47 views of scaffolding

I started off taking pictures of our house as they were building it. Then the scaffolding came to obscure my view.

In this pictues, you can see where they added an airtightness sheet around the upstairs floor, which becomes very difficult to make airtight after the walls have gone in, if your airtight layer is inside the structure.  

Then they started covering the scaffolding with sheets.

This happened all around the house.

Other, greater buildings are blighted with scaffolding. Here is an old building in Athens which I believe Victorian engineers tried very hard to "repair" by straightening all the lines on the structure. It turns out it had been carefully designed with non-straight lines so that it would look right.

I remember visiting Tokyo Disneyland and seeing the centrepiece, Cinderella Castle, surrounded with scaffolding, which had been carefully covered with sheets depicting the castle. I wasn't sure whether this made the building seem more real or more fake.

Here is another view of our house.

Changing groups

Time to shuffle the students a bit.

The lesson on windows went well, and I think the estimation of the room's window U value was not too overwhelming for the students. This was partly because I had structured the problem solving a bit more. Scaffolding is also very helpful in the construction of knowledge. Also I had them change groups at the beginning and tried to get mixed skill sets together.

I had put them into groups in week four and it's a good idea to change after two or three weeks have passed. The dynamics of groups have been characterised by the stages of Forming, Storming and Norming. After norming we hope for performing, but instead it can get boring! Changing groups every week is a bit too disruptive, but leaving the same groups for too long risks unfairness for people who have ended up in a dysfunctional group as well as a missed opportunity for having the students meet more people and make more friends.

The first time I put them into groups, I began by asking everyone these questions:

• What is your major?
• Are you good at maths?
• Are you good at English?
• Are you good at drawing?
• What is your favourite subject?

Then I asked them to make groups of four, with different majors, different favourite subjects, and new friends. I told them that maths was going to be useful, so if they weren't good at maths they should find someone who is. Also, if possible, I wanted different nationalities and mixed genders. The class is about 85% Japanese and 70% male, so this was not going to happen with every group.

In the first couple of weeks, and in previous years, I had tried to have Japanese-speaking and English-speaking groups, but last year I realised that resulted in me having a false sense of the English level of the room, and some parts completely lost. Spreading out the English speakers means they can work more to mediate between my English explanations and instructions, and the Japanese of the students who often have more interest in, and aptitude for, the topic.

Three weeks later they were still more or less in those groups of four, but a group of women had formed in the back corner of the class, and I'm sure the same couple of architecture students had been sitting next to each other every class. It's not really bad to sit next to the same person every week, but a changing environment is conducive to learning since memories are formed by connections and associations. Also they may meet some new people.

So I asked them, within their groups, to first decide who was best at writing. Next, I asked who was best at English. The writer then had to write down those names. Next I asked who was best at communicating. If it was their best English speaker, they should choose the next best English speaker as their English speaker. Finally I asked who was best at mathematics, and if it was their best communicator or best English speaker, they should choose a different best communicator or English speaker.

Then I shuffled the deck by having each writer stay put, each English speaker move around the class clockwise to the next group, the communicator move two groups clockwise, and the mathematician move one group anti-clockwise. I figured the mathematician would be able to handle the negative number. As usual, I had to do some traffic direction, partly because there was one group in the middle of the class, and it wasn't completely obvious which way was clockwise and which was anti-clockwise.

Now I had a high chance of diverse groups. They had all worked in different groups before and would hopefully bring the best experiences into the new group.

When it came to calculating the U value of the windows, I asked them to pick a leader, a designer, a calculator and a checker.


I reminded them of the problem solving steps:
1. Formulate problem, ideally drawing it!
2. Plan a strategy, making sure they write it down!
3. Find equations
4. Find data, but not until they had done the first three steps
5. Calculate
6. Check
7. Check again

After a while I reminded them about surface resistance, then gave them some equations, thermal conductivities and dimensions.

A little later, as I wandered the class looking at their calculations, I noticed a couple of U values of over thirty for the glazing, which looked way out. I went back to check and noticed I'd given them the wrong value for conductivity of air by a factor of ten. It should be 0.024 W/Km but I'd given them 0.24. A great example of how everyone makes mistakes, and how important checking is. Making mistakes is not a problem in itself—everyone does that!—you have to realise when you have made mistakes, and then fix them.

How big a battery would I need?

Our house produces more electricity than we use, so in theory it would be very easy to unplug from the grid and become self sufficient. We don't do this for three reasons:

First, being connected to the grid means that we have electricity when the sun is not shining, its rays are blocked by heavy clouds, or by snow on the roof. We don't need to worry about batteries or generators because the grid is our back-up power supply.

Second, since we produce more energy than we use, we can supply energy to the grid and contribute electricity to the community. We wanted the house to produce more energy than it consumes, and we like to feel that the extra energy is being used and making a difference.

Third, they pay us for any electricity that we supply. They pay us very well: about twice what we pay for day-time electricity and five times the amount for night-time electricity. This is similar to the second reason, since we can see from the negative bills that our electricity is making a difference. We can safely assume that there is more demand and less supply in the day time, so we are filling some kind of need by selling our electricity. It's less safe to assume that our electricity is worth twice as much as their electricity, and easier to see the feed-in-tariff as a boost to the solar industry. Even then it is probably a good thing as the renewable energy industry and its exploitation of a resource that literally falls from the sky still seems to be getting less subsidy than exploiting fossil fuel reserves, and if we are to transition from fossil fuels we will need solar panels.

Regardless of the politics, the highly tangible and easily countable financial considerations mean that we try to sell as much of our day-time electricity as possible, and use their night-time electricity instead. Looking just at energy use this is a bad idea. Our main power consumption is for heating water, which we mostly use in the evening. Currently we are heating hot water at night and it is sat in the tank steadily losing heat for most of the day. Also, the tank is heated by an atmospheric heat pump, getting heat from night-time air, which is colder than the daytime temperature by something like 10 degrees at any time of the year. If we were using electricity in the day time from our own panels, then the heat pump would do a lot less work to get the heat from the outside temperature up to the temperature in the water tank, and the hot water tank would be losing a lot less heat before we use it. This could save us as much as 25% of our electricity, but we don't do it because using our electricity in the day time is over 300% more expensive.

Our contract for selling electricity runs out after ten years and we certainly will not be able to get the same price, but it's not clear yet what the financial calculation will be. If we were to start using daytime electricity, we would also think about trying to use the hot air under the solar panels, which would be even hotter and need even less work to provide us with hot water, but that's another blog post.

Back to the question in the title: If we were to disconnect from the grid and wanted to get a battery to keep us in power, how big would the battery need to be? I have seven years of generation and consumption data to give me an answer.

When I said that we produced more power than we consumed, this has been true for every year and every month. The lowest producing month was October 2017 (670 kWh), which was the least sunny October since 1917 with only 100.9 hours of sunlight. September 2018 had even fewer hours of sunlight (94.4), but we made 800 kWh. That's the same as our highest monthly consumption, 800 kWh, in February 2013.

The longest period when generation stayed above consumption every day was 153 days from 20th April to 20th September, 2016.

The longest period where consumption stayed above generation was for five days between 12th and 17th October, 2017.

If we need a battery to cover all our energy needs, then it may be for these five days. In the simplest calculation, we need a battery of 33.1 kWh (the shortfall between the 70.5 kWh consumption over those five days and the 37.4 kWh generated). That's one or two Nissan Leafs.

There were five times when the consumption stayed above the generation for four days: from 14th January, 18th June, and 23rd October, 2013, from 6th September, 2015 and from 19th October, 2017. Many of these grey-outs are in September or October, when consumption is at its lowest. The snowy days in the middle of January 2013 were at a time of much higher consumption, and for those we would have needed to store 53 kWh to make up the gap between 61 kWh generate and 114.2 kWh consumed. The Teslas have 60kWh batteries.

Although the meteorological data confirms September and October as the months most prone to sunlight shortages, when the roof is covered with 22 cm of snow, our heating needs may also peak.

So the short answer is, we would need a 53kWh battery. Anything smaller and we are still going to need to rely on the grid and pay the monthly connection charge, or we would need some other backup, so the value of a smaller battery is limited.

Since most of our energy is for heating, it may make sense for us to look at storing heat rather than electricity. Phase-change materials may be useful for this.

Also, a more thorough answer would look at charging efficiency, discharging efficiency and electricity leakage. The figures above assume 100% of the electricity goes into the battery, 0% of the charge is lost over time, and 100% of the charge comes out.