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.
Mathematics of Parallel Solar Power System Design
Parallel solar design reduces the number of variables that need attention during solar power system design. Voltage is no longer a factor, so Voc overhead and temperature drift are no longer concerns. We are also freed from worry about the NEC 600V upper limit and its restrictions on the number of modules we can wire together. This simplifies the calculation of wiring loads.
Three basic decisions must be made at the outset: size of the installation in kWh, modules to be used and inverter to be used. With these in mind, we can start to envision the system. As an example, let’s consider a 180 kW building block using 30 kW units with 230 watt solar modules operating at a Vmp of about 40 VDC. The math here is simple: we will need about 132 modules (30,000/230 ‚âà 132). We will assume that the inverter’s peak efficiency point is at about 330 VDC. From this, we can calculate that at maximum power output, we will have to deal with 92 Amps of current into our inverter (132 modules × 230W/330V = 92 amps (P/V=I)).
Thinking about this as a lighting circuit, we can look at using six branches of 15 Amps each, a conservative level for #10AWG PV USE-2 or RHW-2 cable outside of conduit. Each branch would have an inline 20 Amp fuse connecting it to a #4 AWG PV backbone that runs directly into the inverter through a 125 Amp fused DC disconnect.
We can also go a bit larger and design a parallel solar power system for 500kW production capacity: module power density, 230 W; voltage input to the inverter, 330 VDC; total power capacity of system, 550,000 W.
This will tell us the number of modules we want to use: Total System Capacity/Module Power 500,000/230 = 2,174 modules.
To figure the total current the system will need to manage we take the total power and divide it by the voltage. Modules×Module Power in Watts = System Power. System Power/Voltage to Inverter = Current. Thus, 2,174×230/330 = 1,516 Amps.
From here it is a simple matter of working out the number of branches needed to manage the current flow. If we assume use of three of our 180 kW building block circuits (506 Amps each) to connect to our inverter, we can place their terminating points close to the array to minimize our use of conduit. If we want to minimize our terminations, we could use #4 AWG PV wire into our building block combiner units, with each handling 85 Amps.
To minimize I2R losses we can take a conservative approach and use 20-Amp in-line fuses harnessed into the #4 AWG PV backbone, giving us six branches using #10 AWG PV. Each of our three combiners then will have 167kW of power concentrated into a single pair of conductors, handling a total run of 506 Amps into the central inverter. This array would need just six physical field terminations at the combiners, and six at the inverter. If the combiners are placed strategically at the edge of the array, the conduit runs would likewise be limited to three: one from each combiner to the inverter (see figure 1, below).
The difference between parallel and series architecture for solar power system design is as simple as the difference between current and voltage. In a series system, the voltage of the module drives the design and therefore the economics of the installation. Parallel wiring lets the voltage be set as a constant, which allows the system to be driven by current.
Current is a much easier variable to work with on several levels. First, it is a familiar, well-understood design variable for designers and installers; the same one used in all lighting system design. Second, the current variable is much easier to regulate and control with existing safety systems. Third, we can optimize the efficiency of the DC-to-AC conversion by regulating the operational voltage of the solar array to the voltage of the grid that the system is providing power to.
Perhaps most importantly, parallel solar wiring allows different PV technologies to feed a single inverter. This promises to open new vistas for architects and system designers as they search for better ways to integrate solar technology into our everyday lives. It will allow PV manufacturers to optimize products for very specific environmental conditions without having to carry the load of an entire system’s production capacity. It may also make new materials more feasible by isolating each module from the rest of the system, allowing it to work at whatever native voltage is most efficient for that particular technology. All of these new possibilities open the door for innovation in the solar market.