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Author Topic: Designing heating & hot water system for off grid new build  (Read 4420 times)
Rusty55
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« on: October 30, 2020, 05:11:28 PM »

Hello!

I am building an off grid house next year on an island which is low energy sealed/MVHR and will need about 4kw heat input. It will have about 5kw solar PV and a small wind turbine (to be specified) - wind is largely for winter months. Because it is off grid, I need to have max flexibility to deal with low electricity and min. usage. The electricity usage will range from a max of 10kwh/day to as little as 3kwh/day. We will have a propane cooker/hob and WBS with back boiler as backup. My current plan is to have UFH (downstairs only) and 2 drying radiators but want them fed by the either the WBS boiler or ASHP depending on my electricity generation/battery status. I also want an immersion to use as an excess generation dump load + quick(er) hot water as an option. I think the best way to do this may be a thermal store but am keen to hear the war stories + other ideas!! The house is on an island so the solution has to be simple and robust...

Thanks very much for anyone's thoughts/ideas!

Cheers, Rusty.
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desperate
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« Reply #1 on: October 30, 2020, 05:49:35 PM »

Hi Rusty,

if you really do need just 4 kW to heat your place a thermal store may be a good solution IF you can house it within the heated envelope of the house so that heat losses are not an issue. WBS's with boilers in them are great too but the plumbing is not trivial, what about a small dry stove to heat the place in the winter, and a combination of turbine and solar thermal to provide hot water? You could also incorporate PV for the summer months if you have the roof space? If you have spare/stored leccy it would be good to have a heat pump to provide heating when you can't be bothered to light up the stove, personally I would go for an air/air heat pump and keep the hot water seperate to maximise the cop. You can buy a neat little air/air heat pump for less than 500 quid, an air/water version will be loads more.

If you have a turbine and can squeeze in PV and thermal you have loads of options.

Desp
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Bugtownboy
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« Reply #2 on: October 30, 2020, 06:36:48 PM »

Rusty, slightly confused by your heating/power needs - is the propane and WBS included as a contingency/back up ?

We (2 adults) live in a mains connected largeish 3 bed ‘30’s detached house in the SW. Insulation is reasonable.

Our mains energy usage is 1400 kWh electric (oven, lighting, refrigeration, AV), 3000 kWh gas - hob, hot water, minimal CH. Bulk of heating is by two dry WBS - we’re retired and at home ‘pottering/fiddling’ a lot of the time. We also cook - maybe 20% of the time with wood.

We use circa 6-7m3 of hardwood each winter.

We’re comfortable in the house with minimal CH use, relying on ‘dry WBS’ to heat the house. Admittedly, we’ve adjusted to what may be described as a cooler upstairs.
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A.L.
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« Reply #3 on: October 30, 2020, 07:10:22 PM »

hello and welcome,

not for nothing is the unofficial mantra of this forum 'insulate, insulate, insulate'

can I ask for some parameters? floor area/design temp for 4kW/fabric insulation values/infiltration rates

my initial feeling is that 4kW seems quite high for a low energy house, e.g. a design to Passivhaus standards would be 400m2 @ -10°C for  4kW input
« Last Edit: October 30, 2020, 08:12:10 PM by A.L. » Logged
rogeriko
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« Reply #4 on: October 30, 2020, 10:23:05 PM »

I used to live in an off grid house on an island with no mains electric supply. I had solar panels and 3 wind turbines and a massive 48kwh battery bank. There is no way I could have even turned on an ASHP in the winter to heat my house. When you need heating in the winter renewables just dont work unless you are in an incredibly windy place like the outer hebrides or similar. Camillitech of this forum does extremely well at heating and powering his off grid house but it has taken him about 20 years and I hate to imagine how many thousands of pounds and thousands of man hours to achive this. If you look out of your window and all the trees are growing horizontally then you can do it. Forget solar in the winter unless you live way way south of the uk. Get a wood burning stove with a back boiler for hot water with a large hot water tank, 400/500 litres and then a wind turbine and solar panels just for your lighting needs + fridge and kitchen gadgets.

 Check out the link at the bottom of this post for photos of my off grid work on the Greek Islands. Thats where I used to live.



* wind blown tree.jpg (79.14 KB, 800x528 - viewed 238 times.)
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todthedog
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« Reply #5 on: October 31, 2020, 06:37:05 AM »

Hello and welcome

A good old fashioned lister for back up to recharge your batteries.
If logs are available a woodburner.
Read Paul's blog   https://lifeattheendoftheroad.wordpress.com/about/
Read Biffs back catalogue for ideas.

Both have forgotten more than I will ever know.

Good luck we love a good project.

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Kidwelly South Wales
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« Reply #6 on: October 31, 2020, 06:43:46 AM »

If you’re heating your house with towngas (the cheapest form of energy), insulation is financially one of the best investments you can make. Off-grid, when you are paying for/dealing with, bottle gas, biomass or other, this benefit is multiplied. 200mm of PIR in the walls, 400mm in the roof/under the slab, together with a good air-tightness score is a good starting point. Thermal mass is important too.

The key criteria is: where are you? Cornwall is a very different proposition to Scotland. Either way, space heating from solar PV won’t work. The sun just doesn’t shine enough in winter (on Thursday I got 0.3kWh from my 4kWp array). Wind too can be fickle. There are places where a wind turbine can generate consistent energy all winter, but they are very rare. Even then, the turbine needs to be a long way from trees and houses. The reality is that you’ll need a generator if you’re off grid, an over-sized solar array and functioning WT will reduce its use.

Another important criteria is your lifestyle. Are you looking to have the house warm and cosy throughout? Are you expecting to wake up to a toasty-warm bedroom when it is subzero outside? Having an open-plan kitchen/living/dining space heated by a log-burning range cooker is a simple solution (you can cook on electric/bottle gas in summer), but the bedroom(s) would be cooler and you’ll have to tend the fire. Alternatively you could have a WBS heating a thermal store, supplying rads/UFH. This means you’ll be able to light the fire for a few hours in the evening and use the heat all day, which might be more convenient and would heat the whole house. Just be aware you’d get through a lot of wood while you’re loading the TS.


Overall I’m jealous, I’d love to be building an off-grid home! Please keep us up-to-date with how the design/build process progresses.


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« Reply #7 on: October 31, 2020, 07:37:27 AM »

Welcome to the forum Rusty55,
          There is always a limit to what you can achieve  off-grid, I hate to admit that. You just cannot have unlimited power to run anything and everything.
   Sad to say, you have to have diesel backup. We can run for stretches of 6 months + before we start up our diesel charger. We have 3.8kw of PV and a good Chinese  Turbine, (yes there are such things) that fills the clock with such boring regularity that it is now lying resting on its trestle to wait for Aiden to pass over around 12 noon. We have 2 ton of tired lead acid for storage. It works fine but only within a certain weather pattern, we can get by very comfortable with a constant use of 300 to 400 watt daily but need the right weather to drive our washing machine and vacuum cleaner. Washing machine is 2050 watt and the vacuum cleaner is 850 watt. We have a small 75ltr chest freezer converted to a fridge which has worked well for many years . Paul preached loud against our expensive gas fridge and he was right. It was very expensive to run.
 We cook with gas.
  We live comfortably and when the rest of the parish is in outage, I cannot help but turn on every light in the house then sit on a chair and with my pressed hands flat together clenched between my knees, rub them with fiendish glee, ,  well NO , I am not quite as bad as that.
   You can live very comfortably off-grid but there are limits. A big lump of a lister is a must and another smaller, portable generator is a good idea.
   Not every idea that works it's way to the surface will be a good one but don't  give up easy. Take for instance  my latest move in planning to take a lead of one of our dump load  immersion heaters and charging our second battery bank with it. It is / was an ill conceived idea at that especially when the correct  answer was staring me straight in the face,  so I will post pics of the same when this storm blows over.
  We can depend on our WBS for our central heating and of course we have 2 x 2kw immersions, heating a total of 500ltrs of water,  strategically located in the house.
   Yet for all that, we once had weeks of freezing fog, our bank froze and our turbine and Solar PV could not work.
In the - 14 freeze, the roads had 100mm of solid ice but luckily everything came back to normal with the thaw.
  We got by with our diesel standby generator and painful gallons of diesel.
  We have a bowed hawthorn approx 300 mtrs away on the side if hill, a twin for the one on Roger,s pic. You need good wind, and plenty of PV and Insulate,  Insulate  and insulate your dry dwellings. Yes anything is possible if, ,
   Biff
  
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brackwell
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« Reply #8 on: October 31, 2020, 11:20:54 AM »

I have a feeling the OP is a student?
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MR GUS
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« Reply #9 on: October 31, 2020, 12:38:24 PM »

I have a feeling the OP is a student?

Ditto, if so then "Rusty" where are you in reality (which uni / college, online course) & what are the co-ordinates for the island to assess wind & solar. 
Wind is not necessarily a winter only "thing" especially on an island, as Camillatech & Biff can vouchsafe ...otherwise come clean, because what you've written so far reeks of theoretical coursework assignment from others doing the same thing.

Are we wrong?
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« Reply #10 on: October 31, 2020, 03:02:07 PM »

Before actually building the house I suggest.

Excavate an enormous hole directly beneath the plot.  Area to be slightly less than house foot print.  Depth to be 50+ mters.
Fill hole with aggegate and embed 50mm dia pipes in the aggregate, bottom to top. Cap of hole with steel plate with vents in.
Build house on top.
Connect up 25 sq mtr of solar (water) panels to pipes in the hole.
During the summer keep the floor vents closed and allow solar to heat up the aggregate as an energy store, like a giant night storage heater.
During winter open vents to allow slow release of heat to provide warmth for the complete dwelling.
4.5 kw of PV for electric in summer and lazy lister for winter back up.

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« Reply #11 on: November 01, 2020, 07:11:34 AM »

Morning Rusty,

I live on an island too and built an 'off grid' all electric house. First thing you need to do is acquire at least two years of data for wind speed, look for hydro potential and see if there any trees nearby (for firewood and to cut down for your wind turbine to work)  Grin 4kW seems awful high too, I heat my 120m square house with a ASHP that uses 820W and the house is always toasty.

My original plan was to have a big TS and Perge (uses no electricity) batch boiler. However having assessed all my data and electricity requirements I opted to just use the wind, solar and hydro, but I couldn't have done it without the  800W of hydro that matches my heat pump. Well I could but it would mean running the generator for a couple of hours every day to warm the bottom of the TS. https://lifeattheendoftheroad.wordpress.com/2016/10/22/an-off-grid-journey/

It was and is very satisfying, good luck, Paul
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Rusty55
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« Reply #12 on: November 01, 2020, 12:57:20 PM »

Blimey!!! Thanks everyone for the replies. This is the first time I'm been on any kind of forum so quite a surprise to be honest. A lot of you have asked further questions so I'll try to answer them all here with some more background ...

... and no I'm not a student! I am late 50's and planning to retire to the island with which I have a long association. It is an exposed location in the Hebrides with avg. wind speed of 7 to 8 m/s so plenty for a turbine. The problem with turbines is they all break in the extreme climate.

There are no trees harvested on the island (very few actually grow) but lots of people import wood/coal via the ferry. You can actually buy it there for a premium too.

The house is designed to be highly efficient. Sealed/MVHC (heat recovery) Thick SIPS/CPS panels and foundation insulation etc. So the 4kw heating estimate is probably worse than reality.

It is Scotland but the far western isles hardly ever freeze and surprisingly, this island is one of the sunnier places in the UK - the solar yield forecast is similar to Birmingham in spite of the higher latitude. (You can see this on solar maps).

I can get a grid connection but it will cost nearly £20k (!) so I'm trying to go off grid instead. This might not actually be cheaper but it is and good thing to do and hugely interesting. I'm really enjoying the learning and the planning.

There is enough land around the house if needed for solar/turbines etc. but hope that my planned 5kw array and one part-time 1-2kw turbine will do. The 5kw array of solar will produce a min (10 day avg.) of 3kw/day in winter and max (10 day avg.) of 20kw/day in summer. This doesn't mean I can rely on that though. One of you mentioned freezing fog for example - not something I considered.

I intend to build my own turbine (eventually) with the great help of Hugh Piggot's toils from Scoraig but that will be later on. A lot of the islanders have turbines and most of them have had bad turbine experiences. When it gets wild it is really wild! That is why I don't want to rely on one but want to use it opportunistically. I also want to be able to fix my own one when it (and it will!) breaks.

I am also installing rainwater harvesting - there is a burn relatively close by but the water is coloured (peat) and the rain water is really good. I have a static caravan on the plot just now and collect water from the roof - it is clean and clear of any debris so the filtration will be easy - but will include a Uv and Ph-raising stages.

So, back to the heating, my basic assumption is that I might have to exist for a few days at a time without electricity. When we have none or little, we will cook with bottled propane (both hob and oven) and the WBS must be able to heat the house and produce hot water. In the summer, I expect to be able to heat the house and produce hot water from the solar PV generation via some kind of heat pump. If and when I have excess energy generation (turbine/solar), I will probably want to use an immersion as a dump load too. If the turbine works well, it will compensate somewhat for the poor solar yield in winter but I'm nor banking on that.

I really appreciate all your comments and suggestions. I've got about about 4 months to plan, select and order the necessary kit!

Cheers, Rusty.
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renewablejohn
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« Reply #13 on: November 01, 2020, 02:03:08 PM »

Have a look at Virtu PVT which combines normal PV with Thermal Solar.  It still collects heat in cloudy conditions amd can be mounted on vertical walls.
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« Reply #14 on: November 01, 2020, 02:03:30 PM »

Fabulous.... You are going to have a great time setting this up and despite my earlier post you will have plenty of energy. I will post again tonight when I have more time.  Cooking up 100 kilos of pumpkin right now to go in the freezers and used for baking over the next year.
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