◊ This is part of the ‘solar energy’ series of articles ◊
In this article I will cover the basics of solar technology, how the systems work, energy production patterns and some applications.
Solar energy can be harvested using thermal or photovoltaic processes. Photovoltaic (PV) has a wide range of applications and scale that may range from a thumbnail to 4,600 times larger than a football field (Topaz Solar Farm, California). Lets have a look at some basic concepts and how they are applied to enhance our lives.
Solar PV energy basics
There are some basic concepts associated with the use of solar energy that are important to highlight before getting any deeper into the subject.
- Solar radiation from the sun is the source for solar energy. Our atmosphere reflects and absorbs some of that radiation before it reaches ground level. Higher altitudes experience greater levels of solar radiation than lower altitudes.
- Weather patterns have a significant impact on the amount of solar radiation that may reach the ground.
- The angle which solar radiation strikes a surface will determine how much is absorbed and reflected.
- The highest density of radiation on a surface will occur when the radiation is perpendicular to the surface.
- The Earth’s axis is tilted relative to the plane of its orbit around the sun. This orientation is responsible for our seasonal change. The sun appears highest in the sky in summer and lowest in winter. The term for the peaks in the Sun’s excursions is ‘solstice’. In the northern hemisphere the summer solstice occurs on June 21st and the winter solstice is December 21st. The summer solstice provides the most daylight during the year and the winter solstice, the least.
- The farther you are from the equator the lower the amount of solar radiation you will have the potential to receive.
- A PV cell is made of semiconducting material that is sensitive to light which enables it to free-up electrons and generate electricity.
- While a photovoltaic cell produces electricity, the output by itself is direct current (DC) and of very little use. The PV cell’s output is 0.5 volts which is less than 1/3 of a typical AA alkaline battery. In order to produce a higher output voltage, PV modules are manufactured with a large number of cells connected in series and parallel. Generating electricity for grid applications requires alternating current (AC) which requires power electronics to perform the conversion. Effectively the PV modules need an integrated system of components including structures, power electronics, switches and wiring for most applications. Bear that in mind when presented with solar PV data. Some sources will reference the PV modules only and some, the entire system.
- Solar energy conversion efficiency decreases with increasing temperature and age of the PV module.
- Solar energy conversion efficiency varies depending on the cell type. There are two commonly used types for power applications; crystalline and thin-film. Crystalline PVs have higher efficiencies than thin-film but are more expensive to produce.
- Installations can maximize their output through the use of tracking mechanisms that optimize the angle of solar panels based on the position of the sun.
- PV systems have limited capability for providing short duration high current such as would be required by a motor. Motor starting current will typically need to come from the grid or other energy source. Installations that must supply motor loads require careful sizing and overall design in order to avoid equipment damage.
- Solar radiation at ground level has been monitored extensively by many countries for decades using specially designed weather stations. The annual radiation data for many locations (especially North America) is readily available to the public making it easy to estimate the energy production of a PV installation. The National Renewable Energy Laboratory (NREL) in the United States makes this information available.
How do Photovoltaic (PV) systems work?
From the basics we know that we combine cells into modules to produce (dc) electricity. Combining multiple modules determines the total output capacity of a solar facility. Power electronics are used to convert the dc into ac to be compatible with the electricity grid. Electronic controls synchronize and match the solar output voltage to the ac of the grid. When conditions are normal the solar generation operates in parallel with the local grid and can deliver or receive energy as required. Under abnormal conditions the solar output must shut down to ensure the protection and safety of all grid customers and maintenance crews. The solar radiation striking the PV modules will determine how much electricity is produced at any moment in time. Variations in light intensity due to clouds or other obstructions will change the output of the modules instantly. The installation may see wide energy output fluctuations over the daylight hours. The grid connection accommodates the output fluctuations by providing energy from other generating sources that have the ability to respond rapidly to compensate.
Do not take for granted the performance implications for the grid when it has solar generation connected to it. The grid will face voltage regulation, power quality and operational challenges in order to maintain the safety and reliability of all connected customers.
Your friendly Local Distribution Company, Regulator and Supply Planning Authority look after these issues through the connection assessment and approval process.
Without a grid connection the solar system would have difficulty matching it’s output to load without some other mechanism available to compensate for fluctuations. Off grid installations will need another energy source such as batteries to manage transient conditions and variable loads.
PV Solar energy production patterns
While it may be intuitively obvious that solar energy production varies with the sun’s position in the sky it is not at all obvious how large those variations are by geographic location, season and hour of the day. Based on data from Ontario’s Independent Electricity System Operator (IESO) for solar energy produced in southern Ontario, the daily energy pattern under ideal conditions for the summer and winter solstice looks like:
Normalized solar energy production over a year looks like:
These patterns depict ideal conditions for a location in southern Ontario and are to illustrate the concept of solar radiation patterns daily and annually. Although the charts are based on actual IESO data for 2018, adjustments were made to better illustrate the patterns – see note 1 at the bottom of the article. They are ‘normalized’ over the best-case output of the generation source (that is ‘1’ on the charts). The variation in solar output is important when considering the size, cost effectiveness and practicality of an installation. In this case the winter average monthly output of a solar installation in southern Ontario will be approximately 20% of its summer value and the capacity factor is 17%. Should this energy pattern be undesirable it may be mitigated by a connection to the grid, or by utilizing some form of energy storage.
The annual energy patterns for over 3,500 Canadian municipalities is available from Natural Resources Canada (NRCan) here. Based on their average values for Ontario, the normalized output looks like:
In this case the winter average monthly output of a solar installation in southern Ontario will be approximately 45% of its summer value and the capacity factor is 13%. These numbers are from a much larger data-set than the pattern I have presented for the ground-mounted solar farm. According to NRCan, average solar system in Ontario can produce 1,166kWh of electricity per kW of solar panels per year. Individual locations will vary as these are ‘average‘ numbers.
The annual US energy production patterns are available on the Electricity Information Administration (EIA) website as monthly capacity factors.
The farther you get from the equator, the less solar radiation you will receive (ignoring atmospheric effects). To have an appreciation of how much output changes as you move to higher latitudes, here are three examples of actual solar farm capacity factors (CF is explained here):
Solar energy applications – more than just PV
Here are some examples of the application of solar energy beginning with something small and moving to one of the largest installations in the world.
Electronics
The calculator uses thumbnail sized PV cells to power the microprocessr and display.
Infrastructure
Transportation development
Aerospace
Solar modules power NASA spacecraft. Technicians at Astrotech in Titusville, Fla., guide into place the second solar panel to be installed on Messenger spacecraft. Messenger was launched Aug. 3, 2004 aboard a Boeing Delta II rocket from Pad 17-B, Cape Canaveral Air Force Station, Fla. It achieved orbit around Mercury on March 18, 2011. The spacecraft crashed onto the surface of Mercury on April 30, 2015.
Residential electricity
Solar shingles
Solar shingles have been available since 2005 and are currently produced by a number of manufacturers. The technology remains more expensive than commercially available PV modules, however costs are expected to decline as production volumes increase.
Solar power for hydrogen gas
This process leverages solar heat for a thermochemical reaction to produce hydrogen gas from water. The gas can be stored or transported for use as a fuel.
Utility-scale solar thermal plant – Ivanpah Solar Power Facility (California)
The Ivanpah Solar Electric Generating System is a concentrated solar thermal plant in the Mojave Desert. The plant deploys 173,500 heliostat mirrors, focusing solar energy on boilers located on three centralized solar power towers. It uses steam to spin turbines and generate the electricity. Solar thermal installations have an advantage over PV since they produce electricity by rotating machines that produce alternating current which is compatible with our grid. Ivanpah is rated at 392 MW output and is the largest generator of its kind in the world.
Utility-scale solar PV – Topaz Solar Farm (California)
The 550MW Topaz Solar Farm solar photovoltaic power plant located in Southern California occupies 25.6 square kilometers of land and is easily visible from satellite via Google (Co Rd 0, Santa Margarita, CA 93453). It was constructed with 9 million modules based on thin-film PV technology which have approximately 21% efficiency and a 20 year life expectancy. Based on EIA data, the Topaz farm has operated with an average capacity factor of 26% from 2014 to 2018.
I’ll bet there was something here you didn’t know about!
Derek
Note 1 – The normalized energy curves for ground mounted solar are generated from data for a specific solar farm in the IESO generator output files for 2018. Bad data was set to zero and the hourly variations were smoothed using 5th order polynomial trending in Microsoft Excel. The generator outputs were then normalized using the maximum monthly output and manually adjusted for symmetry. The curves are based on a single sample case over the year 2018 and therefore have limitations. They are only intended to illustrate the solar variation phenomenon.
Previous article… A brief history of solar energy
Next article… Solar capacity and capacity factor
