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Heating and cooling: Sizing DHW, Solar input and Tank SIze

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  1.  
    • CommentAuthorborpin
    • CommentTimeFeb 2nd 2012 edited
     
    There have, I know, been a number of similar threads on this but I still cannot get to a point where I know what I need or how to calculate it. The concern right now is DHW rather than space heating.

    Firstly is how big an ST array for size of tank? This I think depends on location (near Edinburgh), shading (about 35% in winter at extremes i.e morn/eve), direction (SSE), inclination (42 deg) and type of array (FP or ET). If I size it for DHW for 9 months say, how do you prevent or cope with over heating in the summer?

    Secondly, how much DHW do you get for a tank of a certain size maintained at a specific temperature? This I think depends on water input temperature, assuming no-recharge. So if I want 40 minutes worth of showers, input temp of 7 deg (?) how big a tank and at what temperature? Is it linear - bigger tank v lower temp?

    Help please.
    • CommentAuthortony
    • CommentTimeFeb 2nd 2012
     
    4 to 6m^2 but depends on how many people are using it too.

    I would rather oversize collectors than oversize cylinder.

    The lower the tank temperature the more can be collected on dull days and the lower the losses.

    The arguments are hugely complex as you are about to find out, but all you get from the sun comes free once the system is in.
    • CommentAuthorpmagowan
    • CommentTimeFeb 2nd 2012
     
    It is complex and I am grappling with this currently. There is just one of me in a house which will hopefully achieve very good levels of insulation and airtightness. I am currently favoring a 58mm 30 tube ET panel connected to a 300L Akvaterm solar tank. My secondary heating will be from a Lohberger wood/pellet range cooker giving about 6kw to the tank at max output. It is mainly (hopefully) for DHW. I am hoping for the ST to do the buisness in summer and the range to sort it in winter (when I will have it on for cooking and a warm glow). I also have a 3000L Akvaterm tank which I had sized for my pre-insulation plans (prior to discovering this forum). I had considered using this in some way. The problem here is that the ST would be very efficient with this tank due to its large size and therefore cold bottom temperature however, it would create lots of luke warm water rather than smaller amounts of hot water. This would be OK for CH but not for DHW, hence the smaller tank. ore complexity comes if you want the ST to do most of the work in the winter and therefore you ideally want a low bottom tank temp and so can not use your auxillary heating without making the ST panels useless. I think you have to take an educated stab in the dark. I will be putting up the fittings for a second 30 tube panel for if it is required at a later date.
  4.  
    I have always heard the rule of thumb of 1m2 of collector per person for DHW as a starting point
    but no recommendations as to the size of tank!!

    Can someone support or reject this thought
    Peter
    • CommentAuthoralec
    • CommentTimeFeb 2nd 2012 edited
     
    Posted By: tonyI would rather oversize collectors than oversize cylinder



    Thats interesting I would, and do, the exact opposite... bigger tanks means lower overall system temperatures and more frequent running of the solar pump...if the tank is at temperature and the system shuts down then there is danger of overheating, and it represents a 0 return on capital invested!
    •  
      CommentAuthorSteamyTea
    • CommentTimeFeb 2nd 2012 edited
     
    Posted By: borpinSo if I want 40 minutes worth of showers, input temp of 7 deg (?) how big a tank and at what temperature

    You really need to estimate the mass flow of the shower. At 11 l/min (what mine does and it is pretty good), you would need 440kg of water at what ever temperature you like your shower, lets call it 37C.
    So to raise 440kg of water by 30C you will need 4.18(kJ)*440(kg)*30(C)
    That is 55176 kJ or energy.
    Now you need to look at the hours of sunlight and the intensity (and reliability).
    This can be found and is usually in kWh/(m^2.Day).

    Knowing that you will need 15.3 kWh for your showing needs (55176kJ/3600s) and assuming that a solar thermal system will have an overall efficiency of 50% (just an assumption mind) you need to calculate (from the daily figures) how many square meters you need.
    Week 11 is the middle of March and Week 42 is the middle of October
    They have total insolation incident on a horizontal surface of between 7.4 and 10.1 (March) and 7.7 and 10 (October) kWh.m^-2.d^-1 to 1 Standard Deviation (or in English, that is what you get about 70% of the time, or how reliable it is), to be on the safe side (not taking over heating into account) assume that 20% of the time you will not get enough water as it is not a normal distribution, the short wave end is positively skewed, or closer to zero, but the long wave is pretty normal)
    Your roof is at 42 deg, so the cosine of that is 0.74 (or 74%)

    So now you need to divide the kWh needed by the Cosine of your roof angle by the kWh.m^-.Day^-1 by the efficiency

    m^2 = 15.3 (kWh.d^-1 / (0.74 (Cosine of Roof) * 7.4 (lower bound kWh.m^-2.d^-1) * 0.5 (efficiency))
    5.6 m^2 (Lower Bound)

    m^2 = 15.3 (kWh.d^-1 / (0.74 (Cosine of Roof) * 10.1 (lower bound kWh.m^-2.d^-1) * 0.5 (efficiency))
    4.1 m^2 (Upper Bound)

    There is little difference at your latitude between horizontal plane and direct beam insolation 6.5 and 8 (March) kWh.m^-2.d^-1 and 6 and 7.3 (October) kWh.m^-2.d^-1

    As for over heating, look at systems that can take a certain amount of time without fluid in them, then plumb accordingly or a large heat dump.
      Edinburgh.jpg
    • CommentAuthorDantenz
    • CommentTimeFeb 2nd 2012
     
    Posted By: alec
    Posted By: tonyI would rather oversize collectors than oversize cylinder


    Thats interesting I would, and do, the exact opposite... bigger tanks means lower overall system temperatures and more frequent running of the solar pump...if the tank is at temperature and the system shuts down then there is danger of overheating, and it represents a 0 return on capital invested!


    Ditto, I would do the same

    Posted By: tony

    I would rather oversize collectors than oversize cylinder.
    The lower the tank temperature the more can be collected on dull days and the lower the losses.

    Contradiction me thinks!
    • CommentAuthorJeff B
    • CommentTimeFeb 2nd 2012
     
    Borpin - I am considering adding a separate DHW cylinder (volume? - perhaps 100 litres) to my existing system which has a 300 litre TS and 4 sq m of FP panels. I have discovered that the Resol controller I have is capable of monitoring/controlling two stores, so it occurs to me that the "solar water" could be sent to the DHW cylinder first and when that achieves the set temperature e.g. 55 - 60C, then the solar water would be diverted down to the 300 litre thermal store. We have the problem that only really in mid summer do we get lashings of hot water from the TS - the rest of the year we simply get a preheating effect (lukewarm water). If we had a smaller tank dedicated to DHW then we could get hotter DHW for more of the year e.g. today with the outside temperature at 3C, the solar water reached 60C at midday. Obviously we would need an immersion heater to top up the DHW tank as/when necessary.

    Does anyone think this is a good idea or not?
    • CommentAuthorborpin
    • CommentTimeFeb 2nd 2012
     
    ST Ok, thanks for that but what does 15.3kWh look like in terms of volume of water. Seem to remember this in another thread somewhere but cannot find it.

    JeffB - Looking at the Akva Solar Plus as that is what it effectively has.
    •  
      CommentAuthorJSHarris
    • CommentTimeFeb 2nd 2012
     
    Water has a specific heat of 4.18 J.g.K, or 4180 J.l.K or 0.0011611 kWh.l.K

    Energy needed to raise by a specific temperature difference depends on the volume.

    15.3 kWh could be 300 litres raised from 20 deg C to 64.5 deg C, for example, or 500 litres raised from 20 deg C to 46.7 deg C.
    •  
      CommentAuthorSteamyTea
    • CommentTimeFeb 3rd 2012 edited
     
    Think of kWh as a box of energy, it can be small and heavy or large and light.
    A Watt is a Joule per second and an hour is 3600 seconds, so a kWh can be written as kJ.s^-1*3600s, the seconds cancel out and you are left with kJ, or the Joules

    With hot water you need a minimum temperature for it to be useful, so in my calculation above I set the temperature at 37C, as this is OK for a shower, water at 25C would feel a bit miserable. Above 37C you can just add cold to it.
    If the tank is above 37C you just add cold and can get more showers out of it, or save the excess for the next day.
    The only problem with this is that as the tank is at a hotter temperature, it does not start to warm up until there is more solar coming in, so sometimes a smaller tank is better.
    It is all a compromise between desired temperature, amount needed, solar inputs, usage patterns, the cost, etc.
  12.  
    Jeff, your proposal of a separate DHW tank seems a good one, this would seem to get over our problem of connecting a gas condensing boiler to the TS and the issues of high return flows to the boiler. Solar would heat DHW tank first, then get diverted to TS which could help with space heating when there is excess, if no solar then gas boiler could heat the DHW at high temp or the TS at lower temp for space heating. Is that what you are thinking?

    Not sure how our possible wood burner would fit into this configuration, am also wondering what advantages the TS are providing (I understand you already have yours) but I just have a boiler right now, maybe better off with two normal tanks with coils?
    • CommentAuthorJeff B
    • CommentTimeFeb 3rd 2012
     
    Borpin – following up on your response I’ve just had a look at the Akva Solar Plus thermal store you suggest. Looks a very nice piece of kit but I don’t think it will do what I’m thinking about. This is the description of how the Akva solar system works (taken from the Stoves on line website):

    “The Solar Plus has an extra solar coil at the top of the tank. The hot water from the solar panels is directed first to the bottom coil until the temperature has gone over 53ºC at which point it starts to be routed to the top coil. By directing the warm (but not hot) solar hot water to the bottom of the tank this heat is captured and, if the top of the tank is not above that temperature already, then this heat will then also spread to the top of the tank through the solar baffle”.

    Apart from mid-summer our solar water is not particularly hot. Yesterday it got up to 60C at one point (briefly!), which sounds pretty good but in summer easily gets up to 90+ for many hours. If the solar water at this time of the year is directed first to the bottom of our 300 litre thermal store its effect is rapidly diluted out so we have lukewarm water at the bottom (around 35C but never 53C). If the same solar water was directed first to a solar coil in a much smaller tank, then we should get hotter water. As Pmagowan says if you want the solar panels to do most of the work in the winter, you ideally want a low bottom tank temp and so can not use your auxillary heating without making the ST panels useless. This is the problem I have because our wood pellet boiler is warming the bottom of the thermal store (even though the heat exchanger coil is mid tank), so even if the solar water WAS getting up to 35 – 40C, the system will of course automatically prevent circulation to/from the panels!

    Admittedly we would need to use an immersion heater to boost the temperature in the winter months (and possibly autumn/spring too) but as we do not use much DHW I am not too worried about that. Definitely not 40 minutes “showering time worth” anyway (440 litres of hot water – wow!), more like 10 minutes for the two of us (5 minutes each!).

    Here, the jury is still out as they say, as I have not made up my mind what to do. I know I cannot afford to replace my existing thermal store, not least because the former is only about 3 years old! As SteamyTea says it is quite a complicated picture and at the end of the day it will have to be a compromise I suppose.
  14.  
    I don't know if this tank is available in the UK but it, or something like it might be what you are looking for.

    http://www.youtube.com/watch?v=vh59WFyzKUs
    • CommentAuthorborpin
    • CommentTimeFeb 3rd 2012
     
    Thanks for all the help. I understood the physics, just could not get my head around the calculations.

    Jeff B, yes I agree that 53C seems a crazy temperature to be set at. Basically as soon as the ST temp is greater than the temperature at the top, it should go there but that is fairly simple to do I think.

    The "40 minutes" come from a max demand (including guests) and 2 teenagers! I currently have a 9l/min shower and everyone seems quite happy with that flow rate.

    I am looking at boosting the DHW output from the tank rather than auxiliary heating *for* the tank. I went back to an old thread where DamonHD posted some thoughts http://www.earth.org.uk/note-on-solar-DHW-for-16WW.html. My trouble had been finding an appliance that would boost from about 37C. Actually I can mix the input to the booster down to 30C (max input temperature for many appliances) and do it that way. So obvious I'd missed it. Needs a few mixer valves and one way valves to prevent back flow, but should work.

    The Finnish folk are a bit further north than us but they do have *similar* isolation to us, so perhaps they know what they are up to with these tanks. Wonder if the favour FP or ET....
    • CommentAuthorJeff B
    • CommentTimeFeb 3rd 2012
     
    Phil.Chaddah-Duke: sorry, our posts have crossed in the ether!

    You wrote:
    Jeff, your proposal of a separate DHW tank seems a good one, this would seem to get over our problem of connecting a gas condensing boiler to the TS and the issues of high return flows to the boiler. Solar would heat DHW tank first, then get diverted to TS which could help with space heating when there is excess, if no solar then gas boiler could heat the DHW at high temp or the TS at lower temp for space heating. Is that what you are thinking?

    Yes to the first part (up to the word “excess”) and no to the second! To simplify things I was not thinking of diverting the flow from the boiler to the proposed new small DHW tank, only the solar hot water. The boiler would just be connected to the thermal store only. I would have to rely on an immersion heater to top up the DHW temperature. I do have solar PV so am feeling slightly less guilty about that!

    You also wrote:
    Not sure how our possible wood burner would fit into this configuration, am also wondering what advantages the TS are providing (I understand you already have yours) but I just have a boiler right now, maybe better off with two normal tanks with coils?

    You are right about wondering what are the advantages of the thermal store – I often have such conversations with myself! At least I do have the flexibility of being able to add one more heat source to the system with very little hassle as the spare coil and the tappings are already there. I was considering an ASHP at one time but not at the moment. Another advantage is that the store provides an instant burst of hot water into the C/H system as soon as the room stat calls for heat. The wood pellet boiler takes an age to get up to temperature compared to gas or oil, so having a “heat bank” is quite useful. The downside is that we are keeping a fairly large volume of water hot at all times but I am assured that this is more efficient than having the boiler cycling all day. Then again I feel the thermal store at 300 litres is too small and I should have had a much larger beast, maybe 2000 litres, so the boiler could be doing one long burn rather than coming on several times a day as it does at present. (How is boiler cycling defined – does every hour or so count as cycling?).

    So I am not really the right person to answer your question about being better off with two normal tanks with coils! There are others on the GBF who are better qualified than me, but if you are proposing to have a log burner then I would say that you ought to go for a very large heat store but also go for a separate smaller DHW tank and divert the boiler feed to that first.

    Jeff B
    • CommentAuthorJeff B
    • CommentTimeFeb 3rd 2012
     
    Posted By: Chris P BaconI don't know if this tank is available in the UK but it, or something like it might be what you are looking for.

    http://www.youtube.com/watch?v=vh59WFyzKUs" >http://www.youtube.com/watch?v=vh59WFyzKUs


    Thanks for the link. Brilliant graphics! Looks good but also highly expensive I suspect. Might be worth making some enquiries though.
    • CommentAuthorborpin
    • CommentTimeFeb 3rd 2012
     
    So, If I have got this right, a 1000L tank @ 80C holds ~400kWh?
    •  
      CommentAuthorJSHarris
    • CommentTimeFeb 3rd 2012 edited
     
    The specific heat of water is about 4.18 J.g.K, which is around 0.001161 kWh.l.K, so 1000l at 80 deg C stores about 92.9 kWh of energy if you could take it down to 0 deg C (i.e. an 80 deg K delta T)..

    In practice you are working between two temperature limits, the upper set by the highest water temperature for reasonable efficiency and the lower set by the lowest temperature at which you can still do something useful. This means you won't be likely to be able to use an 80 deg C delta T, something around 40 to 50 deg is probably the best you can hope for.
    • CommentAuthorborpin
    • CommentTimeMar 8th 2013
     
    I'm going to reboot this thread (in response to the UFH Temp thread) as it is still a problem I grapple with. I favour the Akvaterm Solar Plus tank as I think it will allow me to capture the most energy in winter (I only need low temp water for the UFH but as much as I can gather) but I also want to prevent overheating in the summer.

    2 x 30 x 58mm tube arrays from eco-nomical await a suitable tank!

    All suggestions gratefully received :bigsmile:
    •  
      CommentAuthorSteamyTea
    • CommentTimeMar 9th 2013 edited
     
    I think you have to find out what is the power you can realistically expect to collect, the W/m^2 solar. Then you can match the store size to the variable input to find the most effective size.
    Find your nearest Weather Underground station that has Solar Intensity and work with those figures, you can then decide how much above or below the mean you want to work with (last two years have not been good in the winter).
    Also worth finding out what the efficiency of the panels/tubes are at different inputs and temperatures. This bit is a bit tricky as you need to know the inflow temperature of your transfer fluid, the air temperature to establish the outflow temperature, but it is just like a radiator really.

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