2 Literature Review
2.1 The Sun
The Sun is the most prominent feature in our solar system (AnonA, 2009) It is the largest object in the solar system and contains approximately 98% of the total solar system mass and has a surface temperature of approximately 5,780 K (5,510 °C)
Solar energy is created deep within the core of the Sun. It is here that the temperature in excess of 15,000,000° C and pressure that is 340 billion times Earth's air pressure is so powerful that nuclear reactions happen. The reaction causes four protons or hydrogen nuclei to fuse together to form one alpha particle or helium nucleus (AnonP, 2008). The Sun releases this reaction in the form of light and heat to the entire solar system. In terms of planet earth it is said that the earth absorbs nearly seventy percent (70%) of the energy available via the atmosphere, oceans and land masses, whilst the other Thirty percent (30%) is reflected back (AnonR, 2009).
The Sun produces a vast amount of energy (solar content) about 63MW/m2 irradiance, of which the Earth intercepts only a tiny fraction around 1kW/m2 irradiance because of the sun- earth geometry. Sunlight is the Earth's primary source of energy (Wengenmayr & Buhrke, 2008).
Light and heat from the Sun is the basis for almost all forms of life on planet earth and most energy sources are derived from solar energy.
Solar power is capable of providing many times the world's current energy demand, but with this comes problems, because it is an intermittent energy source, meaning it is not available at all times. Although this problem can be solved, by thermal energy storage or other energy sources such as wind power or hydropower etc (AnonO, 2008).
The Sun's energy can be utilised via a variety of natural and synthetic processes, for instance photosynthesis; plants captures the energy of sunlight and converts it to chemical form and the energy stored in fossil fuels was originally converted from sunlight by photosynthesis in the past due to the discomposure of plants , while direct heating or electrical conversion by solar cells and solar water heaters are used to power equipment and to heat water (AnonN, 2009).
The sun is fundamental to all forms of life. It is the source of our vision, warmth, energy, and the rhythm of our lives. Its movements inform our perception of time and space and our scale in the universe. Guaranteed access to the sun is, thus, essential to energy conservation and to the quality of our lives (Knowles, 2003).
Grade: A. Reason: ready
Renewable sources represent an effective alternative to fossil fuels for preventing resources depletion and for reducing air pollution. However, their diffusion requires huge capital investment and major infrastructure changes, which have to be assessed to verify their effectiveness (Cosmi et al 2002). The main driving force behind need for renewable technologies was the Energy Crisis of 1973 and the pollution caused by the use of fossil fuels thus the need for an alternative energy source (Hester & Harrison, 2003).
2.2 Indirect Solar Energy
The Sun powers the universe and everything on Earth derives its energy from the sun. Over century's the Sun's energy has been converted into useful energy sources such as fossil fuels, biomass, wind, hydropower to name a few and which fossil fuels have been used extensively and irresponsibly to meet the energy needs of the World. Fossil fuels do produce vast amounts of energy but produce it with a considerable amount of pollution and the production of CO2 that endanger the environment and fossil fuels are not considered renewable. These are indirect energy sources as they do not use direct sunlight but uses the sunlight in another form which has an indirect impact on the planet resulting in natural phenomenon. Such indirect renewable energy sources are :
2.2.1 Active
2.2.1.1 Wind
It can today be stated ' that wind energy has become one of the most economically practical, and both technically and environmentally attractive, of all the new renewable energy options' (Sesto & Casale, 1998). Nowadays wind is seen as direct competition to that of a power plant in producing electricity in areas that have good resources. This is all down to the development and research spent designing and manufacturing them.
The earths wind is developed from the movement of atmospheric air masses as a result of variations in atmospheric pressure, which in turn are the direct result of differences in solar heating across the earth's surface (Boyle, 2004)
The wind energy potential on the Earth has enough, in principle, to meet the worlds energy demand all. This is because every country has virtually enough sites with average wind speeds of more than 5 m/s measured at a height of 10 m (Sesto & Casale, 1998).
(up to about 35- 40% conversion efficiency for existing turbines under design working conditions) (Gipe, 1995) States 'a wind turbine is a device that is able to tap some of the wind power to generate electricity'. And have the potential of 35 - 40% conversion efficiency under design conditions. Wind turbine capacities range from as low as 1kW to 7MW the largest being the Enercon E-126 (AnonS, 2009).
Horizontal-axis wind turbines with propeller-type rotors are those which have to date been most developed technologically and are most widely available commercially (Gipe, 1995)
Manufacturers are continually developing these machines to increase efficiency and further reduce manufacturing costs. Manufacturers are trying to achieve this by means of using direct drive systems, passive control of blade pitch and operation of the rotor at variable speed and through a static power convertor which allows a constant grid-connection to be maintained at peak aerodynamic efficiency as wind speeds vary (Sesto & Casale, 1998).
With regards to vertical-axis wind turbines, only a few models have been developed in series by the industry. Particularly, the Darrieus machines which have curved blades are currently the only design being developed to the greatest extent. These machines do not have to be yawed into the wind direction and their mechanical and electrical parts are more easily accessible compared to a horizontal axis turbine. (Boyle, 2004).
Wind energy development has both positive and negative impacts on the environment but more so positives. The scale of the future implementation will rely on successfully maximising the positives and keeping the negatives to a minimum.
The Wind Energy market is constantly growing and will become a major generator of electricity throughout the world in the future. Within Europe, the off shore exploitation of wind energy will make a major contribution in reducing carbon dioxide emissions from the electricity sector (Boyle, 2004).
2.2.1.2 Hydroelectricity
Like many other renewable energy, hydro power is an indirect use of Solar Power as it is derived from the hydrological climate cycle. It is a well established technology which has been available for over a century. Hydro power harnesses the potential energy within falling water and uses basic mechanics to convert energy into electricity (Kaltschmitt et al., 2007).
The resource for hydroelectricity is not a finite quantity of stored energy but the flow of water. The energy becomes available when rain or snow falls on high ground thus because the natural flow of water is back to the source which has the potential to create energy. So in theory the water vapour within the atmosphere is potentially a huge storage of renewable energy (Boyle, 2004).
Hydro power involves the storage of water in a reservoir, then using the gravitational force of falling or flowing water to turn massive turbines thus generating electricity.
Present day hydro stations range in capacity from a few hundred watts till giga watts. The potential for a hydro station depends merely on the location, the potential water available and money.
Hydroelectricity is well established as one of the principal energy-producing technologies around the world, providing some 20% of the world's electricity. In the developing countries, the proportion rises to around 40% (Boyle, 2004).
They are highly efficient, reliable, and long lasting. They are also very controllable and add an element of storage into an electricity supply system, thereby allowing compensation for variations in electricity demand. However, the dams and their large lakes also have major environmental and social impacts (Twidell & Weir, 2006).
2.2.1.3 Tidal
Tidal energy is basically the result of the interaction of the gravitational pull of the moon and to some extent the Sun. The interaction between the earth, moon and the Sun perturbs these forces and motions so that tides are produced (Charlier, 2003)
Energy Schemes that use Tidal energy have to rely on the twice daily tides and up steam and downstream flows in estuaries.
Tidal barrages are built across suitable estuaries and are designed to extract energy from the rise and fall of the sea level using turbines located in water passages, which turn and create kinetic power which is used to drive a generator. (Boyle, 2004).
There is also the introduction of axial turbines, which generate electricity in a similar way to a wind turbine only under water and use the current of water to turn a rotor rather than wind, which turns a generator creating electricity.
The first ever axial turbine was installed by Marine Current turbines in Strangford Lough in Northern Ireland in April 2008. The turbine called SeaGen is capable of generating 1.2MW of electricity (AnonC, 2009). The SeaGen is made up of two axial flow rotors which drive two generators. What makes this turbine extraordinary is that it can generate electricity on both the ebb and flood tides due to blades that can rotate through 1800 (AnonC, 2009).
Tidal power availability is dependable on the site as not ever location is suitable. The development of tidal energy is somewhat slower than other methods of renewable, but has the potential to advance.
2.2.1.4 Wave
Generating electricity from waves is regarded to be a new and promising source of renewable energy (Twidell & Weir, 2006) (De O Falco, 2010).
Wave Energy is the energy generated through the power of the ocean wave. Like most other renewable technologies it derives its energy from the Sun. The Sun heats up the air and atmosphere and the differential heating results in wind. When wind blows over the ocean, it creates the wave energy therefore it is an indirect form of solar energy (Boyle, 2004).
In order to capture energy from waves it is necessary to capture the wave movement with a structure that moves in the same manner as the waves.
There are several implications for capturing wave energy:
2.2.1.4.1 Shoreline
Shoreline devices are fixed or embedded the actual shoreline itself. This has many advantages of easier maintenance and installation. These devices do not require relativity deep water but with there is a lower energy production. (Leijon et al., 2006)
2.2.1.4.2 Near shore
The main prototype device for moderate water depths is the OSPREY developed by Wavegen in the UK. It is an OWC-type WEC designed for deployment on the seabed. It is designed to operate in 15 m of water within 1 km of the shore, generating up to 2 MW of power for coastal consumers (Boyle, 2004).
2.2.1.4.3 Offshore
This class of device exploits the more powerful wave regimes available in deep water (>40 m depth). More recent designs for offshore devices concentrate on small, modular devices, yielding high power output when deployed in arrays (Leijon et al., 2006)
The main converter devices are fixed devices, Floating devices or tethered devices
The majority of devices tested and planned are Oscillating water Column (OWC). In these devices an air chamber pieces the surface of the water and then the contained air is forced out and then into the chamber by the approaching wave crest and troughs. On it passage to and from the chamber the air passes through an air turbine generator thus producing electricity (Boyle, 2004)
There are also devices which take advantage of both as the kinetic and potential energy of a wave to generate hydraulic pressure, which is then transformed into electricity.
Some Successful devices are The Pelamis developed by Ocean Power Delivery Ltd-OPD, Scotland and The LIMPET OWC developed by Wavegen Ltd in Ireland and the Queen's University of Belfast in the UK (Westwood, 2004).
The utilization of this Wave energy has the potential to for fill a significant part of the energy demand in Europe, and, moreover, it could make a substantial contribution to a wide range of the objectives of environmental, social and economic policies of the European Union.
2.2.2 Passive
2.2.2.1 Bio Fuel
Bio fuel is a viable option to help mitigate greenhouse gas emissions and substitute fossil fuels (Ferreira et al., 2009). It is the general term for energy derived from materials such as wood, straw or animal waste, which were previously living matter but is used within a short space of time rather than decomposing it for centuries with respect to fossil fuel . Such materials can be used as a source of heat or power when burned. (Kaltschmitt et al., 2007).
The energy stored within plants is dissipated through a series of conversions involving chemical and physical processes within the plant, soil and surrounding atmosphere and other living matter until it is eventually radiated away from the earth as low temperature heat (Boyle, 2004). Some of this energy will be radiated away within a year but biomass can accumulate over decades in the wood of trees. Some may accumulate over centuries as peat but a minuscule proportion has become fossil fuels over periods measured in millions of years.
Bio-Fuel has two main energy sources:
2.2.2.1.1 Energy Crops
Energy crop in its widest form include plants and crop that are grown specifically for the use as fuel or for conversion into other bio-fuels. Such sources include wood for burning, plants for fermenting to ethanol and crops that yield oils (Ferreira et al., 2009) and offer high output per hectare with a low input.
2.2.2.1.1.1 Short rotation coppice
The most obvious use of SRC is the provision of heat in the form of biomass. Some fast growing tree species such as poplar and willow can be cut down to a low stump above soil level when they are dormantin winter andgo on to produce many new stems in the spring season (AnonW, 2009). The crop is allowed to grow for between 2 - 4 more years and then re-harvested and can yield between 15-30 tonnes of dry mass per hectare. (Scheer, 2002). Once re harvested this is then processed into wood pellets which the most recognisable form of biomass which has relatively high calorific value about 17MJkg-1 (Heinimo & Junginger, 2009).
This practice is well established in the UK and Europe with companies such as Balcas producing a lot of biomass for Northern Ireland.
2.2.2.1.1.2 Agricultural crops
The most common and widely used crops for bio-liquids are sugars, rape seed oil and maize. Conventional vegetable oils, as well as animal fats, can also be used for energy purposes.
Maize products such as (wheat, maize, barley) are in essence starch, which can be hydrolysed to simple sugars using methods that contain acids or enzymes. This can then be fermented in the same way as sugar crops thus producing Bio-ethanol. This substance can be incorporated into petrol which is widely available throughout the UK. In 2005, about 60% of ethanol production was based on sugar crops and is a well developed market (Heinimo & Junginger, 2009).
2.2.2.1.2 Waste
There is a wide range of biomass materials that are produced as by products, residues or wastes from digestion processes or manufacturing processes. Many of these products contain a valuable energy content that can be exploited.
2.2.2.1.2.1 Agricultural waste
The most common forms of agriculture waste are animal manure and dry residues such as straw and silage.
A useful method of recuperating energy from animal waste is by an anaerobic digester. The process generates a useful gas called biogas by the decomposition of organic matter with the absence of air and produces it an effluent that can be used as fertilizer.
This process can also be done with sewerage sludge and has been done in the UK since the first sewage farm were built in the last century (Boyle, 2004)
Another viable option of extracting energy from waste is incineration. Although the water content within the waste must be low the waste can be used for the direct combustion for conventional power generation. There are several power plants throughout the UK running on animal waste and there is an incinerating plant in Belfast belonging to the NI Water which utilises the waste sludge from the sewage works as a means of generating electricity.
2.2.2.1.2.2 Municipal waste.
Municipal Solid Waste (MSW) is waste collected by or on behalf of a local authority. It comprises mostly household waste and it may include some commercial and industrial wastes.
Historically, the quantity of MSW has risen year on year, presenting a growing problem for local authorities particularly as legislation that limits the amount of mixed MSW that can be sent to landfill (AnonB, 2009)
There are several ways of generating power from municipal waste through incineration and the utilisation of the by product produced from the decomposing of the waste.
Extraction of the useful energy is done by anaerobic digestion thus producing gas as a by product which can be used to produce heat or drive an engine. A wet tonne of MSW has the potential to produce about 8GJ of energy (Gregg, 2010).
Incineration of MSW is an effective way of producing energy but many see it as a environmental hazard, by burning plastics and other wastes can produce harmful gases which destroy the ozone.
The is also the production of methane gas from the decomposing of MSW, as a large portion of MSW contains biological material and because deep landfill sites furnish suitable conditions for anaerobic digestion suitable systems can be fitted to the landfill site to collect the methane gas which can be used to produce energy. Such a site can be the North foreshore landfill site used by the Belfast City Council.
Although this means of generating energy cannot be justified as it takes more waste to produce the energy, than gas produced, for instance a typical landfill site produces between 150-300m3 of gas around 60% volume of methane per tonne of waste, which gives an output of 5-6 GJ per tonne of waste.
Assuming a gas to electricity efficiency of 35% it means the overall energy efficiency of a site is around 10%. (AnonB, 2009)
3 Direct
Direct Solar energy has been used for centuries to dry clothes and agricultural products, among other things (AnonR, 2009). With the growing concerns over climate change and global warming the use of solar energy could help minimise consumption of fossil fuels and reduce carbon footprint. Solar radiation can be converted into useful energy directly, using various technologies.
Direct energy can be either in active or passive form by utilising the sun's energy Active solar technologies, convert solar energy into usable heat or electricity. Active solar can use electrical or mechanical equipment to help take advantage of solar energy.
As for passive, it uses direct sunlight to heat water, air thermal mass and can cause air movement without the need for any mechanical or electrical aid and little use of any other energy source (Hoy-Yen Chan et al., 2010).
Passive
Solar Passive heating and Daylighting
All glazed building to some extent are already passively solar heated. Direct gain is the most common form of solar heating. Solar radiation enters the building through the glass during the day. If the there is a temperature difference between inside and outside out temperatures heat will be conducted, convected and radiated either way. The inner thermal mass of the building can also absorbs the energy thus heating the area or releasing it when there is a temperature differential.
In order to take full advantage of solar heating it is recommended that the building are: well insulated, south facing, avoid over-shading and have a good thermal mass to avoid over heating in the summer.
With the use of solar energy as a heating source, it must not be forgotten that this is also a source of light 'Daylighting'. Buildings are usually designed in a way to help utilise natural light but a lot of people take this for granted due to the availability of cheap electricity (Zain-Ahmed et al., 2002). It is a great source of light but people still use artificial lighting when it's not really necessary. Although in the winter the use of artificial lighting can contribute towards a small heat gain.
Daylighting is a combination of energy conservation and passive solar design. Some useful building designs incorporate techniques such as:
Solar Air heaters
Solar air heaters can also utilise direct sunlight by heating up air. They consist of a flat plate collector that contains an absorber plate, insulation and a transparent cover to allow for radiation. Between the top section and bottom section of this structure there is a channel that the air travels through where is gain heat because of the absorber.
These systems can be based on either natural buoyancy (passive) or can be forced (active) using an aid such as a fan. To maximise efficiency it is necessary to make the absorber plate as large as possible to enable as much heat transfer as possible. They must also be inclined at the most efficient incline to maximise solar radiation and also depends on the location (Kothari et al., 2008).
3.1 Solar Drying
In many countries of the world, the use of solar energy in the agricultural area to conserve vegetables, fruits, coffee and other crops has shown to be practical, economical and been around for many of years (Whitfield, 2002).
Two generic groups of solar-energy dryers can be acknowledged, viz passive or natural-circulation solar-energy dryers and active or forced-convection solar-energy dryers (Ekechukwu & Norton, 1999).
3.1.1 Passive Solar drying
In Passive solar drying, solar-energy radiated from the sun, is used as the sole source of the required heat or can be a supplemental source (Ekechukwu & Norton, 1999). Solar drying involves the extraction of moisture from the desired product by heating the surrounding air and carrying away the released vapour (Whitfield, 2002). The air can be generated by natural forces or a forced convection. The major issue with passive drying is the transfer of heat to the moist product by convection and conduction of the surrounding air.
3.1.2 Active Solar drying
With Active solar drying systems they only depend partially on solar energy which is solely for the heat source (Ekechukwu & Norton, 1999). Active solar drying also employ a secondary source of heating using a pump or fan for the circulation of air so therefore all active solar dryers can be classed as forced convection dryers.
Drying is an excellent way to preserve food and solar food dryers are an appropriate food preservation technology for a sustainable world. The main advantages of sun drying are low capital and operating costs and the fact that little expertise is required (Schirmer et al., 1996).
Active thermal
3.2 Solar Water Heating
Solar water heating is a process that heats water by the use of direct solar energy. They utilise solar radiation to heat the fluid. They are generally composed of solar collectors, storage tank and a network of pipes which contain fluids that help transfer the heat from the sun to the water (Wengenmayr & Buhrke, 2008).
Most solar water heating systems are of panel like construction and are generally fixed to a roof or integrated in the roof structure. These panels should be sited on a south-facing pitched roof or mounted on an angled frame on the ground or on a flat roof assuming it is in the Northern Hemisphere, free from any obstruction and unnecessary shade, at an incline between 20 and 50 degrees (Boyle, 2004).
Closed couple
In a close-couple solar hot water system, heat from the sun that has been captured by the collectors is then transferred directly into an integrated tank or to an adjacent storage tank. Water from the tank can then be used for various applications within a building.
Distribution
In comparison, a distributed solar hot water system is based on several components such as thermal collectors , a storage vessel and a network of pipes. The solar collectors are located on a roof with a water storage tank being located in a suitable location in a building. As the sun heats the fluid within the solar collectors, the heat is then transferred to a water storage tank and later distributed to the demanding outlet.
ICS
An integrated collector storage vessel is manufactured on the principles of thermal buoyancy. The design is based on having an outer absorbing sector and an internal perforated inner liner which is made from materials that have a low thermal mass. When capturing solar energy, thermal buoyancy leads to natural circulation within the storage vessel. When the water adjacent to the exterior surface is heated it rises and passes through the perforated inner skin to where it is stored. This can then be used for domestic hot water (Smyth et al., 2003). For further reading and more detail see (Smyth et al., 2003)
3.2.1.1 Flat Plate
In a flat plate collector, direct solar energy passes through a single or double glazed glass covering and strikes a metal plate. This plate is usually made from highly conductive metal and can be treated with either paint or zinc to help with absorbency. The solar energy is then conducted to a fluid usually water that circulates through tubes in direct contact with the plate. A mixture of water and propylene glycol is often used in cold climates to provide freeze protection (Lubitz et al., 2009).
3.2.1.2 Evacuated Tube
Evacuated tube collectors are constructed from glass usually a Pyrex double wall construction. The glass is treated to help absorb energy on the outside and treated to reflect energy on the inside. Within the collector air is removed to make a vacuum and a cooper heat pipe is located in the centre of the tube containing a liquid so the energy can be transferred to it (AnonE, 2009).
As heat rises, hot vapours from the liquid rise up to the top of the heat pipe and transfers it heat to another liquid which is then pumped around a circuit or to the hot water cylinder which in turn gives hot water. This simple system is completely sealed and needs minimal maintenance over its 20+ year's life (AnonD, 2009).
Concentrated Solar water heater
Concentrated solar waters heaters use a reflective device such as mirrors to help focus and redirect more sunlight onto the collector. The most common concentrator application is the parabolic trough. These are usually applied in conjunction with an evacuated tube system either integrated or alongside the tube and yield a higher temperature thus producing more hot water therefore are more thermally efficient. To take full advantage of these types of systems it may be necessary to have them tracking the Sun.
Absorption cooling
Today, Air conditioning of buildings accounts for a large percentage of the release of greenhouse gases, simply because conventional air con systems use harmful refrigerants which are bad for the atmosphere. With today's new legislation there is now a need to introduce new concepts in building air conditioning systems (Tsoutos et al., 2010).with the solution being, Solar cooling systems (SCS) which use harmless working fluids such as water-lithium bromide (LiBr) and ammonia-water (Florides et al., 2002) and are energy efficient and safe.
Open cycles - desiccant cooling systems is the most commonly applied process today which uses rotating desiccant wheels, equipped either with silica gel or lithium-chloride as sorption material.
The process works when hot and humid air passes through the desiccant wheel with the moisture being removed and the air temperature rises. The hotter drier air produced by this process is reduced to the desired comfort condition; the warm and humid air return from the conditioned space is further heated to meet the required regeneration temperature of the desiccant and this regeneration stream of air is passed through the desiccant wheel to remove the moisture from the desiccant (Tsoutos et al., 2010)[8] G.A. Florides, S.A. Kalogirou, S.A. Tassou and L.C. Wrobel, Modeling and simulation of absorption solar cooling system for Cyprus, Solar Energy 72 (1) (2002), pp. 43-51. Article | PDF (287 K) | View Record in Scopus | Cited By in Scopus (25). The operation of such systems is well documented in (Henning, 2007).
For various reasons, LiBr-H2O system is considered to be better suited for most solar absorption air conditioning applications because of a higher coefficient of performance (Tsoutos et al., 2010).
Application of the cycle described above are limited to temperate climates. Away around this is to employ a combination of sorption wheel and cooling coils as the high supply temperatures of the cooling water to the coils are sufficient for the process (Henning, 2007).
Desalination
The use of solar energy in thermal desalination processes has become an attractive application for renewable energies (Delgado-Torres & Garcia-Rodriguez, 2007). By Using Renewable energy it helps overcome the high running costs and reduces greenhouse emissions. Solar desalination can either be in the form of direct or indirect. Direct being the use of solar energy to produce distillate directly in the solar collector and indirectly being conventional desalination techniques, such as multistage flash desalination (MSF), multiple effect humidification (MEH), reverse osmosis (RO) and combining them along with solar collectors for heat generation. Reverse Osmosis is the most common technology as its the most cost effective.
Reverse osmosis is a pressure-driven cycle that forces the separation of fresh water from other constituents through a semi permeable membrane by concentrating solar energy to high temperatures 400C and above, this energy can be used to create steam which can be used to drive a steam turbine, creating electricity that can be used to power reverse osmosis desalination.
The multiple-effect humidification (MEH) techniques use the same process as the natural water cycle but only on a shorter time frame in which it evaporates and condenses water to separate it from other substances. Its driving force is solar thermal energy which is used to produce water vapour.
Solar desalination has both positive and negative effects as they both require large land areas for large scale plants and productivity can be low. On small scale it can be competitive due to low costs and simplicity (Delgado-Torres & Garcia-Rodriguez, 2007).
Active Electricity
4 Photovoltaic PV
Photovoltaic's(PV) are solid state devices that convert direct solar radiation into electricity without the need of any fuel and have no moving parts and during the operational life span generate no greenhouse gases (Sick & Erge, 1996).
Because of the modular structure of a PV cell it is possible to convert solar energy into electricity over a broad power range from milli-watts to megawatts directly at the place of use by using by a p-n (or p-i-n) semiconductor junction device (Miles et al., 2005).
4.1 History
It is said (Komp, 1995) 'that the discovery of the photovoltaic effect, defined as the direct conversion of light energy into electrical energy, is due to the 19th Century French physicist Edmond Becquerel, back in 1839'.
Becquerel discovered the photovoltaic effect while experimenting with an electrolytic cell made up of two metal electrodes. He discovered that certain materials when exposed to light produce small amounts of electric current (Boyle, 2004).
The first application of photocell technology was developed in the form of light meters in 1877 by W,G Adams and R.E Day.
In 1883 Charles Edgar Fritts produced a Selenium solar cell that was very similar to that of silicon cell today. It was inefficient, converting less than 1 per cent of electricity from the available sunlight.
It wasn't till later years, when Einstein and Planck provided a new insight into the nature of radiation and the fundamental properties of the materials. These findings helped rectify low inefficiencies in PV cells (Boyle, 2004).
It wasn't til 1954,the first solar cell was developed at Bell Laboratories were researchers were studying the effect of light on semi conductors (Boyle, 2004). The photovoltaic effect was first used in space applications, for instance to power satellites (Goetzberger & Hebling, 2000)
It was when the first 'oil shock' occurred in 1973 that the development and research into household and industrial photovoltaic energy systems began (Lesourd, 2001).
Semi-Conductor Structure
When light acts upon a material, it can either be reflected absorbed or transmitted. Absorption of light is the conversion of energy that is contained in incident photon to another form of energy, typically electrical. This takes place within a semiconductor in a very complex process.
Semiconductors are constructed of individual atoms which are bonded together in a periodic configuration to form a structure, in order for the atom to be surrounded by 8 electrons (Nelson, 2003).
Each individual atom contains a nucleus that is made up of a core of protons that are positively charged and neutrons, which are negatively charged and surrounded by electrons (Honsberg & Bowden, 2010).
The number of electrons and protons are equal so that the atom is electrically neutral or balanced. The electrons occupy certain energy levels, based on the number of electrons in the atom which can vary depending on the element that is used.
A semiconductor can be either of a single element, such as silicon (Si), a compound, such as Gallium arsenide (GaAs) or an alloy, such as Silicon Germanium (SixGe(1-x)). Silicon is the most commonly used semiconductor material for the use within solar cells (Nelson, 2003).
Within a semiconductor the type of bond structure will determines the material properties. A key effect is to limit the energy levels to which the electrons can occupy and how they shift about the crystal lattice. The electrons surrounding each atom construct a covalent bond. A covalent bond is when two atoms share a single electron and so that each atom is surrounded by 8 electrons (Honsberg & Bowden, 2010). Because electrons are static and need a form of heat to gain any energy to escape from the bond, they cannot participate in current flow, absorption or any process involved in solar cell process, but when the electrons gain enough energy they are free to move about the crystal lattice and participate in conduction (Honsberg & Bowden, 2010) (Nelson, 2003).
These two states 'bound', and 'free' are created when the electrons don't move and are static (bound) and free when they have the minimum amount of energy to free them. The minimum energy is called the 'band gap'. This basic operation is fundamental to the operation of a solar cell (Honsberg & Bowden, 2010).
The Band gap
The minimum energy level required of a semiconductor is called the valence band and the conduction band is where there is enough energy to make an electron free. The band gap is the distance that is between the conduction band and the valence band(Eg) see below ).
The band gap determines what portion of the solar spectrum the cell absorbs. To maximise efficiency it is necessary to match the spectrum of available light to the band gap material (Nelson, 2003). The movement of an electron to the conduction band results in a hole in the valance band and vice versa. Both the electrons and holes can participate in conduction and can be called carriers (Honsberg & Bowden, 2010).
These carriers are called intrinsic carriers when they are concentrated. A semiconductor material that requires no impurities to change the carrier concentration can be called an intrinsic material. The intrinsic carrier concentration is the amount of electrons and holes available in the conduction band and valence band respectively in an intrinsic material (Honsberg & Bowden, 2010).
The number of carriers depends strongly on the temperature and band gap of the intrinsic material. If the band gap is large then the concentration of the carrier is low resulting in less excited carriers, on the other hand, an increase in temperature will excite the electrons in the conduction band which raises the intrinsic carrier concentration (Honsberg & Bowden, 2010) (Nelson, 2003).
In order to move the balance of electrons and holes within a silicon crystal lattice a process called doping can be applied with the use of other atoms (Krieth & Goswami, 2007).
Atoms that contain one more valance electron than a silicon atom is used to create an "n-type" semiconductor material, which increases the number of electrons because it adds electrons to the conduction band. Then Atoms with one less valence electron result in the creation of a "p-type" material (Honsberg & Bowden, 2010). In this material there are more electrons trapped within the bond thus creating more holes. In any doped material there is more of one type of carrier than the other as this is how the solar effect takes place (Krieth & Goswami, 2007).The p-n junction is the most common semiconductor within the solar cell industry.
Characteristics of PV
The photocurrent developed in a PV cell is highly dependant on the intensity of light incident and the wavelength of the incident light. It can be estimated that terrestrial sunlight is in the spectrum of 5800 k blackbody source. Therefore PV cells are constructed of materials for which conversion to electricity of this spectrum is efficient as possible (Messenger & Ventre, 2000).
The p-n junction is the most widely and commonly used device structure in photovoltaic's, as this enables the photovoltaic effect. Selective doping of the different sides of semiconductor wafer, p type and n type which leads to a potential barrier between the two regions (Krieth & Goswami, 2007). The junction consists of a layer of n type Silicon (Si) joined to a layer of p type silicon (Si) with an uninterrupted structure across the junction. The n layer has free electrons and the p layer having free holes. Because of thermal equilibrium conditions, meaning that only temperature is the only variable that can influence the population of free holes and electrons the following relationship can be derived (Krieth & Goswami, 2007):
np=n2i
Where ni is the approximate density of electrons or holes in intrinsic material. Because both holes and electrons are subject to random diffusion within an Si cell structure, they tend to move from high concentration regions to the lower concentrated regions and because of this, its creates a large concentration gradient across the junction resulting in both holes and electrons diffusing across the junction into the n- region and p- region respectively (Krieth & Goswami, 2007).
Because both sides of the p-n junction are electrically neutral before formation of the junction the free electron on the n-side comes from a neutral electron material such as phosphorous and the free holes on the p side come from a neutral hole material such as boron. When the negatively charged electrons drift across the junction they become positively charged as with when the positively charged holes drift across the junction they become negatively charged (Krieth & Goswami, 2007) (Honsberg & Bowden, 2010). When the electrons and holes diffuse on the p-side and n-side they leave behind positively and negativity charge ions which bound to the Si lattice of each side. The diffusion across the junction creates and electrical field thus creating a current. The silver lines within a cell are called contact fingers and are used to make the electrical contact within the cell. These lines are used to draw of the current created from the electrons (Presad & Snow, 2005).
The amount of current and voltage available from a PV cell is dependant upon illumination levels and temperature levels. The I-V characteristics of an actual PV differ from that of a real cell which can be seen below.
The maximum power output of a PV cell is when 1000 W/m2 is incident upon the cell. Such levels of incident are only available when the cell is directly facing the sun and on a clear day. The power output of a PV cell is directly proportional to the incident solar radiation (Lo, 2010).
The quantity of current produced in the photovoltaic cell can be found using the I-V characteristic equation and in an ideal cell, the total current (I) is equal to the current (Il) generated by the photoelectric effect.
I= Il - Io (e qV/kT -1 ) (Messenger & Ventre, 2000)
Where Il is the component of cell current due to photons,
q= 1.6x10-19 (electronic charge)
k= Boltzmann constant
T= cell temperature in K
A PV cell has both limiting voltage and current so that the cell is not damaged due to operating short circuit or open circuit condition. Short circuit current is the current
obtained from the photovoltaic cell when the load resistance is very low and is calculated when the voltage equals 0 (Messenger & Ventre, 2000).
ISC = Il
The short circuit current (ISC) occurs at the beginning of the forward-bias sweep and is the maximum current value in the power quadrant and can be said that its directly proportional to cell illumination (Messenger & Ventre, 2000).
Whereas Open Circuit Voltage (VOC) occurs when there is no current passing through the cell and is the maximum voltage available from a solar cell (Shenck, 2010). The open-circuit voltage corresponds to the amount of forward bias on the solar cell due to the bias of the solar cell junction with the light-generated current and can be seen on Fig ?. If the cell increases in temperature it reduces the performance due to a decrease in open circuit voltage (Messenger & Ventre, 2000). To determine the Voc of a cell set the net current equal to zero in the solar cell equation to give:
V (at I=0) = VOC
(Voc) = nkT/q ln (Il/Io +1) (Messenger & Ventre, 2000)
In order to obtain as much energy from a cell it is necessary to operate the cell at maximum power. The power produced by the cell in Watts can be easily calculated along the I-V sweep by the equation.
Pmax =Im Vm (Messenger & Ventre, 2000)
There is only one point on the on the I-V curve that the cell produces maximum power, this is dependent on illumination levels. it also possible to calculate the maximum power using the following formula:
=FF ISC. Voc
Where FF is the Fill factor. The Fill Factor is a measure of quality of the solar cell. Cells that have large internal resistances tend to have smaller fill factors and the real or ideal cell will have a fill factor of unity. The FF describes the square ness of the I-V curve although a real cell does not have rectangular characteristics (Nelson, 2003). Typical FF figures vary from 0.5 to 0.83 (Messenger & Ventre, 2000) the fill factor then leads to the cell efficiency which is the power density delivered at operating point as a fraction of the incident light power density.
Efficiency n is related to Isc, Voc using the FF and can be expressed as,
n=Isc Voc FF/ Ps
These four quantities Isc, Voc, FF and n are key performance characteristics of a solar cell (Nelson, 2003) and should be defined for specific illumination levels.
4.2 Types of photovoltaic structures
Different PV technologies have demonstrated substantial improvements and transformations over the past 40 years and are expected to undergo developments in the following decades (Makrides et al, 2010). They can be separated into the following structures:
4.2.1 Mono - Crystalline solar cell
The most commonly used cell material is silicon. Mono crystalline cells are made from pure crystalline silicon. The silicon has a single continuous crystal lattice structure which has basically no defects or impurities.
The advantage of this cell structure is that it has a typical efficiency around 15 percent but because the manufacturing process is complicated and exhibit predictable and uniform behaviour making them the most expensive type of device to produce.
The efficiency of a solar cell can be defined as the percentage of the solar energy falling on its surface that can be converted into electrical energy.
There are several approaches to help reduce this cost by introducing various technologies within mono-crystalline such as EFG (edge defined film-fed growth), string ribbon technique and a method which deposits grown films of crystalline silicon on a low cost substrate. (Presad & Snow, 2005)
A number of approaches to reduce costs of crystalline PV cells and modules have therefore been under development. Techniques for the production of multi-crystalline silicon are simpler and therefore cheaper than those required for mono-crystalline material.
4.2.2 Poly-Crystalline
Poly-crystalline or multi-crystalline are manufactured using ingots of multi-crystalline silicon. During the process molten silicon is cast into ingot and allowed to cool so they form a large crystal. This is then cut into shape and assembled to form the new module. New methods in constructing Poly-crystalline cells use grown films of silicon on a low cost substrate such as quartz glass, stainless steel and metallurgical grade silicon (mg-si) sheet (Presad & Snow, 2005).
The reason behind this is the declining shortage of poly-crystalline and high prices of the silicon thus the need to seek alternatives.
Poly-Crystalline cells are not as efficient as mono-crystalline with efficiencies around 12 percent but are cheaper to produce due to a simpler manufacturing process (Miles et al., 2005).
4.2.3 Amorphous Silicon application
Amorphous Silicon (a-Si) also known as thin film solar cells are cells that are composed of silicon atoms in a thin homogenous layer. It is produced by deposition onto a substrate rather than wafer sawing like the other solar cells, so the cell can be thinner hence the name thin-film (Archer & Hill, 2001).
Amorphous silicon can be deposited on a wide range of substrate both flexible and rigid thus giving them more versatility for various applications. Amorphous cells have a much lower efficiency than crystalline cells around 6 per cent but because they require less material they are cheaper to produce (Presad & Snow, 2005).
Amorphous silicon is by no means the only material suited to thin film technologies. Amongst the many other possible thin film technologies some of the most promising are those based on compound semiconductors in particular:
With the major technical progress particularly for those based on CdTe and CIGS (Miles et al., 2005).
An alternative to the all solid state device is the use of photo-electrochemical cell. Dye - sensitised solar Cell (DSG) is the best considered from of this alternative. This type of technology has been dominated by the Gratzel titanium dioxide (TiO2) cell. With this device the top electrode is made by screen printing a layer of TiO2 onto fluorine doped SnO2 coated glass and a dye applied to the TiO2. The bottom counter electrode is made by screen printing a thin layer of pyrolithic platinum onto fluorine doped SnO2 coated glass (Miles et al., 2005). The surfaces of these devices are very rough to help increase the surface area thus improving light absorption.
Efficiencies of this cell are up and around 10 percent and are very effective over a wide range of sunlight conditions.
Multi-junction PV
A possible way to overcome this is by the use of a multi- junction or concentrators(can be read in more detail in ??). A multi-junction is where more layers or added to a PV cell, usually comprised of two or more thin film PV junctions combined on top of one another and each junction is tailored to absorb a particular portion of the spectrum of incident radiation (Lo, 2010). The two vital features of a multi-junction is that cell structure are a stack of alternating polarity n- and p-type layers, and buried contact fingers that electrically connect all like-polarity layers in parallel (Keevers, 2001),
In order to make the cell more efficient, take for example amorphous silicon, by alloying it with carbon will increase the band gap resulting in a better response to the blue end of the incident spectrum, and by Alloying it with germanium this decreases the band gap resulting in a better response to light at the red end of the spectrum (Lo, 2010). It is also possible to create a triple junction so that it is absorbing more colours of the light spectrum.
Multi-junction PV cells are approaching efficiencies of 38% in the laboratory and over 20% commercially (Lo, 2010).
There are many factors which affect PV cell efficiencies, one being the heat captured within the cell which is absorbed from radiation, also known as heat 'sunk'. Secondly a PV's efficiency is also affected significantly when an inverter's rated capacity is much lower than the PV rated capacity. Another factor is the local climate, PV surface orientation and inclination (Yohanis et al., 2006).
Orientation
In the UK, the optimum roof pitch is at an angle of > 25-30° above horizontal, facing due South (Mondol et al., 2007). Although this angle may not always be achievable on the roof of buildings due to the design and locating of the building. PV panels are affectively self cleaning due to rain and wind, if the inclination is any lower there is the risk of shading from leaves and dirt which can have a serious effect on electrical production. During winter seasons it may be necessary to adjust the inclination of the panel to help maximise concentration due to lower sun levels (Lo, 2010).
ARRAY
When a PV cell is charged by the Sun it can typically produce 0.5 to 1 volt dc, although this may sound a reasonable figure it is too small a current for many applications (Messenger & Ventre, 2000). A solution to this is that solar cells can be connected in series and encapsulated to produce a module to obtain a higher voltage. To increase the current output the cells may be wired in parallel (Komp, 1995).
A module typically contains 28 to 36 cells in series to generate a dc current of 12 V. Cells within a module are integrated with bypass and blocking diodes to prevent the complete loss of power when a single cell fails within the module (Nelson, 2003).
These 12V modules can be used singly or connected in parallel or series to create an array which can be used for larger applications that demand a higher voltage or current output. Series connections result in higher voltages, while parallel connections result in higher currents. It is also necessary to connect bypass diodes across each module within the array to prevent failure, which was stated previously (Krieth & Goswami, 2007).
The array is designed to generate power at a certain power and voltage which is some multiple of 12V under standard illumination (Nelson, 2003). The illumination varies constantly, thus not operating efficiently so a means of storage or grid connection is necessary.
4.3 System configuration
A PV system in essence is composed of three subsystems.
The BOS typically consists of structures for mounting the PV arrays or modules and power-conditioning equipment that adjusts and converts the DC electricity to the proper form and magnitude required by an alternating-current (AC) load. The BOS can also include storage devices, such as batteries etc (AnonCC, 2010)
The two main connection types for PV modules are :
A stand-alone system is useful where there is no connection to the local electricity network or where the mains power supply is prone to interruptions. If the installation is for a home that is not connected to the local electricity supply network, then the electricity supply can be configured to directly charge a bank of batteries. The power flow from storage batteries is direct current (DC) that can be converted to alternating current (AC) mains power by the installation of an inverter. A stand-alone BIPV system inherently requires more accurate system sizing than a grid-connected system (Norton et al., 2010) (AnonL, 2009).
A grid connected system is where the energy produce form the PV modules is put back into the national grid. Most PV systems are grid connected with the conventional electricity supply meeting any shortfall between the PV module and building demand. Any access electricity produce can therefore be sold back to the electricity supply company. The performance of grid connected PV system varies; the weather has an influence on it as does each individual component and the connection of the system to the grid (Mondol et al., 2006)
4.4 Costs
Photovoltaic systems have high initial investments. The cost of a system can vary depending on the actual cost of financing the investment (Singh & Singh, 2010), what possible grants are available and what type of system will be installed as the bigger the system the higher the savings.
With the constant increase in costs of fossil fuels, the production of photovoltaic systems will likely increase and be seen as a main stream power option. Although (Schoen, 2001) states that 'It is generally expected that in the next century photovoltaic's will be able to contribute substantially to the mainstream power production, even though PV now is up to five times more than grid power.'
This is based on a price per watt basis and will typically increase or decrease depending on the wattage of the module. Most modules that serve the grid or of 175 watts or bigger. There are also new modules being introduced into the market that are over 200 watts per module (AnonBB, 2010).
The lowest retail price for a mono-crystalline silicon module is 1.55 per watt and thin film module price is €1.28 per watt. Thin film being based on a 130 watt module. (AnonBB, 2010)
From above it can be seen that the generation of electricity from photovoltaic's is very expensive. The main reason behind the high cost is PV cells are expensive to manufacture. Below the graph show the typical cost of processing PV cells from various ingredients.
This shows that the production of cells alone is extremely high and new methods need to be developed to reduce these costs.
There is no hard fact information to accurately forecast the typical payback period but it is assumed that it can exceed 20 years (McIlveen-Wright, 2010). There are calculations in which help derive a payback period but information on the system is required. There are currently very few grants available to help reduce the cost of PV systems but with the increase in energy costs and new legislation on carbon footprint, this makes PV much more attractable.
A PV system can reduce the buildings Carbon foot print and can offset the carbon released during its manufacture within 2-5 years in European conditions. Beyond thisa PV system would be expected to be offsetting emissions made by other energy users (AnonI, 2009).
Another Key factor in reducing costs and making PV systems much more appealing is integrating them with Concentrators or by the Integration of PV systems within a Building. This can read in more detail in section ?? .
PV concentrators
Themost obvious reason for using PV concentrators is to reduce the amount of solar cell material used within a PV system and to increase the cells efficiency or increasing the power output. Increases in efficiency depend largely on the cell design and the cell material used (Lo, 2010). PV cells are the most expensive components of a PV system. A concentrator utilises moderately inexpensive materials such as plastic lenses and a structural frame to capture the solar energy shining on a large area and focusing that energy onto a small aperture area, the PV cell (Avlarez & Zarza, 2007). The optimum Concentrator should have a relatively large, efficient optical surface. One measure of the effectiveness of this approach is the concentration ratio which is how much solar concentration the cell is receiving (AnonDD, 2005).
If concentration ratios are to high this can cause significant problem due to heat, because the operating temperature of cells increases when excess radiation is concentrated, thus generating excess heat. Cells do not operate efficiently under high temperatures thus efficiencies decrease as temperatures increase. This can also affect the long-term stability of PV cells (Krieth & Goswami, 2007) (Sangani & Solank, 2007) (Lo, 2010).
Standard test conditions (STC) for PV modules are;
These are condition at which the PV module produces its specific power output. Although if conditions were to change for instance ambient wind conditions and temperature and reduce the cell temperature below 25 0C the module may produce more power than determined under STC (Lo, 2010).
Concentrators can be categorised in the following manner and types depending on the design scenario.
they can also be broke down further depending on the output of the PV system or concentration required.
Low concentration PV are systems are systems that mainly use conventional or modified silicon solar cells use a solar concentration factor between of 2-100 suns. These systems do not usually need to be actively cooled and are not really suited for high power requirements. This type of system is more commonly fixed and does not track the sun (Sinke, 2007).
Whereas Medium concentration PV has concentration factor between of 100 to 300 suns, it also requires a two-axes solar tracking system which will also need some form of active or passing cooling to dissipate the excess heat generated (Sinke, 2007).
High concentration PV systems use concentrating optics such as a dish reflector or Fresnel lenses which can concentrate sunlight to intensities of 300 suns plus. These types of systems are of high cost and require high-capacity heat sinks to help overcome thermal destruction due to high temperatures (Sinke, 2007).
The artificial increase in the solar intensity incident apon a solar cells using either lenses or mirrors can generate equivalent power with a result of lower production cost. There are two main types of concentration optics which are mirrors or the use of Fresnel lens (Ryu et al., 2006)
The main types of concentration configuration employ either:
Fresnel lens
Fresnel lenses are more so often used in optical systems that require large area, small mass and low image quality. They are a 2D concentrating system in which the incoming solar radiation is concentrated onto a focal point by 1 axis tracking mirrors This fact initiated the study of using Fresnel lens within the solar photovoltaic industry (Ryu et al., 2006). In a conventional Fresnel lens, the facet is shaped circularly which helps refract a collimated beam to a specific point or to an area i.e. a PV cell. A basic analyses of a Fresnel lens is that there is hundreds of concentric rings in one complete lens. It is designed to focus the light to a specific area. Each ring is slightly thinner in size than the next ring in the lens. Each ring is then modified so that it's flat on one side and the same thickness as each concentric rings. To retain the rings' ability to focus the light toward the center, the angle of each ring's angled face will be different (AnonAA, 2009).
Because of the nature of the optical property of the Fresnel lenses, the illumination can be affected strongly by the position of the absorber plane.
Because of this irregularity it can prevent maximum power being extracted from the PV cell. It has also been discovered that non-uniform illumination produces significant local heating and ohmic drops in concentration solar cells (Ryu et al., 2006). To help prevent the cells from overheating there is a finned heat sink integrated into the concentrating device located behind the cell to help dissipate the excess heat. When concentration is combined with multi-junction cells, efficiencies of up to 30% can be achieved (Lo, 2010).
CPC
The principle of a reflecting parabola is that all rays of light parallel to its axis are reflected to a point.Simply a parabolic trough is a linear translation of a two-dimensional parabolic reflector where, the focal point is a receiver that absorbs the illumination and converts it into electrical energy (Stine & Geyer, 2001). These two basic principles, reflection and absorption are the main function of a CPC.
compound parabolic concentrators are derived from the fact that the CPC consists of two parabolic mirror segments that have different focal points.
It can be seen that the focal point for parabola A lies on parabola B where as B lies on A. the parabolic surfaces are symmetrical with respect to reflection through th
