Solar Studies 101: Theory, Array Capacity, and Energy Savings
Solar panels are a great way to offset energy costs for many owners. The fuel may be free, in the sunlight that bears down on our buildings, but the upfront capital needed to build out a fully functional solar panel array can be off-putting to some. While photovoltaic arrays will not be the solution for net-zero, or even net-positive, building energy consumption, it will be a valuable contributor. This is why Aethera Engineers makes the time in all of our projects to conduct an electrical energy generation analysis for photovoltaic systems. Thus, the design team can make decisions based on data with confidence.
Theory
A bit of background on the underlying the photovoltaic (PV) effect is depicted in the following graphic. Essentially, when a photovoltaic cell is exposed to light, a potential difference, i.e. voltage, is created and electrons flow, such that current is induced. The energy produced by the solar modules can then be directly consumed by building systems, stored in batteries for later consumption, or transmitted to an electric utility grid that is connected to the building. This is great for offsetting building consumption during peak times, which has a direct impact on electricity costs for the Owner, providing a net savings over the life of the building.
Solar Array Capacity
We start by requesting a roof plan from the Architect, which often includes explicit areas outlined for solar panel installation. If not, we can infer placements coordinated with roof-mounted building systems equipment, such as air handling units (AHUs), condensing units for direct expansion (DX) or variable refrigerant flow (VRF) systems, and fan enclosures for exhaust, dedicated outside air systems (DOAS), among others.
In the screen capture below, you can see the estimated roof area as determined by an Architect. Notice that there are generous clearances from the edges of the roof as well as ample space to facilitate movement from the roof access hatch. In this particular example there were no roof mounted building systems, which contributed to an increased utilization factor of the roof. Let’s use the value of 7,594 square feet for our calculations.
Screen capture from design of AISD Menchaca Elementary School, Building Roof Plan by LPA, Inc.
Once the roof area is determined, we choose a typical solar panel as a basis of design. Choosing a typical option allows us to understand the dimensions of each solar panel and ultimately provides the quantity of solar panels that will fit within our prescribed roof area. We can then multiply the number of solar panels by the nominal power per solar panel, measured in watts, to arrive at the overall solar array capacity.
Screen capture from REC Solar Peak Energy 72 Series Product Data Sheet, emphasis added.
Given information supplied by a solar panel manufacturer, we can see that a selected typical solar panel is approximately 21 square feet in area. If we use the anticipated area provided earlier (7,594-sf) we find that roughly 360 solar panels can fit within the roof area. We also know the nominal power output of the selected basis of design solar panel is 315-watts. Thus, we can estimate the capacity of the solar array to be 113.4-kW.
Energy Savings
Solar output is only as good as the amount of sunlight that your solar array experiences. We need to estimate the amount of solar radiation, i.e. sunlight, that the solar array will receive, which is based on orientation and physical location. For this, Aethera Engineers uses a software program called RETScreen Expert. This clean energy project analysis software sources its location and climate data from NOAA/NWS or NASA ground stations. For this example we are using the San Antonio International Airport weather station. In the graph below, you can see that daily sunlight is minimal during the winter, and at its peak during summer, as expected. The annual average daily solar radiation is estimated at 4.95 kWh per square meter, each day. This value is higher the closer a location is to the equator.
Screen capture from RETScreen - Energy Model, depicting daily solar radiation data for San Antonio, TX.
Now that we have a value for the average daily solar radiation, our last variable to contemplate is Capacity Factor, which represents the ratio of the average power produced by the solar array over a year to its rated capacity. The U.S. Energy Information Administration (EIA) releases annual data on capacity factors for utility scale generators primarily using non-fossil fuels on a periodic basis. Below is a graph of these data for solar photovoltaic capacity factors from 2009 to 2019, as well as the proposed band of typical capacity factors of 5% - 20%, as recommended by RETScreen.
Data derived from EIA Electric Power Monthly, Table 6.07.B. Graph generated by Aethera Engineers.
While the average photovoltaic capacity factor over the last decade is calculated as 23.2%, we will use the more conservative value of 20.0% for our energy savings calculation. This affords the Owner additional savings in the event that the capacity factor out performs the expected value, which is reasonable to assume given the historical data. After inputting the capacity factor of 20.0% in RETScreen, we arrive at the total expected energy savings, calculated at 198,677-kWh.
Screen capture from RETScreen - Energy Model, depicting energy saved, emphasis added.
To put this number into perspective, the average Texas home consumes roughly 1,174-kWh of energy per month, or 14,088-kWh of energy per year. Therefore, this 113.4-kW example solar array would supply power equivalent to fourteen homes for an entire year. The next installment of Quintessence will pick up here and dive into the initial costs, operations and maintenance, rebates, debt service, and fuel savings, to determine the payback timeline and the overall feasibility of a solar photovoltaic system.
Relevant Links in Solar Sustainability
Below are a couple of links to stories that relate to the overall subject matter of this post, which have been published in the past few weeks.
Wired. (2020, February 22) Family Farms Try to Raise a New Cash Cow: Solar Power. Retrieved from: https://www.wired.com/story/family-farms-try-to-raise-a-new-cash-cow-solar-power/
BBC. (2020, February 19) The World’s Most Unlikely Solar Farms. Retrieved from: https://www.bbc.com/future/article/20200219-the-solar-farms-fighting-climate-change-in-alaska
