Solar cells are made of certain semiconductor materials which produce a voltage when exposed to light. Small wires are placed on the semiconductor to provide a path for the flow of direct current (DC) electricity. As more light falls on a cell, more electricity is generated; therefore, a PV system must not be shaded (i.e. by shadows, snow, or wet leaves). . .because such shading can substantially reduce performance.

A typical solar cell made of crystalline silicon is 4 inches in diameter and 0.010 of an inch thick. In direct sunlight, it generates 2 amperes of direct current at 0.5 volts. By connecting solar cells in series (to increase the voltage), and in parallel (to increase the current), the output of a PV system can match the requirements of the load to be powered. If more power is required, modules can be appropriately connected in series or parallel to form what is called a PV array (see Figure 1).

Figure 1. Residential Photovoltaics System

The total energy output of a PV system can be increased by having the solar cells track the sun as it moves across the sky. Concentrating mirrors and lenses can also be used to increase output. These more complex systems are promising, but the additional cost must be evaluated on a case by case basis.

Most current PV installations are for power requirements in locations remote to existing power lines. In some instances, such as radio communications equipment on top of mountains, photovoltaics may be the only reasonable means of supplying power. However, distance from a power line is not always the controlling factor. For example, even if a power line is located in close proximity to a small load (such as an emergency call box), it is often more economical to use PV power instead of running a special line to the box.

Types of Systems

Stand-Alone Systems

PV installations not connected to a utility power line are referred to as "stand-alone systems." This comprises the majority of PV systems in operation today. The two basic types of stand-alone systems are: (1) direct systems, which utilize the PV electricity as it is produced, and (2) battery storage systems, which have the capability of storing PV generated electrical energy for use when the sun is not shining.

Figure 2. Stand-Alone System with Battery Storage

An example of a direct system is a water pumping facility which pumps water during the (sunny) day to a storage tank for later use. A battery storage system, on the other hand, stores PV electricity to power an electrical device (e.g., an appliance, a light) when the sun is not shining.

The range of applications for stand-alone systems is tremendous. Whenever the economics of providing electricity from the utility grid are in question, photovoltaics should be considered. For large power needs (greater than 500 watts), it has been common to use gasoline, propane, or diesel powered generators. Associated with this option is the high cost of maintenance, as well as the purchase and transportation of fuel. With low maintenance costs and no fuel requirements, PV is an ideal source of power for the less developed areas of the world.

It is also possible to couple a PV system with a fossil-fuel generator. In this type of system, the generator is used to recharge the PV battery system during long periods of cloudy weather. This "hybrid" system requires much less fuel and maintenance for the generator, while extending battery life.

Utility-Interactive Systems

Unlike stand-alone systems, utility-interactive systems are connected to the power line. A typical utility-interactive system can be located at a residence similar to the system at the NCSU Solar House (see Figure 3). This system has PV solar modules on the roof which supply electrical power to the house through a high quality inverter. This inverter converts PV-generated direct current (DC) to high quality alternating current (AC) normally available from the power company. Thus, when a PV system produces more power than is needed in the house, that excess power can be sold back to the utility.

Figure 3. Utility-Interactive System

Selling power back to an electric utility is not quite as attractive as it sounds. Most utilities charge customers retail rates, but offer to buy back PV-generated electricity at a lower wholesale rate. There is usually a significant difference between these two rates. Under current conditions, it is much more cost effective to use a PV system to displace the need for utility power than to generate revenue with it. Until photovoltaic power becomes cheaper relative to utility power, utility-interactive systems should be sized so that very little power has to be sold back to the utility.

In the future, it appears that large-scale PV systems may increasingly be owned by electric utilities or corporations. Due to economies of scale, system costs will be reduced, which will help the industry to grow. While large-scale systems are important to the future of photovoltaics, a major benefit of solar energy is that it is widely distributed. This asset will lead to the installation of more small-scale residential PV systems. Over time, this could reduce the need for costly distribution networks used in centralized generation of electricity.

There are some exciting new developments that are making PV systems more attractive in residential and commercial buildings. One idea that is growing more popular is building-integrated PV whereby the photovoltaics are built into the roof and displace the need for a lot of roofing materials. Another new development is "AC modules." These are PV modules that have small inverters on the back and DC power from the PV is immediately converted to AC so that there is no need to buy a separate inverter.

PV System Sizing and Applications

PV System Output and Sizing

PV modules are typically rated by their peak power output when exposed to a solar radiation of 1000 watts per square meter (317.2 BTU/hr-ft ^²) at a module temperature of 25ºC (77ºF). The former value is referred to as "peak sun" conditions.

During July in North Carolina, there are 14 hours between sunrise and sunset. Much of that time the sun will not shine at peak intensity (1000 W/m²). Clouds or haze may also cause the availability of solar energy to be reduced. The daily output of a PV system is measured in "peak sun hours," which is the number of hours the sun would had to have shone at peak intensity to generate the total solar radiation for the day. Thus, a PV array receiving 7000 watt-hours per square meter (7000 W-hr/m²) per day has received the equivalent of 7 hours of sunshine at peak conditions (i.e., 7 "peak sun hours").

Tables 1 and 2 show the average daily "peak sun hours" on a monthly basis for fixed (Table 1) and tracking (Table 2) arrays for the Raleigh-Durham area.

Table 1. Solar Availability in Raleigh-Durham, NC for South-Facing, Fixed Arrays (kWh/m²)

Table 2. Solar Availability in Raleigh-Durham, NC for One and Two Axis Tracking Arrays (kWh/m²)

Solar cells become less efficient as their temperature rises.

In summer, cells can easily reach 45ºC (113ºF), reducing power output 8% below standard operating conditions. Failure to consider this reduction could cause major design errors.

Example: Estimating Daily Power Output from a 50 Watt Panel in Raleigh N.C.

Thus, to maintain an average system output of 1 kWh would require four 50W panels in July and six panels in December.

Battery Sizing

PV systems typically require high quality "deep-cycle" batteries, such as those used in marine or golf cart applications. Automobile batteries are not designed to be deeply discharged and will wear out in just a few months in a typical PV application. Deep cycle batteries are usually rated by the number of "ampere-hours" available from full charge to total discharge at a specified rate of discharge.

When a battery stores electricity, it loses energy in the form of heat. A typical "turn-around" efficiency for a deep-cycle battery is 70%. This implies that for every 100 amp-hours of energy stored in a battery, only 70 amp-hours can be recovered.

If all the energy generated by a PV system is to be used during non-solar hours, the system sizing must reflect the 30% loss inherent in battery operation. However, if half of the load is carried directly by the PV modules during the day, the system output need only be discounted by 15%.

The amount of energy that a battery can provide to a load is affected by its rate of discharge. For example, a typical 12 volt deep-cycle battery may have a current rating of 100 amp-hours when discharged over 20 hours (i.e., at a discharge rate of 5 amperes). If the discharge rate is reduced to 1 ampere, it may be possible to receive 120 amp-hours from the same 100 amp-hour battery (i.e., a discharge period of 120 Hours).

Battery power (amp-hours) can be converted to photo-voltaic power (watt-hours) by simply multiplying the battery voltage by the amp-hour rating. Thus, a 12 volt, 100 amp-hour battery is capable of providing 1200 watt-hours of energy.

Since batteries are relatively expensive, prolonging battery life is an important aspect of PV system operation. This is best achieved by discharging the battery only slightly before recharging. Good system design suggests that a battery system be sized so that the daily load does not discharge the battery more than 20%. In the event of an extended cloudy period, the battery should never be discharged more than 80%. Most stand-alone systems with battery storage need a good quality charge-controller to insure that the battery is never over- charged or over-discharged. Both extremes can cause permanent damage to a battery.

Battery autonomy refers to the length of time a fully charged battery system can maintain operation of its electrical loads without input from the PV system. In some critical applications, it may be necessary to provide up to seven or eight days of autonomy to insure that the load will never be without power. This typically requires a large (expensive) battery bank. Many applications are not nearly this critical, with two or three days of battery autonomy being sufficient.
 

The following are just a few examples of applications that have been powered with photovoltaics:

• Water Pumping
• Navigational Signals
• Electrification of Remote Facilities
• Communications Equipment
• Sign and Area Lighting
• Residences
• Fence Electrification
• Call Boxes
• Recreational Vehicle Appliances
• Vehicle Battery Charging
• Cathodic Protection

A brief description of two DC system designs is provided below in Table 3. If AC appliances were used, the PV array would have to be increased by 20% to reflect an 85% inverter efficiency. These examples are only intended to demonstrate system configurations and should not be used as a basis for system design. In order to effectively design a system, it is essential to receive some prior training.

The ability to generate electricity wherever the sun shines is a capability unique to photovoltaics. As the value of this capability is better understood, the market for PV will grow significantly. Over the last 25 years, the cost of PV modules has dropped from $100 to $5 per peak watt. This, combined with newer electrical technologies, has resulted in the development of hundreds of thousands of electrical systems that are now powered by PV. As developments are continually made, this truly unique power source is sure to play a key role in our energy future.