Written by Robert Collins, lead Solarcraft Engineer
Solar electric power is a practical, and in some cases, the only solution for powering remote equipment. Reliable, continuous, autonomous operation of end-use equipment is possible when a robust solar-electric system is in place. Proper system design requires essential inputs: careful choices in end-use equipment, a trustworthy estimation of power requirements, an understanding of the available solar resource and a description of the potential site.
The design of the solar power system naturally follows the selection of end-use, or ‘load’, equipment. However, selecting the right load equipment up front can go a long way in reducing the complexity, size, and cost, of a solar-powered system. Selection of load equipment should be driven by high efficiency and low-power consumption. When a solar system is purchased, essentially a 20-30 year supply of energy is being purchased in one shot. Power wasted is money wasted.
Solar-electric power systems are by nature DC systems. At the same time, prospective or desirable pieces of load equipment may require AC power. It is important to know that with few exceptions, such as motors, transformers, and lighting, nearly all electronic devices use DC power internally. This DC power is converted, usually at low efficiency, from the incoming AC power. To compound matters, any AC power required by load devices in a solar system must be provided by a DC-to-AC inverter.
DC-to-AC inverter manufacturers strive for efficiency and 80-90% conversion efficiency, near peak output, can be expected. Power lost while no load, or a small load, is present, termed quiescent power consumption, greatly reduces this efficiency. Worse still, the AC-to-DC conversion inside a piece of load equipment may be as low 50%. When converting from within the same system, the losses compound. The final system may be two to three times larger than an equivalent DC- only system.
To build the most efficient system, AC- powered equipment is only used when there is no other choice. It is advisable to swap AC motors in favor of DC motors, avoid inefficient ‘wall warts’ and other small AC- to-DC power supplies, and obtain the proper DC-input specifications and power the equipment directly. Equipment should be used that is designed to operate from battery supplies at 12, 24, or 48V DC, and operates from a range such as 10-16V, or 20- 32V. This is the sign that the equipment is intended to be operated from batteries and is often a hint that the internal power supply in the device is an efficient switching converter. Equipment with an input voltage listed over a narrow range such as 12.0 +/- 0.2V should be avoided. These loads can be accommodated using a DC-to-DC converter as a voltage stabilizer. Like inverters, DC-to- DC converters carry a penalty in efficiency and quiescent power consumption.
When possible, it is best to choose every piece of load equipment with the same DC operating voltage. A solar system will have one main battery bus voltage, usually 12, 24, or 48V. This voltage should coincide with the input voltage of the largest power consumer in the system. Other lesser loads operating at different voltages can be accommodated, but this will require a DC- to-DC converter, and its associated losses for each separate voltage.
The most important part of reliable solar design is quantifying the load – defining the average power consumption of the end-use equipment. This is often made difficult because manufacturer’s data for power consumption may be misleading or missing entirely. Many types of loads draw varying amounts of power over the course of normal operation. It is then important to know what the various duty cycles are to derive the average power consumption. For a given site location, average power consumption ultimately determines the size, and therefore the cost, of the solar system.
It is often just as important to know the peak-power demand of a load device. Devices such as DC-to-DC converters, DC- to-AC inverters, fuses, and wiring must be sized with sufficient capacity to handle peak-power cycles. Telemetry equipment is a prime example of this. A certain radio may require 50mA at 12.5V DC for receiving or idle operation, but require 8A while transmitting. Perhaps the transmit time is only 10secs/day and will hardly affect the average power consumption of the device. However, this requires that any device powering this radio is rated for 100W.
Underestimating load-power requirements will create a system that will fail in winter, or perhaps not work at all. However, overestimation leads to excess system cost. This may not be an issue when only one or two systems are required and the time and effort required to determine the exact power consumption exceeds the cost of a larger solar system. However, it is usually worth taking the time to derive a reasonably accurate value for power consumption. In the case of systems built in quantity, a thorough profiling of power consumption will always pay off.
Seasonal loads, for example heating and cooling, can pose challenges to system design. These loads must be estimated at least on a monthly basis for proper system sizing. As always is the case, the best strategy is to eliminate the need for them in the first place.
Manufacturers can usually provide more precise power consumption data when asked for it. Some can even offer simulations based on customer-usage patterns for power consumption. Although too often considered a last resort, the best solution may be to construct a system and measure the power consumption.
Once the power requirements of a system are been established, the next task is to determine the available solar resource for a particular location. In general, more sunlight means more energy. There are many variables. Cloud cover varies widely from region to region and day to day, days grow shorter in the winter and longer in the summer, and this variation becomes more extreme as we move farther from the equator. The type of solar array also affects energy production. Fortunately, solar resource maps are available and some difficult work has been done but first it is necessary to establish what type of solar array is being used. Solar modules require full direct sunlight perpendicular to their surface to produce their maximum power. The sun’s angle changes with the time of day and season.
Various devices, so called ‘solar trackers’, are available to track the sun with the solar array and maintain maximum power production. Unfortunately, the increased cost of tracking devices and related structure does not usually offset the value of the increased power production. In most circumstances, it is simply more cost effective to deploy a greater quantity of solar modules in a fixed flat-plate array.
Given a flat-plate array with a clear horizon to the east and west, the best choice is to angle the array toward the equator, which is due south in the northern hemisphere. The angle chosen between the array and the ground depends upon how power is required to be produced during the course of the year. To favor maximum annual power production, an angle of site latitude minus 15° is usually chosen. This makes the solar modules nearly perpendicular (at solar noon) to the sun’s rays during summer.
However, for stand alone applications with fixed, constant loads, we may wish to ensure minimum power production during winter. This is accomplished by mounting the array at angle of latitude plus 15 degrees. It must be kept in mind, if the system has seasonal loads, and the power requirement is something other than the two scenarios outlined above, the system designer can optimize a fixed-array angle to accommodate this.
In equatorial regions, this strategy tends to collapse somewhat. The lines between winter and summer blur. Additionally, solar Solar power modules should never be placed horizontal except in certain mobile or marine applications. A minimum angle of 5-10 degrees must be maintained to prevent build-up of such things as dirt and mold.
With a location, a flat plate array and an angle in mind, it is possible to consult any number of ‘insolation’ databases or maps for a value of peak-sun hours, based on the array type and angle. This value is used by the system designer to determine the quantity of solar modules required to power the load throughout the year. Values for peak sun hours are available in averages or minimums based on 30 years of historical data. Minimums are the norm when designing critical systems; and even this data must be used wisely.
Insolation maps refer by nature to large areas and charts refer to specific locations. Both provide the potential for a serious error in system design, due to the particular micro-climate. Data has not been gathered for every square meter of the planet. A micro-climate is a location with weather that is not typical for the greater area or nearby locations. Certainly a micro-climate with more sunlight than a map or chart would suggest poses no problem because the solar system would not likely be underpowered. This is not usually the case with micro-climates. A valley with fog that does not burn away until noon, and the foothills upwind of a large peak that gather louds are example situations that undermine the value of the insolation data and create the potential for an underpowered system. These conditions must be understood by the user and brought to the system designer’s attention so that compensation can be built in to the design.
Finally, there is the site itself. Mountains, forests, jungles, plains, deserts, swamps, oceans, vehicles, boats, offshore platforms, and spacecraft will require different system enclosures and array structures. A thorough description of the site conditions should include: the ambient temperature extremes, ground and soil conditions or mounting and structural requirements, information about snowfall, rainfall, and extreme winds, the likelihood of lightning, earthquakes, severe storms or flooding, and obstructions that may shade the array, such as antennas, trees and towers. The potential for theft and vandalism needs to be considered also. System design will be influenced by all of these factors and assembling the data will support and speed the design of a reliable solar electric-power system. It is important the solar designer asks these questions or gathers this information for, or from the client – essential outputs, providing power for the equipment, are the product of essential inputs.
Reference: Marion, William and Stephen Wilcox, 1994: Solar Radiation Data Manual for Flat-plate and Concentrating Collectors <http://rredc.nrel.gov/solar/pubs/redbook/> . NREL/TP-463-5607, National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, CO 80401