Residential PV Systems
These designs are printed in the "Stand-Alone Photovoltaic Systems - a Handbook of Recomended Design Practices" available from Sandia. General information is provided in this section. For additional information select from the menu below.
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An increasing number of people living in remote areas are using PV systems or PV-generator hybrid systems because they are clearly the best economic option. Some estimates for utility line extension range up to $30,000 per mile depending on terrain. In such situations, PV systems are the economic choice, even for homeowners who want to maintain their suburban life-style. For the owner of a weekend cabin, recreational vehicle, or boat, the choice of PV is often based on the desire for serenity. PV systems make no noise and fuel delivery is automatic and free. Thousands of 30-200 watt systems are being installed for residential power in developing countries. These small systems are usually dc only and require 12 volt or 24 volt dc appliances. Stand-alone inverters are available from 100 watts to 5,000 watts. An inverter is often used in the larger systems to allow the owner the wider selection of ac appliances.
Stand-alone residential PV systems must handle a diverse set of loads. However, unlike other systems the owner/operator has direct control over the use of the loads and therefore, the power demand placed on the system. Training is an important part of owner satisfaction with system performance.
The following options will conserve energy and minimize initial PV system cost:
As a general rule, the designer should consider using a 12 or 24 volt dc systems for demands less than 1,000 watts. When the ac load is less than 1,500 watts, a 12-volt system with inverter is typically selected. A 24-volt system should be considered for ac loads (120/240 volts) in the 2,500-5,000 watt range, and a hybrid system may be the preferred option for large home power demands.
Arrays should be designed for easy expansion as the needs of the users increase. If the array is at ground level, access should be restricted to authorized personnel. Roof-mounted arrays should use a stand-off mount (>3 inches) and should not face more than 20° from true south. A specially designed support structure will be required if the tilt angle of the roof is not close to the tilt angle determined as optimum for the array. Roof-mounted arrays are less subject to accidental damage but are more difficult to test and maintain. Wiring should be sunlight resistant USE or UF type cable. All connections should be in water-tight junction boxes with strain relief connectors. Array wiring should be laced and attached to support structure with wire ties. Use conduit for output wiring to the controller and batteries. The array should be grounded using bare copper grounding wire (No. 8 or larger) securely attached to each support structure. Array tracking is sometimes used but the economic tradeoff of tracking structure versus more modules should be calculated. All disconnects or circuit breakers should be located in rainproof enclosures. Simple metering of voltage and current is recommended.
Batteries should be installed in a temperature controlled environment in or near the building. Prevent children and pets from getting near batteries and provide adequate ventilation. The batteries should be placed in a nonmetallic enclosure to protect against potential spillage of corrosive electrolyte if flooded batteries are used. Do not place batteries on cold surfaces. Do not expose batteries to flames or electric sparks. Industrial grade deep-cycle batteries are recommended for full-time residences but sealed batteries may be used to minimize the problem of ventilation and corrosion and lower maintenance cost. Check battery availability in the local area. Meters and/or alarms are often used to alert the homeowner to a low battery state-of-charge. An in-line fuse should be installed at the battery output terminal. Follow battery manufacturer's installation and maintenance requirements. Battery charge regulation is critical and directly affects battery life.
Charge controllers are recommended for residential systems and they should be sized to allow for future expansion of the system as the owner's power demand increases. Meters or battery charging indicators are recommended to allow the homeowner to monitor performance. Some system installers tie their warranty to monthly reporting of selected parameters from the homeowner. A competent control technician/engineer should be consulted for hybrid systems controls.
The selection of an inverter is a critical decision in remote home power PV system design as it sets the dc voltage of the system. Before purchasing an inverter, verify that it will be capable of starting and operating the expected loads. Multiple inverters connected in parallel, may be used to power larger loads. Make sure that the battery is large enough to supply the surge current requirements of the loads. A rule of thumb for battery capacity to ampere draw of the inverter is 5:1. All wiring, fusing, etc. should conform to standard electrical procedures as discussed in the National Electrical Code (NEC) for home wiring. NEC Article 690 covers photovoltaics, Article 310 has information on wire types, and Article 250 contains grounding regulations. Check with local authorities for applicable codes.
Ground mounts offer easy installation and maintenance and the possibility of seasonal adjustment of tilt angle. Fencing is recommended to protect the array from animals and children. Roof mounts may give better solar access in areas with a large number of trees or obstructions. Locate the array as close to the batteries as practical to keep wire length to a minimum. Support structures should be anodized aluminum, galvanized or stainless steel designed for maximum anticipated wind velocities. A good ground is required.
Acknowledgment and Disclaimer
