In this lesson, students learn how to evaluate the amount of energy used in a building and the amount of renewable power that can be generated on-site using photovoltaic (PV) panels on its roof (or in other locations on-site).
Using the school campus from the previous lesson as an example, students use the tools in Autodesk® Green Building Studio® software to estimate the energy needed to supply the electrical demands of the lighting fixtures, appliances, and other building equipment.
Then they explore the impact of reducing electrical demand by improving the efficiency of lighting fixtures and mechanical equipment as well as using controls to reduce waste.
Using this estimate of the energy demand as a target, they will then explore ways to determine the total area of PV panels required to meet all or a portion of that demand, focusing on payback analysis and the economic tradeoffs.
Estimating the Electrical Demand Baseline
The orientation of a building and its design features relative to the sun’s path can have a significant impact on the performance of the building and the comfort of its users.
We can use the tools in Green Building Studio to estimate the total electrical demand created by the usage patterns and performance characteristics of the electrical lights, appliances, and equipment in our building model. We can specify the:
- Lighting power density—a measure of the amount of power used to provide lighting per square foot of a building that provides a convenient way to describe the overall efficiency of the lighting system (before the actual lighting fixtures are specified).
- Controls—automated timers and sensors used to reduce unnecessary power consumption by turning off lights when sufficient daylighting is available or when a room is not occupied.
- HVAC system—the type of heating, ventilating, and air-conditioning system that will be used and its efficiency.
While these three measures do not provide a completely accurate model of the power use in the design, they do reflect the characteristics of the major power demands and are sufficient for calculating a quick estimate of the power use for our building type and square footage.
Students can explore the effects of changing the lighting power density, the control systems installed, and the characteristics of the HVAC system to quickly assess the potential energy use impacts of using:
- High-efficiency lighting fixtures, such as compact fluorescent lights (CFLs) and LED lights.
- Automated timers, occupancy sensors, and daylighting sensors.
- Alternate heating, ventilating, and cooling systems with varying efficiencies.
Using high-efficiency fixtures and controls typically has a positive effect of reducing electrical consumption at a low cost. While the fixtures and controls may be more expensive to purchase, the additional investment is typically recovered quickly through their improved efficiency.
The effects of changing the HVAC system are subtler and must be considered carefully. Changing to systems that use a different mix of electrical power and fuel can bring economic savings (if, for example, fuel is less expensive than electricity) but actually have negative environmental effects (by creating a bigger carbon footprint).
Offsetting Power Use Through Net-Zero Measures
Photovoltaic (PV) panels are an excellent source for generating renewable electrical power.
The roof surfaces of a building often provide the best unobstructed views of the sun, so photovoltaic systems are typically placed there. Factors to consider when designing roofs for solar use include:
- Roof area—greater roof area generally provides more potential for placing photovoltaic panels. But each roof surface must be evaluated independently, because the actual power that can be generated will depend upon the direction and slope of the panels.
- Roof slope or panel tilt—panels are typically places to minimize the incident angle with the sun and maximize the current generated. The optimum slope or tilt depends on the project location’s latitude.
- Orientation—panels typically placed to face the direction that maximizes the amount of current that can be generated.
The amount of power that can be generated by PV panels also depends on a number of other factors:
- Insolation—a measure of the solar energy available at specific geographic locations in the world.
- Panel efficiency—a rating that describes the percentage of the available solar energy that can be converted into useful power by a specific type of PV panel.
The power-generating potential of PV panels can be calculated in several different ways. The photovoltaic analysis tools in Green Building Studios perform these calculations and assess the photovoltaic potential of every external surface of the building model and report that potential, so you can choose which surfaces are the best candidates for panel installation.
Often it is not feasible or advisable—due to space and budgetary limitations—to supply 100 percent of the estimated energy demand by generating power through renewal means, such as PV panels.
If the cost of installing panels on a surface exceeds the expected values of the savings that will be realized by reducing power consumption, then it does not make economic sense to do so. To assist with evaluating which surfaces should be used, Green Building Studios calculates a payback period—the period of time required to recover the initial investment through the annual savings that will be realized through the operating life of the system—for each potential surface. You can enter a desired payback period based on your economic objectives (for example, 50 years), and the tool will highlight the surfaces that can be used to meet this objective.
Other noneconomic factors can also enter into design decisions about which surfaces to use. For example, the design team may want to achieve a specific level of power reduction to earn LEED® points. Or one of the building’s design requirements may be net-zero energy—that is, the building can provide all of its own power requirements and essentially be off the grid. In these cases, design teams can override the recommendations made based on the payback period and analyze the power-generating potential of all surfaces, regardless of cost.