ACTIVE AND PASSIVE SOLAR COOLING SYSTEMS IN NIGERIA

ON APPLIED CLIMATOLOGY ARC 810

By

Olaoye, Toba Samuel

Matric. No: ARC/10/3123

Department of Architecture, Federal University of Technology, Akure

tobbyarchy2@yahoo.com

COURSE MENTOR

Professor Ogunsote O.O.

Abstract

Solar energy is the ultimate renewable that originate from the sun and it so amazing that energy can be tapped from sun and converted to solar energy which primary purpose is to cool the building. The architecture of a building includes the knowledge thermal comfort within and the surrounding of a building The technology for the utilization of solar energy is uncommon and yet to be fully tapped in Nigeria may be the reason is because the climate of Nigeria does not require winter heating and little auxiliary energy is needed to maintain thermal comfort. Nature’s energies can be utilised in two ways – passive and active and consequently solar architecture is classified as passive solar and active solar architecture. The issue of solar cooling systems both active and passive systems have not thrived as expected in Nigeria. Active systems are mechanical driven cooling systems. These can range from simple fan to full air-condition. The active part of these systems is the energy required to drive cooling. None of the active systems could run without a source of power driving them but for this paper we are also going to consider how solar energy can be used to power the cooling systems. Moreover, passive system is a system that requires no energy or power to run it. An operable window is part of passive system. Cross ventilation, windmills, evaporating water cooling, wind breakers, thermal mass wall and strategically placed vegetation and soft landscape elements are all part of passive system

This paper will present to us various cooling techniques that could aid thermal comfort either within an envelope called building or outside of it. The use of solar cooling systems especially passive cooling system should be encourage among architects and other designers so as spread the importance to Nigerian as the design of a building the specification is within their capacity only what they need to do is study the climatic characteristics of the proposed site usually know as the microclimate of the site and enlighten their clients on the need to use the best cooling systems for that particular site. However, active solar cooling systems has potential option for energy saving and abatement of greenhouse effect because solar thermal energy is use to drive a refrigeration cycle in order to operate a cooling appliance which will be discussed in this paper

1 INTRODUCTION

Energy from the sun is inexhaustible and sun is considered to be the main source of the earth‘s energy. Solar energy is also the ultimate renewable resource which originates with the thermonuclear fusion reactions occurring in the sun and this represents the entire electromagnetic radiation (visible light, infrared, ultraviolet, x-rays, and radio waves). It is interesting to know that all the chemical and radioactive pollutant of the thermonuclear reactions remain behind the sun, while only pure radiant energy reaches the earth. Energy reaching the earth is incredible. By one calculation, 30 days of sunshine striking the Earth have the energy equivalent of the total of all the planet’s fossil fuels, both used and unused. However, solar energy is a diffuse source and to harness it we must concentrate it into an amount and form that we can use, such as heat, electricity and also for cooling addressed by approaching the problem through collection, conversion and storage

It is so amazing to understand that energy can also be tapped from the sun and converted to solar energy which primary purpose is to cool a building. The architecture of a building includes the knowledge of thermal comfort within and the surrounding of a building. It is necessary for architect to understand the technology involved in the utilization of solar energy as this will help his design in achieving the sustainability of a green building. Basically solar energy is converted for use in building in three ways; biochemical, electrical and thermal. Each of these ways has proved to be efficient and effective in saving energy though require high technology for the conversion and efficient utilisation.

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Figure 1 showing how much solar reach the surface of the earth

The technology for the utilization of solar energy is uncommon and yet to be fully tapped in Nigeria may be the reason is because the climate of Nigeria does not require winter heating and little auxiliary energy is needed to maintain thermal comfort. (Ogunsote, 1988). Moreover, if not for erratic supply of electricity perhaps the recent use of solar panel would not be explored even though the technology has spread across the world. The level of people ‘s response to this technology is slow in developing nation and that is why solar energy which has proffer several solutions to issues relating to power and energy have not been appreciated though this natural resource (sun) is readily available and abundant throughout the year.

In the face of crumbling economy due to unpredictable power supply which has driven potential viable companies away from Nigeria to nearby countries that has constant powers supply and considering the foregoing, solar architecture could offer solution to the current prevalence. However, the term solar architecture refers to an approach to building design that is sensitive to nature and takes advantage of climatic conditions to achieve human comfort rather than depending on artificial energy that is both costly and environmentally damaging. Unlike the conventional design approach that treats climate as the enemy which has to be kept out of the built environment, solar architecture endeavours to build as part of the environment using climatic factors to our advantage and utilising the energy of nature itself to attain required comfort levels. Nature’s energies can be utilised in two ways – passive and active and consequently solar architecture is classified as passive solar and active solar architecture. The issue of solar cooling systems both active and passive systems have not thrived as expected in Nigeria. Active systems are mechanical driven cooling systems. These can range from simple fan to full air-condition. The active part of these systems is the energy required to drive cooling. None of the active systems could run without a source of power driving them but for this paper we are also going to consider how solar energy can be used to power the cooling systems. Moreover, passive system is a system that requires no energy or power to run it. An operable window is part of passive system. Cross ventilation, windmills, evaporating water cooling, wind breakers, thermal mass wall and strategically placed vegetation and soft landscape elements are all part of passive system. Passive cooling techniques can be used to reduce, and in some cases eliminate, mechanical air conditioning requirements in areas where cooling is a dominant problem. The cost and energy effectiveness of these options are both worth considering by homeowner and builders.

2 THERMAL COMFORT AND COOLING SYSTEMS

According to Rebecca White (2003), there is no absolute standard of thermal comfort. This is not surprising, as humans can and do live in a range of climates from the tropics to high latitudes. An internationally-accepted definition of thermal comfort, used by American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) defines it as that condition of mind which expresses satisfaction with the thermal environment (ISO 7330). Perceptions of this environment are affected by air temperature, radiant temperature, relative humidity, air velocity, activity and clothing. More general definitions of comfort include a sense of relaxation and freedom from worry or pain. A controversy between the heat-balance approach and the adaptive approach has dominated the development of thermal comfort science. It has largely been concerned with offices rather than domestic premises, but has implications for the residential sector. The current international thermal comfort standard used by ASHRAE (ISO 7730) is based on experiments in climate chambers, many of which were completed in the 1960s. This approach combines the theory of heat transfer with the physiology of thermoregulation to determine a range of comfort temperatures which

occupants of buildings will find comfortable. The range is determined by a ‘PMV’ (predicted mean vote), derived from studies of individuals in tightly controlled conditions. According to advocates, it is feasible and desirable to architects to provide thermal comfort within the narrow range of temperatures.

The adaptive approach is based on field surveys of thermal comfort and demonstrates that people are more tolerant of temperature changes than laboratory studies suggest: they consciously and unconsciously act to affect the heat balance of the body (behavioural thermoregulation). These actions may change metabolic heat production (changing activity or doing something more or less vigorously), the rate of heat loss from the body (clothing, posture) or the thermal environment (windows, doors, blinds, fans, thermostat adjustment) (Humphreys, 1994). Comfort may therefore be achieved in a wider range of temperatures than predicted by ASHRAE when it is something that individuals achieve for themselves. Adaptive variables are extremely important in ‘free running’ buildings – those without active heating or cooling systems (Nicol, Raja et al. 1999). People in such buildings need to be able to control their immediate environment by opening and closing windows, dressing in such a way as to maximise comfort indoors and outdoors, and using shading as necessary. Research into the comfort levels of sedentary individuals at home, at work and in a climate chamber, shows that simply being ‘at home’, in an environment that is familiar and under control, is conducive to comfort and makes people less sensitive to temperature (Oseland 1995).

The need to understand human body comfort is necessary in order to determine when cooling is required so as to create comfortable shelter. During the development of design project, many task related to attaining to comfort will be taken account by the architect. The human body is capable of living within a fairly wide range of earth’s environmental condition; outside of the poles people inhabit virtually every part of the earth within the range of climatic condition which promotes human productivity called the comfort zone. Shelters constructed or found are the primary source of attaining human comfort. Shelter modifies the natural environment to create a liveable environment. In addition, human body react to hot or cold environment with an attempt to maintain a constant body temperature. Our natural reaction can accommodate a range of temperature and still feel comfortable. There are two set of reaction of reaction that has two extreme conditions at either end of the temperature and humidity scale, from hot humid environment to extreme.

When the body gain more heat that it can use, it tries to shed the excess. This heat must be moved from the body core to the skin to dissipate to the environment. The heart rate increase to move blood flow to the periphery and the blood vessel at the skin dilates to move heat to outer layer of the body. Perspiration occurs to cool the skin however in humid environment it does not evaporate quickly, limiting its effectiveness. Heat exhaustion followed by heat stroke is an extreme case of thermal stress. Thermal comfort in Nigeria means more than keeping the indoor air temperature below 27°c High temperatures, or high humidity (or both) which can lead to excessive discomfort. Fortunately, the regions of high temperatures are quite arid (relative humidity is usually low). The only regions of fairly high humidity, the coastal regions, are also among the coolest parts of the region in Nigeria.

3 THERMAL STRESS AND SOURCES OF HEAT IN A BUILDING

There are three major sources of unwanted heat into a building in Nigeria: direct solar impacts on a building and through windows and skylights; heat transfer and infiltration, of exterior high temperatures, through the materials and elements of the structure; and the internal heat produced by appliances, equipment, and inhabitants. Of the three, the first is

potentially the greatest problem in the Nigeria, but it is usually the easiest to control. Table 1 adapted from the Arizona solar centre passive solar manual (www.azsolarcenter.org) lists approximate heat gains from each source for typical single-family detached homes in a climate where the temperature averages 23.9°c (75°F) on a July day in Arizona. The homes are built to local energy codes and are oriented east-west, and have two-thirds of the total glazing facing south.

Figure 2 showing direct solar into a living room

The remaining glass is located on the east and west walls, and all glass is completely un-shaded. Even assuming that sunlight could be excluded from the interior (a difficult feat), these homes would experience excess heat loads of 250 to 450 thousand BTU. Worse yet, the houses would require about 4-8 tons of air conditioning each to handle peak heat gains and keep the rooms comfortable in the afternoon.

Table 1 adapted from Arizona solar centre manual

Intense direct solar impacts from the sun rising in the east are equal to those of the setting west sun. The reason we feel the setting sun impact more is due to the added thermal impact of the earth reradiating the heat it has gained during the day. The dry season sun is much higher in the sky and has a negative impact on skylights and roof windows and lead to enormous solar heat gains. They should not be used in hot climates unless they are insulated and/or shaded. Vertical south facing glass (windows, clerestories, etc.) with overhangs or shades, present fewer problems but are still adversely affected by exterior air temperature. A horizontal overhang or an awning above a south window is an inexpensive, effective solution. If it protrudes to half the window height (Fig. 2), such an overhang will shade the window completely from early December to April, yet allow for harmattan sun access in Nigeria

Figure 3 showing shading devices

4 SOLAR COOLING DESIGN PROCEDURE IN NIGERIA

To prescribe the best solar cooling system method for a building in any climatic zone in Nigeria, it is important to follow four major steps which will aid the architects design. These steps emanates from intuitive study of procedure of design of shading devices by Ogunsote in his book applied climatology and in chapter four where he explicitly give detail steps to design of shading devices. Shading devices is also one method of achieving thermal comfort by controlling direct sunlight into a building by either vertical, horizontal or egg-crate shading devices. However, for cooling systems steps are discussed below.

4.1 STEP ONE: DETERMINATION OF THE CLIMATIC AND MICROCLIMATIC CONDITION

It is necessary to determine the climatic and microclimatic condition of the proposed site. The involve collection of climatic data for the specific site including outdoor air temperature, humidity or vapour pressure, wind speed and direction, global radiation on a horizontal plane, hours of sunshine, cloudiness and precipitation.

Many different systems of climate classification are in use for different purposes. Climatic zones such as tropical, arid, temperate and cool are commonly referred for representing climatic conditions. For the purposes of building design, a simple system based on the nature of the thermal problem in the particular location is often used as described below:

Cold climates, where the main problem is the lack of heat (under heating), or excessive heat dissipation for all or most parts of the year.

Temperate climates, where there is a seasonal variation between under heating and overheating, but neither is very severe.

Hot-dry (arid) climates, where the main problem is overheating, but the air is dry, so the evaporative cooling mechanism of the body is not restricted. There is usually a large diurnal (day - night) temperature variation.

Warm-humid climates, where the overheating is not as great as in hot-dry areas, but it is aggravated by very high humidity’s, restricting the evaporation potential. The diurnal temperature variation is small.

4.1.1 Climatic zones in Nigeria

There are different climatic zones in Nigeria and knowing the characteristic of each climatic zone will help us to proffer solution to the specific overheating conditions of these zones. The thermal performance of a building is established by code for different climatic zones in Nigeria. The aim of this is to reduce overheating and discomfort of occupant of a building because this is the major problem in Nigeria. In the warm humid climate found near the coastal region, conditioned are uncomfortably hot most months of the year. In this condition thermal storage should be avoided and high insulation should be provided. Basically there are

six architectural climatic zones in Nigeria as opined by Ogunsote (1987) and are discussed below.

4.1.1.1 The Coastal Zone

This includes such cities as Ikeja, Lagos, Ondo, Benin, Warri, Port-Harcourt and Calabar. The climate is characterised by high humidity and hot discomfort for eleven or more months in the year. This makes provision of permanent ventilation essential. The monthly rainfall exceeds 200mm for three or more months making adequate drainage necessary. There is no need for thermal storage as a high diurnal temperature range of more than 10 degrees coupled with low humidity is not experienced for more than one month in the year. The maximum monthly temperature never falls below the comfort limit, thus no special precautions need be taken against cold discomfort.

4.1.1.2 Forest Zone

This covers Ibadan and Oshogbo. There is need for permanent provision for ventilation for ten months of the year as a result of the combination of high humidity and hot discomfort in the day. The monthly rainfall never exceeds 200mm. Despite the hot and humid nature of the climate thermal storage is still needed for two months of the year as a result of the combination of low humidity and high diurnal range of more than 10 degrees Celsius. There is no need to provide outdoor living space and protection against cold is not required.

4.1.1.3 Transitional Zone

This covers Ilorin, Lokoja, Enugu and Makurdi. There is need for cross ventilation and cooling systems for three to nine months in the year. Buildings should be protected from heavy rainfall as a result of downpours exceeding 200mm in some months. There is need for thermal storage for three to five months. High humidity and low diurnal temperature ranges make external sleeping spaces unworkable. Severe cold does not constitute a problem.

4.1.1.4 Savannah Zone

This zone covers a large portion of the country and it includes towns like Yelwa, Sokoto, Gusau, Kano, Potiskum, Maiduguri, Yola, Ibi and other slightly wetter towns like Zaria, Kaduna, Minna, Bida and Abuja. There is need for cross ventilation for three to nine months in the year due to hot day discomfort. There is need to protect buildings against rain as a result of the intensity of downpours when they finally arrive. Cold nights and hot days alternate for six to ten months of the year. This is accentuated during the harmattan and thermal storage is needed for cooling interiors in the day and for providing warmth at night. Outdoor sleeping space should be provided since it is impossible to achieve night comfort during the very hot period.

4.1.1.5 Highland Zone

This is a cool climate to be found at high altitudes. This climate is associated with Jos on the Jos Plateau but it can also be found on the Mambilla plateau and other mountainous regions along the Cameroonian border. There is need for cross ventilation only during one month of the year though good ventilation is desirable during other months. Monthly rainfall exceeding 200 mm for three or more months in the year dictates the need for protection against heavy downpours. Thermal storage is needed for six to ten months in the year to dampen fluctuations in indoor temperatures. Outdoor sleeping spaces are not required and special provision for winter is unnecessary.

4.1.1.6 Semi-desert Zone

This covers Katsina and Nguru. Ventilation is essential for one or two months in the year. It is characterised by low rainfall with monthly readings of more than 200 mm occurring only during one or two months in the year. Hot and humid conditions are experienced during one or two months and thermal storage is needed for more than six months in the year. There should be provision of outdoor sleeping space. Extreme winter conditions are however non-existent.

4.1.2 STEP TWO: DETERMINATION OF WHEN COOLING IS REQUIRED

It is necessary to know when cooling is required especially at what time of the year and during what hour of the day. The thermal stress experienced in a particular city is characterised by the duration of the overheated, the comfortable and the underheated periods. The overheated period is that period when there is hot discomfort and this is when cooling is required while the underheated period represents cold discomfort. In composite climates, there are certain periods of the year, especially during the harmattan months of November to February, when there is underheating characterised by low temperatures in the nights and early morning, the use of solar radiation during this period is welcome. On the other hand, there is serious overheating for a few weeks in March/April and exclusion of sunlight is desirable at this period. The same shading device is used to allow solar heating during the underheated period and block out the sun during the overheated period. The overheated and underheated periods are determined with the aid of a thermal index. Such an index should be able to indicate for given climatic conditions whether there is cold discomfort, comfort or hot discomfort and hence when cooling is required in a building. Thermal indices indicate the simultaneous effect of the six variables on comfort; these are the air temperature, the mean radiant temperature, the air velocity, the relative humidity, the intrinsic clothing and the level of activity. Such indices include the Standard Effective Temperature (SET), the Effective Temperature (ET), the Corrected Effective Temperature (CET), the Resultant Temperature (RT), the Heat Stress Index (HSI) ,the Equivalent Warmth (EW) ,the Equatorial Comfort Index (ECI), the Predicted Four Hour Sweat Rate (P4SR), the Operative Temperature (OT), the Index of Thermal Stress (ITS), the Bioclimatic Chart, the Mahoney Scale and the Evans Scale. Only some of these indices may be applicable in Nigerian conditions.

4.1.3 STEP 3: THERMAL ANALYSIS AND CALCULATION OF COOLING LOAD

This is the analysis of heat balance of the building. The first analysis is done on the basis of the climatic site data to produce a guide. The performance of a building is checked continuously as amendments are made. At the end of the design process the whole building will be analysed to confirm its heat load capacity. This is done by the calculation of the heat load so as to determine of the extent cooling and cooling load that is needed for the building to be comfortable to the occupants.

4.1.3.1 Cooling Load Calculations

Cooling Load is the rate at which heat must be extracted from a space to maintain a desired room condition. Heat gain occurs from a building envelope whenever the exterior temperature exceeds the interior temperature. The rate at which it occurs is affected primarily by the efficiency of the covering materials (glazing, roof, side walls, doors, window frames and end walls). The other source of heat gain is from the lighting, equipments, occupancy, infiltration and the ventilation air. To calculate cooling loads using ASHRAE-based load calculation methodology it is required to prepare cooling load adopting calculations using the

CLTD/SCL/CLF Method, which is based on the cooling load temperature differences (CLTD), the solar cooling load factors (SCL), and the cooling load factors (CLF). Cooling Load Temperature Difference (CLTD): An equivalent temperature difference used for calculating the instantaneous external cooling loads across a wall or roof (CLTD = External Cooling Load/ (U-Value x Area)). When used for glass, the CLTD calculates only the conduction cooling load. All these methods are described in the ASHRAE Handbook Fundamentals. Approved methods for cooling load calculations include:

ASHRAE Fundamentals- Load and Energy Calculations ACCA Manual N - Commercial Load Calculation Computer software such as DOE-2, ASEAM or other non-proprietary software based

upon ASHRAE or ACCA methods A number of proprietary commercial ASHRAE-based software packages such as

TRACE 700, Elite Software, Hevacomp, and software by Carrier Corp etc. Factor in extreme conditions such as arid or humid climates. If necessary, increase the calculated size of equipment and distribution system(s) by up to 10 percent to compensate for morning recovery due to night set forward or by up to 10 percent to compensate for unanticipated loads or changes in space usage. Limit the total combined increase above the size calculated of equipment and distribution system(s) to 15 percent total and divide the building into zones. Always estimate the building peak load and individual zones airflow rate. The building peak load is used for sizing the refrigeration equipment and the individual zone loads are helpful in estimating the air-handling unit capacity. It is however important to avoid cooling loads and not how to cool down the building. If excessive heating can be minimized, then the problem providing sufficient cooling will be half-solved. Cooling loads are due to sunshine through windows or on the outside of walls or roofs, hot air entering the building or heat conducted from hot outside to the inside. Cooling loads can usually be avoided through good design involving the judicious use of shading devices, vegetation, colours and insulation.

4.1.4 STEP 4: DESIGN OF THE PRESCRIBED COOLING SYSTEM.

The choice of a passive solar, active solar cooling system is based on many factors such as client, client demand, cost effectiveness, aesthetics, technology, availability and maintenance.

5 TYPES OF SOLAR COOLING SYSTEM

Basically, there are two types of solar cooling system that can be adopted in Nigeria. They are referred to as passive and active solar cooling system.

5.1 PASSIVE SOLAR COOLING SYSTEMS

These are all natural means used to provide thermal comfort within and around a building. Passive cooling techniques can be used to reduce, and in some cases eliminate, mechanical air conditioning requirements in areas where cooling is a dominant problem. The cost and energy effectiveness of these options are both worth considering by designer and the users. The techniques of passive solar cooling systems were practiced for thousands of years, by necessity, before the advent of mechanical cooling which usually refer to as active solar cooling system. It has remained a traditional part of vernacular architecture in many countries. There is evidence that ancient cultures considered factors such as solar orientation, thermal mass and ventilation in the construction of residential dwellings.

These systems function by either shielding buildings from direct heat gain or by transferring excess heat outside. Carefully designed elements such as overhangs, awnings and eaves shade from high angle summer sun while allowing winter sun to enter the building. Excess heat transfer can be achieved through ventilation or conduction, where heat is lost to the floor and walls. A radiant heat barrier, such as aluminium foil, installed under a roof is able to block up to 95% of radiant heat transfer through the roof.

Water evaporation is also an effective method of cooling buildings, since water absorbs a large quantity of heat as it evaporates. Fountains, sprays and ponds provide substantial cooling to the surrounding areas. The use of sprinkler systems to continually wet the roof during the hot season can reduce the cooling requirements by 25%. Trees can induce cooling by transpiration, reducing the surrounding temperature by 4 to 14 degree F. The various passive cooling methods are discussed below.

5.1.1 Shading

One of the simplest and most effective methods of blocking heat from entering the home is shading. There are many different methods available to provide shading both inside and outside the house. Most are very simple and can easily be retrofitted to an existing structure. In general, exterior shading is more effective than interior because it blocks the heat before it enters the house. Interior shading, while effective at blocking sunlight from reaching the centre of the room, still allows heat to enter the house, where it is trapped between the shade and the window. In addition, some types of exterior shading may be used to shade the walls and roof, as well as windows, thus reducing their temperature and heat transmission to the inside. Interior shading, however, has the advantages of being easily controlled by the occupants of the house while also not being exposed to wind and rain. A combination of both indoor and outdoor shading maximizes both heat reduction and

5.1.1 EXTERIOR SHADING

5.1.1.1 Landscaping and the use of Vegetation

Landscaping is an effective and pleasant means of providing shading for houses. An effectively planned landscape will block out the hot sun, encourage warming sun to enter the house, and channel breezes for cooling. In general, an ideal landscape plan would include trees to the east and west of a building to provide shading. Trees will be most effective if they shade east and west windows, where the most heat can enter, but shading east and west walls and the roof is also important. Even trees which do not directly shade the house, such as those planted to its north, are valuable because they reduce the temperature of the air surrounding the house.

5.1.1.2 Roof Overhangs

A roof overhang is a simple architectural feature which can be used on the south side of the house to block direct sunlight in without reducing the available sunlight. The overhang blocks direct sunlight from entering in summer, while the lower winter sun passes beneath the overhang. Overhangs do not work as effectively on orientations other than due south, however, because the sun is at lower angles in the sky when it shines from the east or west, thus bypassing the overhang. A covered porch or carport on the east or west side may be used, however, to produce the same effect since it would extend out. Overhangs may be a permanent part of the building structure, or may be used seasonally.

5.1.1.3 Awnings

Awnings serve the same general function as an overhang, but are more flexible in their application. Made of lightweight materials such as aluminium, canvas, acrylic, or polyvinyl laminate, it is possible for them to span distances of several meters without the need of extra support, thus making it possible for them to provide adequate shade even on the east or west. They are also frequently designed to extend below the top of the window, increasing their shading effectiveness. Awnings can be custom-made to match the home exterior, making them an attractive design feature for many homes. They may have open or closed sides. Side less awnings can shade east and west windows effectively, but for south windows, awnings with sides will give better protection against the early morning and late afternoon sunlight. To avoid trapping heat underneath the awning next to the window, the awning must have some means of allowing heat to escape, either through open sides or from a vent at the top. To be most effective, awnings should be light in colour. Permanent awnings may be appropriate for use on the east or west side, but awnings on the south side need to be retractable or removable in winter in order to allow in sunlight for heating.

5.1.1.4 Exterior Shade Screens

Solar shade screens are a very effective shading option. Made of a thick fibre glass mesh which absorbs the sunlight, they are effective against diffuse and reflected, as well as direct, sunlight. Consequently, they are capable of blocking up to 70 percent of all incoming sunlight before it enters the windows. Because most varieties can also serve as insect screening, they also allow the use of natural ventilation, unlike some other shading options (such as interior or exterior shades) which block air flow. Shade screens come in a variety of colours. From the outside, most shade screens appear darker than a standard window screen, however, from the inside, most people will not notice an appreciable difference in colour. Shade screens may be ordered to size for a particular window, or the mesh may be purchased by the roll and installed by the homeowner using special hardware that snaps in the window frame. In addition to fibreglass mesh, there is another type of shade screen which uses thin louvered metal fins to reflect the sunlight. This type is more expensive, however, and is used more frequently on commercial buildings than residences.

5.1.1.5 Shutters and Shades

Exterior shutters and shades either hinged or of the rolling blind type, are another option for shading. Although they block sunlight very effectively, they have a few disadvantages: they obscure the view from the window, block day lighting, and may be inconvenient to operate on a daily basis. They are also subject to wear and tear, and may block air flow. Exterior shutters may be operated manually or automatically. Automatic controls are more costly and difficult to maintain, but may be more practical than manual controls when the shutters are at inconvenient locations, such as behind shrubbery or on the second floor. Proper use of the shutters is also more likely when they are automatically controlled rather than depending upon compliance by members of the household. The lifestyle of the family needs to be considered in the decision of whether or not to use exterior shutters. If the house is unoccupied during the day, and the shutters can be easily closed by the occupant as they leave for work and reopened on arriving home, exterior shutters can significantly reduce the amount of heat entering the house during the day. On the other hand, if some of the family is at home occupying those rooms during the day, they may be resistant to the loss of view. Similarly, there will be resistance from family members to opening and closing shutters which are inconvenient to operate.

5.1.2 INTERIOR SHADING

While interior shading is not as effective as exterior shading, since it is unable to block heat until it has already entered the building, it can still be a useful supplement to exterior shading. It should certainly be used where other shading options are unavailable. Interior window treatments are normally considered a necessity for privacy and as part of the house. Proper selection of window treatments can make them an asset for cooling as well. Draperies and curtains are most effective when made of tightly-woven, opaque material of a light or reflective colour. The tighter the curtain fits to the window, the better its ability to trap heat and prevent it from entering the house. Simple white roller shades shade quite effectively when fully drawn, but prevent light and air from entering. Venetian blinds, while not as effective at trapping heat, will allow air and light to pass through, while reflecting some of the sun’s heat. Some newer blinds are coated with special reflective finishes.

5.1.2.1 Reflective Films and Coatings

Reflective coatings which adhere to glass can block up to 85 percent of incoming sunlight. Some coatings may be applied seasonally; others are permanently affixed to the glass surface. Permanent films or coatings are not appropriate for south windows in passive solar homes, since they would block heat from entering all year round. However, they would be practical for unshaded east or west windows. Window films are not recommended for windows which receive partial shading, because the film absorbs the sunlight and will cause the glass to heat unevenly and possibly crack.

5.1.2.2 Radiant Barriers

For roofs which are unshaded, radiant barriers provide another way to block heat from entering your home. A radiant barrier is a layer of aluminum foil placed in an air space between a heat-radiating surface (the roof of your house) and a heat-absorbing surface (the insulation on the floor of your attic). It works to reduce the heat entering your house in two ways: its reflective surface reflects most of the radiant heat striking it, and it will itself emit very little heat. Radiant barriers come in many different forms: single sided or double-sided foils, foil-faced insulation, and multilayered foil systems with air spaces. Any of these products should perform equally well if properly installed, so the cost of the product and its ease of installation should guide your decision between them. To work properly, the shiny side of the radiant barrier must face an air space. In an attic, this is done by stapling the radiant barrier, shiny side down, to the underside of the roof decking or the roof trusses. Although this may seem counter to what your intuition tells you, this is the preferred position. The orientation of the shiny surface itself does not matter; it will reflect heat equally well whether it points up or down. What is important is that the surface remains shiny. Hanging the radiant barrier with its shiny side down prevents dust from accumulating on its surface and reducing its ability to reflect heat. Some dealers recommend laying the radiant barrier on the floor of the attic for ease of installation. This is not a good idea, however, because of the dust accumulation problem, damage from possible traffic and, most important, the possibility of moisture problems being caused by water vapour trapped beneath the radiant barrier

5.1.2.3 Minimize heat generated

Not all of the heat in our homes in summer comes from the sun; much of it comes from the occupants of the home and the appliances they use. By carefully selecting appliances and the times when they are used, members of the household can help keep the house cooler. The first step in minimizing heat generated within the home is choosing energy-efficient appliances throughout the house, from the large appliances like refrigerators down to the smaller ones, like light bulbs. The less efficient an appliance is, the more waste heat it

generates: thus, its inefficiency costs in two ways: the extra energy it costs to run the appliance, and the cooling penalty that comes with having to remove the extra heat it generates.

5.1.2 VENTILATION

Ventilation, or the movement of air, is one of the most powerful means of achieving a cool home. Ventilation has two goals:

To remove heat from the house and To provide air movement within the house to cool its occupants.

There are several different types of ventilation, both natural and mechanical, which meet these goals in different ways. Though mechanical ventilation measures are not strictly passive, they are a much less energy-intensive method of achieving a cool home than air conditioning.

Natural ventilation, or relying upon summer breezes to generate air movement within the house, is the simplest of passive cooling strategies. Due to the variability of wind speed and direction, though, it can also be the least reliable. However, careful selection of windows and their positioning can help enhance the natural ventilation possibilities of building. When determining the type of windows to be used in your home, appearance should not be the only factor; the summer ventilation and winter infiltration potential of the window should also be considered. With the standard double-hung window, where the window is opened by pushing one half of the window in front of the other half, slightly less than half, or about 45 percent, of the total window area is available for ventilation purposes. The same is true of single-hung and horizontal sliding windows. With awning windows, this percentage is 75 percent; with casement windows, the percentage of free vent area is 90 percent. Casement and awning windows are also superior to the single-hung, double-hung, and sliding windows in winter, since they are better able to achieve a tight fit which reduces infiltration. In planning the layout of windows in the house, the important point to remember is that for natural ventilation to succeed there must be both an inlet and an exit for the air. In other words, windows on both the windward side and the leeward side of the house need to be open to promote air flow. If there is not an exit for the air, the house will become pressurized by the addition of incoming air. Once the house is pressurized, the wind will see the open windows of the house as just another obstacle to be bypassed, rather than an inviting gate to enter.

Pathways for airflow within the house also need to be left open. For example, if the door to the bedroom on the windward side of the house is normally left closed, the room will quickly become pressurized and lose its potential to help cool itself and the rest of the house. Rooms with two exterior walls should have windows on both walls, with as much distance between the windows as possible, to maximize the potential for cross-ventilation. Of course, this guideline needs to be considered at the same time as the recommendation to minimize windows on the east and west side.

5.1.3 CONVECTIVE COOLING MODELS

Convective cooling methods are other natural cooling systems that use the prevailing winds and natural, gravity-induced convection to ventilate a house at the appropriate times of the day. This method admits cool night air to drive out the warm air. If breezes are predominant, high vents or open windows on the leeward side (away from prevailing breeze) will let the hottest air, located near the ceiling, escape. The cooler night air sweeping in through low open vents or windows on the windward side will replace this hot air and bring relief. To get

the best cooling rates, leeward openings should have substantially larger total area (50% to 100% larger) than those on the windward side of the house (Fig. 4a).

If there are only light breezes at the site, natural convection can still be used to ventilate and cool a house as long as the outdoor air is cooler than the indoor air at the peak of the house. Since warm air rises, vents located at high points in the interior will allow warm air to escape while cooler outdoor air flows in through low vents to replace it (Fig. 4b). The coolest air around a house is usually found on the north side, especially if this area is well shaded by trees or shrubs and has water features. Cool air intake vents are best located as low as possible on the north side. The greater the height difference between the low and high vents, the faster the flow of natural convection and the more heat mitigation can occur.

Figure 4a Figure 4b

The two basic ways to enhance the convective cooling rate either to increase the volume of air escaping per minute, or bring in cooler air. If Delta T is the temperature difference between exiting indoor air and incoming outdoor air, the overall cooling rate in BTU's per hour is given by the simple equation:

Cooling rate = 1.08 x V x DT, where V is the volume of air escaping.

To a point, increasing the vent area will increase the airflow rate by natural convection. Turbine vents at the roof peak are one way to enhance airflow and improve the cooling rate. Even gentle breezes flowing up and over the roof peak create an upward suction that draws out warm interior air (Fig. 5). An even better approach is to use solar radiation to induce a more rapid flow. One of the many possible approaches, shown in Figure 6, uses a Trombe wall vented to the outside. Sunlight striking the concrete wall will heat the air in the space between glass and wall to temperatures above 150°F. This very hot air rises quickly and escapes, drawing cool air into the house through low vents on the north wall. Additionally, specifically constructed "solar chimneys", composed of passive air heaters with seasonal dampers can be incorporated where solar heated air can be dumped into the building in the winter, and used as a "ventilator driver" in the summer to draw outdoor air through a house and ventilate it. Frequently, they can induce air velocities of 1-2 feet per second.

Another convective cooling strategy is the drawing of outdoor air is drawn through tubes buried in the ground and dumped into the house. Made of material that allows easy thermal transfer, these tubes are buried several feet deep to avoid the warmer daytime surface temperatures. Warm outdoor air entering the tube gives up its heat to the cooler earth, and cools substantially before entering the house (Fig. 7). Thermal saturation of the surrounding earth must be addressed, by means of surface landscaping and watering, thereby removing

the gained thermal energy from the tube/earth transfers. Though condensation is rarely a problem in dry climates, such tubes should be sloped slightly and have adequate drainage to insure that water build-up doesn't block the passage of air. The intake end should be screened and placed in a shady spot away from foot traffic. When properly built and sized, these underground tubes can supply cool air during the peak load daytime even in the hottest climates.

Figure 5 Figure 6

Figure 7

5.1.4 EVAPORATIVE COOLING METHODS

When water evaporates it absorbs a large amount of heat from its surroundings (about 1000 BTU per pound of water evaporated). The most familiar example of this is the cooling effect of evaporating perspiration on the human skin. In arid, hot climates body temperature is partially controlled by the rapid evaporation of perspiration from the surface of the skin. In hot climates with high atmospheric moisture the cooling effect is less because the high moisture content of the surrounding air. In both situations, however, the evaporation rate is raised as air movement is increased. Both of these facts can be applied to natural cooling of structures.

Evaporative methods can be used to enhance the cooling rates in convective cooling systems. One way of doing this is to bring the outdoor air into the house through a moist filter or pad as shown in Figure 8. The familiar evaporative cooler, precursor to the air conditioner, is a mechanical system which uses these principles with a motor to force air movement and distribution. Passive cooling strategies with earth tubes and/or cool towers use the same principles but utilize natural systems for air drivers and distribution. If underground intake pipes are made from a porous material, and ground above them is well cool and watered, some evaporation will occur at the inner surface of the pipe.

Figure 8

Evaporative cooling strategies are well suited to those areas of the southwest with the most severe cooling requirements. In the desert areas of the South, the warm night air (80 degrees+) may impede natural convection heat dissipation from a roof pond cooling system. That is one of the reasons why the cooling rate falls to about 25 BTU/hr/ft^2 in the extreme southeast corner of the state (Fig. 9). Simple introduction of a thin water layer over the water containment surface can increase the overall cooling rate of the roof by 50-100 percent due to the resulting evaporation.

In the most severe climates where night time air temperatures often remain above 90°F in summer, sprays can be used to achieve maximum natural cooling, at standard roofs and roof cooling systems like the roof pond strategy. In the summer, sprays can be used to achieve optimum natural cooling. In the approach shown in Figure 10, water is pumped to sprinklers along the peak of a house and allowed to trickle down a sloping roof. The rate of evaporation is greatly enhanced in such a system because a much larger surface area is exposed to the night air. Roof sprays rely on a little external power to get the water to the roof and hence do not qualify as completely passive systems. But the total amount of energy consumed for pumping is very minimal compared to the energy saved by the added cooling rate attained. Excess water can be captured and reused or used elsewhere on the site.

Figure 9 Figure 10

With all evaporative cooling methods, it is important to maximize airflow across the exposed water. Fresh air must be continually available to replace the humid air being built up near or over the water. Failing this, air will be quickly saturated with water vapor, and the evaporation and cooling rates will decline abruptly. Lips, edges and other structures or buildings that could block or deflect prevailing winds away from the water surfaces should be studiously avoided. Sometimes, a small fan to disturb the air over a pond will greatly aid the evaporation rate on a hot, sultry day or night.

Even with direct, active evaporative cooler systems, provision of interior thermal mass combined with direct evaporative cooling is a combination that works effectively. During the day, the structure can utilize the stored cool in the walls and floors, and maintain an improved

level of comfort while reducing power requirements of direct evaporative cooler system. In many areas of the southwest which are considered hot, arid zones, periods of higher humidity renders mechanical evaporative cooling unsatisfactory even when optimized techniques are used. A solution to this is the two-stage evaporative cooling system, which has been shown to be an effective alternative to direct evaporative cooling or refrigerated air-conditioning.

While not a passive system, two-stage evaporative cooling is an important element to be considered as part of passive cooling strategies. Cooling is accomplished by pre-cooling ambient air without humidification before further cooling by evaporation. The cool air entering the structure is then exhausted, typically through areas of heat gain such as windows or the attic. The pre-cooling may be accomplished by a combined cooling tower, heat exchanger unit, or by nocturnally cooled rock bed through which air is drawn. The second stage, evaporative cooling, is accomplished by a standard commercial evaporative cooling device, or by passive cooling elements of earth tubes or cool towers. Rock bed mechanical cooling has been used extensively in Australia with high degrees of effectiveness.

A typical system consisting of two evaporative coolers and a large rock bed is shown in Figure 11. At night, one evaporative cooler cools the rock bed while the other cools the house using a one-stage evaporative cooler. During the day, hot outside air is drawn through the night-cooled rock bed where it is pre-cooled before entering the main house evaporative cooler. Since no moisture has been deposited in the rock bed, the pre-cooled air has not had moisture introduced into the house. An attractive feature of this type of system is the combining of heating and cooling systems in order to make the best possible use of components during the entire year. An air heater may be used to provide hot air during the heating season to the rock bed where the rock bed, fans, ducts and many of the control systems are used both during the heating and cooling season.

Figure 11

Recuperative and regenerative evaporative cooling options are other methods to produce greater comfort using evaporative cooling. These techniques use the relatively cool air exhausted from the structure to improve the performance of the evaporative cooling device. Evaporative cooled water reduces in temperature the ambient air in the heat exchanger without humidification as it enters the structure. The cool, dry air warms a few degrees as it passes through the structure and exits through the evaporative cooling device or a cooling structure. Since the exiting air is cool and dry, the wet bulb temperature is lower and the water produced by the evaporative cooling device is cooler than if ambient air were used. The rock bed heat exchanger and the evaporative cooling device could be combined into a single unit. If the rock bed is used to store heat in the winter, the cost effectiveness of the system is improved.

5.1.6 Roof Spray The exterior surface of the roof is kept wet using sprayers. The sensible heat of the roof surface is converted into latent heat of vaporisation as the water evaporates. This cools the roof surface and a temperature gradient is created between the inside and outside surfaces causing cooling of the building. A reduction in cooling load of about 25% has been observed. A threshold condition for the system is that the temperature of the roof should be greater than that of air. There are, however, a number of problems associated with this system, which are the cost implication of the maintenance of the system and availability of adequate water.

5.1.7 Roof pond The roof pond consists of a shaded water pond over an non-insulated concrete roof. Evaporation of water to the dry atmosphere occurs during day and nighttime. The temperature within the space falls as the ceiling acts as a radiant cooling panel for the space, without increasing indoor humidity levels. The limitation of this technique is that it is confined only to single storey structure with flat, concrete roof and also the capital cost is quite high.

5.1.8 Earth cooling tubes These are long pipes buried underground with one end connected to the house and the other end to the outside. Hot exterior air is drawn through these pipes where it gives up some of its heat to the soil, which is at a much lower temperature at a depth of 3m to 4m below the surface. This cool air is then introduced into the house. Special problems associated with these systems are possible condensation of water within the pipes or evaporation of accumulated water and control of the system.

5.1.9 Earth-sheltered buildingsit is discovered that soil temperatures at certain depths are considerably lower than ambient air temperature, thus providing an important source for dissipation of a building’s excess heat. Conduction or convection can achieve heat dissipation to the ground. Earth sheltering achieves cooling by conduction where part of the building envelope is in direct contact with the soil. Totally underground buildings offer many additional advantages including protection from noise, dust, radiation and storms, limited air infiltration and potentially safety from fires. They provide benefits under both cooling and heating conditions, however the potential for large scale application of the technology are limited; high cost and poor day-lighting conditions being frequent problems On the other hand, building in partial contact with earth offer interesting cooling possibilities. Sod roofs can considerably reduce heat gain from the roof. Earth beaming can considerably reduce solar heat gain and also increase heat loss to the surrounding soil, resulting in increase in comfort

5.2 ACTIVE SOLAR COOLING SYSTEMS

Active cooling systems are mechanical driven cooling systems. These can range from simple fan to full air-condition. The active part of these systems is the energy required to drive cooling. None of the active systems could run without a source of power driving them. The idea of solar active cooling systems is how we can use solar energy to power these systems. Active cooling systems can either be open cycle or closed cycle. The use of solar panel has found to be effective in generating power from solar energy and this energy could be used to drive the equipment. Another method is the use of absorption cooling systems which transfer a heated liquid from the solar collector to run a generator or a boiler activating the refrigeration loop which cools a storage reservoir from which cool air is drawn into the space.

Rankine steam turbine can also be powered by solar energy to run a compressed air-conditioner or water.

5.2.1 Whole House Fans

Whole house fans allow your house to use outdoor air for cooling even when no breezes are blowing. Whole house fans remove hot room air from the ceiling and exhaust it out through the vents in the attic. At the same time, it pulls in cooler supply air through the windows. A general rule of thumb for sizing whole house fans is that the fan should be able to provide between 0.5 and 1 air changes per minute. For example, consider a 2000 square foot house with 8 foot ceilings. The house volume equals the floor area times the ceiling height, or 16,000 cubic feet. Thus, this house would need a fan that provides between 8000 and 16,000 cubic feet per minute (CFM). You may find it worthwhile to choose a fan rated toward the upper end of this range. This way, you will frequently be able to operate the fan at low speed, where it will run more quietly so that the fan can be set to turn off automatically during the night.

5.2.2 Ceiling Fans

Whole house fans move large volumes of air at moderate speeds in order to exhaust heat from the house. Ceiling fans, on the other hand, don’t remove heat. Instead, they provide localized breezes which blow past your body and help it lose heat more efficiently, giving you the perception that the temperature is about 4 degrees cooler than it actually is. Accordingly, in a house with strategically located fans, the air conditioner thermostat setting may be raised from 2 to 6 degrees above what would otherwise be considered comfortable. To be most effective, fans need to be located throughout the house. If located only in the family room and master bedroom, family members in other rooms are likely to lower the thermostat setting to a point where they are comfortable, too. Portable fans are useful to have around to provide air movement in rooms that are only intermittently occupied.

5.2.3 Exhaust Fans

The kitchen and bathroom come equipped with exhaust fans designed to remove the hot, humid air produced in these areas. Their proper operation is important not just for comfort, but to help prevent the growth of mould and mildew. Install a timer control switch on the bathroom vent fan, so that when the fan is turned on after a shower or bath, it will run only long enough to remove the excess moisture from the room, without having to depend on having someone remember to turn it off. When selecting a fan, choose a quiet fan because experience has shown that people tend to avoid using noisier fans. Exhaust fans need to vent to the outdoors, rather than into the attic, to avoid moisture damage to the insulation or mould growth.

5.2.4 Air- Conditioner

The main purpose of the air conditioning systems is to provide the people inside buildings with "conditioned" air so that they will have a comfortable and safe environment. "Conditioned" air means that air is clean and odour-free, and the temperature, humidity, and movement of the air are within certain comfort ranges. The challenge to system designers and facility executives is achieving goals for improved system performance, efficiency and security without making systems unaffordable or unmanageable. Fortunately there are options that must be explored by designers pursuing high performance buildings.

Here are some basic principles to consider:

Physically smaller air-conditioning equipment requires less overall space and improves core efficiencies as compared to larger equipment.

Moving less air results in lower fan horsepower, less sound attenuation, smaller equipment and smaller ductwork.

Quieter air-conditioning equipment requires less sound attenuation and minimizes special architectural room construction when compared to nosier equipment.

Moving less water will result in smaller piping systems, smaller pumps, and lower pump horsepower.

Smaller pump motors, smaller fan motors, and lower refrigeration horsepower require smaller electrical systems and use less energy.

Concentrating the major electrical loads such as large motors and refrigeration equipment near the electrical utility service entrance is usually less expensive than locating large electrical loads at a greater distance from the electrical service entrance.

Using fewer materials and smaller equipment minimize environmental impact.

6 RECOMMENDATION ON ACTIVE AND PASSIVE SOLAR COOLING SYSTEMS IN NIGERIA

This paper has presented various cooling techniques that could aid thermal comfort either within an envelope called building or outside of it. The use of solar cooling systems especially passive cooling system should be encourage among architects and other designers so as spread the importance to Nigerian as the design of a building the specification is within their capacity only what they need to do is study the climatic characteristics of the proposed site usually know as the microclimate of the site and enlighten their clients on the need to use the best cooling systems for that particular site. However, active solar cooling systems has potential option for energy saving and abatement of greenhouse effect because solar thermal energy is use to drive a refrigeration cycle in order to operate a cooling appliance earlier discussed.

Even though some of these technologies is yet to be available in Nigeria, further study should be done on alternative power generation from solar energy apart from the fact that designer also have to key into the technology for the efficient utilization in Nigeria. Owning to the growing load of cooling load, more and more building now depends on convectional cooling systems globally leading to sharp rise in power consumption. On the other hand, there is erratic supply of power in Nigeria due to insufficient power supply from power station and this problem have forced many people to result the use of alternating generating set that uses fossil fuel to drive the motor which have adverse environmental impact on the atmosphere.

Moreover, passive solar cooling based on bioclimatic strategies such as discussed earlier increase cooling and is widely applied and should be the first step to in cooling a building since such methods are easier and less costly to implement only that they need to be incorporated into the building through the architects design and specification as this decreases the need for additional cooling and therefore reduces the overall energy demand. Sufficient insulation of building also decreases the need for convectional cooling system. However, if the outcome of these measure is not sufficient, a solar assisted cooling system may be an inevitable solution and in that case, the solar thermal system will be require to drive the cooling process for maintaining air conditioning that provide comfort for the occupant of the building instead of using electricity.

Apart from this, it is important to know that Nigeria is located near the equator with intense all-year round solar radiation which makes it suitable for the adoption solar cooling systems. Solar thermal energy can be coupled to any of the above discussed active cooling systems instead of the use of generating set which have been found to release carbon monoxide and other noxious gas which when combined with gases in the atmosphere forms hydro chlorofluorocarbon (HCFCs) and chlorofluorocarbon (CFCs) which tend to deplete the ozone layer thereby causing global warming.

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