IP Erasmus RenoPassCoDe 23th April to 8th of May 2014

IP Erasmus RenoPassCoDe 23th April to 8th of May 2014 Renewable Energy Solar Energy - Fundamentals João Ramos, Polytechnic Institute of Leiria, Portugal

1. A Brief History of Solar Energy • Solar energy is the oldest energy source ever used. The sun was adored by many ancient civilizations as a powerful god. The first known practical application was in drying for preserving food. During the eighteenth century, solar furnaces capable of melting iron, copper, and other metals were being constructed of polished iron, glass lenses, and mirrors. The furnaces were in use throughout Europe and the Middle East. One of the first large-scale applications was the solar furnace, built by the well-known French chemist Lavoisier, who constructed powerful lenses to concentrate solar radiation. This attained the remarkable temperature of 1750 ºC. Solar furnace used by Lavoisier in 1774 Paraboliccollectorpowering, 1878

1.1 Photovoltaics • Photovoltaics(PV) is a methodofgeneratingelectricalpowerbyconverting solar radiationintodirectcurrentelectricityusingsemiconductorsthatexhibitthephotovoltaiceffect. Becquerel discovered the photovoltaic (PV) effect in selenium in 1839. Photovoltaicpowergenerationemploys solar panelscomposedof a numberof solar cellscontaining a photovoltaic material. The conversion efficiency of the silicon cells developed in 1958 was 11%, although the cost was prohibitively high ($1000/W). The first practical application of solar cells was in space, where cost was not a barrier, since no other source of powerisavailable; • Mainstream materialspresentlyused for photovoltaicsincludemonocrystallinesilicon, polycrystallinesilicon, amorphoussilicon, cadmiumtelluride, andcopperindiumgalliumselenide/sulfide. Due to theincreaseddemand for renewableenergysources, themanufacturingof solar cellsandphotovoltaicarrayshasadvancedconsiderably in recentyears.

The two basic types of PV applications are the stand-alone and the grid-connected systems. Standalone PV systems are used in areas that are not easily accessible or have no access to mains electricity grids. A stand-alone system is independent of the electricity grid, with the energy produced normally being stored in batteries. A typical stand-alone system would consist of PV module or modules, batteries, and a charge controller. An inverter may also be included in the system to convert the direct current (DC) generated by the PV modules to the alternating current (AC) form required by normal appliances;

In the grid-connected applications, the PV system is connected to the local electricity network. This means that during the day, the electricity generated by the PV system can either be used immediately (which is normal for systems installed in offices and other commercial buildings) or sold to an electricity supply company (which is more common for domestic systems, where the occupier may be out during the day). In the evening, when the solar system is unable to provide the electricity required, power can be bought back from the network. In effect, the grid acts as an energy storage system, which means the PV system does not need to include battery storage.

1.2 OtherRenewableEnergySystemsWindEnergy • Wind is generated by atmospheric pressure differences, driven by solar power. Of the total of 175,000 TW of solar power reaching the earth, about 1200 TW (0.7%) are used to drive the atmospheric pressuresystem; • In terms of capacity, wind energy is the most widely used renewable energy source. Today there are many wind farms that produce electricity. Wind energy is, in fact, an indirect activity of the sun. Its use as energy goes as far back as 4000 years, during the dawn of historical times. It was adored, like the sun, as a god. For the Greeks, wind was the god Aeolos, the “flying man”; • The exploitation of wind energy today uses a wide range of machine sizes and types, giving a range of different economic performances. Today there are small machines up to about 300 kW and large capacity ones that are in the megawatt range.

The value of wind electricity depends on the characteristics of the utility system into which it is integrated, as well as on regional wind conditions. Some areas, particularly warm coastal areas, have winds with seasonal and daily patterns that correlate with demand, whereas others have winds that do not. Analyses conducted in the United Kingdom, Denmark, and the Netherlands make it clear that wind systems have greater value if numerous generating sites are connected, because it is likely that wind power fluctuations from a system of turbines installed at many widely separated sites will be less than at any individual site; • Technological advances promise continued cost reductions. For example, the falling cost of electronic controls has made it possible to replace mechanical frequency controls with electronic systems. In addition, modern computer technology has made it possible to substantially improve the design of blades and other components.

Passive Solar Buildings • Another area of solar energy is related to passive solar buildings. The term passive system is applied to buildings that include, as integral parts of the building, elements that admit, absorb, store, and release solar energy and thus reduce the need for auxiliary energy for comfort heating. These elements have to do with the correct orientation of buildings, the correct sizing of openings, the use of overhangs and other shading devices, and the use of insulation and thermal mass. • Before the advent of mechanical heating and cooling, passive solar building design was practiced as a means to provide comfortable indoor conditions and protect inhabitants from extreme weather conditions. People at those times considered factors such as solar orientation, thermal mass, and ventilation in the construction of residential dwellings, mostly by experience and the transfer of knowledge from generation to generation.

Biomass • Biomass energy is a generic term applied to energy production achieved from organic material broken down into two broad categories: • Woody biomass. Forestry timber, residues and co-products, other woody material including thinning and cleaning from woodlands (known as forestry arisings), untreated wood products, energy crops such as willow, short rotation coppice, and miscanthus (elephant grass); • Non-woody biomass. Animal wastes, industrial and biodegradable municipal products from food processing, and high-energy crops such as rape, sugarcane, and corn. • Biomass, mainly in the form of industrial and agricultural residues, provided electricity for many years with conventional steam turbine power generators. The United States currently has more than 8000 MWe of generating capacity fueled from biomass. Existing steam turbine conversion technologies are cost competitive in regions where low-cost biomass fuels are available, even though these technologies are comparatively inefficient at the small sizes required for biomass electricity production.

GeothermalEnergy • Measurements show that the ground temperature below a certain depth remains relatively constant throughout the year. This is because the temperature fluctuations at the surface of the ground are diminished as the depth of the ground increases due to the high thermal inertia of the soil. • There are different geothermal energy sources. They may be classified in terms of the measured temperature as low (<100 C), medium (100–150 C), and high temperature (>150 C). The thermal gradient in the earth varies between 15 and 75 ºC per km depth; nevertheless, the heat flux is anomalous in diferentcontinental áreas.

GroundCoupledHeatPumps • In these systems ground heat exchangers (GHE) are employed to exchange heat with the ground. The ground can be used as an energy source, an energy sink, or for energy storage. For the efficient use of the ground in energy systems, its temperature and other thermal characteristics must be known. Studies show that the ground temperature varies with depth; • At the surface, the ground is affected by short-term weather variations, changing to seasonal variations as the depth increases. At deeper layers, the ground temperature remains almost constant throughout the seasons and years and is usually higher than that of the ambient air during the cold months of the year and lower during the warm months. • The ground therefore is dividedintothree zones: 1. The surface zone, where hourly variations of temperature occur; 2. The shallow zone, with monthly variations; 3. The deep zone, where the temperature is almost constant year round

GHE or earth heat exchangers (EHE), are devices used for the exploitation of the ground thermal capacity and the difference in temperature between ambient air and ground. A GHE is usually an array of buried pipes installed either horizontally or vertically into the ground. • They use the ground as a heat source when operating in the heating mode and as a heat sink when operating in the cooling mode, with a fluid, usually air, water or a water–antifreeze mixture, to transfer the heat from or to the ground. They can contribute to the air-conditioning of a space, for water heating purposes and also for improving the efficiency of a heat pump.

Ground-coupled heat pumps (GCHPs) or geothermal heat pumps are systems combining a heat pump with a GHE for the heat exchange process, which improves the heat pump efficiency. Mainly, they are of two types; namely, the ground-coupled (closed-loop) system or the groundwater (open-loop) system.

2. EnvironmentalCharacteristics • Knowledge of the sun’s path through the sky is necessary to calculate the solar radiation falling on a surface, the solar heat gain, the proper orientation of solar collectors, the placement of collectors to avoid shading, and many more factors Sun–earthrelationship • The general weather of a location is required in many energy calculations. This is usually presented as a typical meteorological year (TMY) file

2.1 ReckoningofTime • In solar energy calculations, apparent solar time (AST) must be used to express the time of day. AST is based on the apparent angular motion of the sun across the sky. The time when the sun crosses the meridian of the observer is the local solar noon. It usually does not coincide with the 12:00 o’clock time of a locality. To convert the local standard time (LST) to AST, two corrections are applied; the equation of time (ET) and longitude correction. • The ET arises because the length of a day, that is, the time required by the earth to complete one revolution about its own axis with respect to the sun, is not uniform throughout the year. Over the year, the average length of a day is 24 h; however, the length of a day varies due to the eccentricity of the earth’s orbit and the tilt of the earth’s axis from the normal plane of its orbit. Due to the ellipticity of the orbit, the earth is closer to the sun on January 3 and furthest from the sun on July 4. Therefore the earth’s orbiting speed is faster than its average speed for half the year (from about October through March) and slower than its average speed for the remaining half of the year (from about April through September).

The values of the ET as a function of the day of the year (N) can be obtained approximately from thefollowingequations: ET = 9.87 sin(2B) - 7.53 cos(B) - 1.5 sin(B) [min] and B = (N - 81) 360/364 • The standard clock time is reckoned from a selected meridian near the center of a time zone or from the standard meridian, the Greenwich, which is at longitude of 0. Since the sun takes 4 min to transverse 1º of longitude, a longitude correction term of 4 (Standard longitude [SL] - Local longitude [LL]) should be either added or subtracted to the standard clock time of the locality. This correction is constant for a particular longitude, and the following rule must be followed with respect to sign convention. If the location is east of the standard meridian, the correction is added to the clock time. If the location is west, it is subtracted. The general equation for calculating the AST is: AST = LST + ET +/- 4(SL - LL) - DS where: LST = local standard time; ET = equationoftime; SL = standard longitude; LL = local longitude; DS = daylight saving (it is either 0 or 60 min). • If a location is east of Greenwich, the sign of Eqis minus (-), and if it is west, the sign is plus (+). If a daylight saving time is used, this must be subtracted from the LST. The term DS depends on whether daylight saving time is in operation or not. This term is usually ignored from this equation and considered only if the estimation is within the DS period.

2.2 Solar Angles • The earth makes one rotation about its axis every 24 h and completes a revolution about the sun in a period of approximately 365.25 days. This revolution is not circular but follows an ellipse with the sun at one of the foci, as shown in Figure. Annual motion of the earth about the sun

The eccentricity, e, of the earth’s orbit is very small, equal to 0.01673. Therefore, the orbit of the earth round the sun is almost circular. The sun–earth distance, R, at perihelion (shortest distance, at January 3) and aphelion (longest distance, at July 4) is given by: R = a(1 +/- e) where a = mean sun–earth distance = 149.5985*106km. • The plus sign in Eq. is for the sun–earth distance when the earth is at the aphelion position and the minus sign for the perihelion position. • the Ptolemaic view of the sun’s motion is used in the analysis that follows, for simplicity; that is, since all motion is relative, it is convenient to consider the earth fixed and to describe the sun’s virtual motion in a coordinate system fixed to the earth with its origin at the site ofinterest. Annual changes in the sun’s position in the sky (Northern Hemisphere)

Solar Declination, d • The earth axis of rotation (the polar axis) is always inclined at an angle of 23.45º from the ecliptic axis, which is normal to the ecliptic plane. The ecliptic plane is the plane of orbit of the earth around the sun. As the earth rotates around the sun it is as if the polar axis is moving with respect to the sun. The solar declination dis the angular distance of the sun’s rays north (or south) of the equator, north declination designated as positive. As shown in Figure it is the angle between the sun–earth centerline and the projection of this line on the equatorial plane. Definition of: latitude L, hour angle h and solar declination d

The variation of the solar declination throughout the year is shown in Figure. Declinationofthesun • The declination, d, in degrees for any day of the year (N) can be calculated approximately by the equation: d = 23.45 sin[360/365*(284 + N)]

HourAngle, h • The hour angle, h, of a point on the earth’s surface is defined as the angle through which the earth would turn to bring the meridian of the point directly under the sun. • The hour angle at local solar noon is zero, with each 360/24 or 15º of longitude equivalent to 1 h, afternoon hours being designated as positive. Expressed symbolically, the hour angle in degrees is: h = +/- 0.25 (Number of minutes from local solar noon) where the plus sign applies to afternoon hours and the minus sign to morning hours. • The hour angle can also be obtained from the AST; that is, the corrected local solar time: h = (AST – 12) 15 • At local solar noon, AST = 12 and h = 0. Therefore, the LST (the time shown by our clocks at local solar noon) is: LST = 12 – ET +/- 4 (SL - LL)

Solar Altitude Angle, a • The solar altitude angle is the angle between the sun’s rays and a horizontal plane, as shown in Figure. • It is related to the solar zenith angle, F, which is the angle between the sun’s rays and the vertical. Therefore, F + a= p/2 = 90º The mathematical expression for the solar altitude angle is: sin(a) = cos(F) = sin(L)sin(d) + cos(L)cos(d)cos(h) where L = local latitude, defined as the angle between a line from the center of the earth to the site of interest and the equatorial plane. Values north of the equator are positive and those south are negative.

Solar AzimuthAngle, z • The solar azimuth angle, z, is the angle of the sun’s rays measured in the horizontal plane from due south (true south) for the Northern Hemisphere or due north for the Southern Hemisphere; westward is designated as positive. The mathematical expression for the solar azimuth angle is: sin(z) = cos(d) sin(h)/cos(a) • At solar noon, by definition, the sun is exactly on the meridian, which contains the north–south line, and consequently, the solar azimuth is 0º. Therefore the noon altitude an is:an = 90º - L +d

IncidenceAngle, q • The solar incidence angle, q, is the angle between the sun’s rays and the normal on a surface. For a horizontal plane, the incidence angle, q, and the zenith angle, F, are the same. Solar anglesdiagram: whereb = surface tilt angle from the horizontal; Zs = surface azimuth angle, the angle between the normal to the surface from true south, westward is designated as positive.

The angles shown in Figure are related to the basic angles, with the following general expression for the angle of incidence: cos(q) = sin(L) sin(d) cos(b) - cos(L) sin(d) sin(b) cos(Zs) + cos(L) cos(d) cos(h) cos(b) + sin(L) cos(d) cos(h) sin(b) cos(Zs) + cos(d) sin(h) sin(b) sin(Zs)

2.3 SunPathDiagrams • For practical purposes, instead of using the preceding equations, it is convenient to have the sun’s path plotted on a horizontal plane, called a sun path diagram, and to use the diagram to find the position of the sun in the sky at any time of the year. As can be seen from Eqs, the solar altitude angle, a, and the solar azimuth angle, z, are functions of latitude, L, hour angle, h, and declination, d. • Ina two-dimensional plot, only two independent parameters can be used to correlate the other parameters; therefore, it is usual to plot different sun path diagrams for different latitudes. Such diagrams show the complete variations of hour angle and declination for a full year.

3 Solar Radiation • All substances, solid bodies as well as liquids and gases above the absolute zero temperature, emit energy in the form of electromagnetic waves. • The radiation that is important to solar energy applications is that emitted by the sun within the ultraviolet, visible, and infrared regions. Therefore, the radiation wavelength that is important to solar energy applications is between 0.15 and 3.0 mm. The wavelengths in the visible region lie between 0.38 and 0.72 mm

3.1Solar RadiationMeasuringEquipment • A number of radiation parameters are needed for the design, sizing, performance evaluation, and research of solar energy applications. These include total solar radiation, beam radiation, diffuse radiation, and sunshine duration. Various types of equipment measure the instantaneous and long-term integrated values of beam, diffuse, and total radiation incident on a surface. This equipment usually employs the thermoelectric and photovoltaic effects to m • There are basically two types of solar radiation measuring instruments: the pyranometerand the pyrheliometer.

3.1 The Solar Resource The operation of solar collectors and systems depends on the solar radiation input and the ambient air temperature and their sequences. One of the forms in which solar radiation data are available is on maps. Annualtotal solar irradiationon horizontal surface for Europe

Solar EnergyCollectors • Solar energy collectors are special kinds of heat exchangers that transform solar radiation energy to internal energy of the transport medium. The major component of any solar system is the solar collector. This is a device that absorbs the incoming solar radiation, converts it into heat, and transfers the heat to a fluid (usually air, water, or oil) flowing through the collector. The solar energy collected is carried from the circulating fluid either directly to the hot water or space conditioning equipment or to a thermal energy storage tank, from which it can be drawn for use at night or on cloudy days. • There are basically two types of solar collectors: non-concentrating or stationary and concentrating. A non-concentrating collector has the same area for intercepting and absorbing solar radiation, whereas a sun-tracking concentrating solar collector usually has concave reflecting surfaces to intercept and focus the sun’s beam radiation to a smaller receiving area, thereby increasing the radiation flux. Concentrating collectors are suitable for high-temperature applications. Solar collectors can also be distinguished by the type of heat transfer liquid used (water, non-freezing liquid, air, or heat transfer oil) and whether they are covered or uncovered.

StationaryCollectors • Solar energy collectors are basically distinguished by their motiondstationary, single-axis tracking, and two-axis trackingdand the operating temperature. First, we’ll examine the stationary solar collectors. • These collectors are permanently fixed in position and do not track the sun. Three main types of collectors fall into this category: • 1. Flat-platecollector (FPC). • 2. Stationary compound parabolic collector (CPC). • 3. Evacuated tube collector (ETC).

Flat-plateCollector(FPC) • A typical flat-plate solar collector is shown in Figure. When solar radiation passes through a transparent cover and impinges on the blackened absorber surface of high absorptivity, a large portion of this energy is absorbed by the plate and transferred to the transport medium in the fluid tubes, to be carried away for storage or use. The underside of the absorber plate and the two sides are well insulated to reduce conduction losses. The liquid tubes can be welded to the absorbing plate or they can be an integral part of the plate. The liquid tubes are connected at both ends by large-diameter header tubes. The header and riser collector is the typical design for flat-plate collectors. An alternative is the serpentine design shown in Figure

This collector does not present the potential problem of uneven flow distribution in the various riser tubes of the header and riser design, but serpentine collectors cannot work effectively in thermosiphon mode (natural circulation) and need a pump to circulate the heat transfer fluid. The absorber plate can be a single sheet on which all risers are fixed, or each riser can be fixed on a separate fin. • The transparent cover is used to reduce convection losses from the absorber plate through the restraint of the stagnant air layer between the absorber plate and the glass. It also reduces radiation losses from the collector because the glass is transparent to the shortwave radiation received by the sun, but it is nearly opaque to longwave thermal radiation emitted by the absorber plate (greenhouse effect).

The main components of an FPC, as shown in Figure, are the following: • Cover. One or more sheets of glass or other radiation-transmitting material. • Heat removal fluid passageways. Tubes, fins, or passages that conduct or direct the heat transfer fluid from the inlet to the outlet. • Absorber plate. Flat, corrugated, or grooved plates, to which the tubes, fins, or passages are attached. A typical attachment method is the embedded fixing shown in the detail of Figure. The plate is usually coated with a high--absorptancelow-emittance layer. • Headers or manifolds. Pipes and ducts to admit and discharge the fluid. • Insulation. Used to minimize the heat loss from the back and sides of the collector. • Container. The casing surrounds the aforementioned components and protects them from dust, moisture, and any other material.

CollectorConstruction

CollectorConstruction (cont.)

CompoundParabolicCollector(CPC) • Compound parabolic collectors (CPCs) are non-imaging concentrators. They have the capability of reflecting to the absorber all of the incident radiation within wide limits. • The necessity of moving the concentrator to accommodate the changing solar orientation can be reduced by using a trough with two sections of a parabola facing each other, as shown in the Figure. • Compound parabolic concentrators can accept incoming radiation over a relatively wide range of angles. By using multiple internal reflections, any radiation entering the aperture within the collector acceptance angle finds its way to the absorber surface located at the bottom of the collector. The absorber can take a variety of configurations. It can be flat, bifacial, wedge, or cylindrical.

EvacuatedTube Collector(ETC) • Evacuated heat pipe solar collectors (tubes) operate differently than the other collectors available on the market. These solar collectors consist of a heat pipe inside a vacuum-sealed tube, as shown in the Figure

IP Erasmus RenoPassCoDe 23th April to 8th of May 2014