Tulsa, Oklahoma, United States -- For decades, designers of solar power systems have faced a knotty set of interlocking challenges. Solar panels produce DC at relatively low voltages, but inverters require a relatively high input voltage to be able to convert the power to AC and send it to the grid. Solar panels can be wired in series to sum their voltages, but their combined output fluctuates with even small mismatches among panels on a string.
Striking a balance between these factors is traditionally one of the grand challenges of solar power system design and also a significant element in determining whether a given location is suitable for a solar installation in the first place. However, today new doors are being opened by innovators in a vibrant technology-driven industry and the advent of parallel wiring architectures for solar arrays promises to create new levels of freedom and flexibility for designers.
Series: The Old Way
Series-wired systems are governed by the principles of voltage. A solar array must provide a high enough voltage to enable its inverter to operate at an efficient level; this has traditionally required series wiring, so that panel voltages sum. Similarly it is important to make sure that the system can never go above the maximum voltage permitted by code, usually 600VDC in the U.S.
However, the inverter is sensitive to operating voltage levels. It can suffer major swings in efficiency when the input voltage varies in relation to its fixed output voltage. The larger the variation, the harder it is for the inverter to operate at optimal efficiency. Currently inverter efficiency is shown at a single operating point when actual operating efficiency varies as system voltage changes, real operating efficiencies can be off several percentage points from the optimal operating efficiency.
To accommodate these physical demands, all series-architected solar installations must abide by a set of design rules. The result of these rules is to define the minimum-sized building block (string) used for a given installation. Once this is defined, that exact footprint must be used for the entire array. This can lead to serious challenges, as designers are forced to manage the always-unique geometry of the proposed array location. In many cases, these challenges translate into increased cost of deployment, smaller system sizes or even a decision to forego the installation completely.
The New Parallel Solar Universe
The enabling technology for parallel solar deployment is a new generation of low-cost, high-efficiency electronic devices that allow a solar module to deliver a fixed DC voltage to a DC power bus. This DC power bus can be set to the single best point for the inverter or can float to whatever level the inverter requires, allowing the inverter to concentrate simply on optimizing its AC-to-DC conversion efficiency, as opposed to worrying about what compromises it might need to make to effectively harvest power from the solar modules. This mechanism provides an effective transport of power to a central inverter where AC conversion efficiencies can be optimized.
In this parallel solar paradigm, the PV technology of the module no longer matters, as each module operates with complete independence from its neighbors. Because each module can produce the voltage level needed by the inverter, voltage summing with strings of modules is not needed. This means that a solar array can now be designed and installed just like a lighting system. Each module represents a current source and as long as the array’s wiring is sized appropriately and its branches are capable of handling the current produced, the system will work at optimum efficiency; no other design rules apply.
What does this mean to the system designer? The biggest advantage is that systems can be built using variable-sized blocks of modules ranging from 200 watts to 31,000 watts. This enables designers to maintain installed cost targets while also taking complete advantage of all available space at an installation site. If the geometry or aesthetics of a project require multiple azimuth angles, different angles of tilt or shading, there is no longer a need to incur the costs or design limitations of multiple inverters. The solar power system can accommodate the architecture of the building, rather than requiring the building architecture to provide an ideal platform for the solar array. Different PV module technologies can even be applied to a single inverter (that is, thin film and crystalline).
But this new technology also allows us to think a little further out of the box. We now have a new tool available for optimizing a system’s production capabilities in multiple environments. We are only scratching the surface of what we can achieve with this new capability. For example, rather than using a technology like a tracker, we might use different materials technologies to optimize production across multiple seasons and environmental conditions.