Monday, 15 April 2019

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 literal 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.

Wednesday, 7 November 2018

Squaring the Circle for Traditional Buildings

It often seems that there is a battle going on between traditional building techniques and high-insulation high-airtightness approaches such as Passive House. Advocates and practitioners of traditional buildings have a strong case that years of experience will show how and when buildings fail, and how they can be built to last. They claim natural materials can absorb and release moisture and are free from dangerous chemicals, so they are better for the building and more healthy for the inhabitants.

But traditional buildings do not use a lot of insulation and are not airtight, so here are two questions: 
How do you keep a traditional Japanese building warm in the winter? 
How does ventilation work in traditional Japanese buildings to ensure good air quality?
I'll get to the answers soon.

High airtightness is sometimes achieved with synthetic membranes, but concrete, plaster on stone or brick, and oriented strand board (OSB) can also play a part in a building's airtight layer. Insulation materials are often polymer-based, especially where a high performance is needed. To get the same insulation as ten centimetres of top-grade foam, you need over 30 centimetres of thatch, over 80 centimetres of wood, a similar thickness of clay mixed with straw, or over two metres of rammed earth. Cellulose fibre insulation is better than all those traditional alternatives, but you would still need over twice the thickness to match foam.

It is interesting to note that a mixture of clay and straw has a similar insulation level to wood, which means that a structure of wooden posts and pillars filled with traditional walls may have an even layer of insulation, avoiding cold spots. But a typical passive house wall has something like ten times higher insulation than a traditionally-built house, so for those walls to perform in the same way, they would need to be ten times thicker.

So how do you keep a traditional Japanese building warm in the winter?
Short answer: You don't. 

When it's cold outside, it gets cold inside. The walls are porous so moisture does not tend to build up. If you want the house to be warm you have to start burning stuff. Today that stuff is usually fossil fuel, either directly, or indirectly with electricity generated from fossil fuels. So you certainly can build with traditional, natural materials, but the inhabitants are only going to be comfortable with a steady flow of un-traditional, unnatural fossil fuels. 

Traditional Japanese heating is with wood burnt in an irori open fire or charcoal smouldering under a kotatsu table heater. Irori are open fireplaces in the middle of the room. Traditional Japanese buildings don't have chimneys, so the smoke finds its way up though the house, killing any bugs on the way, and then out through the ample gaps in the structure.

Traditonal kotatsu burn charcoal in a small irori pit, with a table over the top covered in quilts and blankets. The kotatsu just provides a warm space to sit in rather than warming the whole building, which in some ways is a very efficient use of fuel. This 1820 woodblock by Eisen Keisai also hints at other ways couples kept warm on long winter nights. 

Today people do not want open fires because of the risk of the house burning down, and the increased soot and extra cleaning. Charcoal-burning kotatsu are also a carbon monoxide risk so modern kotatsu use electric heating elements. They are still occasionally fatal because of the heat shock when elderly people get in or out of them. Many people in Japan love their kotatsu, but if they start living in an insulated house, they do not miss them!

Most Japanese homes do not have any central heating system, often relying on kerosene fan heaters, electric carpets, or air conditioners in heating mode. Some houses have underfloor heating, but there are frequent stories of people who use it for one year, see the electricity bill, then never switch it on again. None of these heating techniques is traditional or natural. 

Wood burning stoves may be a more natural method, and cast iron stoves from New England or the west coast of Ireland do look very nice in Japanese houses. The rituals of preparing wood and the cleaning and maintenance may not suit everyone's lifestyle, the smoke may not please the neighbours, and unless the house is in the middle of a forest the source of wood may not be sustainable. An increase in wood-burning stoves has been blamed for poor air quality in London, and since London is not a major producer of wood, you also have to wonder about the carbon footprint of transporting the fuel. 

Wood pellets are much more efficient than burning wood directly, which not only means less wood, but also less ash to clear from the stove and less pollution going through the chimney. The first wood pellets were made from sawdust waste from timber mills. However, as demand increases, and efficiency leads to less waste, trees need to be specially cut and grown for wood pellets. Economically speaking, pellets may have started off being made from a waste product with zero cost, but as and demand increases, the price may go up. The impact is not zero and while burning wood pellets may be better than burning fossil fuels, they do not provide a solution to the world's energy problems, and whatever you are burning, it's still better to burn less. Ideally some of the trees in our dwindling forests will be left as habitat, and end up falling to the ground and emerging in a few millennia as a carbon source for future inhabitants of the planet. But I may be digressing from the topic of traditional buildings. On the other hand, preservation of the environment may be exactly what advocates of traditional building want. 

If I may return to more urgent matters of survival, when a building is airtight, it must be ventilated. The solution used in most passive houses is a mechanical ventilation system with heat recovery. Advocates of traditional building techniques often have a visceral reaction to the idea of mechanical ventilation as it is clearly not a traditional way to ventilate buildings. It uses electricity, so how could that ever be natural?

It is not natural. But what exactly does "natural" mean? When people call for natural materials, what are they asking for? Asbestos occurs naturally in the ground, but I'm guessing you wouldn't want that in your natural building! Polyethylene and polypropylene are completely synthetic and harmless to taste and touch.

If you really want nature, you should go and live outside. Buildings are not natural. Rather than asking a binary question whether specific materials or techniques are natural or not, we need to look at health, comfort and energy use, over the lifetime of the building and make the least bad decisions to get the best health and most comfort for the least energy use and lowest environmental impact.

So how do you ventilate a traditional building? 
I'm temped to say that you don't, but of course traditional buildings are ventilated—just not in a very systematic way. If there is a fire in the building then it is also working as a ventilation system by sending hot air up and out of the building while drawing air in through those thoughtfully provided gaps and porous surfaces. When there is no fire, air must find its way in and out through open windows and doors. The amount of natural ventilation then depends greatly on the outside temperature, wind speed and direction. So if a house is designed to always have fresh air, it will usually have too much ventilation. This will lead to uncomfortable drafts and a steady loss of heat. If it is designed to minimise drafts and heat loss, then there won't be enough ventilation for good air quality and control of moisture. 

The traditional builders will usually choose too much ventilation because that is the only way to guarantee there will be no moisture build up. So the house should not be airtight. If the builders do make the house airtight, they need to put in mechanical ventilation. They could ensure ventilation by providing a fire for you to keep stoked, but if they do that, they need to make sure there is no risk of carbon monoxide poisoning, which again will probably mean avoiding airtightness.

Mechanical ventilation does use electricity, but it provides fresh air, takes excess humidity out of the house, and keeps you warm very cheaply by recovering the heat from the expelled air. Heat recovery ventilation will only work if a building is airtight, making sure that air is coming in and out through the heat exchanger. Also, the insulation will only work effectively and without risk of condensation within the walls if the building is airtight. And if the building is airtight, active ventilation is needed because natural ventilation is unreliable.

Without active ventilation and airtightness, extra insulation is a risk as air leaking out of the house in winter drops in temperature and hits the dew point, producing condensation.

So the traditional builders are going to hand you a choice: 
Pay a lot for heating, or be cold. 

On the other hand, a traditional structure can be wrapped in an airtight insulating layer, and include a ventilation system. This will protect the structure and make it last longer, and will make it nice for the inhabitants, who probably do not want to live a traditional life that is not as comfortable and not as long.

In the fight for survival of traditional building, insulation, airtightness and active ventilation are not the enemy. They may be the saviour! 

Emissions from Wood:

Thursday, 11 October 2018

How to Solve Problems

Teaching is a constant learning process. At least it should be. One problem with being a teacher is that you often get into situations where you think you're right, which can make it difficult for you to change what you're doing. In the classical model of the teacher, you can be expected to be right in your knowledge, otherwise you wouldn't be there. But when it comes to how to share that knowledge, or in what order to present it, there is less clear right and wrong and just a whole range of choices.

I believe in the power of problem solving for teaching. This translates to a belief in the power of learners to solve problems, and for them to learn something in the process. The problem is, not all learners are good at solving problems, and many have been through educational systems where they have not been expected to solve them. At least not the kind of problems that I give them. 

So how do I solve this problem?

Given that I want to teach problem solving skills, I probably just have to be a lot more open and transparent about it. I have been mentioning a few things to the students in passing: like suggesting they draw diagrams to help them work out problems, or advising them to write their calculations out carefully and clearly on lots of paper so it's easy to go back later and see what they did. I need to be much more explicit about the steps of the problem solving process, and give them a bit more practice in each step rather than just throwing a problem at them and hoping they'll work it all out. Too often the problem I've been throwing at them is how to solve problems, which is way too abstract.

Here are some steps:
  • Formulate the problem
The first step is to work out exactly what the problem is. Draw a picture. Write down what you know. Draw another picture. Put question marks where you need to find an answer. 
  • Find solutions
Now that you know the problem, you can think about solutions. What strategies are available? Are there different ways to solve the problem. Make a list!
  • Choose a solution
Which is the best way to solve the problem? What are the steps? 
  • Prepare tools
If you are calculating, your main tools are equations. If you are using a computer, the tool is the software. You also needs data. There will be physical properties that need to be looked up from tables, some things may need to be measured. Some will need to be estimated.
  • Calculate
Use lots of paper. Avoid any shortcuts that will not be obvious to someone looking at the calculations later. If you miss out steps on paper, there's a higher chance you'll make a mistake.
  • Check the calculation
Ideally get someone else to check your calculation. It's often difficult to see your own mistakes.
  • Check the answer
Eyeball it. Compare it with your real world experience. So you calculated that this pencil weighs a million tonnes? Maybe you should think again.
  • Check the error
Answers in the real world are never perfect. Their accuracy depends on the accuracy of the numbers going in, and the accuracy of any equations you used. Know how wrong you are!

That's eight steps, and no fancy acronym to go with them. I can start building them into my lessons and watch what happens.

It's probably also worth talking about engineering problems and how they are different to the problems that come up in education. They have often been conditioned to find one correct answer, but over in the real world there is usually more than one answer, and more than one way of finding the answer. Good engineering will find the best solution to a problem, given a range of criteria. The most important considerations are often cost, safety and performance, and the best solution may be optimised between them. Cost itself can be in materials, equipment and construction processes. 

Of course one factor in this optimisation is the length of time the engineer spends on the problem itself, since engineers are a scarce resource and their time precious.

So I think I've written enough on this topic for now.

[Image taken from not sure where they got it from!]

Wednesday, 3 October 2018

Low Energy Building First Class Fact Checking

The first lesson has some background on the energy problem. I have the carbon graph, showing the emissions taking off around 1800, driven by a smooth exponential growth in coal production. Actually the coal was not really being produced—that happened back in the carboniferous period 300 million years ago—it was being moved around and then burnt. The graph shows oil starting, then gas a little later, each in its own exponential variant. Meanwhile, the emissions from coal have still been increasing. 

I've been saying "until this year" with a quizzical optimism, and finally it looks like "production" of coal is down, which is a sign that we will eventually burn less of it. 

This graph from BP is still pretty scary.  That gray coal line definitely seems to be getting thicker. Also alarming is the sandy bit at the bottom, which is biomass. Before coal that was the only source of heat, and it was more or less constant until the middle of the twentieth century. Now that is on the rise. I'm not sure how much is in industrial use of biomass, for example replacing coal in thermal power stations, and how much is domestic use from fuel-poor burning what they can find. 

If you look carefully you can see the thin yellow strip of renewables at the top, like a sprinkling of snow on a mountain top. While this has increased from its previous levels of invisible and insignificant, it is still a long way off replacing any of the behemoths beneath it.

It should be noted that while BP's data can probably be trusted, their main business is still in selling fossil fuels, and their business model is still based on selling more. The graph goes up to 2013. 

The next graph is from the International Energy Agency, and gives us hope that 2013 was around the high point of coal, with production in China and the OECD decreasing. It's tempting to see that as a peak, and look forward to a steady then rapid decline in coal extraction.

However, Dick Van Dyke nostalgia has been strong in the US, and production was up last year. So, once again, it's too early to tell.

I guess it depends on who wins between the people selling fossil fuels, and people promoting energy efficiency and renewable energy. 

While checking figures, I also revised the proportion of Japanese energy that is imported from 80% up to 90%. The lower figure was pre-Fukushima, which had got into my slides at the beginning, and I've now corrected several years late. (Japan was 20.2% energy self-sufficient in 2010, and 8.3% self sufficiency in 2016 according to METI.)

I had been telling student that Japanese houses use 30% of the country's total energy, while in fact its more accurate to say that buildings in Japan use around 30% of the total energy. 

Whichever way you look at it, the amount of energy imported can be reduced if we get serious about low energy buildings.

I also found some interesting changes in energy use, which I may need to mention some time, although should probably work out more carefully first. 

Between 1973 and 2015, residential energy use in Japan increased by 90%, office energy use increase by 140% and industrial energy reduced by 20%. 

I'm not sure to what extent this is a sign that houses and offices have become much less efficient, while industry has become more efficient, or whether it shows that Japanese industry is producing less, and people are spending more time in offices and more money on energy-consuming appliances in their houses. 

(Dick Van Dyke from trailer screenshot - Mary Poppins Trailer, Public Domain,

Wednesday, 26 September 2018

Low Energy Building Course: Every Tuesday Afternoon—Starts 2nd October

Not only can students at my university take the 15-week Low Energy Building course, it's also open to members of the public!

You can read the syllabus below. And find more information about other courses open to the public here.

See you in room 26 half past two!

・Students will learn how basic science affects buildings
・Students will learn how buildings affect the environment and how culture affects building practices
Buildings use over one third of all energy consumed in Japan, as in many other developed countries. In a world of increasing population and limited fossil fuel reserves, reduction in building energy consumption is important. As well as drastically reducing consumption, low energy buildings can be more comfortable, more healthy and less expensive over their lifetime.
This course will introduce students to the principles, the practicalities, and the future of low-energy building.
(2)授業の概要This course will show how simple scientific principles affect buildings, and how insulation, airtightness and good windows can lead to houses with very low energy consumption. We will see how the use of solar power can make buildings that produce energy. We will look at low-energy buildings around the world, including the German Passivhaus standard. We will also consider the design process, including compromise, optimisation and guesstimates.
(4)授業計画1. What is a low-energy building?
2. What is energy?
3. Insulation and thermal envelopes
4. Compound insulation and thermal bridges
5. Why do we feel hot or cold?
6. Air and water
7. Windows
8. Ventilation
9. Windows 2.0
10. Energy standards and low-energy building around the world
11. To zero energy and beyond: Buildings as solar generators
12. Passivhaus
13. Economics and ecology, embodied carbon and life cycle analysis
14. Presentations
15. Review

This plan may change to meet the needs of the class
(5)成績評価の方法Students must complete weekly online activities in eALPs to pass this course. Students will be expected to participate in class and give presentations.
Online quizzes: 80%
Online forums:  10%
Presentations: 10%
(6)成績評価の基準The university policy states that students need 60% to pass, 70% for a B, 80% for an A, and 90% for an S.
(7)事前事後学習の内容Additional information will be made available on eALPS.
(8)履修上の注意The class will mainly be conducted in English. It will be possible for students to ask questions, complete assignments and give presentations in Japanese.