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Solar energy is the source of all energy on earth, available to us in a number of derivatives. Plant matter for example, which relies on solar energy for nutrition, experiences natural compression and decomposition over millions of years to form the the fossil fuels we use today for electrical generation and transportation. Other examples of this can be seen in use of biomass for fuel or the harvesting of wind energy which is reliant on solar heated air for the formation of currents.
We are also able to utilize the solar resource directly. Solar thermal technologies take advantage of this resource to heat a working fluid that can transfer energy to an air stream or water for domestic or commercial use. Solar Photovoltaic or PV devices exploit various materials (principally Silicon) that experience sub-atomic variations when exposed to solar energy in order to induce an electric current. Both solar PV and thermal technologies provide a useful source of energy with little to no moving parts, no pollution and very little embodied energy.
In order to effectively design a solar energy system, an understanding of the available solar resource at the location of interest is required. This article aims to provide readers with an understanding of the earth's solar resource and introduces the tools required to perform a basic analysis. A brief overview of solar radiation measurement techniques and performance modeling will also be provided.
The Sun
All of the energy available on Earth is derived from the sun. We can model the sun's surface as blackbody. At a specific temperature, approximately 5777K for the sun, a blackbody emits energy with a unique radiation spectrum (Table 1).(REFERENCE DUFFMAN)
INSERT TABLE
The spectrum is divided into three broad ranges classified as ultraviolet, visible and infrared which transmit radiation at varying intensities. The highest intensities are found within the visible spectrum, peaking at a wavelength close to 0.5um. Averaged over the entire surface, the power density of the sun is found to be approximately 63 x 10 W/m2.
Earth Bound Radiation The sun emits radiation in all directions, however only a portion of this energy is intercepted by the Earth. The intercepted radiation for any body in space is defined as follows:
FORMULA
For Earth, the distance to the sun is about ???km. This provides an Earth intercepted value of around 1353 W/m2. This number is known as the Earth's solar constant and is often referred to as extra-terrestrial radiation.
Terrestrial radiation, that is radiation hitting the Earth's surface, varies widely with geographical location, local atmospheric conditions, time of day and time of year. Some regions are more suspect to variability in terrestrial radiation than others, depending on the amount of weather fluctuations experienced. For example, desert regions tend to demonstrate consistent weather patterns thereby providing relatively consistent values of terrestrial radiation.
As the radiation passes through the atmosphere the following mechanism determine the amount of radiation that will reach the Earth's surface: Absorption – Certain molecules in the atmosphere posses high photon absorption properties. For example, water vapour (H2O) and CO2 have absorb far infrared radiation and ozone (O3) absorbs ultraviolet radiation. Energy that is absorbed here is unavailable for use by solar panels.
Reflection – In addition to absorption, radiation can also be reflected off of particles in the atmosphere. Radiation that is reflected a number of times before eventually reaching the Earth's surface is known as diffuse radiation. A portion of the incoming radiation may also be lost completely by reflection back into space.
No Interaction – About 70% of the incoming radiation will pass through the atmosphere undisturbed. This known as beam or direct radiation. (FIGURE WITH BREAK DOWN OF LOSSES TO THE RIGHT OF THIS) (REFERENCE PICTURE)
As mentioned above, radiation can reach the Earth's surface as beam/direct radiation or diffuse radiation (Figure with beam vs DIFUSE). Beam radiation is defined as solar radiation that has been received from the sun without any scattering by the atmosphere. Diffuse radiation is solar radiation that has been received from the sun after its direction has been changed by scattering. Diffuse radiation is typically accounted for by adding 10% to the measured beam radiation (REFERENCE PVCDROM).
The losses due to atmospheric effects do not cause any major dips in the radiation spectrum. Rather, the impact is an overall reduction in the intensity of the entire spectrum. Depending on the time of the day, the sun's apparent position in the sky changes and as a result the length of atmosphere that the radiation must travel through also changes. When the sun is directly overhead, this length is referred to as the air mass (AM). Moving away from this overhead position, the travel distance required to hit the collecting surface increases. Figure 2 is simple schematic showing this effect.
INSERT FIGURE 2 – AM drawing
A simple calculation can be performed to quickly determine the air mass:
AM = 1/costheta
In order to account for the curvature of the atmosphere the following formula can be applied:
AM = asdljasdlkajdlkas;djas;lkdj REFERENCE PVCDROM EQUATION
The air mass effect can be visually understood by noting that the sun appears white (high intensity) when it is directly overhead and much redder (low intensity) during the morning and evening hours. REFERENCE PVCDROM If the air mass is known, then the radiation intensity can be calculated as follows: ID IG REF PVCDROM FORMULA
The variability in the radiation received at the Earth's surface requires the identification of a standard to allow for fair testing and comparison of solar energy systems. This is has been defined as AM1.5 and is representative of the total radiation hitting the Earth's surface, which is found to be approximately 1000W/m2 (beam + diffuse). This value is often used to perform preliminary calculations for predicting how a system will perform.
Accounting for the losses discussed above, the amount of energy reaching the Earth's surface every hour is still greater than the amount of energy used by the world's population in an entire year. Herein lies the motivation to design and implement solar thermal and solar PV systems.
Describing the Sun's Location In order to calculate the air mass, the sun's position in the sky needs to be defined. In addition, the apparent position of the sun in the sky will significantly impact the amount of radiation intercepted by our collecting surface. This section will define a number of key terms and outline how to calculate the radiation that is intercepted by a given collector; followed by an example in order to further clarify the methodology.
Solar Noon – This is the time when the sun is directly overhead the position of interest. Each hour away from this position corresponds to a 15o deviation (hour angle).
Zenith Angle (θz) – The angle between the vertical and the line to the sun. This is also the angle of incidence on a horizontal surface for beam radiation. The Solar Altitude angle is defined as the compliment of the zenith angle.
Solar Azimuth Angle – The deviation angle from due South for the projection of the sun's position on the horizontal plane (-180o for Each, +180o for West)
Surface Azimuth Angle – The deviation angle from due South for the surface's normal vector projected onto the horizontal plane (-180o for Each, +180o for West)
Declination Angle – This is the angular position of the sun (at solar noon) with respect to the plane of the equator. The angle varies seasonally due to the Earth's tilt and can be calculated for a given day using:
FORMALA> REFERENCE DUFFIE FORMULA
Slope – Angle between the collecting surface and the horizontal plane. In order to maximize the solar yield over the entire year, this angle should be set equal to the latitude. Steeper angles can be utilized to optimize for winter months. Likewise, shallow angles are used to optimize solar yield during the summer months.
With the angles defined in Table XX@$!?, the sun's position can be calculated as follows:
Example Calculation
Irradiance: W/m2 The rate at which radiant energy is incident on a surface per unit area of surface Irradiation: J/m2 The incident energy per unit area of a surface (integration of irradiance over a specific time) Insolation: Irradiation applying specifically to solar energy
Shading Considerations Shading can be caused by nearby obstructions such as buildings, overhands or other collectors in an array. This can have a drastic impact on system performance, particularly for solar PV panels and it is therefore important to consider when and for how long shading will occur and determine if it is avoidable through design measures. Obstructions can have irregular geometries and be located anywhere in relation to collector surface. By calculating their relative azimuth and altitude angles, a shading profile can be established by overlaying this information onto a solar chart (a solar chart is a graph used to plot the sun's position throughout the year for a given latitude). Figure XS@@! provides an example of theoretical solar chart and shading profile. We can see that the collector will be shading .. bla bla bla
Measuring Solar Resource In addition to providing important meteorological data, measuring solar radiation provides valuable data sets that can be used for modeling and feasibility studies. In field testing of panels also requires accurate monitoring of radiation values. There are three main types of measurement devices: Pyrheliometers : Measures beam radiation by targeting a small portion of the sky around the sun Typically induce a temperature fluctuation onto an enclosed target Requires tracking of the sun Pyranometers Measures beam + diffuse radiation (or diffuse only) using a semi-spherical chamber Device typically operates by utilizing thermopile circuit across selective surfaces Number of models available offering varying degrees of accuracy
PV detectors Spectrally responsive detector Calibration constants available to account for remaining spectrum, however these are unreliable due to variations in actual spectral distribution Relatively Inexpensive REFERNCE DUFFIE A more comprehensive review of solar radiation measurement devices can be found here. LINK TO ROBS PAGE For all radiation measurement devices, a number of factors determines the overall quality and accuracy of retrieved data: Glass Layer: For devices that require an external glass layer, the uniformity of the thickness in this layer is extremely important to avoid refractive effects. The glass should have very low reflective properties Moisture Content: Any moisture within the measurement zone can skew the incoming radiation via reflection and refraction. In addition to proper sealing a number of devices use a replaceable solid desiccant to ensure dry conditions inside the device. Temperature Sensitivity: The response of the circuitry used to output data can vary with temperature fluctuations induced by exposure to radiation and ambient temperature changes. It is difficult to thermally isolate these circuits, however a number of devices use multiple junctions or temperature compensation factors to minimize this effect. Orientation: The semi-spherical chamber used in pyranometers does not maintain a perfect vacuum. As a result, if the device is used to measure radiation on a tilt angle, the heated air inside the chamber can form convective currents which can induce further issues associated with temperature sensitivity and radiation interference. Degradation: All measurements device degrade over time. It is recommended that calibration constants for be re-evaluated every few years to ensure accurate readings.