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New Adventures in 3D Printing

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It's been almost exactly six years since I bought my first 3D printer. In that time I've owned three 3D printers, all of which still work, and two of which I still own and use fairly constantly.

To date all of my printers have been fully-built models from the Chinese manufacturer TierTime. I've decided that it's time to take the next step and build my own printer.

So I've placed a preorder for a Prusa I3 MK3 kit, which I hope to receive around the beginning of February. I've also preordered the multi-material upgrade, which might show up around April.

With my six years of experience 3D printing, I think it's fair to call myself at least a highly competent journeyman. But I'm already learning that the open source world does some things very differently from what I'm used to in TierTime's products.

For example, TierTime's slicer provides only a handful of print settings: layer height, infill, print quality (one of three options), whether you want a raft, and a few parameters for the amount of support material. Slic3r Prusa Edition has around 65 different print settings, not counting the ones under the "Advanced" menu. This clearly represents not just a steeper learning curve, but an entirely different philosophy of how 3D printing should work from the user perspective. While I can see the value of the extra control, I've also managed to get by just fine so far with one tenth the number of parameters to adjust.

Another big, and surprising, difference is how the RepRap world still seems to be struggling with support and rafts. Six years ago, the Up's break-away supports were a major point in their favor (and one of the reasons I didn't go with something like a Makerbot back in 2011). While it's not always perfect, the support material I print with my current printers is generally fairly easy to remove, and the resulting surface of the print after the support is removed usually ranges from pretty good to perfect. But from reading online discussion, it seems that a lot of people still struggle with getting their printers to print supports that are easy to remove and don't leave ugly surfaces behind. I expected the open source community would have figured this out by now--and it's disappointing because not having reliable support and rafts really does limit what you can print and how you can print it.

On the other hand, the limited controls TierTime has given me for the materials (I can only set the extruder and bed temperature on my Up, vs. 15 different material parameters in Slic3r) has limited some of my printing options. I haven't been able to get some interesting filaments (like flexible filament) to work well, or even at all. And being able to print with four different plastics in a single print, as the multi-material upgrade allows, will be a real treat. Even if one of those materials winds up being soluble support because of the problems with break-away supports.

I'm sure I've only just begun to scratch the surface. Being an experienced 3D printer taking my first steps into the world of open source printers is guaranteed to be an interesting adventure.

The Coming Energy Glut

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Renewable energy prices continue to plummet, with a recent solar contract in Mexico coming in well under $0.02 per kWh for power to be delivered to the grid starting in 2020. It's entirely possible that within the next few years we could see a renewable energy contract come in under a penny somewhere on the planet.

Even setting aside the headline-grabbing outliers, solar and wind power are now the cheapest ways to generate electricity in the United States, at least for new capacity.

This fact, plus the unique characteristics of solar and wind power, means that over the next few decades pure economic forces are likely to flip power markets upside down and lead to a glut of energy.

Solar and Wind Will Be Overbuilt

Solar and wind power are different from traditional sources of electricity because:

  1. Nearly all the cost is upfront capital expenditure, and there is no cost savings in curtailing overproduction (vs. coal or gas plants, where the cost of fuel is significant and reducing output when demand is low will save money).
  2. Power output is variable and can't be increased to match demand.
  3. The lifetime cost of power from a solar or wind facility is cheaper than any other power source, and solar and wind are getting cheaper over time.

This combination of factors means that when a power company needs to add generating capacity (whether because of demand growth or because older plants are being retired), it's generally going to be cheaper to build renewables rather than a coal, gas, or nuclear plant. And because of the low cost and variable output of solar and wind, it will be cheaper to overbuild renewable capacity by some percentage, to allow the low cost renewable power to displace more of the (relatively) expensive coal and gas power even when the renewables aren't producing full power.

Once the solar and wind generation capacity is in place, the direct cost of generating power from these sources is very close to zero. The most rational, profit-maximizing approach to building future power generation is one which will inevitably lead to times when more power is being produced than consumed, at zero marginal cost to the utility. If the utility can find any buyer for this power at any price larger than zero, it can make a profit.

In other words, a glut.

(On a side note, utilities overbuild their capacity in traditional power plants, too, so that there's enough power for peak demand or when a power plant goes offline. But this peaking and reserve generation capacity sits idle until it's actually needed, since the fuel costs money. Solar and wind are unique in that they don't cost anything to generate once the plant is built.)

There have already been a few times when wholesale electricity prices have dropped to zero in certain places because of the overproduction of renewable energy. The world is only just getting started in building out the 21st century renewable energy grid, and before we're done, excess power production will be a daily occurrence in many parts of the world.

Demand Response and New Uses

The idea of an energy glut is all very weird to me. I grew up in the 70's and 80's in the shadow of energy crises, high gas prices, and 55 MPH speed limits.

One likely change is that we'll see a lot power use shift to times when there's excess electricity. Even though electricity needs to be generated at the time it's used (unless someone stores it in a battery--which is getting cheaper, but will always be more expensive than using it when generated), it turns out that many uses for electricity don't have to happen at a particular time. Heating and cooling is probably the best example, since heat (or cool) is easy to store for a few hours and a significant fraction of electricity use goes to climate control.

It's easy to imagine a smart thermostat that notices when the price of electricity is low and cranks the thermostat a few degrees (warmer or cooler, depending on where you live) so it doesn't need to run as much the rest of the day. Or a smart water heater that takes advantage of cheap power to heat up some extra hot water in the tank.

The technical term for this is Demand Response, and it's already starting to become a thing. In a few years it's likely to become a really big thing, especially in commercial and industrial applications where user can shift large amounts of consumption and realize substantial cost savings.

It's also going to be interesting to see what new uses for electricity become important. At today's prices, for example, electric cars are more expensive than gasoline but cheaper to drive--over the lifetime of the vehicle, the electric car is still somewhat more expensive. But that might change if you could recharge your EV at one-tenth the retail price of electricity as long as you did it when there's a glut of electricity.

Seasonal Storage

The hard problem in renewable power has been and continues to be, seasonal storage. Batteries can store electricity for a few hours or weeks, but even the cheapest batteries are still many times the cost of generating the power when needed (though this is starting to change: in some electricity markets it is now cheaper to use large batteries to meet peak power demand than to use expensive "peaking" plants powered by natural gas).

In many places renewable power production varies considerably not just throughout the day, but over the course of the year. In Minnesota, on average, we get only around a tenth as much solar power in the darkest month of the year as in the sunniest. It's not uncommon for us to go weeks without seeing the sun in November and December. Batteries just don't have the ability to store electricity from June to use in November.

One exciting possibility for the coming energy glut is that it may enable solutions to the seasonal storage problem. If electricity is cheap enough and plentiful enough during the gluts, it may actually become economical to do things like synthesize liquid fuel using renewable energy. These ideas are being pursued in research labs, but in today's energy markets they are much too expensive to be worthwhile.

The World Is Changing

It seems almost inevitable that, if current trends continue (and there seems to be no obvious reason why they shouldn't), we will find ourselves with intermittent gluts of energy rather than shortages. This is going to be a very different world than the one we live in today, where the challenges will not be in finding enough energy, but in getting the energy to the times and places where it's needed.

Welcome Back to the Frozen North

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I've been neglecting this blog more than a little the past few years. Got busy, life was getting complicated, and so forth. And after a while I got to experience the joy of overwhelming technical debt firsthand, when the version of Drupal I was running became so out of date that it was hard to keep running and a big project to upgrade.

But I finally got around to updating, leapfrogging from Drupal 6 to Drupal 8. I didn't take the time to do much customization: I only did enough to get my basic content moved over and put together a completely vanilla blog site. I'm not perfectly happy with where it stands, but considering the amount of work I didn't do to get here, it's not too bad.

With luck, having an updated and maintainable blog will encourage me to write more often again. Reading through some of my old articles has been interesting. And with a few tweaks here and there, I should be able to gradually get some of the layout and display features closer to what I want.

A lot has changed in the almost three year hiatus this blog has taken. Kids are leaving the nest, business is evolving, and dear God don't get me started on politics.

I write this blog mainly for myself, as way to express my thoughts and ideas. I don't expect anyone has been terribly disappointed, or even noticed, that it hasn't been updated. Nor do I expect anyone will notice or care if I write again. But I care, and perhaps some of the breadcrumbs I leave here might help someone just a little bit down the road.

Half a Year of Solar (almost)

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Our solar panels were activated five months ago, at the end of July. That was a couple months later than we had been hoping, but the modules we ordered were in short supply at the time.

Since then, perhaps the most remarkable thing about living with solar power is just how drama-free the whole thing is. It took a fair amount of effort and planning to get the system installed. But now that it's in place, it just sort of sits there and generates power.

Now that we've had the solar panels in place for almost half a year, here are a few observations in no particular order:

  • Probably the coolest part of the whole system is not the solar panels but the power monitoring system. This lets us view, in real time, how much power we're using and how much we're generating. This turns out to be a great motivator to turn off lights when we leave the room and otherwise look for ways to save energy. We have cut our household energy consumption by about 10% just thanks to having this tool.
  • The TenK modules are performing as advertised when we have partial shade. One reason for selecting this brand was that the roof over our garage (which is where half the solar panels were installed) gets a lot of dappled shade in the winter months, and TenK modules are designed to be highly shade tolerant. Most solar panels will lose a large fraction of their power output if there's even a small patch of shade, but the TenK modules keep generating under these conditions.
  • Speaking of shade....solar panels don't generate much power when it's cloudy, and we're in the middle of the cloudiest stretch of weather in Minneapolis since the 1960's. November and December are normally the cloudiest and darkest months of the year, but we have literally had only two even partly sunny days in the past three weeks.
  • However, despite the clouds we have had relatively little snow cover. Since it's impossible to clear the snow off half of the solar panels, persistent snow cover is also pretty bad for our power production.
  • Speaking of snow, when the panels are covered in snow and the temperature gets above freezing for a few hours, all the snow and ice tends to slide off in a big clump. From inside the house it sounds like being underneath an avalanche (which is pretty much what it is).
  • We've had a few neighbors ask about solar, but it happens that ours is one of the few houses in the neighborhood that's suitable for solar panels. That's the downside to living in an area with a lot of big trees.

Our Solar System Takes Shape

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In the past few months we have finalized the basic design of our solar power installation.

Our system will have two arrays, one over the garage and one on the main part of the house. Each array will have eight 410-watt solar panels from a local manufacturer called  TenK. These will feed 12 microinverters made by Altenergy Power Systems. The total nameplate capacity of the system is 6.56kw, but because the two arrays will face different directions it will never produce that much power at any given time. Instead, with a southwest and a southeast array, one will catch more morning sun, and the other will catch more afternoon sun.

The estimate is that this system will produce, on average, about 5,800 kWh per year. This is relatively low production for a system this size in this area, and the lower production is mostly because of partial shading on the arrays, especially in winter. The garage array, in particular, is estimated to produce almost no power in the month of December because the garage roof will be mostly shaded by the rest of the house. That's not such a great loss, though, since Minnesota gets relatively little solar energy in December anyway.

We chose this system because of a very generous incentive program Minnesota is offering for solar panels made in Minnesota. For the first ten years the system is in production, we will get an incentive payment of $0.29/kWh for all the power it produces. This is in addition to the net metering credit which is currently about $0.12/kWh and will increase as electric rates go up. The Made in Minnesota incentive is paid for through a conservation program established several years ago by the state which requires electric utilities to set aside a small percentage of their revenue towards energy conservation programs.

The Made in Minnesota incentive is so generous that we expect this system to pay for itself in under ten years, despite the shading on our site and the slightly more expensive panels from TenK. Our benchmark for making solar worthwhile is that the system pays for itself within its lifetime (25-30 years), so this system meets that threshold by a large margin.

TenK Modules

The TenK solar modules are a new and innovative product, which was another reason I liked this option. Some people might read "new and innovative" to mean "unproven and risky," especially for a major capital investment expected to last decades. For us, however, since one of our goals is to learn and explore solar energy, the chance to work with a product taking a new approach to solar power is definitely a bonus.

Traditionally, solar panels are very dumb devices. The basic solar module consists of a few dozen photovoltaic cells sealed in a weatherproof enclosure and wired together with a couple diodes. In many cases, the panel manufacturer doesn't even make the solar cells, they just buy the components and assemble them into the final package. That's part of the reason why there are so many solar panel manufacturers and it's such a low margin business. There's been fairly little technology in the module itself, and all the magic happens in manufacturing the photovoltaic cells and in the inverters and controllers.

TenK, on the other hand, takes a very different approach. They sell "smart" panels which incorporate the MPPT electronics (which maximizes the harvest of power from the solar cells) into the module itself, and do a DC-to-DC power conversion to control the output of the module.

This allows them to get more power from the system in situations where a traditional module performs poorly (such as when half the module is shaded and the other half is in the sun). It also allows them to use a power bus for connecting the modules to the inverters, which makes it practical to generate a lot of power but keep the DC voltage at or below 60V.

The low voltage DC bus is important because high voltage DC (traditional photovoltaic strings can operate at hundreds of volts) is dangerous and requires special equipment to manage. The TenK modules also have built in ground fault protection, so if there's a short circuit in the power bus the modules shut down automatically.

So (in theory) the TenK "smart" modules should allow us to get more power from our system (especially in December), and while the modules themselves are more expensive, the rest of the installation is simpler. The total system price quoted by our installer for the TenK system was about 10% higher per watt than what we were quoted for a more traditional system built around "dumb" panels, but it's possible we will actually get 10% more power from this system than from a similarly sized array from another manufacturer

The risk, of course, is that TenK goes out of business and our modules break earlier than expected. With a more complex module there's more risk something will go wrong and the system will need to be repaired; and the solar module business is notoriously brutal.

In the near term, TenK seems fairly stable since they very recently raised a substantial amount of money from investors. I spoke to some of the company's early customers and they were all pleased, so I'm comfortable that they will be around to fix any problems which develop in the first few years.

Next Steps

One downside to the TenK modules is that the product is currently in short supply. Our installer advised us that we can expect the modules to be available in June, which is 2-3 months from now. We're hoping that won't get further delayed, since we want to take advantage of the most productive solar months of the year.

In the meanwhile, we're starting on the paperwork for the utility approvals and the solar incentive program, and looking at what work we can get done in advance so that when the solar panels arrive we can get into production as fast as possible.

Energy Storage: Potential Game-Changer for Renewables

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Solar power has reached the point where, for ordinary consumers, it's generally about the same price as power from the electric company.

Wind energy has reached the point where, for utilities, it's generally about the same price as generating power from fossil fuels.

Not surprisingly, then, both residential solar and utility wind power are growing very fast in the U.S. I've seen some analysis showing that essentially all the net new generating capacity being built in this country is coming from renewable sources. I don't know how credible this is, but whether it's true or not today, it will be true in the not very distant future.

Solar and wind energy can continue to grow like this for many years, since they still represent a very small portion of our total electric generation. But the growth of renewable energy will eventually be limited by the fact that these energy sources are inherently intermittent. The sun doesn't always shine, and the wind doesn't always blow, and there's no way to control when you get power.

The problem is that electricity needs to be generated at the same time it is consumed. The power grid doesn't store power, it just moves it from one place to another.

Right now, storing electricity is a lot more expensive than generating it. In our neighborhood, it costs about $0.12/kWh to buy power from the electric company. Rechargeable batteries, on the other hand, cost (on the cheap end) around $0.50 for every kWh you use because the battery has a limited number of charge cycles before it needs to be replaced.

Given the cost of storage technology today, it is almost never economical to store excess renewable power for later use, even if the power is free (the only exception is if there are no other power generation options available--for example, a cabin in the woods). That means that, with today's technology, wind and solar power can't supply anything close to the majority of our electrical needs, since the power simply won't be generated at the right time.

An inexpensive way to store excess power for later use would radically change the economics of renewable energy. Lots of smart people are working on this problem, and there are several different approaches which could bear fruit.

Improvements in Battery Technology

Traditional batteries are the simplest way to store electricity for future use, but today's technology is simply too expensive for large quantities of power (except in specialty applications like electric cars). There's a lot of research into novel chemistry, better physical designs (including lots of nanotechnology), refinement of approaches like flow batteries, and so forth.

In order to become economical, there needs to be at least an order of magnitude improvement in the cost of large batteries per lifetime kWh (where the lifetime kWh is the capacity of the battery multiplied by the number of charge cycles before the battery has to be replaced). The good news is that there doesn't seem to be any fundamental limitation to getting there--it's possible to build rechargeable batteries from relatively cheap and abundant raw materials. The bad news is that the cost of battery technology seems to be dropping only relatively slowly, and it will take a long time to cut the price by an order of magnitude without a major breakthrough.

Non-Chemical Energy Storage Media

There have also been a lot of novel energy storage approaches proposed, including:

  • Pumping water up a hill and using it to generate hydroelectricity
  • Filling giant underground caverns with compressed air
  • Using large banks of supercapacitors to store electricity
  • Spinning large flywheels

These techniques are certainly able to store energy and make it available on demand. Bringing them up to utility-scale (or even power-a-house scale) is a challenge, though. Pumping water and compressing air are both relatively inefficient and only work in certain geographical locations. Flywheels, compressed air, and supercapacitors have a safety issue, in that if Something Goes Wrong they can release a huge amount of energy uncontrollably fast (that is to say, they can explode). To my knowledge, none of these schemes has made it past small scale pilots, though they sound promising on paper.

Upconverting Excess Electricity to Fuel

One really intriguing approach is to find a chemical process which can be used to produce liquid fuel using electricity, and using the fuel produced to power vehicles or electric generators for times when the renewable power isn't available.

This is attractive for several reasons:

  • It turns excess renewable power into a valuable commodity
  • It allows renewable power for cars, trucks, and airplanes, where renewable power isn't really an option
  • Liquid fuels are easy to store and transport in large quantities, making it possible to use renewable power in times and places where it otherwise wouldn't be available
  • Power-to-fuel plants could be turned up or down as needed to absorb the excess electricity

If I had to guess, I would say that this is the approach most likely to win over the very long term (50+ years). There are a lot of people researching ideas in this space, but to my knowledge nobody has come up with something cheap enough at large scale. On the other hand, there are almost an infinite number of chemical possibilities, and the reward for cracking this puzzle will be immense.

Demand Shifting

The simplest and cheapest way to store power for later use is through demand shifting, adjusting when you use power to match when it's most readily available. One of the biggest consumers of power in a typical home is heating and cooling, including not just the home itself but also hot water, refrigerators, air conditioners, and so forth.

Heat (and cool) are fairly easy to store for up to a day or two. For example, thermal storage heaters (which have been available for decades) use off-peak electricity to heat up a pile of bricks, and then blow the heat into the room throughout the day as needed. Similarly, an off-peak hot water system can heat extra hot water when electricity is cheap for use at other time.

Along the same lines, freezers can get extra cold when there's cheap electricity available (so they don't have to run as much at other times), and an air conditioner could chill a pile of bricks or tank of water to make cool air available at other times.

Using tricks like this, it's probably possible to move 75% (or maybe more) of the electrical use of a typical American home to times when renewable power is available. Other appliances (clothes washers, phone chargers, etc.) can be programmed to mostly run when there's solar or wind.

The beauty of this approach is that it requires no new technology, and has the potential to dramatically increase the amount of our power consumption which could be met with solar or wind power. The downside is that it will require changes to almost any electrical device which can be demand-shifted, and a lot more intelligence in our power systems. But those changes can happen gradually.

It's not unreasonable to think that with aggressive demand-shifting and only a modest amount of battery storage (for lights, computers, and entertainment systems), a typical home could be built with solar power and be off-grid for close to the cost of grid power.

Are Utilities Anti-Solar?

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There's been a bunch of news articles recently about power companies coming into conflict with customers who install solar systems. In Hawaii, where solar power is substantially cheaper than the power company and has become very common, Hawaiian Electric Industries (the local utility) has stopped allowing some new solar systems to be connected to the grid. In Arizona, the power company lobbied (unsuccessfully) to start charging $600/year to customers who install solar. This Bloomberg article is a nice summary of what's been going on in both states.

It would be easy to conclude from this that power companies (or at least, the ones in Hawaii and Arizona) are against solar power. I think the reality is a lot more complicated: I think the power companies are not against solar power, but have let themselves get backed into a corner created by their business model, the net-metering laws in the U.S., and politics.

Traditionally, power companies have built and operated all aspects of the electrical system including power generation and distribution. As regulated monopolies (in the U.S.), power companies' prices are generally set by a governmental agency, which allows the utility to earn a specified return on equity. This formula is supposed to compensate the utility for spending the money to build the infrastructure and allow it a fair profit without taking advantage of its monopoly position.

This system mostly works, though it does have a few quirks. Because the utility's profits are based on the total investment, it's in the best interest of the utility to spend a lot of money on infrastructure and minimize operating costs. Buying power from a third party doesn't help the utility at all, since there's no money invested in that generating capacity. However, since most power companies' rates are directly set by the government--which is ultimately answerable to voters, who don't like to see their power bills go up--they have been somewhat restrained from simply building the most gold-plated power system possible.

Net metering has been around for about 30 years (Minnesota passed the first net metering law in 1983), and requires that utilities buy excess power generated by small customers. The details vary from state to state, but in Minnesota the requirement is that the power company pay full retail for the electricity it buys. In other words, your power bill is based on the "net" amount of power you bought from the utility, not the total amount you used.

Net metering was designed to encourage people to install small solar and wind power systems. It's effectively a subsidy for customers who might need to buy electricity at some times, but generate more power than they need at other times. It's a subsidy because retail electric rates combine the costs of both power generation and transmission into the price per kWh, and the net metering customer gets both the generation and the transmission costs netted out even though the customer is still using the grid to buy and sell electricity. Xcel Energy, our local power company, claims that 45% of our electricity costs are for transmission, so the net metering customer is effectively getting paid double the wholesale cost for excess power generated.

The beauty of net metering is that it encourages connecting small power sources to the grid (where the power can be used more efficiently) and appeals to everyone's sense of fairness. In fact, several power companies voluntarily started offering net metering back in the early 1980's before any states had passed laws requiring it. It has proven a very effective incentive for the adoption of solar power once the price of solar starts to get close to the retail price of electricity.

But as the cost of residential solar power has approached (and in some cases dropped below) the retail price of electricity, net metering has started to create problems for utilities. Net metering is only workable for the power company if a very small percentage of customers sell power back to the grid. If too many customers take advantage of net metering, the subsidy can start eating into the power company's profits (though the power companies prefer to say that "it's too expensive for the other customers," as though the net metering subsidy was somehow automatically added to other customers' bills). Too many net metering customers also takes a percentage of the generating capacity out of the control of the power company, which can create some real problems with keeping the grid functioning smoothly. Power grids, as implemented today, are simply not designed to account for thousands of small power plants constantly coming on- and off-line.

This is where the utilities start to get boxed in by the politics of the situation. Net metering is incredibly popular (at least among people who care about the politics of energy). It seems fair to the average consumer because the subsidy is well-hidden. And it is very effective at encouraging solar installations. But because the utilities can foresee a day when net metering and grid-tied solar will start causing them big problems, they want to get ahead of the issue.

Unfortunately, there is very little a power company can do to change net metering laws or put the brakes on solar installations without looking like the big bad bully out to squash the little guy and slow down the future of energy. And since power companies in the U.S. generally don't have the ability to set prices without government approval, it's going to be very hard for them to adjust to the new reality of widespread adoption of solar power.

At the end of the day, I don't think power companies are anti-solar. I think most power companies would be perfectly fine with generating a lot of their electricity from solar power, as long as they controlled the solar power plants and it was cost-competitive. But right now, solar is cost-effective at retail prices, and ordinary consumers are starting to adopt the technology en masse. This costs the power companies a lot of money and takes a lot of control out of their hands, and that's what they oppose.

What we need is a fundamental restructuring of our electricity markets, to create a system which is fair to everyone but still encourages people to invest in their own solar installations. This is something the power companies are going to fight, since it will likely take away a lot of their control and at least some of their profits. But the politics and the economics of the situation are against them long-term.

Solar Engineering

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We've begun the design process for our solar installation and it turns out to be a lot more complicated than I expected.

Solar cells are very simple devices: light shines on them, they produce a voltage, and you get power. So you would think that designing a solar system would mostly be a matter of deciding how many solar panels you want and where to put them, then plugging a bunch of cables together to hook it all up. Unfortunately, the solar industry is a long way away from that plug-and-play world.

Solar Modules

An individual solar cell is a few inches across and produces just a few watts of power at about 0.5 volts. Even a modest residential solar system will have over a thousand solar cells. To make everyone's life easier, a few dozen solar cells are packaged together in a weatherproof frame with a glass cover to make a module. The module wires together all the solar cells in series so that the module outputs anywhere from 80 to 350 watts at a respectable voltage.

For all the technology that goes into manufacturing solar cells, the module itself is pretty dumb. Nearly all modules just passively wire the cells together and output the resulting power as DC current on a pair of wires. As a result, the voltage and power output of a solar module will vary depending on many different factors, including the amount of light hitting the module, the temperature, and the resistance (load) of the attached circuit.

In order to get the best possible power output from a solar module, the load on the module needs to be constantly adjusted to maintain the optimal current and voltage. This is called Maximum Power Point Tracking, and it's usually the job of the inverter or a specialized device called a DC optimizer.

Inverters

The output of a solar module is not directly usable for most electrical needs. The module produces DC power at a voltage which varies constantly, and most electrical stuff needs AC power at a stable voltage (usually 110V in the U.S.). To make the solar power usable, you need an inverter to convert the DC to AC. The inverter is where most of the intelligence of the system lives: in addition to converting DC to AC power, the inverter will track the Maximum Power Point to optimize the output of the solar modules and monitor the health and output of the system.

If your solar system is connected to the electric grid (as most residential systems are these days), the inverter is the interface between the solar panels and the grid. The inverter will make sure the phase and frequency of your AC power matches the grid, and also shut off the solar if there's a power outage. Shutting off the solar in an outage is important because otherwise your solar system would be feeding power into a dead power grid, with the risk of electrocuting power line workers trying to repair the outage. Unfortunately, that means you can't use solar as backup power (without a fair amount of extra equipment and expense to provide the needed power isolation).

String Inverters vs. Microinverters

Traditionally, a series of solar modules would have their DC outputs wired together and brought into a single centralized inverter. The problem with this is that at any given moment, different modules in the string might have different maximum power points. For example, one module might be partly shaded while the others are in full sun. Or slight differences in manufacturing can lead to slightly different power outputs on the modules.

Since the inverter has to hold a single voltage and load for the entire string, this configuration will always cause some modules to produce less than their maximum power. You also have to make sure all modules in a string are the same model from the same manufacturer--no mixing and matching whatever is cheapest this week. However, since power inverters have traditionally been big and expensive, there's been no economical alternative.

In the past few years there's been a new approach. Instead of a string of modules connected to a central inverter, each module gets its own microinverter physically connected to the backside of the module. The AC output of the microinverters is connected together and wired into the grid.

As the name implies, each microinverter is small, with a capacity of a couple hundred watts instead of the kilowatts more typical for a string inverter. The price for a bunch of microinverters is in the same ballpark as the price for a single string inverter of similar capacity (or anyway, our solar contracter is charging us the same system price whether we go with microinverters or string inverters), and having the modules output grid-ready AC power simplifies some of the design and installation.

The big advantage of microinverters is that it allows each individual module to be held at its own maximum power point, yielding more power from the system as a whole. The manufacturer claims an increase of up to 10% total output over the course of the year, though that depends a lot on the details of the system. Microinverters help more when you have different shade conditions on different parts of the solar array, since a single shaded module in a string can pull down the power output of the whole string.

The biggest disadvantage of microinverters seems to be that there's only one major supplier of them, and because they're a relatively new product, some installers are not comfortable with them yet. String inverters have been in the field for decades and perform well, but microinverters only have a few years of field experience. My solar installer describes them as a bit on the cutting edge, since he's not yet confident they'll perform for the entire 30-year life of the system. For us, though, microinverters make a lot of sense because we will have significant shade issues in some corners of our array.

Mechanical

The solar modules have to be physically attached to whatever surface they will be mounted on (in our case, the roof). This attachment has to be strong enough to hold the weight of the system and keep it from blowing off in a storm. It has to be weathertight so the roof doesn't leak where the solar array is attached. And, ideally, it should be easy enough to install that the labor costs don't get out of hand.

One mechanical problem we won't have in Minnesota is making sure the roof is strong enough to support the weight of the solar arrays. Since our roofs are designed to hold a significant weight of snow and ice in the winter, any roof which was built to code should be strong enough for solar. I'm told this is not always the case in more southerly climates.

It's All Custom

Because every system is a unique combination of modules, inverters, and mechanical components, there's a fair amount of custom design for each installation. There's no question this drives costs up. One would think the industry would move towards "smart" modules with integrated microinverters, standardized connections, and a plug-and-play approach. The closest I've seen is Solarpod, which sells a "system in a box." Solarpod still relies on third-party modules and microinverters (as near as I can tell), rather than an integrated smart module.

Part of the problem is that electrical components which will be connected to the power grid need to be certified for safety, and the certification process is apparently slow and expensive. So where there are hundreds of different solar modules from dozens of manufacturers, and new modules coming on the market all the time, there are fewer companies making inverters (and only one major supplier of microinverters). And since the module manufacturers don't want to be slowed down by the certification process, it looks like for the foreseeable future we will be stuck with dumb modules and separate inverters.

I think the industry recognizes this as a problem. Many people in the solar business have spoken about driving the installed price of a complete solar system under $1/watt. That represents about a 65% to 75% decrease from today's prices. The solar modules are only around a quarter of the total system cost, so there needs to be a lot less expense in inverters, mounting, and installation labor. All that will require a less customized, more integrated approach than we have today.

(Thanks to Charlie Pickard of Aladdin Solar, who has been exceptionally patient with me in answering all my dumb questions.)

Snowblind

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The coldest days in Minnesota also tend to be the sunniest. Those blasts of air from the North Pole bring exceptionally clear weather, with the intensity of the sun making up (somewhat) for the shortness of the days.

Given that, and knowing that solar cells are more efficient when they're cold, you would think that winter in Minnesota would be pretty good for solar production. And it would be except for the snow. A few inches of snow on the solar panels will bring the production to zero, and this time of year we also usually have at least a few inches of snow on the roof pretty much all the time.

There are a lot of solar installations where you can go online and view the production data. This seems to be a common feature of solar monitoring systems, and many people make their systems public--it's sort of a social networking thing for energy nerds. It's cool because you can go online and find solar installations near you and see how much power people are generating.

It's also a little depressing, though, when the weather is intensely sunny but the nearby solar systems are completely dark. We had our first major snowstorm of the year about ten days ago, followed by an extended period of bright sunshine and extremely cold weather. All these snow-covered solar panels are losing a lot of power! I understand that November and December are the darkest months of the year, and the solar contractors take snow cover into account when calculating how much power you should expect to generate.

Nevertheless, it somehow feels wrong to let that much sun go to waste, even though we shouldn't be expecting to generate much power this time of year, and it's dangerous to get up on the roof to clean the panels.

So I've been thinking about ways to safely and cheaply remove snow from solar panels. The "cheap" is important because removing snow from the solar panels will probably only give an extra 10% generation over the year. It's not worth it to spend a lot of money for that amount of gain.

There are three basic approaches to removing snow and ice from a surface: mechanical, chemical, and thermal. It's not necessary to completely clean the solar panels, just get enough snow off so the dark surface can start absorbing light. The heat of the sun will do the rest--even with the most efficient solar panels, over half the sun's energy goes to heat the panel and not generate electricity. It might be good enough to leave up to an inch of snow and ice on the panels, if the intense sunlight defrosts the rest quickly enough. The slope of the roof and the slipperiness of the glass surface mean that if you can break the adhesion between the snow and the solar panel, it should mostly just tubmle off.

  1. Mechanical: Physically removing the snow from the panels is the simplest method, and could be as easy as brushing it off with a roof rake. That could work for the part of our system over the garage, which is relatively close to the ground, but the panels on the roof of the house will be three stories above ground level. Climbing up on a snow- and ice-covered roof is dangerous, so a useful mechanical system has to be something automated. I've seen systems with motorized pushers which move across the solar array, and they definitely work but are too expensive.
  2. Chemical: Spraying some sort of deicing fluid on the solar panels should break the bond between the snow and the glass and allow the snow to fall off. The problem here is finding an effective antifreeze which will be safe for both the solar panels and the environment. Salt is a bad idea, since you've got electrical systems involved. Hot water is also a bad idea, since it will freeze in the tubes and make it a one-use system. An alcohol-based system might be environmentally safe, but alcohol can be corrosive to a lot of stuff and it's not clear if it would damage the solar equipment. Sugar water should be safe for the equipment and the environment, but isn't that great as an antifreeze. Propylene glycol should be safe for the equipment, but maybe not the environment. And so on.
  3. Thermal: Heating the solar modules would certainly work and be environmentally safe. The problem is that it takes a lot of energy to melt snow and ice, and it's possible that it could take more energy to shed the snow than you would generate. Partly it comes down to whether you need to melt all the snow, or just a little bit to make it slide off. Also, this kind of defroster system would probably have to be built into the solar modules at the factory and bonded to the glass, so it's not something you could easily add on after the fact.

So for now, there's no obvious solution to snow on the solar panels other than the one which the professionals advise: wait for spring.

But at some point someone may invent a clever way to clean the solar array which is cheap, safe, and effective. When that happens, I'm guessing it will sell like crazy in these northern climates, just so we can avoid the heartbreak of seeing all those photons go to waste.

Our Solar Adventure Begins

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This week we signed a contract with a solar contractor to install a photovoltaic system on our home in 2014. Ideally it will be operational sometime in the spring, to take advantage of the peak generation through the summer.

Details are still being worked out, but right now the plan is to install 20 modules with a total capacity of 5.4kW. Half of these will be over the garage (facing Southeast), and half over the house (facing Southwest). This is not optimal placement, but it's not too bad. We expect this system will produce around 4,700 kWh/year of electricity, which is enough to offset about half our power consumption.

Our financial projection is that the system will break even after 15 years, and over the 30-year lifetime of the system it will generate an internal rate of return of about 5.7%. That return includes all the current solar incentives, namely the 30% federal tax credit and a production credit from Excel Energy expected to be $0.08/kWh for the first ten years.

It's worth noting that even without the incentive payments--which are substantial--the system would still generate a positive return over its 30-year life. In this scenario, our system would have a 30-year IRR of only about 1.9%. That's not really worth doing on a strictly financial basis (though I think it's still worth it for the environmental benefits); however, it's close enough that solar will pay for itself for people who pay a little more for electricity than we do, or who have somewhat better conditions for generating power.

The next steps will be to finalize the design and submit it to Xcel Energy for approval to participate in their solar program. Then we will be ready to install the system once the snow is off our roof in the spring. If all goes well, we could be starting work in April and be online shortly after.