Post industrial revolution, there has been an increased focus towards utilization of non-renewable sources of energy that consist of fossil fuels like coal, oil and natural gas. However, in recent times, the problem of these depleting resources has led to the search for sustainable energy sources. Moreover, the serious environmental consequences have raised concern regarding their usage. This has resulted in an increased interest in solar energy along with other solar powered resources like wind, wave, hydroelectric and biomass energies.
The significance of solar energy cannot be stressed enough. In ancient times, the sun was an object of worship which only gives a small idea about its importance. The sun is 150 million kilometers away from the earth, and the energy that reaches the earth is, on average, 1.2 x 1017 Watts. This implies that in one hour, enough energy is supplied to the earth's surface to meet the energy needs of the human population for a whole year. [1]
Chapter 2 - Benefits and Drawbacks
2.1 Advantages [2]
The benefits of solar energy are numerous and far-reaching. There are various economic and environmental advantages.
The primary advantage is that there is an inexhaustible supply of the fuel i.e. the sun. The sun is a renewable source of energy and thus the fuel does not cost anything.
Solar energy does not create environmental pollution and it also does not contribute to detrimental environmental effects like global warming, greenhouse effect, acid rain and so on.
Solar cells produce negligible noise as compared to conventional methods and machines that are very noisy.
The cells require little or no maintenance, usually only once or twice a year.
They are easy to install with wires and cords not being required.
Solar power is useful for power generation in remote locations.
Solar energy can be useful for a large variety of devices: from homes to cars and even satellites.
2.2 Disadvantages [2]
Some of the shortcomings of solar energy are stated below.
The initial investment in solar powered panels is relatively high. However, the long term benefits certainly outweigh this cost.
Scores of solar panels are required to produce the desired amount of electricity. Since several solar panels are required, this requires a large area for the installation.
Solar power cannot be collected at night or during cloudy conditions. This setback can be overcome by using rechargeable batteries to store energy.
The power output of the solar cells depends on the climactic conditions of the geographical area of installation. Sometimes an unreliable or less sunny climate can reduce the power output considerably.
Chapter 3 - Universal Applications
The applications of solar energy may be divided into three broad categories. [3]
Thermal Applications
Solar Electric Applications
Other Applications
3.1 Direct Thermal Applications
These applications result from the direct utilization of heat from the absorption of solar radiation. They incorporate
Hot Water Supply Systems
Solar Heating of Buildings
Solar Cooling of Buildings
Heat for Agricultural and Industrial Processes
Solar Distillation
Solar Pumping
Solar Furnace
Solar Cooking
Solar Green Houses
3.2 Solar Electric Applications
The applications comprise methods in which solar energy is directly or indirectly converted into electrical energy. They include
Solar Thermal Electric Conversion
Photovoltaic Electric Conversion
3.3 Other Applications
These consist of different forms of solar energy.
Wind Energy
Ocean Thermal Energy Conversion
Biomass and Bio-gas Energy
Water Power
Solar Production of Hydrogen
3.4 Solar Photovoltaic Systems
There are numerous and wide range of applications of PV systems. Since there are no costs of fuel, and little or no maintenance is required, solar photovoltaic systems are one of the best alternatives for electricity supply.
3.4.1 Rural Electrification
Lighting and power supplies for remote buildings (farms and schools).
Power supplies for remote villages.
Battery charging stations.
Portable power for nomads.
3.4.2 Water Pumping and Treatment Systems
Pumping for drinking water.
Pumping for irrigation.
De-watering and drainage.
Ice production.
Saltwater desalination systems.
Water purification.
Water circulation in fish farms.
3.4.3 Health Care Systems
Lighting in rural clinics.
UHF transceivers between health centres.
Vaccine refrigeration.
Ice pack freezing for vaccine carriers.
Sterilizers.
Blood storage refrigerators.
3.4.4 Communications
Radio repeaters.
Remote TV and radio receivers.
Remote weather measuring.
Mobile radios.
Rural telephone kiosks.
Data acquisition and transmission (river levels, seismographs).
Emergency telephones.
3.4.5 Agriculture
Livestock watering.
Irrigation pumping
Electrical livestock fencing
Stock tank ice prevention
3.4.6 Grid Connected Applications
Distributed applications in buildings.
PV power stations.
3.4.7 Transport Aids
Road sign lighting.
Railway crossings and signals.
Hazard and warning lights.
Navigation buoys.
Fog horns.
Runway lights.
Terrain avoidance lights.
Road markers.
3.4.8 Security Systems
Security lighting.
Remote alarm system.
3.4.9 Corrosion Protection Systems
Cathodic protection for bridges.
Pipeline protection.
Well-head protection.
Lock gate protection.
Steel structure protection.
3.4.10 Miscellaneous Applications
Ventilation systems.
Camper and recreational vehicle power.
Calculators.
Automated feeding systems on fish farms.
Solar water heater circulation pumps.
Path lights.
Yacht/Boat power.
Vehicle battery trickle chargers.
Earthquake monitoring systems.
Battery charging.
Fountains.
Emergency power for disaster relief.
Aeration systems in stagnant lakes.
3.4.11 Income Generation
Battery charging stations.
TV and video pay stations.
Village industry power.
Refrigeration services.
3.4.12 Electric Power for Satellites
Telecommunications.
Earth observation.
Scientific missions.
Large space stations.
Chapter 4 - Applications in the U.A.E.
There are a number of organizations within the U.A.E. that deal with the design, installation, commissioning and maintenance of solar powered systems. Most of them are in the sales business whereas some deal with production as well.
4.1 Enviromena Power Systems
Enviromena is one of the largest solar power based companies operating in the Middle East and North Africa (MENA) region. [4] The following are its projects in the country.
4.1.1 Solar Power Plant
Figure 1 displays a grid - connected solar power plant with a capacity of 10 MW built at Masdar City, Abu Dhabi to power the activities of the city. It is in line with the plans of Masdar City to be the world's first carbon neutral, zero waste city. It consists of a combination of First Solar thin film and Suntech polycrystalline 87,777 thin film and polycrystalline silicon modules that produce 17,500 MWh / year and have a life span of 25 years. SMA SC 560 inverters are used here.
Figure 1 - 10 MW Solar Power Plant at Masdar City, Abu Dhabi
4.1.2 PV Parking Shade
This structure has a capacity of 204 kW. The number of Sunpower 315 Wp solar modules utilized in the design are 650 with 343 MWh of energy produced per year. The inverters used are SMA SMC 7000 HV. This system has the capability to serve a twin function. It not only provides shades to cars, it also produces electricity. It also has an attractive feature of collecting the water on the rooftops and using it to irrigate the plants underneath the installed system. Figure 2 illustrates the photovoltaic parking shade system.
Figure 2 - PV Parking Shade installed at Masdar City, Abu Dhabi
4.1.3 Building Integrated PV System
This system is installed at Shams Tower at the Yas Marina Circuit, Yas Island, Abu Dhabi. It comprises an array of 2500 m2 size. There are 1,120 Suntech polycrystalline silicon modules installed that have a power capacity of 291 kW and produce 450 MWh of energy per year. The inverters used are SMA SMC. Salient features of the system include providing shade to the cars parked and also connection to the grid to use all produced power. Figure 3 displays this system.
Figure 3 - Building Integrated PV System at Shams Tower, Yas Island
4.2 Apex Power Concepts
Apex Power Concepts is one of the leading solar energy solutions organizations in the MENA region. [5] They undertake projects from simple small scale home systems to much more complex large scale off - grid and on - grid systems. Most of its major projects are listed below.
4.2.1 Solar Semi On - Grid System
This system is in place at the top of Abu Dhabi Oil Company (ADOC)'s roof. The IT department of the company is powered by the energy generated through the solar modules. There are no batteries required and the additional power is taken from the grid. Figure 4 shows the semi on - grid system. The components used were as follows.
36 Sharp solar modules of 175 W (NU series).
1 SMA : SMC 6000 Inverter.
Apex Control Gear.
Figure 4 - Solar Semi On - Grid System installed on ADOC's roof, Abu Dhabi
4.2.2 Solar Street Lights
The lights are installed at the premises of Emirates Techno - Casting Company in Hamriya Free Zone, Sharjah. They consist of 120 W solar modules. Figure 5 indicates one of these street lights. The components used were as follows.
1 Sharp polycrystalline solar module of 125 W.
1 Osram SOX 55 W bulb.
1 Philips Driver for bulb.
1 Steca solar charge controller (10 A).
1 Maxima 12 V, 100 A battery.
Figure 5 - Solar Street Lighting at Emirates Techno - Casting Company, Sharjah
4.2.3 Solar Powered Remote Meter
This remote meter is set up for Abu Dhabi Oil & Gas (ADGAS) in Abu Dhabi, where it is used to examine the gas flow in a pipeline. It is powered by solar energy. Figure 6 specifies this meter. The components used were as follows.
4 Kaneka p-type solar modules of 55 W.
1 Steca solar charge controller.
2 BAE USA 140 A batteries.
1 Steca AJ inverter.
Figure 6 - Solar Powered Remote Meter for ADGAS, Abu Dhabi
4.2.4 Solar Water Pump
This pumping system is installed at the campus of the College of Food and Agriculture at UAE University, Al - Ain. In this setup, solar energy is used to pump water from under the ground to a tank situated at ground (or elevated) level. This water is then used for irrigation of plants in the greenhouse. Figure 7 depicts this water pumping system. The components used were as follows.
10 Sharp solar modules of 175 W.
1 Grundfos solar water pump.
1 Grundfos switch for water pump.
Figure 7 - Solar Water Pumping System at UAE University, Al - Ain
4.2.5 Solar and Wind Hybrid System
This is one of the most interesting applications of solar as well as wind energy installed in Ajman. In this system, solar power through modules and wind energy through a wind turbine are utilized together to charge a battery that is then used to drive the load. In case the battery charge level goes down, due to cloudy conditions or less wind, a standby generator is used to meet the load demand. The load regulation and generator switching on and off timing is controlled through relays. This system ensures that reliance on conventional sources of energy is reduced. Figure 8 portrays this system.
Figure 8 - Solar - Wind Hybrid System in Ajman
4.3 PTL Solar
PTL Solar is one of MENA's largest corporations that deal with solar energy products today. It provides solutions to cater to an assortment of applications. Its major projects are listed below. [6]
4.3.1 Solar Powered Tracker
The solar powered tracker is mounted at Dubai International Academic City (DIAC). It has a capacity of 5 kW and is used to power 129 lamps to generate 13.825 kWh of energy per year. Figure 9 represents the tracker.
Figure 9 - Solar Powered Tracker used to run 129 lamps at DIAC, Dubai
4.3.2 Solar Powered Backup System
Figure 10 illustrates a 3 kVA power backup system installed on top of a roof in Ajman. It is associated with inverters to convert DC power into AC. The solar modules are designed to provide energy to home appliances. During the night, power is provided through a generator that is controlled using a timer enabled relay switch.
Figure 11 shows a similar power backup system installed at a camel breeding farm in Abu Dhabi. The solar modules are responsible for the lighting and ensure that electrical power is provided almost instantaneously.
Figure 10 - Solar Powered Backup System installed on a roof in Ajman
Figure 11 - Solar Powered Backup System installed at a camel breeding farm in Abu Dhabi
4.3.3 Solar Powered Windsock
As shown in Figure 12, the solar modules power the LEDs and thus allow the windsock to be lighted at night to indicate wind speed and velocity. This system exists at Yas Marina Circuit, Yas Island, Abu Dhabi.
Figure 12 - Solar Powered Windsock at Yas Island, Abu Dhabi
4.3.4 Solar and Wind Hybrid System
PTL Solar has successfully installed this system for providing electricity to a caravan in Al - Ain as shown in Figure 13. This structure allows the caravan to be powered from the solar modules in summer, and the wind turbines in winter, hence proving to be a hybrid off - grid system.
Figure 13 - Solar and Wind Hybrid System in place at a caravan in Al - Ain
Chapter 5 - The Design of a Solar Thermal Power Plant
The design of a solar thermal power plant requires consideration of various factors, both technical and economic. There are also environmental factors that contribute to the design process.
5.1 Introduction
A solar thermal conversion process[7] involves the conversion of solar energy to mechanical energy. Figure 14 depicts a schematic of such a system. Energy is collected by flat-plate or concentrating collectors, stored, and then used to operate a heat engine. Problems with such a system include the fact that the efficiency of a collector diminishes as its operating temperature rises, while the efficiency of the engine rises as its operating temperature rises. The maximum operating temperatures for flat-plate collectors are low relative to desirable input temperatures for heat engines, and system efficiencies are low if flat-plate collectors are used. The selection of the type of collector will be analyzed in a later section.
Figure 14 - Solar Thermal Conversion System
5.2 Solar Radiation
The sun's structure and characteristics determine the nature of the energy it radiates into space[8]. One of the many things to consider are the characteristics of solar energy outside the earth's atmosphere, its intensity, and its spectral distribution. This is followed by a discussion of solar geometry, that is, the position of the sun in the sky and the direction in which beam radiation is incident on the surfaces of various orientations. After this, comes extraterrestrial radiation on a horizontal surface.
5.2.1 Solar Constant
The solar constant Gsc is the energy from the sun per unit time received on a unit area of surface perpendicular to the direction of propagation of the radiation at mean earth-sun distance outside the atmosphere. The value of the solar constant is 1353 W/m2.
5.2.2 Distribution of Extraterrestrial Radiation
Table 1 shows the WRC spectrum in increments of wavelength while Table 2 shows the same in increments of energy. The average energy Gsc,ï¬ï€ (in W/m2) over small bandwidths centered at wavelength ï¬ï€ is given in the second column. The fraction fï€ï¬ of the total energy in the spectrum that is between the wavelengths zero and ï¬ is given in the third column.
Table 1 - Extraterrestrial solar radiation in increments of wavelength
Table 2 - Extraterrestrial solar radiation in increments of energy
5.2.3 Angles for Tracking Surfaces
Solar collectors track the sun by moving in prescribed ways to minimize the angle of incidence of beam radiation on their surfaces and thus maximize the incident beam radiation. The angles of incidence and the surface azimuth angles are required for these collectors.
Tracking systems are classified by their motions. Rotation can be about a single axis which is usually horizontal east-west, horizontal north-south, vertical, or parallel to the earth's axis or it can be about two axes.
5.2.4 Ratio of Beam Radiation on Tilted Surface to that on Horizontal Surface
It is necessary to calculate the hourly radiation on a tilted surface of a collector from measurements or estimates of solar radiation on a horizontal surface. The geometric factor Rb is the ratio of beam radiation on the tilted surface to that on a horizontal surface at any time.
Rb  cos ï±ï€ ï€¯ï€ cos ï±z
Where Cos ï±ï€ ï€½ï€ cos ï±z cos ï¢ï€ ï€«ï€ sin ï±z sin ï¢ï€ cos (ï§s - ï§ï€©ï€
Where ï±ï€ ï€½ï€ Angle of incidence - the angle between the beam radiation on a surface and the normal to that surface
ï±zï€ ï€½ Zenith angle - the angle between the vertical and the line to the sun, that is, the angle of incidence of beam radiation on a horizontal surface.
ï¢ï€ ï€½ï€ Slope - the angle between the plane of the surface and horizontal. It lies between 00 and 1800.
ï§ï€ ï€½ï€ Surface azimuth angle - the deviation of the projection on a horizontal plane of the normal to the surface from the local meridian, with zero due south, east negative, and west positive. It lies between -1800 and +1800.
ï§s ï€½ï€ Solar azimuth angle - the angular displacement from south of the projection of beam radiation on the horizontal plane. Displacements east of south are negative and west of south are positive.
The declination, ï¤, which is the angular position of the sun at solar noon (i.e., when the sun is on the local meridian) with respect to the plane of the equator is given by the following equation:
ï¤ï€ ï€½ï€ ï€²ï€³ï€®ï€´ï€µï€ sin (360 x ((284 + n)/365))
Its value ranges from -23.450 to +23.450.
The solar radiation incident on a horizontal plane outside of the atmosphere is the normal incident solar radiation divided by Rb:
Go = Gsc ( 1 + 0.033 cos 360n/365 ) cos ï±z
5.3 Available Solar Radiation
This section describes instruments for solar radiation measurements, the solar radiation data that are available, and the calculation of needed information from the available data. Beam and diffuse solar radiation on a horizontal surface, by hours, is useful in simulation of solar processes.
Short-wave radiation is radiation originating from the sun, in the wavelength range of 0.3 to 3ïm. Long-wave radiation is radiation originating from sources at temperatures near ordinary ambient temperatures and at wavelengths greater than 3ïm. Long-wave radiation is emitted by the atmosphere, by a collector, or by any other body at ordinary temperatures.
5.3.1 Instruments Used
A pyrheliometer is an instrument using a collimated detector for measuring solar radiation from the sun and from a small portion of the sky around the sun (i.e., beam radiation) at normal incidence.
A pyranometer is an instrument for measuring total hemispherical solar (beam plus diffuse) radiation, usually on a horizontal surface. If shaded from the beam radiation by a shade ring or disc, a pyranometer measures diffuse radiation.
5.3.2 Calculations
The equation relating monthly average daily radiation to clear-sky radiation is given as follows:
/ = a + b ( / )
Where = monthly average daily radiation on horizontal surface
= radiation for the location averaged over the time period
a,b = empirical constants based on location
= monthly average daily hours of bright sunshine
= monthly average of maximum possible daily hours of bright sunshine
Utilizability is the fraction of the total radiation that is received at intensity higher than the critical level. This fraction is multiplied by the average radiation for the period to find the total utilizable energy. The fraction of an hour's total energy that is above the critical level is the utilizability for that hour:
ï¦hï€ ï€ ï€½ï€ ï€¨IT - ITC) / IT
Where IT = Incident radiation on the tilted surface of the collector
ITC = critical radiation level
ï¦h ranges from zero to unity.
Utilizability for an hour of a month of N days is given by
ï¦ï€ ï€ ï€½ï€ ï€±ï€¯ïŽï€ ∑N) IT - ITC) / IT
The month's average utilizable energy for the hour is the product NITï¦ï€®
5.4 Solar Collectors
A solar collector[9] is a device for collecting solar radiation and transferring the energy to a fluid passing in contact with it. Utilization of solar energy requires solar collectors. These are generally of two types:
Non concentrating or flat plate type solar collectors
Concentrating or focusing type solar collectors
The solar energy collector, with its associated absorber, is the essential component of any system for the conversion of solar radiation energy into more usable form like heat or electricity. In the non - concentration type, the collector area (the area that intercepts the solar radiation) is the same as the absorber area (the area absorbing the radiation). On the other hand, in concentrating collectors, the area intercepting the solar radiation is greater, sometimes almost hundred times greater than the absorber area. By means of concentrating collectors, much higher temperatures can be obtained than with the non-concentrating type. Concentrating collectors may be used to generate medium pressure steam. They use many different arrangements of mirrors and lenses to concentrate the sun's rays on the boiler.
Concentrating collectors use an optical system in the form of reflectors or refractors. A focusing collector is a special form of flat-plate collector modified by introducing a reflecting surface (concentrator) between the solar radiations and the absorber. These types of collectors can have radiation increase from a low value to a very high value. In these collectors radiation falling on a relatively large area is focused on to a receiver of considerably smaller area. As a result of the energy concentration, fluids can be heated up to temperatures of 5000C or more.
An important difference between non-focusing and focusing collectors is that the latter concentrate only on diffuse radiation coming from a specific direction, since diffuse radiation arrives from all directions, only a very small proportion is from the direction for which focusing occurs. Because of the optical system, certain losses are introduced. These include reflection or absorption losses in the mirrors or lenses and losses due to imperfections in the optical system. The combined effect of these factors contributes to the optical efficiency. There are various types of concentrating collectors:
Parabolic trough collector
Mirror strip reflector
Fresnel lens collector
Flat plate collector with adjustable mirrors
Compound Parabolic Concentrator (CPC)
Central Receiver collectors
5.5 Central Receiver Systems
This system[10] constitutes a collection of considerable number of mirrors distributed over an area on the ground. Each mirror, called a heliostat, can be steered independently about two axes so that the reflected solar radiation is always directed towards an absorber mounted on a tower. This type of collector is known as Central Receiver Collector. It is mostly used in tower power plants for generation of electrical energy. Figure 15 illustrates the concept of a central receiver system.
Figure 15 - Concept of a Central Receiver System
In a typical central receiver, the mirror is composed of many small mirrors, each with its own heliostat to follow the sun. The heliostats are generally located in the horizontal plane, but when the situation is favorable, they can simply follow the existing terrain. The heliostat system has a dilute mirror. This means that the entire surface within the system is not covered with mirror surfaces. This is also termed as the fill factor. Figure 16 shows the Solar One power plant in operation at Barstow, California.
Figure 16 - Solar One plant in Barstow, California
Many mirrors are mounted together to act as a paraboloid. The basic problem associated with the central receiver system is that the heliostat mirrors require non-linear drive rate in two co-ordinates to achieve the requirement of keeping the reflected image point on a fixed receiver. The heliostat should also be rugged enough to withstand storms.
Concentration ratio is the ratio of the area of the aperture to the area of the receiver. It is given by:
C = Aa / Ar
Where Aa = Aperture Area
Ar = Receiver Area
For a central receiver system, the maximum concentration ratio is given by
Cmax = (ï¹ sin2 ï¦r) / 4sin2 (0.267 + ï¤/2) - 1
Where ï¦rï€ ï€½ï€ rim angle
ï¹ï€ ï€½ï€ fraction of the ground area covered by mirrors
ï¤ï€ ï€½ï€ dispersion angle
5.5.1 Heliostat
One of the heliostat designs includes an iron heliostat mirror. It is made up of many mirrors combined together to form a single large mirror. Figure 17 illustrates such a heliostat. An important environmental issue is the ability of the heliostat to withstand extreme wind. Typically, heliostats have the ability to operate in wind speeds ranging from 12 m/s to 40 m/s. In extreme wind, the heliostats are designed to stow the heliostats into the face-down position. Each mirror segment is positioned towards a focal point and is concaved slightly. This produces a higher flux density at the main point. Fractional horsepower ratio motors are used to move the heliostats about its axes.
Figure 17 - Heliostat
5.5.2 Receiver Tower
The receiver is responsible for intercepting the absorbed energy at the top of the centrally placed tower. This energy is absorbed from the field of heliostats and is used to heat a working fluid. There are two types of receivers.
5.5.2.1 External Receivers
These receivers are made up of many panels that join together in a circular arrangement to form a cylinder. Figure 18 illustrates such a receiver.
Figure 18 - External Receiver
5.5.2.2 Cavity Receivers
These receivers incorporate the flux absorbing surface inside an insulated cavity, thus reducing the convective losses from the absorber. Figure 19 illustrates such a receiver.
Figure 19 - Cavity Receiver
5.5.3 Tower Design
The design factors incorporate the area and weight of the tower. Cost is a major factor that determines the final height of the receiver. The costs of steel and concrete towers are depicted in Figure 20 below.
Figure 20 - Graph showing tower cost for different heights for steel and concrete
5.5.4 Heat Transfer Fluids
The operating temperature of the fluid and the cost-effectiveness of the fluid are the major factors that determine the operating fluid used. Some of the fluids used are listed below.
Steam is used mostly in central receiver systems with a maximum operating temperature of 540 0C and 10 MPa pressure.
Nitrate salts have operating temperatures of 550 0C.
Sodium in liquid form is used, and it has a maximum operating temperature of 600 0C.
The Brayton cycle uses air or helium as the heat transfer fluid.
5.5.5 Thermal Performance of System
The performance of a system is defined in terms of the efficiency of the system. The efficiency of a central receiver system is given by
ï¨col = useful / InhAh
Where useful = rate of energy addition to the working fluid (measured at the bottom of the receiver tower)
I = solar irradiance
nh = the total number of heliostats in the field
Ah - the total area of the heliostat
The overall efficiency of the system can be broken down into two types.
Field Efficiency
Receiver Efficiency
5.5.5.1 Field Efficiency
These losses contribute the most to the overall efficiency. They include losses due to
The cosine effect
Shadowing and blocking
Reflectance
Attenuation
The field efficiency can be calculated using the equation
η = ηcosηshadowηblockηreflηatten
Where ηcos = efficiency due to cosine losses
ηshadow = efficiency due to shadowing
ηblock= efficiency due to blocking
ηrefl= efficiency due to reflectance losses
ηatten= efficiency due to attenuation losses
5.5.5.2 Receiver Efficiency
The receiver losses can be broken down into the following individual losses.
Spillage
Absorption
Radiation
Conduction
Convection
The receiver efficiency is given by the equation
η = ηspillηabsorpηradηconvηcond
where ηspill = efficiency due to spillage loss
ηabsorp = efficiency due to absorption loss
ηrad = efficiency due to radiation loss
ηconv = efficiency due to convection loss
ηcond = efficiency due to conduction loss
The various receiver losses are indicated in Figure 21.
Figure 21 - The different sources of receiver efficiency
5.6 Energy Storage
The storage of energy[10] is necessary if solar energy is to meet substantial portions of the energy needs. Energy storage is considered in the light of a solar process system, the major components of which are solar collector, storage units, conversion devices (such as engines), loads, auxiliary energy supplies, and others. The performance of each of these components is related to that of the others. The dependence of the collector performance on temperature makes the whole system performance sensitive to temperature. A thermal energy storage system that is characterized by a high drop in temperature between input and output will lead to unnecessarily high collector performance and/or low heat engine inlet temperature, both of which will lead to poor system performance.
The optimum capacity of an energy storage system depends on the expected time dependence of solar radiation availability, the nature of loads to be expected on the process, the way in which auxiliary energy is supplied, and an economic analysis that determines how much of the annual load should be carried by solar and how much by the auxiliary energy source.
For a central receiver system, the energy needs to be stored during the night or due to conditions such as cloudy days.
There are three types of storage mechanisms for thermal energy.
Sensible heat storage
Latent heat storage
Thermo-chemical energy storage
5.6.1 Sensible heat storage
An insulated tank containing cold fluid is a part of the storage. The cold fluid is heated by the high temperatures of the hot fluid from the solar collectors.
In a multi-tank storage system, there exist three tanks, two containing cold fluid and one completely empty. In the morning, the fluid from the first tank is taken to the solar collectors where it becomes hot and then is deposited into the third empty tank. Around midday, the first tank is empty so fluid from the second tank is then taken to the field of collectors, heated and deposited into the first tank.
In a thermocline energy storage system, there exists only one tank for storing both the hot and cold fluids. In the beginning, the storage tank contains only cold fluid. After heat energy becomes available due to the action of solar collectors, cold fluid from the bottom of the tank is withdrawn, taken to the collector field, heated and then deposited at the top of the tank.
5.6.2 Latent heat storage
Latent heat processes allow large quantities of heat within a material to be stored within a small amount of space. For example, when 1kg of sodium hydroxide melts, 156 kJ of thermal energy is absorbed. This reduces the storage space required and thereby the cost.
5.6.3 Thermochemical energy storage
Here, heat energy is used to dissociate water into its components, namely, hydrogen and oxygen. This happens at temperatures greater than 20000C and results in energy storage. The reverse reaction occurs at high temperatures so energy can be stored and utilized when required.
5.7 Power Cycles
A heat engine is a device that is used to produce mechanical energy from thermal energy (produced by solar). It is also known as a power conversion cycle. The two cycles commonly used for the central receiver system are the Brayton and Rankine Cycles.
5.7.1 Brayton Cycle
The engine running on this cycle has the advantage of low operational and maintenance costs. It operates at relatively low pressures and pressurized gas in heated in the central receiver. A disadvantage is high receiver operating temperatures required to get a reasonable efficiency.
5.7.2 Rankine Cycle
This is the most common cycle used in solar power systems. The best working fluid used is steam which is boiled and is then superheated in the receiver.
5.8 System Modeling and Design
A computer-generated model along with weather data for a particular location can be used to find how much solar power can be distributed to a definite load. System modeling is done for the following reasons.
To establish how much energy the system will provide over a period of time.
To verify the economic feasibility of a design.
Chapter 6 - Conclusion
This chapter lists the accomplishments of this project. The following has been achieved:
Analysis of all possible advantages and disadvantages of using solar power in the country.
Study of a broad range of applications of solar energy.
Examination of projects and solar power based solutions of three major corporations in the U.A.E.
Illustrating the design of a solar thermal power plant incorporating a central receiver system.
