Solar hot water panels vs PV Panels

syboit

After reading one of the other threads here "Solar Panels", I was wondering if you can install solar hot water panels for around 4k would it be possible to install a pv system for roughly the same price which would cover the running of the emerisson to give hot water.
Wouldn't there be advantages of running pv panels where electricity can be used around the house, where the solar panels would only be heating water?

mawk

pv might be more versatile, but if you are heating water or rads electrically you would probably see better savings from collectors. heating water is pretty damn costly

Do-more

fclauson will be along in a minute with a spreadsheet to show you how they compare!

I'm pretty sure (from memory) that he will show that PV wins.

Solar hot water has a number of limitations which are related to hot water usage, for example if you are constantly using hot water the bottom of your tank will be at a lower temperature and you will get a good transfer from any solar energy available. On the other hand if you are not using the hot water the bottom of the tank will be at a higher temperature and you won't be able to utilize addition solar energy and it must be dumped via some form of dump load.

Other variables such as the size of the tank, type of panels used and so on will also effect the result, this becomes particularly critical as you try to maximise the fraction of total DHW you produce in the year from solar energy.

My own decision is to produce DHW year round by an ASHP and when the cost of PV has levelled out (hopefully at a good deal lower level than today) I will fill the roof with PV panels.

fclauson

Took me two hours to appear - but hear I am

I have struggled with this one - Quentin is the best man to comment (look at other PV threads for this) - but heating water with electricity is not (today) economic - 1kw electricy will only give around 80% of its energy delivered into the hot water (lots of losses due to cables etc)

So what you propose does not really work

In terms of PV vs Solar thermal - thats a different storey - the decision process is more complex - is it a new build or is it a retro fit, are you on mains gas or are you burning wood as a heat source - all of these can affect the return on investment calc

syboit

fclauson wrote: »

Took me two hours to appear - but hear I am

I have struggled with this one - Quentin is the best man to comment (look at other PV threads for this) - but heating water with electricity is not (today) economic - 1kw electricy will only give around 80% of its energy delivered into the hot water (lots of losses due to cables etc)

So what you propose does not really work

In terms of PV vs Solar thermal - thats a different storey - the decision process is more complex - is it a new build or is it a retro fit, are you on mains gas or are you burning wood as a heat source - all of these can affect the return on investment calc

thanks to you and all the other's for replying. I suppose in our case its a retro fit, and we have an oil boiler for heating/hot water heating.

I suppose as mawk said, PV is more versatile and I suppose the cost of solar vs the cost of a pv install (to guarantee you get a good yield of power) will have a big bearing on the decision.

Yup electricity is probably a more expensive way of heating the tank, but with the prices of oil (and probably gas for others) on the rise it might soon balance out.

quentingargan

You have to bear in mind that KwHr for KwHr, electricity is a premium source of energy. Back at the power station, they typically burn 100KwHrs of oil to produce 40KwHrs of electricity. The rest of the energy is lost as heat. So if you get electricity, using it to heat water is a poor use of this premium energy.

Also, put another way, solar thermal panels are typically about 80% efficient, though that averages out at 50% of so if the cylinder is hot, or due to pipework losses etc. Solar PVs are typically less than 20% efficient.

If you use a lot of hot water in the summer time, solar thermal makes sense - especially if you are heating this using electricity. (Also, some boilers are not very efficient at producing small amounts of heat for the cylinder).

Solar PV was beginning to come into the frame in terms of return on investment, but then ESB / Electric Ireland scrapped their feed in tariff, so you now only get paid 9c per Kw Hr for power exported. That makes it a bit harder to justify.

Do-more

@syboit It's probably a useful exercise for you to try to calculate what the current cost of your DHW is and then try and figure out what return you will possibly get from the investment.

Do you use oil for DHW all year round? If so then you are just looking at an alternative source of heating DHW when your boiler isn't running to heat the house anyway. Probably around 6 months of the year?

A family of 4 is probably going to use around 3000kWhr of DHW in the full year (some more, some less) so assuming that you are looking to produce about 1500kWhr during the 6 months outside the heating season.

If you do that with an electric immersion you are looking at around 1500 X 0.19 = €285.

In my case I will be producing DHW with a air source heat pump and at that time of year the efficiency is very high so I am probably safe enough taking a COP of 3 so my DHW cost by comparison is €285/3 = €95

I don't know what the present cost of heating DHW is for you on oil, it will depend on your boiler and controls but you can probably get a rough figure for it.

But if we use my figure of €95 and I assume that I can do a DIY solar install for €2000 then the payback (before inflation) is over 20 years, putting a finger in the air to judge inflation that might drop to 14-15 years. If the solar installation cost €4000 (easily so if you need a new tank and professional installation) then you can see that the payback time is very long.

All this is very difficult to calculate accurately and a lot of it is dependant on what sort of energy inflation we have in the coming years but hopefully I have given you an idea of what calculations you should make to arrive at your own conclusion.

For Solar PV you can input your own data into this online calculator which will give you an idea of what output you will get from your roof.

Again you have to figure out what's going to happen with feed in tariffs in the future to try to decide on the payback...

fclauson

Do-more wrote: »

...
I don't know what the present cost of heating DHW is for you on oil, it will depend on your boiler and controls but you can probably get a rough figure for it.

check out the SEAI site - its about 8c for oil per kWh
other fuels are also quoted for

http://www.seai.ie/Publications/Statistics_Publications/Fuel_Cost_Comparison/Domestic_Fuel_Costs_Comparison_January_2012_pdf.pdf

quentingargan

fclauson wrote: »

check out the SEAI site - its about 8c for oil per kWh
other fuels are also quoted for

http://www.seai.ie/Publications/Statistics_Publications/Fuel_Cost_Comparison/Domestic_Fuel_Costs_Comparison_January_2012_pdf.pdf

When it comes to getting domestic hot water from oil or gas in the summer, it isn't that straightforward though. In some cases you may be heating a burner and flue in a shed, and then heating the water in underground pipework, routed all the way to a tiny cylinder in the hotpress. If the burner runs for 1/2 an hour, 15 minutes of that might just be warming up the hardware...

Also, bear in mind that the cost of upgrading a water heating system often includes replacing a poorly insulated cylinder with a well insulated unit which will continue to save money year round.

fclauson

all true Quentin - adds to the fun of a return on investment calc and highlights further areas of variation between retrofit and new build choices

fclauson

Do-more wrote: »

... a lot of it is dependant on what sort of energy inflation we have in the coming years but hopefully I have given you an idea of what calculations you should make to arrive at your own conclusion.

..

hope by then the hydrogen fuel cell will be here

ECO_Mental

Just going through the process my self on a new build and trying to figure out which heating and HW combination to use.

Anyway just thinking out loud here,but one other advantage to go down the PV route would be if you think about it, you could spend 3 to 4 weeks a year on holidays out of your house and if you have ST then all that energy is just wasted. BUT if you have PV all that energy can be used to off set the fridge freezer, security system etc kWh.

My current house (3 bed semi) runs about at 100watts on shutdown mode and that's with every light off and every battery charger, sky box switched off at the plug.

Must try and run the numbers myself only just thought of this. It might make a difference on the payback etc. Considering that we all go on holidays during the summer at peak solar time it would be a shame to loose all that energy if you have ST. There is also the risk of over heating your system and trying to dump all that heat

CJhaughey

ECO_Mental wrote: »

Considering that we all go on holidays during the summer at peak solar time it would be a shame to loose all that energy if you have ST. There is also the risk of over heating your system and trying to dump all that heat

Most if not all reasonable solar controllers have a holiday mode where the pump runs at night and dumps heat back through the panels.

heavydawson

Starting a new build in Tipperary in the coming months, and eventually decided upon a building fabric thats 0.12 floor,walls,ceilings. Windows TBD.
We're installing an airtightness membrane in the attic, and going to town on the quality of the plastering for the airtightness in the living areas.

The house is 287 sq m2 and we're designing it to an A2 BER rating (FWIW).
The main heat source will be GSHP, horizontal collector (site is 1.25 acres, so we're lucky enough to be hopefully able to size it appropriately).
We're putting in MVHR, and now it's come to PV.

I've got one quote for a 1.5kwp setup for €5.5k. My southern facing aspect has room for 4-5kwp worth of modules (@250w/module).

It seems to me to make sense to install PV when you're going GSHP or any HP-variant, or is my logic flawed?
I calculated my generation potential to be roughly 2.5kwh/annum @ 3kwp using this tool

My payback time estimate is based on the savings I'd make over the course of a given year @ the current kwh rate from the ESB (18.38c/kwh)
So 2590kwh * 0.1838 = 476€/year saved
Based on what I've been quoted for the 3kwp system (~€8.5k), and assuming I export nothing back to the grid, payback is roughly 17 years for my location with optimal azimuth and orientation.
I'm actually being generous, as I haven't included the mortgage interest that will be added on top of the initial outlay cost.

So if anyone thinks my quote is off-the-wall, and would like to PM me some recommend PV suppliers, I'm all ears.

quentingargan

Hi Heavydawson. I'm afraid your payback time will be quite a bit longer than that because in practice it is unlikely that you will consume all of your power, particularly as you increase the size of array. If you had the system in before February, ESB would have paid you 19c for your exports, but that now has fallen to 9c. There are no particular plans to increase that.

The heat pump will run a lot less in the summer time as well.

All that said, solar PV is cheap as chips at the moment, and you could put in a system hoping that the energy regulator puts the boot in and forces ESB, Airtricity and Bord Gais to pay a reasonable feed in tariff for microgenerators. It is probably cheaper and easier to do that now while the house is under construction.

Whether you use solar thermal as well depends on your hot water use, but it does make sense to do this as your house won't need heating in the summer time, and generally heat pumps aren't great at getting water up to 60 degrees.

Lastly, the price you have been quoted seems a bit high for a new build. If the scaffolding is in place etc., you should be able to do better than that.

heavydawson

quentingargan wrote: »

Hi Heavydawson. I'm afraid your payback time will be quite a bit longer than that because in practice it is unlikely that you will consume all of your power, particularly as you increase the size of array. If you had the system in before February, ESB would have paid you 19c for your exports, but that now has fallen to 9c. There are no particular plans to increase that.

The heat pump will run a lot less in the summer time as well.

All that said, solar PV is cheap as chips at the moment, and you could put in a system hoping that the energy regulator puts the boot in and forces ESB, Airtricity and Bord Gais to pay a reasonable feed in tariff for microgenerators. It is probably cheaper and easier to do that now while the house is under construction.

Whether you use solar thermal as well depends on your hot water use, but it does make sense to do this as your house won't need heating in the summer time, and generally heat pumps aren't great at getting water up to 60 degrees.

Lastly, the price you have been quoted seems a bit high for a new build. If the scaffolding is in place etc., you should be able to do better than that.

Thanks for the feedback. One important detail I forgot to mention is that I'll be working from home 2 or 3 days a week. I'm in IT, so you can expect at least one pretty recent laptop and a monitor (and wifi access point) running during that time. This will surely help offset any surplus capacity?

quentingargan

heavydawson wrote: »

Thanks for the feedback. One important detail I forgot to mention is that I'll be working from home 2 or 3 days a week. I'm in IT, so you can expect at least one pretty recent laptop and a monitor (and wifi access point) running during that time. This will surely help offset any surplus capacity?

Laptops are very modest consumers - about 50W typically. I have seen industrial installations where all power was consumed onsite and that makes sense, but usually geothermal systems are priced on the basis of running at night on electricity that costs about 8c, with a boost in the evening time (just as the sun is going down). You can probably re jig the timers to optimise your own conumption.

LifeBeginsAt40

And for those of us that rent our houses, it is sadly something that can't even be considered!

I'd love to be in the position to heat my water via Solar or PV, but can't see the landlord paying out the €€€ and I won't be in property long enough to get my money back.

Oh well, back to just reading...