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Solar PV v thermal

  • 21-03-2015 10:25AM
    #1
    Registered Users, Registered Users 2 Posts: 234 ✭✭


    Trying to decide on PV or solar thermal. I am leaning towards PV, I want it to heat water in the summer. I am putting in a heat pump. I know I can send the PV power to an immersion to heat water, but would it be more efficient to send it to the heat pump to heat? And can it be done?
    Given that the feed in tariff is gone, is PV still the best option for solar energy?
    I have heard that a 1kw PV system will satisfy part L, but is it too small to heat enough water for two adults and two kids?


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Comments

  • Registered Users, Registered Users 2 Posts: 1,627 ✭✭✭quentingargan


    I think PV has crossed the line on price. A 2kw system will produce over 1800kwHrs of electricity. This is about the same amount of energy as you would get from a 4 sq m solar thermal panel or equivalent tubes. Some of this electricity will save you the 18c retail price of electricity, and the balance will save you about 9c in heating from oil, gas or off-peak electricity. Your heat pump may be slightly cheaper.

    The hardwre for a PV system like this will be about €2500 plus the gizmo to do the immersion switching - about €500. The advantage of this over solar thermal is that there is no need to change glycol every three years, no moving parts or pumps to break down, no expansion vessels to fail and the system is basically "fit and forget".

    The device that does this switches your immersion like a dimmer switch, monitoring your incoming grid and cranking the immersion up and down to prevent free exports. But a good unit to do this is not cheap - an export meter would be cheaper if utilities would buy back your power, so you might spend €500 on one of these gizmos and then find that your provider is willing to buy your electricity again. Environmentally it would be better to export your electricity to the grid and buy it back at night off-peak to heat your water.

    If you could run your heat pump at variable speed, that would be an interesting option. We have done this for variable speed water pumping using solar and wind power, but have not been able to find a heat pump manufacturer who will allow us to interfere with their motor drives. It should be possible to vary the pump speed to keep exports at zero, within certain limits. Anyone out there with a tame heat pump manufacturer willing to allow us to play with that? This would preserve some of the COP of the heat pump and preserve the high primary energy conversion factor of electricity.

    By the way, 2kw solar would need planning - you can get to about 1.78kw on the 12sq m planning exemption. But that would be enough for most hot water use.


  • Registered Users, Registered Users 2 Posts: 234 ✭✭Superdaddy


    Thanks Quentin, this is a new build, with planning for solar. I'd put 6kw up if I thought I'd get paid for it. Any sign of a new scheme? I have to install something to meet building regs part L.
    I see what you mean re dimmer switch, if I understand you correctly I can set it just to use solar power?
    Bit of a loose question but will a 2kw setup heat enough water on an average summer day for 4 people in an 2500sqft A2 rated house? In other words does it work for people out there?
    If it does that's great as it will allow me to add further panels if I wish when it can be sold back to the grid (bring back the Green Party:)). And as you say it is hassle free.


  • Registered Users, Registered Users 2 Posts: 1,116 ✭✭✭freddyuk


    The Immersion diverter "only" uses solar power as it is wired into the PV circuit to measure what is spare free solar power. A switch will just switch on the immersion and drag 3kw from the supply be it solar or grid or both. A diverter measures the free spare power after the house loads are satisfied and balance is sent to the immersion. More PV means more is going to be diverted.


  • Registered Users, Registered Users 2 Posts: 234 ✭✭Superdaddy


    Thanks Freddy. I want cheap hot water and to get the most from my investment. Trying to get my head around it all. Would it be better to heat water at night rate with the heat pump and top it up with the surplus electricity from a small PV system?


  • Registered Users, Registered Users 2 Posts: 1,116 ✭✭✭freddyuk


    The PV is supplying power all day to some degree to your home not just the hot water so that includes the heat pump. You need to design the PV around your usage and budget. If you have 2kw of PV then this will be contributing to the whole house electric bill and with a diverter only free surplus power goes into the cylinder.
    If you use the night rate (which is more expensive) there is no point in filling your cylinder at night and paying for it if the next day is sunny and the PV has nowhere to dump the free energy so manage the hot water according to your habits. You only need enough for the morning peak usage. When you have PV you have power all day but need to use everything you can or it will be sent back to the grid for free. Once you know your base load and average daily usage you can get an idea of how much power you need and when, then design system around that. You can always add more PV.
    If it is a dark day you know you need to override the heap pump for a top up or even watch the forecast and if clearly it is bad then use the night rate for a full cylinder. An immersion heater can always be used for rapid hot water if required urgently but how often is that?
    My wife now watches the weather and if it is clearly sunny for a few hours the washing machine or oven goes on. If the sun disappears it has still saved energy but it is not a 100% free wash. If you want to save energy it takes a bit of effort. Designing from scratch is more difficult but your lifestyle is starting point and how keen you are to adjust so when free power is available you react and use it (or lose it).


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  • Registered Users, Registered Users 2 Posts: 1,627 ✭✭✭quentingargan


    Superdaddy wrote: »
    Thanks Quentin, this is a new build, with planning for solar. I'd put 6kw up if I thought I'd get paid for it. Any sign of a new scheme? I have to install something to meet building regs part L.
    I see what you mean re dimmer switch, if I understand you correctly I can set it just to use solar power?
    Bit of a loose question but will a 2kw setup heat enough water on an average summer day for 4 people in an 2500sqft A2 rated house? In other words does it work for people out there?
    If it does that's great as it will allow me to add further panels if I wish when it can be sold back to the grid (bring back the Green Party:)). And as you say it is hassle free.

    Just to mention that to my mind, the diversion device needs to be installed after you have done your BER assessment. Otherwise, you lose the primary energy conversion factor in DEAP. There are a few devices - the cheap ones use a triac and zero-crossing. These put quite a spike on the sine wave on the grid throughout the house (and usually next door as well...) The dearer ones use PWM and leave the sine wave as clean as a whistle.

    If you have 2kw PV and your heat pump it 3kw, then using your heat pump will result in you importing 1kw or more, depending on the level of light at the time. That may end up costing you more to heat your water... 3kw of PV gives you a bit more wriggle room in terms of running 2kw devices like washing machines etc. But it may be outside your budget, and generally the larger a PV system is, without a feed in tariff, the longer your ROI is as you will not use all the power from it.


  • Registered Users, Registered Users 2 Posts: 2,837 ✭✭✭air


    I'd recommend going with PV unless you use a lot of hot water for some reason or another.
    Put in whatever you can afford now but install enough roof hooks to max out your roof space.
    This means it will be easy to come back later and add more rails and panels when the time comes.


  • Registered Users, Registered Users 2 Posts: 1,504 ✭✭✭BarneyMc


    Not sure if this is a valid question but if going down the route of using PV to power a heat pump & UFH then does it lean towards a thick sand/cement floor or thin screed?


  • Registered Users, Registered Users 2 Posts: 2,837 ✭✭✭air


    BarneyMc wrote: »
    Not sure if this is a valid question but if going down the route of using PV to power a heat pump & UFH then does it lean towards a thick sand/cement floor or thin screed?

    I'd lean towards a thicker screed to allow you store up some heat energy on sunny days to keep you going through subsequent cloudier ones.
    Bear in mind though that the PV will only make a direct appreciable contribution to the heat pump's consumption on rare sunny days in spring and autumn.
    You presumably won't use the heat pump in summer and the PV won't produce any significant energy during the winter months when demand on the heat pump will be greatest.
    In truth the PV / Heat pump combination relies on a feed in tarriff to make economic sense.


  • Registered Users, Registered Users 2 Posts: 1,504 ✭✭✭BarneyMc


    air wrote: »
    I'd lean towards a thicker screed to allow you store up some heat energy on sunny days to keep you going through subsequent cloudier ones.
    Bear in mind though that the PV will only make a direct appreciable contribution to the heat pump's consumption on rare sunny days in spring and autumn.
    You presumably won't use the heat pump in summer and the PV won't produce any significant energy during the winter months when demand on the heat pump will be greatest.
    In truth the PV / Heat pump combination relies on a feed in tarriff to make economic sense.

    I'm leaning towards a 3 inch sand/cement floor so I'm probably on the right tracks.


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  • Registered Users, Registered Users 2 Posts: 234 ✭✭Superdaddy


    Just to mention that to my mind, the diversion device needs to be installed after you have done your BER assessment. Otherwise, you lose the primary energy conversion factor in DEAP. There are a few devices - the cheap ones use a triac and zero-crossing. These put quite a spike on the sine wave on the grid throughout the house (and usually next door as well...) The dearer ones use PWM and leave the sine wave as clean as a whistle.

    If you have 2kw PV and your heat pump it 3kw, then using your heat pump will result in you importing 1kw or more, depending on the level of light at the time. That may end up costing you more to heat your water... 3kw of PV gives you a bit more wriggle room in terms of running 2kw devices like washing machines etc. But it may be outside your budget, and generally the larger a PV system is, without a feed in tariff, the longer your ROI is as you will not use all the power from it.

    I see what you mean with deap. You've lost me with the sine waves, I feel a steep learning curve coming:). So be sure to get a PWM one?
    BarneyMc wrote: »
    Not sure if this is a valid question but if going down the route of using PV to power a heat pump & UFH then does it lean towards a thick sand/cement floor or thin screed?

    I presume you are thinking of thermal mass for slow release of the heat. I think it would very much depend on how much insulation you put under it for starters. I have gone for 300mm of insulation (passive slab) and 100mm slab.


  • Posts: 5,238 ✭✭✭ [Deleted User]


    images?q=tbn:ANd9GcQ8avs7gVLyjWNNUO2SdkGfFZSXlDv1gAOA6x05V2MdrX_GBlTJ
    PWM

    article-2014july-ICs-answer-the-challenge-of-fig1.jpg?la=en
    Triac


  • Registered Users, Registered Users 2 Posts: 1,504 ✭✭✭BarneyMc


    Superdaddy wrote: »
    I presume you are thinking of thermal mass for slow release of the heat. I think it would very much depend on how much insulation you put under it for starters. I have gone for 300mm of insulation (passive slab) and 100mm slab.

    Yes, I guess so. I'll be placing 150mm PIR under the 3 inch slab (or that's the plan anyway). I suppose I'm looking for the ideal slab to be used for slow release but also allow for some control. I don't want to veer too much off topic here so I'll quit!


  • Registered Users, Registered Users 2 Posts: 1,627 ✭✭✭quentingargan


    Sir Liamalot has shown the difference between PWM and TRIAC switching. The resulting sine wave shown at our neighbours house when using a TRIAC on a 2kw load was as per the picture here..


  • Posts: 5,238 ✭✭✭ [Deleted User]


    What value are the scope divisions Q? Could you not filter it with a beefy choke?
    Just curious, I'd go PWM anyways it'll react much faster too.

    Edit: ~15V drop next door?
    Did their appliances notice?


  • Registered Users, Registered Users 2 Posts: 1,627 ✭✭✭quentingargan


    What value are the scope divisions Q? Could you not filter it with a beefy choke?
    Just curious, I'd go PWM anyways it'll react much faster too.

    Edit: ~15V drop next door?
    Did their appliances notice?

    That was 50V divisions. Appliances don't seem to notice, but my wind inverter gets noisier with the grid spikes. My solar inverters don't seem to notice.

    PWM is better of course, but devices on the market are more than double the price for PWM. about €220 for Triac and €480 for PWM. The PWM converts AC to DC and switches DC using mosfets or IGBTs so you can see where the cost comes from.

    Hard to justify 'cause if Electric Ireland starts buying power again, it will make more sense to export your electricity during the day and use cheap off-peak to heat your water. And that option is also more ecologically sound.


  • Posts: 5,238 ✭✭✭ [Deleted User]


    Oh right yeah that's only ¾ of the wave shown, of course, 25V drop then.
    That'd be out of spec of a lot appliances but just milliseconds. Does that affect the RMS voltage much in the offending dwelling?
    If the device was reacting fast enough it shouldn't be visible because it's on the PV end of things.

    I thought your windy was a battery job.


  • Registered Users, Registered Users 2 Posts: 234 ✭✭Superdaddy


    BarneyMc wrote: »
    Yes, I guess so. I'll be placing 150mm PIR under the 3 inch slab (or that's the plan anyway). I suppose I'm looking for the ideal slab to be used for slow release but also allow for some control. I don't want to veer too much off topic here so I'll quit!

    Eliminating any cold bridging is very important, if I was using PIR I'd go for 200mm to improve the u value. Thermal mass is great but a disaster if not done right, you have got to retain the heat.
    Oh right yeah that's only ¾ of the wave shown, of course, 25V drop then.
    That'd be out of spec of a lot appliances but just milliseconds. Does that affect the RMS voltage much in the offending dwelling?
    If the device was reacting fast enough and it shouldn't be visable because it's on the PV end of things.

    I thought your windy was a battery job.

    Ah lads, ye have totally lost me now. 20 years since I left school, knew feck all about sine waves then too.


  • Closed Accounts Posts: 8,722 ✭✭✭nice_guy80


    air wrote: »
    I'd lean towards a thicker screed to allow you store up some heat energy on sunny days to keep you going through subsequent cloudier ones.
    Bear in mind though that the PV will only make a direct appreciable contribution to the heat pump's consumption on rare sunny days in spring and autumn.
    You presumably won't use the heat pump in summer and the PV won't produce any significant energy during the winter months when demand on the heat pump will be greatest.
    In truth the PV / Heat pump combination relies on a feed in tarriff to make economic sense.

    I'm interested in teasing out this statement

    Surely most houses are empty during the day as people are off at work/school and so the PV would generate energy that could be used by the heat pump, washing machine, dish washer or whatever meaning those devices would run free off the PV? Even during the darker winter months?

    The whole area of using PV panels to run a heat pump system is very interesting, and would make a lot of sense


  • Moderators, Home & Garden Moderators Posts: 10,146 Mod ✭✭✭✭BryanF


    nice_guy80 wrote: »
    I'm interested in teasing out this statement

    Surely most houses are empty during the day as people are off at work/school and so the PV would generate energy that could be used by the heat pump, washing machine, dish washer or whatever meaning those devices would run free off the PV? Even during the darker winter months?

    The whole area of using PV panels to run a heat pump system is very interesting, and would make a lot of sense
    And how are you storing the energy, to run these devices? The feedin tariff ?


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  • Registered Users, Registered Users 2 Posts: 1,116 ✭✭✭freddyuk


    You need a device that can switch on AC appliances only using spare solar power. The hot water diverters can only switch resistive loads not normal appliances with motors connected to sockets so this restricts it to heaters basically. Furthermore you only want appliances (washers/dishwashers) to come on when there is sufficient solar to complete the cycle which is nigh on impossible. So the best you can do is measure the solar and program the system to maybe leave it 30 minutes so if there is uninterrupted sun for 30 minutes then start switching on loads without exceeding the solar available. As for heat pumps if they have the variable speed option then the system needs to be programmed to give this priority and get heat into the house and any spare can be dumped into DHW cylinder. If it is a really good day other appliances can be started but in reality you may lose the "free" solar if the appliance completes the cycle and the sun has gone but you have still saved energy by using that free power when it was there.I think most washers need high power early on to heat water and then the rest of the time is cool rinsing. If you do it when you get home it may be 100% paid for as too late for solar.
    What we need is a sophisticated monitoring and control system to achieve this complicated scenario. Or hope there is a new government scheme to pay for excess exported solar which maybe will happen but as every other country is reeling back in the solar payments I feel it is unlikely the payments will be significant especially based on the existing exempted planning limit of sub 2kw of solar input.
    Maybe Wednesday will bring some news of the renewables direction we are going in......


  • Registered Users, Registered Users 2 Posts: 1,627 ✭✭✭quentingargan


    nice_guy80 wrote: »
    The whole area of using PV panels to run a heat pump system is very interesting, and would make a lot of sense
    Running other appliances when there is available power makes sense, but getting heat from solar, whether via a heat pump or not, seldom makes sense.

    If you have a house unoccupied during the day, even running an odd washing machine or dishwasher isn't going to use all your solar output (and you don't run the washing machine every day anyhow).

    I'm afraid our hopes are pinned on the utility companies being obliged to buy back surplus power at an acceptable price. It is ecologically more sound to run discretionary loads such as washing machines, car charging etc., at night using off-peak electricity, and export your solar power to the grid so it can be used by industry and hopefully curtail more carbon emitting turf plants that don't run at night.


  • Registered Users, Registered Users 2 Posts: 2,837 ✭✭✭air


    nice_guy80 wrote: »
    The whole area of using PV panels to run a heat pump system is very interesting, and would make a lot of sense

    In the current environment it makes no sense as has been explained.
    The output of PV panels in the winter months (when you would want to run your heat pump) is negligible.
    You would be far better off spending your money on additional insulation or air tightness. It will last forever effectively with no maintenance and nothing to go wrong.


  • Closed Accounts Posts: 8,722 ✭✭✭nice_guy80


    Ok

    I'm planning on very good levels of insulation, timber frame.

    I was talking recently to a Solar PV company based in NI and they claimed they could run house appliances (in their own homes) such as washing machines, tumble driers for free off the power generated through the solar panels.
    I'm eager to see what systems are out there.

    Is a battery storage system a solution for the excess power generated? as well as using it to heat a water tank.
    I've watched those programs of people going off grid in alaska and canada and they all seem to be at it.


  • Registered Users, Registered Users 2 Posts: 1,627 ✭✭✭quentingargan


    nice_guy80 wrote: »
    Ok
    I was talking recently to a Solar PV company based in NI and they claimed they could run house appliances (in their own homes) such as washing machines, tumble driers for free off the power generated through the solar panels.
    I'm eager to see what systems are out there.

    Is a battery storage system a solution for the excess power generated? as well as using it to heat a water tank.
    I've watched those programs of people going off grid in alaska and canada and they all seem to be at it.

    You can only run a 2kw washing machine for free if you happen to be producing over 2kw of energy at that moment in time. This is the sort of BS you get from sales people. Even if you have a 2kw system, it will only occasionally produce 2kw. You can see a graphs of my system with 1.8kw panels and 1.5kw inverter HERE. It only occasionally got over 1kw today, and this was a good day.

    You would need a clever management system to anticipate whether it will be sunny for long enough to run the washing machine. The point is, that if you were getting, say, 12c for exporting your surplus to the grid, it would be cheaper to run your washing machine off-peak at night for the 8c rate.

    Batteries are an interesting proposition, but it still costs about 11c per KwHr to cycle batteries. Again, exporting to the grid would be a better way to do this.


  • Closed Accounts Posts: 8,722 ✭✭✭nice_guy80


    You can only run a 2kw washing machine for free if you happen to be producing over 2kw of energy at that moment in time. This is the sort of BS you get from sales people. Even if you have a 2kw system, it will only occasionally produce 2kw. You can see a graphs of my system with 1.8kw panels and 1.5kw inverter HERE. It only occasionally got over 1kw today, and this was a good day.

    You would need a clever management system to anticipate whether it will be sunny for long enough to run the washing machine. The point is, that if you were getting, say, 12c for exporting your surplus to the grid, it would be cheaper to run your washing machine off-peak at night for the 8c rate.

    Batteries are an interesting proposition, but it still costs about 11c per KwHr to cycle batteries. Again, exporting to the grid would be a better way to do this.
    but I cannot export
    so that isn't an option at the moment


  • Registered Users, Registered Users 2 Posts: 1,627 ✭✭✭quentingargan


    nice_guy80 wrote: »
    but I cannot export
    so that isn't an option at the moment
    Which is why we're all waiting for this. It would be absurd for Ireland not to offer some fig-leaf to microgenerators


  • Closed Accounts Posts: 8,722 ✭✭✭nice_guy80


    white papers are pie in the sky thinking

    it takes a full government term for things like that to come through the system

    I hope to build next year.


  • Registered Users, Registered Users 2 Posts: 18,989 ✭✭✭✭murphaph


    Running other appliances when there is available power makes sense, but getting heat from solar, whether via a heat pump or not, seldom makes sense
    This is a hotly contested point of view on the German photo voltaic forum I read. I'd say it's about a 50;50 split between those that think it makes sense and those that don't. The assumption however is that a heat pump is used to multiply the effective energy-using resistive loads to do so is always the last resort.

    We're looking at installing just shy of 10kWp (the rules here mean you should stay below this or risk being seen as a business with the goal of making a profit and then different rules apply) on a SSW facing 45° pitched roof (so fairly steep roof angle which optimises for winter when the sun is lower in the sky, consequently we'll produce less power in the summer than the guy next door with a 30° roof pitch....but we'll have an 11kW heat pump to run in the heating season (and in summer too for a little bit of hot water and possibly for cooling on very hot weekend days (during the week we can put the shutters down to prevent the sun over heating the building-not so practical on the weekend)

    Our installation is perhaps much larger than the average Irish one and can realistically provide energy for the heat pump in November and January...though December is probably out. We'll run the heat pump during the afternoon and release the energy from the underfloor thermal mass in the evenings when we need it (and from the storage tank for hot water).


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  • Closed Accounts Posts: 8,722 ✭✭✭nice_guy80


    murphaph wrote: »
    This is a hotly contested point of view on the German photo voltaic forum I read. I'd say it's about a 50;50 split between those that think it makes sense and those that don't. The assumption however is that a heat pump is used to multiply the effective energy-using resistive loads to do so is always the last resort.

    We're looking at installing just shy of 10kWp (the rules here mean you should stay below this or risk being seen as a business with the goal of making a profit and then different rules apply) on a SSW facing 45° pitched roof (so fairly steep roof angle which optimises for winter when the sun is lower in the sky, consequently we'll produce less power in the summer than the guy next door with a 30° roof pitch....but we'll have an 11kW heat pump to run in the heating season (and in summer too for a little bit of hot water and possibly for cooling on very hot weekend days (during the week we can put the shutters down to prevent the sun over heating the building-not so practical on the weekend)

    Our installation is perhaps much larger than the average Irish one and can realistically provide energy for the heat pump in November and January...though December is probably out. We'll run the heat pump during the afternoon and release the energy from the underfloor thermal mass in the evenings when we need it (and from the storage tank for hot water).

    explain that for me please?

    you will use the hot water in the storage tank for underfloor heating?
    and the hot water in the tank is heated using the solar PV system?


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