Article 1: The Beginner's Guide to Solar Energy

August 15, 2018

Solar Industry Growth and Affordability

Solar is playing an increasingly important role in the transition to a world powered by renewable energy. Over the past decade, the number of solar installations has grown at an accelerating rate and with increasing affordability.

Figure 1 illustrates the correlation of the increasing number of installations with the decreasing costs. It shows that the price of a solar installation is now less than a third of what it was in 2009, while annual installations have grown more than tenfold during the same period of time.

Figure 1.

SEIA states that the solar industry is a powerful engine for economic growth. The solar industry currently employs over 200,000 people, twice as many as in 2010 and now employs more people than the coal, or the oil and gas industries. As installed capacity continues to increase, SEIA predicts that the solar workforce will expand to 420,000 by 2020.

Figure 2. The solar industry now employs more workers than coal mining and petroleum extraction. Source: SEIA

Solar Energy, Power, and Irradiance

Solar panels convert the energy of photons, or light particles, from the sun into electricity. Photovoltaic devices, such as solar panels, permit the incoming photons to transfer their energy to electrons. These energized electrons begin to flow, creating an electric current. We use the terms irradiance or insolation to refer to the power density of sunlight on a surface.

Figure 3. Basic schematic of a PV array.

We typically measure energy in kilowatt-hours (kWh), and power (the rate at which energy is produced) in kilowatts (kW).

Energy = Power \cdotTime = 1 k W\cdot1 h o u r = 1 k W h

In solar, we usually define the size of a solar installation in terms of its power (in kW).

Irradiance is typically reported in units of kilowatt-hours per meter squared per day (kWh/m²-d). The amount of irradiance hitting the surface of the earth is often quoted in terms of the number of hours of “full-sun” of solar energy. A "full-sun" is defined as 1 kW/m².

Quantity	Units	Definition
Power	kW	Rate of energy production/output
Energy	kWh	Capacity to do work
Irradiance	kWh/m²-d	Hours of full-sun for a square meter each day

Table 1. Important quantities used for solar energy

Solar Resource of a Rooftop

We can estimate the solar potential of a rooftop using its area and the local irradiance. NREL, the National Renewable Energy Laboratory, publishes irradiance data in its report Solar Radiation Data Manual for Flat-Plate and Concentrating Collectors.

Figure 4. Annual irradiance value for a 150m² roof plane

It is fairly straight forward to calculate rooftop solar potential of a rooftop using this data. For example, a south-facing roof plane of a home (Figure 4) receives an average irradiance of approximately 1,900 kWh/m²/year. Dividing the annual irradiance value by the number of days in a year yields the average daily irradiance.

Average
Daily Irradiance = \frac{Annual Irradiance}{days/year} = \frac{1900 k W h / m^{2} y e a r}{365 d a y s / y e a r} = 5.2 k W h / m^{2} d a y

To calculate the amount of solar energy available on a roof face, multiply its area by the average irradiance value.

Rooftop
Energy [\frac{k W h}{d a y}] = Irradiance [\frac{k W h}{m^{2} \cdotd a y] \times Area [m^{2}]}

If the rooftop has an area of approximately 150m², the solar energy available on the rooftop is as follows.

Rooftop
Energy = 5.2 \frac{k W h}{m^{2} \cdotd a y \times 150 m^{2} = 780 \frac{k W h}{d a y}}

Besides the solar irradiance, Figure 4 also displays information on three additional quantities related to the solar resource: Solar Access, TOF, and TSRF:

Solar Access: This is the ratio of the actual solar energy available — taking into account shading cast by objects in the environment — to the solar energy that would be available in the absence of shading. You can learn more about the effects of shading on PV systems here.

Solar
Access = \frac{Energy with Shade}{Energy
without shade}

TOF (Tilt and Orientation Factor): This is the ratio of the amount of solar energy a location receives to the amount it would receive if the orientation of the roof were optimal.

and orientation

TOF = \frac{Energy with actual tilt and
orientation}{Energy with optimal tilt and orientation}

TSRF (Total Solar Resource Factor): This is the percentage of the available solar resource that a location receives as compared to what it would receive with optimal orientation and without shading. TSRF is equivalent to the Solar Access multiplied by the Tilt and Orientation Factor.

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