Friday, August 14, 2015

Enquiry about the Heat Pump and Degree Days

14 August 2015: I had an email from Shane Slater who has been reading my blog, and he asked some good questions and I said to him that I will probably post them on here.

Here is abridged version of his email:
While looking for Heat Pump diurnal consumption data, I’ve come across your site Charging the Earth. Its excellent.
    Perhaps uniquely, you show graphs of heat pump consumption V degree days. This is a methodology I hope to use in a future model of European housing space heating demand (based on degree days) and consequent Heat Pump demand.
     I am interested in that, while the correlation you show is good, it seems on some of your graphs that in severe cold weather, the heat pump consumption is significantly higher, while in mid-season/summer the opposite is true – the HP load is close to zero even though there is a non-zero Degree day.
     Do you have any thoughts as to why this might be the case? You have said that you turn it off at night but I would have thought that would reduce the HP demand relative to the degree days. 
     I was wondering whether wind effects could play a role i.e. cold but windy weather results in a higher heat loss rate and therefore higher HP consumption? And in summer, perhaps solar gain means the Heat Pump can be switched off?
     I’d be grateful for your help – and in any event congratulations on your website.

My reply:
Hi, thank you for reading the blog, and I apologise that I have not been keeping up-to-date much because most of the work was done five years ago.

1. It seemed logical in 2010 that one should correlate heat pump consumption to degree days so I made that graph. Every winter and summer is slightly different, and the degree days is a perfect way to record this variation. 

2. Degree days go into negative when there are certain days which are so hot that in some countries you would have air-conditioning. In a British load-bearing-wall house there is no need for air-conditioning because there is good insulation and thermal capacity, and openable windows.

3. There is always some consumption in mid summer because we need hot water. The heating function of the heat pump is mostly turned off from May to mid september because the house is warm enough without heating in those months. (Sometimes turned on for a day or two in very cold spells.)

4. In very cold weather, the heat pump consumption goes up, because it goes into 'additional heat mode' if you cannot get enough heat from the ground at a satisfactory rate. This is direct electric heating mode, with no COP. This has not occurred since we had the solar charging. 

5. In cold weather, the cooling of the ground (to feed the HP) is such that it creates a favourable delta T with the solar sunbox panels. Thus, we get some ground recharging even on the cold days if there is a smidgeon of sunshine, or on the more average winter day, if the air temperature is above freezing. The sunbox panels face south, and are vertically positioned, so have some advantage in winter sun angles.

6. We set the clock on the HP to go off during the night (between 930pm and 730am) in both summer and winter. This is not just a desire to improve our metering figures, it is because we have a well insulated house, and most people sleep better if the air temperature drops a few degs C. Nobody can sleep well if they are sweaty and hot, and can hear water pumping around the house at 3 in the morning. My wife often insists on an open window even in winter, so what would be the point of heating? I compromise with her by having the window shut but the bedroom door open so that the roof ventilates to the whole volume of the house. 

7. My wife is disabled, so we have to have the house a degree or two more than we ever had it when she was not disabled. Even at this higher temperature, our consumption figures are satisfactory.

8. Having a HP, we have underfloor heating, so no radiators, and in recent times, I have had to buy a local electric radiant heater for my wife to have in winter. This has increased our house consumption, and must very slightly reduce the HP consumption, but its hard to quantify that. 

9. Wind chill is significant if you have draughty windows, but we do not (house is only 8 years old). Wall insulation is good, and so is the loft insulation. We also wear adaptive clothing, i.e. teeshirts in summer, and woolies in winter. 

August 2015 Update on the Peveril Solar House

14 August 2015: Hi, it is sometime since I made entries into the blog. Mostly, this is because things are going well and the bulk of my solar charging work was done back in 2010 and 2011. My interest since then have moved to the garden, the pond, and to helping on my friends farm. I am continuing to take meter readings and I must enter these and update some of the graphs of house performance.

   I am also preoccupied with the work of my students, during the summer I have 12 Masters dissertations to supervise, and this means going into work almost every day, even though I am meant to be nearly retired. Although I am passed retirement age, I still run three semesters per year of a masters postgraduate course.

   I quite enjoy it when I get cold telephone calls offering me solar panels, or like I had today, offering me a heat pump. I usually thank the caller politely and tell them that I am already living in a carbon zero house, ha ha ha, and then I wonder what they really think, am I winding them up? Some of them appear not to believe it but most of them just ring off immediately, I suppose, treating me like they are mostly treated by the people they telephone.

Sunday, July 26, 2015

More info about the solar dehydrator

25 July 2015: The dehydrator has been at the PIP farm for about six weeks, but the black pipes were not complete. They had come apart and leaked. I went for a couple of hours just to repair it and get everything so that it will never leave again. Meanwhile, John is down in the woodland using his wood butchering techniques to erect a toilet block.

Tuesday, July 7, 2015

Garden ideas: clover and self watering

7.7.15: I am gradually converting my lawn to other herbs.... and doing further experiment with self watering methods. The rain is far too infrequent during July-August for my roofwater based system, and active watering is necessary.

It is now mostly a mixture of clover (left) and chamomile (right), and it has not been done by planting, it has been done by not applying weedkiller and keeping it mowed not too often... And these are the indigenous plants that have taken root. I have dug up the ragwort (ugly and poisonous), and suppressed the dandelions by picking off the flowerheads. Any dandelions remaining, get added to our lunch as part of the salad. The chamomile forms a dense herbal cover with small yellow flowers, and yes, it can be picked for making tea!
The farmer on the field used to grow chamomile as a winter crop and this has drifted over somehow. Also, the clover is in full flower, the bees love it, and I will not mow any of it until the seeds have formed. 

For this to work better, the bottle needs to be stiffer, so that it recovers shape during the night. This one is looking a bit soft. 
I have also been experimenting with the self watering using transparent bottles. You fill the bottle up, and place it heads down with a small hole at the end. It does not run out and the gravity, but when the sun heats up the water or the air in the bottle it squeezes out some water. And at night when it cools, it sucks air back into the bottle.
This is the vertical tube farm (a black conduit). The top funnel catches rainwater or my watering can water and runs down a central perforated pipe. The sideways facing bottle is pointed through a 40mm hole, and there is a 3mm hole in the end that is too small for the water to run directly out. 

Tuesday, April 14, 2015

Open Homes weekend 25th - 26th April, 3rd May)

Open Homes weekend
25th - 26th April (and 3rd May)

Eleven low energy homes open their doors to visitors this spring. 
Some are cutting edge refurbishments with ultra-low bills, other homes have undergone more modest changes to make them more comfortable and cheaper to run.
Details of each home are now up on the websites: (including a timetable of all the opening times)
Every home description includes a link to Eventbrite, to book your free visit(s). Once you have made a booking, further details such as the exact address, bus routes and parking options will be sent to you by email nearer the time.

For further information, contact Tina on or 07962 453037

The date for the Peveril Solar House is May 3rd 2015

Monday, April 13, 2015

Fitting a deep charge battery to PV panel

13 April 2015: I have had a spare PV panel on my extension roof for about two years and let two summers go by without using it. I wasted a lot of time trying to build a battery with NiCad cells, but lost heart, they don't seem to run anything for more than a few hours.

I finally decided to get myself a VICTRON AGM  110AH deep charge battery and this is now connected and charging at 0.7 A when the sun shines.There was spare space on the extension roof. I hope to run something like the glycol pumps off it, and some 12volt lighting down in the dining room.
The last postings on this was in December 2012, in which I felt that the NiCad was a mistake:

I need to elevate the PV panel on short 200mm legs so that the panel is not shaded by the polycarbonate sunbox when the Sun is in the West.

Why the Victron AGM?
You can't use a Car battery, as they are designed for high output for short periods (when starting the engine), so you need a deep charge one : a Caravan/Boat domestic battery which is designed for light continuous loads. It's something to do with the spacing of the plates in the battery.
I currently have a Maplins inverter (in the photo), perhaps I ought to get a Victron Inverter too.

Thursday, April 2, 2015

Progress on the solar dehydrator

March 2015: Work on the solar dehydrator has been progressing well, and it is nearly complete, lacking merely the top surface of the solar tube.

Roof construction. This is designed to allow air to escape but not too quickly, and to prevent water from getting in. It is also a transportable component which is fitted on site.Fitting the Solar Tube -  this sits tidily in the chassis and connects to the hopper below the drying chamber.
Inserting the black plastic tubes. They are on a small aluminium subframe so they can be easily transported or lifted out/in.The front doors are drop-down work surfaces, with chains to keep them level. The light aluminium stool is high enough for safe maintenance of the trays in the upper chamber. 

Friday, March 27, 2015

Devising an optimal arrangement of black tubes

March 2015: We intend to make the solar dehydrator work better using a bit of Thermal Storage, a bit of Eutectics. The solar tube needs a thermal absorber, and it needs to be black, and we have been thinking about some steel rods (too thin), an old radiator (like in my Sunbox), some steel mesh (bothersome to paint black) and some black bricks (too heavy). One of my colleagues has kindly donated some Rubitherm thermal wax from an old project of his. I would have liked wax with a phase change of 25-40 degrees, but this wax is graded 58º.... a bit too high but we do not turn down donations. I decided to buy some 40mm black plastic waste pipe. Easy to cut and join, using push-fit joints.

I have 50 kilos or wax here, and the quantity is enough for two solar dehydrators. Each of these sacks will do about 25 metres of 40mm piping (using a simple calculation of PI*R^2 on the pipe).

The design problem was how to get 25m of piping inside a cuboid volume of 250mm x 900mm x 1400mm. There must be access to sunlight for the pipes, air space around the pipes, and transportability. It must be refillable, and leakproof. Using a combination of Tee-joints, Elbows and 135º joints, I devised a 3-dimensional space frame - but it was too difficult to do on the computer, so I bought a load of joints and some piping, and spent an evening experimenting.

Having devised some sort of piping for one end, it then became clear how to do the other end, and I needed to buy enough joints and straight piping to make the final length correctly.

Still a bit confused and needing yet more joints to make it complete. Another requirement is that when the granules of wax are poured in through the top pipes, they will efficiently fill the whole tube-grid without air gaps. I am not clear about whether the wax melts into a single liquid body, or whether it stays as granules. 

Here is the finished 3-D grid when it lies flat for transportation.  To this has now been added a small aluminium subframe. The final length of pipe is near enough to 25 metres.Here is the grid standing up in 3-D to give a finished height of 260 mm.  The length is designed to allow the bottom louvre to open and close.

Monday, March 16, 2015

Making a rabbit trap for the organic farm

March 2015: My farming friend John Macdonald has a problem of a rabbit in his polytunnel on the PIP organic farm, so I resolved to make a large scale model of the mouse catcher. This is entirely made from plywood from a skip, except for the duct tape and the small quantities of aluminium and polycarbonate which are all recycled from previous solar panel projects.

Forming the basic box shape using 8th inch ply and black duct tape.

 A tilting door at the end composed of polycarbonate and small length of hardwood with aluminium brackets.

When a rabbit walks in to the box, it tilts and closes the door which then locks using small aluminium catches. The window allows one to check to see if there is anybody inside the box ha ha.

As an April afterthought, I thought I would ask John whether he has actually caught his rabbit from this. I will add it to this posting, although when I last asked, he thought the rabbit had actually left the polytunnel. With this one spring weather, it is possibly getting too hot for the rabbit and there is no supply of water in the polytunnel.

Tuesday, March 10, 2015

Solar Dehydrator

9 Mar 2015: For some weeks now, I have been working on building a solar dehydrator. This is far too big for my garden, but it is being built in my garden and will be transported to Woodbro', to the organic farm (PIP) of John Macdonald. The whole device stands 3 m high and is designed for drying out herbs and tomatoes. As the solar tube warms up it draws in from the bottom louvre and that rises up through the drying chamber and out through the roof. As the air rises it passes through as many as 30 perforated trays which can contain herb leaves or drying fruit or drying tomatoes and then exit through the roof.
   In this image, it is fully modelled in ArchiCAD and GDL and is being built precisely according to the model, And if I make small on-site variations in the real thing, I go back to the model and make sure that it represents fully the final object. 
    This will be on a turntable so that it can be rotated to track the sun. The folding mirror will ensure that maximum sunlight enters the solar tube. The tube will contain black painted absorbers of thermal mass phase change wax (Rubitherm) which will cool the tube in hot weather and produce warmth when the sky is grey or the sun has set. The top panel of the solar tube is designed to be an ETFE panel which we are hoping to get from Holscot or Evolve. Holscot kindly donated the front panels for my Surya 3 solar panels. The improved performance of my solar Sunbox has proved the efficacy of ETFE in a solar thermal panel. 

Below is an early 2D drawing of the design, but it has since been 3D modelled, down to the last bolt and bracket, and the final 2D drawings for construction are taken from the model.

I have taken progress photos during the construction and will post a fuller set as a different article in the blog.

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