Photovoltaics

Overview

Photovoltaic (PV) panels, or solar panels, convert sunlight directly into electricity. They generate electricity with no moving parts, operate quietly with no emissions and require very little maintenance. They are also extremely durable and expandable.

The cost of solar panels has fallen rapidly over recent years, however it is still one of the more expensive means of generating electricity. Photovoltaic production has been increasing by an average of more than 20 percent each year since 2002, making it the world’s second fastest-growing energy technology, after wind energy.

Construction

Solar panels are composed of small disks called photovoltaic solar cells. An individual solar cell is usually small and typically produces around 1 or 2 watts. To generate more power, cells can be interconnected to form modules, which can in turn be connected into arrays to produce yet more power, and so on. Because of this modularity, PV systems can be built to meet any electric power need, small or large.

Modules are what is referred to as solar panels. They are usually protected from weather conditions with a sheet of glass and encapsulated in a frame that can be mounted wherever needed. Beneath the glass is an anti-reflective layer to prevent the cell from reflecting the light away.

Technology

Solar cells are electricity-producing devices made of semiconductor materials. To create a solar cell, two layers of semiconductors are combined, one that is positively charged and one that is negatively charged. When light shines on the semiconductor, the electric field across the junction between these two layers causes electricity to flow – the greater the intensity of the light, the greater the flow of electricity. This process of converting light (photons) to electricity (voltage) is called the photovoltaic effect.

Materials presently used for photovoltaics include monocrystalline silicon, polycrystalline silicon, amorphous silicon, cadmium telluride, and copper indium gallium selenide/sulfide. By far, the most common semiconductor material is silicon (Si), which is the main material in 98% of solar cells made today. Although silicon is the second most abundant element in earth’s crust, it has to be purified to an extremely high degree before it can be made into solar cells, making the process somewhat expensive.

Types

There are four main types of solar cells, all of which use silicon.

1. Amorphous (Thin film) – This type has the lowest efficiency but is the cheapest to produce per watt. Amorphous cells are manufactured by placing a thin film of amorphous (non-crystalline) silicon onto a wide choice of surfaces, such as roof tiles and rooftop shingles. While the other solar cell types must be mounted in a rigid frame to protect them, thin film panels can be made flexible if manufactured on a flexible surface.
2. Polycrystalline (Multicrystalline) – Produces a more efficient type of solar cell for a higher cost per watt. Polycrystalline cells are made by combining a large number of silicon crystals. They are easy to spot because they have an uneven color, usually blue.
3. Monocrystalline (Single crystal) – These are the most efficient and the most expensive solar cells that are mass produced. Monocrystalline cells are created by cutting thin slices from a single crystal of silicon. They are easy to spot because they have a smooth texture and even color, usually black.
4. Multi-junction (Tandem) – This type consists of several semiconductor pairs tuned to different light wavelengths. They allow for the greatest efficiency of all solar cell types, but at a much higher cost. For this reason they are not commonly used, other than in space and in concentrating solar systems.

Placement

Photovoltaic arrays for domestic use are often roof mounted, though installations may also be mounted nearby on the ground or integrated into the walls. In order to most effectively capture the solar radiation, the solar installation must be placed where it receives as much light as possible. Therefore, PV modules should be oriented to the true south (for the northern hemisphere) and somewhat inclined. The optimal inclination angle is a number of degrees equal to the latitude from the horizon. However, small divergences from the optimal orientation and inclination result only in even smaller reductions of effect. A steeper angle of up to 15 degrees may be considered to improve the energy yield for the winter at the expense of slightly lower performance in the summer.

Shadows cast by tall trees and neighboring buildings must be kept in mind during the design process. Small areas of shade on a panel can result in significant loss of energy production. This is because the cell with the lowest illumination determines the operating current of the whole solar array in which it is connected.

Reliability

An inherent disadvantage of solar electricity is that it only works when the sun is up. At night, your system will not generate energy. Clouds, fog and precipitation significantly reduce energy production as well. For an off-the-grid solution, one way to compensate for lack of sunlight is by adding an energy storage system that will reserve excess power from a sunny day for later use. Another option is to connect a backup power generator to your solar system to both supply supplemental energy and recharge the energy storage bank when sunlight is unavailable.

Durability

PV systems are among the most durable renewable-energy technology available. A module will properly perform in the field for over 30 years and higher quality panels are expected to survive 40 years or longer. Solar panels are designed to withstand hail, severe wind and weather conditions, assuming proper installation.

Solar panels have no moving parts which can break, however they do suffer a slow degradation in performance due to material aging and photon degradation. Generally, modules degrade between 0.2% to 0.7% each year. Typical guarantees of a solar panel include 5 years workmanship and materials warranty and a 20-25 year performance warranty. The typical PV panel performance warranty will guarantee 90% of rated production for 10-15 years and 80% for 20-25 years.

Maintenance

PV installations can operate for many years with little to no maintenance or intervention after their initial set-up. Therefore, maintenance and operating costs are extremely low compared to other power technologies. Maintenance generally entails simply cleaning the modules.

The degree of soiling will depend on the location. Typically, 5% of the panel’s performance is lost as dust accumulation and self-cleaning reaches a steady state after a few weeks, provided that the array tilt is at least 15 degrees. However, in high pollution areas with infrequent rain, soiling may reduce the performance by up to 25%.

Efficiency

The conversion efficiency of a solar cell is the proportion of sunlight energy that the cell converts to electrical energy. Today’s commercially available PV devices typically convert 6-18% of sunlight into electric energy. Efforts are currently under way to improve photovoltaic cell efficiencies and efficiencies up to 43.5% have been demonstrated in labs using a multi-junction concentrator solar cell. The sunlight that is not converted into electricity is either reflected or turned into heat.

A more efficient panel will either give more power or be smaller in size than a less efficient one. This does not mean that a more efficient panel is more economic. In fact the reverse is often the case. For home owners setting up their own system, efficiency is likely to be less important than the cost per watt output, as long as the system will physically fit in the area it is to be installed at.

Polycrystalline panels are the most popular choice for home solar power systems as they provide a good balance of performance and economy. Recent improvements in polycrystalline panel technology are bringing these modules closer to monocrystalline efficiency and heat tolerance characteristics. Amorphous panels are more economical and should be considered if real estate is plentiful.

Temperature efficiency

The output of a solar cell is temperature dependent. As a result, the power output will be reduced by between 0.25% (amorphous cells) and 0.5% (crystalline cells) for each degree of temperature rise. Solar panels have an output rated at 25 ℃ and due to the solar irradiation heating the panel will typically be 15 ℃ higher than the ambient temperature. For example, in summer time, with an ambient temperature of 30 ℃, the solar panel will be around 45 ℃. This results in a 0.995^(45-25) => 10% reduction in output for a crystalline panel compared to the rated output. Conversely, in winter time the performance of the panels will improve, even beyond the rated output. This comes in handy since the need for electricity tends to be greater during the winter.

Performance

The rated effect of a solar panel states the effect generated when the sun hits the panel vertically at an insolation of 1000 watt/m² with a temperature at 25 ℃. To estimate how much energy the rated effect represents over a year, you can use this formula: 0.8 x kWp x Sol. The variable kWp (kilowatt peak) is the rated capacity of the panel and Sol is the annual solar irradiance, specified in kWh/m²/year. You can estimate this value for your location using a solar radiation map. Here are maps for North America, Europe and Africa.

As an example, suppose you have a house in southern Spain which consumes 10 000 kWh of electricity per year and you want to power the house using only solar panels. Looking at the chart, the annual solar irradiance of southern Spain is 2000 kWh/m². If you took a typical 15% efficient polycrystalline panel measuring 2 m², it will have an output rating of 280 watt. The formula for calculating energy production is:

• Energy produced = Energy available / m² * Panel efficiency * Solar capture factor

Provided that the panel is installed on the roof facing south, properly inclined and with no shading, the solar capture factor is 80% (20% cosine losses). The panel will then generate 0.280 * 2000 * 80% = 450 kWh over the course of a year.

In practice however, certain losses will occur. First of all, the panel will get heated from the irradiation. Assuming an average daytime temperature of 20 ℃ over the year in southern Spain (35 ℃ panel temperature), the performance will be reduced by 5%. Another 5% will be lost due to dirt accumulating on the panels until a self-cleaning steady state is reached. Added to this, 8% will be lost in the inverter and 2% is burnt up as heat in the DC power cables. Finally, 2% is lost from combining PV modules with slightly different current-voltage characteristics.

These factors together add up to 80% and brings the total usable energy down to 450*0.80 = 360 kWh per year. Given this, you can calculate that to cover the house’s electricity needs would require at least 10 000 kWh / 360 kWh = 28 such panels, or 7.84 kWp of solar panel capacity. Each panel measures 2 m², so at least 56 m² of unobstructed roof area will be needed. This also assumes that you have a large enough energy storage system to get by during the rainy days (or season).

Realistically, you would need to oversize the system by at least 10%. Not just to have a margin of error, and because the weather may be worse than average, but also to account for energy storage losses (1-3% for batteries), AC power cable losses (1%) and the solar panel efficiency degradation (2%-7% every 10 years).

Price

The manufacturing cost of solar panels has declined steadily by 3% to 5% per year in recent years, driven by advances in technology and increases in manufacturing scale and sophistication. For example, in 1998 it was estimated that the quantity cost per watt was about $4.50, which was 33 times lower than the cost in 1970 of $150. The lowest cost per watt for the various module types can be seen in the table below.

Source: EnergyTrends.com, 2012

The average capital and installation cost for a residential sized system is about $7.50 to $9.50 per watt. This includes the panels (30% of cost), inverters (10% of cost), mounts and electrical items.

Continuing with the previous example, the house in Spain uses 10 000 kWh annually, which can be generated from 7.84 kilowatts of solar panels. The polycrystalline panels would cost 280*28*1.88 $/w = 15 000 dollars. The full capital and installation cost for the solar system would be around 8.50 $/watt * 7840 watt = 67 000 dollars.

Energy Cost

Continuing with the previous example, the capital and installation (C&I) cost for a 10kW photovoltaic system is about 67 000 dollars. The lifetime of this system is 40 years and the maintenance cost is very low. However, the life expectancy of the inverter is 20 years, so it will need to be replaced at a cost of $3000, bringing the total lifetime cost up to 70 000 dollars.

The panels’ performance will degrade by an average of 0.5% per year. After 40 years in the field the performance will have dropped to approximately 0.995⁴⁰ = 80%. We can therefore assume an average performance of 90% during the system’s lifetime. We also need to factor in the 20% energy losses calculated before. So during 40 years we can expect to generate 10 000 kWh * 90% * 80% * 40 = 288 000 kWh of usable energy.

And so, the total cost per kWh is $70 000 / 288 000 kWh = $0.243/kWh. This is a little more than twice the cost per kilowatt hour for conventionally generated electricity in the US, which is at $0.12/kWh. Note that in this example, the cost of having to lend the money invested was not included. Keep in mind that you may also be eligible for financial incentives, such as grants and preferential feed-in tariffs for solar-generated electricity.

Advantages

Photovoltaic systems have a number of unique advantages over conventional power-generating technologies.

• Widely applicable – Solar energy can be produced anywhere in the world where the sun shines.
• Free fuel – As with all renewables, there is no fuel cost involved.
• No noise – Solar power production is a silent process, as a fixed mounted PV system has no moving parts.
• Easy to install – The equipment is relatively easy to install.
• Expandable – You can start small and add more panels later on.
• Near zero maintenance – Operating costs are extremely low compared to other power technologies.
• High durability – The life expectancy of PV panels is extremely high.

Disadvantages

• High cost – Solar electricity is more expensive than most other power generating technologies.
• Large surface area – The more energy you need, the larger surface area is required.
• Low reliability – Solar electricity is not produced at night and is greatly reduced in cloudy conditions.
• Climate dependent – Energy production is highly weather dependant.
• Performance degradation – Though not a lot per year, it will affect your energy output in the long run.