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Storing energy to save carbon: A personal case study

Michael Fink, Energy Data Analyst -

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Carbon emissions are generally considered at a big picture level. According to the Global Carbon Project, worldwide carbon dioxide (CO2) emissions increased by 2.7 percent in 2018 and fossil-fuel related CO2 emissions were projected to reach record heights of 37.1 billion metric tons by the end of last year. It is a global problem, driven by large industries, entire systems of transportation, big agriculture, and more. For this reason, the levers for reducing carbon emissions can seem impossible to move on an individual level. But upon looking more closely at the intricacies of carbon, it can be surprising to realize just how granular the question of reducing one’s carbon footprint can get. At VEIC we focus on the big and the small picture – pivoting from issues on a macro level to designing and implementing solutions at a local level. In a rapidly evolving industry working to solve critical issues, we are constantly exploring new opportunities to affect change.

Household carbon case study

As an Energy Data Analyst at VEIC, I try to find opportunities for innovation in models and numbers, and my passion for advancing clean energy extends beyond my work, into my home. I have made efficiency upgrades, installed solar panels, and recently installed an in-home, utility-controlled battery. Until this year, my attempts to examine my household’s carbon footprint, specifically from electricity consumption, has been very simple: 1. Did I produce more than I consumed?  2. What is the average carbon footprint of electricity on the grid? This is a good start, and it matches the industry calculation for “net zero” – when the amount of energy produced by a household equals or exceeds the amount consumed by that household over a twelve-month period.

When it comes to translating this calculation to carbon footprint, its simplicity can be problematic. It does not consider the intricacies of the fuel mix of an electric grid at different times of day, or the times of day when electricity is produced, stored, or consumed by the household.

I decided to get more specific about my footprint and drill deeper. The fuel mix and carbon footprint of the grid changes from minute-to-minute. As an exercise, I decided to credit or penalize my footprint based on the exact fuel mix at the time of my consumption or production.  In particular, I wanted to analyze how effective my in-home battery has been at making the grid cleaner.

What's in a day?

Carbon footprint of the grid can change significantly over the course of one day. When looking at the New England grid, usually the early morning features energy with the lowest carbon emissions per unit of energy

 ISO-NE Grid Carbon Footprint - May 29, 2018

When taking these minute by minute changes into account and aggregating them into one full year my home’s electricity carbon footprint changes dramatically. Driven by a solar configuration optimized for summer and lower than average consumption, my footprint has been negative over the last two years in July, displacing an average of just over 213 kg of CO2 emissions from the New England grid.

In the winter, the case is reversed. Heavy use of a cold-climate heat pump, poor solar production, more carbon-intensive grid energy, and short days all contribute to my larger footprint through the winter.  January has been the worst month, with a net responsibility this year for about 208 kg of CO2 emissions. Using this more detailed carbon footprint calculation scheme, over the course of a year, I have averaged out to a net displacement of about 285 kg of CO2 per year.

 Home-Grid Interaction

Battery benefits

When my distribution utility offered me a chance to affordably add battery backup to my house, I leapt at the chance. The battery (a Tesla Powerwall 2.0, capacity 13.5 kW) is controlled remotely by the utility, but it can be used to power the home in the event of an outage and the solar panels can recharge it directly when the grid is offline.

The battery adds another layer of opportunity to reduce carbon footprint, specifically if it is discharged to the grid at a time of day that offsets a dirtier fuel mix. In my analysis I wanted to see if the utility was discharging the battery at times when the grid was particularly carbon-intensive or the market price for electricity, also known as the locational marginal price, was very high. Both appeared to be true: the battery discharges happened when the carbon footprint of the New England grid was almost 20 percent above average (339 g CO2 / kWh vs. 286 g CO2 / kWh average). The battery was also being discharged at times when the locational marginal price (LMP) was unusually high ($106.76 per MWh vs. $38.02 per MWh average).

Another consideration of in-home storage is the ability to address peak issues throughout the year. Peak periods determine electric bill rates and they often feature a more-carbon intensive fuel mix than average.  I was curious how effectively my house battery was being used to lower demand during the peak period. The answer: very effectively. 

In the 17 months preceding installation of the battery, my home was a net consumer of electricity during 15 peak hours (most peak hours happen after sunset, so my solar production was minimal or non-existent). Additionally, in more than half of those months I was a high consumer at the worst possible time.

Over the first seven months since the battery has been installed, I have been a net producer of electricity in five of those seven months’ peak periods, usually producing enough electricity to remove my house from the grid, and two or three houses with similar consumption habits. In only one of the seven months since the battery was installed did my previous pattern of high consumption during a peak time occur.

Optimizing batteries for maximum carbon reduction

While a single-case study is a long way from a defensible strategy for the whole grid, my house demonstrates three valuable uses for distributed storage:

  1. Cleaner energy – even accounting for charging losses, my battery has made my electricity consumption slightly less carbon-intensive. If the distribution utility would wait to recharge the battery until a low-carbon intensity time, my footprint might be as much as 20 percent lower during peak period discharges. This would require a shift to their operations and their contract, which obligates them to recharge the battery as soon as possible after discharging.

  2. Cheaper energy on the spot market – whether the utility is buying or selling electricity at a peak period (when electricity is expensive), it is helped by discharging the battery (which was charged at a much lower price, hours or days ago).

  3. Lower monthly and annual charges for utilities – As long as the distribution utility accurately forecasts the peak period, distributed battery networks could do a lot to lessen demand charges associated with peaks. If each battery is removing itself and two other houses from the grid during a peak period, we’d see a substantial reduction in these charges with just five to ten percent penetration into the residential market.

With minimal utility optimization my in-home battery is only reducing my carbon footprint by less than five percent. However, with a more strategic approach to discharging and recharging the battery, I estimate that my carbon savings could increase to well over 10 percent. Careful deployment of in-home storage has the potential to change the entire carbon emissions profile of an electric grid. The key is to start small – consider the minute by minute fuel mix, seasonal peaks, and annual rates. With the right level of specificity, the impacts could be big.

These insights contribute to our ongoing work to explore and improve renewable energy integration with the grid. Learn more in my paper on solving the Duck Curve.

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