By Mark Snyder and John Connell
If a customer wanted to “go off” the electrical grid, it traditionally meant they had to make lifestyle changes. Off-grid systems were often smaller, with lower energy storage capability and limited ability to scale up or incorporate multiple power sources. A newer alternative—microgrids—combines the benefits of grid-tied and off-grid systems, while offering more options for growth at a substantial savings to customers.
Until recently, grid-connected users have had to rely on the central grid, even when natural disasters leave hundreds of thousands without power. Hurricane Sandy in 2012 led to more than two million power outages at its peak, and many of those without power were outfitted with solar panels and energy storage systems. Microgrids could have helped kept the light on in this and similar situations.
How microgrids work–and their benefits
Microgrids are independent energy systems that combine an off-grid system, an on-grid inverter system, and battery backup (for peak demand, nighttime and/or off-grid use). They’re designed to operate either in grid-connected or off-grid mode. Grid-connected microgrids can also disconnect themselves completely during utility disturbances, a feature called “island mode.”
In a typical microgrid, power is generated from solar panels or wind turbines, with extra demands drawn from batteries. If peak demand greatly exceeds average demand, or in locations where energy production may be low for multiple days, natural gas or diesel generators are often used for backup or additional power. This mix of energy sources makes it possible to operate megawatt-scale systems reliably.
Microgrids slash energy losses from transmission and distribution, because power sources are located closer to the load than in the conventional grid. Microgrids can also more precisely optimize backup power generation for increased efficiency. During peak hours, microgrids can even sell back excess power and reduce their load on the grid, reducing costs and helping prevent utility grid shutdown.
How to get the most from a microgrid
Recent advances in energy storage, AC coupling, high-voltage conversion and energy efficiency can further improve performance, reliability and ROI of microgrids. The following tips will help you build the best systems for clients.
Tip 1: Choose the right advanced-technology batteries
Proper energy storage is one of the keys to microgrid success because the system relies on batteries for off-grid electricity and storing daytime energy surpluses. On paper, batteries with vastly different construction and real-world performance may share the same specifications. Here are a couple guidelines to select the best batteries for a renewable energy system:
- Avoid batteries that use outdated materials and manual assembly, which can shorten lifespan and performance. Such batteries deliver lower ROI and are less reliable.
- Choose batteries that use more lead and heavier grids. These are assembled using cast-on-strap and other robotic assembly methods, while mixing and curing are done by computer and vision systems, ensuring high levels of repeatable quality and a strong ROI. Robotic assembly and automation improve precision and consistency, which means long life, high performance and improved reliability.
- Consider recyclability of the battery. Lead-acid batteries are the most recyclable at 98%, whereas some battery technologies include toxic materials and are not able to be recycled.
Tip 2: Oversize the batteries
Fog and clouds can cripple solar production, and extreme temperatures can increase heating and cooling loads. Thus, it’s recommended to install a larger capacity battery than the project needs. (Two times the client’s true amp-hours is a good rule of thumb.) Be sure not to exceed 50% depth of discharge for a long life. Proper maintenance, coupled with automation and battery management, can further reduce labor and increase lifespan.
Tip 3: Ask your clients to reduce AC and heating loads
Almost 50% of household energy is used for air conditioning and space heating. In addition to changing thermostat settings, adding advanced insulation can reduce loads.
Until recently, high compressor start-up loads made larger microgrids cost-prohibitive, especially in hot or humid climates. But a non-electric compressor, high-efficiency (20-30 SEER), Variable Refrigerant Flow HVAC technology is changing that. It also has the flexibility to operate using natural gas, propane and green biofuels.
Non-electric compressor air conditioning alternatives are the next step because they reduce demand by up to 90% without sacrificing comfort. In low-humidity areas of the southwest United States, pressurization systems that push in cool air and expel hot air at night are highly effective. They turn the building into its own cooling system. Other options include whole-house fans, which improve comfort with low power requirements.
Step 4: Consider AC coupling for improved efficiency
In conventional systems, solar arrays, wind turbines and batteries are DC-coupled. An inverter converts DC to AC power. The system sends electricity into the battery bank and pulls it out again, increasing wear and tear. Systems that are AC-coupled directly connect load and energy sources on the AC side. Because there’s no DC to AC conversion, system efficiency improves and costs decrease. Since batteries aren’t cycled (discharged and charged) as often, this approach reduces battery strain and usage.
In addition, AC-coupled systems allow for high-voltage strings. As current increases, wires consume more power. Thus, by switching to higher voltages, you can reduce current and transmission losses, while increasing efficiency. This also reduces wiring costs. Because the array isn’t connected directly to the battery bank, efficiency increases. Higher-voltage systems can deliver 25% to 50% additional power from the same systems on cloudy and hot days.
John Connell is the vice president of Crown Battery’s SLI Products Group. Crown Battery manufactures all its advanced-technology lead-acid batteries at its ISO 9001:2008-certified plant in Fremont, Ohio. crownbattery.com
Mark Snyder is the owner of Mark Snyder Electric, a microgrid and renewable energy system installer, and factory-certified inverter repair service for RE and mobile systems. marksnyderelectric.com.
Read a case study about a microgrid on a rural Arizona school near a Navajo Nation here https://www.solarpowerworldonline.com/2015/02/remote-charter-school-gets-renewable-retrofit/.