The Sun provides Earth with a staggering amount of energy—enough to power the great oceanic and atmospheric currents, the cycle of evaporation and condensation that brings fresh water inland and drives river flow, and the typhoons, hurricanes, and tornadoes that so easily destroy the natural and built landscape. The San Francisco earthquake of 1906, with magnitude 7.8, released an estimated 1017 joules of energy, the amount the Sun delivers to Earth in one second. Earth’s ultimate recoverable resource of oil, estimated at 3 trillion barrels, contains 1.7×1022 joules of energy, which the Sun supplies to Earth in 1.5 days. The amount of energy humans use annually, about 4.6×1020 joules, is delivered to Earth by the Sun in one hour. The enormous power that the Sun continuously delivers to Earth, 1.2×105 terawatts, dwarfs every other energy source, renewable or non-renewable. It dramatically exceeds the rate at which human civilization produces and uses energy, currently about 13 TW.
Energy is an essential input for economic development and improving the quality of life. At present, most energy is being produced from non-renewable sources, such as fossil fuels, because of the large supply and low cost of production (Kalogirou, 2004).
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It is assessed that, earth is blessed with enormous energy, classified as conventional and non-conventional sources, for electricity generation and its use. Conventional energy sources are fast depleting and scarcity is prioritized at World level, whereas harnessing renewable energy seems to be one of the sustainable ways to meet the increasing global electricity demands.
Emissions from the combustion process have been linked to phenomena such as global warming, acid rain, and photochemical smog. Several international treaties have been made to protect the environment and control emissions: the 1979 Convention on Long-Range Transboundary Air Pollution and its protocols, the 1985 Vienna Convention for the Protection of Ozone Layer, the 1987 Montreal Protocol on Substances that Deplete the Ozone Layer as amended in London in 1990, the 1992 United Nations Framework Convention on Climate Change (UNFCCC), the United Nations Conference on Environment and Development in Rio de Janeiro in 1992, and the Kyoto Protocol in 1997 that extended the UNFCCC with the aim of reducing global warming and manmade CO2 emissions. The energy consumption of residential and other buildings forms a large part of the global and regional energy demand. In the residential sector, almost half of the energy consumption is for space heating. Globally, biomass is the dominant fuel used for heating buildings.
One of the ways to sustainable growth is to generate electricity through solar energy which is cleaner and promising. Solar has the greatest energy potential among the other sources of renewable energy and the amount of energy that earth receives, as assessed by IPCC, theoretically if only a small fraction of this form of energy could be used it can meet our current needs [3] or in other words if we can use only 5% of this energy, it will be 50 times what the World requires.
Direct Solar energy is believed to have the highest potential which can be harnessed for our use in different forms. Table 1 shows the global technical potential for all RES as assessed by IPCC.
A Classification model attempts to draw some conclusion from observed values. Given one or more inputs a classification model will try to predict the value of one or more outcomes. Classification either predicts categorical class labels or classifies data based on the training set and the values in classifying attributes and uses it in classifying new data. Classification models include logistic regression, decision tree, random forest, gradient-boosted tree, multilayer perceptron, one-vs-rest, and Naïve Bayes. Classification belongs to the category of supervised learning where targets are also provided with input data.
Electrical energy and natural gas are extensively used in developed regions. The use of electricity for space heating is showing substantial growth, and it is one of the most resource-intensive forms of consumption in cold climates (Heiskanenet al., 2011). Hannemanet al. (2013) indicated that, in developed countries, residential energy consumption is mostly due to space heating and is a large component of the energy demand. Ahren and Norton (2015) indicated that 27% of the total energy consumption by the European Union (EU) in 2010 was by the residential sector. Fan et al. (2015) indicated that the residential sector is one of the greatest contributors of CO2 emissions in China. They found that the largest portion of carbon emissions is from space heating and cooling.
The countries that signed the Kyoto Protocol set targets to decrease the emission of greenhouse gases. Heiskanenet al. (2011) indicated that there are many cost-effective ways to decrease the resource use and CO2 emissions of space heating, such as the use of heat pumps. For the EU, their target is to reduce greenhouse gas emissions by 20% by 2020. The countries that signed the Kyoto Protocol are trying to meet the 2020 targets; one of the most important aspects in the near future is energy utilisation in a built environment. Policymakers and researchers are searching for cost-effective technologies to reduce energy consumption and CO2 emissions. Using alternative and energy-efficient technologies with better building construction will help decrease the heat demand for space heating and thus reduce resource consumption and emissions. As the global population continues to grow and the limited supply of fossil fuels begins to diminish, it may not be possible to meet the global energy demand by only using fossil fuels.
The rise in fuel costs, exhaustion of fossil fuels, and their adverse effects due to combustion have renewed interest in alternative energy sources such as renewable. Utilising solar energy to heat air, such as through the use of solar air heaters (SAHs), is an effective way to decrease resource consumption and CO2 emissions.
Thesis Motivations and Organisation
Conventional SAHs mainly consist of panels, an insulated hot air duct, and air blowers in active systems. The panel consists of an absorber plate and transparent covers to allow solar radiation to penetrate into the collector. Such heaters have low thermal efficiency because of the low coefficient of the convective heat transfer between the absorber plate and air. Also, a higher temperature of the absorber plate which results in higher heat losses to the surroundings.
The thesis is organised as follows. Chapter 1 introduces the reasons for utilising SAH and the main objectives of this work. Chapter 2 presents SAH along with a literature review discussing research works which are related to the thesis topic. Chapter 3 explains the theoretical model of the proposed SAH in detail. Chapter 4 describes the experimental set up of the solar air collector and the equipment used to collect data. Chapter 5 presents the experimental results obtained from SAHs with different configurations. The collected data on various days of the tests are 6 illustrated with figures and discussed in detail. Chapter 6 presents the general conclusions and recommendations along with future works.
Solar Energy
Solar energy is radiant light and heat from the Sun that is harnessed using a range of ever-evolving technologies such as solar heating, photovoltaics, solar thermal energy, solar architecture, molten salt power plants and artificial photosynthesis.
It is an essential source of renewable energy, and its technologies are broadly characterized as either passive solar or active solar depending on how they capture and distribute solar energy or convert it into solar power. Active solar techniques include the use of photovoltaic systems, concentrated solar power, and solar water heating to harness the energy. Passive solar techniques include orienting a building to the Sun, selecting materials with favourable thermal mass or light-dispersing properties, and designing spaces that naturally circulate air.
The large magnitude of solar energy available makes it a highly appealing source of electricity. The United Nations Development Programme in its 2000 World Energy Assessment found that the annual potential of solar energy was 1,575–49,837 exajoules (EJ). This is several times larger than the total world energy consumption, which was 559.8 EJ in 2012.
In 2011, the International Energy Agency said that 'the development of affordable, inexhaustible and clean solar energy technologies will have huge longer-term benefits. It will increase countries' energy security through reliance on an indigenous, inexhaustible, and mostly import-independent resource, enhance sustainability, reduce pollution, lower the costs of mitigating global warming, and keep fossil fuel prices lower than otherwise. These advantages are global. Hence the additional costs of the incentives for early deployment should be considered learning investments; they must be wisely spent and need to be widely shared'.
Solar Energy Types
Photovoltaic systems
One of the most common ways to use solar power is to use photovoltaic systems or as they are also known solar cell systems, which produce electricity directly from sunlight.
The basic principle behind this technology is similar to what we see in clock or calculators that are powered by the sun!
The semiconductor materials used in these solar energy systems absorb sunlight which creates a reaction that generates electricity – to be exact, the solar energy knocks the electrons loose from their atoms which makes them flow through the semiconductor material and produce energy.
Today, solar panel technology can absorb and convert into energy most of the visible light spectrum and about half of the ultraviolet and infrared light spectrum.
Solar cells are typically combined into modules that hold about 40 cells and as a whole can measure up to several metres on the side. Because of their adjustable size and share, these flat-plate photovoltaic arrays can be mounted at a fixed angle facing south, or they can be mounted on a tracking device that follows the sun, allowing them to capture the most sunlight over the course of a day.
Several of these photovoltaic arrays would be needed to provide enough power for a household; but for a large electric utility or industrial applications, hundreds of arrays would be required and these would be interconnected to form a single, large photovoltaic system [11].
Thin film solar cells
What is more, this type of technology can also be run with thin film solar cells which use layers of semiconductor materials only a few micrometres thick. This has made it possible for solar cells to double as rooftop shingles, roof tiles, building facades, or the glazing for skylights or atria maximising use of the available space from where sunlight would be captured.
Solar water heating system
A second type of solar energy is solar hot water which as the name suggests involves the heating up of water using the sun’s heat. The idea behind this comes straight from nature: the shallow water of a lake or the water on the shallow end of a beach is usually warmer compared to deeper water. This is because the sunlight can heat the bottom of the lake or seashore in the shallow areas, which in turn, heats the water.
So, a system has been developed to imitate this: solar water heating systems for buildings are made up of two parts, the solar collector and a storage tank.
The most common collector is called a flat-plate collector which is mounted on the roof and faces the sun. Small tubes run through the box and carry the fluid – either water or other fluid, such as an antifreeze solution – to be heated. As heat builds up in the collector, it heats the fluid passing through the tubes. The storage tank then holds the hot liquid.
Similar technology is often used to heat swimming pools].
Solar power plants
A third way we can harness the sun’s power for energy is solar electricity; this is usually used in industrial applications. As most of us know, most power plants use non-renewable fossil fuels to boil water.
The steam from the boiling water makes a large turbine rotate which in turn activates the generator to produce electricity [13]. This way of generating electricity is bad for both the environment and our health given the emission of greenhouse gases and air pollutants from the burning of fossil fuels.
However, the good news is that a new generation of power plants is being introduced which rely on solar power! These plants use the sun as a heat source, and they can do so in three different ways:
- Parabolic-trough systems capture the sun’s energy through long rectangular, curved mirrors that are tilted toward the sun. In this way, they help focus sunlight on a pipe that contains oil. The oil is heated and used then used to boil water in a conventional steam generator to produce electricity.
- A dish/engine system uses a mirrored dish resembling in shape a very large satellite dish which collects and concentrates the sun’s heat onto a receiver. This receiver absorbs the heat and transfers it to the fluid within an engine. The heat causes the fluid to expand against a piston or turbine and produces mechanical power. This power is used to run a generator or alternator to produce electricity.
- A power tower system uses a large field of mirrors to concentrate sunlight onto the top of a tower, where a receiver containing molten salt sits. The salt’s heat is used to generate electricity through a conventional steam generator. Molten salt retains heat efficiently, so it can be stored for days before being converted into electricity. That means electricity can be produced on cloudy days or even several hours after sunset.
Passive solar heating
A further way that solar power can be harnessed is through the method of passive solar heating and daylighting. This is not a new concept – indeed, ancient civilisations such as the Anasazi Indians in Colorado had developed passive solar design in their dwelling [14].
The impact of the sun is easy to understand: step outside on a warm sunny day and you can feel the sun. With proper design, buildings can also “feel” the sun’s energy.
For example, south-facing windows will receive more sunlight while buildings can also incorporate materials such as sunlit floors and walls that absorb and store the sun’s heat.
These materials heat up during the day and slowly release the heat at night when heat is most needed. Other design features such as a sunspace, which resemble greenhouses, concentrate a lot of warmth which with the right ventilation can be used to heat an entire building [14]. Such features maximise the direct gains from the sun’s heat but also sunlight itself. The even better news is that on particularly hot days, there are ways to ensure these features do not overheat buildings.
Solar Power Generation
Solar radiation may be converted directly into electricity by solar cells (photovoltaic cells). In such cells, a small electric voltage is generated when light strikes the junction between a metal and a semiconductor (such as silicon) or the junction between two different semiconductors. The power generated by a single photovoltaic cell is typically only about two watts. By connecting large numbers of individual cells together, however, as in solar-panel arrays, hundreds or even thousands of kilowatts of electric power can be generated in a solar electric plant or in a large household array. The energy efficiency of most present-day photovoltaic cells is only about 15 to 20 percent, and, since the intensity of solar radiation is low to begin with, large and costly assemblies of such cells are required to produce even moderate amounts of power.
When sunlight strikes a solar cell, an electron is freed by the photoelectric effect. The two dissimilar semiconductors possess a natural difference in electric potential (voltage), which causes the electrons to flow through the external circuit, supplying power to the load. The flow of electricity results from the characteristics of the semiconductors and is powered entirely by light striking the cell.
Small photovoltaic cells that operate on sunlight or artificial light have found major use in low-power applications—as power sources for calculators and watches, for example. Larger units have been used to provide power for water pumps and communications systems in remote areas and for weather and communications satellites. Classic crystalline silicon panels and emerging technologies using thin-film solar cells, including building-integrated photovoltaics, can be installed by homeowners and businesses on their rooftops to replace or augment the conventional electric supply.
Concentrated solar power plants employ concentrating, or focusing, collectors to concentrate sunlight received from a wide area onto a small blackened receiver, thereby considerably increasing the light’s intensity in order to produce high temperatures. The arrays of carefully aligned mirrors or lenses can focus enough sunlight to heat a target to temperatures of 2,000 °C (3,600 °F) or more. This heat can then be used to operate a boiler, which in turn generates steam for a steam turbine electric generator power plant. For producing steam directly, the movable mirrors can be arranged so as to concentrate large amounts of solar radiation upon blackened pipes through which water is circulated and thereby heated.
Solar energy is also used on a small scale for purposes other than those described above. In some countries, for instance, solar energy is used to produce salt from seawater by evaporation. Similarly, solar-powered desalination units transform salt water into drinking water by converting the Sun’s energy to heat, directly or indirectly, to drive the desalination process.
Solar technology has also emerged for the clean and renewable production of hydrogen as an alternative energy source. Mimicking the process of photosynthesis, artificial leaves are silicon-based devices that use solar energy to split water into hydrogen and oxygen, leaving virtually no pollutants. Further work is needed to improve the efficiency and cost-effectiveness of these devices for industrial use.
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