dnb

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About dnb

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  1. dnb

    Working out a scheme for DHW

    I think you can drop the "domestic" from that statement. Simplicity usually wins. If a system looks complicated then chances are you haven't finished designing it yet! The choice of storage vessel for DHW heat is of little importance to the payback time of the choice of thermal or PV as long as it is large enough. My maths assumes it is something that accepts heat and gives it up with a nominal daily loss. I might win a bit of the loss back with some systems, but this is not the question that I am trying to answer at the moment. I really wanted solar thermal to work out. It is simple - the fewest steps to collect the energy - but the "business case" (how I hate those words!) just does not add up for my situation, even assuming zero servicing. Each ET "bank" removes apprx 900Wp of PV from the system (keeping the collector area constant) and costs more than 3 PV panels in parts alone. It might function better, but unless it can offset the cost disadvantage a simple single technology wins. About the only thing now that will tip the balance is if 3 phase electricity is required for the all PV system but not for any of the alternatives with thermal. (3 phase would be useful in the garage of course. Everyone needs a good machine shop for their own personal use!)
  2. dnb

    Working out a scheme for DHW

    Don't forget that my conclusions are specific to my new house design. I made no assumptions on requiring roof mounting. All I assumed was that there would be a fixed area of collector and it would be filled with the most efficient distribution of PV and ETs for DHW (I am planning no wet UFH - it is a complication that the house does not seem to need if the AC system works like the predictions say). In practice, the roof is the only sane place to put solar equipment so that it is not shaded by the nearby trees. I assumed a 500 litre tank. It doesn't have much bearing on the situation because both the PV and ETs will be feeding it, and sooner or later it will be filled. I had a cunning plan for dealing with thermal over supply so this on its own does not deter me. But the bottom line of payback time for the cost of installation unfortunately does have to be a significant factor in system design. So unless I can find another few percent of efficiency from the ETs (modelling might be pessimistic?) the ET "business case" for want of better words doesn't appear to work for me purely on investment terms. Granted, it's really close. If I have over-estimated ET costs by 20% then they win hands down. Similarly, if I have underestimated the cost of PV panels by 20% then ETs win. Similarly, if I have underestimated my demands of DHW by a significant margin then ETs win. Why does it have to be a photo finish? I wanted a clear winner!!
  3. dnb

    Working out a scheme for DHW

    Getting back to the original topic (the diversions have been very interesting and insightful, but I have to appear vaguely sane when I tell my long suffering architect what I've decided to do next week) The latest version of my modelling tool has led me to the following conclusions: * The difference between PV and ETs is actually quite small when just considering DHW. * If there is a limit on roof space (like my current house - and that's why I'm building another) then ETs are probably the better solution for DHW - ETs are more efficient per m^2, until the store is full. * It's not just the cost of panels and ETs (obvious, I know, but let's follow the logic). If PV (or ETs) are going to be there anyway, then increasing the amount of either is a much smaller delta cost. So there's a big saving when one or other technology achieves zero presence. (and a nice saving in complexity) * I can't remove all the PV - it makes no sense at all from any point of view. So it looks like ETs are not going to be part of my build after all. A chart to justify my position. The X axis represents the number of 3.9m^2 ET assemblies on the roof. The remainder of the roof is used for PV. The roof area is held constant, and the calculations account for quantization of panels. (The numbers differ from previous iterations because I included another kW or so of PV on the SE facing roof having looked at tree heights - and my chainsaw - today) So as can be seen, adding ETs reduces the import costs at every level, even when a lot of the energy collected in summer is waste heat that has no practical use until I build the swimming pool. The best payback however comes from the simple PV system primarily because of the reduction in infrastructure and each addition of a block of ETs always extends the payback time, with the extensions getting gradually larger as the waste heat becomes ever more significant.
  4. dnb

    Working out a scheme for DHW

    It has already included my solar PV plans - it took me to an EPC of 106. Probably 3 more panels would swing it. Good call. Thanks! I'm only looking for a small boost to the numbers, and given the wife and daughter's love of showers it will pay back well over the years.
  5. dnb

    Working out a scheme for DHW

    But hot water is a fairly a useful trick! I take the point though - in the summer, the 9m^2 of ETs are not pulling their weight for much of the day, when their equivilent area of PV could be doing something useful like powering the A/C from my other thread. But of course only if the DHW is charged or it's more cost effective to heat the DHW over night with economy 7 (or whatever other base load incentive I might find) I have come to the conclusion that I can make whatever system I install work and meet my DHW requirements, and all I am going to achieve is to get the last few percent of efficiency out of the system. It should be worth it though. I updated the model with Ed Davies suggested ET efficiency model and used the parameters from the Navitron system I was considering. It makes surprisingly little difference to the conclusions. What I really need is a near infinite capacity thermal store that has next to no loss. Then I can average the energy across the seasons... So, a 10m^3 concrete cube buried deeply ;)in the garden ought to do it. I got the first draft of the SAP calculations this afternoon. It will change a bit, I am sure, because some design details have been entered as guesses. It indicates the house will be annoyingly close to carbon negative, but won't quite achieve it. Still, it's a good A rating so nothing too bad for a first go at house building.
  6. dnb

    Working out a scheme for DHW

    Hmmm. Home made petrol for the classic car collection... now there is an idea.
  7. dnb

    Working out a scheme for DHW

    I had factored in the 30 deg usefulness but had used 85 as an upper limit because there aren't any grown ups around and I assumed a thermal store... I probably should do a run with an upper limit of 65 to properly simulate the UVC case. I was puzzled by the apparent lack of legionella protection too. It is one of the advantages of a thermal store
  8. dnb

    Working out a scheme for DHW

    I suspected you were involved in the industry somehow. My experience is mostly radars. I have been fortunate to sail a lot of miles with the RN and witnessed many missile firing trials to test the systems I have helped to build. It is as you say lots of fun.
  9. dnb

    Working out a scheme for DHW

    Isn't that the truth! I have spent the last 15 years doing mathematical modelling work (amongst other things) in the defence industry. Different subject matter but always the same question of finding the best thing.
  10. dnb

    Working out a scheme for DHW

    A bit more work using an updated version of my house DHW model: I created 6 versions of the house, with various distributions of PV and ST areas on the roof and ran them in parallel for 500 years using randomly generated hot water usage (according to the model I described in the 1st post) and a model of daily solar availability based on recorded data. Two cases of interest are presented here. The all PV case is for 27 300W PV panels with a nominal efficiency of 18%. The second is 18 PV panels of the same spec, and 9m^2 of evacuated tubes. This should serve to illustrate the case scottishjohn discusses - what do 2 different systems do with the same solar input. I modelled the efficiency of these as 80% when the thermal store (or UVC - it makes no difference to the model) temperature is low, rolling off as an exponential decay as the store temperature increased according to [efficiency = nominal * exp(-tank_temperature/constant)]. I set up the constant to give an efficiency of approximately 25% at 60 deg C tank temperature. I assumed the thermal store had a capacity of 500 litres. Heat losses from the tank are modelled as a basic daily loss, applied as part of the water use calculation. All heating from electricity is assumed to be a very simple immersion heater. I know it can be done more efficiently with ASHP etc, but that adds new variables and more system cost that will be looked at in a later model. Obviously take the results with a pinch of salt until I have properly reviewed the maths. I might have got things wrong. Case 1 required the following yearly energy input costs from the grid (a simple histogram of all 500 years worth of data): Case 2 energy input on the same scale: As can be seen, case 2 requires less energy input, so is in theory is more efficient. But it is only £40/year on average, so when system costs are factored in, case 1 may be the best route. This cost data is available and will be presented later. Now here are some specifics. I have taken one year from the data set and plotted internal data from the model for both cases. It concerns "filling" and "emptying" the tank with energy. The model assumes water is consumed just after 6AM and just after 8PM (It simplifies things nicely) and that once the tank is full the thermal energy can't be used. The PV might be used so I don't count it as waste, but it is not used further because it isn't doing anything for the DHW unless I make the modelled system more complex - and that's tomorrow's job. Case 1 Case 2: So on the one-of-one observation it seems the PV system generally wants more energy input throughout the year, but at a low level. Both systems can cope with demand fairly well, so if my model is to be believed, it is a case of finding the cheapest system to install that gives the fewest side effects to the rest of the house rather than DHW running costs. Any thoughts? Does the model look like it represents reality? One last thought - the wife read the comments about water saving and didn't like the idea of re-education. I pointed her at part G of the building regs. Let's just say it could have gone better.
  11. dnb

    Working out a scheme for DHW

    You can. The roof span at 45 deg pitch was just over the maximum span for SIPs panels so 3 14 metre glulam purlins were needed to support the roof. The pitch reduction shortend the span and allowed removal 2 of the purlins and a lot of structural constraints.
  12. dnb

    Working out a scheme for DHW

    I wasn't planning g on any RHI applications unless it was easy. Letting a short term government scheme drive system design is too much like my day job. I will read the posts in detail tonight when I am not at work. Thanks all!
  13. I was, until I joined the forum here, fairly set on solar thermal plus backup electrical heating for my new house DHW. Now I am less sure having read some well written arguments from some forum members. So now I need to revise my models and find what is the most suitable technology. First of all, it's a reasonably large house with 2 ensuites, a family bathroom and a wet room down stairs. I have a near teenage daughter and a wife who both adore water. (I know there is a theoretical limit of 125l/person/day but let's suggest, for argument sake that I have several children and several wives while not admitting to any wrongdoing whatsoever...) I also have elderly parents who like visiting their grand daugher(s!) on the Isle of Wight, and may one day forget to go home. The site has mains water and mains electricity. That's it for services. It also has lots of trees, but as stated in other threads, burning things in houses is a 20th century idea. Oil should be saved for use as petrol in classic cars in my opinion. My airconditioning combined with MVHR thread is going pretty well, so let's make the further (perhaps rash) assumption that this will work. The DHW system will therefore be broadly separate from the space heating. The first picture shows the distribution of energy I expect to need each year for hot water. It is based on simulating water heating loads randomly over a morning and evening. For instance, some mornings there are 2 showers and nothing much in the evening. Other times there is one shower in the morning and 2 showers and a bath in the evening. Duration was modelled as an almost Gaussian distribution centred on 8 minutes for the shower, modified with a long tail to the longer duration to model teenage daughers... So on average I am going to need to find 4,800kWh over a year, preferably mostly out of thin air. Does this seem under done to anyone? Thankfully, I have 40m^s or so (let's be a bit conservative) of south-ish facing roof with a 43.7 degree pitch. (Would have been 45 degrees, but this simple change saved me the best part of £12k in the house structure) So, how do I supply the energy for DHW? I should point out that if at any point my wife decides to make use of hot water and there isn't any then I will be fed to the sharks. Nothing like a bit of pressure is there? As I said, my first thought was solar thermal. It makes grand claims of 80% efficiency etc but these are in optimal conditions, not when feeding a thermal store up to high temperatures. But let's assume I fit 9m^2 of themal evacuated tube "panels" with a 500 litre thermal store and backup electric immersion heater, and the system "works perfectly". (Waste heat will be dumped to a radiator on the north roof without incident - it's a perfect, albeit imaginary, world.) Insolation data comes from several years of daily measurements of intensity and duration from a reputable source. This is run on a daily basis against my random usage model for thousands of years to build a decent statistical sample set. One year of this is presented in the second chart showing how the system responds on a daily basis. This tells me I need to get a decent control system for it all - or write one if all else fails. In the winter, early spring and late autumn, the system needs to use the immersion heater before 6AM in order to ensure I don't get fed to the sharks when the wife wants a shower before going to work - there being no useful sun until 9 or 10AM. This of course means that some days produce waste heat, even in winter. (Telling the wife to plan showers based on the weather involves things that are worse than sharks.) Similarly, we need to add heat with the immersion heater if the day's solar capture is less than we need for the evening baths and showers. The chart therefore shows that much of the time an excess of hotness is generated in order to minimise grid electricity consumption, and that this is a law of diminishing returns because there's no use for heat you can't store and having roof area that is only usefully doing something for 1/10th of the year is not cost effective. Perhaps a PV only system would do better? It may now be cheaper and certainly there would be less waste especially when a battery bank is considered, although currently these don't seem cost effective - but that is another story. Note that the above analysis might just as well be performed on a PV system. I will do so next time I play with Matlab because everything has changed price since 2017 when I first looked at this. So will 9m^2 of solar thermal and 18 300W PV panels do better than simply putting in 24 PV panels? (The Myford is begging for 3 phase, so I need an excuse.) It would appear my model of solar thermal needs to be updated based on the contents of the forum and some links. I wonder if it would be sufficient to use an average efficiency to derate the thermal panels in the model to get something going quickly. NB: The wife isn't the shark queen and doesn't really want me out of the way (at least until the completion certificate for the house arrives) but she does suffer endlessly for my sense of humour.
  14. Thanks all! Lots of good information. The awning is currently serving as the garden shed because the on site garage is full of broken car. Just what I need - another job to do. I am not considering the attic space as bedrooms for the moment, but want to have this as an option in case of any elderly parent care requirement. (Not for them - they get part of down stairs) My architect was very good at helping me to get some nice large half-round windows in the gable ends in the attic to make it all possible. It will be built to all the regs with living space in mind, even though we don't plan to (can't afford to) fit it out fully. I have looked at the SunAmp system and am not yet convinced. I will have a piled foundation with ring beam and beam-and-block floor (probably using some insulating blocks), so relatively little concrete in the grand scheme to store anything - my maths indicates I have more of a cooling problem than a heating problem. I think at the moment it is solar thermal with PV and a battery bank eventually (again depending on cost-benefit analysis) for water heating and taking up the house base load for electricity. I am going for an ASHP - see the thread on integrating the MVHR with air conditioning. GSHPs do work, but are expensive to the point of not paying back in my lifetime in the house and will upset the council over disturbing the trees (even though there are no TPOs).
  15. No problem. I think I might be getting somewhere with the design now I've had sensible people to look at it. Does it look like it might work? As for the noise, it's a good job I'm getting old and deaf! Years of hearing abuse from TVRs! Seriously, the noise is a significant concern so I will be taking great care with duct sizing. This is where there might be a benefit in a "star" based system as I drew badly in my picture - as long as the duct lengths don't cause too much friction loss. This way, the ducts can be kept (generally) smaller because the "trunk" ducting is short and no rooms have to share. I can see a lot of maths and abuse of my Matlab licence in the next few days. Then I need to see what parts are easily available on this little island.