• Cooling India
  • Apr 15, 2017

Stay Cool Without AC

Phase change materials (PCMs) are known for storing thermal energy by virtue of their inherent latent enthalpy. PCM tiles integrated with building envelope promises to offer the ease of construction without encroaching on valuable floor space occupied by building structure. PCM tiles are installed and analyzed in naturally ventilated rooms located in various geographic locations around India. Appropriate indoor and outdoor environmental conditions are logged to provide insights into PCM effectiveness…


India is the world’s third largest greenhouse gas emitter. India pledges to reduce carbon emission per unit of GDP (emission intensity) to 35% by 2030 from 2005 [1]. India’s building sector experiencing unprecedented growth in the past decade. It is expected to grow about five times from 2.1 million square metre in 2010 to about 10.4 million square metre in 2030 [2]. India’s residential sector will fuel energy consumption unless focused policy and market effort is not initiated to provide 30 to 60% savings by 2050 [2]. As per predictions, a significant increase in energy consumption and rapid expansion in the use of space cooling appliances is expected. Figure 1 demonstrates the increasing pattern of energy consumption by service within buildings

Figure 1

Figure 1: Energy consumption by service in rural & urban area (1 Exajoules, EJ = 1018 Joules, J) [1]

2]. This increase will largely be due to outdoor climate change, building construction and materials, operational modes, thermal discomfort levels and increase in resident’s ability to own and use cooling systems. In India, along with a challenge to reduce energy consumption, reduction in peak demand is also necessary for India to focus on. Today, India’s energy consumption per unit area of the exterior wall, the roof and the window are two to five times higher than that in developed countries [3].One of the best ways to reduce energy consumption is to achieve energy efficiency in buildings.

  Solar radiation is the most important direct reason for the air conditioning energy consumption, which accounts for almost 40% electrical peak load in large cities [3]. In summer, for single story buildings the cooling load through the roof has the ratio of 36.7% in the whole building envelop cooling load, while the multi-story building can reach to 8-10% [3]. Back in time, older buildings, churches, castles, etc. had an adequate thermal storage mass for retaining heat energy which ensured cool interiors despite the summer heat outside. But in the current age of lightweight constructions, due to lack of thermal mass in buildings, room temperatures quickly rise to a level that is equal to or even higher than the temperature outside. Conventionally, this problem is solved by installing air-conditioning systems which however, are expensive to buy and operate; their energy consumption is huge and offers the highest electrical peak load today in every residential and commercial space in India. Additionally, the user’s comfort is decreased by draft, noise and dryness of the air. A research showed that air conditioning energy consumption can drop about 10% when inner temperature is decreased by 1°C [3]. Thus, thermal conditioning of the building (roof or wall) cannot only improve the indoor thermal comfort, but can also reduce air conditioning cooling load. One of the approaches has been to use PCMs in buildings that can absorb or release a large amount of heat at a constant or near constant temperature (Fig. 2), which makes it widely potential and has attracted several researchers, companies & technologists over the years in businesses involving precise temperature maintenance.

Figure 2

Figure 2: Effect of using PCMs in buildings: PCM normalizes the temperature to adesired temperature without the use of an active cooling system

  Ice is the most popular example of a phase change material at 0°C. To give you an example, melting of one kilogram of ice at 0°C to produce one kilogram of water at 0°C requires 333 kilojoules of energy. However, the same amount of energy would also be able to heat a kilogram of water from 0°C to approx. 80°C. Hence, these materials can store a large amount of energy at a constant temperature without letting the heat ingress through them. In building applications, PCMs of high heat storage capacity per unit volume (i.e. high energy storage density) is required to increase the thermal mass of the buildings equivalent to 240 mm of concrete with just 10 mm of PCM (refer Fig. 3) in an effort to reduce the amount of energy required to cool or heat the building in summer or winter, respectively. These advanced PCMs can intelligently maintain pleasant temperatures in summers without installing an air conditioning system by increasing the thermal mass of the building structure in least amount of space.

Figure 3

Figure 3: Heat capacities in 4°C interval for different building materials compared to PCM[4].Necessary layer thickness of different building materials to store as much heat as 10 mm layer of PCM

  To understand the potential of PCM for temperature control            (refer Fig. 4), it is necessary to look at the case without PCM as a reference. During the day, the PCM begin to store the heat at the point at which the interior temperature begins to become excessive. The PCM which has a melting point of 24°C begins to absorb heat from the ambient air at this temperature, preventing the interior from heating up any further. This ensures pleasant working conditions and an agreeable temperature to live and work, and there are no large increases in temperature. Apart from enhancing the comfort, a large reduction in costs can also be achieved by dissipating the accumulated heat during the night by allowing cold night air to circulate through them. In developed countries, such a configuration shifts the heat load from expensive daytime to cheaper night time. Additionally, these materials allow more usable space i.e. higher net floor area in the buildings.

Figure 4

Figure 4: The general concept for cooling with PCM integrated in building materials

  Unfortunately, water with its phase transition temperature at 0°C and 100°C is not suitable for use in buildings. Hence, special grades of PCMs is developed which are ideally leak-proof, mechanically-stable and more conductive than ordinary PCMs. One of them is form-stable (FS) PCMs. FS-PCMs are encapsulated in metal coffins of good thermal conductivity to allow accelerated heat exchange with the cold night air from outside. These building tiles can be designed to the usual grid dimensions of the building. The only pre-requisite for the PCM tiles to function is to ensure natural or mechanical ventilation in the night to allow the PCM to release the heat absorbed during the day. This is referred to as the charging of the PCM. Some of the key advantages of PCM building tiles for smart temperature management are:

• Allows huge savings on electricity demand by offering

• Comfortable interior climate without noise and dryness

• Large energy storage density at phase transition temperature

• Occupies limited space and can be retro-fitted in any existing building

• Easy to install and remove in the event of relocation

• Non-combustible, non-toxic and flame-retardant

• No servicing and operating expenses

• Life longer than that of conventional air conditioner

  Currently, a range of PCMs exclusively for building segment are in development, they are: form-stable (FS) PCMs, emulsified PCMs, macro-encapsulated (ME) PCMs. Traditionally, PCMs are of three types: inorganic, organic and eutectic type PCMs. Organic PCMs can be modified to composite PCMs to form FS-PCMs and ME-PCMs, while inorganic PCMs are modified to form emulsified PCMs, refer Fig. 5(a). To prevent leakage and to improve heat transfer characteristics PCMs are encapsulated. Several aspects which govern the selection of an encapsulation are: (a) chemical compatibility of the container wall with the PCM; (b) thickness of the container wall to assure the necessary diffusion tightness; (c) suitability of the thermal characteristics of the container (encapsulating) material with the PCM while in application; and (d) the encapsulation must be designed in a way that it is able to cope up with the mechanical stress acting on the walls caused by the volume change of the PCM. Fig. 5 (b) depicts some of the common industrial scale encapsulation followed for inorganic and organic PCMs for its application in a chosen field ranging from healthcare, pharmaceutical, solar, cold-chain logistics, etc.

Figure 5

Figure 5 (a) New commercialized PCMs for building applications; (b) Encapsulation technology available: a. HDPE Polymer Thermotabs; b. Stainless Steel balls; c. High barrier Aluminum foil; d. HDPE extrusion profiles; d. HDPE pipes; e. PET film; f. HDPE 70mm balls;
g. HDPE 25mm balls; h. Aluminum bottles

  To understand the potential benefits of using PCMs as a building envelop in the form of a wall or a ceiling in any commercial or residential building, a dynamic building simulation using EnergyPlusTMis performed to generate meaningful building performance histograms.The answers about comfort are derived as well as clear economical figures about pay-back time and reduction of cooling capacity is deduced. Here, a 3m x 3m x 3m radio-communication room is chosen in the month of summer in Delhi. The constraints applied are:

PCM of 30°C in the ceiling. The thermo-physical properties of the chosen PCM:

PCM Melting Temperature 28-35 °C
Melting Energy 180 kJ kg-1 = 50 Wh kg-1

• A door facing South and a window facing East

• Standard glazing windows

• Windows and Ventilation System open during the night for natural cooling

• The inside place is lightened with a 10W/m2 and equipment load is 50W/m2

• People scheduled to be in the office during the day fro, 9 am to 6 pm

• Free exchange of air at a rate of 0.2 l/h

• Standard Packaged Terminal Air Conditioner with thermostat at 26°C

• Standard masonry construction with concrete, brick and plaster as shown in Table 1.

TABLE 1

Fig. 6 indicates reduction in day temperature and cooling load over the summer months of New Delhi by the application of a 30°C PCM in the ceiling of a 100 sq. ft. room with and without ventilation.

FIGURE 6

Fig. 6(a) plots the ambient temperature distribution of one whole year (January to December i.e. 0 to 8760 hours) for New Delhi. The highlighted “yellow” band indicates the operating temperature range for 30°C PCM.

FIGURE 6 A

Fig. 6(b) indicates the buildings dimensions2.6 x 2.6 x 2.6 m. They are constructed as per the details given above. It is elevated 0.4 m from the groundand fitted with a door and a window.

FIGURE 6 B

The result, Fig. 6(c) indicates that the interior climate remains balanced even in mid-summer periods of heat and the peaks mitigate. The room with PCM experiences less temperature swing which decreases the peak cooling load and the overall space cooling load.

FIGURE 6 C

In Fig. 6(d) with 30°C PCM, the decrease in cooling load range from 25-40% in the month of March & April (max. 36°C, min 20°C) to 10-20% during May-July (max. 46°C, min 27°C).

FIGURE 6 D

Fig.6 (e) clearly depicts the decrease in the number of hours in a year the building interior will be experiencing temperatures above 34, 36 and 38°C for the cases: without PCM, PCM+night ventilation and PCM+night HVAC. It can be realized that with efficient cooling or charging of the PCM in the night to a temperature below the PCM melting temperature, the day temperatures can very well be moderated to PCM temperature. All in all, there will be annual saving in your electricity consumption compared to other passive cooling systems.

FIGURE 6 E

CASE STUDIES

Name

I. CEPT University, Ahmedabad

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II. DTDC Courier Services, Gurgaon

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Description PCM in the ceiling of a building without any cooling technology was first implemented at CEPT University, Ahmedabad under Indo-US Joint Center for Building Energy Research & Development (CBERD) which is a Partnership for Advancement of Clean Energy – Research (PACE-R) with Oakridge National Laboratory & Lawrence Berkeley Laboratory as partnering organizations. Two rooms were chosen next to each other with one room being an ordinary insulated room while the other room installed with a 30°C PCM. The room was a typical masonry construction with wall and roof assembly without any special insulation. The room was experiencing higher heat ingressions much like the current buildings in India. The experiments were conducted in mid-April where the room interior without PCM was experiencing peak temperatures of 41°C. The weather conditions were studied to identify the most suitable PCM temperature for the months of summer. The max and min temperatures were predicted to be 440C and 270C, respectively. Hence, 35°C PCM was chosen as it will experience enough lower temperature during the night for the PCM to recharge comfortably.
PCM type FS-30 FS-35
PCM volume 0.5 cm thick tile in 3m x 3m ceiling i.e.0.2% volume of the total room volume: 3m x 3m x 3m 0.5 cm thick tile in a 4m x 3m ceiling i.e. i.e. 0.2% volume of the total room volume: 4m x 3m x 2.5m
Saving

Peak temperature during the afternoon reduced from 33°C to 31°C

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Peak temperature during the afternoon reduced from 41°C to 37°C

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III. Greenhouse, Asheville, North Carolina, USA

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IV. Bharti Airtel Telecom Shelter, Delhi

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Name & description

PCM was implemented in the walls of the greenhouse as a PCM curtain. The greenhouse was experiencing very high wall temperatures, much higher than the ambient. To regulate the wall temperature to below ambient temperatures, a 22°C PCM was installed in all the walls as a curtain.

A very effective utilization of PCM as a heat battery to store cold energy during the night for its use during the day was executed on telecom shelters to save on the energy consumption, peak demand and electricity cost. The telecom shelter can be categorized as a lightweight structure with wood and metal sheets constituting the wall and roof structure. To regulate the temperature of the shelter below 25°C, HVAC systems are operational throughout the day and for most part of the night. Additionally, a generator is installed to provide the power backup for operating the HVAC system to maintain the temperature inside. To save on the operation and maintenance cost, PCM was used to provide the backup during power failures by maintaining a constant temperature without additional power source. A 22°C PCM was installed along the walls to give a 6-8 hour backup throughout the day by maintaining the interior temperature around 25°C.
PCM type HS-22 HS-22
PCM volume 1 cm thick curtain in with PCM encapsulated in PVC tubes to form a curtain along the walls. PCM occupied 1% of the total greenhouse space volume PCM occupied 2% of the total telecom shelter space volume. The PCM tiles were 1 cm thick.
Saving

The temperature at the wall with PCM was almost 12°C cooler than the recorded temperature without PCM. The afternoon peak temperature reduced from 47°C to 35.5°C.

(a) Saving on diesel cost for the generator&HVAC running cost;
(b) 8 hour of cold backup without any additional power source or maintenance;
(c) PCM battery as a passive system is automatic – no man power required;
(d) PCM replaced a 7.5 kVA generator by allowing 80% reduction in total cost per year

Conclusion

  Latent heat storage in buildings is important in the reduction of cooling loads and reduction of temperature increases. The placement of the PCM is dependent on the wall/roof/floor surface through which the heat ingression is the highest; they are commonly ceiling and south wall. The amount of PCM is dependent on the specific thickness of PCM to be used for its effective charging and discharging on a daily basis. The natural ventilation during the night in summer and during the day in winter has a great significance in reducing the energy consumption of the space. The application of PCM in buildings has the ability to reduce the peak load of electricity demand throughout the year by reducing the cooling peak load during the day in summer and heating peak load during the night in winter. PCM utilises solar energy continuously, storing it during the day and releasing it at night, particularly for space heating in winter, thus improving the degree of thermal comfort. Additionally, they have the ability to store natural cooling by ventilation at night in summer and to release it to decrease the room temperature at night, thus reducing the cooling load of air conditioning. A most suitable PCM for a given climatic condition can offer decrease in energy consumption ranging from 30 to 40%, however, that is dependent on:set-point temperatures of the space, andvariance between the peak temperatures andthe PCM operating temperature.

References

[1] Ministry of Environment and Forests, GOI , “India’s Intended Nationally DeterminedContribution: Working Towards Climate Justice,” pp. 1–38, 2015

[2] R. Rawal and Y. Shukla, “Residential Buildings in India: Energy Use Projections andSaving Potentials,” Gbpn, pp. 1-50, September, 2014.

[3] Shilei Lu, Yafei Chen, Shangbao Liu, Xiangfei Kong, “Experimental research on a novel energy efficiency roof coupled withPCM and cool materials”,Energy and Buildings, 127,pp. 159–169, 2016

[4] Christina V. Konstantinidou, “Integration ofthermal energystorage in buildings”, The University of Texas at Austin, 2010.


AUTHORS CREDIT & PHOTOGRAPH

ANIRUDDHA CHATTERJEE

Aniruddha Chatterjee
Pluss Advanced Technologies Pvt. Ltd.
Gurgaon