100 days of solar

100dayssolar

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…

Let’s be pro-transit

My latest at streets.mn is all about positivity:

I’ll start by saying I have strong feelings about Southwest LRT. So do some people on this very blog. You probably do too. However, I won’t be contributing further to the gallons of spilled real and virtual ink or weeks of public testimony.  I’d like to talk about how we can set the stage for some other projects that could be really beneficial for transit-dependent and transit-interested communities. Nothing in this post should be interpreted as diminishing the importance of that LRT project, the upcoming decisions that will determine it’s fate/depth of its tunnel, or the correctness of any particular opinion about it.  But I have this urge to start some positive conversations about other projects that need some support.  Weird, right?

Mapping Minnesota’s solar resource

KingfieldSolar

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.

Solar insolation in January

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.

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.

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.

Counting every watt hour

DSC02868This little device is an ethernet to wi-fi adapter. It connects my solar charge controller to my home wi-fi network so I can make fancy graphs.  It uses 1.2 watts per hour.  I know this because I measured its usage using a watt meter.  I do this with everything I power from the solar batteries.

I have a hunch that this is what solar does to you, makes you compulsive about energy use.  Even if (when?) I have a large grid-tied system, I imagine myself checking the daily output, and constantly thinking about how to reduce my usage to match.

On very cloudy days, this little thing has used over 45% of the energy produced by the panels.  I unplugged it.  For now, graphs only on special occasions.

 

Home solar: adding MPPT and marvelous data

MPPT charge controller switched on!

MPPT charge controller switched on!

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!

Streets.mn podcast – Towards a Sustainable Minneapolis

Yours truly spoke recently with Bill Lindeke on the streets.mn podcast about Minneapolis sustainability initiatives (my day job).  We had an interesting discussion about measuring sustainability, greenwashing, and my solar tinkering.

You can find the audio file here, or subscribe to the podcast on iTunes.  You really should, Bill does a great job with it.

Strengthening our region’s response to climate change

My latest at streets.mn is a review of how the draft version of Thrive MSP 2040, the new regional plan for the Twin Cities, addresses climate change.

Our region certainly can’t address this issue alone, but we have a responsibility to do our part.  The science also says we can’t wait another ten years to start addressing the problem.  However, as this plan is currently written, the specifics on climate response are too ambiguous, and risk being watered down during implementation.The regional plan is one of the state’s most significant pieces of land use and transportation policy. By fully embracing state goals and calling for strong response, this could be a document that makes Minnesota a national leader in climate change response.

Read the rest.

Xcel Energy: social cost of carbon is $21 per ton

Old news, but still worth posting. In October, Xcel Energy filed a report with the Public Utilities Commission defending the cost overruns of upgrading the nuclear power plant in Monticello. Via the Star Tribune:

Xcel filed the report in response to the state Public Utilities Commission’s pledge in August to investigate the Monticello investment. The company said that even with the cost overruns, the project benefits customers — saving an estimated $174 million through the remaining 16 years of its license.

Yet that cost-benefit number relies on a “social cost” comparison between keeping the nuclear plant, which emits no greenhouse gases, vs. generating electricity from a plant that does emit them. State law says utility regulators should consider the cost of greenhouse gas emissions, though they’re not currently regulated. Without carbon-emissions savings, the Monticello upgrade would be a losing proposition, costing customers $303 million extra over its life, according to Xcel’s filing.

In interviews, Xcel executives defended the investment, saying they would make the same decision today, even though the utility world has changed since 2008, when the project began. Natural gas, now a favored fuel for power plants, is low-priced thanks to the fracking boom. And electricity demand has lagged since the recession, dampening the need for new plants.

“If we didn’t have our nuclear plants, we would be taking a big step backward in terms of our CO2 accomplishments,” said Laura McCarten, an Xcel regional vice president.

If you dig into the dockets (CI-13-754), you can find that Xcel’s modeling assumptions include a price on carbon of $21.50 per metric ton starting in 2017.

Regardless of your feelings about nuclear power, a utility stating that the externalities of carbon should be priced when making energy planning/financing decisions is significant. The use of a ‘social cost of carbon’ (SCC) metric at the federal level has (not shockingly) been the point of some contention.  The Office of Management and Budget’s SCC is $35/mt in 2015 versus Xcel’s $21 in 2017.

Theoretically, we should start to see this figure or something similar used in all future energy planning decisions (Sherco, cough, cough) in Minnesota.  Unless of course, Xcel was only being selective in order to justify recovering this very large expense (and spare the shareholders).

It would be an interesting exercise to apply this Minnesota SCC to land use and transportation infrastructure and planning decisions.