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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorNoodle
    • CommentTimeJan 16th 2013 edited
    Just a quick question,

    As my house is listed and in Dartmoor, solar thermal or PV on the roof is a big no no. I am planning an approx 4kw pv array on one the outbuildings - direct south aspect.

    Question is can I do the same with solar thermal? Would too much heat be lost over pipe run outside - estimate approx 10 - 15m from outbuilding to proposed location of thermal store. Obviously would propose to highly insulate the pipe run, even bury it 4ft underground it recommended? I assume the solar thermal doesn't "have" to be higher than the store as its pumped round isn't it?

    Perhaps daft questions but would be useful to know the answer.

    • CommentAuthorEd Davies
    • CommentTimeJan 16th 2013
    Let's do the easy one first:
    I assume the solar thermal doesn't "have" to be higher than the store as its pumped round isn't it?

    Yes, it's usually pumped so the relative heights don't matter much. Actually, having the panels below the store helps slightly as a) the thermosyphon effect can help move heat from the panels to the store and b) the lack of night-time reverse thermosyphon can reduce or eliminate the need for none-return valves and careful pipe routing.

    If the panels are well the below store and there aren't drops in between (probably not valid in your case) then use of pure thermosyphon (i.e., without a pump) is possible.

    10 to 15 m is a long way but it might be possible. Losses can be offset by insulation but there's also the volume of water in the pipe which needs to be heated each time the sun comes out to consider. Maybe somebody else will have some good input.
    • CommentTimeJan 16th 2013 edited
    How much room is there on the outbuilding. The reason I ask is that you could always put a bigger area of ST on it to counteract the losses, but may pay for it in the summer when all the losses not so signifcant and the insolence is higher.
    Alternatively, you could up the amount of PV you install, you could put 4.5 kWp (15 x 300Wp modules) through the inverter and off to the grid (get your FIT payments increased on that too) and then have a secondary system feeding an ordinary E7 water tank (say a 2.1 kWp, 7 module and separate inverter). That would be 22 identical modules of 1m by 1.6m (ish). You may have to get a 2 kW heating element made, but that is probably not that hard or expensive.
    Just make sure that the cables are thick as there is only a 1% volt drop allowed between the inverter and the main consumer unit.
    Much easier to fiddle with cables than pipes.
    • CommentAuthorEd Davies
    • CommentTimeJan 16th 2013
    Don't ignore the possibility of using PV to heat your water. The price of PV is currently low enough to make it worth considering:


    You need more roof area for PV with a given output than for solar thermal but if you have the space it might work for you.

    Unfortunately, you can't really just connect the panels directly to an immersion heater:


    Still, worth a thought if you have the space.

    Also, Navitron also have some slightly cheaper panels available:


    Not MCS approved but if you're already at the 4 kW limit for higher rate FITs and don't have the patience or confidence to worry about RHI then that's not a problem.
    • CommentAuthorNoodle
    • CommentTimeJan 16th 2013
    thanks for your replies, ive been looking at this forums discussion:


    who has run solar thermal from ground array and looks interesting.

    Question, over a year do you think the energy produced and FIT payments from a larger PV array would ultimately be a cheaper/ more efficient solution than solar thermal for hot water. I assume by E7 water tank you mean heating a hwc up each night with e7 for the next days use, offset by the money and energy generated by the array?

    I have alot of outbuilding roof space orientated direct south by luck - at least 25m * 3m and more if need be.

    Noted about thermosyphon and potential loses, panels would probably be similar level but pipe runs would have to drop to underground ground (5m) and then rise again in house .

    • CommentTimeJan 16th 2013

    I win :bigsmile:
    • CommentAuthorNoodle
    • CommentTimeJan 16th 2013 edited
    thanks, will have a read through your links!
    • CommentTimeJan 16th 2013
    You have to separate the FITs payment from the Solar Thermal side (this rather assumes that RHI is not going to be worth it)
    I would use an ordinary 300 lt (ish) E7 cylinder. You just directly wire the 7 (or more, you can up it to 10 to make life easy) modules to a 3 kW 'island' inverter and then off to the bottom element in the tank. The top element can be on the ordinary electricity circuit, on a timer or not as that is up to you.
    All this you can DIY if you want to, but would recommend an electrician do the wiring to the tank.
    • CommentAuthorEd Davies
    • CommentTimeJan 16th 2013
    Posted By: SteamyTeaAlternatively, you could up the amount of PV you install, you could put 4.5 kWp (15 x 300Wp modules) through the inverter and off to the grid (get your FIT payments increased on that too) and then have a secondary system feeding an ordinary E7 water tank (say a 2.1 kWp, 7 module and separate inverter).

    Why 4.5 kWp? I thought the price break for FITs was 4 kWp - or has that gone away with the new pricing?

    Anyway, another consideration is the limit of 16 A/phase on your feed in unless you can get the DNO (local electricity network) to agree to allow you more. If you had a total of 6.6 kW then, the moment your water tank thermostat dropped out in bright sunshine you'd be over the 16 A limit.

    Still, the idea of having more than 4 kW of PV, only claiming FITs for 4 kW worth, but being able to use the output of more without importing has some attraction. E.g., a 6 kW array with 4 kW through the total-generation meter on a less than fully bright day so it's only producing half output could run a 3 kW immersion while still claiming 2 kW of FITs.

    Something like this might help:

    • CommentTimeJan 16th 2013 edited
    You can fit 4.5 kWp onto an SMA 4200TL and limit it to 16A. By having more panels you gain when the sun is less powerful. I have been told, though not 100% sure, that you can do this under the MCS/FIT rules. There is no reason electrically why it cannot be done. The DNO will have been notified that there is a G83 'generator' on the line but are not worried about the time it is run (or CHP would be a non starter as would wind turbines).
    • CommentAuthorskyewright
    • CommentTimeJan 17th 2013
    Posted By: SteamyTeaI have been told, though not 100% sure, that you can do this under the MCS/FIT rules.

    IIRC it's complicated, and not necessarily logical...
    • CommentTimeJan 17th 2013 edited
    Yes, I remember reading that and still not 100% sure. If you had a 3 kW inverter and fitted 3.5 kWp of modules would anyone know or care? At the 4 and 4.5 kWp level it is just about within a 10% 'error' margin. Be a different matter if you were pumping 6 KWp in I suspect.
    Though the big gain is at the lower end where it is not so easy to measure the performance as the inverter, MPP and module efficiency are a bit more erratic.
    I have only been involved with 2 oversized installations, one a 3.5 on a Fronius 3kW IG TL (I think) and 4.41 kWp on a SMA 4200 TL. The Fronius soon hit max overpower and the fan kicked in which was noisy and the SMA just sat there as happy as Larry at the 16A limit. Not sure about the reliability as company went bust (but I left before that).
    I would suggest that the inverters can draw in cool air, so no loft installations, or at least put in an air duct from the cold side of the house.

    With the Fronius the reason we went oversize was that it was a single string inverter and needed a minimum voltage to start working, which down here with the clouds caused a lot of under voltage errors. Would not use one again. Though I am rather an SMA fan as they have the monitoring built in and can be accessed by a Bluetooth device at no extra cost.
    • CommentAuthorSteveZ
    • CommentTimeJan 17th 2013
    Hi Steamy - I think the 4kW rule is set in the FIT legislation, or I would have gone for a higher figure to produce more on dull days. The only case I can think of for an exception might be that the power figure is taken at the inverter after losses, but I am pretty sure it is the headline power rating on the panels that count.

    The DNO limit of 16A is locally set and is fixed on my inverter. The actual max output in kW going to the grid is determined by the local line voltage, and can go over the theoretical limit of ~3.7kW, if the line voltage is nearer 250 than 230. Mine is usually around 240 - 244v

    I have seen 4.3kW on my 4kWp system - that seems like a long time ago, when the sun was shining! :cry:

    Hi Noodle - could you use smaller diameter pipes for your ST. Greater pumping effort but less water content to lose heat when switched off
    • CommentAuthorEd Davies
    • CommentTimeJan 17th 2013
    Posted By: SteveZHi Noodle - could you use smaller diameter pipes for your ST. Greater pumping effort but less water content to lose heat when switched off

    Oh, yes, the OP's actual question. :smile:

    Another option for reducing the losses in the relatively large volume of the pipework arising from starting and stopping the flow all the time as the sunlight varies is to have a relatively small tank in the outbuilding and do a “bulk” transfer of heat from that to the main tank in the house when it's hot enough. Needs more antifreeze or that tank could double as a drainback tank.
    • CommentAuthorDon_Munro
    • CommentTimeJan 20th 2013 edited
    When I did the research in to making a block of flats that could run as economically as possible. Solar water heating was one of the things researched. The investors who asked to look into this introduced me to a German marine engineer who had put in a system for his parents in the North of Germany, there house did not sit at the right angle for the panel’s to go on the roof but the outbuilding’s did. He altered the angle of the south face to the right incline for the panel to gain maximum sun, also they were built in their own little green house (so as to speak), and this was backed with reflective panels. The pipes were lagged to slightly over 100 mm all round and this was put inside a large plastic pipe which had glued collars and buried in the yard. The run across the yard was just under 15meters and the house was two story, he used a hydraulic type of pipe that can withstand a lot of heat this took expansion out of the equation. In the house he used a heat exchanger similar to a marine engine, this kept the mixture in the solar panels separate from the central heating system, and the cold water supply to the tank went through the exchanger. He told me that in the summer you could cook on this it was so hot, the pump was a standard central heating pump and it was thermostatically controlled so as not to pump cold water.
    These pipes are designed to be burried underground and would support the high temperatures of solar.

    • CommentTimeJan 21st 2013
    "CASAFLEX is a flexible, self-compensating, endless production, monitorable and heat-insulated pipe system with a special steel medium pipe.

    Operational data
    Excellent heat insulation at high temperatures
    The diffusion barrier and the structure of the CASAFLEX pipe system guarantee minimal heat losses as well as resistance to high temperatures. Operational temperatures up to 160 °C, peak temperatures up to 180 °C, PN 16/25, DN 20-100."

    Shame it says nothing about the thermal properties apart from maximum temperature.
    • CommentAuthorwoodgnome
    • CommentTimeJan 21st 2013 edited
    • CommentTimeJan 21st 2013

    So taking a worse case scenario of 70°C source and 60°C return that will be 15m x (9.4 + 7.8)W/m loss, or 260 W loss.
    I would stick to PV and 6mm cable.
    • CommentAuthorwoodgnome
    • CommentTimeJan 21st 2013
    Steamy, i was considering something similar. as my proposed new build won't be orientated for panels on the roof.
    Do you think a secondary system wired into an E7 tank is the way to go?
    • CommentTimeJan 21st 2013
    It is probably the easiest and cheapest to fit, performance is a bit harder to work out as depends on how much water you want to heat and how much sunlight you get. You would have to contact an inverter expert to see how well they can cope with a 3kW resistance load when the panels are producing less than that, I am sure someone will know.
    Would also be worth looking at cable losses and working out if you are better having an AC or a DC run to the house, not sure how much difference it would make, but easy enough to work out. There are a few online cable loss calculators and the Carbon Trust/EST solar calculator will give you an idea of PV outputs (as will PVGIS).
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