is very vital for any discharging application, to achieve maximum heat retrieval.

5. Conclusions

Understanding the heat exchanger design in a thermal storage helps to design a module better. To enhance this design various parameters can be utilised as discussed above. A heat exchanger with the results is being analysed and it can conclude that the amount of heat charged is easily discharged when the temperature of melting is 89°C. The average charging and discharging efficiencies are maintained high for the HEM tank as the PCM balls present retain high heat capacity and also help to maintain better stratification along with high Richardson number.

Nomenclature

Cp

specific heat of the solidus PCM, J kg–1 K–1

E

Energy level of the storage tank J

g

acceleration due to gravity, m s⁻²

L_pcm

Latent heat of fusion of the PCM, J kg⁻¹

m_pcm

mass of the PCM, kg

TL1 to TL5

Temperatures of all the layers assigned in the storage tank, K

N

Number of temperature measurement nodes

Q

heat absorbed by the PCM, W

Ri

Richardson number

Str

Stratification number

$\frac{\partial T}{\partial y}$

temperature gradient along the height of the storage tank, K m⁻¹

t

instantaneous time, s

T

temperature, K

$\dot{V}$V

flow rate of the HTF, m³ s⁻¹

ν_sf$v_{\mathit{sf}}$

superficial velocity, m s⁻¹

∆Z

distance between the top and the bottom layers, m

Greek symbols

β

Coefficient of thermal expansion K⁻¹

ρ

density, kg m⁻³

Subscript

avg

average

ch

charging

C_-in

Cold incoming water while discharging

C_-out

outgoing hot water while discharging

H_-in

incoming water while charging

H_-out

outgoing water while charging

i

level of the layer

Q_i

Instataneous heat loss

Q_c

Cummulative heat

Q_l

Heat loss

Q_s

Heat stored

ini

initial

k

temperature measurement nodal point counter

Lb

Bottom layer temperature

sf

superficial

Abbreviations

HEM

Heat exchanger module

HTF

Heat transfer fluid

LHTES

Latent heat thermal energy storage

PCM

Phase change material

SHTES

Sensible heat thermal energy storage

SHE

Screw heat exchanger

STHE

Shell and tube heat exchanger

TES

Thermal energy storage

CSP

Concentrating Solar Power