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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorcubbs
    • CommentTimeJan 5th 2017
     
    Hi

    Iv enot been on here in a good while. I finally got our house up and running with an oil boiler heating my thermal store

    I know theres a lot of variables but could it be possible to generate enough electric from Pv panels to run an airsource heat pump here on Isle of Man? Im sure theres some experts out there that would know the figures?

    I am tempted to buy solar thermal panels to connect to my store but id like to keep my options open

    Thanks
    • CommentAuthorEd Davies
    • CommentTimeJan 5th 2017
     
    Without going to complicated extremes PV's unlikely to be able to cover all of the power needed, particularly when you need it most in the middle of the winter.

    Ironically, the better the house is insulated then less likely PV is to contribute much as with good insulation you'll only have a short heating season just when the PV is producing least. An obvious question, therefore, is how much of the year do you need to use your oil boiler for space heating?

    Also of interest is how you do DHW (domestic hot water). If that's off the same thermal store then you'll of course need to keep it hot in the summer in which case PV (either directly to the thermal store or via a heat pump) could contribute a useful amount.

    A good site to play with is PVGIS to give some idea of how much output you'd get from PV panels in different months:

    http://re.jrc.ec.europa.eu/pvgis/apps4/pvest.php

    What inclination the panels are to the horizontal doesn't matter much for total annual production but for an application like this where you want to get the best out of them in winter putting them on a steeper slope can help noticeably. Also, orientation matters more in winter because the sun only pops over the horizon over a relatively narrow arc of directions.
    •  
      CommentAuthorSteamyTea
    • CommentTimeJan 6th 2017 edited
     
    There are two ways to answer this.
    Do you want to run the ASHP directly of the PV. Then the answer is no (except with a very large module array).
    The other way is to off set the electrical usage of the ASHP with electricity generated by the PV, then yes (assuming the sized are reasonable).

    You can use PVGIS to get a production estimate for your location. It does show average energy production and not power though.

    It is worth remembering that when the PV is generating, current it produces will go to the nearest load (by some magic). So if an ASHP was running and the modules were producing a bit of current, that current would be used by the ASHP and your imports would be reduced.
    So one trick is to only allow the ASHP to run during hours of daylight if you can.
    • CommentAuthorcubbs
    • CommentTimeJan 16th 2017
     
    Thanks guys
    • CommentAuthortony
    • CommentTimeJan 16th 2017
     
    On start up heat pumps have very high power demands
    • CommentAuthorGreenfish
    • CommentTimeJan 17th 2017
     
    Most ASHP have a "soft start" designed in these days, but do draw more when the delta T is bigger. For example my nominal 8kW (output) unit was drawing 2.5kW for the first 15mins this morning, then dropped to ~1kW topping up the UFH for the remiander of the hour.

    But at no time today, overcast and grey, has my 3kW PV roof array generated more than 500W. I run my ASHP only during daylight, so any PV generated power is used by it, and on a sunny day it is great to heat more water via ASHP than an immersion would do. But a stand alone PV to ASHP system would not work.
    • CommentAuthordaserra
    • CommentTimeSep 4th 2020
     
    Sorry to bump this old thread but I have a similar question:

    I am in Portugal and am planning to have an extra PV array to run an ASHP during sunny days in winter. I also have a 10kW to water WBS that heats a 300L buffer tank each evening (direct connection) that provides radiators (aluminium 45C flow temp) during the night and lasts until 9-10am in the depths of winter (drops to around 0-5C at night). The house has huge thermal mass (rammed earth) and is well insulated with lots of passive solar heat gain (large west facing windows with electric shutters).

    I would like to heat this tank using something like a Valiant Arotherm high temp ASHP (75C max, R290 gas) during the day using PV when air-temps are around 18C. The ASHP is 3.5 kW so I am estimating around 3 hours to heat the tank. The radiator pump shuts off at 45C so the ASHP is looking to heat the tank from 45-75C each afternoon.

    I am trying to avoid changing any of the existing equipment and keeping the WBS connected for really cold days or cloudy days. I have no space for a larger TS. The aim is to reduce my wood use.

    Can anyone shoot any holes in my idea or come up with a better idea ?
    •  
      CommentAuthorDamonHD
    • CommentTimeSep 4th 2020
     
    What average CoP are you expecting over the 45 to 75C heat delivery?

    Are you significantly constrained on space for PV? Are you happy to use second-hand (thus cheaper) PV?

    What I'm inching towards here is a larger PV array feeding a simpler immersion (ie resistance) based heating might possibly work out better in CAPEX costs, embedded carbon, etc.

    Wouldn't work for me at all on that basis in space-constrained London 'burbs, but I'm still considering a variant once covid is behind us...

    http://www.earth.org.uk/note-on-solar-DHW-for-16WW-UniQ-and-PV-diversion.html#Simple

    Rgds

    Damon
    • CommentAuthortony
    • CommentTimeSep 4th 2020
     
    Heat pumps and high start up currents and PV won’t cope with that

    It will run it but output needs to be guaranteed to be above run demand power which is unlikely

    If connected to the grid it will be ok

    For heating poor as max demand at point of minimum solar insulation
    • CommentAuthordaserra
    • CommentTimeSep 4th 2020
     
    Thanks for your replies.

    I am awaiting the response regarding the COP at high temps, at 55C flow temp and 2C outside it is rated at 4.6. I am not at all restrained on PV space, I've been buying 340W panels for 157€ which seem pretty cheap. I did mull over the idea of using a simple immersion as it would save thousands in equipment costs, embedded carbon and labour/disruption. 12 panels would be able to power a 3kw element for around 3 hours which I think would just about do it. A nice cheap simple idea, I already have a raspberry pi controlling dumping of excess PV to AC in summer !

    Tony, I am grid connected for now (battery prices don't justify off-grid just yet) with a 6.9kw monophase supply. The 300L buffer is the way I am hoping to store the PV energy until night-time. If I had a larger TS it could all happen at lower temps but unfortunately no space as the buffer tank is embedded in the middle of my house.
    •  
      CommentAuthordjh
    • CommentTimeSep 4th 2020
     
    Posted By: DamonHDhttp://www.earth.org.uk/note-on-solar-DHW-for-16WW-UniQ-and-PV-diversion.html#Simple


    The council is determined to pull our place down in a few years, for redevelopment.

    Oops! That seems like a new thread, right there. :bigsmile:

    How much of what you have installed in the house will you be able to take with you and reuse?

    What are your plans for a new place?
    • CommentAuthorEd Davies
    • CommentTimeSep 4th 2020
     
    Do you really need such high temperatures?

    With a high-thermal-mass well-insulated house I'd wonder if you actually need the thermal store at all for a solar-powered system that far south, i.e., with relatively long days even in the depths of winter. In Porto (towards the north of Portugal) even on December 21st the day is just over 9 hours long and the sun gets to an elevation of just over 25° at midday.

    Maybe the best strategy is to just heat the house at lower flow temperatures all the time solar energy is available; either via a ASHP or via direct electric heating of the water (that's a separate question). I.e., use the large mass/low temperature building rather than the relatively low-mass/high temperature TS to store the heat.

    Last January the maximum flow temperature to the radiator in the study of my poorly insulated rather-questionable-thermal-mass study in the north of Scotland was just under 51°C and the average for the whole month was around 30.6°C. That's with the heating on about 16 hours a day and off 8 hours or so overnight. That's from an oil boiler in a quite different situation from yours but still the point is that heating for a longer time at lower temperature works.
    • CommentAuthordaserra
    • CommentTimeSep 4th 2020
     
    Thanks for your reply Ed. The main issue I have is that the PV generates during a narrow band of time at close to its design power, and also I have a small TS/buffer so to store enough for the morning the buffer temp. needs to be high.

    To answer your second point, we already heat with passive solar on the west side and the house gets almost uncomfortably hot late afternoon while a bit parky early morning. The aim is to add a little thermal "momentum" early in the morning. The existing system normally shuts down between 12am and 2am.

    I am erring on the side of a 3kW immersion heater for simplicity and low cost; PV is so cheap now and my roof is pretty big.

    When I have left the house unheated over winter it stabilises around 13C.
  1.  
    Posted By: daserraand also I have a small TS/buffer so to store enough for the morning the buffer temp. needs to be high.

    Could you install a large TS outside, preferably next to the house against a wall in a highly insulated box. This would negate the need for a high temp TS and give the heat storage required.
    PV and motors don't mix well so an ASHP would need the PV to be grid tied or have battery stabilization however a straight forward resistance immersion heater can work and would, as you say be simple and cheaper than an ASHP.
  2.  
    My thought process would be as follows;

    - you have grid power, so running a HP is possible
    - you want it ideally to only run when the sun is out (ie. not behind a cloud)
    - you shouldn't control a HP to cut in/out to match the solar insolation
    - not sure how your Rpi device works, but a regular solar excess energy diverting device will feed down to say 200W of excess PV elect to your chosen device, so it's not a case of 3hrs @ 3kW, it's 8 hours varying from 200W up to 3kW and back down again
    - if you run the HP, sometimes you'll have sufficient PV to cover all the running power, but sometimes you'll be importing, and you won't really have control over that.

    If the above is along the right lines, I would go for a good number of PV panels, and feed the excess PV via the immersion. My own system, and various clients, use this approach (with LBS and therm store), and it is so simple. As I think Ed said above, go for a more vertical PV inclination if you can, to increase the winter capture.
  3.  
    daserra
    Another point to consider is how your grid tied metering is done. Over here the meters are push-pull with an annual reconciliation. This means PV over production in the summer (grid feed in) is balanced by drawing from the grid in the winter. Whilst I accept this does little to help max grid demand (think winter evenings) if you have such a system it would help your situation as you could have an immersion heater in the TS and have enough annual PV generation to match the annual demand.

    BTW why do you want to reduce your wood burning? pollution or supply/storage issues. Just interested!
    • CommentAuthordaserra
    • CommentTimeSep 4th 2020
     
    GreenPaddy,Y
    Your conclusion is similar to mine, an immersion heater. I would be unable to control the inverter in the ASHP as you suggest, I can just turn loads on and off. However I am able to mimic the curve of the PV generation by turning various loads on and off during the day, saving the largest loads for the middle of the day; this immersion would probably be the largest load. Perhaps I could fit a 1kW immersion & a 2kW, there is no fitting for this but there is an access panel at the bottom of the tank I could have drilled and threaded.

    Peter,
    I do feed in however due to Portuguese bureaucracy I receive nothing for this so I try to consume as much as I can. An outdoor TS wouldn't be a preferred option. I am trying to reduce dependence on wood as I spend quite a lot of money on it and although I burn fast and hot, combustion isn't the best for the environment. I'd also like to have a labour free option and probably more so when I get older ! I am pretty sure eventually , with enough PV I could reduce my wood consumption significantly.

    Thanks all for your comments, very helpful to be able to mull these things over with like minded people, Mrs daserra generally glazes over :wink:
  4.  
    Posted By: daserraI do feed in however due to Portuguese bureaucracy I receive nothing for this so I try to consume as much as I can.

    There is very little paid for feed-in here but by balancing the installation against the expected demand with the aim to end the year net zero excess investment in PV is avoided (as there is no financial incentive to over produce) An advantage over the UK system as the FIT payments decrease over time


    Posted By: daserraI am trying to reduce dependence on wood as I spend quite a lot of money on it and although I burn fast and hot, combustion isn't the best for the environment. I'd also like to have a labour free option and probably more so when I get older ! I am pretty sure eventually , with enough PV I could reduce my wood consumption significantly.

    Good reasons IMO
    • CommentAuthordaserra
    • CommentTimeSep 4th 2020
     
    My main array for house electricity is at 45' for exactly this reason, to maximise winter gain and flatten my yield curve, however I have noted I don't meet demand in the early evening so I am planning another array pointing due west, which although it won't be operating in a way that maximises the electricity it produced, will minimise the amount of electricity I have to buy. I do have a long term aim to be off-grid, not just for environmental financial reasons, but the battery payback time is 17 years or so, I think I'll wait 5 years when I hope we will see a decent drop in prices as lithium/iron and flow battery technologies evolve and become cheaper.
    • CommentAuthorEd Davies
    • CommentTimeSep 4th 2020
     
    Posted By: daserraThe house has huge thermal mass (rammed earth) and is well insulated…
    …and the house gets almost uncomfortably hot late afternoon while a bit parky early morning.
    These two statements aren't terribly consistent; if a house is really high thermal mass and well insulated then the temperature should only drop a degree or so, at most, overnight even when the outside temperature is down towards freezing. Perhaps we should understand what's going on with this before going much further?

    Silly question, but the insulation's not on the inside, is it?
    • CommentAuthordaserra
    • CommentTimeSep 4th 2020
     
    Hi again Ed.

    The old core of the house is rammed earth (50cm, unknown u-value), the add-on extensions at either end are a 50cm thick masonry walls (hollow clay pot blocks) with 10cm of compressed cork in the middle. The south facing wall has outer masonry in 20cm Ytong (AAC). The u values of those walls are about 0.35.

    A 1m wide strip of floor is insulated with 4cm XPS around the perimeter, the centre left uninsulated to use the earth as a thermal store (following the Passivhaus guidelines for properties in Portugal). The roof is mainly 9-10cm XPS (u-value of the entire sandwich is about 0.35) although the original rammed earth section roof is only 3cm XPS. For Portugal these are very good levels of insulation.

    I built the place in stages over 20 years and was learning as I went along. We also have built to deal with the hot (40-50C) and extremely sunny dry summers so optimising for heat gain or cooling has been tricky.

    It gets uncomfortably hot in winter because of the west facing glass with direct sunshine and air-temps around 18C, 2/3 of the west facing walls are glass. This is by design in the winter, there are electric shutters for shading when required, I was aiming at 100% passive solar heating but fell short.

    It cools down quickly as our night time temps in the interior of the country are quite low, 0- 5 C , sometimes as low as -5C. The house isn't particularly airtight as we suffer from condensation due to the large day night swing in temps with high humidity, so I am happy with sacrificing some heat to alleviate this. At this point it would be impossible to have an airtight envelope.

    When I say "parky", I am a bit of a softy since my blood has thinned living here, it's usually 18C in of a Dec/Jan morning, 20C in the evening with the WBS running; the house has a pretty even temperature throughout.

    In the last 24 hours it went from 11c to 38C outside, inside from 21 - 24C, so in Summer it's pretty stable. All windows are open at night of course.
    • CommentAuthorgyrogear
    • CommentTimeSep 5th 2020
     
    Posted By: daserraIt gets uncomfortably hot in winter because of the west facing glass with direct sunshine and air-temps around 18C, 2/3 of the west facing walls are glass.


    Any way you could buffer this hot area, to separate it from your main living area, and perhaps divert the heat into a colder part of structure (such as basement if U have got one...)

    gg
    •  
      CommentAuthordjh
    • CommentTimeSep 5th 2020
     
    Posted By: daserraHowever I am able to mimic the curve of the PV generation by turning various loads on and off during the day, saving the largest loads for the middle of the day; this immersion would probably be the largest load. Perhaps I could fit a 1kW immersion & a 2kW, there is no fitting for this but there is an access panel at the bottom of the tank I could have drilled and threaded.

    This sounds unduly complicated. A single 3 kW immersion is capable of absorbing anything from 0 to 3 kW. There's no need for any additional heaters or other devices. The diverter device automatically measures how much power you are generating and sends just that much to the immersion.
    • CommentAuthordaserra
    • CommentTimeSep 5th 2020
     
    Hi gyrogear,

    Half the glass area is open to the rest of the house and that does increase in temp. during the afternoon.

    djh

    The system is already complex but those devices are interesting, "Immersun" right ? I already have a whole house electric meter connected to my Rpi and SMA data from the PV (Domoticz with Zwave relay modules and sensors) so I know when I am exporting.
    • CommentAuthordaserra
    • CommentTimeSep 5th 2020
     
    Looking at the high price of the Immersun I think I'd rather build something else connected to the pi, something like a high power transistr driven by a Zwave dimmer unit. Then I could integrate it myself.
    • CommentAuthorgyrogear
    • CommentTimeSep 5th 2020
     
    Posted By: daserraHalf the glass area is open to the rest of the house and that does increase in temp. during the afternoon


    OK, therefore the intent of a solar buffer zone would be to capture the solar gain, prevent it from invading the living area, and transport the heat elsewhere where it is of greater use (such as to cold side of house etc.) OR to a solar heat dump...

    This vid will give the idea ,if you have the time (IMO, it is worth it !).

    https://www.youtube.com/watch?v=lzGaLw__kdk

    gg
    • CommentAuthorbhommels
    • CommentTimeSep 5th 2020 edited
     
    Posted By: daserraLooking at the high price of the Immersun I think I'd rather build something else connected to the pi, something like a high power transistr driven by a Zwave dimmer unit. Then I could integrate it myself.

    The most efficient unit I have come across for driving an immersion heater using excess PV is the one by Robin Emley:
    https://mk2pvrouter.co.uk/about.html
    There are extensive descriptions for the DIY enthusiast on his website, and the cost is very, very low.
  5.  
    Export minimiser - not sure exactly how your control set up works, but what I understand is you switch loads ON/OFF. The export minimisers, of course vary the power feed, so it's not all or nothing.

    I've fitted quite a few of these, including in my own house. This is super simple, no fancy displays. Fit and forget. I know nothing about elect regs/feeds in Portugal, but can't immediately see why this wouldn't work for you. At ÂŁ200, for me it's a no brainer. (I have no relationship whatsoever with this product or suppliers, though I do LOVE knowing all my excess generated elect is making hot water).

    https://www.earthwiseproducts.co.uk/product/solic-2000/
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