Experimental Analysis of A Vapor Compression Refrigeration System Coupled with A Phase Change Material-Based Condenser for Pre-Cooling

Experimental Analysis of A Vapor Compression Refrigeration System Coupled with A Phase Change Material-Based Condenser for Pre-Cooling

Akshita Singh

Design & Engineering Department, Larsen and Toubro Realty, Mumbai, Maharashtra

Atharva Gandhewar

Design & Engineering Department, Larsen and Toubro Realty, Mumbai, Maharashtra

Suraj Patil

Design & Engineering Department, Larsen and Toubro Realty, Mumbai, Maharashtra

Abstract – in coastal areas like Mumbai, the ambient and wet- bulb temperatures are continuously high, and this data is crucial in evaluating the working of water-cooled air-conditioning systems, which are installed in commercial and residential homes. The cooling tower outlet water temperature exceeding the coolant specifications during the hottest days leads to higher compressor lift and greater electrical power usage in the water-cooled chillers [1, 2]. It uses a shell-and-tube heat exchanger composed of a phase change material (PCM) to contact the cooling tower water line in front of the condenser of an 850 TR water-cooled chiller, which uses R134a refrigerant. The system allows the condenser inlet water temperature to be lowered by some 2°C during peak loads and, consequently, the refrigerant condensing temperature and compressor work. Those PCMs have been selected as calcium chloride hexahydrate (CaCl·6HO) since it melts at approximately 29°C, and the amount of heat it stores (190 kJ/kg) is appropriate based on the weather condition and the temperature in the cooling tower in Mumbai. The environment considered in this paper is the wet-bulb temperature of Mumbai [7]. The thermodynamic study of the vapor compression refrigeration cycle shows that lowering the temperature of the water entering the condenser by 2°C will noticeably decrease the power needed by the compressor during the hottest part of the day. During high ambient conditions, the peak demand is reduced by temporarily absorbing part of the heat load from the condenser into the PCM. At nighttime, the refrigeration cycle is switched off, and the cooling tower water loop only runs to regenerate (re-solidify) the PCM. The cooled water is recirculated back to the condenser, where it removes heat from the refrigerant through the condenser tubes. The extra thermal loading on the cooling tower under regeneration is relatively small compared to its capacity and has a minor impact on the stability of the system. The offered structure will enable shifting peak loads to nighttime-regulated operation without incurring significant auxiliary energy taxes. The results show that using PCM to cool water at the cooling tower during controlled nighttime regeneration can effectively reduce peak demand and help manage energy use in large water-cooled chiller systems in hot and humid coastal weather.

Keywords: vapor compression refrigeration system, phase-change material, condenser pre-cooling, and shell-and-tube. The study also focuses on the heat exchanger, peak load reduction, and thermal energy storage.

1. INTRODUCTION

   The energy used in refrigeration and air-conditioning systems accounts for almost 40-50% of peak electricity consumption in large Indian metropolitan cities, more so in commercial buildings, shopping complexes, hospitals, airports, and data centers that work in hot and humid weather conditions [1, 2]. In cities like Mumbai, where the temperatures of wet bulbs take high values most of the year, cooling tower efficacy is highly affected in water-cooled chillers during daytime operations. Cooling tower outlet water temperatures increase the temperature in the condenser, which causes the temperature to rise further, increases the pressure, and makes the compressor work harder, resulting in lower efficiency (coefficient of performance (COP) of the chillers).

   Common ways to improve how well a water-cooled condenser system works usually involve increasing the cooling tower's capacity; lowering the temperature difference in the tower, which enhances the heat transfer area in the condenser; or increasing the flow rate. These methods may be effective to increase heat rejection; however, they may also cause an increase in pumping power, larger tower size, water usage, and complexity [6]. When managing energy, especially during high- demand times, the first step is to temporarily reduce the heat load on the condenser, and the second step is to make adjustments to this reduction without significantly increasing extra energy use.

   One of the possible solutions is the use of phase change materials (PCMs) to temporarily store thermal energy within a limited time when the service is taken on the other side (the condenser side) applications. At the transition between phases, PCMs can capture high concentrations of latent heat at almost equal temperatures; thus, they can be used to store peak loads

   with minimal temperature increase [10, 11]. In the suggested design, the PCM will be added to the cooling tower's water inlet line in a shell-and-tube heat exchanger. High daytime operation In peak daytime operation, the PCM captures a proportional part of the condenser water heat load and cools down the cooling tower outlet water. Lift the water by some 2°C, then on to the condenser. This decreases the condenser inlet water temperature, which decreases the refrigerant condensing temperature and saturation pressure and decreases compressor lift and power consumption by the compressor [12, 13].

   Common methods or traditional approaches that have been used to increase the surface area for condenser heat transfer or add more cooling tower capacity tend to require more capital investment and further space consumption, which might not be practically achievable in a given installation. Equally, pumping power will rise, and operating expenses will be higher as cooling towers' water flow rates are increased. Despite the possibility of increasing the cooling tower performance through design advancement, it still has a basic limitation of the climate, which is the main issue in hot and humid coastal locations like Mumbai. These real-world constraints show how necessary it is to consider other methods of condenser-side thermal management that are not as much a load of thermal loading as they are importantly simple and do not place such significant requirements on auxiliary energy and complexities of the system.

   Fig. 01: Mumbai climate graph.

2. DESCRIPTION OF VAPOR COMPRESSION REFRIGERATION SYSTEM (VCRS) CYCLE.

   A vapor compression type of refrigeration system functions on a thermodynamic closed loop with four main components, namely, a compressor, a condenser, an expansion valve, and an evaporator [17].

   1. Compression Process:

      The compressor compresses low-pressure refrigerant vapor to produce high-pressure and high-temperature refrigerant vapor. The input compressor work is highly dependent on the ratio of the pressure on the discharge to that of the suction pressure [18].

      Fig 02. Ph Sketch of VCRS Cycle

      COP = Refrigeration effect / work done by compressor

      = R.E/Wc

   2. Condensation Process: –

      The refrigerant vapor that is of high pressure is introduced to the condenser, where heat is rejected to the surrounding media (air or water). A phase change between the liquid and the vapor of the refrigerant occurs at a phase change, following which the pressure is altered nearly constantly [19]. The temperature of the cooling medium is directly proportional to the temperature of the condenser [20].

   3. Expansion Process: –

      This high-pressure liquid refrigerant is introduced through expansion equipment, and at this point, its pressure and temperature are reduced drastically [21].

   4. Evaporation Process: –

      The conditioned room pushes the heat to the circulating chilled water via the air-handling units, and the heat is then collected in the evaporator by the low-pressure refrigerant. As the refrigerant transforms from liquid to vapor, it removes thermal energy content in the chilled water loop and thus sustains the desired conditions in the interior and completes the refrigeration process. [22].

      The condensation process is one of such processes that directly affect compressor work. The decrease in condenser temperature

      decreases the condensing pressure, thus decreasing the power input in the compressor and enhancing the COP. This is the thermodynamic foundation of the condenser pre-cooling in PCM [22, 23].

      2.5. Cooling Tower Working:

      Water-cooled chiller systems reject the heat to ambient air but cool the tower water (rather than it being the rejected heat sink) and then use it as the cooling medium for the cooling towers. This hot condenser water is pumped to the cooling tower, where it releases heat to the atmosphere, which is mainly through evaporative cooling.

      Inside the cooling tower:

      • Heat generated by the condenser is used to spray warm water on fill material to maximize surface area.

      • The tower takes the ambient air into it either via natural draft or mechanical fans.

      • It loses much of the water to evaporation.

      • The leftout water loses its latent heat in the evaporation process.

      • Because of this fact, the bulk water temperature drops.

      • The cooled water is recirculated back to the condenser, where it removes heat from the refrigerant through the condenser heat-exchange surfaces.

      The ambient WBT is determined as the lowest temperature that the cooling tower will cool the water to. Hence, in hot and humid weather conditions in Mumbai, the temperatures of wet bulbs are high, compressor power draw is consequently higher, resulting in high temperatures of condenser water, high temperatures of refrigerant-condensing water, and higher compressor power draw.

      One of the processes that impact directly on compressor work is the condensation process. Reduction in the temperature of the condenser reduces the condensing pressure, which then reduces the amount of power consumed by the compressor and increases the COP. This is the thermodynamic rationale of the pre-cooling of the condenser with PCM [22, 23].

3. PCM-BASED CLIMATIC CONDITIONS IN MUMBAI.

   1. Selection Criteria

      The PCM has been selected as per the following conditions:

      1. Temperature compatibility of melting temperature:

         It was required that the PCM melting temperature be nearly equal to the range of operating water temperature of the cooling tower as it enters the condenser. This will make sure that a

         transition between the phases will be granted in case the inlet refrigerant temperature increase occurs when the peak load conditions are traced. The PCM is merely taking up heat by merely setting the melting point as per the usual high temperature of the refrigerant at the moment when the cooling of heat by the condenser is the least. This lowers the temperature of the refrigerant condensing and the condensing pressure.

         Fig. 03: TS Sketch of VCRS Cycle.

      2. High latent heat of fusion:

         The latent heat storage volume of the phase change material is so immense that it is able to store a lot of latent heat during the time of occurrence of the phase change, which, in any case, is within a limited duration of time [21]. A phase-change material is a material that traps and releases thermal energy, typically by changing physical state. Between solid and liquid states, and at a fixed temperature, although it is at a relatively constant temperature. Unlike sensible material of heat storage, as the heat addition is made, the temperature of the material gradually rises due to the heat addition. PCM materials depend upon the latent heat, and thus when heat is inputted into the material, it will store energy in an effective way, though not with a substantial increase in temperature.

         It is worth noting particularly that the excellent latent heat of the selected PCM (phase change material) is an attribute. Specifically, calcium chloride hexahydrate (CaCl·6HO) was selected because it changes from solid to liquid at a stable temperature, which is excellent for matching the temperature of condenser water, and it can go through many heating and cooling cycles with only a little drop in performance. Important to use in condenser pre-cooling applications in which the intent is not to achieve sustained cooling but to transiently augment the thermal load in peak operating circumstances. The PCM

         eliminates the momentary heat rejection demand on the condenser because the capacity of the storage during melting is large enough to reduce the immediate demand. This allows minimizing the peak thermal load using an amenable, although moderate, quantity of PCM of major importance in the design of systems with extremely small sizes and low space [20]. Further, the near isothermal characteristics of the PCM regarding the heat captivity help in stabilizing the condenser inlet at significantly more stable conditions during the peak hours with better results of the system.

      3. Thermal conductivity is taken into account.

         The thermal conduction is another design parameter that is important in the design of thermal energy storage systems based on PCM, as most of the phase-change heat/cold conductors have low inherent thermal conductivity. The heat transfer is not done between the circulating cooling tower water and the refrigerant in the proposed system but via the PCM. A good thermal exchange of the cooling water with the PCM is, therefore, needed to guarantee the desired reduction in the temperature of the condenser inlet water in its highest- performance conditions.

         With the aim of overcoming this drawback, shell-and-tube heat exchanger design is embraced. With this system, the cooling tower water is directed to flow through the tubes, with the PCM (CaCl*6HO) being placed in the shell side. The high multiple tube setup is vastly increasing the effective heat transfer surface area between circulating water and PCM; therefore, the thermal resistance between the two is reducing and so is the rate of heat transfer.

         The tubular shape also decreases the effective heat diffusion distance inside the PCM, whereby the heat will flow more evenly across the PCM volume. This is to minimize any localized temperature gradients and provide better solidification and melting uniformity in the course of charging and regeneration cycles. A configuration like this is especially useful in peak load operation, where the PCM absorbs heat in the daytime during peak load operation and transfers the stored thermal energy at night in the regeneration process by the cooling tower water loop.

         As a result, the shell-and-tube heat exchanger can improve the overall performance of the PCM and offset the methodically low thermal conductivity possessed by the salt hydrate PCM and thus augment the efficiency of pre-cooling of condenser water in the proposed design [17].

      4. Stability: Cycling and Chemical Stability.

         The PCM also has to be chosen based on the fact that it can be melted and solidified repeatedly without much deterioration in its thermal characteristics. In the case of condenser water pre- cooling, the PCM needs to exhibit phase change roperties in the long-term cyclic operation so that their thermal performance is predictable.

         Stability in the cycling of the PCM is also crucial since the PCM is exposed to repeat charging (melting) throughout the day under peak sunlight conditions and to regeneration (solidification) during the night's super cooling operation. Any segregation, super cooling, or degradation of material property may decrease the heat storage capability and influence the effectiveness of peak load reduction.

         The stability of the chemical and compatibility with the containment material are also important to avoid corrosion or deterioration of the material over the long run. Calcium chloride hexahydrate (CaCl·6HO) was chosen because it has comparatively constant phase change behavior, an appropriate melting temperature to match the temperature of condenser water, and the capacity to experience repeated thermal cycles with only slight degradation of performance [18, 19].

         3.1.5. The suitability and availability of the cost should be evaluated.

         Large-scale thermal energy storage in commercial HVAC systems has significant economic and material factors associated with it. The phase change material chosen must be easily obtainable and affordable and one that, when implemented in high quantities, does not add much to the overall system cost.

         The calcium chloride hexahydrate (CaCl*6HO) meets these criteria because it is commercially available, and its thermophysical characteristics are well-documented in the literature. The material is also easily obtainable as a salt hydrate and can be easily incorporated into heat exchanger systems that are intended to be used to pre-cool condenser water. Since the proposed application implies the incorporation of the water loop into the water-cooled chiller. High capacity, scalability, and cost-effectiveness are critical considerations. This is because the proposed peak-load reduction strategy can be made possible by the use of a low-priced and readily available PCM in large commercial cooling systems [11, 12].

   2. Properties of CaCl*6HO

      The material selected to be used as phase change material in this work is calcium chloride hexahydrate (CaCl·6HO) since its melting range is within the range of temperature of the water

      Specific heat (solid)	1500 J/kg·K
      Specific heat (liquid)	1700 J/kg·K
      Density (solid)	1560 kg/m³
      Density (liquid)	1470 kg/m³
      Volumetric expansion	3.5%
      Viscosity (liquid)	5080 mPa·s
      Thermal diffusivity	2.1 × 10 m²/s

      within the condenser that will be in any water-cooled chiller system working within the climatic conditions of Mumbai. The melting of the PCM is approximately 29°C, and the latent heat of fusion is approximately 190 kJ/kg, and it is appropriate for

      absorbing the heat in the water condenser.

      before it is given to the

      The normal climatic conditions in Mumbai are that of high ambient temperatures and high humidity. In the daytime, under

      normal conditions, the ambient temperature is normally

      between 35 and 38°C, and the water entering the condenser is usually around 32°C in temperature. With this, the PCM will be able to absorb the heat of the inlet water in the condenser and cool it down by approximately 2°C, thus decreasing the condensing temperature and lowering compressor work.

      At nighttime when the ambient temperature is lower and closer to 25-27°C with the cooling tower wet-bulb temperature being lower, the stored latent heat in the PCM can be rejected by the use of the cooling tower water loop. This enables the PCM to solidify and re-form itself to support the next operating cycle and, hence, the repeated daily reduction of peak loads at normal refrigeration cycle functioning without any substantial impact.

   3. Stabilization of PCM

      The selected phase change material also requires the incorporation of nucleating and stabilization agents in order to minimize common drawbacks associated with salt hydrate PCMs, such as super cooling and phase separation. These additives help maintain consistent phase change behavior during repeated melting and solidification cycles. By reducing super cooling effects, the PCM can solidify at a temperature closer to its nominal phase change temperature, ensuring reliable regeneration during nighttime cooling tower operation and maintaining consistent thermal storage performance over long-term cycling.

      Table 01. Properties of PCM (CaCl·6HO)

4. SYSTEM CONFIGURATION

   1. Placement of PCM Heat Exchanger

      The PCM heat exchanger in the suggested system is fitted in the water cooling tower ahead of the condenser in the water-cooled chiller. Water that leaves the tower is collected by the cooling tower, which is then passed through the PCM heat exchanger before it gets into the condenser.

      During peak daytime operation, the relatively warm cooling tower water transfers part of its thermal energy to the PCM contained inside the heat exchanger. As the PCM undergoes phase transition as a solid becomes liquid, it takes up latent heat and consequently cools down the cooling water entering the condenser by some 2°C.

      Property

      PCM

      Melting temperature

      Latent heat

      Thermal conductivity

      Value

      CaCl·6HO

      29 °C

      190 kJ/kg

      0.56 W/m ·K

      This reduction in condenser inlet water temperature lowers the condensing temperature and condensing pressure of the refrigerant, which in turn reduces the compressor pressure ratio and compressor work. As a result, the electrical power consumption of the chiller decreases during peak load periods.

      During nighttime conditions, when ambient and wet-bulb temperatures decrease, the water temperature also decreases. Under these conditions, the stored latent heat in the PCM is rejected through the cooling tower water loop, allowing the PCM to solidify and regenerate for the next operational cycle without interfering with the normal vapor-compression refrigeration cycle.

   2. Shell-and-Tube Heat Exchanger Design

      The PCM heat exchanger is designed using a shell-and-tube configuration, which is commonly used in large-capacity HVAC systems due to its mechanical strength, high heat transfer area, and operational reliability.

      • Shell Side: PCM (CaCl*6HO) is placed in the shell, and it holds the thermal energy during the melting process. When the heat fluxed out of the circulating cooling tower water into the PCM, the material experienced a phase change between solid and liquid at an almost constant temperature.

      • Tube Side: The water from the tower circulates through the tubes of the heat exchanger. When the relatively warm, comparatively warm condenser inlet water enters through the tubes, the heat is exchanged across the tube walls into the surrounding PCM, and the PCM melts to store the thermal energy.

      • Flow Configuration: A counter flow arrangement is preferred because it maintains a larger effective difference in temperature between the cooling water and the PCM at the cost of the length of the heat exchanger. This improves the overall heat transfer effectiveness and ensures more efficient thermal energy absorption during peak operating conditions.

      • Construction Materials: The tubes are typically manufactured from copper or copper alloys because of their corrosion resistance in water-based cooling systems. The shell is coated with an anti-corrosive protective layer to withstand continuous contact with the salt hydrate PCM and repeated thermalcycling during melting and solidification processes.

      • Design Benefits: The shell-and-tube configuration provides high structural integrity and accommodates the volumetric expansion of the PCM during phase change. The modular nature of the design allows the heat exchanger to be scaled by adjusting the PCM quantity or tube bundle size according to system capacity and peak load requirements. This flexibility makes the system suitable for integration into medium- and large-capacity water-cooled chiller installations.

      Fig. 04. Shell and Tube type heat exchanger

   3. PCM regeneration mechanism at night.

      During daytime operation, the PCM absorbs part of the heat from the cooling tower water before it enters the condenser. When the temperature of the inlet water of the cooling tower exceeds the melting point of the PCM (29°C), the PCM begins to melt and absorb latent heat. At the conclusion of peak load, a large portion of the PCM mass is melted.

      To make the system work in a cyclic way on a daily basis, this would have to contain latent heat that would have to be removed so that the PCM can solidify and recover its ability to absorb heat in the next operating cycle. This is done under conditions at night when there are relatively lower ambient and wet-bulb temperatures in Mumbai than there are during the day.

      At night:

      • The cooling tower takes away the heat over the inlet air.

      • The WBT is reduced.

      • The water cooler temperature is reduced.

      • The freezing refrigerant's natural temperature is reduced.

      • In the process of regeneration, the compressor is inactive, whereas the water loop of the cooling tower is the only one running to eliminate the stored latent heat of PCM.

        During nighttime operation:

      • The refrigerant does not go through the PCM.

      • The water flows to the cooling tower and is made to circulate through the PCM heat exchanger.

      On the shell side, there is the PCM:

      When it is late at night, the PCM melting temperature is lower than that of the inlet water of the cooling tower. Since there is such a temperature difference, then the tube walls can conduct heat out of the melting PCM into the cooling tower water. During heating of the PCM, the solidified PCM releases its latent heat at a relatively constant temperature as the heat is transferred to the coolant.

      The rejection heat then goes into the cooling tower, which works in a similar manner to the cooling tower where the heating is staved off to the air through discharging the energy to the water through evaporative cooling. Because the wet-bulb temperature is at a lower, lower temperature at night, the cooling tower is more efficient and is able to reject the additional PCM regeneration load without a significant increase in condensing temperature.

      Thermodynamically, it can be said that:

      • The amount of regeneration load is negligible compared to the overall condenser heat rejection capacity.

      • The incremental thermal load has no great impact on raising condensing pressure.

      • The compressor proceeds to work under good conditions at night.

      • The COP is not inferior to the daytime one.

        Therefore, PCM regeneration does not interfere with the refrigerant cycle, but it is a parallel thermal disposal process accomplished by the cooling tower circuit. The PCM is refreezing prior to the following daytime peak period, which allows daily proportionate changes of loads and peak demand cutbacks.

        This regulated regeneration plan will guarantee:

      • Thermal storage capability it has the capacity to store thermal power on a cyclic basis.

      • Stable condenser operation.

      • Low effects on system performance.

      • Working coordination with traditional water-cooled chiller systems.

      What makes the overall concept of PCM-assisted condenser pre-cooling effective, however, is not only the absorption of heat in daytime but also the ability to ensure nighttime regeneration via the cooling tower loop.

5. CALCULATIONS TABLE

   1. Operating Conditions (Mumbai)

      Parameter	Symbol	Value
      Cooling capacity	QL	850 TR = 2989.45 kW
      wet-bulb temperature (Mumbai peak)	WBT	27 °C
      Temperature of cooling tower water inlet.	T_w,in	32.22 °C
      Temperature of cooling tower water outlet.	T_w,out	37.77 °C
      Target PCM melting temperature.	T_pcm	29 °C
      Refrigerant flow rate. (R134a)		6.55 kg/s
      Specific heat of Water	Cp	4.18 kJ/kg ·°C
      Typical condenser water flow rate.	_water	3 gpm/TR for 850 TR system is 2550 gpm 160.8 kg/s.

      Table 02: Operating Conditions (Mumbai)

6. EXPERIMENTAL CALCULATION

Water-Cooled Chiller 850 TR R134a Day Operation = 1 hour

(Nominal plant operation is 8 hours; the calculation is shown

for 1 hour peak shaving.)

Night Regeneration = 2 hours

Given Data:

• Cooling Capacity QL = 850 × 3.517 QL = 2989.45 kW

• Coefficient of Performance COP (day) = 5.5

• Specific heat of water

  Cp (water) = 4.18 kJ/kg°C

• Latent heat of PCM

  L (PCM) = 190 kJ/kg

  STEP 1 Cooling Load

  QL = 2989.45 kW

  STEP 2 Compressor Power (Without PCM)

  COP = QL / W W = QL / COP

  W = 2989.45 / 5.5 W = 543.54 kW

  STEP 3: Day Energy Consumption (Without PCM for 1 hour of operation)

  E = 543.54 × 1 E = 543.54 kWh

  STEP 4 P-h Based Thermodynamic Analysis (Without PCM)

• Evaporator pressure

  P (evap) = 300 kPa

• From R134a property table:-

  h 395 kJ/kg (compressor inlet)

• Cooling tower water temperatures:

  1. Condenser inlet water temperature =

     32.22°C

  2. Condenser outlet water temperature =

     37.77°C

  3. Condenser approach = 2°C

• Condensing temperature:

  T = 40°C

• Corresponding condensing pressure:

  P 1015 kPa

• From the P-h diagram (isentropic compression):

  p 430 kJ/kg

• Compressor work per kg refrigerant:-

  w = h h

  w = 430 395

  w = 35 kJ/kg

  PART B WITH PCM (2°C LOWER CONDENSING TEMPERATURE)

  STEP 5: New Condensing Condition:-

• New condensing temperature T' = 38°C

• Corresponding pressure

  P' 975 kPa

• From the diagram

  h' 425 kJ/kg

• Compressor work per kg refrigerant

  W' = h' h

  W' = 425 395

  W' = 30 kJ/kg

  STEP 6: Percentage Work

  Work reduction

  w' = 35 30 = 5 kJ/kg

  Percentage reduction

  % redction = (5 / 35) × 100

  % reduction = 14 %

  STEP 7: Day Energy Consumption (With PCM)

  Compressor power with PCM W' = 543.54 × (30 / 35)

  W' 466 kW

  Energy consumption E' = 466 × 1

  E' = 466 kWh

  STEP 8 Daytime Energy Saving

  Day_saving = 543.54 466

  Day_saving = 77.54 kWh

  PART C PCM MASS CALCULATION

  STEP 9 Condenser Water Flow

• Cooling tower flow rate: Flow = 3 gpm/TR

  Total flow

  = 850 × 3

  = 2550 gpm

• Conversion to SI units 1 gpm = 3.785 L/min

  2550 × 3.785 = 9651.75 L/min

  = 9.651 m³/min

  = 0.1608 m³/s

• Water density 1000 kg/m³

• Mass flow rate

  = 0.1608 × 1000

  = 160.8 kg/s

  STEP 10 Heat Removed by PCM

  Heat transfer equation:- Q_PCM = × Cp × T Q_PCM = 160.8 × 4.18 × 2 Q_PCM = 1344.29 kW

  STEP 11 Energy Stored in PCM (1 Hour)

  Q_total = Q_PCM × time Q_total = 1344.29 × 3600 Q_total = 4,839,436 kJ

  STEP 12 PCM Mass Required m_PCM = Q_total / L_PCM m_PCM = 4,839,436 / 190 m_PCM = 25,470 kg

  m_PCM 25.5 tons

  PART D NIGHT REGENERATION (COOLING TOWER ONLY)

• Stored thermal energy:-

  Q (total kWh) = 4,839,436 / 3600 Q (total kWh) = 1344.29 kWh

• Night wet bulb temperature: T_wbt = 24°C

• Cooling tower approach: Approach = 4°C

• Cooling tower outlet temperature: T_out = 28°C

• PCM phase change temperature: T_PCM = 29°C

• Temperature difference:-

  T = 1°C

  STEP 13 Regeneration Time

  Energy balance:-

  M (PCM) × L (PCM) = × Cp × T × t 4,839,436 = 160.8 × 4.18 × 1 × t

  t = 7200 seconds t = 120 minutes t = 2 hours

  STEP 14 Pump Power

• Volumetric flow:- Q = 0.00536 m³/s

• Pump head:- H = 25 m

• Hydraulic pump power:-

  P = × g × H × Q

  P = 1000 × 9.81 × 25 × 0.00536 P = 1.31 kW

• Electrical pump power:-

  P' = P_h /

  P' = 1.05 / 0.75 P' = 1.75 kW

  STEP 15: Pump energy for regeneration.

  E_pump = 1.75 × (120/60) E_pump = 3.48 kWh

  FINAL NET ENERGY SAVING

• Day saving:-

  = 77.54 kWh

  Night regeneration pump energy

  = 3.48 kWh

• Net savings:-

Net saving = 77.54 3.48

Net saving = 74.06 kWh

FINAL RESULT

1. Day compressor saving = 77.54 kWh

2. Night regeneration energy = 3.48 kWh

3. Net energy balance = 74.06 kWh

1. INFLUENCE ON OPERATIONAL HOUR ENERGY CONSUMPTION

   Through the integration of PCM in the cooling tower water line before the condenser. This decrease in condenser inlet water temperature reduces the refrigerant condensing temperature and saturation pressure. [22].

   Any reduction in the condensing pressure reduces the ratio of compressor pressure directly. The compressor work being proportional to the pressure rise across the compressor, a lower pressure ratio across the compressor leads to less mechanical work of compression. This mechanical work reduction corresponds to lower electric power usage in peak-condition daytime when cooling demands are highest and loads of a cooling tower are probably greatest [23].

   This leads to minimization of the total electrical consumption of the water-cooled chiller system during peak hours [9]. Also, the PCM allows shifting partial load by storing thermal energy when the temperature is lower or when off-peak control is active, and it is released when the temperature of the condenser water increases. The approach minimizes the maximum electrical load without the need to add active cooling devices and, hence, promotes demand-side energy control and peak shaving goals [14].

2. EXPERIMENTAL PROBLEMS AND CONSTRAINTS.

   1. Phase separation and supercooling.

      The hexahydrate of calcium chloride is likely to supercool and separate into phases in repeated cycles of melting and solidification, which may lower the optimum use of latent heat and result in non-uniform thermal performance [22].

   2. Requirement of additives.

      The stabilizing or nucleating agents are usually required in order to enhance nucleation and prevent phase segregation. which further complicates material preparation and long-term maintenance [24].

   3. PCM mass requirement.

      Meaningful peak-load reduction can only be achieved with a significant mass of PCM, which results in an increase in system weight, system volume, and support needs [26].

   4. Cost and space of the heat exchanger.

      Shell-and-tube PCM heat exchangers add further capital expenditure and take up large space, thus possibly restricting the possibility of retrofitting existing systems [31].

   5. Requirement of PCM regeneration strategy.

      An efficient PCM regeneration process is needed to restart the continuous operation, and in most cases it re-solidifies the material during off-peak periods and does not incur the extra energy penalty [15].

3. CONCLUSIONS

   Peak cooling load reduction of a water-cooled vapor compression refrigeration system by integrating a heat exchanger with PCM integrated in the condenser cooling tower water line can be important in the Mumbai climate conditions [14, 23]. The PCM heat exchanger will be placed before the water-cooled condenser, and it will pre-cool the condenser inlet cooling water temperature when needed most, which is during peak working periods. The refrigerant condensing temperature

   and the saturation pressure in the condensing water are decreased by making the cooling water entering the condenser cold [22].

   The decreasing condensing pressure lowers the compressor pressure ratio, as a result of which mechanical work of compression directly decreases as well, and this results in a decrease in electricity power consumption [5, 6]. This thermodynamic innovation increases the COP of the chiller at the peak load operation when the condenser water temperatures are generally high because of the high ambient wet-bulb temperatures and higher cooling tower loads.

   It was established that calcium chloride hexahydrate (CaCl·6HO) was the chosen PCM suitable for use in this application, as this compound has a melting temperature similar to the required temperature range (approximately 3240°C) of the condenser inlet water temperature within the operating conditions in Mumbai as well as a relatively high latent heat of fusion [5, 6]. The shell-and-tube design offers efficient heat exchange between the condenser water and the PCM and allows volumetric expansion during phase change. Hence, this design can be applicable in large-scale integration of thermal-energy storage.

   Notably, the system is offered to minimize peak power compressor operation, preventing skewing to extra active cooling systems, including larger cooling tower use or supplemental mechanical refrigeration. In its place, excess heat load is temporarily absorbed during peak hours through the store of thermal energy, which moves some portion of the thermal burden to the off-peak periods. Despite the fact that the latent heat storage capacity of PCM in itself limits the maximum length of the pre-cooling phase, the findings indicate that PCM- based condenser water pre-cooling can be an effective measure in demand-side management when done in a directed and controlled mode.

   On the whole, the result points to thefact that PCM-based condenser refrigerant cooling can be used as the complement to the traditional water-cooled chiller systems in hot and humid environments, which will help to increase the operating efficiency, decrease the peak electrical load, and make systems more stable during peak loading [11-13].

4. FUTURE SCOPE

   • In the long-term cycling performance measurement. Long-term experimental investigations are needed to determine the long-term thermal and chemical stability of the PCM during repeated melting and solidification. especially to determine how

     performance decreases, phase separation, and supercooling behavior change with time [24].

   • More complex encapsulation of PCMs: It can be improved in the future by better encapsulations and containment techniques to improve heat transfer, reduce the risk of leakage, and allow the volumetric expansion of the phase transition [31].

   • Integration with the district cooling system: The condenser pre-cooling concept based on PCM can be expanded on a large scale by incorporating it into the district cooling networks, where centralized thermal storage of energy could have important benefits in reducing peak loads [15, 16].

   • Multi-mode PCM-evaporative cooling methods: The integration The use of PCM-based pre-cooling and evaporative or adiabatic cooling methods can also enhance the condenser performance, especially under hot and humid conditions, and also help in saving water and energy [8].

   • Life-cycle assessment and economic assessment. It should be carefully studied in terms of capital cost, savings in the cost of operation, payback time, and ecological impact of PCM-based condenser pre- cooling systems under the actual performance conditions [9, 14].

5. REFERENCES

1. Kalaiselvam, S., Sureshkumar, K. R., and Sriram, V. (2016). Research on the characteristics of heat transfer and pressure drop in air heat exchangers by using phase-change material in free-cooling operations. Thermal Science, 20(5), 1543-1554. doi:10.2298/TSCI130531096K.

2. Saving of energy during building using a PCM-free cooling system. (n.d.). Research article.

3. Latent heat storage using phase-change material. (n.d.). Review the article.

4. Phase-change material and its selection criteria: An overview. (2020). International Journal of Engineering Research and Technology 9(9).

5. The use of paraffin in the selection of thermal energy storage using latent heat. (n.d.). Research paper.

6. Cabeza, L. F., et al. (2020). Phase change thermal storage materials: Materials and applications. Materials, 13, 2971.

7. Phase-change materials. Literature review. (n.d.). Document on compiled literature review.

8. Bolteya, A. M., Elsayad, M. A., El Monayeri, O. D., and Belal, A. M. (2022). Effects of phase change materials on cooling of a learning institution in Cairo, Egypt. Sustainability, 14, 15956. doi.org/10.3390/su142315956.

9. Al-Obaidi, K. M. (2022). Thermal performance in buildings increased by using PCM. J. Phys. Contr. Serie, 2222, 012005.

10. Ascione, F., et al. (2023). Evaluation of the PCM-integrated building envelopes with respect to performance. Buildings, 13, 1595.

11. The effect of PCM incorporation on the building's cooling energy demand. (2022). Sustainability, 14, 15956.

12. Ali, E., Ajbar, A., & Lamrani, B. (2023). Numerical simulation and modeling of a phase change material tank to power chiller generators in district cooling systems. Sustainability, 15, 10332. doi.org/10.3390/su151310332.

13. Performance evaluation of a district cooling system. (n.d.). Technical research paper.

14. Nogaj, K., Turski, M., & Sekret, R. (2018). Substations using PCM heat accumulators in district heating. MATEC Web of Conferences, 174, 01002. https://doi.org/10.1051/matecconf/201817401002.

15. District cooling storage PCM-based thermal energy storage. (n.d.). Journal article.

16. Development of regulation and practice of the district cooling systems: Changi Business Park case study. (n.d.). Technical report.

17. Shi, Z., Hong, H., Shen, Y., & Cai, J. (2024). Performance of PCM in large space temperature control and cooling release in the environment: Case study of cold storage. Cold energy storage system (PCM-based refrigeration system). (n.d.). Research paper.

18. Du, Y. (2019). Phase change is the storage of thermal energy. PhD dissertation, University of Leeds, United Kingdom.

19. PCM-based thermal energy storage systems modeling and simulation. (n.d.). Research article.

20. PCM-based thermal storage of energy in cooling. (2020). Energies, 13, 3877.

21. PCM systems' thermodynamic behavior. (2020). Processes, 8, 1158.

22. Thermal energy storage systems design manual. (n.d.). Technical design guide.

23. District cooling system applications and commercial developments. (n.d.). Engineering design report.

24. Storage of thermal energy in phase-change material: A review. (2020). arXiv preprint, arXiv:2009.10712.