A dead link

Chronically aware of my lack of posts in the first half of 2015, I've been thinking about adding links to other interesting websites, in the interest of at least posting something.

Instead of that, here is a link that you may not want to visit. It pretends to recommend ideas for recycling and repurposing, and there are one or two interesting ideas, but a lot of them are fairly ridiculous. Some ideas are using things that are not really rubbish. For example they find various uses for bulldog clips. I have never seen cause for throwing bulldog clips away, since they have several uses. The only situation you'd want to throw one away is if it was broken or had a part missing, but in all the ideas here, they need to work properly. 

Some of these don't look like you would need or want them. Would you really want an old bicycle as a basin stand? 

One of them I have tried and tested: producing guitar picks from old credit cards. In the picture, they seem to use a very fancy pick cutter, which I imagine would cost a few hundred times more than a pick. I used a pair of scissors. 

The site calls this "upcycling", which wikipedia defines as "the process of converting waste materials or useless products into new materials or products of better quality or for better environmental value."

http://twistedsifter.com/2012/06/creative-ways-to-repurpose-reuse-and-upcycle-old-things/

A lot of the ideas may be more usefully called "downcycling", since they are converting perfectly good materials and useful products into uses for which they may be less suited, and which will use more envrionmental resources.

The changing cost of solar panels

The LED shop sends me emails from time to time. Usually when I buy stuff online I make sure to switch off all the email notifications, but for some reason I didn't when I got the LEDs for our house, and I have never unsubscribed from their mail magazine. It's not too frequent and is a regular reminder of how much LED lights cost. The last message also had a solar panel, at 25,000 yen for 200 Watts. This seems pretty reasonable and made me wonder about grid parity, and the march of technology.

​Solar panel costs in the US have fallen by a factor of a couple of hundred since the 1970s. That was when Jimmy Carter put panels on the White House roof, and set a target for 20% of US energy to come from renewables by the turn of the century.

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The panels were thermal, rather than photovoltaic, and Carter predicted they would still be producing cheap clean energy in the year 2000.

And they would have been, too, if it hadn't been for Ronald "Trees cause more pollution than automobiles do" Reagan. He quietly removed them in 1986, as he was removing department of energy research into clean energy and removing subsidies for wind and solar power.

This was after receiving 270,000 dollars in 1981 from oil executives in appreciation for deregulation of oil prices, estimated worth 2 billion dollars to the oil industry.

The target to get 20% of energy in the US from renewables would have been met if research and subsidies had carried on. Instead, the research and development has gone into adding power to the energy industry.  

Carter said “a generation from now, this solar heater can either be a curiosity, a museum piece, an example of a road not taken, or it can be a small part of one of the greatest and most exciting adventures ever undertaken by the American people; harnessing the power of the Sun to enrich our lives as we move away from our crippling dependence on foreign oil.” And as it turns out some of them are in museums: the Smithsonian, the Carter Library and the Solar Science and Technology Museum in Solar Valley, Dezhou, China.

New York Times (1981) http://www.nytimes.com/1981/03/26/us/reagan-aide-sought-funds-from-oilmen.html
Scientific American (2010) http://www.scientificamerican.com/article/carter-white-house-solar-panel-array/

"Trees cause more pollution than automobiles do" - Ronald Reagan, 1981

A roof over our heads

We've now paid off the two-year loan for the solar panels. That means that we own the roof over our heads. This is a great thing. We still don't own the walls or the land beneath us, as those are long-term low-interest loans, but at least we own the roof.

Perhaps we should have put the panels in with the builder's contract, but it seemed to make the financing easier to pay some of this up front and get a separate loan for the rest of it. The rate was higher, but since we paid it back in two years rather than thirty-five, the total cost of the loan was a lot less. The first thing the bank advised us when we put the loan application in was to cut the costs by taking some of the panels off the roof. This is strange because it was about the same time they published this report in Japanese which seems positive towards domestic solar.

Over the year and a half of generation, we've earned 47,000 yen per month on average, and paid 7,500 yen for our electricity bill. We sold 89% of what we generated, so without our panels we would have paid another couple of thousand yen on the electricity bills. 

At this rate we'll pay back the investment on the panels in around eight and a half years. I'm not sure who else the bank is lending to, but a return on investments in under nine years seems fairly healthy, and I really don't know why they aren't insisting that all houses they finance put panels on the roof, even offering to fund them in return for the electricity companies paying directly to the bank to repay them. 

In terms of kWh we've generated an average 36 kWh per day, which is a little over twice the 16 kWH we use. In terms of the amount of electricity we are getting for each kilowatt of solar panel we have installed, that's 1450 kWh/kW per year or 4 kWh/kW per day. 

I'm not exactly sure how much of the cost we can attribute to the roof and how much to the panels. I have a back-of-the-envelope estimate from the architect, printed out on an undated piece of A4 with some of the figures to the nearest yen, and some to the nearest 10,000, which compares the option we took with a conventional roof and solar panels installed on top. It compares an older quote from Rooftech for an integrated roof system of 4.44 kW (actually written as kWh) which was around 3 million yen plus an estimate of 1.35 million for the roof work needed underneath their roof, which ends up as 4.35 million; 4.04 million after getting the grant. The other quote was 2.4 million for 4.81 kW of panels, with a roof estimated at 1.81 million coming to 4.21 million; 3.87 million after the grant.

In other words, the integrated roof and panel system was more expensive than installing panels on a conventional roof, but only about 3 or 4 percent. It should be added that this was for around half the roof area covered with solar panels, and it's not clear what kind of roof the alternative was. Anyway, we chose to go for the integrated roof because it seemed well worth the potentially slightly extra cost for the simpler design elegance. 

More revelations from a snowy roof

Another snowy night, and around 20 cm on the roof this morning. This made my trip to the balcony rather exciting, although I was reassured as there was also 20 cm of snow on the terrace below to break my fall.

The sun was trying to poke its head through the clouds soon after it climbed over the mountains, so I cleared the bottom row of panels on the roof before breakfast, then waited for something to happen. There was no generation for quite some time, even when the sky was the blue, and as the sun was out and climbing. By 9:30 large drifts of snow had started sliding off the roof and crashing into the garden. There was still no electrical action, so I went out to look at the roof and around a third of it was clear of snow. Then I went up to the loft to see what the two power conditioners were doing. They weren't doing a lot. I tried switching them off and on again. I think this may have done something but perhaps it was just a coincidence.

I went back up there a little later and the one on the left was generating 0.22 kW while the one on the right was generating over 2. I had assumed that these power conditioners each dealt with half the panels on the roof, the one of the left dealing with the panels on the left, and the one on the right the panels on the right. The roof has 48 panels, Eight high and six wide. The connectors are at the top and bottom, so it makes most sense to connect them in series vertically. I remember they were talking about connecting the panels up in sixes, although I can't find any written evidence of this now. From what I remember, the bottom six panels of each row were connected vertically, then the top two rows of panels were connected in two arrays three wide and two high. A total of eight sets of six panels.

My only explanation for the difference in the two power conditioners is that the top two panels were both connected to the left power conditioner, leading to a lower generation for the panels at the top that were still covered in snow since the snow was falling from the bottom of the roof. Once all the snow melted from the roof, and the generation was higher, the power conditioners were generating around the same amount.

I have noticed that the two power conditioners generated different amounts. This can be explained by three things: differences in the performance of each panel, different lengths of wire and different temperatures.

Each panel produces a slightly different amount of electricity. Although they are rated at 190 watts, they vary in power between 190 and 200. In theory, if all the low generators are in one circuit and the high generators are in the other, there could be 5 Watts difference between the power going into the left power conditioner and the right one, but it's much more likely that the difference will be small and no more than 1 or 2 watts. Even if it is 5 Watts difference, that's only 0.05% out of the 9.12 kW.

The wires to the bottom of the panel are longer. Also, and conversely, there will be more wire in a straight vertical array of six, than an array two high and three wide. More wire means more resistance. Power loss is the resistance times the square of the current. The maximum current of the panels is rated at 5.62 Amps. I'm not sure what gauge they used, but if they used the sums in this EcoWho solar wire sizing calculator , they will have come up with a AWG 12, 2 millimetres gauge, 3 square mm, which according to the Engineering toolbox has a resistance of about 5 milliohms per metre. This would lose 0.17 Watts per metre. Altogether, the panels lower on the roof may have 10 metres more cable, then that's a 1.7 watt difference. This is going to be around 0.05% too.

Since air is flowing under the panels, taking heat off each one, the temperature of the panels at the top of the roof is going to be higher than those at the bottom. The air temperature in the channel gets over 60 degrees in the summer, and there could be a 5 or even 10 degree difference in the panel temperature. Since electrical efficiency drops by around 1% every degree or two, this could drop the output by a 1 or 2% for the circuit with the top arrays compared to the bottom.

Now I don't have any live data for the different generation going through each of each power conditioner, but if you press the right button, rather than the on/off switch, they display the cumulated generation, which is 7,198 kWh for the one on the left, and 7,262 for the one on the right. This is a difference of around 0.8%, so it looks like they connected the two top arrays into the same power conditioner. I suppose the odds of this were even if they had stuck the wires in at random.

When we were talking about the connection of panels I had tried to encourage some kind of optimisation in shortening the wires as much as possible, but the eyes of the contractor started to glaze over and they brushed away my suggestions of optimisation over the next half century of generation in favour of being able to make things easy for the afternoon they were clambering around on my roof.

Knowing what I now know about the rating of the power conditioners being an absolute limit rather than a rough level, so we lose power when the panels get any where near their maximum output, I would have probably pushed more strongly for six vertical arrays of eight panels.

The Power Conditioners will take up to 370 Volts and 24.5 Amps, although it's rated at 250 V, at which it is most efficient. Its maximum power is 4kW at 30 degrees C, and 3.2 kW at 40 degrees C.

The panel maximum voltage is 36.6 Volts. So you could put 10 in series and the voltage would still be under 370 volts. Six panels is only going to be 220 V; less than the rating. Eight panels would be 290 V if they are all producing their maximum voltage. Of course, they're not always going to be producing their maximum voltage, so that's likely to come out around 250 Volts. So I'm not sure why they were putting them together in sixes.

Six circuits of eight rather than eight circuits of six would have reduced the amount of wire on the roof by about 20 metres, which would account for about 0.1% of the power. This doesn't sound a lot, but when you multiply by 50,000 yen, it's one lunch per month. It would also have made the fitting substantially easier, but perhaps they had a good reason for doing it in that way. I can't see any charge per metre of wire used on the invoice, so that's not it.

Maybe when the ten year contract with the electricity company runs out and we look at alternative heat generation, we can fix the wiring on the panels at the same time.

Another possibly meaningless experiment in solar snow clearing

There was about a centimetre of snow outside this morning, making the ground crisp and clear beneath the blue sky. I postulated that there would be a coating on the roof too. This postulation, at least, was correct.

I also postulated that clearing this snow off the roof would increase the generation, and sure enough it did. Before I cleared the snow, it was generating 1.5 kW, which is not bad for 8 o'clock on a winter morning. I cleared the bottom  row of panels of its thin covering of snow, and it went up to 1.8 kW, an increase of 300 watts. The array is 8 x 6, so I'd cleared 1/6, which presumably had been generating 250 watts before, more than doubling when I removed the snow. Another way of looking at it is that the panels generate a little less than half as much electricity when covered with a thin layer of snow.

They will also be absorbing half as much heat, so, as before, clearing these bottom panels speeds up the clearing of the whole roof. Ten minutes later, we were generating over 5 kW.

One interesting thing was that the cosmetic panels, which run up and down each side of the roof to make up the difference between the width of the roof and the dimensions of the panels, were already completely clear of snow from the melting effect of the sun. 

I should probably warn you not to try clearing snow off your solar panels yourself. The only reason it's easy and relatively safe in our house is that we have a balcony running along the south side of the house, so I can step onto it from upstairs and easily reach the roof from there with a brush. The biggest danger is bits of snow falling down my neck.

Cost of solar panels in the US drops by 80% in five years

Yes, that means it was five times more expensive five years ago. And apparently there is 14 times more installed solar generating power since 2007. That sounds much more impressive as 1400%. Interesting how percentages and rates seem different. 

I learnt this from Proud Green Home who were announcing a report "Tracking the Sun" from Lawrence Berkeley National Labs. Berkeley labs have apparently won 13 nobel prizes, so I'm reluctant to doubt their science, and there should be no need to worry that this wishy-washy propaganda from the solar lobby. They do have an infographic though.

Apparently, "The report also finds that the installed price of residential PV systems on new homes has generally been significantly lower than the price of similarly sized systems installed as retrofits to existing homes, that building integrated PV systems have generally been higher priced than rack-mounted systems, and that systems installed on tax-exempt customer sites have generally been priced higher than those installed at residential and for-profit commercial customer sites." So, in terms of cost, we got two out of three right.  

I still think whole-roof solar panels are a better bet, aesthetically, when the south facing roof of an average house is going to produce around 10 kW. 

But this is not America, and I'm not sure how this translates to Japan. Costs have certainly been trending downwards, but a number of factors probably mean that the reductions are more modest. For a start, Japan has been doing solar for longer, so many of the US gains are probably comparing the frontier times of solar cowboys with developed businesses. Also, Japan has a lot more protection in its markets, so to a large extent costs depend on what the local solar giants are charging. Having said this, our panels came from China, so the Japanese companies do not have a stanglehold. But we did not get Turkish hybrid PVT panels because installation grants were not available and we wouldn't have been able to sell them into the grid, because the company in Turkey had not paid the millions of yen demanded for a license to sell in Japan.

One thing that is difficult to find in the report, although is perhaps somewhere in the small print, is the percentage of total solar generation that this represents. In the absence of a number, I suspect is it a very small drop in a large ocean, which is dirty and getting gradually warmer with fossil fuel emmisions, and glowing slightly from nuclear radiation. At least the solar drop is relatively clean and growing.

Let it snow!

I love snow.

In England when it snows everything stops. People miss school and don't go to work. Traffic systems close down and people can't get to the airport to go on their skiing holidays. Sometimes power cables are brought down and the food in people's freezers is spoiled.

Here in Japan people are used to snow. Matsumoto is not in Snow Country, but is surrounded by it. The snowiest city in the world is apparently Aomori, where it comes in from the sea like waves over the winter. In Hokkaido they have a different front door upstairs, and neighbourhood battles go on as people clear snow into each other's parking spaces as soon as they vacate them.

Clearing snow here, for the most part seems to me a waste of time, although everyone gets very busy doing it. Sooner or later it's going to melt, so why bother? While it's snowing, you can walk on it or drive in it as everyone changes into snow tires from some time in early November when the first hint of white hits the highest mountain peaks. I don't really like driving in snow, so I don't unless I really have to. Like if I'm going skiing. Usually I don't really have to drive. I cycle to work, but can walk or get a bus if necessary.

As a house owner, my view of snow should have changed a little from this childish enthusiasm, but it hasn't yet. My house is effectively at the end of a cul-de-sac, so I don't have any responsibility to clear the thoroughfare in front of it. The biggest worry is the roof and the balcony on the south. Snow is rather heavy. It may be much lighter than water, but you don't get fifty centimetres of water building up all across the top of a house. Actually we don't get snow building up over our roof either, so far. It slides off the southern solar roof well. Some of it does hit the balcony on the way down making a ridge there that seems to sit happily, although most of if falls straight onto the terrace below. Probably a good idea to clear that off in the interest of balcony longevity, but it is not doing too much harm, and we wouldn't need to worry if it snowed while we were out of the country. 

Snow is more likely to stay on the north roof as it's shallower, and part of it is in the shade. Snow goes through a melting freezing cycle, and bits of ice come crashing down to where we keep the bicycles. 

Obviously snow impedes the performance of the solar panels. When the roof panels are covered, not much sunlight gets through, although of course the radiation that does get through heats the panels, melts the snow on top of it, and helps the snow slide off. Also, when it's actually snowing, few rays get through the clouds and snow. Even so, it was producing something while it was snowing. Admittedly only a hundred watts or so--in the battery charger ball park--and half the 200 watts that the house seems to consume even when everything is switched off. Over a day when it snowed pretty much all the time, we still made 2.3 kWh, which is about 10% of what we used. 

I was hoping we'd hit 1000 kWh within the first month of living in the house, but it's snowing again today, so it's looking unlikely.

More mind-numbing numbers about solar panels

I was thinking of using all the bits of paper we've received somewhere in the house. They could wall paper most of it... Probably two layers in some places.

Of course some of the information has come electronically.

One of the more interesting files has a list of the power ratings for each panel. Although they are nominally rated at 190 Watts each, the test results for each panel all come over 190, and average 195. So while the array is rated at 9.12 kW, it's actually 9.36. All extra kilowatts over their lifetime. The panels are guaranteed for 25 years. More specifically, at 95% for 5 years, 90% for 12 years, 85% for 18 years and 80% for 25 years, so I guess their lifetime will pretty much correspond to the rest of my lifetime. Anything we get out of them (or me) after another quarter century is going to be a bonus. Unless something drastic happens, the panels will still be there producing electricity long after I've stopped consuming it.

Keeping those panels cool

The people from Caname, the roof makers, and Rooftech, the roofers, came to visit the other day. My concern was with the air channel under the panels, which seems to me to be just too small. I've started measuring the temperature of the air coming out of the top, and it was getting up to 70 degrees centigrade. You can see a graph of the temperatures below, showing also for references the ambient temperature, the temperature inside the house and the temperature at the bottom of the slab, which comes pretty much to a straight line. The heat of the actual panels is going to be more than the temperature of air in the channel. 

So what? I hear you ask. 

I can hear some of you replying that solar panels produce less electricity as they get hotter. With the Suntech STP190S-24/Ad+ panels we are using, the efficiency drops half a percent with each one degree increase in temperature. 

As a thermodynamic system, incoming heat is beating down in solar radiation. This heat is lost in four ways: the panels are losing some heat to the wind from the top of the panels by convection, they are losing some heat by convection to air passing through the channel between the panels and the roof, they are losing some heat that is converted to electricity and they are losing some through the top by radiation. A small amount of heat will be conducted from the panels to the roof, but the roof is well insulated, so this heat is not really going to go anywhere very quickly. Heat that is not lost will make the panels hotter, and the efficiency will go down.

Directly fitting panels onto roofs as solar tiles is a bad idea because of this heat loss. Even conventional arrays that have been added onto a roof with a gap underneath will suffer efficiency loss of around 20% in the summer. 

Caname have done some research with panels fitted directly onto a roof with no air channel, some fitted in the conventional way, where air can flow north to south and east to west, and some fitted in their roof system where air only flows south to north. Their results over a year peg the conventional roof at 100%, find that fitting the panels directly to the roof with no gap drops to around 80%, while their roof is 99.5%. In terms of average panel temperatures, the conventional panels and their panels averaged around 65 degrees, while the tile style was over 70.   

In my opinion, rather than treating the conventional way as 100%, they should be treating ideal output as 100%, and ideal output means either full rating of the panels, or the projected power output at ambient temperature, in other words with perfect cooling.

The fact that they are building a roof system should mean that they can do better than panels that are added to an existing roof. Setting this as their target seems to be aiming too low. For example, they mount the panels on corrugated steel. Corrugated steel roofing may sound like a really bad idea, but it should work very effectively to cool the panels, both by channeling the air from bottom to top of the roof, and by increasing the surface area to conducting the heat from the panels.

Sparks start to fly

The electrician turned up at the building site on Friday. I asked about getting the solar panels connected to the power conditioners so that it would be possible to use the free electricity I've paid a lot to get, rather than the builders having to pay for electricity brought to the site. He proceeded to give reasons why this was not a good idea, which mostly annoyed rather than illuminated. I'm already more or less resigned to the fact that the panels will be sat on the roof soaking up a whole summer's sun from the beginning of June, and I won't be able to sell any to the electricity company until I move in and start a contract with them in October.

Actually, first I asked what would happen in the event of a power cut, and he said that, in the case of an earthquake or disaster, a signal would be sent from the electricity company to switch off the power conditioners. This was also a bit of shock (though fortunately not an electric shock) but seems sensible as there may be a loose wire somewhere after an earthquake and it's best to shut off the power. A plug is available on each power conditioner for emergency use.  

Anyway, the first reason he gave for not connecting the panels was that it would be dangerous. Apparently there's a lot of electricity in those wires, and it would be dangerous to connect them. Somebody might die. The panels are there soaking up sunlight now, sending frustrated electrons and holes in opposite directions, but destined never to meet on the other side of a circuit. So, I asked, if the wires are going to be connected to a power conditioner, then what's the difference? 

But somebody might cut the wires. 

Well, surely there's a lot of electricity in the wires now anyway, and somebody might cut the wires now? 

Next, he seemed to think there wouldn't be enough electricity. It's rated at over 9 kilowatts. If it's running at half power, that's still over 4 kilowatts. At 100 volts, that's 40 amps. I know they're running a workshop, but that seems like quite a lot of electricity. As long as there's more coming into the power conditioner than going out, it shouldn't cause a problem. Well, electricians like to work at night time. 

Couldn't they bring a torch?

This makes me wonder how much the idea of a low-energy house has got through to the people making it. 

By the way, he told me, it didn't make any difference to the electricity bill how much they used. There was a standard rate, paid regardless of how much power was used.

But, actually, really, the main reason for not connecting the power conditioners was that they didn't want to start using the them as they will start wearing out. They want to hand everything over in pristine order. Also, they didn't want the power conditioner to get damaged. If they put it in place, there's a chance that it'll be damaged by the workmen. 

In and among, they were talking about new government plans for houseowners with solar panels to be able to sell all the electricity they produce, at the premium rate, and buy all the electricity they use at lower rates. One argument they had against this was that it would not encourage people to save electricity. The current system means that you're much better off selling your electricity than using it, so people will generally try to switch things off, especially during the day when electricity demand is higher. 

I may be over sensitive or paranoid, but I can't help feeling from their general tone that they think all this energy saving and solar power is complete nonsense, and they may of course be right, but more about that another day. 

I was looking, later, at the 230 watt incandescent bulb that the carpenter has brought to illuminate the workspace. Just wondering what that was doing in a house that's trying to reduce carbon emissions. This one bulb will probably use more electricity than all the lights in the completed house. It would be a great heater on cold winter nights. I'm sure the carpenter has heard of low energy lighting, but it comes down to economics. This kind of bulb is cheaper to buy, and he doesn't have to pay for the electricity. The builders pay for the electricity and the carpenter is just their subcontractor. In fact not even the builders really pay for the electricity, because there's a flat rate regardless of how much is used. 

Energy efficiency is not just a question of technology. Energy efficient technology must be used. In fact, that may not lead to energy efficiency; the important thing is that energy-inefficient technology is not used. Even energy efficient technology should be used as little as possible. These are not questions of technology but of design, economics and politics. The technology is actually not so difficult! Anyway, it may be a little optimistic to think that energy efficiency is going to reduce energy consumption.

Roof going round in circles

The foundation is racing ahead, but it's still not clear what is happening on the roof.  This needs to be decided so that the order can go to pre-cut the wood, and until that happens there will be no pillars, which will be needed to hold the roof up.

The South-facing roof is solar. Until recently solar roofs meant building a normal roof (compliant with the regulations for fireproofing and waterproofing) and then solar installers coming along and drilling holes in it to fit on their panels. Drilling holes is fair enough for retro-fits, but for new builds this seems foolish, especially if you're trying to get a highly insulated, airtight roof. Using a different construction does not meet their installation requirements and you lose their guarantee, and the panels themselves don't qualify as roofing. 

Last July one roof maker, Caname, who started out making roofs for temples, introduced a solar roof that meets the regulations for roofing, so there is no need for a double roof and, at least in terms of design, a more elegant solution is possible. Economically this should also make sense, but design simplicity does not always correlate with economics.

So, all we have to do it build up to the rafters, then Kaname will put their roof on top of it.  As long as they've got something they can put a waterproof sheet over, and into which they can screw the corrugated steel roofing which they mount the panels onto, everything will be fine.  The Kaname construction allows air flow under the panels, which is important to keep temperature down on the panels, which keeps efficiency up. Corrugated steel should work well for this, increasing the surface area and taking heat away by convection and conduction.

It's usually practice in Japanese building to have an air gap between the insulation layer and the external wall or roof.  As Kaname are putting corrugated steel on top, as well as providing air flow for the panels, this should also provide an air gap for the insulation. The reason for the air gap is in case humidity builds up and needs somewhere to go. Not everyone believes this is a good idea, and any gaps potentially can attract insects or even bats. The practice seems to have evolved to cover up for any problems with humidity, although, in theory at least, a well designed wall should not allow humidity to build up to turn to condensation. Also in theory, the gaps around the corrugated sheet should work both ways, but I've been battling both the roofer and the insulator (two different contracters) who seem keen to have things done their own way, and are looking at an assembly of parts rather than the whole, which was the main reason why Kaname seemed so appealing. 

However, there's also the west wall. In our wisdom http://minuszeroeco.blogspot.com/2010/04/west-wall.html, we decided that the house should not be square.  As the plot is not square, and non-square rooms do interesting things with space perception, this seems to be the correct decision long term, but when it comes to this issue, (and no doubt countless others that will emerge) it's causing problems. Kaname only make square roofs, so there is going to be a one metre overhand at the South west corner that needs to be supported somehow. 

One possibility is to get the rafters to stick out of the side of the house to support the overhand.  There are two problems here. In the original plan the rafters run up and down, north-south. Changing all of the rafters to run side to side, east-west seems to have been too difficult for the architect to adapt to without changing the whole structure. More seriously the rafters sticking out will lead to thermal losses and thermal bridge effects, sucking the heat out of the house. 

So we reached a plan to put the waterproof roofing sheet on top of the rafters (where the insulation layer ends) then put horizontal beams on top of that, which can stick out to the west and take the load of the overhanging corner. The panels need to be fitted with screws at horizontal intervals of 160mm and vertical intervals of 830 mm. Horizontal beams can cope with this, but if they are mounted horizontally, there are fears of beams twisting and the roof rolling off. The thinner the beams are, the less of a problem this is, but the beams need to all be the same thickness as the solar roof is flat, and they need to be thick enough to support the overhang. Extra beams can be added to the west to make this stronger. 

Another problem is that the solar roof installers need to add the roofing sheet, which would mean them making two trips, with some carpentry in between, rather than just coming and doing the whole job in one day. It's also not clear how they would feel about people making holes in the waterproof layer. 

The builder seemed much happier with vertical beams and some construction board mounted on top, although that will mean another air space and another layer of tyvec or some barrier sheet to stop any moisture that gets in there.  They pointed out that this would stop overheating in the summer, but we already have solar panels on the roof, and almost half a metre of insulation, so over-engineering is a much bigger concern to me than overheating!

So far all attempts to keep the design simple seem to be riddled with complications, and "simple" means very different things to different people!