Keeping Tabs on Your PV System

Intermediate

Inside this Article

Monitoring System Flow Chart
Use this flow chart to help find the monitoring system best for you.

Until recently, consumers had few choices to help them keep tabs on their grid-tied PV systems. Most PV monitoring products were inverter-specific data loggers, which uploaded data to the manufacturer’s Web portal. Homeowners had access to raw data—inverter energy production, simple alerts, and approximations of the CO2 saved—but little more.

Now, monitoring products are providing deeper insight into PV performance and beyond—including whole-house energy management via energy generation and consumption monitoring. Newer monitoring systems can tell you how much money is saved on electric bills; report on whole building, branch circuit, and individual loads; and illustrate the effects of energy conservation steps taken.

The value of solar energy and energy conservation is best shown if their daily effects can be readily monitored. According to a February 2009 study by the Electric Power Research Institute, residential electricity usage feedback tools—such as monitoring devices—are effective at  encouraging conservation. The study showed that using monitoring systems resulted in up to an 18% reduction in energy use, and that more direct, detailed information leads to higher levels of conservation. Being able to examine data can yield enough savings for the monitoring equipment to pay for itself—and more—over the life of the home.

“The future of residential PV in the United States is dramatically improving—playing a bigger role in the energy mix,” says James Bickford of Tigo Energy, manufacturer of the Module Maximizer and its monitoring software. “The monitoring component will be a critical piece, allowing for control and management of distributed power sources and integration with the utilities.”

There are many inverter and whole-house monitoring devices on the market—this article discusses hardware and software solutions that support monitoring of solar generation for residential systems.

The Pitfalls of Data Communication

A big part of monitoring is getting the data where it needs to go. We can break data transfer into two parts of the communications process: from the solar equipment to the data logger, and from the data logger to the Internet.

A cable is the simplest means of getting the data from the solar equipment to the data logger—but running cables can be difficult or impossible due to the paths across land, in trenches, through walls, and in attics. This has driven the industry to introduce radio technologies such as wireless computer networking (WiFi); power line communications (PLC), which modulate data over AC lines; and radio technologies such as ZigBee, a wireless home area networks (WHAN) standard. These technologies have their own issues with distance, obstructions, and interference.

Moving the data from the logger to the Internet normally involves connecting with the homeowner’s always-on Internet service, which is typically cable modem, DSL, or fiber optic network. Cat5 cables or WiFi are common but also share the above issues. Cellular routers allow for an upload strategy that’s independent of the homeowner’s service, but they can pose a significant one-time cost of about $200, as well as ongoing monthly data service fees. The cellular industry has recently recognized the need for residential data service and more affordable service plans are available, from $10 to $40 a month.

Careful review of the data communications requirements of the products, and assessing your situation, should lead to successful monitoring. Most products offer several options, so you’ll need to research which data communication solution is best for your needs.

Inverter-Tied Monitoring

Manufacturers of residential-sized inverters provide proprietary, basic monitoring equipment. There is little compatibility between different manufacturers’ systems, but there is movement to standardize cables, wireless communications, and data protocols. Most inverter-paired monitoring systems offer data loggers that upload to the manufacturer’s Web site, and some offer wireless displays that communicate over radio, such as Bluetooth. With wireless options, carefully evaluate the distances and obstacles to ensure a strong signal.

Inverter-Independent Monitoring

Several monitoring solutions are inverter-independent. The basic technology is to use CTs in the homeowner’s circuit breaker panel to obtain data. One pair of CTs (for 240 V utility service) monitors the conductors from the inverter to a double-pole breaker to keep track of the home’s PV generation. Another pair reads the conductors from the utility meter to the main breaker to track net building load—the amount of energy that comes from the utility. More CTs can be used to monitor individual 120 V branch circuits for appliances or other loads, or in pairs for 240 V branch circuits, such as for air conditioners. With an array of CTs installed in the circuit breaker panel, the raw data can reflect a home’s full energy profile.

While this level of monitoring is powerful, installing CTs into a breaker panel presents some issues. First is jurisdiction approval and adherence to National Electrical Code standards. One issue is tapping into a branch circuit breaker—this enables the device to measure voltage and calculate power. Since this creates a small branch circuit inside the breaker panel, NEC Section 210.19 can apply. Some authorities also insist that the complete assembly of breaker panel and monitoring unit be UL-listed—an impossibility. UL Standard 1244 and 916 govern electrical monitoring devices, and vendors can choose to test and certify against one or both of these. Some permitting offices may be unfamiliar with this technology, so you may have to educate the staff on UL listing and NEC requirements that apply.

The second issue is physical space. CTs and the other equipment take up room in the typically already-crowded breaker panel. Locating additional equipment may necessitate mounting a suitable enclosure to the side of the breaker panel.

The third issue is data communication. Most CT-based systems transmit their data using PLC over the home’s power lines to a data logger. Signal integrity can be compromised by branch circuit length and “noisy” devices near the logger, such as computers, home electronics, or motorized appliances such as refrigerators, blenders, and air conditioning compressors—anything that causes radio frequency interference (RFI). Despite the challenges, most CT-based systems have few problems.

Module-Level Monitoring

PV systems can suffer from a range of energy-limiting problems, like module mismatch, partial shading, complex layouts, and subarrays on different roof planes. To address these issues, single-module AC and DC maximization technologies have been developed. Module-level monitoring is a beneficial side effect, and customers have taken a more active interest in their systems due to the level of information available.

Microinverters mimic the function of a string inverter, but for individual modules. Mounted on the racking system or the back of each module, a microinverter takes DC power from its partner module and produces AC power at its individual maximum power point (MPP), squeezing the most out of each module and, therefore, the maximum power out of the whole system. Microinverters allow more freedom of design since modules can be placed on different roof surfaces and angles, without degrading the overall power generation. Since there is intelligence built into each microinverter, the separate data can be collected and sent to a data logger. Data is normally transported from the microinverters to a logger via a PLC, removing the need for a separate data cable. 

DC maximizers adjust the DC voltage and current for some or all of the modules in a string to generate more DC power than would have otherwise been available. With a standard string inverter, underperforming modules drag down the whole system’s power production. DC maximizing allows for more freedom of design, since a roof with partial shading risk can have more modules installed than otherwise—partially shaded modules won’t sabotage the whole system. Since it’s difficult to transmit data over DC wiring, most DC maximizers use either a separate data wire or wireless communication with the data logger.

With both single-module technologies, data is uploaded through a gateway and the homeowner’s Internet connection to the manufacturer’s Web servers displaying a visual representation of system performance. These Web sites display individual module performance, with the module’s graphic representation laid out in the same pattern as the physical modules. The module displays normally show a numerical and color indication of power—brighter colors indicate more power generation. Users can trigger a time-lapse display or move a slider back and forth, showing dawn-to-dusk performance. The color cue can help detect an underperforming module or string, which could be due to shading, soiling, module mismatch, blown fuses, or broken wires. The installer can troubleshoot the system, armed with a great deal of information about the system’s issues.

At this point, quantifying the actual additional power generated is difficult, since module-level monitoring is only available with microinverters and maximizers installed—you cannot see what the individual modules are producing without the distributed MPPT equipment. However, the non-quantifiable aspect of these systems—the benefits of module-level monitoring—should be taken into account in the overall decision process.

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Michael Brown worked in software architecture, development, and support at IBM for 22 years. He joined REC Solar Inc. shortly after having the company install a solar-electric system at his house, and is in the process of converting a Porsche 914 to full electric drive (porsche914e.blogspot.com).

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