Living with Renewable Energy
Types of Photovoltaic Systems
1) Small Stand-Alone DC System
The small stand-alone system is very efficient and an excellent way to power lights, water pumps, refrigerator, freezer fans and lights in a remote home, cabin boat or recreational vehicle. The size of the photovoltaic (PV) array and battery will depend upon individual requirements. The actual sizing methods will be discussed on page 5. The PV array charges the battery during the daylight hours and the battery supplies power to the DC loads as needed. The charge controller terminates the charging when the battery reaches full charge. The load center may contain meters to monitor system operation and fuses to
2) PV - Generator Combination
The PV - Generator Combination system is an economical alternative to a large stand-alone PV system, because the PV array does not have to be sized large enough for worst case weather conditions. A gasoline, propane or diesel generator combined with a battery charger can supply power when the PV array falls short. If the PV array is sized for average conditions, then during extended overcast situations or periods of increased load usage, the generator can be started. When batteries are low, the generator will power the AC loads in the house as well as a battery charger to help recharge the batteries. If the charger is adequately sized it can also power any DC loads in the system at the same time. Generator and battery bank size must be chosen carefully for reliable system operation. See the sizing section for more details on equipment choices.
3) Utility Intertie System
The Utility intertie system is used in an electrical grid connected home, shop or office. This system type can power loads directly and/or charge batteries and/or supply the utility grid. The utility intertie system employs a special type of inverter, which inverts DC power from the PV array into low distortion AC, acceptable for distribution by the utility company. The inverter may also act as a battery charger if the system is designed to power the home independently from the utility grid. In either case, the generated power can be delivered to the grid through a kilowatt-hour (kWh) meter. (Get your utilities approval before intertieing.) A second kWh meter is used to measure the power consumed from the grid during periods of insufficient generation by the PV system. The user of this type of system will notice no difference except lower utility bills or possibly payments from the power company for excess electricity generated. If batteries and a PV charge control are integrated into the system, the home will automatically begin operating on battery/inverter during a power outage from the utility lines.
The size of a solar electric system depends on the amount of power that is required (watts), the amount of time used (hours) and the amount of energy available from the sun in a particular area (sun hours per day). The user has control of the first two of these variables, while the third depends on the location.
Conservation (Systems Load Worksheets)
Conservation plays an important role in keeping the cost of a photovoltaic system down. The use of energy efficient appliances and lighting as well as non-electric alternatives wherever possible can make solar electric a cost competitive alternative to gasoline generators and in some cases, power from a utility company, especially if it is not already on site.
Cooking, Heating & Cooling
Conventional electric cooking, space heating and water heating equipment use a prohibitive amount of electricity. Electric ranges use 1500 watts or more per burner, so bottled propane or natural gas is a realistic alternative to electricity for cooking. A microwave oven has about the same power draw, but since food cooks more quickly, and are often only warmed, the amount of kilowatt hours used may not be nearly as large. Propane, wood, coal, etc. are better alternatives for space heating. Good passive solar design and proper insulation can reduce the need for heat. Evaporative cooling is an alternative to air conditioning in locations with low to moderate humidity, the results are almost as good. One plus for cooling—the largest amount of solar energy is usually available when the temperatures are the highest.
Lighting requires the most study since so many options exist in type, size, voltage and placement. The type of lighting that is best for one system may not be right for another.
The first decision is whether your lights will be run on low voltage direct current (DC) or conventional 110 volt alternating current (AC). In a remote home, RV, or a boat, low voltage DC lighting is usually the best. DC wiring runs can be kept reasonably short, or can be wired for DC current using the `BUSS` wiring method. Since an inverter may not be required, the system cost can be much lower. If an inverter is part of the system, and DC lighting still used, the house will not be dark if the inverter fails since the lights are powered directly by the storage battery. DC lights are not subject to lower power conversion efficiency, like their 110 VAC counterparts operated by an inverter.
In addition to conventional size medium base low voltage bulbs, the user can choose from a large selection of 12 & 24VDC fluorescent lights, which have 3 to 4 times the light output per watt of power used compared with incandescent types. Halogen bulbs are 30% more efficient and actually seem twice as bright as similar incandescent because of the spectrum of light they produce.
In a very large installation or one with many lights, the use of an inverter to supply AC power for conventional lighting can some times be cost effective. In a large stand alone system with AC lighting, the user should have a backup inverter or (at the least) a few low voltage DC lights in case the primary inverter fails. AC light dimmers will not function from inverters unless they have pure sinewave output. Small fluorescent lights may not turn on with some “load demand start“ type inverters.
Gas powered absorption refrigerators are a good choice in small to medium sized systems if bottled gas is available. Modern absorption refrigerators consume 23 to 46 litres of LP gas per month. If an electric refrigerator will be used in a stand-alone system, it should be a high efficiency type. Twelve and 24 volt powered refrigerator and freezer kits, which come precharged with environmentally friendly refrigerant, are available for use with your own home-built super efficient box, or can be used to retrofit existing refrigerators and freezers to DC operation by a handy do-it-yourself person. This can be much less costly and can rival the efficiency of manufactured appliances costing thousands of dollars more. SunFrost refrigerators use 300 to 400 watt hours of electricity per day while conventional AC refrigerators use 3000 to 4000 watt hours per day at a 210 C (700 F) average air temperature. The higher cost of good DC refrigerators is made up many times over by savings in the number of solar modules and batteries required.
Standard AC electric motors in washing machines, larger shop machinery and tools, pumps etc. (usually 1/4 to 3/4 horsepower) require a large inverter. Often, a 2000 watt or larger inverter will be required. The inverter will get warm or hot when running these loads, which may shorten its life. These electric motors are sometimes hard to start on inverter power because of a high surge requirement. They consume relatively large amounts of electricity, and they are very wasteful compared to high-efficient motors, which use 50% to 75% less electricity. A standard washing machine uses between 350 and 500 watt-hours per load. If the appliance is used more than a few hours per week, it is cheaper to pay more for a high-efficiency appliance, rather than make your electrical system larger to support a low-efficiency load. For many belt-driven loads (washers, drill press, etc.), their standard electric motor can be easily replaced with a high-efficiency type. These motors are available in either AC or DC, and come as separate units or as motor-replacement kits.
Vacuum cleaners usually consume 600 to 1000 watts, depending on how powerful they are, about twice what a washer uses, but most vacuum cleaners will operate on inverters larger than 1000 watts because they have low surge motors.
Many small appliances such as irons, toasters and hair dryers consume a very large amount of power when they are used but by their nature require very short or infrequent use periods, so if the system inverter and batteries are large enough, they may be usable.
Electronic equipment, like stereos, televisions, VCR`s and computers have a fairly small power draw. Many of these are available in low voltage DC as well as conventional AC versions, and in general, DC models use less power than their AC counterparts. A portable stereo “boom box“ that runs on 8 or 10 “D-cell“ batteries will usually work on 12 volts DC. Some have a DC input, or you can connect wires from the battery contacts to the 12 volt system. This should be done by someone experienced in electronics repair.
Many appliances are instant-on types which use electricity even when they are supposedly off. Back lights, built in clocks, programmability and memory features are indicative of appliances with "phantom loads" which can rob you of useful energy and bleed your power system. They are poor choices for incorporation into RE systems. Watch for them.
Keys to Your System's Performance
As you proceed from here to design your system we have these 5 guidelines to keep "top of mind" to ensure you are pleased with the performance of your renewable energy system
- Efficiently use your solar energy
- Fully recharge your batteries
- Prevent overcharging and excessive gassing of your batteries
- Prevent over discharging and sulphation of your batteries
- Invest in status information (metering) for load management