Jack Shandle, Mouser Electronics
The distributed intelligence that has been the defining characteristic of the Smart Grid can do more than enable smart homes and help utilities distribute power more efficiently. These two well-known advantages will be enhanced by two revolutionary changes at the edge of the power grid.
The first change is that the large fossil-fuel generating stations that public utilities use to pump out energy to the grid will be challenged by smaller power-generation networks – mostly solar initially – that leverage collaborative energy exchange.
It is likely that the center of these collaborative networks will be solar power aggregators such as Sunrun Corp., Solar City and the aptly named SunEdison – three of the larger companies involved in home solar power installation, financing and leasing. Leaders in smart meter technology such as Itron Corp. will also be key players.
Aggregating solar energy requires intelligence at every level, including smart meters, but it also has to include solar inverters “upping their game” so that they can assist in the critical task of estimating how much power can be generated at any given moment and what to do with it.
The second change will be to accommodate – and exploit – the emergence of electric vehicles. As more electric cars come online, one natural consequence will be more 240V charging stations in homes, which will radically change the load profile of the average residential customer. Charging electric vehicles may also stress the capacity of existing power distribution systems. Under the right circumstances, however, car batteries may also be used as a local energy resource. Vehicle-to-home adapters can be designed to discharge their energy to power a home during a brownout.
The essence of collaborative networking is that smart homes communicate with each other to create a community – as opposed to each smart home communicating independently with the public utility. Solar aggregators and other suppliers of local power manage the community’s energy resources.
Potentially, any local entity with internal energy production capability – a business or business park, factory, university, or hospital could participate. Information will be transmitted not only over the utility’s power grid but also using social media and – importantly – the Internet of Things (IoT).
This scenario opens a wide range of electronic design opportunities, which include energy monitoring at a more granular level and – at the other end of the business model – handling the financial and business aspects of small-scale energy exchange.
It’s important for both the energy consumer and energy producer to measure and understand the bi-directional flow of both energy and information. Financial transactions such as taking advantages of credits and incentives for clean energy generation, behavior modification and carbon emission reductions will rely heavily on measurement and collaboration among multiple stakeholders.
In a collaborative network, utilities would be able to identify customers with solar panels and which of those customers also qualify as a “smart home.” For their part, consumers would know more about incentives the utility might be offering and how they can benefit by using home automation software. At the local level, energy services companies would be better able to control generation in the aggregate so they could participate proactively in energy markets for grid stability services. Cost savings derived from collaboration could be passed on to consumers in more attractive financing for solar panel installation.
Cooperative power management
Highly accurate measurements of bi-directional energy flow and voltage monitoring will be required for energy-producing equipment such as solar inverters and battery/energy storage because they will now be directly involved in market transactions. The sensors and processors in these products must deliver high quality, but still be relatively inexpensive.
On the other hand, the role played by equipment such as load control switches, thermostats, pumps, water heaters, and smart appliances will be to help collaborators understand the cycling behavior of their equipment and provide estimates of energy consumption over time, such as with HVAC systems. These products can work successfully with sensors that don’t require revenue-grade measurement.
Although collaborative power networks are just getting off the ground, semiconductor companies have already developed strategies, ecosystems and product portfolios that make designing for collaborative networks easier. For the most part, the chips designed for the traditional version of the Smart Grid and smart home will work just as well in collaborative networks.
Solar panels are the linchpin of collaborative networks because they provide the basic commodity: energy. Photovoltaic conversion has come a long way in the past decade, but using it as the primary power source for a community makes reliability and predictability essential (see Figure 1). Solar energy efficiency depends on many variables including the amount of sunshine available for harvesting, the temperature of the solar panels, and their theoretical peak efficiency. For design engineers, another problem is the fact that photovoltaic cells produce voltages that vary erratically around a half volt. This has many implications, such as choosing a power conversion topology. It is possible, for example, for a poorly implemented power conversion technology to consume a high percentage of the power-harvested photovoltaic power.
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Figure 1: Solar panels supply the base level power for collaborative networks
To accommodate the inconvenient fact that the sun does not shine 24 hours a day, batteries are required for delivering power when the panels are not producing enough energy. Adding batteries to the system makes DC-DC conversion circuits a necessary. Battery management and monitoring circuits are also required so that the batteries can be charged efficiently.
In both collaborative and traditional power distribution networks, solar systems interface with the power grid, which requires phase synchronization and power factor correction. There are also several use scenarios that require sophisticated control. For example, fault projection must be built in to guard against events such as brownouts and blackouts.
Typically, solar panels installed in a residential setting have a single power inverter, which basically handles the conversion of unregulated DC power from all the solar panels in the system, to high-quality AC power that can be used by the consumer. The downside of a single-inverter power conversion topology is significant. Most striking is the reliability issue: if the inverter malfunctions, the energy being generated by all the panels is lost until the inverter is fixed or replaced. Single-inverter topologies also are limited in efficiency because each panel has different operating characteristics that are “averaged out” during conversion.
The problems of single-inverter topologies can be addressed by using a micro inverter for each individual solar panel. In many applications, using micro-inverter topology can significantly improve overall system efficiency. Energy continues to be converted even if one micro inverter malfunctions. On the other hand, micro inverter topologies require a dedicated MCU and other components for each solar panel. Additional MCUs can be added to improve monitoring and for metrology.
Another important benefit is the ability to adjust conversion parameters on each panel to accommodate the amount of sunlight, shade, rain, and other variables. In order to handle this task, micro inverters typically execute sophisticated algorithms known as maximum power point transition (MPPT) algorithms. This computation requirement can add another feature to the MCU: a DSP core. At the panel level, a 30% increase in efficiency is easily attainable.
Communication is key
Collaborative networks obviously require communication between panels to keep loads in balance, and allow advance planning for how much power will be available, and what to do with it. Since each subsystem will be part of the IoT, the first communication requirement is the use of Internet protocols.
To simplify interfacing with other micro inverters, the MCU should also integrate on-chip communications peripherals such as CAN, SPI, and UARTs. Supporting power line communications protocols is another important option because it would eliminate some of the wires dedicated to communication. But this requires a high-performance PWM capability to be built into the MCU.
Semiconductor companies have comprehensive product portfolios for micro-inverter designs. Freescale recommends its MC56F84xxx digital signal controllers and its Kinetis KV3x microcontrollers. Texas Instruments offers a C2000 Solar Micro Inverter Development Kit based on the company’s Piccolo TMS320F28035 microcontroller. Microchip Technology offers a reference design and development tools for micro-inverter development.
Smarter home, smarter grid
Collaborative power networks based on sharing solar generation with a traditional utility grid for support share many characteristics of the Smart Grid. Internet-enabled appliances and other products as small as light bulbs feed their information into a Home Area Network (HAN) using ZigBee, Bluetooth, Wi-Fi or some other wireless protocol. A gateway transports the aggregated information though a smart meter and into the solar aggregator, which has now taken the place – at least in part – of the public utility (see Figure 2).
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Figure 2: Block diagram of a typical smart meter
Advanced energy applications leverage this data into actionable events. Metering data is stored in a variety of data formats so it can be used by multiple communications interfaces. For the consumer, downloadable apps are being developed to extract value from local metering data. Calculating baseline consumption locally in a device provides consumers and utilities with indications of typical usage, and the chance to reduce load during demand-response events. Similarly, rate calculation code can estimate the cost of battery charging – including electric vehicles – or for energy conservation. All this helps consumers select the best choice of tariff given their energy-consumption behavior.
Collaborative power networks that combine energy utilization, generation and storage by end consumers are a new spin on the Smart Grid. Collaborative networks require accurate measurement and communications, however, which means that intelligence and control must be extended to multiple parties in order to create mutual value.