Something’s become more and more apparent to me lately as I ponder our off-grid setup—being grid-tied is inherently more efficient than being off-grid. I know, nothing too sexy here with this technical point, but it’s an important realization, and it has implications for the larger systems that nations need to be working toward.
In my case, as I add generation to approach net-zero, each additional kilowatt of capacity will be needed less and less. Some numbers to illustrate—we are off-grid, and have about 3 kw of solar PV installed in two large arrays. On a sunny day in the summer, when the days are long and the sun is high, the system can produce over 20 kwh’s of power. We tend to use about 7 kwh a day, which means that in the summer we’re often making about three times the amount that we use or can effectively store. The batteries tend to be full by 10:30 in the morning on such days, and then the panels do very little for the rest of the day. In December, however, it’s quite the opposite, with much shorter days and a lower sun angle. At that time of year we only average about 4 or 5 kwh’s of generation each day, which is a bit shy of what we need, and so we run the gas-powered generator off and on, especially in November and December. Usually by mid-January the skies are clearer and the days start lengthening a bit, and we start breaking even again, and continue that way for the next ten months.
So, our house is close to net-zero, but I’d like to completely eliminate those hours where we need to run the gas-powered generator. If I added 2 more kw’s of PV capacity, I’d probably get really close. BUT—that investment (probably $4,000 if I did it myself) would only be needed for about two months a year. Thus, for about 80% of the year it would just sit there essentially unused, which would equate to hundreds of kwh’s of uncollected, and therefore lost, power. In short, the closer I approach being fully net-zero in the off-grid setup, the less efficient the total system becomes. Needless to say, this isn’t good—in addition to the expense, everything has environmental costs when produced, even technology that we need more of like solar PV, so it seems like it would be a case of not using our resources wisely.
If our house was grid-tied (which it never has been, due to the potential expense, because we’re something like 1,500 feet from the power lines), the story would be dramatically different. All those hundreds of kwh’s that I currently am forced to waste would flow into the grid, which would enable to power company to generate less. At other times, when our demand exceeded our production, I would draw these “banked” hours back from the grid. This is actually another of those win/win/win situations. It would be more efficient—it would keep my solar production from being wasted, the losses incurred by transforming power to and from a chemical state in the batteries would be avoided, and when generation is required, it would be done by the power company’s much-more-efficient stationary natural gas plants, or by grid-scale wind or hydro.
The power company benefits as well—peak solar hours often overlap with peak grid demand, so grid-tied solar inputs tend to reduce peak demand on the grid. The opposite tends to be true when grid-tied homes are pulling from the grid, say, in the middle of the night to charge EV’s, during times of very low demand. The net effect is that grid-tied systems help level the grid. Many power companies, like Green Mountain Power (GMP) here in Vermont, seem to be embracing distributed generation for another reason—taking the long view, they seem to recognize that the role of power companies is and will be changing, away from the old idea of generating power and distributing it in one direction for a single price, and to the model of the power company as a manager of a complex grid that buys power from many sources and distributes it, as needed, in all directions, perhaps with time-of-use (TOU) pricing. (See earlier post “Cloudy Day Pause” for more about how grids might function in the future.)
Remarkably, being grid-tied would probably be a better choice even if I had to use a power company, like some in the Midwest, that rely nearly 100% on coal. Being grid-tied does not change the total amount of fossil fuel that is burned—the companies burn less when grid-tied homes are feeding power in, and then burn more later, when such homes are pulling power out.
Now, while being grid-tied is more efficient when viewed system-wide, what would happen if everyone was grid-tied, in a future situation where fossil fuels might be nearly totally phased out? It’s easy enough to see how the grid can work as a virtual (and unlimited) “battery” for a small proportion of customers, but where is the upper limit? The short version—we’re not quite sure. One thing is for sure, though—U.S. power companies are nowhere close to this limit. In Vermont the electric utilities are currently allowed by law to have up to 4% of their generation from grid-tied systems, but that number was established somewhat arbitrarily in years past, and the legislature is currently expected to soon raise it to 15%, a move that is being welcomed by most of the power companies. A better case study of high RE penetration would be the situation in Germany, though it’s complicated enough that the topic really warrants its own post. Short version—their solar feed-in is around 35% on sunny days, and due to vagaries in the international coal market (coal has become cheaper due to plentiful supplies of natural gas in the U.S.) it has caused disruption in the business models of German power companies, which has had economic costs and, as of yet, fewer than expected CO2 reductions (see Economist article, “How to Lose Half a Trillion Euros“. I personally think The Economist is quite one-sided in this article, but that, again, would probably be a whole other post.) Eventually grids worldwide will need to move toward 100% RE generation as we phase out fossil fuels, and much of this will be distributed generation from point sources. But, 1) we’re not even close enough to worry about it now, at least in the U.S., and 2) power companies will change their business models over time. Indeed, companies like GMP have already started. Fortunately, moving to a smarter grid isn’t an all-or-nothing propostion, but rather evolutionary change over time. (Again, previous post “Cloudy Day Pause” discusses some of this in more detail).
So, back to where I started—it isn’t sexy, but the higher efficiency of grid-tied systems is an important point as we work out our workable vision of the future. We’ll eventually need a smart, flexible grid that efficiently connects renewable generation from million of sources to millions of destinations. In the much shorter term for me, tying to the grid might be the easiest, if not the cheapest, way to achieve net-zero. Much to ponder…