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    We are currently designing an off-grid dwelling. The system we are proposing:

    1) PV array to provide 3.2kW per annum
    2) 5kW Biofuel Generator to run alongside
    3) 35kWh High capacity Battery storage
    4) Heat Recovery system harnessing heat from the generator and house

    We have considered alternative CHP systems however the majority of domestic solutions are gas powered which we are currently not considering an option.

    The benefit of this system is that during the summer the PV array can supply the majority of the required electricity. Heating load will be lower, allowing the heat recovery system plus intermittent use of the bio-fuel generator to provide enough hot water.

    During the winter when the solar panels are less efficient and heating load is higher the generator will supply the electricity and heating required.

    Although we have undertaken our own research to get to this stage we would like to hear from others about their experience using systems such as this. The last thread I have seen on this forum dates back to 2011, are there any:
    - new recommended suppliers?
    - technological breakthroughs?
    - recommended off-grid specialists who may be interested in assisting?

    Thanks in advance for the advice.
    • CommentTimeMar 2nd 2016
    When you say a 35 kWh battery bank, is that the name plate rating, which may only be 7 kWh of usable energy or you cook your batteries.
    It is something I always forget when thinking about batteries.

    As you have opted for a bio-fuel generator, is your aim to reduce carbon emissions.
    Burning just about anything adds to a saturated system, it does not reduce it.
    • CommentAuthorEd Davies
    • CommentTimeMar 2nd 2016
    Posted By: Studio_Bark1) PV array to provide 3.2kW per annum
    I assume you just mean a 3.2 kW array (ie, the “per annum” bit is spurious). Why so small? I'm planning roughly 6 kW which I still think is about the minimum to see how things go. But then I'm trying to avoid as far as possible the use of a generator - I hope to be able to keep that to a few runs each winter and maybe eventually get rid of even that with a wind turbine.
    The recommended battery capacity we would need is 35kWh, with an output power of around 5kW.

    Yes, we had originally been in search of a woodburning CHP, but from our research these are not yet available for domestic scale use. Therefore we consider a bio-fuel generator, over a gas CHP, a more environmental solution here.
    • CommentTimeMar 2nd 2016
    I would reassess the assumption about being environmental with respect to your fuel choice.
    You may well find that there is no benefit compared to LPG.
    Thanks, that is food for thought. Could you recommended any bio-gas compatible LPG CHPs?

    We will probably do a comparison between the 2 systems, but if the LPG is more efficient, as you say, it may well be the right option here.

    Further to this, any advice on batteries would be great, there are many different options. We are looking for value for money, high capacity and longevity.
    • CommentTimeMar 2nd 2016
    No experience with CHPs at all.
    Someone I know was involved with Baxi's small CHP, he raved about them, but was getting paid to do so.

    Battery technology is still a long way off, is get a bit cheaper and a bit more energy dense every year, but it is not going to be like PV modules (which are about as cheap as they will get in my opinion as you are paying for glass and alloy and a tiny bit of cell material).
    Have you looked at NiFe, old technology, not very energy dense but long lived and reliable.
    Hi Studio

    Why are you looking to go off grid? Tell us a bit about the house you're planning?

    we've just moved into a 120m square 'off grid' new build that we designed using 25 years of 'off grid' experience. Our house is all electric and we're aiming for zero generator use. We have run it 44 hours over the three months of winter but by next year we hope to have it down to nil when we install a bigger wind turbine.


    As Ed says, your PV is waaaaaaaaaaaay too small and why no solar hot water? I'm guessing you may have planning issues or no wind to not be having a turbine either?

    No experience with CHP, I dismissed it a long while ago as we can only get hold of diesel out here in the sticks.

    As for batteries, well there I can help, I've had hundreds and look after several banks both domestic and commercial from a few hundred Ah to a whopping .75mW LiFePO4 one on a hybrid ferry. My own house uses 48V 800ah of Rolls FLA batteries (40kWh on the label but 20kWh to 50% DoD) but part of the reason for that is my transport issues. I would personally steer clear of NiCad, NiFe, Gel, and Lithium technologies (at the moment anyway) and go for trusty fork lift cells. If you are wanting to extract 35kWh out of them then you need to be looking at a bare minimum of 1600Ah ideally more.

    Good luck, Paul
    Thanks camillitech,

    The house is a paragraph 55 approved project. For more information http://www.studiobark.co.uk/portfolio-item/black-barn/

    We have a stretch of oak trees to the east with a root protection zone, this is the main point of access to the grid and we do not want lay any underground services in this zone. Additionally we feel a new home located in the rural countryside should not need to rely on the grid.

    I think we will be looking at increasing our solar array, this may need to be a re-submitted to the planning authority but they have been very supportive of the scheme so far, so shouldn't be a problem. We have not included solar hot water as the array is located away form the house and would need insulated pipes which can be quite expensive. Any advice on this would be very welcome.

    We have not included wind as it was predicted it would not generate enough electricity in the location.

    Battery wise we have been recommended the Rolls FLA, we had hoped battery technology would have developed further. We also know our anrticipated capacity at 35kWh is very high, so we will have to have a look at this to see whether we can bring it down.

    SteamyTea I have been looking for LPG CHPs but have not been able to find any that do not require a connection to the grid. If you can let me know which system you were referring to that would great.

    We have been lucky that the proposed house is located next to Midas a Bio-fuel distributor. (http://midas-uk.co.uk/services/biodiesel/) The back-up bio-fuel generator still seems a good option here if we can harness the heat recovery.

    Thanks all for the advice so far, it has definitely helped a lot.
    • CommentAuthorSteveZ
    • CommentTimeMar 3rd 2016
    Looking a bit further into the future, have you considered the following technologies?

    ReDox Flow Battery. Now available in small capacities from Redflow, as a package with a suitable Victron inverter for linking with your PV array.
    Fairly small footprint
    No DoD to worry about, you use all the rated capacity
    Can be charged at a high rate
    Low maintenance
    Expensive initially
    Still not very friendly electrolytes, but better than Vanadium
    There are better electrolytes in development but this one is least bad at the moment!

    Microturbine and high speed direct drive generator - see Capstone as an example
    Can run on biofuel, kerosene or any normal gas supply
    Supplies heat and power with very low harmful emissions
    Expensive initially
    Low maintenance compared with usual IC generating sets
    Wastes heat if needed in summer unless you have a pool, but used with PV could provide year round energy, DHW and heating
    Commercially available as a small CHP unit, but probably too big for one house unless it is a big one!
    Hi SB,

    lovely project, I can see why you wouldn't want to put panels on the roof, in which case I would forget solar hot water, it's far easier to move electricity than water :-)

    Battery technology HAS moved in in leaps and bounds recently but there's still nothing that comes close to FLA in terms of cost, durability and ease of maintenance. If you have deep pockets and space is a premium then LiFePO4 is the way to go they're light can be deeply cycled and have good life expectancy. If you believe in global financial meltdown and the collapse of society as we know it then get NiFe, they'll last forever but are horribly inefficient.

    Rolls are great batteries, I use them and look after another 'off grid' property that does, but if I lived near civilization I'd go for fork lift cells.


    Not sure why you would need to be on the grid to use CHP, how does it know you're on the grid??? When you're 'off grid' you create your own 'mini grid'. Mine extends over a mile and used to service 3 properties, I use grid tied inverters and they are quite happy with my grid. If you're 'off grid' inverter/charger 'bi directional' (not transformerless) and is of sufficient size it should run anything.

    Really love you barn, Paul
    Thank you Paul and SteveZ,

    I have been looking at these Flow batteries and thought they weren't available in UK, I have now emailed Redflow to see whether there is enough budget! I have also found this company http://www.greenacornenergy.co.uk/#!redt-flow-battery-for-energy-storage/c2k5 who supply a similar product REDT flow. I am going to do a comparison chart for all these batteries, which I can post up once I have collated all the information.

    Just to clarify the domestic CHPs that we have found marketed in the UK connect to mains gas for the initial installation, you can then run them on LPG but they need that initial connection (you don't need to connect to the electricity mains). The Microturbine seems to be what we are looking for but is too large for a singular dwelling.

    Thanks for the advice!
    There is a 5kw biomass CHP made by Inresol in Sweden but I think there having difficulty getting it approved for Fits and RHI on the UK market as its to big for the 2kw micro chp exemption.
    • CommentTimeMar 3rd 2016
    Posted By: Studio_BarkJust to clarify the domestic CHPs that we have found marketed in the UK connect to mains gas for the initial installation, you can then run them on LPG but they need that initial connection

    That sounds rather strange, do any of the suppliers publish the information on the web?
    • CommentAuthorringi
    • CommentTimeMar 3rd 2016
    The 1st generation CHP boilers are likely to give you a lot more heat then you need. Fuel Cell CHP seem to be only being put into "test sites" in the UK. Even fuel cells seem to give out more heat then electric, so it would be interesting to know the best level of insulation.

    Personally I would be trying very hard to get a grid connection, as PV gives such low output in winter.
    • CommentAuthorEd Davies
    • CommentTimeMar 3rd 2016
    Posted By: Studio_Bark: “…we had hoped battery technology would have developed further.”

    Battery technology has moved on a lot since the likes of those Rolls batteries. What there isn't, though, for the new battery types is an off-the-shelf solution for managing them, particularly not off the grid. On grid things are a bit better with the likes of the Tesla PowerWall [¹]. I'm not sure but I think those and/or similar boxes can be configured for off-grid operation (the Tesla can be used as a UPS when the grid's down, for example) but setting that up will need somebody who knows what they're up to.

    [¹] https://www.teslamotors.com/en_GB/powerwall

    They're not intrinsically more complicated than lead-acid other than that you have to monitor the individual cells (or more precisely: you're better off monitoring the individual cells with all technologies but you can get away without it for lead-acid). It's just that lead-acid is so much better known and more common (in boats and so on as well as houses) that the chargers to suit are easily available together with understanding how to set them up.

    Even then, people don't necessarily get it right:


    OTOH, for domestic energy storage the advantages of the new technologies (more energy per m³ or kg, primarily) are less important so the choice of lead-acid is not terrible. The main advantage of, say, lithium ion batteries is that they can be left in a partly charged state which suites off-grid use and partial charges on dull days better. Lead-acid's life is significantly reduced by being left partly discharged.

    For my house I plan to use LiFePO₄ cells. However, that'll need me to sort out the battery management myself. In some ways it's the least of my worries but it's one area where it'll need practical experience and experimentation to get things right. It'll be a while before things settle down and dealing with these things becomes common knowledge.
    Good luck with the LiFePO4's Ed, the efficiency is staggering and we cycle them to 80% DoD daily but the BMS is a feckin' nightmare. Two or more years of regular daily cycling and they still show no sign of degradation but I've lost count of times we've had to sort the BMS out. Fine if you are that way inclined but FLA is still more cost effective and 'idiot proof' at the mo. It's a house, not a car or airplane so why worry about the weight or space they'll take up.

    Cheers, Paul
    Thanks All,

    Following the suggestion from renewablejohn I have been speaking to http://www.omnihp.co.uk/ who are working with Inresol in Sweden, among others to bring a biomass CHP to the UK for domestic projects. They are expecting to start installing these over the next year with a demonstration site at staffordshire university.

    The main issue is they produce more heat than is required, for a 5kwh power generator it produces 15kw thermal output (triple that needed for a single dwelling). It is also currently not qualifying for domestic FIT incentives but is hoping to have that sorted by next year.

    Further to this they are looking at pairing this up with a sceptic tank to harness the anaerobic gasses to produce bio-gas.

    Battery wise Inresol have developed a battery similar to a Tesla Powerwall which is integrated into the CHP.

    The CHP itself comes ready to run off bio-mass wood pellets, however it can be fueled by most sources, diesel, bio-diesel, gas, bio-gas.

    As an off-grid solution it seems very neat.
    • CommentTimeMar 7th 2016
    Posted By: Studio_BarkThe main issue is they produce more heat than is required

    I once read that the best way to look at a CHP system is as a heat generator of the required capacity that happens to generate some additional electrical power, rather than the other way around. In particular, a house may require 5 kW of heat in winter (although it would need to be pretty large if built to a good standard of insulation) but it won't need any heat in summer, so there won't be any additional electric power then either.

    Or else you need to recalculate all the theoretical efficiencies when comparing it against other technologies.
    • CommentTimeMar 7th 2016
    Posted By: Studio_Barkfor a 5kwh power generator it produces 15kw

    kWh, not kwh and kW, not kw.

    Posted By: Studio_BarkFurther to this they are looking at pairing this up with a sceptic tank to harness the anaerobic gasses to produce bio-gas.
    Good luck with that one.
    • CommentTimeMar 7th 2016
    Posted By: not SteamyTeasceptic tank

    You missed this one. I'd love one of those.
    • CommentTimeMar 7th 2016
    Life is like a poo sandwich, the more bread you have, the less poo you have to eat :shocked:
    If your looking at chp using septic (sceptic) tank then this landfill system has a good track record but really more suitable to a small farm.

    • CommentAuthorEd Davies
    • CommentTimeMar 8th 2016
    Posted By: djhbut it won't need any heat in summer
    Says the man who often points out, quite rightly, that DHW is more of a problem than space heating for properly done new build.

    Posted By: Studio_Barkfor a 5kwh power generator it produces 15kw thermal
    Proper capitalization, as Steamy mentions, would be nice but its lack is not really a likely cause of misunderstanding. Comparison of kWh against kW is a bit more suspect - I imagine that either could be used here if done consistently and that this is just a typo (the numbers look about right for that) but it would be good to be sure that there's not a more serious confusion going on.
    • CommentTimeMar 8th 2016 edited
    Posted By: Ed Davies
    Posted By: djhbut it won't need any heat in summer
    Says the man who often points out, quite rightly, that DHW is more of a problem than space heating for properly done new build.

    Indeed, and I confess that I also make the assumption that a sensibly done build will also have enough PV or solar thermal that DHW won't be a problem in summer. No need to burn things in summer, apart from on the BBQ.
    • CommentAuthorEd Davies
    • CommentTimeMar 8th 2016
    Posted By: djhIndeed, and I confess that I also make the assumption that a sensibly done build will also have enough PV or solar thermal that DHW won't be a problem in summer.
    Indeed - but an assumption not entirely consistent with the original post and some of the further discussion.
    • CommentTimeMar 8th 2016
    Well I do hope the OP listens to you and the others as well as me that have pointed out the merits of increasing the size of the PV array.
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