DC Power Production, Delivery and Utilization
times overburdened grid may not be the best way to manage the resource. Net-metering agreements and the meters that they require can be expensive.
One solution is to couple a DC power delivery system with equipment such as DC-powered lighting ballasts. A few DC- ready products are commercially available, as noted previous- ly. For instance, the Nextek system, featured in the case study of a distribution warehouse in Rochester, NY on p.20 consists of a special controller/conditioner with an AC power port and a DC input. The controller combines as much PV DC power into the mix as is available or needed to power the lighting system. If more DC power is needed than is available, some grid power can be converted to DC to supplement the local PV source. Like a hybrid car, the “hybrid” building uses two forms of power.
DC power delivery to optimize PV system economics
To evaluate the economics of a PV-DC system, consider the cost components of conventional PV systems employing an inverter. The current lifecycle cost of PV system energy is in the range of 20–40 cents per kilowatt-hour, based on capital costs of PV systems without energy storage in the range of $5 to $10 per watt-AC of capacity. These costs include the PV modules, inverter, AC interconnection equipment, and installation.
The inverters and equipment associated with AC power sys- tem interconnection represent 25% or more of the total system capital cost. In addition, inverters may have a life of only 5–10 years prior to needing replacement or significant repair.
Direct current applications of PV can avoid the need for invert- ers (and inverter repairs) and associated AC interconnection equipment. This could reduce cost, improve reliability, and increase usable power output since the rated inverter losses of 5–10% are avoided. A DC application can also avoid the need for sometimes costly and time-consuming AC interconnection reviews since DC installations are not able to feed AC power back into the utility distribution system. Furthermore, a DC system can continue operating during a grid outage.
Of course, not all PV-DC applications are suitable or can result in lower cost. Careful matching of load and source are needed.
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For example, by confining a PV system to a single DC load, the advantage of the diverse loads found on the AC system are lost and it becomes more critical that the PV system size and out- put cycle be optimally matched to end-use requirements. Too large a PV panel on a given load could result in underutiliza- tion of the PV energy source and actually raise the effective cost of PV energy compared to the AC inverter approach. Fur- thermore, DC applications won’t eliminate the need for some control and conditioning of the PV energy.
A DC-to-DC voltage converter as well as various load switch- ing controls may be necessary for many PV-DC applications. However, despite these issues, a well-designed PV-DC applica- tion can have a significant cost advantage over a conventional inverter approach. This is, in part, because DC-DC convert- ers are more efficient than inverters and can be lower in cost. Furthermore, for some applications, DC-DC converter func- tion can actually be integrated into the end-use equipment, further optimizing the PV-DC approach.
Overall, even though the DC delivery approach comes with some issues, if considering the potential equipment-cost re- duction, efficiency enhancements, and value of reducing inter- connection concerns, EPRI research indicates that that the to- tal lifecycle cost of PV energy for certain DC applications could be 25% lower than using a conventional inverter approach.14
As shown in the section “Powering Equipment and Appliances with DC,” many loads can operate with DC power and some are even better suited to DC than AC. These include variable speed motors, lighting technologies, resistive heating ele- ments, and electronic switch-mode power supplies found in various office equipment.
Schematics illustrating how selected AC loads can be sup- plemented with DC energy from PV are shown in Figures 12 and 13. In the applications shown in Figures 12 and 13, 60-Hz utility system power is combined with DC from the solar array on a common DC collector bus. By appropriately controlling the DC voltage level from the solar array (using a DC-DC con- verter) with respect to the power supplied by the AC system, it is possible to make sure that PV energy is utilized when it is available but that 60-Hz utility power will “pick up” the load entirely if there is no PV power output. To insure full utiliza-