Photovoltaic System Types

Photovoltaic systems can be configured in many ways. For example, many residential systems use battery storage to power appliances during the night. In contrast, water pumping systems often operate only during the day and require no storage device. A large commercial system would likely have an inverter to power AC appliances, whereas a system in a small cabin would likely power only DC appliances and wouldn't need an inverter. Some systems are linked to the utility grid, while others operate independently.

1. Day Use Systems

The simplest and least expensive photovoltaic systems are designed for day use only. These systems consist of modules wired directly to a DC appliance, with no storage device. When the sun shines on the modules, the appliance consumes the electricity they generate. Higher insolation (sunshine) levels result in increased power output and greater load capacity.

Examples of day use systems include:

• Remote water pumping for a storage tank
• Operation of fans, blowers, or circulators to distribute thermal energy for solar water heating systems or ventilation systems

2. Direct Current Systems Powering Alternating Current Loads

Photovoltaic modules produce DC electrical power, but many common appliances require AC power. Direct current systems that power AC loads must use an inverter to convert DC electricity into AC. Inverters provide convenience and flexibility in a photovoltaic system, but add complexity and cost. Because AC appliances are mass-produced, they are generally offered in a wider selection, at lower cost, and with higher reliability than DC appliances. High quality inverters are commercially available in a wide range of capacities.

3. Direct Current Systems with Storage Batteries

To operate loads at night or during cloudy weather, PV systems must include a means of storing electrical energy. Batteries are the most common solution.  System loads can be powered from the batteries during the day or night, continuously or intermittently, regardless of weather.

In addition, a battery bank has the capacity to supply highsurge currents for a brief period, giving the system the ability to start large motors or to perform other difficult tasks. A simple DC system that uses batteries is illustrated below. This system’s basic components include: PV modules, charge controllers storage, batteries, and appliances (the system’s electrical load).

A battery bank can range from small flashlight size batteries to dozens of heavy-duty industrial batteries. Deep-cycle batteries are designed to withstand being deeply discharged and then fully recharged when the sun shines. (Conventional automobile batteries are not well suited for use in photovoltaic systems and will have short effective lives). The size and configuration of the battery bank depends on the operating voltage of the system and the amount of nighttime usage. In addition, local weather conditions must be considered in sizing a battery bank. The number of modules must be chosen to adequately recharge the batteries during the day.

Batteries must not be allowed to discharge too deeply or be overcharged - either situation will damage them severely. A charge controller will prevent the battery from overcharging by automatically disconnecting the module from the battery bank when it is fully loaded. Some charge controllers also prevent batteries from reaching dangerously low charge levels by stopping the supply of power to the DC load. Providing charge control is critical to maintaining battery performance in all but the simplest of PV systems.

4. Hybrid Systems

Most people do not run their entire load solely off their PV system. The majority of systems use a hybrid approach by integrating another power source. The most common form of hybrid system incorporates a gas or diesel-powered engine generator, which can greatly reduce the initial cost. Meeting the full load with a PV system means the array and batteries need to support the load under worst-case weather conditions. This also means the battery bank must be large enough to power large loads, such as washing machines, dryers, and large tools. A generator can provide the extra power needed during cloudy weather and during periods of heavier than normal electrical use, and can also be charging the batteries at the same time. A hybrid system provides increased reliability because there are two independent charging systems at work.

Another hybrid approach is a PV system integrated with a wind turbine. Adding a wind turbine makes sense in locations where the wind blows when the sun doesn't shine. In this case, consecutive days of cloudy weather are not a problem, so long as the wind turbine is spinning. For even greater reliability and flexibility, a generator can be included in a PV/Wind system. A PV/Wind/Generator system has all of the advantages of a PV/Generator system, with the added benefit of a third charging source for the batteries.

5. Grid-Tied Systems

We offer extensive experience and the highest quality components for grid-tied solar systems, a system connected to the electrical grid, allowing the customer to use the electricity from the grid as a back-up. Should your customer’s needs be unique, our team can design a system that reflects customer requirements and site specifications.

Photovoltaic systems that are connected to the utility grid (utility-connected, grid-tied, or line-tied systems) do not need battery storage in their design because the utility grid acts as a power reserve. Instead of storing surplus energy that is not used during the day, the homeowner sells the excess energy to a local utility through a specially designed inverter. When homeowners need more electricity than the photovoltaic system produces, they can draw power from the utility grid.

If the utility grid goes down, the inverter automatically shuts off and will not feed solar-generated electricity back into the grid. This ensures the safety of line persons working on the grid. Because utility-connected systems use the grid for storage, these systems will not have power if the utility grid goes down. For that reason, some of these systems are also equipped with battery storage to provide power in the event of power loss from the utility grid.

The Public Utilities Regulatory Policies Act (PURPA) of 1978 requires electric utilities to purchase power from qualified, small power producing system owners. The utilities must pay the small power producers based on their "avoided costs," or costs the utility does not have to pay to generate that power themselves. Additional terms and conditions for these purchases are set by state utility commissions and vary from state to state. While this law allows homeowners in areas with utility power to purchase photovoltaic systems and sell their excess power to an electric utility, people contemplating doing so should remember that this is rarely a profitable venture at the present time.

Some utility companies offer “net metering” to their customers, where a single meter spins in either direction depending upon whether the utility is providing power to the customer or the customer is producing excess power. The customer or independent power producer pays or collects the net value on the meter. Net metering is very desirable to the independent power producer because he/she can sell power at the same retail rate that the utility charges its customers.

6. Off-Grid

Just as our experience is extensive in grid-tied solar, so it is in off-grid, a stand-alone solar electricity generating system. We provide the knowledge, as well as the components, for off-grid solar systems. We can also design systems that accommodate virtually every type of remote location.

Off-Grid Introduction

By definition an off-grid power system is any system that provides power where utility power is unavailable. Off-Grid systems typically make financial sense any place where the utility would have to run lines more than one half of a mile for grid connection. In addition, the new federal PV incentive does not distinguish between grid-tied and off-grid, so any system should be eligible for a federal tax rebate.

A typical off-grid system typically consists of an off-grid inverter, batterybank, generator, and a DC power source (PV, Wind, Micro Hydro, etc.). If a PV array is used as a DC power source then a charge controller would also be used to harvest energy from the solar array and protect the batteries from overcharging.

System sizing is much more important on an off-grid system than a gridtied system. Here are the questions that need to be answered:

• How many kWh do you expect to consume?
• How many hours/days of autonomy do you want to be able to run without PV (or other energy source)?
• What is the largest load that you need to run? How much power is required to start this load?
• What is your budget?

Off-Grid Inverters

There are a number of things to consider when choosing an off-grid inverter:

Tare Losses

Tare losses are the power that is required to run the system in standby mode. Every watt is precious in an off-grid system and reducing power wasted is critical. This is a specification to look at very closely since there is a wide variance among different inverter manufacturers for tare losses. In addition, some inverter companies have the ability to turn off inverters entirely in multiple inverter systems to further reduce tare losses.

Surge Capability

The ability of an inverter to surge to a higher level than its rated output for a short duration to start large loads like well pumps is critical. The specifications that should be looked at are the Maximum Output Current and the AC Overload capability. If there are large loads a good number to look for is a five second surge capability of at least 1.5 times the rated output of the inverter.

System Information

It is very helpful to have good, reliable information about your battery’s state of charge. In many systems, generators are started automatically when the batteries get down to a certain state of charge. Usually this is accomplished through an external DC monitor. The best systems give you true battery state of charge which is a more accurate reading of the capacity of your batteries than battery voltage.

Field Serviceability

Often systems are installed in very remote locations. The ability to service the product in the field without having to take down the system is very important.

Source: “PHOTOVOLTAICS - Design and Installation Manual” by Solar Energy International.

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