the skinny on solar hot water

In 2004, the average UK household energy bill was £522. Last year, it was £1334.That’s a jump of over 150% in under a decade (1, 2).
If you’ve started your attack on these numbers, then you know that insulation and smart usage are the first steps. With these measures out of the way, next you can look into generating your own electricity, maybe with a wind turbine, maybe with a solar photovoltaic array.
But you may have skipped a step!
For the UK and similar latitudes, a properly-sized solar hot water system can supply around 75% of a family’s hot water over the course of a year. That’s hot water you don’t have to pay to generate, knocking a solid 15-20% off your annual energy usage (3).
Solar hot water (a.k.a. solar thermal) uses sunlight to heat your water. Some systems use no electricity at all. Because the technology is daylight-dependent, an installation is best used in conjunction with a pre-existing water heater. The idea is to lighten the heating load rather than to make a full-on conversion. Going entirely solar thermal can be done, but it would probably require a change in lifestyle.
As is the case with any renewable technology, you need to make careful cost-benefit analyses before you begin shopping. There currently aren’t many subsidy or FIT options out there for solar thermal, although there still is a £300 rebate on your installation. If your hot water usage is light — say, under £150/year — then you could be looking at a payback time of over 25 years, which isn’t desirable. In fact, it seems wrong, but a solar photovoltaic array might currently be more cost effective than a solar thermal installation, even though PV is a less-efficient technology and requires a higher upfront investment. I’ll explore this issue in a future article. Keep in mind that the renewables industry is rapidly growing and morphing, and this claim wouldn’t have been true three years ago.
There is a plan in the works to give solar thermal a generation tariff, though. In summer 2014, the RHI is slated to take effect, which will bring in 17.3 p/kWh — a higher rate than solar PV, and one that promises to make solar thermal much more competitive (15). Keep your fingers crossed, though, because it’s already been pushed back two times now.
That said, usage-heavy applications stand lots to gain even without the aid of FITs. If you’re looking into solar thermal on behalf of a college dormitory, restaurant, laundromat, etc., then your payback time could be under 3 years. If you’re looking to cover the four hot showers your family takes every evening, then your payback time could be desirable as well.
How expensive your system is depends not only on its size (i.e., the surface area of the panels), but also on the kind of system you choose. This, in turn, is largely predetermined by your site and your usage. For a technology so simple, there are quite a few variations to solar thermal, but by the end of the article, you should have an understanding of which one is right for you.

how it works

There are 2 main ways to go about it.

  1. You can route your plumbing through a solar panel so that the sun heats your potable water directly. This is called an open or open-loop system.
  2. You can run a freeze-resistant fluid from a solar panel to a heating coil within your water tank. This is called a closed loop system.

closed-loop solar system

Schematic of a closed-loop solar thermal system. Image from AcuaCare Solar.  Disclaimer: these are meant to be application examples, not connection diagrams.

An open- or closed-loop system can be either active or passive.

  1. Active systems use electric pumps to circulate the fluid.
  2. Passive systems are designed so that the fluid circulates on its own. Heated fluid expands, loses density, and rises, pulling cooler fluid from below to take its place. More details in the next section.

So, your system will be either closed- or open-loop, and either active or passive.
Mostly, solar thermal is simple and intuitive. No matter what kind of system you choose, you’ll have more hot water in the summer, less in the winter; more at the end of the day, less at the beginning. Unlike solar PV installations, shading is not a crippling issue, and cloudy days still make an appreciable amount of energy. Storage options exist, but aren’t common. This is why it’s a good idea to use solar thermal in conjunction with your normal water heater.
In order to understand the further variations, we need to review some properties of liquids.

some quick science

As the pros and cons of system varieties are discussed, you’ll find that all solar thermal systems have to come to terms with the fact that as temperature varies, pressure varies too. Taken to extremes, this can be very bad news for plumbing: a hard freeze can split pipes open and ruin your system. Boiling as well can be a threat in hot climates.
Note, however, that solar thermal wouldn’t be able to work without this law. As the sun heats a fluid within a solar panel, the fluid loses density and slowly rises out of the panel, pulling cold fluid from below. An active system uses a pump to move the fluid from here, but passive systems are designed so that the heated fluid continues to rise as it enters the storage tank, which must be above the solar panel. So you can see that the laws of temperature and pressure make solar thermal possible as much as they threaten an installation’s health!

So how do you get around freezing?

The main reason to use a closed-loop system is that you can use a freeze-resistant fluid, the most common of which is food-grade propylene glycol. The drawback is maintenace: glycol becomes sluggish over time and will probably need to be replaced every 5 years (6).
Another way to avoid freezing trouble is to use a drainback system. This is a kind of closed-loop system in which the transfer fluid (usually treated water) has a reservoir into which it can retreat when temperatures reach freezing levels. There’s usually a sensor that triggers the drain automatically, and you’ll probably need a pump. It should be noted that using water as a transfer fluid is not without its maintenance issues, i.e., potential legionella buildup. You might look into treating your water before installation, though this problem can also be avoided by maintaining the system at a specific minimum temperature (4).

collector types

Now let’s explore the business end of the system. A solar thermal panel is called, appropriately, a collector (as opposed to a PV panel, which is called a module).The most common solar thermal setups follow one of these two designs:

  1. A flat plate collector resembles a traditional solar panel. It is an insulated case with piping inside. The sun heats an absorber plate on the surface of the casing; the absorber plate transfers the heat to the pipes that hold the transfer fluid (closed-loop) or household water (open-loop).
  2. Evacuated tube collectors run the pipes through tubes that are insulated internally by a thin layer of vacuum space. As long as the vacuum stays pressurized, it is an excellent insulator. Evacuated tubes are capable of reaching very high temperatures. See the photo for an example:

A roof-mounted evacuated tube collector. Credit: Alfred Twu

These two varieties don’t account for all solar thermal installations, though. In warm climates, it is possible to have the sun heat your water storage tank directly. This is called solar batch water heating. Batch heaters can be as simple as a cylindrical tank mounted to the roof of a house, painted black for better heat absorption… an age old technology for sure! Note that batch heaters qualify as open-loop systems, because the sun heats the potable water directly.
Add a collector to a storage tank, and you have a form of batch heating called integrated collector storage, or ICS. Some ICS units are merely flat plate collectors with oversized tubes inside, meaning the collector itself can hold enough hot water for your shower. The advantage of batch systems is that you won’t need to install a separate cylinder*, as the collector also acts as the storage tank. You’ll need to make sure your roof can take the weight of a full batch heater, though.
Unfortunately, batch systems won’t work in climates with freezing winters, and it becomes necessary to use a flat plate or evacuated tube collector with the storage tank located indoors. This is currently the typical setup.
*When it comes to integrating solar thermal with your pre-existing system, you’re in luck if you have a twin coil cylinder (storage tank). This means that the cylinder can accept two heating coils, one from your conventional heater and one from a solar thermal application. If 1) your cylinder is not twin coil and 2) you want a closed-loop system, then you may have to install a separate storage tank dedicated to your solar thermal system. “Preheated” water would leave this tank and then enter your pre-existing heating cylinder.
Still, some types of storage tank can’t accept pre-heated water at all, i.e. a combi_boiler or an electric water heater. If this is the case, then you could look into solar PV to power your existing setup, or you could replace your cylinder with a twin coil (4).

pros, cons, & tips

As you can see, there are many options, and it quickly gets complicated! To narrow things down, I suggest you answer these questions in order:

  1. a) Do I use enough hot water to justify an installation? b) Have I insulated my current cylinder?
  2. Is my existing water heater compatible with solar thermal?
  3. Do the winters here experience hard freezes?
  4. Do I have hard_water?

Answering those questions can narrow things down for you. For the remaining questions, below is an amalgam of unorganized tips. I’m sure there’s information missing, so if you’ve got a question about your specific system (or system-to-be), please leave a comment.

+ Open-loop systems are best suited for non-freezing climates with soft water. Hard water (see above) makes buildup along the pipes! Normal open loop systems can withstand some light freezes, especially ICS units because there’s enough mass to keep the water warm overnight (4). Open-loop drainback systems can withstand cold climates.

+ If you decide to go with a passive system, your collector will need to be mounted below the storage tank, usually by at least 2 feet (5). You can mount the storage tank above the panels on the roof (if your beams can support the weight), or you can mount your collector on the ground (if there’s enough sun). Both options are used.

+ Remember that the simpler your system is, the fewer the headaches. Short pipes are good, too.

+ How well-insulated is your water tank? Wrapping it in a thermal blanket might save you a chunk of change. Insulated piping saves some heat loss as well, but might not be worth the effort if you’re not installing a system from scratch.

+ When installing solar thermal, you don’t need to be as stringent about sun exposure as you do with solar PV. Just keep it facing southeast to southwest.

+ In case you’re worried about the small amount of electricity required to run the pumps in an active system, know that it’s common to install a small PV panel dedicated to powering the pump(s). You won’t need an inverter either, as DC-powered pumps are available (5).

+ Finally: are you handy? Then consider building a flat plate system yourself! Just be confident in your ability to keep the system sealed so everything will siphon well, especially if you’re designing a passive system.


  4. Solar hot water buyer’s guide. Energy Saving Trust, February 2012.
  5. Chiras, Dan. The Homeowner’s Guide to Renewable Energy: Achieving Energy Independence Through Solar, Wind, Biomass, and Hydropower. Edition 2, revised. New Society Publishers, 2011. Accessed on google books.
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