Solar water heating
Last update: January 2016
No doubt about it: solar water heating systems definitely save money in the long run, but typical payback time in the U.S. is around 7 years (vs. an electric heater) to 16 years (vs. a gas heater). And of course, once it pays for itself, your hot water is free for years after that. As you might suspect, Mr. Electricity's household enjoys solar hot water. As I write this, in the middle of winter, our system is making our water nice and toasty.
Yes, solar works even when it's cold, as long as there's sun. Even if it's freezing outside, your system can still use the sun to heat the water. When there's no sun, your traditional tank heater will act as a backup to handle the heating.
Note that in probably most cases, it's a better deal to install solar PV panels to make electricity, and then use the electricity to power a heat-pump water heater. (The only reason I didn't do that myself was that I had only 40sf of sunny roofspace, vs. the 110sf I'd need for a solar PV system.) See more about this at GBA. Before you have a solar water heating system installed, I strongly suggest you consider whether going with solar PV + a heat-pump heater will be better for your situation.
With solar PV, you can generate 100% of your water heating needs with solar. With a solar thermal system, you won't be making all of it because the heater will use the electric backup when the sun's not shining. The amount of heating supplied by solar is called the solar fraction, and it varies from about 50% to 85% depending on how much sun your area gets. (NREL, pp. 8,31,32)
The rest of the page is for those who are sure that solar thermal is better for them than solar PV.
Solar thermal systems
There's a bewildering variety of solar heating systems. The easy way is to deal with this is to choose a reputable, well-reviewed installer, and go with their recommendation. The harder way is to learn all about the different kinds of systems yourself. This page is for those of you who want to go the hard way.
So, let's compare the different kinds of systems. Of course, before doing that, we'll need to learn some basic vocabulary:
- Collector. This is the part that absorbs the sun's solar energy. It's usually a flat-plate collector or a rectangular grid of tubes (both of which you probably called a solar panel). Your installer will call it a collector, so that's what we call it. In some batch systems (more on those below) a standard water heater tank is used as the collector, but those aren't very popular.
- Tank. Most systems use your normal water heater tank for storage. This lets you heat your water the normal way (gas or electricity) when the sun's not shining.
- Direct/Indirect (aka, Open/Closed). Direct systems heat the water you'll actually use, while indirect systems constantly recirculate the same water or antifreeze in a closed loop between the collector and the tank. Direct is also called "open", and indirect is also called "closed".
- Active/Passive. An active system is simply one that uses pumps. Most do. The less common passive (pumpless) systems are cheaper but not good for freezing climates, and most places do freeze at least sometimes. If we combine the two ways of classing systems, then we have Direct Active, Indirect Active, and Direct Passive. (There's no such thing as Indirect Passive.)
- HTF. HTF is heat transfer fluid, which is the water or antifreeze solution that constantly recirculated in a closed loop in an indirect system.
| Types of solar water heating systems | |||||
| System name |
D/I | A/P | Description | Pros | Cons |
| Batch (integral, ICS), cylindrical tank | Direct | Passive |
The collector is the tank. It's either a standard cylindrical tank like you're used to, or a panel of tubes. You'll still need a separate tank as a backup, for when the sun's not shining. Sometimes batch systems just function to pre-heat the water for the conventional heater. | Simple, cheap | • Freeze risk (in freezing climates) • Overheating risk • Scale can build up, decreasing capacity • Most of these systems have small capacity • Batch systems also have the cons of losing lots of heat at night (not so with CHS) • Batch systems with cylindrical tanks make less hot water than batch w/tube collectors or thermosiphon. |
| Batch (integral, ICS), tube collector | Simple, cheap | ||||
| Thermosiphon (CHS) | The tank sits above the collector, and the hot water rises into it naturally since it's less dense than cold water. | • Simple • Unlike batch systems, the collector tank can be insulated, reducing heat loss an night |
|||
| Stereotypical |
Active |
Cold water is pumped up to the collector,then down to the tank. (I don't know why the house's water pressure isn't sufficient to push the water up to the collector.) I call this one "stereotypical" because it's probably how most people imagine a solar water heater, although it's actually not the most popular kind. | • More efficient than indirect systems • As with all Active systems, a traditional tank is used, which can heat the water with gas or electricity when there's no sun |
• Freeze risk (in freezing climates) • Overheating risk • Scale can build up, decreasing capacity • Pump uses ~$10 of electricity per year |
|
| Water | Indirect | The same distilled water is constantly circulated in a closed loop of piping between the collector and the tank. (That water never comes out of a faucet; it's permanently stuck in the loop.) While in the collector, the water absorbs solar heat, then it's pumped into the tank where its heat radiates through the piping to heat the tank water, then the heat-removed water is pumped back up to the collector. | • No scale buildup in the collector or
the piping between the collector and the tank, unlike
direct systems • Pump uses less energy than drainback system |
• Freeze risk (in freezing climates) • Overheating risk • Pump uses ~$10 of electricity per year |
|
| Glycol | Same as with the Water system above, except it's a glycol solution (60% water, 40% glycol) that's pumped through the loop, which offers some freeze and overheat protection. | • Glycol systems typically circulate some hot water
from the tank through the collector for extra freeze
protection in cold weather; this wastes some of the
heat that was generated • If glycol overheats, it acidifies: the acid shortens the life of the system, and acidic glycol no longer provides freeze protection • Glycol must be replaced every 3-8 years, at a cost of $150+ • Pump uses ~$10 of electricity per year |
|||
| Drainback | When the tank has reached its set temperature, the pump turns off and the HTF falls "drains back" into a separate, small drainback tank, which empties the loop of HTF fluid. This elegantly prevents both freezing and overheating. | Overcomes all problems of other systems (no sediment
buildup as with direct systems, no worries about
freezing/overheating, HTF never needs to be replaced
as with glycol systems) |
• Less efficient than a stereotypical system • Pump uses more energy than water- or glycol-based system (~$24/yr. is my estimate based on figures here) |
||
How some of the "Cons" in the table are sometimes addressed
Direct systems:
- Freezing. Take your pick:
- Freeze-tolerant collectors (but that doesn't protect the piping).
- Using a pump to circulate hot tank water through the system (wastes some of the heat that was generated).
- Draining the system in freezing weather (inconvenient)
- Overheating: Heat export pump, or covering the collectors with a blanket
- Scale buildup: Ion-exchange softener
Freezing in indirect water-based systems: Draining the system in freezing weather, or using glycol instead of water as the HTF, or using a drainback system in the first place
Overheating in glycol system:
- Covering the collector with a blanket, which is inconvenient if the collector is on the roof.
- Setting the tank temperature higher, which will keep the pump circulating until that higher temp is reached, if ever. This has the added benefit of increasing hot water storage, though this adds the downside of increased risk of scale buildup in the tank. Also, if the tank does reach the max temp (easy in the summer), then you're right back to the risk of overheating the glycol.
- Running the pump continuously to keep the glycol from stagnating. But this is a waste of electricity for running the pump.
Which system for which part of the country
Solar Direct has a U.S. map which shows which kind of system they recommend for which part of the country.
Figures I couldn't find
- the advantage in efficiency for a "stereotypical" system (as listed above) vs. an indirect system
- the reduction in efficiency in a direct system due to sediment buildup. The closest I could get was this, which doesn't help much. I also found the "solscale" calculator, which I couldn't get to actually run.
To-Do List
Reminder to self: Next time I update this page, run through the math to show the savings.