The Power of Solar Energy

Lesson 7: How is PV Used?

Objectives

Background

Activity 1: Sizing A PV System

Extension

Wrap-up

Objectives

• Students will demonstrate an understanding of the different applications in which PV systems are used.
  
• Students will analyze and calculate how much energy typical household appliances and devices use and calculate the size of a basic PV system.

Background

Photovoltaics—commonly referred to as PV—is used in a variety of applications. Some smaller applications are quite common, such as PV-powered calculators and watches. There also are numerous large-scale PV applications, including:

• Water pumping for small-scale remote irrigation, stock watering, residential uses, remote villages, and marine sump pumps;
  
• Lighting for homes, billboards, security, highway signs, streets and parking lots, pathways, recreational vehicles, remote villages and schools, and marine navigational buoys;
  
• Communications by remote relay stations, emergency radios, orbiting satellites, and cellular telephones;
  
• Refrigeration for medical and recreational uses;
  
• Corrosion protection for pipelines and docks, petroleum and water wells, and underground tanks;
  
• Utility grids that produce utility- or commercial-scale electricity; and
  
• Household appliances such as ventilation fans, swamp coolers, televisions, blenders, and stereos.

Virtually any power need can be met with photovoltaics, but some situations are more cost-effective than others. (Cost-effectiveness is discussed further in Lesson 8.) PV systems are well-suited to locations where accessing an electrical grid (the system through which your utility company supplies electricity to its customers) is either not feasible or expensive.

Many small household appliances can be cost-effectively powered with PV. In general, though, PV is not used to generate electricity for hot water, space heating, electric cook stoves or ovens, refrigerators, or other applications with high power needs. Propane is a more cost-effective fuel for these applications. In the case of refrigerators, super-efficient models that use considerably less electricity than conventional models are available and are often used instead of propane models.

Activity 1: Sizing A PV System

Sizing a PV system requires analysis of many factors. Perform a simple sizing calculation to determine how many PV modules a system will need.

Teacher’s Notes: Set the stage for this activity as follows: You are designing a PV system to meet the electrical needs of a remote home in Montana (one that is located a great distance from the utility grid). In this activity, you will determine how many PV modules your system will require. This activity is for illustrative purposes only. Sizing and designing a complete PV system requires analyses of many more issues than are addressed here.

Remind your students that PV will be the only source of electricity for this house. If they fail to account for any electrical use when sizing a system, they may not have enough electricity to meet their demands. While, in a real situation, a single PV system would be designed to meet the needs of the entire house, for purposes of this activity, you will treat each area in the house as though it is a separate, stand-alone building.

Materials:

• Computer with Internet access
• Calculator for each group

Method:

1. Separate the class into four groups. Assign one group to each of the following areas of the house:

• Kitchen
  
• Living Room
  
• Garage (including any power tools that might be used)
  
• Bedroom/Study

1. Instruct each group to determine their daily energy use using the following steps:

• Identify all electrical devices that will rely on the system for power.

• Determine each device’s power usage in watts (Hint: you can find wattage charts for common appliances and devices at http://thermomax.com/load.htm or http://www.atlantasolar.com/wattage.html).

• Estimate the average daily use of each device in hours per day.

• Multiply each device’s wattage by the hours of daily use to get watt-hours per day.

• Add together the watt-hours for all devices to get the total energy requirement.

(Note: if the energy requirement varies from season to season, it must be calculated for each season to determine the largest requirement. Residences tend to use more energy in winter when the days are shorter, since lights and other appliances are on longer.)

1. Adjust the load for system losses, module output and average winter sunlight by multiplying your estimated daily load by 1.4
   
2. Assuming you will use 50-watt modules (12-volt), calculate how many watt-hours of electricity each module will provide. To do this, multiply the module rating (50 watts) by 3 (Montana's solar multiplier). Divide the result of Step 3 by the result of Step 4. This is how many modules you will need to meet your electricity needs. If your result is not an even number, round up to the next number."
   
3. Combine the number of modules from the four groups.

Extension

Instruct your students to expand this activity by sizing the system’s battery storage and inverter. They can find help at websites such as http://aaasolar.com/design/pvsizing/PVSIZING.htm

Wrap-up


Lead a class discussion about how one might reduce the size of a PV system to make it more cost-effective. How would energy efficiency and energy conservation affect a PV installation? Your students might note such things as more energy-efficient appliances and lighting will use less energy and thus allow a smaller PV system; that eliminating unnecessary electrical items (such as stereos, color televisions, or electric garage door openers) will reduce electrical load; or that switching to other fuels (propane-fired stove, for example) would reduce electricity needs.

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Next Section: Lesson 8 – Are PV Systems Cost-effective?