Batteries

One of the running costs for the house is batteries for the thermometers. There are 16 thermometers around the house with data loggers collecting their temperature all the time. Each one has a battery in. They are lithium, not sure of the original serial number but I've been replacing them with CR2s, since they sell them at the local electrical shop. Two for a thousand yen.

The thermometers are not essential to the running of the house, but useful in seeing how heat is flowing around it.

The batteries seem to last around six months, but I had to change a lot of them recently and I wondered, why not rechargables? I should have thought of it much earlier, but they didn't sell them at the electrical shop so I hadn't pursued it. 

I found two types on the net. One was selling with a charger, so I got that. Unfortunatlely the batteries didn't seem to work very well. The data loggers came on when I put them in, but were still flashing "low battery". Not wanting to give up straight away, I ordered a couple of the other type, Ultrafire, that did not come with a charger. These were a little over half the price of the CR2s from the elctrical shop.

The Ultrafire batteries are 800 mAh, while the other ones were only 250. Not necessarily bad batteries but just smaller capacity. The voltage of batteries drops steadily as they loose charge, so a high capacity 3 volt battery may start at about 3.6 while a low capacity one just around 3. The data loggers, like many other semi-sentient electronic devices, judge the battery life by measuring its voltage.

The Ultrafire CR2s are still working a couple of months later, and I just ordered another four, in anticipation of having to change some batteries when I did my monthly data download. In the event, none of the batteries ran out this time, so I still have four ready for next time, and over the next six months I'll need to get 10 more so I can replace all of them.

The site I bought them from on Rakuten only ever says they have one or two packs of two in stock, although this may be because people usually only want one or two for their camera and a spare, so I'll have to ask them to let me order 5 packs.

Heat pumps from cold night air

The bath talks to us.

We press a button or set the timer, and a little later it tells us when the bath is ready. If the bath's empty it's easy. It knows how much water is in the tank, it knows how hot it is, and it knows how much needs to go into the bath. If the bath has some water in it, then it must decide how much water needs adding and how much to heat up the water. Either it can add water from the tank, which will be relatively hot, or it can send water from the bath through the tank, from which it will take heat, in accordance with the second law of thermodynamics. Adding hot water is a lot more effective than circulating the existing water since the exchange has inefficiencies and there are heat losses as the water goes through the pipes between bath and boiler, even though we insulated them and kept them as short as possible. This is where the problems start.

Sometimes, there is not really enough heat in the tank to effectively heat up the bath. The bath brain probably starts off by adding hot water to the bath, then thinks that it's still not hot enough, so it starts circulating the heat to get it warmer. What it doesn't realise is that the heat from the boiler isn't going to get to the bath, or the part of the tank where the pipes from the bath are circulating may not be as hot as the part with the thermometer in. Rather than heat going into the bath, it may start leaving the bath, and dissipating into the larger thermal system following the inevitable fate of entropy. Then the bath's brain thinks "sod it", and gives up, without saying anything.

The bath talks to us, but it doesn't know how to say sorry.

This lukewarm bath situation is most likely to happen after a very cold night, when the heat pump is so inefficient that the energy would have been better spent drilling for oil in the Japan Sea. Obviously, I could just switch the boiler to be on all the time, or to "o-makase" (trust me) mode and let it switch on when it's warmer in the day time, but I don't really trust it. 

I would like to change the heating system so that the atmospheric heat source for the boiler is not night-time air, but the air flowing under the solar panels in the daytime.

This is the situation on a couple of clear, cold days in winter, midnight at the beginning of 13th January to midnight at the end of 14th. The black line is the temperature outside. The air temperature drops around 9 below zero on the first night, then up to around 2 degrees in the heat of the next day's sun. Then it plummets to -14 the following night and up to about 2 degrees again the following day. This leaves pretty cold temperatures for the Eco Cute to squeeze heat out of, leading to great inefficiency. 

The blue line is the temperature of air flowing through the channel under the solar panels on the roof. At night it gets just as cold as outside temperature. In fact on the night of the 13th, it was over 2 degrees colder because of the effect of the roof radiating heat to the stratosphere on a cloudless night. In the daytime, it gets over 30 degrees. If we could use the air from the channels in the middle of the day, rather than the air outside in the middle of the night, the air temperature could be 40 or 50 degrees higher. At this higher temperature, the heat pump would be much more efficient. 

The coldest nights and the warmest days are when it is clear, and of course it's not sunny every day. For example, this is the temperature on the panels the day it snowed. You can see the 14th and 15th January here, and how the temperature below the panels is just above freezing. In this case we have nothing to lose. The following day, with some snow still left on the roof, the temperature still got up above 10 degrees in the day time. You can see the temperatures from 12th January to 17th February below, perhaps averaging 25 or 30 degrees under the panels in the hottest part of the day, compared to minus 3 or 4 degrees outside at night. 

The main use of hot water is probably the bath, which we usually run at night time, so another advantage of switching from night time heat pumping to daytime heat pumping would be that the hot water would have less time to cool down, so we would need to use less of it. In terms of economics, this would mean a switch from using bought night time electricity to using our own generated electricity. Since we're buying off-peak electricity at 9 yen, but selling our electricity at 48 yen, the system would have to be 5 times more efficient to be worth changing. However, we only have a 10 year contract for selling our electricity at 48 yen, and no idea what may happen after that. 

This graph may give us an idea of how much energy we would save. Heat pumps are rated by COP, coefficient of performance, rather than efficiency. This corresponds to the amount of heat coming out compared to the amount of energy put in. So if you used 1 kW of electricity and got 5 kW of heat, that would be a COP of 5. The COP is affected by the temperatures inside and outside, so if you're trying to heat up luke warm water with hot air, you're going to get a much higher COP than trying to make hot water very hot from cold air. I've seen that Eco Cutes have a COP of 3.8, but this is a meaningless figure, unless you know precisely what operating conditions that was under. In the real world, not the world of advertising and eco-posturing, the COP for a particular heat pump is on a curve, dependent on the temperature of condensation, which happens outside, and the temperature of evaporation, which happens inside. Many heat pump manufacturers do not publish COP charts. From the graph above I estimate that if the temperature outside is 20 degrees higher, the COP will double. 

(The COP graph was stolen from Science Direct's website, from a paper which I did not pay $36 to read, by Forrest Meggers and Hansjurg Leibundgut of  ETH Zurich, Faculty of Architecture, Institute of Technology in Architecture, Building Systems Group, "The potential of wastewater heat and exergy: Decentralized high-temperature recovery with a heat pump". I hope they don't mind.)

Too bloody hot

December and January could be the hottest months in the house. At least, somewhat counterintuitively, they are the months with the highest solar gain. It's not that the sun is hotter in December and January. In fact, the sun is more or less the same temperature all the time, and cares little whether it is winter or summer in the northern hemisphere on Earth, but of course there is a difference in how much of that heat reaches the surface of our planet.

In terms of the radiation from the sun, there is more in the summer than in the winter. There are two reasons for this. First, the days are longer, so there are more hours of sunlight. More hours of sunlight mean more heat. Second, the angle of the sun is higher. This has two benefits. First, more sunlight is going to hit a given area of the earth. If the sun is directly above, a square metre of sunlight is going to hit a square metre of the earth. If the sun is 60 degrees below vertical, 30 degrees above the horizon, a square metre of sunlight will be elongated over two square metres of the earth so the incident radiation is halved. Also, the higher the sun is, the less atmosphere it has to get through, so the rays are stronger when they reach the ground.

The point with a house is that the windows are on the walls, so we aren't really interested in how much sunlight reaches a square metre of the ground. We want to know how much reaches a square metre of window. And this, almost by some divine intervention, means that in the winter, when we may expect it to be coldest outside, we get the most heat coming in through the windows. And when it gets warmer in the summer, less heat comes in. If we are careful with balconies and eaves, then we can try to keep this radiation to a minimum. Reflection is another thing that may lead one to believe that God invented windows, or at least that God was a double glazing salesman. The smaller the angle between solar rays and glass, the more is reflected and the less heat comes in. This means that more of the low winter sun will get through, and more of the high summer sun will be reflected.

So this is why it got up to 28 degrees centigrade in the living room at lunch time on 12th January, even though it was only one degree above freezing outside. The bottom line on this graph of temperatures over the first few weeks of our residence shows outside temperature (green - averaging more than one degree below zero). The highest temperature is inside temperature south (red at the top), and inside temperature upstairs north is pinkish below that, but dancing to the same tune. The others are slab temperatures. The big leap in inside ambient temperature was when we closed the windows and switched on the ventilation system on 23rd December, but you can see the jump in the temperature at the middle of the floor (light blue) as the underfloor heating started working on 26th December three days later. The effect at the bottom of the foundation slab (middle - dark blue) is slower, with about a three-day delay. At the North West corner of the foundation, the temperature change is much slower.   

Obviously it would be churlish to complain about the house being too hot, when all around are pouring gallons of oil into theirs and still freezing, and of course there are a few things that we can do before resorting to opening windows and letting the heat out. According to the thermometer in the upstairs north room, it is significantly cooler there, so if we open the inside windows from the atrium into the bedroom, the heat should go in there. Also we can open the door into the genkan and washitsu, which are to the north and significantly cooler. 

Part of the reason the north side is cooler is that the slab is much cooler there. This is by design. Kind of. The underfloor heating passes from the boiler to the south side of the floor, then to the north side of the floor, then back to the boiler, so the south side is being heated more effectively. Eventually the slab will probably have a constant temperature, but it actually seems like a good idea to have some temperature difference in the house. It would be nice to be able to control it a bit better, and I'm sure there is something we could do with the ventilation system. At the moment we are using a fan to blow air from the cooler northern parts of the house. 

But, going back to emissivity, I can't help feeling that it may have been a good idea to have had a higher emissivity for the floor and the walls so that they would have been absorbing more heat. What I guess is happening is that the radiation is just bouncing around the floor and the walls and getting the air really hot. The white terrace outside is probably helping by reflecting more sun into the house.

We're going to get some blinds soon anyway. I'd really like Venetian blinds with white on one side and black on the other, but I'm not sure if they are available or aesthetically pleasing. 

The Crookes radiometer shows the difference between black and white, invented by the eponymous Victorian chemist, William Crookes, who was pleased with himself for being able to make vacuum tubes. It was supposed to work as a kind of light mill, the white sides of each panel reflecting the sunlight, the black sides not reflecting anything, and spinning accordingly. When it started spinning, it went the wrong way; the black panels going away from the sunlight. The simple explanation is that the black sides get hotter than the white sides, and heat up the air molecules next to them, because actually the vacuum was far from perfect, which push the wheel around. A more detailed and accurate definition can be found 

here on wikipedia, unless the US government has shut it down. The difference the vacuum makes is to greatly reduce the resistance, so the effect of the heat becomes more significant. 

The temperature is being logged

Some data loggers have just arrived for the thermometers we put into the slab at a couple of levels. The recording all started back in the middle of winter at the bottom of the foundation, when we added sensors into the rebar.

Then we had all these sensor plugs growing like flowers from the wet concrete.

Most of them survived the layer of aggregate, then another sensor was added in the metal grid for the screed floor. I tried to get them to move the sensors as far as possible from the underfloor heating pipes, outlined in red.

Hopefully the sensor in the middle is around this position, although it may have moved when the concrete was poured.

After the screed floor, there's a few centimetres of wire to the plug. This is at the back of the house where the store room floor is a few centimetres lower and the slab a few centimetres thinner.

To avoid another decapitation, the carpenters very quickly made little boxes to cover the protruding sockets.

Tsukanaka-san arrived from T&D, and fixed the one we broke when the aggregate was poured in. I had tried to fix it, and stripped back the cover of the wire only to find three identical wires inside. I tried to fix them together, hoping for the best, but when Tsukanaka-san and the company president Morizumi-san plugged in the sensor, there was no reading. The president pulled my work apart, and reconnected two of the wires. One is apparently a dummy. He got the right two wires first time!

When the data loggers arrived they had to make bigger boxes.

You can see from the readings that there's already over 2 degrees difference between the temperature at the top of the slab to the bottom. There was also a difference from the west side of the slab, which gets some sunlight in the evening, while the other parts are in the shade. We can track the temperature over the next few months and get some idea of its reaction time, which should help when we start operating the underfloor heating. I'm hoping to some extent we can build up heat at the end of summer, and then release it over the winter so the slab is as cool as possible when summer hits. Probably not enough thermal inertia though. 

Each logger has an index number from 1 to 10, so we have started in the middle of the house with number 1 at the bottom of the foundation, inside the insulation, and number 2 in the concrete floor. Each logger can store 16,000 readings, or around 110 days' worth of data. The batteries last six months, so we need to fix the loggers where we can get to them. They have extension cables, but three pairs are positioned in or under cupboards, one is under the stairs and the other is in the storeroom. Maybe the cupboards should have removable floors or something. The data can be collected by a radio collector, which can then put the information into a computer.

There are fifteen channels, so we can record the room temperature and humidity in a few places, possibly outside as well. Also I was wondering about recording the temperature in the air channel under the solar panels, which I think is going to get quite hot and impair the power generation.

How warm is it going to be?

It should be warm, but to find out we're putting thermometers in each corner and in the middle, in the bottom slab and in the floor slab, a few centimetres higher.