(Due to broad interest in this subject and my post, I have extensively expanded it on Monday, 12^th July 2021. Also, Canary Media carried a story from #EnergyTwitter about the New York Times article which is worth looking at. I’ve added a bit about that at the end of the post.)

From The New York Times : More Power Lines or Rooftop Solar Panels: The Fight Over Energy’s Future.

Yeah, okay, distributed. It has many advantages. It needs some way of sharing excess energy among neighboring microgrids, and some of the better ways of doing that don’t involve transmission lines and seem to exceed some people’s imaginations. But it could work.

Nevertheless, to match the amount of displaced fossil fuel power required, particularly supporting manufacturing, the rollout of distributed energy on rooftops needs to be massive. The people who propose the idea really ought to do some calculations on what it will require, both to inform themselves and be more transparent and honest about the prospects for the proposal. I have calculated below what this would take and how little take-up of solar installation there actually is in some municipalities in Massachusetts using the resources of Project Sunroof.

But if this is or ends up being the consensus, one thing states and the country need to do is to create rules and laws that override stupid local solar bylaws, including prohibitions on ground mounted solar, or solar on any structure whatsoever. And they need either to demand, with compensation, that utilities support connection of all these prosumers, or strongly incentivize prosumers co-installing enough battery or other storage that their demand for grid electricity is vastly smaller than it is now. And this needs to consider the inevitable domination of EVs. That second choice — prosumers with storage — is the preferable route. And maybe the answer isn’t microgrids at all. Maybe it’s energy islands.

How much rooftop solar is available in selected municipalities? How much is installed?

I used the resources of Project Sunroof to obtain these numbers. Their methodology is documented here.

As of 2018, the fraction of potential rooftop solar installed is pathetic. Whatever the long term potential of rooftop solar as a solution for generating electrical energy in lieu of putting solar farms on agricultural lands and felling forests, the ambition exhibited is vastly inadequate.

How much electrical energy from solar does Massachusetts need?

According to the Massachusetts Decarbonization Roadmap, 23 GW of (just) solar energy is needed from an electrical system balancing perspective by 2050 (page 59). This figure actually came from the Massachusetts Energy Pathways report, although there is no specific citation in the Decarbonization Roadmap as to where it was cited. I couls not find it after a careful search either.

However, Figure 7 of the MEP report allows some estimates. This presents a number of Sankey diagrams showing alternative scenarios for Massachusetts electrical energy in 2050. They are reproduced below.

These all show various energy mixes and, except for the DER Breakthrough, Regional Coordination, and No Thermal scenarios, show demand for solar generation as about 135 trillion BTUs. The exceptions have 113, 121, and 281 trillion BTUs of solar, respectively. Use of the BTU is unfortunate, even if common in utilites planning. A trillion BTUs is 293.071 GWh (giga Watt-hours). I also infer these represent are annual consumption, even if the diagrams does not label them as such. I conclude that by comparing the reference diagram at the top for 2020 with the 2019 table of Massachusetts energy consumption by the U.S. EIA. Accordingly, the annual demands projected for solar are:

According to the the MEP report, the DER Breakthrough scenario employs the highest amount of rooftop solar rather than utility scale solar, balancing needs by having a maximal amount of variable end-use loads (hence “DER”). This is of interest, because this minimizes transmission and open space land use, which is the key question of this blog post. What amount of all rooftop solar is needed to supply this scenario, assuming no additional ground mount utility scale solar?

As of 2020, Massachusetts generated 1,565 GWh in a year from utility scale solar, and 256 GWh from small scale (“rooftop”, although it’s not all roofs) PV. These data come from the U.S. EIA Electricity Data Browser. So that leaves an additional 34,000 GWh to be generated. The additional requirements for rooftop installations expressed in terms of multiples of whole towns, assuming all of the eligible rooftops in the towns have solar installed to generate this amount is given below:

Project Sunroof has not yet mapped all areas of Massachusetts. For example, many of the towns on Cape Cod are not mapped at all. While numbers of roofs go roughly in proportion to population, urban centers have high population densities, and roof area per person goes down, as well as proportion of eligible roof area. I don’t know to the degree to which commercial roof area goes down as urban centers are approached or, offhand, the proportion of roof area which is commercial area, such as big box mall stores. Generally speaking, though, there is a tradeoff between sitability of solar PV and population density, whether on roofs or not.

Accordingly, Springfield, MA is an interesting case. It is reasonably dense and large, having an estimated population in 2019 of 153,606, being the “fourth most populous city in New England after Boston, Worcester, and Providence, and the 12th most populous in the Northeastern United States” (same Wikipedia article). But it also has large open areas. If it is assumed that the solar eligible roof density is typical of that of the Massachusetts average, and that goes in proportion to population, then, as Springfield has a population 1/46 of Massachusetts there should be about 24 GWh of generation from rooftops available from Massachusetts rooftops, assuming all eligible rooftops participated. That’s about as much as the total solar requirement cited by the Decarbonization Roadmap but is less than the amount of solar needed by some of the MEP pathways.

This of course assumes that the other part of zero Carbon energy is coming from offshore wind and Quebec hydropower. To the degree to which some parties may object to transmission lines from offshore wind, offshore wind placement itself, Quebec hydropower expansion, or transmission lines from Quebec hydropower, the requirements are accordingly made much larger. In fact, the requirements are double these because the Massachusetts Decarbonization Roadmap says roughly the same amount of wind is needed as solar.

Wrapping Up

My conclusion is (a) even if all eligible roofs are equipped with solar, it isn’t enough to either meet our needs or match the requirements of the 2050 Decarbonization Roadmap, and (b) it is unlikely anything like all eligible roofs will be equipped because of local opposition, such as recent complaints about an Episcopal Church in Framingham which wants to put up a solar canopy over a parking lot or ridiculously restrictive bylaws limiting solar on residence properties in Westwood, or a two town protest scheduled for Onset and Wareham claiming:

“Our coalition is holding a statewide rally to protect our lands and waters from large ground-mounted solar generation utilities and battery storage systems,” organizers wrote in a press release. “These are not clean, green or renewable energy but a dangerous false solution to the climate crisis.”

wareham.theweekday.com article, 11th July 2021

Miscellaneous

Of interest in the Massachusetts Commonwealth reports is that “Solar shows less day‐to‐day variability than offshore wind in New England, which is the primary reason for the large overbuild of solar in the No Thermal pathway” (MEP report, page 61). Footnote 43 on page 59 of the Decarbonization Roadmap found that:

The Energy Pathways Report determined that the amount of solar power needed by 2050 exceeds the full technical potential in the Commonwealth for rooftop solar, indicating that substantial deployment of ground-mounted solar is needed under any circumstance in order to achieve Net Zero.

That MEP report also stated (on page 5):

Very high rooftop solar deployment significantly reduced the land‐use required for ground‐mounted renewables, but also increased capital cost. In general, because the resources have similar attributes, the relative share of rooftop and ground‐mounted solar did not have a large impact on decarbonization results.

This is consistent with my assessment above.

National experience shows local opposition to siting of solar does not stop its placement. However it increases the per kWh cost for it by about 20% (MEP report, page 26, Table 5).

Update, 15^th July 2021

Canary Media and #EnergyTwitter reported a great takedown of the Times‘ dialectical approach to transmission versus rooftops, a manner of discourse which is simply bad engineering.