OBJECTIVES 1. Understand the underlying principles of active photovoltaic systems and factors that effect their efficiency. 2. List the advantages and

OBJECTIVESOBJECTIVES

1. Understand the underlying principles of active photovoltaic systems and factors that effect their efficiency.2. List the advantages and disadvantages of various PV system configurations.3. Discuss the factors affecting solar cell efficiency.4. List the various types of solar PV panels and the advantages and disadvantages of each.5. Explain the differences between the various types of waveform inverters.6. Understand and list all the components of a typical PV system and their function within the system.7. Explain what is meant by Solar Thermal systems. 8. List and discuss the function of the various parts of a solar thermal system.9. Understand the difference between a direct (open ) and closed loop system.10. Discuss the various types of solar thermal collectors and the advantages and disadvantages of each.11. Determine the proper sizing of a solar thermal system.12. Understand the maintenance and safety issues surrounding solar thermal systems.13. Discuss the economics of a solar thermal system.

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OBJECTIVES (CONTINUED)OBJECTIVES (CONTINUED)

14. Relate the advantages and disadvantages of utility-scale solar electric (PV) systems.15. Discuss several technologies employed by utility-scale PV generators.16. Understand the barriers the PV industry faces in utility-scale electrical generation.17. Explain the purpose of Renewable Energy Certificates, and how they are used.

Figure 5-1: A Simple PV System

Stand alone

Grid-Tied

Hybrid

There are three major classes of PV systems:

Figure 5-2: Stand-Alone System with Batteries Added

Figure 5-3: A Grid-Tied System

Figure 5-4: A Stand-Alone Hybrid System

No Pollution: Energy generated by most other power sources emits greenhouse gases and/or toxins as they are burned to produce electricity. PV technology produces no waste products (other than heat).

Reliability: PV systems are relatively simple and extremely reliable (assuming there is sunlight).

Low Maintenance: Maintenance of power systems in remote locations can be quite expensive. Since there are no moving parts in a typical PV system, maintenance is minimal.

The advantages of PV technology:

(continued)

Durability: Most systems manufactured today can be expected to perform well for 30 or more years.

No Fuel Cost: At present, the sunlight is free.

Quiet: Unlike generators or turbines, PV systems are silent.

The advantages of PV technology: (continued)

(continued)

Scalable: Because systems are designed and installed in modules, they can be easily expanded as power needs increase.

Decentralized Electrical Grid: From a security and reliability perspective, a decentralized grid is much less prone to disruption.

High Altitudes: PV systems actually perform better at high altitude (less light diffusion). Power sources that rely on burning (oxygen availability) actually perform less well at high altitude.

The advantages of PV technology: (continued)

Cost: In today’s power market, the initial cost of a PV system is relatively high compared to more traditional alternatives.

Available Sunlight: PV systems only generate power when the sun is shining. This makes them unreliable in some locations, and of course means no power generation at night.

Energy Storage: Battery systems to store excess energy remain expensive and require a great deal of maintenance.

The disadvantages of PV technology:

Figure 5-5: Cross Section of a Solar Cell

Load Resistance: The load of a system is the amount of power that is being used at any one point in time. When the switch of a vacuum cleaner is turned on, the load of the electric system is increased by the amount of power needed to operate the vacuum. Turn on too many appliances, and the system can “over load.” Too much power is being drawn through the system and the system shuts down. In a PV system, this load demand can come from appliances or from charging batteries.

Other factors affecting solar cell performance include:

(continued)

Anti-reflective coating: Pure silicon is very shiny. In fact, it is so shiny that about 35% of the sun’s light energy will simply bounce off, and be reflected into space if the material is not treated with an anti-reflective coating. Again, there is a balance to be struck. The cell must be coated in a way to allow for the absorption of photons, but not covered or masked (which will restrict or block photons from reaching the silicon wafers).

Other factors affecting solar cell performance include: (continued)

(continued)

Temperature: As the temperature of the solar cells increase, their efficiencies decrease. For most cells operating between 175-195°F (80– 90ºC), there will be a 0.5 decrease in efficiency for every 2°F (1º C) increase in temperature. For this reason, panels should be installed in a way that provides for adequate ventilation to help reduce operating temperatures.


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Physical Size: With solar cells (and modules), there is a one-to-one relationship between surface size and the amount of power generated. A solar panel that is 20 square feet (or two square meters) will generate twice as much power as a panel that is 10 square feet (or one square meter) in size.


(continued)

Series resistance: By silicon’s very nature (as a semi-conductor) it resists the flow of electricity (as opposed to a good conductor such as copper). Some of the efficiency of the solar cell is lost due to the resistance inherent in the material used to make the solar cells and to connect these cells together. To minimize this loss, metallic connectors are used at the top and bottom of the solar cell to carry the flow of electricity through the circuit. But even with excellent conductors, some power loss is inevitable.


(continued)

Size and placing of electrical contacts: Small metallic wires are typically connected to each of the solar cells, which transfer the electrical energy along the circuit. Because half of the wires are connected to the top layer, they will block a portion of the sun that might otherwise strike the PV cell. Reducing the area blocked by these wires can help to increase the efficiency of the cell.


(continued)

Shading: Aside from the electrical contacts, any number of other external items (such a trees, telephone poles, etc) can block the PV cell, resulting in shading. The effect of this shade can be dramatic upon the performance of the system. Even blocking just one cell on a typical solar panel can reduce the entire panel’s output by as much as 75%.


So the surface area of the solar cell (Ac) has to be taken into account (if the solar cell was one square meter, this could be ignored, but generally they are much smaller than that. So the equation for solar cell efficiency would be:

Light irradiance (E) is measured in watts per square meter (W/m2).

==PmPm

E E xx Ac Ac

So a solar cell that has a surface area measuring 0.01 m2 (100 cm2) that produces1.5 W of power under these conditions will

have a 15% efficiency rating.

= = = = .15= = = = .15PmPm

Ex Ex xx Ac Ac 1.5W1.5W10W10W

1.5W1.5W1000 W/m1000 W/m2 2 xx 0.01 m 0.01 m22

To convert this ratio to a percentage, we just multiply it by 100.

.15 x 100 = 15%.15 x 100 = 15%

Figure 5-6: Inverter

Figure 5-7: Square Wave, Modified Sine Wave and Sine Wave

Figure 5-8: Electricity vs. Renewable Energy Certificates

Figure 5-9: Complete PV System

Junction Box – A place where the wires from the various panels come together, effectively connecting them into an array.

Beginning at the solar array, these BOS components may include:

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Combiner – This device combines the electrical current strength of the array. For instance, the current from four arrays producing at 7 amps each, can be combined in this device to carry a 28-amp signal to the next portion of the system (typically either the controller or the inverter).

Battery System (optional) - In a system that incorporates a storage system (batteries), there are more than just batteries involved. There is the charge controller, that will control the charge coming from your PV array, controlling how much charge they receive and when they receive it. There is also a low-voltage disconnect device that will make sure the batteries are not depleted to such a level as to damage them. There are also disconnect switches and wire that complete the system.

Beginning at the solar array, these BOS components may include: (continued)

(continued)

Inverter – In nearly every system there is an inverter which converts the DC current into an AC signal that can then be used by standard household electrical devices. These devices usually incorporate a basic monitoring system to indicate the power produced. Additional monitoring capacity (such system lifetime energy production, AC power output, temperature, operating status) may also be incorporated or added into the system.


(continued)

Maximum power point tracking device - This device may also be incorporated in the charge controller or the inverter or as a separate component to make the system more efficient.


Disconnects – Every piece of equipment in the PV system must be able to be disconnected from all sources of external power.

Figure 5-10: Basic Solar Hot Water System

Figure 5-11: Simple Direct Compact System

Figure 5-12: Diagram of a Closed Loop System

Figure 5-13: Batch Water Heater

Figure 5-14: Flat Plate Collector

Figure 5-15: Evacuated Tube Water System, with Cross Section of Tubes Integrated

Figure 5-16: Sunlight on a Tube

Open or closed loop systems

The various elements within a solar hot water heating system allow for quite a variety of configuration options:

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Water or other antifreeze within the closed loop system

Drain down options (where water is used in the collectors)

Pumps, thermosiphon systems or mains water pressure to move the water

Internal (inside the storage tank) or external heat exchangers

The various elements within a solar hot water heating system allow for quite a variety of configuration options: (continued)

High pressure or low pressure systems

Formed plastic, flat plate, or evacuated tube collectors

Weather conditions: Is the home subject to freezing temperatures?

With so many options, the variety of systems available are many and varied. The configuration selected will depend on a number of factors:

Available sunshine: The efficiency of the system may be critical.

Home use: Is it a vacation cabin or for year-round use?

Size of the system: How much hot water is required?

Figure 5-17: Efficiency of Water Heating System

Table 5-1: Sizing Chart

In most locations, plan for 1.5 to 2 gallons of storage capacity for each square foot of collector area. For instance, if 40 sq ft (3.7 square meters) of solar collectors are installed, the system should incorporate an 60-gallon (225 liter) water tank (this system should adequately supply a typical family of three).

When designing or selecting a solar hot water system, a few things to keep in mind:

(continued)

Two-tank systems are generally more efficient, but require more space and cost more.

When designing or selecting a solar hot water system, a few things to keep in mind: (continued)

(continued)

Utilize glycol-based antifreeze closed loop systems in hard freeze areas.

Simple is better. Fewer pumps and other components will minimize potential problems.

Use components that are certified and are obtained from reputable dealers and manufacturers.

Pumps and controllers that rely on AC power from the grid will not operate when the grid is down. This may subject your system to potential freezing (such as during a winter storm that results in a power outage). Incorporating a manual drainage system or a battery backup power alternative may be appropriate in some instances.

When designing or selecting a solar hot water system, a few things to keep in mind: (continued)

National and local incentives may be available that may offset a large portion of the installation price of a solar water system.

Table 5-2: Solar Thermal System Advantages and Disadvantages

Electrical Protection: In active systems, electricity is used to control and operate the system. These electrical components must be properly sized, rated, grounded in compliance with the National Electrical Code. Additionally, adequate over-current and ground fault protection should be in place in.

Solar thermal systems have some inherent risks that should be avoided:

(continued)

System Failure Protection: Controls should be in place that prevents the system from reaching temperatures and pressures that might damage the system, the building or people. Each portion of the system that can be isolated must have separate protection.

Solar thermal systems have some inherent risks that should be avoided: (continued)

(continued)

Combustibility: These systems operate at high temperatures. Materials should be used that will not catch fire at these high temperatures.

Fluid Safety Labeling: Closed loop systems use antifreeze to assist in heating water for domestic consumption. The system must have very clear labeling telling users what liquid is in which portion of the system (to avoid unfortunate mistakes). Toxic fluids should be avoided. For this reason most closed-loop antifreeze systems utilize food grade glycol is used.


(continued)

Also, solar thermal systems should clearly be labeled that the fluid can be released at a high pressure and/or temperature.

Contamination of Potable Water: Materials which come in direct contact with potable water should not adversely affect the taste, odor or quality and appearance of the water. Standards for drinking water are defined by the National Sanitation Foundation.


(continued)

Exposed Surfaces: Any part of the system that is exposed to traffic (where people may touch them) must be insulated to avoid exposed surface temperatures in excess of 140°F (60°C).

Table 5-3: Installed US Solar Capacity

With an initial investment of $1,500 and an annual investment of $672, it will take 8.63

years to pay back the cost of the solar hot water system.

$1,500 + (8.63 $1,500 + (8.63 xx $672) = $7,300 $672) = $7,300(give or take a few pennies)(give or take a few pennies)

First, in many parts of the world the government wants to encourage people to invest in solar

power, so they provide incentives (grants or tax breaks) that help pay for the initial installation.

These can be pretty significant. Our system might only end up costing us about $3,800 after we

figure in all these government deals.

$1,500 + (3.4 $1,500 + (3.4 xx $672) = $3,785 $672) = $3,785

In that case it will pay for itself in just 3.4 years.

The cost of solar electric decreases

and/or,

11

The price increases for all other power sources

22

One of two things may occur that will make PV systems more attractive.

Figure 5-18: Module Prices

Figure 5-19: Electric Prices Going Up

Non-Polluting: PV systems generate no pollution. In fact, typically, the only waste byproduct of a PV system is heat. By definition, the amount of heat energy lost through inefficiencies will be less than the amount of heat energy generated if the sunlight had simply hit a rock rather than a solar panel. So PV systems are effectively non-polluting.

Advantages of Solar Energy:

(continued)

Resource Neutral: PV systems utilize a portion of the sunshine that falls upon them. For practical purposes, it can be safely assumed that the sun will continue to shine (for millions of years, anyway), so this source of power is effectively unlimited.

Advantages of Solar Energy: (continued)

(continued)

Easier to Construct: Unlike traditional fossil fuel power plants, or nuclear power plants, a PV power plant can be constructed in a relatively short period of time. Huge amounts of water are not required to cool the system, so they do not need to be located near rivers or other sources of large amounts of fresh water. As the environmental damage is much less, there is less resistance by the community than is typically associated with fossil fuel or nuclear plants.


(continued)

Scalable: Unlike traditional power plants, PV power plants can increase the size of electrical output simply by adding more panels to the system. This allows for a great deal of flexibility to supply more energy as demand increases. It also helps lower initial construction costs, since the system needs only be built to meet existing demand (and not anticipate increase in the distant future).


(continued)

Flexible Location: Because PV systems can be scaled in size, and located in places otherwise deemed unsuitable for more traditional power plants, smaller PV systems can be installed closer to where the energy is consumed. A great deal of the power generated by a traditional power plant is lost during transmission (due to resistance and other voltage drop factors). The further this energy must be transported, the greater this loss.


(continued)

Carbon Credits: Utility-scale PV power plants qualify for a tremendous number of carbon credits under many proposed plans. These credits are an asset that can be sold under a cap and trade scheme, providing a lucrative revenue source for PV utilities.


Figure 5-20: Power Sources (2007)

Cost: PV systems are still relatively expensive (as compared to traditional fossil fuel power plants). There are many who would argue that this is not really the case if all the externalized costs of production are included in the cost of the electricity. However, under current regulatory and accounting methods, PV is still considered a relatively expensive power source in most locations.

Disadvantages of Solar Energy:

(continued)

Sunshine: PV systems only generate power when the sun is shining. This means that even under the best of circumstances, they are only producing power a fraction of each day. An electrical utility, however, must be prepared to generate power 24 hours a day, 365 days a year.

Disadvantages of Solar Energy: (continued)

(continued)

Resource Depletion: It was stated earlier that PV systems are resource neutral - which is not quite true. While the energy powering these systems (sunshine) can be considered an unlimited resource, PV systems use materials in their manufacture. These materials are limited.

Disadvantages of Solar Energy: (continued)

Figure 5-21: Power Costs

Parabolic Trough

Solar Dish

Solar Power Towers

Three broad design categories are typically used in the generation of solar thermal utility-scale power. These include:

Figure 5-22: Parabolic Trough

Figure 5-23: Collector Array Tracking the Sun

Figure 5-24: Dish Concentrators

Figure 5-25: Solar Power Tower

REVIEW QUESTIONSREVIEW QUESTIONS

Describe a very basic PV system. What are the major components and what function do they serve?

11


Discuss the various classes of PV systems (stand-alone, grid-tied, and hybrid). In what situation might a stand-alone system be the only practical solution?

22


Discuss how electricity is generated using a solar cell.

33


List four factors that can affect the performance of a solar cell. Which is most critical?

44


Relate the difference between a square-wave and a sine wave inverter. Which is preferred and why?

55


What are the major components of a solar thermal system?

66


List three common solar thermal systems (direct pump, for example) and give an advantage and disadvantage of each system listed.

77


List four important safety considerations that should be kept in mind when working with solar thermal. Why are these inherent dangers with solar thermal systems?

88


Discuss the economics of solar power. What economic factors will affect the growth of this market segment of energy production?

99


Discuss the concept of carbon credits and cap and trade systems. How will this affect the price of PV?

1010


Explain the difference between a direct and a closed loop solar thermal system. What are the relative advantages and disadvantages of each system?

1111


A family wants to install an inexpensive solar thermal water heating system at their vacation cabin located near San Jose, Costa Rica. What system would you recommend and why?

1212


List five advantages and two disadvantages of current PV technology.

1313


Define grid-tied, stand-alone and hybrid PV systems and give the relative advantages and disadvantages of each system.

1414


List five environmental factors that may effect the performance of an installed PV system.

1515

Documents

OBJECTIVES 1. Understand the underlying principles of active photovoltaic systems and factors that effect their efficiency. 2. List the advantages and