Integrating Solar Electric Systems into Roofing Design
Understanding Power vs. Energy
A review of basic electrical concepts and terminology is helpful in understanding the design implications of a BIPV system. The word Power is used to describe the generic measure of raw electricity and is identified as the amount of Current x the Voltage of that current (P = I x V measured in watts. A thousand watts of power is simply termed a kilowatt (1 kW) and a million watts (or 1000 kW) is termed a megawatt (1 MW). The word Energy is used to infer the amount of Power produced over Time (E = P x T) and is usually measured in watt-hours (Wh). A thousand watts of power produced over the course of one hour is 1 kilowatt-hour (kWh) of energy while a million watt-hours (or 1000 kWh) is a megawatt-hour (1 MWh) of energy.
Elements of a BIPV system
In order to include BIPV in a building design, first an understanding of the separate elements of the system is in order.
- Photovoltaic (PV) Cells: Sometimes also called solar cells, these are the basic building blocks of any BIPV system. An individual PV cell is the electricity-producing device made of silicone semiconductor materials similar to those used in computer chips. These cells are usually quite small, ranging in size from smaller than a postage stamp to several inches across and typically produce about 1 or 2 watts of power each. There are two basic commercial PV cell technologies available on the market today with differing characteristics, advantages and disadvantages: crystalline based products and thin-film products.
- Crystalline Systems: one of the original and a commonly seen type of PV technology, these solid silicone crystal cells require a very controlled backing and covering. Crystalline systems are typically used in pre-manufactured panels, hence the common term "solar panels." They require some type of grounded rack system to support the panels which may in turn require either roofing or wall penetrations or a ballasted system. They tend to work best with flat or low slope roofs and standing seam metal roofing. By incorporating them with lower tilt angles, they avoid the compromise of shading from adjacent panels and wind forces being transferred to the building.
Some commonly available crystalline PV cells |
Advantages of crystalline systems compared to thin film systems include generally higher efficiencies in electrical production and higher power density. As a result, crystalline systems require a fewer total number of cells. There are also a large number of manufacturer choices available. Disadvantages include higher manufacturing cost per watt of power produced and higher weights (approx 2.5 lbs per square foot). Crystalline systems also perform poorly in high temperatures, in shaded situations, or when sunlight is diffuse.
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Thin film flexible PV cells
Advantages of thin film systems compared to crystalline systems include lower manufacturing cost per watt of power produced and lower weight of the system (less than 1 lb per square foot) due in part to avoiding the need for racking or mounting systems and grounding. Thin film systems also provide superior performance in high temperatures, good shade tolerance, and good performance in ambient/diffuse light. Thin film systems are usually more easily worked into the aesthetics of a building as well. Disadvantages of these systems include lower efficiencies and lower power density meaning that the system may require more total cells. They also require a bit of time for the power output to stabilize and are available from a smaller number of manufacturers currently.
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PV Cells are clustered into modules which are further clustered into arrays
- Building Integration Components: In some fashion, the PV modules and arrays need to be attached and supported to some part of the building. It may be that a rack and mounting system frame or structure needs to be accommodated for a crystalline solution or it may literally be a "peel and stick" thin film solution to a conventional building material. Some manufactured systems come with the PV system already incorporated making the coordination easier and the integration more complete.
- Balance of System: By themselves PV modules or arrays do not represent an entire PVsystem, in fact the modules typically make up about 50% of the total system cost with racking and mounting systems accounting for another 12% if required. There are other elements to take into account and allow for in the design of a building all of which are collectively referred to as the "balance of system" (BOS) components:
Beyond (1) the PV modules and arrays, a complete BIPV system typically includes (2) a racking / mounting system, (3) a DC Combiner Box, (4) DC to AC Inverter (5) Power meter/disconnect/distribution panel, and (6) a monitoring system.
- Wiring, Installation and Electrical Connections: All PV systems need to be installed, wired and connected to junction or connector boxes, circuit breakers and other parts of the system. In designing the system into the building, these elements need to be accommodated. The cost of this part of the system will vary, but can be estimated to be around 20% of the total system.
- Power Inverter: The electricity coming from the PV array is naturally generated as direct current (DC) while most equipment used in buildings is based on machine generated alternating current (AC). Hence, some power conversion equipment such as an inverter will be needed to convert the DC output of the PV modules into AC. This equipment will need to be selected, sized and located in the building to be compatible with the electrical requirements of the building and the utility grid and could account for another 7 − 8 % of the system cost.
- Electrical System Interface Equipment: The converted electrical power needs to properly and appropriately connect to the building's overall electrical system. This will include work at the power meter, disconnect and distribution panel for the building along with a power regulator to control the PV power level and keep it predictable. Since the most popular installation is to have the building also tied to the utility grid, the entire interface will need the proper engineering and clearance space to meet the standards and requirements of the prevailing Codes, the local utility, and good electrical engineering practices, all of which may account for around 10% of the total cost. Adding an inexpensive power monitor for the system (< 1% of the system cost) is always a good and recommended idea.
- Stand-alone Alternative: While considerably less popular for full building design at the moment, small PV installations may be designed to "stand alone" without any connection to the utility grid. These systems will still require everything above (except the utility connection equipment), but will also need a battery storage bank with a charge controller to regulate the power into and out of the batteries. Additionally, a back up electrical system may be required such as a stand alone generator.