Light Emitting Diodes (LED)

Light Emitting Diodes (LED)

Introduction

Light-emitting diode (LED) is a two-lead semiconductor light source. It is a p–n junction diode that emits light when activated. When a suitable current is applied to the leads, electrons are able to recombine with electron holes within the device, releasing energy in the form of photons.   This effect is called electroluminescence, and the color of the light (corresponding to the energy of the photon) is determined by the energy band gap of the semiconductor. LEDs are typically small (less than 1 mm²) and integrated optical components may be used to shape the radiation pattern. Unlike a laser, the color of light emitted from an LED is neither coherent nor monochromatic, but the spectrum is narrow with respect to human vision, and for most purposes the light from a simple diode element can be regarded as functionally monochromatic

  LEDs are like tiny light bulbs. However, LEDs require a lot less power to light up by comparison. They’re also more energy efficient, so they don’t tend to get hot like conventional light bulbs do   this makes them ideal for mobile devices and other low-power applications The LED is a light source which uses semiconductors and electroluminescence to create light.  LED   uses a small semiconductor crystal with reflectors and other parts to make the light brighter and focused into a single point. Basically, LEDs are just tiny light bulbs that fit easily into an electrical circuit. But unlike ordinary incandescent bulbs, they don't have a filament that will burn out, and they don't get especially hot. They are illuminated solely by the movement of electrons in a semiconductor material, and they last just as long as a standard transistor. The lifespan of an LED surpasses the short life of an incandescent bulb by thousands of hours. Tiny LEDs are already replacing the tubes that light up LCD HDTVs to make dramatically thinner televisions.

History

LEDs appeared as practical electronic components in 1962, the earliest LEDs emitted low-intensity infrared light. Infrared LEDs are still frequently used as transmitting elements in remote-control circuits, such as those in remote controls for a wide variety of consumer electronics. The first visible-light LEDs were of low intensity and limited to red. Modern LEDs are available across the visible, ultraviolet, and infrared wavelengths, with very high brightness.

Working

The LED consists of a chip of semiconducting material doped with impurities to create a p-n junction. As in other diodes, current flows easily from the p-side, or anode, to the n-side, or cathode, but not in the reverse direction. Charge-carriers—electrons and holes—flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level and releases energy in the form of a photon.

The wavelength of the light emitted, and thus its color, depends on the band gap energy of the materials forming the p-n junction. In silicon or germanium diodes, the electrons and holes usually recombine by a non-radiative transition, which produces no optical emission, because these are indirect band gap materials. The materials used for the LED have a direct band gap with energies corresponding to near-infrared, visible, or near-ultraviolet light.

LED development began with infrared and red devices made with gallium arsenide. Advances in materials science have enabled making devices with ever-shorter wavelengths, emitting light in a variety of colors.

LEDs are usually built on an n-type substrate, with an electrode attached to the p-type layer deposited on its surface. P-type substrates, while less common, occur as well. Many commercial LEDs, especially GaN/InGaN, also use sapphire substrate.

If you connect an LED directly to a current source it will try to dissipate as much power as it’s allowed to draw, and, like the tragic heroes of olde, it will destroy itself. That’s why it’s important to limit the amount of current flowing across the LED.. Resistors limit the flow of electrons in the circuit and protect the LED from trying to draw too much current. 

LEDs create light by electroluminescence in a semiconductor material. Electroluminescence is the phenomenon of a material emitting light when electric current or an electric field is passed through it - this happens when electrons are sent through the material and fill electron holes. An electron hole exists where an atom lacks electrons (negatively charged) and therefore has a positive charge. Semiconductor materials like germanium or silicon can be "doped" to create and control the number of electron holes. Doping is the adding of other elements to the semiconductor material to change its properties. By doping a semiconductor you can make two separate types of semiconductors in the same crystal. The boundary between the two types is called a p-n junction. The junction only allows current to pass through it one way, this is why they are used as diodes. LEDs are made using p-n junctions. As electrons pass through one crystal to the other they fill electron holes. They emit photons (light).

Applications

Early LEDs were often used as indicator lamps for electronic devices, replacing small incandescent bulbs. They were soon packaged into numeric readouts in the form of seven-segment displays and were commonly seen in digital clocks. Recent developments have produced LEDs suitable for environmental and task lighting. LEDs have led to new displays and sensors, while their high switching rates are useful in advanced communications technology. . Light-emitting diodes are used in applications as diverse as aviation lighting, automotive headlamps, advertising, general lighting, traffic signals, camera flashes, lighted wallpaper and medical devices. They form numbers on digital clocks, transmit information from remote controls, light up watches and tell you when your appliances are turned on. Collected together, they can form images on a jumbo television screen or illuminate a traffic light.Oter uses incuude :Indicator lights ; LCD panel backlighting Specialized white LEDs are used in flat-panel computer displays ;Fiber optic data transmission Ease of modulation allows wide communications bandwidth with minimal noise, resulting in high speed and accuracy ; Remote control Most home-entertainment "remotes" use IREDs to transmit data to the main unit; Optoisolator Stages in an electronic system can be connected together without unwanted interaction.

,LEDs serve as the foundation of networked lighting systems that also provide information to other systems connected to an internal building management network. For example, the heating, ventilation, and air conditioning systems could be alerted that certain parts of the building are empty, and could adjust the temperature or shut off the air conditioning entirely.

Energy savings

LEDs use about 85% less electricity than incandescent bulbs and as much as 50% less than fluorescents. The amount saved with fluorescents will vary depending on whether you’re using a fluorescent tube or a compact fluorescent bulb (CFL). The efficiency of a light bulb is measured in lumens per watt and currently LED technology is providing higher lumens per watt than fluorescent, but is not practical for every application a fluorescent source is being used.

One of the main problems with incandescent bulbs is they emit a great deal of energy as heat (that’s why you can burn yourself if you touch a lit incandescent bulb), and this heat signifies that some energy is being wasted instead of converted to light. On the other hand, if you touch an LED light it is typically cool to the touch and will likely not notice any heat at all. This is because more electricity is being converted to light — about 85% more.

A few of the major advantages of LEDs over CFLs are they don’t produce any UV rays, last 5 times longer, save 20% in energy costs and are lead and mercury free.

By design LEDs can last as much as 50 times longer than other bulbs and have lifetimes ranging from 30,000 to 100,000 hours or more at constant operation. Depending on how many hours a day your facility is lit, this can equal a lifespan of anywhere from 6 to 30 years. In comparison, incandescent bulbs only last an average of 1000 to 5000 hours, CFLs last 8,000 to 10,000 hours, and fluorescent tubes have lifetimes of 20,000 to 50,000 hours.

Part of the reason LEDs last so long is because they are durable (no glass components) and do not have a filament (like incandescent bulbs) that can break our burn out. Their illumination comes exclusively from the movement of electrons in a semiconductor material.

Lighting accounts for about 20 percent of the total energy usage worldwide, approximately 1,944 terawatt hours. Because lighting is relatively simple to upgrade, the trend in recent years is to switch to LEDs. Doing so greatly improves energy efficiency; mitigating global warming’s impact and reducing energy dependence on other countries.

Globally, LEDs make up less than 10 percent of lighting systems, according to the U.S. Department of Energy Solid-State Lighting R&D Plan, published in June 2016. But the DOE forecasts LEDs to become the predominant source of illumination—for indoor and outdoor spaces—over the next decade. The DOE’s “Energy Savings Forecast of Solid-State Lighting in General Illumination Applications,” released in September, predicts that by 2035, LED lamps and luminaires will be used in 86 percent of all lighting products in the United States, compared with 6 percent in 2015. That translates into an annual savings of 1,495 terawatt hours over traditional lighting systems, nearly the total annual energy consumed by 45 million U.S. homes today. This adds up to nearly $52 billion in energy costs savings.

Because LEDs are semiconductor devices, integrating additional electronics in the bulbs, such as occupancy and daylight sensors connected to a network interface, provides the bulbs with new functions. This design enables, for example, the automatic dimming of lights when there’s sufficient natural light in the room, and it senses the presence of people in a room and adjusts overhead lighting accordingly. These capabilities not only provide the optimal amount of lighting throughout a building but also save energy and money.

LEDs also help lower electricity costs in exterior lighting systems. Those used in parking lots, garages, and walkways can be configured to automatically dim or turn lights off when sensors detect an unoccupied area, and much faster than today’s systems. This type of automation is an especially attractive cost-saving option for cities. About half of a city’s monthly electricity budget is devoted to keeping the streetlights on.

Other Benefits  

LEDs also are changing the way architects and interior designers use lighting. Because the bulbs are smaller and weigh less than traditional ones, LED products allow for greater flexibility and creativity in lighting venues such as concert halls, museums, and retail stores, where aesthetics are especially important, while reducing energy usage.

LEDs in malls can motivate people to linger in a store longer and therefore boost sales. The ability to adjust from warmer to cooler shades of white, referred to as color tuning—along with precise light distribution—allows the retailer to create an inviting space that could appeal to the shopper’s sense of comfort, safety, and familiarity. In the home, LED lights could automatically mimic a bright, sunny day even if it is gloomy out. Such lighting systems will allow us to adjust our environments in ways that older technologies never could.

High Upfront Cost

Although LED prices have dropped dramatically, and continue to do so as technology advances, they typically cost more than other lighting systems. However, their high level of efficiency means you can recoup your upfront costs in a relatively short period of time. Besides requiring less electricity, LED’s long lifespan also saves you money on maintenance and replacement bulbs,  .

 Advantages:

The U.S. Department of Energy sees LEDs as the lighting source with the greatest potential for the future. They predict that, with widespread adoption of LEDs, by 2025 the country will:

·         Lower electricity demands for lighting by 62%.

·         Reduce carbon emissions by 258 million metric tons.

·         Diminish amount of materials in landfills.

·         Prevent construction of 133 new power plants.

·         Save $280 billion.

Although LEDs aren’t practical for every application (yet), they are certainly the lighting choice of the future and offer huge benefits, including energy savings, for your facility and the country as a whole.

LEDs have many advantages over incandescent light sources, including lower energy consumption, longer lifetime, improved physical robustness, smaller size, and faster switching. They are also significantly more energy efficient and, arguably, have fewer environmental concerns linked to their disposal.

Benefits of LEDs and IREDs, compared with incandescent and fluorescent illuminating devices, include: Low power requirement,  Most types can be operated with battery power supplies; High efficiency Most of the power supplied to an LED or IRED is converted into radiation in the desired form, with minimal heat production; Long life When properly installed, an LED or IRED can function for decades.

The evolution of LEDs as a source of white light has progressed dramatically in the last two decades, especially in terms of efficacy, or how well a light source produces visible light, measured in output lumens/input watts. The increase in efficacy has been achieved primarily through advances in LED chip technology, and is helping to drive LED adoption. LED products are also more affordable than incandescent bulbs over their life cycle, because their power requirements are lower and the bulbs themselves can last more than 10 years, compared with today’s incandescent bulbs they are replacing, which often last up to a year or so. Advntgaes of LEDs include:

-Energy efficient source of light for short distances and small areas. The typical LED requires only 30-60 milliwatts to operate
-Durable and shockproof unlike glass bulb lamp types
-Directional nature is useful for some applications like reducing stray light pollution on streetlights

Disadvantages:

-May be unreliable in outside applications with great variations in summer/winter temperatures, more work is being done now to solve this problem
-Semiconductors are sensitive to being damaged by heat, so large heat sinks must be employed to keep powerful arrays cool, sometimes a fan is required. This adds to cost and a fan greatly reduces the energy efficient advantage of LEDs, it is also prone to failure which leads to unit failure
-Circuit board solder and thin copper connections crack when flexed and cause sections of arrays to go out
-Rare earth metals used in LEDs are subject to price control monopolies by certain nations
-Reduced lumen output over time

Efficiency

Typical indicator LEDs are designed to operate with no more than 30–60 milliwatts (mW) of electrical power. Around 1999, Philips Lumileds introduced power LEDs capable of continuous use at one watt. These LEDs used much larger semiconductor die sizes to handle the large power inputs. Also, the semiconductor dies were mounted onto metal slugs to allow for greater heat dissipation from the LED die.

One of the key advantages of LED-based lighting sources is high luminous efficacy. White LEDs quickly matched and overtook the efficacy of standard incandescent lighting systems. In 2002, Lumileds made five-watt LEDs available with luminous efficacy of 18–22 lumens per watt (lm/W). For comparison, a conventional incandescent light bulb of 60–100 watts emits around 15 lm/W, and standard fluorescent lights emit up to 100 lm/W.

As of 2012, Philips had achieved the following efficacies for each color. The efficiency values show the physics – light power out per electrical power in. The lumen-per-watt efficacy value includes characteristics of the human eye and is derived using the luminosity function.

	Color	Wavelength range (nm)	Typical efficiency coefficient	Typical efficacy (lm/W)
	Red	620 < λ < 645	0.39	72
	Red-orange	610 < λ < 620	0.29	98
	Green	520 < λ < 550	0.15	93
	Cyan	490 < λ < 520	0.26	75
	Blue	460 < λ < 490	0.35	37

Red and Infrared LEDs are made with gallium arsenide
Bright Blue is made with GaN -gallium nitride
White LEDs are made with yttrium aluminum garnet

There are also orange, green, blue, violet, purple, ultraviolet LED

In September 2003, a new type of blue LED was demonstrated by Cree. This produced a commercially packaged white light giving 65 lm/W at 20 mA, becoming the brightest white LED commercially available at the time, and more than four times as efficient as standard incandescent. In 2006, they demonstrated a prototype with a record white LED luminous efficacy of 131 lm/W at 20 mA. Nichia Corporation has developed a white LED with luminous efficacy of 150 lm/W at a forward current of 20 mA. Cree's XLamp XM-L LEDs, commercially available in 2011, produce 100 lm/W at their full power of 10W, and up to 160 lm/W at around 2 W input power. In 2012, Cree announced a white LED giving 254 lm/W, and 303 lm/W in March 2014. Practical general lighting needs high-power LEDs, of one watt or more. Typical operating currents for such devices begin at 350 mA.

These efficiencies are for the light-emitting diode only, held at low temperature in a lab. Since LEDs installed in real fixtures operate at higher temperature and with driver losses, real-world efficiencies are much lower. United States Department of Energy (DOE) testing of commercial LED lamps designed to replace incandescent lamps or CFLs showed that average efficacy was still about 46 lm/W in 2009 (tested performance ranged from 17 lm/W to 79 lm/W). 

Energy Efficient Homes and Buildings

Energy Efficient Homes and Buildings

Introduction

EU households, heating and hot water alone account for 79% of total final energy use (192.5 Mtoe  Heating and cooling account for about 48% of the energy use in a typical U.S. home, In the East it is the cooling load that dominates the households energy utilization . 

Heating and cooling of buildings do not get the same attention as climate change, but these should be paid attention as, both are carried out in a very inefficient manner. Buildings are a long way away from efficient thermal insulation and are not leak proof. .Much of heating and cooling is lost through leaky floors, poorly sealed windows, improperly insulated roofs and walls.

The growing use of air conditioners in homes and offices around the world will be one of the top drivers of global electricity demand over the next three decades, according to new analysis by the International Energy Agency that stresses the urgent need for policy action to improve cooling efficiency. Using air conditioners and electric fans to stay cool already accounts for about a fifth of the total electricity used in buildings around the world – or 10% of all global electricity consumption today. But as incomes and living standards improve in many developing countries, the growth in AC demand in hotter regions is set to soar. AC use is expected to be the second-largest source of global electricity demand growth after the industrial sector, and the strongest driver for buildings by 2050.

Regulation

 The Developed countries have issued regulations for cooling and heating of buildings and homes, this is both an economic waste and cause of emissions. The regulations, however, are applicable to new construction only;, there is need to require renovation of excising structures as well. In the developing world, there is frequently no such regulation, and only very few people pay attention to efficient cooling and heating. It is only high energy prices that induce some people to go for thermal insulation and sealing of windows and doors.

Orientation

Properly insulated and sealed houses and buildings seldom need central heating Walls are the main source of heat loss or cooling loss in homes and buildings. Thermal insulating sheets installed between double walls assist in reducing heat and coiling losses, this, however, is only possible at time of construction. The roof similarly can be insulated by placing a layer of thermal insulating sheets at the time of construction. Insulating sheets can also be placed after construction but this will involve the cost of removing existing materials on the roof.

Windows are also the means of significant heat losses or cooling losses. For starters the placement of windows should be such that the sun should provide the heat and lighting needs for at least a part of the time, this requires pooper placement and directions of windows. The first consideration for energy efficient homes should always be the location and orientation of the building. You should attempt to maximize the use of passive solar gain while reducing heat gain during summer months. Simple directional and design related tweaks can make a big difference to enjoy the summer sun without overheating the house. Good design control of passive solar gain helps reduce heating loads during winter and cooling loads during the summer.

Windows

 The maximum losses occur due the losses from the frame and the fittings. Double or triple glazing of windows help, in severe climate placing inert gasses between the panes also assist in arresting heat losses. Adopting low U-value frames and Low-E (low emissivity) glazing appropriately for the climate and direction is another critical design consideration. For example, higher altitude locations benefit from good UV rays all year round except cold winters. Proposed glazing for any site should take account of occupants' comfort level on completion.

Insulation as a solution

Reducing the heat loss from building elements such as walls and floors is imperative for designing an energy efficient home. A good design of these composite components minimizes the u-Value and R-Value, which provides a passive and long lasting benefit to the buildings' lifetime costs. There are many energy efficient systems and materials available such as ICFs (Insulated Concrete Forms), thicker wall constructions and roof insulations. Additionally, blown-in foam is also a well recognized, viable solution. It's easier to design a well insulated, efficiently lit, correctly orientated and efficiently heated building than attempt to retroactively improve it. Taking good consideration of the local climate and geography, as well as supplementing accordingly with passive shading strategies will pay dividends in the long run for your dream home. Energy efficient design should always be a key consideration for any design team for any building.

Air tightness

Air tightness assist in achieving energy efficiency Windows and doors frequently have gaps that allow heat and cooling losses, simply closing these gaps results in a significant improvement in arresting heat losses.

Ventilation

Air tight and sealed home, the recommended solution is to use heat exchangers as simple fans would defeat the purpose of sealing, will, however, need ventilation, as sealing will starve the home of fresh air. Heat exchangers use heat transfer from the leaving air to the entering air so that either heat of cooling is transferred from the outgoing to incoming air. Heat recovery should be another integral part of the building design for all energy efficient homes.   there are excessive amounts of technologies for ventilation systems. Although the fact that they can now further tweak your home's energy efficiency is a relatively new technology for the domestic market. Technologies such as Flue Gas Heat Recovery (FGHRS) or Waste Water Heat Recovery systems can provide additional cost savings.

Air tightness or leakage has an enormous impact on the energy efficiency of any building. Energy efficient homes should have proper sealing of joints, sills, ducts, doors and vents. This will significantly reduce heating costs for the final building. "Build tight, ventilate right" is a good adage to follow. Clearly, some areas need mechanical ventilation e.g. wet rooms, kitchens, etc.Ventilation systems have become very sophisticated and often include heat recovery technology.

Trees

Engineering the landscape around the building can also play a major role in energy efficiency. Planting deciduous trees on the west and south sides (depending on your geographical location of course) can help provide shade for the building during summer months. On the other hand, in autumn the trees lose their canopies and allow winter sun to heat your home passively.

Heating design

Heating costs tend to comprise at least 50 percent of a home's energy bills. Choosing the most efficient heating system is an essential design consideration and will affect the lifetime running costs of the building. Another consideration should be the addition of controls such as thermostats, weather compensators, etc. to provide an autonomous control  of the heating plant. More sophisticated control systems, Building Management Systems, can actively manage the building heating schedules.

 Lighting Issues

Lighting design is another key factor for improving your home's energy efficiency. Although fluorescent lighting is great, LED's are taking over . The technology has come a long way over the last decade and will save you hundreds of dollars (or any currency) over 10-20 years before needing replacement. LED's are one of the quickest returns on investment and can fit most of the existing light fittings.

Quantify losses

Traditionally, hot water used to be either generated or stored in a cylinder or a tank. Hot water accounts for around 15-20 percent of most domestic energy bills. When designing your domestic hot water system you should seriously consider tank-less water technology systems such as combination boilers. Clearly, the size of the house, number of occupants and hot water capacity requirements of the final home will ultimately dictate the design. A series of combination boilers instead of a large storage tank should provide the volume and supply rate needed for most situations. Additional venting and installation costs would be incurred but, this method of providing instant hot water eliminates storage or standing heat losses in the long run. Solar heating is at apr with gas heating of water , it is suggested that use of solar water heaters be mandated .

 Utilize by the sun

 Being able to generate your own electricity or supplement heating/hot water generation systems using PV or solar thermal systems is a good design consideration. In most cases, you can sell your excess electricity to the grid. However, future technology systems are also expected to enable you to store your self-generated power.

It’s not easy to calculate the return of investment as the market shifts, but it’s a fact that energy costs increase. Correspondingly, so does the cost of solar installations and your cost-savings. The applicability of this technology is of course latitude-dependent and the system size, design and orientation may not be flexible. Solar water heating should be mandated as this is at par with gas water heating. There is no economic sense in not using solar for eater heating

Efficient building Materials

Every homeowner should take advantage of the new eco-friendly technological advances in home building, because they're affordable, more efficient and greener these include: Scrap . steel It could take as many as 40 - 50 trees to build an average house. If recycled steel is used it will take just 6 scrap cars to serve the same purpose. Steel beams can be used as a replacement for wooden ones and can be ordered to fit a specific design. Steel is a very durable material and particularly useful in areas where there are earthquakes and high winds; Concrete is poured between two insulating layers and left in place. It can be used for free-standing walls and building blocks; Plant-based Polyurethane Foam. Plant-based polyurethane foam is usually made from natural materials such as bamboo, hemp and kelp. Used as insulation it offers high resistance to moisture and heat and protects against mold and pests. It insulates better than fiber glass or polystyrene. It's not really a surprise that nature once again has provided us with a better solution to our insulation problems than artificial science; Straw Bales, this material actually is. It's been used for centuries for various purposes (beds, roofing) but nowadays it can help us with its excellent insulation properties. If kept dry they can last for hundreds of years and they bond well to plaster and external render; Cool Roof, Cool roofing technology   will improve the heat dissipation and will lower temperatures in your home during summer A lot. It's also safe for the environment because it lowers heat in the atmosphere.  , it’s their reflectiveness which gives them the name. They reflect the sunlight and thus reducing the heat in your home; Structural Insulated Panels, Manufactured from a layer of foam insulation which is sandwiched between plywood or cement panels. It is fire resistant and suitable for floors, basements, foundations as well as load bearing walls. You can choose from a variety of materials but the principle remains the same. This material will help you reduce your energy bill greatly. You can consult a handyman services company if you want to know more about this; Plastic Composite Lumber, Often manufactured from waste plastic and wood fiber it is more durable and less toxic than conventionally treated wood. It is resistant to mold and rot and more rigid in the cold and pliable in the heat than purely plastic building materials. Also the one in the picture is the anti-slip variety which is suitable for bathrooms and outside decks; Low-E Windows, Low-E windows or also known as "high performance" windows are another great substitute for normal glass which will help you reduce heat during summer and block infrared radiation. They have a clear coating of metal oxide. It also helps keep the heat in during the winter. They can reduce heat flow by up to 50%; Vacuum Insulation Panels, Vacuum insulation panels or VIP (even the name sounds important) are a quick glimpse in the future of home building. Currently only used for commercial refrigeration units they could become available for general home building in the future. They comprise of a textured silver rectangle that encloses a core panel in an airtight envelope. All of this means heat loss will be reduced to a minimum and we'll have much greener homes; Earth, Earth walls.   Have many advantages over other building materials. Mainly, earth is practically everywhere around us, meaning it's pretty cheap. Walls made from earth provide an excellent thermal mass and it is up there with other renewable sources of building materials. 

Cooling Efficiencies in Pakistan

The energy, consumed in a building, can be reduced by adopting simple methods through the use of suitable building design and energy efficient strategies, such as, passive cooling strategies. Passive cooling eliminates the use of mechanical equipment and provides cooling through the use of passive processes. To improve the efficiency of the building envelope, passive cooling strategy reduces heat gains from the external sources and helps heat loss to the natural sources of cooling, such as, cool air, earth coupling and evaporation. Passive cooling is based on the principle of preventing heat from getting into a building during a hot day and bringing in external cool air into the building when the external temperature falls. There are various parameters that affect the thermal behavior of buildings, such as the climatological ones, which are environmental variables and which are not subject to human control. The other type of parameters is the design variables, which are under control at the design stage. Inadequate attention to the aspect of a building’s thermal behavior at the initial stages of its design can lead to an unwelcoming internal environment. During summer, buildings located in hotter regions often face overheating conditions due to exposure to intense amount of solar radiation and high temperature. When these overheating conditions inside the buildings surpass the threshold of thermal comfort; cooling them becomes extremely significant. Energy consumption of buildings both in the developed countries as well as in the developing countries for cooling the building has increased tremendously over the past few decades  . The reason for this is the extensive use of mechanical air conditioning for cooling the buildings. One reason for heavy reliance on mechanical cooling is due to affordable cost and easy availability of electricity as well as cooling equipment. Now, the world realizes that eventually there would be running out of fossil fuel; the main source of energy in buildings at present thereby creating problems in fulfilling the energy demands. Extensive use of fossil fuel is also causing an irreparable damage to the environment. One of the solutions to address the above issues is to build energy efficient buildings using passive heating and cooling strategies. Passive cooling techniques not only offer energy and environmental benefits but they are also very economical. Materials and Methods Methods of passive cooling Passive cooling can utilize several heat sinks and a variety of climatic influences to create thermal comfort in warm regions, unlike the passive heating, which is driven by sun only. Traditionally, passive cooling has been in use in indigenous buildings   The first step towards achieving thermal comfort conditions is to take preventive measures against the radiation from the sun, by shading and reflective barriers and also by heat transfer through the envelope (by insulation and infiltration and infiltration sealing). Minimizing the need for mechanical cooling and extending the range of passive cooling is dependent on good control over thermal gains. The thermal gains can be due to multiple sources such as infiltration of outside warm air, heat conduction through building structure, solar heat gains through windows, heat gains from the occupants and equipment inside the building   Evidently, the more these gains are, the higher will be the cooling load to achieve a desired thermal condition in the building. It is, therefore, important to control these gains in summer months in order to reduce the energy requirements for cooling. Besides the preventive measures to attain passive cooling other methods include the evacuation of heat from the building to the heat sinks. The natural heat sinks of the planet are the atmosphere, the sky and the earth. The main techniques of natural cooling according to the mode of heat transfer and fluid flow can be classified   as follows: • Cooling with natural ventilation • Radiative cooling • Cooling by evaporation • Earth cooling Some of the techniques provide a direct instantaneous cooling effect, in others the coolness is collected during night time and is released the next day, thus smoothing the effect to the accumulated heat inside the building. Natural ventilation is the movement of outside air into a space without mechanical support. One of the oldest cooling methods in buildings is ventilation. Purpose made openings in buildings, such as, doors, windows and non-powered ventilators can be used to control natural ventilation, that provide a certain degree of ventilation besides infiltration. Other methods may include, wind towers, solar chimneys and atrium (Khan et al., 2008). In each case, the system is designed to take advantage of prevailing driving forces. An air movement is also important aspect of ventilation cooling since it offsets increase in temperature while maintaining comfort ventilation cooling. When heat transfers from a hotter surface to a colder surface or external space, it is known as radiative cooling and the basic principle for radiative cooling is that a hot body emits heat energy in the form of electromagnetic radiations to the cold body that faces it. Similarly, the envelope of the building absorbs heat during the day and becomes warm. During the night when the temperature drops, the building emits this heat to the atmosphere due to which the building gets cooled down. The radiative potential of a roof/horizontal surface is greater than a vertical surface. The radiative potential of a building is reduced during hot summer nights because the hot air adds heat to the building by convection. When the sensible heat in air exchanges with latent heat of water droplets on moistened surface, it is called evaporation. In evaporation, the state changes into vapour from liquid. This is accompanied by release of huge quantities of heat (sensible) from the air that comes in contact with the wet surface where evaporation takes place. When comparatively dry air is passed over a moistened surface then direct evaporative cooling occurs. For example, when a draft or wind blows through a fountain or over a pool of irrigated field, it is cooled by direct evaporation. These landscape features have aesthetic benefits in addition to a relatively automatic control of the process. Passive cooling strategies: The bioclimatic chart tells us that conditions are comfortable in the shade and in still air; if the plotted point lies within the comfort zone. If the points lie outside the comfort zone, we need to take corrective measures to get the conditions into the comfort area. A brief description of different passive strategies shown on the chart for bringing conditions into comfort zone is elaborated in the subsequent paragraphs. Natural ventilation is the movement of fresh air into a space without mechanical assistance. Deliberate openings in buildings, such as, doors, windows, etc., can be used to control natural ventilation. In natural ventilation the movement of outdoor air across the building is caused by pressure difference. Buoyancy effect or the wind can be used to create pressure difference, which is created by humidity difference or difference in temperature. Ventilation based on the buoyancy effect utilises stacks, which are tall spaces inside the buildings. The cooler outside air moves into the building from openings near the ground, whereas the hot air leaves the building through openings close to the top of the stack. In ventilation, in order to allow to airflow through buildings, we need to keep the building open during the day. Ventilation in buildings is needed not only to provide cooling in summer but also provides fresh air for occupants to dilute and exhaust pollutants. A good ventilation design not only caters the residents comfort by making the spaces inside the building ‘airy’ (not draughty) and ‘fresh’ (not stuffy) but also ensures good air quality that has low levels of pollutants. We need to have optimum ventilation, because excessive and unnecessary ventilation during the heating season incurs an energy penalty while too small ventilation can adversely affect the health and comfort of the residents. Ground cooling, evaporating cooling, radiative cooling and convective cooling utilises the heat dissipation techniques for cooling the buildings. Dissipation of the additional heat mainly relies on two conditions: presence of a suitable environmental heat sink; and creation of a suitable thermal coupling between the sink and the building, besides adequate temperature difference required for the heat transfer. For the above referred techniques the following heat sinks are used: • Sky is used as the heat sink in radiative cooling • Air and water are utilized as the heat sink in evaporative and convective cooling • Soil and the ground are used as the heat sink in ground cooling Materials, like, brick and concrete, have high thermal mass because of their capacity to store both cold and heat. Materials having high specific heat capacities and high densities are ideal for thermal mass.. Any matter that has mass whether solid, liquid or gas will have some thermal mass. Not only soil, earth or concrete has thermal mass but the air has a thermal mass as well, though it is substantially low. Various materials are used for thermal mass, but the most common ones are mud brick or adobe brick, mud, earth, natural rocks and stone concrete, clay bricks, water, etc. In these commonly used materials, the volumetric heat capacity of water is the highest. Normally, large containers are used for keeping water. The heat capacity of other materials, such as, earth, dirt and mud, depends on a number of factors, such as, its density, particle shape, moisture content, composition and temperature. The type of thermal mass in buildings varies from climate to climate. The prevalent climatic conditions in a region decide the right use and application of thermal mass. The internal temperature peaks inside a building can be reduced by the use of the correct thermal mass which, in turn, minimizes the requirements for mechanical ventilation. The use of thermal mass to decrease temperature peaks during daytime normally needs ventilation cooling during night time to decrease the mass temperature. High thermal mass together with ventilation at night, depends on the daily heat storage of thermal mass along with night time ventilation which lowers down the temperature of the mass. During the daytime the buildings need to be closed, whereas during night time they need to be opened to remove the heat. Advantages of passive cooling: Passive cooling decreases the need for conditioned cooling by minimizing or eliminating the periods in which cooling is required. The energy requirement for heating and cooling of buildings is around 6.7% of the total world energy consumption   Out of this, we may save around 2.35% of the world energy output, just by making appropriate environmental design. The cooling energy requirements are normally two to three times higher than the heating energy requirements on an annual basis in hot climates. Utilisation of the basic principles of heat transfer coupled with the local climate and exploitation of the physical properties of the construction materials could make possible the control of the comfort conditions in the interior of buildings. A proper building design may be used to achieve thermal comfort inside the buildings, even in regions, that have average maximum ambient temperature around 31.7 °C and helps to eliminate the use of air conditioning in buildings. For example, suitable orientation with respect to the sun and the use of adequate insulation material in the construction of dwellings will not only reduce the summer and winter discomforts but also decrease the noise complaints. Passive cooling helps in protecting the environment because air conditioning is associated with various environmental problems, such as, ozone depletion etc. Studies carried out for various cities in Queensland, Australia,  show that the passive cooling strategies are very suitable for these hot and humid subtropical climatic conditions   Results and Discussions Climate: The climate has clear effects on human thermal environment. A strong understanding of the environmental features that affects a building site is extremely important for designing an energy conscious building. While designing an energy efficient building, we must incorporate the useful factors that the environment and the climate have to offer and guard against those that are unfavorable to comfort. Passive cooling utilizes the processes fundamentally related to climate, air temperature, relevant humidity, velocity and direction of wind   Different climatic variables, such as, atmosphere, sky and the earth, act as a heat sink when we are rejecting heat from the building to the atmosphere. The applicability of passive cooling strategies could be limited by insufficient information to designers and building users on the potential of passive cooling. Hence, it is important to have knowledge about climate and different climatic variables while designing energy efficient buildings. The average daily temperature of Peshawar greatly varies in winter and summer, for example in January the average temperature is around 4°C whereas it rises up to 42°C in June. The majority of the rainfall happens in the months of July to August and March to April  while it is quite low in other months of the year. Rainfall in other months of the year is very low. The humidity level is not very high and hailstorms are common in the spring   Suitability of the weather of Peshawar for passive cooling strategies The traditional architecture of Pakistan was very much environment friendly. In the hot areas of Pakistan, the buildings were of massive construction. Fountains, pools and vegetation could still be seen around some of the old buildings. They would help in passive cooling apart from giving aesthetic beauty to the surrounding area of the building. But during the past few decades, due to fast and speedy advancement in science and technology, the use of active air conditioning systems in buildings has increased tremendously. As a result, the cooling load of buildings has increased by a high percentage. To check which weather conditions are appropriate for a certain design strategy, we can use bioclimatic charts for an early examination. When we mark areas on a psychometric chart to help matching the design solutions to climatic conditions, it is known as a Bioclimatic chart   Hence this method is quite helpful in anticipating early low design approaches on the basis of existing climatic conditions. This is achieved in two ways; first by using the sun, wind and night time cooling and secondly only when these are insufficient, by selecting appropriate mechanical equipment. To check which passive strategy is suitable for a specific area, G.Z. Brown in 1985 used bioclimatic chart. On the basis of temperature and relative humidity the bioclimatic chart offers four passive cooling strategies   The bioclimatic chart is normally used for residential and light commercial buildings that have low rates of inside heat gains from lighting, equipment and people, etc. Conclusions The climate of Pakistan has a lot of potential for passive cooling. There is a need for creating more awareness amongst the people for adopting passive cooling strategies. Bioclimatic chart can be used for preliminary investigation of the weather appropriateness of weather conditions for a building design strategy. Adopting passive cooling strategies for Pakistan would not only help to reduce the building cooling loads by a significant amount but also help to build a green Pakistan. Although the climate of Pakistan has a lot of potential for passive cooling but there is a need to remove this misconception from the minds of the people that passive cooling strategies cannot be adopted in modern building. More awareness programs need to be created amongst the people for adopting passive cooling strategies in building design.

Efficient Air conditioners  

In general, the more efficient the equipment is, the more costly it is compared to the regular ones. Here are some steps that you can take when choosing energy efficient air conditioners to purchase.

1. Cooling Capacity

Determine the cooling capacity that is required of the room  Buying an oversize air conditioner is not a good choice as it is more costly and does not necessarily provide better comfort level.

2. Inverter Vs Non-Inverter

Choose an inverter model as it will be definitely more efficient than a non-inverter unit. The inverter compressor's rotation can be varied according to the requirements of the load hence the power savings is there.

On the other hand, the non-inverter compressor is only able to turn ON or OFF. It is not able to vary its speed according to the load. The frequent turning ON and OFF will consume more energy. Choose also a DC inverter compressor as it is more efficient compared to the AC inverter.  

Infra-Red Sensor

Some manufacturers   have built-in infra-red sensor that is able to detect the presence or absence of the occupants in the room. If it does not detect any movement for a certain period of time, it will adjust the set temperature higher automatically to reduce the temperature in the room. This will help to save your electricity bill.