Solar photovoltaic (PV) energy sits at the forefront of renewable energy system implementation. The International Finance Corporation’s report[i] on utility-scale solar PV describes how the cost of solar is going down while panel efficiency is going up. In developed and developing economies alike, the push for solar is gaining momentum.
Solar energy[ii] is collected by arrays of PV cells that are connected in series and parallel to form the solar panels we see. Solar panels are classified as either crystalline or thin-film. Crystalline panels are more efficient (particularly mono-crystalline), but are also more costly. Thin-film panels will be ignored for the sake of this study, as they are not practical in residential settings due to lower efficiency and high demand for space. Crystalline panels are made with silicon as the semiconductor that carries out the photovoltaic effect, as shown in Figure 2. Panels convert energy from the sun into DC power by generating a current from the electrons excited by the light that hits it. The excited electrons are gathered by metal plates and transferred to wires as DC current.
Equipment required for energy generation and collection extends beyond just solar panels. According to the National Renewable Energy Laboratory (NREL)[iii], typical solar PV systems contain equipment listed below. Figure 3 shows these elements plus other optional elements (generator, critical load subpanel) that are often integrated in larger-scale buildings such as multifamily residential complexes or commercial and office facilities.
Figure 3: Elements and connections in a typical solar PV system[iv]
- Panels: there are a variety of options for panels as described above. These typically last for about 25 years, depending on weather conditions and type of panel.
- Inverters: PV cells generate DC current, but utility power is alternating current (AC). Thus, inverters are used to make each power source compatible.
- Meter(s): Metering is used to see financial payback from a solar system. If panels produce more than the load demands at a given time, the excess feeds back into the utility grid through a meter. The owner of the PV array gets compensated for the energy that is sent back into the grid, according to utility company policy.
- Mounting systems: Mounting systems can be at a fixed angle or rotate around one or two axes. In the northern hemisphere, a south-facing tilt is optimal.
- Storage: Battery storage is optional for PV systems. On a smaller residential scale, a 16 kWh battery (standard size) can power a small- to mid-sized home during the evening and night when there is no solar production.
Solar PV can be installed on a residential, commercial, or industrial scale. Based on facility demand and the amount a consumer wishes to supplement utility usage with renewables, anywhere from a few panels to hundreds of kilowatts (kW) of solar capacity can be installed. Though cost generally decreases as you scale, urban areas pose technical challenges that inhibit massive solar arrays.
Solar in Urban Environments
In urban settings, space restrictions and increased shading make rooftop solar the more desirable solar array design. Space constraints limit the size of an array, while other factors such as roof material, roof angle, and amount of shade present determine how effective a solar array would be in an urban setting. A study[v] simulated different types of panels in various configurations to find the optimal choice for urban solar arrays in Phoenix, AZ. The data collected can be seen in Table 1. Since this 2010 study, similar work has been done in other cities across the United States.
[i] Utility-Scale Solar Photovoltaic Power Plants. International Finance Corporation, 2015.
[ii] Pukhrem, S. How Solar Cells Work – Components and Operation of Solar Cells. Solar Love, 2013.
[iii] Connecting Your Solar Electric System to the Utility Grid. NREL, 2002.
[iv] Photovoltaic (solar electric) Systems with Battery Backup. Florida Solar Design Group, 2015.
[v] Bryan, H. et al. Methodology for estimating the rooftop solar feasibility on an urban scale. SOLAR 1 (2010), 476-505.