How Much Energy Could Minneapolis Get from Solar?

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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.

773,000 MWhs would represent about 18% of Minneapolis’ total annual electricity consumption (based on 2010 figures).  It would be the equivalent of reducing 392,684 metric tons of CO2 (also based on 2010 figures), which is equal to the emissions from the energy usage of almost 36,000 average American homes each year.

Explore the results for individual buildings in Minneapolis.

There are some limitations to this calculation, and some additional interesting findings, but first a brief description of how I came up with these numbers.

Methodology

Annual solar insolation values shown in a black-white color ramp.

Annual solar insolation values in the Wedge neighborhood shown in a black-white color ramp.

I briefly covered how to calculate solar potential in a previous post, and the process for this analysis was similar.  I was able to get my hands on the solar insolation raster for the whole city thanks to the excellent work of some students in Dr. Elizabeth Wilson’s capstone class at the University of Minnesota’s Humphrey School.  Solar insolation represents a measure of the total energy from the sun reaching any particular point (each square meter in this case) on a building, tree, earth, etc.  To calculate this, ArcGIS has a complex tool called Solar Radiation Analysis.  It takes in to account things like how trees shade buildings, and how the sun moves across the sky at different times of year based on the latitude of a particular point on earth.  It spits out a measure of solar energy hitting that location over the course of a year,  measured in watt-hours per square meter. This gives you a good idea of where exactly on each building a suitable spot might be for a solar PV system.

LiDAR data can also be used to calculate the slope of roofs, another important piece of information to understand solar potential.  This allows a user to pick out areas of flat or south-facing roofs.

Red hatched areas represent areas of rooftop that are good for solar

Red hatched areas represent areas of rooftop that are good for solar

Finally, Minneapolis supplies building footprints, so I knew approximately what was a roof. I confined my analysis to building roofs, assuming we don’t want any of our precious open space filled with solar panels.  I also buffered the roof edges, since I’m told OSHA requires some open space between the panels and the roof edge for safety, at least for flat roofs.  I also considered 1,000 watts to be the minimum size that would warrant an installer to climb onto a roof.

Combine all this with some assumptions about the space needed for installations on flat and sloped roofs (the students helped with that too) and information on the size and power output of panels, and you get a measurement of the total “good” roof area and associated potential energy production from each roof.

That’s enough how-to, here are more interesting findings.

Findings

The 100 buildings (0.14 percent of the total building with solar) with the largest solar potential would provide 14 percent of the total production, or over 109,000 MWhs annually. The 1,000 buildings (1.4 percent of the total buildings with solar) with the largest solar potential would provide 43 percent of the total production, or over 333,000 MWhs annually. Targeting these structures for further analysis and possibly incentives would probably make sense to achieve the largest economies of scale for installation costs.

Buildings symbolized by their total solar energy potential

Buildings symbolized by their total solar energy potential – warmer colors represent higher potential

The 100 highest-potential buildings are geographically concentrated in roughly three areas: the northeast industrial area – roughly north and east of the U of M campus, the Lake Street/Greenway Corridor, and extending from the North Loop along the river into north and northeast Minneapolis.  Unsurprisingly, these are areas that still have many large, flat-roofed warehouse and industrial buildings.  If Minneapolis wants to maximize its solar resource, we may want to think about the trade-offs in redeveloping these areas or developing high density near them that may shade existing rooftops.

Commercial, industrial and single-family residential structures (based on parcel data) each account for almost exactly 23% of the total roof-top solar potential in the city.  The next largest potential was among apartment properties at 9%, and duplexes at 7%.  While the top three were evenly split potential-wise, single-family residences with good solar potential included over 46,000 structures, while commercial and industrial together was about 4,300.  See economies of scale note above.

The fact that 46,000 residential structures have good solar potential means that lots of homeowners, even in leafy Minneapolis, could be empowered to go solar.  This would be a more powerful political constituency than a small number of commercial property owners.  Obviously some would face the trade-off between more trees and their benefits and electricity from solar.

Suburban areas are much more likely to approach energy production equal to energy usage.  With its high density commercial core, Minneapolis uses a lot more energy than it can produce on its roofs.  Residential structures are also smaller and more shaded than many suburban areas.  This isn’t necessarily a bad thing, as density brings many other environmental benefits, like the ability to use transit cost-effectively.

Limitations

Xcel Energy limits the size of solar installations they allow to be connected to their system.  An interconnected solar PV system cannot be designed to produce more than 120 percent of the customer’s total usage from the previous year.  Many homes in Minneapolis, and possibly low-energy warehouse buildings, could accommodate systems larger than that.  This analysis limited system size only based on roof/sun conditions, and not electricity usage in the structure since that wasn’t known.  In some cases, this means this analysis over-represents solar potential.

This analysis includes no information on roof age or structural integrity.  Some flat-roofed buildings aren’t structurally able to accommodate solar without expensive retrofits.  Residential structures may need to have old roofs replaced before putting on a solar energy system (which are typically designed to last 20 years).  Some structures, like parking ramps and stadiums, would require additional structural supports to be added before a solar energy system could be added.  These factors could all further limit solar potential on Minneapolis buildings.

There was a geometry problem I couldn’t solve in GIS.  While I could calculate the size of a roof area that got good sun and had the correct slope, I couldn’t quickly figure out how many solar panels of a certain shape (defined length and width) fit in that area.  I only used total square footage divided by the square footage of a standard solar panel.  Internet forums are filled with many people better at GIS than I discussing this problem (but not providing me with easy solutions).  If anyone reading this wants to take a crack at it, let me know in the comments.

Originally posted at netdensity.net.

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7 Responses to How Much Energy Could Minneapolis Get from Solar?

  1. Elliot Altbaum
    Elliot Altbaum July 21, 2014 at 3:42 pm #

    I can’t think of an easy way to do this in ArcGIS. My experience with the raster software IDRISI has me thinking of a couple options. With good iterative and modeling tools there might be a way to iterate solar panels on each roof until the optimal solution is found. There are no off the shelf tools in IDRISI that could do it, so it would involve some digging.
    From a very different angle, a physicist working on how materials pack (atoms or objects) might have insight into a methodology.
    That’s my 2 cents.
    Good luck!

  2. Eric Anondson
    Eric Anondson July 21, 2014 at 3:50 pm #

    A building room can realistically be either a green roof or solar, not both. You would need to tune this to compare roofs with plant life.

  3. Rosa July 22, 2014 at 12:38 am #

    This is such a great resource, thank you!

  4. Brian July 22, 2014 at 10:00 am #

    Any idea what the utilization rate for the solar panels might be?

    Based off of the specs a 280 watt panel would generate 2,452.8 kWh per year per per panel. Assuming 2.8 M Panels are installed, the max generating potential, assuming the sun shines 24 hours per day/ 365 days per year, yields 6,867,840 MWHs per year.

    You are calculating that the panels would generate 773,000 MWHs per year.

    (773,000 / 6,867,840) = 11% Utilization

    Just threw this together quick, is this correct or am I missing a step?

  5. Mya July 24, 2014 at 1:01 pm #

    Great article. Would be interesting to calculate solar potential for freeway corridor space or covering soundwalls too, public open space that is not so precious or well-utilized.

  6. Nathanael July 29, 2014 at 11:28 am #

    In the long run this is an underestimate. This is an estimate of the solar potential of *existing roofs*. However, when blocks are redeveloped, when buildings are replaced, the brand new building gets a brand new roof (obviously).

    Perhaps the biggest bang for the buck is in integrating solar into all *new* building designs. Or at least making them solar-ready.

  7. Matt August 3, 2014 at 1:45 pm #

    I have crafted a small article of my own. I go by Matt and I have years of research into what I am saying here. (I am going to be impressed if this forum can actually handle my comments here)

    I start with addressing a simple issue with this article. I then venture off into all things green, renewable and snake oil. This may not be the best place for my opinions but its Sunday and I had nothing better to do so here it is.

    PLAN WITH NO COST ESTIMATE?

    I would like to see a cost estimate on this. When would the public see a return on investment? It seems that this would be large scale solar (everyone cooperating) vs small scale solar. (each building would consume that power generated) What about transmission costs? I am going to deep dive into the reality of the current ‘green power’ initiative and get off topic a bit but I wanted to first address that this is more of a pipe dream than an actual plan with no cost estimate or ROI mentioned. The scary part of this is that undereducated people are going to read this a go into renewable energy crazy mode. (keep reading, I promise it gets better)

    I am as green as it gets but after doing tons of research online with the goal of using solar and possibly wind to create energy for my use I found that these technologies do not produce enough power to pay for itself even with no maintenance costs which are inevitable. Even with government grants and subsidies this is a losing proposition. I will start with discussing small scale solar as it pertains to houses or single buildings.

    SMALL SCALE SOLAR (maybe that is what is discussed in this article, maybe not)

    The best uses of small scale solar I have seen involve storing the energy in DC (Direct Current) batteries. To use it as AC (Alternating Current) power we must use an inverter.
    AC power is the type of power we use in our homes.

    These devices (inverters) get hot and that heat is created by the energy wasted in the conversion process. The best uses of small scale solar seem to be using the power as DC power for things like lighting (LED preferably) and devices that would otherwise convert AC to DC. (all of those DC power adapters you use for charging your phone, laptop or other electronics) Of course in Minnesota you would need to remove snow and ice from panels in the winter which would be a lot of work and you need a backup plan as it is not always sunny in this part of the country.

    WHAT REALLY USES A LOT OF POWER?

    In reality, the biggest consumers of power in our homes run on AC. The air conditioner is a huge draw on the grid. (this is why CenterPoint has the saver switch for your air conditioner and not for a small light bulb) Refrigerators have a compressor that uses quite a bit of power and the washer and dryer use a lot of power. If these things could be run off of small scale solar you would see some savings and it would make economic sense to put solar in your home. If all you can do is light up specific (off grid) devices using DC power, it is an expensive proposition that would not significantly lower your energy usage.

    LEFTOVER TOPICS AND QUESTIONS ON THIS ARTICLE (after this the rest will have little to do with the actual article and will be more of my opinion based on research)

    Of course this article discusses solar use in the downtown area. I do like how the article actually addresses the roof issues and the lifespan of the roof. Many roofs are in trouble on their own without the additional weight and other considerations needed to facilitate solar panels. It is unclear whether each building needs a grid connect system or transmission lines will be run and it will as large scale solar.

    COST AND CARBON SCARE TACTICS

    We DO need to actually consider costs in the equation. (although many of us do not want to allow this factor into the equation) Clean coal and fossil fuels are used extensively in this country. The biggest concern we are being made aware of is “Carbon”. This sounds bad and would lead you to believe that Carbon means CO or carbon monoxide which is odorless and poisonous. What we really mean by carbon is CO2 which is harmless to humans. We need oxygen to live and CO2 doesn’t help humans but it does help plants. Plants convert CO2 to oxygen and plants thrive with higher CO2 levels. I am not discounting that higher carbon levels may have a negative effect on the climate at some point. I am pointing out that scare tactics have been used to ‘educate’ a population that refuses to do its own fact checking. CO2 levels are used to discuss this with people who are in the know. Carbon levels are used to discuss this with people who have no idea. We used to say global warming but since the overall warming has changed to leveling or slight cooling we now say climate change. (you should be offended by this if you have done your research)

    The fact is that solar is not ready to take over much of our power needs. Fossil fuels do create the energy we need to survive in Minnesota. Without them right now, humanity would suffer and many would die. (the term fossil fuel indicates that it came from dinosaur remains is an artifact from the past that we know is not true but the name has stuck so we don’t argue it anymore….we do not currently know where it comes from)

    COAL – EVIL OR CLEAN AND ABUNDANT ENERGY?

    Clean coal is good for the economy and our future. In other countries the use of boilers that produce heat by burning coal for individual homes is still popular. This is by no means clean coal. Centralized power production using coal and ‘scrubbers’ to remove contaminants from the air is much cleaner. When burning coal, the byproduct is fly ash which is used in concrete to make it stronger and better. Certain environmentalists are trying to make fly ash a hazardous material due to the inclusion of substances that are basically sand and the existence of heavy metals that are lower than the existence of heavy metals in our normal soil. (Erin Brockovich is one such environmentalist) We need to handle the waste in a responsible way, making concrete out of it at the plant makes sense to me. (it isn’t green if a truck drives it across the countryside)

    The public already wants to believe that solar can power the country. Obama and supporters want to kill the coal industry and the jobs that go along with it. (no new coal plants will be able to exist with the tax increases placed on them) We will mine and ship our coal oversees using diesel since that is a more ‘green’ way to handle our energy needs.

    NUCLEAR POWER

    Why not increase modern nuclear. (pronounced NU-CLEAR not NUKE-U-LAR, forget how Bush and several other presidents have pronounced it) I know we are scared of this technology but it can actually produce a lot of power with little environmental impact. Fukushima is something that I have followed closely and we didn’t end up with radioactive waste on our shores as predicted initially. Lets not be afraid, lets advance as a society and shut down plants near rivers and flood plains that actually are a risk of incident and create newer, modern plants that are safer. We built a lot 30-40 years ago and a few recently. Learn about this potent technology and forget about the scare mongering that is stopping us from progressing in this field.

    WHAT WE WANT TO BELIEVE IS EASY TO BELIEVE (and made up statistics)

    The need that the public has to ‘believe’ in renewable energy without the supporting facts is unbelievable in this country. I use an example of the Great River offices in Maple Grove MN. They have always had rooftop solar panels and a single windmill on the property. I have been told by many locals that the windmill produces enough energy to power the entire building and 100 more homes. (this is not true) The building is the most environmentally friendly building in the state. It has earned the LEED platinum version 2.2 award. This did not come without a cost though. The building uses 40ish percent less energy that other 160,000 square foot buildings. It is north/south facing with white paint on it to deflect the sunlight without absorbing it. It has very few windows on the east/west sides. The pond is used for irrigation and a 20,000 gallon tank below the building holds rain water from the roof for use in the toilets. They save 66% of the water that would be used. (that btw has nothing to do with energy) They save 40 something percent of the energy costs by using geothermal (which is a great technology in my opinion) along with the other design aspects of the building. (fly ash was used in the concrete which is not surprising as it is used in 50 percent of all concrete anyway)

    So with the lower consumption what percentage of the energy used comes the rooftop solar and the windmill? I have been told so many inflated numbers from 80% to 500% but in reality it is up to 14%. Don’t believe me, watch Great River’s video here. http://www.youtube.com/watch?v=zYc6dgMFq6g

    At a minimum, anyone who wants to debate on solar and renewable energy should at least understand the difference between DC and AC power. (my estimate is that 25% of people I debate with understand this simple topic)

    Like I said earlier, I have had dozens of people tell me that this building produces more power than it can use. Most were unaware of the rooftop solar. They just believed that the smallish windmill was amazingly efficient.

    Recently, Great River installed more solar panels at the street level. This has changed opinions even further. Now it produces many times more energy than it can use. (not in reality….only in people’s minds though) The truth is that government grants paid for the additional panels. They used several types of technologies and it is a test bed for ways to integrate solar into mix. (at no point were these going to actually pay for themselves, the company most likely would not have invested its own money into the panels)

    I am in awe of this building and it is a great example. I want to someday tour it. It doesn’t make economic sense and nobody that I know of is following in their footsteps but the fact that it does exist is an accomplishment in and of itself. (though government subsidies made part of that possible)

    EUROPE IS WAY AHEAD…..OH WAIT MAYBE NOT

    We should try to learn from Europe. Many countries gave huge subsidies out for years without actually changing the use of fossil fuels much. There are 100’s of articles on this to read on the internet. I won’t give you a single link but google, “renewable energy madness” to see what I mean.

    We in America have reduced out carbon levels by using our abundant supply of natural gas. All of the efforts in Europe haven’t come close to it with their clean energy policies. (which are on the way out) Europe is envious of our ability to reduce carbon by simply burning natural gas. (from fracking btw)

    PIPELINES ARE BAD, Umkay.

    We need to think smarter now. I cannot understand why we cannot get the keystone pipeline approved. Obama has a war on fossil fuel going on that is costing us money and polluting the environment. The pipeline, once completed can bring oil from Canada and the Bakken oil fields in ND and such to refineries creating zero carbon emissions in the process. By refusing to allow it we are saying we would rather use diesel to power trains and trucks to move the oil. Pipelines are green. (and cost efficient) The first pipeline was created because the railroad industry was raising the price on freight for oil companies. Once in place the railroad was no longer a concern.

    MINNESOTA, THE PERFECT PLACE FOR SOLAR (after all of the states in the south that is)

    Please do your research as I have. Please realize that solar, especially in Minnesota is not near as potent as fossil fuels. I am not saying we should abandon clean energy. I would love to see a world were we use less fossil fuel but it needs to make economic sense. (and be realistic)

    PROGRESS IN SPECIFIC PLACES NEAR YOU BUT NOT TO THE LEVEL THIS ARTICLE WOULD INDICATE

    This article, if you skim through it sounds rather promising. Great River’s LEED building is absolutely amazing though only 14% (max annually) of the energy needs of this very expensive, high tech wonder of man will come from solar and/or wind. (it is a very small windmill btw) The mall of america is even better though when you consider that all (or almost all) of the heat in the Nickelodeon Universe park is solar. (the entire area is a large skylight) Real trees live in that area. Things that that will help. Better insulation will help. LED lighting will help and being aware of our consumption will help even more.

    HOW NOT TO HAVE A CONVERSATION WITH SOMEONE EDUCATED ON THE TOPIC

    Before you argue with someone based on ideals, speculation and emotions please make sure that you have a little bit of knowledge sprinkled with a few actual facts. That is what real debates in this country are missing: knowledge and facts.

    MY SPECULATION ON THE AUTHOR

    After reading the credits for this author, I believe that he obviously knows more than I do about city planning and such. I also get the feeling after reading the realistic part about how many roofs would require expensive upgrades to be able to support solar that knows more about roofs and buildings than I do. I get a sense that he may lean a bit in the favor or solar as a solution as 18% would be an incredible number given the current state of available technology. The thing missing is the cost estimate.

    CAN WE GREEN OURSELVES OUT OF ALL OF OUR GREEN?

    We can all be much poorer and pay exorbitant fees for energy if we continue to pass laws without facts, logic and reason to satisfy the uneducated and brainwashed masses of people who fancy themselves ‘green’. Alternately, we can be realistic and move forward into the future doing things that make sense.

    OBAMA SAVES THE PLANET, AT LEAST IN WORDS

    Our president made one of the most arrogant speeches I have ever heard when he was first elected. He said, ” this was the moment when the rise of the oceans began to slow and our planet began to heal”

    I was enthralled by this speech. I loved every second of it. (I have to admit) In hindsight, it is arrogant at best for a president to think that he can save the world single highhandedly. In fact, Obama’s plan was to pipe money into companies who supported him. (read: crony capitalism) One such example was Solyndra. Obama dumped money into a company that he mentioned in several of his speeches. The company is now bankrupt as are many of the other companies who received a windfall from the government. The problem is that photovoltaic is evolving rapidly. Their improvement to the manufacturing process was great for a short period of time but a competitor quickly came up with a better mousetrap. http://en.wikipedia.org/wiki/Solyndra

    The government takes tax dollars (by force) and does with them what it wishes. Sometimes great things come of this but often lots of waste comes with this. Regardless, Obama plays the game that all politicians are pretty much forced to and gives money to his cronies. The government isn’t going to save the planet, the free market is.

    I AM A GREENIE (at least to the extent that it makes any sense)

    As a person who has not put any lawn or garden waste into a landfill and composts almost everything that can be composted on his own property, I consider myself a green person. (its easy, just make a pile and do a little research) Not only do I have over $100 per year in waste disposal, I make about $100 per year in free compost. (great stuff btw) I will be amongst the first to buy a grid connect system and use solar and/or wind or anything else once the technology is available at a price that makes economic sense. I am truly green but I have done the research and know the facts. That being said I will compost the rest of my life. I won’t make trucks burn Diesel to take my grass clippings to a landfill or even the Hennepin County compost site. I will however use my gas to bring your yard waste to my house in the form of free compost. If I build a home it will most likely have geothermal. At some point I will invest in other technologies as it makes sense.

    SUMMARY

    My point is that what you think is true and what you want to believe is true isn’t always true. Be careful about jumping on bandwagons because they make you feel good about yourself. (it is very easy to do) Most of the TV news programs are left leaning and ultra green. Most union teachers in America are also left leaning and ultra green. Our kids get an education with somewhat of an agenda. It is reinforced by Hollywood and the media. It feels good to be green and if you are realistic you can actually do your part. My other points are that fossil fuels are relatively inexpensive and necessary to sustain life as we know it. (at least for now) People need to understand how much more it costs to produce solar than it does to produce energy from fossil fuels. (and be realistic) This is a difficult read but Wikipedia has some information if you can learn to read it: http://en.wikipedia.org/wiki/Cost_of_electricity_by_source

    Pipelines are not bad, in fact they are green!! Why can’t people see that using no energy is better than using rail cars and tankers? Can we apply a little common sense to this?

    I dream of a day when a simple grid connect system is available that costs under $5,000 to purchase and install in your home. A meter would keep track of your power generation and credit you for your good work. It doesn’t exist today for a reasonable price. (though it will someday)

    Obama cannot save the world, though he can get away with promising to. The truth of the matter is that we had a hole in the ozone layer. We changed out habits and it went away. We used to have very polluted waterways but we changed our habits and they are now much cleaner for it. By producing power in centralized plants we have greatly reduced our pollution levels especially in highly populated areas. (think of what it is like in Poland where they burn coal in their homes)

    Continue to dream of a cleaner day….it is coming but be realistic. Make compost and stop the diesel waste trucks from having to haul your grass clippings. (mulching is good too)

    Lets continue to use fly ash from coal plants for concrete and maybe other purposes. (stop the government from changing the laws just to fight against the coal industry)

    Lets produce more nuclear power. It is potent and safer than ever. Some people have an irrational fear of bugs or air travel. The public at large seems to be deathly afraid of this very viable solution to our energy problems. (assuming we have a problem that is)

    PLEASE DO NOT IGNORE FACTS, LOGIC and REASON (the internet is your friend)

    I invite other Americans to please do the same. I love to have intelligent conversations on the topic but I have too many conversations with people who refuse to do any research.

    Lets be entertained by politicians speeches and come away feeling all warm and fuzzy but don’t stop there, let’s demand actual results and check the facts after the high has worn off. There is almost nothing better than the positive feeling you get after hearing a seductive, well educated speech. Don’t turn the TV off though, wait for the opposition’s response and listen with an open mind. You need both sides to be a fair minded person.

    I believe debate is way to educate others and advance our society. In Europe politicians debate each other in real time on camera and sometimes in an emotional fashion. They are getting ideas out faster than we are here because of this. The problem we have is that most Americans want to debate based on their feelings and opinions with no facts to back any of it up. Most Americans believe something is fact if they have heard it 3 times. (whether it is true or not) Learn about the technology, the cost, the benefit and be realistic. That is my entire point in this. Don’t believe that government and subsidies are going to change the world. Edison invented the lightbulb for his own personal gain. That is how technology evolves and that is going to be what changes the world.

    I apologize to those offended by the opinions shared here. I fully expect a flame war to develop even though this site probably does not get a ton of traffic. If you know the difference between AC and DC and can speak intelligently to any of the information shared please do comment with a response.

    Lets preserve this planet for future generations in a practical and realistic way. Thank you.

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