Is it a fair COP?

Just looking at the spec for our eco cute in search of hard data on the temperature performance of the heat pump, it reveals very little. 

The spec is for two models, the CHP-H4619AT, recommended for "Region III" and the CHP-H4619ATK, recommended for Region I. The regions corresponds to the Next Generation Energy Efficiency standards, where Region I is Hokkaido, the coldest part of Japan, and Region III is Miyagi, Yamagata, Tochigi and Niigata in the lower North East of Japan, and the warmer parts of Nagano, which is central but mostly high in the mountains. 

Then it shows annual hot water efficiency of 3.1, which apparently is the ratio of the heat coming out in hot water to the electricity going in. So for a kWh of electricity, you get 3.1 kWh of heat. The note next to this says it is the situation using low energy mode, under certain conditions referred to in another note. The other note says that this is the average performance between Tokyo and Osaka, which are neither in Region I nor III but in Region IV.

Further down the spec, the mid-range rating for the heat pump COP is given as 4.5, which has no notes attached, but there are several notes to the numbers around it, which show the mid-range output heat and mid-range power consumption. "Mid-range" means an ambient temperature of 16 degrees (12 degrees wet bulb) and mains water temperature of 17 degrees with an output water temperature of 65 degrees. The electric power consumption is 1.33 kW and the hot water output is 6 kW; a ratio of 4.51. It also gives a summer figure with ambient temperature 25 degrees (21 degrees wet bulb) and mains water temperature of 24 degrees. The power consumption is then 0.97 kW and hot water output is 4.5 kW; a slightly higher ratio of 4.63). For the winter it gives ambient temperature 7 degrees (wet bulb 6 degrees) and mains water temperature 9 degrees with hot water at 90 degrees. The electric power in the winter is then 1.99 kW producing 6 kW of hot water; a significantly lower ratio of 3.0.

Two things leap out of this data when you actually look at the small print. The mid-range performance is suspiciously close to the summer performance. The winter performance is much worse. Since hot water use is going to be less in the summer, and more in the winter when the heating is on, the performance at the middle of the actual range is going to be between what it calls the mid-range performance and the winter performance. Also, the ambient temperature for the winter performance is about 5 or 10 degrees warmer than the actual ambient temperature we get in the winter. That's in Region III, let alone in Hokkaido in Region I.

It does give another note that under cold conditions, the performance will be lower. 

So there you go. 

The COP is 4.5...

Or 3.1.

Or some other number...

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.)

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.

Extractor fan hot water units

The more I think about it, the more sensible seems the idea of pumping heat out of extracted air into hot water tanks. Given a reasonably well sealed thermal envelope, the places you want to extract air from a house are kitchens, bathrooms and toilets. These are also places where hot water is used. 

And if you don't have a well-sealed thermal envelope, then extracting air is not an issue.

If you extracted 50 cubic metres and dropped the temperature by 20 degrees, 1,300 kJ would be available. If you did this every hour, you'd get about one kWh every three hours, 8 kWh per day. According to Without Hot Air by David Kay, in Sustainability without the hot air, a bath takes about 5kWh and a shower 1.4 kWh. He estimates 12 kWh of hot water per day per person, although he seems to include cooking, refrigerating and freezing in his sums. 

The problems, of course, are in economies of scale and system complexity.

In the summer, rather than cooling the air going out, you would want to cool the air coming in, but you probably wouldn't want to be drawing air into the house via the kitchen, bathroom and toilet! 

Air conditioners are now pretty much standard fittings in Japanese houses and models are available that heat water as they cool the air, but these are not widespread, and in installation they work out more expensive than buying separate units for heating water and cooling air, and since the air conditioner is not on for most of the year, another means of water heating is necessary anyway.

Useful physical characteristics of air: 
Air holds 1 kJ per kg per degree change in temperature. 
In cubic metres, that's about 1.3 kJ per cubic metre kelvin. 

Bog standard

Until relatively recently a lot of the houses in this city would have had dry lavatories, not connected to the town sewage. The one in our old house seemed to have moved three times, most recently from the North East to the South East corner. Where they had running water, they were squatters and I imagine for the first half of the twentieth century, sit-down toilets were about as common as thrones, and no doubt considered with the same admiration. In fact around a hundred years ago a sit-down toilet was installed in a nearby town in anticipation of a visit by the Meiji Emperor.

From the second half of the twentieth century there has been a dramatic movement from squatters to sitters, and the toilet seat has burst into Japanese culture. Japanese toilet seat design lacks the wondrous assortment of colours, materials and patterns available in the UK market, but makes up for it with the availability of gadgets. Many houses may not have running hot water, but their toilet seats do. They are on display and available in electric shops, since toilet seats seem to be more electrical devices than plumbing. 

The first attraction for the consumer is perhaps warmth. Most houses are not centrally heated, and buildings are often assembled in a modular way, rather than rooms being fit into the thermal envelope of a house. The lavatory itself has not completely been accepted as a room in the house anyway, as evidenced by the toilet slipper. This humble piece of footwear will surprise the visitor to Japan and often embarrass him as he walks back into the rest of the house with "toilet" written on each foot. I believe the toilet slippers are there because the toilet remains conceptually "outside" in the complex world of uchi to soto (inside and outside) that informs a lot of Japanese culture but very little of its thermal efficacy. When it is freezing outside, it is freezing in many lavatories.

The result is that the seats can get pretty cold. With a squatter this is not an issue, since there is no seat and no contact, and not so much heat is lost through convection or radiation, but electrical heating elements can soften the thermal shock of conducting heat away from the fleshy behind. Since these heating elements are electrical, these devices must also be very attractive for electricity companies, as they put several hundred yen on each month's electricity bill for each house. Now they are advertising low-energy seats, but they still use substantially more than a seat with no plug on it.

The hi-tech toilet seat that we have has a low-energy function, which makes a light come on saying "low energy". It has a "super low energy" function too. This makes the light flash. 

There are also elaborate washing and bidet functions with hot and cold running water, elevating the humble toilet seat further and further above a hole in the floor. These are not considered particularly luxurious and you can find such toilets in shops and restaurants and even some public conveniences.

Of course the seat heater in our house is switched off, but if we ever need it I suspect it would do a good job heating the whole room.

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.