By Alan Scott — Senior Associate at WSP — Solar Oregon Board Member
According to the Solar Energy Industry Association (SEIA), solar installation has seen an average annual growth of 59 percent in the U.S. over the past 10 years, with steady expansion in residential, commercial and utility-scale solar installations. SEIA also reports that the cost to install solar has dropped by more than 70 percent since 2010. The Energy Information Administration (EIA) projects that solar deployment will triple by 2030. Additionally, advances in solar technology including frameless photovoltaics (PV) and new, thin-film applications have increased opportunities for architects to integrate renewable energy into buildings. These are all very positive trends as we look to boost energy independence, create clean energy jobs and reduce carbon emissions to curb global climate change. However, the growth of grid-connected solar has introduced some challenges.
As we all know, PV solar energy systems produce electricity when the sun shines, with peak production in the middle of the day. Daily electricity use patterns can vary between residential, commercial and industrial buildings, with additional variations based on climate zone and season. However, the sum of all electricity users aggregates into a fairly consistent (with seasonal variation) daily utility load curve that utility companies respond to when managing generating capacity on their grids. This curve typically ramps up to a mid-morning peak as the business day begins, dips in an afternoon trough, and ramps up more dramatically toward an evening peak as people return home from work.
As more and more solar generation is added to the grid, it shaves off the morning peak and creates an ever-deepening mid-day trough in the net load, and a steeper and higher afternoon ramp-up as solar production drops off just as the evening peak is coming. The California Independent System Operator (CASIO) first graphed this phenomenon in 2013 and dubbed it the “duck curve” because its profile resembles that of the common waterfowl.
Duck curve graph (Courtesy of CASIO)
The duck curve presents two challenges for utilities. Traditionally, utilities have generating resources like hydroelectric, coal or nuclear plants to produce the electricity to meet the base load, and then rely on electricity purchases from neighboring utilities and supplemental production from peaker plants (usually natural gas) when the load on the grid increases. They cannot ramp up or down the production of base load plants quickly, so peaks are handled with more quickly dispatchable sources. A steep ramp up in load requires more dispatchable capacity than a utility can affordably maintain with typical resources. Likewise, as the belly of the duck curve gets lower and lower due to increased mid-day solar generation, the need for base load generation drops closer to zero, requiring the curtailment of generation to maintain grid stability.
The challenge of the duck curve is most evident in states like Hawaii and California, where growth in installed PVs is (or soon will be) overproducing energy in mid-day hours, requiring curtailment of generation if the solar energy cannot be stored for later use. Hawaii may have already reached the distributed solar saturation point where there is no longer a benefit to additional daytime generation, and California, Massachusetts and Arizona are fast approaching this point.
Energy storage matching PV production to load curve.
Rather than limiting future deployment of solar, the best approach is to add energy storage to save the mid-day peak production and release it later to meet the evening peak. While some utility-scale solutions like pumped hydro can be viable, building- and utility-scale battery storage present one of the best opportunities to continue to expand grid-connected solar generation while maintaining grid balance, thus killing the duck curve. Shifting stored solar for later use can help utilities to avoid ramping up additional generating capacity.
In addition to these system-level benefits, solar-plus-storage offers other advantages for both residential and commercial building owners. It allows utility customers to maximize their own use of on-site generated power, avoiding any disadvantages in net-metering rate structures. This may become increasingly important as electric vehicles grow in popularity. For commercial buildings, storage allows utility customers to avoid spikes in energy use that result in demand charges. In addition to day-to-day benefits, solar-plus-storage offers businesses and homeowners greater resilience in the face of disruptions, from prolonged power outages to natural hazards, supporting comfort and security as well as business continuity until grid power is restored. Some jurisdictions are beginning to consider battery storage as a viable and reliable alternative to maintaining a diesel generator and fuel storage on-site for emergency power. Not only is it clean and quiet compared to a gen set, it takes less space and maintenance, and could better support continuity of operations during prolonged power outages.
The increasing investment, research and development, and commercial production of behind-the-meter battery energy storage that we are currently seeing will lead to better performance and lower costs, and will further power this trend toward solar-plus-storage. The cost of lithium-ion battery packs has dropped over 70 percent since 2012, and this is expected to accelerate resulting in costs below $200/kWh by 2019.
Currently, batteries have a life expectancy or five to 15 years, and just like your cell phone battery, their capacity will decrease over that life span. The two main battery choices currently available are lead-acid and lithium-ion. Lead-acid batteries have a much lower first cost, but have a shorter life, require regular maintenance, and need more space for the same capacity. Lithium-ion batteries have a higher cost, offset with a much longer life, no maintenance and a compact size, making them a favorable choice. A newer technology, saltwater batteries, offers a cost and life span in between lead-acid and lithium-ion, with the added benefit of containing no heavy metals, making them easier to recycle. All batteries have a warranty for the number of cycles (daily charge/discharge) or years of life, and the percent of original capacity retained at the end of the warranty period.
Utilities and energy regulators in many locations are considering policies for new net energy metering rate structures (time-of-use rates and demand charges) and other incentives to encourage installation of solar-plus-storage. With these incentives and the downward trend in both PV and battery costs, the economics of solar-plus-storage are shifting rapidly. With these emerging cost advantages combined with other benefits such as resilience, I predict that solar-plus-storage will soon be a common feature in both commercial and residential projects. Including batteries is a key part of the continued growth of renewable energy and the decarbonizing and stabilizing our utility grid.