Commercial PV System Data Monitoring, Part Two
Inside this Article
In Part One of this article (October/November 2011, SolarPro magazine), we discussed the value of monitoring commercial PV systems and the benefits to investors, integrators, system owners and operators. We also discussed the options and components involved in the selection of a monitoring system.
In Part Two, we concentrate on the best practices for successful integration, including predesign planning with the monitoring provider, site-specific considerations and conduit routing. We provide a case study showing the financial effects of system downtime, which potentially can be alleviated via sufficient data monitoring. We also explore the trend toward inverter-integrated monitoring and its advantages and disadvantages compared to third-party monitoring.
MONITORING SYSTEM SELECTION AND SPECIFICATION
According to the subject matter experts we interviewed for this article, the most common mistakes made when specifying and implementing PV monitoring systems are generally traceable to a lack of planning, foresight or communication. Planning for a monitoring system after the PV system is built has obvious flaws. It may also be problematic to add a monitoring system after the PV system is quoted, let alone sold.
“The most common mistake integrators make is waiting too long to decide upon and implement a monitoring solution,” observes Robert Smith, director of marketing at ArgusON, a provider of site monitoring and management services. This often adds costs that could have been avoided. “When the monitoring platform is the last item to be selected on a commercial PV site,” he says, “expenses can rise substantially due to an inefficient system design.”
Fortunately, there is general agreement on how to avoid these problems: Plan ahead, do your homework, and communicate with the customer and the monitoring solutions provider early and often.
Customer needs ultimately determine the best data monitoring system configuration specific to each project. According to Benjamin Compton, COO at meteocontrol North America, which provides solar management solutions to large PV power assets: “In any measurement and verification program, a balance must be struck between the level of detail required for an accurate evaluation and the associated costs relative to the overall energy generation value.”
Site survey. Once you have determined that a commercial system will be monitored, the conceptual design must be tailored to meet the needs of the customer, system operator or both. The best way to make sure that you are asking the right questions and getting the right answers is to complete a monitoring installation site-survey form as early in the sales or design process as possible.
Typically, PV monitoring companies provide a site-survey document to their customers upon request. Site-survey responses are useful for specifying the right equipment, optimizing the system design and providing an estimated price for the goods and services. A site-survey form seeks to answer as many basic questions as possible about the site and desired functionality of the monitoring system. Typical questions include:
- What data do you plan to collect?
- Who will be using the web portal and how?
- How will Internet access be achieved?
- How will you power components that need a dedicated power supply?
- What are the conduit and cable requirements?
- Where will major monitoring system components be located?
The other major component of the monitoring site survey is a one-line electrical drawing. If this is provided along with an approximate or to-scale site layout, the monitoring provider can use this information to create a customized site-specific single-line drawing, showing the location of power and communication connections to the monitoring equipment. Sharing the system design with the monitoring provider also helps verify that all components are compatible, including inverters and meters.
Gathering this information during the planning stage of the project is crucial. The addition of a monitoring system inserts another layer of electrical equipment to the design, including power supply circuits, communication circuits, meters and IT equipment that must be located, sized and considered in advance to be fully incorporated into the PV plan set. The predesign process for determining the monitoring system should be considered just as important as sizing the correct inverter or choosing a racking system.
Monitored data. The most basic inverter-level data acquisition system typically captures dc volts, amps and power on the input side of the inverter, and ac volts, amps, power and energy harvest on the ac side of the inverter. These data are frequently reported graphically via bar charts for various time periods (hour, day, month or year).
More advanced monitoring systems can capture environmental conditions or detailed data from subarrays, strings or even individual modules. It is also possible to monitor utility grid-power quality and stability by tracking grid voltage and frequency variations outside the IEEE 1547 windows and report the duration of any outages.
Monitoring instantaneous building demand and net energy consumption is another option. If the building load is monitored, it is possible to compare the ac energy consumption at the facility against the PV array production. This may have value both for PR purposes and as an educational tool. It may also allow facility managers to become aware of, investigate and monitor other energy-saving opportunities at the facility.
Note that if a building has a preexisting automation system, the PV data acquisition system may need to have network compatibility with that system. This depends on the level of functionality and integration desired by the customer.
Graphical interface. It is a good idea to “walk through” a vendor’s web portal before making purchasing decisions. If an interface is difficult to navigate, lacks detail or does not allow for managing work orders, the monitoring solution may not prove useful for O&M purposes. Once you have selected a provider, it may not be possible to overcome issues like this— short of finding another provider.
Another thing to consider is the degree of personalization possible for a web portal’s user interface and the relative effort involved. The ability to customize web portals merits consideration: If it is not possible to brand a portal with your logo, the monitoring system may not meet your marketing needs. Your customers may have similar branding needs. Some third-party data monitoring companies provide software customization free of charge. Some products, such as the SMA Sunny Portal, allow customers to modify the appearance of the website. Other solutions allow little to no customization.
“As the market continues to mature, costs are coming down and customizable energy monitoring is becoming available all the way down to the residential level,” notes Robert Schaefer, CEO at AlsoEnergy, a provider of renewable energy monitoring solutions. “All users want a direct relationship with their purchase,” he explains. “The user base is getting more sophisticated in its purchasing decisions.”
Consider whether the content being displayed can be varied easily, based on whether the viewer is the facility owner, system owner, financial backer or O&M provider. If the intended viewer is the general public, consider what kiosk or wallmounted displays are offered, as well as the cost and connectivity requirements.
If the web portal is primarily used for O&M purposes, you may want to select a system that allows for crosschecking performance at a variety of sites in the same general vicinity. Data acquisition systems themselves can and do fail, so this feature may provide additional insight into potential data acquisition problems or into other PV system problems. It may also be important to select a portal that includes performance ratio indicators or other quick indicators of general system health.
If the system is designed to generate error or alarm signals, consider how this feature operates. Faulty alarms and excessive email or text messages often prompt integrator complaints. To the extent that alarm triggers are configurable, it is possible to reduce or eliminate false alarms. If the source of the alarm requires corrective action, it may be possible to schedule service calls and keep maintenance logs from the O&M portal. Companies interested in managing a fleet of PV systems should consider a monitoring system with this functionality.
Contractual terms also vary. Some web portals are free with equipment purchase; others incur annual fees. Some vendors offer multiple levels of service and tiered fee structures. Be sure to determine the cost and terms of service before selecting a monitoring solution.
Internet access. Since monitoring systems are generally cloud-based, Internet access is essential. If a hardwired Internet connection is used, coordination with the host facility’s IT team is a prerequisite. Communication is key, relates Chuck Wright, principal at PowerDash: “If the customers’ Ethernet is to be used, verify their understanding that it is their responsibility to keep the communication channel open for a period of years, even decades, and that they agree to do so.” Important questions to ask relate to the requirement for static versus dynamic IP settings, firewall setup for inbound traffic and other general network security or compatibility issues. The goal is to gather as much network data as possible so that the monitoring company can preconfigure the datalogger to automatically communicate with the network right out of the box.
If the site is remote or access to the existing network is prohibited due to network security systems and firewalls, a cell modem must be included with the monitoring system to push the data onto the Internet. If cellular communication is necessary, be sure to determine whether there is coverage during the site survey. In some rare cases, a satellite modem must be included. Note that adding a cell modem does not just add a onetime cost for the hardware component; a cell service contract must be included and renewed as long as the system is to be monitored.
According to Solectria Renewables CTO Michael Zuercher-Martinson: “Solectria Renewables recently added a 3G cellular network communication option to its monitoring product line, which has quickly turned into a best seller. Not having to deal with the local IT infrastructure and associated firewalls and not having to run additional communication conduits in many cases pays for the cost of the cell modem and the long-term service contract.”
Power supply. The power requirements for each monitoring package are different. A critical part of the site analysis is identifying available power sources for dataloggers, energy meters, smart combiner boxes and other equipment. While it is possible to purchase small self-contained and self-powered dataloggers and weather stations, they are generally more expensive than tapping into a preexisting load-branch circuit or installing a new load-branch circuit.
In systems covered by the NEC, ac power cannot be pulled directly from the inverter output, and a different circuit must be used. The power source does not have to be a dedicated circuit: Power can typically be pulled from an existing load-branch circuit, if one is available. In the event that there are no existing branch circuits in the vicinity, be sure to size any new ac branch circuit conductors for voltage drop when making long-distance runs to combiner boxes. In systems with inverter output direct to medium voltage, installation of an additional small transformer might be necessary to provide 120 Vac, which can also be used to power on-site service receptacles.
In addition, verify that the monitoring components receive the type of power they require. Smart combiners and data acquisition units might actually require a 24 Vdc power supply. In some cases, a 120 Vac/24 Vdc power supply is integrated into the component enclosure; however, sometimes an external power supply must be provided. The location of power supplies must be incorporated into the total system design and the conductors must be sized for voltage drop.
Note that while NEC Section 300.3(C)(1) allows ac and dc circuits rated 600 volts or less to occupy the same equipment wiring enclosure, cable or raceway, Section 690.4(B) specifies that PV system circuits “shall not be contained in the same raceway, cable tray, cable, outlet box, junction box or similar fitting as conductors, feeders or branch circuits of other non-PV systems, unless the different systems are separated by a partition.”
Depending upon your interpretation—or that of your engineer or the inspector for the AHJ—power system conductors for the PV data monitoring system may be considered non-PV system conductors. If they are considered PV system conductors, then they can share conduits with other PV system circuits; if not, then they cannot. Of course, this is relevant only for the power system conductors.
Communication circuits. Low-voltage monitoring equipment has its own requirements, as described in NEC Chapter 8, “Communications Systems.” All communication cables in PV data monitoring systems carry low-voltage signals, typically under 10 volts. The insulation rating on most communication cables is limited to 150 Vac. Check with the manufacturer or the NEC for specific cable insulation ratings. Cable insulation must be appropriate for the installation conditions—burial rated, UV resistant and so on.
As a result of this low insulation rating, communication cables cannot share conduits, raceways or enclosures with high-voltage power conductors—either ac or dc—unless proper physical separation is provided. Conduits carrying communication cables can share trenches, provided that adequate separation of the conduits is maintained. The additional conduit, wire and installation cost for running lowvoltage communication cables must be considered during system design and construction planning.
Conduits and circuit routing. The addition of a monitoring system can add a significant amount of conduit and circuits that must be planned and accounted for in the initial design. Verify that placing the cables in conduit does not exceed the conduit-fill allowance. This is especially true for weather sensors that have unique connectors.
When evaluating the Internet connection, keep in mind the distance limitations of the various protocols and bus drivers. TCP communication on a CAT 5 or 5e cable is limited to 300 feet. If the distance from the base station to the Internet connection exceeds this distance, consider using fiber optic cables, which have distance allowances that are counted in miles, or cellular modem connections if within range of a cellular signal.
To keep costs down in large array fields or custom structures, identify all of your conduit requirements before hardscapes like roads or parking lots are completed. This lets you get your conduit into trenches before they are backfilled and is much more cost effective than finding alternative installation solutions afterwards.
COST ANALYSIS CASE STUDY
Since financials are the biggest driver in determining monitoring decisions, the following analysis attempts to demonstrate the financial effects of varying degrees of system downtime. Washington, DC, was chosen for the location due to its mature and developed solar market.
As of June 2011, the SREC value in this market with a 3-year term is averaged at $365/MWh or $0.365/kWh. In addition, as of February 2011, the average commercial electrical offset rate for this area is $0.1358/kWh, according to the US Energy Information Administration. Therefore, the total value to the investor of the PV-generated energy in this market is $0.5008/kWh.
The example system consists of 500 kW of 14% efficient modules, roof-mounted at a 10° tilt and a 180° orientation and organized into 152 strings that feed two 250 kW inverters. Using PVsyst, we can predict an average annual energy production of approximately 697,000 kWh.
Table 1 demonstrates the effects of different levels of system failures over typical durations of time. The effects are quantified in total energy production loss, percentage of annual production loss, and the total and daily financial repercussions.
These scenarios are not uncommon. As the annual production loss calculation indicates, they can have a significant impact on the performance ratio of a system. Time is money in these cases, and the integration of a data acquisition system can significantly increase system uptime.
It is not an exaggeration to estimate a 2-week time period to get an inverter back online. In fact, without data monitoring, it could be up to a month or longer before a problem is detected. Fixing the problem after it is identified typically involves time for travel, troubleshooting, shipping replacement parts, yet more travel, and replacing the component and recommissioning the subarray.
With inverter-level monitoring in place, an alarm and accompanying error code is received instantly via web portal, text message or email as soon as the inverter failure occurs. This allows for a more timely and focused response. In many cases, good inverter communication allows problems to be resolved on the first trip to a site, instead of merely being diagnosed.
Losing the input from a single string into a 250 kW inverter would not be noticeable at the system or inverter level. Without string or combiner box monitoring, it could be many months before the problem is noticed. By then, a lot of potential energy would have been lost. The loss of a single string could eventually lead to bigger problems, depending upon the causality. (See “The Bakersfield Fire,” February/March 2011 SolarPro magazine.)
Although a single string represents a small fraction of total system production, over time the value of the lost energy adds up. In the example system, using a $/kWh escalator of 4% and a discount rate of 8%, the net present value of having that string functioning properly over 25 years is around $35,000. The internal rate of return for the system also shows a drop from 17.5% to 17%. This drop alone might justify the up-front cost of providing a more sophisticated monitoring system.
For comparison purposes, a system in Oakland, California, would have a performance-based incentive value of $0.05/ kWh for 5 years and an average electricity cost offset of $0.1723/kWh, as of March 2011, for a total value of $0.2223/kWh. Based on this lower value of energy and the relatively higher insolation in Oakland compared to Washington, DC, the economic value of the production losses in Table 1 would decrease by about 50%, even though the amount of energy lost would increase by about 10%.
Location. For best results, show the locations of datalogger enclosures, energy meters, weather sensors and other monitoring system components in your site plans and elevation drawings. Once again, not including these items in the plans leads to wasted time and money. Sometimes inverter rooms or pads are small, and the designer must plan for the additional wall or pad space needed for the datalogger or external revenue-grade meter.
Distance considerations between components must also be taken into account for data accuracy. For example, the distance between the energy meter(s) and the datalogger may have limitations depending on the communication protocol and bus drivers used. Weather sensors have limits on how far they can be located from the datalogger or analogto- digital converter. RS-485 cable should not be run farther than 4,000 feet from the first device to the last.
Just as the power system equipment is sensitive to environmental conditions, so is the monitoring equipment. Although most systems are housed in NEMA 3 or better enclosures, the equipment should be kept out of direct sun.
Other considerations. When evaluating monitoring services providers, consider whether their product features enable you to successfully operate your plant today and provide the flexibility to meet your needs tomorrow. Besides applicationspecific criteria, consider long-term compatibility. Here are some questions to ask:
- Is automatic REC-compliance reporting an option?
- What are the configurations for downloadable system performance reports, including time periods available and data views?
- How can you troubleshoot the data monitoring system?
- What service contracts and warranties are available, and at what cost?
- What kinds of support services are available?
- How confident are you that the monitoring company will still be around in 20 or 25 years?
Some criteria, like product features, are relatively self-evident; others, like level of customer support, are more nuanced. Support services might include any or all of the following: predesign support, installation support, commissioning support, predelivery system testing, host customer training and preconfigured equipment.
Because PV systems have such long service life expectancies, choosing data monitoring components and service providers is a significant exercise. “Convince yourself that your data monitoring partner will still be around in 25 years and supporting its equipment with boots on the ground,” advises Solectria’s Zuercher-Martinson.
INVERTER INTEGRATED OR THIRD PARTY?
For a decade or more following the inception of the gridconnected PV industry, inverter manufacturers generally provided minimal inverter-integrated faceplate data— usually just ac energy, ac power, and dc and ac volts and amps. When installers and system owners wanted verifiable in-depth data reporting, they relied upon specialized hardware and software from providers of third-party data monitoring solutions.
In recent years, central inverter manufacturers have increasingly begun to offer more sophisticated inverter-integrated data acquisition systems. The latest inverterintegrated monitoring systems have a much wider ability than those of the past to track, log and display PV system data and error alerts. Revenue-grade metering and reporting is generally an option.
This trend toward more sophisticated inverter-integrated monitoring is likely to continue. As an example, Power-One— which is currently the second largest solar inverter manufacturer in the world—purchased the assets of Fat Spaniel Technologies, a leading US provider of PV monitoring solutions, in October 2010. Subsequently, Power-One acquired monitoring business assets from National Semiconductor, which included solutions developed by Energy Recommerce.
Ultimately, there are advantages and disadvantages to both inverter-integrated and third-party monitoring approaches. Generally speaking, over a period of years, system designers, integrators and EPC contractors specify various brands of inverters. There are several reasons why: Inverter product availability and pricing is dynamic; new inverters are always coming to the market; some inverters are a better fit for different climates or with different modules.
Whatever the reason, it can be impractical or even undesirable to specify the same inverter manufacturer’s products for all of your projects. However, if you specify products on a case-by-case basis, and always rely on inverter-integrated monitoring packages, your data monitoring hardware and software will vary from site to site. This complicates your overall monitoring strategy. Security access may be more challenging, as multiple hosting platforms may require multiple passwords. There is a learning curve associated with each new platform, both in equipment installation in the field and in managing the interface back at the office.
One of the main selling points for choosing a single supplier for third-party monitoring services is the promise of being able to monitor and manage different inverters and different sites from a single-user interface. Errors, alerts, alarms, reports and work orders can be standardized across every installation site within one web-hosted platform. Ideally, all of the data for every site is easy to access and track. Another benefit of using thirdparty monitoring services is that the monitoring system operates independently of the inverter, which may prove especially valuable in the event of an inverter failure.
Of course, before deciding to use a single provider for all of your monitoring needs, you need to evaluate the company’s stability, reliability and experience. Working with a small thirdparty monitoring solutions provider may not instill the same level of confidence as working with a major inverter manufacturer, such as SMA. Of course, not all third-party providers are necessarily small companies. For example, ArgusON is backed by the financial resources of SPX Corporation, a Fortune 500 company, and maintains a national call center that is open 24 hours a day, seven days a week.
Another consideration is the degree of inverter-level functionality that the monitoring solution enables. Inverters have lists of alarm and error codes that vary by manufacturer, and inverter-integrated data monitoring systems interpret these codes correctly. Third-party monitoring providers rely on the inverter manufacturers to share these alarm codes, and some manufacturers are not forthcoming with this information. Furthermore, if you are using an inverter-integrated data acquisition system, it may be possible to remotely resolve and reset some inverter alarms. While some third-party monitoring suppliers may be able to provide this capability, it is less common.
Installing cost-effective and efficient PV data acquisition systems means that monitoring solutions providers and PV system integrators must work together as partners—through the sales cycle, the design and installation processes and onward— to keep PV systems producing as intended for decades. Choosing the best data monitoring system and partner for your needs means thoroughly researching the options and correctly valuing data monitoring as an investment in future returns.
Special thanks to Bill Reaugh at Draker Laboratories for providing expert technical review services and input during the preparation of this article. Commercial Monitoring
Kyra Moore / Southern Energy Management / Durham, NC / southern-energy.com
Rebekah Hren / 02 Energies / Durham, NC / 02energies.com
RESOURCES Third-Party Commercial PV Monitoring Providers
AlsoEnergy / 866.303.5668 / alsoenergy.com
ArgusON / 866.459.4103 / arguson.com
DECK Monitoring / 503.224.5546 / deckmonitoring.com
Draker Laboratories / 866.486.2717 / drakerlabs.com
Fat Spaniel Technologies (Power-One) / 408.785.5200 / fatspaniel.com
Locus Energy / 877.562.8736 / locusenergy.com
meteocontrol North America / 510.764.6494 / meteocontrol.com
Solar-Log / 203.702.7189 / solar-log.com