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

The other alternative to electrical resistance heat is an air source heat pump water heater. It uses one quarter of the energy compared to resistance heat. You would have to compare cost to know which is the most economical choice between thermal solar or adding a HPWH.

After owning an maintaining a solar direct water heating system, I would never recommend one unless there was no better choice. Way too many things to go wrong including overheating in the summer, freezing in the winter, nuisance leaks, corrosion, controller failure, expensive tank replacement at 5 years, etc. After 7 years I'd had enough of that. I tore it all out, sold the copper plumbing, gave away the panels and went back to electric as we had no access to natural gas at the time. These days, we have natural gas available and use that but I would choose a heat pump water heater (HPWH) if I had to heat water with electricity in the future.

1.) Given well designed and fit for purpose equipment, day/day efficiency of heating water with PV will be something of the order of 15-17 % when measured as irradiance input per m^2 of panel delivered to a water tank.

2.) A well designed and fit for purpose solar thermal flat plate system will, within some measure accounting for ambient temp., deliver something like ~ 35 - 40% or so of the irradiance input per m^2 of collector surface.

3.) Both efficiency measures are exclusive of tank standby thermal losses.

4.) The solar flat plate thermal efficiency is also before or exclusive of thermal piping losses.

5.) Both estimates are a rough average over a year.

6.) The PV efficiency will be slightly lower than average over summer. The solar thermal efficiency will probably be noticeably lower over the winter.

7.) in either case summer energy collection will be greater than winter energy collection.

8.) On a per square area basis, while the solar thermal system will be approx. 2X as efficient at turning cold(er) water into hot(ter) water using solar energy, The solar thermal system will require more maintenance and baby sitting than a PV system. Many folks find that gets old quickly and if neglected caused problems. Solar thermal systems are demanding mistresses and are vengeful when ignored.

9.) On a delivered unit of heat basis, the systems' costs may or may not be comparable depending on local availability and such things as government or utility co. incentives.

9A.) Note: I'm mostly ignorant of either systems' costs in the UK. Comparson shop but be informed before you interview vendors and don't rely on them for information. Know the answers to the questions you ask - you'll learn 2X that way, once for product knowledge and again when you find out what vendors don't know (but should). Spend more time and effort on vendor quality than on equipment quality. Beyond some basic level, most solar equipment is a commodity. A good vendor is worth a slight premium and can make the difference between a mostly trouble free job and a nightmare. Don't skimp on vendor quality.

10.) As Ampster notes, a PV/air source heat pump combination may be one option. Doing so will enable the heat pump to operate at a year long average C.O.P. of 2.0 or greater. That will bring the PV/heat pump system's overall efficiency at turning sunlight into hot water to operate at something similar to or perhaps greater than the solar thermal system efficiency for a lot less maintenance and hassle. The cost of a heat pump/PV system will be more than PV/resistance heating but probably not much more than a solar thermal system operating on its own. Also, the existing piping would not need modification.

11.) My usual pitch that conservation is more cost effective than adding generating capacity applies here. Less water use and lower thermostat settings will produce more savings in energy and its cost than more solar equipment.

Welcome to the neighborhood.

This string brought to mind something I'm considering installing. Based upon research utilizing the phrase "hot water recirculating pump" I found an article on the Plumbing and Mechanical's website titled "What you need to know about hot-water recirculation" that provides lots of useful information including electrical costs associated with hot water recirculation.

My initial starting point was determining the time and water used when turning on the hot water spigot on the kitchen sink (the furthest hot water source from the water heater). The findings were:

• Time = 52 seconds
• Volume = 1.2 gallons

After completing the test, my initial thought, demonstrating impatience, was it always seems like a long wait time (mindful of the phrase "it always take longer for water to boil in a watched pot"). Turning to calculate the cost of that 1.2-gallon use, last month my water usage was approximately 600 cubic feet. at a total cost of $33.00. 600 cubic feet represents 4,488 gallons. The resulting cost per gallon is $0.00735294.Keeping things simple, if we have 10 hot water waits per day, the cost would be approximately $0.07 per day of water usage or $25.55 per year.

On the cost side of this equation, the article above indicates a potential electrical energy cost of $1.40 to $1.59 per year, based on a rate of $0.12/kWh. The rate I pay here in Southern California averages 3 times that rate (estimating $0.36/kWh). That increase has me estimating $4.50 in energy cost per year to operate the circulating pump. From a cost-recovery standpoint, moving forward with this effort after adding the installation costs may not make sense unless it is personally worthwhile saving my time as I stand in front of the shower of the kitchen sink.

While I understand and appreciate the importance of not wasting water, perhaps the best approach would be to have an empty one-gallon bottle next to the kitchen sink and use that colder water to water plants.

Thoughts?

I have a completely different opinion on solar thermal water heating.
As JPM stated the efficiency is at least double of the best PV panels. Flat plates panels with a stainless steel drainback tank and stainless storage/ water heater should last past 25 yrs. without any problems. No issues with overheating, or freezing in winter. The newer evacuated tube systems have given solar thermal a bad reputation with extremely high failure rates because of inferior Chinese components.
The drainback system can't get any simpler. A controller, pump, panels and storage, no glycol and the system shuts down automatically on power failure. I 've installed over a dozen drain back systems in the last 15 years with few minor problems, all minor issues were covered under the manufacturers warranty. Selecting quality components and a competent installer makes all the difference in the longevity of any mechanical system.
Just as an FYI the projected life of a standard glass lined steel water heater whether it's electric, gas, or heat pump is 7 years depending on the water quality.

When I moved here, it was quite a delay to get hot water. I did not like
waiting, running so much water down the drain, or losing the energy
of the parially heated water. I re arranged the pipes so hot water flowed
past all the spigots and then to my (very energy efficient) recirculating
pump and the water heater. A control runs the pump if the differential
between the heater and the extreme end reaches 7 deg F. I did my
best amateur job of insulating the pipes, limited by the quality at the
big box stores. A meter tells the temp at both ends.

Now heat is very quick, not running a lot of water down the drain. The
downside is, the water heater must run more to make up heat lost from
the always hot pipes. Not a terrible loss, the basement tends toward
chilly. It will no longer matter once my heat pump water heater is
functional, the rest of the HVAC is already solar powered heat pumps.
Circulators are for sale, but I built my own.

It is pretty annoying to replace water heaters so often, ought to be a
better way. Bruce Roe

Replacing the anode rods on standard water heaters helps increase the lifespan.
There are stainless steel, or stone lined water heaters available with lifetime warranties, but expect to pay a premium for that. Rigid fiberglass pipe insulation is available in various wall thicknesses to decrease losses in water piping. It works great but I've only used it in commercial / industrial buildings because of the total install cost. There's always an energy saving solution but will the customer accept the additional cost ? Usually not.

Should last 25 years but not my experience. Maybe newer stuff is better?

My system was a drain-back. When the drain valve failed one cold winter evening, the tubes in the flat-plate panels on the roof froze solid that night and split open in multiple places. The controller had a "snap-freeze" function that was supposed to circulate water as a failsafe to prevent a freeze in such possibilities but it did not function correctly. My neighbors called me at work the next day asking if I was aware of all the water spilling off my roof after the sun melted the ice in the tubes. The company repaired them in-place instead of replacing them. I think the bonds between the flat plates and the tubes were broken in many areas from the tubes swelling so much but I had no easy way to prove that the freezing and repair reduced the panel efficiency. The company refused to replace the panels of course.

When the water reached boiling point one summer afternoon, the pressure relief valve opened but then failed to close and dumped water for nearly a week because my wife did not notice the issue and I was out of town. The special 120 gallon solar tank with a 5-year warranty failed at just under 6 years, flooding my garage and bonus room. The cost to replace the tank even 10 years ago was over $1200 plus labor. I repaired all the water damage to the carpet, baseboards and drywall on my own to avoid an insurance claim and returned to a standard electric water heater.

Rough estimates tell me I lost at least $4000 (not counting flood repair cost) compared to if I had simply stayed with a normally electric water heater.

Since you seem to be asking:
Thoughts:

1.) The subject of domestic hot water recirculation seems pretty far removed from the OP's questions, and stated situation and intentions. Seems close to a thread hijack. Besides, we've exhausted this topic several times in the past.

2.) Still, and briefly, an energy analysis that takes all added energy use associated with residential recirc. schemes will usually and commonly show them to be an energy waster as well as being very costly to retrofit if not part of original construction. In spite of the peddler's hype, such systems are a profligate waste.

3.) Better cost effectiveness and less energy waste is usually possible while still achieving the goal of instant point of use hot water by installing small point of use tank type elec. heaters of 2 - 6 gal. capacity at/under sinks w/size determined by pipe run length. Results: Hot water in less than 5 sec. at point of use. No recirc. loss of hot water which also BTW = uncontrolled heat gain to the dwelling. No, or much less water waste. Much easier installation.

What I have found is on solar forums they can't heat water with PV to save their lives. PV resistive gets a bad rap. I heat water with PV and at low use, 20 gallons a day, it is really cost competitive compared to buying a heat pump. I have a little board that does it which has arc interrupt so standard thermostats can be used and heaters daisy chained, it can track, it can be used off the same panels as a charge controller and divert excess energy, no batteries needed. Last panels I picked up were $300 for a KW which wasn't all that cheap. Compare that to the cost of a heat pump. Summer, more than enough power. In winter, PG&E study said the actual COP of a HPWH dropped 44% when natural gas was used. In supplemental heating at least all the PV energy is used. I have a 40 gallon tank just for the laundry and stratification is a great strategy. Top 15 gallons is always hot. As you go lower in the tank it is pre heated and that heat loss is very low. I've seen the studies and some people must be washing elephants. But there are a number that actually don't use that much water. And when you get into low use the COP of HPWH drops quite a bit. The trouble with thermal transfer systems is that they are a window to the world when they aren't active.

For DHW loads that low, I'd agree that a heat pump may not be the best solution, but PV/resistance ain't that much better, if at all. I'd also agree that the C.O.P. of an air source HPWH drops a lot in winter making those devices less than ideal for cold(er) climates depending on just how cold.

For small DHW loads like 20 gal./day or so, I'd consider a passive batch heater w/a resistive heating element for the tank and use heat tape with a lot of pipe insulation for freeze protection. Cheap, fit for purpose and not much maintenence.

I saw some data from a guy in S Africa with a solar thermal system. Every morning he supplements it with 50 minutes of electric heat, 4KWH. That is how much I put into my system all day and that is only from the excess my off grid system does not use. If people can just throw a black barrel on their roof, great. I would have to move that heat 150 feet, wires are a lot easier. Purely an engineering problem. I have a guy in MN with a 650W array. He just added a diversion control from his system to a 6 gallon water heater and when that is up to temperature it switches over to space heater which warms his camp thru the week when he isn't there. That is a small system. I see so many living in small camps like refugees. It is nice having hot water and not having to expand the system. Give me a wire anytime over a pipe.

Small and often DIY systems often have different needs and priorities than more permanent and commonly found (in the U.S. anyway) systems.

I have yet to win a battle against entropy. Making (or buying) low entropy electricity and then intentionally turning it into high(er) entropy heat for a simple task is costly.

The inherently inefficient round trip of sunlight (high entropy due to being diffuse in nature) to electricity (low entropy and so versatile by nature) to heat (back to high entropy) is a round trip that is a longer trip than that of sunlight directly to heat (less entropy increase, even when the heat losses in piping are considered).

Those resultingly small losses (because the systems are small) are probably more acceptable and may not be considered important or considered acceptable because PV water heating is easier to achieve and maintain than a bunch of plumbing.

But, the dumb task of heating water, or heating anything else for that matter, with a highly versatile energy source like electricity is a waste of the electricity that is better used for tasks thermal energy cannot perform. Direct water heating with the sun about 2x+ more efficient than turning sunlight into energy for a reason. The reason is entropy. It takes a lot of dumb (thermal) energy to make versatile energy like electricity. That versatility comes at a price. Why then turn that versatile energy back into dumb energy ?

As an example of the versatility of low entropy electricity, try running the monitor you're looking at on heat. Heating water with electricity is like using a gold brick for a paperweight when a rock is as fit for purpose. Things are easier to make more efficient (read less costly) when the energy source is a closer entropic match to the desired result.

That SA system was professionally installed and totally unsuitable for the application. All it did was heat water to lose it overnight. The tank was too large and no attention was paid to time of use. It is what you get for the cost and get it when you need it, period. I understand your narrow view. But, Just about everyone in solar wastes a lot of power because they have to have excess to recover. If you don't use it, your stupid. I can see you don't have anything worthwhile to contribute.