Solar PV Power Immersion Heater - Have I Lost The Plot?
Discussion
We will be using solar water heater on our build. Theory is the stainless tank will sit on the roof garden and just suck up the suns rays all afternoon, and keep the water reasonably warm all the time. If it is not as efficient as guesstimated I may paint it flat black.
And if that don't work I may make some sort of glass greenhouse enclosure device to go over the tank.
We don't live in rainy old England either.
It'll sit somewhere near the end here, in the direct sun all afternoon.
And if that don't work I may make some sort of glass greenhouse enclosure device to go over the tank.
We don't live in rainy old England either.
It'll sit somewhere near the end here, in the direct sun all afternoon.
If my maths are correct, please somebody else check:
An average PV day in December (97KWh / 30 = 3.2KWh per day) could increase 60l's of water by 40 deg C
That's if you could push all the PV power produced in 1 day into an immersion heater direct from the Inverter.
From March to October you could heat 200l's a day as you are producing 12.6 to 10.7 KWh per day
Therefore could you heat enough hot water for free + additional electricity paid for via a mains powered immersion in the evening to top up a 200l tank at a reasonable cost in December to February?
I think you could and not have the cost of Solar hot water on your roof. For an extra cost of paid for electricity of approx £45 per year at current charges ( that's on topping up a 200l tank for December,January and February per year at 50p per day ie 3KWh per day)
Any ideas?
Could there be a hybrid choice of Solar Hot water and Immersion and have no gas boiler?
I'm working on these questions because I'm building a passive property that requires very little heat and a gas boiler is not practical to drive a 200l hot water tank only for two peoples requirements for hot water per day.
An average PV day in December (97KWh / 30 = 3.2KWh per day) could increase 60l's of water by 40 deg C
That's if you could push all the PV power produced in 1 day into an immersion heater direct from the Inverter.
From March to October you could heat 200l's a day as you are producing 12.6 to 10.7 KWh per day
Therefore could you heat enough hot water for free + additional electricity paid for via a mains powered immersion in the evening to top up a 200l tank at a reasonable cost in December to February?
I think you could and not have the cost of Solar hot water on your roof. For an extra cost of paid for electricity of approx £45 per year at current charges ( that's on topping up a 200l tank for December,January and February per year at 50p per day ie 3KWh per day)
Any ideas?
Could there be a hybrid choice of Solar Hot water and Immersion and have no gas boiler?
I'm working on these questions because I'm building a passive property that requires very little heat and a gas boiler is not practical to drive a 200l hot water tank only for two peoples requirements for hot water per day.
johnuready said:
An average PV day in December (97KWh / 30 = 3.2KWh per day) could increase 60l's of water by 40 deg C
It'll be a damn big panel to produce 3.2kWh per day in December - not sure where you've got that figure from? The trouble with moving to a small tank (60l is very small) is that, once you've exhausted it, you've got no more hot water. A good shower will loose virtually all useable heat from your store. We keep our tank at 60 degrees, and that's marginal.johnuready said:
I'm working on these questions because I'm building a passive property that requires very little heat and a gas boiler is not practical to drive a 200l hot water tank only for two peoples requirements for hot water per day.
If you use a thermal store, you can use a variety of heat sources - including a gas boiler in a 'batch' mode - ie. run it every couple of days for a long hard burn to heat up your store. Tuna
I see you are a self builder, I'm just down the road from you and working on retro fitting a 1970's bungalow to passive house standard. hence the questions on the use of PV / electricity to provide hot water as a separate or combined with solar thermal. My CH requirement is so low I'm using a 2Kw electric boiler to provide heat to 2 No. 840 BTU towel rails only as a gas boiler would not get into condensing mode with such a small load. That leaves me with a Hot Water issue to resolve as economically as possible. What are you using on your build?
I will have 4K of PV next week hopefully.
I see you are a self builder, I'm just down the road from you and working on retro fitting a 1970's bungalow to passive house standard. hence the questions on the use of PV / electricity to provide hot water as a separate or combined with solar thermal. My CH requirement is so low I'm using a 2Kw electric boiler to provide heat to 2 No. 840 BTU towel rails only as a gas boiler would not get into condensing mode with such a small load. That leaves me with a Hot Water issue to resolve as economically as possible. What are you using on your build?
I will have 4K of PV next week hopefully.
John - I agree with Tuna:
http://www.navitron.org.uk/forum/index.php
You'll need a 'proper' email address not webmail to register. There's a LOT of helpful people there, and the search works well
Tuna said:
If you use a thermal store, you can use a variety of heat sources - including a gas boiler in a 'batch' mode - ie. run it every couple of days for a long hard burn to heat up your store.
And for loads of information on solar power, both Thermal and PV look here:http://www.navitron.org.uk/forum/index.php
You'll need a 'proper' email address not webmail to register. There's a LOT of helpful people there, and the search works well
johnuready said:
Tuna
I see you are a self builder, I'm just down the road from you and working on retro fitting a 1970's bungalow to passive house standard. hence the questions on the use of PV / electricity to provide hot water as a separate or combined with solar thermal. My CH requirement is so low I'm using a 2Kw electric boiler to provide heat to 2 No. 840 BTU towel rails only as a gas boiler would not get into condensing mode with such a small load. That leaves me with a Hot Water issue to resolve as economically as possible. What are you using on your build?
I will have 4K of PV next week hopefully.
We used a 3kW immersion heater to heat our whole house and water last winter. Whilst that sounds impressive, it ran 24 x 7 in the really cold bits and cost a FORTUNE. Ouch.I see you are a self builder, I'm just down the road from you and working on retro fitting a 1970's bungalow to passive house standard. hence the questions on the use of PV / electricity to provide hot water as a separate or combined with solar thermal. My CH requirement is so low I'm using a 2Kw electric boiler to provide heat to 2 No. 840 BTU towel rails only as a gas boiler would not get into condensing mode with such a small load. That leaves me with a Hot Water issue to resolve as economically as possible. What are you using on your build?
I will have 4K of PV next week hopefully.
With solar, you have to allow for the variability in winter. If you get an average of 2kWh a day, that can mean one day of 6kWh (great, but you can't use all that heat), and two days of nothing (suddenly you're cold). That's why we have a thermal store, to even things out. Sizing down to your theoretical 'minimum average' has problems that you struggle when you have guests, extremes of weather and can make it harder to sell (people can't run your house like they'd run a normal house).
We have a 6kW solar thermal array - oversized for the house, but it faces west, so the two sort of balance out. We do really well up to mid October, then output drops to nearly nothing. We also have a wood stove with a back boiler, so in the winter months that can contribute to the heating. As a family with two young kids, we use plenty of hot water.
I'm impressed that you're doing a passive renovation - that's a very challenging job. Traditional builds are really difficult to convert to the required levels of insulation and airtightness. The detailing can be very hard work as that's where you get most of the losses. Good luck!
Thanks for that note. I find the detail of a passive refurb ideal, I love detail. Spend my working life as a consultant, PM, running business and hands on property developer I now spend all day on site as PM, designer, specify and provider of money etc and have two chaps who work with me. The air tightness and build is going great and with the triple glazed windows in last month it feels as its coming on. That level of detail only works with staff that understand or my presence to expalin and coach. If ever you are Bishops Stortford way pop in.
I think I'm getting on with the issues that need bottoming out. The CH, as it’s a very low requirement I'm going to use the 2Kw hot water boiler running on a timer and thermostat per bathroom towel rail, that will give me instant heat as required. Boiler cost £250 and come complete with pump, pressure tank etc. Really plug and play.
The HW, I'm going for the Vailllant Solar system. This will give me a 200l tank with 2 coils, direct system. One coil for Solar Thermal and one for another heat source when / if required. The Vaillant roof panel comes with all the controls etc and integrates.
That leaves an immersion to play with on the diversion of PV power V load on house or just 100% PV during the winter combined with Solar Thermal as see where we get to.
It also leaves me the spare coil just in case it all goes wrong and I fit an Air Source / something in the future.
Im looking at MVHR units and thinking about the Vaillant unit next week, another issue to resolve.
John
I think I'm getting on with the issues that need bottoming out. The CH, as it’s a very low requirement I'm going to use the 2Kw hot water boiler running on a timer and thermostat per bathroom towel rail, that will give me instant heat as required. Boiler cost £250 and come complete with pump, pressure tank etc. Really plug and play.
The HW, I'm going for the Vailllant Solar system. This will give me a 200l tank with 2 coils, direct system. One coil for Solar Thermal and one for another heat source when / if required. The Vaillant roof panel comes with all the controls etc and integrates.
That leaves an immersion to play with on the diversion of PV power V load on house or just 100% PV during the winter combined with Solar Thermal as see where we get to.
It also leaves me the spare coil just in case it all goes wrong and I fit an Air Source / something in the future.
Im looking at MVHR units and thinking about the Vaillant unit next week, another issue to resolve.
John
The idea was good but Inverters do not allow switching to happen that easy. SMA inverters must see the grid, measuring impedance, voltage and frequency. Therefore you must have a grid connection, that means using grid power if an immersion load was on line, the grid making up the difference between the very small winter output and power required.
The SMA unit can switch loads with soft programmable function on the power coming of the PV for a set period of time which would be very handy in the summer but in the summer my Solar Thermal kicks in, that function probably designed in to charge batteries and switch out when PV output drops below a pre set threshold. Its all soft that switching with an additional relay.
The SMA unit can switch loads with soft programmable function on the power coming of the PV for a set period of time which would be very handy in the summer but in the summer my Solar Thermal kicks in, that function probably designed in to charge batteries and switch out when PV output drops below a pre set threshold. Its all soft that switching with an additional relay.
johnuready said:
Im looking at MVHR units and thinking about the Vaillant unit next week, another issue to resolve.
There is a company called GENVEX who I'm sure do, or at least did make a MHRV unit that incorporated an air source heat pump, that could heat the air directly and or heat hot water.I'll try and get more info if i can find it, if you want.
I have just come across this Topic.
I would like to comment/question as follows:
1) Johnuready - I believe you are correct in your post of 18th Nov "3.2KWh per day" in December - This is based on the figures from Paul Drawmer's 3.88 kWp system.
2) Tuna - So I DO think you can "get a decent bath full of hot water" in winter! - Your 18th Nov post.
3) Tuna - I do not think your post of 18th Nov "it makes the most sense to export it all," is correct.
The FIT is 3.1p /kWh and Gas price is currently somewhere around 3p / kWh.
BUT you need to consider also the energy factor (EF) for water heaters, which indicates the energy efficiency based on the amount of water consumed throughout a day. This factor takes into consideration how efficient the heat source, electric or gas, is transferred to the water; the standby loss, which is how much heat and energy is lost just by storage; and the cycling loss, which is how much energy is lost as the water circulates.
Natural gas hot water heaters have a rating of 0.60 for conventional units and 0.65 for high-efficiency units and 0.86 for condensing. Electric hot water heaters typically have an EF of 0.90 for minimum efficiency and 0.95 for high-efficiency. (These figures from: http://www.ehow.com/about_5325392_natural-electric...
So in fact the cost for gas assuming a high-efficiency boiler is 4.6p / kWh of heated water. For Electricity not sent via FIT it is 3.3p / kWh of heated water assuming minimum efficiency. So theoretically it is better to use the PV to run an Immersion Heater by at least 1.3p / kWh unless you have a Condensing Boiler and are running it at optimum efficiency.
Please correct me if I am wrong or have missed the point somewhere.
Regards
Henry
I would like to comment/question as follows:
1) Johnuready - I believe you are correct in your post of 18th Nov "3.2KWh per day" in December - This is based on the figures from Paul Drawmer's 3.88 kWp system.
2) Tuna - So I DO think you can "get a decent bath full of hot water" in winter! - Your 18th Nov post.
3) Tuna - I do not think your post of 18th Nov "it makes the most sense to export it all," is correct.
The FIT is 3.1p /kWh and Gas price is currently somewhere around 3p / kWh.
BUT you need to consider also the energy factor (EF) for water heaters, which indicates the energy efficiency based on the amount of water consumed throughout a day. This factor takes into consideration how efficient the heat source, electric or gas, is transferred to the water; the standby loss, which is how much heat and energy is lost just by storage; and the cycling loss, which is how much energy is lost as the water circulates.
Natural gas hot water heaters have a rating of 0.60 for conventional units and 0.65 for high-efficiency units and 0.86 for condensing. Electric hot water heaters typically have an EF of 0.90 for minimum efficiency and 0.95 for high-efficiency. (These figures from: http://www.ehow.com/about_5325392_natural-electric...
So in fact the cost for gas assuming a high-efficiency boiler is 4.6p / kWh of heated water. For Electricity not sent via FIT it is 3.3p / kWh of heated water assuming minimum efficiency. So theoretically it is better to use the PV to run an Immersion Heater by at least 1.3p / kWh unless you have a Condensing Boiler and are running it at optimum efficiency.
Please correct me if I am wrong or have missed the point somewhere.
Regards
Henry
HKafeman:
It takes about 5-6kWh to heat a bath up, so 3.2kWh is not enough.
I may be wrong, but my understanding is that the FIT is ten times as much as you're estimating.
At the 'realistic' amounts you're quoting, it's worth pointing out that a 3.88kwp array is (a) huge and (b) costs around ten grand. That's a ten grand investment to replace £4.50 worth of fuel in December (97kwh * 4.6p). Paul's system produced 3,960kWh for the year, so that's equivalent to £182 worth of gas, or a 1.8% return on investment if you use it to replace mains gas heating. Without feed in tariffs, PV doesn't (yet) make financial sense.
Remember also that Paul's numbers are average. You might get two grey days when you effectively get no power, then one when you do.
It takes about 5-6kWh to heat a bath up, so 3.2kWh is not enough.
I may be wrong, but my understanding is that the FIT is ten times as much as you're estimating.
At the 'realistic' amounts you're quoting, it's worth pointing out that a 3.88kwp array is (a) huge and (b) costs around ten grand. That's a ten grand investment to replace £4.50 worth of fuel in December (97kwh * 4.6p). Paul's system produced 3,960kWh for the year, so that's equivalent to £182 worth of gas, or a 1.8% return on investment if you use it to replace mains gas heating. Without feed in tariffs, PV doesn't (yet) make financial sense.
Remember also that Paul's numbers are average. You might get two grey days when you effectively get no power, then one when you do.
Henry's post for the FIT rate is referring to the actual rate for feeding into the grid, not the rate for everything generated. NOTE: what we call FIT should be GT for Generated Tariff.
However, it isn't as simple as that, 'cos many installations don't even measure the amount fed in, there is a 'deemed amount' for those without export meters which is set at 50% of generated.
So, when you decide to use the electricity you generate, the calculation will only be valid if you have an export meter, and the electricity you use is stopping you exporting.
And yes, there are plenty of nil production days in the winter! There is no doubt that solar thermal is much more efficient at producing heat than PV is, and even a whole roof of thermal panels can't guarantee hot water all the time - we just don't get enough sun in the winter.
However, it isn't as simple as that, 'cos many installations don't even measure the amount fed in, there is a 'deemed amount' for those without export meters which is set at 50% of generated.
So, when you decide to use the electricity you generate, the calculation will only be valid if you have an export meter, and the electricity you use is stopping you exporting.
And yes, there are plenty of nil production days in the winter! There is no doubt that solar thermal is much more efficient at producing heat than PV is, and even a whole roof of thermal panels can't guarantee hot water all the time - we just don't get enough sun in the winter.
herewego, Tuna and Paul Drawmer
Thank you for your comments.
Sorry for any confusion - My terminology and explanantion were lacking!
My understaning is as follows to clarify for all:
The Generated Tariff for "Solar photovoltaic with total installed capacity of 4kW or less, where attached to or wired to provide electricity to a building" is 43.3p/kWh reducing to 18.8p over 10 years. With the recent proposed change to reduce to 21.0p. This is for every kWh of Power Generated irrespective of how used.
The other element is the Feed In Tariff for power exported to the grid. This is assumed at 50% (the "deemed amount") of the Generated Power or is measured by a meter and is at 3.1p/kWh currently.
So if not metered any Power used rather than exported is free. If metered then any Power used rather than exported reduces the Feed In Tariff payment by 3.1p/kWh - hence my earlier details.
My calculation was only referring to using the PV power for heating water compared to the alternative of using mains gas. In fact any PV Power generated that is used for electrical appliances (lights, fridges, kettles, washing machines, et al) will be saving the cost of that Electricity from the grid - Say around 12p/kWh. Hence the payback is not as bad as Tuna says.
BUT yes I do agree that PV is not currently viable without the Generated Tariff!
Okay looking at the bath again - For an average (?) bath of 150 litres then it would take 90 litres at 60C and 60 litres at 10C to result in a 40C bath. So it would take 5.23 kWh to heat the 90 litres of hot water by 50C from 10C.
Regards
Henry
Thank you for your comments.
Sorry for any confusion - My terminology and explanantion were lacking!
My understaning is as follows to clarify for all:
The Generated Tariff for "Solar photovoltaic with total installed capacity of 4kW or less, where attached to or wired to provide electricity to a building" is 43.3p/kWh reducing to 18.8p over 10 years. With the recent proposed change to reduce to 21.0p. This is for every kWh of Power Generated irrespective of how used.
The other element is the Feed In Tariff for power exported to the grid. This is assumed at 50% (the "deemed amount") of the Generated Power or is measured by a meter and is at 3.1p/kWh currently.
So if not metered any Power used rather than exported is free. If metered then any Power used rather than exported reduces the Feed In Tariff payment by 3.1p/kWh - hence my earlier details.
My calculation was only referring to using the PV power for heating water compared to the alternative of using mains gas. In fact any PV Power generated that is used for electrical appliances (lights, fridges, kettles, washing machines, et al) will be saving the cost of that Electricity from the grid - Say around 12p/kWh. Hence the payback is not as bad as Tuna says.
BUT yes I do agree that PV is not currently viable without the Generated Tariff!
Okay looking at the bath again - For an average (?) bath of 150 litres then it would take 90 litres at 60C and 60 litres at 10C to result in a 40C bath. So it would take 5.23 kWh to heat the 90 litres of hot water by 50C from 10C.
Regards
Henry
I started this thread and now far further down the line. The EMMA box is expensive and yes you can buy gas and heat your water that way.
I think its all down to the individual. You will find much cheaper boxes to push spare energy to an immersion or units that re pre set to switch at a pre set level. See http://www.solarhotwaterswitch.co.uk and only about £350.
There are many blogs out there showing different ways of switching. I have spoken with people who have sold on their designs for manufacture. The clever ones are sensing the output from the PV and the load on the house and use variable power switching to squeeze the last drop of spare electricity.
I think hold your breath and they will appear or the hobby electronics magazine will print the circuit.
What you also have to take into the mix, the cost of that new gas boiler and installation etc. What will the cost of Air Source heat pumps come down to? Will they be subsidised by cheap loans one day?
I think its all down to the individual. You will find much cheaper boxes to push spare energy to an immersion or units that re pre set to switch at a pre set level. See http://www.solarhotwaterswitch.co.uk and only about £350.
There are many blogs out there showing different ways of switching. I have spoken with people who have sold on their designs for manufacture. The clever ones are sensing the output from the PV and the load on the house and use variable power switching to squeeze the last drop of spare electricity.
I think hold your breath and they will appear or the hobby electronics magazine will print the circuit.
What you also have to take into the mix, the cost of that new gas boiler and installation etc. What will the cost of Air Source heat pumps come down to? Will they be subsidised by cheap loans one day?
We have now purchased our PV Immersion switch box, I have to build it from a box of components but its a different approach where it only has one clamp. This clamps the cable between your meter and the grid. Clever software looks at the current flow and switches with a solid state relay when enough power is coming off the PV v the house load.
I'll update the forum when I'm further down the road.
I'll update the forum when I'm further down the road.
I'm new to this forum, but have been looking to find out who makes this box. The owners of the house weren't in when we were doing some wiring, but this thing controls a standard 3kW immersion heater directly. The panels are only 2kW capacity, so my guess is that some electronics must turn down the mains to the heater element. There is nothing between the box and the immersion heater, which is rated at 3kW.
How can you run a 3kW heater with a 2kW solar panel, and still not draw grid power? Beats me.
But does anyone know who makes this little solar immersion heater controller?
cheers,
fred
Sorry about the image quality- from my phone...
How can you run a 3kW heater with a 2kW solar panel, and still not draw grid power? Beats me.
But does anyone know who makes this little solar immersion heater controller?
cheers,
fred
Sorry about the image quality- from my phone...
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