Low-carbon energy, land use and community planning

At Energy Collective, Jesse Jenkins looks at the land use impacts of three low-carbon energy sources: solar, wind and nuclear. On solar:

According to the MIT authors, powering 100 percent of estimated U.S. electricity demand in 2050 with solar energy would require roughly 33,000 square kilometers (sq-km) of land. That’s if we spread solar panels evenly across the entire country. If we concentrate solar production in the sunniest regions, the total land footprint falls to 12,000 sq-km.

Those sound like big numbers. On the one hand they are. Massachusetts (where I reside) spans about 27,000 sq-km, for comparison.

On the other hand, the United States apparently devotes about 10,000 sq-km of land just to golf courses. And as the infographic illustrates, it’s agriculture and forestry that truly drives humanity’s footprint on the natural landscape.

In reality, no one is calling for 100 percent solar energy. Even the most bullish renewable energy advocates typically envision solar providing less than half and usually no more than a quarter of U.S. electricity. (See: “Is There An Upper Limit to Variable Renewables”)

If solar provided one-third of Americans’ electricity, it would require just 4,000-11,000 sq-km.

In other words: with an area no larger than the amount of land currently devoted to golf courses, we could power a third of the country with solar energy.


The above is only assuming greenfield development. And wind:

Powering one-third of the country with wind farms would thus truly impact only on the order of 1,800 sq-km, of which only roughly 600 sq-km would be permanently removed from production.

That’s an almost trivially small amount of land, equal to only 6 percent of the land area wasted, er, devoted to golf in this country.

If well sited and co-located on already disturbed and productive agricultural lands, wind farms could thus fuel a sizeable fraction of America’s energy demand without expanding the human footprint on the land in any meaningful way, except aesthetically.

So, on a national scale, the potential land use impacts of really high proportions of renewable energy doesn’t look like a barrier. But most energy projects will have neighbors, and they may not like the look of solar panels, or the loss of borrowed views.

Additionally, putting solar near the busiest parts of the electrical grid, where it can have the most benefit, may conflict with local communities’ plans for future development. A solar farm will probably pay far less in property taxes than residential, commercial or industrial development.

Both of these local conflicts may mean that a significant portion of future solar needs to be co-located with buildings (on the roof), parking lots and other developed areas to minimize land use conflicts and/or reduce transmission costs.

Wind and solar now provide 10% of growth in China’s energy consumption

I got into a twitter discussion about a blog post that was challenging the idea that China is seeming a “renewables revolution”. I do not claim to be a China expert, and I hope I do not qualify as an insta-expert/pundit. However, I can read a spreadsheet.

The blog post compared the growth in solar and wind in 2014 to the average growth in total primary energy consumption averaged over the last decade. I argued that one data point was not a good way to judge a “revolution”, much less discern a trend.

Trying to be proactive, rather than argumentative, I produced the data I thought should be in the post from the same data source.


This chart shows the percentage of the growth in China’s primary energy consumption that is being met by new wind and solar sources (according to BP’s Statistical Review of World Energy 2015)

Before 2005, effectively none of the growth in energy use in China was being met by new wind and solar generation. In 2013, about 12 percent of the growth was provided by new wind and solar resources. In 2014, that figure was about 11.5 percent. So the share of China’s growing energy consumption that is provided by new wind and solar is definitely increasing, and hence so is total wind and solar production as a share of total consumption (1.4 percent in 2014 according to BP).

Here’s another way to look at it: in 2000, wind and solar production in China was basically zero. In 2014, production from wind and solar sources in China was more than the total annual energy consumption of twenty six countries listed in BP’s data book (including developed countries like Kuwait, Austria, Switzerland, New Zealand and Denmark). So the solar and wind generators in China can now provide the equivalent of all the energy (including equivalent of fuels for transportation and heating, not just electricity) required to power a small, modern western European country.

Is this a “revolution”? I’ll leave that to others. I’d say the growth of solar and wind production in China is very, very strong. Of course, the growth in total energy consumption in China is very, very strong also.

Home solar update – to the roof!

Home solar installMy small experiment with solar started with one 100 watt, battery-connected panel resting on the ground in the backyard. I soon added a second and eventually a third panel. I learned a lot about every component, as well as the seasonal and weather-related variability of production.

Over the last few weeks, the hobby was turned over to the professionals, panels went up on the roof, and most of the system is now “grid-tied” instead of going to a battery. We now have 3,840 watts on the roof! Thanks to a very attractive financing option from Innovative Power Systems, our total costs should be similar or slightly lower than normal pre-solar electricity bills. The system should produce very close to the amount of electricity we use in an average year. We’re still waiting for the Xcel Energy engineer to sign-off on the install and switch on the grid-tied portion, but are told that will happen in days/weeks.

Of course, copious data will flow, as solar production AND total home usage will be monitored in real time. I look forward to trying to match usage to production curves.

I kept a piece of the system battery-tied for my hobbyist tendencies. The two smaller panels on the right connect to the battery charge controller and small battery bank in the basement. Both the panels and the batteries have been upgraded in size from the backyard setup, and of course the panels will no longer be shaded by the neighbor’s walnut tree. The usability of the battery system should be way up, and this should allow an interesting comparison between battery- and grid-tied systems.

Thoughts on Xcel’s 2030 Resource Plan

Xcel Energy, the state’s largest electric utility, has filed their 2016-2030 Resource Plan with the Public Utilities Commission. This begins a long process of commenting and modification until their plan is approved by that body (which can take years). The Resource Plan details what trends in usage Xcel expects, and what resources (like new power plants, etc) are needed to meet that demand. The plan is important because it identifies the infrastructure investments the utility will need to make, and also the resulting environmental performance, among many other details.

I’m slowly making my way through it, both for professional and personal interest, and hope to highlight some thoughts for you, my dozens of readers.

There are a lot of things to like in the plan, the first being that Xcel is planning to meet State greenhouse gas emissions reduction goals within their own system. This is unlike the previous plan, which showed emissions increasing between 2015 and 2030. The chart below, from Appendix D, compares the two plans. (State goals include a reduction of 15 percent by 2015, 30 percent by 2025 and 80 percent by 2050)

2030 CO2 Emissions Xcel

Most of the planned reductions in carbon pollution come from the addition of renewable energy resources to their system, as the chart below shows. By 2030, Xcel plans for 35 percent of their energy portfolio to be renewables.

Sources of CO2 reductions

However, I think the plan’s assumptions about the future cost of the solar portion of those renewables is probably too high.

Xcel plans to add over 1,800 MW of utility-scale solar to their system by 2030 (up from basically zero in 2015). This is a significant increase from the “reference case”, a ten-fold increase in fact. However, this slide was presented at a public meeting at the Public Utilities Commission:

Renewable Price ForecastXcel says this in Appendix J about their assumption:

As solar technology is still not fully mature, and costs are expected to decline and conversion efficiency to improve, it was assumed that the $95/MWh price holds throughout the study period. In effect, the assumption is that fundamental cost driver improvements will offset inflation.

So the rate of decrease in solar prices will match the inflation rate? Many sources have documented the dramatic decline in solar PV prices over recent years. Lazard seems to be an oft-cited source, and their 2014 Levelized Cost of Energy Analysis shows the price of energy from solar has dropped 78% since 2009. According to usinflationcalculator.com, the cumulative rate of inflation between 2009 and 2014 was about 10%. So, at least looking historically, this seems way off.

Of course, current precipitous declines probably won’t continue forever (most of the cost is now not modules). NREL says costs have been dropping on average 6 to 8 percent per year since 1998. If we assume just half of that decline per year (4 percent), solar energy would be around $51 per MWh in 2030. Using some very back-of-envelope calculations, a price difference of $46 per MWh in 2030 means costs for new solar energy shown in the Plan’s “Preferred Plan” scenario could be over-estimated by $97 million.

This is significant not just because the price estimates of the Preferred Plan may be too high. In preparing the plan, Xcel also ran seemingly dozens of other scenarios, some including CO2 reductions of over 50% in 2030 (compared with 2005). The price difference, according to Xcel, between the Preferred Plan scenario and the scenario with the largest CO2 benefit is $172 million (from Appendix J). These other scenarios which seem too costly may actually be more in line with what Xcel is currently asking to spend once dropping technology costs are factored in.

November solar doldrums

cloud gif

I made this gif of visible satellite imagery from the NOAA’s Geostationary Satellite Image Archives. It basically shows cloud cover over the last 12 days at about 1 pm (19:15 zulu) each afternoon. This is a high-tech way of saying we’ve barely seen the sun for the last two weeks.

The implications for my 300 watt off-grid solar project are that almost nothing is being produced, and I’m not running anything from the batteries. With no sun in the forecast, I’m concerned about them sitting at a low stage of charge for days (or weeks at this point), which can reduce the life of lead-acid batteries.

In Minnesota in the winter, solar needs a backup, or at least a supplement. It’s great to have a grid. If I were truly off-grid, I would need some other kind of backup unless I was willing to significantly overbuild batteries or panels.

How much energy could Minneapolis get from solar?

Solar PV seems to be the current darling of the renewable energy world.  But how much “resource” is really out there?  How much should cities rely on the development of local solar resources to meet their climate and energy goals?  What trade-offs should urban cities make between desirable things like tree canopy and maximizing solar energy resources?  GIS tools and new data resources can help begin to answer that question.

Counties and states are beginning to produce LiDAR data more regularly, which provides the building block information needed to analyze solar resources on buildings and elsewhere (see my previous post for a brief intro to LiDAR, or see here).  Minnesota happens to have LiDAR for the whole state, and Minneapolis has a climate action goal that references local renewable development, so I’ll focus there.

So how much solar electric potential does Minneapolis have?  Enough to supply 773,000 megawatt-hours (MWHs) each year, at the upper bound.  That would mean covering every piece of rooftop with good sun exposure and appropriate pitch (southeast to southwest facing or flat) with the best modern PV panels.  It would also mean solar installations on 68,351 structures, consisting of over 2.3 million individual panels. Continue reading

100 days of solar


Watt hours to the battery, first 100 days

Today I noticed that my solar charge controller has been running for 100 days (it logs this among many other data points).  Here are some highlights from the first 100 days:

  • The system has produced 32 kWhs from two 100-watt panels.  This is roughly 2% of the total electricity consumption we saw over the same period last year.
  • Converting from DC current to AC current at low wattages is wildly inefficient.  I usually run the wifi router and cable modem continuously off the battery and I lose about 40% of my produced energy to the inverter.  It is much happier running closer to its peak (1000 watts).  We should probably convert to DC.
  • Something happened to my charge controller settings when I converted to 24 volts.  Although the controller was still charging, I lost about 10 days worth of data (hence the gap in the chart) and wasn’t able to communicate with it over that time.  A firmware reboot fixed this.
  • Although very cold, clear days are when the panels perform their best, the sun just doesn’t shine for that long each day in January and February in Minnesota.  The panels being on the ground doesn’t help either.  Just from the middle of March to the middle of April I’ve about doubled my daily output.
  • All that said, this chart doesn’t really show total potential of the panels on a given day.  If I didn’t use much of the battery the day before, panel production the next day was curtailed by the controller to avoid overcharging the battery.  I’m trying to match the loads I put on the battery with the “capacity” of the season, but that’s sometimes tricky.
  • I recently learned we were accepted into the Minnesota solar rebate program for 2014!  So with the help of a friendly solar installer, we should have a 2.8 kW grid-tied system installed sometime this year. Along with the grid-tied panels, the installer will be adding two panels on the roof dedicated to battery charging.  Now I just have to wait…

Mapping Minnesota’s solar resource

Boston, New York City, Denver, Cambridge and other cities have created solar potential maps to help their residents understand that solar photovoltaic systems are viable in dense urban areas, and to demonstrate the potential that exists on rooftops.

Of course, I had to try this myself.

Minnesota produces LiDAR data, which is basically micro-scale elevation data produced by flying a plane back and forth in a grid and shooting the ground with lasers a bajillion times.  Skilled/obsessive GIS users can clean from this data information that can be used to make a fairly accurate model of everything on the ground (buildings, trees, etc).  GIS software also makes it easy to produce daily, monthly or annual solar insolation maps.  By taking the position of the buildings and trees, knowing the latitude, and projecting how the sun moves across the sky throughout the year, the software calculates a total amount of solar radiation that will hit a point after shading, angle and other factors are taken into account.

Solar insolation in January

Solar insolation in January

After much tinkering, the Kingfield Solar Energy Potential map was born.  The extreme density of the LiDAR data limits how large an area I could process (there were 4.9 million individual data points in this one small section of Minneapolis), but you get the idea.  This map shows the area of each roof that might be appropriate for solar, how many panels could fit in that area, and an estimate of the annual production from those panels.

The Kingfield Solar Map

The Kingfield Solar Map

Some roofs are wholly inappropriate for solar, whether due to tree or building shading, orientation or size.  But there is significant potential.  If solar was installed on every appropriate piece of roof in this one-quarter square mile area, it would produce an estimated 2.2 megawatt hours of electricity each year, and avoid 2.9 million pounds of carbon dioxide emissions.

Home solar: adding MPPT and marvelous data


As part of my plan for the eventual expansion of my off-grid solar energy system, I recently added a new charge controller with Maximum Power Point Tracking (MPPT).  Besides being much more efficient, this controller is capable of producing reams and reams of wondrous data, and is network-connected, meaning I can geek out on battery voltage and array current from anywhere in the house!  The charge controller I had was great, but it wouldn’t handle anything beyond a few more small panels.  Now I should be able to go all the way up to 750 watts of panels (my goal).  So, thanks Santa!

While installing the controller, I also took the opportunity to install a breaker box, which should bring me closer to code, and upgrade to larger diameter battery cable, which should reduce efficiency losses.

The MPPT advantage

MPPT is a fancy way of saying the charge controller is able to send significantly more energy to the batteries from the same panels.  How much more? After only a few days of testing, I estimate 40 – 60% more than the Pulse-Width Modulation (PWM) controller on days when the battery is low.  (If you want to know the details of how MPPT works, I found this explanation helpful.)

Here’s some actual data from my system which I think illustrates the MPPT advantage well:

1-5 plain graph

The blue line is the amps, or current, coming from the panels.  The red line shows the amps the controller is putting in to the battery.  It’s higher!  The magical MPPT doohicky converts excess voltage into amperage (remember, amps X volts = watts) so less of your panel’s potential is wasted.  On this particular day, I estimate the charge controller may have been able to wring an extra 100 – 150 watt-hours from the panels.

There are other interesting things going on here, so here’s a little annotation:

1-5 annotated graph

Here’s the next day, when the battery starts out the day almost totally full.  It was very sunny.

1-6 amps graph

The controller limits the array current and current to the battery significantly because the battery is almost fulled charged.  The gentle downward slope in the amperage is a function of battery charging called absorption.  Less current is pushed into the battery as it reaches capacity.

I can track hundreds of days of watt-hour production, so I’ll do another update when I can show some seasonal changes.  How I yearn for the days when the panels get more than 4 hours of sun per day!

Power surge

solar farm

My back yard solar farm has doubled in size to a massive 200 watts!

If you remember from the last post, the wooden frame was half empty (who built it like that?), and was feeling really lopsided without another panel.  The frame is now full (Amazon is a marvel of the modern age), so any further expansion will have to happen on the roof or elsewhere.


Parallel wiring.

Under optimal conditions I think this will produce 480 watt-hours per day.  For reference, that’s about 60 percent of the power needed to run our chest freezer.  These panels are connected in parallel, meaning the voltage (12) is the same, but the amperage is doubled (to about 10 amps).  I ordered a cheap ammeter, which I’m excited to hook up so I can see real time results.

The last few weeks have been very cloudy, so I think output has been down.  The next week doesn’t look that great either.  When you’re a solar farmer, you start caring about the weather a lot more.

After adding some additional wiring, I now have access to a solar-powered power strip in the living room, rather than just having access in the basement where I could run an extension cord from the inverter.  Now I’m regularly running some lights and the stereo from the battery.  The cable modem and router are small, consistent loads, so I may move those downstairs to become battery-powered.  I’ve become somewhat obsessed about matching load to the production, since I don’t want to “waste” any electricity produced (and also don’t want to run my battery below 50%).

Stay tuned for another update when the ammeter arrives.