Lighting Research Center Lighting Research Center

Volume 2 Number 4
Copyright @1998 Rensselaer Polytechnic Institute

Solar Power is Hot!

Most of us don't think twice about flipping a switch,clicking a remote control, or even clapping our hands when we want light, entertainment, or communication in our homes and offices. The electricity is there, just waiting for us to command it for our convenience, safety, warmth, and access to the outside world.

For those times that we are without power we can be hard-pressed to find ways to cope. We light candles, cozy up under warm comforters, or in summer, sip cold drinks (for as long as the ice lasts). We worry about refrigeration; we struggle to survive without television; we fret, along with our children, about the dark.

The other end of the tunnel

More than two billion people worldwide, including pockets of underdeveloped or remote areas in the United States, have no electric power and no hope of being put on a national utility grid.

Now, however, a 20- to 30-year-old technology that was perhaps overmarketed in the past is making inroads into places where it was once thought that electricity would never be available.

Power of the Sun

Photovoltaics (PV) are solid-state semiconductor devices that take radiant energy from the sun and convert it into direct-current (dc) electricity. According to Sarah Howell, Media and Government Relations Coordinator for Solarex, a major manufacturer of photovoltaic modules, it works this way:

"The heart of a PV power system is the cell, which converts energy [from the sun] directly into electricity. Typical Solarex polycrystalline cells consist of at least two layers of semiconductor material such as silicon. One layer is doped with a small quantity of boron to give it a positive electrical charge. A thin layer on the front of the cell is doped with a phosphorous material to give it a negative character. The interface between these two layers contains an electric field and is called a junction.

"When particles of energy (known as photons) enter the cell's positive layer, some photons are absorbed in the region of the junction, which frees electrons in the cell's negative layer to flow from the semiconductor material through an external circuit and back into the positive layer. This flow of electrons is an electric current," said Howell. "The PV process is completely solid-state and self-contained. It involves no moving parts and no materials are consumed or emitted."

Add-on modules expand power

A single solar cell produces about 1/2 volt (V). This can vary with the intensity of the sun, which changes because of cloud cover and seasonal position; higher voltages are achieved by connecting solar cells in series. A typical photovoltaic module contains 36 four-inch square silicon solar cells, connected in a series, to provide enough voltage to charge a 12-V battery. Multiple modules wired together form an "array" which can produce from a few watts to several megawatts of power.

This ability to expand systems to meet power requirements is one of the biggest advantages of PV systems. Other major pluses include:

  • Reliability. PV systems have no moving parts and operate unattended. They are not affected by on-grid power outages and can withstand severe weather conditions including snow and ice.
  • Unlimited resource and convenience. They consume no fossil fuels which eliminates the need to transport fuel to a remote site.
  • Ease of maintenance. Typical batteries are warranteed for 3- to 5- years, and PV panels can last for 15 to 20 years.
  • Non-polluting. No detectable emissions or odors during operation.

Let the sun shine in

Because maximum exposure to sunlight changes with the seasons, the array tilt (or angle at which the solar array faces the sun) is important for the most uniform system performance. Although the best tilt varies with the site, season, and weather conditions, latitude plus 15 degrees is widely accepted as the rule of thumb for best mean performance with minimal seasonal variation for a non-movable PV array.

A window to the world

A typical PV system for a home includes a 20 to 100 watts peak (Wp) PV array. The capacity of a PV module is defined as watts peak of output. The rated peak output is measured under standard test conditions of 1,000 watts per meter squared (W/m2) solar radiation, and 25o C cell temperature, according to Best Practices for Photovoltaic Household Electrification Programs, published by The World Bank.

Because array size and available sunlight determine the amount of electricity available for daily use, even systems of the same size will vary in output. In Indonesia, for example, a 50-Wp system can provide enough energy to operate four 6- to 10-W fluorescent lights and a 15-inch direct current (dc) black and white television for up to five hours. In an area with longer hours of sunlight, a smaller system could provide a similar amount of power.

Storage batteries collect energy for use at night or in times of little sunlight, such as during heavy cloud cover, thus providing an uninterrupted power supply. Systems include a charge controller to avoid overcharging the battery. An inverter may be added to the system if alternating current (ac), the type most Americans use in their homes, is required.

Batteries are trickle-charged; in other words, even when appliances are using power, cells are converting solar energy to electricity and more power is being stored, so the battery is continually recharging.

PV systems are either connected to the grid as supplemental power (for use during peak demand or in emergencies), or are used off-grid where no utility lines are available. In off-the-grid areas where sunshine is minimal in some seasons, PV systems may require a fuel-powered backup generator for continuous power use. Many system dealers provide pre-packaged systems that include all necessary components.

Forecast: cloudy and cold

PV systems perform even if the day is cloudy or cold. Under bright overcast, PV systems generate about 50 to 70 percent of their rated output; in dark overcast, the output is proportionately diminished to perhaps only 5 to 10 percent.

Cold actually enhances power generation because PV cells operate more efficiently at cooler temperatures. Less power is generated in the winter, but this is because of shorter days and lower sun angles. Because the panels are mounted at an angle, snow often melts before accumulating enough to completely block sunlight from the panel's face.

Let there be light

Photovoltaic systems typically operate at 12- to 24-V dc and serve as power for lighting and appliances, according to Roger Hill at Sandia National Laboratories. While most systems use fluorescent lamps because they require less energy than incandescents for similar light output, PV systems can power most other types of lamps as well. The lamps can be controlled with timers, photocells, or motion sensors for added energy efficiency.

Direct-current lamps are more reliable and efficient than ever before. Coupled with PV modules, these lamps are providing light throughout the world. They illuminate portable highway information signs, off-the-grid billboards, public-use facilities, bus stops, parking lots, marinas, remote facilities, homes, and vacation cabins.

The Market

With about one-third of the world's population without access to electricity, the potential market for PV is enormous.

Scott Sklar, Executive Director of the Solar Energy Industries Association (SEIA) told Lighting Futures: "1996 was the biggest year so far for photovoltaics. Three new, or expanded, automated PV manufacturing plants were put into use, and another six will be functional in 1997. The PV market is booming, with 70 percent of production exported to Third World countries. And that's just the PV modules. When you add in controls, batteries, and luminaires, the market is $1 billion per year in [PV systems for] lighting alone."

Sklar said the three main reasons for this market expansion are:

  • Modernization. New countries are evolving at a rapid rate, and they are striving to improve the quality of life of people who have been ignored. While more than two billion people worldwide have no electricity, an additional billion have access for less than five hours a day.
  • Privatization. Many national governments presently own electric utilities, but they are following the lead of the United States and Great Britain by allowing the private sector new markets and energy services.
  • Environmental concerns. PV is clean and does not put a burden on limited natural resources. It is quiet and has virtually no moving parts, so PV panels can last for about 20 years with almost no maintenance.

Sklar thinks the market will boom for at least the next two decades. "Multilateral lending institutions have already loaned $1 billion to PV suppliers and consumers, signifying the lenders' confidence in the market's growth. Domestic utility deregulation in the United States will also increase the demand for PV products as energy consumers increasingly request efficient, reliable, midday power and off-grid services.

"Of the remaining 30 percent of PV systems sold domestically, about one-half are for off-grid lighting. This includes lighting for streets, remote parks, bus stops, and marine sites, and transportation signage for railroads, highways, and airports," said Sklar.

Off-grid applications

PV systems provide practical solutions for power in remote areas where a full infrastructure of power lines and plants, or accessibility to the existing grid, is prohibitively expensive.

Some ways that PV power in remote locations could be (and in many cases already is being) implemented include:

  • Water purification
  • Pumping water to irrigate crops or for cattle
  • Highway, railroad, and airport lighting, signs, flashing signals, traffic counters, and calling boxes
  • Lights and refrigeration to operate a hospital in an underdeveloped country
  • Power to run lights, fans, and computers at remote park stations

On the other hand

When compared with the cost of grid-provided electricity, PV systems are still very expensive. At the average price for electric power in the U.S., it takes exceptional reasons to rely on PV as an alternative energy source for many applications. "It just doesn't make economic sense right now," said Jennifer Harvey, Senior Project Manager, Energy Resources at the New York State Energy Research and Development Authority (NYSERDA). "The payback for installing a PV system can be 10 years or more and that's just not acceptable."

According to Solarex, the cost of PV power depends greatly on the application. However, as a general guideline, using typical borrowing costs and equipment life, the life-cycle cost of PV generated power generally ranges from $.30 to $1.00 per kWh. Therefore, the cost usually prohibits using PV in areas where an existing grid is available.

There are, however, many cases where PV systems can play a vital role even when the grid is nearby. These include temporary power requirements that avoid running power lines through sensitive areas (such as through private property, beneath paved parking areas, or over busy highways), and the need to install a system quickly. Under a time-of-day pricing structure for electricity, PV is competitive during the peak demand part of the day. (A time-of-day structure prices power according to demand; power is more expensive during peak periods and less costly when demand is lower.)

In addition, because of the reliability of the system, stored power can be accessed for critical loads, such as for telecommunications and computers, or as a dependable backup to a regular power supply.

Some utilities are using solar systems to help offset more expensive peak power. Another utility is considering the merits of placing a PV system near the end of distribution lines that become heavily stressed because of summer air conditioning loads. Even though the economics of operating a PV system are not cost effective as yet, the extra power provided by the system may help the utility to avoid the expense of upgrading the line to a heavier voltage-and, as PV power becomes more economical, the benefits and costs may become more balanced.

Birds and bucks

Ecology concerns are the most compelling reasons for off-grid use in places where energy from the grid is available. "Companies and individuals concerned about the environment may choose to pay more for environmentally friendly power," said William Seils, Vice President of Marketing and Sales for SunWize Technology, a company that specializes in providing PV engineered solutions on a world-wide basis.

"Ecotourism is also starting to come into its own, with resorts built around the philosophy of providing recreation while preserving and protecting the environment. This is a perfect opportunity for PV technology," he added. "Irrigation is also a natural for PV technology. Think about it: solar power is plentiful in dry hot weather, which is exactly when water is most needed."

Upfront planning

Jeffrey Peterson, Program Manager, Energy Resources, at NYSERDA emphasized that the key to overcoming the perceptions that PV is unreliable or too expensive happens in the design stage. PV technology needs expert designers at the outset who know how to minimize the power loads and wiring costs. A designer who understands the needs of the system can effectively plan a PV modular or hybrid system (one that includes an electric generator to switch on when solar energy is insufficient to supply the load) to provide sufficient power to meet the required demands in the most economical way possible.

According to Harvey, "There are two main reasons why PV will be successful: reliability and small load requirements. Customers are interested in protecting their power supply for critical applications such as computers. Also, single small loads, such as a street light at the end of a person's property line, are perfect examples of how this technology can be used near the grid in the United States."

"Applications that are grid-independent are the most cost effective," said Peterson. "For example, suppose you want to put a light next to your backyard pool. You don't want to dig up the yard, and you need the power only about three or four hours in the evening. A solar-powered pool light can provide light for the required time without the expense and inconvenience of major reconstruction."

Meanwhile, back at the ranch

One sign of potential growth in the marketplace is the formation of new companies whose goal is servicing the PV industry. Texas-based Planergy, Inc., one of the nation's largest independent energy service companies, recently announced that it will manage a coalition of rural and municipal electric utilities called the Texas PV Coalition (TxPVC). This coalition was formed to accelerate the commercial availability of PV systems as a service alternative to extending utility lines, and is focusing on developing self-standing photovoltaic systems.

In areas where installing electric service can mean constructing line extensions for several miles at a cost of up to $20,000 per mile depending on the type of terrain, potential customers usually choose to live elsewhere or manage without electricity. According to John Hoffner, Director of Alternative Energies for Planergy, PV systems "will provide a solution to the problem of extending power lines into hard-to-reach areas, increasing customer satisfaction and bringing new revenue to utilities."

"PV systems are becoming increasingly viable as an energy option," said Hoffner. "The cost of photovoltaic services is decreasing at the same time the efficiency of photovoltaic energy technology is increasing. For many applications, PV systems are now the lowest-cost power alternative."

Seils agrees. "A PV system might have an upfront cost of $25,000, but it has a life expectancy of 20 years or more with minimal maintenance. Compared with the cost of installing power lines in a remote area, a PV system will provide the power needed at a comparable cost."

Sunny side up

Douglas A. Koop, President of SunWize, put on his fortune-teller's cap and told Lighting Futures where he sees the market going. "PV systems are stand-alone systems. They power the batteries, which power other things. Once information about the technology flows, demand for other products will grow. As people in remote villages start to know about the world that surrounds them, urban and rural boundaries will blur.

"Communications will be the key that opens the door to the external world for these people. Satellites will offer cellular-phone service and PV will provide the power. At first, there will probably be only one phone in a village, but eventually, there will also be one computer with a modem. And once villagers start to communicate with the rest of the world, a whole new market expansion will be seen," said Koop.

NYSERDA's Peterson added other possible uses: portable PV systems could be driven to disaster areas to provide emergency power; on the highway, deer-crossing warning paths could be set up; and fiber optic guardrails could be powered by PV modules to help illuminate highway lanes during construction, smog, fog, or blizzards.

A beacon in the night

Rick Lewandowski, President and CEO of Direct Global Power, Inc. (DGP), a solar energy systems development and marketing company, told Lighting Futures about a PV system his company installed for school lighting in a remote area of Sierra Leone. "It was the only source of night lighting in the village other than kerosene or candle. The school became the social community center for the village and residents would use its beacon as a guidelight to find their way home at night."

DGP has designed a new outdoor luminaire that appears perfect for the home market. Smaller than a solar-powered street light, but larger than a luminaire designed for marine or pool areas, this system has the PV panel built into the top of the l4minaire (like a roof). The lamps are underneath the solar panel and provide light downward all night through transparent openings. The battery is encased inside the pole, so the entire unit is attractive and functional.

A house in the woods

Between December 1993 and 1995, NYSERDA, the New York Power Authority (NYPA), and SunWize developed and demonstrated a photovoltaic and generator hybrid system to meet the total electric needs of a home in upstate New York.

The system comprises a 48-V PV array, rated at 1 kW, with an area of about 9 square meters. For supplemental power, a two-cylinder propane-fueled generator is available to produce 3.6 kW at 120-V ac single phase. Twenty-four 450 Amp hours, 2-V lead-acid batteries, connected in series make up the battery pack. In addition, two 2.5 kW inverters convert the energy to 120-V ac.

The combined PV system and propane-fueled generator provided all electricity for heating and appliance use for the home. The generator was switched on when the amount of stored solar energy was insufficient. The PV array produced 13 percent of electricity used in the winter and 48 percent in the summer. Overall, it produced 974 kWh in 1994, with the propane-fueled generator producing 2,520 kWh. (See photo on back cover.)

Is sunlight really free?

According to NYSERDA, a cost comparison found that the system's installation and operation for this house compared favorably with a utility line extension. A similar system's costs could vary depending upon the area of the country, distance from the grid, interest rates, and other variables.

Assumptions use a 20-year life cycle. Investment Rate was 12 percent; Net Discount Rate, 7.28 percent; Inflation Rate, 4.4 percent. The installed cost of the PV system was $26,500. 2100 liters of propane were used per year at $0.32/liter. Estimated power consumption was 37,020 kWh total, or 185 kWh/yr. Using these assumptions, the estimated total life-cycle cost of the PV system would be $44,400. In comparison, the cost of installing a utility line extension is estimated at $45,860.

This compares lifecycle costs for a PV hybrid system to the cost of a utility line extension. The PV system, which provides more expensive power per kilowatt hour than on-grid electricity, enables access to electricity in an area that would otherwise be without power. The operating cost for on-grid electricity is not included. A simple calculation using $0.10 per kWh would add $3702 to the on-grid total for the 20 year period.

What's next?

NYSERDA is considering testing the effectiveness of a portable PV system that will be used on a farm cooperative for irrigation and light for both the barn and birthing animals in the field. Because the system is a mobile source of power, other practical uses are expected to be discovered as the system is put into use. "The results from this research, as with all NYSERDA findings, will be available to the public and the PV industry for use in moving PV technology further along the path to commercial success," noted NYSERDA's Peterson.

"This is a young industry," said Harvey. "Its higher potential is not yet fully realized. We need to convey the value of the product and its efficiency, and better distribution channels need to be implemented."

To that end, NYSERDA is planning a conference, tentatively scheduled for 1998. "We want to bring together potential customers, energy groups, and the PV industry to increase awareness and understanding of the values, needs and specifications of the marketplace," said Peterson.

As consumers and companies become more aware of the potential of photovoltaic technology, Lighting Futures thinks this clean, cost-effective, and environmentally friendly technology should become one of the mainstays of power production and provide a better way of life for people who now have little or no access to electric power.

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