The following was written in response to a recent item in the Press regarding the village of Rantoul’s solar policy.
Consumers currently are dependent on fossil fuel energy and nuclear energy resources that could be replaced or supplemented by solar energy. Solar energy may be used for structural heating, hot water and electricity generation.
Solar energy requires a collector to utilize the sun’s non-ionzing radiation. The Sun’s radiant energy is called insolation with the usable isolation varying by hour, day, and month due to the 23 degree tilt of the earth’s axis as the earth revolves around the sun and rotates on its own axis.
As seen from the earth’s surface the sun follows what is called the ecliptic path rising in the east and setting in the west. During the winter the sun is lower to the horizon than during the summer, resulting in shorter days and less insolation. This means we can collect and utilize more isolation during the summer in comparison to the winter because of the length of the day based on the sun’s location.
The sun’s location along the ecliptic in the sky is measured by its right ascension (azimuth) and declination (altitude). Solar radiation (ray) acts as a packet of particles moving in a straight line from the sun until it impacts the earth. What this means is that the collector surface must be at a right angle to the sun’s rays throughout the day, month, and year for optimal isolation utilization.
Some solar contractors tell consumers that the direction or azimuth the collector surface faces and the collectors tilt are irrelevant. This ignores basic astronomy principles and reduces collector efficiency but simplifies design, construction, and operation to reduce cost.
Structural integrity is essential. The roof or ground mount must be able to carry the total collector weight and resist varying wind speeds without collapsing. The collector also must be in an accessible location to permit maintenance. The collector should be made of materials impervious to the weather, mechanical, and thermal degradation.
Photovoltaic electrical generation is currently being promoted as a way to solve our electrical energy problems. Photovoltaics is the direct generation of electricity for any electrical power need. Sunlight is converted into electricity by using thousands of photo cells mounted on photovoltaic panels for electricity generation.
These panels may be mounted on the house roof or placed on the ground near the house. The sun always must shine on the panels to collect and convert insolation into electricity.
Therefore, the sun as seen from the collector always must be visible along the entire ecliptic. This means that there can be no trees, bushes, other buildings, or any portion of the building on which the collectors are mounted that blocks the solar insolation from reaching the collector surface. If the collector is in shade at any time its efficiency will be reduced.
A photovoltaic cell converts solar insolation into 12 or 24 volt direct current electricity (VDC) that must rectified into 120 volt alternating current (VAC) electricity to provide electrical power. It can also be used to provide an electrical power source for recharging hybrid vehicles.
A photovoltaic system includes: 1. Photovoltaic panels mounted on the roof or the ground made up of thousands of individual cells that transform non-ionizing radiation into 12 volts direct current electrical energy. 2. A solid state electronics rectifier that converts 12 volts of direct current (VDC) into 120 volts of alternating current (vac), 3. Connecting cables, safety interrupts, and switching boxes to hook it all together. 4. A step-up transformer to convert 120 vac into 240 vac electrical power. 5. A continuous line supplied 240 vac to enable photovoltaic system operation. 6. In some instances batteries to store generated electricity.
The conversion of sunlight or solar insolation into 120 vac electricity is a very low efficiency process due to cumulative losses. Photovoltaic cells are approximately 17 percent efficient. VDC to VAC rectifiers are approximately 80 percent efficient. This means that the overall photovoltaic system efficiency that cannot be altered or increased is approximately only 14 percent before insolation availability is even considered. This very low system cumulative efficiency effects system design, operation, and overall costs.
Solar energy and consequently electrical power load requirements are both measured in watts. The sun provides approximately 342 watts per square meter under ideal conditions. That quantity requires the collector surface to always be perpendicular to the incoming solar rays (insolation). The sky must also be completely clear of clouds or any other obstructions.
Simply the normal vector or an arrow that is perpendicular to the collector surface must point directly at the sun at its exact location on the ecliptic at all times. If the sun’s rays are not perpendicular to the collector surface we lose available insolation.
The easiest way to visualize the great losses is to shine a flashlight beam onto a white piece of paper. You will see a circular pattern then tilt the flashlight at different angles and see how much light is dispersed all over the paper. What this means is all photovoltaic cells will lose available insolation reducing overall efficiency.
Preventing this loss requires that the collector be able to move in both azimuth (right ascension) and elevation or tilt (declination) to always point at the sun as it moves along the ecliptic path. This is referred to as “tracking”.
A collector that does not move is referred to as “fixed.” Tracking collectors are complex and require axels, motors, and gears to allow the collector surface to rotate in two dimensions. A fixed collector will usually be pointed south and tilted at an angle equal to your specific latitude.
Champaign County’s latitude is 40 degrees north. Therefore contractors usually install the fixed collector systems at 40 degrees tilt or declination pointing south. That assumes that the fixed collector tilted at 40 degrees and pointed south as measured by a compass will be able to collect the greatest amount of solar insulation when averaged over a year and consequently make the entire system economically feasible. That is a false assumption.
It is crucial to understand that magnetic north as detected by a lensatic compass and polar or astronomical north and thus south are not the same direction. The earth’s magnetic north pole as detected from North America is located in the vicinity of Hudson’s Bay Canada.
Consequently photovoltaic collector design and installation must be based on complex synergistic trigonometry relationships. It also requires excellent comprehension and application of geometry skills in addition to standard mathematics.
Finally collector system design requires sizing based on calculation of your actual total structural load or power consumption in watts or kilowatts. This load is determined by adding up the power consumption of every electrical device you will use. Every device has an attached data plate that specifies the wattage that device will use. Average household power use today is approximately 8000 watts to 10000 watts or 8 kw to 10 kw.
Current cost for installation of a complete operational fixed solar collector array for Illinois is approximately $3.25 per watt. Consequently the initial cost may be $28,000 for an 8kw system. Please remember a PV system only produces power when the sun shines.
A PV system always requires line or power company supplied 220 vac 30 amp for its own operation. If line power fails then you do not have any solar generation available. If you are able to produce 50 percent of your electrical power needs from solar and your monthly electrical bill is $150 you may save $75 per month. Simple division $28,000 / $75 per month quickly reveals that it will take 374 months before you break even. Sadly equipment reliability especially for the expensive inverter has a lifespan of maybe 10 or 12 years before it must be replaced thus extending your breakeven point even further into the future because of maintenance costs.
In conclusion if you have money that is spare and not borrowed and you do not need to recoup your investment, then a pv array may be viable but if you are looking for short term gains and consequently free power it will not happen. We can’t change the laws of physics and astronomy no matter how much we wish.
Dr. Doug Rokke is a resident of rural Thomasboro.