The European Union (EU) has already tuned its energy policy into achieving maximum carbon dioxide (CO2) emissions reduction from power generation plants. In this context, it has already set out a strategic objective of achieving at least a 20% reduction of greenhouse gases by 2020 compared to 1990 levels. This strategic objective represents the core of the new European energy policy. Recognizing the positive effects of renewable energy sources (RES) technologies towards achieving this goal, the EU has taken a range of specific actions in the direction of enhancing the integration of RES in the existing European power generation system as a major step towards the reduction of global warming and climate change phenomena. Specifically, an action plan in the form of an EU Directive (2009/28/EC) on the promotion of the use of energy from renewable sources has been introduced by the EU whereby a target of renewable energy share of 20% out of the gross final energy consumption of the EU has been set to be reached by the year 2020. The RES Directive sets out specific national targets to be achieved by each individual Member State, regarding the share of RES generated in each Member State by the year 2020. For Cyprus, the national target states that the share of energy produced from RES must be at least 13% out of the gross national final consumption of energy in 2020.
In light of the above, the Cyprus Government has launched a number of financial measures in the form of governmental grants and/or subsidies. These financial measures are realized as RES Grant Schemes prepared by the Ministry of Energy, Commerce, Industry and Tourism which aims to provide, among others, support and incentives for the promotion of RES-E utilization in Cyprus. The main types of RES technologies which are promoted under these measures for integration in the Cyprus power system are the following:
Solar energy
Wind energy
Biomass
1. Solar energy
Solar energy is the energy force that sustains life on earth for all plants, animals and people. The earth receives this energy from the sun in the form of electromagnetic waves, which the sun continually emits into space. The earth can be seen as a huge solar energy collector receiving large quantities of this energy which takes various forms, such as direct sunlight, heated air masses causing wind, and evaporation of the oceans resulting as rain which can form rivers. This solar potential can be trapped directly as solar energy (concentrated solar power and/or photovoltaics) and indirectly as wind, biomass and hydroelectric energy.
The solar energy industry is divided into mainly two markets, the photovoltaic (PV) market and the concentrated solar power (CSP) market. The CSP technology uses the heat radiated from the sun, for purposes such as heating water or power generation. On the other hand PV solar cells use the properties of particular semiconducting materials to convert sunlight energy to electricity. The PV industry is far larger than the CSP market.
1.1 Photovoltaics
Electricity can be produced from Global Horizontal Irradiance through a process called “photovoltaics”, which can be applied, in either a centralized or decentralized way. “The term describes a solid-state electronic cell that produces direct current (DC) electrical energy from the radiant energy of the sun. The basic steps from the photovoltaic (PV) solar cell to a fully operating PV system are presented in Figure 1. PV solar cells are made of semi-conducting material, most commonly silicon, coated with special additives. When light strikes the cell, electrons are knocked and become loose from the silicon atoms and flow in an in-built circuit producing electricity. Individual solar cells can be connected in series and in parallel to obtain desired voltages and currents. These groups of cells are packaged into standard modules that protect the cells from the environment. PV modules are extremely reliable since they are solid state and there are no moving parts
Figure 1: Basic steps from PV cell to PV system
PV systems are made up of a variety of components, which aside from the modules, may include conductors, fuses, batteries, inverters, etc. Components will vary, however, depending on the application. PV systems are modular by nature, meaning that systems can be expanded and components easily repaired or replaced if needed. PV systems are cost effective for many remote power applications, as well as for small stand-alone power applications in proximity to the existing electricity grid. There are two main PV technologies:
The technology used to make most of the crystalline silicon solar cells, fabricated so far, borrows heavily from the microelectronics industry and is known as silicon wafer technology. The silicon source material is extracted from quartz, although sand would also be a suitable material. The silicon is then refined to very high purity and melted. From the melt, a large cylindrical single crystal is drawn. The crystal, or “ingot”, is then sliced into circular wafers, less than 0.5mm thick, like slicing bread from a loaf.
The first step in processing a wafer into a cell is to etch the wafer surface with chemicals to remove damage from the slicing step. The surface of crystalline wafers is then etched again using a chemical that etches at different rates in different directions through the silicon crystal. This leaves features on the surface, with the silicon structure that remains determined by crystal directions that etch very slowly. The p-n junction is then formed. The impurity required to give p-type properties (usually boron) is introduced during crystal growth, so it is already in the wafer. The n-type impurity (usually phosphorus) is now allowed to seep into the wafer surface by heating the wafer in the presence of a phosphorus source. There are two types of crystalline silicon solar cells that are used in the industry:
The monocrystalline silicon cell is made using cells saw-cut from a single cylindrical crystal of silicon. The main advantage of the monocrystalline silicon cells is the high efficiency which is around 15%. The multicrystalline silicon cell is made by sawing a cast block of silicon first into bars and then into wafers. Multicrystalline cells are cheaper to manufacture than monocrystalline ones due to the simpler manufacturing process. However they are slightly less efficient than the monocrystalline with average efficiency of approximately 12%.
Another alternative technology for the manufacturing of PV solar cells using cheaper materials is the thin-film solar cells technology. In the thin-film technology approach, thin layers of semiconductor material are deposited onto a supporting substrate, or superstrate, such as a large sheet of glass. Typically, less than a micron (μm) thickness of semiconductor material is required, 100-1000 times less than the thickness of silicon wafer. Reduced material use with associated reduced costs is a key advantage. Another advantage is that the unit of production, instead of being a relatively small silicon wafer, becomes much larger, for example, as large as a conveniently handled sheet of glass might be. This reduces manufacturing costs. At present, solar cells made from different thin-film technologies are either available commercially, or close to being so, such as,
Cadmium telluride (CdTe),
Copper indium diselenide (CIS),
Amorphous silicon (a-Si),
Thin-film silicon (thin film-Si).
Thin-film panels have several important drawbacks. What they gain in cost savings and flexibility they lose in efficiency resulting in the lowest efficiency of any current PV technology at approximately 6-7%. The main interest in these technologies rises from the fact that they can be manufactured by relatively inexpensive industrial processes, in comparison to crystalline silicon technologies yet they offer typically higher module efficiency than amorphous silicon.
The primary article of commerce in the PV market is the PV module. PV modules are integrated into systems designed for specific applications. The components added to the module constitute the “balance of system” or BOS. Balance of system components can be classified into four categories:
Batteries which store electricity to provide energy on demand at night or on overcast days,
Controllers which can manage the energy storage to the battery and deliver power to the load,
Inverters which are required to convert the DC power produced by the PV module into alternate current (AC) power,
Structure which is required to mount or install the PV modules and other components.
Not all systems will require all these components. For example in systems where no AC load is present an inverter is not required. For grid connected systems, the utility grid acts as the storage medium and batteries are not required. Some systems also require other components which are not strictly related to PVs. Some stand alone systems, for example, include a fossil fuel generator that provides electricity when the batteries become depleted; and water pumping systems require a DC or AC pump.
The use of PV systems provides a number of key benefits:
Energy security: Solar energy provides reliable access to energy where it is used. It can also supplement energy needs during blackouts and disaster recovery for electricity, water pumping and hot water,
Energy independence: Solar energy can be used to reduce our independence on fossil fuels imported from foreign countries,
Eco-friendly: Solar energy is a non polluting source of energy. The significant adaptation to PV electricity could further reduce CO2 emissions in the environment,
Economic benefits: When installed properly in homes, businesses it can begin to save money immediately,
Job creation: New jobs are created in manufacturing, distribution, and also many building related jobs for electricians, plumbers, roofers, designers and engineers.
1.2 Concentrated solar power
Concentrated solar power (CSP) technologies are suitable in areas with high solar irradiance. Contrary to PV technology, which utilizes Global Horizontal Irradiance, they utilize only Direct Normal Irradiance (DNI). CSP plants offer a noticeable advantage when compared to PVs, as large amounts of thermal energy can be stored easily with minimal losses and thus they can provide energy on demand (day and night). In this manner, CSP plants are capable of both reliably producing large quantities of power and, if they are part of a network with other renewable energies, compensating fluctuations of wind and PV energy. Hence they also contribute towards stabilizing the electricity grid. As a result, CSP plants allow greater use of fluctuating RES within the electricity mix.
The CSP technologies are:
1.2.1 Parabolic trough technology
A parabolic trough, which is the most commonly used CSP technology, is a long, trough-shaped reflector with a parabolic cross-section, as indicated in Figure 2. As a result of this cross-section, sunlight reflected within the trough is focused along a line running the length of the trough. In order to collect this heat, a pipe is positioned along the length of the trough at its focus and a heat collection fluid is pumped through it. The tube (or receiver) is designed to be able to absorb most of the energy focused onto it and must be able to withstand the resultant high temperature. Typical receivers for this purpose are made of steel tubing with a black coating and surrounded by a protective glass cover with the space between the two evacuated to reduce heat loss. An anti-reflective coating may be added to the outer glass surface to increase efficiency further.
Figure 2. Principle of operation of parabolic trough system
The solar array of a parabolic trough power plant consists of several parallel rows of parabolic reflectors. The heat collecting fluid which is pumped through the pipes along the length of each solar trough is typically synthetic oil, similar to engine oil, capable of operating at high temperature. During operation it is likely to reach between 300ºC and 400ºC. After circulating through the receivers the oil is passed through a heat exchanger where the heat it contains is extracted to raise steam in a separate sealed system and the steam is then used to drive a steam turbine generator to produce electricity. The heat collecting fluid is then cycled back through the solar collector field to collect more heat. Parabolic solar troughs are usually aligned with their long axes north south and they are mounted on supports that allow them to track the sun from east to west across the sky.
1.2.2 Solar tower technology
Solar towers (often called solar central receiver power plants) offer an alternative method of exploiting the energy from the sun in a solar thermal power plant. In this case the collector field consists of an array of heliostats (mirrors) at the centre of which is a tower, as illustrated in Figure 3. At the top of the tower is a receiver designed to collect the heat from the sun. In operation each heliostat has an individual two axis tracking system and all are aligned so that the sunlight striking them is directed onto the receiver atop the central tower. As the sun moves across the sky, each mirror must be moved too if high collection efficiency is to be maintained. The receiver itself is designed to absorb the energy from the sunlight incident upon it and transfer it to a heat transfer fluid. Depending on system design, this heat transfer may be water, molten salt or air. Solar towers are normally designed with energy storage capability.
Figure 3. Principle of operation of solar tower system
1.2.3 Linear Fresnel technology
Effectively, linear Fresnel reflector single axis tracking technology follows the principles of parabolic trough technology, but replaces the curved mirrors with long parallel lines of thin, shallow curvature (or even flat) mirrors or reflectors. These mirrors track the sunlight and are arranged in such way so that the direct solar irradiation is reflected and concentrated onto a stationary, single linear receiver (or absorber tube) located at a common focal point of the reflectors, several meters directly above them. The mirrors are capable of concentrating the sun’s energy to approximately 30 times its normal intensity. On top of the receiver, a small parabolic mirror can be attached (called secondary reflector) for further focusing the light, as shown in Figure 4. The receiver contains an absorber pipe where typically water is converted to steam in a process called direct steam generation, to drive a turbine to produce electricity. Typical steam conditions that can be generated by this technology at the steam turbine inlet of a Fresnel CSP plant are 270°C at 55bar. Optical efficiency of a typical linear Fresnel reflector system can reach 70%, and it is, however, still inferior to that of a parabolic trough system which is in the range of 75 – 80%.
Figure 4. Principle of operation of linear Fresnel reflector solar system
A key component that makes all linear Fresnel reflector systems more advantageous than traditional parabolic trough mirror systems is the use of Fresnel reflectors. These reflectors make use of the Fresnel lens effect, which allows for a concentrating mirror with a large aperture and short focal length while simultaneously reducing the volume of material required for the reflector. This greatly reduces the system's cost since curved glass parabolic reflectors are typically very expensive. Essentially, linear Fresnel technology systems aim to offer lower overall investment and operational costs, compared to parabolic trough technology.
A fundamental disadvantage of typical linear Fresnel reflector systems, apart from the lower plant efficiency, is that the receiver row is shared among several rows of mirrors. This can lead to the increased effect of shading of incoming solar radiation and blocking of reflected solar radiation by adjacent reflectors. The use of Compact Linear Fresnel Reflector (CLFR) technology, where the inclination of mirrors is alternated to focus solar energy on multiple linear reflectors, has been able to alternate this advantage and theoretically improve system efficiency.
1.2.4 Solar dish technology
Solar dish technology is the oldest of the solar technologies with a number of installations and operation of demonstrated solar dish systems between mid 80s and mid 90s. Solar dish systems high efficiency, high power densities, modularity, versatility, hybrid operation and their potential for long term low maintenance operation, continues to interest developers and investors in investing in solar dish technology and thus reducing their capital cost.
Solar dish systems convert the thermal energy of solar radiation to mechanical energy and then to electrical energy similar to the way that conventional power plants convert thermal energy from the combustion of a fossil fuel to electricity. Solar dish systems use a mirror array, in the shape of a parabolic dish, to reflect and concentrate incoming direct solar irradiation to a receiver, in order to achieve the temperatures required to efficiently convert heat to work, as illustrated in Figure 5. In order to achieve maximum efficiency, these concentrators are mounted on a structure with two axis tracking system, so that the dish can track the sun. The concentrated solar irradiation is absorbed by the receiver and transferred to an engine. The engine (usually Stirling engine) converts the heat to mechanical power by compressing the working fluid and then expanding the fluid through a turbine or with a piston to produce work. The engine is coupled to an electric generator to convert the mechanical power to electric power.
Figure 5. Solar dish system
Solar dish systems have demonstrated the highest solar to electric conversion net efficiency. Therefore, solar dish systems have the potential to become one of the least cost solar thermal technologies. The modularity of these systems allows them to be deployed individually for remote applications, or by groups for distributed generation applications. These systems can also be hybridized with a fossil fuel to provide dispatchable power, a technology which is in the engineering development stage.
2. Wind energy
Winds are caused by the rotation of the earth and heating of the atmosphere by the sun. Due to the heating of the air at equatorial regions, the air becomes lighter and starts to rise, and at the poles the cold air starts sinking. The power in the wind is proportional to the cube of the wind speed. It is, therefore, essential to have detailed knowledge of the wind and its characteristics if the performance of wind turbines is to be estimated accurately.
Wind energy converts the power available in moving air into electricity. Wind turbines turn in by the moving air and drive an electric generator. The generator then supplies the electric current. Wind energy is renewable and environmentally benign. Wind-driven electric generators could be utilized as an independent power source and for purposes of augmenting the electricity supply from grids. Wind energy potential increases very rapidly with increasing wind speed. The annual wind speed at a location is useful as an initial indicator of the value of the wind resource.
Large, modern wind turbines operate together in wind farms to produce electricity. Small turbines are used by homeowners and farmers to help meet localized energy needs. Wind turbines capture energy by using propeller-like blades that are mounted on a rotor. These blades are placed on top of high towers, in order to take advantage of the stronger winds at 30 meters or more above the ground. This is illustrated in Figure 6. The wind causes the propellers to turn, which then turn the attached shaft to generate electricity. Wind can be used as a stand-alone source of energy or in conjunction with other renewable energy systems.
Figure 6. The wind turbine technology
There are many onshore wind farms around the world. Offshore wind farms in coastal waters are being developed because winds are often stronger blowing across the sea. It is important to mention that more than 83% of the world-wide wind capacity is installed in only five countries: Germany, USA, Denmark, India and Spain. Hence, most of the wind energy knowledge is based in these countries. The use of wind energy technology, however, is fast spreading to other areas in the world.
Wind turbines reach the highest efficiency at the designed wind speed, which is usually between 12m/s to 16m/s. At this wind speed, the power output reaches the rated capacity. Above this wind speed, the power output of the rotor must be limited to keep the power output close to the rated capacity and thereby reduce the driving forces on the individual rotor blade as well as the load on the whole wind turbine structure. Three options for the power output control are currently used:
Stall regulation,
Pitch regulation and
Active stall regulation.
The wind energy advantages can be summarized as:
No fuel is used,
No wastes or greenhouse gases are produced,
The land beneath can usually still be used for farming,
Can supply energy to remote areas and
Maintenance requirements are minimal.
The wind energy disadvantages can be summarized as:
The wind is not always predictable (e.g., some days have no wind),
Some people feel that covering the landscape with these towers is unsightly,
Can kill birds since migrating flocks tend to like strong winds,
Can be noisy since wind generators have a reputation for making a constant, low, "swooshing" noise day and night and
Suitable areas for wind farms are often near the coast, where land is expensive.
3. Biomass
Biomass covers a wide range of products, by-products and waste streams from forestry and agriculture (including animal husbandry) as well as municipal and industrial waste streams. Biomass thus includes trees, arable crops, algae and other plants, agricultural and forest residues, effluents, sewage sludge, manures, industrial by-products and the organic fraction of municipal solid waste.
There are three ways to use biomass as shown in Figure 7. It can be burned to produce heat and electricity; changed to gas-like fuels, such as methane, hydrogen, and carbon monoxide; or changed to a liquid fuel. Liquid fuels, also called bio-fuels, include mainly two forms of alcohol: ethanol and methanol. The two most common bio-fuels are ethanol and bio-diesel. The most commonly used bio-fuel is ethanol, which is produced from sugarcane, corn, and other grains. A blend of gasoline and ethanol is already used in cities with high air pollution. However, ethanol made from biomass is currently more expensive than gasoline on a gallon-for-gallon basis. Ethanol is mostly used as a fuel additive or oxygenate to enhance the octane and to cut down a vehicle’s carbon monoxide and other smog-causing emissions. So, it is very important for scientists to find less expensive ways to produce ethanol from other biomass crops.
Figure 7. Biomass conversion pathways
Bio-diesel can be used as a diesel additive to reduce vehicle emissions or in its pure form to fuel a vehicle. Concerns about the depletion of diesel fuel reserves and the pollution caused by continuously increasing energy demands make bio-diesel an attractive alternative motor fuel for compression ignition engines. Heat is used to convert biomass into a fuel oil, which is then burned like petroleum to generate electricity. Biomass can also be burned directly to produce steam for electricity production or manufacturing processes. In industrialized countries, the main biomass processes utilized in the future are expected to be the direct combustion of residues and wastes for electricity generation. The future of biomass electricity generation lies in biomass integrated gasification/gas turbine technology, which offers high-energy conversion efficiencies. The electricity is produced by the direct combustion of biomass; advanced gasification and pyrolysis technologies are almost ready for commercial-scale use. Biomass power plants use technology that is very similar to that used in coal-fired power plants.
The biomass advantages can be summarized as:
Waste materials are used,
The fuel, in some cases, tends to be cheap and
Less demand on the Earth's resources.
The biomass disadvantages can be summarized as:
Collecting the waste in sufficient quantities can be difficult,
Fuel is burned, so greenhouse gases are emitted and
Some waste materials are not available all year round.
