Active and Passive Solar Cooling Systems in Nigeria

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

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

1INTRODUCTION

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 isbecause 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 appreciatedthough 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 – passiveand 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. Crossventilation, 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.

2THERMAL 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 rateincrease 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.

3THERMAL 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 1adapted from the Arizona solar centre passive solar manual ( 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

4SOLAR 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.

4.1STEP 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.

4.1.1Climatic 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.

4.1.1.1The 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.

4.1.1.2Forest 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.