The current trend in world oil and fuel prices signal the urgency for alternative sources of energy. Reminiscent of the oil crisis during 1970s, solar energy is once again in the limelight.
The benefit in the use of the clean and renewable power of the sun is often overshadowed by the expensive cost of installation. This is mainly due to the price of solar panels and storage batteries. Currently, the efficiency of an average solar panel is only 10-15%. That means that only a fraction of solar energy striking the panel is converted into electricity. The development of much more efficient panels and better electrical storage devices are crucial factors in making solar energy affordable and gain widespread use.
An average complete solar energy electrical generating system for the household may range from $15,000 to $70,000. However, the cost of the total installation will eventually pay for itself through the money saved from paying electrical utilities. Information and learning some basic skills is the first step in the planning of a solar energy system.
There are four common set-ups in a solar energy system. They differ in the way that the generated power is used.
Back-up power – using the solar system as a source of emergency power when regular electrical service is interrupted
Partial use – wiring only some of the household circuits to run directly off the solar system
Grid interactive or Grid-tied system – excess electricity generated by the solar system is fed onto the grid. This has the effect of turning back the meter or offsetting power. When there is insufficient power generated by the system, electricity is drawn from the grid.
Off-grid system – electricity is entirely generated by the solar system. Self-sufficient in terms of electricity and is independent from the grid. This is the most expensive.
In any case, the feasibility of any solar energy system is determined by your commitment and how much you are willing to spend.
Things to consider
1)How much electricity do you use?
Your electric bill indicates your monthly electrical usage including fluctuations depending on the season. The highest monthly consumption in a year is the peak power consumption and is an ideal reference in designing an off-grid solar system.
Power (P) is computed as current (I) multiplied by voltage (V) or P=IV. Appliances usually list the power in watts or current in amperes to indicate their electrical consumption. If its in watts you simply multiple its “wattage” to how long you use it (in hours) to get your usage. A 50-watt bulb that is on for 10 hours results in 500 watt-hours. This can be written as 0.5 kilowatt-hours (kwh) because 1-kw equals 1000 watts. If it is in amperes you multiply the “amperage” to the voltage. A regular line supplies 120-volts of electricity. Therefore, an appliance using 10 amperes can use 1.2 kilowatts in just an hour. The sum of all appliance usage for a whole month indicates your monthly electrical usage.
2)Can you find ways to cut and limit your electrical consumption?
Find ways to decrease your electrical consumption by changing to fluorescent lights and replacing old appliances to newer efficient ones. Exercise discipline when to use electricity and limit the use of “luxuries” like air conditioners. Avoid appliances that utilize heating elements such as irons, water heaters, electric stoves, etc. These are “power hungry” machines that consume too much electricity. Switch to gas or propane equivalents. Your monthly electrical bill is dependent on how much electricity your appliances require and how long you use them. A decrease in either one or both will lessen total electrical usage.
3)Do you have the right location?
To harness solar energy adequate exposure to the sun throughout the year is required. The electricity produced by your panels is directly proportional to the solar energy striking it. The intensity of solar energy is affected by the time of day, season of the year and weather conditions such as rain, snow and clouds. There is on average 5 to 6 hours of effective peak sun hours per day. Determine the most effective place for the solar panels and avoid shaded areas.
A Typical Solar Energy System
A typical solar energy system (Figure 1) is composed of the solar panels, charge controller, storage batteries and inverter. Many brands are commercially available that can provide a range of solutions. In any system always consider your peak power requirement in choosing the individual components.
A solar panel is made up solar cells “wired” together. There are three types of solar panels – monocrystalline, polycrystalline and amorphous.
Monocrystalline – highest efficiency, most expensive due to complicated production methods
Polycrystalline – less efficient, less expensive
Amorphous – least efficient, the cheapest, versatility allows it to be shaped into different forms
A typical solar panel can lasts for more than 20 years and require very little maintenance except for occasional cleaning. Solar panels have different voltage and current generating capacities. A solar panel rated at 17 v and 6 amperes has a power output of 100 watts (P=IV=17.1v x 6 A=102) at peak hours. With 6 peak hours that yields 600 watt-hours of power a day. Solar panels can be wired in series (positive to negative) or parallel (positive to negative/negative to positive) to increase their power output. (Figure 2) Wiring solar panels in series or parallel increases voltage and current, respectively. (Figure 3)
A good rule of thumb to follow in choosing solar panels is “dollars per watt”. This will provide maximum power at the least expense. Using second hand solar panels is also definitely cheaper compared to new ones.
The function of a charge controller or regulator is to prevent the overcharge and discharge of storage batteries. Charging current is supplied by the solar panels to the batteries. Depending on voltage set points, the charge controller allows the correct amount of current (charging rate) to flow when it detects that the batteries’ charge is low. At a preset level it cuts-off the flow to avoid overcharging that will otherwise damage the batteries. Voltage set points are different charging rates (fast, gradual, trickle) relative to battery temperature. Maintaining nominal battery temperature in varying charging rates ensures longer battery life and safety. Some controllers with temperature compensation have sensors that can vary the set points depending on battery temperature. Charge controlling through set points allows a wider range of battery types to be used. The charge controller also prevents the discharge of batteries through reverse current flowing back to the solar panel especially during nighttime.
As a rule, choose a charge controller that is intended for your type of battery. Low Voltage Protection (LVP) or Overload Protection may be a separate or built-in feature.
The batteries are the components that store the electricity generated by the solar panels. They can be connected in series or parallel to achieve the desired input voltage and power for the inverter. (Figure 4) The most common type used is the deep cycle lead acid battery. This is further classified into the flooded type and sealed type.
The flooded type or wet cells, as its name implies, is filled with fluid. It is economical but bulky and is commonly used for solar battery banks. These battery banks provide the energy required by off-grid systems or high power loads. It is usually made up of large 2-volt cells connected together. 6 and 12-volt cells are also available.
Sealed type known as “maintenance-free” or “absorbed gas mat” (AGM) batteries have pads between the plates that are “soaked” in fluid. They are much smaller and used for lower power needs such as in back-up systems. Sealed types are charged at a voltage a little lower than the big flooded types due to the limited fluid between its plates.
Deep cycle batteries must not be confused with car batteries. A car battery provides limited high current used for starting and not designed for “deep discharge cycles”. Deep cycling means that more than half of the energy stored is used up or discharged before recharging. Lead acid batteries are designed for thousands of cycles however flooded types have more cycles compared to sealed types. The remaining charge of the battery must not fall below 20% of the battery capacity. That means a maximum of 80% percent of the charge can be used before it needs to be recharged. When the charge falls below than 20% the battery is irreversibly damaged. Overcharging also causes damage. Proper charging is maintained with the use of charge controllers and regulators.
Battery capacity is measured in terms of ampere-hours (Ah) and may range up to 2000 Ah for lead acid batteries. This means that a 50 Ah battery can be used at 5 amperes for 10 hours. However, recommended use is at 5 amperes for less than 8 hours only. (10 Ah or 20% remaining capacity) Nickel Cadmium (Nicad) batteries offer up to 50000 cycles, charging stability and longer life but are very expensive.
Make sure that the number of solar panels is able to recharge the batteries even during off-peak hours (nighttime) and that the system is not overloaded. By doing this, deep cycling is avoided thus increasing the overall life of your battery banks. Remember that overloading means that too much power is used than the system can provide or that the maximum time that power can be used has been exceeded.
The inverter is the final component of a solar energy system. There are numerous companies that sell different types of inverters. Limit your choices to companies with a proven track record in the design and building of inverters. Inverters convert the direct current (DC) power from the batteries to alternating current (AC) electricity. This is accomplished through complicated electronics that simulate the 60-hertz sine wave output of a typical 120-volt line. Early inverters used modified square waves or synthesized “stepped” sine waves. Currently, inverters that range from 1500, 2500, 3600, 4,000 and 5,500-watts are available that use dc input voltages of 24 or 48 volts.
Many inverters have a “transfer relay” that can be connected to a sub-panel to power essential appliances in case of “blackouts”. The inverter provides back-up power from the batteries until normal power is restored. The ac voltage is detected at the “ac input” and the relay transfers the load back to the grid.
Partial use or transferring some circuits to be run from your system may be done though the transfer relay. AC input monitoring is disregarded and the sub-panel is reserved only for those circuits. Usually inverters with AC inputs also have a built-in battery charger that can be used to charge the batteries using the grid. Be careful to balance the voltage set points of the battery charger with that of the charge controller used by the solar panels.
Grid-tie inverters are used in grid-interactive systems. This system may use storage batteries. During daytime, the storage batteries are kept at full charge and excess power generated by the solar panels is fed by the inverter to the grid. During nighttime the inverter uses the storage batteries to provide the electricity needed and no power is drawn from the grid. When there are no storage batteries, the solar panels are wired to produce higher dc voltages (300 to 400-volts) that is fed back by the inverter to the grid. Much cheaper but hazardous due to the higher dc voltages involved that may cause shock and/or fire. Proper “grounding” must be observed.
High power robust inverters are used for off-grid systems to provide the reliability for full time use. A panel for dc monitoring or remote display helps keep an eye on battery bank levels and overall system status. The inverter may include a control relay to turn on a generator to be used in conjunction with the system when battery levels are low or power requirements increase.
In the design of a solar energy system inherent losses through component inefficiencies must be expected. Considering these losses in the design will guarantee that the system can produce the required output power. It is highly recommended to seek the advice and help of professional solar installers and electricians to ensure the safety and quality of the installations.