diy solar

diy solar

Water heating. PV panels or their water filled cousins for heating tanks in the loft.

David in the UK

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Apr 19, 2022
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Hi all, newbie here with plenty of questions and in need of some advise and guidance.
I live in the London region of the UK and currently have on the house 19 solar panels (7kw) which charge 4kw batteries and also provide some power to the mains, this is all fine. I have an additional clear area on the roof of approx 4 feet (1.2 m) x 23 feet (7m) which faces the sun (southfacing tiled roof, about 35- 40 degree pitch) and would like to use this area solely for heating water.

My questions given the area, how does solar PV compare with water filled panels in terms of efficiency, cost and installation issues? Plus any other issues I've not thought of?

My intention is to heat one or 2 tanks in the loft quite close to the panel location with a total capacity of about 100 gallons. So whether it is heated by an immersion element or 2 (powered from PV panels) or heated by solar water filled panels is the choice to be made. What would be the best decision?

Our water heating currently is just using electricity and has been like this for the last 50+ years as the house had a gas fired warm air unit installed. I have got a replacement warm air unit I'll be installing shortly that has a built in water heater that can also be plumbed into these water tank(s) but is only about 3.3Kw so not hugely powerful on it's own.

Any other considerations or issues?
 
I would go with panels for several reasons.
You don’t have to worry about servicing calcification build up in the solar water heater panels pipes.
Which would not be easy to get cleaned out on a roof.
Secondly you have the ability to use the grid to run the heater if the weather is bad and lastly extra pv power could be used for running more things in the house.
 
Europe has a lot of oddball tanks, is yours vented? I use PV for my hot water and it can be very cost effective. Supplemental PV resistive heating can be more cost effective than a heat pump water heater in low water use situations. In winter, the utility PG&E commissioned a study and found that actual COP dropped 44% in homes with natural gas heat. Trouble is there is very little equipment on the market for efficient stand alone heating and without a second element blending grid power is difficult. You will get a lot of bad advice here on how to do it. Don't even consider direct connect panels in your climate.
 
Currently using both, I can tell you that from an efficiency (and equipment) standpoint the hot water panels win hands down over PV. All you need is an inexpensive (Delta T) solar controller and a small circulation pump (that can also be run off of PV solar) to circulate the hot water. In terms of square feet, it takes 2-3x the amount of area in PV panels (and more expensive controllers) to heat the same water.

I made my system stand alone and it requires nothing but sunshine to function and provides WAY more* hot water than we can use. In fact we can go up to 3 days without sun and still have (at least) warm water to use on that 3rd day.

*In both of my systems, I heat my tanks up to 185F and use a tempering valve to automatically reduce the water down to 125F (normal household Hot Water Temperature). You may or may not need to do this based on your storage capacity and water use.

NOTE: If it EVER freezes in your area, you will need to have a way for the water in the panels to drain out completely or the freezing water will burst the pipes and destroy the panels. I use what is called a "drain back" system to avoid this problem all together. When the pump cuts off, all of the water in the pipe and panels automatically (gravity) drains back into the tank which is stored in the house.

Also, I have my incoming water filtered and put through a water softener so there are no calcium buildup issues to worry about. You can test your water hardness to see if that is even necessary for your location.
 
Many thanks for all the replies, all appreciated.

EfficientPV, my current tanks in use are vented, I do however have in the garage an unused 210 litre (45 gallons) unvented tank and I was considering purchasing another one to go over to an unvented system. What advise would you give?

JP, can I ask why would you not use an antifreeze type mixture in the system rather than draining back?
 
Many thanks for all the replies, all appreciated.

EfficientPV, my current tanks in use are vented, I do however have in the garage an unused 210 litre (45 gallons) unvented tank and I was considering purchasing another one to go over to an unvented system. What advise would you give?

JP, can I ask why would you not use an antifreeze type mixture in the system rather than draining back?
Either will work. I just choose the drain back option because I felt it had the most benefits and least drawbacks. You can look at this page to see some of what I am talking about. https://www.sunnyhotwater.com/closedloop.html
 
With PV water heating you have limited space and I imaging less than optimal sun conditions. That means you would have to use a power point controller like the ACTii to optimize panel output. That costs about $250US although it comes from Poland and whatever VAT would be added. I build my own for about $10 from old parts and it appears that is not an option for you. So, you are left without enough power or spending too much for what power you get. PV water heating is a great project for technical individuals or those who have tons of cheap panels they have no idea what to do with. Tall stratified tanks are a way to dump energy on an unknown basis. I have a 40 gallon tank for laundry that only gets 1-2KWH a day and that is enough for laundry several times a week. For me it is free heat just using excess solar for almost nothing in electronics. It is not yet something which can reasonably be ordered from amazon. If you have any technical friends, contact me.
 
I have a DIY glycol system ( 2 pumps and heat exchanger). I have used it for 15 years, replaced 1 pump. 2 pumps x 60 watts. Pumps power from my PV system. Some winter days use 2 kw. I would say it works great about 9 months of the year. I only have room for a 50 gallon tank though. Panels are the glass from sliding glass doors so not as big as some but free parts.
 
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