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 How much energy could Minneapolis get from solar?→
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…
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.
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.
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:
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:
Here’s the next day, when the battery starts out the day almost totally full. It was very sunny.
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!
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.
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.
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.
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.
Being carbon-conscious, naturally inclined to tinker, and seeing the falling costs for components, I was curious to know whether I could put together a small solar PV system on my own. Having experienced a few blackouts this summer and expecting more in the future, I was also curious about providing a small amount of backup power for essential items. Here’s the story of my first foray into off-grid renewables.
This post at Do The Math (an excellent blog you should read regularly) in which Tom Murphy describes his small, off-grid system really got me started on the whole thing. I’m not a physics professor, but after reading it and doing some additional google searching, it seemed easy enough for a lay-person armed with a small amount of reading. A valuable resource (also provided by Tom’s blog) is the Solar Living Sourcebook, available at your local library, which provides all the basics on what solar PV is, how it works, important safety tips, and options for setup. I also learned a few things from various youtube videos and generally google searching.
The system I put together is 12 volts, which seems very common for small, off-grid installations. It’s basically only six things: a solar panel, a charge controller, a battery, an inverter, and assorted wires and fuses. The solar panel provides the electrons, the charge controller controls how those electrons flow to the battery (and makes sure it doesn’t overcharge), the battery stores electrons, and the inverter turns the battery’s 12 volt DC power into 110 volt AC power so it can be used with regular household electronics. The wires and fuses connect things together and provide safety.
You can now purchase relatively affordable panels from Amazon or Home Depot in many wattages and sizes. I chose a 100 watt panel that seemed to receive good reviews and a website that suggested that the company might be around for a while.
The other pieces of the system (inverter and charge controller especially) come in a huge range of prices. After some reading, I decided that it might be better to spend a little more on a charge controller, as many people had complaints about cheap versions, and keeping your battery well-maintained is important (the function the charge controller plays). I purchased a 30 amp controller from Morningstar, which I think could power up to 300 400 watts of panels if I expand the system in the future. The battery is rated at 80 amp hours, and is sealed lead acid. I purchased it from a local battery store, and its a discount version.
So what can this thing power you ask? That’s a function of how much the panel produces, how much the battery stores, and how much amperage I can draw at one time from the battery and inverter.
According to some assumptions I pulled from NREL’s PVWatts tool, the panel might generate 400 watt hours per day (100 watts X 4 hours equivalent of 100% production) in the peak season and maybe 210 in the low season (November), although I’ve seen higher numbers in other places. The battery is large enough to store all that daily production and more (80 amp hours X 12 volts = 960 watt hours). In fact, it would probably take one and a half to two two and a half days of sun to fully charge the battery.
Even in the winter, the daily production of the panel would probably be enough to power a few lights (the LED variety), an efficient laptop, a fan, and a small TV for a few hours. It won’t run a hotplate, anything but the smallest air conditioner, a heater, or a refrigerator, at least not continuously. The battery and the inverter could probably handle it (one of these things), but the panel wouldn’t be able to keep up. As far as a back-up power source, this set up would power my refrigerator for about 12 8 hours, and our 8.8 cubic foot chest freezer for about 24 16 hours. That’s assuming a fully charged battery, the panel couldn’t keep up with the draw from those appliances for more than a day. These are just my estimates, I don’t have any real world results yet, but will report back soon. Right now I’ve got just the chest freezer plugged into the inverter and I’m going to time how long until I get the low battery warning.
Things I’ve learned so far:
It’s all the other stuff that costs money. At this stage, the panel itself only accounts for 20% of the cost. If I added two three more panels (which would probably max out the charge controller) to economize, the panels would still only be 40% 51% of the cost.
I need scale to “save” money. Right now, my costs per watt are about 76% higher than what I have been quoted to put a grid-tied, full size system on my roof. If I maxed out the charge controller with two more panels and got another battery, I could bring my costs in line with the pros (again, on a per watt basis). Whether this would continue to scale up I kind of doubt, since batteries get expensive and I would get into more serious electrical work pretty quickly.
I need another battery (or two) for large stuff. High amperage appliances, like a vaccuum, seem to be within the wattage range of the inverter, but my battery is only 80 Ah. The internet tells me I should only run things that are 10-12% in amps of that capacity to avoid shortening the batteries life, and indeed I got a low battery warning when trying to run the shopvac.
You should think hard about where to locate a panel before you embark on this kind of project. I’m still squeamish about getting into roofing for fear I will cause a leak, and others in my household disagree about the aesthetics of a home-built wooden frame. My goal in the long run is to get this on a roof somewhere.
Solar panels aren’t just for tree-huggers. If you’ve ever searched youtube for videos about solar back-up systems, you’ll be a lot less surprised about news items like the Atlanta Tea Party teaming with the Sierra Club to promote more solar. Many of the instructional videos I watched were clearly made by folks of the conservative persuasion who were into solar because they feared the grid would go down or they weren’t comfortable being beholden to utilities/the government. Maybe there is more common ground here than we thought.
One of the key issues is the idea that utilities want to avoid “stranded assets”, or infrastructure they still have to pay to maintain with a shrinking pool of customers. As some customers get more power from solar, sales of electricity shrink, leaving utilities with the same distribution infrastructure to maintain using less revenue. Some utilities, the latest being a municipal utility in San Antonio profiled by David Roberts, argue they shouldn’t pay customers the “market” rate for electricity their customers generate with rooftop solar, but instead should pay them a wholesale rate, or the same as they pay for other electricity on the grid.
The thinking here is that paying the wholesale price will put renewable energy on an even playing field, and help keep the old utility model more financially whole, since wholesale prices are typically much lower than market prices. For example, the 5-year average wholesale price for electricity in the grid area that serves Minnesota was $53.62 per MWh for the period ending in 2010, according to FERC. This is for the “peak” time of day, meaning the afternoon, which is also the time solar is most productive. That’s equal to roughly 5 cents per kWh, which is the unit at which typical household sales are measured. Last month I paid about 11 cents per kWh to Xcel before taxes, fees and other charges like WindSource.
At 5 cents/kWh, rooftop solar would take a very long time to pay off. Many fewer people would likely choose to install it. However, those in the renewable energy world will tell you that 5 cents/kWh doesn’t pay the owner of a system for some of the benefits solar energy has over wholesale electricity. We should actually be looking at a “value of solar” that includes not just the wholesale energy price, but reimbursement for other values. There is movement right now in Minnesota to legislate that a true “value of solar” be computed for future projects. So what other value does solar energy have that utilities might value?
For one, it can be more efficient. Whenever you transmit electricity or long distances, you lose some due to resistance (heat). EIA estimates these loses at 7% nationally and 7.4% in Minnesota. That means utilities are generating more kWhs than are needed to make up for the losses, and thus the customer is paying more for each kWh. If you’re generating power very close to where you use it, you minimize these losses and the extra generation. Distributed solar energy should actually be valued 7% above wholesale prices by a utility if you think it will reduce these line losses. If you include that 7% bump, 5 cents becomes almost 6 cents per kWh.
The other value is the reduced environmental cost of solar generation. There is plenty of discussion about what the optimal cost of carbon should be, and it all depends on what you adopt as your discount rate. Here is a must-read on discount rates, also by David Roberts. If you think that climate change will have a net drag on the economy in the future, your discount rate is likely low, and the optimal cost of carbon gets up into the $50 to $100/ton range. Carbon levels per unit of electricity produced vary quite a bit across the county, but in Minnesota and parts of the upper Midwest, they averaged 0.738 metric tons per MWh in 2009 (the latest year for which EPA has data). At that rate, a high carbon tax might add between 3.5 and 4.5 cents per kwh.
If you add all this up, (an economically optimal price on carbon, savings from transmission losses, and a wholesale price consistent with the 5-year peak average), you get a value of solar energy between 9.5 and 13 cents per kWh. That’s at or above the market rate I’m paying in Minnesota right now. Check out my extremely messy spreadsheet if you want to see the math.
Keep in mind there are other values of solar energy I haven’t considered in my calculus. The Minnesota House legislation includes the savings from delaying capital investments in distribution infrastructure, savings from not having to build more generation, fuel price hedge value savings (not having to bet on fuel costs), and the value of local employment generated by manufacture and installation of solar energy.
This week a bill was introduced to the Minnesota legislature to establish a 10 percent solar energy standard by 2030. This would be on top of the existing requirements for utility renewable energy, bringing the total amount of energy coming from renewables in the state to at least 35 percent in 2030.
This bill is being promoted for it’s job creation aspects, but clearly a key benefit is the reduction in greenhouse gas emissions from the electricity generation sector (which currently produces 32% of the state’s greenhouse gas emissions). So, by how much would a 10% solar standard reduce Minnesota’s emissions? Would it allow us to meet our greenhouse gas reduction goals?
The first (and easier) part of trying to put some numbers to this is estimating how much electricity Minnesota will use in 2030. EIA summaries tell us that Minnesota consumed a little under 68 million megawatt hours in 2010. Power projections produced for the Annual Energy Outlook tell us that in MRO West (our electricity grid region), the annual growth in electricity consumption will be very modest through 2030, typically under 1% annually. If you assume these growth rates apply to Minnesota, we may consume over 73 million MWh in 2030, or 8 percent growth over 20 years.
10 percent of that is 7.3 million MWhs in 2030. Figuring out exactly how much greenhouse gas this would save is trickier. In 2010, electricity generation accounted for 32% of the total 155.6 million metric tons of CO2 equivalent emissions. Rough math using EIA consumption figures provides a greenhouse gas coefficient for Minnesota electricity of 0.73 metric tons of CO2e per MWh. However, this figure will surely go down over the next 20 years as utilities work to meet the existing renewable energy mandates already on the books. Xcel Energy, which has to meet a more aggressive renewable energy standard then the rest of the state, already has a coefficient closer to 0.5 mt/MWh, which will be declining (see slide 17) to something like 0.42 mt/MWh by 2025.
So, assume the state’s net greenhouse gas coefficient for electricity is somewhere around 0.5 mt/MWh in 2030 (assuming other utilities and imported electricity are both dirtier than Xcel). If 10% of our electricity demand is met by solar energy, this would be a savings of 3.6 million metric tons of CO2e. 3.6 m metric tons is about 2.3% of our 2010 emissions total, or about 7% of emissions from the electricity sector in 2010.
Using an net coefficient average emissions factor for calculation may be too simplistic, but it’s the best I’ve got right now. Those more in the know say that renewable energy like solar will most frequently replace natural gas production, rather than coal or nuclear, as gas is easier to cycle on and off. I’m not sure whether this would increase or decrease the benefit of this level of installed solar (but I’m working on it).
Update: I was pointed to this journal article by Carbon Counter, which attempts to calculate “marginal emissions factors”, rather than average factors. It turns out, since the Midwest is coal-heavy, usually an “intervention” (adding solar, for example) would displace coal power first, rather than gas. The marginal emissions factor they calculate for the Midwest is about 13% higher than the 0.73 mt/MWh I mention above. The Midwest is somewhat unique in this regard, as most regions show gas as the most common “marginal fuel source”. It also has the highest marginal emissions factor of all the regional electricity generators looked at in the study. A 12% increase over 3.6 million mt is 4.03 million mt.
At something near 3 or 4 million metric tons of emissions saved, would a 10% solar standard help us meet our state emissions reduction goals? Nowhere near on its own, but it would be a significant step in the right direction, especially when combined with strong action in other sectors like transportation and agriculture. Think of it as part of the Minnesota version of the wedge game.