Home  5  Books  5  GBEzine  5  News  5  HelpDesk  5  Register  5  GreenBuilding.co.uk
Not signed in (Sign In)


Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

PLEASE NOTE: A download link for Volume 1 will be sent to you by email and Volume 2 will be sent to you by post as a book.

Buy individually or both books together. Delivery is free!

powered by Surfing Waves

Vanilla 1.0.3 is a product of Lussumo. More Information: Documentation, Community Support.

Welcome to new Forum Visitors
Join the forum now and benefit from discussions with thousands of other green building fans and discounts on Green Building Press publications: Apply now.

    Hi All

    Charenteman in France here.
    I have been busying myself during the respective lockdowns in framing and insulating barn roofs and walls in readiness for 1st fix plumbing and electric works prior to plaster boarding the lot.

    During this time I have been trying to firm up a heating and hot water scheme using a heat pump solution.
    Earlier thread discussions had suggested that integrating pv panels would be a good idea - connected straight into the Hot Water Cylinder with any excess electricity (when HW is at desired temperature) stored in back up batteries to power the heat pump?).
    The trouble is that when I explain this objective to local plumbers I get glazed looks. They all propose heat pump packs (Hitachi, De Dietrich, Mitsubishi) with outdoor unit and indoor unit (with HW storage) serving both central heating and hot water demands from a single tall unit.
    Connecting PV panels is not an option they say.
    Can anyone out there set me straight on this and suggest some tried and tested component combinations that I could refer them to?
    Any help appreciated!

    • CommentAuthorEd Davies
    • CommentTimeNov 13th 2020
    I'm not sure what have in mind regarding PV panels “connected straight into the Hot Water Cylinder”. Do you mean a DC connection to an immersion heater? That would be tricker than it might seem for various reasons and would be an odd choice, particularly for an on-grid setup.

    In general, the only “integration” that's likely to make sense is to try to arrange for the heat pump to run mostly when the PV panels are producing.

    Storing energy in batteries for any later use is difficult to justify on grid. Using batteries to store energy to later use for heating seems a complete waste to me; it's much cheaper to turn the energy into heat immediately then store that.

    That doesn't apply for longer-term storage of larger quantities of energy such as inter-seasonal heat stores for district heating systems.

    Back-of-the-envelope calculation says the electro-chemical energy storage of a lead-acid battery is only twice that of just heating it from 20 to 95 °C and you could use any knackered old battery for that. That's assuming a specific energy of 41 Wh/kg ( https://sinovoltaics.com/learning-center/storage/energy-density-and-specific-energy-of-battery/ ) and a specific heat capacity of 1000 J/kg (apart from water, most things are around that). Even at a CoP of 4 you'd only need something like 16 [¹] times the mass of the batteries to store the same amount of energy at temperatures reasonable for heat pumps. Bricks are less than 1/16th of the price of batteries and water's even cheaper but you need something to keep it in.

    [¹] × 2 for the lower heat capacity than electo-chemical energy capacity, × 4 for the CoP then × 2 again for the reduced temperature.
    • CommentAuthortony
    • CommentTimeNov 13th 2020
    Ed, I am totally with you on batteries , we are swimming against the tide though

    Re Solar PV, big problem is that there is the least amount of solar PV when it is needed the most

    Excess solar pv can be diverted via a black box to make hot water - better would be divert to a heat pump and for that to make hot water, uneconomic unless you have heat pump already

    Heat pumps often have very high start up currents and solar pv cant cope with that so need to be grid connected
    • CommentTimeNov 13th 2020
    I expect that connecting PV panels to a hot water cylinder means just using a diverter connected to the mains to power an immersion as Tony says. It's difficult to use PV panels to power a complicated thing like a heat pump, because the complicated thing needs a constant reliable source of power and PV panels don't provide that. Heat pumps should be (need to be) mains-powered.

    I don't know what diverters are available as CE-marked products in France. If there are some, then you can use one of those connected to an extra immersion in the heat pump's water tank, assuming it is possible to fit one.
    • CommentAuthorEd Davies
    • CommentTimeNov 13th 2020
    Posted By: tonyHeat pumps often have very high start up currents and solar pv cant cope with that so need to be grid connected
    That used to be the case but is nothing like as much of a problem as it used to be with modern inverter heat pumps.

    Posted By: tonyRe Solar PV, big problem is that there is the least amount of solar PV when it is needed the most
    Indeed, but what can be missed is that the angle of the panels makes a big difference. If the panels are near vertical then there's a small reduction of overall annual production but a substantial increase in winter generation.

    However, being as far south as the Charente makes this a bit moot. For a point at the western end of the main runway of Angouleme-Cognac airport [¹] (45.731, 0.207) PVGIS reckons the “optimum” angle of the panels is 38° with annual production of 1234 hours (i.e., kWh per kW peak) with January production of 55.48 h whereas increasing the angle to 70° only increases the January production to 61.14 h while knocking back the annual production to 1091 h (and also significantly reducing the July production to the point that it's less than in some of the shoulder months).

    At this location, though, the difference between summer and winter is less marked than further north. For 38° panels the January production, as noted above, is 55.48 h whereas the July production is 136.77 h so only a 1:2.47 difference.

    [¹] I like to use airports for this sort of thing as they often have relatively clear horizons for some reason.
    Thanks all for your comments.
    From what I have gathered, the credit from EDF for returning pv electric to the grid has now plummeted to about 5p/kwh (was 65p to start with) and their admin system is designed to be impenetrable!
    A payback against set up costs would seem to be unachievable.

    That is why I was interested in any tried and tested options to harness and utilise the solar pv (we have plentiful and strong sunshine in this location - save when it rains!) in the property, in some practicable combination with an ashp heating and hot water combination.
    It seems that my ideas are technically flawed!

    Given that the ashp big boys don’t seem to have seen a need for an indoor unit with integrated hw cylinder that allows pv connection through an immersion heater connection gizmo (unless anyone knows otherwise) I perhaps need to look at whether I could add a standalone cylinder heated by pv fed immersions that could serve my domestic hot water when at a hotter temperature than the ashp integrated cylinder, by way of some thermostatically controlled diverting valves?

    Any final thoughts?
    • CommentAuthorphiledge
    • CommentTimeNov 14th 2020
    Ive no experience with heat pumps but Id be very surprised if you cant find a standalone heat pump to connect to your own cylinder. Cylinders can be built to your spec with as many immersions and indirect input coils as you want to cover boiler, heat pump, woodburner, solar thermal etc. Obviously youd need a pretty big cylinder to get them all in. Newark copper cylinders built a custom cylinder for us and well worth discussing what you want with them. Other suppliers are available, probably!

    If you find local plumbers a bit resistant to something they are not familiar with, it may be worth tracking down a French custom cylinder supplier as that may help get your plumbers on side if they are installing something home grown. Alternatively get a Newark cylinder and stick some French labels on it:)
    • CommentTimeNov 14th 2020
    Posted By: philedgeAlternatively get a Newark cylinder and stick some French labels on it

    Come January that might actually be required? Dunno. :devil:
    • CommentAuthorGreenPaddy
    • CommentTimeNov 15th 2020

    sometimes people think that using PV to heat water means that you have to feed from the PV directly. It's much simpler than that. PV doesn't know or care what it's feeding, it all goes to your dist board, and then to what ever happens to be running, or back to the grid. (sorry if you already knew all that).

    Point being, you need to be able to switch things on, when PV is generating adequately. So a little black box which confirms that you are making excess PV, will then feed power to the piece of kit that black box is connected in line with.

    Here's one I often fit (I have no other link to this company apart from being a satisfied customer).


    You simply insert this in-line in the power feed from your dist board to the immersion in your cylinder, near to the dist board, so you can clip the sensors onto the main incomer, so it can determine if you are starting to export.

    No idea if this can be used in France, thought I suppose you can just fit it yourself after everyone else has left

    The second question is...does it make sense to use this direct electricity user, when you can utilise a heatpump and get a COP of say 3. The points have been made by others above about trying to coincide PV excess generation with running a complex kit, rather than a dumb immersion (which can take the varied voltage to give power inputs of down to about 200W).

    To add a little complexity, it may not matter whether you use the PV, or send it to the grid (financially), since you'll get 5p /kWh from the grid, but if you buy that back later at say 15p/kWh, using it to run your heatpump will drop that actual cost to 5p/kWh due to the CoP.

    So, you might as well just run the heat pump, and if it coincides with excess PV generation, each kWh will cost you say 2p, (opportunity cost of not exporting it 5p divided by CoP). If it doesn't coincide, then it's costing you 5p/kWh.

    That may be moot, however, if you go for air-air heat pump, and have no "wet" element to that system, in which case the use of immersion and excess PV diverter would be ideal.

    Confused? You have been.:devil:
    Posted By: GreenPaddySo a little black box which confirms that you are making excess PV, will then feed power to the piece of kit that black box is connected in line with.......

    No idea if this can be used in France, thought I suppose you can just fit it yourself after everyone else has left

    It could depend how the import / export of PV and grid is calculated.
    Over here we use push/pull meters with an annual reconciliation so there is not much point in using kit to divert excess PV to say DHW, you just export to the grid and import later when you need it.

    I don't know what happens in France.
    • CommentTimeNov 15th 2020
    Posted By: GreenPaddyrather than a dumb immersion (which can take the varied voltage to give power inputs of down to about 200W).

    Just to note that whilst it is true that an immersion can handle reduced voltages, which heat pumps etc can't, that isn't actually the situation with the power supplied by one of these devices (known as 'diverters'). They supply power at mains voltage but continually interrupt it so that the average power supplied matches that available rather than the normal situation where the power supplied through an ordinary switch matches that demanded by the immersion.
    • CommentAuthorEd Davies
    • CommentTimeNov 15th 2020
    For a grid-connected heat pump with PV it seems to me that you ideally want a little box which monitors PV generation, grid import/export, amount of energy in the various thermal stores (DHW tank, slab, whatever), future space and DHW demand (weekday/weekend, number of visitors, whatever), likely sunshine over the rest of the day and the next day or two and any other factors you can think of and decides then when to turn the HP on and off avoiding short cycles.

    Similar things would happen in houses with variable tariff, ones which are off-grid and so on.

    This sort of thing is not entirely trivial. On the other hand, it's really just a SMOP (small matter of programming) and just the sort of thing something like a Raspberry Pi is ideal for.

    Something like, every five minutes:

    1) Look at the slab and DHW tank temperatures and work out some number indicating how desperate for heat you are, i.e., the amount of grid electricity you're willing to take in order to get some more heat.

    2) Look at the time of day and amount of PV generation which has happened over the last hour or so and is happening now to make an estimate of how much generation is likely in the next half hour and look at domestic consumption now and recently to see how much generation is spare.

    3) If the effect of running the heat pump will not cause more import than you're willing to accept for the heat you need then run the heat pump for half an hour then go back and start the calculation again. If that results in the decision to turn off the pump then leave it off for a time before doing another calculation.
    • CommentTimeNov 15th 2020
    Posted By: Ed Davieslikely sunshine over the rest of the day and the next day or two

    Good luck with getting that info :devil:
    • CommentAuthortony
    • CommentTimeNov 16th 2020
    Off grid the pv have a lot of trouble with start up currents - that happens each time the pump kicks in :(
    • CommentAuthorEd Davies
    • CommentTimeNov 16th 2020
    Has charenteman said he's off-grid?

    Also, I'm not convinced that the start-up current is a problem with reasonably modern inverter-driven heat pumps. A while ago I had a look at the specifications for some and they seemed well within the capabilities of standard off-grid inverters to deal with short overloads. You'd probably not want the HP to start (or run) while you're cooking but that shouldn't be difficult to arrange.
    • CommentAuthorEd Davies
    • CommentTimeNov 16th 2020
    Re sunshine forecasts, djh posted: “Good luck with getting that info”

    Yep, it's annoying that a reasonably authoritative basic forecast isn't easily available. Still, screen scrapping, e.g.:


    should go some way. Even:


    should give some clues.
    • CommentTimeNov 16th 2020
    if [ "sunny" != "`wget --retr-symlinks -q -O - https://weather-broker-cdn.api.bbci.co.uk/en/forecast/rss/3day/kt1 | awk '/<title>.*emperature/ { if(++count==2) {if($0 ~ /[Ss]unny/){printf("sunny");}exit;} } '`" ] ; then rm -f /var/run/FORECAST_PV_GEN_GOOD.flag; else touch /var/run/FORECAST_PV_GEN_GOOD.flag; fi
Add your comments

    Username Password
  • Format comments as
The Ecobuilding Buzz
Site Map    |   Home    |   View Cart    |   Pressroom   |   Business   |   Links   

© Green Building Press