MAILBOX: PV-Direct Water Heating Vs. Solar Thermal

A Willis style immersion heater is plumbed in parallel with a standard water heater to provide a small amount of hot water quickly, while also contributing to the main tank temperature.

In Mailbox HP179, Hugh Piggott cited some advantages that make PV-powered water heating preferable to solar thermal technology. Here are a few more:

  • Reliability—There’s no heat-transfer fluid that can leak or deteriorate; and no circulating pump means no moving parts that can fail.
  • Simplicity—The systems are easier to install, test, and commission. A tank water heater having one ordinary and one solar immersion element is no big deal, compared to a special “solar tank” with its necessary heat exchanger.
  • Autonomy—Many solar thermal systems need grid electricity for the pump controller and the circulation pump. PV systems can be entirely self-powered—ideal for those in developing countries, as well as “off-gridders.”
  • Layout flexibility—It’s easier and cheaper to run cables than plumbing. There are no airlock worries, either.
  • Guaranteed energy harvest—Lower solar input in early spring, late autumn, and winter, can compromise a solar thermal system—will the collectors achieve the necessary temperature to input heat to the tank? With PV, if there’s any energy available, it’ll be harvested. Plus, PV modules are more efficient in cold weather.
  • High-temperature capability—Even if the available PV power is low, it can directly heat the hot water supply tank; there’s no need for a preheat tank.
  • Instant delivery—By using a Willis-type external immersion heater arrangement (popular in Northern Ireland, but universally applicable), hot water can be drawn almost as soon as your PV array starts generating a surplus.
  • Greater end-use flexibility—With a PV-heat arrangement, you aren’t just restricted to heating water. With a little creativity and electrical know-how, you can use conventional switches and relays to readily swap between water heating, background space heating (very useful in spring and autumn), or greenhouse soil heating. You can also cook with PV power—think of the deforestation, the fuel-collecting time, and the smoke-related illnesses this could prevent. Meanwhile, even Scots, Alaskans, or Canadians can—using a conventional AC hot plate fed from a dedicated “PV surplus” socket, on sunny days—be bulk-bottling fruit or juice, jam-making, or brewing beer in large batches, and enjoying full tanks of hot water!

Christopher Jessop • Pembrokeshire, West Wales, UK

Comments (3)

Captron's picture

Chris, as I intimated previously, I appreciate your consideration of solar PV-Thermal but still believe that while technically correct you still are not giving the whole story. And, as given you leave a reader with a severely slanted view of the realities of a PV heating system. Asa principle, I spent many years in the Solar PV industry, and now I am in the Solar Thermal (ST) industry also as a principle again for many years and understand both systems intimately.

Let me answer your questions and let’s move on.

PC: You said: “energy harvested per $1,000 of investment.” Let’s Use your number of $2k for the PV system. You harvest energy at a rate of roughly 66% of the size of your panels so let’s say on average your solar array produces 1kW. Over a month, on average you might generate 1x10x30 or 300kW based on an 84SqFt panel footprint .
ST: Qty 2 x 30 tube ST panel, with a smaller aperture of only 58SqFt, generates ~4.8 kWt, therefore with an 84% efficiency over a month you might expect to harvest closer to 1,200kWt. The performance envelop comparing both is closer to 4:1 at peak and yet when the variabilities of weather are factored in the numbers skew well in favour of ST.

Zero Thermal mass: Very true and it does take a few seconds for a true solar thermal system to begin full operation. Let me stress here, a few seconds. Does a DC based PV system lose any energy to transportation, absolutely. If someone wants a HW system with no moving parts then PV is the way to go, sort of. Remember when dealing with DC and heat you also need to consider corrosion, not mentioned, as well are critters.

Let’s use your PV example for comparison: You have 4 solar panels connected to a HWT (Hot water tank) and let’s say you are wired using DC. This does save some conversion losses. Thus while rated at 340W (An awesome solar PV output panel BTW), with a total potential output of 1360W on a perfect insolation day, applied it to a typical 2500W resistance element, then I agree you will get a low but decent thermal rise if the sun is out all the time providing this peak energy output. It would be like trickle charging a Battery, it just takes time.
We both know perfect insolation performance is rare over a given month, and that EvT (Evacuated Tube) solar thermal panels perform quite well in subdued visual light and with cross shadows, due in no small part to their comparative visual acuity spectrum association. Thus if you compared a solar thermal setup in a similar fashion to your PV setup, your EvT performance would get roughly >3.5X more heat energy instilled into the HWT for the same solar panel area.
Having said that, if we were talking only about a HWT, you make some fair points, assuming you are willing to live with the moderate to poor performance. If you want performance over a limited roof area, you can do much better with Solar Thermal [ST].
The way I see it is you have put a lawnmower engine into a small car. Will it work; yes. Will it get you where you want to go; perhaps. Is it the best option, because the engine is small and uses less gas; you decide, because just getting there is only part of the equation.

Yes your $2k in solar PV would cost more as a Solar Thermal [ST] system, although to be honest we would need only 1 panel to match your comparative poor performance. If we used your same solar aperture, then your hot water set-point would be hugely more reliable with ST, especially in cloudy environs where PV is poor and ST functions well, or with a high hot water demand situation such as with a large family.

ST overheating during power outages: not sure where you got this but it does not happen with a ST Drainback system.

Remember, a HWT is a fraction of an average single family dwelling’s energy footprint. We (Digital Solar Heat) are now heating whole homes, including their hot water with solar thermal, even in cold regions like yours, some are heated using solar for 100% of the year.
I did a quick calculation here for a standard fully insulated home in the 139SqM or 1500SqFt range, and there is not enough roof space to put a PV system out there to come close to the ST performance envelope. I also calculated it would cost about $27/yr in electrical energy to run the system if a PV panel was not installed. I do acknowledge you are more focused on a single HWT but the same principles apply: The right tools for the job.

Double your PV system size to more accurately and reliably heat your HWT and your costs have increased to where it is cost compatible with ST.

So in conclusion, I am not saying this PV HW heating system will not work as described, and yet I am not saying PV is better or has more going for it other than no physically moving parts, what I am saying is if you have limited space, want thermal performance and temperature reliability, while maintaining a reasonable price point, I would take ST any day of the week. It’s simply the right tool for the job.

Ron Theaker CD, Calgary, Canada.

Peter Gruendeman_2's picture

Hi Captron:
In order to have a meaningful discussion it is necessary to speak the same language. The language I speak when I teach PV for DHW is that of energy harvested per $1,000 of investment. It's true that solar thermal products harvest more energy per square meter of sunshine but square meters of sunshine are not in short supply. It's dollars that are in short supply, so making better use of them via PV for DHW is the solution of choice.
There are several aspects of PV for DHW that make it less expensive than solar thermal for hot water: It's a very simple system that more people can install themselves. There are no moving parts, no heat transfer fluid or pumps to maintain. There is exactly zero thermal mass in the system so the instant the clouds clear, energy is collected. Or the clouds don't clear and at least some Watts are still flowing into storage. No Watts are wasted heating the panels or tubes, or piping that delivers the heated fluid. No Watts are wasted to cold ambient air, no matter what the temperature and sky conditions are.
Where most people get hung up is they don't know that PV, which now costs 0.70-0.75 USD per nameplate Watt is only slightly more expensive than flat plate solar thermal and PV costs less than Apricus evacuated tube systems per Watt of heat. The other confusing area is that people are unaware that electric resistance water heaters work perfectly well on DC directly off the PV panels. The array needs to be wired to match the impedance of the heating element, or swap the heating element to match your PV array. It's pretty simple stuff.

My PV for hot water system is four SolarWorld 340 Watt panels, which has reduced my use of electricity for hot water by an estimated 97%, to a measured cost of just over $3 USD/ year. This is at La Crosse, Wisconsin, not exactly the solar capital of the USA. My PV panels cost 1,200 USD two years ago. Can you provide a complete solar hot water system, with a 25 year guarantee and no maintenance for $2,000 USD? I can.

Potential buyers of domestic hot water systems should ask themselves:
Do I want a system with zero moving parts and a maintenance-free working fluid?
Do I want a system where all the components are guaranteed by their makers for 25 years?
Do I want a system that can provide backup power (with the addition of batteries + inverter), or do a want a system where the glycol is overheated and ruined during a prolonged power failure?

Learn more by seeing my lecture at the MREA energy fair in June.
Pete Gruendeman
La Crosse, Wisconsin, USA

Captron's picture

Chris, it's not that the comments are technically wrong, but they do give you the wrong impression. This article reads like someone trying to convince me walking is better than driving a car. Yes if you are walking locally this may be a good idea, but if more than that is involved, such as distance, carrying capacity, infirmity, and braving the -40c elements, then a car is a much better idea. You can scrounge up all the minor details to support your conclusion but in the end it’s a disservice even if some of the some of the words can be construed to be correct.

Christopher, this article while barely accurate is so far removed from the reality of solar thermal that you leave readers with the impression that this is a valid premise when in fact it is not. The only person who might garner some confidence from this is a solar tinkerer, not a valid homeowner or business owner who wants to make a real difference with respect to heating or hot water.

I live in a country where heating is life or death, nothing you have illustrated would make me think this option of PV powered thermal hot water for this use is even close to valid. At one point I thought a PV powered heat pump might fit this bill but in testing this I found it too was poor substitute for the real thing. Not only did you need twice as many panels to get the thing started but locations with ambient temps of -25c can’t efficiently operate in an air-to-water environment. On top of that you needed 4 times as many PV panels for the same thermal energy output. I am focused here on quality EvT panels not pipes or flat plate panels.

In addition some of the comments made are categorically inaccurate. Low angle solar is just as bad for PV as it is for EvT panels. There is a scientific reason for that, it’s called solar angle of inclination and if solar energy is drawn through thousands of KM of earth’s atmosphere it will be attenuated or degraded. And, an EvT solar panel will outperform a PV panel in this insolation regime over 3:1. Also the EvT panels are not susceptible to random partial shadow events, or angular orientation or even bird droppings.

This article needed an intelligent and thoughtful counterpoint otherwise it is just not good journalism even if the title forces you down one path: Change the title.

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