Home solar: adding MPPT and marvelous data

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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.

leverBigCorners

Planet levers we can pull at home

leverBigCorners

At Ensia, Jon Foley explores how we can break the cycle of climate inaction:

Frankly, we cannot afford to waste more time in a state of denial, saying that maybe this time our national leaders will wake up and take the problem seriously. We need to look for leadership and solutions elsewhere.

More importantly, we need to match our climate solutions to situations where leadership is still effective. We need to find targeted, strategic opportunities to reduce emissions, matching solutions to effective leadership.

But just where are those targeted opportunities?

In the search for effective climate solutions, we need to look for what I call planet levers: Places where relatively focused efforts, targeted the right way, can translate into big outcomes. Just like a real lever, the trick is to apply the right amount of force in just the right place, with little opposition.

In the search for planet levers to address climate change, we should look for ways to significantly cut emissions that don’t require grand policy solutions, such as carbon taxes or global cap-and-trade schemes, or the approval of the U.S. Congress or the United Nations. We need practical solutions to substantially cut emissions that work with a handful of nimble actors — including a few key nations, states, cities and companies — to get started.

Focusing on cities presents a particularly good set of levers to address climate change. Cities represent a nexus point of critical infrastructure — for electricity, communications, heating and cooling, and transportation — that are already in desperate need of improvement, and shifting them toward low-carbon “climate smart” technologies is a natural progression. Done right, most of these investments would improve the health, economic vitality, efficiency and livability of cities. Most important, most cities largely avoid the partisan gridlock of our national (and some state) governments, making them an excellent place for making progress.

I agree with Jon that cities are a good place to focus, not only because they have “functioning governments” that aren’t deadlocked, but because they have some key policy levers that can be pulled without a great deal of opposition, without getting a huge number of actors involved (creating potential for gridlock or slow movement), and that could have significant emissions impacts in a short time period.

Here are some of the local climate levers I think we can lean on locally, mostly at the city level.

Community choice aggregation (CCA)

The deregulation of electric utility markets is usually associated with some bad outcomes.  However, it can have positive benefits as well.  Since July of this year, over 58,000 residents and over 7,000 small business customers in Cleveland have received a 21% savings on their electricity bill AND received electricity from 100% green sources (50% wind, 50% hydro) through the Cleveland Municipal Aggregation Program.

This type of program is made possible by the fact that in deregulated electricity markets, cities can act as bulk purchasers for all or many of their community’s electrical customers.  This large buying power allows cities to negotiate good terms – like low rates and high renewable percentages.  These programs also don’t require the dismantling or purchasing of local investor-owned utilities.  Six states allow CCAs, and to date eight cities have used this authority to secure cleaner, more affordable power for their residents.  Most allow customers to opt-out and stay with their existing utility if they choose.

A program that requires electric customers to basically do nothing and could reduce the city’s greenhouse gas footprint 3% seems like a pretty good lever.

Note: state legislation is required to make CCA a reality.

Community solar (solar gardens)

Most people in Minnesota (some say only a third) have a roof that is good for collecting solar energy.  Shading, orientation, structural integrity, and ownership structure are just a few of the potential barriers to putting solar on roofs.  Matching the demand for solar with the supply of best locations, developed at a large scale for efficiencies, is something community solar or solar gardens can do.  These programs could be a powerful climate lever.  According to Midwest Energy News:

Minnesota's first community solar project in Rockford, MN. Image courtesy Wright-Hennepin Cooperative.
Minnesota’s first community solar project in Rockford, MN. Image courtesy Wright-Hennepin Cooperative.

The idea is to let customers who can’t or don’t want to install solar panels on their own rooftop instead buy individual panels in a nearby solar development. The electricity generated by a customer’s panels is credited to their utility bill as if they were installed on their home or business.

New legislation makes this possible in Minnesota.  In Colorado, where the program has been in place since 2012, 9 megawatts of solar was sold out in 30 minutes.   That’s roughly the equivalent of 3,000 single family home-sized systems.  Time will tell if this demand by project developers translates into strong demand by consumers.

Solar gardens generally require state policy change (except in the case of a municipal or cooperative utility), but don’t require thousands of people making individual installation decisions, hiring contractors, finding financing, etc.  A smaller number of experienced installers can do big projects with (theoretically) lower costs, supported by community interest.  Customers can buy-in to solar projects at whatever level they choose (usually bound by a minimum and maximum) but can skip all the installation headaches.

Capturing waste heat from the sewer

This one is my favorite.  There is a large supply of wasted heat flowing directly beneath our feet all day because we’ve literally flushed it down the drain.  One estimate says we’re flushing away 350 billion kWh of energy each year.  That’s more than 35 Minneapolis’ worth of energy every year.

Neighborhood Energy Utility, City of Vancouver
Vancouver’s Southeast False Creek Neighborhood Energy Utility

Sewer waste heat recovery systems, or “sewer thermal”, work just like ground-source heat pumps to pre-condition air or water before they are used for heating and cooling (don’t worry, no sewer water or gas gets into your air conditioner).  In the Olympic Village neighborhood of Vancouver, sewer waste heat provides 70% of the annual energy demand of a district heating system (natural gas provides the rest).  National Geographic has a good overview of the growing attention being paid to sewer thermal.

All major cities have large sewer mains collocated with the highest density development. Tapping this waste heat resource would require digging up those pipes, but it can be done much more easily in conjunction with large new redevelopment projects.  And generally, there are few actors: wastewater utilities control the pipes, cities control the right of way.

Making energy use transparent

According to the EPA, the commercial and residential sectors were responsible for 40% of US greenhouse gas emissions from the burning of fossil fuels (which is itself responsible for 79 percent of emissions) in 2011.  And in most major cities, it’s the large buildings (usually commercial buildings) that are associated with half or more of the energy consumption and associated greenhouse gas emissions.  Making these buildings more energy efficient could be a significant climate lever, but that requires knowing how they are performing now and motivating action from their owners and managers.

Nine cities in the US (and many more internationally) are addressing building energy use by making energy usage information more transparent.  Building rating and disclosure policies (typically enacted by cities) require large buildings to use widely adopted benchmarking tools to measure their energy performance, and generally require them to disclose this information, along with a score, to the public.

NYCenergyuse

In New York City, one million residents can now see how much energy and water their apartment buildings consumed.  In total, over 2 billion square feet of real estate in New York City is now benchmarking building energy and water performance each year.  This information isn’t just for tenants, building owners and managers, real estate professionals, and energy service providers can all use this information to improve the performance of the building stock.  In 2012, in their first report on benchmarked buildings, New York City estimated that:

If all comparatively inefficient large commercial buildings were brought up to the median energy use intensity in their category, New York City consum­ers could reduce energy consumption in large buildings by roughly 18% and GHG emissions by 20%. If all large buildings could improve to the 75th percentile, the theoretical savings potential grows to roughly 31% for energy and 33% for GHG emissions. Since large buildings are responsible for 45% of all citywide carbon emissions, this translates into a citywide GHG emissions reduction of 9% and 15% respectively. Much of this improvement could be achieved very cost-effectively through improved operations and maintenance.

An EPA study also showed that buildings doing benchmarking reduce their energy usage.  An analysis of 35,000 large buildings over three years showed that these buildings showed a 7 percent average energy savings.  Many of these policies are very new (NYC has only reported results for two years), so time will tell how increased public scrutiny of energy performance influences energy use.  But ask any building professional, and they will tell you that the first step to improving efficiency is measuring what is currently being used.

LED streetlights

Streetlights typically account for a significant portion of the electricity used by a city government enterprise.  For Minneapolis, its 31 percentNavigant says up to 40% can be typical.  Water treatment (for drinking) and wastewater treatment are two other major sources of energy use for cities or regional government entities.

Streetlight retrofits can often be done by a city itself, if they own the lights, or by the utility, which is also sometimes the owner.  Retrofits can be quick (a few years), and the paybacks, both in greenhouse gas emissions and cost, can be significant.

LA's Hoover Street before and after LED lighting retrofit. Image via Los Angeles Bureau of Street Lighting
LA’s Hoover Street before and after LED lighting retrofit. Image via Los Angeles Bureau of Street Lighting

Los Angeles recently completed the world’s largest swap-out, replacing 140,000 lights.  Los Angeles estimates it will save $7 million in energy costs and $2.5 million in avoided maintenance costs (LEDs can last as long as 20 years, versus the standard lights 6).  The project will be paid off in seven years.  New York City, Las Vegas, Austin Texas, San Antonio, and Eden Prairie, Minnesota are all switching.  For cities in the metro that don’t own their own lights, Xcel Energy is testing 500 LED lights in West Saint Paul, and could set a new rate for cities once the test is complete.  Initial results are showing energy use is cut in half, while the quality of the light has improved.

These are some examples of “levers” I think can be pulled relatively quickly, and without a great deal of political wrangling. And maybe more importantly, they can be done at the local level, usually by cities.  Cities are demonstrating they can and will move on climate, breaking what Jon calls the “cycle of climate inaction”.

There may be other strategies which are essential to addressing climate change, but which require engaging many more stakeholders and/or take significantly more time (an example might be residential building energy retrofits).  These strategies may be just as critical, often because they may address issues besides energy and climate – like environmental equity.  But if we want to work on a timetable that’s anything close to what they experts call for, we should identify and prioritize these short timeframe, high-impact levers we can pull at home.

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.

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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.

Car2Go first impressions

Over at streets.mn, I detail some of my first impressions of one of Minneapolis’ newest car-sharing options, Car2Go.

According to MPR, by 2014 there will be over 400 vehicles available to car-sharing customers.

The bulk of these will be in the fleet of Car2Go, which operates on a different business model than the rest, offerring “point to point” service.  Walk to a car anywhere in Minneapolis, drive where you want to go, then park it at any curb space in Minneapolis (expect rush hour zones).  Other companies require that you return their cars to a specified location at a specified time.  The trade-off is that their rates are generally cheaper.

Read the rest.

On estimating solar output and voltage drops

12.9 volts?! I want 18.9!
12.9 volts?! I want 18.9!

When I started tinkering with off-grid solar one of the first questions I asked myself was “how long is it going to take to charge this battery?” Or, similarly, “how much power will I produce in a day”?  Initially, the answer seemed easy: I’ve got a 100-watt panel, Minnesota gets about 4 hours of peak sun on average per day, so I’ll get 400 watt hours per day!  A 960 watt hour battery should be charged in two and a half days!

Wrong.  Way off.  Maybe more like 250 watt hours per day, and more like four days to fully charged.

I realized this quickly when attempting to charge a fully depleted battery.  In reality, it took into the fourth day for the charge controller to switch to battery maintenance mode (showing it was completely full).

Under perfect operating conditions and when grid-tied, you may actually get close to that nameplate 100 watts.  However, when your system is connected to a battery the voltage drops.  Many charge controllers (except for the really good expensive ones) will match the voltage of the panel to that of the battery to facilitate charging, which is almost always a lower voltage than the panels potential peak.  The rest of that potential is wasted.  So while my panel will produce 5.29 amps at 18.9 volts under optimum conditions (5.29 amps x 18.9 volts = 100 watts), when connected to my battery, it will probably only produce 5.29 amps at between 11 and 13 amps (5.29 amps x 12 volts = 63 watts).

Panels are built this way on purpose to make sure power can continue to flow to the battery even during overcast conditions when voltage may drop a little (gotta make sure that water flows downhill!).  Grid-tied panels don’t have this problem, since the grid can usually accommodate your voltage, and in a grid-tied system you’ll probably opt for one of the fancier MPPT controllers (see link above).

So, in summary, I’m probably getting 65 – 70% of my nameplate wattage (by design), and a realistic estimate for charging a fully-depleted 80 amp-hour battery from my 100 watt panel is 3.5 – 4 days.

Another takeaway for this amateur: it’s about amps, not watts.  The websites where you shop for panels always have the watts in large font, but the small print tells you the optimum operating current, or amps.  This number times hours of sun gives a much better estimate of the output (5.29 amps x 4 hours = 21 amp hours per day) for a battery-tied system.

Home solar update

Dead battery!
Dead battery!

The battery was totally dead this morning (11.4 volts) after about 16 hours powering the freezer.  The charge controller indicated the panel was charging the battery for about 4-5 of those hours, but it was extremely cloudy.  I turned off the inverter and reconnected the freezer to grid power (it’s nice to have a backup to the backup!)  After charging all day today, the battery is up to about 60%.  I hope I didn’t do any permanent damage to the battery.

My estimate of the charge time was pretty close, but my estimate of running time for the freezer was pretty far off.  I don’t think I calculated any loses from the inverter, which the internet tells me can be 15% or more.  It was also a hot day, hotter than when I measured my freezers usage initially, which could have had an impact. Next time I’ll definitely be measuring the total watt hours used (I forgot to hook up the meter) so I can try to estimate what was lost to the inverter.

One more day of charging before I do any more experimenting.