a contribution by Deborah Lycan and Rich Farrington
There has always been, and always will be, a need to match the generation of electric power with the consumption of electric energy, as neither is constant throughout the course of a day. If there is excess generation, the system voltage will rise to higher than 120 V at a typical wall outlet, threatening to damage devices (“loads”) such as lightbulbs and microwave ovens. Conversely, if demand exceeds generation, the voltage in a regular outlet will fall below 110 V, causing a “brown-out” that also will ruin microwave ovens, refrigerators, etc. (though not most light bulbs). Traditionally, each electric customer consumed grid power without consideration of this issue, as the power generating plant monitored the grid conditions at the power plant closely, and turned off generators (allowed them to spin without generating any juice) whenever there was a surplus of power and added generators when there was a shortage. But with the rise of renewable sources such as rooftop solar and windmills, matching power demand and supply has gotten a lot more complicated.
For one thing, energy generation is now occurring at hundreds or thousands of places, many of which are not easily monitored by the company that is responsible for matching supply and demand. For another, some of the generation goes up and down in a matter of seconds (rather than hours), due to clouds intervening between the sun and the solar panels, or due to changing wind speeds. In addition, the voltage at a power plant is no longer sufficient to predict the voltages at each “end” user, because one end user might be close to a big solar generator, whereas another might be a long distance from any generator (power diffuses throughout the grid, and the further the power spreads through the branching connections of the grid from a generator, the more the voltage is influenced by other users and generators. Thus the voltage in the line might be excessive next door to a big generator, but average and therefore just fine a few blocks away. I’m simplifying matters to just voltage (trust me, you don’t want to know about power factors, phase and other electrical complexities), but the tree-like structure of the electric grid formerly provided a predictable pattern of local voltages in relation to the voltage at the power plant. It no longer does, for the reasons outlined in this paragraph.
In response to these challenges, La Plata Electric Association (LPEA) – the local power company in La Plata and Archuleta counties in Colorado – flags certain neighborhoods as “red” zones, where homeowners are not permitted to send power from rooftop solar panels into the grid at certain times of day (times when the grid is up against the maximum voltage limit). This frustrates those homeowners, especially if they aspire to becoming “net zero”. Net zero means different things in different contexts, but for many people (such as the family mentioned below), it means generating as much electrical energy in a year as they consume. But if they live in a red zone, their panels may be unable to send energy to the grid during the daytime, which is of course the only time when solar panels are inclined to do so.
LPEA is toying with the idea of installing stationary storage batteries in red zone neighborhoods, such that excess production can be stored at the times of day when voltages rise too high, and depleted at other times of day. But to my knowledge, they have not yet done so. Instead, they have placed “export restrictions” on homeowners with solar panels in red zones. These automatically disable the solar panels at certain times of day when voltages threaten to become excessive.
Should they choose to do so, homeowners can address this problem by purchasing and installing stationary storage batteries in their home or garage, as our featured homeowners have done (see below). These batteries are not cheap (Tesla Powerwalls can store about 13.5 kWh each, at a cost of about $10K), but they can absorb excess generation until they are full, and discharge it when local grid voltage drops.
Let’s put that storage capacity into perspective: a plug-in hybrid like a Chevy Volt has about a 10 kWh battery, an EV with a modest-sized battery (e.g., a Chevy Bolt, such as that mentioned by the homeowners below) has about a 65 kWh capacity, and a really big EV battery (e.g., the Rivian maxPack) can store up to 180 kWh. Most EV pickup trucks now or soon-to-be on the market advertise that they can be used for a home’s backup power. As long as they are not used for “high-frequency” power exchange (reversing the direction of energy flow many times per second – not considered in this post) or charged to an excessive charge state (above 80%) or excessively depleted (below 20%), the effect on the vehicle’s battery life should be minor, according to Rivian and other leading EV producers. Note that in the narrative below, the homeowners’ EV battery is never discharged to the grid, an additional option not yet implemented.
How much grid energy does a house use in an average day? According to the Energy Information Agency, the average American residence consumes about 30 kWh per day, but an all-electric home (no use of gas/fuel oil/propane/ for heating) would use substantially more, and a really efficient home might use substantially less (our home’s average daily use is 6 kWh).
This means that car batteries have the potential to both absorb and consume a substantial share of a family’s daily energy needs, potentially moderating much or even all of the grid’s supply/demand mismatch occurs during a day. Suffice it to say that the infrastructure to do this is just now being developed and perfected. The electric utilities are thrilled at the prospect, but they have many details to be worked out. The homeowners described below worked it out themselves, in a LPEA red zone no less.
Their solution depends on a number of features that might not be practical for everyone: 1) they work from home and have the option of charging their 65 kWh EV battery at home in the middle of some days, and 2) they were willing to purchase about 20 kWh of stationery batteries for their home. Nonetheless, they have found a practical solution and shown the way for LPEA and others to incorporate home storage and EV batteries to eliminate operational carbon emissions using 100% renewable energy sources, the holy grail of renewable power generation.
Here is their story (courtesy of Debra Lycan and Rich Farrington):
If you find yourself in an LPEA red zone, where you are not allowed to export solar energy during the day, don’t give up on solar panels. When we moved to Durango, we were passionate about using the sunny climate here to make our home net zero – where we create as much energy as we use. When we found out that we were in a red zone, we were pretty discouraged. But we solved the problem in two ways. First, we purchased two 10kW storage batteries that we use to store the excess electricity we generate during the day. We then export energy from those batteries to the grid at night, if we haven’t used everything stored to run the house. On a typical Colorado day, the batteries are filled by about 11:30 in the morning. Without more consumption, the panels quit producing [in early afternoon] because there is nowhere for those electrons to go. [The first figure shows a typical day without plugging in the EV, from midnight on the left edge, to the following midnight on the right edge, in which energy consumption exceeds generation until about 7 AM – red shaded area – after which the stationery storage batteries are charged – blue line in lower graph shows charge state in percentage – by the sun shining on the home’s solar array filling up the batteries by early afternoon – solar cell production shown in light blue and gray shaded area in upper graph – is shut down around 2 PM once the stationery storage batteries – lower graph – reach 100%].
The second part of the solution was an EV. To use more of the potential of our panels, we needed more consumption, and we realized that an electric vehicle, with its giant battery, could be another, multipurpose, storage battery. Our current system is to use the panels in the morning to fill the [stationery] batteries, and then plug the [partially depleted] Chevy Bolt in around noon. The Bolt battery takes so much electricity that it can absorb everything the panels can produce plus it draws from the stationery batteries. Charging the car for about 3 hours depletes the stationery storage batteries down to about 50% full, so that when we unplug the car, the solar panels keep generating electricity and start refilling the stationery batteries. By sunset we have usually refilled the storage batteries to 100% so that we have plenty of stored electricity going into the evening.
The addition of the EV to the equation means that we can use the full potential of the solar panels and generate electricity for most of the day. During the summer, we can thus generate sufficient excess electricity that we export [to the grid] at night that allows us to get to net zero [for the year]. When we found out that we had an export restriction, we thought our dream of being net zero was impossible, but with a little creativity we managed to make it work. We hope this story might help others figure out how to make solar panels work for them. It is a great feeling to know that your car and your home are running on sunshine. It’s a fuel whose cost isn’t going up anytime soon.
