CHAPTER 1 INTRODUCTION

Transcription

1 1 CHAPTER 1 INTRODUCTION In the past, researchers were interested in reducing the cost of energy and save the depleting fossil fuels. However, recently, the motivation has changed from these goals towards minimizing the carbon dioxide emission, from the environmental perspective. Energy efficiency, and the use of clean and renewable energy, has gained the major attention of the developed and developing countries. Hence in the present energy and environment scenario there is absolutely no need to justify the need for energy conservation. The energy consumption pattern of China and India, the latest emerging economies, shows a rapidly increasing trend. Focusing on the energy sectors of these two countries, where one third of the population of the world is present will constitute one of the first in a series of steps on smooth energy demand growth. Greener technologies become important for both these countries while they consume energy. India s share of residential, commercial and agricultural sector energy consumption is more, as shown in Figure BACKGROUND AND MOTIVATION A comfortable home with minimum energy consumption is the dream of the common man, governments and researchers. The energy consumption for mechanical cooling forms a major share within the

2 2 residential and commercial sectors and the trend showing the energy share of building cooling in the domestic sector is shown in Figure 1.2. The use of mechanical cooling requires complicated refrigeration systems that consume high grade energy. The demand for mechanical cooling equipments is expected to rise in the future as shown in Figure 1.3. This share is projected to increase in the future with an increase in the energy demand proportionate to industrial growth. Reduction in energy consumption for building cooling is the need of the hour. This can be done by various methods. Figure 1.1 Sector wise energy consumption of the energy consuming countries Source: IEA

3 3 (a) In absolute values (b) In percentage Figure 1.2 Trend showing the energy share of building cooling in the domestic sector Source : Can and Stephane (2009)

4 4 Figure 1.3 Projected rise in air conditioners and air coolers in India Source : Can and Stephane (2009) Cooling done by natural methods is called passive cooling. Passive cooling is considered as an alternative to mechanical cooling. By employing passive cooling techniques in modern buildings, mechanical cooling can be eliminated or at least the size and cost of the equipment can be brought down. Hence, it is planned to explore the various passive cooling options, and to further carry out research on free cooling potential and technologies for various climatic conditions. 1.2 METHODS OF REDUCING ENERGY CONSUMPTION FOR BUILDING COOLING The ambient temperature of a place is dependent on the seasonal climate and its geographical location. The indoor temperature of a building depends on the climatic condition of a place (outdoor temperature, wind velocity, solar radiation etc), building structural geometry (wall thickness, area ratio of window to wall) and the thermo- physical properties of the building materials (thermal conductivity and specific heat of wall material

5 5 etc), indoor heat load and air change rate. The comfort temperature of a room ranges from 21 C to 25 C. If the room temperature is above 25 C cooling will be required and if the room temperature is below 21 C heating will be required. The temperature inside the room can be reduced and controlled by perfect thermal insulation and controlling the amount of ventilation air. Also, passive cooling techniques can be used to reduce the indoor temperature of the building for comfort. Building cooling can be done by two methods Envelope design. Passive cooling systems Envelope Design Building envelope is a term which describes the roof, walls, windows, floors and walls of a home. The envelope controls the heat gain into the building in summer and the heat loss from the building in winter. Envelopes reduce the climatic extremes greatly by passive design. A carefully designed envelope maximizes cooling air movement and reduces the solar heat in summer. In winter, envelopes trap and store heat from the sun and minimize the heat loss to the external environment. i) Insulation Insulation acts as a barrier to heat flow and is essential to keep a building warm in winter and cool in summer. A well insulated and well designed building will provide year-round comfort, and not only reduce cooling and heating energy consumption, but will also reduce greenhouse emissions. Local climatic conditions will influence the selection of the appropriate level and type of insulation. It should be decided whether the

6 6 insulation will be needed to keep the heat out in summer or let it in, in winter (or both). Insulation must meet the requirements of seasonal as well as daily variations in the temperature of the air. Insulation in the ceiling can assist with weatherproofing and eliminate moisture problems such as condensation by prevention, in the reduction of the indoor temperature. Some types of glass wool insulation also have the sound proofing property. The most economical time to install insulation is during construction. The latest materials for building construction like aerated concrete blocks, hollow expanded polystyrene blocks, straw bales and rendered extruded polystyrene sheets have a good insulation property. ii) Thermal mass Controlling the thermal mass is the ability of the building material to absorb heat energy. More amount of heat energy is required to change the temperature of high density materials like concrete, bricks and tiles, and they are therefore said to have high thermal mass. Lightweight materials such as timber have low thermal mass. Thermal mass acts like a thermal battery. In cold regions, during the day time it absorbs heat from the sun, and at night the same thermal mass may release the heat keeping the house comfortable. A high mass building needs to gain or lose a large amount of energy to change its internal temperature, whereas a lightweight building requires only a small energy gain or loss. Thermal mass is particularly beneficial where there is a big difference between day and night outdoor temperatures. The correct use of thermal mass can delay the heat flow through the building envelope for as much as 10 to 12 hours producing a warmer house at night in winter and a cooler house during the day in summer. The correct use of thermal mass throughout the building increases its comfort and reduces the energy costs. The same concept can be extended for a warm region by storing the cool energy available during the early morning hours and retrieving the same

7 7 during the day time to cool the building space. Poor use of thermal mass can result in the radiation of heat all night during a summer heat wave, or absorb all the heat which is produced on a winter night. iii) Shading The shading of the building and outdoor spaces reduces summer temperatures, improves comfort and saves energy. Direct radiation from the sun can generate the same amount of heat as a single bar radiator over each square meter of a surface. Shading can block up to 90 percent of this heat. The shading of glass to reduce unwanted heat gain is critical. Unprotected glass is often the greatest source of unwanted heat gain in a building. Radiant heat from the sun passes through glass and is absorbed by the building elements and furnishings, which then re-radiate it. Re-radiated heat has a different wavelength and cannot pass back out through the glass as easily. In most climates, trapping radiant heat due to the green house effect is desirable for winter heating, but must be avoided in summer. The shading of wall and roof surfaces is important to reduce summer heat gain, particularly if they are dark colored and/or heavyweight. Shading requirements vary according to climate and house orientation. iv) Reflective coatings Recent research on highly reflective material for outdoor wall painting has led to the development of advanced and cheap materials for solar cooling load reduction. Reflective material when compared to the conventionally used material of the same color results in a reduction in the surface temperature of about 15 C. The solar control of transparent components comes from the switchable glazing technology. Electro chromic glazing has been considerably improved for load reduction and it is available

8 8 in the market. A combination of the cool material with green space and heat sinks can decrease the inside building temperature (Santamouris 2007) Passive Cooling Techniques Passive cooling techniques utilize the careful design of the micro climate, shading and thermal capacitance, which can contribute to the decrease of the cooling load. Heat sinks like atmospheric air, sea, earth, etc, are required to dissipate the excess heat of a building to a natural heat sink by the convective, evaporative and radiative principles and by ground cooling (Agas et al 1991). Passive cooling techniques contribute to the cause of reducing the green house gas emissions. techniques are Three different types of passive and hybrid cooling systems and i) Earth to air heat exchangers (buried pipes). ii) Evaporative cooling components. iii) Ventilation techniques. i) Earth to air heat exchangers Air is sucked from the environment using an electric fan and cooled by circulating it underground. The coldness of the earth is transferred to the air and it is injected into the building. It was verified experimentally that a 2-5 C reduction of peak indoor temperature can be obtained as the depth of earth to air heat exchanger ranges between 1.5 to 6.5m respectively. Many buildings have been designed and monitored and the performances of these buildings has been proved.

9 9 ii) Evaporative cooling A direct evaporating cooling device uses the evaporative principle to cool the air entering the building. Droplets of water when evaporated absorb the heat of evaporation from the air, and hence the air gets cooled. This method works well in the interior and desert regions due to their lesser humidity, but is less effective in the coastal regions. An indirect evaporative cooling system provides efficient cooling of buildings without increasing the moisture content of the indoor air. Plate type indirect evaporative cooling systems are more efficient compared to other types of cooling systems. iii) Ventilation techniques The outdoor air is pumped into the building when the ambient temperature is less for cooling purposes and also to meet the fresh air requirement based on the occupancy. Ventilation techniques contribute to a more comfortable and healthy indoor environment. The types of ventilation techniques for a low energy building are: a. Natural ventilation : There will be natural air flow around these buildings. Here the ventilation is controlled by the occupants by opening the windows. b. Advanced natural ventilation: Here, the flow and the direction of the ventilating air are controlled by natural forces other than windows, such as thermal chimneys and wind towers. Solar chimneys are the natural draft components using the solar energy to build up stack pressure, and thus driving the air flow through the chimney. In a wind tower, air enters the towers of the wind ward facade and leaves at the lower part to the inside of the building. Air may be cooled by the convective or the evaporative principle through the tower.

10 10 c. Mechanical Ventilation : These buildings usually have a central fan or local fans that provide ventilation air. d. Mixed Mode of Ventilation : These buildings have one of the above type ventilation systems with an additional mechanical cooling system. The mechanical cooling system will not be categorized as a passive cooling technique. Ventilation techniques requires a better understanding of the air flow phenomena and expected comfort benefits, particularly in the urban environment. iv) Night cooling The daily variation in air temperature is controlled by the incoming energy (primarily from the sun) and outgoing energy from the earth's surface due to sky radiation during the night. The air temperature rises, in a place where the incoming solar energy exceeds outgoing energy (orange shade), The air temperature falls, in a place where the outgoing energy due to sky radiation exceeds incoming solar energy (blue shade) as shown in Figure Figure 1.4 Atmospheric air temperature energy balance

11 11 A building with a high thermal mass with proper utilization is sufficient for year round thermal management. There are various methods adopted for the passive cooling of buildings. Night ventilation is one such method by which the structural components are cooled, thus providing reduced temperature of indoor air conditions for the following day. In places where the daily variations of the ambient temperature are high, night ventilation is highly suitable. In free cooling, apart from a sensible storage system, the latent heat thermal energy storage system (LHTES) is also used as a storage medium which stores the coldness of the ambient air during early morning and supplies it with a time delay during the day. Phase change materials become the natural storage options because of the small temperature difference between the day indoors and night outdoors. i) Night ventilation Night ventilation is a passive cooling method in which the structural components are cooled using the cold night air, thus providing a reduced temperature of indoor air conditions for the following day. The suitable sites for night cooling/night ventilation are the places where the diurnal temperature variation is high and the night temperature is very low. This ventilation system uses a fan to enable accelerated night cooling using ambient air for ensuring sufficient night cooling. However, in the urban location due to the increase in air temperature and decrease in wind velocity, the efficiency of the night ventilation is decreased. ii) Free Cooling Free cooling is a method of storing the outdoor coldness during early morning time, and supplying it to the indoor air during the day. Phase change storage is the best among storage options, because of the small temperature difference between the day indoors and night outdoors. Free-

12 12 cooling is the method of storing the outdoor coolness during the night, and supplying it to the indoor air during the day. This system is best suited for places where the diurnal temperature variation (difference between maximum and minimum temperature of the place), is at least 15 C. The free cooling concept is site specific and climate dependent. Free cooling is suitable for the less humid interior and desert regions. The benefit is less in the coastal area because the temperature moderation is done by the sea and land breeze. 1.3 OTHER METHODS OF REDUCING ENERGY CONSUMPTION IN BUILDINGS An entirely different type of approach called adapting lifestyle can be utilized to reduce the energy consumption; this involves adapting living, sleeping, cooking and activity patterns to work with the climate rather than using mechanical cooling to emulate an alternative climate. This method is applicable in all climates, especially hot humid and hot arid. Hot humid climate conditions pose the greatest challenge in achieving thermal comfort because high humidity levels reduce evaporation rates. Acclimatizing is living comfortably by adopting appropriate living patterns to maximize the outdoor lifestyle opportunities during extreme climates. The majority of people living in tropical climates choose to do so. Sleeping comfort at night during the hottest and most humid periods is a significant thermal comfort issue for many people living in tropical climates. Unlike cooler climates, sleeping comfort is a high priority when choosing, designing or building a home. Different members of a house will have different thermal comfort thresholds. Children often adapt to seasonal changes more easily than adults do. Understanding the sleeping comfort requirements of each member of the

13 13 household can lead to better design, positioning or allocation of bedrooms, resulting in increased thermal comfort for all and less dependence on mechanical cooling. Live outside when the time of day and seasonal conditions are suitable, particularly in the evenings. Radiation by the body to cool night skies is an effective cooling mechanism, particularly in the early evening when the daytime heat load is not allowed to escape from the interior of the house. 1.4 COOL ENERGY STORAGE SYSTEM DESIGN AND MODELING Energy Storage System There are many types of energy storage systems and they are broadly classified as below. i) Mechanical Energy Storage Hydro storage (pumped storage) Compressed air storage Flywheels ii) Chemical Energy Storage Electrochemical batteries Lead acid batteries Lithium iron sulfide batteries Sodium sulfur batteries Organic molecular storage Chemical heat pump storage

14 14 iii) Biological Storage iii) iv) Magnetic Storage Thermal Energy Storage. Among these storage methods, Thermal Energy Storage (TES) is one of the key technologies for energy conservation, and therefore, is of great practical importance. TES systems can contribute significantly to meet the society s needs for more efficient, environmentally benign energy use in building heating and cooling, aerospace power, and utility applications. TES is perhaps as old as civilization itself. Since recorded time people have harvested ice and stored it for later use. Large TES systems have been employed in recent years for numerous applications, ranging from solar hot water storage to building air conditioning systems. The TES technology has only recently been developed to a point where it can have a significant impact on modern technology. Thermal Energy Storage systems have an enormous potential to increase the effectiveness of energy conversion equipment use and for facilitating large scale fuel substitutions in the world s economy Cool Thermal Storage Systems in Buildings- Necessity and Advantages The major technical constraint that prevents the successful implementation of energy storage systems in buildings, is matching the cool energy availability (during early mornings) and the time for the demand of cool energy (around afternoons). In order to overcome the above constraint, building systems should be integrated with energy storage units. There are two types of thermal energy storage system.

15 15 Sensible Heat Storage System: In the sensible heat storage system, thermal energy is stored by raising or lowering the temperature of the solid or liquid. The sensible heat storage system utilizes the heat capacity and change in temperature of the material during the process of charging and discharging. The sensible heat storage system uses rocks or water as the storage medium. Latent Heat Storage System: Latent heat storage is based on heat absorption or release, when the storage material undergoes a phase change from solid to liquid, or liquid to gas, or vice-versa. Latent heat storage systems store energy in phase change materials. A latent heat thermal energy storage system stores the coolness of the ambient air during the night and supplies it with a time delay during the day Importance of Numerical Modeling A good understanding of the heat transfer process involved in an LHTES unit is essential for accurately predicting the thermal performance of the system and in avoiding costly system overdesign. Solidification and melting are important phenomena in thermal storage applications. Accurate prediction of the temperature field through internal probes by experiment is a difficult task in complicated designs with a small temperature range of operation. The complexity in the phase change problems arises due to the moving boundary of the solid liquid interface, the location of which is an unknown function of time. Further, practical situations involve complex boundary conditions and are multidimensional in nature. The recent advancement in high capacity computers has attracted the scientist to use numerical modeling for such complicated problems like phase change, turbulence and two phase flow phenomena. The numerical approach of solving phase change problems is categorized as Temperature based models or variable domain model and

16 16 Enthalpy based models or fixed domain models. In the temperature based models, the volume of each region changes with respect to time and the temperature is the sole dependent variable. The energy conservation equations are written separately for each region and the solutions of these equations are coupled through the energy balance at the interface. The major disadvantage in the temperature based model is the continuous tracking of the interface by solving simultaneously all the three energy equations. The enthalpy method which was introduced in the 1940s is widely employed in modeling phase change problems. The advantage is that it can accommodate materials that change their phase over a temperature range. In addition, the phase change problem can be reduced to a single equation in terms of enthalpy. There is no boundary condition to be satisfied at the interface and the total domain volume does not change with respect to time. An alternative formulation called the apparent heat capacity method is employed to solve the melting and solidification problem by including the effect of the latent heat value through apparent heat capacity values by approximating the values of the DSC analysis by rectangular or triangular shapes within the phase change temperature region. 1.5 CONCEPT OF THE FREE COOLING SYSTEM The concept of the free cooling system proposed in the present study is shown in Figures 1.5 and 1.6. The set up consists of a PCM regenerative heat exchanger with a series of bulk cylindrical disc modules containing the PCM on the shell side and passages for the flow of air through the tubes. These modules are stacked one over the other with air spacers between each module. The cool air available in the early morning is made to pass through the PCM regenerative heat exchanger and this cool energy is stored by freezing the PCM. The stored cool energy is retrieved during the day time for space cooling.

17 17 Figure 1.5 Concept of free cooling with regenerative heat exchanger Night time operation Figure 1.6 Concept of free cooling with regenerative heat exchanger - Day time operation

18 18 During the night time when dampers 2 and 3 are in the closed position and 1 and 4 are in the open position as shown in Figure 1.5, the cool energy available in the atmospheric air is made to pass through the modular heat exchangers using a fan or blower. As the cold air is passing through the PCM regenerative heat exchanger, the PCM in the modules will freeze and store its cool energy. A fan is used for air circulation during the night time and the dampers are adjusted to control the flow rate of air. During the day time when dampers 2 and 3 shown in Figure.1.6 are in the open position and 1 and 4 are in the closed position, the hot air from the room goes to the PCM module by natural circulation or a small capacity fan and the PCM releases the stored cool energy to the room. During the day time, circulation is done by using a small capacity fan or by natural convection so that the discharge of cool energy is as low as possible to cater to the cooling need throughout the day. The free cooling concept is best suited for places where the diurnal temperature variation is at least 15º C. 1.6 OBJECTIVES OF THE PRESENT WORK The present work aims to evaluate the feasibility of the free cooling system and to design and construct an experimental set up to study the performance in regions with low diurnal temperature variation, using the phase change material-based storage system. The major objectives of the present work are i) The selection of a suitable PCM for free cooling based on a material survey. ii) The numerical investigation for the solidification characteristics of an HTF in tube and the PCM in shell arrangement for designing a single module of a modular heat exchanger. The only difference between the melting and solidification is the additional natural

19 19 convection effect during the melting process. However since the temperature difference involved in the present heat exchange process is very less that the melting characteristics is not separately studied. iii) iv) Flow and heat transfer studies on a modular type heat exchanger for analyzing the pressure drop and storage characteristics and to conduct parametric studies to finalize the operating parameters. To design a PCM based modular heat exchanger suitable for free cooling based on numerical and CFD analysis. v) The construction of an experimental set up to study the performance characteristics of such a system.